FR2872279A1 - Optical and electrical pulses measuring device for e.g. telecommunication domain, has laser source and Kerr cell for forming optical samples from replica pulses, and detector for forming electric signal from optical samples - Google Patents

Abstract

The device has a directional coupler (1) and a single mode optical fiber (F) to create a series of replica pulses (Ri) from an initial optical pulse (I). A feedback loop regulates amplitudes of the replica pulses to values approximately identical to each other. A laser source (7) and a Kerr cell (Ech) form optical samples (Ei) from the replica pulses. A detector (D) forms an electric signal from the optical samples. The feedback loop consists of a three port coupler (2), a control circuit (3) and an adjustable gain amplifier (4).

Description

PROFILE MEASURING DEVICE

  TECHNICAL FIELD AND PRIOR ART The invention relates to a one-shot pulse profile measuring device of very short duration.

  The invention applies to the measurement of laser pulses with durations of between a few picoseconds and a few tens of nanoseconds, in typical analog frequency bands included, taking into account the current technology of optical fibers and associated components, between 10 GHz and 300 GHz.

  The invention is also applicable to the measurement of single-shot electrical pulses with durations of between a few tens of picoseconds and a few tens of nanoseconds, in analog frequency bands included, given the current technology of electro-optical modulators, between 10GHz and 100GHz.

  Advantageously, the invention also applies to the measurement of optical and electrical pulses with a low repetition rate.

  Measuring the short single-pulse pulse pattern is generally difficult and requires expensive instrumentation. Depending on the desired performance, different types of instruments are used, such as, for example: - single-shot power oscilloscopes, for pulses whose typical analog frequency bands vary, for example, from 0 to 7 GHz (the technology is then that of Flash converters), slot scanning cameras (CBF cameras) for pulses whose analog frequency bands typically vary from a few GHz to 100 GHz (the technology used is that of vacuum electronic tubes, with deflection linearized lateral to restore the axis of time); Slot scan cameras are complex and expensive equipment that requires frequent recalibration, low optical sensitivity and high sensitivity to environmental disturbances, autocorrelators for pulses typical analogue frequency bands vary. from a few hundred GHz to several tens of THz (this equipment only provides a relative measure of the duration of the pulse, which measurement is obtained taking into account assumptions about the shape of the profile); the technology used is that of nonlinear optics with implementation of a time / space conversion coupled with image analysis and signal processing.

  Compared to the single-shot pulse measuring devices of the known art, the measuring device of the invention is an optimal combination of simple and original means that make it possible to extend very significantly the analog frequency bands of the measured pulses.

  SUMMARY OF THE INVENTION Indeed, the invention relates to a one-shot or low repetition optical pulse profile measuring device which comprises: means for creating, from an initial optical pulse, a series of replica pulses of the initial optical pulse and of period i, servocontrol means for adjusting the amplitudes of the replica pulses to substantially identical values to each other, means for sampling the replica pulses to deliver optical samples to from the replica pulses, detection means for forming an electrical signal from the optical samples delivered by the sampling means, the optical sampling means comprising a laser source which emits laser pulses of frequency 1 / TB and of duration very substantially less than the duration of the initial optical pulse, the period t of the replica pulses and the period e To laser pulses being connected by the relation: t = To - 5t, where 5t is a temporal slip of duration very much shorter than the duration of the initial optical pulse, and an optical sampler illuminated by the replica pulses and by the laser pulses, the optical sampler being made substantially transparent to the replica pulses under the effect of the laser pulses.

  According to a first variant of the invention, the optical sampler is a Kerr cell which comprises an active Kerr effect medium placed between two crossed polarizers.

  According to a second variant of the invention, the optical sampler comprises: a divider for dividing a replica pulse into a first replica pulse fraction and a second replica pulse fraction; a saturable absorber comprising a first zone; a second zone illuminated, respectively, by the first and second replica pulse fractions, the first zone being made substantially transparent under the effect of the pulses of the sampling laser source, and - an interferometric optical subtraction device which delivers a optical sample as a result of the difference between a sampled signal outputted from the first saturable absorber area and a signal outputted from the second saturable absorber area.

  According to a further feature of the invention, the measuring device comprises an interferometric device placed at the output of the optical sampler to reduce the width of the optical samples.

  According to a preferred embodiment of the invention, the means for creating a series of replica pulses of the initial optical pulse comprise a four-port coupler and a waveguide, for example a monomode optical fiber, looped back on the coupler.

  The invention also relates to a one-shot or low-repetition electric pulse profile measuring device, characterized in that it comprises an electro-optical modulator in series with an optical pulse profile measuring device according to the invention. 'invention.

