EP4367475A1

EP4367475A1 - Fibre-optic interferometer based on a monofrequency laser source and interferometry method corrected for parasitic reflections

Info

Publication number: EP4367475A1
Application number: EP22737505.2A
Authority: EP
Inventors: Eric Ducloux
Original assignee: Centre National dEtudes Spatiales CNES; Exail SAS
Current assignee: Centre National dEtudes Spatiales CNES; Exail SAS
Priority date: 2021-07-07
Filing date: 2022-07-07
Publication date: 2024-05-15
Also published as: FR3125120B1; FR3125120A1; WO2023280992A1

Abstract

The invention relates to an interferometer (100) comprising a light generator (1), a fibre-optic coil (4), an optical coupler/splitter, a photodetector (5) and an electronic signal-processing system (6). The light generator (1) comprises a laser source (8) able to emit a source beam (10) split into two secondary beams (11, 12) passing through the coil with a travel time τ defining a natural frequency (A). According to the invention, the laser source (8) is a monofrequency laser source and the light generator (1) comprises modulation means (7, 9) designed to modulate the source beam (10) at a modulation frequency equal to (B), where n is an integer greater than or equal to 1, and the photodetector (5) and the electronic signal-processing system (6) are configured to acquire and process a signal representative of the interference beam (15) at a demodulation frequency equal to (C).

Description

Fiber optic interferometer based on a single frequency laser source and method of interferometry corrected for parasitic reflections

Technical area

The present invention relates to the technical field of interferometric systems.

[0002] It relates more particularly to an interferometric system of the Sagnac interferometer type comprising a loop or coil of optical fiber. Such an interferometric system finds applications in particular in fiber optic gyroscopes (or FOG for fiber-optic gyroscope), electric current sensor or magnetic field sensor (see “The Fiber-Optic Gyroscope”, H. C. Lefèvre, Artech House, Second Publishing, 2014).

[0003] The invention relates most particularly to an optical fiber interferometer based on a laser source, the interferometer being insensitive to parasitic reflections from the laser source and a method for interferometric measurement corrected for the parasitic reflections induced by the laser source.

Prior technique

[0004] There are different types of interferometers based on the use of an optical fiber coil. In a conventional interferometer, intended for example for an application of fiber optic gyroscope, electric current sensor or magnetic field sensor, a source beam is separated into two light beams which are each injected at one end of the fiber optic coil to browse this reel in counter-propagative direction. After having traversed the coil each one only once, the beams are extracted at the two ends of the coil and recombined to form an interferometric beam.

[0005] There also exist fiber optic gyroscopes of the resonator type (or RFOG) (cf. US2013/0107271). Unlike a conventional fiber optic gyroscope, the RFOG comprises a device, for example a fiber optic coupler, which redirects the light beams that have already traveled through the fiber optic coil to reinject them and circulate in the coil, so that that they make multiple passes through the coil. In addition, the RFOG includes means for modulating or shifting the frequency of each of the counter-propagating beams so as to observe the resonance frequencies of the coil. The resonance frequency corresponds to a condition such that the beams having traversed the coil several times form constructive interference at any point of the coil. The RFOG measures an optical frequency difference between the two counter-propagating beams to deduce a rotation speed.

The present invention does not relate to an RFOG, but to a conventional fiber optic interferometer.

[0007] In most fiber optic interferometers, for example for current fiber optic gyroscope applications, the light source is a so-called broadband source, that is to say a source emitting a light beam extending over a spectral band generally comprised between a few nanometers and a few tens of nanometers. For example, the source is conventionally a so-called ASE (“Amplified Spontaneous Emission”) source or else a so-called SLED (“Superluminescent Light Emitting Diode”) source. The use of such sources has on the one hand the advantage of limiting the undesirable non-linear effects in the fiber, for example the Kerr effect, which induce a bias in the measurements, and on the other hand has the advantage to limit spurious optical interference and Rayleigh retro-reflection which are sources of noise disturbing the measurements.

[0008] On the contrary, with a monofrequency source, for example a laser source or a laser diode, parasitic reflections are a major problem for the noise of the FOG, due to the long coherence length of a monofrequency source. In this case, a parasitic reflection interferes with the main signal and generates particularly high noise on the measured power.

[0009] In this document, monofrequency source, otherwise called monomode source, means a source configured to emit at most 4 longitudinal modes, or emitting in the emission spectral band of width less than or equal to 1 GHz, at any time t. Laser diodes, in particular diodes of the DFB type or with distributed feedback (“Distributed Feed-Back”) are examples of single-frequency sources. Fabry Pérot type laser diodes, which emit over a very few modes, leaving much less powerful secondary modes, are here also considered as sources having a monofrequency source operation.

