CN110829167B - 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; an optical isolator 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; the PDH frequency stabilization module is used for carrying out sideband modulation processing and resonance locking processing on the second laser beam so as 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 adjustment signal; and the laser controller is used for outputting a control signal to control the laser, adjusting the control signal according to the frequency adjusting signal so as 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 suppressing the phase noise of the laser can effectively suppress 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 the laser in the 60 s of the last century, laser related technology has evolved rapidly. The laser has good monochromaticity, long coherence time and good directivity, and is widely applied to the fields of scientific research and industry. In some spectrum experiments, the fields such as spectrum detection, optical frequency scale, quantum computation and the like also put higher requirements on the performance of the laser, including narrow linewidth, single longitudinal mode and the like. However, it has been found during use that the laser light emitted by the laser is not an ideal single frequency oscillation, but rather is noisy, i.e. fluctuates randomly in frequency, phase and amplitude. The presence of noise can have negative effects on many areas of research such as atomic clocks, quantum computing, optical sensing, coherent communication, gravitational wave detection, and the like. The suppression of laser noise is of great importance.
The noise of the laser includes optical power noise and phase noise, and the method for suppressing the phase noise is more commonly used: 1. locking the frequency of the laser to the atomic absorption spectrum; 2. the laser is locked to a stable optical resonator. Based on the two methods, the line width of the laser can be narrowed to the mHz level, and when the line width is narrowed to a certain degree, certain noise components can be reflected. In the laser types such as titanium sapphire lasers, an etalon is used to stabilize the frequency of the output laser, wherein the length of the etalon needs to be modulated, noise caused by the modulated signals has a single frequency, and the amplitude exceeds that of common noise, so that the precision spectrum experiment is greatly influenced. Experiments find that the noise suppression technology has no ideal noise suppression effect.
For measuring the phase noise of laser, the following methods are mainly adopted at present: for lasers with linewidths above the megahertz scale, the measurement can be performed by scanning an F-P interferometer. While there are generally three methods for laser measurement with narrower linewidths: one method is to convert the optical frequency signal to the electrical frequency band by optically heterodyning two lasers with close frequency and performance. The two lasers can only estimate the line width of the laser; to measure the noise spectrum, three lasers are required to perform beat frequency two by two, and the spectral components of each laser can be obtained through correlation analysis. The scheme of multiple lasers is high in cost and is not suitable for the condition of a common laboratory. The second method is implemented by a time-lapse self-timer technique. The method mainly utilizes long optical fiber, optical fiber ring and delay self-beat frequency based on Michelson interferometer to measure laser noise. This approach is relatively sensitive to environmental influences. The current feasible schemes for noise suppression are few, and one method is to utilize the transmission light of the optical resonant cavity to carry out injection locking and further carry out optical amplification on the basis of the optical resonant cavity frequency stabilization technology to realize the output of laser. Since the transmission power of the ultra-stable cavity is usually not large, the method needs to adopt a multi-stage amplifying structure and is complex.
Disclosure of Invention
The present invention aims to solve at least one of the technical problems in the related art to some extent. To this end, it is an object of the present invention to provide a method of suppressing phase noise of a laser.
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, comprising:
A laser for generating a laser beam;
An optical isolator allowing the laser beam to be input unidirectionally 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 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 adjustment signal;
and the laser controller is used for outputting a control signal to control the laser, adjusting the control signal according to the frequency adjusting signal so as to correct the frequency error of the laser and enabling the laser to output frequency-stabilized laser.
Further, the phase noise suppression module is electrically connected with the laser controller to collect a control signal of the laser controller and output the control signal as a source signal.
Further, the phase noise suppression module includes:
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 amplifying 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 stabilization module and is used for carrying out phase-locked amplification and operation on the frequency error signal output by the PDH frequency stabilization 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 correction signal;
And the first electro-optical modulator carries out anti-phase modulation on the first laser beam output by the optical isolator according to the deviation correcting 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 includes:
The first photoelectric detector is used for detecting the second laser beam so as to acquire an intensity signal;
The frequency 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 obtaining 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;
a second electro-optical modulator driven by the frequency modulation signal, for modulating the second laser beam, so that the second laser beam carries a sideband signal to become a third laser beam output;
The ultra-stable cavity is used for receiving the third laser beam, resonating the laser beams in the cavity and reflecting the third laser beam to output a fourth laser beam;
the second photoelectric detector is used for detecting the fourth laser beam so as to acquire an interference signal;
And the second mixer is used for carrying out mixing processing on the frequency modulation signal generated by the modulation signal generator and the interference signal detected by the second photoelectric detector so as to obtain the frequency error signal output.
