INTERFEROMETER WITH A LOOP-SHAPED OR STRAIGHT OPTICAL FIBER

Description

technical field

[0001] The present invention relates generally to the field of interferometric systems.

[0002] It relates more specifically to a loop or line fiber optic interferometric system. Such an interferometric system finds applications in fiber optic gyroscopes (FOGs, see "The Fiber-Optic Gyroscope," H.C. Lefèvre, Artech House, Second Edition, 2014). It also finds applications in fiber optic current sensors (FOCSs) and magnetic field sensors.

[0003] It relates in particular to a high-precision optical fiber interferometric system and process. Previous technique

[0004] There figure 1This schematically represents a Sagnac loop fiber optic interferometric system according to the prior art. This fiber optic interferometric system comprises a light source 20 emitting a source beam 100, a source-receiver splitter 22, a multifunction integrated optical circuit 14 (MIOC), an optical fiber reel 17, a photodetector 18, and a signal processing system 900. The integrated optical circuit 14 includes optical waveguides preferably formed by annealed proton exchange (APE) on a planar electro-optical substrate, for example, lithium niobate. Proton exchange on lithium niobate leads to the formation of single-mode waveguides. The input-output waveguide thus forms a single-mode waveguide polarizer 24 that guides only a single linear polarization.The integrated optical circuit 14 also includes a Y-junction type coil splitter 15 formed by dividing the input-output waveguide into two single-mode secondary branches. Advantageously, the integrated optical circuit 14 also includes electrodes connected to an electrical generator to form an electro-optical modulator or phase modulator 16 suitable for modulating the phase difference ΔΦ between two counter-propagating beams. The planar substrate of the multi-function integrated optical circuit 14 can easily be connected on one side to the two ends of the optical fiber coil 17 and on the opposite side by a section of optical fiber 23 to the source-receiver splitter 22.

[0005] The coil splitter 15 spatially separates the source beam 100 into a first single-mode wave 101 and a second single-mode wave 102, which propagate in opposite directions in the optical fiber coil 17. At the output of the coil, the coil splitter 15 recombines these two single-mode waves to form an interferometric beam 300. The source-receiver splitter 22 guides the interferometric beam 300 to the photodetector 18. The detector 18 receives the interferometric beam and generates a detected signal 80.

[0006] The signal processing system 900 includes, for example, an analog-to-digital converter 19, a digital processor 30, for example of the DSP (Digital Signal Processor), FPGA (Field Programmable Gate Array), or ASIC (Application Specific Integrated Circuit) type, and a digital-to-analog converter 31. The digital processor 30 extracts a signal of a parameter to be measured 90, for example, rotational speed, to a digital output. The digital-to-analog converter 31 applies a modulation voltage 60 to the electrodes of the optical phase modulator 16.

[0007] When the interferometric system is at rest, the two split beams emerge from the optical fiber coil in phase, due to the reciprocity of optical paths in the optical fiber coil.

[0008] However, in the presence of physical phenomena that can produce non-reciprocal effects on the optical path of the two counter-propagating beams in the optical fiber coil 7, a phase difference appears in the detected interferometric beam. The open-loop response of an interferometric system as described above is a function of the phase difference ΔΦ related to the quantity to be measured, according to the following equation, where P is the power of the return interferometric beam, and P0 is the maximum return power when ΔΦ = 0. P = P 0 2 1 + cos ΔΦ

[0009] Among the main physical phenomena inducing non-reciprocal effects, the rotation of the interferometric system around the axis of the optical fiber coil induces a phase difference proportional to the rotation speed. From this property, called the Sagnac effect, stems the primary application of a Sagnac loop interferometer to a gyroscope for measuring the rotation speed around the axis of the optical fiber coil. Indeed, during rotation of the interferometer around the axis of the optical fiber coil, a phase difference ΔΦs is induced by the parameter to be measured. In the presence of the Sagnac effect in a FOG (Front of Optical Gyroscope), the phase difference ΔΦs is proportional to the rotation speed.

[0010] The Faraday effect, or collinear magneto-optical effect, is also known to produce non-reciprocal effects. Loop or line fiber optic interferometers have applications as magnetic field sensors or as electric current sensors (see the publication J. Blake et al. "In-Line Sagnac Interferometer Current Sensor" IEEE Transactions on Power Delivery, Vol. 11, No. 1, pages 116-121, 1996).

[0011] A conventional Sagnac interferometer, called a loop interferometer, uses a closed optical path, with the same optical beam splitter component separating and recombining the two waves. The two separated waves travel along the closed optical path in opposite directions. In a loop fiber optic interferometer, the two separated waves use the same polarization state along the closed optical path. In a loop fiber optic gyroscope, the two waves have the same linear polarization. In a loop current sensor, the two waves have the same circular polarization in the optical fiber loop. In an in-line fiber optic interferometer, a mirror is positioned at one end of the optical fiber coil, and the closed optical path is traversed by the two waves in the same direction and in orthogonal polarization states that reverse on the return (see the publication GM Muller et al.).“Inherent temperature compensation of fiber-optic current sensors employing spun highly birefringent fiber", Optics Express, Vol. 24, No 10, 2016).

[0012] Compared to a loop system, such an inline fiber optic interferometric system is insensitive to variations in many environmental parameters but works equivalently with regard to phase modulation.

[0013] Phase modulation techniques, well known to those skilled in the art in fiber optic interferometers, are used to improve the sensitivity and linearity of the interferometer's response to a phase difference due to a non-reciprocal effect, such as the Sagnac effect or the collinear magneto-optical Faraday effect. In this document, the quantity to be measured is defined as a phase difference induced by a non-reciprocal effect in a loop or line fiber optic interferometer.

[0014] There figure 2 represents a phase modulator 16 in a prior art loop fiber optic interferometer.

[0015] In the above-mentioned domain, it is known to apply a modulated electrical voltage Vm(t) between the electrodes of the phase modulator 16 to modulate the phase difference ΔΦm(t) of the measured interferometric signal. This modulation introduces a bias that increases the sensitivity of the interferometric system, particularly for low-amplitude rotation measurements. More precisely, the phase modulator 16 generates a phase shift Φm(t) that is reciprocal, meaning it is perfectly identical in both directions of propagation. However, there is a propagation time difference, denoted Δτ, between the longest optical path, which passes through the optical fiber reel 17, and the shortest optical path, which exits directly at the splitter-combiner 15. This propagation time difference Δτ is related to the group velocity vg of the waves and not to their phase velocity vΦ.This results in a modulation of the phase difference ΔΦm(t) according to the following equation. ΔΦ m t = Φ m t − Φ m t − Δτ

[0016] This modulation of the phase shift Φm(t) is obtained by applying a modulated electrical voltage Vm(t)60 to the electrodes of the phase modulator 16.

[0017] It is also possible to have a second modulator placed at the other end of the coil and to connect it electrically in reverse to double the modulation efficiency in a so-called push-pull configuration. Circuit 14 of the figure 1 uses such a push-pull setup for modulator 16 which is placed on the two branches of the Y junction.

[0018] In particular, it is known to apply a so-called 2-state modulation, by modulating the modulation voltage Vm in a square wave between two step values, so as to produce a modulation of the phase difference on two levels, for example of ΔΦb(t) = ±π / 2, called the bias phase difference, at the natural frequency fp of the optical fiber coil. The natural frequency fp is defined such that T / 2 = 1 / (2fp) = Δτ where T represents the period of the square wave modulation. Thus, the half-period of the modulation T / 2 corresponds to the difference in group delay Δτ between the long optical path passing through the coil and the short optical path connecting the phase modulator 16 to the splitter 15. The detection system acquires the power of the interferometric beam at the output of the interferometer according to the two modulation states.The signal processing system digitizes the detected interferometric beam and demodulates the detected signal at fp by sampling two power measurements during each modulation period, assigning a negative sign to the first level and a positive sign to the next. This modulation-demodulation scheme, based on a square wave modulation voltage generating two states at the frequency fp, results in improved sensitivity of the interferometric system and greater stability of the measurements around zero, independent of variations in output power. It should be noted that peaks are observed between successive measurements of the detected signal.

[0019] It is also known to apply a 2-state square modulation with a phase difference modulation ΔΦm(t) greater than ±π / 2, such as ±3π / 4 or ±7π / 8. This overmodulation decreases the sensitivity but improves the signal-to-noise ratio of the interferometric system.

[0020] To extend and linearize the response dynamics of an interferometric system, it is also known to apply a feedback signal. The demodulated signal is used as an error signal in a feedback loop to generate an additional phase difference ΔΦFB that opposes the phase difference ΔΦS of the quantity to be measured. The total phase difference ΔΦFS + ΔΦS is set to zero, and -ΔΦFB, which is equal to ΔΦS, becomes the measurement. This allows for a linear response and good stability, independent of variations in power or gain of the detection system.

