CHARACTERIZATION OF THE NONLINEAR BEHAVIOR OF A WAVEGUIDE

Abstract

A device (100) for characterizing the nonlinear behavior of a waveguide (101) is based on a Sagnac interferometer (2). The waveguide is designed to carry two parts of radiation in opposite directions of propagation. Advantageously, an interference state produced by the interferometer, which is used to deduce coefficient values ​​characterizing the nonlinear behavior of the waveguide, can be measured by synchronous detection. Automated operation of the device is also possible, allowing for variation of the power of the radiation injected into the interferometer. The characterization of the nonlinear behavior of the waveguide can thus be fast, precise, and inexpensive. (Shorthand figure: Figure 1)

Description

Title of the invention: 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 performing such characterization. Previous technique

[0002] A waveguide is said to exhibit linear behavior when the complex field amplitude of a spectral component of electromagnetic radiation transmitted through the waveguide at its output is proportional to the complex field amplitude of the same spectral component when injected into the waveguide at its input. The index value that is effective in determining the optical path length in the waveguide between its input and output is then independent of the intensity of the spectral component considered. However, several mechanisms of interaction of electromagnetic radiation with a propagation medium are known that impart nonlinear behavior to a waveguide constructed from that propagation medium. The nonlinear Kerr effect, two-photon absorption, and free-carrier absorption are among these mechanisms that produce nonlinear behavior.Thus, variations in a local power and local phase of a spectral component of electromagnetic radiation propagating in a waveguide are governed by the following two differential equations: . 9P _ p Ptpa p2 t*FCA 2Ê - v. p _ L . . p2In these equations which 3x - -a- r- • r - -r dx y 2 are known to a person 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, <e>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 effect coefficient of the waveguide, [3Tpa is the two-photon absorption coefficient of the waveguide, qpcA is the free-carrier absorption coefficient of the waveguide, Aeff is the effective mode cross-sectional area of ​​the spectral component of electromagnetic radiation in the waveguide, and q is a dimensionless parameter that quantifies the effect of dispersion caused by free carriers in the propagation medium inside the waveguide. If necessary, one can refer to one of the available scientific publications that explain these two differential equations.

[0003] Many applications use waveguides and require sufficiently precise knowledge of their nonlinear behavior. Examples of such applications include high-power laser radiation transfer, neuromorphic or quantum optical computers, environmental sensors, and so on. However, most of the methods currently available for characterizing 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 a waveguide's nonlinear behavior generally appear superimposed on the effects of its linear behavior, even though the latter may be more significant or much more significant than those of the nonlinear behavior.In interpreting measurements characterizing the behavior of a waveguide, it is necessary to distinguish between contributions of non-linear behavior and contributions of linear behavior. In particular, a variation that the phase undergoes. <e>The radiation between the waveguide inlet and outlet is the sum of a linear behavioral contribution, generally the larger of the two, and a nonlinear behavioral contribution, generally smaller. Because of this, each nonlinear behavioral contribution evaluated from phase measurement results is subject to 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. However, this characterization method has the following drawbacks: - because it is necessary to use a very pulsed radiation source To achieve sufficient levels of instantaneous light power, it is essential to adjust the optical path length of the interferometer in which the waveguide is not inserted so that it is approximately 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 through the two optical paths of the Mach-Zehnder interferometer. However, this optical path length adjustment is tedious, difficult, and time-consuming, requiring ultrafast optoelectronic equipment and measuring instruments. Because of this, this characterization method is laborious and expensive; and - as mentioned above, subtracting the contribution of the linear behavior of the waveguide from the measurement results of phase variations reduces the accuracy 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, one aim of the present invention is to allow the non-linear behavior of a waveguide to be characterized by reducing or eliminating at least some of the difficulties which have just been mentioned for prior methods.

