Optical stability transfer device, associated optical system and associated optical stabilization method

Abstract

The present invention relates to an optical stability transfer device (16), which comprises an interferometer (20) including: an acousto-optic filter (26), configured to receive a reference optical signal (Soref) and at least one target optical signal (So1, So2), to emit a plurality of acoustic signals (Saref, Sa1, Sa2), and to diffract each optical signal to form zero-order reference and target optical signals (Soref(0), So1(0), So2(0)) and first-order reference and target optical signals (Soref(1), So1(1), So2(1)); and an optical coupler (34), configured to combine the optical signals into recombined optical signals (Soref(comb), So1(comb), So2(comb)). The optical stability transfer device (16) also comprises a photodetector (22), configured to convert each recombined optical signal into output electrical signals (Seref, Se1, Se2); and a control unit (24), configured to keep the optical delay (T) of the interferometer (20) constant and to keep the frequency (fo1, fo2) of the or each target optical signal constant.

Description

[0001] TITLE: Optical stability transfer device, optical system and associated optical stabilization method

[0002] The present invention relates to an optical stability transfer device, as well as an optical system and an optical stabilization method.

[0003] High-stability and high-coherence optical sources are of great interest for applications involving measurements over long integration or interrogation times. This is the case, for example, in inertial sensors or for memories, particularly those using quantum states. To achieve the desired levels of stability, optical sources must be stabilized at a few reference frequencies, which generally correspond to an atomic transition of an element or molecule in its gaseous state, such as rubidium or acetylene.

[0004] Devices exist that transfer the frequency stability of a reference light source to a light source whose frequency, and therefore wavelength, is suitable for the intended application. For example, devices using a frequency comb, a Fabry-Pérot transfer cavity, or fiber-reinforced cavity devices are known to be effective. However, optical frequency comb devices are complex and expensive systems with a limited intrinsic spectral width, meaning the range of wavelengths that can be stabilized. Devices using Fabry-Pérot cavities are difficult to implement because they are sensitive to external disturbances, such as vibrations or temperature variations, which generally necessitate a monolithic cavity whose length cannot be finely adjusted to compensate for long-term drift.On the other hand, the spectral width of such a device is also reduced, generally less than 50 nm. While fiber-cavity devices are less complex to implement, particularly with regard to length control, their spectral width remains limited to 100 nm at best.

[0005] The aim of the invention is therefore to propose an optical stability transfer device, enabling the stabilization of light sources with wavelengths far removed from the wavelength of the reference light source, which is robust and simple to implement.

[0006] To this end, the invention relates to an optical stability transfer device comprising: - an interferometer comprising:

[0007] an acousto-optical filter, configured for:

[0008] ■ receive a reference optical signal, emitted by a reference light source, and at least one target optical signal, the target optical signal or each target optical signal being emitted by a respective target light source, a frequency of the target optical signal or each target optical signal being distinct from a frequency of the reference optical signal;

[0009] ■ emit a plurality of acoustic signals of distinct frequency and amplitude, each acoustic signal being independently associated with a single optical signal from among the reference optical signal and the target optical signal(s); and

[0010] ■ independently diffract each optical signal to form respectively zero-order and one-order reference and target optical signals, o a first arm, configured to receive and propagate the reference and target optical signals of the same given order among zero-order and one-order;

[0011] o a second arm, configured to receive and propagate the reference and target optical signals of the other given order, the first and second arms being configured to impose a predetermined optical delay between the optical signals propagating in the first arm and the optical signals propagating in the second arm; o an optical coupler, disposed at the output of the first and second arms, and configured to combine the zero- and first-order reference optical signals into a recombined reference optical signal on the one hand, and the zero- and first-order target optical signal(s) into a recombined target optical signal(s) on the other hand;

[0012] - a photodetector, configured to convert the recombined reference optical signal and each recombined target optical signal(s) respectively into an output reference electrical signal and an output target electrical signal(s); and

[0013] - a control unit, configured to convert the output reference electrical signal and the output target electrical signal(s) respectively into a reference error signal and a target error signal, so as to keep the optical delay of the interferometer constant and to keep the frequency of the target optical signal(s) constant, depending on the reference and target error signals.

[0014] Thanks to the invention, it is possible to stabilize light sources whose wavelength is far removed from the wavelength of the reference light source. In particular, the wavelengths of the target light signals are separated, for example, by several hundred nanometers from the reference optical signal.

[0015] The use of an acousto-optical filter in an unbalanced interferometer-type discriminator advantageously extends the operating range to typically one octave, and provides a resolved acousto-optical coupling, allowing a correspondence between a particular wavelength and a particular acoustic frequency, and an independent diffraction efficiency optimization for each wavelength involved.

[0016] Furthermore, the device allows for the simultaneous stabilization of multiple target optical signals, meaning it stabilizes the frequency, and therefore the wavelength, of each target optical signal without significantly affecting the device's performance. Specifically, it is possible, for example, to stabilize more than two target light sources at different wavelengths. The wavelengths of the target light signals can also be separated by more than 100 nm.

