In-phase and quadrature demodulation phase modulation optical heterodyne frequency stabilization system with RAM suppression

Abstract

The invention discloses an in-phase and quadrature demodulation phase modulation optical heterodyne frequency stabilization system for suppressing RAM, and relates to the technical field of laser frequency stabilization, comprising a laser, an electro-optic modulator, a circulator, an optical resonant cavity, a photodetector, a phase shifter, a mixer, an adder, a low-pass filter, a signal generator and a signal acquisition and processing module; laser light emitted by the laser enters the circulator after being modulated by the electro-optic modulator, the circulator receives the reflected light of the optical resonant cavity and emits it into the photodetector, and all error signals are obtained through the mixer and the low-pass filter, all error signals are added through the adder, and then input into the signal acquisition and processing module and fed back, so that the frequency of the laser light emitted by the laser is locked to the resonant frequency of the optical resonant cavity; the frequency stabilization system modulates the laser phase in the electro-optic modulator by introducing multiple modulation frequencies, so as to suppress the influence of RAM noise on the frequency stabilization.

Description

Technical Field

[0001] The invention relates to the technical field of laser frequency stabilization, and in particular to an in-phase, quadrature demodulation phase modulation optical heterodyne frequency stabilization system for suppressing RAM. Background Art

[0002] Through a variety of frequency stabilization technologies, the current best laser frequency stability has reached The limiting factor is mainly the thermal noise of the reference cavity. To achieve a higher stability level, it is necessary to reduce thermal noise through advanced reference cavity technology and further suppress the frequency noise caused by RAM.

[0003] RAM is the abbreviation of Residual Amplitude Modulation. RAM is the main source of frequency noise in the ultra-stable laser PDH (Pound-Drever-Hall) frequency stabilization. There are many reasons for its generation, including the misalignment of laser polarization and the main axis direction of the electro-optic modulator, temperature disturbance of the electro-optic modulator (EOM), parasitic standard tool effect, the spot size received by the photodetector, the angle of the incident laser, vibration, RF source noise, and the uniformity of the electric field strength in the electro-optic crystal.

[0004] RAM noise affects the stability and accuracy of laser frequency locking. Currently, many technologies have been proposed for RAM suppression. For spatial EOM, the birefringence light of EOM can be separated spatially by designing the shape of the electro-optical crystal to avoid birefringence RAM in which the laser polarization does not coincide with the main axis direction of the electro-optical modulator; for fiber EOM, in addition to the conventional careful adjustment of the optical path, making the optical device more ideal, vibration isolation, and selecting the appropriate crystal temperature operating point, it is also proposed to control the DC bias and temperature of the EOM operating point and use multi-frequency modulation for RAM suppression; other devices such as AOM and spatial light modulator can also be used instead of EOM to modulate the laser.

[0005] The scheme of controlling the DC bias and temperature of the EOM enhances the dynamic compensation capability of the system, but adds a servo loop to the system, making the system more complex and increasing the instability factor. At the same time, for the RAM active suppression scheme, if there are optical fiber devices such as polarization-maintaining fiber, fiber coupler, circulator, etc. with polarization-related losses and birefringence between the different optical paths corresponding to the RAM detection in front of the optical resonator and the PDH error signal generation, the RAM detected in front of the cavity is not proportional to the RAM at the error signal generation position when the polarizer extinction ratio is limited, affecting the RAM active suppression effect. At this time, the error signal feedback of orthogonal demodulation can be used for RAM suppression.

[0006] For multi-frequency modulation schemes, the laser can be dual-frequency modulated using frequencies Ω and 2Ω at the same time. The phase and amplitude of the modulation frequency can be appropriately adjusted to weaken the RAM of the photocurrent with a frequency of 2Ω. Three adjacent modulation frequencies with equal frequency differences can also be used for modulation. The amplitude and phase relationship of the traditional PDH carrier and sideband can be simulated at the sideband on one side of the carrier frequency. The laser can be locked on the sideband, the phase difference between the modulation signals can be adjusted, and the RAM at the difference frequency of adjacent modulation frequencies can be suppressed. The three frequencies can be modulated simultaneously using AOM to simulate the amplitude and phase relationship of the carrier, positive and negative first-order sidebands obtained by single-frequency modulation using EOM, and RAM can be suppressed by feeding back the amplitude and phase of the high-frequency modulation signal.

[0007] The dual-frequency modulation scheme suppresses the RAM at the original modulation frequency by adding a modulation signal with half the original modulation frequency, but it has high requirements on the amplitude and phase relationship of the dual-frequency modulation, and the error signal obtained is still demodulated at a single frequency. In the EOM's triple-frequency modulation scheme, the degree of freedom of modulation is increased, and the size and phase of the three modulation frequencies can be independently controlled, but the sideband locking inevitably reduces the sensitivity of the error signal, does not suppress the light intensity noise and servo noise, and does not naturally expand more modulation frequencies. Summary of the invention

[0008] Based on the technical problems existing in the background technology, the present invention proposes an in-phase and orthogonal demodulation phase modulation optical heterodyne frequency stabilization system that suppresses RAM, thereby suppressing the influence of RAM noise on frequency stabilization.

[0009] The in-phase and quadrature demodulation phase modulation optical heterodyne frequency stabilization system for suppressing RAM proposed by the present invention comprises a laser, an electro-optical modulator, a circulator, an optical resonant cavity, a photodetector, a phase shifter, a mixer, a low-pass filter and a signal acquisition and processing module;

[0010] The electro-optic modulator acquires a modulation signal to phase-modulate the laser light emitted by the acquired laser, and transmits the modulated laser light to the circulator;

[0011] The circulator transmits the transmitted light into the optical resonant cavity to obtain the laser light reflected by the optical resonant cavity;

[0012] The photodetector receives the reflected light emitted by the circulator, obtains a voltage signal proportional to the power of the reflected light, divides the voltage signal into N parts, and transmits them to N mixers respectively;

[0013] The M fast modulation frequencies and NM slow modulation frequencies emitted by N signal generators are respectively input into N mixers through N phase shifters, the electrical signals output by the N mixers are respectively output as error signals through N low-pass filters, and the N error signals are input into a signal acquisition and processing module after passing through an adder to control a laser or an optical resonant cavity so that the laser emitted by the laser is locked to the resonant frequency of the optical resonant cavity.

