GENERATOR FOR GENERATING AN ANTI-KERR-EFFECT MODULATED LIGHT SIGNAL, INTERFEROMETRY MEASURING DEVICE COMPRISING SUCH A GENERATOR, AND METHOD FOR MODULATING A LIGHT SIGNAL

Abstract

Disclosed is a generator for generating an anti-Kerr modulated light signal, including a primary light source having four longitudinal modes or fewer and configured to generate a light signal and a modulator configured to modulate the power of the light signal by way of a square-wave or rectangular-wave control signal the duty cycle of which is less than or equal to 50%, and which are adapted such that the modulated light signal is periodic and has: at a first point of the signal, a first power value equal to the product of its average power and a gain between 1.6 and 2.4; at a second point of the signal, a second, non-zero power value, that is different from the first power value. Also disclosed is a modulation method and to a measuring device.

Claims

1. A generator for generating an anti-Kerr modulated light signal, including a primary laser light source having four longitudinal modes or fewer and configured to generate a light signal, and means for modulating the light signal configured to modulate the power of the light signal by way of a square-wave or rectangular-wave control signal the duty cycle of which is lower than or equal to 50%, and which are adapted such that the modulated light signal is periodic and has: at a first point of the signal, a first power value equal to the product of its average power by a gain between 1.6 and 2.4, at a second point of the signal, a second, non-zero power value, that is different from the first power value.

2. The generator according to claim 1, wherein the duty cycle of the control signal is strictly lower than 50%.

3. The generator according to claim 1, wherein the light source is a laser diode.

4. The generator according to claim 1, wherein the gain is equal to 2.

5. The generator according to claim 1, wherein the modulation means include means for adjusting the second power value of the light signal.

6. The generator according to claim 1, wherein the modulation means (4) include means for adjusting the duty cycle of the control signal.

7. The generator according to claim 1, wherein the first power value is the maximum power value of the modulated light signal.

8. The generator according to claim 1, comprising a feedback control module producing a feedback signal representative of the difference between the first value and the average power of the modulated light signal multiplied by the gain, the modulation means being controlled by the feedback signal to create a feedback loop.

9. The generator according to claim 8, wherein the feedback control module includes a photodetector configured to receive part at least of the modulated light signal and to produce a first signal representative of the power of the modulated light signal, a first filter configured to produce a second signal equal to the average value of the first signal multiplied by the gain, a second filter configured to produce a third signal equal to the first value, a feedback module configured to determine the difference between the second signal and the third signal and to produce the feedback signal representative of the difference between the second signal and the third signal.

10. The generator according to claim 1, wherein the primary light source comprises an integrated photodiode.

11. The generator according to claim 1, wherein the modulation means are adapted to modulate the power supply for the primary light source.

12. The generator according to claim 1, wherein the modulation means comprise an optical modulator located downstream from the primary light source.

13. A method for modulated a light signal emitted by primary light source having four longitudinal modes or fewer, including a step of modulating the light signal by way of a square-wave or rectangular-wave control signal the duty cycle of which is lower than or equal to 50% in such a way that the modulated light signal is periodic and has: at a first point of the signal, a first power value equal to the product of its average power by a constant between 1.6 and 2.4, at a second point of the signal, a second power value that is different from the first power value and non-zero.

14. The method according to claim 13, wherein the duty cycle of the control signal is strictly lower than 50%.

15. The method according to claim 13, wherein the gain is equal to 2.

16. The method according to claim 13, wherein the modulation step includes an adjustment of the second power value of the light signal.

17. The method according to claim 13, wherein the modulation step includes an adjustment of the duty cycle value of the modulated light signal.

18. The method according to claim 13, wherein the modulation step includes a feedback control of the modulation by a feedback signal (Sas) representative of the difference between the first power value and the average power of the light signal multiplied by the gain.

19. The method according to claim 13, wherein the modulation step includes a modulation of the power supply for the primary light source.

20. The method according to claim 13, wherein the modulation means includes an optical modulation of the light signal emitted by the primary light source.