  The optical pulse measuring device according to the invention covers frequency bands currently inaccessible to known oscilloscopes which measure single-shot optical pulses. Only CBF cameras provide access to frequencies around 100 GHz, but with a dynamic encoding much less than 8 bits, dynamic made accessible by the device of the invention.

  The pulse measuring device according to the invention has an extended coding dynamic associated, by construction, with a very high temporal resolution. The dynamics is essentially limited by the noise factor of the servo loop and the optical / electrical conversion output. The transfer function of the device is linear, which allows to maintain optimal dynamics. This is not the case in the nonlinear conversion mechanisms of the parametric amplification or optical frequency conversion type, or in the autocorrelator type configuration.

  According to the invention, the measurement sensitivity is very high. At identical temporal resolution, one can quote a higher sensitivity of several decades than that of the CBF cameras. It is also advantageous to compare the performances of the device according to the invention with those of the autocorrelators, in which the nonlinear optical conversion process is incompatible with the conversion of low-flux optical signals.

  The implementation of the device of the invention using a fiber loop coupled to a saturable absorber has the advantage of great technological simplicity, with a very small number of active components. A consequence of this simplicity with regard to accessible performance is the possibility of realization at a reduced cost. A very advantageous comparison can be made, from this point of view, with all-optical oscilloscopes for applications in the field of telecommunications and with CBF cameras. The need for signal processing and calibration, essential for the operation of measurements, are reduced. If one refers to the constraints of implementation of a CBF camera, which periodic recalibration is essential, the device of the invention has a significant advantage in terms of use.

  The device of the invention is particularly robust. Indeed, all the critical optical components of the sampling function are preferably fibered, resulting from proven technologies in the telecommunications and laser sectors. Similarly, the electronic part downstream of the device consists of components that are specified in frequency bands well below the high analog frequencies of the optical part. The fastest components are, for example, the photodiodes of electrical / optical conversion, which function naturally in integrator mode and whose response time must simply remain less than T. In the case of the measurement of electrical pulses, the device of the invention advantageously allows an analysis of pulse profiles in a wide range of frequencies. Despite the reduction in the analog bandwidth resulting from the insertion of an electro-optical input converter, the electrical pulse measuring device according to the invention offers much better performances than those of known equipment such as, for example, for example, CBF cameras whose encoding dynamics are very limited when used on their fastest time bases.

  The basic concept of the measuring device according to the invention is very flexible and scalable, as it can be adapted to a wide range of compromises to be optimized with regard to the desired measurement performance.

Brief description of the figures

  Other features and advantages of the invention will appear on reading a preferred embodiment of the invention with reference to the appended figures among which: FIG. 1 represents a single-shot optical pulse measuring device according to FIG. 'invention; Figs. 2A-2B show timing diagrams that illustrate the operation of the measuring device shown in Fig. 1; FIG. 3 represents a first variant of a particular circuit of the measuring device according to the invention; FIG. 4 represents a second variant of the particular circuit of which a first variant is represented in FIG. 3; FIGS. 5A-5C represent time diagrams that illustrate the operation of the circuit represented in FIG. 4; FIG. 6 represents an improvement of the measuring device represented in FIG. 1; FIG. 7 represents a detailed view of a circuit of the measuring device according to the improvement shown in FIG. 6; Figs. 8A-8B show timing diagrams that illustrate the operation of the circuit shown in Fig. 7; FIG. 9 represents another embodiment of the measuring device according to the invention; FIG. 10 represents an improvement of the measuring device represented in FIG. 9; FIG. 11 represents a single-shot electrical pulse measuring device according to the invention.

  In all the figures, the same references designate the same elements.

  Detailed description of modes of implementation of

  FIG. 1 shows an optical pulse measuring device according to the preferred embodiment of the invention.

  The device comprises a four-port coupler 1, a single-mode optical fiber F, a three-port coupler 2, a control circuit 3, an adjustable gain amplifier 4, a source 7 of recurrent laser pulses of very short duration, an optical sampler Ech, a detector D, an analog / digital converter CN and a memory M. The optical fiber F, of length L, is of monomode type in order to prevent mode dispersion effects appearing, which would widen pulse signals that propagate. Coupler 1 has four ports a, b, c, d. The port a constitutes the input of the measuring device, the port b is connected to the input of the three-port coupler 2, the port c is connected to the input of the amplifier 4 via a first section of the fiber F. and the port d is connected to the output of the amplifier 4 via a second section of the fiber F. The four-port coupler 1 is a directional coupler which operates as follows: a signal which enters the port a is transmitted for A fraction of a%, for example 5%, on the port b and for the remaining fraction of (1-a)%, for example 95%, on the port c, and - a signal entering the port d is transmitted for a fraction of a% on port b and for the remaining fraction of (1-a)% on port c.

