JPH0227831A - Multiple information transmission optical network - Google Patents

Abstract

PURPOSE:To avoid the deterioration in the S/N by using carriers having different center frequencies so as to modulate plural sets of information and sending the result respectively. CONSTITUTION:A radiated light from a light source 1 is led to a 1st optical modulator 51. A driving signal from the 1st optical modulator 31 is a 1st information modulated by a modulator by using an output of an oscillator 51 oscillated at a center frequency f1 as a carrier. Similarly, the lights subjected to serial optical modulation and passing in series through all modulators 31-3i are given to an optical demodulator 7, where optical demodulation is applied and converted into an electric signal. Then a required component is extracted by band pass filters 81-8i corresponding to center frequencies f1-fi, demodulated by demodulators 91-9i and the original information is recovered. Since the transmission reception for relaying is not repeated, even when the information is sent from lots of points, the deterioration in the S/N due to relaying is not caused.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to an optical network that transmits information from multiple points in series to one location using optical fibers, and is useful for monitoring multiple locations. It is used in systems and monitoring systems that monitor many points using sensors.

Conventional technology Conventionally, methods for transmitting a large amount of different information from many places to one place in series using optical fibers include (1)
A set of optical transceivers is installed at each location by connecting each location with a fiber, converting the optical signal from the previous location into an electrical signal, and then electrically multiplexing the signal to be sent and converting it into light. The so-called bus-type transmission method uses a light source with a different wavelength at each point and combines it into a transmission fiber using a multiplexer, and uses a demultiplexer at the time of reception to send each signal to the next point. A so-called wavelength multiplex transmission system was used, which separates wavelengths and regenerates the original signal.

Problems to be Solved by the Invention As mentioned in the conventional example, in order to serially transmit a lot of different information from many places to one place using optical fibers, it is necessary to Since the same number of optical transceivers are required, the complexity and cost of the entire device increases.

Furthermore, in bus-type transmission systems, digital transmission is generally used to prevent deterioration due to relaying, so TDM (time division multiplexing) must be adopted as a multiplexing system.

This method has relatively little flexibility, and the entire relay device becomes quite large.

In order to avoid this, analog transmission using frequency multiplexing may be considered, but if you try to transmit frequency multiplexed signals, (1) there will be deterioration in transmission quality such as modal noise and distortion during fiber transmission; , high-quality analog transmission is not possible. (2) In analog transmission, if transmission and reception are repeated for relaying, S/N deterioration occurs each time, so there are problems such as multistage relaying being impossible.

Furthermore, the wavelength multiplexing method requires light sources with different wavelengths corresponding to each point, and elements for multiplexing and demultiplexing the light sources. As the number of transmitted information increases, it is necessary to prepare a corresponding number of light sources with different wavelengths and a corresponding number of multiplexers/demultiplexers with high resolution, which is currently extremely difficult.

Means for Solving the Problems The present invention provides a plurality of modulators that modulate a plurality of pieces of information using carrier waves each having a different center frequency, and a plurality of modulators each having a small optical modulation degree and each driven by the output of the modulator. an optical modulator, one light source, a transmission medium arranged so as to guide the emitted light from the light source to each optical modulator in series, and optical demodulation corresponding to the optical modulation of the output light from the transmission medium. This is a multi-information transmission optical network that includes an optical demodulator that converts the signal into an electrical signal, a bandpass filter that extracts a signal corresponding to the center frequency, and a demodulator that demodulates the extracted signal.

Function: Only one light source is used in the present invention. The emitted light is sequentially modulated and modulated by a large number of optical modulators arranged in series.

At this time, the signals that drive each optical modulator are modulation signals having carrier waves with different center frequencies, and the optical modulation degree of the optical modulator is set small. The light that has passed through the all-optical modulator in series is converted into an electrical signal by the optical demodulator. Since the optical modulation degree of each optical modulator is small, it can be used in a region where the optical modulator transmittance is high, so the loss due to the modulator is small. (2) High-order components other than the fundamental wave of the demodulated electrical signal can be ignored. It has this feature.

