Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
  • G01J9/00—Measuring optical phase difference; Determining degree of coherence; Measuring optical wavelength
• • G—PHYSICS
  • G02—OPTICS
  • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
  • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
  • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
  • G02F1/21—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  by interference
  • G02F1/225—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  by interference in an optical waveguide structure
• • G—PHYSICS
  • G02—OPTICS
  • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
  • G02F2203/00—Function characteristic
  • G02F2203/25—Frequency chirping of an optical modulator; Arrangements or methods for the pre-set or tuning thereof

Definitions

• the invention relates to a device and a method for determining the "chirp" of an electrooptical modulator. It has applications in the industrial and research fields where optical components and more particularly Mach-Zehnder type modulators must be characterized by their operating parameters including the "chirp". These modulators are more specifically used in fiber optic telecommunications applications.
• Fiber optic broadband optical telecommunications systems use external optical modulators connected to a monochromatic laser source. These modulators can be of two types. On the one hand, semiconductor-based modulators such as indium phosphide in which the modulation of light is obtained by injection of carriers in a junction: we speak of electroabsorbent modulators.
• the other category of modulators is based on Mach-Zehnder type interferometers which can be integrated in blocks of electro-optical materials. This latter property is characterized by the fact that the refractive index of the material varies with the electric voltage applied to the electrode terminals arranged on either side of one or two optical waveguides forming the arms of a interferometer.
• the most widely used electro-optical materials for making Mach-Zehnder modulators for optical telecommunications are lithium niobate, indium phosphide, gallium arsenide, and certain orientable polymers.
• the major interest of the Mach-Zehnder modulator compared to the electro-absorption modulator is based on the fact that the parasitic phase modulation associated with the desired light intensity modulation, property called "chirp", can be very weak or even zero under certain conditions, which is not the case with electro-absorbing modulators. This is a great advantage because an optical modulation devoid of "chirp" presents a great robustness for the propagation of pulses in the dispersive media which constitute the optical fibers with silica core conventionally used in current telecommunications networks.
• Mach-Zehnder modulator consists of.
• the first large type corresponds to an X-section crystal and the second to that of a Z-section crystal. These two types correspond to the broadband modulator devices most frequently encountered with this type of material.
• These components have on their surface a structure of optical waveguides obtained by metallic diffusion and reproducing the circuit of a Mach-Zehnder interferometer.
• the modulators also include microwave coplanar electrodes comprising a line, a narrow central one carrying the electrical signal, and two lateral ground planes.
• section X the optical guides are centered on either side of the central line, substantially halfway between the latter and each lateral ground plane.
• the electric field induced in the crystal is of opposite sign in each of the arms, which makes it possible to double the modulation efficiency when the waves are recombined.
• the amplitude of phase variation in each of the arms is the same. After recombination, it can be shown that in theory, this results in an amplitude modulation of the light without modulation of associated residual phase.
• the component in section X is said to have low “chirp” or even to “chirp” zero.
• the central line is centered on one of the arms of the interferometer while the other arm is positioned at the edge of one of the lateral ground planes.
• the modulation efficiency is not the same in each of the arms, in particular due to the difference in overlap between the guided light fields and the interacting electric field.
• the amplitude modulation is associated with a residual phase modulation.
• This component is said to be non-zero "chirp".
• a variant of this type of modulator exists which makes it possible to control the resulting “chirp” using a double set of electrodes arranged on each of the arms. If the electrodes are powered by variable gain amplifiers, this allows the value of the overall "chirp" to be adjusted very finely.
• the “chirp” is generally defined as the normalized ratio connecting the phase variation and the intensity variation produced by the modulator:
• n represents the index of the material, r its electro-optical coefficient, ⁇ the working wavelength, l the length of the electrodes, g the distance between electrodes. V z is the applied voltage.