Further, the PDH frequency stabilization module further includes:
A 1/2 wave plate for adjusting the linear polarization direction of the third laser beam;
The polarization beam splitter is used for polarization beam splitting of the third laser beam and the fourth laser beam incident on the polarization beam splitter, so that the third laser beam is transmitted, and the fourth laser beam is reflected;
The 1/4 wave plate is arranged between the ultra-stable cavity and the polarization beam splitter and is used for adjusting the polarization states 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 invention is to provide a method of suppressing 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:
outputting a regulating signal through a laser controller to control a laser and outputting 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 carrying out sideband modulation processing and resonance locking processing on the second laser beam so as to acquire a frequency error signal;
calculating the frequency error signal through a first servo loop module to output a frequency adjustment signal;
and adjusting a control signal by a laser controller according to the frequency adjusting signal so as to correct the frequency error of the laser and enable the laser to output frequency-stabilized laser.
Further, the eliminating the single-frequency phase noise of the first laser beam includes:
the method comprises the steps of collecting a modulation signal of the laser controller through a noise sampling circuit module and outputting a source signal;
the source signal is subjected to amplitude modulation and phase shift through an amplitude modulation phase shift amplifying module so as to output a phase error signal;
The phase-locked amplifying circuit module performs phase-locked amplification and operation on the source signal and the frequency error signal 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 so as 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;
And adjusting the amplitude modulation value and the phase shift value of the source signal through an amplitude modulation phase shift amplification module until the phase noise displayed in the spectrum analyzer is zero, so that the first electro-optical modulator carries out constant amplitude and opposite phase modulation on the noise of the first laser beam, and single-frequency phase noise is eliminated.
Further, the step of sideband modulating and resonance locking the second laser beam to output a frequency error signal further comprises:
Driving a second electro-optic modulator by 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 transmitted through the 1/2 wave plate, the polarization beam splitter and the 1/4 wave plate to be polarized, is transmitted to the ultra-stable cavity to be subjected to resonance locking treatment, and is reflected to output a fourth laser beam;
Detecting the fourth laser beam through the second photoelectric detector to output a coherent signal;
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 invention are as follows:
The invention adopts the error signal input spectrum analyzer using PDH to stabilize frequency to observe the phase noise spectrum, and carries out anti-phase modulation by phase-shifting the modulated source signal into the first electro-optic modulator to restrain the phase noise.
In addition, the phase-locked amplifying module is adopted to extract the phase noise amplitude signal as an amplitude error signal so as to inhibit the change of the phase noise amplitude. The outgoing light signal is detected by a photodetector and input to a spectrum analyzer for observation of the amplitude noise spectrum. The residual amplitude modulation noise of the first electro-optic modulator can be suppressed by using temperature control, bias voltage addition, and the like.
Drawings
Fig. 1 is a schematic diagram of a system for suppressing phase noise of a laser according to a first embodiment of the present invention.
Fig. 2 is a flow chart diagram of suppressing phase noise of a laser according to a second embodiment of the present invention.
Description of the reference numerals
| Name of the name | Reference numerals | Name of the name | 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 |
| Ultra-stable 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, without conflict, the embodiments of the present application and features of the embodiments may be combined with each other.
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 invention. The embodiment of the invention provides a system for inhibiting phase noise of a laser, which comprises the following components: 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; a phase noise suppression module 6, configured to cancel 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 for outputting a frequency error signal by performing sideband modulation processing and resonance locking processing on the second laser beam 12; a first servo loop module 4, configured to operate on the frequency error signal to output a frequency adjustment signal; the laser controller 2 is electrically connected with the PDH frequency stabilization module and is used for outputting a control signal to control the laser 1, adjusting the control signal according to the frequency adjusting signal so as 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 collect a control signal of the laser controller 2 as a source signal to output.
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 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 phase-lock 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 an amplitude error signal; a first mixer 64 for mixing the phase error signal and the amplitude error signal to output a correction signal; the first electro-optical modulator 65 performs anti-phase modulation on the first laser beam 11 output from the optical isolator 3 according to the deviation correction signal to eliminate single-frequency phase noise of the first laser beam 11, and outputs the second laser beam 12.