[0017] Finally, because the device maintains a constant optical delay for the interferometer, it is adaptable to varying environmental conditions, such as temperature variations or vibrations. It is therefore robust and compatible with applications in embedded systems.

[0018] According to other advantageous aspects of the invention, the device comprises one or more of the following features, taken individually or in all technically possible combinations:

[0019] - the control unit is further configured to adjust the amplitude of the acoustic signals according to the reference and target error signals;

[0020] - the first arm has a predetermined length, and in which the control unit includes an interferometer correction module, configured to receive the reference error signal and to adjust at least one of the following parameters as a function of the reference error signal: the length of the first arm, a temperature of the interferometer and a phase of the acoustic signals emitted by the acousto-optic filter, in order to keep the optical delay of the interferometer constant;

[0021] - the control unit includes a correction module for the target light source or each target light source, configured to receive the target error signal or each target error signal and to adjust at least one of the following parameters according to the target error signal or each target error signal: a temperature of the target light source or at least one target light source, an electrical intensity of an electrical supply current to the target light source or at least one target light source, and a voltage of the electrical supply current to the target light source or at least one target light source, to maintain the frequency of the target optical signal or each target optical signal constant;

[0022] - the first arm includes a waveguide, configured so that the reference and target optical signals of the same given order among zero order and first order propagate through the waveguide;

[0023] - the first arm is configured to receive and propagate zero-order reference and target optical signals and the second arm is configured to receive and propagate first-order reference and target optical signals.

[0024] The invention also relates to an optical system comprising:

[0025] - a reference light source, configured to emit a reference optical signal

[0026] - at least one target light source, the target light source or sources being configured to emit a target optical signal, the target optical signal or signals having a frequency distinct from a frequency of the reference optical signal; and

[0027] - an optical stability transfer device, the acousto-optical filter being configured to receive the target optical signal or signals emitted by the target light source or sources and the reference optical signal emitted by the reference light source.

[0028] According to other advantageous aspects of the invention, the optical system comprises one or more of the following features, taken individually or in all technically possible combinations:

[0029] - the reference and target light sources are laser sources; - comprising a plurality of target light sources, preferably more than two target light sources, each target light source being configured to emit a target optical signal of a frequency distinct from the other target light sources;

[0030] - the target optical signal from each target light source is an auxiliary target optical signal, each target light source being configured to further emit a main target optical signal, of the same frequency as the frequency of the auxiliary target optical signal emitted by the target light source, and further comprising an optical combining module, configured to combine the main target optical signals into a combined optical signal, of a combined frequency, distinct from the frequencies of the main target optical signals;

[0031] - The optical combining module is a frequency summing or subtracting device. The invention also relates to an optical stabilization method, implemented by a device, the method comprising the following steps:

[0032] - reception of a reference optical signal and at least one target optical signal, by the acousto-optical filter;

[0033] - emission of the plurality of acoustic signals by the acousto-optical filter;

[0034] - diffraction of each optical signal by the acousto-optical filter to form the zero-order and first-order reference optical signals, and the zero-order and first-order target optical signal(s);

[0035] - reception and propagation of reference and target optical signals of the same given order among the zero order and the first order in the first arm and of reference and target optical signals of the other given order in the second arm;

[0036] - combination of zero-order and first-order reference optical signals into the recombined reference optical signal, and of zero-order and first-order target optical signals into the recombined target optical signal(s), by the optical coupler;

[0037] - conversion of the recombined reference optical signal into the output reference electrical signal, and of each recombined target optical signal(s) into the output target electrical signal(s) by the photodetector; and

[0038] - conversion of the electrical reference output signals and of each electrical target output signal(s) respectively into the reference error signal and the target error signal(s), so as to keep the optical delay of the interferometer constant and to keep the frequency of each optical target signal(s) constant, depending on the reference and target error signals.

[0039] The invention will become clearer upon reading the following description, given solely by way of non-limiting example, and made with reference to the drawings in which:

[0040] [Fig. 1] Figure 1 is a diagram of an optical system according to the invention;

[0041] [Fig. 2] Figure 2 is a diagram of a stability transfer device according to the invention; and

[0042] [Fig. 3] Figure 3 is a flowchart of an optical stabilization method according to the invention. Throughout the description, "equal to" means a relationship of equality plus or minus the specified range of variation, or uncertainty. In the absence of a specified range of variation, the range of variation is plus or minus 5%.

[0043] Figure 1 schematically represents an optical system 5. The optical system 5 advantageously comprises a reference light source 10, as well as two target light sources 11 and 12.

[0044] Light sources 10, 11, and 12 are advantageously monochromatic. In particular, light sources 10, 11, and 12 are laser sources.

[0045] The reference light source 10 is advantageously a source configured to emit a reference optical signal S ore f, originating from a reference atomic transition, which allows us to obtain a reference optical signal S ore f of frequency f ore f constant, that is to say whose relative variation is less than or equal to 10' 12 over measurement times ranging from 1 to 10,000 s. Advantageously, the reference atomic transition is that of acetylene. The reference light source 10 is then configured to emit a reference optical signal Soref having a frequency f ore f equals 195.337 THz, corresponding to a wavelength λ ore f of 1542.38 nm. Of course, other reference atomic transitions besides acetylene can be used to generate the reference optical signal S ore f.