  An initial laser pulse I which enters the measuring device via the port a of the coupler 1 splits accordingly into a first pulse fraction (a%) delivered on the port b and a second pulse fraction ((1 a)%) issued on the port c. The first pulse fraction is directed towards the coupler 2 and the second pulse fraction is looped back, via the fiber F and the amplifier 4, onto the port d of the coupler 1. The propagation speed v of the second fraction d pulse on the fiber F of length L defines a loop time i such that: T = L / v When it enters the port d, the second pulse fraction thus looped is itself divided into a third fraction d pulse (a%) which is delivered on port b, to coupler 2, and a fourth pulse fraction ((1-a)%) which is looped back on port d. At each loop turn of a pulse fraction, the coupler 1 thus returns a fraction a% of the power it receives on the output port b and a fraction of (1-a)% on the port c.

  An initial laser pulse which enters the measuring device via the port a of the coupler 1 is thus at the origin of a series of replica pulses of the initial pulse which are separated from each other by the duration T. G gain of amplifier 4 which is located in the loop is set to compensate for coupling losses. The following condition is then realized: a x G = 1 (1) The relation (1) above is obtained using a servo loop. For this purpose, a fraction of each replica is taken using the coupler 2. The coupler 2 is a three-port coupler e, f, g. The port e is connected to the port b of the coupler 1, the port f is connected to the input of the divider 5 and the port g is connected to the input of the control circuit 3. A signal which enters the coupler 2 by the port e is divided into a first fraction delivered on the port g and a second fraction delivered on the port f. The fraction delivered on the port g can represent, for example, 3% of the signal that enters the port e. This first fraction of the signal constitutes the input signal of the control circuit 3. The control circuit 3 evaluates the amplitude of the signal fraction that it receives on its input and consequently delivers a gain control Cg which, applied at amplifier 4, provides a loop gain equal to 1 (condition (1) above) so that all the replicas have substantially the same amplitude.

  The pulses collected on the output port f of the coupler 2 then constitute a series of replica pulses Ri (i = 1, 2, ..., N) of period T having substantially the same amplitude. The replica pulses Ri are transmitted to the input of the optical sampler Ech, which samples them under the action of the laser pulses IL delivered by the laser pulse source 7. The laser pulse source 7 is preferably a blocking source of modes that provides laser pulses of To period and TML duration much less than T (TML T).

  Figures 2A and 2B show timing diagrams for illustrating the sampling of replica pulses.

  FIG. 2A illustrates the pulses of width TML and of period To which are delivered by the laser source 7.

  FIG. 2B shows a succession of replicas R1, R2, R3,..., RN sampled on the basis of the pulses represented in FIG. 2A. The first replica R1 is sampled at a first time position Ti, the time position Ti measuring a duration that separates a time that marks the beginning of the replica R1 and the sampling time. The replicas Ri of rank i greater than 1 (i> 1) are sampled at the respective time positions T1 + (i-1) 5t. Thus, the sampling instant slips progressively from the quantity 5t inside the profile of the successive replicas. This sampling principle is similar to that of electronic sequential sampling used for recurrent pulses. The configuration implemented in the context of the optical pulse measurement according to the invention is however very different from that used in the context of the measurement of recurrent electrical pulses. The method of the invention indeed has the advantage of restoring a true pulse profile and not an averaged time profile as is the case for the measurement of recurrent electrical pulses.

  The samples Ei (i = 1, 2, ..., N) delivered by the optical sampler Ech are transmitted to a detector D, for example a photodiode. The detector D can operate, for example, in analog frequency bands between 10Ghz and 60Ghz, as currently permitted by telecommunications technologies. The optical / electrical conversion implemented by the detector D operates in integrator mode. The integrating mode consists in assimilating the amplitude of a given optical sample to the value integrated in the time of the electrical response of the detector. The only errors made, to be corrected eventually, are those resulting from the effects of desaturation of the detector, the shifting of the baseline induced by a stacking effect of successive electrical pulses, etc. The detector D delivers electrical samples which are transmitted to the analog-digital converter CN which codes the level of the signals which it receives. The coding may be, for example, an 8 or 10 bit coding. The coding time is not critical. It is only limited by the value of T. The digital data delivered by the analog-digital converter CN is then transmitted to the memory M whose access time is at least equal to T. Figure 3 shows the optical sampler Ech according to a first variant of the invention.