[0006] More specifically, the invention aims to characterize the non-linear behavior of a waveguide, with an accuracy that is improved compared to previous 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 goals 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: - a two-path interferometer, each path 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 such that two parts of a radiation which is injected into the optical input, they propagate respectively one-to-one in the two optical paths and 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 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 optical path portion in which the waveguide to be analyzed is intended to be inserted for its characterization, and these two optical paths are such that the portions of the radiation propagating respectively in each of the two optical paths travel in the waveguide to be analyzed in opposite directions of propagation. In other words, the interferometer used according to the invention to characterize the nonlinear behavior of the waveguide to be analyzed is a Sagnac interferometer.

[0011] Because the two interfering parts of the radiation have each propagated through the waveguide being analyzed, the linear behavior of the waveguide equivalently affects the respective phases of these two parts of the radiation between the optical input of the interferometer and the superposition zone. For this reason, contributions to the phase shift of the interference state generated by the linear behavior of the waveguide being analyzed self-compensate between the two parts of the radiation. The phase shift of this interference state is then essentially determined by the nonlinear behavior of the waveguide being analyzed. Therefore, this nonlinear behavior of the waveguide being analyzed, in particular its nonlinear Kerr effect, can be accurately determined numerically from the interference state measurements performed using the photodetection system.

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

[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 fast.

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

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

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

[0018] For these reasons, the cost of each use of the device of the invention can be low. This device is therefore particularly suitable for metrological application of waveguide characterization.

[0019] According to a first improvement of the invention, a first of the two optical paths may include a modulator adapted to generate a variable phase shift for the portion of the radiation propagating in this first optical path, according to an electrical modulation signal received at a control input of this modulator. The device may further include: - an alternating current 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 portion of the radiation propagating in the first optical path; and - a synchronous detection system, which is connected on the one hand to the alternating signal electrical 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 phase shift of the interference state of the radiation which exists in the superposition zone of the interferometer. Thanks to such synchronous detection, the amplitude and phase shift of the interference state of the radiation existing in the interferometer's superposition zone can be easily extracted in real time, and using a standard equipment for analyzing detection signals.

[0020] For this first improvement of the invention, the optical path other than the first one designated above may advantageously include a delay device configured to reproduce, at least in part, a constant transfer delay produced by the modulator in the first optical path. Such use of the delay device facilitates the use of the device of the invention with a short-pulse or ultrashort-pulse radiation source. Indeed, thanks to the delay device, the two optical paths have substantially equal optical lengths, so that the two parts of the same radiation pulse produced by the source, which propagate separately in the two optical paths, arrive at the overlap zone substantially at the same time 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 form respective segments of the two optical paths. The two segments are thus separated and regrouped by the two optical circulators, and traversed by the radiation particles in opposite directions of propagation from one optical circulator to the other during the characterization of the waveguide to be analyzed. Such an arrangement of the two optical paths is easily implemented 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 dimmer. The optical power dimmer is then interposed between the primary source and the optical input of the interferometer, to produce the variable power value of the radiation injected into the interferometer. Such use of an optical power dimmer in the device of the invention makes it possible to characterize the nonlinear behavior of the waveguide to be analyzed in a more complete and precise manner.

[0023] For this second improvement of the invention, the device may advantageously further include a system for measuring the variable power of the radiation 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 existing in the superposition zone for each measured power value of the radiation injected into the interferometer. Thanks to such automated operation, the characterization of the waveguide to be analyzed is possible with a reduced device operating time.

[0024] Generally, for the invention, each optical path of the interferometer can be made up of polarization-maintaining components. Improved contrast interference, that is to say an amplitude of the interference state which is greater, can thus be obtained in the superposition zone of the interferometer.

[0025] Generally speaking, for the invention, the device of the invention may consist, at least in part, of optical fiber components and / or components implemented as integrated optical circuits. The entire device can thus be particularly compact and easy to use. In particular, the optical alignment of these components with respect to each other can be achieved initially, without requiring subsequent readjustment.

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

[0027] Finally, and again generally for the invention, the device may further include a computing unit which is configured to determine at least one of a value of the nonlinear 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 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 that 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 - to simultaneously activate the radiation source and the photodetection system, to measure the amplitude and phase shift of the interference state of the radiation that exists in the overlap zone.