[0010] Although single-frequency sources have already been considered for the production of interferometers, their use has been greatly reduced in favor of broadband sources, in particular because of the parasitic interference generated by the reflections on the interfaces encountered by the different bundles.

One of the aims of the invention is to propose a system and method for optical fiber interferometric measurement based on a monofrequency source and which is insensitive to parasitic reflections.

Disclosure of Invention

Of course, the different characteristics, variants and embodiments of the invention can be associated with each other in various combinations insofar as they are not incompatible or exclusive of each other.

More specifically, the invention relates to an optical fiber interference meter comprising a light generator, an optical source splitter, an optical fiber coil, an optical coupler-splitter, a photodetector and an electronic signal processing system. , the light generator comprising a laser source capable of emitting a source beam, the source splitter being capable of transmitting the source beam to the optical splitter-coupler, the optical splitter-coupler being capable of splitting the source beam into two secondary beams and injecting each secondary beam at one end of the optical fiber coil so that the two secondary beams propagate in mutually opposite directions in the optical fiber coil, each secondary beam traversing the coil with a travel time τ defining a natural frequency of the coil, the optical coupler-splitter being capable of recombining the two secondary beams at the output of the optical fiber coil to form an interference beam and the source splitter being capable of transmitting the interference beam to the photodetector. According to the invention, the laser source is configured to emit at most 4 longitudinal modes at each instant t and in that the light generator comprises modulation means adapted to modulate the source beam at a modulation frequency equal to where n is a natural number greater than or equal to 1 and the photodetector and the electronic signal processing system are configured to acquire and process a signal representative of the interference beam at a demodulation frequency equal to

[0015] The laser source is configured to emit at most 4 longitudinal modes at each instant t. Equivalently, the laser source is configured to present a spectral width less than or equal to 1 GHz at each instant t. In other words, a monofrequency source is used at each instant t.

[0016] According to a first embodiment, the modulating means are adapted to power modulate the source beam, so that the source beam has a non-zero power at a time t- τ and the source beam has a power zero at times t and t-2 τ.

[0017] According to a second embodiment, the modulating means are suitable for tuning the laser source in wavelength, so that the laser source emits a first longitudinal mode at a time t- τ and at least one second longitudinal mode, distinct from the first longitudinal mode, at times t and t-2 τ, and in which the electronic signal processing system comprises a filter adapted to suppress a signal detected at a beat frequency between the first longitudinal mode and said at least a second longitudinal mode.

[0018] According to a particular aspect of the second embodiment, the modulating means comprise means for adjusting the temperature and/or electric current of the laser source.

[0019] According to a particular aspect of the first or the second embodiment, the modulating means are suitable for modulating the power of the source beam in a square pattern so that the power value at the measurement point is equal to two times the average value of the power over a range of duration 2τ around the measurement point.

According to another particular aspect of the first or second embodiment, the interferometer comprises control means configured to balance the power of the two secondary beams.

[0021] Particularly advantageously, the laser source comprises a laser diode, a distributed feedback laser diode or a Fabry-Pérot laser, or any other single-frequency optical source that can be modulated directly or by adding an external modulation. For example, mention is made of laser diodes, of the DBR type, with direct or external filtering.

According to a particular aspect, the interferometer comprises an optical phase modulator configured to modulate a phase difference between the two secondary beams at a phase modulation frequency equal to

Advantageously, the optical phase modulator is configured to modulate the phase difference according to a modulation with M states, per phase modulation period, where M is an even integer less than or equal to 20.

The invention also relates to an interferometry method comprising the following steps: emission of a source beam comprising at most 4 longitudinal modes at each instant t; optical separation of the source beam into two secondary beams; injection of each secondary beam at one end of a coil of optical fiber so that the two secondary beams propagate in mutually opposite directions in the coil of optical fiber, each secondary beam traversing the coil with a travel time τ defining a natural frequency of the coil; optical recombination of the two secondary beams at the output of the optical fiber coil to form an interference beam; detection of the incident interference beam on a photodetector, characterized in that the method comprises a step of modulating the source beam at a modulation frequency equal to where n is a natural integer greater than or equal to 1; the detection of interference beam being carried out at a demodulation frequency equal to The interferometric method and system of the invention make it possible to suppress the effects of parasitic reflections in an optical fiber interferometer based on a monofrequency source.

Brief description of the drawings

In addition, various other characteristics of the invention emerge from the appended description made with reference to the drawings which illustrate non-limiting forms of embodiment of the invention and where:

Figure 1 is a view of an interferometric system according to the invention,

[0028] FIG. 2 illustrates an example of interferometric phase shift measurement with a two-state phase modulation;

FIG. 3 illustrates a classic example of interferometric phase shift measurement with two-state phase modulation;

Figure 4 is a first example of modulation of the source beam according to the present disclosure and measurement of interferometric phase shift with the two-state phase modulation

FIG. 5 is a second example of modulation of the source beam according to the present disclosure and of interferometric phase shift measurement with the two-state phase modulation.