[0021] In the above-mentioned field, patent FR2654827_A1 proposes applying a so-called 4-state modulation voltage that generates 4 successive levels of ΔΦm(t) over each modulation period T equal to 2Δτ. figure 3 illustrates an example of 4-state modulation. On the figure 3The following are represented respectively: in the lower left, the modulation of the phase difference ΔΦ as a function of time t; in the upper left, the power P of the interferometric beam as a function of the phase difference ΔΦ; and, in the upper right, the power P of the interferometric beam as a function of time. Over a modulation period of 2Δτ, the four states i=1, 2, 3, 4 correspond to the four successive levels of ΔΦ m(t) respectively: i=1 for π - alpha; 2 for π + alpha; 3 for -π + alpha; 4 for -π - alpha. In the example illustrated on the figure 3 For alpha = π / 4, the 4 levels of ΔΦm(t) are as follows: 3π / 4, 5π / 4, -3π / 4, -5π / 4. This modulation can be decomposed into a superposition of a first modulation of ±π at the natural frequency fp (represented by a dashed line on the figure 3 bottom left) and a second modulation of ±π / 4 in quadrature (represented by dashes on the figure 3bottom left). The modulation resulting from the superposition of the first modulation of ±π and the second modulation of ±π / 4 is represented by a solid line on the figure 3 bottom left. This modulation ΔΦm(t) has four levels per modulation period. In practice, this can be achieved when the phase modulator generates a phase shift modulation Φm(t) of ±π / 2 at the natural frequency fp and ±π / 8 in quadrature. A digital feedback phase ramp can be added. The steps of duration Δτ of this digital ramp are equal to ΔΦFS and compensate for the signal phase difference ΔΦS. The four states corresponding to the four modulation levels are represented by points on the curve P as a function of the phase difference ΔΦ. On the figure 3In the upper right, the detected power P(t) is represented as a function of time. Four power measurements are sampled over each modulation period. In other words, Pi denotes the power measurement detected by the detector receiving the interferometric beam corresponding to the four states i = 1, ..., 4 over one modulation period. To extract the signal of the quantity to be measured, the signal processing system demodulates the detected power signal into four states by multiplying by +1 the two states corresponding to +alpha and by -1 the two states corresponding to -alpha, regardless of the sign of the ±π of these states. The signal of the quantity to be measured is also demodulated here at the natural frequency fp. In 4-state modulation, the signal of the quantity to be measured, for example the Sagnac signal, exhibits a square wave modulation at the natural frequency fp and is in phase with the ±alpha modulation and in quadrature phase with the ±π modulation.In 4-state modulation, the signal phase difference ΔΦS of the quantity to be measured is calculated from the signal SS according to the following expression, where the power measurements P i for states i=1, ..., 4 are acquired over a modulation period T equal to 2Δτ. S S = − P 1 + P 2 + P 3 − P 4

[0022] In 4-state modulation, it is also possible to extract a signal, called Vπ, modulated at 2fp. The signal Vπ represents the transfer function of the phase modulator, that is, the ratio between the voltage applied Vm to the modulator and the induced phase shift Φm, with Vπ / π = Vm / Φm. However, this signal Vn fluctuates with the environment, for example, with temperature. The demodulation of the signal Vπ is obtained by multiplying the power measurements Pi sampled at each state by the sign of the product of the signs of the modulations ±π and ±α. ​​In other words, the + sign is applied to measurements corresponding to the states +π +α and -π -α, and the - sign is applied to measurements corresponding to the states +π -α and -π +α. The 4-state modulation allows the phase difference to be locked to zero simultaneously and the Vπ signal to be locked.In 4-state modulation, the error signal of the modulator's transfer function, therefore of Vπ, is calculated according to the following expression. S V π = − P 1 + P 2 − P 3 + P 4

[0023] In the field of optical fiber interferometric systems, patent EP2005113_B1 describes a so-called 6-state modulation, based on 4 levels of phase difference for biasing. figure 4 illustrates an example of 6-state modulation modulated at 3fp. On the figure 4The following are represented respectively: in the lower left, the modulation of the phase difference ΔΦ as a function of time t; in the upper left, the power P of the interferometric beam as a function of the phase difference ΔΦ; and, in the upper right, the power P of the interferometric beam as a function of time t over a modulation period equal to 2Δτ. This 6-state modulation can be decomposed into a superposition of a first modulation of the phase shift Φm(t) of ±π / 2 at the natural frequency fp and a second modulation of the phase shift Φm(t) of ±α / 2 at 3fp. The second modulation is synchronized with the first modulation. In other words, we obtain a modulated phase difference ΔΦm(t) of ±π at the natural frequency fp (represented by a dashed line on the graph). figure 4 bottom left) and from ±alpha to 3f p (represented by dashes on the figure 4bottom left). The modulation of the phase difference resulting from the superposition of the modulation of ±π and the modulation of ±alpha is represented by a solid line on the figure 4 bottom left. This modulation ΔΦm(t) has four levels per modulation period. More generally, a modulated phase difference ΔΦm(t) of ±π is generated at the natural frequency fp and of ±α at (2k+1)fp, where k is a natural number greater than or equal to 1. The four modulation levels are represented by points on the curve P as a function of the phase difference ΔΦ. On the figure 4In the upper right corner, the detected power P(t) is represented as a function of time. Six power measurements are sampled during each modulation period. With 6-state modulation, the signal of the parameter to be measured, for example the Sagnac signal, is extracted at 3fp by applying a demodulation that multiplies by +1 the power measurements corresponding to the +alpha states and by -1 the other power measurements corresponding to the -alpha states, regardless of the sign of ±π in these states. In 6-state modulation, the states are numbered: i=1 for π - alpha; i=2 for π + alpha; i=3 for π - alpha; i=4 for -π + alpha; i=5 for -π - alpha; and i=6 for -π + alpha. The signal of the quantity to be measured SS is calculated according to the following expression, where the power measurements P i for the states i = 1, ..., 6 are acquired over a modulation period T equal to 2Δτ. S S = − P 1 + P 2 − P 3 + P 4 − P 5 + P 6

[0024] In 6-state modulation, it is also possible to extract a Vπ signal. The demodulation of the Vπ signal is obtained by multiplying the successively sampled power measurements by 0, +1, and -1. Indeed, as in 4-state modulation, the demodulation of the Vπ signal is obtained by multiplying the power measurements Pi, sampled at each state, by the sign of the product of the signs of the ±π and ±alpha modulations, but while maintaining only the same number of states multiplied by the + sign and states multiplied by the - sign. In 6-state modulation, the error signal of the modulator's transfer function is calculated using the following expression. S V π = P 2 − P 3 + P 5 − P 6

[0025] Patent EP2005113_B1 also describes the use of an 8-state, 8-level modulation over a total period T equal to 4Δτ. According to this conventional 8-state modulation, the modulation is performed first on 4 high states corresponding to ± (alpha + beta) and then on 4 other low states corresponding to ± (alpha - beta). The figure 5 The graph schematically represents, in the upper right, the power P(t) detected at the output of the interferometric system as a function of the modulation phase difference ΔΦm(t) (curve in the lower left). This modulation ΔΦm(t) exhibits 8 levels over a modulation period T equal to 4Δτ. The 8 modulation states are numbered from 1 to 8 on the power measurement curve P(t), in the upper right, according to their order of appearance over a modulation period. On the figure 5In the lower left, a 4-state modulation of ±π and ±(alpha-beta) at the natural frequency fp is shown (dashed line), along with an additional modulation (dashed line) of -2beta, -2beta, +2beta, +2beta, zero, zero, zero, and zero over the period 4Δτ; and the modulation of the total phase difference resulting from the superposition of these two modulations is shown in solid line. This total modulation therefore corresponds to the succession of a 4-state modulation of ±π and ±(alpha+beta) on 4 high states for one half-period 2Δτ and a 4-state modulation on 4 low states of ±π and ±(alpha-beta) for the following half-period.

[0026] On the upper left part of this figure 5 The different modulation levels are indicated. a +< corresponds to the modulation level ΔΦ a+ = π + alpha + beta a -< corresponds to the modulation level ΔΦ a- = π + alpha - beta b +< corresponds to the modulation level ΔΦ b+ = π - alpha + beta b -< corresponds to the modulation level ΔΦ b- = π - alpha - beta c +< corresponds to the modulation level ΔΦ c+ = -π + alpha +beta c corresponds to the modulation level ΔΦ c- = -π + alpha - beta d +< corresponds to the modulation level ΔΦ d+ = -π - alpha + beta d -< corresponds to the modulation level ΔΦ d- = -π - alpha - beta.

