CN110829167A - Method and system for inhibiting single-frequency phase noise of laser - Google Patents
Method and system for inhibiting single-frequency phase noise of laser Download PDFInfo
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Abstract
The invention discloses a system for inhibiting single-frequency phase noise of a laser, which comprises: a laser for generating a laser beam; the optical isolator outputs the first laser beam to isolate the first laser beam to return 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 frequency stabilization module for outputting a frequency error signal by performing sideband modulation processing and resonance locking processing on the second laser beam; 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. The invention also discloses a method for inhibiting the phase noise of the laser. The method and the system for inhibiting the phase noise of the laser can effectively inhibit the phase noise of the laser.
Description
Technical Field
The invention relates to the field of laser noise, in particular to a method and a system for inhibiting single-frequency phase noise of a laser.
Background
Since the advent of lasers in the 60's of the last century, laser-related technology has rapidly evolved. Because the laser has good monochromaticity, long coherence time and good directivity, the laser is widely applied in the scientific research and industrial fields. In some spectrum experiments, fields such as spectrum detection, optical frequency calibration, quantum computation and the like also put higher requirements on the performance of the laser, including narrow line width, single longitudinal mode and the like. However, during use, it has been found that the laser light emitted from the laser is not ideally monochromatic but rather noisy, i.e. random fluctuations in frequency, phase and amplitude occur. The presence of noise can have a negative impact on many areas of research, such as atomic clocks, quantum computing, optical sensing, coherent communication, and gravitational wave detection. The suppression of laser noise is of great significance.
The noise of the laser includes optical power noise and phase noise, and the methods for suppressing the phase noise are commonly used as follows: 1. locking the frequency of the laser to the atomic absorption spectrum; 2. the laser is locked to a stable optical cavity. Based on the two methods, the line width of the laser can be narrowed to the level of mHz, and when the line width is narrowed to a certain degree, certain noise components can be reflected. In a laser type such as a titanium sapphire laser, an etalon is used to stabilize the frequency of an output laser, wherein the length of the etalon needs to be modulated, and noise caused by such a modulation signal has a single frequency, and the amplitude exceeds that of general noise, so that the noise has a large influence on a precise spectrum experiment. Experiments show that the noise suppression technology has no ideal noise suppression effect.
For measuring the phase noise of the laser, the following methods are mainly used at present: for laser with line width above megahertz magnitude, a scanning F-P interferometer method can be adopted for measurement. While there are generally three methods for laser measurement with narrower line widths: one method is to convert the optical frequency signal to the electrical frequency band by optical heterodyne mixing of two lasers with close frequency and performance. The two lasers can only estimate the line width of the laser; three lasers are needed to carry out pairwise beat frequency to measure the noise spectrum, and the spectrum components of the lasers can be obtained through correlation analysis. The scheme of multiple lasers is relatively expensive and is not suitable for the condition of a common laboratory. The second method is realized by a delay self-timer frequency-taking technology. The method mainly utilizes the technologies of long optical fibers, optical fiber rings, delay self-beat frequency based on the Michelson interferometer and the like to measure the laser noise. This method is sensitive to environmental influences. For noise suppression, the currently available schemes are few, and one is to realize laser output by using a method of injection locking by using transmitted light of an optical resonant cavity and further performing optical amplification on the basis of an optical resonant cavity frequency stabilization technology. Because the transmission power of the ultrastable cavity is usually not large, the method needs a structure with multistage amplification and is complex.
Disclosure of Invention
The present invention is directed to solving, at least to some extent, one of the technical problems in the related art. To this end, it is an object of the present invention to provide a method of suppressing laser phase noise.
The technical scheme adopted by the invention is as follows:
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 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 outputting a frequency error signal by subjecting the second laser beam to a sideband modulation process and a resonance locking process;
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.
Furthermore, the phase noise suppression module is electrically connected with the laser controller to acquire a control signal of the laser controller as a source signal to be output.
Further, the 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.
Further, the 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.