[0014] Furthermore, it also includes a signal generator and an adder, N signal generators send out fast modulation signals and slow modulation signals with different modulation frequencies, and the fast modulation signals and the slow modulation signals are passed through the adder to obtain a modulation signal.

[0015] Furthermore, N phase shifters, N mixers, N signal generators, and N low-pass filters are arranged in one-to-one correspondence, and N is greater than or equal to 2.

[0016] Furthermore, when N is equal to 2, the two signal generators respectively emit a slow modulation frequency and a fast modulation frequency, the slow modulation frequency is half of the optical resonant cavity linewidth, the fast modulation frequency is more than one order of magnitude greater than the optical resonant cavity linewidth, and the slow modulation frequency is greater than the feedback bandwidth of the fast modulation frequency.

[0017] Furthermore, when N is equal to 2, the electric field intensity of the modulated laser emitted by the electro-optic modulator is The calculation is as follows:

[0018]

[0019] , , ;

[0020] , ;

[0021] in, is the laser electric field amplitude, is the carrier frequency of the laser emitted by the laser, are the slow modulation and fast modulation depths of o light, They are the slow modulation depth and fast modulation depth of e-light, are the slow modulation frequency and the fast modulation frequency respectively, represents the first process parameter, represents the second process parameter, is an imaginary unit, For time, represents the disturbance signal, i.e., the birefringence phase difference, represents the third process parameter, represents the fourth process parameter, represents the angle between the incident laser polarization and the EOM optical axis, represents the angle between the outgoing laser polarization and the EOM optical axis, represents the phase difference of light entering and exiting the birefringent region of the electro-optic modulator (2), Represents the phase difference of e-light entering and exiting the birefringence region of the electro-optic modulator (2).

[0022] Furthermore, the electric field strength The formula is expanded by Bessel, and the frequency of the laser after modulation is obtained as follows: , is the laser frequency, is an integer laser modulated sideband signal.

[0023] Furthermore, when M is greater than 2, the fast modulation frequency satisfies formula (4) or formula (5). When formula (5) is satisfied, all The value of contains an odd number of odd numbers:

[0024] (4);

[0025] (5);

[0026] in, is the total number of modulation frequencies, They are modulation frequency numbers. Indicates indivual The value of Respectively The modulation frequency and The modulation frequency, is any symbol, For the existence symbol.

[0027] Furthermore, when the modulation depth is adjusted by laser beat frequency, the in-phase and quadrature demodulation phase modulation optical heterodyne frequency stabilization system performs the following steps:

[0028] The laser light emitted by the laser is split into two, one laser light is used to obtain the error signal, and the other laser light is used for the beat frequency;

[0029] The electro-optic modulator is heated or cooled, and the laser frequency is locked by error signal feedback. The direction of change of the laser beat frequency after locking is observed, and the demodulation phase of the slow modulation frequency is adjusted by the phase shifter so that the direction of change of the beat frequency when the slow modulation frequency locking is performed alone is opposite to that when the fast modulation frequency locking is performed alone, and the peak-to-peak value of the frequency fluctuation is maximized;

[0030] Then the adder simultaneously inputs the fast modulation signal and the slow modulation signal into the electro-optic modulator, and simultaneously performs fast and slow modulation on the laser. The adder is used to sum all the error signals and then perform feedback frequency locking, adjust the modulation depth of the fast modulation and the slow modulation, minimize the beat frequency change, and select the laser intensity.

[0031] Furthermore, when the laser does not adjust the modulation depth by laser beat frequency, the in-phase and quadrature demodulation phase modulated optical heterodyne frequency stabilization system performs the following steps:

[0032] The slow modulation frequency is used as the first slow modulation frequency, and another slow modulation frequency that is mutually prime with the first slow modulation frequency is selected as the second slow modulation frequency. The laser is modulated by two slow modulation signals and one fast modulation signal at the same time, and demodulated respectively to obtain the first slow modulation error signal, the second slow modulation error signal and the fast modulation error signal, and the discrimination slopes of the three error signals are measured. ;

[0033] Heating or cooling the EO modulator and measuring the RAM amplitude in the first slow modulation error signal when locked using only the fast modulation error signal , measure the RAM amplitude in the first slow modulation error signal when the fast modulation error signal and the second slow modulation error signal are summed and locked through the combiner ;

[0034] Measuring the RAM amplitude in a second, slower modulating error signal while locked with a faster modulating error signal ;

[0035] Calculate the RAM voltage from the three error signals ;

[0036] By changing the demodulation phase of the first slow modulation by the phase shifter, The positive and negative signs of On the contrary, The positive and negative signs of same;

[0037] Change the modulation voltage of the first slow modulation frequency or fast modulation frequency to make and Equal in size, and finally turn off the second slow modulation frequency.

[0038] Furthermore, the RAM voltage The calculation formula is as follows:

[0039] ;

[0040] ;

[0041] ;

[0042] in, They are the RAM voltages in the first slow modulation error signal, the second slow modulation error signal, and the fast modulation error signal, respectively. is the gain of the combiner.

