CN110829167A - Method and system for suppressing single-frequency phase noise of laser - Google Patents

Abstract

The invention discloses a system for suppressing single-frequency phase noise of a laser, comprising: a laser for generating a laser beam; an optical isolator for outputting a first laser beam to isolate the first laser beam and returning to the laser; a phase noise suppression module, Used to eliminate the single-frequency phase noise of the first laser beam, and output the second laser beam; PDH frequency stabilization module, by performing sideband modulation processing and resonance locking processing on the second laser beam to output a frequency error signal; The first servo The loop module is used to operate the frequency error signal to output the frequency adjustment signal; the laser controller is used to output the control signal to control the laser, and adjust the control signal according to the frequency adjustment signal to correct the frequency error of the laser and make the The laser outputs frequency-stabilized laser light. The invention also discloses a method for suppressing the phase noise of the laser. The method and system for suppressing the phase noise of the laser disclosed in the present invention can effectively suppress the phase noise of the laser.

Description

Method and system for suppressing single-frequency phase noise of laser

technical field

The invention relates to the field of laser noise, in particular to a method and system for suppressing single-frequency phase noise of a laser.

Background technique

Since the birth of lasers in the 1960s, laser-related technologies have developed rapidly. Due to the good monochromaticity, long coherence time and good directionality of lasers, lasers have been widely used in scientific research and industrial fields. In some spectral experiments, such as spectral detection, optical frequency scaling, quantum computing and other fields, higher requirements are also placed on the performance of lasers, including narrow linewidth, single longitudinal mode, etc. However, during use, it was found that the laser light emitted by the laser is not an ideal single-frequency oscillation, but has noise, that is, random fluctuations in frequency, phase and amplitude. The existence of noise has a negative impact on many research fields such as atomic clocks, quantum computing, optical sensing, coherent communication, and gravitational wave detection. Therefore, the suppression of laser noise is of great significance.

The noise of the laser includes optical power noise and phase noise. Among them, the commonly used methods to suppress the phase noise are: 1. Lock the frequency of the laser to the atomic absorption spectrum; 2. Lock the laser to a stable optical resonant cavity. On the basis of these two methods, the line width of the laser can be narrowed to the level of mHz. When the line width is narrowed to a certain extent, some noise components will be reflected. In laser types such as Ti:sapphire lasers, the etalon is used to stabilize the frequency of the output laser, in which the length of the etalon needs to be modulated. The noise caused by this modulation signal has a single frequency, and the amplitude exceeds the general noise, which is useful for precision spectroscopy experiments. greater impact. Experiments show that the above noise suppression techniques are not ideal for noise suppression.

For the measurement of the phase noise of the laser, there are mainly the following methods: For the laser with the line width above the megahertz level, the method of scanning the F-P interferometer can be used for measurement. There are generally three methods for laser measurement with narrower linewidth: one method is to convert the optical frequency signal to the electrical frequency band by performing optical heterodyne mixing of two lasers with similar frequency and performance. Two lasers can only estimate the linewidth of the laser; to measure the noise spectrum, three lasers are required to perform two beat frequencies, and the spectral components of each laser can be obtained through correlation analysis. The solution of multiple lasers is expensive and not suitable for general laboratory conditions. The second method is achieved through the time-lapse selfie frequency technology. This method mainly uses long optical fiber, optical fiber loop and time-lapse Selfie frequency based on Michelson interferometer to measure laser noise. This method is sensitive to environmental influences. For noise suppression, there are few feasible solutions at present. One is to use the transmitted light of the optical resonator to perform injection locking on the basis of the frequency stabilization technology of the optical resonator, and further perform optical amplification to realize the output of the laser. Since the transmission power of the ultra-stable cavity is usually not large, this method requires a multi-stage amplification structure, which is relatively complicated.

SUMMARY OF THE INVENTION

The present invention aims to solve one of the technical problems in the related art at least to a certain extent. To this end, an object of the present invention is to provide a method for suppressing phase noise of a laser.

The technical scheme adopted in the present invention is:

In a first aspect, the present invention provides a system for suppressing single-frequency phase noise of a laser, including:

a laser for generating a laser beam;

an optical isolator, allowing one-way input of the laser beam and outputting a first laser beam to isolate the first laser beam and return to the laser;

a phase noise suppression module for eliminating the single-frequency phase noise of the first laser beam and outputting a second laser beam;

PDH (Pound-Drever-Hall) frequency stabilization module, by performing sideband modulation processing and resonance locking processing on the second laser beam to output a frequency error signal;

a first servo loop module, configured to operate on the frequency error signal to output a frequency adjustment signal;

The laser controller is used to output a control signal to control the laser, and adjust the control signal according to the frequency adjustment signal to correct the frequency error of the laser and make the laser output stable frequency laser.