Further, the PDH frequency stabilization module includes:
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 to enable the second laser beam to carry sideband signals to become a third laser beam to be output;
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 the frequency error signal output.
Further, the PDH frequency stabilization module further includes:
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 that the third laser beam is transmitted, and the fourth laser beam is 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.
To this end, a second object of the present invention is to provide a method of suppressing laser phase noise.
In a second aspect, an embodiment of the present invention provides a method for suppressing single-frequency phase noise of a laser, including:
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;
the phase noise suppression module is used for eliminating single-frequency phase noise of the first laser beam and outputting a second laser beam;
the PDH frequency stabilization module is used for performing sideband modulation processing and resonance locking processing on the second laser beam 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 according to the frequency adjusting signal through a laser controller so as to correct the frequency error of the laser and enable the laser to output frequency-stabilized laser.
Further, the canceling the single frequency phase noise of the first laser beam includes:
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.
Further, the step of performing sideband modulation and resonance locking on the second laser beam to output a frequency error signal further 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 subjected to polarization processing through 1/2 wave plates, transmission of the polarization beam splitter and 1/4 wave plates, is incident to the super-stable cavity to be subjected to resonance locking processing, and is reflected to output a fourth laser beam;
outputting a coherent signal by detecting the fourth laser beam by a second photodetector;
the coherent signal and the frequency modulated signal are mixed by a second mixer to output a frequency error signal.
The invention has the beneficial effects that:
the invention adopts an error signal stabilized by PDH to input into a spectrum analyzer for observing a phase noise spectrum, and a modulation source signal is injected into a first electro-optical modulator in a phase-shifting manner for inverse phase modulation so as to inhibit phase noise.
In addition, the invention also adopts the phase-locked amplification module to extract the phase noise amplitude signal as the amplitude error signal so as to inhibit the change of the phase noise amplitude. The amplitude noise spectrum is observed using a photodetector to detect the outgoing optical signal input to a spectrum analyzer. In addition, residual amplitude modulation noise of the first electro-optical modulator can be suppressed by using temperature control, bias voltage application and the like.
Drawings
Fig. 1 is a schematic structural diagram of a system for suppressing phase noise of a laser according to a first embodiment of the present invention.
Fig. 2 is a schematic diagram of a process for suppressing the phase noise of the laser according to a second embodiment of the present invention.
Description of the reference numerals
| Name (R) | Reference numerals | Name (R) | Reference numerals |
| Laser device | 1 | Laser controller | 2 |
| Optical isolator | 3 | First servo loop module | 4 |
| PDH frequency stabilization module | 5 | Phase noise suppression module | 6 |
| Modulation signal generator | 51 | Second electro-optic modulator | 52 |
| Ultrastable cavity | 53 | Second photodetector | 54 |
| Second mixer | 55 | 1/2 wave plate | 56 |
| Polarization beam splitter | 57 | 1/4 wave plate | 58 |
| Noise sampling circuit module | 61 | Amplitude modulation phase-shift amplifying module | 62 |
| Phase-locked amplifying 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 Description
It should be noted that the embodiments and features of the embodiments in the present application may be combined with each other without conflict.
Referring 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. The embodiment of the invention provides a system for inhibiting phase noise of a laser, which comprises: a laser 1 for generating a laser beam; an optical isolator 3 allowing unidirectional input of the laser beam and outputting a first laser beam 11 to isolate the first laser beam 11 from returning to the laser 1; the phase noise suppression module 6 is configured to eliminate single-frequency phase noise of the first laser beam 11 and output a second laser beam 12; a PDH (Pound-Drever-Hall) frequency stabilization module 5 that outputs a frequency error signal by performing sideband modulation processing and resonance locking processing on the input second laser beam 12; the first servo loop module 4 is used for calculating the frequency error signal so as to output a frequency adjusting signal; and the laser controller 2 is electrically connected with the PDH frequency stabilization module and used for outputting a control signal to control the laser 1 and adjusting the control signal according to the frequency adjustment signal to correct the frequency error of the laser 1 and enable the laser 1 to output frequency stabilized laser.
