Laser Signal Modulation System and CPT Maser Atomic Clock

Abstract

The present application discloses a laser signal modulation system and a CPT Maser atomic clock, which are applied to the field of atomic frequency standards. The system includes an optical signal emission module, a double-pass frequency shift module, an amplitude noise suppression module, and an optical signal modulation module; the double-pass frequency shift module is configured to divide the original optical signal emitted by the optical signal emission module into a first light beam and a second light beam for generating the CPT effect, and perform a double frequency shift operation on the second light beam to obtain a frequency-shifted light beam. The amplitude noise suppression module is configured to obtain the electrical signal corresponding to the combined optical signal of the optical beam modulated by the optical signal modulation module and the frequency-shifted light beam, and obtain an error signal of the double-sideband amplitude value through demodulation processing and differential amplification processing of the electrical signal, and at the same time, based on the error signal, feedback control the modulation depth of the optical signal modulation module at the current moment. The optical signal modulation module performs real-time modulation on the first light beam based on the adjusted depth, thereby effectively reducing the double-sideband amplitude difference.

Description

Technical Field

[0001] This application relates to the technical field of atomic frequency standards, and particularly to a laser signal modulation system and a CPT Maser atomic clock. Background Art

[0002] CPT (Coherent Population Trapping) atomic frequency standard is a new type of atomic frequency standard realized by using a quantum interference phenomenon generated by the interaction between atoms and coherent lasers. Due to its small size and low power consumption, it is widely used in various time-frequency systems. The frequency stability that can be achieved by a CPT maser atomic clock is 2-3 orders of magnitude higher than that of a passive CPT atomic clock, and it has very important practical value in places with high requirements for time accuracy. According to the basic principle of the CPT effect, to realize a CPT atomic clock, it is necessary to modulate the clock laser to generate a pair of ±1 order sidebands with equal amplitude and opposite phase. After the ±1 order sidebands interact with the two hyperfine energy levels of rubidium atoms, the dark state effect occurs, and the frequency discrimination signal is detected by a photodetector and the frequency is stabilized. In practical applications, the amplitudes of the laser sidebands generated after being modulated by a phase modulator are often not equal, resulting in the conversion of amplitude noise into frequency noise after demodulation, thereby reducing the frequency stability of the CPT atomic clock.

[0003] To solve the problem that the amplitudes of the laser sidebands generated after phase modulation are not equal, related technologies usually achieve it by adjusting the parameters of the phase modulator once. For example, the optimal modulation depth of the phase modulator can be obtained through theoretical calculation. At the initial state of system operation, the symmetry of the sideband amplitudes generated by modulation is observed, and then the modulation depth of the phase modulator is manually adjusted. It is also possible to control the temperature of the phase modulator to reduce the amplitude fluctuations of the sidebands caused by temperature drift.

[0004] However, when the laser and other optical devices in the optical path are affected by the environment, such as fluctuations in temperature and air flow, environmental vibrations, and sound waves, etc., they will all cause amplitude fluctuations of the laser sidebands; and the etalon effect existing in the optical path will also reduce the symmetry of the sideband amplitudes. Related technologies simply adjusting the depth and temperature of the phase controller once cannot effectively reduce the amplitude difference between the two sidebands.

[0005] In view of this, how to effectively reduce the amplitude difference between the two sidebands is a technical problem that needs to be solved by those skilled in the art. Summary of the Invention

[0006] This application provides a laser signal modulation system and a CPT Maser atomic clock, which can effectively reduce the amplitude difference between the two sidebands.

[0007] To solve the above technical problems, the embodiments of the present invention provide the following technical solutions:

[0008] An embodiment of the present invention provides a laser signal modulation system on the one hand, including: an optical signal transmitting module, a double-pass frequency shift module, an amplitude noise suppression module, and an optical signal modulation module;

[0009] The double-pass frequency shift module is used to divide the original optical signal emitted by the optical signal transmitting module into a first light beam and a second light beam for generating the CPT effect, and perform a double frequency shift operation on the second light beam to obtain a frequency-shifted light beam;

[0010] The amplitude noise suppression module is used to obtain the electrical signal corresponding to the combined optical signal of the light beam modulated by the optical signal modulation module and the frequency-shifted light beam, and obtain the error signal of the double-sideband amplitude value through demodulation processing and differential amplification processing of the electrical signal. At the same time, based on the error signal, feedback control is performed on the modulation depth of the optical signal modulation module at the current moment;

[0011] The optical signal modulation module is used to perform real-time modulation on the first light beam based on the modulation depth to generate double sidebands with equal amplitudes.