21. An interferometric measurement device including a generator according to claim 1.

22. The interferometric measurement device according to claim 21, the device being a fibre-optic gyroscope.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0063] Moreover, various other features of the invention emerge from the appended description made with reference to the drawings that illustrate non-limiting embodiments of the invention, and wherein:

[0064] FIG. 1 described hereinabove illustrates a conventional architecture of a fibre-optic gyroscope,

[0065] FIG. 2 is a block diagram illustrating a modulated light signal generator according to an embodiment of the invention,

[0066] FIG. 3 is a chronogram illustrating the output power of the modulated light signal of a generator according to the invention as well as the value of the modulation control signal,

[0067] FIG. 4 illustrates an alternative embodiment of the light signal generator according to the invention, wherein the generator includes a feedback loop for controlling the minimum power of the light signal,

[0068] FIG. 5 illustrates an alternative embodiment of the light signal generator according to the invention, wherein the generator includes a feedback loop for controlling the duty cycle,

[0069] FIG. 6 illustrates an alternative embodiment of the light signal generator according to the invention, wherein the generator includes an optical modulator,

[0070] FIG. 7 illustrates a fibre-optic gyroscope including a modulated light signal generator according to the invention.

[0071] It is to be noted that, in these figures, the structural and/or functional elements common to the different alternatives can have the same references numbers.

[0072] Various other modifications may be made to the invention within the scope of the appended claims.

DETAILED DESCRIPTION

[0073] The modulated light signal generator 1 illustrated in FIG. 2 includes a primary light source 2 configured to generate a light signal having a single longitudinal mode. The primary source 2 is here supplied by a current source 3 and produces a light signal Smod modulated by modulation means 4 or modulation circuit, in such a way as to have a first power value, or high value, and a second power value, or low value.

[0074] In this example, the primary light source 2 is an integrated laser diode of the DFB type, conventionally including a PN junction, a waveguide and an optical resonance cavity with a Bragg grating. A DFB-type diode is configured to produce a single-frequency light signal.

[0075] The current source 3 here includes a voltage supply module 7 producing, at an intermediate supply terminal 8, an intermediate voltage and a voltage-current conversion circuit 9 producing, at a main supply terminal 10, the supply current Ic for the primary light source 2.

[0076] The voltage supply module 7 includes a first supply terminal 11 configured to produce a first supply voltage V1, for example here 3 volts, a second supply terminal 12 configured to produce a second supply voltage V2, which can here take for example any value between 0 and 1 volt, and a two-way switch 13 coupled between the two supply terminals 11, 12 and the intermediate terminal 8.

[0077] The two-way switch 13 is configured to be either in a first configuration in which the current-voltage conversion circuit 9 is coupled to the first supply terminal 11, or in a second configuration in which the conversion circuit 9 is coupled to the second supply terminal 12. For example, the two-way switch 13 is here a semi-conductor integrated circuit. The two-way switch 13 is electrically controlled by a control signal 50, or a modulation control signal.

[0078] The modulation means 4 are here configured to adjust the modulation parameters, i.e. here the value of the second supply voltage V2 and hence the second power value, and are configured to adjust the duty cycle by controlling the two-way switch 13.

[0079] The modulation means 4 are here configured to produce the control signal 50, which here controls the two-way switch 13 and which is a square-wave signal liable to be either in a high state or in a low state. For example, here, the modulation means 4 include a communication interface 60, for example a connector or a terminal board, making it possible to receive the control signal 50 from the outside, for example from centralized control means MC of an interferometer to which the light signal generator 1 is coupled.

[0080] The control means MC of the gyroscope are configured to produce the first control signal 50 having here a fixed duty cycle lower than 50%, for example 47%.

[0081] The two-way switch 13 is configured to be in its first configuration when the control signal 50 is in the high state and in its second configuration when the control signal 50 is in the low state.

[0082] The modulation means 4 include means 61 for adjusting the value of the second supply voltage V2. For example, the adjustment means 61 comprise a mechanically adjustable potentiometer to adjust the value of the second supply voltage between 0 volt and 1 volt. In FIG. 2, the action of the adjustment means 61 on the second voltage V2 is symbolized by an arrow marked 51.

[0083] Therefore, by varying periodically the voltage produced by the intermediate supply terminal 8, the control signal 50 modulates the intensity value of the supply current Ic and thus the power value of the modulated light signal Smod emitted by the primary light source 2.

[0084] The modulated light signal Smod emitted by the primary light source 2 has thus the first power value, for example here equal to 2 mW, when the primary light source is supplied by the first voltage V1, and the second power value lower than the first power value when the primary light source 2 is supplied by the second voltage V2. The second power value is adjustable by adjustment of the second voltage V2. Because none of the two voltages V1 and V2 is zero, none of the two power values of the modulated signal is zero and the modulated light signal Smod is not extinguished.