The phase noise suppression module 6 is electrically connected with the laser controller 2 to acquire a control signal of the laser controller 2 and output the control signal as a source signal.
The phase noise suppression module 6 includes: the noise sampling circuit module 61 is configured to collect single-frequency phase noise of the first laser beam 11 and output a source signal; an amplitude modulation and phase shift amplification module 62 for amplitude modulating and phase shifting the source signal to output a phase error signal; the phase-locked amplifying circuit module 63 is configured to perform phase-locked amplification on the frequency error signal output by the PDH frequency stabilizing module 5 and the source signal output by the noise sampling circuit module 61, and perform operation to output an amplitude error signal; a first mixer 64 for mixing the phase error signal and the amplitude error signal to output a deviation correction signal; the first electro-optical modulator 65 performs inverse phase modulation on the first laser beam 11 output by the optical isolator 3 according to the deviation correcting signal to eliminate single-frequency phase noise of the first laser beam 11 and output the second laser beam 12.
The phase noise suppression module 6 further comprises: a first photodetector 66 for detecting the second laser beam 12 to obtain an intensity signal; and the spectrum analyzer 67 is electrically connected with the PDH frequency stabilization module 5 and is used for analyzing the frequency error signal output by the PDH frequency stabilization module 5 and the intensity signal detected by the first photodetector 66 and acquiring the phase noise spectrum of the second laser beam 12.
The PDH frequency stabilization module 5 includes: a modulation signal generator 51 for generating a frequency modulation signal; the second electro-optical modulator 52 is driven by the frequency modulation signal, and modulates the second laser beam 12, so that the second laser beam 12 carries sideband signals to become a third laser beam 13 and output the third laser beam; the super-stable cavity 53 is incident by the third laser beam 13, resonates by multiple beams in the cavity, and reflects and outputs a fourth laser beam 14; a second photodetector 54 for detecting the fourth laser beam 14 to obtain an interference signal; and a second mixer 55 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 a frequency error signal output.
The PDH frequency stabilization module further includes: 1/2 wave plate 56 for adjusting the linear polarization direction of the third laser beam 13; a polarization beam splitter 57 for polarization beam splitting the third laser beam 13 and the fourth laser beam 14 incident thereon, so as to transmit the third laser beam 13 and reflect the fourth laser beam 14; 1/4 wave plate 58 is disposed between the meta-stable cavity 53 and the polarization beam splitter 57 for adjusting the polarization states of the third laser beam 13 and the fourth laser beam 14, so that the third laser beam 13 before being adjusted is perpendicular to the polarization of the fourth laser beam 14 after being adjusted.
The second laser beam 12 is modulated by a second electro-optical modulator 52 to add sideband signals in addition to phase noise to the third laser beam 13.
By obtaining a 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 a spectrum analyzer 67, and when the amplitude modulation and phase shift amplification module 62 is used for inverse modulation, the sideband signal varies with the adjustment until the amplitude reaches 0, and if the amplitude varies, the phase shift amplification module is locked to generate an error feedback signal, and the error feedback signal is transmitted to the first mixer 64 through the second servo loop module to perform feedback control on the amplitude of the inverse phase modulation.
The first mixer 64 mixes the phase error signal and the amplitude error signal to form a deviation-correcting signal, and transmits the deviation-correcting signal to the first electro-optical modulator 65 to adjust the first laser beam 11. The modulation signal and the noise signal are in equal amplitude and opposite phase by controlling the adjusting parameters of the amplitude modulation and phase shift amplification module 62, so as to eliminate the phase noise in the laser beam.
In other embodiments, temperature control, bias voltage application, etc. may be used to suppress the remaining amplitude modulation noise of the first electro-optic modulator 65.