[0012] Optionally, the double-pass frequency shift module includes a first beam splitter, an acousto-optic modulator, a reflector, a quarter-wave plate, and a signal generator;

[0013] The first beam splitter is used to split the original optical signal into the first light beam and the second light beam;

[0014] The signal generator is used to generate an acousto-optic modulation signal; the acousto-optic modulation signal is used for the acousto-optic modulator to generate diffracted light for frequency shift;

[0015] The quarter-wave plate is arranged between the first beam splitter and the acousto-optic modulator;

[0016] The reflector is used to return the ±1 order diffracted light of the second light beam passing through the quarter-wave plate and the acousto-optic modulator along the original path;

[0017] The acousto-optic modulator is used to modulate the second light beam passing through the quarter-wave plate and the ±1 order diffracted light returned along the original path to obtain a frequency-shifted light beam with a frequency shift amount reaching 2ω.

[0018] Optionally, the optical signal modulation module includes an electro-optic modulator and a second signal generator;

[0019] The electro-optic modulator is used to concentrate the first light beam on the ±1 order sidebands by changing the modulation depth;

[0020] The second signal generator is used to generate an electro-optic modulation signal; the electro-optic modulation signal is used for the electro-optic modulator to perform phase modulation.

[0021] Optionally, the optical signal transmitting module includes a vertical surface emitting laser with a central wavelength of 795 nm and a linewidth of less than 100 MHz.

[0022] Optionally, the amplitude noise suppression module includes a beat frequency component, a demodulation component, a differential amplification component, and a controller;

[0023] The beat frequency component is configured to perform photoelectric detection on the combined optical signal of the modulated light beam and the frequency-shifted light beam to obtain a microwave band signal carrying amplitude information;

[0024] The demodulation component is configured to double the frequency of the signal with a frequency of ω and mix it with the signal with a frequency of Ω to obtain demodulation signals with central frequencies of Ω - 2ω and Ω + 2ω, and mix each demodulation signal with the microwave band signal respectively to obtain DC voltage change signals corresponding to the double sidebands;

[0025] The differential amplification component is configured to perform differential amplification processing on the DC voltage change signal to obtain an error signal of the amplitude values of the two sidebands;

[0026] The controller is configured to adjust the modulation voltage of the electro-optic modulator in the optical signal modulation module based on the error signal.

[0027] Optionally, the beat frequency component includes a 45° reflector, the second beam splitter, and a first photodetector;

[0028] The first photodetector is configured to detect the combined optical signal of the transmitted light of the modulated light beam passing through the 45° reflector and the second beam splitter and the frequency-shifted light beam passing through the second beam splitter.

[0029] Optionally, the demodulation component includes a first low-pass filter, a power divider, a first band-pass filter, a second band-pass filter, a mixer, and a frequency doubler;

[0030] The first low-pass filter is configured to filter the microwave band signal to obtain a carrier with a frequency of 2ω and two sidebands with central frequencies of Ω - 2ω and Ω + 2ω;

[0031] The power divider is configured to divide the microwave band signal after low-pass filtering into a first beat frequency signal and a second beat frequency signal;

[0032] The first band-pass filter has a central frequency of Ω - 2ω and a bandwidth of 2ω, and is configured to filter the first beat frequency signal to obtain a first sideband amplitude signal;

[0033] The center frequency of the second band-pass filter is Ω + 2ω, and the bandwidth is 2ω. It is used to filter the second beat frequency signal to obtain a second sideband amplitude signal;

[0034] The frequency multiplier is used to multiply the frequency of the signal with frequency ω;

[0035] The mixer is used to mix the first sideband amplitude signal and the local first oscillation signal with the same center frequency, and the second sideband amplitude signal and the local second oscillation signal with the same center frequency respectively.

[0036] Optionally, the differential amplification component includes a third low-pass filter, a fourth low-pass filter, and a differential amplifier;

[0037] The third low-pass filter is used to remove the high-frequency noise of the mixed signal of the first sideband amplitude signal and the local first oscillation signal;

[0038] The fourth low-pass filter is used to remove the high-frequency noise of the mixed signal of the second sideband amplitude signal and the local second oscillation signal;

[0039] The differential amplifier is used to differentially amplify the output signals of the third low-pass filter and the fourth low-pass filter.

[0040] Optionally, the controller includes an amplifier, a PI control circuit, a loop filter, and a voltage-controlled oscillator;

[0041] The amplifier is used to amplify the error signal;

[0042] The loop filter is used to filter the phase noise in the amplified error signal and change the output frequency of the voltage-controlled oscillator;

[0043] The PI control circuit is used to feedback and adjust the modulation voltage of the electro-optical modulator to change the modulation depth.

[0044] Another aspect of the embodiments of the present invention provides a CPT Maser atomic clock, including the laser signal modulation system and the CPT effect physical system as described in any one of the foregoing items.

