Apparatus and method for optical interferometric noise signal measurement reflectometry

Abstract

The present invention relates to a method and apparatus for reflectometry (100) by measuring optical interferometric noise signals. According to the invention, the reflectometry apparatus comprises a light source (1), an optical beam splitter (4) spatially separating the modulated source beam (10) between a measuring optical arm (5) and a reference optical arm (7), and a detection system (30) adapted to acquire, as a function of time and as a function of a sweep of the modulation frequency over several modulation periods, the power of an interferometric signal of the reflected beam (23) in at least one temporal overlap region (25) between a pulse of a first reflected beam (21) at a reflection point (16) of the measuring optical arm and a pulse of a second reflected beam (22) by reflection of the second portion of the source beam (12) on the optical reflector (8) of the reference optical arm. Figure 1

Description

Title of the invention: Apparatus and method for optical interferometric noise signal reflectometry. Technical field

[0001] The present invention relates to the technical field of apparatus and methods for metrologically measuring an optical device by reflectometry. The present invention finds applications in the detection of the position and amplitude of optical reflections distributed within an optical device. Previous technique

[0002] There already exist a number of methods for carrying out measurements of parasitic reflections in an optical device to be tested (or DUT, acronym for Device Under Test in English).

[0003] Optical Time Domain Reflectometry (OTDR) involves sending an ultrashort-duration light pulse (generally between 10 ns and 50 ns) to one end of the optical device under test and continuously measuring stray reflections as a function of time. The time between the emission of the light pulse and the reception of a reflection indicates the position of the reflection point (in optical length). To perform several measurements, the OTDR emits several successive pulses, the time interval between two successive pulses being greater than the round-trip time in the DUT. The greater the optical length of the DUT, the lower the acquisition rate. OTDR allows measurements over very long distances, such as in optical fibers.However, this method suffers from a blind spot at the beginning of the DUT and has temporal resolution limitations that translate into spatial resolution limitations for the position of spurious reflections. The OTDR apparatus and method are relatively inexpensive.

[0004] White light interferometry relies on the use of a source with a very short coherence length, denoted Lco, on the order of 50 pm to 500 pm. The white light interferometer comprises a measuring arm in which the optical device to be tested is placed and a reference arm of variable length, for example, based on a movable, translating mirror. The optical return of the optical device is made to interfere with that of the reference arm. The measurement range is limited by the amplitude of the movable reference arm. However, white light interferometry allows for very precise mechanical positioning accuracy. The cost of such an interferometer is very high.

[0005] Optical reflectometry in the frequency domain (or OFDR for Optical Frequency Domain Reflectometry (OFDR) relies on the use of a single-frequency source with a tunable wavelength and a long coherence length, whose wavelength is linearly scanned. In OFDR, the return beam from the DUT is typically made to interfere with a portion of the source beam. The position difference between the emitted and reflected beams is transformed into an optical frequency difference, generating a beat frequency through interferometry at a frequency corresponding to the difference in optical frequencies. A time-domain harmonic analysis (FFT, Fast Fourier Transform) is then performed on a large sample with very rapid acquisition. OFDR is an expensive technique that allows access to a large measurement range, slightly beyond 100 meters of optical length, with good resolution around 50 pm.

[0006] It is desirable to propose a method and system for carrying out measurements of parasitic reflections in an optical device to be tested, which is accurate over a large measurement range, without blind zone and inexpensive. Description of the invention

[0007] In order to remedy the aforementioned drawbacks of the prior art, the present invention proposes an optical interferometric noise signal reflectometry apparatus, comprising: a light source capable of emitting a monochromatic source beam having a spectral width of less than 20MHz; a modulation system adapted to modulate the source beam temporally and periodically and to form a modulated source beam composed of source pulses modulated in power and wavelength at a given modulation frequency defining a modulation period, the modulation system being capable of varying the modulation frequency;an optical beam splitter arranged and configured to spatially separate the modulated source beam into a first portion of the source beam directed towards a measuring optical arm comprising an optical device to be measured in reflection and a second portion of the source beam directed towards a reference optical arm, the reference optical arm having a determined optical length L, L being greater than or equal to 20 meters and the optical device having an optical length less than L / 2, the reference optical arm comprising an optical reflector disposed at a distal end of the reference optical arm relative to the optical beam splitter, a difference in optical length between the measuring optical arm and the reference optical arm being greater than L / 2;the optical beam splitter being capable of optically recombining a first reflected beam formed by reflection of the first portion of the source beam at at least one reflection point of the optical measuring arm and a second reflected beam formed by reflection of the second; portion of source beam on the optical reflector of the reference optical arm so as to form a reflected beam, the optical beam splitter being capable of transmitting the reflected beam to a detection system, the detection system being adapted to acquire, as a function of time and as a function of a sweep of the modulation frequency over several modulation periods, a power of an interferometric signal of the reflected beam in at least a temporal superposition zone between a pulse of the first reflected beam and a pulse of the second reflected beam, an electronic system for demodulating the power of the acquired interferometric signal, the electronic system being adapted to extract an optical interferometric noise signal based on a standard deviation of a power difference at the modulation period as a function of the modulation frequency,and a processing system comprising a computer adapted to process the optical interferometric noise signal as a function of the modulation frequency and extract a reflectometry measurement as a function of the optical distance between at least one reflection point in the optical device to be measured and the optical reflector of the reference optical arm.

[0008] The device allows, at low cost, the measurement of distributed optical reflections using a modulated optical source, the separation into two of the signal with an arm towards the DUT and a reference arm using a mirror at the end thus allowing the recombination of the signal reflected by the DUT and that reflected by the reference arm.

[0009] Unlike the white light interferometer, the return of the reference arm is time-shifted to avoid direct interference. This shift is achieved by using a long reference arm. Similarly, the reference arm is fixed, thus significantly reducing the cost of the device, and no mechanical elements are used in the proposal to determine the position of the spurious reflection points in the DUT.

[0010] According to a particular and advantageous aspect, the calculator is adapted to extract from the reflectometry measurement a reflection amplitude of at least one reflection point in the optical device to be measured.

[0011] According to another particular and advantageous aspect, the light source comprises a distributed feedback laser diode.

[0012] According to one embodiment, the modulation system includes means for modulating an electrical current supply to the light source.

[0013] Advantageously, the modulation system is adapted to perform a modulation frequency sweep with a frequency resolution of at least Ippm.

[0014] According to one embodiment, the modulation frequency varies from 650 kHz to 800 kHz with a variation step between 1 Hz and 100 Hz.

[0015] Advantageously, the modulation period has a duty cycle of between 5% and 90%, for example, of 25%.

[0016] According to a particular and advantageous aspect, the modulated source pulses have a constant or time-varying power in a monotonic, exponential, linear or quasi-linear manner, over each modulation period and / or in which the modulated source pulses have a time-varying wavelength in a continuous and monotonic, exponential, linear or quasi-linear manner, over each modulation period.

[0017] According to another particular and advantageous aspect, the detection system is adapted to sample the power of the interferometric signal at a sampling frequency higher than the modulation frequency, so as to obtain a number N of measurement points of the interferometric noise signal at each modulation period over a number M of successive modulation periods, with N ranging from 1 to 100 and M greater than 1000.

[0018] According to one embodiment, the detection system includes an electronic low-pass filter having a cutoff frequency greater than or equal to the spectral width of the source.

[0019] Advantageously, the optical reference arm includes an optical fiber reel.