The phase noise suppression module 6 further includes: a first photodetector 66 for detecting the second laser beam 12 to obtain an intensity signal; the spectrum analyzer 67 is electrically connected to the PDH frequency stabilization module 5, and is configured to analyze the frequency error signal output by the PDH frequency stabilization module 5 and the intensity signal detected by the first photodetector 66, and obtain a 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; a second electro-optical modulator 52, driven by the frequency modulation signal, for modulating the second laser beam 12, so that the second laser beam 12 carries a sideband signal to be a third laser beam 13 for output; an ultra-stable cavity 53, which 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 modulated signal generated by the modulated signal generator 51 with the interference signal detected by the second photodetector 54 to obtain a frequency error signal output.
The PDH frequency stabilization module further includes: a 1/2 wave plate 56 for adjusting the linear polarization direction of the third laser beam 13; a polarization beam splitter 57 for polarization-splitting the third laser beam 13 and the fourth laser beam 14 incident thereon, transmitting the third laser beam 13, and reflecting the fourth laser beam 14; the 1/4 wave plate 58 is disposed between the ultra-stable cavity 53 and the polarization beam splitter 57, and is used for adjusting the polarization states of the third laser beam 13 and the fourth laser beam 14, so that the polarization of the third laser beam 13 before adjustment is perpendicular to the polarization of the fourth laser beam 14 after adjustment.
The second laser beam 12 is modulated by a second electro-optical modulator 52 to add a sideband signal to the third laser beam 13 in addition to carrying phase noise.
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 phase-shift amplification module 62 is used for inverse modulation, the sideband signal changes along with the adjustment, the amplitude is up to 0, if the amplitude changes, the phase-lock phase amplification module generates 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 correction signal and sends the correction signal to the first electro-optic modulator 65 to condition the first laser beam 11. The modulation signal and the noise signal are in constant amplitude phase inversion by controlling the adjustment parameters of the amplitude modulation phase shift amplification module 62 so as to eliminate phase noise in the laser beam.
In other embodiments, temperature control, bias voltage, etc. may be used to suppress residual amplitude modulation noise of the first electro-optic modulator 65.
In the process of phase-reversing 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 back 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)
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 is derived from the frequency modulation inside the laser and is the noise that is mainly suppressed in this embodiment), and β is the noise modulation depth.
In the initial stage of the phase noise cancellation, since the 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-optic modulator 52, the second laser beam 22 will add the sideband signal modulated by the second electro-optic modulator 52 for PDH frequency stabilization, the sideband is typically 5MHz to several tens MHz, and the laser beam fluctuation is expressed as
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 sidebands modulated 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 is partially reflected back along the original optical path. The light beam oscillated via the ultra-stable cavity 53 interferes with the light beam not directly reflected by the ultra-stable cavity 53 to form coherent light and is received by the second photodetector 54.
The following equation for the change before and after reflection of the third laser beam 13 in the ultra-stable cavity 53 shows that if the electric field before incidence of the third laser beam 14 is E oeiωt and the electric field emitted by the fourth laser beam 14 is E 1eiωt, the reflection coefficient is given
R is the reflectance of the cavity mirror in the ultra-stable cavity 53 and Deltav fsr is the free spectral range of the ultra-stable cavity 53. And the laser electric field of the third laser beam 13 which enters the ultra-stable cavity and is modulated by the second electro-optic modulator 52 is obtained by 1-order expansion of the formula (2)
Wherein the electric field of the interference light formed by the interference of the light beam which does not enter the ultra-stable cavity 53 and the light beam which is reflected after entering the ultra-stable cavity 53 is
The light field of the interference light formed by the total reflected beam is detected by the second photodetector 55, which detects its intensity,The optical frequency band to which the second photodetector 54 is not responsive is removed, wherein the frequency components of the detected current would include Ω, Ω 2、Ω2+Ω、Ω2 - Ω,2Ω 2, and so forth.
Furthermore, since the phase noise of the laser 1 is mainly in the low frequency band, i.e., Ω 2 > > Ω is satisfied. 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 components are filtered out, leaving only the frequency components much smaller than Ω 2. Therefore, frequency components in the photocurrent are adjacent to the target frequency such as omega 2、Ω2+Ω、Ω2 -omega frequency components at omega 2, and the frequency components are mixed with a modulating signal source with the frequency of omega 2 and high-frequency signals are filtered, so that an error signal for stabilizing the PDH frequency is obtained. Wherein the current signal component related to the noise modulation β detected by the second photodetector 54 is:
When the laser beam is located in the resonant cavity, the amplitudes of the (omega 2 -omega) component and the (omega 2 +omega) component in the above formula are equal, the optimal phase phi is selected, the signal is mixed with sin [ omega 2 t+phi ], low-pass filtering is carried out, and the signal of the omega component is demodulated. The amplitude of the omega component is analyzed by a spectrum analyzer under otherwise stable conditions, which is proportional to the magnitude of the phase noise J 1 beta.