[0045] The advantages of the technical solution provided by this application are as follows: The two sidebands are frequency-shifted to the same frequency, and the optical frequency signal is frequency-converted to the microwave band to obtain the voltage value corresponding to the amplitude of the double sideband. By comparing the amplitude difference of the double sidebands, the corresponding error signal is determined, and the best modulation depth can be obtained by feedback-adjusting the modulation depth of the modulator. By actively controlling the modulation depth of the modulator, the asymmetry of the sideband amplitude can be reduced in real time, making the reduction of the laser amplitude noise real-time, with strong anti-environmental interference ability, and the double sidebands with equal amplitudes can be stably obtained. Furthermore, the signal-to-noise ratio of the error signal of the CPT Maser atomic clock can be effectively improved, and the frequency stability of the CPT Maser atomic clock, especially the short-term stability, can be improved.

[0046] In addition, the embodiment of the present invention also provides a corresponding application system, namely the CPT Maser atomic clock, for the laser signal modulation system, further making the system more practical, and the CPT Maser atomic clock has corresponding advantages.

[0047] It should be understood that the above general description and the following detailed description are only exemplary and do not limit the present disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] In order to more clearly illustrate the technical solutions of the embodiments of the present invention or related technologies, the following will briefly introduce the drawings required for use in the description of the embodiments or related technologies. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0049] Figure 1 It is a schematic structural framework diagram of a laser signal modulation system provided by an embodiment of the present invention in an alternative embodiment;

[0050] Figure 2 It is a schematic diagram of an optical path for suppressing laser amplitude noise provided by an embodiment of the present invention;

[0051] Figure 3 It is a schematic diagram of an optical path provided by an embodiment of the present invention in which two local oscillator signals are respectively used for laser phase modulation and frequency shift, and a signal for down-converting the sideband frequency is generated through mixing;

[0052] Figure 4 It is a schematic diagram of a feedback control for suppressing laser amplitude noise provided by an embodiment of the present invention;

[0053] Figure 5 It is a schematic diagram of a laser signal modulation process provided by an embodiment of the present invention;

[0054] Figure 6Schematic structural framework diagram of a CPT Maser atomic clock provided by an embodiment of the present invention under an optional implementation manner;

[0055] Figure 7 Schematic diagram of the working principle of a CPT Maser atomic clock provided by an embodiment of the present invention. Specific implementation manners

[0056] In order to enable those skilled in the art to better understand the solution of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and specific implementation manners. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0057] The terms "first", "second", "third", "fourth", etc. in the specification and claims of the present application and the above-mentioned drawings are used to distinguish different objects, rather than to describe a specific order. In addition, the terms "comprising" and "having" and any variations thereof are intended to cover non-exclusive inclusion. For example, a process, method, system, product or device that includes a series of steps or units is not limited to the listed steps or units, but may include steps or units not listed.

[0058] After introducing the technical solutions of the embodiments of the present invention, various non-limiting implementation manners of the present application will be described in detail below.

[0059] First, please refer to Figure 1 , Figure 1 Schematic structural framework diagram of a laser signal modulation system provided by an embodiment of the present invention under an optional implementation manner. The embodiments of the present invention may include the following contents:

[0060] The laser signal modulation system may include an optical signal transmitting module 1, a double-pass frequency shifting module 2, an amplitude noise suppression module 3, and an optical signal modulation module 4.

[0061] Among them, the optical signal transmitting module 1 is used to emit an optical signal. Since the laser signal modulation system of this embodiment is used for atomic frequency standards, optionally, the optical signal transmitting module 1 may include a laser, and the laser may be a vertical surface emitting laser with a central wavelength of 795 nm and a line width less than 100 MHz.

[0062] The dual-pass frequency shift module 2 of this embodiment is used to receive the optical signal emitted by the optical signal transmitting module 1, and divide the optical signal, which is also called the original optical signal in this embodiment, into two optical signals. For the convenience of description, they can be called the first light beam and the second light beam. The first light beam is used to generate the CPT effect, that is, the first light beam is sent to the optical signal modulation module 4 for modulation to generate a pair of ±1 order sidebands with equal amplitude and opposite phase to be applicable to the CPT atomic clock. The second light beam is used as a reference to reflect the change of the optical signal caused by the environmental influence and the etalon effect existing in the optical path itself. In this embodiment, the dual-pass frequency shift module 2 performs a dual-frequency shift operation on the second light beam. The so-called dual-frequency shift means passing through the frequency modulation device such as an acousto-optic modulator twice, and also performing two frequency shifts on the second light beam to obtain a frequency-shifted light beam. If the single-frequency shift amount is ω, the frequency-shift amount of this frequency-shifted light beam can reach 2ω.