[0046] The target light sources 11 and 12 are, for example, laser diodes. The target light sources 11 and 12 are configured to emit target optical signals. Advantageously, the target light source 11 is configured to emit an auxiliary target optical signal Soi and a main target optical signal Soi', of frequency f o i. Similarly, the target light source 12 is configured to emit an auxiliary target optical signal S02 and a main target optical signal S02' of frequency f o2 Advantageously, in practice, the target light sources 11 and 12 each emit a single initial optical signal of frequency f o i, respectively f o2 , which is then separated to form the auxiliary optical target signal Soi, respectively S02, and the main optical target signal Soi', respectively S02'.

[0047] The target light sources 11 and 12 are less stable than the reference light source, i.e., a relative variation of the frequencies f oi and f o2 is much greater, that is to say at least twice greater than 10' 12 over measurement times ranging from 1 to 10,000 s. The frequencies f oi and f o2 are distinct from the frequency f ore f of the reference optical signal S ore f. The frequencies f oi and f o2 are advantageously distinct from one another.

[0048] In the example described here, the frequency f oi is equal to 187.4 THz, corresponding to a wavelength A oi of 1.6 pm, and the frequency f o2 is equal to 305.9 THz, corresponding to an ΔO2 wavelength of 980 nm. Wavelengths Δ oi and A02 are thus separated by several hundred nanometers, and the wavelengths A oref and A02 are also separated by several hundred nanometers.

[0049] In an alternative not shown, system 5 comprises a single target light source, or more than two target light sources. System 5 further comprises an optical stability transfer device 16, associated with the light sources 10, 11 and 12. The optical stability transfer device 16 comprises an interferometer 20, a photodetector 22, disposed at the output of the interferometer 20, and a control unit 24, electrically connected to the photodetector 22. System 5 is configured to stabilize frequencies within a frequency range or spectral range, also called the operating frequency range, which is as wide as possible, corresponding for example to an optical octave, that is to say, for a given frequency, an interval going from that frequency to twice that frequency.

[0050] The interferometer 20 includes an acousto-optical filter 26, a first arm 31 and a second arm 32, the acousto-optical filter being disposed at the input of arms 31 and 32, and an optical coupler 34, disposed at the output of arms 31 and 32.

[0051] Interferometer 20 is advantageously a Mach-Zehnder type interferometer.

[0052] The acousto-optic filter 26 is advantageously a broadband acousto-optic filter. The acousto-optic filter 26 advantageously comprises an acoustic wave emitter 36, visible in Figure 2, for example a piezoelectric transducer, in contact with a filtering element 38 made of a material having acousto-optic properties, for example a paratellurite crystal. The acousto-optic filter 26 is advantageously connected to the reference light sources 10 and target light sources 11 and 12 by optical fibers 33a, 33b, and 33c, respectively. The acousto-optic filter 26 is configured to receive optical signals and to emit a plurality of acoustic signals of distinct and predetermined frequencies and amplitudes in order to diffract the optical signals it receives. More specifically, it is the transmitter 36 which is configured to emit the plurality of optical signals, which then propagate into the filtering element 38.To this end, each acoustic signal emitted by the acousto-optical filter 26 is associated with a single optical signal received by the same filter. In other words, a single acoustic signal, of predetermined frequency, is independently associated with a single optical signal. By "independently," we mean that the association of an acoustic signal with an optical signal is independent of other acoustic and / or optical signals.

[0053] Arms 31 and 32 are configured to receive and propagate transmitted and diffracted optical signals from the output of the acousto-optical filter 26, and to impose a predetermined optical delay T between the optical signals propagating in arm 31 and the optical signals propagating in the second arm 32. For this purpose, arm 31 advantageously has a predetermined length. Advantageously, arm 31 includes a waveguide 40 of predetermined length L. The waveguide 40 is, for example, an optical fiber.

[0054] Advantageously, the arm 31 further includes a piezoelectric stretcher 42, configured to stretch the waveguide 40.

[0055] Advantageously, the arm 31 further includes an injection module 44, connected at the input of the waveguide 40, and an output module 45, connected at the output of the waveguide 40. The injection module 44 allows optical signals to be injected into the input of the waveguide 40 in such a way as to minimize losses, and the output module 45 allows optical signals to be extracted from the output of the waveguide 40, also in such a way as to minimize losses.

[0056] The second arm 32, for example, is made in free space, that is to say, the signals which propagate through the arm 32 propagate in the air.

[0057] Optical coupler 34 is for example a polarization splitter cube or a broadband coupler, advantageously with a coupling ratio of 50 / 50.

[0058] Photodetector 22 is for example a photodiode, possibly a plurality of photodiodes.