Further, the phase noise suppression module is electrically connected with the laser controller to collect the control signal of the laser controller as a source signal and output.

Further, the phase noise suppression module includes:

a noise sampling circuit module for collecting the single-frequency phase noise of the first laser beam and outputting a source signal;

an amplitude modulation and phase shift amplifying module for performing amplitude modulation and phase shifting on the source signal to output a phase error signal;

The lock-in amplifier circuit module is electrically connected with the PDH frequency stabilization module, and is used for lock-in amplification of the frequency error signal output by the PDH frequency stabilization module and the source signal output by the noise sampling circuit module, and operation to output the amplitude error signal;

a first mixer, configured to mix the phase error signal and the amplitude error signal to output a deviation correction signal;

The first electro-optic modulator performs anti-phase modulation on the first laser beam output by the optical isolator according to the deviation correction signal to eliminate the single-frequency phase noise of the first laser beam, and outputs a second laser beam output .

Further, the phase noise suppression module also includes:

a first photodetector for detecting the second laser beam to obtain an intensity signal;

A spectrum analyzer, electrically connected to the PDH frequency stabilization module, for analyzing the frequency error signal output by the PDH frequency stabilization module and the intensity signal detected by the first photodetector, and obtaining the first Phase noise spectrum of two laser beams.

Further, the PDH frequency stabilization module includes:

Modulation signal generator for generating frequency modulation signal;

The second electro-optic modulator, driven by the frequency modulation signal, modulates the second laser beam, so that the second laser beam carries the sideband signal to become the third laser beam output;

an ultra-stable cavity, which is incident by the third laser beam, resonates with multiple beams in the cavity, and reflects and outputs a fourth laser beam;

a second photodetector for detecting the fourth laser beam to obtain an interference signal;

The second frequency mixer is configured to perform frequency mixing processing on the frequency modulation signal generated by the modulation signal generator and the interference signal detected by the second photodetector, so as to obtain the frequency error signal output.

Further, the PDH frequency stabilization module also includes:

1/2 wave plate, used to adjust the linear polarization direction of the third laser beam;

a polarizing beam splitter, which is used for polarizing and splitting the third laser beam and the fourth laser beam incident thereon, so that the third laser beam is transmitted and the fourth laser beam is reflected;

A 1/4 wave plate, placed between the superstable cavity and the polarization beam splitter, is used to adjust the polarization states of the third laser beam and the fourth laser beam, so that the third laser beam before adjustment It is perpendicular to the polarization of the adjusted fourth laser beam.

To this end, the second object of the present invention is to provide a method for suppressing the phase noise of a laser.

In a second aspect, an embodiment of the present invention provides a method for suppressing single-frequency phase noise of a laser, including:

The control signal is output through the laser controller to control the laser and output the laser beam;

The laser beam generated by the laser is isolated by the optical isolator, and the first laser beam is output;

The phase noise suppression module is used to eliminate the single-frequency phase noise of the first laser beam, and output the second laser beam;

The PDH frequency stabilization module is used to perform sideband modulation processing and resonance locking processing on the second laser beam to obtain a frequency error signal;

The frequency error signal is operated by the first servo loop module to output a frequency adjustment signal;

The laser controller adjusts the control signal according to the frequency adjustment signal, so as to correct the frequency error of the laser, and make the laser output frequency-stabilized laser light.

Further, eliminating the single-frequency phase noise of the first laser beam includes:

Collect the modulation signal of the laser controller through the noise sampling circuit module, and output the source signal;

The source signal is amplitude-modulated and phase-shifted by the amplitude-modulation and phase-shift amplifying module to output a phase error signal;

The source signal and the frequency error signal are phase-locked, amplified and operated by a phase-locked amplifier circuit module to output an amplitude error signal;

Mixing the phase error signal and the amplitude error signal by a first mixer to output a deviation correction signal;

According to the deviation correction signal, the first laser beam is modulated by the first electro-optic modulator to output the second laser beam;

Detecting the second laser beam by a first photodetector to output an intensity signal;

Analyze the intensity signal and the frequency error signal by a spectrum analyzer to obtain a phase noise spectrum of the second laser beam;

The amplitude modulation value and phase shift value of the source signal are adjusted by the amplitude modulation phase shift amplification module until the phase noise displayed in the spectrum analyzer is zero, so that the first electro-optic modulator can perform noise reduction on the first laser beam. Equal-amplitude inverse modulation to eliminate single-frequency phase noise.

