Characterization of the non-linear behaviour of a waveguide

Abstract

The disclosed device (100) for characterizing a non-linear behaviour of a waveguide (101) is based on a Sagnac interferometer (2). The waveguide is intended to be traversed by two portions of an emission in opposite directions of propagation. Advantageously, an interference state, which is produced by the interferometer and which is used to deduce values of coefficients characterizing the non-linear behaviour of the waveguide, may be measured by synchronous detection. Automated operation of the device is also possible, with a view to varying a power of the radiation injected into the interferometer. The non-linear behaviour of the waveguide may then be characterized quickly, accurately and inexpensively.

Description

Description Title: CHARACTERIZATION OF THE NON-LINEAR BEHAVIOR OF A WAVEGUIDE Technical field

[0001] The present description relates to the characterization of the non-linear behavior of a waveguide, by providing a device and a method for carrying out such characterization. Prior art

[0002] A waveguide is said to have linear behavior when the complex field amplitude of a spectral component of electromagnetic radiation that is transmitted by this waveguide, at its output, is proportional to the complex field amplitude that this same spectral component has when it is injected at the input into the waveguide. The index value that is effective in determining the length of the optical path in the waveguide between its input and its output is then independent of the intensity of the spectral component considered. But several mechanisms of interaction of electromagnetic radiation with a propagation medium are known, which provide a non-linear behavior to a waveguide that is made from this propagation medium. The non-linear Kerr effect, two-photon absorption and free-carrier absorption are among these mechanisms that produce non-linear behavior.Thus, variations of a local power and a local phase of a spectral component of electromagnetic radiation propagating in a waveguide are governed by the following two differential equations:. In these equations which are known to those skilled in the art: x is a longitudinal coordinate of the waveguide, parallel to the direction of guided propagation of the radiation in this waveguide, P is the local power of the spectral component of electromagnetic radiation, which is a function of the longitudinal coordinate x, is the local phase of the spectral component of electromagnetic radiation, which is also a function of the longitudinal coordinate x, a is the linear absorption coefficient of the waveguide, y is the non-linear Kerr coefficient of the waveguide, PTPA is the two-photon absorption coefficient of the waveguide, PFCA is the free-carrier absorption coefficient of the waveguide, Aeff is the effective mode cross-sectional area of ​​the spectral component of the electromagnetic radiation in the waveguide, and p is a dimensionless parameter that quantifies the effect of dispersion that is caused by free carriers in the propagation medium inside the waveguide. If necessary, one can refer to one of the available scientific publications which explain these two differential equations.

[0003] However, many applications use waveguides requiring knowledge of their nonlinear behavior with sufficient accuracy. Such applications include the transfer of high-power laser radiation, neuromorphic or quantum optical computers, environmental sensors, etc. However, most of the methods currently available to characterize the nonlinear behavior of a waveguide provide insufficient levels of characterization accuracy. One of the main difficulties stems from the fact that the effects of the nonlinear behavior of a waveguide generally appear in superposition with the effects of its linear behavior, while the latter can be more important or much more important than those of the nonlinear behavior.In the interpretation of measurements characterizing the behavior of a waveguide, it is then necessary to distinguish contributions from non-linear behavior from contributions from. linear behavior. In particular, a variation that the phase undergoes <t>of the radiation between the waveguide input and the waveguide output, is the addition of a linear behavior contribution, usually the largest, and a nonlinear behavior contribution, usually smaller. Because of this, each nonlinear behavior contribution that is evaluated from phase measurement results is affected by a significant uncertainty.

[0004] In particular, the article entitled “Heterodyne interferometry applied to the characterization of nonlinear integrated waveguides”, Optics Letters, Vol. 45, No. 18, pp. 5053-5056, September 15, 2020, describes characterizing the nonlinear behavior of a waveguide by inserting this waveguide into one of the two optical paths of a Mach-Zehnder interferometer. But this characterization method has the following drawbacks: - because it is necessary to use a very short pulse radiation source to achieve sufficient levels of instantaneous light power, it is essential to adjust the length of the optical path of the interferometer in which the waveguide is not inserted so that it is substantially equal to that of the other optical path in which the waveguide is inserted. Such an adjustment allows two parts of the same light pulse to interfere with each other after propagating separately in the two optical paths of the Mach-Zehnder interferometer. But this adjustment of the optical path length is tedious, difficult and time-consuming, requiring optoelectronic equipment and ultra-fast measuring devices. Because of this, this characterization method is laborious and expensive in cost; and - as indicated above, subtracting the contribution of the linear behavior of the waveguide from the phase variation measurement results reduces the precision that is accessible to characterize the contribution of the non-linear behavior of the waveguide to these phase variations. Technical problem

[0005] From this situation, an aim of the present invention is to make it possible to characterize the non-linear behavior of a waveguide by reducing or eliminating at least some of the difficulties which have just been recalled for the previous methods.

[0006] More specifically, the invention aims to characterize the non-linear behavior of a waveguide, with a precision that is improved compared to prior methods.