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

[0030] When the radiation source includes the primary source and the optical power dimmer, the method may include recording the measured values ​​respectively for the amplitude and 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 include determining at least one of the values ​​of the nonlinear 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 values ​​measured respectively for the amplitude and 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 carried out: - 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 that is injected into the interferometer, this first sequence of measured values ​​for the phase shift of the interference state extending towards a zero value for the power of the radiation that is injected into the interferometer; - calculate 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 injected into the interferometer, this second sequence of measured values ​​for the amplitude of the interference state also extending towards zero for the power of the radiation injected into the interferometer. In an optimized manner, this second sequence uses the same values ​​for the power of the radiation 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 nonlinear behavior of the waveguide, using the values ​​already calculated for the nonlinear Kerr effect coefficient and for the two-photon absorption coefficient. The selected value for the free carrier absorption coefficient is the one that produces the best agreement between the measured and calculated values ​​for the interference phase shift, for several values ​​of the power of the radiation injected into the interferometer. Brief description of the figures

[0032] The features and advantages of the present invention will become more apparent from the following detailed description of a non-limiting embodiment, with reference to the accompanying figures, among which:

[0033] [Fig-1] is a synoptic 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 that shows variations in the phase shift of the interference state 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 variations in amplitude of the interference state. Detailed description of the invention

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

[0038] With reference to [Fig.1], the meanings of the following numerical references are: 101: waveguide to be analyzed, noted ANALYS, which extends along the longitudinal coordinate x between its two opposite optical input ends Eioi and output ends S ioi; 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 dimmer 12; 2: Sagnac interferometer; 3: photodetection system, noted PD for photodetector; 4: system for measuring the power of the radiation that 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 passing through it; 7a and 7b: optical circulators; and 8a and 8b: polarization adjustment systems.

[0039] The waveguide to be analyzed 101 can be of any type, including a segment of optical fiber or a waveguide incorporated into an integrated optical circuit. It can be optically connected by its two optical input ends E101 and output ends Sioi to the interferometer 2, using optical connection means each capable of injecting and recovering portions of radiation through either of the optical ends E101 and Sioi, so that the two portions of radiation propagate in the waveguide 101 in two opposite directions. For example, and in a way known to those skilled in the art, each optical connection means can understand a coupling network which has a focus located on one of the optical ends EiOi, Sioi of the waveguide 101. The polarization adjustment systems 8a and 8b allow 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, shown in [Fig. 2], is implemented as a silicon-on-insulator (SOI) integrated optical circuit. 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 supported by the substrate 102 and has a rib N on one side opposite the substrate. As is known, the rib N and the portion of the layer 103 intermediate between this rib and the substrate 102 constitute the waveguide 101, with the longitudinal direction x of the rib N as the radiation guidance direction.Waveguide 101 can have the following dimensions: width of the integrated optical circuit: 3.05 µm (micrometer), length L of rib N, corresponding to the length of waveguide 101: 20 mm (millimeter), width of rib N: 450 nm (nanometer), height of rib N: 160 nm, and thickness of layer 103 outside rib N: 150 nm. Such a waveguide is effective for conducting radiation with a wavelength in a vacuum of 1550 nm. The cross-sections of the guided transmission mode are symbolically indicated by mixed dashed lines in the figure at the optical input Eioi and output S^i ends 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 constitutive medium of this waveguide is approximately equal to 7.5, for radiation having a wavelength of 1550 nm when propagating 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 may also optionally comprise the radiation power measurement system 4, the modulator 5 and the retarder 6. The optical circulators 7a and 7b allow the modulator 5 and the retarder 6 to be added to 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 radiation pulses all at a vacuum wavelength of 1550 nm, with an individual pulse duration that The pulse duration can be approximately 1 ps (picosecond), and the pulse repetition rate can be approximately 80 MHz (megahertz). These values ​​are given only as examples to facilitate the reproduction of the invention, but without limitation. The radiation thus produced by the primary source 11 can have an average power of approximately 1 mW (milliwatt), as calculated by taking into account the pulse durations and also the intervals between successive pulses.

[0043] The optical power dimmer 12 can be of the electro-optical modulator type, denoted MEO. It is intended to apply variable attenuation to the radiation produced by the primary source 11.