Further, the step of performing sideband modulation and resonance locking on the second laser beam to output a frequency error signal further includes:

The second electro-optic modulator is driven by the frequency modulation signal generated by the modulation signal generator to perform sideband modulation on the second laser beam and output a third laser beam;

Through 1/2 wave plate, polarization beam splitter transmission, 1/4 wave plate to carry out polarization processing on the incident of the third laser beam, incident to the ultra-stable cavity for resonance locking processing, and reflected and output the fourth laser beam;

outputting a coherent signal by detecting the fourth laser beam via the second photodetector;

The coherent signal and the frequency modulated signal are mixed by a second mixer to output a frequency error signal.

The beneficial effects of the present invention are:

The present invention adopts the PDH frequency-stabilized error signal to input the spectrum analyzer to observe the phase noise spectrum, and the modulation source signal is phase-shifted and injected into the first electro-optic modulator for anti-phase modulation to suppress the phase noise.

In addition, the present invention also uses the lock-in amplifier module to extract the phase noise amplitude signal as the amplitude error signal to suppress the change of the phase noise amplitude. A photodetector is used to detect the outgoing light signal and input the spectrum analyzer to observe the amplitude noise spectrum. In addition, the residual amplitude modulation noise of the first electro-optic modulator can be suppressed by using methods such as temperature control and bias voltage application.

Description of drawings

FIG. 1 is a schematic structural diagram of a laser phase noise suppression system according to the first embodiment of the present invention.

FIG. 2 is a schematic flowchart of a laser phase noise suppression according to the second embodiment of the present invention.

Description of reference numerals

name label name label laser 1 Laser Controller 2 Optical isolator 3 The first servo loop module 4 PDH frequency stabilization module 5 Phase Noise Suppression Module 6 Modulation Signal Generator 51 second electro-optic modulator 52 Super stable cavity 53 second photodetector 54 second mixer 55 1/2 wave plate 56 Polarizing Beamsplitters 57 1/4 wave plate 58 Noise sampling circuit module 61 AM/Phase Amplifier Module 62 Lock-in amplifier circuit module 63 first mixer 64 first electro-optic modulator 65 first electro-optical detector 66 Spectrum Analyzer 67 first laser beam 11 second laser beam 12 third laser beam 13 Fourth laser beam 14

Detailed ways

It should be noted that the embodiments in the present application and the features of the embodiments may be combined with each other in the case of no conflict.

Please refer to FIG. 1. FIG. 1 is a schematic structural diagram of a system for suppressing single-frequency phase noise of a laser according to an embodiment of the present invention. An embodiment of the present invention provides a system for suppressing phase noise of a laser, which includes: a laser 1 for generating a laser beam; an optical isolator 3 for allowing the laser beam to be input in one direction and outputting a first laser beam 11 for isolation The first laser beam 11 returns to the laser 1; the phase noise suppression module 6 is used to eliminate the single-frequency phase noise of the first laser beam 11 and output the second laser beam 12; PDH (Pound-Drever-Hall ) frequency stabilization module 5, by carrying out sideband modulation processing and resonance locking processing to the input of the second laser beam 12, and outputting a frequency error signal; the first servo loop module 4 is used to calculate the frequency error signal, Adjust the signal with the output frequency; the laser controller 2, electrically connected with the PDH frequency stabilization module, is used to output a control signal to control the laser 1, and adjust the control signal according to the frequency adjustment signal to correct the frequency error of the laser 1, and Make laser 1 output frequency-stabilized laser light.

The phase noise suppression module 6 is electrically connected with the laser controller 2 to collect the control signal of the laser controller 2 as a source signal and output.

The phase noise suppression module 6 includes: a noise sampling circuit module 61 for collecting the single-frequency phase noise of the first laser beam 11 and outputting a source signal; an amplitude modulation and phase shift amplifying module 62 for amplitude modulation and shift of the source signal phase, to output the phase error signal; the lock-in amplifier circuit module 63 is used to lock-in amplify and operate the frequency error signal output by the PDH frequency stabilization module 5 and the source signal output by the noise sampling circuit module 61 to output the amplitude error signal The first frequency mixer 64 is used to mix the phase error signal and the amplitude error signal to output the deviation correction signal; The first electro-optical modulator 65 carries out the first laser beam 11 output by the optical isolator 3 according to the deviation correction signal. Inverse phase modulation to eliminate the single-frequency phase noise of the first laser beam 11 and output the second laser beam 12 .

The phase noise suppression module 6 also includes: a first photodetector 66 for detecting the second laser beam 12 to obtain an intensity signal; a spectrum analyzer 67, electrically connected to the PDH frequency stabilization module 5, for connecting the PDH frequency stabilization module The output frequency error signal and the intensity signal detected by the first photodetector 66 are analyzed, and the phase noise spectrum of the second laser beam 12 is obtained.