[0043] The advantages of the in-phase and quadrature demodulation phase modulation optical heterodyne frequency stabilization system for suppressing RAM provided by the present invention are: by introducing other modulation frequencies to modulate the laser together, different modulations share the same spatial optical path, and suppress the influence of RAM noise in the fast modulation error signal on the frequency stabilization. The introduced modulation frequency signal and the original modulation frequency signal are summed through an adder and then input into the electro-optic modulator, and laser modulation is actually performed in the EOM, thereby suppressing the influence of RAM noise on the frequency stabilization. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 It is a structural schematic diagram of the present invention;

[0045] Figure 2 RAM in the error signal when the laser is locked to the optical resonator for simultaneous slow and fast modulation; the slow modulation frequency is half the line width of the resonator, curve (1) is the RAM of slow modulation in-phase demodulation, and curve (3) is the RAM of slow modulation quadrature demodulation; the fast modulation frequency is 11 times the line width of the optical resonator, curve (2) is the RAM of fast modulation in-phase demodulation, and the amplitude is normalized to the maximum value of curve (2);

[0046] Among them, 1-laser, 2-electro-optic modulator, 3-circulator, 4-optical resonator, 5-photodetector, 6-phase shifter, 7-mixer, 8-low-pass filter, 9-signal acquisition and processing module, 10-adder, 11-signal generator. DETAILED DESCRIPTION

[0047] Below, the technical solution of the present invention is described in detail through specific embodiments. Many specific details are set forth in the following description to facilitate a full understanding of the present invention. However, the present invention can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific implementation disclosed below.

[0048] like Figure 1 and 2As shown, the in-phase, quadrature demodulation phase modulation optical heterodyne frequency stabilization system for suppressing RAM proposed by the present invention includes a laser 1, an electro-optical modulator 2, a circulator 3, an optical resonant cavity 4, a photodetector 5, a phase shifter 6, a mixer 7, a low-pass filter 8, a signal acquisition and processing module 9, an adder 10 and a signal generator 11; N phase shifters 6, N mixers 7, N signal generators 11, and N low-pass filters 8 are arranged in a one-to-one correspondence, and N is greater than or equal to 2;

[0049] The electro-optic modulator 2 receives the modulation signal output by the adder 10 to phase-modulate the laser light emitted by the acquired laser 1, and transmits the modulated laser light to the circulator 3. The modulation signal is obtained by the fast modulation signal and the slow modulation signal through the adder 10. The fast modulation signal and the slow modulation signal are respectively emitted by N signal generators 11.

[0050] The circulator 3 transmits the transmitted light to the optical resonant cavity 4 to obtain the laser light reflected by the optical resonant cavity 4;

[0051] The photodetector 5 receives the reflected light emitted by the circulator 3 and obtains a power proportional to the reflected light power. The voltage signal , the voltage signal Divided into N parts, and respectively transmitted to N mixers 7;

[0052] The M fast modulation signals and (NM) slow modulation signals emitted by the N signal generators 11 are respectively input into the N mixers 7 through the N phase shifters 6, the electrical signals output by the N mixers 7 are respectively output as error signals through the N low-pass filters 8, the N error signals are input into the signal acquisition and processing module 9 after passing through the adder to control the laser 1 or the optical resonant cavity 4, so that the laser emitted by the laser 1 is locked to the resonant frequency of the optical resonant cavity 4.

[0053] The existing method is to modulate the laser by a single frequency modulation signal. This embodiment introduces other modulation frequencies to modulate the laser together, thereby suppressing the influence of RAM noise in the fast modulation error signal on the frequency stabilization. The introduced modulation frequency signal is summed with the original modulation frequency signal through an adder and then input into an electro-optic modulator (EOM), where laser modulation is actually performed. The following describes the in-phase and orthogonal demodulation phase modulation optical heterodyne frequency stabilization system with N equal to 2. When N is greater than 2, the introduced modulation frequencies can be set and connected in sequence.

[0054] When N is equal to 2, the laser light emitted by the laser 1 enters the electro-optic modulator 2 through the incident isolator, the fast modulation signal emitted by a signal generator 11 and the slow modulation signal emitted by another signal generator 11 are summed by the adder and enter the RF end of the electro-optic modulator 2, the laser light is modulated by the two modulation frequency signals and then transmitted to the circulator 3, the transmitted light of the circulator 3 is incident on the optical resonant cavity 4, the circulator 3 receives the laser light reflected by the optical resonant cavity 4 through the reflection effect of the optical resonant cavity 4, and the laser light is incident on the photodetector 5, thereby obtaining a power proportional to the reflected light power. The voltage signal , the voltage signal It is divided into two parts and respectively transmitted to the RF ends of the two mixers 7 for demodulation. At the same time, the fast modulation frequency emitted by one signal generator 11 and the slow modulation frequency emitted by the other signal generator 11 are respectively transmitted to the LO ends of the corresponding mixers 7 through their own phase shifters 6 to demodulate the voltage signal. The IF end of one mixer 7 obtains a slow modulation error signal, and the IF end of the other mixer 7 obtains a fast modulation error signal. The slow modulation error signal and the fast modulation error signal are transmitted to the signal acquisition and processing module 9 after passing through an adder to control the output of the laser 1, so that the laser emitted by the laser 1 is locked to the resonance frequency of the optical resonant cavity 4.

[0055] The laser frequency can be locked to the resonance peak of the resonant cavity by the feedback of the PDH fast modulation error signal alone, but the locking process will introduce a large RAM noise. The main RAM noise comes from the misalignment of the laser polarization and the EOM main axis direction, the EOM temperature change, the parasitic standard in the optical path, etc. The first two affect the phase of the laser modulation sideband, and the last one affects the phase and amplitude of the laser modulation sideband. Based on the PDH fast modulation technology, this embodiment generates another modulation error signal to compensate for the birefringence phase difference change caused by the misalignment of the laser polarization direction and the main axis of the electro-optical modulator 2, the temperature change of the electro-optical modulator 2, the electric field inhomogeneity in the electro-optical modulator 2, vibration, etc., so as to perform RAM suppression.

