Digitally controlled quadrature modulation laser single sideband frequency shifting and phase locking apparatus and method

Abstract

The application relates to a digital control quadrature modulation laser single sideband frequency shift and phase locking device and method, wherein a field programmable logic gate array is connected with four digital-to-analog converters, one quadrature modulator is connected with every two digital-to-analog converters, two quadrature modulators are further connected to an optical quadrature modulator, laser source output is subjected to single sideband frequency shift through the optical quadrature modulator, part of the optical signal is converted into an electrical signal through a third beam splitter, and the optical modulator parameters are adjusted in real time through a voltage control port; another laser passes through a first beam splitter and interferes with the frequency-shifted optical signal at a beam combiner, phase information is fed back after demodulation through a quadrature down-conversion mixer, and the phase information is fed back to the field programmable logic gate array through a three-channel analog-to-digital converter to form a closed loop control. The application reduces the influence of the sideband signal generated by adjustment; the frequency shift bandwidth can be higher, the influence of the spatial light path is reduced, and the stability is improved.

Description

Technical Field

[0001] This invention belongs to the field of quantum optoelectronic device technology, and particularly relates to a digitally controlled orthogonal modulation laser single-sideband frequency shifting and phase locking device. Background Technology

[0002] In the field of cold atom precision measurement, laser technology plays a crucial role. Specifically, lasers of specific frequencies can effectively cool atoms and even cause atomic clusters to undergo arbitrary projectile motion at predetermined speeds and directions. Furthermore, coherent light sources with atomic energy level frequency differences can precisely manipulate the internal states of atoms. Frequency shifting and phase locking of lasers are particularly critical laser technologies.

[0003] Currently, traditional direct laser modulation frequency shifting mainly relies on acousto-optic modulators and electro-optic modulators. Acousto-optic modulators can transfer most of the optical power to diffracted light through the acousto-optic effect, thus achieving laser frequency shifting. Applying a periodic radio frequency signal with a suitable voltage to an electro-optic modulator can generate frequency-shifted laser light through electro-optic phase modulation. However, acousto-optic modulators inevitably require a relatively large and less stable spatial optical path, and the multi-sideband effect generated by electro-optic modulation can easily introduce some system effects.

[0004] Optical phase-locked loop (PLL) systems, derived from traditional PLL systems, utilize optical beat frequency devices to obtain the difference frequency signal between two light beams. This difference frequency signal is then introduced into a phase detection system. After obtaining the phase detection error signal, negative feedback control is used to ultimately stabilize the relative phase of the optical difference frequency signal. Optical PLLs can achieve relative laser frequency shifting of one laser beam at the frequency of another reference laser, simultaneously generating a coherent light source. However, optical PLLs require at least two laser sources and a relatively complex control system.

[0005] With the development of quantum precision measurement, the demands for industrialization, application, and miniaturization are becoming increasingly urgent. Furthermore, there are high requirements for high-bandwidth dynamic frequency shifting of lasers and relatively phase-stable coherent light sources. Therefore, a method is needed that is simple to implement and easy to control, enabling high-bandwidth single-sideband frequency shifting of lasers while simultaneously generating low-phase-noise coherent light sources.

[0006] A prior art patent document entitled "Laser Frequency Stabilization and Shifting System and Laser," with authorization publication number CN 114221206 B, discloses a laser frequency stabilization and shifting system and laser. It uses an orthogonal electro-optic phase modulator to achieve single-sideband frequency shifting of the laser, and then uses an atomic spectral frequency locking method to lock the absolute frequency of the laser. Frequency stabilization of the shifted laser is achieved by feeding back the frequency error to the output laser. This invention's technical means can perform high-bandwidth frequency shifting of the laser, and the high sideband suppression ratio due to orthogonal electro-optic phase modulation makes the atomic spectrum excited by the frequency-shifted laser more stable and unaffected by other sidebands. However, the orthogonal signal source used in this invention is implemented using a 90-degree bridge or phase shifter, resulting in a limited range of frequency shifts that the system can adapt to. It is not easy to perform frequency hopping conversion between different laser frequencies, and it also limits the system's versatility and makes it inconvenient to incorporate into a laser phase-locked loop system.