The above-mentioned PDH frequency stabilization module 5 includes: a modulation signal generator 51 for generating a frequency modulation signal; a second electro-optical modulator 52, driven by the frequency modulation signal, modulates the second laser beam 12 to make the second laser beam 12 carries the sideband signal to become the output of the third laser beam 13; the ultra-stable cavity 53 is incident by the third laser beam 13, and is reflected by the multi-beam resonance in the cavity to output the fourth laser beam 14; the second photodetector 54 is used for The fourth laser beam 14 is detected to obtain the interference signal; the second mixer 55 is used for mixing the frequency modulation signal generated by the modulation signal generator 51 and the interference signal detected by the second photodetector 54 to obtain Frequency error signal output.

The above-mentioned PDH frequency stabilization module also includes: a 1/2 wave plate 56 for adjusting the linear polarization direction of the third laser beam 13; a polarizing beam splitter 57 for the incident on the third laser beam 13 and the fourth laser beam The beam 14 is polarized and split, so that the third laser beam 13 is transmitted and the fourth laser beam 14 is reflected; the 1/4 wave plate 58 is placed between the ultra-stable cavity 53 and the polarizing beam splitter 57 for The polarization states of the third laser beam 13 and the fourth laser beam 14 are adjusted so that the polarization of the third laser beam 13 before adjustment and the polarization of the fourth laser beam 14 after adjustment are perpendicular.

The second electro-optical modulator 52 is used to modulate the second laser beam 12, so that the third laser beam 13 not only carries phase noise, but also adds a sideband signal.

By obtaining the noise source from the laser controller 2, the use of a phase locked loop can be avoided.

The intensity signal received by the first photodetector 66 is analyzed by the spectrum analyzer 67. When the amplitude modulation and phase shift amplifying module 62 is used for inverse modulation, the sideband signal changes with the adjustment until the amplitude reaches 0. If the amplitude changes, the lock The phase amplifier module generates an error feedback signal, and sends the error feedback signal to the first mixer 64 through the second servo loop module to feedback control the amplitude of the anti-phase modulation.

The first mixer 64 performs frequency mixing processing on the phase error signal and the amplitude error signal to form an offset correction signal and transmits it to the first electro-optic modulator 65 to adjust the first laser beam 11 . By controlling the adjustment parameters of the amplitude modulation and phase shift amplifying module 62, the modulation signal and the noise signal are equal in phase inversion, so as to eliminate the phase noise in the laser beam.

In other embodiments, methods such as temperature control and bias voltage application may be used to suppress the residual amplitude modulation noise of the first electro-optic modulator 65 .

In the process of inverse-phase modulation of the phase noise by the first electro-optic modulator 65, phase matching and amplitude matching are performed. Phase matching is achieved by means of feedforward, and amplitude matching is achieved by negative feedback. Please refer to Figure 1 again. After the laser beam with phase noise passes through the isolator, its incident light can be expressed by the following equation

E _inc =E _o e ^{i(ωt+βsinΩt)} (1)

where E ₀ is the electric field strength of the laser, ω is the frequency of the laser carrier, the frequency of the Ω sideband noise (which originates from the frequency modulation inside the laser and is the main noise suppressed in this embodiment), and β is the noise modulation depth.

In the initial stage of eliminating phase noise, since a good phase error signal and amplitude error signal are not obtained, the phase noise is not suppressed, and the first electro-optic modulator 65 does not modulate the first laser beam 11 .

When the second laser beam 22 with phase noise passes through the second electro-optical modulator 52 , the second laser beam 22 will increase the sideband signal modulated by the second electro-optical modulator 52 in addition to its own phase noise. , to be used for PDH frequency stabilization, the frequency of this sideband is generally 5MHz to tens of megahertz, and the expression of the laser beam fluctuation is:

where β ₂ is the frequency modulation depth, Ω ₂ is the modulation frequency of the second electro-optic modulator 52 , and the phases of the two modulated sidebands are opposite. After the third laser beam 13 passes through the polarization beam splitter 57 , the third laser beam 13 is incident on the ultra-stable cavity 53 , oscillates in the ultra-stable cavity 53 , and partially reflects back along the original optical path. The light beam reflected after oscillating through the ultra-stable cavity 53 interferes with the light beam that is not directly reflected by the ultra-stable cavity 53 to form coherent light and is received by the second photodetector 54 .