  The Ech optical sampler is a Kerr cell. The Kerr cell comprises an active Kerr effect medium K placed between two crossed polarizers P1, P2. When applying a laser pulse IL delivered by the source 7, the medium K has a transient cooling sufficient to move the device from a blocked state (non-depolarizing medium placed between two crossed polarizers) to a on state ( transmission of the optical polarization component in the plane of the output polarizer).

  The optical sampler in the form of a Kerr cell has excellent temporal resolution. It is indeed possible to deliver optical samples whose width is of the order of two hundred femtoseconds.

  The Kerr effect active medium may be, for example, a CS2 type liquid requiring sealed enclosure packaging. The Kerr-effect active medium can also be a glass-based solid medium which then requires a higher laser intensity at the output of the source 7.

  FIG. 4 represents the optical sampler Ech according to a second variant of the invention.

  The sampler Ech then comprises a divider by two 5, a fast saturable absorber 6, an interferometric optical subtraction device DISO and collimation lenses L1, L2, L3, L4. The interferometric optical subtraction device DISO comprises two input arms B1, B2, the arm B2 being provided with a phase shifter Dph, and a recombination junction J with two inputs and an output. The input and the output of the sampler are respectively the input of the divider by two and the output of the junction J. The arms B1 and B2 and the junction J are made using any type of guide. optical waves, for example optical fibers.

  A replica pulse Ri received at the input of the sampler Ech is divided into a first pulse fraction la and a second pulse fraction Ib of amplitude substantially identical to the amplitude of the pulse fraction Ia. The first pulse fraction 1a is transmitted to a first zone Z1 of the saturable absorber via a first collimation lens L1 and the second pulse fraction Ib is transmitted to a second zone Z2 of the saturable absorber via a second lens collimation L2.

  The operation of a saturable absorber is based on the saturation phenomenon of the absorption. Low intensity signals that pass through a saturable absorber undergo intrinsic absorption of the absorbing medium and are, therefore, partially absorbed. On the other hand, signals of high intensities whiten the absorbent medium through which they pass until reducing the absorption thereof to a value substantially lower than that of the initial absorption. High intensity signals can then be partially transmitted.

  From a mathematical point of view, the transfer characteristic of a saturable absorber is expressed by the following relation:

AT

  A (_ (3), where (Dsat A (0) is the absorption coefficient of the absorbent 20 as a function of the intensity 0 of the signal which illuminates the absorbent, Ao is the attenuation coefficient of the absorbent in the absence of illumination, and Osat is the proper saturation intensity of the absorbent (W / cm 2).

  In the context of the invention, the zone Z1 is illuminated by the pulse fractions Ia and by the pulses IL while the zone Z2 is illuminated by the only pulse fractions Ib. The peak power PML of the pulses IL originating from the 1 + mode 7 blocking laser source (source SML) is chosen to saturate the transmission of the absorbent completely or at least very significantly during the TML periods of the laser pulses supplied by the source 7. The beam of IL pulses and the beam of pulse fractions la are not collinear. The beam of the pulses IL is collimated on the zone Z1 of saturable absorber, via a collimation lens Lc, along an axis offset by a few degrees to a few tens of degrees of the axis along which the beam of the pulse fractions is collimated. the. The signal collected at the output of the zone Z1 and which is collimated, via the collimation lens L3, on the input of the arm B1 then consists of the only pulse fractions la which have passed through the zone Z1.

  For a pulse fraction la which enters the zone Z1, the zone Z1 behaves like a sampling gate which operates at the frequency Fo of the mode-locking laser source (Fo = 1 / To). The opening time Tech of the sampling gate is approximately equal to the recovery time Tsat of the saturable absorber. The recovery time Tsat has a duration greater than the pulse duration TML.

  The quantities t 1 and T 0 are connected so that: t i = (To - St) (2) where St represents a time slip of the sampling instant between two loop turns (see FIG. 2B). The sampling rate of the measuring device is then equal to St, which is greater than or equal to Tsat, but which is much smaller than the duration TI of the input pulse. The maximum number of samples that can be acquired is N such that: N = T / 5t (3) The number N can also be written: N = 1 / ((To / T) -l) (4) The saturable absorber according to the invention can be made according to several different technologies. A liquid solution based on solid matrix dyes may be envisaged, or a solid saturable absorbent based on carbon nanotubes or semiconductors. The solid absorbers of this type currently the fastest have Tsat recovery times of the order of 1 to 5 picoseconds. From a spectral point of view, the saturable absorber is also chosen, preferably, to be compatible with an optical operation in the near infrared which is a frequency band useful for lasers based on neodymium, ytterbium or Erbium.

