Orthogonal linear polarization dual-frequency laser phase modulation method

Abstract

This invention discloses a method for phase modulation of orthogonally linearly polarized dual-frequency lasers. A coaxially transmitted beam of orthogonally linearly polarized dual-frequency laser light is output from a dual-frequency laser and enters a single electro-optic phase modulator. A sinusoidal signal output from a local oscillator serves as the modulation signal for the electro-optic phase modulation system. This signal drives the electro-optic phase modulator to modulate the phase of the incident dual-frequency laser light. By adjusting the rotation angle of the electro-optic phase modulator around the laser propagation axis, the two polarization directions of the orthogonally linearly polarized dual-frequency laser light are made parallel to the two principal axes of the electro-optic crystal. The orthogonally linearly polarized dual-frequency laser light incident on the electro-optic crystal is polarized along the two principal axes. An applied electric field does not change the polarization state of the output dual-frequency laser light, thus achieving phase modulation of the orthogonally linearly polarized dual-frequency laser light. Equally spaced multi-order sideband components are generated on both sides of the modulated dual-frequency laser carrier, and the frequencies of the sideband components are equal to the frequencies of the modulation signal.

Description

Technical Field

[0001] This invention belongs to the field of laser modulation technology, specifically relating to a method for phase modulation of orthogonally linearly polarized dual-frequency lasers. Background Technology

[0002] Dual-frequency lasers have broad application prospects in laser sensing, laser communication, and laser interferometry. Currently, commonly used dual-frequency lasers mainly include Zeeman dual-frequency He-Ne lasers, dual-longitudinal-mode He-Ne lasers, and birefringent dual-frequency He-Ne lasers. In recent years, laser diode (LD)-pumped 1064nm dual-frequency Nd:YAG lasers have also developed rapidly, and are particularly suitable as ideal light sources for synthetic wave absolute distance interferometry systems, possessing significant application value. For example, in 2003, VG Gudelev et al. of the National Academy of Sciences of Belarus proposed a tunable dual-frequency Nd:YAG laser with a laser diode (LD) end-face pumped coupling cavity. The two end faces of the Nd:YAG crystal and the spherical output coupling mirror (OC) constitute three resonant cavities, and the mutual coupling characteristics between these three cavities are used for laser longitudinal mode selection. By applying an external force F to the output coupling mirror within the laser resonant cavity to induce a photoelastic effect, the laser longitudinal modes are split, resulting in the output of an orthogonally linearly polarized 1064nm continuous-wave tunable dual-frequency laser with a frequency difference tuning range of 50MHz to 8.4GHz. A 12mW dual-frequency laser output was achieved at a pump power of 240mW. This dual-frequency laser has a simple structure, high device efficiency, convenient frequency difference tuning, and is easily miniaturized, making it a promising dual-frequency laser for applications. In 2014, Zhang Shulian et al. from Tsinghua University designed a birefringent-Zeeman dual-frequency He-Ne laser based on birefringent mirror technology. This laser consists of a concave mirror M... c And plane mirror M f A resonant cavity is formed, and an antireflection coating is deposited on the inner side of the output plane mirror, while a high-reflection coating is deposited on the outer side. Therefore, the output plane mirror is equivalent to a birefringent element embedded within the cavity, and the internal stress transforms it into an anisotropic medium. Applying a magnetic field perpendicular to the axis of the dual-frequency laser yields a dual-frequency laser output with a frequency difference of 3.9 MHz. In 2022, Jiao Mingxing et al. from Xi'an University of Technology designed a Fabry-Perot (FP) etalon mode-selective dual-cavity dual-frequency Nd:YAG laser. This laser uses a polarization beam splitter to polarize and split the 1064 nm oscillating laser, forming a linear cavity and a right-angle cavity. An FP etalon is inserted into each cavity, allowing the p-polarized and s-polarized components of the 1064 nm laser to oscillate simultaneously in a single longitudinal mode within the linear and right-angle cavities, respectively, thus obtaining orthogonally linearly polarized dual-frequency laser output. By finely adjusting the tilt angle of the FP etalon inside the cavity, the frequency difference of the dual-frequency laser was tuned, with a tuning range of 16–76 GHz.

