Device and method for generating bright low-frequency compressed light

Abstract

The invention provides a bright low-frequency compressed light generation device and method, and belongs to the technical field of non-classical light field generation. The problems that an existing bright compressed light generating device still has technical defects in the aspects of optical power, compression degree, stability, preparation device complexity and the like are solved. According to the technical scheme, laser generated by a single-frequency laser is divided into two beams after passing through a laser preparation module, one beam enters a frequency doubling cavity to generate frequency doubling light, fundamental frequency light reflected by the frequency doubling light is subjected to passive noise suppression, and fundamental frequency reflected light is subjected to further noise suppression in an active feedback control loop; the frequency doubling light serves as pump light to be injected into an optical parametric oscillator, a coherent control technology is used to generate a stable amplitude compression vacuum state light field, and the compression vacuum state light field and fundamental frequency reflection light after passive and active noise suppression are coupled on a 99: 1 beam splitter to generate bright low-frequency amplitude compression light; the method is applied to bright low-frequency compressed light generation.

Description

Technical Field

[0001] This application relates to the field of optical technologies, and particularly to a device and method for generating bright low-frequency squeezed light. Background Art

[0002] As an important continuous variable non-classical light source, squeezed light exhibits important application values in the fields of quantum precision measurement, gravitational wave detection, quantum communication, and basic physical research. And bright squeezed light, as a squeezed light source with both coherent amplitude and quantum noise squeezing characteristics, its core advantage lies in being able to break through the shot noise limit of classical measurement while maintaining high light intensity characteristics, providing key technical support for systems such as high-sensitivity interferometers, atomic clocks, and quantum imaging. However, existing bright squeezed light generation devices still have technical deficiencies in terms of optical power, squeezing degree, stability, and complexity of the preparation device, and it is difficult to directly generate highly stable bright low-frequency squeezed light for practical applications, which restricts its further application in high-precision scenarios. Summary of the Invention

[0003] To solve the above technical problems, this application proposes a device and method for generating bright low-frequency squeezed light.

[0004] The technical solution adopted in this application is as follows: A device for generating bright low-frequency squeezed light includes a laser preparation module, a frequency doubling cavity, an optical parametric oscillator, and a beam splitter. The laser preparation module outputs two beams of laser. One beam of laser enters the frequency doubling cavity to generate frequency-doubled light and passively suppress the noise of the reflected fundamental frequency light. The fundamental frequency reflected light undergoes further noise suppression in the active feedback control loop. The frequency-doubled light is injected into the optical parametric oscillator as the pump light. The other beam of laser is used as the frequency-shifted light and injected into the optical parametric oscillator after acousto-optic modulation. The coherent control technology is used to make the optical parametric oscillator generate a stable amplitude-squeezed vacuum state light field. The squeezed vacuum state light field and the fundamental frequency reflected light after passive and active noise suppression are coupled on the beam splitter to generate bright low-frequency amplitude-squeezed light.

[0005] Furthermore, a first Faraday isolator and a first dichroic mirror are arranged between the laser preparation module and the frequency doubling cavity. An electro-optic phase modulator, a second Faraday isolator, a first phase shifter, and a second dichroic mirror are arranged between the frequency doubling cavity and the optical parametric oscillator. A beam of laser output by the laser preparation module enters the frequency doubling cavity after passing through the first Faraday isolator and the first dichroic mirror. The frequency-doubled light generated by the frequency doubling cavity enters the electro-optic phase modulator after passing through the first dichroic mirror. After passing through the electro-optic phase modulator, the frequency-doubled light serves as pump light and enters the optical parametric oscillator after passing through the second Faraday isolator, the first phase shifter, and the second dichroic mirror. The polarization beam splitter prism in the second Faraday isolator injects the reflected light of the optical parametric oscillator into the first photodetector. The first photodetector is electrically connected to the second mixer. The second mixer is electrically connected to a first signal source and a second proportional integral derivative controller. The second proportional integral derivative controller is electrically connected to the piezoelectric ceramic in the optical parametric oscillator.