[0063] In this embodiment, after the second light beam passes through the dual-pass frequency shift module 2, it will be emitted to the amplitude noise suppression module 3, and the amplitude noise suppression module 3 processes it to finally obtain the error information that can reflect the amplitude difference between the two sidebands. The first light beam will pass through the optical signal modulation module 4 for modulation to obtain a modulated light beam. For the convenience of implementation, the modulated first light beam can be divided into two modulated optical signals. The first modulated optical signal is used to interact with the atomic gas in the cell to generate the CPT effect, and the second modulated optical signal is used to obtain the error signal. The amplitude noise suppression module 3 can first combine the modulated light beam, that is, the second modulated optical signal and the frequency-shifted light beam, to obtain a combined light signal, and then convert the combined light signal into an electrical signal carrying amplitude information through an optoelectronic conversion method such as optoelectronic detection technology, and convert the optical frequency signal to the microwave frequency band, that is, obtain the electrical signal corresponding to the conversion of the combined light signal. This electrical signal is a microwave frequency band signal. By demodulating and differentially amplifying the microwave frequency band signal, an error signal of the amplitude value of the two sidebands is obtained. At the same time, based on the error signal, the modulation voltage of the modulator in the optical signal modulation module at the current moment is feedback-controlled to change the modulation depth, and the amplitude difference between the two sidebands can be eliminated in real time, so that the amplitude noise of the laser can be actively suppressed.

[0064] The optical signal modulation module 4 of this embodiment includes at least one phase modulator such as an electro-optic modulator, and uses this phase modulator to modulate the light beam to generate high-order sidebands. By selecting an appropriate modulation depth, the amplitude of the high-order sidebands can be reduced, so that the optical power is mainly concentrated in the ±1 order sidebands, that is, two sidebands with equal amplitude can be obtained. This appropriate modulation depth is feedback-controlled by the amplitude noise suppression module 3 based on the error signal of the amplitude of the two sidebands. Optionally, the optical signal modulation module 4 may include an electro-optic modulator and a second signal generator; the electro-optic modulator is used to concentrate the first light beam on the ±1 order sidebands by changing the modulation depth; the second signal generator is used to generate an electro-optic modulation signal; the electro-optic modulation signal is used for the electro-optic modulator to perform phase modulation.

[0065] In the technical solution provided by the embodiment of the present invention, two sidebands are frequency-shifted to the same frequency, and the optical frequency signal is frequency-converted to the microwave band to obtain the voltage value corresponding to the amplitude of the double sideband. By comparing the amplitude difference of the double sidebands, the corresponding error signal is determined, and the modulation depth of the modulator is feedback-regulated to obtain the optimal modulation depth. By actively controlling the modulation depth of the modulator, the asymmetry of the sideband amplitude can be reduced in real time, so that the reduction of the laser amplitude noise has real-time performance, strong anti-environmental interference ability, and the double sidebands with equal amplitudes can be stably obtained. Furthermore, the signal-to-noise ratio of the error signal of the CPT Maser atomic clock can be effectively improved, and the frequency stability of the CPT Maser atomic clock, especially the short-term stability, can be improved.

[0066] In the above embodiment, no limitation is imposed on the optical components and the optical path structure included in the double-pass frequency-shifting module 2, as long as it can achieve double frequency-shifting of the light beam. As an optional implementation manner, this embodiment also provides an optional optical path structure of the double-pass frequency-shifting module 2, which may include the following contents:

[0067] The double-pass frequency-shifting module 2 may include a first beam splitter, an acousto-optic modulator, a mirror, a quarter-wave plate, and a signal generator.

[0068] The first beam splitter is used to split the original optical signal into a first light beam and a second light beam;

[0069] The signal generator is used to generate an acousto-optic modulation signal; the acousto-optic modulation signal is used for the acousto-optic modulator to generate diffracted light for frequency-shifting;

[0070] The quarter-wave plate is arranged between the first beam splitter and the acousto-optic modulator;

[0071] The mirror is used to return the ±1st order diffracted light of the second light beam passing through the quarter-wave plate and the acousto-optic modulator along the original path;

[0072] The acousto-optic modulator is used to modulate the second light beam passing through the quarter-wave plate and the ±1st order diffracted light returning along the original path to obtain a frequency-shifted light beam with a frequency shift amount reaching 2ω.

[0073] In this embodiment, the first beam splitter can be any device that can achieve the beam splitting function, such as a polarization-sensitive beam splitter PBS. The quarter-wave plate can cause an additional optical path difference of λ / 4 between the o light and the e light, that is, it can change the polarization direction of the light beam. The mirror can be, for example, a 0° mirror. Of course, other types of mirrors can also be used, which do not affect the implementation of this application. After the optical signal passes through the acousto-optic modulator AOM, the +1st order diffracted light returns along the original path by the mirror, and the double-pass through AOM completes the double frequency-shifting.