  While the zone Z1 behaves like a sampling gate, the zone Z2 has an absorption of coefficient Ao. The signal delivered by the zone Z2 is transmitted via the collimation lens L4 to the input of the arm B2.

  FIG. 5A represents the sampled signal E (B1) collected at the output of zone Z1 and FIG. 5B represents the signal E (B2) collected at the output of zone Z2. The signal E (B1) shows a sampling peak Pech of width Tech within a signal that reproduces, by attenuating, the pulse fraction Ia. The signal E (B2) reproduces, by attenuating it, the pulse fraction Ib.

  The B2 arm is equipped with a voltage controlled Dph phase shifter. The voltage V1 applied to the phase shifter Dph is adjusted so that the optical signal delivered at the output of the arm B1 is in phase opposition with the signal delivered at the output of the arm B2. The J junction which sums the signals delivered by the arms B1 and B2 then delivers a sample Ei of width Tech (see FIG.

  FIG. 6 represents an improvement of the measuring device represented in FIG. 1. The improvement of the invention applies as well in the case of the first variant as of the second variant.

  In addition to the elements shown in FIG. 1, the measuring device comprises a dispersion compensator 15, an optical window 16 and an interferometric fitness device 17. It is for the sake of convenience that the three devices 15, 16 and 17 are represented in the same figure. In fact, the devices 15, 16 and 17 are independent of each other and can be integrated with the measuring device together or separately.

  The compensator 15 is inserted in the loop between the output of the amplifier 4 and the port d of the coupler 1. Its function is to compensate for the effects of chromatic dispersion of the loop which would otherwise lead to a deformation of the pulse who is spreading. The compensator 15 may be, for example, a Bragg grating device of spatially variable pitch (chirped in English) or a fiber section whose dispersion coefficient is opposite to the dispersion coefficient of the fiber constituting the loop.

  The optical window 16 is placed between the source 7 of pulses IL and the sampler Ech. It allows to define an elementary sampling sequence for a duration OTE such that: ATE = N x 5t, where N is the maximum number of samples that can be acquired (see equation (3)). The duration TI of an optical pulse must then necessarily be less than ATE. The function of the optical gate 16 is to define a time window for the application of the pulse train emitted by the source 7. This makes it possible, inter alia, to prevent the saturable absorber from being permanently illuminated.

  According to a particular embodiment of the second variant of the invention (in the case where the optical sampler comprises a saturable divider and absorber), the four-port coupler 1, the waveguide which loops over the four-port coupler, the three-port coupler 2, the amplifier 4 and the divider 5 are integrated on an implanted waveguide semiconductor substrate. The amplifier 4 is then produced in semiconductor technology. In the context of this particular embodiment, the compensator 15 relating to the improvement of the invention then consists of a Bragg grating implanted on the waveguide.

  FIG. 7 shows the interferometric shaping device 17 in the case of free space propagation and FIGS. 8A and 8B show time diagrams which illustrate the operation of the interferometric device 17.

  The interferometric device 17 comprises a balanced separator 18, an externally biased adjustable delay circuit 19, a combiner 20, two mirrors 21, 23 and a fixed delay circuit 22 (delay Td). The adjustable delay circuit 19 is controlled by a bias voltage Vb. An optical sample Ei delivered by the optical sampler Ech is transmitted to the balanced separator 18. The balanced separator 18 then transmits a first subsample to the polarization-adjustable delay circuit 19 in a small range of values and a second subsample to the mirror 21. The sub-sample which is transmitted on the mirror 21 is reflected by the latter and transmitted to the second mirror 23 through the delay circuit 22. The mirror 23 reflects the sub-sample that it receives and transmits the to the combiner 20 which receives, moreover, the pulse sub-sample transmitted through the polarization-adjustable delay circuit 19. The combiner 20 combines the two sub-samples that it receives and transmits a signal of sample reshaped towards photodiode 8.

  The delay circuit 22 creates a delay Td between the first sub-sample and the second sub-sample. The second sub-sample which passes through the delay circuit 22 is then delayed by the quantity Td with respect to the first sub-sample which passes through the delay circuit 19. When a sample signal arrives on the separator 18, both that the duration Td has not elapsed, the signal delivered at the output of the interferometric device 17 then consists only of the first sub-sample, that which passes through the adjustable delay circuit 19. When the duration Td has elapsed, the delivered signal by the interferometric device 17 consists of the combination of the first and second subsamples (combiner 20). The adjustable delay circuit 19 is set to a bias voltage Vb equal to the voltage demionde. It follows that the first and second subsamples that combine are in phase opposition and are destroyed by interference. The signal delivered by the interferometric device 17 then consists of a single sub-sample of duration Td, a duration beyond which only small residuals of extinction Rext remain (residues of the interferences between the first and second sub-samples ). FIG. 8B illustrates the signal SB delivered by the interferometric device 17. FIG. 8A illustrates the various signals whose combination leads to obtaining the signal SB illustrated in FIG. 8B. The signal S1 represents the first subsample that passes through the circuit 19 and the signal S2 represents the second subsample that passes through the delay circuit 22.