[0020] The invention also relates to a reflectometry method by optical interferometric noise signal measurement, comprising the following steps: - emission by a light source of a monochromatic source beam having a spectral width of less than 20MHz; - periodic time modulation of the source beam to generate a modulated source beam composed of source pulses modulated in power and wavelength at a given modulation frequency defining a modulation period, the modulation frequency being variable; - optical separation of the modulated source beam between a first portion of the source beam directed towards a measuring optical arm including an optical device to be measured in reflection and a second portion of the source beam directed towards a reference optical arm of determined optical length L, L being greater than or equal to 20 meters, the reference optical arm including an optical reflector disposed at a distal end of the reference optical arm, the optical device having an optical length less than L / 2; - optical recombination of a first beam reflected by the optical measuring arm and a second beam reflected by the optical reference arm, so as to form a reflected beam; - detection, as a function of time and as a function of a sweep of the modulation frequency over several modulation periods, of the power of an interfero- signal metric of the reflected beam in at least one temporal overlap zone between a pulse from the first reflected beam and a pulse from the second reflected beam, - demodulation of the power of the acquired interferometric signal to extract an optical interferometric noise signal based on a standard deviation of a power difference at the modulation period as a function of the modulation frequency, - processing of the optical interferometric noise signal as a function of the modulation frequency to extract a reflectometry measurement as a function of an optical distance between at least one reflection point in the optical device to be measured and the optical reflector of the reference optical arm.

[0021] Of course, the various features, variants, and embodiments of the invention can be combined with one another in various ways, provided they are not incompatible or mutually exclusive. Brief description of the drawings

[0022] In addition, various other features of the invention become apparent from the attached description made with reference to the drawings which illustrate one, non-limiting, embodiment of the invention and where:

[0023] [Fig.1] is a schematic view of a reflectometry device according to an example embodiment;

[0024] [Fig.2] is a schematic of a power curve of a time-modulated source beam;

[0025] [Fig.3] represents, as a function of time, a curve, in solid line, of power of a modulated beam reflected by the reference optical arm and a curve, in dashed lines, of power of a modulated beam formed by reflection in the measuring optical arm of a reflectometry device, the hatched areas indicating the areas of temporal superposition between these two modulated reflected beams;

[0026] [Fig.4] represents, as a function of time, an example of measuring the power of the signal detected by the detection system over a modulation period;

[0027] [Fig.5] is an example of periodic sampling of the received signal;

[0028] [Fig.6] is another example of periodic sampling of the received signal;

[0029] [Fig.7] is an example of variation in wavelength of the source beam;

[0030] [Fig.8] is an example of wavelength variations respectively for a part of a pulse reflected in the measuring optical arm and a part of a pulse reflected in the reference optical arm;

[0031] [Fig.9] represents, as a function of time, a solid line curve of periodic power modulation of a source beam, and respectively, a dashed line curve of periodic wavelength modulation of the same source beam;

[0032] [Fig. 10] represents, as a function of time, a continuous curve of power of a beam reflected by the reference optical arm, a dashed curve of the power of a beam formed by reflection in the measuring optical arm of a reflectometry device and indicates the corresponding modulation period numbers for each reflected beam, in the case of a partial temporal overlap between the pulses reflected in the two arms;

[0033] [Fig. 11] represents, as a function of time, a continuous curve of power of a beam reflected by the reference optical arm and a dashed power curve of a beam formed by reflection in the measuring optical arm of a reflectometry device in the case where there is no temporal overlap between the pulses of the two arms;

[0034] [Fig. 12] schematically represents, as a function of time, a curve, in line continuous, power of a beam reflected by the reference optical arm and a dashed curve of the power of a beam formed by reflection in the measuring optical arm in the case of perfect temporal overlap between the pulses of the two arms;

[0035] [Fig. 13] schematically represents, as a function of time, the power signal reflected detected electronically in the case of perfect temporal overlap between the pulses of the two arms;

[0036] [Fig. 14] schematically represents, as a function of time, an example of a signal of reflected power detected over three periods after electronic low-pass filtering in the case of perfect temporal overlap between the pulses of the two arms;

[0037] [Fig. 15] represents, as a function of time, a continuous curve of a signal of reflected power before electronic low-pass filtering, in the case of partial temporal overlap between the pulses of the two arms;

[0038] [Fig. 16] represents, as a function of time, a power curve, in solid line, of a reflected power signal detected after electronic low-pass filtering, in the case of a partial temporal overlap between the pulses of the two arms;

[0039] [Fig. 17] represents two measurement curves of a signal representative of a deviation of reflected power at the modulation period as a function of the optical distance from the optical reflector of the reference arm: a dashed curve with, in the measuring arm, a cleaved optical fiber section, and a solid line curve with, in the measuring arm, a complex optical device comprising several interfaces;

[0040] [Fig. 18] represents a zoom on a part of the solid line curve of [Fig. 17].

[0041] It should be noted that in these figures the common structural and / or functional elements the different variants may have the same references.

[0042] Of course, the different characteristics, variants and embodiments of The inventions can be combined with each other in various ways, provided they are not incompatible or mutually exclusive. Detailed description

[0043] Figure 1 schematically represents a reflectometry apparatus 100 according to an exemplary embodiment. The reflectometry apparatus 100 is based on a Michelson interferometer architecture, while operating outside the usual conditions of interferometry. As detailed below, the reflectometry apparatus 100 does not measure interference but rather an interferometric noise signal.

[0044] We seek to detect at least one reflection point in an optical device 6 or DUT. The optical device 6 is capable of propagating light along at least one optical propagation axis. The optical device 6 includes, for example, a waveguide-based device, such as one or more sections of optical fibers, one or more integrated optical components (also called PLCs for Planar Lightwave Component), free-field devices consisting of an assembly of lenses and / or mirrors, or any other optical component operating in transmission or reflection, the presence of interfaces or defects of which causes unwanted or intentional reflections. The optical device 6 thus comprises one or more optical components arranged in series. A reflection point corresponds, for example, to an optical defect or an interface between two materials within the optical device 6.A reflection point is likely to back-reflect at least part of a light beam propagating in the DUT along the optical axis of propagation, in the opposite direction to the incident light beam. The aim is to detect the existence of such reflection points and, if so, to determine the position of each reflection point along the optical axis of propagation in the DUT. The amplitude of the reflection at each reflection point is also to be measured.

[0045] The reflectometry device 100 comprises a light source 1, also called the light source, a modulation system 2, an optical beam splitter 4, a detection system 30, an electronic signal demodulation system 40, and a processing system 50. Demodulation, as defined here, is the subtraction between the signal detected at time t and the signal detected at time tT, where T is a modulation period of the source modulation. The reflectometry device 100 also comprises an electronic synchronization system 3, including, for example, a clock common to the source / modulation system and the detection and demodulation system. Alternatively, the electronic synchronization system 3 comprises a control edge detection system for the modulation system to define the acquisition time(s) of the detection system.As an example, the detection system includes a standard PIN photodiode detector associated with an amplifier. transimpedance converter.

[0046] The light source 1 is adapted to emit a single-frequency (or monochromatic) source beam. The light source 1 comprises, for example, a DFB laser diode or distributed feedback laser diode. The light source emits a source beam having a certain spectral bandwidth, generally between 1 MHz and 10 MHz, and therefore with a coherence length on the order of a few tens of meters in air. The wavelength of the emitted light is chosen, for example, around 1550 nm to allow the use of standard telecom components. Unlike a white light interferometer, a broadband spectral light source is not used. Unlike an optical frequency domain reflectometer (OFDR), an extremely coherent light source, i.e., one with a coherence length generally much greater than 1 km, does not necessarily have to be used.

[0047] The modulation system 2 comprises an optical modulator external to the light source 1 or a system capable of directly modulating the light source 1. The modulation system 2 is adapted to temporally modulate the source beam and form a modulated source beam 10 composed of power-modulated source pulses at a modulation frequency F corresponding to a modulation period T. Furthermore, the modulation system 2 is capable of varying the modulation frequency F. In one example, the light source 1 comprises a single-frequency DFB laser diode, and the modulation system 2 is configured to periodically modulate the electrical current supplying the DFB laser diode so as to generate the source pulses. In another example, the light source 1 emits a continuous light beam, and the modulation system 2 comprises an optical modulator, external to the source, disposed downstream of the light source 1.In this case, the optical modulator forms the source pulses.