[0007] It also aims to provide a characterization method that is faster, easier and less expensive to implement than previous methods.

[0008] In particular, the invention may aim to provide numerical values ​​for the non-linear Kerr effect coefficient of a waveguide, and if possible also for its two-photon absorption coefficient and / or its free-carrier absorption coefficient. Summary of the invention

[0009] To achieve at least one of these aims or another, a first aspect of the invention proposes a new device for characterizing a non-linear behavior of a waveguide, called the waveguide to be analyzed, which comprises: - an interferometer with two optical paths each extending between an optical input of the interferometer and a superposition zone, this interferometer being adapted so that the waveguide to be analyzed is inserted into at least one of the optical paths for characterization of the waveguide to be analyzed, and the interferometer comprising a beam splitter arranged so that two parts of a radiation which is injected into the optical input propagate respectively one by one in the two optical paths then interfere with each other in the superposition zone, having respective power values ​​which are different; - a radiation source, adapted to produce radiation which has a variable power value, and arranged to inject the radiation into the optical input of the interferometer during the characterization of the waveguide to be analyzed; and - a photodetection system, arranged to measure an amplitude and a phase shift of an interference state of the radiation which exists in the superposition zone during the characterization of the waveguide to be analyzed.

[0010] According to the invention, the two optical paths have a common portion of optical path in which the waveguide to be analyzed is intended to be inserted for its characterization, and these two optical paths are such that the parts of the radiation which propagate respectively in one and the other of the two optical paths, circulate in the waveguide to be analyzed according to respective directions of propagation which are opposite. In other words, the interferometer which is used according to the invention to characterize the non-linear behavior of the waveguide to be analyzed is a Sagnac interferometer.

[0011] Due to the fact that the two interfering parts of the radiation have each propagated in the waveguide to be analyzed, the linear behavior of the latter affects the respective phases of these two parts of the radiation equally between the optical input of the interferometer and the superposition zone. For this reason, contributions to the phase shift of the interference state which are generated by the linear behavior of the waveguide to be analyzed self-compensate between the two parts of the radiation. The phase shift of this interference state is then essentially determined by the non-linear behavior of the waveguide to be analyzed. For this reason, this non-linear behavior of the waveguide to be analyzed, in particular its non-linear Kerr effect, can be determined numerically with precision from the measurements of the interference state which are carried out by means of the photodetection system.

[0012] Furthermore, also thanks to the fact that the two parts of the radiation which interfere have each been transmitted by the waveguide to be analyzed, they undergo respective propagation times in which the waveguide to be analyzed intervenes in the same way for these two parts of the radiation. Thus, their respective propagation times in the two optical paths of the interferometer are substantially equal, and only require propagation or transit delay compensation which is independent of the waveguide to be analyzed. Such propagation delay compensation for using radiation which is in the form of successive pulses can then be carried out initially during the manufacture of the characterization device, without being repeated subsequently for each new waveguide to be analyzed.The use of a Sagnac interferometer, as proposed by the invention, is therefore particularly suitable for radiation consisting of successive pulses which are very short or ultra-short, to achieve significant values ​​of instantaneous radiation power.

[0013] A first advantage of the device of the invention is therefore that its use to successively characterize several waveguides does not require optoelectronic devices or ultra-fast measuring devices.

[0014] A second advantage is that the characterization of a waveguide to be analyzed using the device of the invention can be particularly rapid.

[0015] A third advantage is that the radiation source of the device of the invention can have a radiation emission power which is limited.

[0016] A fourth advantage is that the device of the invention does not require particularly expensive optical, optronic or electronic components.

[0017] Finally, a fifth advantage is that the device of the invention can be produced in a compact and ready-to-use form.

[0018] For these reasons, the cost price of each use of the device of the invention can be low. This device is then particularly suitable for metrological exploitation of characterization of waveguides.

[0019] According to a first improvement of the invention, a first of the two optical paths may comprise a modulator adapted to generate a variable phase shift for the part of the radiation which propagates in this first optical path, in accordance with an electrical modulation signal which is received at a control input of this modulator. Then, the device may further comprise: - an electrical alternating signal generator, which is connected to the control input of the modulator, so that during the characterization of the waveguide to be analyzed, the modulator produces a frequency shift for the part of the radiation which propagates in the first optical path; and - a synchronous detection system, which is connected as an input on the one hand to the alternating electrical signal generator and on the other hand to an output of the photodetection system, and which is adapted to provide during the characterization of the waveguide to be analyzed, measured values ​​respectively for the amplitude and the phase shift of the interference state of the radiation which exists in the superposition zone of the interferometer. Thanks to such use of synchronous detection, the amplitude and the phase shift of the interference state of the radiation which exists in the superposition zone of the interferometer can be extracted easily and in real time, and using standard detection signal analysis hardware.

