CN114336261B - Digital PDH error resolving module and laser system - Google Patents
Digital PDH error resolving module and laser system Download PDFInfo
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Abstract
The application discloses digital PDH error resolving module for laser output frequency adjusts, include: DDS, which generates intermediate frequency quadrature signals sin and cos; the input signal is divided into two paths, and is multiplied by sin and cos respectively and then subjected to second-order low-pass filtering to obtain an in-phase branch signal I and a quadrature branch signal Q; the signal I and the signal Q respectively pass through a squarer, and the output signals are added and then pass through an squarer; when the I path signal is greater than 0, the squarer outputs a first control signal; when the I-path signal is smaller than 0, the squarer outputs a first control signal in an inverted mode. The application also includes a laser system of the ultra-digital PDH error resolving module. The method and the device solve the problem that the laser frequency control precision is not high.
Description
Technical Field
The application relates to the field of lasers, in particular to a digital PDH error resolving module and a laser system.
Background
The narrow linewidth laser has important application in the aspects of time frequency transmission, optical frequency standard, ultra-stable microwave signal generation, precise laser spectrum, gravitational wave detection, basic physical experiment and the like. The method of round-Drever-Hall (PDH) is to compare the output frequency of the laser with the resonant frequency of the optical reference cavity to obtain an error signal, and adjust the output frequency of the laser by feedback, so as to lock the output frequency of the laser at the resonant frequency of the reference cavity. The method has the characteristics of good frequency discrimination, high signal-to-noise ratio, quick servo response and the like.
Currently, the PDH error signal is typically generated and resolved using discrete analog devices. By adopting a coherent resolving method, two paths of same-frequency sinusoidal signals are generated through a signal generating source or a DDS, one path of the sinusoidal signals is input into an electro-optical modulator to perform optical heterodyne modulation on laser, the photoelectric detector receives an optical reference cavity reflected optical signal and converts the optical reference cavity reflected optical signal into an electric signal, an error signal with a modulated carrier wave is obtained, the error signal is mixed with the other path of sinusoidal signals after phase shift, and then the error signal is resolved through a low-pass filter.
The coherent calculation requires that the carrier phases of the electric signals obtained by the local oscillation signal phase photoelectric detector are strictly matched, in practice, the phase shift of two paths of sinusoidal signals of a signal source is adjusted, meanwhile, the waveform of an error signal of an oscilloscope is observed, and the maximum amplitude of the error signal is obtained by adjusting the phase shift, so that the phase shift quantity is determined. The method ignores the change of the phase in the transmission line along with time, inevitably introduces noise, and reduces the control precision of the system.
Disclosure of Invention
The invention aims to provide a digital PDH error resolving module and a laser system, which solve the problem of low laser frequency control precision.
The embodiment of the application provides a digital PDH error resolving module which is used for adjusting output frequency of a laser, wherein the digital PDH error resolving module comprises a DDS, a multiplier, a squarer, an squarer, a low-pass filter, a discriminator and an inverter. Wherein, DDS generates intermediate frequency quadrature signals sin and cos; the input signal is divided into two paths, and is multiplied by sin and cos respectively, and then is subjected to second-order low-pass filtering to obtain an in-phase branch signal I and a quadrature branch signal Q. The signal I and the signal Q respectively pass through squarers, and output signals are added and then pass through squarers. When the I path signal is greater than 0, the squarer outputs a first control signal; when the I-path signal is smaller than 0, the squarer outputs a first control signal in an inverted mode.
Preferably, the digital PDH error resolution module further includes an ADC and a DAC. The ADC is positioned at the input end of the digital PDH error resolving module; the DAC is positioned at the output end of the digital PDH error resolving module.
Further, the digital PDH error resolving module also outputs a second control signal and a third control signal. The sin signal is output as a second control signal for adjusting the output frequency of the laser. When the digital PDH error resolving module comprises a triangular wave generator, the triangular wave generator is used for generating a triangular wave signal as a third control signal, and the third control signal is used for controlling the laser to sweep.
Further, the digital PDH error resolving module further includes a signal conditioning unit. The signal conditioning unit is configured at the input end and/or the output end of the digital PDH error resolving module. The signal conditioning unit is used for adjusting signal amplitude and bias and performing low-pass filtering on the analog signal, and the filtering bandwidth is smaller than half of the sampling rate of the ADC or the DAC.