[0048] The optical beam splitter 4 is connected to four branches: a first input branch connected to the light source 1, a second input-output branch connected to a measuring optical arm 5 comprising the optical device 6 to be measured by reflection, a third input-output branch connected to a reference optical arm 7, and a fourth output branch connected to the detection system 30. The optical beam splitter 4 is, for example, based on a 2x2 optical fiber coupler. In the forward direction of the modulated source beam, the optical beam splitter 4 divides the optical power received on the first input branch between the second input-output branch and the third input-output branch. In the reverse direction, after reflection of the divided beams in the measuring arm and the reference arm, the optical beam splitter 4 operates as a beam coupler or combiner.

[0049] The reference optical arm 7 comprises an optical reflector 8 disposed at a distal end of the reference optical arm 7 relative to the optical beam splitter 4. The reference optical arm 7 and the measuring optical arm 5 thus form the two arms of a Michelson type interferometer.

[0050] The reference optical arm 7 has a predetermined optical length L, the optical length being measured between the optical beam splitter 4 and the optical reflector 8. The optical length L is greater than or equal to 20 meters and preferably greater than or equal to 100 meters, for example, 150 m. Advantageously, the reference optical arm 7 includes a spool of optical fiber 9, which makes it possible to obtain such an optical length L in a compact size, the spool of optical fiber having, for example, a diameter of approximately 5 cm. By way of example, the optical reflector 8 includes a plane mirror. According to this disclosure, the optical length L of the reference optical arm 7 is fixed. Furthermore, the reference optical arm 7 is manufactured so as to avoid any unwanted reflections between the optical beam splitter 4 and the optical reflector 8.A particularly advantageous feature is that the reference arm incorporates a thermal regulation device to prevent thermal drift.

[0051] The optical measuring arm 5, comprising the optical device 6, has an optical length, denoted Ll, measured between the optical beam splitter 4 and the distal end of the optical device 6. The optical length Ll is much shorter than the optical length L of the reference arm. Advantageously, Ll is less than L / 2. The difference in optical length between the optical measuring arm 5 and the optical reference arm 7 is preferably greater than 100 m, and thus generally greater than the coherence length of the light source. This configuration prevents the formation of stable interference between the beams reflected by the measuring arm 5 and by the reference arm 7.However, in this configuration, a source with a significant coherence length can be retained if the source is directly modulated, the modulation periodically applying a zero emitted power condition and allowing the coherence between two successive pulses to be eliminated.

[0052] The detection system 30 includes a detector adapted to acquire, as a function of time, the optical power of the beam propagating from the fourth branch of the optical beam splitter 4. The detector is, for example, a PIN photodiode associated with a transimpedance amplifier for converting the photodiode current into a voltage. Advantageously, the detection system 30 includes an electronic low-pass filtering device having an adjustable cutoff frequency.

[0053] The electronic demodulation system 40 is arranged to receive the power signal acquired by the detection system 30, and preferably filtered. In particular, the electronic demodulation system 40 is adapted to convert the signal from The power is acquired by sampling it at the modulation period and subtracting the values ​​obtained at two successive periods. The electronic demodulation system thus makes it possible to obtain a periodic demodulation signal synchronized with the modulation of the source, whose time standard deviation defines the "interferometric noise signal".

[0054] The processing system 50 includes a computer adapted to process the optical interferometric noise signal as a function of the modulation frequency and thus form a reflectometry signal as a function of the optical distance (x) between at least one reflection point 16 in the optical device 6 to be measured and the optical reflector 8 of the reference optical arm.

[0055] We will now describe in detail the operation of the reflectometry device illustrated in [Fig.1].

[0056] Figure 2 shows an example of the power, denoted P(t), of the modulated source beam 10 as a function of time, denoted t. The duration of a source pulse is generally between 0.05 microseconds (ps) and 1 ps, and the modulation frequency F is between 100 kHz and 2 MHz, in other words, the modulation period Te is between 0.5 ps and 1 ps. The shape of the power-modulated pulses is, for example, rectangular, with a high level and a low level, the low level preferably being of zero power. Other power modulation shapes are also considered, such as sawtooth or sinusoidal pulses, generally with a low level, preferably of zero power. The duty cycle of the power modulation is generally between 5% and 50%.Unlike a time-domain optical reflectometer (OTDR), this method does not use a single ultra-fine pulse duration to determine the delay of optical returns from that initial pulse, but rather a series of periodic pulses whose juxtaposition at the return allows for the measurement of parasitic reflections.

[0057] The optical beam splitter 4 receives the modulated source beam 10 on its first input branch. The optical beam splitter 4 spatially separates the modulated source beam 10 into a first portion of the source beam 11 directed towards the measuring optical arm 5 and a second portion of the source beam 12 directed towards the reference optical arm 7. The optical beam splitter 4 is advantageously configured to transmit approximately 50% of the modulated source power to each of the two optical arms 5 and 7, especially since the beams pass through this optical beam splitter 4 twice. Alternatively, to increase the return level of the DUT, the coupler can be unbalanced to transmit, for example, 10% of the modulated source power to the reference optical arm 7 and 90% of the modulated source power to the measuring optical arm 5, but this configuration is not considered optimal.

[0058] The first portion of the source beam 11 is transmitted to the measuring optical arm 5 and propagates through the optical device 6 to be tested. Consider a reflection point 16 located in the optical device 6 at an optical distance x from the optical reflector 8 of the reference arm 7. For clarity, only one reflection point 16 is considered here. However, as described below, the device is adapted to simultaneously detect several reflection points located in the optical device 6 at different optical distances. A reflection point 16 is understood here to be a surface, an interface, an optical defect, or any element capable of generating light reflection. The reflection point 16 is at an optical distance Lx from the optical beam splitter 4.This reflection point 16 receives the first portion of the source beam 11 and contributes to forming a first reflected beam 21 propagating towards the optical beam splitter 4, in the opposite direction to the first portion of the source beam 11. Thus, the first portion of the source beam 11 travels the optical length Lx before reaching the reflection point 16 and the first reflected beam 21 also travels the optical length Lx before reaching the optical beam splitter 4. As previously stated, the optical length (Lx) is here much smaller than the optical length L.

[0059] The second portion of the source beam 12 is transmitted through the reference optical arm 7 to the optical reflector 8 and forms a second reflected beam 22, propagating in the opposite direction to the second portion of the source beam 12, towards the optical beam splitter 4. As previously stated, the reference optical arm 7 is free of any stray reflections. Thus, the second portion of the source beam 12 travels the optical length L before reaching the optical reflector 8, and the second reflected beam 22 also travels the optical length L before reaching the optical beam splitter 4.

[0060] The optical beam splitter 4 receives the first beam reflected 21 by the measuring optical arm 5 having traveled the optical length 2(Lx) and the second beam reflected 22 by the reference optical arm 7 having traveled the optical length 2L. The optical beam splitter 4 is capable of optically / spatially recombining / superimposing the first reflected beam 21 and the second reflected beam 22 to form a reflected beam 23 propagating from the fourth output branch to the detection system 30.

[0061] Figure 3 represents, as a function of time, the optical power of the first beam reflected 21 by the measuring optical arm 5 and, respectively, of the second beam reflected 22 by the reference optical arm 7, for a modulated source beam 10 as illustrated in Figure 2. The two reflected beams 21 and 22 are also time-modulated in the form of pulses at the modulation frequency F. The first beam reflected 21 by the measuring optical arm 5, for example, contains... successive pulses 211, 212, 213 at the modulation period T. Similarly, the second beam reflected 22 by the reference optical arm 7 has successive pulses 221, 222, 223 with the same modulation period T. By sweeping the modulation frequency F, the temporal overlap between the pulses 211, 212, 213 reflected by the measuring arm 5 and the pulses 221, 222, 223 reflected by the reference arm 7 is changed. The pulses reflected by the two arms overlap exactly when the modulation period T of the modulated source beam 10 satisfies the following equation, where c represents the speed of light in a vacuum.