• ⁇ is a coefficient between zero and one, which results from the calculation of the normalized recovery integral between the spatial distribution of the light field in the guide and that of the applied electric field, the latter not having a uniform distribution in the space between the edges of coplanar electrodes.
• “Chirp” measurement systems are already known and an example is the system for measuring the time profile and the phase (“chirp”) of short duration pulses which has been the subject of a request for Patent FR2,774,762 under the title: "Method and device for measuring the complex spectrum (amplitude and phase) of optical pulses".
• This system makes it possible to measure pulses emitted by an optical source at a given frequency.
• the principle of this device is based on an over-modulation by a Mach-Zehnder modulator of the pulses produced by an optical pulse source.
• the modulator used in the device is adjusted to an operating point corresponding to a phase shift of ⁇ radians and controlled by a frequency half the frequency of emission of the pulses.
• a variable delay line is introduced between the microwave generator driving the light source and the modulator which is preceded by a frequency divider. Detection and processing is done using an optical spectrum analyzer. To access the phase and the profile of the pulses, this involves measuring the variations in the amplitude of each peak or line of the optical spectrum measured as a function of the phase introduced by the microwave delay line.
• a Fourier transform makes it possible to trace the time profile and the instantaneous phase (or frequency) variation as a function of time, that is to say the "chirp".
• This system can lend itself to the characterization of pulses or signals produced by Mach-Zehnder modulators under test, but the principle of data acquisition does not allow a determination of the "chirp" at all frequencies or, only, at price of a very long measurement time and involving the availability of specific and relatively expensive components (frequency dividers) and measuring instruments (optical spectrum analyzer) in order to allow exploration from a few MHz to frequencies of the order of 50 GHz.
• Mach-Zehnder is desirable in the characterization chain of a designer or manufacturer of components to quantify the value of this parameter.
• Such a device is of even greater interest if it is possible for it to measure the frequency dispersion of the value of the "chirp".
• the object of the invention is therefore to propose such a device, which, moreover, is simpler materially and in use than the devices of the prior art.
• the invention therefore relates to a device for determining the “chirp” of an electro-optical interferometric modulator of the Mach-Zehnder type with two arms by optical spectral analysis of a modulated optical signal, an optical signal of predetermined frequency ⁇ 0 produced by an optical source being modulated into an optical signal modulated in the modulator by an electrical modulation signal produced by at least one means for generating an electrical signal, said electrical signal comprising a periodic component of predetermined frequency ⁇ and a continuous bias component, the " chirp ”weighting the amplitude of the lateral optical modulation lines shifted in frequency by
• the device comprises at least one optical measurement means of at least one of the lateral lines and a control and calculation means making it possible, in a first measurement step, to send a continuous component such as the difference of optical phase ⁇ between the two arms is substantially zero and, in a second measurement step, of sending a continuous component such that the difference in optical phase ⁇ in absolute value between the two arms is substantially equal to ⁇ , the means of calculation allowing at least to calculate for a given lateral optical line frequency the ratio of the measurement at
• the following means can be combined according to all the technically possible possibilities, are used:
• the optical measurement means also makes it possible to measure the modulated optical signal of frequency ⁇ 0 and in that, in addition, the control and calculation means in the first step makes it possible to adjust the DC component so that said optical signal modulated is maximum and, in the second step, makes it possible to adjust the DC component so that said modulated optical signal is minimal,
• the optical measurement means comprises a quadratic detector measuring the optical power, the ratio then corresponding to the square of the "chirp", the value of the "chirp” is determined by calculating the square root of the report in the case of a detector quadratic,
• the optical measurement means comprises a system capable of measuring the optical amplitude, the ratio then corresponding to the "chirp", the calculation of the ratio is carried out with the lateral lines of frequency ⁇ o- ⁇ ,
• control and calculation means make it possible to vary the periodic component between a low frequency ⁇ and a high frequency ⁇ h , measurements and calculations being carried out for each of said frequencies between ⁇ and ⁇ h , - the periodic component is sinusoidal,
• control and calculation means make it possible to determine the “chirp” for each of the frequencies of the periodic component variable in frequency, - the optical source is adjustable in frequency,