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

detailed description

FIG. 1 represents an optical fiber interferometric system 100 according to an exemplary embodiment. The interferometric system 100 comprises a light generator 1, an optical source splitter 2, a coil of optical fiber 4, an integrated optical circuit 3 comprising an optical coupler-splitter and an electro-optical modulator 31, a photodetector 5 and a system signal processing electronics 6, preferably digital.

The light generator 1 comprises a monofrequency laser source 8 which emits a source beam 10. For example, the monofrequency laser source 8 comprises a DFB type laser diode. The light generator 1 also includes an electrical power supply 7, for example a current source, which supplies the laser diode. Advantageously, as illustrated in FIG. 1, the laser diode is fiberized. The light generator 1 emits the source beam 10 at a source wavelength, λ, for example of 1550 nm with a spectral width of 10 MHz, over at most 4 longitudinal modes. Such a monofrequency source has a very long coherence length, of the order of 50 m but which nevertheless remains less than the optical length of the coil. The light power emitted by the source, or source power P0 in this document, is generally constant and between 0.2 mW and 20 mW. The monofrequency laser source 8 is generally a highly polarized source. The management of the polarization is taken into account in a conventional manner in the interferometer, but is not shown in the figures so as not to complicate the description.

The optical source splitter 2 is for example a 2×2 fiber optic coupler or a circulator. Alternatively, the optical source splitter 2 comprises an integrated optical waveguide device. The source optical splitter 2 receives the source beam 10 and transmits it in the direction of the integrated optical circuit 3 via a monomode optical fiber.

In the example represented, the integrated optical circuit 3 comprises an optical coupler-splitter formed by a Y junction with a waveguide integrated on a planar substrate. Advantageously, the flat substrate is made of lithium niobate or consists of a glass-niobate hybrid assembly. In the example illustrated in FIG. 1, electrodes are deposited along the two secondary branches of the Y junction to form an optical phase modulator 31. The main branch of the Y junction is connected for example via an optical fiber to the output of the source splitter 2. Advantageously, the main branch of the Y junction forms a waveguide polarizer which linearly polarizes the source beam 10. Each of the two secondary branches of the Y junction is connected to one end 41, 42 of the fiber optic coil 4.

The optical coupler-splitter receives the source beam 10 which it separates into two secondary beams 11, 12. The two secondary beams 11, 12 are also linearly polarized. Each secondary beam 11, respectively 12, is injected at a end 41, respectively 42, of the optical fiber coil 4. In this way, the two secondary beams 11, 12 propagate in mutually opposite directions in the optical fiber coil 4. Each secondary beam 11, 12 traverses the coil only once times with travel time τ. The natural frequency, fp, of the coil is defined according to the known formula fp=1/2τ. The optical coupler-splitter recombines the two secondary beams 11, 12 at the output of the optical fiber coil 4 to form an interference beam 15. The interference beam 15 propagates from the main branch of the Y junction in the direction of the optical source splitter 2.

In the return direction, the optical source splitter 2 transmits the interference beam 15 to the photodetector 5. The photodetector 5 detects a signal representative of the power of the interference beam 15 as a function of time.

A signal processing system 6 records the detected signal, which is digitized. The signal processing system 6 controls the voltage applied to the terminals of the electrodes of the optical phase modulator 31. The signal processing system 6 also makes it possible to control the power of the laser source and/or to adjust the length of the waves the source beam 10, as detailed below in connection with the first and second embodiment.

In Figure 1, the interface areas likely to generate parasitic reflections are represented by circles 21, 22, 41, 42. It is observed that the parasitic reflections occur mainly on the interfaces relatively close to the laser source 8. The length of the optical fiber coil is usually several hundred meters or several kilometers. As indicated above, the travel time of the light waves in the coil is denoted t. The reflection point 21 is located at the input interface on the source optical splitter 2. The reflection point 22 is located at the input interface of the integrated waveguide forming the main branch of the Y junction The reflection points 41 and 42 are located, respectively, at the interface between each secondary branch of the Y junction and the corresponding end of the optical fiber coil 4. Each reflection point 21, 22, 41,

42 contributes to spurious reflection. [0041] Compared to the length of the coil, the distance between the laser source 8 and each point of reflection 21, 22, 41, 42 is very small. For simplification in the rest of the description, it is considered here that the laser source 8 and the parasitic reflection zones are almost all at the same point.