[0085] Here, the transfer functions of the current source 3 and of the primary light source 2 are such that the modulated light signal Smod is distorted and has not a duty cycle identical to that of the control signal 50. In particular, the modulated signal Smod is distorted in such a way that it is not a square-wave signal; it is therefore difficult to determine a duty cycle thereof.

[0086] The generator 1 further includes a photodetector 24 configured to receive a part of the modulated light signal Smod and to produce a measurement signal Sm representative of the power of modulated light signal Smod. Here, the photodetector is an integrated photodiode.

[0087] Here, the photodetector 24 is integrated to the bottom of the laser diode cavity and receives all the power of the light signal emitted on this side. As an alternative, the photodetector can be located outside the laser diode 2, downstream from the laser diode 2 relative to the direction of propagation of the light signal in such a way as to receive part of the optical power of the light signal, for example 5%.

[0088] The second power value can thus be adjusted here, in particular depending on the value of the measurement signal and of operations performed on it.

[0089] FIG. 3 is a chronogram showing the evolution of the value of the control signal 50 and of the power value of the modulated light signal Smod produced by the light signal generator 1, or output power Ps. The control signal 50 is here a square-wave signal having a duty cycle of 47%.

[0090] The modulated light signal Smod is here substantially distorted with respect to the control signal 50. This distortion is in particular due to the architecture of the light signal generator 1, to the materials used and to the conditions in which the generation of the modulated signal Smod is implemented. The modulated light signal Smod is not extinguished or, in other words, has a non-zero low (or minimal) state.

[0091] Here, taking into account the shape of the modulated signal Smod, the average power <P> of the signal Smod is equal to 1 mW. Here, the equality P.sub.Mes??<P>=0 is respected. The light signal generator 1 thus advantageously allows the Kerr effect to be reduced.

[0092] In the alternative embodiment illustrated in FIG. 4, the light signal generator 1 includes a feedback loop including a feedback control module 15 configured to produce a feedback signal Sas for adjusting the value of the second supply voltage V2.

[0093] The feedback control module 15 is here configured to produce a feedback signal Sas representative of the integration of the difference between the power of the modulated signal Smod having the first power value and the average value of the modulated light signal Smod multiplied by the gain, here equal to 2.

[0094] Here, the adjustment means 61 have an analog potentiometer controlled by the feedback signal Sas and are configured to feedback control the value of the second supply voltage V2 based on the cancellation of the difference between the power of the modulated signal Smod having the first power value and the average value of the modulated light signal Smod multiplied by the gain.

[0095] The feedback control module 15 includes a first branch 16 and a second branch 17, each configured to receive a first signal, here the measurement signal Sm. The feedback control module 15 further includes a subtractor 18 including a first input coupled to the first branch 16, a second input coupled to the second branch 17, and an integrator 19.

[0096] The first branch 16 is configured to perform operations on the measurement signal Sm in order to send to the first input of the subtractor 18 a second signal S2 the value of which is representative of the average power of the modulated signal Smod multiplied by the gain, here equal to 2. For example here, the first branch includes a low-pass filter 20 and an amplifier 21 with the gain, coupled in series between the photodetector 24 and the subtractor 18.

[0097] The second branch 17 is configured to perform an operation on the measurement signal Sm in order to send to the second input of the subtractor 18 a third signal S3 the value of which is representative of the maximum power value of the modulated signal Smod. For example here, the second branch 17 includes a peak detector 22 that conventionally includes a resistive-capacitive circuit and at least one diode.

[0098] The subtractor 18 is here configured to establish the difference between the values of the signals S2, S3 from the first and second branches 16, 17, and to produce a fourth signal S4 representative of this difference. For example here, the subtractor 18 includes a differential amplifier.

[0099] The fourth signal S4 is transmitted to the integrator 19 that integrates the fourth signal S4 in such a way as to generate the feedback signal Sas controlling the value of the second supply voltage V2.

[0100] As an alternative, as illustrated in FIG. 5, it would be possible that the feedback control module 15 is configured to feedback control the duty cycle value of the control signal 50, the value of the second supply voltage V2 being constant or manually adjustable.