• control and calculation means make it possible to determine the "chirp" for each of the frequencies of the adjustable optical source
• control and calculation means make it possible to determine the "chirp" for each of the frequencies of the periodic component variable in frequency and for each of the frequencies of the adjustable optical source,
• control and calculation means make it possible to determine an average of "chirp" from a set of measurements and calculations
• the periodic component has a peak-to-peak amplitude less than the half-wave voltage V ⁇ of the modulator, and preferably less than or equal to 60% of V ⁇ ,
• the two-arm interferometric electro-optical modulator is of the Mach-Zehnder type
• control and calculation means is a programmable computer means, in particular a microcomputer, a microwave signal generator producing the periodic component, a controlled voltage generator producing a continuous component,
• the "chirp" is determined in the form of a set of digital data in a file
• control and calculation means comprise at least one interface for entering operating parameters and an output for monitoring and recovering the “chirp” determination results
• the optical measurement means comprises a monochromatic band pass filter centered on the frequency of a lateral line, the determination being made for a given frequency of the source and a given frequency of the periodic component,
• the optical measurement means comprises a spectrometer
• the spectrometer comprises a controlled orientable diffraction grating
• the spectrometer includes a Fabry-Pérot interferometer, one of the mirrors of which is movable and controlled.
• angle values for the phase shift also include integer multiples since the transfer function of the modulator is periodic.
• the invention also relates to a method for determining the "chirp" of an electro-optical interferometric modulator of the Mach-Zehnder type with two arms by optical spectral analysis of a modulated optical signal, an optical signal of predetermined frequency ⁇ 0 produced by an optical source being modulated into an optical signal modulated in the modulator by an electrical modulation signal produced by at least one means for generating an electrical signal, said electrical signal comprising a periodic component of predetermined frequency ⁇ and a continuous bias component, the " chirp ”weighting the amplitude of the lateral optical modulation lines shifted in frequency by
• At least one optical measurement means of at least one of the lateral lines and a control and calculation means are used, and, in a first measurement step, a continuous component such as the difference is sent of optical phase ⁇ between the two arms is substantially zero, and, in a second measurement step, a continuous component is sent such that the optical phase difference ⁇ in absolute value between the two arms is substantially equal to ⁇ , and, is carried out with the calculation means, for a lateral optical line frequency given, the ratio of the measurement at
• a periodic component which has a peak-to-peak amplitude lower than the half-wave voltage V ⁇ of the modulator, and preferably less than or equal to 60% of V ⁇ , - the periodic component is varied between a low frequency ⁇ b and a high frequency ⁇ h using the control and calculation means, and measurements and calculations are made for each of said frequencies between ⁇ b and ⁇ h ,
• the “chirp” is determined for each of the frequencies of the periodic variable frequency component in the form of a curve of a set of measurements and calculations
• the measurement device allows the measurement of the absolute value of the "chirp" of optical modulators of the Mach-Zehnder type.
• the device and the implementation are simplified because specifically dedicated and of lower cost in material and use.
• the system applies to all types of Mach-Zehnder modulators, whatever the substrate materials used for integration: gallium arsenide, lithium niobate, polymer ...
• the measurement is self-calibrated since it is done by calculating the ratio between two successive measurements for each frequency. It therefore does not depend on the microwave power applied or on the optical power of the source and it suffices that the measurement is made in small signals.
• the invention allows analysis over the entire operating frequency range of the modulator and not at one or two predetermined frequencies.
• the invention can be implemented at a fixed and predetermined frequency, which allows rapid measurement compared to existing systems and allows the use of an even more simplified device and of even lower cost, with a fixed oscillator such as microwave source and an optical filter preset on a side line at the output. There is no need for a delay line unlike previous systems.
• the device of the invention can be integrated into a single instrument to which the modulator to be tested is connected. The choice of a high resolution spectral filter makes it possible to carry out the measurement at low frequencies. Statistical data processing can also achieve a high signal-to-noise ratio.