The following describes the formula of the change before and after the reflection of the third laser beam 13 in the ultra-stable cavity 53. If the electric field before the third laser beam 14 is incident is E _o e ^iωt , and the electric field when the fourth laser beam 14 is emitted is E ₁ e ^iωt , then with reflection coefficient

r is the reflection coefficient of the cavity mirror in the ultra-stable cavity 53 , and Δv _fsr is the free spectral path of the ultra-stable cavity 53 . The laser electric field of the third laser beam 13 modulated by the second electro-optic modulator 52 entering the ultra-stable cavity is obtained by the first-order expansion of equation (2)

The electric field of the interference light formed by the interference between the light beam that does not enter the ultra-stable cavity 53 directly reflected and the reflected light after entering the ultra-stable cavity 53 is:

除去第二光电探测器54无法响应的光频段，其中探测电流的频率成分会包括Ω、Ω₂、Ω₂+Ω、Ω₂-Ω、2Ω以及2Ω₂等。The light field of the interference light formed by the total reflected light beam can be detected by the second photodetector 55, and its light intensity is detected,

Excluding the optical frequency band to which the second photodetector 54 cannot respond, the frequency components of the detection current may include Ω, Ω ₂ , Ω ₂ +Ω, Ω ₂ −Ω, 2Ω, and 2Ω ₂ .

In addition, since the phase noise of the laser 1 is mainly located in the low frequency band, Ω ₂ >>Ω is satisfied. In the PDH frequency stabilization stage, the signal of the second photodetector 55 is mixed with the modulation signal of the electro-optical modulator by the second mixer 54 to form a mixed signal, and the mixed signal is mixed by a low-pass filter (not shown in the figure). After the mixed signal passes through the low-pass filter, the high-frequency components are filtered out, leaving only the frequency components much smaller than Ω ₂ . Therefore, the frequency components in the photocurrent are adjacent to Ω ₂ such as Ω ₂ , Ω ₂ +Ω, Ω ₂ -Ω frequency components are the target frequencies, they are mixed with the modulation signal source with frequency Ω ₂ and filter out high-frequency signals, Obtain the error signal for PDH frequency stabilization. The current signal components related to the noise modulation β detected by the second photodetector 54 are:

When the laser beam is located in the resonator, the amplitudes of the (Ω ₂ -Ω) and (Ω ₂ +Ω) components in the above formula are equal, select the optimal phase φ, mix the signal with sin[Ω ₂ t+φ] and combine Perform low-pass filtering to demodulate the Ω component signal. The magnitude of the Ω component, which is proportional to the magnitude of the phase noise J ₁ [β], is analyzed by a spectrum analyzer under otherwise stable conditions.

As shown in FIG. 1 , the source signal F*sin(Ωt+φ) containing the modulated noise signal Ω is extracted from the laser 1 through the noise sampling circuit module 61 and transmitted to the amplitude modulation phase shift amplifier circuit module 62 and the lock-in amplifier respectively. (not shown). The source signal is adjusted by the amplitude modulation and phase shift amplifier circuit module 62, and the first electro-optic modulator 65 is acted on by the first mixer 64 to directly reverse modulate the laser beam and eliminate the phase noise sideband. Check the signal containing Ω through the spectrum analyzer 67, and adjust the phase and amplitude of the first electro-optical modulator 67, so that the Ω signal on the spectrum analyzer 51 is reduced, and phase noise can be suppressed. The phase matching is achieved by means of feedforward, and the amplitude matching is achieved by negative feedback.

The source signal F*sin(Ωt+φ) is transmitted to the A channel of the lock-in amplifier circuit module 63, and D*sinΩt in the mixing signal generated by the second mixer 55 is injected into the B channel of the lock-in amplifier circuit module 63 , when the phase noise is reduced, the value of D will be smaller; when the phase noise becomes larger, D will change, and the change can be extracted by the lock-in amplifier module and injected into the other end of the first mixer 64. An electro-optic modulator 65 is modulated in magnitude to compensate for changes in the amplitude of the noise.

As shown in FIG. 2 , a flowchart of a method for suppressing single-frequency phase noise of a laser provided by the implementation of the present invention. Please refer to FIG. 1 together. The present invention also provides a method for suppressing phase noise of a laser, which includes: outputting a regulation signal through a laser controller 2 to control the laser 1 and output a laser beam; The laser beam generated by the laser 1 is isolated, and the first laser beam 11 is output; the single-frequency phase noise of the first laser beam 11 is eliminated by the phase noise suppression module 6, and the second laser beam 12 is output; through the PDH The frequency stabilization module 5 performs sideband modulation processing and resonance locking processing on the second laser beam 12 to obtain a frequency error signal; the first servo loop module 4 operates on the frequency error signal to output a frequency adjustment signal; through the laser controller 2. Adjust the control signal according to the frequency adjustment signal to correct the frequency error of the laser, and make the laser 1 output a frequency-stabilized laser.

