CN115513761A - A super-large capture band laser phase-locking device and method based on dual-channel digital phase detection - Google Patents

Abstract

The invention discloses a super-large capture band laser phase locking device and method based on two-way digital phase discrimination, and belongs to the technical field of optical coherence. The device comprises an optical beat frequency system and a digital phase frequency and phase detection system. The optical beat frequency system comprises a master-slave laser, a first BS, a second BS and a high-speed detector. The digital phase frequency and phase discrimination system comprises a power amplifier, a power divider, a mixer, a low-frequency amplifier, a coupler, a frequency divider, a first digital phase discrimination plate, a second digital phase discrimination plate, a PID module, a first external reference source, a second external reference source and a third external reference source. The invention adopts two-time phase locking by an heterodyne method, provides two paths of fast and slow feedback signals, realizes semi-automatic locking of laser phase change in a short time within a large capture band range, and can change the difference frequency between laser beams by changing external reference frequency. The invention has the advantages of high locking precision, compact and simple structure, large laser tuning range, capture bandwidth and the like.

Description

A super large capture band laser phase-locking device and method based on dual-channel digital phase detection

technical field

The invention relates to an ultra-large capture band laser phase-locking device and method based on dual-channel digital phase discrimination, and belongs to the field of optical coherence technology.

Background technique

With the continuous research and development of laser technology, the use of laser technology to cool atoms has become more and more mature. The application field of cold atoms is constantly growing, and cold atom interference is one of its popular applications. The Raman process is the core technology of the atomic interference process. It links the laser field with the atomic state and becomes an important means of manipulating and observing atoms. In this type of interferometer, by using the two-photon Raman transition between atomic fine-structure energy levels, one can greatly increase the time interval between light fields, and ultimately increase the speed of the earth's rotation, gravity gradient, and gravity acceleration. The measurement accuracy of the series data.

The process of atomic interference manipulation requires a pair of phase-coherent Raman lasers, and the phase noise of Raman lasers will directly introduce interference phase shift and affect the measurement resolution of gravitational acceleration. At this time, the phase quality of Raman lasers is particularly important. important. At present, there are three common methods for generating coherent Raman laser: the frequency shift method of the acousto-optic modulator, the modulation sideband method of the electro-optic modulator, and the optical phase-locked loop method.

The first acousto-optic modulator frequency shift method uses the Bragg diffraction effect of the acousto-optic modulator to diffract the incident laser light. The diffracted light and the undiffracted zero-order light come from the same beam of incident laser light, and the two are phase-coherent. This method is simple and convenient to use, and is very suitable for generating coherent beams with a difference frequency below 1 GHz. The disadvantage is that the diffraction efficiency decreases with the increase of the diffraction frequency. The diffraction efficiency of a typical commercial acousto-optic modulator with a frequency shift of 3.4 GHz is only 2 %, which is unacceptable in atomic interference experiments.

The second electro-optic modulator modulation sideband method is to use the electro-optic modulator to modulate the main Raman laser to generate sidebands, and then use Fabry-Perot cavity or external cavity semiconductor laser as a frequency selection tool to filter out useless sidebands and zero At this time, the selected sidebands form coherent Raman light with the main Raman laser. The disadvantage of this method is that unwanted sidebands and zero-order light that spatially coincide with the desired sidebands can interfere with the experiment.

The third optical phase-locked loop method is to compare the beat frequency signals of two lasers that have not been phase-locked with the standard reference signal to obtain an error signal, and use the electrical feedback method to lock the frequency of the beat frequency signal to the frequency reference point. , can produce coherent Raman light. The performance of coherent Raman light in this method depends on the performance of the phase-locked circuit. Generally, when the frequency of the beat signal changes greatly, the circuit must be readjusted to ensure the phase-locked effect. The frequency long-term drift of semiconductor lasers is generally at the level of 1GHz/day, while the capture band of the existing optical phase-locked loop method is generally only at the level of 100MHz, so it is necessary to carefully adjust the laser current and cavity length parameters before locking the laser each time. Only when the laser beat frequency signal falls within the capture band can the subsequent phase-locking process be carried out, which increases the difficulty of experimental maintenance and reduces the stability of the system. More importantly, when the laser loses lock due to external vibration, temperature drift and other factors, it is easy to exceed the existing capture band range, which requires readjustment of laser parameters, making it difficult to realize the semi-automatic locking method of the optical phase-locked loop.