In the process of performing the inverse phase modulation of the phase noise by the first electro-optical modulator 65, phase matching and amplitude matching are performed. The phase matching is realized by a feedforward means, and the amplitude matching is realized by negative feedback. Referring to fig. 1, after the laser beam containing phase noise passes through the isolator, its incident light can be represented by the following formula
Einc＝Eoei(ωt+βsinΩt)(1)
Wherein E0Is the intensity of the laser beam, ω is the frequency of the laser carrier wave, and Ω sideband noise (which originates from the frequency modulation inside the laser and is the noise mainly suppressed in this embodiment)) And β is the noise modulation depth.
At the initial stage of eliminating the 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-optical 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 may add a sideband signal modulated by the second electro-optical modulator 52 for PDH frequency stabilization, in addition to the phase noise itself, where the sideband signal generally has a frequency of 5MHz to several tens of MHz, and the expression of the laser beam fluctuation at this time is
β therein2Is the frequency modulation depth, Ω2The two sidebands modulated are in opposite phase for the modulation frequency of the second electro-optic modulator 52. After the third laser beam 13 passes through the polarization beam splitter 57, the third laser beam 13 enters the unstable resonator 53, oscillates in the unstable resonator 53, and is partially reflected back along the original optical path. The light beam reflected after oscillating through the unstable resonator 53 interferes with the light beam directly reflected without entering the unstable resonator 53 to form coherent light and is received by the second photodetector 54.
The following formula for the change before and after reflection of the third laser beam 13 in the super-stable cavity 53 is illustrated, where the electric field before incidence of the third laser beam 14 is EoeiωtThe electric field emitted by the fourth laser beam 14 is E1eiωtThen has a reflection coefficient
r is the reflection coefficient of the cavity mirror in the metastability chamber 53, Δ vfsrThe free spectral path of the metastable cavity 53. And the laser electric field of the third laser beam 13 modulated by the second electro-optical modulator 52 and entering the super-stable cavity is obtained by 1-order expansion of formula (2)
Wherein the electric field of the interference light formed by the interference between the light beam not directly reflected by the super stable cavity 53 and the light beam reflected by the super stable cavity 53 is
The light field of the interfering light, formed by the total reflected light beam, is detected by the second photodetector 55, the intensity of which,
eliminating the optical frequency bands to which the second photodetector 54 is not responsive, wherein the frequency components of the detection current would include Ω, Ω2、Ω2+Ω、Ω2- Ω, 2 Ω and 2 Ω2And the like.
Furthermore, since the phase noise of the laser 1 is mainly located in a low frequency band, i.e., satisfies Ω2>>Omega. In the PDH frequency stabilization phase, 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 filtered by a low-pass filter (not shown), after which the high-frequency component is filtered out, leaving the frequency much less than Ω2The frequency component of (1). The frequency component in the photocurrent is therefore Ω2Adjacent to e.g. omega2、Ω2+Ω、Ω2The-omega frequency components are target frequencies, and the passing frequency of the target frequencies is omega2And filters the high frequency signal to obtain an error signal for PDH frequency stabilization, wherein the current signal component detected by the second photodetector 54 associated with the noise modulation β is:
when the laser beam is located in the resonant cavity, the upper formula is middle (omega)2- Ω) and (Ω)2Amplitude of + omega) componentAre equal, the optimum phase phi is chosen, the signal is summed with sin omega2t+φ]Mixing and low-pass filtering are carried out, and signals with omega components are demodulated. The amplitude of the omega component is analyzed by a spectrum analyzer in the case of otherwise stable conditions, which is proportional to the magnitude J of the phase noise1[β]。
As shown in fig. 1, a source signal F × sin (Ω t + Φ) containing the modulated noise signal Ω is extracted from the laser 1 by a noise sampling circuit module 61 and transmitted to an amplitude modulation and phase shift amplifier circuit module 62 and a lock-in amplifier (not shown), respectively. The source signal is modulated by the am-pa module 62 and applied to the first electro-optic modulator 65 by the first mixer 64 to directly back-modulate the laser beam to eliminate phase noise sidebands. The phase noise can be suppressed by viewing the omega-containing signal through the spectrum analyzer 67 and adjusting the phase and amplitude of the first electro-optic modulator 67 so that the omega signal on the spectrum analyzer 51 is reduced. Wherein the phase matching is realized by a feedforward means, and the amplitude matching is realized by negative feedback.