[0007] A prior art patent document entitled "An Optical Phase-Locked Loop Device," publication number CN114371582A, discloses an optical phase-locked loop device that utilizes two lasers for relative phase locking. First, an optical beat frequency device is used to obtain the radio frequency signal of the frequency difference between the two lasers. Then, an up-conversion mixer is used to obtain a microwave reference signal and a sum-frequency signal from the signal source. The optical difference frequency signal and the obtained up-mixed signal are mixed again before entering a phase detection system. The phase detection system performs digital and analog phase detection with the radio frequency reference signal, respectively. The resulting error signals are then subjected to digital and analog feedback on the piezoelectric ceramic of one of the lasers to achieve relative phase locking between the two lasers. This invention optimizes the performance of the optical phase-locked loop by combining digital and analog control, enabling the optical phase-locked loop to operate at its optimal point and improving the phase-locking bandwidth. However, this invention requires two lasers and a complex frequency control and feedback system to obtain a coherent light source.

[0008] Currently, quantum precision measurement technology has entered the industrial application stage, which places higher demands on the miniaturization and integration of core optical components, ease of operation, and system stability. Therefore, a highly integrated and easily controllable laser broadband single-sideband frequency shifter is needed, which can also generate a coherent light source with low phase noise. Summary of the Invention

[0009] To address the aforementioned technical problems, the first objective of this invention is to provide a digitally controlled orthogonal modulation laser single-sideband frequency shifting and phase-locking device. This device features highly integrated and easily controlled wideband single-sideband laser frequency shifting, while simultaneously generating a coherent light source with low phase noise. The second objective of this invention is to provide a digitally controlled orthogonal modulation laser single-sideband frequency shifting and phase-locking method.

[0010] To achieve the first objective mentioned above, the present invention adopts the following technical solution: A digitally controlled quadrature modulation laser single-sideband frequency shifter and phase-locking device includes a field-programmable gate array, a digital-to-analog converter, a quadrature modulator, an optical quadrature modulator, a laser, a first beam splitter, a second beam splitter, a third beam splitter, a beam combiner, and a quadrature down-conversion mixer. The field-programmable gate array (FPGA) is connected to four digital-to-analog converters (DACs) via a data bus. Each pair of DACs is connected to a quadrature modulator (QM), and both QMs are connected to a local oscillator. The two QMs are also connected to an optical quadrature modulator. A laser beam output from the laser is input to an optical quadrature modulator. After being modulated by the optical quadrature modulator, the laser beam is input to a third beam splitter. The third beam splitter is connected to a field-programmable gate array (FPGA) via a second photodiode and a three-channel analog-to-digital converter (ADC). The FPGA is connected to the voltage control port of the optical quadrature modulator via a three-channel digital-to-analog converter (DAC). Another laser beam output from the laser is input to the first beam splitter. After being modulated by the optical quadrature modulator, the laser beam is input to the second beam splitter. The two laser beams, after passing through the first and second beam splitters, are then combined by the beam combiner and connected to the quadrature down-conversion mixer via the first photodiode. The quadrature down-conversion mixer is connected to the local oscillator and is also connected to the field programmable gate array via a three-channel analog-to-digital converter.

[0011] As a preferred embodiment, the four digital-to-analog converters are a first digital-to-analog converter, a second digital-to-analog converter, a third digital-to-analog converter, and a fourth digital-to-analog converter, and the two quadrature modulators are a first quadrature modulator and a second quadrature modulator, respectively. The first digital-to-analog converter and the second digital-to-analog converter are connected to the first quadrature modulator, and the third digital-to-analog converter and the fourth digital-to-analog converter are connected to the second quadrature modulator.

[0012] As a preferred embodiment, the digital-to-analog converter is a 14-bit parallel high-sampling-rate DAC, model AD9764, with a sampling rate of 125 MHz and an analog bandwidth greater than 40 MHz.

[0013] As a preferred embodiment, the field-programmable gate array includes a digitally controlled oscillator, four digital phase shifters, and four digital amplifiers. The digitally controlled oscillator generates a digital signal of the desired frequency and inputs the digital signal into the four digital phase shifters respectively. The digital phase shifters make the output signal form two pairs of mutually orthogonal digital signals. The digital amplifiers then change the frequency of the corresponding signals and output them to the parallel data bus to drive an external digital-to-analog converter.