  The interferometric device 17 is well suited to quasi-exponential monotonic decay profiles, as are the profiles characterizing the transmission overlap phase of the saturable absorbers (see FIGS. 8A-8B).

  The value of the delay Td depends on the optical path difference between the two arms of the interferometric device 17. It must be generally stable and precise. In the case of an embodiment in free space, a delay circuit 22 may be made, for example, by a plate of index and thickness calibrated placed between the two reflecting mirrors 21 and 23. The length of the medium is then chosen according to the desired value for the duration Td. In the case of guided propagation, the interferometric device 17 may be integrated in a waveguide component implanted on an electro-optical substrate, in a technology similar, for example, to that of modulators for telecommunications.

  As mentioned previously, in the absence of interferometric device 17, the temporal resolution is limited by the recovery time Tsat of the saturable absorber which must then be as short as possible. Advantageously, the interferometric device 17 makes it possible to obtain sampling signal durations shorter than the recovery time Tsat. It is then possible, for example, to reduce the duration Tech of a value Tsat between 1 and 5ps to a value Td equal to a few hundred femtoseconds. The presence of the interferometric device 17 thus makes it possible to significantly improve the temporal resolution limit of the measurements and, consequently, to extend the scope of the fields of application of the invention.

  Apart from the fact that the shaping of the rear transition must be adjusted in a very fine way to eliminate any signal residue after the rear transition, another phenomenon must be taken into account: the shaping performed by the interferometric device modifies the energy balance. A specific calibration coefficient can be used to account for the change in signal shape. A preliminary calibration phase is then necessary.

  Although not restrictive, some characteristic figures make it possible to quantify the basic quantities and the range of the performances accessible within the framework of the invention. Consider the case of standard SML sources, with TML = 100 to 200 fs, and a loop time i = 10 ns (ie L = 2 m) which corresponds to the usual value Fo = 100 MHz. The maximum number of samples is in the range N = 103 to 109 samples, insofar as the amplifier 4 of the loop has sufficient saturation power and the coupling rate at the output of zone Z1 remains low. By way of example, a resolution value 8t = 3ps on the profile of a pulse of duration TI = 30 ns implies N = 10000 points. N is also the minimum memory depth needed to save the acquired samples. Considering a complete configuration including the interferometric device 17, it is possible to evaluate the fields of application and metrological performance covered by the invention.

  In the current state of the art optical components available for pulses whose TI duration can vary from a few 10 picoseconds to a few 10 nanoseconds, the following values provide an overview of the level of expected performance: a range of resolutions 8.sup.-picosecond and a few picoseconds, a dynamic encoding amplitude greater than 8 bits, a repetition rate of the profile measurement between the one-shot and a few gigahertz. The 8-bit encoding is compatible with the performance of commercial fiber amplifiers and standard photodiode amplifiers. The best current amplifiers have a noise factor F = 4 to 5 dB. The usual photodiodes offer a dynamic of the order of 30 dB. For all these figures, compromises are to be assessed according to the metrological needs. As regards the illumination requirements in the picosecond regime at the level of the saturable absorber, for example in the case of carbon nanotubes whose total whitening occurs at around 10 MW / cm 2, it should also be noted that they are quite in the field of accessible fluences with existing SML source technologies: at F0 = 100 MHz and PML = 10 MW peak / cm2 corresponding to a laser sample energy of 10 pJ on a section of 10 pm2, an average power source equal to 1 mW.

  Figure 9 shows another embodiment of the measuring device according to the invention.

  The measuring device comprises a four-port coupler 1, a single-mode optical fiber F, a three-port coupler 2, a control circuit 3, an amplifier 4, a divider by two 5, a saturable absorber 6, a source 7 laser pulses, two detectors 8, 9, two analog / digital converters 10, 11, a differential circuit 12, a memory 13 and a display device 14.