[0020] For this first improvement of the invention, that of the two optical paths other than the first as designated previously may advantageously comprise a retarder configured to reproduce at least in part a constant transfer delay which is produced by the modulator in the first optical path. Such use of the retarder facilitates the use of the device of the invention with a radiation source with short or ultra-short pulses. Indeed, thanks to the retarder, the two optical paths have optical lengths which are substantially equal, so that the two parts of the same radiation pulse produced by the source, which propagate separately in the two optical paths, arrive substantially at the same time at the superposition zone and can therefore interfere with each other there.

[0021] When the retarder is used, the device may further comprise two optical circulators, the modulator and the retarder being arranged in parallel between the two optical circulators to constitute respective segments of the two optical paths. The two segments are thus separated and grouped by the two optical circulators, and traversed by the parts of the radiation in directions of propagation from one optical circulator to the other which are opposite during the characterization of the waveguide to be analyzed. Such an arrangement of the two optical paths is easy to implement in the Sagnac interferometer.

[0022] According to a second improvement of the invention, the radiation source may comprise a primary source and an optical power variator. The optical power variator is then intermediate between the primary source and the optical input of the interferometer, to produce the variable value of the power of the radiation which is injected into the interferometer. Such use of an optical power variator in the device of the invention makes it possible to characterize more completely and more precisely the non-linear behavior of the waveguide to be analyzed.

[0023] For this second improvement of the invention, the device can advantageously further comprise a system for measuring the variable value of the power of the radiation which is injected into the interferometer. It can then be adapted to automatically vary this power value, and to output the amplitude and phase shift of the interference state of the radiation that exists in the superposition zone for each measured value of the power of the radiation that is injected into the interferometer. Thanks to such automated operation, the characterization of the waveguide to be analyzed is possible with a reduced operating time of the device.

[0024] Generally for the invention, each optical path of the interferometer can be constituted by polarization-maintaining components. A better interference contrast, i.e. a greater amplitude of the interference state, can thus be obtained in the superposition zone of the interferometer.

[0025] Also generally for the invention, the device of the invention may be constituted at least in part by fiber optic components, and / or by components produced in the form of integrated optical circuits. The entire device may thus be particularly compact and easy to use. In particular, the optical alignment of these components relative to each other may be carried out initially, without requiring readjustment thereafter.

[0026] Still generally for the invention, the radiation source may be of the pulsed source type, and adapted to produce the radiation in the form of successive pulses. Higher instantaneous power values ​​may thus be provided for the radiation, which make it possible to measure the interference state in the superposition zone even when significant optical losses are likely to occur during use of the device. Such losses may occur in particular at the level of optical connections which are used to insert the waveguide to be analyzed in the common portion of the optical path.

[0027] Finally, and again generally for the invention, the device may further comprise a calculation unit which is configured to determine at least one of a value of the non-linear Kerr effect coefficient, a value of the two-photon absorption coefficient and a value of the free-carrier absorption coefficient, which are relative to the waveguide to be analyzed, from the values ​​which have been measured respectively for the amplitude and the phase shift of the interference state which exists in the superposition zone.

[0028] A second aspect of the invention proposes a method for characterizing the non-linear behavior of a waveguide to be analyzed, this method comprising the following steps: - provide a device which conforms to the first aspect of the invention; - insert the waveguide to be analyzed into the common portion of the optical path of the device; and - simultaneously activate the radiation source and the photodetection system, to measure the amplitude and phase shift of the radiation interference state that exists in the superposition zone.

[0029] This method of the invention is compatible with any type of waveguide to be analyzed, whatever its technology, in particular of the optical fiber type or of a type of waveguide which is produced in the form of an integrated optical circuit.

[0030] When the radiation source comprises the primary source and the optical power variator, the method may comprise recording the measured values ​​respectively for the amplitude and the phase shift of the interference state of the radiation which exists in the superposition zone, for several values ​​of the power of the radiation injected into the interferometer which are produced successively. Then, the method may further comprise determining at least one of the value of the non-linear Kerr effect coefficient, the value of the two-photon absorption coefficient and the value of the free-carrier absorption coefficient, which are relative to the waveguide to be analyzed, from the measured values ​​respectively for the amplitude and the phase shift of the interference state, and relative to several measured values ​​for the power of the radiation injected into the interferometer.

[0031] In particular implementations of the characterization method of the invention, at least one of the following steps may be implemented: - calculate the value of the non-linear Kerr effect coefficient for the waveguide to be analyzed from a first sequence of measured values ​​for the phase shift of the interference state as a function of measured values ​​for the power of the radiation which is injected into the interferometer, this first sequence of measured values ​​for the phase shift of the interference state extending in the direction of a zero value for the power of the radiation which is injected into the interferometer; - calculating the value of the two-photon absorption coefficient for the waveguide to be analyzed from a second sequence of measured values ​​for the amplitude of the interference state as a function of measured values ​​for the power of the radiation that is injected into the interferometer, this second sequence of measured values ​​for the amplitude of the interference state also extending in the direction of the zero value for the power of the radiation that is injected into the interferometer. Optimally, this second sequence uses the same values ​​for the power of the radiation that is injected into the interferometer as the first sequence above; and - select the value of the free carrier absorption coefficient for the waveguide to be analyzed, from among several values ​​used to numerically simulate the non-linear behavior of the waveguide to be analyzed, using the values ​​already calculated for the non-linear Kerr effect coefficient and for the two-photon absorption coefficient, the value selected for the free carrier absorption coefficient being the one that produces a better coincidence between the measured values ​​and calculated values ​​for the interference state phase shift, for several values ​​of the power of the radiation that is injected into the interferometer. Brief description of the figures