[0059] The control unit 24 advantageously includes a correction module 46 for the interferometer 20 and a correction module 48 for each target light source 11, 12, shown in Figure 2. The correction module 46 for the interferometer 20 and the correction module 48 for each target light source 11, 12 are advantageously each made in the form of a programmable logic component, such as an FPGA (Field Programmable Gate Array), or an integrated circuit, such as an ASIC (Application Specific Integrated Circuit).

[0060] In an alternative not shown, the control unit 24 includes an information processing unit consisting, for example, of a memory and a processor associated with the memory. The correction module 46 of the interferometer 20 and the correction module 48 of each target light source 11, 12 are then advantageously each implemented as software, or a software component, executable by the processor.

[0061] The memory of control unit 24 is then capable of storing interferometer correction software and correction software for each target light source.

[0062] When the control unit is implemented as one or more software programs, that is, as a computer program, also called a computer program product, it is also capable of being stored on a computer-readable medium (not shown). A computer-readable medium is, for example, a medium capable of storing electronic instructions and being connected to a bus of a computer system. Examples of such a readable medium include an optical disc, a magneto-optical disc, ROM, RAM, any type of non-volatile memory (e.g., FLASH or NVRAM), or a magnetic card. A computer program containing software instructions is then stored on this readable medium.

[0063] The correction module 46 of the interferometer 20 advantageously comprises a demodulator 52a, a filter 55a and a corrector 56a.

[0064] The correction module 46 of the interferometer 20 is advantageously connected to the piezoelectric stretcher 42, in order to control the piezoelectric stretcher 42 and to control the length L of the waveguide 40.

[0065] Alternatively or in addition, the correction module 46 of the interferometer 20 is advantageously connected to the acousto-optic filter 26 in order to control the phase of the acoustic signals emitted by the acousto-optic filter 26. The correction module 46 of the interferometer 20 is advantageously connected to a temperature control module 58, included in the optical system 5. The temperature control module 58 is, for example, a Peltier module, on which the interferometer 20 is placed. The correction module 46 of the interferometer 20 is advantageously configured to control a temperature in the temperature control module 58, and thus, a temperature of the interferometer 20.

[0066] The correction module 48 for each target light source 11, 12 advantageously comprises a demodulator, a filter, and a corrector for each target light source 11 and 12. In other words, the correction module 48 for each target light source 11, 12 comprises a demodulator 52b, a filter 55b, and a corrector 56b associated with the target light source 11, and a demodulator 52c, a filter 55c, and a corrector 56c associated with the target light source 12. The filters 55a, 55b, and 55c are advantageously low-pass filters, and the correctors 56a, 56b, and 56c are advantageously PID-type correctors.

[0067] The correction module 48 is advantageously connected to each target light source 11, 12, in order to control the frequency f o i, f02 of the optical signals Soi and S02 emitted by the target light source 11 and the target light source 12, respectively.

[0068] The system 5 further includes an optical combining module 60, external to the optical stability transfer device 16. The optical combining module 60 is, for example, a frequency adder or a frequency subtractor. If the optical combining module 60 is a frequency adder, it is configured to combine optical signals into a combined optical signal, whose frequency is the sum of the frequencies of the optical signals. If the optical combining module 60 is a frequency subtractor, it is configured to combine optical signals into a combined optical signal, whose frequency is the difference between the frequencies of the optical signals. The optical combining module 60 is, for example, a nonlinear crystal.

[0069] In an alternative not shown, the optical combination module 60 includes a frequency modulator, in order to modulate an optical signal, and a frequency summing or subtracting device, which adds or subtracts the optical signal modulated by the modulator, with another optical signal.

[0070] The operation of system 5 is now described in detail. In this context, an optical stabilization method, implemented by device 16, is also described.

[0071] The reference light source 10 is powered by an electric supply current and emits the reference optical signal S oref. The target light source 11 is powered by an electric supply current and emits the auxiliary target optical signal S01 and the main target optical signal S02'. The target light source 12 is powered and advantageously emits the auxiliary target optical signal S02 and the main target optical signal S02' in a similar manner to that described for the target light source 11, i.e., by separating a single optical signal. Advantageously, the reference optical signal S ore f, and the auxiliary target optical signals Soi and S02 propagate respectively in the optical fiber 33a, 33b, 33c linking respectively the reference light source 10 and the target light sources 11, 12 to the acousto-optic filter 26.

[0072] The acousto-optical filter 26 receives the reference optical signal S re f and the auxiliary target optical signals Soi and S02 during a reception stage S102.

[0073] The acousto-optic filter 26 emits a plurality of acoustic signals of distinct frequencies and amplitudes during an emission stage S104. Each acoustic signal emitted during the emission stage S104 is associated with a unique optical signal from among the reference optical signal Soref and the target optical signals Soi and S02, as described previously. Thus, in the example in Figures 1 and 2, the acousto-optic filter 26 is configured to emit three distinct acoustic signals, a reference acoustic signal S are f, associated with the reference optical signal S ore f, a target acoustic signal S a i, associated with the target optical signal Soi and a target acoustic signal S a 2, associated with the target acoustic signal SO2. In particular, the reference acoustic signal S are f, the target acoustic signal S ai and the target acoustic signal S a 2 have respective frequencies f aref, fai and f a 2 distinct from each other. The frequencies f are f, fai and f a 2, for example, are between 10 and 100MHz.