[0101] According to this embodiment, the modulation means 4 include an adjustment module 62 of the duty cycle, coupled between the communication interface 60 and the two-way switch 13 controlled by the feedback signal Sas and configured to adjust the duty cycle of the control signal 50 and to produce an adjusted control signal.

[0102] For example, the adjustment module 62 includes an RC circuit, an adder and a Schmitt flip-flop.

[0103] In another embodiment, the adjustment module 62 is not feedback controlled but makes it possible to manually adjust the duty cycle value. For example, the adjustment module includes a potentiometer.

[0104] In the embodiment illustrated in FIG. 6, the light signal generator 1 does not include electronic modulation means, but optical modulation means 25 located downstream from the primary light source 2 relative to the direction of propagation of the light signal emitted by the source 2. The optical modulation means 25 here include an optical intensity modulator configured to modulate the light signal generated by the primary light source 2 in such a way as to produce the modulated light signal Smod. Here, the optical modulator 25 is a modulator of the Mach Zehnder type. It is to be noted that, in this embodiment, the photodetector 24 is not integrated into the cavity of the laser diode 2 but consisted of a separate component. For example, 5% of the power of the modulated light signal Smod is here directed towards the photodetector 24.

[0105] The current source 3 does not include here a two-way switch and the current Ic provided has a constant value. For example, the intermediate supply terminal 8 is configured to produce the first supply voltage V1.

[0106] Here, the feedback signal Sas is transmitted to the control means that are in this example designed to adjust the duty cycle and/or the second power value of the light signal.

[0107] FIG. 7 illustrates an interferometric measuring device 1b, here a fibre-optic gyroscope including [0108] a Sagnac interferometer 3b, in which propagate a first light signal and a second light signal, which are mutually counter-propagating, said Sagnac interferometer 3b comprising [0109] an input/output port 4b receiving, in a forward direction 5b, an input light signal, [0110] an optical splitter 6b connected, on the one hand, to said input port 4b and, on the other hand, to a first arm 7b and to a second arm 8b of said Sagnac interferometer, [0111] a fibre-optic loop 9b, a first end of which is coupled to the first arm 7b and a second end of which is coupled to the second arm 8b, configured in such a way that a rotation of the gyroscope in the plane of FIG. 7 generates a phase shift between the counter-propagating signals propagating in the loop, [0112] the input/output port 4b transmitting, in a return direction 10b, opposite to the forward direction 5b, an output light signal having a output light power that is function of the phase shift between the two counter-propagating signals, [0113] a photodetector 11b, configured to receive said output light power and to produce an electrical signal representative of the output light power, [0114] an optical coupler 12a that couples, in the forward direction 5b, the light source to said input/output port 4b and, in said return direction, the input/output port 4b to the photodetector 11b, [0115] a light signal generator 2b according to the invention, [0116] the control means MC in particular configured to produce a control signal Sx here acting as a synchronisation signal, the characteristics of which are calculated by the control means MC as a function in particular of the time of circulation of the light in the sensor 9b. Thus, the generation of the modulated light signal is made synchronously with the gyroscope operation. The control means MC are further configured to perform, synchronously, operations of phase and power modulation of the light signals circulating in the gyroscope 1b, in particular at the optical splitter 6b.

[0117] In this device, the consequences of the Kerr effect are greatly reduced thanks to the presence of the generator according to the invention.

[0118] The invention is not limited to the embodiments described hereinabove in relation with FIGS. 1 to 7.

[0119] In particular, it has been described hereinabove a generator including a primary light source that is a laser diode of the DFB type. However, the invention is compatible with any type of light source having a number of modes smaller than four. Preferentially, the invention is compatible with a single-frequency light source or the operation of which is similar to a single-frequency operation. For example, the laser diodes of the Fabry Perot type, which emit in very few modes, leaving much less powerful secondary modes, are compatible with the invention.

[0120] Moreover, the value of 47% of the duty cycle of the control signal 50 described hereinabove is not limiting, and the control signal can have any duty cycle lower than or equal to 50%.

[0121] The maximum power of the modulated light signal Smod can take preferentially any value between 0.5 mW and 20 mW, as a function of the configuration of the primary light source and of the value of the first supply voltage V1. The minimum power depends on the value of the maximum power and of the duty cycle, and is then obtained by adjustment of the second supply voltage as described hereinabove.