[0042] Therefore, the photodetector 5 receives simultaneously at a time t the following three contributions: the main signal from the interference beam 15 described above, corresponding to the source beam emitted at a time t-τ, having crossed (after optical separation) the entire coil of optical fiber before being recombined and transmitted to the photodetector 5; a direct reflection corresponding to the source beam emitted at time t and which is reflected on the point(s) of reflection 21, 22, 41, 42, without passing through the optical fiber coil, to return directly to the photodetector 5; and a reflection from the coil corresponding to the source beam emitted at time t-2τ, having crossed the optical fiber coil for the first time, then being reflected on an interface (on the reflection point(s) 21, 22, 41, 42) to form a parasitic reflected signal propagating a second time in the optical fiber coil, before being transmitted to the photodetector 5.

A finding forming part of the present disclosure is that, in a conventional continuous-emitting single-frequency source interferometric system, the main signal and the parasitic reflection signals are received simultaneously by the photodetector while being emitted at different instants. each other.

It is assumed, for the clarity of the presentation, that the modulation applied to the optical phase modulator 31 is a two-state modulation, illustrated in FIG. 2. This phase modulation is periodic and at the natural frequency fp. However, a 4-state, 6-state, 8-state or more modulation can be envisaged without departing from the scope of the present disclosure.

We will briefly describe the conventional operation of an optical fiber interferometric system based on a monofrequency laser source, with a two-state phase modulation.

For two-state modulation, a modulated electric voltage V _m (t) is applied between the electrodes of the optical phase modulator 31 to modulate the difference in phase ΔΦ _m (t) of the measured interferometric signal. This modulation allows biasing which increases the sensitivity of the interferometric system, in particular for low amplitude rotation measurements. More precisely, the optical phase modulator 31 generates a phase shift Φ _m (t) on each secondary beam 11 and 12. A modulation of the phase difference ΔΦ _m (t) is thus obtained according to the following equation.

[0047] [Math 1]

ΔΦ _m (t) = Φ _m (t) -Φ _m (t- τ)

This modulation of the phase shift Φ _m (t) is obtained by applying a modulated electrical voltage 61 V _m (t) to the electrodes of the optical phase modulator 31.

It is observed that the modulation of the phase shift at the repetition frequency fp or period 2τ, implies that the phase differences are exactly opposite (or present an alternation symmetry) to difference τ, as in the following equation .

[0050][Math 2]

ΔΦ > _m (t + τ) = Φ _m (t - τ) - Φ _m (t + τ - τ) = Φ _m (t - τ) - Φ _m (t) = - ΔΦ _m (t)

This modulation alternation symmetry appears in FIG. 2, which shows a symmetrical phase difference as a function of time. This alternating symmetry is verified whatever the configuration of the electrodes: two push-pull electrodes or a single electrode on one arm of the optical phase modulator 31.

In particular, it is known to apply a so-called 2-state modulation, by modulating the square modulation voltage V _m between two step values, so as to produce a modulation of the phase difference, called the phase difference. biasing phase, on two levels, Δφ ₁ and Δφ ₂ symmetrical. For example, with a laser source, a modulation of ΔΦ _b (t)=±3π/4 is applied. The 2-state modulation is applied to the natural frequency f _p of the optical fiber coil. The natural frequency f _p is defined in such a way that T/2 = 1/(2.f _p ) = τ where T represents the period of the square modulation.

Thus, the modulation half-period T/2 corresponds to the difference in propagation time T between the long optical path passing through the coil and the short optical path which connects the optical phase modulator 31 to the optical coupler-splitter 3. In Figure 2, there is shown at the top the power Ps of the interferometric signal detected as a function of the phase shift Δφ between the two secondary beams propagating in mutually opposite directions in the coil. The modulation of the two-state phase difference as a function of time has also been represented. A modulation voltage is applied to the terminals of the electrodes of the optical phase modulator 31, so as to modulate the phase difference between the two secondary beams between two symmetrical levels Δφ ₁ and Δφ ₂ .

In Figure 3, there is shown at the top, an example of modulation of the two-state phase difference as a function of time, and, at the bottom, the power P0 of the laser source, as well as the instants of detection t and t-τ. The power P0 of the laser source is assumed to be constant. The curved lines illustrate the link between the laser signal emitted at time t-τ and the main interferometric signal detected at time t when only the main signal is considered.

As illustrated in Figure 3, the detection system 5 acquires the power Ps of the interferometric beam at the output of the interferometer at times t and t-τ according to the two modulation states, here at ±π/2. The signal processing system 6 digitizes the detected interferometric beam and demodulates the detected signal at f _p by sampling two power measurements over each modulation period and assigning a negative sign to a first level and a positive sign to the following level. This modulation-demodulation scheme based on a square modulation voltage generating 2 states at the frequency f _p makes it possible to obtain a better sensitivity of the interferometric system and a better stability of the measurements around zero, independently of the variations of the output power Ps .