[0003] Using lasers as a tool for information technology, electro-optic modulation technology has important application value in fields such as laser communication, laser spectroscopy, laser sensing and laser ranging. Among them, electro-optic phase modulation technology has advantages such as high speed and large extinction ratio, which can significantly improve the stability and transmission efficiency of laser communication. Therefore, it has broad development prospects in future high-speed and high-capacity laser communication.

[0004] The physical basis of electro-optic phase modulation technology is the Pockels effect, which states that the refractive index of an electro-optic crystal changes linearly with the intensity of an applied electric field. An electro-optic phase modulation system consists of a polarizer and an electro-optic crystal. The polarization direction of the polarizer is parallel to the principal axis of the electro-optic crystal. Therefore, linearly polarized light incident on the crystal does not decompose into two components along the principal axis, but rather remains polarized along one direction. Thus, the applied electric field does not change the polarization state of the output light, only its phase. The modulated light output from the right end face of the electro-optic crystal has additional sideband components on both sides of its laser carrier, determined by the modulation frequency of the applied electric field.

[0005] Currently, researchers both domestically and internationally have applied electro-optic phase modulation technology in many fields. For example, in 2019, the team of Marko Loncar at Harvard University and the team of Joseph M. Kahn at Stanford University developed an integrated electro-optic (EO) modulated optical frequency comb generator that can produce a stable broadband optical frequency comb. Researchers built an integrated EO optical frequency comb generator using a thin-film lithium niobate photonics platform, specifically using EO phase modulation in a strongly second-order nonlinear resonant cavity to generate the optical frequency comb. A continuous laser beam is coupled into the reference cavity, and the phase modulation frequency is equal to the free spectral range of the FP reference cavity. Therefore, the input light is modulated within the cavity to generate sidebands, which are then cyclically modulated again to output a stable broadband EO optical frequency comb. While ensuring dispersion control, high EO response, extremely low optical loss, and highly colocalized microwave and optical fields were achieved. Furthermore, the optical frequency comb generator exhibits high tolerance to modulation frequency detuning and can effectively control the spacing of the optical frequency teeth across a frequency band spanning seven orders of magnitude (10Hz–100MHz). The frequency range of the experimentally measured EO optical frequency comb exceeds the entire L-band (more than 900 optical frequency teeth, corresponding to a span of ~10 GHz). The advantage of this method is the excellent stability and controllability of the resulting optical frequency comb, and the tooth width of the device is increased by nearly two orders of magnitude. Pound-Drever-Hall (PDH) frequency stabilization technology combines electro-optic phase modulation technology and optical heterodyne detection technology. It utilizes the phase modulation signal to carry frequency drift error information, and then obtains this information through phase demodulation, exhibiting advantages such as fast servo response, low noise, and resistance to lock-up. In 2022, Jiao Mingxing et al. from Xi'an University of Technology designed a dual-cavity dual-frequency Nd:YAG laser orthogonal demodulation PDH (QD-PDH) frequency difference stabilization system. Orthogonally linearly polarized dual-frequency lasers output from the laser's linear cavity and right-angle cavity enter two sets of electro-optic phase modulation systems for phase modulation. The modulated dual-frequency laser carrier generates first-order sidebands with consistent amplitude and opposite phase on both sides. Two 10MHz signals carrying error information were synchronously demodulated using an orthogonal demodulation method, enabling the dual-frequency laser to simultaneously stabilize at two different resonant frequencies of the FP reference cavity. Experiments showed that the frequency difference stability of the dual-frequency laser was better than 4.2 × 10⁻⁶. -7 .