As shown in fig. 1, a source signal f×sin (Ω+Φ) containing a modulated noise signal Ω is extracted from the laser 1 by a noise sampling circuit block 61, and is transmitted to an amplitude modulation phase shift amplifying circuit block 62 and a lock-in amplifier (not shown), respectively. The source signal is conditioned by an amplitude modulation phase shift amplification circuit module 62 and applied to a first electro-optic modulator 65 by a first mixer 64 to directly reverse modulate the laser beam to eliminate phase noise sidebands. Looking at the signal containing Ω through the spectrum analyzer 67, the phase and amplitude of the first electro-optical modulator 67 are adjusted so that the Ω signal on the spectrum analyzer 51 is reduced, and phase noise can be suppressed. Wherein phase matching is realized by a feedforward means and amplitude matching is realized by negative feedback.
The source signal f×sin (Ω+Φ) is transmitted to the a channel of the lock-in amplifying circuit module 63, and d×sin Ω in the mixed signal generated by the second mixer 55 is injected into the B channel of the lock-in amplifying circuit module 63, and when the phase noise is reduced, the D value is smaller; when the phase noise becomes large, D changes, which can be extracted by the lock-in amplification module and injected into the other end of the first mixer 64, the amplitude change of the noise is compensated by adjusting the modulation size of the first electro-optic modulator 65.
As shown in fig. 2, the embodiment of the invention provides a flowchart of a method for suppressing single-frequency phase noise of a laser. Referring to fig. 1, the present invention also provides a method for suppressing phase noise of a laser, which includes: outputting a regulating signal through a laser controller 2 to control the laser 1 and outputting 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; the single-frequency phase noise of the first laser beam 11 is eliminated through the phase noise suppression module 6, and a second laser beam 12 is output; the PDH frequency stabilization module 5 performs sideband modulation processing and resonance locking processing on the second laser beam 12 to acquire a frequency error signal; calculating the frequency error signal through the first servo loop module 4 to output a frequency adjustment signal; the control signal is adjusted by the laser controller 2 according to the frequency adjustment 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 noise sampling circuit module 6 is used for collecting the modulation signal of the laser controller 2 and outputting a source signal; amplitude and phase shifting the source signal by the amplitude and phase shifting amplification module 62 to output a phase error signal; the phase-locked amplifying circuit module 63 performs phase-locked amplification and operation on the source signal and the frequency error signal to output an amplitude error signal; mixing the phase error signal with the amplitude error signal by a first mixer 64 to output a 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 phase shift amplification module 62 until the phase noise displayed in the spectrum analyzer is zero, so that the first electro-optical modulator 65 carries out the constant amplitude and opposite phase modulation of the noise on the first laser beam 11, and single-frequency phase noise is eliminated.
The step of sideband modulating and resonance locking the second laser beam 12 to output a frequency error signal comprises:
The frequency modulation signal generated by the modulation signal generator 51 drives the second electro-optical modulator 54 to sideband-modulate the second laser beam and output a third laser beam; the third laser beam 13 is transmitted to the ultra-stable cavity 53 for resonance locking treatment through the 1/2 wave plate 56, the polarization beam splitter 57 and the 1/4 wave plate 58, and the fourth laser beam 14 is reflected and output; by detecting the fourth laser beam 14 via the second photodetector 54, an interference signal is output; the interference signal and the frequency modulated signal are mixed by a second mixer 55 to output a frequency error signal.
The second laser beam 12 is modulated by a second electro-optical modulator 52 to add a sideband signal to the third laser beam 13 in addition to carrying phase noise.
Referring back to fig. 2, after the laser beam containing phase noise passes through the optical isolator 3, its incident light can be represented by the following formula
Einc＝Eoei(ωt+βsinΩt) (1)
Where E 0 is the electric field strength 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 this embodiment), and β is the noise modulation depth.