[0014] As a preferred embodiment, the field-programmable gate array further includes a digital proportional-integral-derivative (DPDA) controller and a digital phase detector. The digital phase detector receives orthogonal signals transmitted from a digitally controlled oscillator and a three-channel analog-to-digital converter (ADC), and calculates the relative phase error between the two pairs of signals. The DPDA controller then calculates the error signal to obtain the phase amount that needs to be compensated for in the four orthogonal intermediate frequency (IF) signals. The feedback amount is added to the four output digital phase shifters, thereby changing the four IF signals generated by the ADC and performing digital feedback control of the relative phase of the laser beat frequency signal.

[0015] As a preferred embodiment, the four digital phase shifters are designated as a first digital phase shifter, a second digital phase shifter, a third digital phase shifter, and a fourth digital phase shifter, and the four digital amplifiers are designated as a first digital amplifier, a second digital amplifier, a third digital amplifier, and a fourth digital amplifier, respectively; the first digital phase shifter is connected to the first digital amplifier, the second digital phase shifter is connected to the second digital amplifier, the third digital phase shifter is connected to the third digital amplifier, and the fourth digital phase shifter is connected to the fourth digital amplifier; the digital proportional-integral-derivative controller is connected to the first digital phase shifter, the second digital phase shifter, the third digital phase shifter, and the fourth digital phase shifter, respectively.

[0016] To achieve the second objective mentioned above, the present invention adopts the following technical solution: A digitally controlled method for single-sideband frequency shifting and phase locking of orthogonal modulated lasers, employing any of the devices described above, comprises the following specific steps: S1. The local oscillator outputs a radio frequency signal to two quadrature modulators. The field-programmable gate array outputs two pairs of quadrature intermediate frequency signals through a digital-to-analog converter. The two pairs of quadrature intermediate frequency signals are respectively input to the two quadrature modulators to perform quadrature modulation on the radio frequency signal to obtain two pairs of quadrature high-frequency radio frequency signals. The two pairs of quadrature high-frequency radio frequency signals are used to modulate the optical quadrature modulator. S2. The laser generates two laser beams. One beam is output directly, and the frequency of the laser source is locked by frequency locking of this laser beam. The other beam is input to an optical quadrature modulator for modulation. The modulated laser is a single-sideband frequency-shifted laser. The frequency shift of the laser is controlled by changing the frequency of the signal output by the local oscillator. S3. After frequency shifting, the laser passes through the third beam splitter. A small portion of the split light source is collected by the second photodiode to obtain an electrical signal. This signal is then acquired by one channel of the three-channel analog-to-digital converter and transmitted to the field-programmable gate array for monitoring optical power. S4. The field-programmable gate array calculates the appropriate voltage through the feedback quantity, and controls the three voltage control ports of the optical quadrature modulator by driving a three-channel digital-to-analog converter. By scanning the voltage values ​​of the three voltage control ports and recording the photoelectric signal on the second photodiode, the control voltage is obtained, so that the output optical power remains efficient and stable. S5. One laser beam output from the laser passes through the first beam splitter, and the other laser beam output passes through the optical quadrature modulator and then through the second beam splitter. The two split laser beams are combined by the beam combiner, and the resulting optical beat frequency signal is collected by the first photodiode. The resulting electrical signal is input to the quadrature downconversion mixer and is quadraturely downmixed with the signal output from the local oscillator. The mixed signal is collected by the other two channels of the three-channel analog-to-digital converter and then enters the field programmable gate array for digital demodulation and digital feedback.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention uses Field-Programmable Gate Array (FPGA) technology to control two pairs of orthogonal intermediate frequency (IF) signals, and then obtains orthogonal radio frequency (RF) signals by orthogonally modulating a local reference signal. Compared to traditional phase-shifting methods for obtaining orthogonal signals, this invention can produce orthogonal signals with higher bandwidth, allowing for a wider laser frequency shift range. Furthermore, this invention utilizes a digital feedback system to lock the relative phase of the laser before and after frequency shifting with a single light source, resulting in a simpler structure compared to traditional optical phase-locked loop (PLL) systems that require two laser sources.