[0006] Currently, orthogonally linearly polarized dual-frequency laser phase modulation systems generally contain two independent electro-optic phase modulation units, meaning each single-frequency laser beam enters a separate electro-optic phase modulator for phase modulation. The modulated dual-frequency laser carrier generates multiple equally spaced sidebands on both sides. While this dual-frequency laser phase modulation method has a clear objective, its system structure is relatively complex and it does not fully utilize the polarization characteristics of the dual-frequency laser. Summary of the Invention

[0007] The purpose of this invention is to propose a phase modulation method for orthogonally linearly polarized dual-frequency lasers, which uses a single electro-optic phase modulator to achieve phase modulation of orthogonally linearly polarized dual-frequency lasers; after modulation, multiple sideband components with equal spacing are generated on both sides of the dual-frequency laser carrier, and the frequency of the sideband components is equal to the frequency of the modulation signal.

[0008] The first technical solution adopted in this invention is an orthogonal linearly polarized dual-frequency laser phase modulation system, including a dual-frequency laser, the output end of which is connected to the input end of an electro-optic phase modulator, and a local oscillator connected to an electro-optic phase modulator driver. The electro-optic phase modulator driver is connected to the electro-optic phase modulator, the electro-optic phase modulator is connected to a rotary stage, and the output end of the electro-optic phase modulator outputs the phase-modulated dual-frequency laser.

[0009] The invention is further characterized by:

[0010] The output of the electro-optic phase modulator is connected in sequence to a scanner interferometer and an oscilloscope.

[0011] The rotary table rotates around the output beam of the dual-frequency laser.

[0012] The local oscillator is a direct digital frequency synthesizer.

[0013] The second technical solution adopted in this invention is an orthogonal linearly polarized dual-frequency laser phase modulation method, the specific process of which is as follows:

[0014] A dual-frequency laser is used to output orthogonally linearly polarized dual-frequency lasers via coaxial transmission, and the dual-frequency lasers enter a single electro-optic phase modulator;

[0015] The local oscillator outputs a sinusoidal signal as a modulation signal, which enters the electro-optic phase modulator driver to generate a drive signal.

[0016] The driving signal drives the electro-optic phase modulator to perform phase modulation on the incident dual-frequency laser. By controlling the rotation stage to adjust the rotation angle of the electro-optic phase modulator around the propagation axis of the dual-frequency laser beam, the two polarization directions of the incident dual-frequency laser are parallel to the two principal induction axes of the electro-optic crystal in the electro-optic phase modulator, thereby realizing phase modulation of the orthogonally linearly polarized dual-frequency laser.

[0017] The output of the electro-optic phase modulator is connected in sequence to a scanner interferometer and an oscilloscope. The modulation mode spectrum of the dual-frequency laser is observed through the oscilloscope. At the same time, the rotation angle of the electro-optic phase modulator is adjusted by controlling the rotating stage around the propagation axis of the dual-frequency laser beam until equal-spaced multi-order sidebands are observed on both sides of the dual-frequency laser carrier.

[0018] The third technical solution adopted in this invention is an orthogonal linearly polarized dual-frequency laser phase modulation method, which is applied to frequency stabilization in a QD-PDH frequency difference stabilization system.

[0019] The fourth technical solution adopted in this invention is the application of an orthogonal linearly polarized dual-frequency laser phase modulation method to form multi-order sidebands in an orthogonal linearly polarized dual-frequency comb generator.

[0020] The beneficial effects of the orthogonal linearly polarized dual-frequency laser phase modulation method of the present invention are:

[0021] Compared to traditional electro-optic phase modulation systems, this invention eliminates the need for a polarizer. By rotating the electro-optic phase modulator, the two principal axes of the electro-optic crystal in the electro-optic phase modulator are made parallel to the two polarization directions of the incident dual-frequency laser beam, respectively. Therefore, a single electro-optic phase modulator can be used to simultaneously perform phase modulation on orthogonally linearly polarized dual-frequency lasers.