[0074] The acousto-optical filter 26 then diffracts the reference optical signal S ore f, and the auxiliary target optical signals S01 and S02 during a diffraction step S106. The type of diffraction is advantageously Bragg diffraction, each optical signal S ore f, Soi, SO2 being diffracted through its interaction with the acoustic signal S are f, S a i, S a 2 with which it is associated in the acousto-optical filter 26. The diffraction stage S106 allows the formation, for each optical signal, of a zero-order and a first-order optical signal. In addition, the first-order optical signal is frequency-shifted by the Doppler effect, by the frequency of the acoustic signal with which the optical signal interacted. More precisely, the reference optical signal Sore f is diffracted to form a zero-order optical reference signal S ore f(0), and a first-order optical reference signal S ore f(1). The zero-order optical reference signal has a frequency equal to f ore f, and the first-order optical reference signal has a frequency equal to f O The target optical signal Soi is diffracted to form a zero-order target optical signal S o i(0), of frequency f oi and a first-order optical target signal Soi(1), of frequency f o i+f a i. Similarly, the target optical signal SO2 is diffracted to form a zero-order target optical signal S O 2(0), of frequency f o2 , and a first-order optical target signal SO2(1), of frequency f O 2+f a 2. The diffraction of each optical signal S oref, Soi, SO2 is independent of other optical signals. In particular, the diffraction efficiency of an optical signal S ore f, Soi, S02 given depends only on the frequency and amplitude of the acoustic signal S are f, S ai , S a 2.

[0075] At the output of the acousto-optical filter 26, arm 31 receives and propagates optical signals of the same order (zero-order and first-order), and arm 32 receives and propagates reference and target optical signals of the other order, during a reception and propagation step S108. In the example of Figures 1 and 2, the zero-order reference optical signal S ore f(0), and the zero-order target optical signals S o i(0) and S O 2(0) are received and propagated in the arm 31, more precisely injected into the waveguide 40 via the injection module 44.

[0076] The first-order reference optical signal S oref(1), as well as the first-order target optical signals Soi(1) and S02(1) are received and propagated in arm 32, advantageously in free space.

[0077] The length L of the fiber 40 induces the optical delay T between zero-order optical signals propagating in arm 31 and first-order optical signals propagating in arm 32. The optical delay T creates a spectral transfer function periodic in 1 / T. L is advantageously between 2 and 100 m. It is thus possible to discriminate optical signals with frequencies spaced proportionally to 1 / T, advantageously every 2 to 50 MHz, depending on the value of the optical delay T chosen. The presence of the waveguide 40 in arm 31 allows for easy adjustment of the optical delay T, simply by changing the length L, without needing to modify the spatial arrangement of the various components of the interferometer 20.Moreover, the 40 waveguide, especially if it is a polarization-maintaining fiber, makes it possible to obtain a delay T of several tens of meters with very low propagation losses, over a spectral range covering several hundred nanometers, optimal spatial overlap, and conservation of polarization.

[0078] Zero-order and first-order optical signals are combined in a combination step S110. Advantageously, the zero-order optical signals S ore f(0), S o i(0) and S O 2(0) propagate from the waveguide 40 to the output module 45 before being sent to the optical coupler 34. The first-order optical signals S ore f(1), S o i(1) and SO2(1) propagate advantageously directly on the optical coupler 34.

[0079] Zero-order optical reference signals S ore f(0) and an S oref(1), the zero-order target optical signals S o i(0) and an S o i(1), and the zero-order target optical signals S O 2(0) and an S02(1) are respectively combined into a combined reference optical signal Soref(comb), into a recombined target optical signal S o i(comb) and in combined target optical signal S O 2(comb). Furthermore, due to the frequency shift of first-order optical signals, in addition to the interference caused by recombination, the recombined signals Soref(comb), S o i(comb) and S O 2(comb) form optical beats, with a frequency equal to the frequency difference between the zeroth and first-order optical signals. In other words, the recombined reference optical signal Soref(comb) has a beat frequency faref, the recombined target optical signal S o i(comb) has a beat frequency f ai and the recombined target optical signal S O 2(comb) has a beat frequency f a2. The recombined optical signals Soref(comb), Soi(comb) and S O 2(comb) thus have a distinct beat frequency.

[0080] An amplitude of the beat signal, in other words, of the recombined optical signals Soref(comb), Soi(comb) and S O 2(comb) is independent of the other recombined optical signals, and depends in particular on the amplitude of the acoustic signals S are f, S ai and S a 2, in other words, the power of the acoustic signals S are f, S ai and S a 2.