The steps of eliminating single-frequency phase noise for the first laser beam 11 include: collecting the modulation signal of the laser controller 2 through the noise sampling circuit module 6 and outputting the source signal; phase, to output the phase error signal; the source signal and the frequency error signal are phase-locked and amplified by the lock-in amplifier circuit module 63 and operated to output the amplitude error signal; the phase error signal and the amplitude error signal are analyzed by the first mixer 64 Perform frequency mixing to output a deviation correction signal; according to the deviation correction signal, modulate the first laser beam 11 through the first electro-optic modulator 65 to output the second laser beam 12; detect the second laser beam through the first photodetector 66 The beam 12 is used to output the intensity signal; the intensity signal and the frequency error signal are analyzed by the spectrum analyzer 65 to obtain the phase noise spectrum of the second laser beam 12; the amplitude modulation value of the source signal is adjusted by the amplitude modulation phase shift amplifying module 62 and the phase shift value until the phase noise displayed in the spectrum analyzer is zero, so that the first electro-optic modulator 65 performs equal-amplitude inverse modulation of noise on the first laser beam 11 to eliminate single-frequency phase noise.

The step of performing sideband modulation and resonance locking on the second laser beam 12 to output a frequency error signal includes:

The second electro-optic modulator 54 is driven by the frequency modulation signal generated by the modulation signal generator 51 to perform sideband modulation on the second laser beam and output the third laser beam; The mirror 57 and the 1/4 wave plate 58 are used to polarize the incident third laser beam 13, incident to the ultra-stable cavity 53 for resonance locking processing, and reflect and output the fourth laser beam 14; through the second photodetector 54 The fourth laser beam 14 is detected to output an interference signal; the interference signal and the frequency modulation signal are mixed by the second mixer 55 to output a frequency error signal.

The second electro-optic modulator 52 is used to modulate the second laser beam 12, so that the third laser beam 13 not only carries phase noise, but also adds a sideband signal.

Please refer to Figure 2 again, after the laser beam with phase noise passes through the optical isolator 3, its incident light can be expressed by the following formula

E _inc =E _o e ^{i(ωt+βsinΩt)} (1)

where E ₀ is the electric field strength of the laser beam, ω is the frequency of the laser carrier, Ω is the frequency of the sideband noise (which originates from the frequency modulation inside the laser 1, and is the main noise suppressed in this embodiment), and β is the noise modulation depth .

In the initial stage of eliminating phase noise, since a good error signal is not obtained, the phase noise is not suppressed, and the first electro-optic modulator 65 does not modulate the first laser beam 11 .

When the second laser beam 12 with phase noise passes through the second electro-optic modulator 52, the third laser beam 13 will increase the sideband modulated by the second electro-optic modulator 52 in addition to its own phase noise. for PDH frequency stabilization,

The frequency of the sideband is generally 5MHz to tens of megahertz, and the expression for the fluctuation of the laser beam at this time is:

where β ₂ is the frequency modulation depth, Ω ₂ is the modulation frequency of the second electro-optic modulator, and the two modulated sidebands have opposite phases.

After the third laser beam 13 passes through the polarization beam splitter, the third laser beam 13 is incident on the ultra-stable cavity 53, and resonates and oscillates in the ultra-stable cavity 53 and is partially reflected and received by the second photodetector 55. The reflected interference The light beam is received by the second photodetector 55 to detect the interference signal.

The following describes the formula of the change before and after the reflection of the fourth laser beam 14 in the ultra-stable cavity 53. If the electric field of the third laser beam 13 is E _o e ^iωt and the electric field of the fourth laser beam is E ₁ e ^iωt , there is a reflection coefficient

r is the reflection coefficient of the cavity mirror in the ultra-stable cavity 53 , and Δv _fsr is the free spectral path of the ultra-stable cavity 53 . And the laser electric field of the third laser beam 13 modulated by the second electro-optic modulator 54 entering the ultra-stable cavity 53 is obtained by the first-order expansion of equation (2)

Then the electric field of the interference light formed by the interference of the total reflected light is

除去第二光电探测器54无法响应的光频段，其中探测电流的频率成分会包括Ω、Ω₂、Ω₂+Ω、Ω₂-Ω、2Ω以及2Ω₂等。The light field of the interference light can be detected by the second photodetector 54, and its light intensity is detected,

Excluding the optical frequency band to which the second photodetector 54 cannot respond, the frequency components of the detection current may include Ω, Ω ₂ , Ω ₂ +Ω, Ω ₂ −Ω, 2Ω, and 2Ω ₂ .

