CN1761857A - Method and apparatus for generation and transmission of high energy optical pulses for long range measurements - Google Patents

Abstract

A method and apparatus for generating and transmitting high energy optical pulses are described. Distributed temperature sensors usually use Raman scattering in optical fibres as the means to determine the temperature. Here, light from a laser source is sent down a fibre and the small amount of light that is scattered back towards the source is analysed. As the fibre length increases, the resolutions of the temperature and loss measurements become poorer. This is because losses in an optical fibre attenuate the signal. An obvious solution to this problem is to launch more light into the fibre to compensate for the losses but stimulated Raman scattering limits how much light may be launched. The present invention solves this problem by using a pulse conversion method to maximise the resultant pulse energy while the power is kept below SRS threshold.

Description

Method and apparatus for generating and transmitting high-energy optical pulses for long-distance measurements

technical field

The present invention relates to a sensing system, relates to a method and device for generating and transmitting high-energy light pulses for sensing, especially for long-distance measurement, more specifically, for measured objects, such as temperature and long-distance Distributed optical fiber measurement of upper loss, and also relates to a transmitter for the system, a receiver for the system, and a pulse generating device and method for the system.

Background technique

There is a need in industry to measure the temperature at all points over long distances. Typical applications are monitoring long fiber optic cables and pipes. Because these structures can be very long, sensing systems that can work over very long distances are required. Loss measurements along optical fibers are required in communications and sensing. Because these fibers are very long, a loss measurement system that operates over very long distances is required.

Distributed temperature sensors typically use Raman scattering in optical fibers as a means of determining temperature. Note here that light from a laser source is sent down an optical fiber, and the small amount of light that is scattered back to the source is analyzed. By using pulsed light and measuring the return signal as a function of time, the backscattered light produced at all points along the fiber can be determined. The backscattered light contains components that are shifted up and down in frequency from the source (Raman's and Brillouin's inverse Stokes light and Stokes light, respectively), and elastically scattered light (Rayleigh light). The power of the returned Raman and Brillouin signals is related to temperature, so the temperature can be obtained from the analysis of these components. Usually the Raman Stokes and anti-Stokes signals are used to determine the temperature, but sometimes the Rayleigh light is used as a reference, and sometimes the Brillouin component is also used. Rayleigh light, and sometimes Raman Stokes light, are often used to measure loss along optical fibers.

An example is known from US Patent 5,194,847 which deals with fiber optic intrusion sensing. In this example, to enable sensing to encroach on a predetermined perimeter, coherent pulsed light is injected into a light-sensing optical fiber positioned along the predetermined perimeter. In response to receiving the coherent light pulse, backscattered light is generated and coupled into a light-receiving fiber. The backscattered light is detected by a photodetector, and intrusion can be detected by a change in the backscattered light. To increase the sensitivity of the device, reference fibers and interferometers can also be used.

As the fiber length increases, the resolution of temperature and loss measurements becomes poorer. This is because there are losses in the fiber that cause signal attenuation. As the length of the fiber increases, the overall loss over the length of the fiber also increases, so the return signal from the far end is smaller, resulting in more noise. An obvious solution to this problem is to inject more light into the fiber to compensate for the loss, but there is a limit to how much light can be injected. This is because, when sending high-power light along an optical fiber, nonlinear effects become more pronounced as the length of the fiber increases. The most problematic of these nonlinear effects is the stimulated Raman effect. This stimulated Raman effect absorbs power from the light of the source and frequency shifts the light towards the wavelength of the Raman Stokes signal. It is generally this stimulated Raman scattering (SRS) that limits the length over which this type of distributed sensor can operate. It is known that in an attempt to alleviate this problem, "Optical Time Domain Reflectometry", Hartog, Arthur, Harold, and WO 1998GB 0000028 of January 8, 1997, proposes a fiber that causes the SRS threshold to occur at higher input powers, so that the Much power can be used in such fibers. However, this solution is limited because it requires the technology to be used in specialized optical fibers, which can be expensive. This solution also does not allow the system to be used on already installed common fiber optics.

Another reason why temperature and loss resolution becomes worse with fiber length is the longer round-trip delay per pulse. Typically, only one pulse is useful in the fiber at any one time, otherwise it is impossible to determine where the return signal originated (the backscattered signals from multiple pulses will overlap). The time for optical pulses to travel along the fiber is proportional to the length of the fiber, so as the length of the fiber increases, the time between pulses that can be injected must increase. Since many averaging are usually required to measure a signal with reasonable accuracy, the necessity to reduce the pulse repetition rate means that the measurement becomes less accurate as the sensing length increases. However, the SRS threshold cannot be increased indefinitely, and the pulse repetition rate is still limited by the round-trip time of the pulse through the full length of the fiber.

Another source of error is differential loss at different wavelengths. This is a property of the most common types of fiber and is a source of error since the calculation of temperature involves determining the ratio of the returned optical power at different wavelengths. The Raman Stokes and anti-Stokes components are frequency shifted to different wavelengths and thus experience different amounts of loss. This source of error becomes more pronounced as the distance along the fiber to which the point needs to be measured increases. In some cases, it may be necessary to reduce this error by choosing the injection wavelength to minimize the differential loss for a given fiber. It is also possible to use two sources, or a single tunable source, emitting at two different wavelengths, with measurements made at the Stokes wavelength from one source and the anti-Stokes wavelength from the other to cancel out differential losses. This method is described in US Patent 4,767,219. This approach can cancel differential loss errors, but it will be difficult or expensive to ensure that the two sources have sufficiently identical operating lifetimes without introducing other errors. A single tunable light source is less practical because it is difficult to tune and pulse the light source.

Contents of the invention

It is an object of the invention to provide improved apparatus and methods. According to a first aspect of the present invention, there is provided:

A sensing system for sending an electromagnetic signal along a waveguide to sense a state at one or more sensing locations along the waveguide, the system comprising transmitter means for emitting a pulse of suitably shaped The process of stimulated inelastic scattering before the position causes a controlled transformation of this pulse into different wavelengths. The system includes receiving means to receive the signal returned from the sensed position and, based on the received signal, to determine these states .

The present system makes clever use of non-linear effects which have been avoided or minimized to date. After a short transition length, all of the original light is converted to this new wavelength, which then illuminates the remaining length of the fiber. One advantage is that the present system can increase this distance limit for a given sensing accuracy or resolution, because the injected power can be much higher, thus enabling the detection of returning optical signals from greater distances along the fiber. Another advantage is that the present system provides a self-optimizing process, since as distance increases, the optical frequency shifts to longer wavelengths, and longer wavelengths result in lower fiber scattering losses. The electromagnetic signal may be an optical signal or other wavelength. The shape refers to, for example, the shape of a power versus time curve. The peak power is part of the shape, which has a strong influence on where the SRS wavelength conversion takes place, but the pulse width and the characteristics of the transmission medium also play a part.

As an additional feature, the injection power can be configured so that different portions of the fiber are illuminated by more than one different wavelength, and the receiving means can be configured so that it distinguishes the returning optical signal at each different wavelength. An advantage of configuring this receiver arrangement is that it enables more than one pulse to propagate along the fiber at the same time. These pulses can be timed so that when the first pulse has been switched to a different wavelength, subsequent pulses are sent. Now, with appropriate filtering, it is possible at the receiver to distinguish between the backscatter from the subsequent pulse and the backscatter needed at a different wavelength from the first pulse. For a given accuracy and spatial resolution, this discrimination capability can increase the repetition rate of pulses, further improving the accuracy of sensing by enabling more averaging or allowing faster sensing.