  Elements 1, 2, 3, 4, 5, 6 and 7 are interconnected in the same way as before. The signal delivered by the zone Z1 of the absorbent is here transmitted to a first detector 8 and the signal delivered by the zone Z2 is transmitted to a second detector 9. The detectors 8 and 9 are, preferably, photodiodes whose operating time climb is included, for example, between ten and a few hundred picoseconds. The photodiodes 8, 9 are placed behind the zones Z1 and Z2 in a collimated coupling configuration. The photodiodes 8 and 9 can operate, for example, in analog frequency bands between 10GHz and 60 GHz, as currently authorized technologies telecommunications networks. Although this is not essential, a full-length single-mode fiber transport is possible between the outputs of the zones Z1 and Z2 and the respective detectors 8 and 9. The bandwidth constraints behind the sampling function are indeed strongly relaxed and the electrical / optical conversion is operated in integrator mode.

  The electrical pulses delivered by the photodiodes 8 and 9 are transmitted to the respective analog-digital converters 10 and 11 which numerically code the level of the signals they receive. The coding may be, for example, 8 to 10-bit coding. The coding time is not critical. It is only limited by the value of T. The digitized data are then exploited by a calculation electronics (differential circuit 12) whose role is to determine the difference AP between the signal delivered by the analog-digital converter 10 and the delivered signal. by the digital-to-analog converter 11.

  The difference operation previously carried out optically is here carried out digitally.

  FIG. 10 represents an improvement of the measuring device represented in FIG. 9. In addition to the elements represented in FIG. 9, the measuring device comprises a compensator 15 and an optical window 16. The compensator 15 and the optical window 16 are placed at the same place than in the development shown above in Figure 6 and have the same advantages as before.

  Advantageously, the invention is applicable not only to optical pulses but also to electrical pulses. It then suffices to convert into an optical pulse the electrical pulse to be measured. An electro-optical modulator is then placed on the port a of the coupler 1 to ensure the conversion. The electro-optical modulator may be a direct modulation modulator or an indirectly modulating modulator. In the latter case, which is the one that allows the largest bandwidths, the modulator is fed at its input by a low power continuous laser source. In the case of direct modulation, the modulator may be, for example, a diode. In the case of the measurement of electrical pulses, a certain degradation of the frequency performances is however to be considered, the transfer function of the electro-optical modulator acting as a limiting factor in terms of the analog bandwidth of the device. On the other hand, the temporal resolution of the sampling is preserved.

  The electro-optical modulator may be, for example, a modulator derived from electroabsorbent or polymer technologies or a traveling wave modulator. FIG. 11 illustrates an electrical pulse measuring device according to the invention. The device comprises an electro-optical modulator 24 in series with an optical pulse profile measuring device according to the invention 25. The electro-optical modulator 24 converts an electric pulse IE into an optical pulse I.

Claims (26)