[0062] [Math.l] 2x - cT

[0063] The pulse 21 emitted at time t overlaps with the pulse 22 emitted at time tT.

[0064] However, due to the large difference in optical length between the two arms and / or the return to 0 of the source power when it is modulated directly with zero low power, the reflected pulses which overlap temporally are not coherent with each other in terms of optical phase.

[0065] In the case of a partial temporal superposition of the pulses reflected by the two arms illustrated in [Fig.3], a pulse 211 of the first reflected beam 21 and a pulse 221 of the second reflected beam 22 only partially overlap in a temporal superposition zone 25 of duration less than the duration of the high level of the modulation. In a portion 24 of each modulation period T, only a part of the pulse 221 of the second reflected beam 22 is received by the optical beam splitter 4. In another portion 26 of each modulation period T, only a part of the pulse 211 of the first reflected beam 21 is received by the optical beam splitter 4. In yet another portion 27 of each modulation period T, neither part of the pulse 211 of the first reflected beam 21 nor any part of the pulse 221 of the second reflected beam 22 is received by the optical beam splitter 4.

[0066] The detection system 30 receives the reflected beam 23 formed by the temporal superposition of the first reflected beam 21 and the second reflected beam 22. As indicated above, the superimposed pulses from the two reflected beams 21, 22 are not coherent with each other and are generally time-shifted with respect to each other.

[0067] The reflectometry measurement according to the present disclosure is very different from a white light interferometric measurement. Indeed, in white light interferometry, the measurement point of the DUT is made to be at the same distance The optical length of the coupler-splitter is similar to that of the reflecting mirror of the reference arm (which would be equivalent to using an optical length of L close to Lx in the diagram in [Fig. 1]). The white light interferometer uses a mechanical displacement of the reference mirror for this purpose, and the spatial resolution is given by the coherence length of the source. When the two arms of the white light interferometer have the same optical length, such an interferometer provides a direct interferometric measurement.

[0068] On the contrary, according to the present disclosure, the lengths of the two arms 5, 7 are fixed and very different from each other.

[0069] The coherence length of a single-frequency laser source is represented in a simplified manner as a sequence of segments of optical length Lco in which the phase is constant, and which exhibit a random variation of this phase from one segment to the next. In the reflectometry apparatus and method 100 described here, in the case of a DFB-type source, the segments that overlap with the return from the two arms have no coherence with each other but interfere, resulting in a stable bias over a maximum time tco, corresponding to the length Lco. In the case of a source with a long coherence length, it is the direct modulation of the source, with a low modulation level of zero power, that causes the loss of optical phase coherence between two successive pulses.

[0070] Figure 4 shows, as a function of time, an example of a power measurement of the signal received by the detection system over one modulation period by modulating the constant source signal with an external modulator and balancing the power sent to the two arms. For a visual example, the signal representing the return of the DUT is much higher than it is generally in real-world applications. Several curves are shown over the same modulation period T, here equal to approximately 16.5 microseconds (ps): a dashed curve indicating the average detected power or average value, denoted 31, and a solid curve indicating the standard deviation, denoted 32, of the power measurement. This average is calculated over 1 ms, therefore averaged over 60 periods, with an acquisition step of 1 ns. The standard deviation represents the noise amplitude, also referred to in this document as the interferometric noise signal 32.During portion 27 of the modulation period T, the detection system 30 receives no pulse portion from the first reflected beam 21 nor any pulse portion from the second reflected beam 22, so the power average 31 of the detected signal is zero and the standard deviation 32 of the detected power is zero. During portion 24 of the modulation period T, the detection system 30 receives a pulse portion from the first reflected beam 21 and no pulse portion from the second reflected beam 22, so the power average 31 of the detected signal is non-zero. During the . In portion 26 of the modulation period T, the detection system 30 receives a portion of the pulse from the second reflected beam 22 and no portion of the pulse from the first reflected beam 21, so that the power average 31 of the detected signal is non-zero. However, in portions 24 and 26 respectively, the standard deviation 32, in other words, the noise signal, remains low and characteristic of the intrinsic noise of the source. On the other hand, in the temporal overlap region 25 of the modulation period T, the detection system 30 receives a portion of the pulse from the first reflected beam 21 and a portion of the pulse from the second reflected beam 22, so that the signals add together, the power average 31 of the detected signal is non-zero, and the standard deviation is very high: a non-zero interferometric noise signal 32 is observed.In other words, in the temporal superposition zone 25, the measured signal consists of a series of segments of interferometric signals, each having a duration tco, and which exhibit a random variation of this interferometric signal from one segment to the next.

[0071] The detection system 30 and the demodulation system 40 are adapted and configured to extract a representative measurement of the interferometric noise signal 32 as a function of time, for example based on the standard deviation or an equivalent of the standard deviation to the modulation period.

[0072] In the example illustrated in [Fig. 4], an optical modulator external to the light source 1 is used, and the wavelength is identical at every point of each power-modulated source pulse. Furthermore, the lower power is not perfectly zero here, as can be obtained by direct modulation of the source, but the difference between the two arms is sufficient to reject any direct interferometry, as can be seen approximately in the figure by the fact that the sum of the average powers of each beam given respectively at 24 and 26 is equal to the average power of the portion 25.

[0073] The reflectometry device 100 is adapted and configured to detect, as a function of time and as a function of a sweep of the modulation frequency, a signal representative of the standard deviation of the power difference detected in particular in the time overlap zone 25 between successive modulation periods.

[0074] For this purpose, an electronic demodulation system 40 is used with the detection system 30 and the modulation system 2. The measurement of the interferometric noise signal 32 and the modulation of the source 1 are perfectly synchronized via the electronic synchronization system 3 based either on a common clock or on the detection of the edges of the modulation control to define the acquisition instant(s).

[0075] Figure 5 represents an example of demodulation based on periodic sampling of the detected signal power as a function of time, over several periods modulation, for example here three modulation periods, is used to extract the interferometric noise signal. The detected signal has an arbitrary shape. In this example, a measurement point is sampled at each modulation period T, each measurement point being denoted respectively i at one modulation period, i-1 at the previous modulation period, and i+1 at the following modulation period. In other words, in this example, the acquisition frequency is equal to the modulation frequency. The calculation of the demodulation standard deviation is expressed as a summation according to the following equation:

[0076] [Math.2] J? — ■" y- (' / j- - / ----

[0077] where Pii represents the instantaneous power of the signal detected at time i-1, Pi the instantaneous power of the signal detected at time i, and Pi+1 the instantaneous power of the signal detected at time i+1, and B represents interferometric noise. On an interference-free signal, such demodulation gives a constant and very low value B representing the intrinsic noise of the source, which is low.

[0078] In equation Math2, we obtain the value B. To extract the interferometric noise signal 32, we calculate for example the standard deviation of the interferometric noise, based on the square root of the value of B divided by the number of cumulative values.

[0079] Advantageously, an electronic low-pass filter is used on the signal which preserves variations in the coherence time tco of the source. This is equivalent to having a cutoff frequency beyond the spectral width of the source, which is less than 10 MHz, without, however, exceeding this value in order to limit noise at higher frequencies. In practice, since the pulse length is from 0 μps to a few μs, it is useful to oversample the pulse duration, which is generally greater than the coherence time of the light source μ.

[0080] Furthermore, in order not to have to precisely define the measurement point in relation to the shape of the pulse, we choose to make the measurement uniformly over the whole of the modulation period.