[0022] An external electric field does not change the polarization state of the emitted dual-frequency laser, but only causes a phase delay, thus realizing phase modulation of the orthogonally linearly polarized dual-frequency laser; after modulation, multiple sideband components with equal spacing are generated on both sides of the carrier of the dual-frequency laser, and the frequency of the sideband components is equal to the frequency of the modulation signal. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of a dual-frequency laser phase modulation system based on a single electro-optic phase modulator.

[0024] Figure 2 This is a schematic diagram of the orthogonal linear polarization dual-frequency laser phase modulation principle based on a single electro-optic phase modulator;

[0025] Figure 3 This is a schematic diagram of the QD-PDH frequency difference stabilization system based on the single modulator dual-frequency laser phase modulation system of the present invention.

[0026] Figure 4 This is the orthogonal linear polarization dual-frequency laser phase modulation mode spectrum obtained using the orthogonal linear polarization dual-frequency laser phase modulation method of this invention;

[0027] Figure 5 This invention relates to an orthogonal linear polarization dual optical frequency comb generator based on a single modulator dual-frequency laser phase modulation system.

[0028] In the diagram, 1: Dual-frequency laser; 2: Electro-optic phase modulator; 3: Electro-optic phase modulator driver; 4: Local oscillator; 5: Fiber-coupled LD; 6: LD pigtail; 7: Coupling lens a; 8: Coupling lens b; 9: Nd:YAG crystal; 10: Polarizing beam splitter a; 11: FP etalon a; 12: Output coupling mirror a; 13: Piezoelectric ceramic tube a; 14: FP etalon b; 15: Output coupling mirror b; 16: Piezoelectric ceramic tube b; 17: Mirror a; 18: Mirror b; 19: Polarizing beam splitter b; 20: Optical isolator; 21: Electro-optic phase modulator a; 22: Direct digital frequency converter. Synthesizer a; 23: First electro-optic phase modulator driver; 24: Polarizing beam splitter c; 25: Reflector c; 26: Polarizing beam splitter d; 27: Optical beam splitter; 28: Quarter-wave plate; 29: FP reference cavity mirror a; 30: FP reference cavity mirror b; 31: Photodetector a; 32: Feedback control unit a; 33: Photodetector b; 34: Feedback control unit b; 35: Dual-frequency laser a; 36: FP reference cavity mirror c; 37: FP reference cavity mirror d; 38: Electro-optic phase modulator b; 39: Direct digital frequency synthesizer b; 40: Second electro-optic phase modulator driver. Detailed Implementation

[0029] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0030] The orthogonal linearly polarized dual-frequency laser phase modulation system of the present invention, such as Figure 1 As shown, the output of the dual-frequency laser 1 is connected to the input of the electro-optic phase modulator 2. It also includes a local oscillator 4, which is connected to the electro-optic phase modulator driver 3. The driver 3 is connected to the electro-optic phase modulator 2, which is connected to a rotary table. The output of the electro-optic phase modulator 2 outputs a phase-adjusted dual-frequency laser. The output of the electro-optic phase modulator 2 is sequentially connected to a scanner interferometer and an oscilloscope. According to... Figure 1 To illustrate, a coaxially transmitted p-polarized and s-polarized dual-frequency laser beam is output from dual-frequency laser 1. This dual-frequency laser beam simultaneously enters electro-optic phase modulator 2, which is fixed on a rotating stage. The modulation switch of electro-optic phase modulator driver 3 is turned on, and a sinusoidal signal is output from local oscillator 4 as the modulation signal for electro-optic phase modulation. This signal enters electro-optic phase modulator driver 3 to generate a driving signal, which drives electro-optic phase modulator 2 to perform phase modulation on the incident dual-frequency laser. The modulated dual-frequency laser enters a Fabry-Perot (FP) scanning interferometer. The modulation mode spectrum of the dual-frequency laser is observed using an oscilloscope connected to the FP scanning interferometer. At the same time, the rotation angle of electro-optic phase modulator 2 is adjusted using a rotating stage until equal-spaced multi-order sidebands are observed on both sides of the dual-frequency laser carrier.