  1. Device for measuring single-shot optical pulse or low repetition rate, characterized in that it comprises means (1, F) for creating, from an initial optical pulse (I), a series of replica pulses (Ri) of the initial optical pulse and of period z, servocontrol means (2, 3, 4) for adjusting the amplitudes of the replica pulses (Ri) to values substantially identical to each other, sampling means (7, Ech) of the replica pulses (Ri) for delivering optical samples (Ei) from the replica pulses (Ri) and detection means (D) for forming an electrical signal from the optical samples (Ei) delivered by the sampling means (7, Ech), the sampling means comprising a laser source (7) which emits laser pulses (IL) of frequency 1 / TB and of duration (TML) which is very much lower to the duration of the initial optical pulse, the i and the period To, laser pulses being connected by the relation: T = To - 5t, where 5t is a temporal slip of duration very much shorter than the duration of the initial optical pulse, and an optical sampler ( Ech) illuminated by the replica pulses (Ri) and the laser pulses (IL), the optical sampler (Ech) being made substantially transparent to the replica pulses (Ri) under the effect of the sampling laser pulses.   2. Device according to claim 1, characterized in that the optical sampler (Ech) is a Kerr cell which comprises an active medium Kerr effect (K) placed between two crossed polarizers (P1, P2).   3. Device according to claim 1, characterized in that the optical sampler (Ech) comprises a divider (5) for dividing a replica pulse (Ri) into a first replica pulse fraction (Ia) and a second fraction of replica pulse (Ib), a saturable absorber (6) comprising a first zone (Z1) and a second zone (Z2) illuminated, respectively, by the first and second replica pulse fractions, the first zone (Z1) being rendered substantially transparent under the effect of the laser pulses (IL) and an interferometric optical subtraction device (DISO) which delivers an optical sample (Ei) as a result of the difference between a sampled signal outputted from the first zone (Z1) and a signal delivered at the output of the second zone (Z2).   4. Device according to claim 3, characterized in that the interferometric optical subtraction device (DISO) comprises a first arm (B1) having an input for collecting the sampled signal delivered at the output of the first zone (Z1), a second arm (B2) for collecting the signal delivered at the output of the second zone (Z2), a voltage-controlled phase shifter (Dph) placed in series on the second arm, to establish a phase shift of n between the optical signal delivered at the output of the first arm (B1) and the optical signal delivered at the output of the second arm (B2) and a junction (J) with two inputs and an output which collects on a first input the signal delivered at the output of the first arm (B1) and on a second input the signal delivered at the output of the second arm (B2) and which delivers the optical sample (Ei) on its output.   5. Device according to any one of claims 3 or 4, characterized in that the saturable absorber is a liquid solution based on dyes.   6. Device according to any one of claims 3 or 4, characterized in that the saturable absorber comprises nano-carbon tubes.   7. Device according to any one of claims 3 or 4, characterized in that the saturable absorber is a semiconductor absorbent.   8. Device according to any one of the preceding claims, characterized in that it comprises an interferometric device (17) placed at the output of the optical sampler (Ech) to reduce the width of the optical samples (Ei).   9. Device according to claim 8, characterized in that the interferometer device (17) comprises: a balanced separator (18) which divides an optical sample into a first subsample and a second subsample, - a first arm which comprises a first mirror (21), a second mirror (23) and a delay circuit (22) placed between the first mirror and the second mirror to form a first optical path for the first sub-samples, - a second arm which comprises an externally bias adjustable delay circuit (19) biased at a voltage (Vb) substantially equal to a half-wave voltage to provide a second optical path for the second subsamples, and - a combiner (20) for combining the first and second subsamples which propagate respectively on the first and second arms.   10. Device according to any one of the preceding claims, characterized in that the means (1, F) for creating a series of replica pulses of the initial optical pulse comprise a four-port coupler (a, b, c, d) and a waveguide (F), the four-port coupler having two input ports (a, d) and two output ports (b, c), a first input port (a) constituting the input through which the initial optical pulse enters the measuring device and a first output port (b) constituting an output by which the replica pulses are output, the waveguide (F) connecting the second output port ( c) at the second port of entry (d).   11. Device according to claim 10, characterized in that the servocontrol means (2, 3, 4) comprise a three-port coupler (2) whose input port (e) is connected to the output by which the replica pulses are provided, a control circuit (3) and an amplifier (4) placed in series on the waveguide (F), the three-port coupler (2) for taking a replica pulse fraction to transmit said pulse fraction replicates to the control circuit (3) which delivers a control signal (Cg) according to the amplitude of the replica pulse fraction taken, the control signal (Cg) being applied as a gain control of the amplifier (4).   12. Device according to claim 10 or 11, characterized in that the waveguide (F) is a monomode fiber.   13. Device according to claim 12, characterized in that it comprises a dispersion compensator (15) connected in series on the monomode fiber (F).   14. Device according to claim 13, characterized in that the compensator (15) is a Bragg grating of spatially variable pitch.   15. Device according to claim 13, characterized in that the compensator (15) is a piece of fiber whose dispersion coefficient is opposite to the dispersion coefficient of the monomode fiber (F).   Device according to claim 11, with the exception of claims 1 and 2, characterized in that the four-port coupler (1), the waveguide, the three-port coupler (2), the amplifier ( 4) and the divider (5) are integrated on a semiconductor substrate with implanted waveguides.   17. Device according to claim 16, characterized in that it comprises a dispersion compensator (15) consisting of a Bragg grating implanted in series on the waveguide.   18. Device according to any one of the preceding claims, characterized in that it comprises an optical window (16) placed in the path of the laser pulses (IL), between the laser pulse source (7) and the sampler Optical (Ech).   19. Device according to any one of the preceding claims, characterized in that the detection means (D, CN) comprise at least one photodiode (D) in series with an analog / digital converter (CN).   20. Device according to claim 19, characterized in that the analog / digital converter (CN) is an 8-bit or 10-bit converter.   21. Device according to any one of claims 19 or 20, characterized in that it comprises a memory (M) which stores the signals delivered by the analog / digital converter (ON).   22. Device according to any one of the preceding claims, characterized in that the laser pulse source (7) is a mode-locked source.   23. Device for measuring electric pulse profile single shot or low repetition rate, characterized in that it comprises an electrooptic modulator in series with an optical pulse profile measuring device according to any one of claims 1 to at 22.   24. A single-shot electric pulse measuring device according to claim 23, characterized in that the electro-optical modulator is an indirectly modulated modulator.   25. One-shot electric pulse measuring device according to claim 23, characterized in that the electro-optical modulator is a modulator with direct modulation.   26. One-shot electrical pulse measuring device according to claim 25, characterized in that the electro-optical modulator is a diode.