In addition, since the phase noise of the laser 1 is mainly located in the low frequency band, Ω ₂ >>Ω is satisfied. In the PDH frequency stabilization stage, the signal of the second photodetector 54 is mixed with the modulation signal of the electro-optical modulator by the second mixer 55 to form a mixed signal, and the mixed signal is mixed by a low-pass filter (not shown in the figure). After the mixed signal passes through the low-pass filter, the high-frequency components are filtered out, leaving only the frequency components much smaller than Ω ₂ . Therefore, the frequency components in the photocurrent are adjacent to Ω ₂ such as Ω ₂ , Ω ₂ +Ω, Ω ₂ -Ω frequency components are the target frequencies, they are mixed with the modulation signal source with frequency Ω ₂ and filter out high-frequency signals, The error signal of PDH frequency stabilization is obtained. The current signal components related to the noise modulation β detected by the second photodetector 54 are:

When the laser beam is located in the cavity resonance, the amplitudes of the (Ω ₂ -Ω) and (Ω ₂ +Ω) components in the above formula are equal, select the optimal phase φ, mix the signal with sin[Ω ₂ t+φ] and combine Perform low-pass filtering to demodulate the Ω component signal. The magnitude of the Ω component, which is proportional to the magnitude of the phase noise J ₁ [β], is analyzed by a spectrum analyzer under otherwise stable conditions.

The source signal F*sin(Ωt+φ) containing the modulated noise signal Ω is extracted from the laser 1 through the noise sampling circuit module 61 , and transmitted to the amplitude modulation phase shift amplifier circuit module 62 and the lock-in amplifier circuit module 63 respectively. The source signal is adjusted by the amplitude modulation and phase shift amplifier circuit module 62, and the first electro-optic modulator 65 is acted on by the first mixer 64 to directly reverse modulate the laser light and eliminate the phase noise sideband. Check the signal containing Ω through the spectrum analyzer 67, and adjust the phase and amplitude of the first electro-optic modulator 65, so that the Ω signal on the spectrum analyzer is reduced, and phase noise can be suppressed.

The source signal F*sin(Ωt+φ) is transmitted to the A channel of the lock-in amplifier circuit module, and D*sinΩt in the mixing signal generated by the second mixer 55 is injected into the B channel of the lock-in amplifier. When the phase noise After decreasing, the value of D will be smaller; but the phase noise becomes larger, and D changes, which can be extracted by the lock-in amplifier module 63 and injected into the other end of the first mixer 64 through the servo loop. An electro-optic modulator 65 is modulated in magnitude to compensate for changes in the amplitude of the noise.

The above is a specific description of the preferred implementation of the present invention, but the present invention is not limited to the described embodiments, and those skilled in the art can also make various equivalent deformations or replacements on the premise that does not violate the spirit of the present invention , these equivalent modifications or substitutions are all included within the scope defined by the claims of the present application.

Claims (9)

1. A system for suppressing single frequency phase noise of a laser, comprising:

a laser for generating a laser beam;

an optical isolator allowing unidirectional input of the laser beam and outputting a first laser beam to isolate the first laser beam from returning to the laser;

the phase noise suppression module is used for eliminating single-frequency phase noise of the first laser beam and outputting a second laser beam;

a PDH (Pound-Drever-Hall) frequency stabilization module for performing sideband modulation processing and resonance locking processing on the second laser beam to output a frequency error signal;

the first servo loop module is used for calculating the frequency error signal so as to output a frequency adjusting signal;

and the laser controller is used for outputting a control signal to control the laser, and adjusting the control signal according to the frequency adjusting signal to correct the frequency error of the laser and enable the laser to output frequency-stabilized laser.

2. The system for suppressing laser single frequency phase noise according to claim 1, wherein the phase noise suppression module is electrically connected to the laser controller to collect the control signal of the laser controller as a source signal output.

3. A system for suppressing laser single frequency phase noise according to claim 1, wherein said phase noise suppression module comprises:

the noise sampling circuit module is used for collecting single-frequency phase noise of the first laser beam and outputting a source signal;

the amplitude modulation phase shift amplification module is used for carrying out amplitude modulation and phase shift on the source signal so as to output a phase error signal;

the phase-locked amplifying circuit module is electrically connected with the PDH frequency stabilizing module and is used for performing phase-locked amplification and operation on the frequency error signal output by the PDH frequency stabilizing module and the source signal output by the noise sampling circuit module so as to output an amplitude error signal;

a first mixer for mixing the phase error signal and the amplitude error signal to output a deviation correction signal;

and the first electro-optical modulator is used for performing anti-phase modulation on the first laser beam output by the optical isolator according to the deviation rectifying signal so as to eliminate single-frequency phase noise of the first laser beam and output a second laser beam.