Contents of the invention

In order to solve the problems that the existing optical phase-locked loop cannot be locked directly when the drift is too large, it must be adjusted in a large range, the frequency sweep range is too small, the capture band is narrow, and the locking accuracy is not high. The ultra-large capture band laser phase-locking device and method of one-way digital phase discrimination adopts two heterodyne phase locks to provide fast and slow two-way feedback signals to realize semi-automatic locking of laser phase changes in a short period of time with a large capture range, and at the same time The difference frequency between laser beams can be changed by changing the external reference frequency, so that the controlled difference frequency can be from tens of MHz to several GHz. The invention can reduce the influence of Raman phase noise on the measurement accuracy, has the advantages of high locking accuracy, compact and simple structure, large laser tuning range, capture bandwidth, etc., and is suitable for the use requirements of the atomic interferometer.

The purpose of the present invention is achieved through the following technical solutions.

The invention discloses a dual-channel digital phase discrimination-based ultra-large capture band laser phase-locking device, which includes an optical beat-frequency system and a digital frequency-discrimination and phase-discrimination system. The optical beat frequency system includes a master-slave laser, a first BS, a second BS, and a high-speed detector. Digital frequency and phase detection system includes power amplifier, power divider, mixer, low frequency amplifier, coupler, frequency divider, first digital phase detection board, second digital phase detection board, PID module, first external reference source , the second external reference source, and the third external reference source. All devices between the digital frequency and phase detectors are connected by coaxial cables to form a control circuit system. The first digital phase detector board and the second digital phase detector board respectively include a digital comparator, a digital phase detector, and a differential amplifier.

Preferably, the power amplifier is a 6.5-7.2G high-power amplifier.

The invention also discloses a super-large capture-band laser phase-locking method based on dual-channel digital phase discrimination, which is realized based on the super-large capture-band laser phase-lock device based on dual-channel digital phase discrimination. Described a kind of ultra-large capture band laser phase-locking method based on two-way digital phase discrimination is:

The main laser is used as a reference laser. After frequency locking, a beam of light is split out through the first BS, and a small part of the slave laser is selected through the second BS. After the two beams are combined, they enter the high-speed detector to measure their beat frequency signals. After the beat frequency signal passes through the amplifier, it is divided into two paths by the power divider, which are used as fast feedback and slow feedback RF signals respectively. Fast feedback all the way First, the first external reference source provides a standard 6.8GHz local oscillator signal and the beat frequency signal for mixing to obtain a difference frequency signal with a frequency of about 50M, which is amplified and then input to the coupler. The coupler will A small part of the difference frequency signal is coupled for monitoring, the main part is input to the digital phase detector, and the phase is compared with the standard 50M signal provided by the second external reference source to obtain the final error signal, which is sent to the PID module for PID adjustment , and then control the laser current, because of the existence of zero frequency and the limitation of the phase detector board, its capture band range is in the range of -50MHz to 400MHz; the slow feedback path first divides the frequency by 100 times through the frequency divider, and obtains a frequency of about 68MHz The difference frequency signal is input to the second digital phase detector board and compared with the standard 68MHz signal provided by the third external reference source to obtain the final error signal, which is sent to the PID module for PID adjustment, and then the cavity length is controlled through the PZT port of the laser Change the laser frequency, because the slow feedback is a low-frequency signal obtained by direct frequency division of the beat frequency signal, and its capture band range is directly extended to -6.8GHz to 6.8GHz.

Because of the existence of slow feedback and the long-term drift of the laser at 1GHz/day, the capture band can completely cover the drift range of the laser, so the laser current and PZT are adjusted respectively through the fast and slow feedback inputs, and finally the laser is directly connected to the The reference laser is phase-locked, eliminating the need to adjust the laser current to a narrower trapping band.

Beneficial effect

1. The present invention discloses an ultra-large capture band laser phase-locking device and method based on dual-channel digital phase discrimination. It uses two heterodyne phase locks to provide fast and slow two-way feedback signals to realize laser capture with a large capture range. Semi-automatic locking of phase changes in a short period of time. At the same time, the difference frequency between laser beams can be changed by changing the external reference frequency, so that the controlled difference frequency can range from tens of MHz to several GHz. It has high locking accuracy and laser tuning. Large range, capture bandwidth and other advantages.

2. The present invention discloses an ultra-large capture band laser phase-locking device and method based on dual-channel digital phase discrimination. The optical system only needs to provide one beat frequency signal, and the circuit system uses a set of circuit boards to lock the laser phase. The invention transfers the higher controlled frequency to the lower frequency for control, and by changing the second external reference frequency, it can realize laser phase locking in a large range ranging from dozens of MHz to 7 GHz, and can greatly improve the capture band, from The level of 100MHz is increased to 6GHz, which has the advantages of large laser tuning range and capture bandwidth.