[0074] To make those skilled in the art more clearly understand the implementation manner of this embodiment, in combination with Figure 2 , the process of the optical signal from emission to the generation of the frequency-shifted beam is given as follows:

[0075] The laser light emitted by the laser of the optical signal emission module 1 is divided into two parts by the PBS, namely, the transmitted light for generating the CPT effect and the local light for beat frequency. The local light for beat frequency is frequency-shifted by the AOM after being refracted by the PBS, and the modulation frequency of the AOM is LO2 = ω. Since the linewidth of the laser is generally 100 MHz, in order to separate the double sidebands using a filter after frequency down-conversion, the laser passes through the AOM twice for frequency shifting, so that the frequency shift amount of the laser reaches 2ω. The implementation method is to place a quarter-wave plate between the AOM and the PBS. After the laser passes through the AOM and the quarter-wave plate for the first time, the +1 order diffracted light is taken. At this time, the frequency of the laser is υ + ω. A reflector is placed behind the quarter-wave plate to return the +1 order diffracted light along the original path. After passing through the AOM again, the +1 order light of the second pass is taken. At this time, the frequency of the laser is υ + 2ω, that is, the frequency shift target of 2ω is achieved. At the same time, since the polarization of the laser optical signal changes by 90° after passing through the quarter-wave plate twice, when passing through the PBS again, it changes from refraction to transmission. Therefore, the optical path does not return along the direction of PBS refraction, which is beneficial to the subsequent beam combination and beat frequency with the modulated light.

[0076] In the above embodiment, no limitations are imposed on the optical components and the optical path structure included in the amplitude noise suppression module 3, as long as it can calculate the error information of the double sideband amplitude and select the most appropriate modulation depth based on this error information. As an optional implementation manner, this embodiment also provides an optional optical path structure of the amplitude noise suppression module 3, which may include the following content:

[0077] The amplitude noise suppression module 3 may include a beat frequency component, a demodulation component, a differential amplification component, and a controller. The beat frequency component is used to perform photoelectric detection on the combined optical signal of the modulated beam and the frequency-shifted beam to obtain a microwave band signal carrying amplitude information; the demodulation component is used to double-frequency the signal with a frequency of ω and mix it with the signal with a frequency of Ω to obtain demodulation signals with center frequencies of Ω - 2ω and Ω + 2ω, and mix each demodulation signal with the microwave band signal respectively to obtain the DC voltage change signals corresponding to the double sidebands; the differential amplification component is used to perform differential amplification processing on the DC voltage change signals to obtain the error signals of the amplitude values of the two sidebands; the controller is used to adjust the modulation voltage of the electro-optic modulator in the optical signal modulation module based on the error signals.

[0078] Among them, for the beat frequency component, it may include a 45° reflector, a second beam splitter, and a first photodetector. The 45° reflector is used to transmit the modulated light beam, that is, the light beam obtained after the first light beam is modulated by the optical signal modulation module 4, to the second beam splitter. The second beam splitter can be used to combine the modulated light beam and the frequency-shifted light beam for subsequent beat frequency. Furthermore, the first photodetector is used to detect the transmitted light signal of the modulated light beam passing through the 45° reflector and the second beam splitter, and the combined light signal of the frequency-shifted light beam passing through the second beam splitter.

[0079] For the demodulation component, it can double the frequency of the signal with frequency ω and mix it with the signal with frequency Ω to generate demodulation signals with frequency centers of Ω - 2ω and Ω + 2ω. The demodulation signals of Ω - 2ω and Ω + 2ω are respectively mixed with the two sideband signals demodulated from the microwave band signal to obtain the DC voltage change signals corresponding to the two sidebands. In combination with actual optical components, it may include a first low-pass filter, a power splitter, a first band-pass filter, a second band-pass filter, a mixer, and a frequency doubler. The first low-pass filter is used to filter the microwave band signal to obtain a carrier with frequency 2ω and two sidebands with center frequencies of Ω - 2ω and Ω + 2ω; the power splitter is used to divide the microwave band signal after low-pass filtering into a first beat frequency signal and a second beat frequency signal; the first band-pass filter has a center frequency of Ω - 2ω and a bandwidth of 2ω, and is used to filter the first beat frequency signal to obtain a first sideband amplitude signal; the second band-pass filter has a center frequency of Ω + 2ω and a bandwidth of 2ω, and is used to filter the second beat frequency signal to obtain a second sideband amplitude signal; the frequency doubler is used to double the frequency of the signal with frequency ω; the mixer is used to mix the first sideband amplitude signal with the same center frequency and the local first oscillation signal, and the second sideband amplitude signal with the same center frequency and the local second oscillation signal respectively. Among them, the mixer may include a first mixer and a second mixer. The first mixer mixes the first sideband amplitude signal with the same center frequency and the local first oscillation signal; the second mixer is used to mix the second sideband amplitude signal with the same center frequency and the local second oscillation signal. For the generation of the local first oscillation signal and the local second oscillation signal, in combination with Figure 3 as shown Figure 3 in the schematic example where ω = 200, the local first oscillation signal LO3 = Ω - 2ω and the local second oscillation signal LO4 = Ω + 2ω can be obtained by mixing two local oscillators LO1 and LO2 and then filtering them respectively with band-pass filters with center frequencies of Ω - 2ω and Ω + 2ω.