[0081] The combined reference optical signal Soref(comb), the combined target optical signal Soi(comb), and the combined target optical signal S O 2(comb) are respectively converted into an output reference electrical signal S ere f, in target electrical signal S ei and in target electrical signal S e2 by the photodetector 22 during a conversion step S112. Advantageously, the recombined optical signals Soref(comb), S o i(comb) and S O 2(comb) are sent to the same photodetector 22, without being spatially separated. Indeed, the beat frequencies are distinct for the three combined optical signals Soref(comb), S o i(comb) and S O 2(comb), they can be converted into three independent electrical output signals, without the need to separate them spatially to avoid noise phenomena, or crosstalk.

[0082] The output reference electrical signal S ere f is converted into the reference error signal Eref, and the target electrical signals S ei and S e 2 are converted into target error signals Ei and E2 by control unit 24 during a conversion step S114.

[0083] Advantageously, the output reference electrical signal S ere f is received by the correction module of interferometer 46. Advantageously, the output electrical reference signal S ere f is received by the demodulator 52a which converts the output reference electrical signal S ere f in the reference error signal E re f. Advantageously the reference error signal E re f is filtered by the low-pass filter 55a and then received by the corrector 56a.

[0084] Similarly, the target electrical signal S ei and the target electrical signal S e 2 are received by the correction module 48 from each target light source 11, 12. Advantageously, the target electrical signal S eiis received by the demodulator 52b which converts the target electrical output signal Sei into the target error signal E1, and the target error signal E1 is filtered by the low-pass filter 55b and then received by the corrector 56b. The target electrical signal S e 2 is received by the 52c demodulator which converts the target electrical output signal S e 2 in the target error signal E2. Advantageously, the target error signal E2 is filtered by the low-pass filter 55c and then received by the corrector 56c.

[0085] Advantageously, after receiving the error signals E re f, E1 and E2, the correctors 56a, 56b and 56c emit control signals during a control step S116. The corrector 56a emits a control signal in order to control the piezoelectric stretcher 42, so as to adjust the length of the arm 31. For example, the piezoelectric stretcher 42 tensions the waveguide 40 to maintain its length L constant.

[0086] Alternatively or in addition, the compensator 56a outputs a control signal to control the temperature control module 58, for example, to maintain a constant temperature in the temperature control module 58, and therefore a constant interferometer temperature. The control signal is represented by a dashed arrow in Figure 2. This allows, in particular, compensation for changes in the optical delay T due to thermal expansion.

[0087] Alternatively or in addition, the corrector 56a emits a control signal to control the acousto-optical filter 26, more precisely to adjust a phase of the acoustic signals S are f, S ai and S a 2 emitted by the acousto-optical filter 26. The control signal is represented by a dashed arrow in Figure 2. In particular, the phase of the acoustic signals Saref, S ai and S a2 can be chosen to compensate for changes in the optical delay T. Thus, the control unit 24 adjusts at least one of the following parameters according to the reference error signal: the length L of the arm 31, a temperature of the interferometer 20 and a phase of the acoustic signals S are f, S ai and S a 2 emitted by the acousto-optical filter 26, in order to maintain the optical delay T of the interferometer 20 constant.

[0088] The corrector 56b outputs a control signal to adjust at least one of the following parameters of the target light source 11: the temperature of the target light source 11, the current intensity of the power supplying the target light source 11, and the voltage of the power supplying the target light source 11, in order to maintain the frequency of the target optical signal Soi constant. Alternatively, the control signal allows adjustment of other parameters of the target light source 11, for example, in the case where the light source includes a cavity, the length of the cavity, or the voltage applied to an electro-optical crystal, if the light source 11 includes such a crystal.

[0089] Alternatively, the corrector 56b emits several control signals in order to adjust several parameters of the target light source 11 simultaneously.

[0090] Similarly, the corrector 56c emits a control signal in order to adjust at least a temperature of the target light source 12, an electrical intensity of the electrical current supplying the target light source 12, and a voltage of the electrical current supplying the target light source 12, to maintain the frequency of the target optical signal SO2 constant.

[0091] Alternatively, as described for corrector 56b, corrector 56c emits several control signals in order to adjust several parameters of the target light source 12 simultaneously.

[0092] The frequency f oi and f o2 The optical signals Soi and SO2 are thus kept constant, that is to say, the relative variation of the frequencies f oi and f o2 is less than or equal to 10' 12 over measurement times between 1 and 10000 s, which is the same as the relative variation of the frequency f oref of reference optical signal S ore f from the reference light source 10.