3. An ultra-large capture band laser phase-locking device and method based on dual-channel digital phase discrimination disclosed in the present invention, because of the existence of slow feedback and the long-term drift of the laser at 1GHz/day, the capture band of the present invention can completely cover the drift of the laser Therefore, the laser current and PZT are adjusted respectively through the fast and slow feedback inputs, and finally the phase interlocking of the direct laser and the reference laser is realized after power-on, and there is no need to adjust the laser current to a narrower capture band.

4. A super-large capture band laser phase-locking device and method based on dual-channel digital phase discrimination disclosed in the present invention, after the capture band is raised, it is significantly greater than the long-term drift of the laser's own frequency (DFB, DBR lasers, the daily drift is 100MHz, and the monthly drift is 100MHz. Drift/annual drift is generally less than 1GHz), enabling it to achieve semi-automatic locking.

Description of drawings

FIG. 1 is a schematic structural diagram of a super-large capture band laser phase-locking device based on dual-channel digital phase detection provided by an embodiment of the present invention.

Among them, 1-master laser (reference laser); 2-slave laser (laser to be locked); 3-first BS; 4-second BS; 5-high-speed detector; 6-6.2~7.2G power amplifier; 7- Power divider; 8-first external reference source; 9-mixer; 10-low frequency amplifier; 11-coupler; 12-second external reference source; 13-first digital phase detector board; 14-frequency divider , 15-the third external reference source; 16-the second digital phase detector board; 17-PID module.

detailed description

In order to better illustrate the purpose and advantages of the present invention, the content of the invention will be further described below in conjunction with the accompanying drawings and examples.

Example 1:

As shown in FIG. 1 , it is a dual-channel digital phase detection-based ultra-large capture band laser phase-locking device disclosed in this embodiment, including an optical beat frequency system and a digital frequency-frequency detection system. Optical beat frequency system includes master laser 1, slave laser 2, first BS3, second BS4, high-speed detector 5; digital frequency discrimination and phase discrimination system includes 6.5-7.2G high-power amplifier 6, power divider 7, first external Reference source 8, mixer 9, low frequency amplifier 10, coupler 11, second external reference source 12, first digital phase detector board 13, frequency divider 14, third external reference source 15, second digital phase detector board 16. PID module 17, etc. All devices between digital frequency and phase detectors are connected by coaxial cables to form a control circuit system. The digital phase detector board includes a digital comparator, a digital phase detector, and a differential amplifier.

The master laser 1 acts as a reference laser whose frequency is locked via the MTS. The slave laser 2 is the laser to be locked, its frequency is not locked, and its frequency can be changed by changing its current and cavity length. After the master and slave lasers pass through the first BS3 and the second BS4 respectively, a small part is selected as the input of the frequency discrimination and phase discrimination laser, and the two beams combine and enter the high-speed detector 5 to measure the beat frequency signal. After the beat frequency signal is amplified by the 6.5-7.2G high-power amplifier 6, it is divided into two paths by the power divider 7, which are respectively used as radio frequency signals for fast feedback and slow feedback.

The fast feedback path first provides a standard 6.8GHz local oscillator and beat frequency signal from the first external reference source 8 and mixes it through the mixer 9 to obtain a difference frequency signal with a frequency of about 50M, which is amplified by the low frequency amplifier 10 Input to the coupler 11, the coupler 11 couples out a small part of its difference frequency signal and connects it to the spectrum analyzer for monitoring, monitoring its frequency and phase changes, and observing whether it is unlocked; the other main part is input to the first Digital phase discriminator 13, and compare the phase with the standard 50M signal provided by the second external reference source 12 to obtain the final error signal, send it to PID module 17 for PID adjustment, and then control the laser current through the laser ModDC port to change the laser frequency, because Due to the existence of zero frequency and the limitation of the phase detector board, its capture band ranges from -50MHz to 400MHz.

The slow feedback path first divides the frequency by 100 times through the frequency divider 14 to obtain a difference frequency signal with a frequency of about 68MHz, which is input to the second digital phase detector board 16 and phased with the standard 68MHz signal provided by the third external reference source 15 Comparing, the final error signal is obtained, which is sent to the PID module 17 for PID adjustment, and then the laser frequency is changed by controlling the cavity length through the PZT port of the laser, because the slow feedback is a low-frequency signal obtained by direct frequency division of the beat frequency signal, and its capture band range is directly pulled As large as -6.8GHz to 6.8GHz.