Therefore, each piece of information can be extracted from the demodulated electrical signal by using a bandpass filter corresponding to each center frequency.

Example A basic configuration diagram of one embodiment of the present invention is shown in FIG.

- Emitted light from the light sources 1 is guided to the first optical modulator 31 by the first transmission medium 21. The drive signal for this first optical modulator is modulated by the modulator 81 using the output of the oscillator 51 that oscillates the first information 41 to be transmitted at the center frequency f1 as a carrier wave. In the present invention, basically any modulation method of the optical modulator and any information modulation method may be used. The emitted light from the first optical modulator is guided to the second optical modulator by the second transmission medium 22. Thereafter, optical modulation is performed in series one after another in the same manner, and the light is guided to the first optical modulator 31 by the first transmission medium 21.
The first information 41 to be transmitted is modulated by a modulator 61 using the output of an oscillator 51 that oscillates at a center frequency fi as a carrier wave, and drives an optical modulator. The light that has passed through all the modulators in series enters the optical demodulator 7, undergoes optical demodulation corresponding to the modulation method of the optical modulator, and is converted into an electrical signal.

Let's consider the components of this electrical signal.

If the i-th information is expressed as al(t), its central angular frequency is 0 kan (=2πfl), the input light of the optical modulator is Pi-11, the output light is PI, and the optical modulation degree is m, then P1= (1
+rrl Persimmon 1 (t) book 5 In (ωth t)) Bunch P1-1
(1) becomes. The light P to the final optical demodulator is p=pokyrz (++m1 bundle al(t) bundle 5ln(
ω-th t)) (2) 1=1 (However, PO represents the light emitted from the light source.) N is the total number of optical modulators.

Depending on the optical modulation method, if P represents light intensity, amplitude, frequency, etc., it can basically be expressed by such an equation, although there may be some changes.

The frequency components included in this P include not only ω1 but also many other components. However, if the degree of optical modulation is small, the higher-order terms in equation (2) can be ignored, so p=po (l+)22 m1 lines ai(t) less51n
(ω tit)) (3) It is expressed as 1:1. This indicates that it is the same as a normal frequency multiplexed signal.

The output electrical signals of such optical demodulators are passed through bandpass filters (BPF) 81 and 81, respectively, corresponding to the center frequencies.
...8j... extracts the necessary components, and demodulator 91...
・91... demodulates and the original information is reproduced. When all information on each point is required at the same time, it is necessary to provide all numbers of BPFs as in this embodiment. However, −
Sometimes, when not all the information is needed, only the necessary information can be extracted using a so-called tuner with an electrical tuning function.

The transmission medium used in this embodiment may be any medium as long as it can transmit light in a concentrated manner.

For example, it is possible to transmit the light beam through the air or use an optical fiber.

Next, let's discuss the operation of the optical modulator.

Figure 2 shows the characteristics of the in-shell optical modulator. As light modulation,
If intensity modulation is performed, the horizontal axis represents the voltage applied to the modulator, and the vertical axis represents the transmittance. Normally, the operating point of an optical modulator is often used at point A in the figure from the viewpoint of modulation efficiency and linearity of modulation. In this case, as a matter of course, a minimum of 3 dB is required as a loss due to the photometer II in order to set the operating conditions to this point. By the way, in the present invention, since the modulation degree of the optical modulator is required to be small, it is possible to operate the optical modulator in a region such as point B, which has poor modulation efficiency and linearity but has high transmittance. can. The high transmittance of an optical modulator is very important because it allows a large number of optical modulators to be used in series.

By the way, when the optical modulator is used in such a state, the modulation distortion of the optical signal by the optical modulator becomes a big problem. If optical modulation is performed centering around point A in FIG. 2, linearity is good and distortion does not occur much. However, if it is used at point B, the transmittance increases, but modulation distortion increases. In order to reduce the influence of distortion of the optical modulator, it is effective to make the maximum frequency of each center frequency twice or less than the minimum frequency.