[0093] Advantageously, control unit 24 also adjusts the amplitude of the acoustic signals S are f, S ai and S a 2, in other words, the acoustic power of the S signals are f, S ai and S a 2, in order to optimize the amplitude of the recombined optical signals Soref(comb), S o i(comb) and S O 2(comb), via a control signal. This optimization is performed independently for each of the three recombined optical signals Soref(comb), S o i(comb) and S O 2(comb). It is performed by adjusting the acoustic power of the S signals are f, S ai and S a 2 in order to adjust the diffraction efficiency of optical signals S ore f, Soi, S02. Thus, the power between the zero-order reference optical signals S oref(0) and an S ore f(1), the zero-order target optical signals S o i(0) and an S o i(1), and the zero-order target optical signals S O 2(0) and SO2(1) are equal at the time of their recombination on the photodetector 22. This allows optimization of the power of the error signals E ref, Ei and E2 and to ensure that the stability transfer is effective regardless of the frequency of the target optical signal, within the operating frequency range of device 16. In particular, such optimization makes it possible to correct imperfections of the interferometer 20, related to the use of a wide frequency range. The reference source 10 thus ensures that the interferometer 20 has the same operation over time, regardless of external conditions, and therefore allows it to be used to precisely measure the frequency variations of the target optical signals Soi, S02 in order to stabilize their frequency f oi and f o2 .

[0094] Frequency stabilization f oi and f o2 , from the auxiliary optical signals Soi and S o2 In particular, it allows for the stabilization of the frequency of the main target optical signals Soi' and S o2'Indeed, advantageously, the control signal emitted by the correction module 48 of each target light source 11, 12 allows the frequency of the initial optical signal emitted by each target light source 11, 12 to be adjusted, an initial signal which is separated into principal optical signals Soi' and S o2 ' and auxiliaries Self and S o2 as described previously.

[0095] The main optical signals target Soi' and S o2 ' are advantageously sent to the optical combining module 60. The optical combining module 60 combines the main target optical signals Soi' and S o2 'into a combined optical signal S oc In the case where the optical combining module 60 is a frequency adder, it is possible to obtain a combined optical signal S oc whose frequency f oc is in the visible range by summing the main target optical signal Soi' of frequency f oiequal to 187.4 THz and the main target optical signal S o2 ' of frequency f o2 equal to 305.9 THz.

[0096] The combined optical signal S oc thus has a frequency f oc stability comparable to a reference light source, because the frequencies f oi and f o2 are stabilized by device 16. The combined optical signal S oc is used for example to perform measurements requiring high stability of the light source enabling the measurement, for example because of a long integration time, such as in inertial navigation applications or in gyroscopic sensors, or in applications involving quantum states, for example in the interrogation of quantum memories.

[0097] Any feature described for an embodiment or variant in the foregoing may be implemented for the other embodiments and variants described above, provided that it is technically feasible.

Claims

DEMANDS 1. Optical stability transfer device (16) comprising: an interferometer (20) comprising: an acousto-optical filter (26), configured for: ■ receive a reference optical signal (S ore f), emitted by a reference light source (10), and at least one target optical signal (Soi, SO2), the target optical signal(s) (Soi, SO2) being emitted by a respective target light source (11, 12), a frequency (f o i, fa) of each target optical signal (Soi, SO2) being distinct from a frequency (f ore f) of the reference optical signal (S ore f); ■ emit a plurality of acoustic signals (S are f, S ai , Sa2) of distinct frequency (faref, fai, fa) and amplitude, each acoustic signal (Saref, S ai , Sa2) being independently associated with a single optical signal among the reference optical signal (S oref) and the target optical signal(s) (Soi, SO2); and ■ diffract each optical signal independently (S ore f, Soi, S02) to form respectively zero-order (Soref(0), Soi(0), So2(0)) and first-order (S) reference and target optical signals ore f(1), S o i(1 ), S02(1)), o a first arm (31), configured to receive and propagate the reference and target optical signals of the same given order among zero order and one order; o a second arm (32), configured to receive and propagate the reference and target optical signals of the other given order, the first and second arms (31, 32) being configured to impose a predetermined optical delay (T) between the optical signals propagating in the first arm (31) and the optical signals propagating in the second arm (32); o an optical coupler (34), disposed at the output of the first and second arms (31, 32), and configured to combine the zero-order reference optical signals (Soref(0)) and one (Soref(1)) into a recombined reference optical signal (Soref(comb)) on the one hand, and the target zero-order optical signal(s) (S o i(0),S O 2(0)) and a (Self(1),S O 2(1)) into recombined target optical signal(s) (S o i(comb), So2(comb)) on the other hand; a photodetector (22), configured to convert the recombined reference optical signal (Soref(comb)) and the recombined target optical signal(s) (S o i(comb), S O 2(comb)) respectively in output reference electrical signal (S ere f) and in target electrical output signal or signals (S e i, S e 2); and a control unit (24), configured to convert the output reference electrical signal (S eref) and the target output electrical signal(s) (S e i, S e 2) respectively in a reference error signal (E re f) and a target error signal (E1, E2), so as to keep the optical delay (T) of the interferometer (20) constant and to keep the frequency (foi, fo2) of the target optical signal or signals (Soi, SO2) constant, as a function of the reference error signals (E re f) and target (E1, E2).

2. Device (16) according to claim 1, wherein the control unit (24) is further configured to adjust the amplitude of the acoustic signals (S are f, S a i, Sa2) as a function of the reference error signals (E re f) and target (E1, E2).