[0056] In this embodiment, a slow modulation frequency is introduced to obtain a slow modulation error signal, that is, a fast modulation error signal and a slow modulation error signal are used to simultaneously modulate the laser at two frequencies in the electro-optical modulator 2. Slow modulation frequency Half the line width of the optical resonator 4, the fast modulation frequency It is more than one order of magnitude larger than the line width of the optical resonant cavity 4; the slow modulation frequency is half of the line width of the optical resonant cavity 4 in order to maximize the peak-to-peak value of the slow modulation error signal. The slow modulation frequency can be adjusted according to actual conditions to suppress the influence of RAM noise in the fast modulation error signal on frequency stabilization.

[0057] Using two modulation frequencies simultaneously Modulation is performed, considering that the main axis directions of the laser polarization EOM do not completely coincide, and the electric field intensity of the laser after modulation for:

[0058] (1);

[0059] , , ; , (2);

[0060] in, is the laser electric field amplitude, is the carrier frequency of the laser emitted by laser 1, are the slow modulation and fast modulation depths of o light, They are the slow modulation depth and fast modulation depth of e-light, are the slow modulation frequency and the fast modulation frequency respectively, represents the first process parameter, represents the second process parameter, is an imaginary unit, For time, represents the disturbance signal, i.e., the birefringence phase difference, represents the third process parameter, represents the fourth process parameter, represents the angle between the incident laser polarization and the EOM optical axis, represents the angle between the outgoing laser polarization and the EOM optical axis, represents the phase difference of light entering and leaving the birefringence region of electro-optic modulator 2, Represents the phase difference of e-light entering and exiting the birefringence region of electro-optic modulator 2.

[0061] Formula (1) can be expanded by Bessel to obtain the modulated laser frequency: , is an integer laser modulated sideband signal, and its complex amplitude is .

[0062] The electro-optic modulator 2 transmits the modulated laser to the circulator 3, and the electric field of the reflected light of the optical resonator 4 is ,in is the optical resonator 4 at the laser frequency The reflected light power of the laser about the optical resonant cavity 4 is detected by the photodetector 5 (i.e. light intensity) as in formula (3), retaining the first-order modulation sideband of each modulation frequency:

[0063] (3);

[0064] The structure of formula (3) is as follows:

[0065]

[0066] in, for The complex conjugate of is used to calculate the power of the reflected light, because the power is proportional to the square of the modulus of the electric field, To take the real part of a complex number, and The laser phase changes with time, After modulation Slow modulation, The laser sideband amplitude of the fast modulation is , , , , , , wait; for The complex conjugate of , , , , , , .

[0067] The photodetector 5 detects the reflected light power , to convert into a voltage signal , and then use the demodulation frequency signal with the same modulation frequency and the split voltage signal By mixing and low-pass filtering, and selecting the appropriate demodulation signal phase, the corresponding error signal can be obtained, that is, the photodetector 5 obtains a signal proportional to the reflected light power. The voltage signal , the voltage signal It is divided into two parts and respectively transmitted to two mixers 7 for demodulation; the fast modulation frequency emitted by the first signal generator 11 passes through the first phase shifter 6 and then enters the first mixer 7, and the slow modulation frequency emitted by the second signal generator 11 passes through the second phase shifter 6 and then enters the second mixer 7. The first mixer 7 demodulates the fast modulation signal after phase shifting to obtain a fast modulation error signal, and the second mixer 7 demodulates the slow modulation signal after phase shifting to obtain a slow modulation error signal.

[0068] The laser sidebands generated by slow modulation and fast modulation share the same spatial optical path. In different modulations, non-ideal factors such as laser polarization, EOM temperature, and EOM internal electric field inhomogeneity affect the angles in multiple modulations. ,angle , Birefringence phase difference The influence is the same and can be suppressed by this method. At the same time, for the traditional RAM active suppression scheme, the different optical paths corresponding to the pre-cavity RAM detection and the PDH (quadrature demodulation phase modulated optical heterodyne) error signal generation, due to the polarization-related losses and birefringence of optical fiber devices such as polarization-maintaining fiber, fiber coupler, circulator, etc., lead to the polarization extinction ratio of the polarizer being finite. The RAM of the pre-cavity detection is not proportional to the RAM at the error signal generation position, which affects the RAM active suppression effect. Since fast modulation and slow modulation share the same spatial optical path, this situation does not affect the locking of the fast modulation and slow modulation schemes. When the laser is simultaneously fast modulated and slow modulated in the electro-optic modulator 2, it is equivalent to the presence of a carrier of , The fast modulation and carrier are , Slow modulation, where the carrier The error signal is close to zero and is far detuned from the optical resonator 4. It is ignored in formula (3). or The demodulation only obtains the carrier The error signal; carrier Near resonance with the optical resonator 4, its fast modulation error signal is not zero, using The demodulated carrier is , This increases the linear range of the error signal while having little effect on the sensitivity of the error signal.

[0069] In the above demodulation process, while the error signal is obtained, there is also a RAM signal, which comes from the non-ideal amplitude or phase of the same-order high-frequency and low-frequency sidebands of the laser modulation. The laser polarization direction and EOM temperature will affect the formula (2) , which is proportional to The RAM signal in both the slow modulation and fast modulation error signals has a RAM effect proportional to .

[0070] like Figure 2 , without considering the effect of parasitic etalon on the amplitude of laser modulation sideband, due to the fast modulation frequency The reflectivity of the optical resonant cavity 4 is approximately one order of magnitude larger than the line width of the optical resonant cavity 4 at the modulation sideband frequency. Therefore, when the demodulated signal and the modulated signal are in phase, the demodulated signal obtains an error signal with the largest peak-to-peak value and a RAM signal with the largest peak-to-peak value. When the demodulated signal and the modulated signal are orthogonal in phase, the demodulated error signal and RAM signal are both zero.