[0081] Figure 6 illustrates another example of demodulation based on periodic oversampling of the received signal at each modulation period, to extract a value B representing the interferometric noise. Figure 6 shows only a portion of the measurement points, which are sampled at an acquisition frequency generally equal to a multiple of the modulation frequency, the multiple being, for example, from 10 to 100, which is achievable with current analog-to-digital converter components. In this example, several measurement points are sampled at each modulation period T, each measurement point being denoted respectively i, j, k at one modulation period, i-1, j-1, k-1 at the previous modulation period, and i+1, j+1, k+1 to the next modulation period. Consecutive measurement points are spaced temporally by the sampling period denoted TE, which is generally equal to an integer fraction of the modulation period T. The demodulation calculation is written as a cumulative process according to the following equation:

[0082] [Math.3] ^ =... + / vi / + + C^ j - / / / + - a /

[0083] where P; represents the instantaneous power of the signal detected at time i, Pj the instantaneous power of the signal detected at time j and Pk the instantaneous power of the signal detected at time k... More compactly, the above equation can be written as follows:

[0084] [Math.4] B = - Pfidt - r))2

[0085] where P(idt) represents the sampled power at time idt and P(idt-T) the sampled power at time idt-T, N being the number of cumulative points, timed by the user sampling frequency (for example, 100 Hz), considering each time an integer number of modulation periods. In the example given in [Fig. 4], the number of periods over which the mean and standard deviation were calculated is 60 for practical reasons, but it is generally around 1000 to 10000.

[0086] In equation Math3 or Math4, differences are calculated at the modulation period T. Consequently, the value of B allows us to recover the only source / electronic noise assumed to be constant (which is easily verified) when there is no interference, i.e. outside the temporal superposition zone 25. Another advantage of the oversampling technique is to avoid searching for the optimal measurement point over the period T since all points are measured.

[0087] In equations Math3 or Math4, we obtain the value B representing the interferometric noise. To extract the interferometric noise signal 32, we calculate, for example, the standard deviation of the power difference at the period, by dividing B by the number of measurement points and taking the square root of this result.

[0088] The interferometric noise signal 32 allows us to obtain a good dynamic range for measuring optical reflection at the measured point of the DUT, as with interferometric devices using direct interference since interference measures reflection in amplitude and not in power.

[0089] We will now describe how to obtain good spatial resolution in exploiting a wavelength modulation of the modulated source beam 10. For example, a DFB laser light source is used. If the two reflected beams 21, 22 which are added together consist of two wave trains superimposed temporally while being shifted in wavelength, the signal detected over the duration tco is modulated at a beat frequency Af corresponding to the difference in optical frequency of the two contributions.

[0090] Consider the two reflected beams 21, 22 having, at time t, a different wavelength, that is, a different optical frequency. Applying to the detection system an electronic low-pass filter whose cutoff frequency is lower than the beat frequency Af makes it possible to eliminate the noise generated by interference. The difference or gap in optical frequency Af is expressed as a function of the difference or gap in wavelength AX as follows:

[0091] [Math.5]

[0092] With an optical frequency deviation Af of 10 MHz and an average wavelength of 1550 nm, equation Math5 yields a wavelength deviation AX of 0.08 pm. Therefore, the interferometric noise signal can be considered not to be detected for wavelength deviations AX greater than 0.08 pm if electronic filtering is applied above 10 MHz. For this purpose, the detection system 30 or the demodulation system 40 includes an electronic low-pass filter. Electronic low-pass filtering thus improves spatial resolution when the wavelength changes periodically over the duration of the pulse.

[0093] An example of wavelength variation of power-modulated source pulses is described below. In the example illustrated in conjunction with Figures 7 and 8, a modulated source beam 10 is used, composed of power- and wavelength-modulated source pulses at the modulation frequency. Figure 7 schematically represents the wavelength of a modulated source beam 10 as a function of time. In this example, the source pulses are power-modulated so that they have a non-zero power during a portion 28 of each modulation period and zero power during another portion 29 of each modulation period. The sum of portions 28 and 29 is equal to the modulation period T. Furthermore, as illustrated in Figure 7, the source pulses are wavelength-modulated at least during the portion 28 of each modulation period where the power is non-zero.Advantageously, wavelength modulation has a linear form of periodic square waves at the power modulation period. In the example in [Fig.7], the wavelength modulation is linearly increasing over each portion 28. of modulation period T. In portion 29 where the power of the source pulses is zero, which appears in grey on [Fig.7], the shape of the variation in wavelength is arbitrary.

[0094] Figure 8 shows, as a function of time, the wavelength variations of the first beam reflected 21 by the measuring optical arm 5 and the second beam reflected 22 by the reference optical arm 7, respectively, for a beam of source pulses modulated in power and wavelength as described in relation to Figure 7. In the time overlap region, the pulses reflected by the two arms therefore exhibit a wavelength shift when they do not correspond exactly to the position defined by the modulation frequency according to the Mathl equation. The wavelength variation illustrated in Figures 7 and 8 is linear with a slope equal to dX / dt.If the two pulses of the interfering beams 21, 22 are separated by an optical distance AL (modulo the modulation period T), the wavelength difference between the first reflected beam 21 and the second reflected beam 22 is constant at every point in the temporal superposition zone, at each modulation period. We obtain the following relation where c represents the speed of light in a vacuum: .

[0095] [Math.6]

[0096] For example, a DFB laser source emitting around 1550 nm with a spectral width of 10 MHz and an electronic low-pass filter with a cutoff frequency also around 10 MHz are used. In this case, the interferometric noise signal tends towards zero if the following relationship is satisfied:

[0097] [Math.7] 0 ÛSwm -------OÂ

[0098] In one embodiment, a wavelength variation dX / dt of 100 pm / ps is used for each pulse. Applying equation Math7, the resolution limit for optical distance x is approximately 24 cm in a single pass along the optical axis of the DUT. To obtain a resolution / accuracy over the optical distance x of 1 cm, it is necessary to apply a wavelength variation on the order of 2400 pm / ps. It is assumed here that the wavelength modulation (or chirp) continuously changes the phase of the source pulse thus modulated in wavelength, which is in practice the case, particularly for a current-modulated DFB laser source as detailed below. The wavelength variation is clas This is strictly controlled by the temperature and / or the electric current of the laser source. The chromatic dispersion of the optical fiber of the reference arm has little impact on the choice of configuration.

[0099] Various examples of such power and wavelength modulation are obtained with a DFB laser source. The DFB laser source is supplied with electric current by a current source.

[0100] According to a first embodiment, a linear current ramp, for example increasing, is applied over the duration of the portion 28 of each modulation period where the power is non-zero. This results in a power that varies linearly with each modulation period and a monotonic variation of the wavelength with each modulation period. For example, a sawtooth-modulated electrical current is used directly. A current modulation amplitude on the order of 20 mA at a modulation frequency of 3 MHz makes it possible to achieve a wavelength variation of approximately 800 pm, or on the order of 2400 pm / ps. Such a sawtooth current modulation thus provides the variation necessary to obtain a resolution of 1 cm.

[0101] Figure 9 shows a second example of power and wavelength modulation. This example is based on the observation that a rectangular square wave modulation of the current supplying the DFB laser source simultaneously produces a square wave modulation of the source pulse power, reproducing the shape of the electric current squares, and a wavelength modulation that increases until it reaches an asymptotic value—depending on the pulse length—with each source pulse. Indeed, the current variation instantaneously induces a variation in the delivered optical power. An external cooling device thermally regulates the temperature of the DFB laser source. However, the response time of the external cooling device is much longer than the temperature variation of the DFB laser source induced by the current variation for modulation frequencies exceeding 100 kHz.The wavelength variation arises from temperature variations in the DFB source induced by current variations, which are faster than the externally applied thermal regulation of the DFB source. The wavelength variation is monotonic and continuous, i.e., increasing, during the portion 28 of each modulation period where the power is non-zero. In the portion 29 where the source pulse power is zero, the wavelength decreases rapidly. By reducing the duration of the portion 28 of each modulation period where the power is non-zero, i.e., by reducing the modulation period T and / or the modulation duty cycle, the wavelength variation tends to be linear or quasi-linear over the portion 28 of each modulation period. A slope of is easily obtained. wavelength variation dX / dt greater than or equal to 100 pm / s by modulating a DFB laser diode with current between 0mA and 100mA with a modulation period from 0.5ps to 1Ops and a modulation duty cycle from 5% to 50%.