[0031] The rotating stage rotates about the direction of laser propagation output from dual-frequency laser 1 as an axis, such as... Figure 2 The z-axis is the rotation axis, which can be adjusted during rotation so that the p and s polarization directions of the incident dual-frequency laser are parallel to the induction principal axes x′ and y′ of the electro-optic crystal, respectively.

[0032] The orthogonal linearly polarized dual-frequency laser phase modulation method, the specific process is as follows:

[0033] A dual-frequency laser 1 is used to output coaxially transmitted orthogonally linearly polarized dual-frequency laser, and the dual-frequency laser enters a single electro-optic phase modulator 2;

[0034] The local oscillator 4 outputs a sinusoidal signal as a modulation signal, which enters the electro-optic phase modulator driver 3 to generate a drive signal.

[0035] The driving signal drives the electro-optic phase modulator 2 to perform phase modulation on the incident dual-frequency laser. By controlling the rotation stage to adjust the rotation angle of the electro-optic phase modulator 2 around the propagation axis of the dual-frequency laser beam, the two polarization directions of the incident dual-frequency laser are parallel to the two principal induction axes of the electro-optic crystal in the electro-optic phase modulator 2, thereby realizing phase modulation of the orthogonally linearly polarized dual-frequency laser.

[0036] like Figure 2 As shown, the incident light is a coaxially propagating, orthogonally polarized p- and s-polarized dual-frequency laser beam. This dual-frequency laser beam enters an electro-optic crystal for phase modulation. By adjusting the rotation angle of the electro-optic crystal around the laser propagation axis, the p and s polarization directions of the incident dual-frequency laser are made parallel to the principal axes x′ and y′ of the electro-optic crystal, respectively, thereby achieving phase modulation of the dual-frequency laser. New sideband components are generated on both sides of the modulated dual-frequency laser carrier.

[0037] The orthogonally linearly polarized dual-frequency laser phase modulation method, applied in a QD-PDH frequency difference stabilization system, can stabilize the frequency of a dual-cavity dual-frequency Nd:YAG laser. Specifically, the entire frequency difference stabilization system comprises two frequency stabilization subsystems, both sharing the same electro-optic phase modulation unit and the frequency stabilization reference cavity (FP). The system composition is as follows: Figure 3As shown, the system includes an optical fiber coupled LD5, which serves as a pump source connected to an LD pigtail 6. The LD pigtail 6 is coaxially fitted with coupling lenses a7 and b8. The 808nm pump light emitted from the LD pigtail 6 is converged by coupling lenses a7 and b8 onto the left end face of an Nd:YAG crystal 9. This end face is coated with a bichromatic dielectric film that provides high reflectivity to the 1064nm oscillating laser while simultaneously reducing the transmittance of the pump light, serving as a rear reflector for the resonant cavity. The right end face of the Nd:YAG crystal 9 is coated with a 1064nm antireflection film. A polarizing beam splitter a10 and a FP are coaxially arranged along the optical axis of the lens group and on the right side of the Nd:YAG crystal 9. An etalon a11 and an output coupling mirror a12 are used. The output coupling mirror a12 is coated with a 1064nm partial transmission medium film (transmittance 2.2%). The bicolor medium film of the Nd:YAG crystal 9 and the partial transmission medium film of the output coupling mirror a12 form a linear cavity. An FP etalon b14 and an output coupling mirror b15 are placed sequentially along the optical axis perpendicular to the lens group and at the corresponding positions of the polarizing beam splitter a10. The output coupling mirror b15 is coated with a 1064nm partial transmission medium film (transmittance 2.2%). The bicolor medium film of the Nd:YAG crystal 9 and the partial transmission medium film of the output coupling mirror b15 form a right-angle cavity. The two resonant cavities share the same gain medium Nd:YAG crystal 9 and respectively contain laser longitudinal mode selection elements FP etalon a11 and FP etalon b14, enabling the 1064nm laser p-component and s-component to oscillate simultaneously in a single longitudinal mode in the linear cavity and the right-angle cavity, respectively, thereby obtaining a 1064nm orthogonally linearly polarized dual-frequency laser output. Piezoelectric ceramic tubes a13 and b16 are bonded to output coupling mirrors a12 and b15, respectively. By changing the voltage between the electrodes on the inner and outer walls of the piezoelectric ceramic tubes, the cavity lengths of the linear cavity and the right-angle cavity can be adjusted, thereby achieving frequency stability of the dual-frequency laser. The frequency difference of the laser is mainly determined by the tilt angle of the FP etalon within the laser cavity. The s-polarized light output from the right-angle cavity is reflected by mirrors a17 and b18 and then combined with the p-polarized light output from the linear cavity at the polarization beam splitter b19, thus obtaining coaxially transmitted orthogonally linearly polarized dual-frequency laser with a frequency difference of 24 GHz.