[0071] The slow modulation frequency It is half of the line width of the optical resonator 4. The reflectivity of the optical resonator 4 has both real and imaginary parts at the modulation sideband frequency. The magnitude of the real and imaginary parts is affected by the optical carrier frequency. and slow modulation frequency Therefore, when the demodulated signal is in phase or orthogonal to the modulated signal, the demodulated error signal and RAM signal are not zero. By selecting a suitable demodulation phase, The slow modulation and demodulation RAM signal has opposite positive and negative signs to the fast modulation and demodulation RAM signal. By selecting appropriate slow modulation and fast modulation depth, the slow modulation and demodulation RAM signal amplitudes are equal to the fast modulation and demodulation RAM signal amplitudes. Under disturbance, the influence of RAM signal on frequency stabilization can be suppressed by summing the error signals (fast modulation error signal and slow modulation error signal) through an adder.

[0072] For slow modulation, the phase of the demodulation signal can also be changed, and the photodetector 5 can be used to detect the intensity signal of the transmitted light of the optical resonant cavity 4 for demodulation.

[0073] The suppression of RAM by this method depends on the refractive index of the electro-optic modulator 2 , , electro-optic coefficient, modulation signal voltage, phase accuracy and stability. At the same time, this method can also perform a second suppression of RMA based on other frequency stabilization schemes, where are the refractive indices for e-light and o-light respectively.

[0074] The selection of modulation depth depends on the intensity of the birefringence effect of the electro-optic modulator 2. The selection is not unique. The RAM suppression effect depends on the accuracy and stability of the modulation signal voltage, the modulation signal phase, and the EOM electro-optic coefficient.

[0075] In order to prevent the PDH fast modulation from affecting the slow modulation, the slow modulation frequency needs to be greater than the feedback bandwidth of the fast modulation, which is usually in the MHz order. The condition for generating the PDH slow modulation error signal is that the modulation frequency is in the same order of magnitude as the line width of the optical resonant cavity 4. Therefore, this embodiment can be directly applied to optical resonant cavities with a line width in the MHz order, including multi-wavelength cavities, fiber ring cavities, etc.; and if the laser has been pre-stabilized by another optical resonant cavity, the laser line width is narrow and can be stabilized with a lower feedback bandwidth. In this case, this embodiment can also be applied to fast and slow modulation stabilization.

[0076] The above description is made in detail for N being equal to 2. This embodiment is still applicable to multiple fast feedbacks corresponding to M being greater than 2. In the multiple fast feedbacks, the multiple signal generators 11 provided will respectively send out at least one fast modulation frequency and at least one slow modulation frequency. If there are multiple fast modulation frequencies, the selection of the fast modulation frequency satisfies formula (4) or formula (5). are modulation frequency numbers, that is, integer laser modulation sideband signals, is the total number of modulation frequencies. When formula (5) is satisfied, all The value of contains an odd number of odd numbers. When the fast feedback modulation depth is equal, the error signal of PDH fast feedback is proportional to , has nothing to do with the modulation frequency. Indicates plural The imaginary part of Indicates the frequency The reflection coefficient of the laser incident on the resonant cavity. As long as it is much larger than the line width of the optical resonant cavity 4, almost the same error signal can be obtained using different modulation frequencies.

[0077] (4);

[0078] (5);

[0079] in, is the total number of modulation frequencies, They are modulation frequency numbers. Indicates indivual According to steps (4) and (5), from Take the value in Respectively The modulation frequency and The modulation frequency, is any symbol, For the existence symbol.

[0080] The specific implementation process of this embodiment is as follows:

[0081] First, according to Figure 1 Build an optical path, adjust the surface direction of adjacent devices in the optical path, and suppress the RAM caused by parasitic standard tools. Use the demodulation signal phase that maximizes the peak-to-peak value of the fast modulation error signal plus π / 2 to demodulate the output of the photodetector 5. If the demodulation result is close to zero, it is judged that the parasitic standard tool effect in the system is weak. Select half of the line width of the optical resonator 4 and a frequency value that is more than one order of magnitude greater than the line width of the optical resonator 4 as the modulation frequency of the input electro-optic modulator 2. First, the laser is fast modulated and slow modulated by the electro-optic modulator 2, respectively, and the modulation depth of each modulation frequency is set to be equal. The slow modulation signal is used to demodulate the reflection signal or transmission signal of the laser relative to the optical resonator 4, and the fast modulation signal is used to demodulate the reflection signal of the laser relative to the optical resonator 4. The signal acquisition and processing module 9 is used to control the laser 1 or the optical resonator 4 to perform triangular wave scanning. The cavity length of the optical resonant cavity 4 can be controlled by changing the temperature of the optical resonant cavity 4 through a piezoelectric ceramic (PZT) installed on a cavity mirror or a thermoelectric cooler (TEC) installed on the cavity. The phases of the slow modulation and demodulation signals and the fast modulation and demodulation signals are adjusted respectively to obtain a slow modulation error signal and a fast modulation error signal with the same polarity and the maximum sensitivity.

[0082] When the modulation depth is adjusted by laser beat frequency, the laser light emitted by the laser 1 is divided into two, one laser is used to obtain the error signal, and the other laser is used for the beat frequency; the electro-optic modulator 2 is heated or cooled, and the direction of change of the laser beat frequency after frequency locking is observed. The demodulation phase of the slow modulation signal is adjusted by the phase shifter 6 so that the direction of the beat frequency change when the slow modulation is locked alone is opposite to that when the fast modulation is locked alone, and the peak-to-peak value of the frequency fluctuation is the largest. Then the laser is modulated fast and slow at the same time, and the modulation depth of the fast modulation and slow modulation is adjusted to minimize the change of the beat frequency. The two error signals are summed by an adder for feedback frequency locking, and the appropriate laser light intensity is selected.