[0080] For the differential amplification component, it demodulates the DC signals of the amplitudes of the two sidebands, performs low-pass filtering on them, and then performs differential amplification to obtain the error signals of the amplitudes of the two sidebands. In combination with actual optical components, it may include a third low-pass filter, a fourth low-pass filter, and a differential amplifier; the third low-pass filter is used to remove the high-frequency noise of the mixed-frequency signal of the first sideband amplitude signal and the local first oscillation signal; the fourth low-pass filter is used to remove the high-frequency noise of the mixed-frequency signal of the second sideband amplitude signal and the local second oscillation signal; the differential amplifier is used to perform differential amplification on the output signals of the third low-pass filter and the fourth low-pass filter.

[0081] For the controller, it may include an amplifier, a PI control circuit, a loop filter, and a voltage-controlled oscillator; the amplifier is used to amplify the error signal; the loop filter is used to filter out the phase noise in the amplified error signal and change the output frequency of the voltage-controlled oscillator; the PI control circuit is used to feedback and adjust the modulation voltage of the electro-optic modulator to change the modulation depth. Of course, other types of control circuits can be used, which does not affect the implementation of this application.

[0082] To make those skilled in the art more clearly understand the implementation manner of this embodiment, in combination with Figure 4 and Figure 5 , the generation process of the error signal is given:

[0083] The signal collected by the first photodetector mainly has three frequency components after passing through the low-pass filter, the carrier wave with a frequency of 2ω, the double sidebands with frequencies of Ω - 2ω and Ω + 2ω, and the two sidebands differ by 4ω. When the modulation frequency ω ≥ 100 MHz, the frequency difference between the two sidebands is much larger than the laser linewidth. Use a power splitter, that is, a power divider, to divide the beat frequency signal, that is, the microwave band signal, into two beams, and respectively use band-pass filters with center frequencies of Ω - 2ω and Ω + 2ω and a bandwidth of 2ω for filtering to obtain the voltage values corresponding to the amplitudes of the two sidebands. Mix the two local oscillators LO1 and LO2 and then filter them respectively with band-pass filters with center frequencies of Ω - 2ω and Ω + 2ω to obtain the oscillation signals LO3 = Ω - 2ω and LO4 = Ω + 2ω. Mix the two sidebands with LO3 and LO4 with the same center frequencies respectively, and use a low-pass filter to filter out the high-frequency noise, only retaining the DC voltage signal. Use a comparison amplifier to obtain the difference between the two sidebands, that is, the error signal, and feedback-control the EOM with this error signal to change its modulation depth, thereby reducing the amplitude difference between the two sidebands in real time.

[0084] Based on the above embodiments, the embodiments of the present invention also provide a CPT Maser atomic clock. Please refer to Figure 6 , which may include:

[0085] First, in combination with Figure 7Describe the basic working principle of the CPT Maser atomic clock. It is necessary to use an electro-optic modulator to modulate the phase of the laser to generate a pair of sidebands with opposite phases and equal amplitudes, which are used to produce the CPT effect with the two hyperfine energy levels of the atom. Usually, a vertical-cavity surface-emitting laser (VCSEL) is used as the light source, and the emitted laser of this laser is in a divergent form. After collimating the laser with a lens, it is coupled to the electro-optic modulator. A half-wave plate set in front of the electro-optic modulator can adjust the polarization direction of the optical fiber coupled into the electro-optic modulator. Rotate the wave plate to make the laser polarization consistent with the fast axis of the polarization-maintaining fiber. After being modulated by the electro-optic modulator, the laser generates a pair of sidebands with opposite phases and equal amplitudes. After exiting from the other end, a quarter-wave plate is used to generate circularly polarized light. After the laser enters the atomic bubble, a pair of sidebands with a frequency difference of 6.8 GHz interact with the two hyperfine energy levels of the atom to produce the dark state effect. The signal detected by the PD is divided into two parts by a power splitter. One part is used to lock the laser frequency, and the other part is used to produce the CPT effect.