The source signal F × sin (Ω t + Φ) is transmitted to the channel a of the lock-in amplifier circuit module 63, and D × sin Ω t in the mixing signal generated by the second mixer 55 is injected into the channel B of the lock-in amplifier circuit module 63, and when the phase noise is reduced, the value of D is smaller; when the phase noise becomes large, D changes, which can be extracted by the lock-in amplifier module and injected into the other end of the first mixer 64 to compensate for the amplitude change of the noise by adjusting the modulation level of the first electro-optical modulator 65.
As shown in fig. 2, a flowchart of a method for suppressing single-frequency phase noise of a laser according to an embodiment of the present invention is provided. Referring to fig. 1, the present invention also provides a method for suppressing phase noise of a laser, including: outputting a regulation signal through the laser controller 2 to control the laser 1 and output a laser beam; the laser beam generated by the laser 1 is isolated by the optical isolator 3, and a first laser beam 11 is output; eliminating single-frequency phase noise of the first laser beam 11 through a phase noise suppression module 6, and outputting a second laser beam 12; performing sideband modulation processing and resonance locking processing on the second laser beam 12 through a PDH frequency stabilization module 5 to obtain a frequency error signal; the frequency error signal is calculated through the first servo loop module 4 to output a frequency adjusting signal; the laser controller 2 adjusts the control signal according to the frequency adjusting signal to correct the laser frequency error and enable the laser 1 to output frequency-stabilized laser.
The step of single frequency phase noise cancellation of the first laser beam 11 comprises: the modulation signal of the laser controller 2 is collected through a noise sampling circuit module 6, and a source signal is output; amplitude and phase shifting the source signal by an amplitude modulation and phase shifting amplification module 62 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 63 to output an amplitude error signal; mixing the phase error signal and the amplitude error signal by a first mixer 64 to output a deviation correction signal; modulating the first laser beam 11 by a first electro-optical modulator 65 according to the deviation correcting signal to output a second laser beam 12; detecting the second laser beam 12 by the first photodetector 66 to output an intensity signal; analyzing the intensity signal and the frequency error signal by a spectrum analyzer 65 to obtain a phase noise spectrum of the second laser beam 12; the amplitude modulation value and the phase shift value of the source signal are adjusted by the amplitude modulation and phase shift amplification module 62 until the phase noise displayed in the spectrum analyzer is zero, so that the first electro-optical modulator 65 performs equal-amplitude reverse-phase modulation on the first laser beam 11 to eliminate single-frequency phase noise.
The step of sideband modulating and resonance locking the second laser beam 12 to output a frequency error signal comprises:
driving a second electro-optical modulator 54 by a frequency modulation signal generated by a modulation signal generator 51 to perform sideband modulation on the second laser beam and output a third laser beam; the third laser beam 13 is incident through an 1/2 wave plate 56, a polarization beam splitter 57 and a 1/4 wave plate 58 to be subjected to polarization processing, is incident to the super-stable cavity 53 to be subjected to resonance locking processing, and is reflected to output a fourth laser beam 14; outputting an interference signal by detecting the fourth laser beam 14 via the second photodetector 54; the interference signal and the frequency modulation signal are mixed by the second mixer 55 to output a frequency error signal.
Which is used by the second electro-optical modulator 52 to modulate the second laser beam 12 such that the third laser beam 13 adds a sideband signal in addition to carrying phase noise.
Referring to fig. 2 again, after the laser beam containing phase noise passes through the optical isolator 3, its incident light can be expressed by the following formula
Einc＝Eoei(ωt+βsinΩt)(1)
Wherein E0Is the electric field intensity of the laser beam, ω is the frequency of the laser carrier, the frequency of the Ω sideband noise (which originates from the frequency modulation inside the laser 1, which is the noise mainly suppressed in the present embodiment), and β is the noise modulation depth.