[0083] The laser beat frequency system can perform laser beat frequency based on the existing method.

[0084] When the modulation depth is not adjusted by laser beat frequency, the above slow modulation frequency is used as the first slow modulation frequency, and another slow modulation frequency that is coprime with the first slow modulation frequency is selected as the second slow modulation frequency. The laser is modulated by two slow modulation frequencies and one fast modulation frequency at the same time, and demodulated respectively to obtain the first slow modulation error signal, the second slow modulation error signal and the fast modulation error signal, and the demodulation slopes of the three error signals are measured. ; Heat or cool the electro-optic modulator 2 and measure the RAM amplitude in the first slow modulation error signal when locked using the fast modulation error signal alone , measure the RAM amplitude in the first slow modulation error signal when the fast modulation error signal and the second slow modulation error signal are summed and locked through the combiner ;Measure the RAM amplitude in the second slow modulation error signal when locked using the fast modulation error signal alone ;Measure the RAM voltage in three error signals ; The demodulation phase of the first slow modulation frequency is changed by the phase shifter 6, so that The positive and negative signs of On the contrary, The positive and negative signs of Same; change the modulation voltage of the first slow modulation frequency or fast modulation frequency to make and Equal in size, and finally turn off the second slow modulation frequency.

[0085] RAM Voltage The calculation formula is as follows:

[0086] (6);

[0087] (7);

[0088] (8);

[0089] in, They are the RAM voltages in the first slow modulation error signal, the second slow modulation error signal, and the fast modulation error signal, respectively. is the gain of the combiner.

[0090] Embodiment 1

[0091] When using an optical resonator 4 (line width of about 24MHz, free spectrum range of about 2.48GHz) with a cavity length of 4.04cm, a refractive index of 1.5, and a reflection coefficient R of 0.97 to lock a laser with a wavelength of 780nm, first adjust the surface of the adjacent device in the optical path to suppress the RAM caused by the parasitic standard. The power of the modulated laser before entering the optical resonator 4 can be detected, and the fast modulation signal with the phase of the demodulation signal with the maximum peak-to-peak value of the fast modulation error signal plus π / 2 is used for demodulation. If the demodulation result is close to zero, it is judged that the parasitic standard effect in the system is weak, and it is judged that the system noise mainly comes from the spectrum line broadening caused by the spontaneous radiation of the laser, the thermal noise of the optical resonator 4, the cavity length drift of the optical resonator 4, the electronic detection noise of the photoelectric tube, etc. The PDH slow modulation frequency is selected to be 12MHz, which is half of the line width of the optical resonator 4, and the fast modulation frequency is ten times the line width of the optical resonator 4 (240MHz). At the same time, it is ensured that the optical power of the laser entering the optical resonator 4 is less than the damage threshold of the optical resonator 4.

[0092] First, connect each device according to the specific implementation method, use the signal generator 11 to generate a modulation signal of each frequency under the same external frequency reference, sum it through the adder and input it into the electro-optical modulator 2 to modulate the laser and demodulate it respectively, scan the frequency of the laser or the resonant frequency of the optical resonator 4, adjust the phase of the phase shifter 6, make the slow modulation and fast modulation error signals have the same polarity and maximum sensitivity, heat or cool the electro-optical modulator 2, observe the change direction of the laser beat frequency after the single fast modulation and the single slow modulation are locked, select the phase of the slow modulation demodulation signal, make the laser beat frequency change direction when the slow modulation is locked (corresponding to the laser locking), and the signal peak-to-peak value is the largest, and select the appropriate laser light intensity. Use the adder to sum the two error signals and transmit them to the input signal acquisition and processing module 9 to feed back to the laser 1 or the optical resonator 4, then change the amplitude of the two modulation signals, make the RAM amplitudes in the slow modulation error signal and the fast modulation error signal equal and opposite, and select the appropriate laser light intensity.

[0093] Embodiment 2

[0094] When using an optical resonant cavity 4 (line width of about 24 MHz, free spectrum range of about 2.48 GHz) with a cavity length of 4.04 cm, a refractive index of 1.5, and a reflection coefficient R of 0.97 to lock a laser with a wavelength of 780 nm, first adjust the surface of the adjacent devices in the optical path to suppress the RAM caused by the parasitic standard tool. The power of the modulated laser before entering the optical resonant cavity 4 can be detected, and the fast modulation signal with the phase of the demodulation signal with the peak-to-peak value of the fast modulation error signal maximized plus π / 2 is used for demodulation. If the demodulation result is close to zero, it is judged that the parasitic standard tool effect in the system is weak, and it is judged that the system noise mainly comes from the spectral line broadening caused by spontaneous radiation of the laser, thermal noise of the optical resonant cavity 4, cavity length drift of the optical resonant cavity 4, and electronic detection noise such as photoelectric tubes. The PDH slow modulation frequency is selected as 12 MHz, which is half of the line width of optical resonator 4. The fast modulation frequency is ten times the line width of optical resonator 4 (240 MHz). The original slow modulation frequency is used as the first slow modulation frequency. In addition, the second slow modulation frequency is selected, and at the same time, the optical power of the laser entering the optical resonator 4 is ensured to be less than the damage threshold of the optical resonator 4.