[0086] Based on the above principle, the CPT Maser atomic clock of this embodiment may include the laser signal modulation system 61 and the CPT effect physical system 62 described in any one of the above embodiments. Among them, the CPT effect physical system may include an atomic cell, a microwave cavity, a temperature controller, and a photodetector. After the laser signal modulated by the laser signal modulation system 61 enters the atomic cell, the CPT effect occurs, and the photodetector detects the dark state error signal and then feeds back for control. The modulation process of the laser signal modulation system 61 may be, for example: The laser with a frequency of υ is emitted from the laser and is divided into two paths A and B by a PBS. After the laser in path A passes through a double-pass frequency shift module (the single frequency shift amount is ω), the laser frequency is υ + 2ω. The laser in path B generates high-order sidebands after phase modulation. The phase modulation frequency is Ω, and the laser frequency after modulation is υ ± nΩ, where n is a positive integer. By selecting an appropriate modulation depth, the amplitude of the high-order sidebands can be reduced, so that the optical power is mainly concentrated in the ±1st order sidebands, that is, the main frequency of the modulated light is υ ± Ω. The modulated laser is divided into two beams after passing through the PBS. One of the modulated light beams is used to interact with the atoms in the cell to produce the CPT effect, and the other modulated light beam and the frequency-shifted light can detect the amplitude signal of this laser in the microwave band after beat frequency processing. The laser frequency in the microwave band is mainly 2ω ± Ω. Further, after demodulating this signal and then passing it through a differential amplifier, an error signal of the amplitudes of the two sidebands is obtained. After amplifying and filtering this error signal again and passing it through PI control, the modulation depth of the phase modulator is feedback-controlled. The amplitude difference between the two sidebands is eliminated in real time, so as to achieve the purpose of actively suppressing the amplitude noise of the laser of the CPT maser atomic clock.

[0087] As can be seen from the above, the present application actively controls the modulation depth of the electro-optic modulator to reduce the asymmetry of the sideband amplitude in real time, and can achieve real-time reduction of the laser amplitude noise of the CPT Maser clock, with strong anti-environmental interference ability. It can effectively reduce the laser amplitude noise, improve the signal-to-noise ratio of the error signal of the CPT Maser atomic clock, and improve the frequency stability of the CPT Maser atomic clock, especially the short-term stability. Since the acquisition of the laser sideband amplitude noise is placed before the atomic cell, the amplitudes of the positive and negative first-order sidebands are filtered after frequency down-conversion, excluding the local noise mixed with the higher-order sidebands after passing through the atomic cell, and the acquisition of the amplitude noise is more real and reliable, and more truly reflects the amplitude difference between the two first-order sidebands.

[0088] In this specification, each embodiment is described in a progressive manner. The key point of each embodiment is to illustrate the differences from other embodiments. For the same or similar parts among the embodiments, reference can be made to each other. For the CPT Maser atomic clock disclosed in the embodiments, since it corresponds to the relevant content of the laser signal modulation system disclosed in the embodiments, the description is relatively simple, and reference can be made to the description of this part for the relevant parts.

[0089] Those skilled in the art can further realize that the units and algorithm steps of each example described in combination with the embodiments disclosed in this article can be implemented by electronic hardware, computer software, or a combination of the two. To clearly illustrate the interchangeability of hardware and software, the composition and steps of each example have been generally described according to functions in the above description. Whether these functions are executed in a hardware or software manner depends on the specific application and design constraints of the technical solution. Those skilled in the art can use different methods to implement the described functions for each specific application, but such implementation should not be considered to exceed the scope of the present invention.

[0090] The above has introduced in detail a laser signal modulation system and a CPT Maser atomic clock provided by the present application. Specific examples are used in this article to elaborate on the principles and implementation manners of the present invention. The description of the above embodiments is only used to help understand the method and its core idea of the present invention. It should be noted that for those of ordinary skill in the art in this technical field, without departing from the principle of the present invention, several improvements and modifications can be made to the present application, and these improvements and modifications also fall within the protection scope of the claims of the present application.