[0018] This invention utilizes an optical quadrature modulator to perform quadrature modulation on a laser, enabling single-sideband frequency shifting. Compared to traditional laser electro-optic modulation frequency shifting, it reduces the influence of sideband signals generated by the adjustment; compared to traditional acousto-optic modulation frequency shifting, it achieves a higher frequency shift bandwidth and reduces the influence of spatial optical paths, thus improving stability. Attached Figure Description

[0019] The accompanying drawings, which form part of this application, are used to provide a further understanding of this application. The illustrative embodiments of this application and their descriptions are used to explain this application and do not constitute a limitation thereof.

[0020] Figure 1 This is a schematic diagram of the overall structure of the device of the present invention; Figure 2 This is a schematic diagram of the internal structure of the field-programmable gate array of the present invention.

[0021] The attached figures are labeled as follows: 101, Field-Programmable Gate Array (FPGA); 102, First Digital-to-Analog Converter (D / A converter); 103, Second D / A converter; 104, Third D / A converter; 105, Fourth D / A converter; 106, First Quadrature Modulator; 107, Second Quadrature Modulator; 108, Local Oscillator; 109, Optical Quadrature Modulator; 110, Laser; 111, First Beam Splitter; 112, Second Beam Splitter; 113, Third Beam Splitter; 114, Beam Combiner; 115, Three-Channel Analog-to-Digital Converter; 116, Three... 117. Channel digital-to-analog converter; 118. Quadrature down-conversion mixer; 119. First photodiode; 110. Second photodiode; 201. Digitally controlled oscillator; 202. First digital phase shifter; 203. First digital amplifier; 204. Second digital phase shifter; 205. Second digital amplifier; 206. Third digital phase shifter; 207. Third digital amplifier; 208. Fourth digital phase shifter; 209. Fourth digital amplifier; 210. Digital proportional-integral-derivative controller; 211. Digital phase detector. Detailed Implementation

[0022] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of this application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains.

[0023] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments according to this application. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0024] Furthermore, in the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "clockwise," and "counterclockwise," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0025] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, unless otherwise stated, "a plurality of" means two or more, unless explicitly defined otherwise.

[0026] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0027] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can include direct contact between the first and second features, or contact between the first and second features through another feature between them. Furthermore, "above," "over," and "on top" of the second feature includes the first feature directly above or diagonally above the second feature, or simply indicates that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature includes the first feature directly below or diagonally below the second feature, or simply indicates that the first feature is at a lower horizontal level than the second feature.

[0028] The present invention will be further described below with reference to the accompanying drawings and embodiments: like Figure 1 As shown, a digitally controlled quadrature modulation laser single-sideband frequency shifting and phase-locking device includes a field-programmable gate array (FPGA) 101, a digital-to-analog converter (DAC), a quadrature modulator, an optical quadrature modulator 109, a laser 110, a first beam splitter 111, a second beam splitter 112, a third beam splitter 113, a beam combiner 114, and a quadrature down-conversion mixer 117. The FPGA 101 is connected to four DACs via a data bus. The DAC used is an AD9764, a 14-bit parallel high-sampling-rate DAC with a sampling rate up to 125 MHz and an analog bandwidth greater than 40 MHz. Considering the need to change the signal frequency during modulation, the initial output frequency of the DAC is set to 20 MHz.

[0029] Each pair of digital-to-analog converters is connected to a quadrature modulator, and both quadrature modulators are connected to a local oscillator 108. Both quadrature modulators are also connected to an optical quadrature modulator 109. The RF signals modulated by both quadrature modulators are output from the local oscillator 108. The LO signal comes from a chip-level programmable frequency generator with a phase-locked loop, capable of providing RF signals up to 20 GHz. After quadrature modulation of the LO signal, a pair of quadrature high-frequency RF signals are obtained, which are then modulated by the optical quadrature modulator 109.

[0030] The four digital-to-analog converters are a first digital-to-analog converter 102, a second digital-to-analog converter 103, a third digital-to-analog converter 104, and a fourth digital-to-analog converter 105, and the two quadrature modulators are a first quadrature modulator 106 and a second quadrature modulator 107. The first digital-to-analog converter 102 and the second digital-to-analog converter 103 are connected to the first quadrature modulator 106, and the third digital-to-analog converter 104 and the fourth digital-to-analog converter 105 are connected to the second quadrature modulator 107.