• the measurement is quick because two frequency scans or two fixed frequency acquisitions are enough to create two measurement files which allow calculations to be carried out to determine the "chirp". Also simplicity of the treatments and calculations on the data by subtraction of two files expressed in decibels.
• the measures are used immediately for processing and calculations and these are “chirp” data which are stored in one or more files.
• the device of the invention and the method can also directly or in combination with other elements or programs allow determinations of parameters other than only the "chirp" and, for example, the bandwidth, the curve of answer, transfer function ...
• FIG. 1 represents a system for measuring the chirp according to the prior art
• FIG. 2 which represents examples of Mach-Zehnder type modulators seen from above and in section
• Figure 3 which represents the result of the spectral analysis of an optical signal modulated by a Mach-Zehnder type modulator for a maximum and a minimum of transmission
• FIG. 4 which represents an example of a device for determining the “chirp” according to the invention
• FIG. 5 which represents an example of a sequence of the steps of the method for determining the “chirp” according to the invention.
• phase ⁇ is a difference in static phase between the arms which can be applied using a DC voltage, either directly on the active electrodes of the Mach-Zehnder, or on an additional set of electrodes placed in series of the so-called active electrodes.
• Equation (6) can then take a new form making it possible to separate the phase modulation from the amplitude modulation:
• any deviation in the modulation efficiency between the arms produces a term of pure phase modulation, here as a factor in equation (10) before what corresponds to the amplitude modulation part proper placed in square brackets .
• the objective of the measurement will be to access ⁇ o and ⁇ separately.
• Eft I. ei ⁇ 0t U ( ⁇ o + P) ⁇ o cos ⁇ t + é ⁇ foo - ⁇ ) o cos ⁇ t-j ⁇
• the signal is applied around one of the transmission maxima of the modulator transfer curve connecting the light intensity transmitted to the voltage applied to the electrodes.
• This transfer function resulting from constructive or destructive interference at the output of the interferometer arms, is sinusoidal.
• the amplitude of the field is then expressed according to the following relation:
• the spectral analysis of this field provides 3 terms.
• the first term, power E 0 2 is centered at ⁇ 0, of the optical pulse laser source, the following two optical power (E 0/7 0 0 K / 2) 2 are centered ⁇ o- ⁇ and in ⁇ 0 + ⁇ .
• ⁇ t) ⁇ -e j ⁇ o t
• 2 ⁇ 0 K 0 cos ⁇ t] or also: E (t) ⁇ ⁇ 0 ⁇ n e J + ⁇ ) tj _ j ( ⁇ 0 - ⁇ ) t ⁇ n e-
• the decomposition of the signal thus makes it possible by spectral analysis to measure the amplitude of the lateral modulation lines and to access 77 0 and ⁇ . It therefore makes it possible to extract from it the value of the parameter of “chirp” ⁇ . To do this, it suffices to use an optical spectrum analyzer or any spectral light decomposition system to go back to this “chirp” parameter.
• the spectral distribution of optical power shows the different lateral modulation lines as a function of the case of static phase difference ⁇ considered.
• This ratio is equal to the square of the “chirp” ( ⁇ ).
• the system comprises an optical source, preferably of monochromatic laser type. This source is connected to the optical input of the component to be tested.
• the entire measurement device can be controlled by a unique analysis and processing control system which controls each subsystem, orders the different measurement phases, collects the data and performs the processing.
• This system can be the operator of the device itself or can preferably be an automated system.
• the component to be tested is controlled by a microwave signal generator producing an electrical signal of peak-to-peak peak voltage modulation. The amplitude of modulation must be low compared to the half-wave voltage V ⁇ .
• the small signal modulation condition can be obtained by arbitrarily choosing a peak-to-peak voltage less than or equal to, for example, 60% of the value of the half-wave voltage V ⁇ .
• the frequency supplied by the microwave generator can vary for each measurement point from a minimum frequency to the maximum operating frequency of the modulator. This scanning is controlled by the control and calculation means. Two successive scans corresponding to each of the cases of. figure described above are required to perform a full measurement.
• a DC voltage source is connected to the modulator electrodes. It is either a separate set of electrodes, or the microwave electrodes themselves. In the latter case, a polarization tee will be used.
• the DC voltage source makes it possible to apply a voltage such that the two arms of the modulator are in phase to within k2 ⁇ . In this way the modulation will operate on an operating point corresponding to a maximum transmission of the transfer function of the modulator.
• the DC voltage source makes it possible to apply a voltage such that the two arms of the modulator are in phase opposition to within k2 ⁇ , ie a voltage equal to V ⁇ compared to the first situation. In this way the modulation will operate on an operating point corresponding to a minimum of transmission of the transfer function of the modulator.