[0032] The characteristics and advantages of the present invention will appear more clearly in the detailed description below of a non-limiting exemplary embodiment, with reference to the appended figures among which:

[0033] [Fig. 1] is a block diagram of a device according to the invention;

[0034] [Fig. 2] a perspective view of a waveguide to be analyzed which can be characterized using the device of [Fig. 1];

[0035] [Fig. 3a] is a diagram showing interference state phase shift variations as detected using the device of [Fig. 1] for the waveguide to be analyzed of [Fig. 2]; and

[0036] [Fig. 3b] corresponds to [Fig. 3a] for amplitude variations of the interference state. Detailed description of the invention

[0037] For the sake of clarity, the elements represented in [Fig. 1] are only symbolically. Furthermore, the dimensions in [Fig. 2] do not correspond to real dimensions or to real dimensional ratios.

[0038] With reference to [Fig. 1], the meanings of the following reference numerals are: 101: waveguide to be analyzed, noted ANALYS. and which extends according to the longitudinal coordinate x between its two opposite optical ends of input E101 and output S101; 100: device according to the invention, for characterizing the non-linear behavior of the waveguide 101; 1: radiation source, comprising a primary source 11 and an optical power variator 12; 2: Sagnac interferometer; 3: photodetection system, noted PD for photodetector; 4: system for measuring the power of the radiation which is injected by source 1 into interferometer 2, system 4 also being noted PD for photodetector; 5: modulator, adapted to generate a variable phase shift for radiation passing through it; 6: retarder, noted DL for “delay line” in English, and adapted to generate a propagation delay for radiation which passes through it; 7a and 7b: optical circulators; and 8a and 8b: polarization adjustment systems.

[0039] The waveguide to be analyzed 101 may be of any type, in particular a segment of optical fiber or a waveguide which is incorporated in an integrated optical circuit. It may be optically connected by its two optical input E101 and output S101 ends to the interferometer 2, using optical connection means which are each capable of injecting and recovering parts of radiation by one and the other of the optical ends E101 and S101, so that the two parts of radiation propagate in the waveguide 101 in two opposite directions. For example and in a manner which is known to those skilled in the art, each optical connection means may comprise a network of coupling which has a focus located on one of the optical ends E101, S101 of the waveguide 101. The polarization adjustment systems 8a and 8b make it possible to increase an amplitude of variation of the interference state by achieving a polarization match between the interferometer 2 and the waveguide to be analyzed 101.

[0040] The waveguide to be analyzed 101 which is shown in [Fig. 2] is produced in the form of an integrated optical circuit of the SOI type, for "Silicon On Insulator" in English or silicon on insulator. This circuit comprises a substrate 102, for example made of silica (SiO2), and an upper layer 103, for example made of silicon (Si), which is carried by the substrate 102 and comprises a rib N on a side opposite the substrate. In a known manner, the rib N and the part of the layer 103 which is intermediate between this rib and the substrate 102 constitute the waveguide 101, with the radiation guiding direction being the longitudinal direction x of the rib N.The waveguide 101 may have the following dimensions: width of the integrated optical circuit: 3.05 pm (micrometer), length L of the rib N, corresponding to the length of the waveguide 101: 20 mm (millimeter), width of the rib N: 450 nm (nanometer), height of the rib N: 160 nm and thickness of the layer 103 outside the rib N: 150 nm. Such a waveguide is effective for conducting radiation which has a wavelength in vacuum equal to 1550 nm. The cross-sections of the guided transmission mode are symbolically indicated in broken lines in the figure at the input optical ends E101 and output S101 of the waveguide 101. This waveguide 101 of [Fig. 2] will be used in the remainder of this description as an example to explain the operation of the device 100 of [Fig. 1] as well as the principle of the characterization carried out according to the invention.For such a waveguide 101, the parameter p which quantifies the effect of the dispersion caused by the free carriers of the medium constituting this waveguide is substantially equal to 7.5, for radiation having the wavelength of 1550 nm when it propagates in a vacuum.

[0041] Returning to [Fig. 1], the device 100 of the invention comprises at least the radiation source 1, the Sagnac interferometer 2 and the photodetection system 3. In improved embodiments, it can also optionally comprise the radiation power measurement system 4, the modulator 5 and the retarder 6. The optical circulators 7a and 7b make it possible to add the modulator 5 and the retarder 6 in the interferometer 2, as described later.