4. A system for suppressing laser single frequency phase noise according to claim 3, wherein said phase noise suppression module further comprises:

a first photodetector for detecting the second laser beam to obtain an intensity signal;

and the spectrum analyzer is electrically connected with the PDH frequency stabilization module and is used for analyzing the frequency error signal output by the PDH frequency stabilization module and the intensity signal detected by the first photoelectric detector and acquiring the phase noise spectrum of the second laser beam.

5. The system for suppressing laser single frequency phase noise according to claim 1, wherein the PDH frequency stabilization module comprises:

a modulation signal generator for generating a frequency modulation signal;

the second electro-optical modulator is driven by the frequency modulation signal and modulates the second laser beam so that the second laser beam carries a sideband signal to become a third laser beam and output the third laser beam;

the ultrastable cavity is incident by the third laser beam, resonates by multiple beams in the cavity and reflects and outputs a fourth laser beam;

a second photodetector for detecting the fourth laser beam to obtain an interference signal;

and the second mixer is used for mixing the frequency modulation signal generated by the modulation signal generator and the interference signal detected by the second photodetector so as to obtain a frequency error signal output.

6. The system for suppressing laser single frequency phase noise according to claim 5, wherein the PDH frequency stabilization module further comprises:

1/2 wave plate for adjusting the linear polarization direction of the third laser beam;

the polarization beam splitter is used for carrying out polarization beam splitting on the third laser beam and the fourth laser beam which are incident on the polarization beam splitter so as to enable the third laser beam to be transmitted and the fourth laser beam to be reflected;

1/4 wave plate, which is placed between the super stable cavity and the polarization beam splitter, and is used to adjust the polarization state of the third laser beam and the fourth laser beam, so that the polarization of the third laser beam before adjustment is perpendicular to the polarization of the fourth laser beam after adjustment.

7. A method for suppressing single frequency phase noise of a laser, comprising:

outputting a regulation signal through a laser controller to control a laser and output a laser beam;

isolating the laser beam generated by the laser through an optical isolator, and outputting a first laser beam;

eliminating single-frequency phase noise of the first laser beam through a phase noise suppression module, and outputting a second laser beam;

performing sideband modulation processing and resonance locking processing on the second laser beam through a PDH frequency stabilization module to obtain a frequency error signal;

calculating the frequency error signal through a first servo loop module to output a frequency adjusting signal;

and adjusting the control signal through the laser controller according to the frequency adjusting signal to correct the frequency error of the laser and enable the laser to output frequency stabilized laser.

8. The method of suppressing laser single frequency phase noise according to claim 7, wherein the step of performing single frequency phase noise cancellation on the first laser beam comprises:

acquiring a modulation signal of the laser controller through a noise sampling circuit module, and outputting a source signal;

amplitude modulation and phase shift are carried out on the source signal through an amplitude modulation and phase shift amplification module to output a phase error signal;

performing phase-locked amplification and operation on the source signal and the frequency error signal through a phase-locked amplification circuit module to output an amplitude error signal;

mixing the phase error signal and the amplitude error signal through a first mixer to output a deviation correcting signal;

modulating the first laser beam through a first electro-optical modulator according to the deviation correcting signal to output a second laser beam;

detecting the second laser beam by a first photodetector to output an intensity signal;

analyzing the intensity signal and the frequency error signal by a spectrum analyzer to obtain a phase noise spectrum of the second laser beam;

the amplitude modulation value and the phase shift value of the source signal are adjusted through an amplitude modulation and phase shift amplification module until the phase noise displayed in the spectrum analyzer is zero, so that the first electro-optical modulator performs equal-amplitude reverse modulation on the noise of the first laser beam, and single-frequency phase noise is eliminated.

9. The method of suppressing laser single frequency phase noise according to claim 7, wherein the step of performing a sideband modulation process and a resonance lock process on the second laser beam to output a frequency error signal comprises:

driving a second electro-optical modulator through a frequency modulation signal generated by a modulation signal generator to perform sideband modulation on the second laser beam and output a third laser beam;

the third laser beam is incident through an 1/2 wave plate, a polarization beam splitter and a 1/4 wave plate to be subjected to polarization processing, is incident to the super-stable cavity to be subjected to resonance locking processing, and is reflected to output a fourth laser beam;

detecting the fourth laser beam through a second photoelectric detector to output an interference signal;

the interference signal and the frequency modulation signal are mixed by a second mixer to output a frequency error signal.

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