[0095] First, connect each device according to the specific implementation method, use the signal generator 11 to generate the modulation signal of each frequency under the same external frequency reference, select the second slow modulation frequency 17MHz, and select a smaller modulation depth of 0.1rad. The signals of the three modulation frequencies (such as the above-mentioned fast modulation frequency, the first slow modulation frequency, and the second slow modulation frequency) are summed by an adder and input into the electro-optical modulator 2 to modulate the laser and demodulate them respectively, scan the frequency of the laser 1 or the resonant frequency of the optical resonator 4, adjust the phase of the phase shifter 6, so that the first slow modulation error signal and the fast modulation error signal have the same polarity and maximum sensitivity, and measure the frequency discrimination slope of the three error signals. Heat or cool the EO modulator 2 and measure the RAM amplitude of the first slow modulation error signal when locked using the fast modulation error signal alone. , measure the RAM amplitude in the first slow modulation error signal when the fast modulation error signal and the second slow modulation error signal are summed and locked through the combiner ;Measure the RAM amplitude in the second slow modulation error signal when locked with the fast modulation error signal , calculate the RAM voltage from the three error signals ; The demodulation phase of the first slow modulation frequency is changed by the phase shifter 6, so that The positive and negative signs of On the contrary, The positive and negative signs of Same; change the modulation voltage of the first slow modulation frequency or fast modulation frequency to make and Equal in size, and finally turn off the second slow modulation frequency.

[0096] RAM Voltage The calculation formula is as follows:

[0097] (9);

[0098] (10);

[0099] (11);

[0100] in, They are the RAM voltages in the first slow modulation error signal, the second slow modulation error signal, and the fast modulation error signal, respectively. is the gain of the combiner.

[0101] The first slow modulation error signal and the fast modulation error signal are summed by an adder and transmitted to the signal acquisition and processing module 9 and fed back to the laser 1 or the optical resonant cavity 4. Then, the amplitudes of the first slow modulation frequency and the fast modulation frequency are changed so that the RAM amplitudes in the slow modulation error signal and the fast modulation error signal are equal and opposite in sign, and a suitable laser light intensity is selected.

[0102] Embodiment 3

[0103] When using an optical resonator 4 (line width of about 24MHz, free spectrum range of about 2.48GHz) with a cavity length of 4.04cm, a refractive index of 1.5, and a reflection coefficient R of 0.97 to lock a laser with a wavelength of 780nm, first adjust the surface of the adjacent device in the optical path to suppress the RAM caused by the parasitic standard tool. The power of the modulated laser before entering the optical resonator 4 can be detected, and the fast modulation signal with the phase of the demodulation signal that makes the peak-to-peak value of the fast modulation error signal maximum plus π / 2 is used for demodulation. If the demodulation result is close to zero, it is judged that the parasitic standard tool effect in the system is weak, and it is judged that the system noise mainly comes from the spectrum broadening caused by the spontaneous radiation of the laser, the thermal noise of the optical resonator 4, the cavity length drift of the optical resonator 4, the electronic detection noise of the photoelectric tube, etc. The PDH slow modulation frequency is selected to be 12MHz, which is half of the line width of the optical resonator 4, and the fast modulation frequency is 231MHz, 240MHz, and 253MHz, which are about ten times the line width of the optical resonator 4. At the same time, it is ensured that the optical power of the laser entering the optical resonator 4 is less than the damage threshold of the optical resonator 4.

[0104] First, various devices are connected according to the specific implementation method. Under the same external frequency reference, a signal generator 11 is used to generate a signal of each modulation frequency. After summing through an adder, the signals are input into the electro-optical modulator 2 to modulate the laser and demodulate them respectively. The frequency of the laser or the resonance frequency of the optical resonator 4 is scanned, and the phase of the phase shifter 6 is adjusted so that the slow modulation and fast modulation error signals have the same polarity and maximum sensitivity. The electro-optical modulator 2 is heated or cooled, and the direction of change of the laser beat frequency after locking with fast modulation alone and slow modulation alone is observed. The phase of the slow modulation demodulation signal is selected so that when slow modulation locking is performed alone (corresponding to laser locking), the direction of change of the laser beat frequency is opposite to that when fast modulation locking is used alone (corresponding to laser locking), and the peak-to-peak value of the signal is maximized, and a suitable laser light intensity is selected.

[0105] The above three embodiments can use analog devices such as mixer 7, low-pass filter 8, phase shifter 6, adder, etc. to demodulate the frequency error signal, and then use analog servo control circuit for feedback; or a full digital solution can be used to digitally synthesize the required modulation signal in a field programmable gate array (FPGA), and after being converted into an analog signal by a high-speed DAC, the laser is modulated; the signal reflected from the optical resonant cavity 4 is detected by the photodetector 5, converted into a digital signal by a high-speed ADC, and sent to the FPGA for digital demodulation to obtain a modulation frequency error signal, and after performing servo operations such as PID, the feedback signal is output by the DAC (digital-to-analog converter) to adjust the laser frequency.

[0106] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. RAM-suppressing in-phase and quadrature demodulation phase-modulated optical heterodyne frequency stabilization system, characterized in that: It includes laser, electro-optic modulator, circulator, optical resonator, photodetector, phase shifter, mixer, low-pass filter and signal acquisition and processing module; The electro-optic modulator acquires a modulation signal to phase-modulate the laser light emitted by the acquired laser, and transmits the modulated laser light to the circulator; The circulator transmits the transmitted light into the optical resonant cavity to obtain the laser light reflected by the optical resonant cavity; The photodetector receives the reflected light emitted by the circulator, obtains a voltage signal proportional to the power of the reflected light, divides the voltage signal into N parts, and transmits them to N mixers respectively; The M fast modulation frequencies and NM slow modulation frequencies emitted by N signal generators are respectively input into N mixers through N phase shifters, the electrical signals output by the N mixers are respectively output as error signals through N low-pass filters, and the N error signals are input into a signal acquisition and processing module after passing through an adder to control a laser or an optical resonant cavity so that the laser emitted by the laser is locked to the resonant frequency of the optical resonant cavity.