[0042] The primary source 11 is preferably a laser source, and advantageously of the pulsed type to provide high instantaneous radiation power values. It produces successive pulses of radiation all at the vacuum wavelength of 1550 nm, with an individual pulse duration that may be equal to approximately 1 ps (picosecond), and with a pulse repetition frequency that may be approximately 80 MHz (megahertz), these values ​​being given only as an example to enable the invention to be reproduced, but without limitation. The radiation that is thus produced by the primary source 11 may have an average power of approximately 1 mW (milliwatt), as calculated by taking into account the pulse durations and also the separation times between successive pulses.

[0043] The optical power variator 12 may be of the electro-optical modulator type, denoted MEO. It is intended to apply variable attenuation to the radiation which is produced by the primary source 1 1 .

[0044] The photodetection system 3 may be a photodiode. It is placed in a superposition zone of the interferometer 2, designated by S in [Fig. 1 ] and inside which interfere parts of radiation which have propagated by the two optical paths of the interferometer 2.

[0045] The radiation power measuring system 4 can also be a photodiode.

[0046] Modulator 5 may be of the acousto-optic modulator type, denoted MAO. However, liquid crystal or oscillating mirror optical modulators may be used alternatively.

[0047] The retarder 6 may be of any type, for example of an adjustable liquid crystal type or a mechanical delay line type. It is intended to approximately reproduce a radiation transfer delay that is generated by the modulator 5 between the two optical circulators 7a and 7b. A criterion for adjusting the delay value that is produced by the retarder 6 is to detect a non-zero, or even maximum, signal by the photodetection system 3, meaning that two parts of each radiation pulse produced by the source 1 that propagate through the two optical paths of interferometer 2 are temporally superimposed on the photodetection system 3, or arrive at this system 3 at the same time.

[0048] The interferometer 2 comprises two optical paths CH1 and CH2 which each extend between an optical input E of this interferometer and the photodetection system 3. In accordance with the so-called Sagnac configuration, the two optical paths CH1 and CH2 of the interferometer 2 comprise a common portion in which the waveguide to be analyzed 101 is inserted. Furthermore, the two optical paths CH1 and CH2 are arranged to transmit, in directions of propagation which are opposite inside the waveguide to be analyzed 101, the two parts of radiation which are intended to interfere at the level of the photodetection system 3.

[0049] Two portions of optical paths are separated from each other from the optical input E of the interferometer 2 by a beam splitter 9, then the two portions of optical paths each connect one of the opposite ends E101, S101 of the waveguide to be analyzed 101. Thus, the two portions of optical paths complement each other to constitute the optical paths CH1 and CH2 which correspond to opposite directions of propagation of the radiation in the waveguide 101. The output of the radiation source 1 is optically connected to a first input of the beam splitter 9, and the latter has a second input which is used to transmit to the photodetection system 3 the parts of radiation which have traveled the two optical paths CH1 and CH2.The beam splitter 9 can be selected to separate between its two outputs to the optical paths CH1 and CH2, the radiation which is transmitted by the source 1, according to an optical power separation ratio which can be 90:10 for example, or 99:01, or even 60:40. This beam splitter 9 can thus be selected to send an optical power into the optical path CH2, which is greater than that sent into the optical path CH1.

[0050] Advantageously, the entire interferometer 2, and also possibly the radiation source 1, can be made from optical fibers and / or integrated optical components. These fibers and / or integrated optical components are preferably polarization-maintaining, in which case the polarization adjustment systems 8a and 8b are two linear polarization direction rotators. In particular, the beam splitter 9 may be a 2 x 2 evanescent field coupler. The device 100 may then have a construction that is compact, and be contained in a closed housing with a ready-to-use presentation. The waveguide to be analyzed 101 may then be optically connected in a removably manner to dedicated optical inputs / outputs that are provided on the housing.

[0051] In such an embodiment, the 2 x 2 coupler which constitutes the beam splitter 9, at the optical input E of the interferometer 2, has two optical outputs which are each connected to the input of one of the optical paths CH1 and CH2, simultaneously constituting the output of the other optical path. A first of the two optical inputs of the 2 x 2 coupler is optically connected to the output of the optical power variator 12, to receive the radiation which is produced by the source 1. According to an economical and efficient arrangement, the second optical input of the same 2 x 2 coupler can be optically connected to the photodetection system 3 to transmit a superposition of the radiation parts which have propagated separately by one and the other of the two optical paths CH1 and CH2.The overlapping zone S of the interferometer 2 is then the optical path segment which extends from the second optical input of the 2 x 2 coupler forming the beam splitter 9 to the photodetection system 3.

[0052] Advantageously, the measuring system 4 can be arranged to receive a fixed fraction, for example 1%, of the power of the radiation which is produced by the source 1, for example by using another 2 x 2 optical coupler with evanescent fields as designated by the reference 13 in [Fig. 1]. Thus, the measuring system 4 provides a measurement signal which is representative of the radiation power Pin which is injected into the interferometer 2 by its optical input E.