At the initial stage of eliminating the phase noise, since a good error signal is not obtained, the phase noise is not suppressed, and the first laser beam 11 is not modulated by the first electro-optical modulator 65.
After the second laser beam 12 with phase noise passes through the second electro-optical modulator 52, the third laser beam 13 will add the sidebands modulated by the second electro-optical modulator 52, in addition to the phase noise itself, for PDH frequency stabilization,
the general frequency of the sideband is 5MHz to dozens of MHz, and the expression of the laser beam fluctuation at this time is:
β therein2Is the frequency modulation depth, Ω2The two sidebands modulated are in opposite phase for the modulation frequency of the second electro-optic modulator.
After the third laser beam 13 passes through the polarization beam splitter, the third laser beam 13 is incident to the metastable cavity 53, and resonates and partially reflects in the metastable cavity 53 and is received by the second photodetector 55, and the reflected interference beam is received by the second photodetector 55 to detect an interference signal.
The following formula for the change before and after reflection of the fourth laser beam 14 in the super-stable cavity 53 is explained, if the electric field of the third laser beam 13 is EoeiωtThe electric field of the fourth laser beam is E1eiωtThen has a reflection coefficient
r is the reflection coefficient of the cavity mirror in the metastability chamber 53, Δ vfsrThe free spectral path of the metastable cavity 53. And the laser electric field of the third laser beam 13 entering the super stable cavity 53 modulated by the second electro-optical modulator 54 is obtained by 1-order expansion of formula (2)
The electric field of the interference light formed by the interference of the total reflected light is
The light field of the interfering light, the intensity of which is detected,
eliminating the optical frequency bands to which the second photodetector 54 is not responsive, wherein the frequency components of the detection current would include Ω, Ω2、Ω2+Ω、Ω2- Ω, 2 Ω and 2 Ω2And the like.
Furthermore, since the phase noise of the laser 1 is mainly located in a low frequency band, i.e., satisfies Ω2>>Omega. In the PDH frequency stabilization phase, 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 filtered by a low-pass filter (not shown), after which the high-frequency component is filtered out, leaving the frequency much smaller than Ω2The frequency component of (1). The frequency component in the photocurrent is therefore Ω2Adjacent to e.g. omega2、Ω2+Ω、Ω2The-omega frequency components are target frequencies, and the passing frequency of the target frequencies is omega2And filtering the high frequency signal to obtain an error signal stabilized by PDH, wherein the current signal component detected by the second photodetector 54, which is related to the noise modulation β, is:
when the laser beam is at cavity resonance, in the above equation (omega)2- Ω) and (Ω)2The + omega) component is equal in amplitude, the optimum phase phi is chosen, and the signal is summed with sin omega2t+φ]Mixing and low-pass filtering are carried out, and signals with omega components are demodulated. The amplitude of the omega component is analyzed by a spectrum analyzer in the case of otherwise stable conditions, which is proportional to the magnitude J of the phase noise1[β]。
The source signal F × sin (Ω t + Φ) containing the modulated noise signal Ω is extracted from the laser 1 by the noise sampling circuit module 61, and is transmitted to the amplitude modulation phase shift amplifier circuit module 62 and the phase lock amplifier circuit module 63, respectively. The source signal is modulated by the am-pa module 62 and applied to the first electro-optic modulator 65 by the first mixer 64 to directly reverse modulate the laser and eliminate the phase noise sidebands. The phase and amplitude of the first electro-optical modulator 65 are adjusted by the spectrum analyzer 67 looking at the omega-containing signal so that the omega signal on the spectrometer is reduced and phase noise can be suppressed.
The source signal F × sin (Ω t + Φ) is transmitted to the channel a of the lock-in amplifier circuit module, and D × sin Ω t in the mixing signal generated by the second mixer 55 is injected into the channel B of the lock-in amplifier, and when the phase noise is reduced, the value of D is smaller; however, the phase noise becomes large, 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, to compensate the amplitude change of the noise by adjusting the modulation of the first electro-optical modulator 65.
While the preferred embodiments of the present invention have been illustrated and described, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.
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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