2. The in-phase and quadrature demodulation phase modulation optical heterodyne frequency stabilization system with RAM suppression according to claim 1 is characterized in that: It also includes a signal generator and an adder. N signal generators send fast modulation signals and slow modulation signals with different modulation frequencies. The fast modulation signals and the slow modulation signals are passed through the adder to obtain a modulation signal.

3. The in-phase and quadrature demodulation phase modulation optical heterodyne frequency stabilization system with RAM suppression according to claim 1, characterized in that: N phase shifters, N mixers, N signal generators, and N low-pass filters are arranged in one-to-one correspondence, and N is greater than or equal to 2.

4. The in-phase and quadrature demodulation phase modulation optical heterodyne frequency stabilization system with RAM suppression according to claim 1, characterized in that: When N is equal to 2, the two signal generators emit a slow modulation frequency and a fast modulation frequency respectively. The slow modulation frequency is half of the optical resonant cavity line width, and the fast modulation frequency is more than one order of magnitude greater than the optical resonant cavity line width. The slow modulation frequency is greater than the feedback bandwidth of the fast modulation frequency.

5. The RAM-suppressing in-phase and quadrature demodulation phase-modulated optical heterodyne frequency stabilization system according to claim 4, characterized in that: When N is equal to 2, the electric field intensity of the modulated laser emitted by the electro-optic modulator is The calculation is as follows: ， ， ； ， ； in, is the laser electric field amplitude, is the carrier frequency of the laser emitted by the laser, are the slow modulation and fast modulation depths of o light, They are the slow modulation depth and fast modulation depth of e-light, are the slow modulation frequency and the fast modulation frequency respectively, represents the first process parameter, represents the second process parameter, is an imaginary unit, For time, represents the disturbance signal, i.e., the birefringence phase difference, represents the third process parameter, represents the fourth process parameter, represents the angle between the incident laser polarization and the EOM optical axis, represents the angle between the outgoing laser polarization and the optical axis of the electro-optic modulator, represents the phase difference of light entering and exiting the birefringent region of the electro-optic modulator (2), Represents the phase difference of e-light entering and exiting the birefringence region of the electro-optic modulator (2).

6. The RAM-suppressing in-phase and quadrature demodulation phase-modulated optical heterodyne frequency stabilization system according to claim 5, characterized in that: Electric field strength The Bessel expansion of the formula gives the laser frequency after modulation: , is the laser frequency, is an integer laser modulated sideband signal.

7. The RAM-suppressing in-phase and quadrature demodulation phase modulation optical heterodyne frequency stabilization system according to claim 1, characterized in that: When M is greater than 2, the fast modulation frequency satisfies formula (4) or formula (5). When formula (5) is satisfied, all The value of contains an odd number of odd numbers: （4）； （5）； in, is the total number of modulation frequencies, They are modulation frequency numbers. Indicates indivual The value of Respectively The modulation frequency and The modulation frequency, is any symbol, For the existence symbol.

8. The in-phase and quadrature demodulation phase modulation optical heterodyne frequency stabilization system with RAM suppression according to claim 2, characterized in that: When the modulation depth is adjusted by laser beat frequency, the in-phase and quadrature demodulation phase modulation optical heterodyne frequency stabilization system performs the following steps: The laser light emitted by the laser is split into two, one laser light is used to obtain the error signal, and the other laser light is used for the beat frequency; The electro-optic modulator is heated or cooled, and the laser frequency is locked by error signal feedback. The direction of change of the laser beat frequency after locking is observed, and the demodulation phase of the slow modulation frequency is adjusted by the phase shifter so that the direction of change of the beat frequency when the slow modulation frequency locking is performed alone is opposite to that when the fast modulation frequency locking is performed alone, and the peak-to-peak value of the frequency fluctuation is maximized; Then the adder simultaneously inputs the fast modulation signal and the slow modulation signal into the electro-optic modulator, and simultaneously performs fast and slow modulation on the laser. The adder is used to sum all the error signals and then perform feedback frequency locking, adjust the modulation depth of the fast modulation and the slow modulation, minimize the beat frequency change, and select the laser intensity.

9. The in-phase and quadrature demodulation phase modulation optical heterodyne frequency stabilization system with RAM suppression according to claim 2, characterized in that: When the laser does not adjust the modulation depth by laser beat frequency, the in-phase and quadrature demodulation phase modulated optical heterodyne frequency stabilization system performs the following steps: The slow modulation frequency is used as the first slow modulation frequency, and another slow modulation frequency that is mutually prime with the first slow modulation frequency is selected as the second slow modulation frequency. The laser is modulated by two slow modulation signals and one fast modulation signal at the same time, and demodulated respectively to obtain the first slow modulation error signal, the second slow modulation error signal and the fast modulation error signal, and the discrimination slopes of the three error signals are measured. ; Heating or cooling the EO modulator and measuring the RAM amplitude in the first slow modulation error signal when locked using only the fast modulation error signal , measure the RAM amplitude in the first slow modulation error signal when the fast modulation error signal and the second slow modulation error signal are summed and locked through the combiner ; Measuring the RAM amplitude in a second, slower modulating error signal while locked using a faster modulating error signal ; Calculate the RAM voltage from the three error signals ; By changing the demodulation phase of the first slow modulation by the phase shifter, The positive and negative signs of On the contrary, The positive and negative signs of same; Change the modulation voltage of the first slow modulation frequency or fast modulation frequency to make and Equal in size, and finally turn off the second slow modulation frequency.

10. The RAM-suppressing in-phase and quadrature demodulation phase-modulated optical heterodyne frequency stabilization system according to claim 9, characterized in that: RAM Voltage The calculation formula is as follows: ； ； ； in, They are the RAM voltages in the first slow modulation error signal, the second slow modulation error signal, and the fast modulation error signal, respectively. is the gain of the combiner.

Images