[0053] Within the radiation source 1, the optical power variator 12 can be controlled to vary the radiation power Pin, for example with a new value of Pin every eight thousand successive pulses of the radiation as a non-limiting example. The discrete values ​​which are thus adopted one after the other for the power Pin can increase within a characterization interval of the waveguide 101, and each value of the power Pin is maintained for a duration of measurement of the interference state which then exists on the photodetection system 3. A programmable control unit 20 can be used to produce such a variation of Pin. Thus, a new interference state is measured for each new value of the power Pin of the radiation which is injected into the optical input E of the interferometer 2. As explained later, the measurement results of the interference state which exists on the photodetection system 3 can be delivered by a synchronous detection system 33. An acquisition system which is synchronized with the variations of Pin can make it possible to record the measurement results of each interference state which exists during the duration of maintaining a power value Pin in association with this power value Pin as measured by the system 4. Such an acquisition system is designated by the reference 43 in [Fig. 1] and noted LOCK-IN. It can be a synchronous detection system, but which is distinct from that designated by the reference 33 and dedicated to measuring the interference state which exists while each power value Pin is maintained.

[0054] The optional improvement of the invention which is now described makes it possible to obtain more easily, from the detection signal which is delivered by the photodetection system 3, the amplitude and the phase shift of the instantaneous interference state which exists in the superposition zone S. The loop of the optical paths CH1 and CH2 is divided according to the direction of propagation of the radiation in this loop into two separate portions of optical paths between the optical circulators 7a and 7b, independently of the waveguide to be analyzed 101 which is in a common portion of the two optical paths CH1 and CH2. Then, the modulator 5 is inserted into one of the two separate portions of optical paths between the optical circulators 7a and 7b, for example in the optical path CH1 in accordance with [Fig. 1], and the retarder 6 is inserted into the other separate portion of optical path, that of the optical path CH2.The retarder 6 can be selected to compensate between the two optical paths CH1 and CH2, a delay in the transfer of parts of pulses produced by the modulator 5. This adjustment is independent of the waveguide to be analyzed 101, and can be initially executed only once during the assembly of the device 100.

[0055] The detection signal which is delivered by the photodetection system 3 is an interference signal between a part of each pulse which has traveled through the waveguide 101 in the direction of propagation of the optical path CH1, and another part of the same pulse which has also traveled through the waveguide 101 but in the direction of propagation of the optical path CH2. The field of the radiation part which propagates in the path optical path CH1 is denoted Ei, and that of the part of radiation which propagates in the optical path CH2 is denoted E2. Taking into account the operation of the optical circulators 7a and 7b and as has just been described, the modulator 5 belongs to the optical path CH1 and the retarder 6 belongs to the optical path CH2. The modulator 5 therefore applies a variable phase shift to the field E1, which depends on an electrical modulation signal received at the input by this modulator. The electrical modulation signal is transmitted to the modulator 5 by an electrical alternating signal generator 30. When this modulation signal is sinusoidal at a frequency denoted E, the modulator 5 produces a frequency shift of E for the part of the radiation which propagates in the optical path CH1.

[0056] Furthermore, due to the non-linear behavior of the waveguide 101, the radiation propagating therein undergoes absorption and propagation delay which depend on the power of this radiation. Thus, and in particular due to the optical power division ratio which is produced by the beam splitter 9, the two parts of radiation which propagate in the optical paths CH1 and CH2 undergo absorptions and propagation delays which are different. The complex amplitudes of the respective fields of these parts of radiation are then, in the superposition zone S and for the configuration of the device 100 of [Fig. 1], and as a first approximation considering only the non-linear Kerr effect: where X is the optical power division ratio which is produced by the beam splitter 9, for example X is equal to 0.90, q is an energy loss coefficient of the set of the two optical circulators 7a and 7b, combined with the modulator 5 or the retarder 6, the value of this coefficient q being assumed to be identical for the two optical paths CH1 and CH2 and being able to be calculated from data provided by the manufacturer of these optical components, T101 is the modulus of the field transmission coefficient of the waveguide 101 , for the linear behavior of this waveguide 101 , t denotes time, and Left is the optical length of the waveguide 101, for its linear behavior. In these expressions, terms and factors which are not essential to the understanding of the invention have been omitted for reasons of clarity, but those skilled in the art will be able to easily restore them in light of the present description.

[0057] Then, as is known, the variable part of the interference state which is produced on the photodetection system 3 is: 2-| Ei c | -| E2 C | ■cos(E-t+ <t> i- <t>2). The detection signal produced by system 3 therefore varies sinusoidally as a function of time, with an amplitude of temporal variation of this interference state which is noted A and equal to 2-| Ei c | -| E2 C |, and a phase shift of time variation of the interference state with respect to the modulation signal which is transmitted to modulator 5, which is noted AT> and equal to <t> i- <t>2. The amplitude A and the phase shift AT> of variation of the interference state can then be measured simply by carrying out a synchronous detection of the signal which is output by the photodetection system 3, with respect to the sinusoidal modulation signal which is transmitted by the electric generator 30 to the modulator 5. Such a synchronous detection is carried out by the system 33, denoted SYNCH R., which outputs the values ​​of the amplitude A and the phase shift A <t>. For approximate expressions of complex field amplitudes Ei c and E2 C which were given above: A4> = y ■ L eff ■ ■ P in ■ [X — r|(l — X)]. Thus, the phase shift AT> is then proportional to the non-linear Kerr effect coefficient, with a proportionality factor that can be determined experimentally.