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

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

[0046] The modulator 5 can be of the acousto-optic modulator type, denoted AOM. However, optical modulators with liquid crystals or oscillating mirrors can be used alternatively.

[0047] The delay 6 can be of any type, for example, an adjustable type based on liquid crystals or a mechanical delay line. It is intended to reproduce approximately a radiation transfer delay generated by the modulator 5 between the two optical circulators 7a and 7b. One criterion for adjusting the delay value produced by the delay 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 propagating through the two optical paths of the interferometer 2 are temporally superimposed on the photodetection system 3, or arrive at this system 3 simultaneously.

[0048] The interferometer 2 comprises two optical paths CH1 and CH2, each extending 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 have 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 opposite directions of propagation within the waveguide to be analyzed 101, the two portions of radiation intended to interfere at 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 Eioi, Sioi of the waveguide wave to be analyzed 101. Thus, the two portions of optical paths complement each other to form optical paths CH1 and CH2, which correspond to opposite directions of radiation propagation in 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 used to transmit to the photodetection system 3 the portions of radiation that have traveled along the two optical paths CH1 and CH2. The beam splitter 9 can be selected to separate the radiation transmitted by source 1 between its two outputs to optical paths CH1 and CH2, according to an optical power separation ratio that can be, for example, 90:10, 99:1, or 60:40. This beam splitter 9 can thus be selected to send optical power into optical path CH2 that is greater than that sent into optical path CH1.

[0050] Advantageously, the entire interferometer 2, and possibly also the radiation source 1, can be made of 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 can be a 2 x 2 evanescent field coupler. The device 100 can then have a compact design and be contained in a closed housing with a ready-to-use presentation. The waveguide to be analyzed 101 can then be removably connected optically to dedicated optical inputs / outputs provided on the housing.

[0051] In such an embodiment, the 2x2 coupler, which constitutes the beam splitter 9 at the optical input E of the interferometer 2, has two optical outputs, each connected to the input of one of the optical paths CH1 and CH2, simultaneously constituting the output of the other optical path. One of the two optical inputs of the 2x2 coupler is optically connected to the output of the optical power inverter 12 to receive the radiation produced by the source 1. According to an economical and efficient arrangement, the second optical input of the same 2x2 coupler can be optically connected to the photodetection system 3 to transmit a superposition of the radiation portions that propagated separately along each of the two optical paths CH1 and CH2.The superposition zone S of the interferometer 2 is then the optical path segment which extends from the second optical input of the 2x2 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 eva- nescents as designated by reference 13 in [Fig. 1]. Thus, the measurement 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 control 12 can be controlled to vary the radiation power Pin, for example, by a new value of Pin every eight thousand successive pulses of the radiation, as a non-limiting example. The discrete values ​​thus adopted one after another for the power Pin can grow within a characterization interval of the waveguide 101, and each value of the power Pin is maintained for a measurement duration of the interference state that 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 that is injected into the optical input E of the interferometer 2.As explained later, the measurement results of the interference state present on the photodetection system 3 can be provided by a synchronous detection system 33. An acquisition system synchronized with the Pin variations can record the measurement results of each interference state present for the duration that a Pin power value is held, in association with that Pin power value as measured by the system 4. Such an acquisition system is designated by reference numeral 43 in [Fig. 1] and denoted LOCK-IN. This can be a synchronous detection system, but one that is separate from the one designated by reference numeral 33 and dedicated to measuring the interference state present while each Pin power value is held.

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

[0055] The detection signal delivered by the photodetection system 3 is an interference signal between a portion of each pulse that has traveled through the waveguide 101 in the direction of propagation of the optical path CH1, and another portion of the same pulse that has also traveled through the waveguide 101 but in the direction of propagation of the optical path CH2. The field of the portion of radiation propagating in the optical path CH1 is denoted Eb and that of the portion of radiation propagating in the optical path CH2 is denoted E2. Given the operation of the optical circulators 7a and 7b and as just 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 EB which depends on an electrical modulation signal received at its input.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 that propagates in the optical path CH1.