By doing so, the harmonic distortion components generated in each optical modulator are outside the band of the necessary signal components, so that they can be prevented from adversely affecting signal transmission.

In addition, when an optical fiber is used as a transmission medium, and when an optical modulator that utilizes polarization is used, a polarization-maintaining fiber is used, or the plane of polarization is separated into two orthogonal components. If you use the so-called polarization diversity method, which modulates each and then adds them together,
Loss due to modulation can be reduced.

In the method of using intensity modulation as an optical modulation method, the modulator itself and the method of optical demodulation are very simple, and are formed into a film. Other modulation methods, such as amplitude, frequency, and phase modulation, have been actively researched and developed in recent years for coherent transmission, and although they are not yet mainstream, they are just as fundamental as intensity modulation. It can be used for inventions. In that case, it is naturally necessary to adopt a method corresponding to the optical demodulation.

Next, a case will be described in which an optical fiber is used as the transmission medium. When a fiber is used as a transmission medium, there is a high possibility that very good transmission can be achieved due to its light weight, flammability, noise resistance, etc. In this case,
As a light source, it is necessary to use a semiconductor laser due to fiber input power and optical spectrum. However, in this case, there is a big problem of noise generation due to transmission. This is caused by reflected light emitted from the end face of the transmission fiber, and can be prevented by making all the fiber end faces oblique. Moreover, further noise reduction is possible by using a distributed feedback laser (DFB-LD) as the semiconductor laser.

In the present invention, the modulation (not optical modulation) performed on the information to be transmitted is only required to separate specific signals by frequency, and therefore does not need to be a specific modulation method. , AM, FM.

Any PM is fine. However, in the present invention, since it is necessary to suppress the degree of optical modulation to a small value, C
There is a problem that /N cannot be taken large. However, a feature of this system is that the transmission band can be wide because it is optical transmission, so when these are taken into account, it is optimal to adopt FM modulation with a large modulation index.

More than the effects of the invention, according to the present invention, since it is an analog transmission, the degree of freedom of the entire system is increased.Furthermore, there is no need to repeat transmission and reception for relaying, so information can be transmitted from many points. The major advantage is that there is no S/N deterioration due to relaying.

Therefore, it is possible to construct a network with a very high degree of freedom and low system cost as a monitoring system that monitors many places or a monitoring system that monitors many points using sensors.

[Brief explanation of the drawing]

FIG. 1 is a basic configuration diagram of an embodiment of the present invention, and FIG. 2 is a general characteristic diagram of an optical modulator. 1... Light source, 2... Transmission medium, 3... Optical modulator,
4... Information to be sent, 5... Oscillator, 6... Modulator, 7... Optical demodulator, 8... Bandpass filter,
9... Demodulator.

Claims (5)

[Claims] (1) A plurality of modulators that modulate a plurality of pieces of information using carrier waves each having a different center frequency, a plurality of optical modulators each having a small degree of optical modulation each driven by the output of the modulator, and one light source. , a transmission medium arranged so as to guide the emitted light from the light source to each optical modulator in series, and a light beam that performs optical demodulation corresponding to the optical modulation of the output light from the transmission medium and converts it into an electrical signal. A multiple information transmission optical network comprising: a demodulator; a bandpass filter that extracts a signal corresponding to the center frequency; and a demodulator that demodulates the extracted signal. (2) The driving condition of the optical modulator according to claim 1 is set to a region of high light transmittance, and the maximum frequency among the different center frequencies is set to be less than twice the minimum frequency. A multi-information transmission optical network. (3) A multi-information transmission optical network according to claim 1 or 2, characterized in that a fiber is used as the transmission medium and all fiber end faces are oblique with respect to the fiber optical axis. (4) A multiple information transmission optical network characterized in that a distributed feedback laser is used as a light source according to claim 1, 2 or 3. (5) A multiple information transmission optical network characterized in that an FM modulator is used as the modulator according to claim 1, 2, 3 or 4.