[0102] It should be noted that in these two examples of power and wavelength modulation, the wavelength variation amplitude is sufficiently small to maintain a monotonic and continuous variation on each pulse and periodicity. The stability of the periodic wavelength response is important, but required only for the duration of each noise measurement, between 1 ms and 10 ms.

[0103] We will now describe more precisely how to determine the position x of a reflection point 16 in the optical device 6 to be tested.

[0104] As disclosed above, particularly in relation to [Fig. 4], a signal representative of the interferometric noise is measured, based on the standard deviation of the power of the interferometric signal detected by the detection system 30. In other words, the value of the measured signal is the noise value, the spatial resolution in x being achieved by the wavelength shift between the two reflected beams. The aim is to measure the absolute or relative position in x of the measured reflection point(s) with respect to the reflector 8 of the reference arm 7.

[0105] This utilizes advances in electronic components that allow, for example, the use of low-cost, programmable PLL (phase-locked loop) components, enabling the control of a clock with variations smaller than ppm and an equally precise absolute value. The clock allows the modulation frequency of the modulated source beam to be varied with very good resolution and very high precision.

[0106] The positional difference between the optical length L of the reference arm and the optical length, Lx, at the measurement point 16 from the beam splitter coupler 4 is given simply by the Mathl equation, indicated previously. In other words, the optical distance difference x is given by the following equation:

[0107] [Math.8]

[0108] Since the modulation frequency F is controlled with very high precision by the PLL, the determination of the optical distance x is also obtained with very high precision. The optical length L of the reference optical arm 7 is fixed and varies only with temperature. A thermal regulation device for the reference arm, for example based on a spool of optical fiber, makes it possible to obtain a measurement of the optical distance x with very high precision. Of course, the distance optics x depends on the refractive index of the material of DUT 6.

[0109] We will now describe the operating range of the reflectometer, as a function of a sweep of the modulation frequency F. An upper limit of the modulation frequency is set by half the optical length of the reference arm. Indeed, when the modulation frequency corresponds to half the length L of the reference arm 7, the detection system 30 receives the superposition of two pulses for comparison: on the one hand, a pulse reflected from the part starting at the length of half the reference (between the DUT reflection and the first subsequent pulse) and on the other hand, a pulse reflected from the beginning of the DUT (between the DUT reflection and the following pulse), which is difficult to discriminate. The upper limit, denoted Fmax, of the modulation frequency is therefore equal to twice the characteristic frequency of the reference length, that is, Fmax = 2c / 2L = c / L.We therefore consider a modulation frequency range less than or equal to Fmax.

[0110] In summary, the optical interferometric noise signal reflectometry apparatus operates under the following conditions. The modulation of the source pulse beam is configured to produce a linear or quasi-linear variation in wavelength in each pulse, either naturally or by current control, preferably by limiting the power variations in each source pulse. The measurement range is swept by changing the modulation frequency of the source pulse beam. Sampling is preferably performed over the entire modulation period. Advantageously, sampling is preferably performed with a sampling period TE close to the coherence length of the source and equal to an integer multiple of the modulation period. The demodulation of the received power is representative of the standard deviation of the power over the modulation period.The detection system 30 includes an electronic low-pass filter having a cutoff frequency greater than or equal to the spectral width of the source, preferably close to the spectral width of the source, for example on the order of 10 MHz.

[0111] Figures 10 to 16 allow for a more visual description of the modulation / demodulation method. Figure 10 represents, as a function of time, the power, denoted Ptest, of the first beam reflected 21 by the measuring optical arm 5 and, respectively, the power, denoted Pr, of the second beam reflected 22 by the reference optical arm 7, for a modulated source beam 10 as illustrated in Figure 9. The two reflected beams 21 and 22 are also time-modulated as pulses at the modulation frequency F. During a reflection at a reflection point of the DUT 6, a reflected pulse is added to the reflected pulse from the reference arm. For clarity, only one reflection point is considered here in the measurement arm 5. The successive pulses are numbered N=0, 1, 2. Depending on the modulation frequency, one of the three cases illustrated in figures 10, 11 and 12 occurs: the optical path difference between the reflection point of the DUT and the reference arm is so large that there is no time overlap between the pulses of the two arms ([Fig. 11]); the optical path difference between the reflection point of the measurement arm and the reference arm is exactly equal to half the modulation period (equation Math 1) and interferometric noise is observed in the bandwidth of the detection system ([Fig. 12]); or the optical path difference between the measurement arm and the reference arm is slightly different from half the modulation period and interferometric noise is observed outside the bandwidth of the detection system ([Fig. 10]).

[0112] Figure 11 schematically represents the case where there is no temporal overlap between the pulses of the two arms. In this case, the power difference between two successive periods is zero, because the periodicity of the successive pulses remains perfect. The signal representing the interferometric noise, based on a standard deviation of the power difference received between two successive periods, is therefore zero.

[0113] Figure 12 schematically represents the case where there is perfect temporal overlap between the pulses of the two arms. In this case, as illustrated in Figure 13, a randomly varying detected power level is observed. The variation in the power level is measured and is expressed as follows: 4^'

[0114] where Pr represents the power of the second beam reflected 22 by the reference optical arm 7, and Ptest respectively represents the power of the first beam reflected 21 by the return of the measuring optical arm 5. This variation in power level is added to the sum of the two powers according to the classical rules of interferometry. However, since there is no coherence between two successive pulses, the phase shift between the pulses differs between two successive modulation periods and the signal appears in time rather like in [Fig. 14].

[0115] In [Fig. 14], the average value of the detected power, denoted Pm, averaged over several portions 28 of successive modulation periods, is represented in dashed lines, and the average power calculated over each portion 28 of each modulation period is represented in solid lines, assuming that the time tco is of the same order of magnitude as the pulse time in this case, with the same temporal overlap between the pulses of the two arms as illustrated in Figures 12-13. It can be observed that the difference in average power between two successive modulation periods produces a detectable interferometric noise signal. The measurement of this noise signal interferes- This metric allows us to determine an amplitude that is proportional to the amplitude of the reflection in the optical device 6 under test. When the modulation frequency satisfies equation Math8, the method described here allows us to measure the reflection of the measurement branch 5 at the so-called synchronous reflection point, with respect to the modulation frequency. In practice, the power levels exhibit a series of variations whose length is the effective coherence length and which intentionally corresponds to the detection bandwidth.

[0116] As indicated above, the measurement is performed by scanning the modulation frequency. Figure 15 represents the power received on the detector in the case where the time offset between the pulses of the two arms is not synchronous with the modulation frequency (as illustrated in Figures 3 and 10).We distinguish different time zones or portions of each modulation period T: the portion 24, where only part of the pulse 221 of the second reflected beam 22 is received on the detector; the time superposition zone 25 where part of a pulse 211 of the first reflected beam 21 and part of a pulse 221 of the second reflected beam 22 are superimposed and are received on the detector; the portion 26, where only part of the pulse 211 of the first reflected beam 21 is received on the detector; and the portion 27, where neither part of the pulse 211 of the first reflected beam 21 nor part of the pulse 221 of the second reflected beam 22 is received on the detector. In the temporal overlap zone 25, due to wavelength modulation (see description in relation to [Fig.8]), the wavelengths between pulses 211 and 221 are sufficiently far apart to generate a very high-rate beat.

[0117] Figure 16 represents the power curve representative of the inter- noise signal. ferometric in the configuration of [Fig. 15], after electronic filtering of the detected power. The electronic filter here is an electronic low-pass filter, having a cutoff frequency above the modulation frequency, the cutoff frequency being greater than or equal to the spectral width of the source, preferably close to the spectral width of the source.

[0118] In [Fig. 16], it can be seen that the electronic low-pass filter has suppressed the high-frequency beats. A periodic pattern is observed, and the difference at the modulation period T is zero. In this time-shift configuration between the two received pulses, since the optical frequency difference is greater than the bandwidth of the electronic system, the interferometric noise signal is therefore zero.