[0056] Furthermore, due to the nonlinear behavior of the waveguide 101, the radiation propagating through it undergoes absorption and propagation delay that depend on the power of that radiation. Thus, and particularly because of the optical power division ratio produced by the beam splitter 9, the two radiation particles propagating in the optical paths CH1 and CH2 undergo different absorptions and propagation delays. The complex amplitudes of the respective fields of these radiation particles 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 nonlinear Kerr effect: EjC = / ÜX • ' TW1 ' with = y. Leff. . n( 1 -X) . Pin for CH1, and E2C = • Vn ' T101 • ei <p2 avec y. Lrff . . x . p.^ pour CH2, where X is the optical power division ratio produced by beam divider 9, for example X is equal to 0.90, q is an energy loss coefficient of the combination 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, Tioi is the magnitude of the field transmission coefficient of waveguide 101, for the linear behavior of this waveguide 101, t denotes time, and Leff is the optical length of the 101 waveguide, for its linear behavior. In these expressions, terms and factors which are not essential to understanding the invention have been omitted for the sake of clarity, but a person skilled in the art will be able to easily restore them in the light of this description.

[0057] Then, in a known manner, the variable part of the interference state which is produced on the photodetection system 3 is: 2«IEicl»IE2cl«cos(e«t+ i- 2) The detection signal produced by system 3 therefore varies sinusoidally with time, with a temporal variation amplitude of this interference state, denoted A, equal to 2√(IEcI√(IE2cI)), and a temporal variation phase shift of the interference state relative to the modulation signal transmitted to the modulator 5, denoted A <1> and equal to <e> i- <e>2. The amplitude A and the phase shift A <1> The variation in the interference state can then be measured simply by performing synchronous detection of the signal delivered at the output of the photodetection system 3, relative to the sinusoidal modulation signal transmitted by the electrical generator 30 to the modulator 5. Such synchronous detection is performed by the system 33, denoted SYNCHR., which outputs the values ​​of the amplitude A and the phase shift A <1> For the approximate expressions of the complex field amplitudes Eic and E2C given above: = y. L ff • ' • P - [Xr|( 1 - X) ]• Thus, the phase shift A <1> is then proportional to the non-linear Kerr effect coefficient, with a proportionality factor that can be determined experimentally.

[0058] The modulation frequency e is preferably greater than the rates of change of the phase shift A <1> which are caused by thermal and dimensional fluctuations that can affect interferometer 2. Typically, the modulation frequency e can be between 40 MHz (megahertz) and 200 MHz, without limitation. Under these conditions, the phase shift A <1> can be determined with an accuracy on 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 is simply a straightforward and accurate way of determining the amplitude A and the phase shift. <e>of the interference state that exists in the superposition zone S, by making this interference state vary periodically with time. However, alternative methods exist for determining the amplitude A and the phase shift A <1> of this interference state, which do not necessarily use to vary this interference state as a function of time for each value of the power P in 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 <1> which were measured using device 100 of [Fig. 1] with waveguide 101 of [Fig. 2], as a function of Pin power values ​​as measured by system 4. The horizontal axis marks the Pin power values ​​expressed in watts (W), and The vertical axis represents the phase shift values ​​AO expressed in degrees (°). The value of the nonlinear Kerr effect coefficient y can be calculated by extrapolating the slope of the AO(Pin) curve towards zero. For waveguide 101, this gives y = 39 m * W1 (unit: per meter per watt).

[0060] The diagram in [Fig. 3b] shows the interference state amplitude values ​​A that were measured simultaneously with the phase shift values ​​AO in [Fig. 3a]. The horizontal axis of [Fig. 3b] similarly represents the power values ​​Pin expressed in watts (W), and the vertical axis represents the amplitude values ​​A, also expressed in watts. The value of the two-photon nonlinear absorption coefficient [3TPA] can be calculated from extrapolating the slope of the curve A(Pin) towards zero. For the 101 waveguide, this gives [3TPA] = 4.39 * 10^12 m * W^1 (meters per watt).