[0038] like Figure 3 In the middle, the components in the left frame I form a dual-frequency laser, which outputs dual-frequency laser light.

[0039] Specifically, it can be used for PDH frequency stabilization of dual-cavity dual-frequency solid-state lasers, dual-cavity dual-wavelength single-longitudinal-mode fiber lasers, and dual-cavity dual-frequency semiconductor lasers.

[0040] The coaxially transmitted orthogonally linearly polarized dual-frequency laser beam passes through optical isolator 20 and then enters electro-optic phase modulator b21 for phase modulation. Direct digital frequency synthesizer 22 generates three sine and cosine signals with amplitudes of 500mV and frequencies of 10MHz. One sine signal first enters the first electro-optic phase modulator driver 23 to drive electro-optic phase modulator b21 to generate modulation sidebands. The other two sine and cosine signals are used as demodulation reference signals. The modulated dual-frequency laser beam is split at polarization beam splitter c24: p-polarized modulated light passes through polarization beam splitter c24 and s-polarized modulated light is reflected by polarization beam splitter c24. The p-polarized modulated light, after passing through optical beam splitter 27 and quarter-wave plate 28, is perpendicularly incident on the FP reference cavity. The FP reference cavity consists of FP reference cavity mirrors a29 and b30. After being reflected by the FP reference cavity, it passes again through quarter-wave plate 28 and optical beam splitter 27, and is reflected by polarization beam splitter c24 before entering photodetector a31 for optical heterodyne interference. The output interference signal enters feedback control unit a32 for frequency-selective amplification, quadrature demodulation, phase-sensitive detection, and PI control calculation to obtain an error control signal. This control signal is then input to piezoelectric ceramic tube a13 to adjust the cavity length of the linear cavity, thereby locking the output laser frequency of the linear cavity to a certain resonant frequency of the FP reference cavity. The s-polarized modulated light, after passing through mirror c25, polarizing beam splitter d26, optical beam splitter 27, and quarter-wave plate 28, is perpendicularly incident into the FP reference cavity. Reflected by the FP reference cavity, it passes again through quarter-wave plate 28 and optical beam splitter 27, and is transmitted through polarizing beam splitter d26 before entering photodetector b33 for optical heterodyne interference. The output interference signal enters feedback control unit b34 for frequency-selective amplification, quadrature demodulation, phase-sensitive detection, and PI control calculation to obtain an error control signal. This control signal is then input to piezoelectric ceramic tube b16 to adjust the cavity length of the right-angle cavity, thereby locking the output laser frequency of the right-angle cavity to another resonant frequency of the FP reference cavity.