[0031] The laser generated by the laser 110 will be split into two beams. One beam is input to an optical quadrature modulator, and the modulated laser is a single-sideband frequency-shifted laser. The frequency shift of the laser can be controlled by simply changing the frequency of the LO signal. The other beam is output directly, and the frequency of the laser source can be locked by frequency locking of this laser beam.

[0032] A frequency-shifted laser beam output from laser 110 is input to optical quadrature modulator 109. After being modulated by optical quadrature modulator 109, the laser beam is input to third beam splitter 113. Third beam splitter 113 is connected to field-programmable gate array 101 via second photodiode 119 and three-channel analog-to-digital converter 115 for monitoring optical power. Field-programmable gate array 101 is connected to the voltage control port of optical quadrature modulator 109 via three-channel digital-to-analog converter 116. Field-programmable gate array 101 calculates a suitable voltage based on the feedback and drives a three-channel digital-to-analog converter 116 to control the three voltage control ports of optical quadrature modulator 109. By scanning the voltage values ​​of the three voltage control ports and recording the photoelectric signal on second photodiode 119, the optimal control voltage can be obtained, enabling the output optical power to maintain high efficiency and stability.

[0033] Another laser beam output from laser 110 is input to the first beam splitter 111. After being modulated by optical quadrature modulator 109, the laser beam is input to the second beam splitter 112. The two laser beams, after passing through the first beam splitter 111 and the second beam splitter 112, are then combined by beam combiner 114. The resulting optical beat frequency signal is collected by the first photodiode 118. The resulting electrical signal is input to quadrature down-conversion mixer 117 for quadrature down-mixing with the LO signal. The quadrature down-conversion mixer 117 is connected to the local oscillator 108. Since the LO signal is quadraturely modulated by the intermediate frequency signal, the frequency difference between the RF signal used for optical quadrature modulation and the LO signal is the frequency of the intermediate frequency signal, i.e., the frequency of the optical beat frequency. Therefore, the quadrature signal obtained after quadrature down-mixing has the same frequency as the intermediate frequency signal. The quadrature down-conversion mixer 117 is also connected to the field programmable gate array 101 through a three-channel analog-to-digital converter 115 for digital demodulation and digital feedback.

[0034] like Figure 2 As shown, the field-programmable gate array 101 includes a digitally controlled oscillator 201, four digital phase shifters, and four digital amplifiers. The digitally controlled oscillator 201 generates a digital signal of the required frequency and inputs the digital signal into the four digital phase shifters respectively. The digital phase shifters make the output signal form two pairs of mutually orthogonal digital signals. Then, the digital amplifiers change the frequency of the corresponding signals and output them to the parallel data bus to drive an external digital-to-analog converter.

[0035] The field-programmable gate array 101 also includes a digital proportional-integral-derivative (DPID) controller 210 and a digital phase detector 211. The digital phase detector 211 receives orthogonal signals transmitted from the digitally controlled oscillator 201 and the three-channel analog-to-digital converter 115, and calculates the relative phase error between the two pairs of signals. The DPID controller 210 then calculates the error signal to obtain the phase amount that needs to be compensated for in the four orthogonal intermediate frequency signals. The feedback amount is added to the four output digital phase shifters, which correspondingly changes the four intermediate frequency signals generated by the digital-to-analog converter, thereby providing digital feedback control for the relative phase of the laser beat frequency signal.

[0036] The four digital phase shifters are designated as first digital phase shifter 202, second digital phase shifter 204, third digital phase shifter 206, and fourth digital phase shifter 208, respectively. The four digital amplifiers are designated as first digital amplifier 203, second digital amplifier 205, third digital amplifier 207, and fourth digital amplifier 209, respectively. First digital phase shifter 202 is connected to first digital amplifier 203, second digital phase shifter 204 is connected to second digital amplifier 205, third digital phase shifter 206 is connected to third digital amplifier 207, and fourth digital phase shifter 208 is connected to fourth digital amplifier 209. The digital proportional-integral-derivative controller 210 is connected to first digital phase shifter 202, second digital phase shifter 204, third digital phase shifter 206, and fourth digital phase shifter 208, respectively.