[0027] The output power P is sampled into 8 measurements Pi corresponding to the 8 states i = 1, ..., 8 per modulation period. The modulation levels corresponding to these states are: d- for state 1; b- for state 2; a+ for state 3; c+ for state 4; d+ for state 5; b+ for state 6; a- for state 7; c- for state 8. The demodulation of the signal of the parameter to be measured (Sagnac, for example) and that of the signal Vπ are performed on the 8 states in a manner analogous to that of the 4-state modulation. In this 8-state modulation, the signal of the quantity to be measured is calculated according to the following expression, where the power measurements Pi for the eight successive states i = 1, ..., 8 are acquired over a modulation period T equal to 4Δτ. S S = − P 1 − P 2 + P 3 + P 4 − P 5 − P 6 + P 7 + P 8

[0028] In this 8-state modulation, the error signal of the modulator's transfer function is calculated according to the following expression. S V π = P 1 − P 2 + P 3 − P 4 + P 5 − P 6 + P 7 − P 8

[0029] In a fiber optic interferometric system such as the one described above, it is desirable to adjust the output power of the detector. To this end, in the 8-state modulation scheme described above, it is known to extract the transfer function of the interferometer's detection system, also called the open-loop response, and denoted as the signal ΔP, in order to control this open-loop response, for example, by adjusting the power of the light source. This measurement is performed by detecting the power difference ΔP between the four high states (d ≤ ; b ≤ ; a ≤ ; c ≤ ) and the four low states (d ≤ ; b ≤ ; a ≤ ; c ≤ ) using the following formula. S ΔP = P 1 + P 2 + P 3 + P 4 − P 5 − P 6 − P 7 − P 8

[0030] It is desirable to improve the performance of a loop or line fiber optic interferometric system and in particular to increase the accuracy of measurements, stability, linearity and / or response dynamics of such a system. Description of the invention

[0031] In order to overcome the aforementioned drawbacks of the prior art, the present invention proposes a loop or line optical fiber interferometer comprising a light source adapted to generate a source beam, an optical separation device adapted to separate the source beam into a first single-mode wave and a second single-mode wave, an electronic system adapted to apply a modulating electrical voltage Vm(t) to a phase modulator capable of inducing the same phase shift Φm(t) on the first single-mode wave and the second single-mode wave, an assembly of optical fibers adapted to receive and propagate the first single-mode wave along a first optical path and, respectively, the second single-mode wave along a second optical path inverse to the first optical path, and to form, after a propagation time difference Δτ, a first output wave and, respectively, a second output wave.having a modulated phase difference ΔΦm(t) = Φm(t) - Φm(t-Δτ), the optical fiber assembly having a natural frequency fp equal to the inverse of twice the propagation time difference Δτ, the optical separation device being adapted to recombine the first output wave and the second output wave to form a time-modulated interferometric beam, and a detection system adapted to detect a power P(t) of the interferometric beam as a function of time.

[0032] More particularly, the invention proposes an interferometer in which the modulated phase difference ΔΦ m (t) is equal to the sum of a first periodic phase difference ΔΦ π (t) of level equal to ± π, a second periodic phase difference ΔΦ alpha (t) of level equal to ±alpha and a third periodic phase difference ΔΦ beta (t) of variable level between -beta and +beta, alpha and beta having different predetermined values, so that the modulated phase difference ΔΦ m (t) has a modulation period T equal to an odd multiple (2M+1) of twice the propagation time difference Δτ, where M is a natural number, the modulated phase difference ΔΦ m (t) having per modulation period T at least eight modulation levels among the following twelve modulation levels: ΔΦ a+ = π + alpha + beta; ΔΦ a- = π + alpha - beta; ΔΦ a = π + alpha; ΔΦ b+ = π - alpha + beta; ΔΦ b- = π - alpha - beta; ΔΦ b = π - alpha;ΔΦ c+ = -π + alpha +beta ; ΔΦ c- = -π + alpha - beta ; ΔΦ c = -π + alpha ; ΔΦ d+ = -π - alpha + beta ; ΔΦ d- = -π - alpha - beta ; ΔΦ d = -π - alpha and this modulated phase difference being such that ΔΦ m (t + T / 2) = - ΔΦ m (t) at each instant t between 0 and T.;

[0033] According to a particular and advantageous embodiment, the modulation period T is equal to twice the propagation time difference Δτ, the first phase difference ΔΦ π (t) has a modulation frequency equal to the natural frequency fp and the second phase difference ΔΦ alpha (t) and the third phase difference ΔΦ beta (t) have the same modulation frequency equal to an odd multiple (2N+1) of the natural frequency fp, where N is a non-zero natural number, the second phase difference ΔΦ alpha (t) being synchronized with the first phase difference ΔΦ π (t), the third phase difference ΔΦ beta (t) being in quadrature phase with respect to the second phase difference ΔΦ alpha (t).

[0034] According to another particular and advantageous embodiment, the modulation period T is equal to twice the propagation time difference Δτ, the third phase difference ΔΦ beta (t) having a modulation frequency equal to the natural frequency fp and the first phase difference ΔΦ π (t) and the second phase difference ΔΦ alpha (t) have the same modulation frequency equal to an odd multiple (2N+1) of the natural frequency fp, where N is a non-zero natural number, the second phase difference being in quadrature phase with respect to the first phase difference, the third phase difference ΔΦ beta (t) being synchronized with the first phase difference or the second phase difference.

[0035] According to yet another particular and advantageous embodiment, M is a non-zero integer, the first phase difference ΔΦ π (t) and the second phase difference ΔΦ alpha (t) have the same modulation frequency equal to the natural frequency fp, the second phase difference being in quadrature phase with respect to the first phase difference and the third phase difference ΔΦ beta (t) having a modulation period equal to the modulation period T, this third phase difference being synchronized with the first phase difference or the second phase difference.

[0036] Other non-limiting and advantageous features of the interferometer according to the invention, taken individually or in all technically possible combinations, are as follows.

[0037] The detection system includes an electronic demodulation system adapted to extract a signal representative of a quantity to be measured, a transfer function signal from the phase modulator and / or a transfer function signal from the detection system from a series of at least 12 power measurements of the detected interferometric beam per modulation period.

[0038] The signal representing the quantity to be measured is equal to a sum of the power measurements of the interferometric beam acquired per modulation period, each power measurement being multiplied by -1 for levels corresponding to - alpha and by +1 for levels corresponding to + alpha.

[0039] The phase modulator transfer function signal is equal to a sum of the interferometric beam power measurements acquired per modulation period, each power measurement being multiplied by the sign of the product of the sign of the first modulation at ±π and the sign + or - of the second modulation at ± alpha, or by zero so as to maintain the same number of states multiplied by the sign + and states multiplied by the sign -.

[0040] The transfer function signal of the detection system is equal to a sum of the power measurements of the interferometric beam acquired per modulation period, each power measurement being multiplied by the sign of the product of the sign of the second modulation at ± alpha and the sign of the third modulation at ± beta when the level of this last modulation is +beta or -beta, and by 0 when the level of this third beta modulation is zero.

[0041] The modulated phase difference ΔΦ m (t) further comprises a ramp composed of phase steps ΔΦ FS opposed to a phase difference ΔΦ S of the signal representative of the quantity to be measured.

[0042] The optical separation device is adapted to spatially separate the source beam into the first single-mode wave and the second single-mode wave, and the optical fiber assembly includes an optical fiber reel adapted to receive the first single-mode wave at one end of the optical fiber reel and, respectively, the second single-mode wave at a second end of the optical fiber reel, the first single-mode wave and the second single-mode wave propagating in opposite directions in the optical fiber reel.

[0043] The first single-mode wave and the second single-mode wave are linearly polarized and the optical fiber coil is linearly polarization maintained, the interferometer being adapted to measure a phase difference representative of a rotation around an axis of the optical fiber coil.

[0044] The optical fiber assembly comprises a linear polarization-maintaining optical fiber section, a circular polarization-maintaining optical fiber reel, and another linear polarization-maintaining optical fiber section, with a quarter-wave plate disposed between the optical fiber section and one end of the optical fiber reel, and another quarter-wave plate disposed between the other optical fiber section and the other end of the optical fiber reel, the interferometer being adapted to measure a phase difference induced by an electric current passing through the optical fiber reel.

[0045] The optical fiber assembly comprises a linear polarization-maintaining optical fiber section and a circular polarization-maintaining optical fiber reel, the optical fiber section being connected to one end of the optical fiber reel, a mirror being disposed at a second end of the optical fiber reel, the interferometer being adapted to measure a phase difference induced by an electric current passing through the optical fiber reel.

[0046] The interferometer includes a feedback system adapted to control the measurement of the signal representing the quantity to be measured, the transfer function signal of the phase modulator and / or the transfer function signal of the detection system.

[0047] Of course, the different features, variants and embodiments of the invention can be combined with each other in various ways as long as they are not incompatible or mutually exclusive.