As yet an additional feature, the launch means adjusts the injected power to move the transition length where the transition takes place to a different portion of the fiber. This is useful to avoid gaps in the sensing footprint due to transition lengths.

As yet another additional feature, the receiving means is further configured to receive signals returned from one or more additional sensing locations prior to transformation. In this way, higher power pulses before conversion can be used for shorter distance sensing, and wavelength converted pulses can be used for longer distance sensing.

As yet another additional feature, the sensing system is configured to compensate for different fiber losses. As yet an additional feature, the transmitting device includes a source for generating the pulses, a splitter for splitting the pulses into two or more streams of pulses, and a converter for converting the wavelength of one of the streams of pulses before sending the two streams along the optical fiber to achieve differential fiber loss compensation.

As a further additional feature, the receiving means is configured to detect an upwardly frequency-shifted component returning from one of the pulses at the first wavelength, and to detect a downwardly frequency-shifted component returning from one of the pulses transmitted at the second wavelength to the first wavelength The downward frequency shift component of , compensates for the differential loss. This shows that the two components are already experiencing the same loss regardless of the amount of differential loss, so the ratio of the two components will cancel out any differential loss.

As a further additional feature, the system has a controller for controlling the repetition rate of the pulses. The controller is useful for maximizing the repetition rate, eg, to the limit of one pulse per segment between transition regions at any time. The controller helps to take advantage of the possibility of higher repetition rates if the power is changed to cause more wavelength conversion along the fiber.

In a second aspect of the present invention, there is provided a transmitting device as part of a sensing system for transmitting and receiving optical signals along an optical fiber in order to sense conditions along the optical fiber, the transmitting device has a source for generating a pulse stream, a A splitter that splits a pulse into two or more pulse streams, and a converter that converts the wavelength of one of the pulse streams before sending the two pulse streams down an optical fiber.

The advantage of creating two pulse streams from a single source is that it does not require precise control of the wavelength spacing of the two lasers, allowing precise correction for differential fiber losses.

Each pulse stream may be of sufficient power so that the signal wavelength can be converted by the SRS and illuminate the sensing fiber at at least two different wavelengths to perform measurement and correction for differential fiber loss, wherein the at least two different wavelengths are determined by the Other wavelengths are frequency shifted apart from the Stokes wavelength.

As an additional feature, the source and resolver are adapted to provide sufficient power to the pulses reaching the transducer to effect transformation by stimulated inelastic scattering.

As an additional feature, the transmitting device further includes a pulse shaper for lengthening the pulses prior to transmission along the optical fiber.

As an additional feature, the pulse shaper includes a dispersive unit. This is one way to achieve pulse stretching so that peak power and pulse width can be controlled while maintaining pulse energy. Longer pulse widths reduce spatial resolution, but higher pulse energies can improve detection and sensing accuracy, or eg improve distance for a given accuracy. The dispersive element is optionally incorporated into the converter in order to keep the number of elements to a minimum.

As an additional feature, the pulse shaper includes a burst generator for converting each pulse into a burst, and a dispersion unit for shaping the burst. An advantage of this feature is that, for a given amount of dispersion, the pulse can be spread wider in time. This produces pulses that spread more flatly and reduces the expense, delay, and loss incurred when large amounts of dispersion are required. For a given power level, broadly extended pulses are useful to give more energy and thus greater distance.

According to a third aspect of the present invention, there is provided a device for initiating the generation of one or more stimulated Raman lights, so that different parts of the fiber are irradiated with different wavelengths, so that high input powers can be used and the There is more than one pulse; the device is also used to measure the power of different components of backscattered light in order to achieve distributed measurement of temperature and/or loss over long distances, the device includes: at least one optical fiber providing a scattering medium, A light source, optical filtering means, and means for converting an optical signal to an electrical signal, which may include a single detector or an array of detectors.

This aspect provides a sensing system for sending and receiving optical signals along the optical fiber in order to sense the state along the optical fiber. The system has a transmitting device and a receiving device. The transmitting device is used to activate one or more stimulated The generation of Raman light enables different parts of the fiber to be irradiated with different wavelengths, and is also used to simultaneously transmit more than one pulse into the fiber; the receiving device is used to measure the power of different components of the backscattered light, so that in the long distance Sensing is performed on an optical fiber, and the receiving device has an optical filtering device and one or more detectors for converting optical signals into electrical signals.

As an additional feature of the dependent claims, the receiving arrangement is adapted to distinguish Raman Stokes or Raman anti-Stokes or Rayleigh wavelengths scattered back from a forward propagating pulse. These components can be used to correct for losses in the fiber, and to calculate the temperature along the sensing fiber.

As yet an additional feature, the receiving means has a filter configured to collect the desired backscattered signal at a wavelength slightly offset from the transmitted pulse wavelength prior to wavelength conversion. This filter helps to avoid crosstalk between the anti-Stokes wavelength, which is returned from the pulse after the transformation, and the Rayleigh scatter, which is at the same wavelength, from any remaining portion of the start pulse that is still propagating Returned before the transformation.

As yet an additional feature, the measurements are repeated at different pulse powers and the measurements are used to correct for non-linearities.

As yet an additional feature, the system includes optical fibers.

As yet an additional feature, the system allows the power of the pulses to be controlled to vary the position along the fiber at which the wavelength conversion occurs along the transition length.

As yet an additional feature, the transmitting device includes a laser, a modulator for generating pulses from the output of the laser, and an optical amplifier for amplifying the pulses. These devices help make pulse shaping more controllable.

As yet another additional feature, the receiver has filters for collecting Raman Stokes, and/or Raman anti-Stokes, and/or Rayleigh signals backscattered from one section of fiber that do not correlate with those backscattered from other sections. Signal components in which equivalent components overlap significantly.

As yet an additional feature, the receiving means is operable to compare one, both, or all of the returned Raman Stokes, Raman inverse Stokes, and Rayleigh signal components to correct for any non-linearity in the signal.

As yet an additional feature, the receiving device compensates for non-linearity using a combination of returned Raman Stokes and Raman inverse Stokes components that are in a saturated, non-linear region relative to the sensed state.

In a fourth aspect of the invention, the pulse transformation method is used to optimize the final pulse energy while keeping the power below the SRS threshold. This pulse conversion method is useful for systems utilizing SRS wavelength conversion as described above. According to this aspect of the invention, there is provided a pulse shaper for shaping an optical input pulse, the shaper having a pulse train generator for generating a pulse train from an input optical pulse, and a dispersive unit located either before or after the generator, the generator The device includes a device for decomposing input pulses and a combiner, which decomposes the input optical pulses between two or more paths with different delays to generate delayed pulses; the combiner combines the delayed pulses , to generate a pulse train; and the dispersion unit expands the input pulse or the pulse train respectively.

This enables the light pulse to modify its shape (for example in terms of power curves over time or distance) for various purposes. This is often easier to implement than trying to control the light source to produce the desired shape. Especially where the pulses must be high power and have a tightly controlled wavelength. One advantage of generating pulse trains is that for a given amount of dispersion, the input pulses can be spread wider in time or space. This produces pulses that spread more flatly and reduces the expense, delay, and loss incurred when large amounts of dispersion are required. For a given power level, broadly extended pulses are useful to give more energy and thus greater distance.