[0119] The apparatus and method of this disclosure thus make it possible to selectively detect a reflection point in the measuring arm 5 by measuring a non-zero interferometric noise signal at a specific modulation frequency. The apparatus Optical interferometric noise signal reflectometry offers several advantages, including very low cost, relative compactness, and no moving parts, while delivering impressive performance in terms of measurement range, longitudinal spatial resolution, positional accuracy of the reflection point(s) within the DUT, and measurement dynamics without blind zones in the optical device under test. Indeed, adjusting the modulation frequency allows for the detection of a reflection point anywhere within the DUT.

[0120] Let us now consider the case where the optical device 6 to be tested in the measuring arm 5 has several points of reflection at different distances x from the optical reflector 8 of the reference arm 7. As soon as the pulses reflected by the different points of reflection are spaced temporally by more than the effective coherence length of the modulated DFB source, their contributions add up linearly with the following rule: only the synchronous part (between the measuring arm and the reference arm) of the different reflected beams is detected, thanks to the detection system and the low-pass electronic filtering system.

[0121] The physical phenomena involved in this disclosure are described below in a simplified representation. P0(t) denotes the optical power of the source beam emitted by the light source 1. The optical beam splitter 4 splits the modulated source beam by dividing, for example, by two the power injected onto each arm 5, 7. On the return, the optical beam splitter 4 superimposes the first beam reflected 21 by the measuring optical arm 5 and the second beam reflected 22 by the reference optical arm 7 to form a reflected beam 23 propagating towards the detection system 30. It is assumed here that the polarization of the beams is conserved, which allows for scalar addition, and that the losses in the fibers are negligible. The power detected by the detection system is given as follows:

[0122] [Math.9] P = 0.25^®^' T = û.25Pq(1. 4- J? + 2r case(Atù i + $>))

[0123] Where cp is a random phase shift in time, co represents the angular frequency (2p times the optical frequency) of the source beam, r represents an amplitude reflection coefficient at a reflection point 16 in the DUT 6 and R its power value (i.e., R = r²), Aco the angular frequency difference due to wavelength modulation at each modulation period described above in relation to [Fig. 8]. We simplify by assuming the wavelength shift AX is linear and therefore Aco is constant.

[0124] The first step consists of filtering with an electronic low-pass filter. By choosing a high-order filter, the average value of the cross term (cos) can be considered to be zero above the cutoff frequency and does not affect the signal below the cutoff frequency. When the two signals are sufficiently close (modulo the modulation period T), we obtain a variable power on each period and a difference at the period, that is to say a power difference AP between two successive periods, which is written as follows, where q> and q>i represent the phase shifts on the two successive periods:

[0125] [Math. 10] AP = 0.5rP8(cos(â&> (t 4- T) — œs(A&* t +• ^))

[0126] Since the two phases q> and q>i are random with respect to each other and vary with each period, the term function of time (which hardly varies over the pulse time in this case) is no longer relevant. We can therefore retain the generic expression with two random phases:

[0127] [Math. 11] AP = 0,SrPa(ços

[0128] It follows that the amplitude reflection coefficient, r, at the reflection point 16 in the DUT 6 is proportional to the standard deviation of AP measured at the modulation period T. The method also allows for calibration of the device using a reference optical device instead of the DUT.

[0129] A processing system 50 receives from the electronic power demodulation system 40 the interferometric noise signal 32 detected by measuring the power deviation at the modulation period, as a function of the modulation frequency sweep. The processing system 50 includes a computer adapted to process the signal thus obtained as a function of the modulation frequency and extract an optical distance deviation x between a reflection point 16 in the optical device 6 to be measured and the optical reflector 8 of the reference optical arm 7. The modulation frequency sweep is translated into a distance sweep x by applying the formula Math8.

[0130] Figures 17 and 18 illustrate an example of reflectometry measurement results by measuring the optical interferometric noise signal as a function of the optical distance x between the entry point in the DUT and the measurement point in the same arm, which corresponds approximately to the "Lx" in [Fig. 1]. A device 100, as illustrated in [Fig. 1], is used for these measurements. The optical fiber 9 on the reference optical arm 7 is wound to a length of 150 m (220 m optical length), which corresponds approximately to the length L in [Fig. 1]. The light source 1 is a DFB laser source emitting at around 1550 nm and providing a power of approximately 1 mW, which is current-modulated by applying 60 mA current pulses. The source beam has a wavelength of 1550 nm. Frequency modulation is performed over a range from 650 kHz to 800 kHz with a step The sweep frequency variation ranges from 1 Hz to 100 Hz, corresponding to an x-axis resolution of less than 1 mm to a few centimeters. The modulation duty cycle is around 25%, and the wavelength variation is a few tens of millimeters over each modulation period.

[0131] First, a measurement (dashed curve in [Fig. 17]) of the parasitic return is taken with, in the measurement arm, for DUT, a section of optical fiber that terminates with a right-angled cleave (with a reflection coefficient R of 4%). Then, a measurement is taken (solid line curve in Figures 17 and 18) with, in the measurement arm, a much more complex optical device 6, with different interfaces distributed along the longitudinal propagation axis, i.e., at different optical distances x from the optical reflector 8 of the reference arm 7. The distance is in optical length. It is noted in [Fig. 17] that the measurement dynamic range is 60 dB with an optical distance resolution x on the order of 2 cm.

[0132] The dashed curve of [Fig. 17] shows a very intense reflection on the cleaved end of the optical fiber segment and allows the optical distance between the cleaved interface of the optical fiber segment and the optical reflector to be determined, which is on the order of 6 m.

[0133] The solid curve in [Fig. 17] shows a series of low-intensity reflections on the different interfaces of the complex optical device 6, each interface forming a reflection point. The main reflection points are indicated in [Fig. 17] by arrows at respective optical distances of approximately: 27 m, 32 m, 35 m and 37 m.

[0134] Fig. 18 shows a zoom on the solid line curve of Fig. 17, around the point of reflection at the optical distance of approximately 37 m. The good spatial resolution in optical distance is observed here.

[0135] Different variants of the optical interferometric noise signal reflectometry apparatus are considered here. Naturally, it is important to ensure that the reference arm does not have any spurious reflection points other than the reflection from the mirror 8. According to a first variant, the optical fiber reel 9 is moved from the reference arm 7 to the measuring arm 5 where it is placed in series with the DUT 6 (the reel being closer to the beam splitter 4 than the DUT), the optical reflector being arranged at the output of the optical beam splitter 4. According to a second variant, the DUT 6, the optical fiber reel 9, and the optical reflector 8 are arranged in series on the measuring arm 5.

[0136] Unlike the white light interferometer, the return of the reference arm is time-shifted to avoid direct interference. This shift is achieved by using a long reference arm. Similarly, the reference arm is fixed, thus significantly reducing the cost of the device, and no mechanical elements are used in the proposal to determine the position. extraneous points of reflection in the DUT.

[0137] The periodicity of the signal and the use of high modulation frequencies allow us to consider that the properties of this signal are identical between two successive periods, with the important exception of the optical phase, the value of which changes randomly in response to the decoherence obtained by the time difference between each pulse. The present disclosure uses the signal resulting from the superposition of two successive periods that will intersect on the return path, the first from the reference and the second from the DUT, due to the difference in propagation length between the arm of the DUT and that of the reference.The phase decorrelation between the reference signal and the DUT return signal no longer allows for stable interference, but the coherence length of the source is sufficient for the superposition of the two beams to cause random, yet measurable, variations in power when the time difference reaches the modulation period of the signal. This results in interference noise whose amplitude characteristics allow for a very high measurement dynamic range, similar to direct interferometry used, for example, in white light interferometry.