[0048] The fiber optic interferometer described in this disclosure improves the accuracy of measurements of the quantity being measured, such as the Sagnac phase shift, by eliminating errors induced by the RC time constant of the phase modulator's control circuit, which notably affects measurements obtained with conventional 8-state modulation. The fiber optic interferometer described in this disclosure also allows for the measurement of the interferometer's transfer function by measuring the power difference ΔP between high and low states. Brief description of the drawings

[0049] Furthermore, various other features of the invention become apparent from the attached description made with reference to the drawings which illustrate non-limiting embodiments of the invention and where: [ Fig. 1 ] schematically represents a Sagnac interferometric system with a looped optical fiber for application to a fiber optic gyroscope according to the prior art; [ Fig. 2 ] represents a phase modulator in a looped fiber optic interferometric system, for generating a modulated phase difference ΔΦm(t) for signal biasing according to the prior art; [ Fig. 3 ] schematically represents an example of a modulated phase difference ΔΦm(t) applied to a phase modulator, following a prior art 4-state modulation, the position of the 4 modulation states on the interferometer response curve and the 4 power measurements P(t) detected as a function of time here over three modulation periods; Fig. 4 ] schematically represents an example of a modulated phase difference ΔΦm(t) applied to a phase modulator, following a prior art 6-state modulation, the position of the 6 modulation states on the interferometer response curve, and the 6 power measurements P(t) detected as a function of time over one modulation period; Fig. 5 ] schematically represents an example of a modulated phase difference ΔΦm(t) applied to a phase modulator, following a prior art 8-state modulation, the position of the 8 modulation states on the interferometer response curve, and the 8 power measurements P(t) detected as a function of time; Fig. 6] schematically represents the modulated phase difference applied in the presence of an RC time constant in the phase modulation chain, in a conventional 8-state modulation, the effect of the RC time constant on the power P(t) of the interferometric system and the residual spurious signal in the absence of rotation of a Sagnac interferometric system obtained by classical demodulation of this 8-state modulation; Fig. 7 ] illustrates a first embodiment based on an 8-level modulation of ΔΦm(t) and 12 states of the corresponding detected power P(t); [ Fig. 8 ] illustrates a second embodiment based on an 8-level modulation of ΔΦm(t) and 12 states of the corresponding detected power P(t); [ Fig. 9 ] illustrates a third embodiment based on an 8-level modulation of ΔΦm(t) and 12 states of the corresponding detected power P(t); [ Fig. 10] schematically represents an example of modulation according to the third embodiment and the power P(t) detected in the presence of a Sagnac signal; [ Fig. 11 ] schematically represents a looped fiber optic interferometric system for application to an electric current sensor according to this disclosure; [ Fig. 12 ] schematically represents an online fiber optic interferometric system for application to an electric current sensor according to this disclosure; [ Fig. 13 ] schematically represents another online fiber optic interferometric system for application to an electric current sensor according to this disclosure.

[0050] 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

[0051] In a phase-modulated interferometric system such as described in connection with the figure 1The phase modulator 16 is powered by a control circuit whose RC response time, also called the time constant, is related to the load resistance R between the electrodes of this phase modulator and the electrical capacitance C of these electrodes. The resistance R is on the order of 50 to 500 ohms. The electrical capacitance of a 10 mm long electrode of an integrated optical circuit modulator (for example, lithium niobate) is on the order of 3 pF, corresponding to a capacitance C of approximately 12 pF for a pair of 20 mm long push-pull electrodes. In this case, the RC time constant of the phase modulator can be estimated to be approximately 1 to 10 ns. Generally, the RC time constant of the control circuit of a phase modulator is between 0.5 and 50 ns.It can be noted that with certain electrical setups, there is no load resistance between the electrodes and this time constant is then given by the gain-bandwidth product of the amplifier in the control circuit.

[0052] This disclosure shows that this RC time constant can influence the performance of an interferometric system and proposes different modulation and demodulation schemes to reduce or even cancel the negative effects induced by the RC time constant of the phase modulator.

[0053] There figure 6 This illustrates the case of a classic 8-state modulation, highlighting the effect of the RC electrical response time of the phase modulator. To graphically demonstrate the effect of the RC electrical response time on the figure 6 We chose a high value for RC: RC = Δτ / 12. The figure 6schematically represents the power P(t) detected at the output of the interferometric system as a function of, on the one hand, the phase difference ΔΦ (curve in the upper left) and, on the other hand, the time t (curve in the upper right). figure 6 It also represents the modulated phase difference ΔΦm(t) as a function of time (curve at the bottom left). On this curve of the modulated phase difference ΔΦm(t) as a function of time, we observe that this phase difference does not follow the ideal square shape but follows an exponential curve that reaches each phase difference level with a delay related to the time constant RC.

[0054] Finally, the figure 6represents, at the bottom right, a time curve of power difference between states 1 and 3, and 5 and 7 as well as 2 and 4 and 6 and 8, as they are demodulated to measure the signal phase difference ΔΦ S with conventional demodulation (see equation Math 7) in an 8-state interferometer in the absence of rotation of this Sagnac interferometer.

[0055] Indeed, when the digital processor 10 and the digital-to-analog converter 11 generate a square-modulated control signal Cm(t) switching between two levels, the electrical response time RC means that the control voltage Vm(t) actually applied to the modulator and the modulated phase difference ΔΦm(t) it generates do not instantaneously reach the desired level. More precisely, each modulated phase difference level ΔΦm(t) follows an exponential curve in (1 - exp(-t / RC)) that starts from the previous level and tends asymptotically towards the desired value for that level. On the power P-time curve, this results in two states of the measured interferometric signal theoretically corresponding to the same power level but starting from different previous levels not actually being identical because they do not have the same history.For 8-state modulation, the classical demodulation of the signal relative to the measured quantity is based on differences between the powers measured for the pairs of states 1 and 3, 5 and 7, 2 and 4, 6 and 8 (see equation Math 7). In particular, state 8, which precedes state 1 (modulo T), is of a lower level than state 2, which precedes state 3, so states 1 and 3 are not perfectly identical, not having the same history, and their difference, which is calculated in the demodulation, is not perfectly zero in the absence of rotation, i.e., when ΔΦ S = 0. Similarly, state 4, preceding state 5, is of a higher level than state 6, preceding state 7, so states 5 and 7 are also not identical, not having the same history.However, it is fundamental that the demodulation of the signal of the quantity to be measured gives zero and does not generate any defect when the parameter to be measured in the interferometer is zero, in particular no defect in a Sagnac interferometer in the absence of rotation.

[0056] On the curve at the bottom right of the figure 6 It is clearly observed that the power difference calculated in the demodulation of the quantity to be measured is not zero, particularly for the difference between states 1 and 3, as well as 5 and 7. Such a prior art interferometric system therefore generates defects. The order of magnitude of these defects can correspond to a parasitic phase difference on the order of 10⁻⁴ to 10⁻⁵ radians, whereas a zero stability on the order of 10⁻⁸ to 10⁻⁹ radians is sought.

[0057] This disclosure proposes various modulation and demodulation techniques suitable for mitigating or even eliminating defects induced by the RC time constant of the phase modulator control circuit in an interferometric system generating at least 8 levels per modulation period and 12 states per demodulation period.

[0058] There figure 7 illustrates a first embodiment based on a modulation with 8 levels of modulation and 12 states.

[0059] In the remainder of this document, the term "level" (or "modulation level") refers to the asymptotic value of the different modulated phase difference values ​​ΔΦm for each modulation step. The term "modulation states" refers to the different measured power values ​​P corresponding to the modulation levels that occur sequentially during each modulation period. Several states may use the same modulation level within a modulation period.

[0060] In the first embodiment, in connection with the figure 7A modulation voltage is applied at 8 levels over a modulation period T equal to 2Δτ. More precisely, a control signal Cm(t) is applied, consisting of the sum of three square wave modulations. The first square wave modulation is matched to induce a first phase difference ΔΦπ(t) equal to ±π. This first phase difference ΔΦπ(t) is periodic at the natural frequency fp. The second square wave modulation is matched to induce a second phase difference ΔΦα(t) equal to ±α. This second phase difference ΔΦα(t) is periodic and has a modulation frequency equal to an odd multiple (2N+1) of the natural frequency fp, where N is a natural number greater than or equal to 1. The second phase difference ΔΦα(t) is synchronized with the first phase difference ΔΦπ(t). The third square modulation is suitable to induce a third phase difference ΔΦ beta (t) equal to ± beta.The third phase difference ΔΦ beta(t) is periodic and has a modulation frequency equal to the same odd multiple (2N+1) of the natural frequency fp. The third phase difference ΔΦ beta(t) is in quadrature phase with respect to the second phase difference ΔΦ alpha(t), in other words, delayed by T / 12 with respect to the second phase difference ΔΦ alpha(t) in the case of the figure where 2N+1=3. In the general case, it is a delay of T / (4(2N+1)). The modulated phase difference ΔΦ m(t) resulting from this modulation is equal to the sum of the first periodic phase difference ΔΦ π(t), the second phase difference ΔΦ alpha(t), and the third phase difference ΔΦ beta(t) according to the following equation. ΔΦ m t = ΔΦ π t + ΔΦ alpha t + ΔΦ beta t

[0061] In the example shown on the figure 7The number N is equal to 1, the frequency of the second phase difference ΔΦ alpha (t) and the third phase difference ΔΦ beta (t) is equal to 3f p, and the following values ​​were chosen for alpha and beta: alpha = 3π / 8 and beta = 3π / 128. The period T is 2Δτ, knowing that Δτ is on the order of 5 µs per kilometer. The constant RC has been exaggerated compared to reality to make the figure 7 more readable. RC here is approximately 1 / 20 of Δτ.