In the initial stage of the phase noise cancellation, since a good error signal is 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 12 with phase noise passes through the second electro-optical modulator 52, the third laser beam 13, in addition to the phase noise itself, also increases sidebands modulated by the second electro-optical modulator 52 for PDH frequency stabilization,
The typical frequency of the sidebands is 5MHz to several tens of MHz, where the laser beam fluctuation is expressed as:
Where β 2 is the frequency modulation depth, Ω 2 is the modulation frequency of the second electro-optic modulator, and the phases of the two sidebands modulated are opposite.
After the third laser beam 13 passes through the polarization beam splitter, the third laser beam 13 is incident to the ultra-stable cavity 53, resonantly oscillates in the ultra-stable cavity 53 and is partially reflected and received by the second photodetector 55, and the reflected interference beam is received by the second photodetector 55 to detect the interference signal.
The following equation for the change in reflection of the fourth laser beam 14 in the ultra-stable cavity 53 shows that if the electric field of the third laser beam 13 is E oeiωt and the electric field of the fourth laser beam is E 1eiωt, the reflection coefficient is given
R is the reflectance of the cavity mirror in the ultra-stable cavity 53 and Deltav fsr is the free spectral range 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 and entering the ultra-stable cavity 53 is obtained by 1-order expansion of the formula (2)
Then the electric field of the interference light formed by the total reflected light interference is
The optical field of the interference light, the intensity of which is detected,The optical frequency band to which the second photodetector 54 is not responsive is removed, wherein the frequency components of the detected current would include Ω, Ω 2、Ω2+Ω、Ω2 - Ω,2Ω 2, and so forth.
Furthermore, since the phase noise of the laser 1 is mainly in the low frequency band, i.e., Ω 2 > > Ω is satisfied. 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 components are filtered out, leaving only the frequency components much smaller than Ω 2. Therefore, frequency components in the photocurrent are adjacent to the frequency components such as omega 2、Ω2+Ω、Ω2 -omega in omega 2 as target frequencies, and the frequency components are mixed with a modulating signal source with the frequency of omega 2 and high-frequency signals are filtered, so that a PDH frequency stabilization error signal is obtained. Wherein the current signal component related to the noise modulation β detected by the second photodetector 54 is:
When the laser beam is at cavity resonance, the amplitudes of the (omega 2 -omega) component and the (omega 2 +omega) component in the above formula are equal, the optimal phase phi is selected, the signal is mixed with sin [ omega 2 t+phi ] and subjected to low-pass filtering, and the signal of the omega component is demodulated. The amplitude of the omega component is analyzed by a spectrum analyzer under otherwise stable conditions, which is proportional to the magnitude of the phase noise J 1 beta.
The source signal f×sin (Ω+Φ) containing the modulated noise signal Ω is extracted from the laser 1 by the noise sampling circuit module 61, and transmitted to the amplitude modulation phase shift amplifying circuit module 62 and the phase lock amplifying circuit module 63, respectively. The source signal is modulated by the amplitude modulation phase shift amplification circuit module 62 and acts on the first electro-optic modulator 65 by means of the first mixer 64 to directly reverse modulate the laser light, eliminating phase noise sidebands. Looking at the signal containing omega by the spectrum analyzer 67, the phase and amplitude of the first electro-optic modulator 65 are adjusted so that the omega signal on the spectrometer is reduced, and phase noise can be suppressed.
The source signal f×sin (Ω+Φ) is transmitted to the a channel of the lock-in amplifying circuit module, and d×sin Ω in the mixed signal generated by the second mixer 55 is injected into the B channel of the lock-in amplifier, and when the phase noise is reduced, the D value is smaller; but the phase noise becomes large and D changes, which can be extracted by the lock-in amplifying module 63 and injected into the other end of the first mixer 64 through the servo loop, the amplitude change of the noise is compensated by adjusting the modulation size of the first electro-optic modulator 65.
While the preferred embodiment of the present application has been described in detail, the present application is not limited to the embodiments, and those skilled in the art can make various equivalent modifications or substitutions without departing from the spirit of the present application, and the equivalent modifications or substitutions are included in the scope of the present application as defined in the appended claims.
