A dual-path digital phase discrimination based ultra-wide capture range laser phase-locked loop device and method

Abstract

The application discloses a kind of based on two-way digital phase discrimination's ultra-large capture band laser phase-locked device and method, belong to optical coherence technical field.The device of the application includes optical beat frequency system and digital frequency discrimination phase discrimination system.Optical beat frequency system includes master-slave laser, first BS, second BS, high-speed detector.Digital frequency discrimination phase discrimination system includes power amplifier, power divider, frequency mixer, low-frequency amplifier, coupler, frequency divider, first digital phase discrimination board, second digital phase discrimination board, PID module, first external reference source, second external reference source, third external reference source.The application adopts two heterodyne method phase-locked, provides fast, slow two-way feedback signal, realizes the semi-automatic locking of the change of laser phase in a short time in large capture band range, simultaneously by changing external reference frequency, the beat frequency between laser beams can be changed.The application has the advantages of high locking precision, compact structure, large laser tuning range, capture bandwidth etc..

Description

TECHNICAL FIELD

[0001] The application relates to a kind of super large capture band laser phase-locked device and method based on two-way digital phase discrimination, and belongs to the field of optical coherence. BACKGROUND

[0002] With the continuous research and development of laser technology, the use of laser technology to cool atoms becomes more and more mature. The application field of cold atoms is growing, and cold atom interference is one of the popular applications. The Raman process is the core technology of atomic interference process, which links the laser field and the atomic state, and becomes an important means of manipulating and observing atoms. In such an interferometer, by using the two-photon Raman transition between the fine structure energy levels of atoms, the time interval between the light fields can be greatly improved, and ultimately the measurement accuracy of a series of data such as the rotation speed of the earth, the gravity gradient, and the gravity acceleration can be improved.

[0003] The atomic interference control process requires a pair of phase-coherent Raman lasers to achieve, and the phase noise of the Raman laser will directly introduce an interference phase shift and affect the measurement resolution of the gravity acceleration. At this time, the phase of the Raman laser is particularly important. There are three common methods for generating coherent Raman lasers: acousto-optic modulator frequency shift method, electro-optic modulator modulation sideband method, and optical phase-locked loop method.

[0004] The first acousto-optic modulator frequency shift method uses the Bragg diffraction effect of the acousto-optic modulator to make the incident laser produce diffraction. The diffraction light and the undiffraction zero-order light come from the same incident laser, and they are phase-coherent. This method is simple and convenient to use, and is very suitable for generating coherent light beams with a difference frequency below 1GHz. The disadvantage is that as the diffraction frequency increases, the diffraction efficiency decreases. The diffraction efficiency of a typical commercial acousto-optic modulator with a frequency shift of 3.4GHz is only 2%, which is unacceptable in atomic interference experiments.

[0005] The second electro-optic modulator modulation sideband method is to use an electro-optic modulator to modulate the main Raman laser to generate sidebands, and then use a Fabry-Perot cavity or an external cavity semiconductor laser as a frequency selection tool to filter out unwanted sidebands and zero-order light. At this time, the selected sideband and the main Raman laser form a coherent Raman light. The disadvantage of this method is that the unwanted sidebands and zero-order light that coincide with the required sidebands in space will affect the experiment.

[0006] The third optical phase-locked loop method is to compare the beat signal of two lasers which have not been phase-locked with a standard reference signal to obtain an error signal, and to use an electrical feedback method to lock the frequency of the beat signal to the frequency reference point, so that the coherent Raman light can be generated. In this method, the performance of the coherent Raman light is determined by the performance of the phase-locked loop circuit. Generally, when the frequency of the beat signal changes greatly, in order to ensure the phase-locked effect, the circuit must be re-adjusted. The frequency long drift of the semiconductor laser is generally at the level of 1GHz / day, and the capture band of the existing optical phase-locked loop method is generally only at the level of 100MHz, so before locking the laser each time, the current and cavity length parameters of the laser need to be carefully adjusted to make the laser beat signal fall within the capture band, and then the subsequent phase-locked process can be carried out, which increases the difficulty of experimental maintenance and reduces the stability of the system. More importantly, when the laser is out of lock due to external vibration, temperature drift and other factors, it is easy to exceed the existing capture band range, so the laser parameters need to be re-adjusted, which makes it difficult to realize the semi-automatic locking of the optical phase-locked loop. SUMMARY