The application also provides a laser system for resolving the digital PDH error, which uses the digital PDH error resolving module according to any one embodiment of the application, and further comprises
The device comprises a laser, an optical fiber isolator, an acousto-optic modulator, an optical fiber coupler, an electro-optic modulator and a circulator which are sequentially connected;
the bypass of the optical fiber coupler outputs stable laser;
light of the output port of the circulator enters an optical reference cavity through an optical fiber coupling mirror, and reflected light is generated and returned to the output end of the circulator;
the light returned by the output port of the circulator is output to the photoelectric detector through a third port to obtain an error signal which is used as an input signal of the digital PDH error resolving module;
the first control signal controls the modulation end of the acousto-optic modulator through the driving circuit of the acousto-optic modulator.
Preferably, the sin signal is used as a second control signal and is input to the modulation end of the electro-optical modulator.
Preferably, the triangular wave signal is used as a third control signal and is transmitted to a PZT adjusting end of the laser, so that the laser sweep frequency is realized.
The above-mentioned at least one technical scheme that this application embodiment adopted can reach following beneficial effect:
the invention integrates laser control, sinusoidal signal generation and error signal calculation on a single digital board, and has the characteristics of complete function, excellent performance and small volume. All functions of generating a signal generation source and generating an error signal required by a Pound-Drever-Hall feedback control method in a laser frequency stabilization system are realized by using an FPGA, and a layer of judgment logic is nested in an orthogonal resolving logic, so that the problem of error signal symbol information loss caused by squaring and opening root numbers of in-phase branch signals and orthogonal branch signals in an orthogonal resolving scheme is solved, and on the premise of functional integrity, the integration level and the system precision of the system are improved, and the operation complexity is reduced.
Drawings
The accompanying drawings, which are included to provide a further understanding of the application and are incorporated in and constitute a part of this application, illustrate embodiments of the application and together with the description serve to explain the application and do not constitute an undue limitation to the application. In the drawings:
FIG. 1 is a block diagram of a digital PDH error signal quadrature calculation module according to an embodiment of the invention
Fig. 2 is an embodiment of a laser system including a digital PDH error signal quadrature resolution module.
Detailed Description
For the purposes, technical solutions and advantages of the present application, the technical solutions of the present application will be clearly and completely described below with reference to specific embodiments of the present application and corresponding drawings. It will be apparent that the described embodiments are only some, but not all, of the embodiments of the present application. All other embodiments, which can be made by one of ordinary skill in the art without undue burden from the present disclosure, are within the scope of the present disclosure.
The optical adjusting frame of the all-fiber optical assembly and five degrees of freedom is utilized to achieve miniaturization of the optical part of the ultra-stable narrow linewidth laser system, complexity of system installation is reduced, and repeated adjustment work is reduced.
The following describes in detail the technical solutions provided by the embodiments of the present application with reference to the accompanying drawings.
Fig. 1 is a block diagram of a digital PDH error signal quadrature resolution module embodiment of the present invention.
The embodiment of the application provides a digital PDH error resolving module which is used for adjusting output frequency of a laser, wherein the digital PDH error resolving module comprises a DDS, a multiplier, a squarer, an squarer, a low-pass filter, a discriminator and an inverter. Wherein, DDS generates intermediate frequency quadrature signals sin and cos; the input signal is divided into two paths, and is multiplied by sin and cos respectively, and then is subjected to second-order low-pass filtering to obtain an in-phase branch signal I and a quadrature branch signal Q. The signal I and the signal Q respectively pass through squarers, and output signals are added and then pass through squarers. When the I path signal is greater than 0, the squarer outputs a first control signal; when the I-path signal is smaller than 0, the squarer outputs a first control signal in an inverted mode.
As shown in fig. 1, for example, the sinusoidal signal is generated by the DDS IP core inside the FPGA into two paths of signals sin and cos that are 10MHz orthogonal. Outputting a sin signal; the input signal is subjected to digital first-order low-pass filtering to remove high-frequency spurious, then is multiplied by sin and cos respectively in two paths, in-phase branch signal I and quadrature branch signal Q are obtained through second-order low-pass filtering, and the square sum root number of the two paths of signals is calculated to obtain output OUT; and taking the I-path signal as a reference, outputting OUT if the I-path signal is greater than zero, outputting-OUT if the I-path signal is less than zero, obtaining an error signal, and obtaining a laser frequency feedback control signal after passing through an internal digital PID module.