[0062] On the curve ΔΦ m (t) of the figure 7The modulation, shown in dashed lines, represents the 6-state / 4-level modulation resulting from the sum of the first phase difference ΔΦπ(t) modulated at fp and the second phase difference ΔΦα(t) modulated at 3fp. The third phase difference ΔΦβ(t), modulated at 2 levels at 3fp in quadrature phase relative to the second phase difference ΔΦα(t), is shown in dashed lines. Finally, the modulated phase difference ΔΦm(t), or total modulation resulting from the sum of the dashed and dashed modulations, is shown in solid lines. The modulated phase difference ΔΦm(t) has 8 levels per period T=2Δτ. However, this 8-level modulation differs from the 8-level modulation of the prior art (illustrated, for example, in the...). figures 5 and 6). We observe the effect of the time constant RC on the modulated phase difference curve ΔΦ m (t). Each modulated phase difference level follows an exponential curve in (1 - exp(-t / RC)) which starts from the previous level.

[0063] At each period T, this modulation of the phase difference ΔΦm(t) generates the following 8 modulation levels ΔΦm(t) = ±π ±alpha ±beta. These eight modulation levels correspond to the points marked a +<, a -<, b +<, b -<, c +<, c -<, d +< and d -< on the power curve as a function of the phase difference.

[0064] On the figure 7 ΔΦ a + = π + alpha + beta a -< corresponds to the modulation level ΔΦ a − = π + alpha − beta b +< corresponds to the modulation level ΔΦ b + = π − alpha + beta b -< corresponds to the modulation level ΔΦ b − = π − alpha − beta c +< corresponds to the modulation level ΔΦ c + = − π + alpha + beta c -< corresponds to the modulation level ΔΦ c − = − π + alpha − beta d +< corresponds to the modulation level ΔΦ d + = − π − alpha + beta d -< corresponds to the modulation level ΔΦ d − = − π − alpha − beta .

[0065] On the output power curve P as a function of time, the 8 levels of modulated phase difference ΔΦ m (t) follow one another in a sequence of 12 states per modulation period T in the following order: b -< b +< a +< a -< b -< b +< c +< c -< d -< d +< c +< c -< .

[0066] The detector receiving the interferometric beam acquires 12 power measurements Pi per modulation period T, corresponding to the twelve states i = 1, ..., 12. In other words, the detector samples the power signal P at the frequency 12fp. More generally, for a modulation of the second phase difference ΔΦα(t) and the third phase difference ΔΦβ(t) at (2N+1)fp, the sampling frequency is 4(2N+1)fp. On the power-time curve, the effect of the RC time constant of the phase modulator control circuit on the detected power measurements is clearly visible. The value of each power measurement Pi reaches a plateau following an exponential curve that depends on the difference between the two successive asymptotic power values.

[0067] Depending on the signal being sought, a specific demodulation is applied. More precisely, to extract the signal of the quantity to be measured, for example the Sagnac signal, a demodulation of the 12 acquired states is used. Over the modulation period T equal to 2Δτ, the signs are applied in the following order to the 12 power measurements Pi: - - + + - - + + - - + +. In other words, the power measurement Pi is multiplied by -1 for the levels corresponding to -alpha and by +1 for the levels corresponding to +alpha, regardless of the sign of the modulations at ±π and ±beta. Thus, the demodulation of the signal of the quantity to be measured, modulated over 12 states, is expressed as follows in the first embodiment. S S = − P 1 − P 2 + P 3 + P 4 − P 5 − P 6 + P 7 + P 8 − P 9 − P 10 + P 11 + P 12

[0068] We observe, on the power-time curve, that the 8-level, 12-state modulation exhibits, for each state demodulated with the + sign, an identical state, that is to say, a state with the same history, demodulated with the - sign. Thus, state 1 corresponding to level b -< is identical to state 7 corresponding to level c +<; state 2 corresponding to level b +< is identical to state 8 corresponding to level c -<; state 3 corresponding to level a +< is identical to state 9 corresponding to level d -<; state 4 corresponding to level a -< is identical to state 10 corresponding to level d +<; state 5 corresponding to level b -< is identical to state 11 corresponding to level c +<; state 6 corresponding to level b +< is identical to state 12 corresponding to level c -<.In other words, to each state of the first half-period T / 2, demodulated to extract the quantity to be measured, in + or -, corresponds a state with the same history, demodulated with the opposite sign, in the second half-period. We observe that the modulated phase difference ΔΦ m (t) according to the 8-level and 12-state modulation of the first embodiment satisfies the following equation at any instant t of a modulation period T, here between 0 and 2Δτ. ΔΦ m t + T 2 = − ΔΦ m t

[0069] It follows that the demodulation of a signal modulated according to 8 levels and 12 states as described above, to extract the signal of the quantity to be measured, for example the Sagnac signal, does not present a defect induced by the RC time constant, unlike a demodulation of a signal modulated according to a conventional modulation of 8 levels and 8 states.

[0070] In the first embodiment, the demodulation of the phase modulator's transfer function, denoted Vπ, is performed by multiplying the power measurements Pi for i = 1, ..., 12 acquired over a modulation period T, by the sign of the product of the + or - sign of the ±π modulation and the + or - sign of the ±α modulation, regardless of the sign of the ±β modulation, or by zero so as to retain only as many states multiplied by + as states multiplied by -. The demodulation of the phase modulator's transfer function, denoted Vπ, is expressed as follows in the first embodiment. S V π = + P 3 + P 4 − P 5 − P 6 + P 9 + P 10 − P 11 − P 12

[0071] In the first embodiment, the demodulation of the transfer function of the detection system, or open-loop response, denoted ΔP, is performed at the frequency 6fp by multiplying the 12 power measurements Pi for i = 1, ..., 12 acquired over a modulation period T, by the sign of the product of the + or - sign of the ±alpha modulation and the + or - sign of the ±beta modulation, independently of the sign of the ±π modulation. In the example illustrated on the figure 7To extract ΔP, we sample at 12f p. The high levels at a +< and c +< corresponding to a (+alpha+beta) modulation and the high levels at b -< and d -< corresponding to a (-alpha-beta) modulation are demodulated by multiplying by +1, while the low levels at a -< and c corresponding to a (+alpha-beta) modulation and the low levels at b +< and d +< corresponding to a (-alpha+beta) modulation are demodulated by multiplying by -1. More precisely, the demodulation of the transfer function of the detection system, therefore the power difference ΔP between high and low states, modulated over 12 states, is expressed as follows in the first embodiment. S ΔP = + P 1 − P 2 + P 3 − P 4 + P 5 − P 6 + P 7 − P 8 + P 9 − P 10 + P 11 − P 12

[0072] The first embodiment, based on modulation of ±π at fp, ±alpha at 3fp, and ±beta at 3fp, resulting in 8 modulation levels and 12 states per period T, allows for the extraction, through appropriate demodulation, of the signal of the quantity to be measured, the signal Vπ, and the open-loop response signal ΔP. The signal of the quantity to be measured is corrected for defects induced by the RC time constant of the phase modulator control circuit. This modulation and demodulation scheme improves the performance of an interferometric system without modifying its structure and enables the upgrading of existing interferometric systems.

[0073] We now describe a second embodiment related to the figure 8 . Similar to the first embodiment, the second embodiment is based on a modulation with at least 8 modulation levels and 12 states per modulation period T equal to 2Δτ.

[0074] The modulation is also applied at 8 levels per modulation period T. The control signal Cm(t) is also composed of the sum of three square wave modulations. In this second embodiment, the first square wave modulation induces a first phase difference ΔΦπ(t) with a level equal to ±π. The first phase difference is periodic and has a modulation frequency equal to an odd multiple (2N+1) of the natural frequency fp, where N is a natural number greater than or equal to 1. The second square wave modulation voltage is adapted to induce a second phase difference ΔΦalpha(t) with a level equal to ±α. The second phase difference ΔΦ alpha (t) is periodic and has a modulation frequency equal to the same odd multiple (2N+1) of the natural frequency fp , where N is a natural integer greater than or equal to 1. The second phase difference ΔΦ alpha (t) is in quadrature with respect to the first phase difference ΔΦ π (t).The third square wave modulation voltage is designed to induce a third phase difference ΔΦ beta (t) with a level equal to ± beta. The third phase difference ΔΦ beta (t) is periodic and has a modulation frequency equal to fp, synchronized with the first phase difference ΔΦ π (t).