[0138] As with other existing techniques, the aim here is to perform distributed reflection measurements along the DUT, and resolution is a key factor in the proposed device. This resolution is significantly increased by adding a variation in the optical wavelength of the modulated source signal, this variation being periodic and synchronized with the power variation. Unlike OFDR, which uses wavelength variations (over a range of nearly 100 nm) to measure the beat between the reference and the DUT reflection points, the invention proposed here uses wavelength variations over a very narrow range to increase selectivity and reject contributions near the measurement point. Selectivity on the order of centimeters can thus be achieved.The small wavelength range required in the proposed device allows the use of standard, very compact and low-cost laser sources, such as DFBs. This is a significant advantage over OFDRs, which generally require the use of expensive, tunable laser cavities. The proposal described here therefore eliminates any moving mechanical components at the source level.

[0139] After dynamics and resolution, the third important element in the realization of a device for measuring distributed optical returns is the determination of the position of these returns. The proposal described here therefore does not rely on temporal detection of the returns, as in OTDR, nor on determining the beat frequency between the return signal of the DUT and that of the reference, as in the case of OFDR. As with white light interferometry, one The position is not determined by analyzing measurements; instead, the measurement is taken on the DUT at a chosen position, providing greater measurement flexibility. Indeed, the position of the reference mechanical arm in the white light interferometer provides the measurement at the equivalent point of the DUT. In the case of the proposed invention, it is the choice of the source's modulation period that identifies the measurement point on the DUT, and it is the quality of the clock used by the electronic board that allows for the precise determination of this absolute position. Advances in the quality, cost, and control capabilities of electronic clocks enable absolute precision down to the ppm level in defining the modulation frequency. This allows for millimeter-precise positioning of the DUT reflections while maintaining a very wide modulation frequency range, resulting in a measurement range that can extend well beyond 100 meters.

[0140] The apparatus and method of the present disclosure thus make it possible to achieve a significant dynamic range, a resolution around the cm with a measurement range well above 100m and positioning to the mm, while remaining very low cost with the notable absence of any mechanical parts, whether at the source or the reference arm.

[0141] Using the modulation frequency to select the measurement point differentiates the various measurement zones within the DUT only by the frequency selection, with a maximum variation of 2 in the value of that frequency. The measurement is therefore very homogeneous in its execution for all points within the DUT, and there is no concept of a blind spot in the measurement. Furthermore, as with the white light interferometer, but without the mechanical constraints, focused measurements can be performed on a reduced area of ​​the DUT, thus enabling potentially very rapid temporal monitoring of the evolution of reflection in the area concerned.

[0142] Of course, various other modifications can be made to the invention within the scope of the annexed claims.

Claims

Demands

1. Optical interferometric noise signal measuring apparatus (100), comprising: - a light source (1) capable of emitting a monochromatic source beam having a spectral width of less than 20MHz; - a modulation system (2) adapted to modulate the source beam temporally and periodically and to form a modulated source beam (10) composed of source pulses modulated in power and wavelength at a given modulation frequency defining a modulation period, the modulation system (2) being capable of varying the modulation frequency; - an optical beam splitter (4) arranged and configured to spatially separate the modulated source beam (10) between a first portion of the source beam (11) directed towards a measuring optical arm (5) comprising an optical device (6) to be measured in reflection and a second portion of the source beam (12) directed towards a reference optical arm (7), the reference optical arm (7) having a determined optical length L, L being greater than or equal to 20 meters and the optical device (6) having an optical length less than L / 2, the reference optical arm (7) comprising an optical reflector (8) disposed at a distal end of the reference optical arm (7) relative to the optical beam splitter, a difference in optical length between the measuring optical arm (5) and the reference optical arm (7) being greater than L / 2; - the optical beam splitter (4) being capable of optically recombining a first reflected beam (21) formed by reflection of the first portion of source beam (11) at at least one reflection point (16) of the optical measuring arm (5) and a second reflected beam (22) formed by reflection of the second portion of source beam (12) on the optical reflector (8) of the reference optical arm (7) so as to form a reflected beam (23), the optical beam splitter (4) being capable of transmitting the reflected beam (23) to a detection system (30), - the detection system (30) being adapted to acquire, as a function of time and as a function of a sweep of the modulation frequency over several modulation periods, a power of an interferometric signal of the reflected beam (23) in at least one super- temporal position (25) between a pulse of the first reflected beam (21) and a pulse of the second reflected beam (22), - an electronic system (40) for demodulating the power of the acquired interferometric signal, the electronic system (40) being adapted to extract an optical interferometric noise signal (32) based on a standard deviation of a power difference at the modulation period as a function of the modulation frequency, and - a processing system (50) comprising a computer adapted to process the optical interferometric noise signal (32) as a function of the modulation frequency and extract a reflectometry measurement as a function of an optical distance (x) between at least one reflection point (16) in the optical device (6) to be measured and the optical reflector (8) of the reference optical arm (7).

2. Apparatus according to claim 1 in which the calculator is adapted to extract from the reflectometry measurement a reflection amplitude of at least one reflection point in the optical device (6) to be measured.

3. Device according to claim 1 or 2 in which the light source (1) comprises a distributed feedback laser diode.

4. Apparatus according to any one of claims 1 to 3 in which the modulation system (2) comprises means for modulating an electrical current supply to the light source (1).

5. Apparatus according to any one of claims 1 to 4 in which the modulation system (2) is adapted to perform a modulation frequency sweep with a frequency resolution of at least Ippm.

6. Device according to claim 5 wherein the modulation frequency varies from 650 kHz to 800 kHz with a variation step between 1 Hz and 100 Hz.

7. Device according to any one of claims 1 to 6 wherein the modulation period has a duty cycle of between 5% and 90%, for example 25%.

8. Apparatus according to any one of claims 1 to 7 wherein the modulated source pulses have a constant or time-varying power in a monotonic, exponential, linear or quasi-linear manner, over each modulation period and / or wherein the modulated source pulses have a time-varying wavelength in a continuous and monotonic, exponential, linear or quasi-linear manner, over each modulation period.

9. Apparatus according to any one of claims 1 to 8 wherein the detection system (30) is adapted to sample the power of the interferometric signal at a sampling frequency greater than the modulation frequency, so as to obtain a number N of measurement points of the interferometric noise signal at each modulation period over a number M of successive modulation periods, with N ranging from 1 to 100 and M greater than 1000.

10. Device according to any one of claims 1 to 9 in which the detection system (30) comprises an electronic low-pass filter having a cutoff frequency greater than or equal to the spectral width of the source.

11. Apparatus according to any one of claims 1 to 10 wherein the optical reference arm (7) comprises an optical fiber reel (9).

12. A method for optical interferometric noise signal reflectometry, comprising the following steps: - emission by a light source (1) of a monochromatic source beam having a spectral width of less than 20 MHz; - periodic time modulation of the source beam to generate a modulated source beam (10) composed of source pulses modulated in power and wavelength at a given modulation frequency defining a modulation period, the modulation frequency being variable;- optical separation of the modulated source beam (10) between a first portion of the source beam (11) directed towards a measuring optical arm (5) comprising an optical device (6) to be measured in reflection and a second portion of the source beam (12) directed towards a reference optical arm (7) of determined optical length L, L being greater than or equal to 20 meters, the reference optical arm (7) comprising an optical reflector (8) disposed at a distal end of the reference optical arm (7), the optical device (6) having an optical length less than L / 2; - optical recombination of a first beam reflected (21) by the measuring optical arm (5) and a second beam reflected (22) by the reference optical arm (7), so as to form a reflected beam (23);- detection, as a function of time and as a function of a sweep of the modulation frequency over several modulation periods, of a power of an interferometric signal of the reflected beam (23) in at least one temporal superposition zone (25) between a pulse; of the first reflected beam (21) and an impulse from the second reflected beam (22), - demodulation of the power of the acquired interferometric signal to extract an optical interferometric noise signal (32) based on a standard deviation of a power difference at the modulation period as a function of the modulation frequency, - processing of the optical interferometric noise signal (32) as a function of the modulation frequency to extract a reflectometry measurement as a function of an optical distance (x) between at least one reflection point (16) in the optical device (6) to be measured and the optical reflector (8) of the reference optical arm (7).

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