3. Device (16) according to any one of claims 1 to 2, wherein the first arm (31) has a predetermined length (L), and wherein the control unit (24) comprises a correction module (46) for the interferometer (20), configured to receive the reference error signal (E re f) and to adjust at least one of the following parameters according to the reference error signal (E re f): the length (L) of the first arm (31), a temperature of the interferometer (20) and a phase of the acoustic signals (Saref, S a i, Sa2) emitted by the acousto-optical filter (26), in order to keep the optical delay (T) of the interferometer (20) constant.

4. Device (16) according to any one of claims 1 to 3, wherein the control unit (24) comprises a correction module (48) for the target light source(s) (11, 12), configured to receive the target error signal(s) (E1, E2) and to adjust at least one of the following parameters as a function of the target error signal(s) (E1, E2): a temperature of the target light source(s) (11, 12), an electrical current intensity supplying the target light source(s) (11, 12), and a voltage of the supplying current to the target light source(s) (11, 12), to maintain the frequency (f o i, f02) of each target optical signal (Soi, S02) constant.

5. Device (16) according to any one of claims 1 to 4, wherein the first arm (31) comprises a waveguide (40), configured so that the reference and target optical signals of the same given order among zero order and first order propagate through the waveguide (40).

6. Device (16) according to any one of claims 1 to 5, wherein the first arm (31) is configured to receive and propagate zero-order reference and target optical signals (S ore f(0), Self(0), S O 2(0)) and the second arm (32) is configured to receive and propagate the first-order reference and target optical signals (S ore f(1), S o i(1 ), S02(1)).

7. Optical system (5) comprising: a reference light source (10), configured to emit a reference optical signal (S ore f); at least one target light source (11, 12), the target light source or sources (11, 12) being configured to emit a target optical signal (Soi, SO2), the target optical signal or signals (Soi, SO2) having a frequency (f oi , fa) of the or each target optical signal (Soi, SO2) distinct from a frequency (f ore f) of the reference optical signal (S ore (f) ; and an optical stability transfer device (16) according to any one of claims 1 to 6, the acousto-optical filter (26) being configured to receive the target optical signal(s) (Soi, SO2) emitted by the target light source(s) (11, 12) and the reference optical signal (S ore f) emitted by the reference light source (10).

8. System (5) according to claim 7, wherein the reference and target light sources (11, 12) are laser sources.

9. System (5) according to any one of claims 7 to 8, comprising a plurality of target light sources (11, 12), preferably more than two target light sources (11, 12), each target light source (11, 12) being configured to emit a target optical signal (Soi, SO2) of frequency (fa, fa) distinct from the other target light sources (11, 12).

10. System (5) according to claim 9, wherein the target optical signal (Soi', SO2) of each target light source is an auxiliary target optical signal (Soi, SO2), each target light source (11, 12) being configured to further emit a main target optical signal (Soi', SO2'), of frequency (fa, fa) identical to the frequency (fa, fa) of the auxiliary target optical signal (Soi, SO2) emitted by the target light source (11, 12), and further comprising an optical combining module (60), configured to combine the main target optical signals (Soi', SO2') into a combined optical signal (S oc ), of frequency (fa) a combined frequency, distinct from the frequencies of the main target optical signals (fa , fa).

11. System (5) according to claim 10, wherein the optical combining module (60) is a frequency summing or subtracting unit.

12. Optical stabilization method, implemented by a device (16) according to any one of claims 1 to 6, the method comprising the following steps: reception (S102) of a reference optical signal (S ore f) and at least one target optical signal (Soi, S02), by the acousto-optical filter (26); emission (S104) of the plurality of acoustic signals (S are f, S ai , S a 2) by the acousto-optical filter (26); diffraction (S106) of each optical signal (Soref, Soi, S02) by the acousto-optic filter (26) to form the zero-order reference optical signals (S ore f(0)) and of order one (Soref(1)), and the target optical signal(s) of order zero (S o i(0),S O 2(0)) and of order one (Self(1),S o2 (1)); reception and propagation (S108) of reference and target optical signals of the same given order among zero-order and first-order signals in the first arm (31) and of reference and target optical signals of the other given order in the second arm (32); combination (S110) of zero-order reference optical signals (S ore f(0)) and a (Soref(1)) in the recombined reference optical signal (Soref(comb)), and zero-order target optical signals (S o i(0),S O 2(0)) and a (S0i(1),S02(1)) in the recombined target optical signal(s) (S o i(comb), S O 2(comb)), by the optical coupler (34); conversion (S112) of the recombined optical reference signal (Soref(comb)) into the electrical output reference signal (S ere f) and of each recombined target optical signal (S0(comb), S02(comb)) into the target electrical output signal(s) (S e i, S e 2) by the photodetector (22); and conversion (S114) of the output reference electrical signals (S ere f) and of each target electrical output signal (S e i, S e 2) respectively in the reference error signal (E re f) and in the target error signal(s) (Ei, E2), so as to keep the optical delay (T) of the interferometer (20) constant and to maintain a frequency (f oi , fa) of each target optical signal (Soi, SO2) constant, as a function of the reference error signals (E re f) and target (E1, E2).

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