In a prior art interferometer, with two-state phase modulation, the power of the source P ₀ is generally constant as a function of time.

In FIG. 3, the power of the source P ₀ is normalized to 1. The interferometric signal is acquired at times t and t − τ, at each modulation period.

However, as indicated above, the monofrequency source emitting continuously, the main signal and the parasitic reflection signals are received simultaneously by the photodetector while being emitted at different times from each other. We will now describe two embodiments making it possible to suppress the contribution of parasitic reflections in the detected signal.

FIG. 4 illustrates the operation of an interface based on a monofrequency source, and with a two-state phase modulation, according to an example of the first embodiment.

In this first embodiment, instead of emitting a continuous source beam of constant power, the intensity or the power of the source beam 10 is modulated. On the one hand, the light generator 1 is configured in such a way so that the monofrequency laser source 8 emits the source beam 10 of non-zero power P ₀ at time t - τ. On the other hand, the light generator 1 is configured so that the source beam 10 has a zero power P ₀ at times t and t - 2τ. Advantageously, the source beam 10 is modulated at a frequency equal to 2fp/(2n+1), where n is a natural number greater than or equal to 1. Preferably, the source beam 10 has a frequency equal to 2fp/ 3 (i.e. n=1). The duration of the source signal at P0=1 is generally around τ /2 without however having to be limited to this value. The transmitted power pattern of the source beam is not necessarily defined on the other times and can be arbitrary.

In this first embodiment, the two-state phase modulation remains at the natural frequency fp. The signal processing system 6 digitizes the detected interferometric beam and demodulates the detected signal at the frequency fp/(2n+1), here fp/3, by sampling two power measurements over three phase modulation periods.

In Figure 4, the source beam 10 is here power modulated at the frequency 2fp/3. The power P ₀ of the source beam 10 is equal to 1 (in arbitrary units) at times t −τ and t −4τ. The duration of each high level (at 1) of power P0 is here for example of the order of t. The power P ₀ of the source beam 10 is zero at times t, t - 2τ, t - 3τ and t - 5τ. Moreover, the interferometric signal is modulated in phase, according to a two-state modulation, at the frequency fp. The detected signal is demodulated at the frequency fp/3. More precisely, the interferometric signal is detected at times t and t−3τ. In Figure 4, the curved lines illustrate the link between the laser signal emitted at the instant of t-τ, respectively t-4τ, and the main interferometric signal detected at instant t, respectively t-3τ. The interferometric signal detected at instant t indeed corresponds to the source beam emitted at instant t-τ , of power equal to 1 (ua). The spurious signals likely to be detected at time t and which correspond to source beam 10 emitted at time t or t-2τ are impaired, since the power of the source beam emitted at time t and t-2τ is zero. Similarly, the interferometric signal detected at time t-3τ corresponds to the source beam emitted at time t-4τ, of power equal to 1 (ua). The parasitic signals likely to be detected at time t - 3τ and which correspond to the source beam 10 emitted at time t - 3τ or t - 5τ are damaged, since the power of the source beam emitted at time t - 3τ and t - 5τ is zero.

The signal detected at time t corresponds to the high modulation state, for example +τ/2. The signal detected at time t −3τ corresponds to the low modulation state, for example −τ/2. The two phase shift states are well measured with this modulation/demodulation scheme.

In this example of the first embodiment, the source beam 10 is modulated on two different power levels (0 and 1 in a.u.). To this end, the laser source 8 can be modulated directly via a signal 62, or even via a modulation 63 of the current source 7 which supplies the laser diode 8 so that the laser diode emits by pulse. As a variant, the laser diode 8 emits continuously and an external optical modulator 9, of the Mach-Zehnder type, is arranged downstream of the laser diode 8 to modulate the power of the source beam 10.

According to a particular aspect of the first embodiment, the power of the source beam 10 is modulated in a slot with a slot at P0=1 of duration close to or equal to τ so that the power value at the measurement point is equal to twice the average value of the power over a range of duration 2τ around this measurement point. This particular configuration also makes it possible to attenuate or even eliminate the harmful Kerr effects liable to appear in the optical fiber with a monofrequency laser source. According to another particular aspect of the first embodiment, there is added, at the level of the optical phase modulator SI or on the input fibers of the coil, an electro-optical device making it possible to balance the power of the two beams secondary 11, 12. This balancing of the secondary beams also makes it possible to attenuate or even eliminate the harmful Kerr effects likely to appear in the optical fiber coil with a monofrequency laser source.

In the example illustrated in FIG. 4 of the first embodiment, the phase modulation is at fp, while the source power modulation is at 2fp/3. The demodulation of the detected signal is at fp/3. The entire modulation/demodulation scheme is therefore at fp/3.