[0075] In the example shown on the figure 8 , the first phase difference ΔΦ π (t) and the second phase difference ΔΦ alpha (t) are at the frequency 3f p and we choose the following values ​​for alpha and beta: alpha = 3π / 8 and beta = 3π / 128, and RC = Δτ / 20.

[0076] On the curve ΔΦ m (t) of the figure 8The modulation, shown in dashed lines, represents four states resulting from the sum of the first periodic phase difference ΔΦπ(t), modulated at 3fp, and the second phase difference ΔΦα(t), modulated at 3fp in quadrature with respect to ΔΦπ(t). The third phase difference ΔΦβ(t), modulated at two levels at frequency fp, is shown in dashed lines. Finally, the modulated phase difference ΔΦm(t), or total modulation, resulting from the sum of the dashed and dashed modulations, is shown in solid lines. The modulated phase difference ΔΦm(t) also exhibits eight levels. The effect of the time constant RC on the curve of the modulated phase difference ΔΦm(t) is observed. Each modulated phase difference level follows an exponential curve in (1 - exp(-t / RC)) which starts from the previous level.

[0077] As in the first embodiment, the modulated phase difference ΔΦ m (t) has 8 levels and this modulated phase difference ΔΦ m (t) on 8 levels generates 12 modulation states on the output power curve as a function of time, in the following order: b +< a +< c -< d -< b -< a -< c -< d -< b +< a +< c +< d +< corresponding to the phase differences ΔΦ a+ to ΔΦ d- indicated above.

[0078] The detector acquires 12 power measurements P i per modulation period T corresponding to the twelve states i = 1,..., 12. On the power-time curve, we can also clearly observe here the effect of the time constant RC of the phase modulator on the detected power measurements.

[0079] In the second embodiment, the demodulation of the signal of the quantity to be measured, for example the Sagnac signal, is analogous to that of the first embodiment insofar as the power measurement Pi for i = 1, 2, ..., 12 over one modulation period is multiplied by -1 for levels corresponding to -alpha and by +1 for levels corresponding to +alpha, regardless of the sign of the modulations at ±π and ±beta. This demodulation scheme is expressed as follows in the second embodiment. S S = − P 1 + P 2 + P 3 − P 4 − P 5 + P 6 + P 7 − P 8 − P 9 + P 10 + P 11 − P 12

[0080] Thus, each state of the first half-period T / 2, demodulated in a positive or negative direction, corresponds to a state with the same history in the second half-period, demodulated in the opposite direction, either negative or positive: the pairs 1-7, 2-8, 3-9, 4-10, 5-11, and 6-12 allow the suppression of the effects induced by the RC of the phase modulator. Indeed, in the second embodiment, we observe on the figure 8that the modulated phase difference ΔΦ m (t) following the 8 level and 12 state modulation satisfies the Math 12 equation at any instant t of the modulation period, here between 0 and 2Δτ.

[0081] Similarly, the demodulation of the phase modulator's transfer function, denoted Vπ demodulation, is carried out by multiplying the 12 power measurements Pi for i = 1, ..., 12 acquired over a modulation period T, by the sign of the product of the sign + or - of the modulation at ±π and the sign + or - of the modulation at ±alpha, independently of the sign of the modulation at ±beta.

[0082] Thus, the demodulation of the transfer function of the phase modulator, denoted demodulation of V π, over 12 states is expressed in the second embodiment by the following expression. S V π = − P 1 + P 2 − P 3 + P 4 − P 5 + P 6 − P 7 + P 8 − P 9 + P 10 − P 11 + P 12

[0083] Finally, the demodulation of the transfer function of the detection system, noted as ΔP demodulation, is carried out by multiplying the 12 power measurements P i for i = 1, ..., 12 acquired over a modulation period T, by the sign of the product of the sign + or - of the modulation at ± alpha and the sign + or - of the modulation at ± beta, independently of the sign of the modulation at ± π.

[0084] Thus, the demodulation of the transfer function of the detection system, denoted ΔP, over 12 states is expressed as follows in the second embodiment. S ΔP = − P 1 + P 2 − P 3 + P 4 + P 5 − P 6 − P 7 + P 8 − P 9 + P 10 + P 11 − P 12

[0085] In the second embodiment, the signal of the quantity to be measured is also corrected for the defects induced by the RC time constant of the phase modulator.

[0086] We now describe a third embodiment related to the figure 9The third embodiment is based on a 12-state modulation with 8 modulation levels per modulation period T.

[0087] The control signal Cm(t) also consists of the sum of three square modulations. In the third embodiment, the first square modulation induces a first phase difference ΔΦπ(t) with a level equal to ±π. The first phase difference is periodic and has a modulation frequency equal to the natural frequency fp. The second square modulation is adapted to induce a second phase difference ΔΦalpha(t) with a level equal to ±α. The second phase difference ΔΦalpha(t) is periodic and has a modulation frequency equal to the natural frequency fp, in quadrature with respect to the first phase difference ΔΦπ(t). In other words, the sum of the first phase difference and the second phase difference ΔΦalpha(t) produces a 4-state modulation (shown as a dashed line on the time-modulated phase difference curve). figure 9The third square wave modulation is suitable for inducing a phase shift Φbeta(t) of a level equal to ±β / 2. This modulation Φbeta(t) is periodic and has a modulation frequency equal to an odd subharmonic of the natural frequency: fp / (2N+1), where N is a natural number greater than or equal to 1. The modulation period is then equal to 2(2N+1)Δτ. The third modulation Φbeta(t) is synchronized with the first phase difference ΔΦπ(t) or the second phase difference ΔΦalpha(t). The third modulation Φbeta(t) of the phase shift induces a phase difference ΔΦbeta(t) = Φbeta(t) - Φbeta(t - Δτ) on 6 levels that switch all Δτs according to the following sequence: +beta, 0, 0, -beta, 0, 0 (represented by dashed lines on the time-modulated phase difference curve). figure 9 ).

[0088] In the example shown on the figure 9The modulation frequency of the phase shift Φ beta (t) is equal to fp / 3 and the modulation period T is equal to 6Δτ. In the example illustrated on the figure 9 We choose the following values ​​for alpha and beta: alpha = 3π / 8 and beta = 5π / 128. The value of RC is exaggerated compared to reality to make the figure 9 more readable and here is equal to Δτ / 10.

[0089] The modulated phase difference ΔΦm(t), or total modulation resulting from the sum of the dashed and dashed modulations, has a modulation period T equal to (2N+1)2Δτ. In the example of the figure 9 , the modulated phase difference ΔΦ m (t) induces 8 levels per modulation period T. These 8 modulation levels produce on the power measurement curve a sequence of 12 states, which switch all Δτ / 2 in the following order of appearance over a modulation period T: b +< a +< cdbac -< d -< bac d.

[0090] On the figure 9, a+ corresponds to the modulation level ΔΦ a+ = π + alpha + beta ; a corresponds to the modulation level ΔΦ a = π + alpha ; b +< corresponds to the modulation level ΔΦ b+ = π - alpha + beta ; b corresponds to the modulation level ΔΦ b = π - alpha ; c corresponds to the modulation level ΔΦ c = -π + alpha ; c -< corresponds to the modulation level ΔΦ c- = -π + alpha - beta ; d corresponds to the modulation level ΔΦ d = -π - alpha ; d -< corresponds to the modulation level ΔΦ d- = -π - alpha - beta.

[0091] On the output power curve P as a function of the phase difference ΔΦ of the figure 9 The modulated phase difference levels ΔΦm(t) generate 12 modulation states per modulation period. On the figure 9 , we observe that the modulated phase difference ΔΦ m (t) following the 8 level and 12 state modulation of the third embodiment satisfies the Math 12 equation at any instant t of the modulation period T, period here equal to 6Δτ.

[0092] As in the first and second embodiments, during the demodulation of the signal of the quantity to be measured, for example the Sagnac signal, each state of the first half-period T / 2, demodulated to extract the quantity to be measured, as + or -, corresponds to a state with the same history, demodulated with the opposite sign, in the second half-period. This demodulation scheme is represented by the following expression in the third embodiment. S S = − P 1 + P 2 + P 3 − P 4 − P 5 + P 6 + P 7 − P 8 − P 9 + P 10 + P 11 − P 12

[0093] The demodulation of the phase modulator transfer function, denoted Vπ, is carried out by multiplying the 12 power measurements Pi for i = 1, ..., 12 acquired over a modulation period T, by the sign of the product of the sign + or - of the modulation at ±π and the sign + or - of the modulation at ±alpha, independently of the sign of the modulation at ±beta.

[0094] Thus, the demodulation of the transfer function of the phase modulator V π modulated on 12 states is expressed for this third embodiment according to the same equation (Math 16) as for the second embodiment.