[0037] A digitally controlled method for single-sideband frequency shifting and phase locking of orthogonal modulated lasers, employing any of the devices described above, comprises the following specific steps: S1. The local oscillator 108 outputs radio frequency signals to two quadrature modulators. The field programmable gate array 101 outputs two pairs of quadrature intermediate frequency signals through a digital-to-analog converter. The two pairs of quadrature intermediate frequency signals are respectively input to the two quadrature modulators to perform quadrature modulation on the radio frequency signals to obtain two pairs of quadrature high-frequency radio frequency signals. The two pairs of quadrature high-frequency radio frequency signals are used to modulate the optical quadrature modulator 109. S2. The laser 110 generates two laser beams. One beam is output directly, and the frequency of the laser source is locked by frequency locking of this laser beam. The other beam is input to the optical quadrature modulator 109 for modulation. The modulated laser is a single-sideband frequency-shifted laser. The frequency shift of the laser is controlled by changing the frequency of the signal output by the local oscillator 108. S3. After frequency shifting, the laser beam passes through the third beam splitter 113. A small portion of the split light source is collected by the second photodiode 119 to obtain an electrical signal. The signal is then collected by one channel of the three-channel analog-to-digital converter 115 and transmitted to the field programmable gate array 101 for monitoring the optical power. S4. The field programmable gate array 101 calculates the appropriate voltage through the feedback quantity, and controls the three voltage control ports of the optical quadrature modulator 109 by driving a three-channel digital-to-analog converter 116. By scanning the voltage values ​​of the three voltage control ports and recording the photoelectric signal on the second photodiode 119, the control voltage is obtained, so that the output optical power remains efficient and stable. S5. One laser beam output from the laser 110 passes through the first beam splitter 111, and the other laser beam output passes through the optical quadrature modulator 109 and then through the second beam splitter 112. The two split laser beams are combined by the beam combiner 114, and the resulting optical beat frequency signal is collected by the first photodiode 118. The resulting electrical signal is input to the quadrature downconversion mixer 117 and is orthogonally downmixed with the signal output from the local oscillator 108. The mixed signal is collected by the other two channels of the three-channel analog-to-digital converter 115 and then enters the field programmable gate array 101 for digital demodulation and digital feedback.

[0038] This invention has the following key advantages: It generates two pairs of intermediate-frequency orthogonal signals using digital technology to orthogonally modulate a higher-frequency radio frequency (RF) signal. Then, it optically orthogonally modulates the laser using these orthogonal RF signals, enabling single-sideband frequency shifting of the laser and effectively reducing the sideband effect caused by laser modulation frequency shifting. Furthermore, it achieves relative phase locking between the two laser beams through analog-to-digital conversion and digital feedback, allowing for the generation of two or more coherent beams even with a single laser source.

[0039] Compared to the laser frequency stabilization and shifting system and laser with authorization announcement number CN 114221206 B, this invention generates two pairs of orthogonal intermediate frequency signals with controllable phase, frequency, and amplitude using an FPGA. Then, it orthogonally modulates the signal from the local oscillator using an orthogonal modulator to obtain orthogonal radio frequency signals for optical orthogonal modulation. Compared to the method in that invention that obtains orthogonal signals using a 90-degree bridge or phase shifter, the parameters of the modulated signal are more flexible and controllable, and the modulation bandwidth is higher. This invention, through digital demodulation and digital proportional-integral-derivative (PID) control within the FPGA, can more easily control the laser frequency shifting frequency.

[0040] Compared to the optical phase-locked loop device (CN 114371582 A), this invention uses an optical quadrature modulator to control the phase, amplitude, and frequency of a laser. Compared to the optical phase-locked loop scheme in that invention, which uses two laser sources, one as a reference laser and the other for relative phase locking via digital phase detection and feedback control, this invention offers greater control flexibility. Furthermore, the device requires only one laser source to generate two relatively phase-stable light sources, making the device composition simpler.