The first embodiment has several advantages. First, this source power modulation scheme allows the total suppression of interferometric spurious reflection. It allows the elimination of local interferometers (Michelson type) measured by the interferometric system, for example for a FOG application or electric current sensor or magnetic field sensor and sources of problems. Finally, it makes it possible to eliminate potential disturbances of the source by a parasitic return, which makes it possible to dispense with an optical isolator (or optical circulator) between the laser source 8 and the source splitter 2.

Compared to the conventional configuration of modulation and demodulation at the frequency fp, it is observed that there is no longer any crossover between transmission and reception in this first embodiment. However, only 2 measurement points remain at the frequency fp/3, whereas there are 6 measurement points in the classic configuration. This first embodiment therefore has the drawback of increasing the measurement noise by a factor √3.

This first embodiment is therefore advantageous when the noise reduction generated by the parasitic reflections is greater than the increase in noise resulting from the change of sampling, or when one of the other advantages indicated above is sufficiently therefore.

This first embodiment can be generalized to a phase modulation with 4 states, 6 states, 8 states or 12 states or 20 states, without departing from the scope of the present disclosure. More generally, the invention applies to any phase modulation with M states, where M is an even integer less than or equal to 20.

[0072] FIG. 5 illustrates the operation of an interface based on a monofrequency source, and with a two-state phase modulation, according to an example of the second embodiment.

In this second embodiment, instead of emitting a monofrequency source beam of constant wavelength, the wavelength of the source beam 10 is modulated with a modulation frequency. However, at each instant of the phase modulation period, the source beam remains monofrequency. In other words, the source emits only one wavelength at a time. In this second embodiment, the source beam 10 is modulated in wavelength (or in optical frequency) at a frequency equal to 2fp/(2n+1), where n is a natural number greater than or equal to 1. Preferably, the source beam 10 is modulated in wavelength at a frequency equal to 2fp/3 (in other words n=1). Thus, for example, the light generator 1 is configured so that the laser source 8 emits the source beam 10 at a first wavelength λ ₁ at time t -τ (modulo 3τ), at a second wavelength wavelength λ ₂ at time t - 2τ (modulo 3τ) and at a third wavelength λ ₃ at time t (modulo 3τ). The minimum difference between each pair of optical frequencies corresponding to the wavelengths λ ₁ , λ ₂ and λ ₃ is 100 MHz, to be above the power measurement bandwidth, with an emission duration of each wavelength (λ ₁ , λ ₂ and λ ₃ ) around τ/2. The wavelength of the source beam is not necessarily defined on the other times and can be arbitrary. The source can transmit continuously. As a variant, the source emits pulses at different wavelengths.

In this second embodiment, the phase modulation, for example with two states, remains at the natural frequency fp. The signal processing system 6 digitizes the detected interferometric beam and demodulates the detected signal at the frequency fp/3 by sampling six power measurements over three phase modulation periods, therefore with a sampling corresponding to the conventional approach. In this second embodiment, the source beam 10 is here modulated in wavelength, for example via a modulation of the electric current 70 generated by the current source 7 which supplies the laser diode 8. The constraint of cancellation of the power of the source beam at a measurement instant is not necessary here, unlike the first embodiment. In this second embodiment, the wavelength of the source beam 10 is modulated at the frequency 2fp/3 as indicated above. In this way, emission and detection are never simultaneously at the same wavelength. At time t-τ (modulo 3t), a signal is transmitted at the first wavelength λ ₁ which is detected at time t. At time t-2τ (modulo 3τ), a signal is transmitted at the second wavelength λ ₂ which is detected at time t-τ. At time t or t-3τ (modulo 3x), a signal is transmitted at the third wavelength λ ₃ which is detected at time t-2τ.

In this second embodiment, the detected signal is demodulated at the frequency fp. More precisely, the interferometric signal is detected at times t, t -τ, t - 2τ, t - 3τ, t -4τ and t - 5τ and a demodulation scheme is applied which is a duplication of the demodulation scheme at fp, therefore +-+-+- in this case.

The interferometric signal detected at time t indeed corresponds to the source beam emitted at the first wavelength at time t-τ. The spurious signals likely to be detected at time t and which correspond to the source beam 10 emitted at the third wavelength at time t and/or to the source beam 10 emitted at the second wavelength at time t instant t - 2τ generate spurious interference at a high beat frequency, corresponding to the difference in optical frequencies, which is easily electronically filtered. For a source wavelength difference of 1pm, the optical frequency difference is greater than 100MHz.