Preferably, the digital PDH error resolution module further includes an ADC and a DAC. The ADC is positioned at the input end of the digital PDH error resolving module; the DAC is positioned at the output end of the digital PDH error resolving module.
Further, the digital PDH error resolving module also outputs a second control signal and a third control signal. The sin signal is output as a second control signal for adjusting the output frequency of the laser. When the digital PDH error resolving module comprises a triangular wave generator, the triangular wave generator is used for generating a triangular wave signal as a third control signal, and the third control signal is used for controlling the laser to sweep.
Further, the digital PDH error resolving module further includes a signal conditioning unit. The signal conditioning unit is configured at the input end and/or the output end of the digital PDH error resolving module. The signal conditioning unit is used for adjusting signal amplitude and bias and performing low-pass filtering on the analog signal, and the filtering bandwidth is smaller than half of the sampling rate of the ADC or the DAC.
As the best embodiment of the application, a high-speed digital Pound-Drever-Hall error signal quadrature calculating circuit is manufactured, and the circuit comprises three parts, namely an analog signal conditioning circuit, a digital-analog-digital conversion circuit and an FPGA module. The analog signal conditioning circuit scales and additionally biases the amplitude of an input signal, so that an external input analog signal is sampled in the full range of the ADC as much as possible, anti-aliasing filtering (low-pass filtering, which is cut off according to the Nyquist sampling law and is less than half of the sampling rate of the ADC, and one third to one fourth of the engineering) is performed, high-frequency noise is filtered, and signal distortion caused by frequency spectrum aliasing is avoided; and scaling, biasing and smoothing the DAC output signal to enable the output signal to meet the input requirements of the peripheral. The module has an input and three outputs. The output 3 (third control signal) is used for generating a triangular wave signal and transmitting the triangular wave signal to a PZT adjusting end of the laser, and is used for realizing laser frequency sweep; an output 2 (second control signal) outputs a 10MHz sinusoidal signal, which is sent to the radio frequency input end of the electro-optic modulator, heterodynes the laser to obtain an error signal for feedback control of the laser frequency, the magnitude of the error signal is proportional to the deviation between the output laser frequency and the frequency of the optical reference cavity, and the error signal is a dispersion-like frequency discrimination curve (only using the middle linear part) under the sweep frequency; and outputting an error signal calculated by the step 1 (first control signal), and passing through a digital PID module in the FPGA to compensate the response of a feedback system, wherein the error signal is used for accessing an AOM driver to control the laser frequency. The input receives an electric signal converted from an optical reference cavity reflected light signal received by a photoelectric detector, the signal is an error signal with modulation, and the error signal is obtained by calculation processing of an internal quadrature resolving module after internal digital filtering.
Since the error signal has a negative value, the quadrature solution algorithm involves a sum of squares root operation, leaving only the absolute value of the error signal. The invention solves the problem of symbol information loss in error signal calculation by embedding a layer of judgment logic.
Fig. 2 is an embodiment of a laser system including a digital PDH error signal quadrature resolution module (in the figure, the solid line in the optical path is a fiber optic connection; the dashed line is free space propagation).
The embodiment of the application provides an ultra-stable narrow linewidth laser system, which comprises a laser, an optical fiber isolator, an acousto-optic modulator, an optical fiber coupler, an electro-optic modulator, a circulator, an optical fiber coupling mirror, an optical reference cavity arranged at the downstream of the optical fiber coupling mirror, and a photoelectric detector connected to a third port of the circulator.
And the bypass of the optical fiber coupler outputs stable laser. And the light of the output port of the circulator enters the optical reference cavity through the optical fiber coupling mirror, and reflected light is generated and returned to the output end of the circulator. The light returned by the output port of the circulator is output to the photoelectric detector through a third port to obtain an error signal which is used as an input signal of the digital PDH error resolving module;
for example, in order to achieve the above object, the present invention employs: waveguide-based acousto-optic modulator (AOM), waveguide-based electro-optic modulator (EOM), tunable fiber optic coupling mirror. The first photodetector is fiber-coupled.