[0095] Finally, the demodulation of the detection system's transfer function, denoted ΔP, is performed by multiplying the 12 power measurements Pi for i = 1, ..., 12 acquired over the modulation period T = 6Δτ, by the sign of the product of the + or - sign of the modulation at ±alpha and the + or - sign of the modulation at ±beta when beta is non-zero, and by 0 when beta is zero, regardless of the sign of the modulation at ±π. In particular, beta is zero for states a, b, c, and d.

[0096] Thus, the demodulation of the transfer function of the detection system ΔP over 12 states is expressed as follows in the third embodiment. S ΔP = − P 1 + P 2 − P 7 + P 8

[0097] In the third embodiment as well, the signal of the quantity to be measured is corrected for the defects induced by the RC time constant of the phase modulator.

[0098] There Figure 10 illustrates an example of the third embodiment in the presence of a signal of the quantity to be measured. In the example shown on the Figure 10 We choose the following values ​​for alpha and beta: alpha = 3π / 8 and beta = 3π / 128. RC is equal to Δτ / 12. The phase difference induced by the quantity to be measured is here equal to ΔΦS = 3π / 32. The measurement of ΔΦS is corrected for the defects induced by the time constant RC of the phase modulator. Consequently, the measurement exhibits better stability.

[0099] Advantageously, 8-level, 12-state modulation according to any of the embodiments described above is used to control the signal of the parameter to be measured (for example, the Sagnac signal), the adjustment of the Vπ signal and / or the open-loop response (or ΔP signal).

[0100] In summary, the table below indicates the demodulation rules for the three embodiments described above. [Tables 1] Mod. π Mod. π and alpha Mod. π, alpha and beta Modulation levels Demodulation parameter to be measured Demodulation V π ΔP Demodulation +π +π+alpha +π+alpha+beta a +< + + + +π+alpha a + + 0 +π+alpha-beta a -< + + - +π-alpha +π-alpha+beta b +< - - - +π-alpha b - - 0 +π-alpha-beta b -< - - + -π -π+alpha -π+alpha+beta c +< + - + -π+alpha c + - 0 -π+alpha-beta c + - - -π-alpha -π-alpha+beta d +< - + - -π-alpha d - + 0 -π-alpha-beta d - + +

[0101] The invention applies to a Sagnac loop optical fiber interferometer for measuring a rotational speed around the axis of the optical fiber spool, for example as illustrated in the figure 1In the Sagnac loop fiber optic interferometer, the first single-mode wave 101 and the second single-mode wave 102 are linearly polarized, and the optical fiber coil 17 maintains linear polarization. The signal processing system 900 applies a modulation voltage 60 to the electrodes of the phase-shifting optical modulator 16 to generate a phase-difference modulation with at least 8 states and 12 levels, according to any of the embodiments described above. The signal processing system 900 applies a demodulation adapted to the detected signal 80, depending on the chosen modulation.

[0102] The invention also applies to a loop or line optical fiber interferometer for applications as a magnetic field sensor or as an electric current sensor.

[0103] By way of non-limiting example, the figure 11represents a looped fiber optic interferometer intended for use as an electric current sensor. The same reference symbols designate the same elements as on the figure 1 In this application, an optical fiber assembly comprises an optical fiber section 71, an optical fiber reel 73, and another optical fiber section 72 arranged in series. The optical fiber 73 is wound around an axis. The optical fiber 73 is preferably circularly polarized. The optical fiber section 71 is preferably linearly polarized. The other optical fiber section 72 is also preferably linearly polarized. An electrical conductor 120 is arranged along the axis of the optical fiber reel 73. I denotes an electric current flowing through the optical fiber reel 73. The integrated optical circuit 14 is analogous to that described in connection with the figure 1. At the output of the integrated optical circuit 14, the first single-mode wave 101 and the second single-mode wave 102 are linearly polarized according to the same polarization state. The first single-mode wave 101 propagates in the optical fiber section 71. The second single-mode wave 102 propagates in the other optical fiber section 72. A quarter-wave plate 32 receives the first linearly polarized single-mode wave 101 and transmits a first circularly polarized single-mode wave 111, for example right-hand circular, to one end of the optical fiber reel 73. Another quarter-wave plate 33 receives the second linearly polarized single-mode wave 102 and transmits a second circularly polarized single-mode wave 112, here for example also right-hand circular, to the other end of the optical fiber reel 73. The first right-hand circular single-mode wave 111 and the second right-hand circular single-mode wave 112 propagate in opposite directions in the optical fiber reel 73.At the output of the optical fiber reel 73, the quarter-wave plates 32, 33 transform the circularly polarized waves into linearly polarized waves, which recombine to form the interferometric beam 300. The signal processing system 900 applies any one of the modulation-demodulation schemes with at least 8 states and 12 levels to extract an electrical current measurement corrected for the RC time constant of the phase modulator. The propagation time difference Δτ to be considered for the phase modulation ΔΦm(t) is then the propagation time in the optical fiber 73 and in the fiber sections 71 and 72.

[0104] As another, non-exhaustive example, the figure 12This represents an in-line fiber optic interferometer intended for use as an electric current sensor. In this example, a polarizer 24 linearly polarizes the source beam 100. The integrated optical circuit 34 comprises only a waveguide formed, for example, by titanium diffusion in a lithium niobate substrate. The phase modulator electrodes 16 are deposited along the sides of the waveguide. The waveguide of the integrated optical circuit 34 is birefringent. The optical axes of the polarizer 24 are preferably oriented at 45 degrees to the birefringence axes of the waveguide of the integrated optical circuit 34 at the input / output 25 of the integrated optical circuit 34.In this way, the polarizer 24 and the integrated optical circuit 34 separate the source beam 100 into two polarizations and generate the first single-mode wave 101, polarized according to a linear polarization state, and the second single-mode wave 102, polarized according to an orthogonal linear polarization state. The waveguide of the integrated optical circuit 34 guides the two polarizations. Since the phase modulator 16 has a different efficiency depending on the polarization, it effectively generates a modulation differential of the phase shift between the two waves, and will allow the same phase modulations as in the loop configuration. This differential modulator is often referred to as a birefringence modulator. In this embodiment, the optical fiber assembly comprises an optical fiber section 74 and an optical fiber reel 73 arranged in series. The optical fiber 73 is wound around an axis. The optical fiber 73 is preferably circularly polarization-preserving.The optical fiber section 74 is preferably linear polarization maintaining. The first single-mode wave 101 and the second single-mode wave 102 propagate in the optical fiber section 74. A quarter-wave plate 42 receives the first linearly polarized single-mode wave 101 and transmits a first circularly polarized single-mode wave 111, for example right-hand circular, to one end of the optical fiber reel 73. The quarter-wave plate 42 receives the second single-mode wave 102, polarized according to another orthogonal linear polarization state, and transmits a second circularly polarized single-mode wave 112, here left-hand circular, to the same end of the optical fiber reel 73. A mirror 26 is arranged at the other end of the optical fiber reel 73. After a first pass through the optical fiber reel 73, the two orthogonally circularly polarized single-mode waves 111, 112 are reflected by the mirror 26.Upon reflection from the mirror, their polarization states are reversed. The two single-mode waves make a second pass in the opposite direction, and with their polarizations reversed, through the optical fiber coil 73. The quarter-wave plate 42 receives the two single-mode waves with orthogonal circular polarizations and transforms them into two waves with orthogonal linear polarizations. The integrated optical circuit 34 and the polarizer 24 recombine these two waves and form the interferometric beam 300. The signal processing system 900 applies any one of the modulation-demodulation schemes with at least 8 states and 12 levels to extract an electric current measurement corrected for the RC time constant of the phase modulator. In this case, the propagation time difference Δτ to be considered for the phase modulation ΔΦm(t) is the round-trip propagation time in the optical fiber section 74 and the optical fiber coil 73.

[0105] There figure 13 represents another example of an online fiber optic interferometer intended for use as an electric current sensor. In this example, the integrated optical circuit 14 includes a polarizing waveguide 24 and a Y-junction separator 15, analogous to that described in connection with the Figures 1 And 11The optical fiber assembly comprises an optical fiber section 71, another optical fiber section 72, an optical fiber section 74, and an optical fiber reel 73. The optical fiber 73 is wound around an axis. The optical fiber 73 is preferably circularly polarized. The optical fiber sections 71, 72, and 74 are preferably linearly polarized. The waveguide 24 linearly polarizes the source beam 100. The splitter 15 separates the linearly polarized source beam 100 into a first linearly polarized single-mode wave 101 and a second linearly polarized single-mode wave 102, both with the same linear polarization state. The first single-mode wave 101 propagates in the optical fiber section 71. The second single-mode wave 102 propagates in the other optical fiber section 72.The other optical fiber section 72 is oriented so as to rotate the linear polarization of the second single-mode wave 102 by 90 degrees, thus making it a second linearly polarized single-mode wave 122 with a polarization orthogonal to the first single-mode wave 101. A polarization coupler-separator 27 recombines the first single-mode wave 101 and the second single-mode wave 122, with orthogonal linear polarizations propagating in the optical fiber section 74. A quarter-wave plate 42 transforms the orthogonal linear polarizations into orthogonal circular polarizations 111, 112. This is analogous to the embodiment described in connection with the... figure 12Mirror 26 reflects the two single-mode waves 111 and 112 and reverses their polarizations. In this way, the two single-mode waves travel through the entire optical fiber with inverted polarization states. The signal processing system 900 applies any one of the modulation-demodulation schemes with at least 8 states and 12 levels to extract an electrical current measurement corrected for the RC time constant of the phase modulator. The propagation time difference Δτ to be considered in this case for the phase modulation ΔΦm(t) is the propagation time in fiber sections 71 and 72 and the round-trip propagation time in fiber section 74 and optical fiber 73.