As an additional feature, the shaper has one or more optical switches for selecting which of the delayed pulses forms the burst. This is a way of controlling the burst formation in order to control the output shape, eg the length of the burst.

The SRS limit depends on pulse power rather than energy, and the returned signal depends on pulse energy rather than power. In order to extend the sensing length without using SRS wavelength conversion, it is necessary to maximize the pulse energy while keeping the pulse power below the SRS threshold. This requirement can be achieved by using long, low power pulses. The factor governing the maximum useful length of a pulse is the spatial resolution of the system. The pulse width must not exceed twice the required spatial resolution. Therefore, to maximize the return signal for a given SRS limit, the pulse width should be equal to twice the required spatial resolution, and the pulse power must not exceed the SRS threshold.

On the other hand, the SRS power threshold increased with decreasing pulse duration. The reason is that SRS requires the input light to co-propagate with Raman Stokes light. The different wavelengths of these components indicate that the components travel at different speeds in the glass due to the dispersion of the glass in the fiber. If the input pulse is short, then the pulses will "spread out" over a distance and no longer overlap. Then the stimulated Raman process stops growing.

Although, for short pulses, the SRS power threshold can be higher, but the pulse energy is not optimal when the pulse width is significantly shorter than the spatial resolution. This fourth aspect is able to overcome this limitation by using a long pulse train comprising a series of short pulses. The advantage of this fourth aspect is that there is a long pulse train width, and therefore high energy, and that there are short component pulses which allow discrete occurrences.

According to the fourth aspect of the present invention, a device for generating and transmitting high-energy light pulses is provided to maximize the energy of the excited light while suppressing the stimulated Raman light, and to measure the power of different components of the backscattered light in order to be able to To achieve distributed measurement of temperature and/or loss over long distances, the device includes at least one optical fiber providing a scattering medium; a light source; a pulse selection device or conversion device that converts a single high-power pulse into a pulse train through a modulation device; optical filtering means; and one or more detectors that convert optical signals into electrical signals.

The use of pulse trains in this system has advantages in extending the sensing distance because it can reduce the SRS effect, which will be explained below. Trains of short pulses can be generated by directly or externally modulating the light from a laser source. The pulse train can then be amplified by an optical amplifier so that the final pulse power is optimized for a given measurement length. Alternatively, a mode-locked laser with a pulse picker can be used to select trains of fast pulses.

However, high-power, short-pulse lasers using Q-switched technology are more readily available. These lasers typically produce very high peak power, capable of generating SRS over very short distances. In the prior art, the SRS is suppressed by significantly attenuating the power of the laser. This suggests that a large amount of available pulse energy is not being utilized, reducing the distance that can be efficiently reached.

Where a single high power pulse is transformed into a train of low power pulses, each of which will not exceed the SRS threshold, although the power may be reduced, the pulse energy is the same and light wasted is minimal. The length of the burst should be approximately twice the required spatial resolution. For short input pulses, SRS can be further suppressed by allowing gaps between component pulses in the pulse train. This approach suppresses SRS by allowing the scatter to occur and by allowing the Raman scattered light to decay significantly before encountering the next pulse in the train.

As an additional feature of the dependent claims, the fiber comprises different lengths of fiber for providing different Raman frequency shifts along the length, or for changing the modal properties of the propagating light. This can differentiate the backscatter from different lengths at the receiving device, or reduce the amount of interference. In this way, more than one pulse can be introduced into the fiber at a time, increasing the pulse repetition rate, and thus increasing the accuracy for a given measurement time.

As another such additional feature, the input optical pulses are transformed so that the duration of the resulting pulse train matches the required spatial resolution.

As another such additional feature, the pulse transformation means includes one or more sets of decomposing and recombining means, each of which sends the input optical pulse along two or more paths of different optical lengths and divides the light combined into pulse trains. This is one way of generating pulse trains, and in some cases it is more practical than generating individual pulses at the source.

As another such additional feature, the pulse transformation device includes an optical fiber and fiber optic components. Such components can be robust and relatively inexpensive.

As another such additional feature, the burst duration and burst pattern resulting from the pulse transformation may be fixed or controlled by the use of optical switches which select or eliminate the number of pulse transformation devices used, and and/or the number of paths used in the Pulse Transformer.

As another such additional feature, when deriving temperature and/or loss data or other sensed conditions, the burst width and/or mode and/or energy can be varied to help correct for non-linearities.

As another such additional feature, when calculating temperature and/or loss, one, both, or both of the returned Raman Stokes, Raman inverse Stokes, and Rayleigh signal components can be compared to correct for any nonlinearity in the signal .

According to the fifth aspect of the present invention, a sensing system for sending and receiving optical signals along a waveguide is provided for sensing the state along the waveguide, the system has a transmitting device for sending pulses along the waveguide, a receiving device, and a waveguide, The waveguide includes a number of segments with different properties, causing the backscatter of the pulse to have different optical properties, and the receiving device uses these different optical properties to distinguish the backscatter of the different segments.

An advantage of this system is that it enables different segments of backscatter to be differentiated at the receiving device, or reduces the amount of interference between different segments of backscatter. This enables more precise sensing, or the ability to introduce more than one pulse into the fiber at a time, increasing the pulse repetition rate, and thus increasing the accuracy for a given measurement time.

As an additional feature of the dependent claims, the segments have waveguides providing different Raman frequency shifts for providing backscattered components at different wavelengths. These wavelength differences can be distinguished relatively easily on the receiving device without the need for expensive additional components.

As another such additional feature, the segments have waveguides that cause different changes in the modal properties of the propagating light, providing backscattered components with different modal properties. In addition or in combination with other means, this is another way to achieve the same advantages.

Typically, existing sensing systems discard 90% of the energy of the laser pulse output in order to stay within the SRS limits. By providing wavelength discriminatory filtering at the receiving device, it is possible to allow power in certain lengths of the waveguide to exceed the SRS limit, while reducing the interference that would normally be caused. For example, to make measurements over longer distances, the power can be set so that the initial 10km is non-linear, while the readout is performed over the next 1-5km.

As an additional feature, timing gates are provided to differentiate the backscatter returning from the desired position along the waveguide. At the receiver, the advantage of timing gates combined with wavelength discrimination is the ability to use higher repetition rates and improve signal averaging.

Other aspects include using a reference segment of known temperature to receive the scatter component and to calibrate the amplitude of the scatter component.

Still other aspects include using a switch to send pulses to at least two sensing fibers and or to both ends of the sensing fibers. Means of calibrating the modal dependence of the transmission wavelength and switching, eg using at least one reference segment of known temperature, can also be used to improve the accuracy of temperature measurements between different sensing fibers.

Other aspects include corresponding methods of sensing using such devices, methods of providing sensing services, methods of manufacturing sensing systems, and methods of generating sensing signals.

Other advantages of the present invention will be apparent to those skilled in the art, especially in connection with other prior art not familiar to the inventors. It will naturally be apparent to those skilled in the art that any additional features of the invention may be combined together and combined with any aspect of the invention.