[0041] In this embodiment, the dual-frequency laser phase modulation process based on a single electro-optic phase modulator is as follows: The orthogonally linearly polarized dual-frequency laser beams output from the laser simultaneously enter the electro-optic phase modulator b21 (Newport, type 4003), and the electro-optic phase modulator b21 (Newport, type 9071M) is fixed on a rotating stage. The modulation switch of the first electro-optic phase modulator driver 23 (Newport, type 3363B) is turned on, and a sinusoidal signal with an amplitude of 500mV and a frequency of 10MHz is output by the direct digital frequency synthesizer 22 as the modulation signal of the electro-optic phase modulation system. This sinusoidal signal first enters the first electro-optic phase modulator driver 23 to generate a driving signal, which drives the electro-optic phase modulator b21 to perform phase modulation on the incident dual-frequency laser. The modulated dual-frequency laser enters a FP scanning interferometer (with a corresponding free spectral range of 375MHz). The scanning interferometer is connected to an oscilloscope, and the modulation mode spectrum of the dual-frequency laser is observed through the oscilloscope. Simultaneously, the rotation angle of the electro-optic phase modulator b21 is adjusted via a rotary stage until first-order sidebands with consistent amplitude and opposite phase appear on both sides of the dual-frequency laser carrier. The phase modulation mode spectrum of the orthogonally linearly polarized dual-frequency laser is as follows: Figure 4 As shown in the figure, the upper part represents the sawtooth wave voltage signal, and the lower part represents the modulation mode spectrum of the dual-frequency laser. It can be seen that within one rising edge of the sawtooth wave voltage signal, the two different modulation modes are scanned twice; the higher amplitude represents the p-modulation mode, and the lower amplitude represents the s-modulation mode. Simultaneously, it can be clearly observed that first-order sidebands with consistent amplitude and opposite phase are generated on both sides of the p-polarized and s-polarized laser carrier.

[0042] This embodiment employs a single electro-optic phase modulator to modulate the phase of a coaxially transmitted orthogonally linearly polarized dual-frequency laser. Two independent heterodyne signal detection units and feedback control units simultaneously stabilize the output laser frequencies of the linear cavity and right-angle cavity to two different resonant frequencies of the same FP reference cavity, thus achieving frequency difference stabilization of the dual-frequency laser. Experiments show that the laser frequency stability of the linear cavity and right-angle cavity is better than 1.6 × 10⁻⁶. -11 and 2.0×10 -11 The frequency difference stability of the dual-frequency laser is better than 2.9×10. -7 This frequency-stable dual-cavity dual-frequency Nd:YAG laser can be used as an ideal light source for synthetic wave absolute distance interferometry systems.

[0043] like Figure 5As shown, the orthogonal linearly polarized dual-frequency laser phase modulation method, applied in an orthogonal linearly polarized dual-frequency comb generator, can be used to generate an orthogonal linearly polarized dual-frequency comb. Specifically, the coaxially transmitted orthogonal linearly polarized dual-frequency laser output from the dual-frequency laser is coupled into an FP reference cavity with a built-in electro-optic phase modulator. This allows the dual-frequency laser to achieve phase modulation within the FP reference cavity, generating multiple sidebands and outputting an orthogonal linearly polarized dual-frequency comb. Specifically, the dual-frequency laser a35 outputs a coaxially transmitted orthogonal linearly polarized dual-frequency laser beam, the frequency difference of which is an integer multiple of the free spectral range of the FP reference cavity. This dual-frequency laser beam is coupled into the FP reference cavity, which consists of FP reference cavity mirrors a36 and b37, and an electro-optic phase modulator a38 is placed inside, fixed on a rotating stage. A sinusoidal signal is output from a direct digital frequency synthesizer 39 as the modulation signal for the electro-optic phase modulation system, and the frequency of the modulation signal is an integer multiple of the free spectral range of the FP reference cavity. The signal enters the second electro-optic phase modulator driver 40 to generate a driving signal, which drives the electro-optic phase modulator a38 to perform phase modulation on the incident dual-frequency laser. The rotation angle of the electro-optic phase modulator a38 is adjusted around the laser propagation axis so that the two polarization directions of the orthogonally linearly polarized dual-frequency laser are parallel to the two principal induction axes of the electro-optic crystal. At this time, the orthogonally linearly polarized dual-frequency laser incident on the electro-optic phase modulator a38 is polarized along the two principal induction axes, resulting in equally spaced multi-order sidebands appearing on both sides of the dual-frequency laser carrier. These multi-order sidebands are then phase-modulated again in the FP reference cavity, generating new higher-order sidebands, and this cycle repeats, thus outputting an orthogonally linearly polarized dual-frequency comb.