[0007] In order to solve the problems that the existing optical phase-locked loop cannot be directly locked when the drift is too large, a large range of adjustment is required, the sweep frequency range is too small, the capture band is narrow, and the locking precision is low, the purpose of the present application is to provide a super large capture band laser phase-locked device and method based on double channel digital phase discrimination, which adopts twice heterodyne phase-locked method to provide fast and slow feedback signals, realizes semi-automatic locking of the laser phase change in a short time in a large capture band range, and can change the difference frequency between the laser beams by changing the external reference frequency, so that the controlled difference frequency can be from tens of MHz to several GHz. The present application can reduce the influence of Raman phase noise on the measurement accuracy, has the advantages of high locking precision, compact and simple structure, large laser tuning range, wide capture band, etc., and meets the use requirements of the atomic interferometer.

[0008] The purpose of the present application is realized by the following technical solutions.

[0009] The super large capture band laser phase-locked device based on double channel digital phase discrimination disclosed by the present application comprises an optical beat system and a digital frequency and phase discriminator system. The optical beat system comprises master and slave lasers, a first BS, a second BS and a high-speed detector. The digital frequency and phase discriminator system comprises a power amplifier, a power divider, a frequency mixer, a low-frequency amplifier, a coupler, a frequency divider, a first digital phase discriminator board, a second digital phase discriminator board, a PID module, a first external reference source, a second external reference source and a third external reference source. All devices between the digital frequency and phase discriminators are connected through coaxial cables to form a control circuit system. The first digital phase discriminator board and the second digital phase discriminator board respectively comprise a digital comparator, a digital phase discriminator and a differential amplifier.

[0010] Preferably, the power amplifier is a 6.5-7.2G high-power amplifier.

[0011] The application also discloses a laser phase-locked method based on a double-path digital phase discrimination and a large capture band, and is realized based on the laser phase-locked device based on the double-path digital phase discrimination.

[0012] The main laser is taken as a reference laser, first frequency-locked, and then a light beam is branched out through the first BS, a small part is selected from the laser through the second BS, and the two are combined to enter a high-speed detector to measure a beat signal. The beat signal is amplified by an amplifier and then is divided into two paths by a power divider, and is taken as a radio frequency signal of fast feedback and slow feedback respectively. The fast feedback path is first mixed with a standard 6.8GHz local oscillator signal provided by a first external reference source to obtain a difference frequency signal with a frequency of about 50M, the signal is amplified and input to a coupler, the coupler couples a small part of the difference frequency signal for monitoring, and the main part is input to a digital phase discriminator and compared with a standard 50M signal provided by a second external reference source to obtain a final error signal, which is input to a PID module for PID adjustment, and then the laser current is controlled, because of the existence of zero frequency and the limitation of the phase discriminator, the capture band range is-50MHz to 400MHz; the slow feedback path is first divided by 100 by a frequency divider to obtain a difference frequency signal with a frequency of about 68MHz, which is input to a second digital phase discriminator and compared with a standard 68MHz signal provided by a third external reference source to obtain a final error signal, which is input to a PID module 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 directly dividing the beat signal, the capture band range is directly enlarged to-6.8GHz to 6.8GHz.

[0013] Because of the existence of the slow feedback and the long drift of the laser is 1GHz / day, the capture band can completely cover the drift range of the laser, so the laser current and the PZT are adjusted by the fast and slow feedback inputs respectively, and finally the phase interlocking between the laser and the reference laser is realized after starting, without the need to adjust the laser current to a relatively narrow capture band.

[0014] Advantages

[0015] 1. The laser phase-locked device and method based on the double-path digital phase discrimination disclosed by the application adopt a double heterodyne method for phase locking, provide fast and slow feedback signals, realize the semi-automatic locking of the laser phase change in a large capture band range in a short time, and can change the difference frequency between the laser beams by changing the external reference frequency, so that the controlled difference frequency can be different from tens of MHz to several GHz, and has the advantages of high locking precision, large laser tuning range, wide capture band and the like.