Description of drawings

Below with reference to accompanying drawing, by way of example, illustrate embodiment of the present invention and how to implement these embodiments, accompanying drawing has:

Figures 1 and 2 illustrate an embodiment of the present invention involving SRS wavelength conversion,

Fig. 3 and 4 draw the embodiment of the present invention, show the disposition of launching device,

Figure 5 depicts a possible configuration of a pulsed laser,

Fig. 6 shows a view of a receiving device according to one embodiment, for use by the system of Fig. 1 or 2 or 3 or other embodiments,

Fig. 7 shows a schematic diagram representing the working principle of the receiving device according to an embodiment of the present invention,

Figures 8, 9, and 10 illustrate an embodiment of the present invention in which a pulse is converted into a pulse train using a transform before the pulse train is sent along the optical fiber, and

Figures 11 and 12 schematically show how a pulse is shaped, or transformed into a pulse train.

Detailed ways

figure 1

Figure 1 depicts an embodiment of the present invention. Transmitting device 22 is coupled to sensing fiber 30 and sends a pulse of _λ1 along the fiber. The pulses are of sufficient power to establish SRS along the fiber and cause a wavelength shift over the lambda transition length 34. The backscatter at all points of the sensing fiber is separated from the forward traveling light and fed to receiving device 20 using conventional techniques. This backscatter has Raman Stokes, Raman anti-Stokes, and Rayleigh components derived from the two forward-going wavelengths _λ1 and _λ2 .

The primary purpose of the receiving device is to output sensed or state values for a sensing location 36 at a given distance along the sensing fiber, these values being derived from the backscattered signal. This step requires distinguishing the useful part of the backscattered light and calculating the sensed value. In the example shown, the optical receiver 24 has an optical filter for separating the _λ2 backscatter from other unwanted backscatter.

The electrical output is output from the receiver to timing gate 26 . The backscatter from each point of the sensing fiber returns to the receiver after a different amount of time. The timing of the start and end of the timing gates can be finely tuned to the injection timing of the corresponding pulses so that only the backscatter from the location to be sensed is passed. As shown, and described in more detail below, a processor 28 is used to compute sensed values from the filtered and selected backscatter components. Typically, this step involves averaging many pulses to reduce noise effects. Many variations on this configuration principle are conceivable. For example, if there is only one pulse in the fiber at any one time, then the optical receiver can be used without a filter. With a filter, subsequent pulses can be injected very quickly because there can be a pulse before the transition length and a pulse after it, and the filter can separate the backscattered component from each pulse.

figure 2

Figure 2 shows a schematic diagram of another embodiment of the invention in which a pulsed laser is used and only accessed at one end of the fiber. The diagram shows how the SRS illuminates different parts of the fiber with different wavelengths of light. A pulsed laser light source (1) whose power, pulse repetition rate, and pulse duration can be controlled is connected to a decomposition device (3) such as an optical coupler. Optical amplifiers and/or attenuation units (2) can be used to control the amount of optical power entering the system. This optical power can be monitored with a photodetector (4) in order to monitor the incoming power. The photodetector (4) can have variable gain. The light from the decomposing device (3) is fed into a length of optical fiber (5), which may be single-mode or multi-mode, polarization-maintaining, and may include lengths of different fiber connections. Initially, the wavelength of the forward propagating light in the optical fiber (5) is the same as that of the light source, namely λ ₁ (10). At some distance in the fiber, the SRS is activated and at some transition distance (11) the forward propagating light is shifted to a new wavelength _λ2 . The forward propagating light then continues to propagate through the fiber at wavelength _λ2 (12).

At some further point, the SRS can be activated again over some transition distance (13), producing forward propagating light (14) of a new wavelength. The backscattered light produced by all forward propagating components in the fiber (5) returns to the splitting device (3) and is then sent to the attenuator and/or amplifier (6). This light is then sent to a filter unit (7), which is a device that selects one of many filters to put into the light path. The light selected by the filtering unit is detected by a photodetector (8), which may have a variable gain. The processing unit (9) analyzes the signal from the signal photodetector (8) and controls the filter selected by the filtering unit (7). The processing unit can also analyze the signal from the monitoring photodetector (4), and/or control the gain of the monitoring photodetector, and/or control the gain of the signal photodetector (8), and/or control the laser (1) power, and/or repetition rate, and/or pulse width, and/or control the amplification and/or attenuation of one and/or both of the amplification and/or attenuation units (2 and 6).

In use, the laser sends pulses into the fiber to generate SRS in at least one location in the fiber. For each wavelength selected by the filter unit, the return signal is collected and recorded as a function of time. Each selected wavelength corresponds to a signal of interest in the backscatter spectrum, and by analyzing these signals, temperature and/or loss are determined at all points along the spectrum.

Using nonlinear effects in the fiber, it is possible to optimize the measurement of weak signals generated throughout the fiber. As the input optical power increases, at some point in the fiber, after a certain interaction length, the SRS shifts the input light to a longer wavelength. This process can be repeated again along the fiber, so that different parts of the fiber are illuminated with different wavelengths. The transition point along the fiber can be adjusted by changing the input power.

The pulsed light of the laser is sent by the decomposition device and enters the optical fiber. This light travels down the fiber, and as it travels, it produces Raman Stokes and anti-Stokes, and Rayleigh backscattered light. The backscattered light is separated into its spectral components, for example using a filter, a wavelength division multiplexing coupler, or a combination of a coupler and filter, which are then detected and analyzed. Analysis of the data can lead to temperature distribution and/or loss distribution or other conditions along the fiber.

Over a certain distance in the fiber, the Raman process becomes nonlinear and the Stokes light propagating forward becomes excited. It should be noted here that after a short transition length, the optical power of the source is transferred to the Stokes wavelength, which then illuminates the remaining length of the fiber. The device then processes this new wavelength as it does the light from the source, and collects and analyzes the Raman Stokes and anti-Stokes, and Rayleigh wavelengths of this new second generation light. The new forward-propagating Stokes signal, itself, can become stimulated, and so the process repeats further down the fiber. There, the new wavelength is treated as a light source, and the Raman Stokes and anti-Stokes, and Rayleigh wavelengths for that new wavelength are collected and analyzed. This process can be repeated many times along the length of the fiber.

In this way, the fiber is in effect split such that each split has a different source wavelength. Therefore, only one pulse in each division (as opposed to one pulse in the entire fiber) is required to avoid or reduce interference. Ideally, data is collected independently from each segment, but it should be noted that this is only necessary if there is one pulse in the segment currently under investigation. Also, because the system is deliberately pushed beyond the SRS limit, the returned signal has higher power than if the SRS limit were not exceeded.

When the light frequency shifts from the original wavelength to the new Stokes wavelength (which now becomes the new wave source), the original wavelength now coincides with the new anti-Stokes wavelength. This causes new anti-Stokes wavelengths to interfere with any Rayleigh scattering of the original source light that is still propagating. This interference can be eliminated in the receiver by selecting an appropriate filter which, in this case, collects the new anti-Stokes signal at a slightly different wavelength than the old Rayleigh signal. This solution works because while the stimulated Stokes light becomes very narrow in frequency spectrum, the spontaneous emission driven at much lower power becomes very broad in frequency spectrum. If necessary, light need only be collected in certain parts of the spectrum, so a part can be chosen that does not overlap with another component of the backscattered signal.

The power of the backscattered component will depend on the temperature at the location where the backscattered component is generated, the degree of attenuation of the light from the source, and any non-linear growth of the signal itself. Algorithms can be used to combine Raman Stokes and Raman anti-Stokes, and Rayleigh signals to deconvolute these effects in order to determine temperature and/or loss data. It is useful to analyze data collected for different input powers during deconvolution of temperature and/or loss data. In regions where the Stokes signal becomes stimulated (these regions are usually relatively short), it may be difficult to use these algorithms reliably. To overcome this difficulty, these regions can be moved along the fiber by changing the input power. By making many measurements at different input powers, the system can thereby correct for non-linearities and thereby reliably obtain temperature and/or loss data.