Claims (5)
1. A system for suppressing single frequency phase noise of a laser, comprising:
A laser for generating a laser beam;
An optical isolator allowing the laser beam to be input unidirectionally 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;
The PDH frequency stabilization module is used for carrying out sideband modulation processing and resonance locking processing on the second laser beam so as 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 adjustment signal;
The laser controller is used for outputting a control signal to control the laser, adjusting the control signal according to the frequency adjusting signal so as to correct the frequency error of the laser and enable the laser to output frequency-stabilized laser;
Wherein 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 amplifying 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 stabilization module and is used for carrying out phase-locked amplification and operation on the frequency error signal output by the PDH frequency stabilization 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 correction signal;
the first electro-optic modulator carries out 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;
the phase noise suppression module further includes:
The first photoelectric detector is used for detecting the second laser beam so as to acquire an intensity signal;
The frequency 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;
The PDH frequency stabilization module comprises:
A modulation signal generator for generating a frequency modulation signal;
A second electro-optic modulator driven by the frequency modulation signal to modulate the second laser beam so that the second laser beam carries a sideband signal to become a third laser beam output;
The ultra-stable cavity is used for receiving the third laser beam, resonating the laser beams in the cavity and reflecting the third laser beam to output a fourth laser beam;
the second photoelectric detector is used for detecting the fourth laser beam so as to acquire an interference signal;
And the second mixer is used for carrying out mixing processing on the frequency modulation signal generated by the modulation signal generator and the interference signal detected by the second photoelectric detector so as to obtain a frequency error signal output.
2. The system of claim 1, wherein the phase noise suppression module is electrically connected to the laser controller to collect a control signal of the laser controller as a source signal output.
3. The system for suppressing single-frequency phase noise of a laser of claim 1, wherein said PDH frequency stabilization module further comprises:
A 1/2 wave plate for adjusting the linear polarization direction of the third laser beam;
the polarization beam splitter is used for polarization beam splitting of the third laser beam and the fourth laser beam incident on the polarization beam splitter so as to enable the third laser beam to transmit and enable the fourth laser beam to reflect;
The 1/4 wave plate is arranged between the ultra-stable cavity and the polarization beam splitter and is used for adjusting the polarization states 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.
4. A method of suppressing single frequency phase noise of a laser, the method employing a system for suppressing single frequency phase noise of a laser as defined in claim 1, comprising:
outputting a regulating signal through a laser controller to control a laser and outputting 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 so as to acquire a frequency error signal;
calculating the frequency error signal through a first servo loop module to output a frequency adjustment signal;
adjusting a control signal through a laser controller according to the frequency adjusting signal so as to correct the frequency error of the laser and enable the laser to output frequency-stabilized laser;
The step of performing single-frequency phase noise elimination on the first laser beam comprises the following steps:
the method comprises the steps of collecting a modulation signal of the laser controller through a noise sampling circuit module and outputting a source signal;
the source signal is subjected to amplitude modulation and phase shift through an amplitude modulation phase shift amplifying module so as to output a phase error signal;
The phase-locked amplifying circuit module performs phase-locked amplification and operation on the source signal and the frequency error signal 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 so as 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;
And adjusting the amplitude modulation value and the phase shift value of the source signal through an amplitude modulation phase shift amplification module until the phase noise displayed in the spectrum analyzer is zero, so that the first electro-optical modulator carries out constant amplitude and opposite phase modulation on the noise of the first laser beam, and single-frequency phase noise is eliminated.
5. The method of suppressing single frequency phase noise of a laser as recited in claim 4, wherein the step of performing a sideband modulation process and a resonance lock process on the second laser beam to output the frequency error signal comprises:
Driving a second electro-optic modulator by 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 to the ultra-stable cavity to be subjected to resonance locking treatment through the 1/2 wave plate, the polarization beam splitter and the 1/4 wave plate, and the fourth laser beam is reflected and output;
detecting the fourth laser beam by a second photoelectric detector to output an interference signal;
the interference signal and the frequency modulated signal are mixed by a second mixer to output a frequency error signal.
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| CN109167246A (en) * | 2018-08-07 | 2019-01-08 | 中山大学 | A kind of dual signal exploring laser light frequency stabilization integrated optical circuit device |
| CN210693007U (en) * | 2019-10-21 | 2020-06-05 | 中国人民解放军国防科技大学 | System for inhibiting single-frequency phase noise of laser |
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| CN109167246A (en) * | 2018-08-07 | 2019-01-08 | 中山大学 | A kind of dual signal exploring laser light frequency stabilization integrated optical circuit device |
| CN210693007U (en) * | 2019-10-21 | 2020-06-05 | 中国人民解放军国防科技大学 | System for inhibiting single-frequency phase noise of laser |
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