[0016] 2. The application discloses a kind of based on dual-path digital phase discrimination's super large capture band laser phase-locked device and method, optical path system only needs to provide one way beat frequency signal, circuit system uses a set of circuit board, and laser phase can be locked.The application transfers higher controlled frequency to low frequency to control, and the laser phase locking in the large range of several tens of MHz to 7GHz can be realized by changing second external reference frequency, and the capture band can be greatly improved, from 100MHz level to 6GHz, with the advantages of large laser tuning range, capture bandwidth etc.

[0017] 3. The application discloses a kind of based on dual-path digital phase discrimination's super large capture band laser phase-locked device and method, because of the existence of slow feedback and the long drift of laser is 1GHz / day, the capture band of the application can completely cover the drift range of laser, so by fast, slow two-way feedback input respectively adjusting laser current and PZT, finally, laser and reference laser are realized phase interlock after starting, and laser current does not need to be adjusted to the capture band of relatively narrow again.

[0018] 4. The application discloses a kind of based on dual-path digital phase discrimination's super large capture band laser phase-locked device and method, after capture band is promoted, it is significantly greater than the long drift (DFB, DBR laser, daily drift is 100MHz, monthly drift / year drift is generally less than 1GHz) of laser itself, so that it can realize semi-automatic locking. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 The structure schematic diagram of a kind of based on dual-path digital phase discrimination's super large capture band laser phase-locked device provided by the embodiment of the application.

[0020] Wherein, 1-main 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 discrimination board;14-frequency divider, 15-third external reference source;16-second digital phase discrimination board;17-PID module. DETAILED DESCRIPTION

[0021] In order to better illustrate the purpose and advantages of the application, the content of the application is further described below in combination with the drawings and examples.

[0022] Example 1:

[0023] As Figure 1As shown, the laser phase-locked device based on two-way digital phase discrimination and ultra-large capture band disclosed in the embodiment includes an optical beat frequency system and a digital frequency discrimination and phase discrimination system. The optical beat frequency system includes a master laser 1, a slave laser 2, a first BS 3, a second BS 4, and a high-speed detector 5. The digital frequency discrimination and phase discrimination system includes a 6.5-7.2G large-power amplifier 6, a power divider 7, a first external reference source 8, a frequency mixer 9, a low-frequency amplifier 10, a coupler 11, a second external reference source 12, a first digital phase discrimination board 13, a frequency divider 14, a third external reference source 15, a second digital phase discrimination board 16, a PID module 17, etc. All devices between the digital frequency discrimination and phase discrimination boards are connected through a coaxial cable to form a control circuit system. The digital phase discrimination board includes a digital comparator, a digital phase discriminator, and a differential amplifier.

[0024] The master laser 1 serves as a reference laser, and its frequency is locked via an MTS. The slave laser 2 serves as a laser to be locked, and its frequency is not locked and can be changed by changing its current and cavity length. The master and slave lasers select a small part as a frequency discrimination and phase discrimination laser input after passing through the first BS 3 and the second BS 4, respectively. The two are combined and enter the high-speed detector 5 to measure the beat frequency signal. The beat frequency signal is amplified by the 6.5-7.2G large-power amplifier 6, and then divided into two paths by the power divider 7 as radio frequency signals for fast feedback and slow feedback, respectively.

[0025] The fast feedback path first mixes the standard 6.8GHz local oscillator provided by the first external reference source 8 with the beat frequency signal through the frequency mixer 9 to obtain a difference frequency signal with a frequency of about 50M, which is amplified by the low-frequency amplifier 10 and then input to the coupler 11. The coupler 11 couples out a small part of the difference frequency signal to a spectrum analyzer for monitoring the frequency and phase changes and observing whether the lock is lost. The other main part is input to the first digital phase discrimination board 13 and compared in phase with the standard 50M signal provided by the second external reference source 12 to obtain the final error signal, which is sent to the PID module 17 for PID adjustment, and then the laser current is controlled through the laser ModDC port to change the laser frequency. Because of the existence of zero frequency and the limitation of the phase discrimination board, the capture band range is-50MHz to 400MHz.