[0041] In summary, this invention utilizes FPGA technology to implement a digitally controlled oscillator, phase shifter, and digital gain, generating two pairs of mutually orthogonal intermediate frequency (IF) signals. These signals are then driven by an external digital-to-analog converter via a parallel data bus to generate two pairs of analog quadrature signals with a bandwidth up to 25 MHz. These two pairs of orthogonal IF signals are used to orthogonally modulate a higher-frequency radio frequency (RF) signal, resulting in orthogonal RF signals. Because the orthogonal signals used for RF modulation are generated by digital circuitry, the phase, amplitude, and frequency parameters of the orthogonal RF signals can be flexibly and controllably adjusted. Furthermore, by using these orthogonal RF signals for optical quadrature modulation, the phase, amplitude, and frequency of the modulated laser can be controlled, achieving laser frequency shifting. Simultaneously, due to the advantages of quadrature modulation, single-sideband frequency shifting can be achieved, reducing sideband effects caused by modulation. By monitoring the beat frequency signals of two laser beams and then using orthogonal downmixing and digital orthogonal demodulation techniques, the relative phase error between the laser beat frequency signal and the digital oscillator can be obtained. By controlling the error through digital feedback on the two pairs of output orthogonal intermediate frequency signals, the relative phase between multiple laser beams can be locked.

[0042] In the description of this specification, references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0043] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention without departing from the principles and spirit of the present invention. Any simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention shall still fall within the scope of the technical solution of the present invention.

Claims

1. A digitally controlled single-sideband frequency shifting and phase-locking device for orthogonal modulation laser, characterized in that: It includes a field-programmable gate array (101), a digital-to-analog converter, a quadrature modulator, an optical quadrature modulator (109), a laser (110), a first beam splitter (111), a second beam splitter (112), a third beam splitter (113), a beam combiner (114), and a quadrature downconversion mixer (117). The field-programmable gate array (101) is connected to four digital-to-analog converters via a data bus. Each pair of digital-to-analog converters is connected to a quadrature modulator. Both quadrature modulators are connected to a local oscillator (108). The two quadrature modulators are also connected to an optical quadrature modulator (109). A laser beam output from the laser (110) is input to an optical quadrature modulator (109). After being modulated by the optical quadrature modulator (109), the laser beam is input to a third beam splitter (113). The third beam splitter (113) is connected to a field-programmable gate array (101) via a second photodiode (119) and a three-channel analog-to-digital converter (115). The field-programmable gate array (101) is connected to the voltage control port of the optical quadrature modulator (109) via a three-channel digital-to-analog converter (116). Another laser beam output from the laser (110) is input to the first beam splitter (111). After being modulated by the optical quadrature modulator (109), the laser beam is input to the second beam splitter (112). The two laser beams after passing through the first beam splitter (111) and the second beam splitter (112) are then combined by the beam combiner (114) and connected to the quadrature down-conversion mixer (117) through the first photodiode (118). The quadrature down-conversion mixer (117) is connected to the local oscillator (108) and the quadrature down-conversion mixer (117) is also connected to the field programmable gate array (101) through the three-channel analog-to-digital converter (115).

2. The digitally controlled orthogonal modulation laser single-sideband frequency shifting and phase-locking device according to claim 1, characterized in that, The four digital-to-analog converters are a first digital-to-analog converter (102), a second digital-to-analog converter (103), a third digital-to-analog converter (104), and a fourth digital-to-analog converter (105), and the two quadrature modulators are a first quadrature modulator (106) and a second quadrature modulator (107), respectively. The first digital-to-analog converter (102) and the second digital-to-analog converter (103) are connected to the first quadrature modulator (106), and the third digital-to-analog converter (104) and the fourth digital-to-analog converter (105) are connected to the second quadrature modulator (107).

3. The digitally controlled orthogonal modulation laser single-sideband frequency shifting and phase-locking device according to claim 1, characterized in that, The digital-to-analog converter is a 14-bit parallel high-sampling-rate DAC, model AD9764, with a sampling rate of 125 MHz and an analog bandwidth greater than 40 MHz.

4. The digitally controlled orthogonal modulation laser single-sideband frequency shifting and phase-locking device according to claim 1, characterized in that, The field-programmable gate array (101) includes a digitally controlled oscillator (201), four digital phase shifters, and four digital amplifiers. The digitally controlled oscillator (201) generates a digital signal of the required frequency and inputs the digital signal into the four digital phase shifters respectively. The digital phase shifters make the output signal form two pairs of mutually orthogonal digital signals. The digital amplifiers then change the frequency of the corresponding signals and output them to the parallel data bus to drive an external digital-to-analog converter.