Optical fibers can be single-mode, multi-mode, or a mixture of single-mode and multi-mode for different wavelengths, and can have specially specified doping levels, or different doping layers, to optimize interaction.

To help correct for non-linearities, the source power can be varied when deriving temperature data and changing the location or locations in the fiber where the Stokes signal becomes excited. The filtering action may be chosen so as to collect signal components from the Stokes and/or anti-Stokes and/or Rayleigh signals of each section of fiber that do not substantially overlap with equivalent components produced by other sections.

One, both, or both of the returned Raman Stokes, Raman anti-Stokes, and Rayleigh signals can be compared to correct for any non-linearities in the signals, and to correct for differences between the signal from one segment and the signal from the other segment any overlap and calculate temperature and/or losses.

image 3

FIG. 3 shows another embodiment in schematic form, showing an example of a transmitting device used in the embodiment of FIG. 1 or 2. FIG. It should be noted that in order to transmit pulses at two or more wavelengths, it is necessary to correct for differential losses in the fiber in the receiving device. Differential loss represents the different losses along the fiber for different wavelengths, and it is significant in the case of long distances, eg involving several kilometers of fiber, and where sensing depends on backscattering at more than one wavelength. This is the case, for example, for sensing temperature, where the temperature value is computed from the backscattered components of a single forward-going wavelength comprising any two or more of Raman Stokes, Raman anti-Stokes, and Rayleigh backscattering component.

To correct for such differential losses, pulses of two or more wavelengths are used. If the Raman Stokes backscattering component of the longer wavelength pulse is compared with the Raman anti-Stokes backscattering component of the shorter wavelength pulse, then the respective loss components can be canceled out. Therefore, the error caused by the differential loss can be eliminated. To generate pulses at two or more wavelengths, two separate light sources can be used, such as two-wavelength microchip lasers, but the example drawn in Figure 3 shows how a single laser can be used to generate pulses at two wavelengths. The laser 110 generates pulses under the control of the pulse timing control section 100 . The timing control part can be implemented with a conventional electronic circuit, such as a microprocessor. Generated pulses of relatively high power, eg 10 kW peak power, <1 nsec long, and 10 μJ of energy in this region, are used to initiate SRS transitions. The pulse is broken down by a splitter 120 into two pulse streams. 90% of the energy is passed to the first pulse flow and 10% to the second pulse flow. This is done to overcome the greater loss of the first pulse stream caused by the converter and shaper 130 .

The converter and shaper are used to convert the pulse wavelength of the first pulse stream from λ ₁ to λ ₂ . As described in other embodiments, these wavelengths may be 1054 nm and 1115 nm, respectively. The converter may be implemented as a length of fiber, eg 500m, or any length sufficient to allow SRS to occur, which length will depend, among other factors, on power and fiber properties. Pulse shaping is to stretch the pulse, making it longer and keeping more energy in the pulse at lower peak power. Such pulse shaping can be implemented with dispersive elements that provide modal dispersion, in other words, different delays for different propagating modes. Dispersion elements can be incorporated into the properties of the same fiber or, following established practice, by setting elements such as step-index multimode fibers. The filter 140 is optionally placed before or after the two pulse streams are combined by a combiner, which may be in the form of a switch 150, or another type of combiner. Filters are useful for selecting scattered wavelengths and removing other unwanted components.

The output of this combiner is a stream of pulses at two different wavelengths. These pulses should have the same power and other characteristics. The switch can control the pulse sequence in various ways, each wavelength should have an equal number of pulses, and the receiving device 120 should alarm the sequence and time the pulses by, for example, the timing control section 100 shown. Other approaches are conceivable. Some parts are not shown or can be selected. For example, not shown are attenuators that control the power level of the pulses. The attenuator can be installed anywhere in the light path and is controlled eg by a microprocessor. The advantage of a single light source is that the timing and power of the two or more wavelength pulses are inherently linked. There is no need to match or correct for long-term and short-term variations in laser characteristics, resulting in a simpler and less cost-effective setup, or higher quality output.

Splits off a fraction of the output pulses for timing triggering. This small fraction of the output pulses, as shown, can be fed to a receiving device to synchronize timing gates in the receiving device. Alternatively, this small fraction of the output pulses can be tapped elsewhere, or driven electrically by the pulse control device, but with an optical tap behind the optical signal component in the transmitter device, it can be more accurate and also avoid the necessary calculations. and delays in transmitters. The optical tap can feed back to the pulse control device, providing some feedback on the timing and shape of the output pulses for monitoring or control purposes if required.

Figure 4

FIG. 4 shows another configuration of a transmitting device, according to one embodiment. The aim is again to provide pulses at two wavelengths so that the receiving device (not shown) can correct for differential losses. In this example, two lasers 110 are provided to output pulsed light under the control of the pulse control device 100 . The pulse control means can be implemented with, for example, a conventional microprocessor. Initially, the pulses are on the same wavelength, but a transformer and shaper 130 is provided to change the wavelength of one of the pulse streams. As shown in Figure 3, a filter 140 can optionally be provided to remove unwanted scatter component signals. Then, for shaping the pulses, a burst generator 220 and a shaper unit 135 are provided. The pulse train generator transforms each pulse into a pulse train through a series of resolvers, delayers, and combiners. Further details will be given below with reference to FIGS. 8 to 12 . This step spreads the pulse in time. A shaper can then expand each pulse in the burst, remerging the pulses into a single long pulse. The goal is still to make the pulse longer so that it can hold more energy, either for use in sensing fibers below the SRS limit, or for a system such as that shown in Figure 1. By virtue of the use of the pulse train generator, the shaper needs to be extended less so that its optical dispersion properties are not too extreme. Thus, the shaper can be easier to make and/or the final pulse flatter. As shown in Figure 3, a switch is provided for combining the two pulse streams. As previously indicated, the optical timing trigger signal is dropped.

Extending short pulses, typically less than 1 nsec, to pulses of tens of nsec is an unusual step, but facilitates the use of short-pulse solid-state microchip lasers. This laser is excellent at efficiently generating sufficient peak power (eg, 10 kW), but its short cavity means it is difficult to generate long pulses. Another is to use active Q-switched lasers instead of passive devices. But a passive unit is more recommended as it is stronger and less expensive.

Figure 5

Figure 5 shows an example of an externally modulated laser. This externally modulated laser can be used as an alternative directly modulated microchip laser for pulse generation. To form pulses from a continuous wave output, a modulator 220 is provided, controlled by the pulse control section 100 of, for example, FIG. 3 or 4 . An optical amplifier 230 is provided at the output of the modulator to increase the power and actively control the power level so that the power is the same at both wavelengths. The pulse timing signal is fed to the modulator, while the pulse power can in principle be controlled by any of the three components. Although the configuration of Figure 5 is more complex, it has some advantages in terms of easy control of the pulse shape.