Similarly, the interferometric signal detected at time t-3τ corresponds to the source beam emitted at the first wavelength at time t-4τ. The parasitic signals likely to be detected at time t - 3τ and which correspond to the source beam 10 emitted at the third wavelength at time t - 3τ and/or at the second wavelength at instant t - 5τ are also electronically filtered at the beat frequency. The interferometric signal detected at time t -τ corresponds to the source beam emitted at the second wavelength at time t - 2τ. The spurious signals likely to be detected at time t -τ and which correspond to the source beam 10 emitted at the first wavelength at time t -τ and/or at the third wavelength at instant t - 3τ are also electronically filtered at the beat frequency.

The interferometric signal detected at time t - 2τ corresponds to the source beam emitted at the third wavelength at time t - 3τ. The spurious signals likely to be detected at time t - 2τ and which correspond to the source beam 10 emitted at the second wavelength at time t - 2τ and/or at the first wavelength at instant t - 4τ are also electronically filtered at the beat frequency.

The interferometric signal detected at time t - 4τ corresponds to the source beam emitted at the second wavelength at time t - 5τ. The spurious signals likely to be detected at time t - 4τ and which correspond to the source beam 10 emitted at the first wavelength at time t - 4τ and/or at the third wavelength at instant t - 6τ are also electronically filtered at the beat frequency.

The interferometric signal detected at time t - 5τ corresponds to the source beam emitted at the third wavelength at time t - 6τ. The spurious signals likely to be detected at time t - 5τ and which correspond to the source beam 10 emitted at the second wavelength at time t - 5τ and/or at the first wavelength at instant t - 7τ (modulo 6x, in other words at t- τ) are also electronically filtered at the beat frequency.

In Figure 5, the signal detected at times t, t - 2τ and t - 4τ corresponds to the low modulation state, for example -π/2. The signal detected at times t-τ, t-3τ and t-5τ corresponds to the high modulation state, for example +π/2. The two phase shift states are well measured with this modulation/demodulation scheme.

In this second embodiment, the source is modulated with a periodicity of 2fp/3. The basic periodicity of the overall modulation/demodulation scheme is therefore here also fp/3.

The second embodiment has the advantage of totally eliminating, by electronic filtering, the interferometric parasitic reflection. Moreover, the second embodiment does not induce any degradation of the measurement noise by sampling defect. In addition, this modulation/demodulation scheme makes it possible to maintain isolation of the source under certain conditions, without requiring an optical isolator.

However, this second embodiment has drawbacks. First of all, it does not make it possible to eliminate the effect of local interferometers (Michelson type) which can be substantial for the interferometric system, for example for a FOG application or electric current sensor or magnetic field sensor. Moreover, it requires an additional means to allow a variation of the wavelength of the source beam between the emissions at the different instants.

Let us consider the case of a laser source 8 of the DFB laser type. Mention is made here of various examples enabling the implementation of the second embodiment.

According to a first example, the electric power is modulated via the injection of the electric current 70 which supplies the DFB laser diode, in order to vary the wavelength of the source beam emitted. A current variation of the order of 1mA easily makes it possible to make a sufficient wavelength shift since this corresponds to an optical frequency difference of several hundred MHz on standard DFBs.

In a second example, the duty cycle is modulated (in other words the duration of the source pulses emitted successively), so as to modify the heating of the DFB laser diode and the emission wavelength. A variation of 5% of the duration of the pulse (for plateau durations around 1μs) between the pulses emitted at times t, t - τ, t - 2τ makes it possible in practice to obtain a beat between the lengths d wave well above 100MHz.

In a third example, the pulses emitted successively are shifted. In each pulse, the effect of increasing temperature makes it possible to vary the wavelength of the emitted source beam. The successive pulses being shifted with respect to each other, one thus obtains different wavelengths at the instants considered for the measurement. For reasons very similar to those used in the previous example, an offset of 5% of the duration of the pulse (for plateau durations around 1μs) between the pulses emitted at times t, t-τ , t - 2τ makes it possible in practice to obtain a beat between wavelengths well above 100MHz.

According to a particular aspect of the second embodiment, the power of the source beam 10 is modulated in a slot with a slot at P0=1 of duration close to or equal to τ/2 so that the power value at the point of measurement is equal to twice the average value of the power over a range of duration 2τ around this measurement point. This particular configuration also makes it possible to attenuate or even eliminate the harmful Kerr effects liable to appear in the optical fiber with a monofrequency laser source.

According to another particular aspect of the second embodiment, is added, at the level of the optical phase modulator 31 or on the input fibers of the coil, an electro-optical device making it possible to balance the power of the two beams secondaries 11, 12.

This balancing of the secondary beams also makes it possible to attenuate or even eliminate the harmful Kerr effects likely to appear in the optical fiber coil with a monofrequency laser source.

The first and second embodiments are described here in the context of a two-state phase modulation at the frequency fp. However, those skilled in the art will easily apply the principle of the invention to a phase modulation with 4 states or more.