5. The digitally controlled orthogonal modulation laser single-sideband frequency shifting and phase-locking device according to claim 4, characterized in that, The field-programmable gate array (101) also includes a digital proportional-integral-derivative controller (210) and a digital phase detector (211). The digital phase detector (211) receives the quadrature signals transmitted by the digitally controlled oscillator (201) and the three-channel analog-to-digital converter (115) and calculates the relative phase error between the two pairs of signals. The digital proportional-integral-derivative controller (210) then calculates the error signal to obtain the phase amount that needs to be compensated for in the four quadrature intermediate frequency signals. The feedback amount is added to the four output digital phase shifters, and the four intermediate frequency signals generated by the digital-to-analog converter are changed accordingly to digitally control the relative phase of the laser beat frequency signal.

6. The digitally controlled orthogonal modulation laser single-sideband frequency shifting and phase-locking device according to claim 5, characterized in that, The four digital phase shifters are the first digital phase shifter (202), the second digital phase shifter (204), the third digital phase shifter (206), and the fourth digital phase shifter (208), respectively. The four digital amplifiers are the first digital amplifier (203), the second digital amplifier (205), the third digital amplifier (207), and the fourth digital amplifier (209), respectively. The first digital phase shifter (202) is connected to the first digital amplifier (203), the second digital phase shifter (204) is connected to the second digital amplifier (205), the third digital phase shifter (206) is connected to the third digital amplifier (207), and the fourth digital phase shifter (208) is connected to the fourth digital amplifier (209). The digital proportional-integral-derivative controller (210) is connected to the first digital phase shifter (202), the second digital phase shifter (204), the third digital phase shifter (206), and the fourth digital phase shifter (208), respectively.

7. A digitally controlled method for single-sideband frequency shifting and phase locking of orthogonal modulated laser, characterized in that, The specific steps of using the apparatus described in claim 5 or 6 are as follows: S1. The local oscillator (108) outputs radio frequency signals to two quadrature modulators. The field programmable gate array (101) outputs two pairs of quadrature intermediate frequency signals through a digital-to-analog converter. The two pairs of quadrature intermediate frequency signals are respectively input to the two quadrature modulators to perform quadrature modulation on the radio frequency signals to obtain two pairs of quadrature high-frequency radio frequency signals. The two pairs of quadrature high-frequency radio frequency signals are used to modulate the optical quadrature modulator (109). S2. The laser (110) generates two laser beams. One beam is output directly, and the frequency of the laser source is locked by frequency locking of the laser beam. The other beam is input to an optical quadrature modulator (109) for modulation. The modulated laser is a single-sideband frequency-shifted laser. The frequency shift of the laser is controlled by changing the frequency of the signal output by the local oscillator (108). S3. After frequency shifting, the laser beam passes through the third beam splitter (113). A small portion of the split light source is collected by the second photodiode (119) to obtain an electrical signal. The signal is then collected by one channel of the three-channel analog-to-digital converter (115) and transmitted to the field programmable gate array (101) for monitoring the optical power. S4. The field programmable gate array (101) calculates the appropriate voltage through the feedback quantity and controls the three voltage control ports of the optical quadrature modulator (109) by driving a three-channel digital-to-analog converter (116). By scanning the voltage values ​​of the three voltage control ports and recording the photoelectric signal on the second photodiode (119), the control voltage is obtained, so that the output optical power remains efficient and stable. S5. One laser beam output from the laser (110) passes through the first beam splitter (111), and the other laser beam output passes through the optical quadrature modulator (109) and then through the second beam splitter (112). The two split laser beams are combined by the beam combiner (114), and the resulting optical beat frequency signal is collected by the first photodiode (118). The resulting electrical signal is input to the quadrature downconversion mixer (117) and is quadraturely downconverted with the signal output from the local oscillator (108). The mixed signal is collected by the other two channels of the three-channel analog-to-digital converter (115) and then enters the field programmable gate array (101) for digital demodulation and digital feedback.

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