• the optical output of the modulator under test is connected to two measurement subsystems.
• the first subsystem can be omitted but it will preferably be used to guarantee the stability and reliability of the measurement. It allows, thanks to a photo-detection and a suitable electronic control looped on the DC voltage source, to control the operating point of the modulator. This control is carried out successively on a maximum transmission during the first scan then on a minimum of transmission during the second frequency scan. The order of switching from one voltage to the second is controlled, between two successive frequency scans, by the central control system.
• the second measurement subsystem is an optical frequency discriminator.
• This discriminator can be an optical spectrum analyzer based on an orientable diffraction grating, or else a tunable spectral filter such as a Fabry-Pérot interferometer, one of the mirrors of which is mobile and equipped with a controlled actuator.
• a diffraction grating-based system exhibits a high sensitivity in photo-detection, therefore a high signal-to-noise ratio. However, it allows a resolution typically limited to 2.5 GHz in frequency for the most efficient systems. In this case, this defines the low limit frequency.
• Fabry-perot interferometer systems descend lower in frequency: typically a few hundreds of MHz for a free spectral interval of ten GHz. They therefore allow a more extensive exploration in frequency.
• the photodetection sensitivity is lower and the signal to noise ratio lower than in the first case.
• the line width of the source must therefore be chosen accordingly so as not to limit the performance of the optical frequency discriminator chosen.
• a quad ratic photo-detector makes it possible to measure the optical power present in one of the lateral modulation lines.
• the role of the control system is to permanently position the frequency discriminator on a position corresponding to the lateral modulation line produced by the frequency emitted by the microwave generator.
• the description which follows relates to the case of a frequency sweep for a measurement of the dispersion of the "chirp". It is obviously possible to also carry out the measurement on a single and predetermined frequency.
• the measurement can be carried out in the following sequence. -
• the control system switches the continuous voltage source for the first time so as to place the operating point on a maximum transmission of the modulator transfer curve. During this entire first phase, the control and servo subsystem will maintain the modulator at this operating point.
• the latter is previously set to a fixed peak-to-peak amplitude of small amplitude compared to the half-wave voltage of the modulator. If the measurement takes place on a fixed frequency, a sweep is not necessary, and the microwave generator can simply be a preset frequency oscillator.
• the tunable filter synchronizes in optical frequency so that its transmission is permanently set on one of the side lines centered either on ⁇ o- ⁇ , or on ⁇ o + ⁇ . If the measurement takes place on a fixed frequency, the filter is pre-positioned on the lateral modulation line.
• the photo-detector detects the transmitted light intensity and present on one of the lateral lines ⁇ o- ⁇ or ⁇ o + ⁇ , this for each value of pulsation ⁇ supplied by the microwave generator.
• the control system recovers the optical power data measured in a first file, converted beforehand for example into decibels. Alternatively, the measurement data is kept in a random access memory for direct use in the calculation (s) for determining the "chirp".
• the control system then switches the continuous voltage source so as to place the operating point on a minimum transmission of the modulator transfer curve. During this entire second phase, the control and servo subsystem maintains the modulator at this operating point.
• the tunable filter synchronizes in optical frequency so that its transmission is permanently set on one of the side lines centered on ⁇ o- ⁇ or ⁇ o + ⁇ .
• the photo-detector detects the light intensity transmitted and present on one of the lateral lines ⁇ o- ⁇ , or ⁇ o + ⁇ , this for each value of pulsation ⁇ supplied by the microwave generator.
• the control system recovers the optical power data measured in a second file, previously converted for example into decibels.
• control system can extract the “chirp” parameter ⁇ for each value of ⁇ , by subtracting the two stored files then converting decibels to linear unit and extracting the square root of the result.
• FIG. 1 there is shown a short optical pulse measurement system of the prior art to extract the time form and phase of the modulated pulses and to determine the "chirp".
• This system therefore comprises the source of light pulses on the one hand and the analysis system on the other hand.
• the generation of light pulses is obtained by using a radiofrequency generator 1 connected to the light source 2.
• the latter can for example be a semiconductor laser directly modulated by the generator or any other type of pulsed light source to be tested.