Figures 6 and 7

6 and 7 show the configuration of a possible embodiment of the receiving device and the operating steps of the receiving device, respectively. The backscattered signal of the incoming light is fed to a WDM (Wavelength Division Multiplexing) demultiplexing device 300 . The demultiplexing device separates various components by wavelength. These devices can be implemented with commonly available devices. Four separate outputs are shown, although there could be more. The four outputs enable the separation of the three backscattered components of pulses of two different wavelengths. Attenuator 310 is used to attenuate the Rayleigh component since it is much brighter than the other components so as not to damage the detector. The attenuator can be switched depending on which wavelength pulses are to be received.

O/E 320 performs light-to-electricity conversion for each wavelength through photodetectors. The analog electrical signal is amplified by amplifier 330 before being fed to a digitizer and timing gate 340 . The digital converter and the timing gate output a digital value sampled at a given instant in the time window set by the timing trigger signal sent by the transmitting device, and the sampling frequency is at least twice the highest frequency of the signal. The digital stream of data is sent to a buffer and/or averaging section 350 which feeds the value or average to a processor 360 . The processor can be implemented in the form of a PC (personal computer), for example, or can be implemented by other commonly used hardware circuits located locally or remotely. The processor outputs, for example, sensed values and measurement confidence indications.

As shown in FIG. 7, at step 400, a pulse of _λ1 is sent. At step 410 or earlier, the attenuators are switched to ensure that only _λ1 is attenuated. At step 420, the returned Raman Stokes and Raman anti-Stokes components at λ ₀ , λ ₂ are detected. In steps 430 to 450, the corresponding step is to detect the pulse of λ ₂ and the Raman Stokes and anti-Raman Stokes components on λ ₁ , λ ₃ . These steps are all repeated to obtain the average value of the backscattered component. These measurements can be repeated at different pulse power levels to achieve some correction for some non-linearities.

In step 470, the ratio of _{λ1 pulses from λ2} to _{λ2 pulses from λ1} is obtained by the processor. As mentioned above, this ratio enables the determination of temperature with differential losses that cancel. The processor can also determine other ratios or perform calculations. For example, if the pulse power is high enough that the backscattered component varies in a non-linear fashion, then the two components, which vary in opposite ways, can be combined mathematically to give a value that does have an approximately linear relationship to the state being measured. In this way, at step 480, a temperature value or other state value according to the above-mentioned ratio is output, and various errors, such as non-linear errors, are corrected.

In step 490, optionally, a second ratio can be determined to estimate the degree of error, such as the amount of differential loss. This second ratio is useful for determining whether a fiber or other component has deteriorated, and for confidence in the output measurement. It is especially useful in situations where fiber access is not possible, and where measurements become dangerous, eg for safety reasons. For this purpose, for example, the ratio of λ1 _{pulses from λ2} to λ0 _{pulses from λ1} can be used.

Figure 8

Figure 8 shows an embodiment of the invention for distributed measurement, where light from a laser source is converted into a pulse train and entered into only one end of the fiber. The laser light source (51) is connected to a pulse conversion device (52), which converts the input light into a pulse train. The pulse train is then sent to a decomposition device (54), such as an optocoupler, which may be incorporated into the pulse conversion device. Optical amplifier and/or attenuator units (53) can be used to control the amount of optical power entering the system. This optical power can be monitored with a photodetector (55) in order to monitor the incoming power. The photodetector (55) can have variable gain. Light from the decomposing device (54) is fed into a length of optical fiber (56), which may be single-mode or multi-mode, polarization maintaining, and may include lengths of different fiber connections. The backscattered light generated by the forward propagating pulse train in the optical fiber (56) returns to the decomposition device (54) and can also be sent to the attenuator and/or amplifier unit (57).

This light is then sent to a filtering unit (58), which may be a device that selects one of many filters into the light path, or may include a splitting device, such as a coupler, that splits the light into Two paths to separate filtering and detection. Light selected by the filter unit is detected by one or more photodetectors (59), which may have variable gain. The processing unit (60) analyzes the signal from the one or more signal photodetectors (59) and may also control the filter selected by the filtering unit (58). The processing unit can also analyze the signal from the monitoring photodetector (55), and/or control the gain of the monitoring photodetector (55), and/or control the gain of one or more signal photodetectors (59), and/or Or control the power, and/or repetition rate, and/or pulse width of the laser (51), and/or control one of the amplification and/or attenuation units (53 and 57) and/or both amplification and/or attenuation , and/or control the width and shape of the pulse train with pulse transformation (52).

In use, light from the laser is fed into a pulse transforming device which generates pulse trains which are fed into a sensing fiber and these pulse trains generate backscattered signals in the fiber. The backscattered signal for each wavelength selected by the filtering unit is collected and recorded as a function of time. Each selected wavelength corresponds to a signal of interest in the backscatter spectrum, and by analyzing these signals, temperature and/or loss are determined at all points along the spectrum.

Pulse trains are efficiently generated by sending input light pulses to a splitting device which directs a fraction of the light into two or more paths, each of different length. Then, at the combining device, the pulses propagating along these paths are recombined and directed to a common path. Because each pulse travels a different distance along its respective path, these pulses recombine to form a pulse train. By varying the length difference and the number of paths, wide variations can be obtained in the duration and shape of the final burst. Furthermore, as explained here, a number of pulse conversion means can be combined one after the other. The advantage of this is that, since the nature of the splitting means can be used, pulses can be sent along fewer paths at each splitting unit to achieve the same purpose, thus minimizing losses at the recombining means. Also, optical switches may be included in the configuration so that the number of paths and/or the number of switching devices can be selected to produce a specially specified final burst duration and shape. This enables the burst width to be adjusted to match the required spatial resolution, and also enables pulses to be added to or removed from the burst if desired, e.g. increasing the spacing between pulses can further attenuate the SRS. It is also possible to remove the pulses to make the pulse trains have more complex pulse patterns. The pattern may eg reflect some known periodicity of the structure to be measured, giving a higher signal power than would be produced by a single pulse, but without adding any uncertainty to the measurement.

For distributed measurements, the light from the laser is converted into pulse trains with the pulse train width optimized for the required spatial resolution. It is also possible to form pulse trains with allowable discrete intervals between component pulses. The pulse train is sent through the decomposition device and enters the optical fiber. This light travels down the fiber, and as it travels, it produces Raman Stokes and anti-Stokes, and Rayleigh backscattered light. The backscattered light is separated into its spectral components, for example using a filter, a wavelength division multiplexing coupler, or a combination of a coupler and filter, which are then detected and analyzed. Analysis of the data can lead to temperature distribution and/or loss distribution along the fiber.

Optical fibers can be single-mode or multimode, and can have specially specified doping levels, or many different doping layers, to optimize the interaction. The fiber can be polarization maintaining to provide a more controllable interaction. The optical fiber may comprise various lengths of different fibers in order to provide different Raman frequency shifts along these lengths, or to alter the modal properties of the propagating light.

The splitting ratio of the splitting means may be wavelength-dependent or mode-dependent in order to improve power transfer between input light and backscattered light. Return light can be amplified and/or attenuated to enhance detection. The light source can be Q-switched, model-locked, directly pulsed, and externally pulsed. The input light pulses can be transformed so that the duration of the resulting pulse train matches the required spatial resolution.

The pulse transformation means may comprise one or more sets of decomposing and recombining means, each of which sends the input light pulses along two or more paths of different optical lengths and combines the light into pulse trains. The decomposing and recombining device used in the pulse conversion device may be one of a common coupler, a single-mode coupler, a multi-mode coupler, or a polarization mode coupler, or a combination thereof.