FIGS. 4 and 5 illustrate exemplary embodiments with modulation of the source beam (in power or in wavelength) at a frequency of 2fp/3.

More generally, the invention applies to any phase shift modulation pattern at a frequency fp/(2n+1) with alternating symmetry, combined with a modulation of the source (in terms of power and/or of wavelength) at the frequency 2fp/(2n+1), with n positive integer greater than or equal to 1.

Of course, various other modifications can be made to the invention within the scope of the appended claims.

Claims

1. Optical fiber interferometer (100) comprising a light generator (1), an optical source splitter (2), an optical fiber coil (4), an optical coupler-splitter, a photodetector (5) and a system signal processing electronics (6), the light generator (1) comprising a laser source (8) able to emit a source beam (10), the source splitter (2) being able to transmit the source beam (10) to the optical coupler-splitter, the optical coupler-splitter being able to separate the source beam (10) into two secondary beams (11, 12) and to inject each secondary beam (11, 12) at one end (41, 42) of the optical fiber coil (4) so that the two secondary beams (11, 12) propagate in mutually opposite directions in the optical fiber coil (4), each secondary beam (11, 12) traversing the coil with a travel time τ defining a natural frequency of the coil, the optical coupler-splitter being able to recombine the two beams secondaries (11, 12) at the output of the optical fiber coil (4) to form an interference beam (15), the source splitter (2) being capable of transmitting the interference beam (15) to the photodetector (5), characterized in that the laser source (1) is configured to emit at most 4 longitudinal modes at each instant t and in that the light generator (1) comprises modulation means (7, 9) adapted to modulate the source beam (10) at a modulation frequency equal to where n is a natural integer greater than or equal to 1 and in that the photodetector (5) and the electronic signal processing system (6) are configured to acquire and process a signal representative of the interference beam (15) at a demodulation frequency equal to

2. Fiber optic interferometer according to claim 1, in which the modulation means (7, 9) are adapted to power modulate the source beam (10), so that the source beam (10) has a power not zero at a time t- τ and the source beam (10) has zero power at times t and t-2 τ.

3. Fiber optic interferometer according to claim 2 wherein the modulation means (7, 9) comprise an optical modulator (9) disposed between the laser source (8) and the source splitter (2), the optical modulator (9) being configured to modulate the power of the source beam (10).

4. Fiber optic interferometer according to claim 1, in which the modulation means (7, 9) are suitable for tuning the laser source (8) in wavelength, so that the laser source (1) emits a first longitudinal mode at a time t- τ and at least a second longitudinal mode, distinct from the first longitudinal mode, at times t and t-2 τ, and in which the electronic signal processing system (6) comprises a matched filter to suppress a detected signal at a beat frequency between the first longitudinal mode and said at least one second longitudinal mode.

5. Fiber optic interferometer according to claim 4, in which the modulation means (7, 9) comprise means for adjusting the temperature and/or electric current of the laser source.

6. Fiber optic interferometer according to one of claims 1 to 5, in which the modulating means (7, 9) are adapted to modulate the power of the source beam (10) in a slot such that the power value at the measurement point is equal to twice the average value of the power over a range of 2t duration around the measurement point.

7. Fiber optic interferometer according to one of claims 1 to 6 comprising control means configured to balance the power of the two secondary beams (11, 12).

8. Fiber optic interferometer according to one of claims 1 to 7 wherein the laser source (1) comprises a laser diode, a distributed feedback laser diode or a Fabry-Pérot laser.

9. Fiber optic interferometer according to one of claims 1 to 8 comprising an optical phase modulator (31) configured to modulate a phase difference between the two secondary beams (11, 12) at an equal phase modulation frequency at

10. Fiber optic interferometer according to claim 9, in which the optical phase modulator (8) is configured to modulate the phase difference according to a modulation at M states, per phase modulation period, where M is an even integer less than or equal to 20.

11. Interferometry method comprising the following steps: emission of a source beam (10) comprising at most 4 longitudinal modes at each instant t; optical separation of the source beam (10) into two secondary beams (11, 12); injection of each secondary beam (11, 12) at one end (41, 42) of a coil of optical fiber (4) so that the two secondary beams (11, 12) propagate in mutually opposite directions in the optical fiber coil (4), each secondary beam (11, 12) traversing the coil with a travel time τ defining a natural frequency of the coil; optical recombination of the two secondary beams (11, 12) at the output of the coil of optical fiber (4) to form an interference beam (15); detection of the interference beam (15) incident on a photodetector (5), characterized in that the method comprises a step of: modulating the source beam (10) at a modulation frequency equal to where n is a natural integer greater than or equal to to 1; the detection of the interference beam (15) being carried out at a demodulation frequency equal to