• This pulse source 2 is connected to an electro-optic modulator 3 of the Mach-Zehnder type which is part of the measurement system.
• This modulator is also controlled by the electrical signal supplied by the radiofrequency generator 1, but the frequency of which is divided by two by a frequency divider 4.
• a delay line 5 placed downstream of the frequency divider 4 makes it possible to adjust the phase of the radio frequency signal between the optical pulse generator and the modulator.
• the modulator 3 is set to a fixed operating point corresponding to a minimum of its electro-optical transfer curve.
• a tunable spectral filter 6 performs the spectral analysis of the modulation lines produced by the optical source.
• the spectrum of the source presents an envelope containing a certain number of lines.
• the difference between the lines (inter-lines) corresponds to the modulation frequency of the radio frequency generator 1.
• Each line is then split by the electro-optical modulator controlled by a frequency half. In this way, each line can interfere with its direct neighbor.
• the variable delay line can thus vary the amplitude of the inter-line interference signal.
• the photoreceptor 7 detects these variations in light energy for each line and the acquisition system can extract the relative phase for each line. By inverse Fourier transform, this gives access to the shape of the light pulses produced as well as to the time phase variation.
• the modulators shown in Figure 2 are two examples of prior art Mach-Zehnder type modulators.
• the modulator 2. (a) in top view and 2. (b) in sectional view of Figure 2 corresponds to a modulator structure which may have a zero chirp and the modulator 2. (c) in top view and 2 (b) in section view of Figure 2 corresponds to a structure which may have a reduced "chirp".
• These two examples of modulators are manufactured industrially and have different "chirp" parameters. These modulators are produced on a lithium niobate crystal.
• the first configuration 2. (a), 2, (b) is obtained with a crystal 9 in section X and the second configuration 2. (c), 2. (d) with a crystal 10 in section Z.
• Such modulators have mainly a Mach-Zehnder type optical circuit produced from optical waveguides 11 scattered in the substrate.
• the optical circuit consists of a straight input section 12, a straight output section 13 connected to the respective ends of the two arms 14 of the interferometer by couplers 15 in Y.
• the modulators also include a set of coplanar line type electrodes 16. These electrodes have a central line 17 and two lateral ground planes 18.
• the major difference between section X and section Z is shown in section in FIGS. 2. (b) and 2. (d ). Indeed, in the case of cutting X shown in Figure 2.
• Part (b) of Figure 3 represents the spectrum of a modulated optical signal in the case of a maximum transmission of the modulator and part (a) of this same Figure, the corresponding transfer curve.
• Part (d) of Figure 3 represents the spectrum of an optical signal modulated in the case of a minimum transmission of the modulator and part (c) of this same Figure, the corresponding transfer curve.
• Figure 3. (a) shows the principle of small signal modulation and 3. (b) the spectral decomposition of the resulting optical signal.
• the variation in transmitted light intensity 20 has a period half that of the signal incident, which corresponds to a double frequency 2f.
• FIG 3. we observe a peak (or line, the two terms being equivalent) 21 centered on the pulsation ⁇ 0 , and of optical power Eo 2 , corresponding to the emission wavelength of the source of monochromatic light, and two lateral lines 22 and 23 of modulation centered respectively on the pulses ⁇ o- ⁇ and ⁇ o + ⁇ and of optical power Eo 2 . KB 2 . ⁇ 2/4.
• Figure 4 describes the entire “chirp” measurement system of a Mach-Zehnder modulator under test.
• the assembly comprises a monochromatic source 27 connected for example by means of an optical fiber 28 to the modulator 29 under test.
• the latter is supplied by a hyper-frequency generator 30, the frequency sweep of which is controlled by the central control and calculation system 31 which performs, according to a program, the control of the various devices and the analysis and processing of the data collected. during measurements and from the devices.
• the modulator is also connected to a source 32 of direct voltage VB, intended to switch the modulator, on command of the control and calculation means 31, and in particular to pass from a phase shift ⁇ equal to zero to a phase shift equal to - ⁇ radians for example (+ ⁇ radians may also be suitable).
• the modulator has an optical output which, in a preferred mode of implementation, is connected by optical fiber 33 to a detection subsystem 34 having a feedback electronics making it possible to control the operating point of the modulator during the duration of each measurement via the DC voltage source 32. In a simplified version of implementation, no feedback is applied to control the operating point of the modulator.