The pulse conversion device may include optical fibers and fiber optic components. The burst duration and burst pattern resulting from the pulse transformation may be fixed or may be controlled by an optical switch that selects or eliminates the number of pulse transformation devices used and/or the number of paths used in the pulse transformation device . The space-to-mark ratio between pulses in the pulse train can be chosen such that stimulated Raman scattering is further suppressed by the induced dispersion between the pulses and the scattered Raman light. To help correct for non-linearities when deriving temperature and/or loss data, the burst width and/or mode and/or energy can be varied.

The power of the light source can be varied directly, or by means of attenuation or amplification. Filtering can be achieved with optical filters, variable optical filters, wavelength division multiplexing couplers, or a combination of couplers and filters.

When calculating temperature and/or loss, one, both, or both of the returned Raman Stokes, Raman inverse Stokes, and Rayleigh signals can be compared to correct for any nonlinearities in the signals. The fiber can be better calibrated with measurements from both ends of the fiber. The fiber ends may be index matched and/or angle cleaved to prevent end reflections.

Using a photodetector, photodetector array, avalanche photodiode, single photon counting detector, or multiphoton counting detector, the optical signal is converted to an electrical signal, and the electrical signal is fed to a processor and recorded in the desired form. A processor may control the operation of the components of the system.

Figure 9

Figure 9 shows an embodiment of a pulse transformation device used as a pulse shaper. The pulse shaper in this example can use a pulsed laser as a light source, and has two delay lines connected in series to convert the input pulse into a pulse train of four pulses. Referring now to FIG. 7, the high power pulse (21) is sent to a splitting device, such as an optocoupler (22), where the coupler introduces light into two paths, one of which (23) has more power than the other (24). Longer optical path length. Here, the split ratio is such that an equal portion of the input light is sent into each path with a difference in path length greater than the length of the input pulse. The light pulses in each of the two paths are half as high as the input pulses but have the same width (25 and 26). These two pulses then pass through a recombination device (27), which here is an optocoupler. The coupler (27) then splits the light into two paths (28 and 29), and the process repeats for each path that emits a pulse train consisting of two pulses, each of the same width , the height is one quarter of the input pulse (30 and 31). Now, the difference in the lengths of these two paths, is more than twice the width of the input pulse, so that when the two paths are recombined, the resulting pulse trains (32 and 33) each consist of four pulses, each of which has the width of the input pulse , while the height is one-eighth of the input pulse. The resulting pulses can be combined together and fed into a single fiber using a recombiner (36) which can be identical to the recombiner (34) or which can effectively combine two or more paths coupler for optical combination. For example, it could be a modal coupler that combines the output of several small cores into a larger core, or it could be a polarizing beam coupler. The pulse train (41) obtained in the output optical fiber (35) has a height of 1/4 that of the input pulse (21), and a width wider than four times the width of the input pulse (21). More transform units than the two shown in Figure 2 can be used if necessary. Additionally, switches (37, 38, 39, and 40) can be used to prevent light from passing through one or more paths. As a result, by removing component pulses, it is possible to control the width of the pulse train and/or change the shape of the pulse train. For example, if the switch (40) is set to block light, then the output of the transforming device will be a train of pulses one-fourth the height of the input pulse and wider than twice the width of the input pulse. Whereas if the switch (40) is set to transmit light, then the output will be a pulse train with a height one-fourth that of the input pulse and a width more than four times the width of the input pulse.

Figure 10

Figure 10 shows another example of a pulse shaper that creates a burst of pulses from a single input pulse. Using a single-mode fiber at the input and coupling to three fibers that are also single-mode causes the incoming pulse to be broken up into three approximately equal parts that propagate on separate paths. Of course, there could be more paths, depending on how much the pulse needs to be spread. Each path is provided with a different delay by delayers 600 , 610 , and 620 . These delays can be implemented in different lengths of fiber and can optionally be arranged to have a controlled amount of dispersion in order to achieve some pulse spreading. An optical switch 630 is provided in each branch. The single-mode fiber serving as each path is coupled into a multimode fiber to recombine pulses with different delays. This enables control of the optical switch to control the length of the burst, and to control the amount of overlap or gap between pulses of the burst. This method of using single-mode fiber and multimode fiber enables disassembly and recombination with minimal loss. Further control of the pulse shape can be obtained, if necessary, by adding controllable attenuators or amplifiers in some or all branches. Each branch can itself be decomposed into many sub-branches, or a cascade of branches can be built. The configuration of Figure 9 can be combined with the configuration of Figure 10 to create any type of cascaded or sub-branched topology.

Figures 11 and 12

Figure 11 shows a graphical representation of how an input pulse is spread by the action of a dispersive element, which is known. The amount of spread is limited, the output is not spread flat, more power is concentrated near the center. Figure 12 graphically shows how the pulses are extended by the configuration of Figures 9 and 10, or other embodiments of the pulse transforming means, or pulse shapers. As a first step in the figure, draw the effects of decomposition, delay, and combination to create a pulse train. As a second step on the diagram, draw the effect of providing some dispersion to spread the pulse. Dispersion can for example be provided before decomposition, or after decomposition, or after combination, the end result being the same. In a typical example, an input pulse with a width of less than 1 nsec is transformed into a pulse with a width of tens of nsec. Stretching individual pulses, and fusing them, transforming the pulse train into a single long pulse, can be achieved through multimodal effects, for example, by using step-index multimode fibers in which different modes have different paths. By filling all the gaps in the pulse train and forming a single long pulse, more energy is provided for a given peak power than if no gaps were filled. By generating bursts, less extension is required, so for example 5km of fiber is replaced by a dispersion unit with only 500m of fiber. This can reduce volume and cost, and also reduce loss.

conclusion

Although it has been described with reference to light as an example, other parts of the electromagnetic spectrum should be considered, although clearly, to be included, which have similar characteristics and achieve the same advantages as those described above. Although it has been described with reference to sensing of temperature or loss, it should be apparent that these applications should not be considered limited, and it is believed that aspects of the invention can be used in other applications as well.

Methods and apparatus for generating and delivering high energy light pulses have been described. Distributed temperature sensors typically use Raman scattering in optical fibers as a means of determining temperature. The point here is that light from a laser source is sent down an optical fiber and the small amount of light that is scattered back to the source is analyzed. As the fiber length increases, the resolution of temperature and loss measurements becomes poorer. This is because there are losses in the fiber that cause signal attenuation. An obvious solution to this problem is to inject more light into the fiber to compensate for the loss, but how much light can be injected is limited by stimulated Raman scattering. The present invention solves this problem by using a pulse transformation method to maximize the resulting pulse energy while keeping the power below the SRS threshold.

Claims (39)

1. sensing system, be used for sending electromagnetic signal along waveguide, and sensing is along the state on the one or more sense position of waveguide, native system comprises emitter, be used to launch the pulse of suitable shape, the inelastic scatter of being excited by before arriving sense position makes the controlled conversion of this pulse generation, becomes different wavelength, native system comprises receiving trap, be used to receive the signal that returns from sense position, and, determine these states according to the signal that receives.

2. according to the system of claim 1, the power of pulse can make different wave length stand the one or many conversion once more, become other different wave length, the different piece of waveguide is shone by different wave length, and this receiving trap is used to distinguish the light signal that returns on each different wave length.

3. according to the system of claim 1 or 2, controller is arranged, be used to adjust pulse power, the transition length that conversion is taken place moves to the different piece of optical fiber.