Because the long-term drift of the laser is 1GHz/day, the existence of slow feedback in this method enables the capture zone to completely cover the drift range of the laser, so the laser current and PZT are adjusted respectively through the fast and slow feedback inputs, and finally no need to re- Adjust the laser current to a narrower capture band, first close the slow feedback loop to reach the laser frequency near 6.8 GHz we need, then close the fast feedback loop, feed back the error signal, and perform phase adjustment between the laser and the reference laser Precise locking to achieve semi-automatic phase locking.

The specific description above further elaborates the purpose, technical solution and beneficial effect of the invention. It should be understood that the above description is only a specific embodiment of the present invention and is not used to limit the protection of the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims (4)

1. The utility model provides a super large capture band laser phase locking device based on two-way digital phase demodulation which characterized in that: the system comprises an optical beat frequency system and a digital frequency and phase discrimination system; the optical beat frequency system comprises a master laser (1), a slave laser (2), a first BS (3), a second BS (4) and a high-speed detector (5); the digital phase frequency and phase detection system comprises a power amplifier (6), a power divider (7), a first external reference source (8), a mixer (9), a low-frequency amplifier (10), a coupler (11), a second external reference source (12), a first digital phase detection plate (13), a frequency divider (14), a third external reference source (15), a second digital phase detection plate (16) and a PID module (17); all devices between the digital phase frequency detector are connected through coaxial cables to form a control circuit system; the first digital phase detection plate (13) and the second digital phase detection plate (16) respectively comprise a digital comparator, a digital phase detector and a differential amplifier.

2. The ultra-large capture band laser phase locking device based on two-way digital phase discrimination as claimed in claim 1, characterized in that: the power amplifier is a 6.5-7.2G high-power amplifier.

3. A two-way digital phase discrimination based laser phase locking method for an ultra-large capture band is realized based on the two-way digital phase discrimination based laser phase locking device for the ultra-large capture band as claimed in claim 2, and is characterized in that:

the main laser (1) is used as a reference laser, and the frequency of the main laser is locked through MTS; the slave laser (2) is used as a laser to be locked, the frequency of the slave laser is not locked, and the frequency of the slave laser is changed by changing the current and the cavity length of the slave laser; a small part of the master laser and the slave laser is selected as a frequency discrimination and phase discrimination laser to be input after passing through the first BS (3) and the second BS (4) respectively, and the two lasers enter a high-speed detector (5) to measure a beat frequency signal after being combined; after being amplified by a 6.5-7.2G high-power amplifier (6), the beat frequency signal is divided into two paths by a power divider (7) and respectively used as a fast feedback radio frequency signal and a slow feedback radio frequency signal;

in the fast feedback path, a first external reference source (8) provides a standard 6.8GHz local oscillator and beat frequency signals, the local oscillator and the beat frequency signals are subjected to frequency mixing through a frequency mixer (9) to obtain difference frequency signals with the frequency of about 50M, the difference frequency signals are amplified through a low-frequency amplifier (10) and then input into a coupler (11), the coupler (11) couples out a small part of the difference frequency signals to be connected to a frequency spectrograph for monitoring, monitoring the change of the frequency and the phase of the signals, and observing whether the signals are unlocked or not; the other main part is input into a first digital phase detection plate (13), and is subjected to phase comparison with a standard 50M signal provided by a second external reference source (12) to obtain a final error signal, the final error signal is sent into a PID module (17) for PID adjustment, and then the laser current is controlled through a laser Mod DC port to change the laser frequency, and because of the existence of zero frequency and the limitation of the phase detection plate, the capture band range is in a range from-50 MHz to 400 MHz;

one path of slow feedback is firstly subjected to frequency division by 100 times through a frequency divider (14) to obtain a difference frequency signal with the frequency about 68MHz, the difference frequency signal is input into a second digital phase detection plate (16) and is subjected to phase comparison with a standard 68MHz signal provided by a third external reference source (15) to obtain a final error signal, the final error signal is sent into a PID module (17) for PID adjustment, and further the laser frequency is changed through controlling the cavity length through a PZT port of a laser, because the slow feedback is a low-frequency signal obtained by directly dividing the frequency of a beat signal, and the capture band range of the slow feedback is directly enlarged to-6.8 GHz to 6.8GHz.

4. The ultra-large capture band laser phase locking method based on two-way digital phase discrimination as claimed in claim 3, characterized in that: the laser current and the PZT are respectively adjusted through the fast feedback input and the slow feedback input, finally, the direct laser and the reference laser realize phase interlocking after the starting, and the laser current does not need to be adjusted to a narrower capture band.

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