[0058] The modulation frequency £ is preferably greater than the variation speeds of the phase shift AT> which are caused by thermal and dimensional fluctuations likely to affect the interferometer 2. Typically, the modulation frequency £ can be between 40 MHz (megahertz) and 200 MHz, without limitation. Under these conditions, the phase shift AT> can be determined with an accuracy of the order of 1° (degree) by the synchronous detection system 33. Those skilled in the art will understand that the use of the modulator 5 and the synchronous detection system 33 constitutes only a simple and precise way of determining the amplitude A and the phase shift <t>of the interference state that exists in the superposition zone S, by making this interference state vary periodically as a function of time. However, alternative methods exist to determine the amplitude A and the phase shift AT> of this interference state, which do not use not necessarily to vary this interference state as a function of time for each value of the Pin power of the radiation which is injected into the interferometer 2.

[0059] The diagram in [Fig. 3a] shows the values ​​of the interference state phase shift A <t>which were measured using the device 100 of [Fig. 1] with the waveguide 101 of [Fig. 2], as a function of values ​​of the Pin power as measured by the system 4. The horizontal axis marks the values ​​of the Pin power expressed in watts (W), and the vertical axis marks the values ​​of the phase shift A <t>expressed in degrees (°). The value of the non-linear Kerr effect coefficient y can be calculated from the extrapolation towards the zero value of the power Pin, of the slope of the curve A <t>(Pin). For the waveguide 101, we thus obtain y = 39 nr 1 W' 1 (unit: per meter and per watt).

[0060] The diagram in [Fig. 3b] shows the values ​​of the interference state amplitude A that were measured simultaneously with the phase shift values ​​A <t>of [Fig. 3a]. The horizontal axis of [Fig. 3b] also marks the values ​​of the power Pin expressed in watts (W), and the vertical axis marks the values ​​of the amplitude A also expressed in watts. The value of the two-photon non-linear absorption coefficient £TPA can be calculated from the extrapolation in the direction of the zero value of the power Pin, of the slope of the curve A(Pin). For the waveguide 101, we thus obtain £TPA = 4.39-10' 12 nrW' 1 (meter per watt).

[0061] The numerical resolution of the differential equations which were recalled at the beginning of this description, concerning the local power P and the local phase <t>of the radiation propagating in a waveguide, makes it possible to determine the value of the free carrier absorption coefficient PFCA of the waveguide 101. In practice, these differential equations are solved numerically for several potential values ​​of the PFCA coefficient, and each time using the values ​​calculated previously for the non-linear Kerr effect coefficient and for the two-photon absorption coefficient. Then the variation of the phase shift A <t>as a function of the Pin power is calculated for each of these potential values ​​of the FCA coefficient. Those of these potential values ​​for which the variation A <t>(Pin) which was calculated to coincide most closely with the curve of [Fig. 3a] is selected as the actual value of the free carrier absorption coefficient PFCA of the waveguide 101. Such a method of selecting values ​​is known as designation of "best-match" in the jargon of the person skilled in the art. For the waveguide 101, we thus obtain |1FCA = 2.4-10' 26 m 3 -W' 2 (cubic meter per square watt).

[0062] Such obtaining of the values ​​of the coefficients y, PTPA and PFCA which characterize the non-linear behavior of the waveguide 101 can be executed by the calculation unit which is designated by the reference 50 in [Fig. 1], and noted CPU for “Central Processing Unit” in English.

[0063] Values ​​which are determined as described above for the non-linear behavior coefficients y, PTPA and PFCA of a waveguide to be analyzed have uncertainties of less than 10%. This level of uncertainty is compatible with metrological use of the characterization method of the invention.

[0064] It is understood that the invention may be reproduced by modifying secondary aspects of the embodiment which has been described in detail above, while retaining at least some of the advantages cited. In particular, the positions of several of the optical components of the device of the invention may be exchanged or modified within this device, while retaining an operation of the device which is equivalent. Furthermore, all the numerical values ​​that have been cited have been cited for illustration purposes only. Finally, although the waveguide to be analyzed 101 that is shown in [Fig. 2] is made in the form of an optical circuit, it can be replaced by another waveguide to be analyzed that is of the optical fiber type, or of any other type.< / t> < / t> < / t> < / t> < / t> < / t> < / t> < / t> < / t> < / t> < / t> < / t> < / t> < / t>