[0061] The numerical solution of the differential equations recalled at the beginning of this description, concerning the local power P and the local phase <e>The radiation propagating in a waveguide allows us to determine the value of the free-carrier absorption coefficient qPCA of the waveguide 101. Practically, these differential equations are solved numerically for several potential values ​​of the qPCA coefficient, each time using the values ​​previously calculated for the nonlinear Kerr effect coefficient and the two-photon absorption coefficient. Then, the variation of the phase shift A <1> Depending on the power, Pin is calculated for each of these potential values ​​of the qPCA coefficient - those of these potential values ​​for which the variation A <e>The value (Pin) that was calculated most closely matches the curve in [Fig. 3a] is selected as the actual value of the free carrier absorption coefficient qPCA of waveguide 101. Such a method of value selection is known as "best-match" in the jargon of those skilled in the art. For waveguide 101, we thus obtain pPCA = 2.4*1026 m³*W² (cubic meters per square watt).

[0062] Such a determination of the values ​​of the coefficients y, [3TPA and qPCA which characterize the non-linear behavior of the waveguide 101 can be carried out by the computing unit which is designated by reference 50 in [Fig.1], and noted CPU for "Central Processing Unit" in English.

[0063] Values ​​determined as described above for the nonlinear behavior coefficients y, [3TPA and qPCA 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 can be reproduced by modifying secondary aspects of the embodiment described in detail above, while retaining at least some of the advantages mentioned. In particular, the positions of several of the The optical components of the device of the invention can be exchanged or modified within this device, while maintaining equivalent device functionality. Furthermore, all numerical values ​​cited are for illustrative purposes only. Finally, although the waveguide to be analyzed 101 shown in [Fig. 2] is implemented as an optical circuit, it can be replaced by another waveguide to be analyzed, whether of the optical fiber type or any other type.< / e> < / e> < / e> < / e> < / e> < / e> < / e>

Claims

Demands

1. A device (100) for characterizing a non-linear behavior of a waveguide, referred to as the 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-to-one in the two optical paths and then interfere with each other in the superposition zone, having respective power values ​​which are different; - a radiation source (1), adapted to produce radiation having 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 phase shift of an interference state of the radiation that exists in the superposition zone (S) during the characterization of the waveguide to be analyzed (101), in which the two optical paths (CH1, CH2) have a common optical path portion 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 that propagate respectively in each of the two optical paths circulate in the waveguide to be analyzed in opposite directions of propagation, the device (100) being characterized in that it further comprises: - a computing unit (50) configured to determine at least one of a value of a nonlinear Kerr effect coefficient,a value of a two-photon absorption coefficient and a value of a free-carrier absorption coefficient, relative to the waveguide to be analyzed (101), from values ​​that were measured respectively for the amplitude and phase shift of the interference state that exists in the superposition zone (S).

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 portion of the radiation propagating in said first optical path, according to 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 portion of the radiation propagating in the first optical path (CH1);and - a synchronous detection system (33), which is connected on the one hand to the alternating signal electrical 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 phase shift of the interference state of the radiation which exists in the superposition zone (S) of the interferometer (2).;

3. 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. A device (100) according to any one of the preceding claims, wherein the radiation source (1) comprises a primary source (11) and an optical power dimmer (12), the optical power dimmer being intermediate between the primary source and the optical input (E) of the interferometer (2), such that said optical power dimmer determines the variable value of the power of the radiation that is injected into the interferometer.

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

7. Device (100) according to any one of the preceding claims, wherein each optical path (CH1, CH2) is made up of polarization-maintaining components.

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

9. A method for characterizing a non-linear behavior of a waveguide, referred to as the waveguide to be analyzed (101), the method comprising the following steps: - providing a device (100) that conforms to 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 interference state of the radiation that exists in the superposition zone (S).

10. A method according to claim 9, wherein the device (100) conforms to claim 6, and the method comprises recording measured values ​​respectively for the amplitude and phase shift of the interference state of the radiation existing 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 nonlinear Kerr effect coefficient, a value of a two-photon absorption coefficient, and a value of a coefficient absorption by free carriers, relative to the waveguide to be analyzed, from values ​​measured respectively for the amplitude and phase shift of the interference state, and relative to several values ​​measured for the power of the radiation injected into the interferometer.

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