[0026] The slow feedback path is first divided by 100 times by the frequency divider 14 to obtain a difference frequency signal with a frequency of about 68MHz, which is input to the second digital phase discrimination board 16 and compared in phase with the standard 68MHz signal provided by the third external reference source 15 to obtain the final error signal, which is sent to the PID module 17 for PID adjustment, and then the cavity length is controlled through the laser PZT port to change the laser frequency. Because the slow feedback is a low-frequency signal obtained by directly dividing the beat frequency signal, the capture band range is directly enlarged to-6.8GHz to 6.8GHz.

[0027] Because the long drift of the laser is 1GHz / day, the slow feedback loop can cover the whole drift range of the laser, so the laser current and PZT are adjusted by the fast and slow feedback loops respectively, and finally the laser current does not need to be adjusted to a narrow capture band after the start, and the slow feedback loop is closed first to reach the required 6.8GHz, and then the fast feedback loop is closed to achieve the precise locking of the laser and the reference laser phase, and the semi-automatic phase locking is realized.

[0028] The above detailed description further describes the purpose, technical scheme and beneficial effects of the application. It should be understood that the above description is only a specific embodiment of the application and is not used to limit the protection scope of the application. Any modification, equivalent replacement, improvement, etc. within the spirit and principle of the application should be included in the protection scope of the application.

Claims

1. A dual-path digital phase-discriminated ultra-wide capture range laser phase-locked loop device, characterized in that: It includes optical beat system and digital frequency discriminator phase discriminator system; the optical beat system includes main laser (1), slave laser (2), first BS (3), second BS (4), high-speed detector (5); the digital frequency discriminator phase discriminator system includes 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 discriminator board (13), frequency divider (14), third external reference source (15), second digital phase discriminator board (16), PID module (17); all devices between the digital frequency discriminator phase discriminator system are connected through coaxial cable, and constitute control circuit system; the first digital phase discriminator board (13) and the second digital phase discriminator board (16) respectively include digital comparator, digital phase discriminator and differential amplifier; The power amplifier is 6.5-7.2G high-power amplifier; The main laser (1) is used as reference laser, and the frequency thereof is locked via MTS; The slave laser (2) is used as laser to be locked, and the frequency thereof is not locked, and the frequency thereof is changed by changing current and cavity length; the main laser and the slave laser are selected by first BS (3) and second BS (4) respectively, and a small part is selected as frequency discriminator phase discriminator laser input; after the two are combined, the beat signal is measured by high-speed detector (5); after the beat signal is amplified by 6.5-7.2G high-power amplifier (6), the beat signal is divided into two paths by power divider (7), and the two paths are used as radio frequency signals of fast feedback and slow feedback respectively; The fast feedback path is first mixed with the beat signal by first external reference source (8) providing standard 6.8GHz local oscillator, and the difference frequency signal is amplified by low-frequency amplifier (10) and input to coupler (11); the coupler (11) couples a small part of the difference frequency signal to a spectrum analyzer to monitor the frequency and phase changes and observe whether the lock is lost; another main part is input to the first digital phase discriminator board (13), and the standard 50M signal provided by the second external reference source (12) is compared in phase to obtain the final error signal, which is sent to the PID module (17) for PID adjustment, and then the laser current is controlled through the Mod DC port of the laser to change the laser frequency; because of the existence of zero frequency and the limitation of the phase discriminator board, the capture band range is-50MHz to 400MHz; The slow feedback path is first divided by 100 times by frequency divider (14) to obtain the difference frequency signal, which is input to the second digital phase discriminator board (16) and compared in phase with the standard 68MHz signal provided by the third external reference source (15) to obtain the final error signal, which is sent to the PID module (17) for PID adjustment, and then the cavity length is controlled through the PZT port of the laser to change the laser frequency; because the slow feedback is a low-frequency signal obtained by directly dividing the beat signal, the capture band range is directly enlarged to-6.8GHz to 6.8GHz.

2. A dual-path digital phase discrimination based ultra-wide capture range laser phase-locked loop device as claimed in claim 1, wherein: The laser current and PZT are adjusted by fast and slow feedback inputs respectively, and finally the phase interlock between the laser and the reference laser is realized after starting, without the need of adjusting the laser current into a relatively narrow capture band.

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