Claims

1. A laser signal modulation system, characterized in that, Including: An optical signal transmitting module, a double-pass frequency shifting module, an amplitude noise suppression module, and an optical signal modulation module; The double-pass frequency shifting module is configured to divide the original optical signal emitted by the optical signal transmitting module into a first light beam and a second light beam for generating the CPT effect, and perform a double frequency shifting operation on the second light beam to obtain a frequency-shifted light beam; The amplitude noise suppression module is configured to obtain the electrical signal corresponding to the combined optical signal of the optical beam modulated by the optical signal modulation module and the frequency-shifted light beam, and obtain the error signal of the bilateral band amplitude value through demodulation processing and differential amplification processing of the electrical signal. At the same time, based on the error signal, feedback control is performed on the modulation depth of the optical signal modulation module at the current moment; The optical signal modulation module is configured to perform real-time modulation on the first light beam based on the modulation depth to generate bilateral bands with equal amplitudes; Among them, the double-pass frequency shift module includes a first beam splitter, an acousto-optic modulator, a reflector, a quarter-wave plate, and a signal generator; the first beam splitter is used to split the original optical signal into the first optical beam and the second optical beam; the signal generator is used to generate an acousto-optic modulation signal; the acousto-optic modulation signal is used for the acousto-optic modulator to generate diffracted light for frequency shifting; the quarter-wave plate is arranged between the first beam splitter and the acousto-optic modulator; the reflector is used to return the ±1st order diffracted light of the second optical beam passing through the quarter-wave plate and the acousto-optic modulator along the original path; the acousto-optic modulator is used to modulate the second optical beam passing through the quarter-wave plate and the ±1st order diffracted light returning along the original path to obtain a frequency-shifted optical beam with a frequency shift amount reaching 2 ω of the frequency-shifted optical beam; Wherein, the amplitude noise suppression module includes a beat frequency component, a demodulation component, a differential amplification component, and a controller; the beat frequency component is configured to perform photoelectric detection on the combined optical signal of the modulated optical beam and the frequency-shifted light beam to obtain a microwave band signal carrying amplitude information; the demodulation component is configured to double-frequency the signal with a frequency of ω and mix it with the signal with a frequency of Ω to obtain demodulation signals with center frequencies of Ω-2ω and Ω+2ω, and mix each demodulation signal with the microwave band signal respectively to obtain the DC voltage change signals corresponding to the bilateral bands; the differential amplification component is configured to perform differential amplification processing on the DC voltage change signals to obtain the error signals of the two side band amplitude values; the controller is configured to adjust the modulation voltage of the electro-optic modulator in the optical signal modulation module based on the error signal.

2. The laser signal modulation system according to claim 1, wherein, The optical signal modulation module includes an electro-optic modulator and a second signal generator; The electro-optic modulator is used to concentrate the first beam on the sidebands of the The second signal generator is configured to generate an electro-optic modulation signal; The electro-optic modulation signal is used for the electro-optic modulator to perform phase modulation.

3. The laser signal modulation system according to claim 2, characterized in that, The optical signal transmitting module includes a vertical surface emitting laser with a central wavelength of 795 nm and a line width less than 100 MHz.

4. The laser signal modulation system according to claim 1, characterized in that The beat frequency component includes a 45° reflector, a second beam splitter, and a first photodetector; The first photodetector is configured to detect the transmitted light of the modulated optical beam through the 45° reflector and the second beam splitter, and the combined optical signal of the frequency-shifted light beam passing through the second beam splitter.

5. The laser signal modulation system according to claim 1, wherein, The demodulation component includes a first low-pass filter, a power divider, a first band-pass filter, a second band-pass filter, a mixer, and a frequency multiplier; The first low-pass filter is configured to filter the microwave band signal to obtain a carrier with a frequency of 2ω, and two side bands with center frequencies of Ω-2ω and Ω+2ω; The power divider is configured to divide the microwave band signal after low-pass filtering into a first beat frequency signal and a second beat frequency signal; The first band-pass filter has a center frequency of Ω-2ω and a bandwidth of 2ω, and is configured to filter the first beat frequency signal to obtain a first side band amplitude signal; The center frequency of the second band-pass filter is Ω + 2ω, and the bandwidth is 2ω. It is used to filter the second beat frequency signal to obtain a second sideband amplitude signal; The frequency multiplier is used to multiply the frequency of the signal with a frequency of ω; The mixer is used to mix the first sideband amplitude signal and the local first oscillation signal with the same center frequency, and the second sideband amplitude signal and the local second oscillation signal with the same center frequency respectively.

6. The laser signal modulation system according to claim 5, characterized in that, The differential amplification component includes a third low-pass filter, a fourth low-pass filter, and a differential amplifier; The third low-pass filter is used to remove the high-frequency noise of the mixed signal of the first sideband amplitude signal and the local first oscillation signal; The fourth low-pass filter is used to remove the high-frequency noise of the mixed signal of the second sideband amplitude signal and the local second oscillation signal; The differential amplifier is used to perform differential amplification on the output signals of the third low-pass filter and the fourth low-pass filter.

7. The laser signal modulation system according to claim 1, wherein The controller includes an amplifier, a PI control circuit, a loop filter, and a voltage-controlled oscillator; The amplifier is used to amplify the error signal; The loop filter is used to filter out the phase noise in the amplified error signal and change the output frequency of the voltage-controlled oscillator; The PI control circuit is used to feedback-adjust the modulation voltage of the electro-optic modulator to change the modulation depth.

8. A CPT Maser atomic clock, characterized in that, It includes the laser signal modulation system and the CPT effect physical system according to any one of claims 1 to 7.

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