Claims

1. An orthogonal linearly polarized dual-frequency laser phase modulation system, characterized in that, The system includes a dual-frequency laser (1), the output of which is connected to the input of an electro-optic phase modulator (2), and a local oscillator (4), which is connected to an electro-optic phase modulator driver (3). The electro-optic phase modulator driver (3) is connected to the electro-optic phase modulator (2), which is connected to a rotary table. The output of the electro-optic phase modulator (2) outputs a phase-adjusted dual-frequency laser. The driving signal drives the electro-optic phase modulator (2) to perform phase modulation on the incident dual-frequency laser. By controlling the rotation stage to adjust the rotation angle of the electro-optic phase modulator (2) around the propagation axis of the dual-frequency laser beam, the two polarization directions of the incident dual-frequency laser are parallel to the two sensing principal axes of the electro-optic crystal in the electro-optic phase modulator (2), thereby realizing the phase modulation of the orthogonally linearly polarized dual-frequency laser.

2. The orthogonal linearly polarized dual-frequency laser phase modulation system according to claim 1, characterized in that, The output of the electro-optic phase modulator (2) is connected in sequence to a scanner interferometer and an oscilloscope.

3. The orthogonal linearly polarized dual-frequency laser phase modulation system according to claim 1, characterized in that, The rotary table rotates about the direction of laser propagation output by the dual-frequency laser (1).

4. The orthogonal linearly polarized dual-frequency laser phase modulation system according to claim 1, characterized in that, The local oscillator (4) is a direct digital frequency synthesizer.

5. A method for phase modulation of orthogonally linearly polarized dual-frequency lasers, characterized in that, The specific process of using the orthogonal linear polarization dual-frequency laser phase modulation system as described in claim 1 is as follows: A dual-frequency laser (1) is used to output coaxially transmitted orthogonally polarized dual-frequency laser, which enters a single electro-optic phase modulator (2); The local oscillator (4) outputs a sinusoidal signal as a modulation signal, which enters the electro-optic phase modulator driver (3) to generate a driving signal; The driving signal drives the electro-optic phase modulator (2) to perform phase modulation on the incident dual-frequency laser. By controlling the rotation stage to adjust the rotation angle of the electro-optic phase modulator (2) around the propagation axis of the dual-frequency laser beam, the two polarization directions of the incident dual-frequency laser are parallel to the two sensing principal axes of the electro-optic crystal in the electro-optic phase modulator (2), thereby realizing the phase modulation of the orthogonally linearly polarized dual-frequency laser.

6. The orthogonal linearly polarized dual-frequency laser phase modulation method according to claim 5, characterized in that, The output of the electro-optic phase modulator (2) is connected in sequence to a scanner interferometer and an oscilloscope. The modulation mode spectrum of the dual-frequency laser is observed through the oscilloscope. At the same time, the rotation angle of the electro-optic phase modulator (2) is adjusted by controlling the rotating stage around the propagation axis of the dual-frequency laser beam until the appearance of multi-level sidebands with equal intervals on both sides of the dual-frequency laser carrier is observed.

7. The orthogonal linearly polarized dual-frequency laser phase modulation method according to claim 5, characterized in that, Frequency stabilization application in QD-PDH frequency difference stabilization system.

8. The orthogonal linearly polarized dual-frequency laser phase modulation method according to claim 5, characterized in that, Application of forming multi-order sidebands in orthogonal linearly polarized dual-frequency comb generators.

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