• the output of the modulator is connected to a tunable filter 35 constituted for example, either by a diffraction grating of angle adjustable by external control, or by a Fabry-Pérot interferometer of which a mirror can be mobile and controlled externally.
• the filter 35 can be any other type of tunable spectral filter of sufficiently high resolution to transmit only the line or lines which are to be measured.
• the transmission of the filter 35 on a lateral modulation line is controlled by the control system 31 synchronously with the frequency scanning of the generator 30.
• a quadratic photo-detector 36 measures the variations in optical power of the line transmitted for each value of the frequency delivered by the generator 30.
• the control and calculation means 31 is preferably a programmable computer means and for example a computer with a program.
• the means 31 can also be an electronic card with digital signal processor (DSP) and / or microprocessor.
• Figure 5 gives an example of a flow diagram of the different sequences required to perform a measurement complete. In the case of a frequency sweep, the steps are referenced by:
• spectral filter performs a wavelength increment scan of its transmission function on a lateral peak 22 and / or 23 of modulation resulting from the spectral analysis of the signal 20 transmitted by the modulator 29 under test; - 42 where the control system 31 controls the frequency sweep of the microwave generator 30 on the wavelength sweep of the tunable filter 35;
• the DC voltage VB supplied by the low voltage source 32 is adjusted for operation on a minimum of the transfer function 18, corresponding to a static phase shift from to - ⁇ radians (for example) between the arms of the interferometer under test 29;
• the spectral filter performs a wavelength incremental scan of its transmission function on a peak lateral 25 and / or 26 of modulation resulting from the spectral analysis of the signal 20 transmitted by the modulator 29 under test;
• control system 31 controls the frequency sweep of the microwave generator 30 on the wavelength sweep of the tunable filter 35;
• the two frequency scanning sequences are terminated and the control and calculation means 31 performs by calculation the extraction of the “chirp” factor a for each value of the modulation frequency.
• the frequency scanning steps are omitted.
• the repetition of measurements can be carried out and statistical calculations carried out in order to reduce the variability of the results.
• the monochromatic source is of variable frequency and the “chirp” is also determined as a function of this.
• the measurement system therefore preferably comprises a monochromatic source 27, an adjustable microwave generator 30 and allowing scanning in frequency or adjustable on a fixed frequency, an adjustable continuous low voltage supply 32, a possible means of controlling the operating point of the modulator.
• a tunable optical spectral filter 35 or adjustable on a fixed or non-fixed optical line under test 34, a tunable optical spectral filter 35 or adjustable on a fixed or non-fixed optical line, a quadratic photodetector for measuring optical power 36, a control and calculation system 31 for control, analysis and processing.
• the invention can be implemented both with a single dedicated device comprising the means necessary for determining the chirp, and with independent devices but under the control of a means of control and calculation.
• the measurement is therefore preferably carried out according to the following steps: Adjustment of the modulator 29 under test to a maximum of its electro-optical transfer function 18. Generation of a harmonic signal 19 of variable frequency. Application of this signal to the modulator under test 29. Measurement, through a tunable optical spectral filter 35, of the optical power present on the lateral modulation lines 22 and / or 23 for each value of the applied frequency. Adjustment of modulator 29 under test to a minimum of its electro-optical transfer function 18. Generation of a harmonic signal of variable frequency. Application of this signal on the modulator under test 29. Measurement, through a tunable optical spectral filter 35, of the optical power present on the lateral modulation lines 25 and / or 26 for each value of the applied frequency. Extraction of the “chirp” parameter ⁇ from the measurements made by frequency sweeping for each of the two operating points.

Landscapes

• Physics & Mathematics (AREA)
• Spectroscopy & Molecular Physics (AREA)
• General Physics & Mathematics (AREA)
• Optical Modulation, Optical Deflection, Nonlinear Optics, Optical Demodulation, Optical Logic Elements (AREA)
• Optical Communication System (AREA)

Applications Claiming Priority (3)

Publications (1)

Family

ID=28800040

Family Applications (1)

Country Status (4)

Families Citing this family (3)

Family Cites Families (3)

• 2002
  • 2002-04-29 FR FR0205393A patent/FR2839151B1/fr not_active Expired - Fee Related
• 2003
  • 2003-04-28 AU AU2003246867A patent/AU2003246867A1/en not_active Abandoned
  • 2003-04-28 EP EP03747468A patent/EP1509755A2/de not_active Withdrawn
  • 2003-04-28 WO PCT/FR2003/001332 patent/WO2003093777A2/fr not_active Ceased

Non-Patent Citations (1)

Also Published As

Similar Documents

Legal Events