4. according to each system of claim 1 to 3, be used to compensate the differential loss on the different wave length.

5. according to the system of claim 4, this emitter comprises: produce the light source of pulse, pulse is decomposed into the resolver of two stream of pulsess and at two stream of pulsess along the transducer that makes one of stream of pulses conversion wavelength before optical fiber sends, this receiving trap uses the signal that returns from two stream of pulsess, the differential loss in the compensation waveguide.

6. according to the system of claim 5, this receiving trap is by detecting the upwards frequency-shifted components that one of pulse from first wavelength is returned, return with one of pulse of launching from higher wavelength, to the downward frequency-shifted components of downshift to the first wavelength, the compensation differential loss.

7. according to each system of front claim, this receiving trap also is used for from one or more other sense position, receives the signal before the conversion of returning.

8. according to each system of front claim, controller is arranged, be used for the repetition rate of gating pulse.

9. sensing system, be used for sending electromagnetic signal along waveguide, and sensing is along the state on the one or more sense position of waveguide, and native system has: receiving trap and the emitter of the electromagnet source that produces stream of pulses is arranged, pulse is decomposed into the resolver of two stream of pulsess and at two stream of pulsess along the transducer that makes one of stream of pulses conversion wavelength before optical fiber sends.

10. according to the system of claim 9, this electromagnet source and resolver, the process that is used for arriving in pulse transducer provides enough power, so that realize conversion by being excited inelastic scattering.

11. according to each system of front claim, also comprise pulse shaper, be used to make pulse edge optical fiber to send before, pulse is extended.

12. according to the system of claim 11, this pulse shaper comprises the chromatic dispersion unit.

13. according to the system of claim 11 or 12, this pulse shaper comprises train of impulses transducer and chromatic dispersion unit, this train of impulses transducer is each impulse transfer a train of impulses, and this chromatic dispersion unit is used for the shaping of paired pulses string.

14. sensing system, be used for sending electromagnetic signal along waveguide, and sensing is along the state on the one or more sense position of waveguide, native system has emitter apparatus, be used to start the generation of one or more stimulated Raman light, the different piece of optical fiber can be shone by different wave length, and be used for simultaneously in the light pulse of optical fiber emission more than one; Also have receiving trap, be used for the sensing on the long-distance optical fiber, measure the power of the different components of rear orientation light, this receiving trap has optical filtering apparatus and one or more detecting device, and being used for converting optical signals is electric signal.

15. according to each system of front claim, this receiving trap is used to distinguish from being excited backward scattered Raman Stokes or the anti-Stokes or the Rayleigh wavelength of propagated forward signal.

16. according to each system of front claim, this receiving trap has wave filter, is used for before wavelength conversion, on the wavelength that the pulse wavelength than emission is offset slightly, collects the backscatter signal that needs.

17., be used for different pulse power duplicate measurementss and proofread and correct non-linear with measurement result according to each system of front claim.

18., also comprise optical fiber according to each system of front claim.

19. according to each system of front claim, be used for gating pulse power, to change position along Optical Fiber Transmission length generation wavelength conversion.

20. according to each system of front claim, this emitter comprises laser instrument, produces the modulator of pulse and amplify the image intensifer of pulse from laser instrument output.

21. according to each system of front claim, this receiving trap has wave filter, be used for collecting from one section backward scattered Raman Stokes of optical fiber and/or anti-Stokes of Raman and/or Rayleigh component of signal, these component of signals are not with significantly overlapping from the backward scattered component of equal value of other sections.

22. according to each system of front claim, this receiving trap, one of the anti-Stokes of Raman Stokes, Raman that is used for relatively returning and Rayleigh component of signal, two or all, any non-linear with in the correction signal.

23. according to the system of claim 22, this receiving trap uses with sensed state relation to be in saturated, the Raman Stokes that returns of inelastic region and the combination of the anti-Stokes component of Raman, comes compensating non-linear.

24. pulse shaper, be used for the shaping of light input pulse, this pulse shaper has the pulse series generator that produces train of impulses from input optical pulse, with the chromatic dispersion unit that is positioned at this generator front or back, this generator comprises device and the combiner that decomposes input pulse, the device of this decomposition produces delay pulse having the light pulse of importing between the path of different delayed time to decompose two or more; This combiner produces train of impulses to the pulse combined of time-delay; And this chromatic dispersion unit is respectively input pulse or train of impulses expansion.

25. according to the equipment of claim 24, one or more photoswitches are arranged, are used for selecting to form train of impulses with any delay pulse.

26. equipment that produces and transmit high energy optical pulses, this equipment makes the luminous energy maximization that excites, suppress stimulated Raman light simultaneously, this equipment also is used to measure the power of the different components of rear orientation light, so that can realize the distributed measurement of temperature and/or loss on long distance, this equipment comprises at least one optical fiber that scattering medium is provided; Light source; Be transformed to the pulse-selecting apparatus or the converting means of train of impulses by modulating device, pulse-selecting apparatus, single high power pulse; Optical filtering apparatus; Be converting optical signals one or more detecting devices of electric signal.

27. according to the equipment of claim 26, optical fiber wherein comprises the length of different fiber, so that provide different Raman frequency displacements along length, or the characteristics of mode of change propagates light.

28. according to the equipment of claim 26 or 27, wherein, the light pulse of input is transformed, and makes the spatial resolution coupling of the train of impulses duration that obtains and requirement.

29. according to each equipment of claim 26 to 28, impulse transfer device wherein comprises that one or more groups decomposes and the recombinant device, and each input optical pulse sends along the path of two or more different optical length, and, form train of impulses the light recombinant.

30. according to each equipment of claim 26 to 29, impulse transfer device wherein comprises optical fiber and optical fiber components.

31. according to each equipment of claim 26 to 30, wherein, the train of impulses duration and the pulse string mode that obtain from impulse transfer, can fix, also can control by photoswitch, these photoswitches are selected or are eliminated the impulse transfer device number that uses, and/or the number of path of using in the impulse transfer device.

32. according to each equipment of claim 26 to 31, burst width wherein and/or pattern and/or energy can change, so that when deriving temperature and/or lossy data, help to proofread and correct non-linear.

33. according to each equipment of claim 26 to 32, wherein, when accounting temperature and/or loss, one of the anti-Stokes of Raman Stokes, Raman that can relatively return and Rayleigh component of signal, two or all, any non-linear with in the correction signal.

34. sensing system, be used for sending electromagnetic signal along waveguide, and sensing is along the state on the one or more sense position of waveguide, native system has emitter apparatus, receiving trap and the waveguide that sends pulse along waveguide, this waveguide comprises many sections that different qualities is arranged, making from the back scattering of pulse has different optical properties, and this receiving trap uses this different optical property, distinguishes the back scattering from different sections.

35. according to the sensing system of claim 34, these sections have the waveguide that different Raman frequency displacements are provided, so that the back scattering component is provided on different wave length.

36. according to the sensing system of claim 34 or 35, these sections have the waveguides that cause the different variations of propagates light characteristics of mode, to provide the back scattering component of different modalities characteristic.

37. according to the sensing system of claim 1 to 23 or 26 to 36, the pulse shaper of have the right requirement 24 or 25 is used for the shaping pulse on the emitter.

38. one kind is used each equipment of front claim that the sensing service method is provided.

39. method of using claim each equipment in front to produce sensing signal.

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