Claims

Claims

1. Device (100) for characterizing a non-linear behavior of a waveguide, called waveguide to be analyzed (101), said device comprising: - an interferometer (2) with two optical paths (CH1, CH2) each extending between an optical input (E) of the interferometer and a superposition zone (S), said interferometer being adapted so that the waveguide to be analyzed (101) is inserted into at least one of the optical paths for a characterization of said waveguide to be analyzed, and the interferometer comprising a beam splitter (9) arranged so that two parts of a radiation which is injected into the optical input propagate respectively one-by-one in the two optical paths then interfere with each other in the superposition zone, having respective power values ​​which are different; - a radiation source (1), adapted to produce radiation which has a variable power value, and arranged to inject the radiation into the optical input (E) of the interferometer (2) during the characterization of the waveguide to be analyzed (10); and - a photodetection system (3), arranged to measure an amplitude and a phase shift of an interference state of the radiation which exists in the superposition zone (S) during the characterization of the waveguide to be analyzed (101), the device (100) being characterized in that the two optical paths (CH1, CH2) have a common portion of optical path in which the waveguide to be analyzed (101) is intended to be inserted for the characterization of said waveguide to be analyzed, and said two optical paths are such that the parts of the radiation which propagate respectively in one and the other of the two optical paths, circulate in the waveguide to be analyzed according to respective directions of propagation which are opposite.

2. Device (100) according to claim 1, wherein a first (CH1) of the two optical paths comprises a modulator (5) adapted to generate a variable phase shift for the part of the radiation which propagates in said first optical path, in accordance with an electrical modulation signal which is received at a control input of said modulator, and the device (100) further comprises: - an electrical alternating signal generator (30), which is connected to the control input of the modulator (5), so that during the characterization of the waveguide to be analyzed (101), the modulator produces a frequency shift for the part of the radiation which propagates in the first optical path (CH1); and - a synchronous detection system (33), which is connected as an input on the one hand to the alternating electrical signal generator (30) and on the other hand to an output of the photodetection system (3), and which is adapted to provide during the characterization of the waveguide to be analyzed (101), measured values ​​respectively for the amplitude and the phase shift of the interference state of the radiation which exists in the superposition zone (S) of the interferometer (2). [Claim s] Device (100) according to claim 2, wherein the one of the two optical paths (CH1, CH2) other than said first optical path (CH1) comprises a retarder (6) configured to reproduce at least in part a constant transfer delay which is produced by the modulator (5) in said first optical path.

4. Device (100) according to claim 3, further comprising two optical circulators (7a, 7b), and wherein the modulator (5) and the retarder (6) are arranged in parallel between the two optical circulators to constitute respective segments of the two optical paths (CH1, CH2), the two segments being separated and grouped by the two optical circulators, and traversed by the parts of the radiation in directions of propagation from one optical circulator to the other which are opposite during the characterization of the waveguide to be analyzed (101).

5. Device (100) according to one of the preceding claims, wherein the radiation source (1) comprises a primary source (11) and an optical power variator (12), the optical power variator being intermediate between the primary source and the optical input (E) of the interferometer (2), so that said optical power variator determines the variable value of the power of the radiation which is injected into the interferometer.

6. Device (100) according to claim 5, further comprising a system (4) for measuring the variable value of the power of the radiation which is injected. in the interferometer (2), and the device (100) is adapted to automatically vary said power value of the radiation which is injected into the interferometer (2), and to output the amplitude and the phase shift of the interference state of the radiation which exists in the superposition zone (S) for each measured value of the power of the radiation which is injected into the interferometer.

7. Device (100) according to one of the preceding claims, in which each optical path (CH1, CH2) is constituted by polarization-maintaining components.

8. Device (100) according to one of the preceding claims, in which the radiation source (1) is of the pulsed source type, and adapted to produce the radiation in the form of successive pulses.

9. Device (100) according to one of the preceding claims, further comprising a calculation unit (50) configured to determine at least one of a value of a non-linear Kerr effect coefficient, a value of a two-photon absorption coefficient and a value of a free-carrier absorption coefficient, relating to the waveguide to be analyzed (101), from values ​​which have been measured respectively for the amplitude and the phase shift of the interference state which exists in the superposition zone (S).

10. Method for characterizing a non-linear behavior of a waveguide, called waveguide to be analyzed (101), the method comprising the following steps: - providing a device (100) which is in accordance with any one of the preceding claims; - inserting the waveguide to be analyzed (101) into the common portion of the optical path of the device; and - simultaneously activating the radiation source (1) and the photodetection system (3), to measure the amplitude and phase shift of the radiation interference state which exists in the superposition zone (S).

11. A method according to claim 10, wherein the device (100) is in accordance with claim 6, and the method comprises recording measured values ​​respectively for the amplitude and the phase shift of the interference state of the radiation which exists in the superposition zone (S), for several values ​​of the power of the radiation injected into the interferometer which are produced successively, and the method further comprises determining at least one of a value of a non-linear Kerr effect coefficient, a value of a two-photon absorption coefficient and a value of a free-carrier absorption coefficient, relating to the waveguide to be analyzed, from measured values ​​respectively for the amplitude and the phase shift of the interference state, and relating to several measured values ​​for the power of the radiation injected into the interferometer.

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