[0106] Of course, various other modifications can be made to the invention within the scope of the attached claims.

Claims

1. A fiber-optic loop or in-line interferometer comprising a light source (20) adapted to generate a source beam (100), an optical splitting device (15, 24, 34) adapted to split the source beam into a first single-mode wave (101) and a second single-mode wave (102), an electronic system (900) adapted to apply a modulation electric voltage Vm(t) to a phase modulator (16) adapted to induce a same phase shift Φm(t) on the first single-mode wave and the second single-mode wave, a optical fiber set (17, 71, 72, 73, 74) adapted to receive and propagate the first single-mode wave along a first optical path and the second single-mode wave along a second optical path, reverse of the first optical path, respectively, and to form after a propagation time difference Δτ a first output wave and a second output wave, respectively, having a modulated phase difference ΔΦm(t) = Φm(t) - Φm (t-Δτ), the optical fiber set (17, 71, 72, 73, 74) having an eigen frequency fp equal to the inverse of the double of the propagation time difference Δτ, the optical splitting device (15, 24, 34) being adapted to recombine the first output wave and the second output wave and to form a temporally modulated interferometric beam (300), a detection system (18) adapted to detect a power P(t) of the interferometric beam (300) as a function of time, characterized in that the modulated phase difference ΔΦm(t) is equal to the sum of a first periodic phase difference ΔΦπ (t) of level equal to ±π, a second periodic phase difference ΔΦalpha(t) of level equal to ±alpha and a third periodic phase difference ΔΦbeta(t) of variable level between -beta and +beta, alpha and beta having predetermined different values, in such a way that the modulated phase difference ΔΦm(t) has a period of modulation T equal to an odd multiple (2M+1) of the double of the propagation time difference Δτ, where M is a natural integer, the modulated phase difference ΔΦm(t) having, per period of modulation T, at least eight modulation levels among the twelve following modulation levels: ΔΦa+ = π + alpha + beta; ΔΦa- = π + alpha - beta; ΔΦa = π + alpha; ΔΦb+ = π - alpha + beta; ΔΦb- = π - alpha - beta; ΔΦb = π - alpha; ΔΦc+ = - π + alpha + beta; ΔΦc- = - π + alpha - beta; ΔΦc = - π + alpha; ΔΦd+ = - π - alpha + beta; ΔΦd- = - π - alpha - beta; ΔΦd = - π - alpha; and this modulated phase difference being such that: ΔΦ m t + T / 2 = − ΔΦ m t at each time t comprised between 0 and T.

2. The fiber-optic loop or in-line interferometer according to claim 1, wherein the period of modulation T is equal to the double of the propagation time difference Δτ, the first phase difference ΔΦπ(t) has a modulation frequency equal to the eigen frequency fp and wherein the second phase difference ΔΦalpha(t) and the third phase difference ΔΦbeta(t) have a same modulation frequency equal to an odd multiple (2N+1) of the eigen frequency fp, where N is a non-zero natural integer, the second phase difference ΔΦalpha(t) being synchronized with the first phase difference ΔΦπ(t), the third phase difference ΔΦbeta(t) being in phase quadrature with respect to the second phase difference ΔΦalpha(t).

3. The fiber-optic loop or in-line interferometer according to claim 1, wherein the period of modulation T is equal to the double of the propagation time difference Δτ, the third phase difference ΔΦbeta(t) having a modulation frequency equal to the eigen frequency fp and wherein the first phase difference ΔΦπ(t) and the second phase difference ΔΦalpha(t) have a same modulation frequency equal to an odd multiple (2N+1) of the eigen frequency fp, where N is a non-zero natural integer, the second phase difference being in phase quadrature with respect to the first phase difference, the third phase difference ΔΦbeta(t) being synchronized with the first phase difference or with the second phase difference.

4. The fiber-optic loop or in-line interferometer according to claim 1, wherein M is a non-zero integer and wherein the first phase difference ΔΦπ(t) and the second phase difference ΔΦalpha(t) have a same modulation frequency equal to the eigen frequency fp, the second phase difference being in phase quadrature with respect to the first phase difference and the third phase difference ΔΦbeta(t) having a period of modulation equal to the period of modulation T, this third phase difference being synchronized with the first phase difference or the second phase difference.

5. The fiber-optic loop or in-line interferometer according to any one of claims 1 to 4, wherein the detection system (18) includes an electronic demodulation system adapted to extract a signal representative of a quantity to be measured, a transfer function signal of the phase modulator and / or a transfer function signal of the detection system from a series of at least 12 power measurements of the detected interferometric beam per period of modulation.

6. The fiber-optic loop or in-line interferometer according to claim 5, wherein the signal representative of the quantity to be measured is equal to a sum of the interferometric beam power measurements acquired per period of modulation, each power measurement being multiplied by -1 for the levels corresponding to -alpha and by +1 for the levels corresponding to +alpha.

7. The fiber-optic loop or in-line interferometer according to claim 5 or 6, wherein the transfer function signal of the phase modulator is equal to a sum of the interferometric beam power measurements acquired per period of modulation, each power measurement being multiplied by the sign of the product of the first ±π modulation sign and the second ±alpha modulation + or - sign, or by zero in such a way as to keep a same number of states multiplied by the sign + and states multiplied by the sign -.

8. The fiber-optic loop or in-line interferometer according to any one of claims 5 to 7, wherein the transfer function signal of the detection system is equal to a sum of the interferometric beam power measurements acquired per period of modulation, each power measurement being multiplied by the sign of the product of the second ±alpha modulation sign and the third ±beta modulation sign when the level of this last modulation is +beta or -beta, and by zero when the level of this third beta modulation is zero.

9. The fiber-optic loop or in-line interferometer according to any one of claims 5 to 8, wherein the modulated phase difference ΔΦm(t) further includes a ramp composed of phase steps ΔΦFB opposite to a phase difference ΔΦS of the signal representative of the quantity to be measured.

10. A fiber-optic loop interferometer according to any one of claims 1 to 9, wherein the optical splitting device (15) is adapted to spatially split the source beam into the first single-mode wave (101) and the second single-mode wave (102) and wherein the optical fiber set (17, 71, 72, 73, 74) includes an optical fiber coil (17, 73) adapted to receive the first single-mode wave at a first end of the optical fiber coil and the second single-mode wave at a second end of the optical fiber coil, respectively, the first single-mode wave and the second single-mode wave propagating in reverse direction in the optical fiber coil (17, 73).

11. The fiber-optic loop interferometer according to claim 10, wherein the first single-mode wave and the second single mode-wave are linearly polarized and the optical fiber coil (17) is of the linear polarization maintaining type, the interferometer being adapted to measure a phase difference representative of a rotation about an axis of the optical fiber coil (17).

12. The fiber-optic loop interferometer according to claim 10, wherein the optical fiber set (17, 71, 72, 73, 74) includes a linear polarization maintaining optical fiber section (71), the circular polarization maintaining optic fiber coil (73) and another linear polarization maintaining optical fiber section (72), a quarter-wave plate (32) being arranged between the optical fiber section (71) and an end of the optical fiber coil (73), another quarter-wave plate being arranged between the other optical fiber section (72) and the other end of the optical fiber coil (73), the interferometer being adapted to measure a phase difference induced by an electric current passing through the optical fiber coil (73).

13. A fiber-optic in-line interferometer according to any one of claims 1 to 9, wherein the optical fiber set (17, 71, 72, 73, 74) includes a linear polarization maintaining optical fiber section (74) and a circular polarization maintaining optic fiber coil (73), the optical fiber section (74) being connected to one end of the optical fiber coil (73), a mirror (26) being arranged at a second end of the optical fiber coil (73), the interferometer being adapted to measure a phase difference induced by an electric current running through the optical fiber coil (73).

14. A fiber-optic loop or in-line interferometer according to claim 5 alone or combined to one of claims 6 to 13, comprising a feedback system adapted to control the measurement of the signal representative of the quantity to be measured, of the modulator transfer function signal and / or of the detection system transfer function signal.