When the ultra-stable narrow linewidth laser system works, laser is transmitted in an optical fiber, a free-running laser passes through an isolator and an AOM, then a fiber coupler divides a laser beam into two paths, one path is used for laser output, the other path is used for generating an error feedback signal, one path for generating the error feedback signal is transmitted to a second port through a first port of a circulator after passing through the EOM, then the laser is converted into a free-space laser beam through a fiber coupling mirror to be coupled into an optical reference cavity, and meanwhile, the coupled light is transmitted to a third port through the second port of the circulator to be output and connected into a photoelectric detector, so that the error feedback signal for controlling the frequency of the laser is obtained.
The first control signal controls the modulation end of the acousto-optic modulator through the driving circuit of the acousto-optic modulator. Preferably, the sin signal is used as a second control signal and is input to the modulation end of the electro-optical modulator. Preferably, the triangular wave signal is used as a third control signal and is transmitted to a PZT adjusting end of the laser, so that the laser sweep frequency is realized.
When the laser system is controlled, the light path and the circuit of the ultra-stable narrow linewidth laser system are connected, and the light path coupling is regulated. The laser is coupled into an optical reference cavity along an optical fiber through an isolator, an acousto-optic modulator (AOM), an optical fiber coupler, an electro-optic modulator (EOM), a circulator and an optical fiber coupling mirror, and an optical signal reflected by the optical reference cavity is connected into a photoelectric detector through the circulator to be converted into an electric signal which is connected into an input end of a high-speed digital PDH error signal quadrature resolving module. The output of the digital module, the output 3 (third control signal) and the output triangular wave signal are connected to the PZT control end of the laser, so as to realize laser sweep; 2 (second control signal) outputs 10MHz sine signal to the electro-optical modulator to realize heterodyne modulation of laser; and the output 1 (first control signal) is an error signal which is finally solved by the digital PID module, and is connected to an AOM driver to realize laser frequency control.
It should also be noted that the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one … …" does not exclude the presence of other like elements in a process, method, article, or apparatus that comprises an element
The foregoing is merely exemplary of the present application and is not intended to limit the present application. Various modifications and changes may be made to the present application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc. which are within the spirit and principles of the present application are intended to be included within the scope of the claims of the present application.
Claims (5)
1. A digital PDH error resolving module for adjusting the output frequency of a laser, which is characterized by comprising a DDS, ADC, DAC and a triangular wave generator;
DDS, which generates intermediate frequency quadrature signals sin and cos;
the input signal is divided into two paths, and is multiplied by sin and cos respectively and then subjected to second-order low-pass filtering to obtain an in-phase branch signal I and a quadrature branch signal Q;
the signal I and the signal Q respectively pass through a squarer, and the output signals are added and then pass through an squarer;
when the I path signal is greater than 0, the squarer outputs a first control signal; when the I-path signal is smaller than 0, the inverting output of the squarer is a first control signal;
the sin signal is output as a second control signal and is used for adjusting the output frequency of the laser;
the ADC is positioned at the input end of the digital PDH error resolving module; the DAC is positioned at the output end of the digital PDH error resolving module;
and the triangular wave generator is used for generating a triangular wave signal serving as a third control signal and controlling the laser to sweep.
2. The digital PDH error resolution module as defined in claim 1, further comprising a signal conditioning unit,
the signal conditioning unit is configured at the input end and/or the output end of the digital PDH error resolving module;
the signal conditioning unit is used for adjusting signal amplitude and bias and performing low-pass filtering on the analog signal, and the filtering bandwidth is smaller than half of the sampling rate of the ADC.
3. A digital PDH error resolving laser system, comprising the digital PDH error resolving module according to any one of claims 1 to 2, and further comprising a laser, an optical fiber isolator, an acousto-optic modulator, an optical fiber coupler, an electro-optic modulator, and a circulator, which are sequentially connected;
the bypass of the optical fiber coupler outputs stable laser;
light of the output port of the circulator enters an optical reference cavity through an optical fiber coupling mirror, and reflected light is generated and returned to the output end of the circulator;
the light returned by the output port of the circulator is output to the photoelectric detector through a third port to obtain an error signal which is used as an input signal of the digital PDH error resolving module;
the first control signal controls the modulation end of the acousto-optic modulator through the driving circuit of the acousto-optic modulator.
4. A digital PDH error resolved laser system as in claim 3,
and taking the sin signal as a second control signal and inputting the second control signal into a modulation end of the electro-optic modulator.
5. A digital PDH error resolved laser system as in claim 3,
and the triangular wave signal is used as a third control signal and is transmitted to a PZT adjusting end of the laser, so that the sweep frequency of the laser is realized.
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