[0006] Further, the active feedback control loop includes an electro-optic amplitude modulator, an adjustable beam splitter, an inner-loop photodetector, and a first proportional integral derivative controller. The fundamental frequency light reflected by the frequency doubling cavity is reflected and output through the polarization beam splitter prism in the first dichroic mirror and the first Faraday isolator, and is incident on the beam splitter after passing through the electro-optic amplitude modulator and the second phase shifter. The reflected beams of the beam splitter enter the outer-loop photodetector and the adjustable beam splitter respectively. The reflected beams of the beam splitter are incident on the inner-loop photodetector and the third photodetector respectively after passing through the adjustable beam splitter. The electro-optic amplitude modulator is electrically connected to a first proportional integral derivative controller. The first proportional integral derivative controller is electrically connected to the inner-loop photodetector. The third photodetector is electrically connected to a third mixer. The third mixer is electrically connected to a fifth signal source and a fourth proportional integral derivative controller. The fourth proportional integral derivative controller is electrically connected to the second phase shifter. The outer-loop photodetector is electrically connected to a spectrum analyzer.

[0007] Further, another beam of laser output by the laser preparation module is reflected to the third Faraday isolator after passing through the first acousto-optic modulator and the second acousto-optic modulator. The laser output by the third Faraday isolator is injected into the optical parametric oscillator. The polarization beam splitter prism in the third Faraday isolator injects the laser reflected by the optical parametric oscillator into the second photodetector. The second photodetector is electrically connected to a second mixer. The second mixer is also electrically connected to a fourth signal source and a third proportional integral derivative controller. The third proportional integral derivative controller is electrically connected to the first phase shifter. The first acousto-optic modulator is electrically connected to a second signal source. The second acousto-optic modulator is electrically connected to a third signal source.

[0008] Further, the laser preparation module includes a single-frequency laser and a mode cleaner. The free-running laser output by the single-frequency laser filters out the laser noise above the cavity linewidth of the mode cleaner in the laser preparation module.

[0009] Further, the frequency doubling cavity adopts a two-mirror cavity or a multi-mirror cavity, and at least one cavity mirror in the frequency doubling cavity is provided with a piezoelectric ceramic for changing the cavity length; The frequency doubling cavity consists of a meniscus concave mirror and a nonlinear crystal. The front end face of the nonlinear crystal is flat, and the rear end face is convex. The meniscus concave mirror serves as both the input and output mirror of the frequency doubling cavity. The convex surface of the nonlinear crystal is coated with a highly reflective film for both the fundamental frequency light and the second harmonic frequency light, the flat surface of the nonlinear crystal is coated with an antireflection film, the output surface of the meniscus concave mirror is coated with a film that partially reflects the fundamental frequency light and reduces the reflection of the second harmonic frequency light, and the input surface of the meniscus concave mirror is coated with a film that reduces the reflection of both the fundamental frequency light and the second harmonic frequency light.

[0010] Furthermore, the structure of the optical parametric oscillator is the same as that of the frequency doubling cavity, and the coating on the nonlinear crystal of the optical parametric oscillator is the same as that of the frequency doubling cavity. The coating on the meniscus concave mirror of the optical parametric oscillator reflects both the fundamental frequency light and the second harmonic frequency light.

[0011] Furthermore, the beam splitter adopts a 99:1 beam splitter. A combination of a half-wave plate and a polarization beam splitter prism that can change the laser power incident on the 99:1 beam splitter is also provided between the electro-optic amplitude modulator and the 99:1 beam splitter.

[0012] Furthermore, both sides of the first dichroic mirror and the second dichroic mirror are coated. One side is highly transmissive to the fundamental frequency light and highly reflective to the second harmonic frequency light, and the other side is highly transmissive to both the fundamental frequency light and the second harmonic frequency light.

[0013] A method for generating bright low-frequency squeezed light, using the above-described device for generating bright low-frequency squeezed light, includes the following steps: Step 1: Use the laser preparation module to perform preliminary noise suppression on the initial laser. Step 2: Divide the laser output by the laser preparation module into two beams. Inject one of the laser beams into the frequency doubling cavity. The generated second harmonic frequency light is used as the pump light to be injected into the optical parametric oscillator after passing through the electro-optic phase modulator. Step 3: Use the first photodetector to receive the pump reflected light of the optical parametric oscillator. The modulation signal generated by the first signal source and the signal received by the first photodetector are mixed and demodulated at the first mixer to extract the error signal. Use the second proportional integral derivative controller to use this error signal to control the piezoelectric ceramic in the optical parametric oscillator to lock its cavity length. Step 4: Reflect and output the fundamental frequency light reflected by the frequency doubling cavity through the polarization beam splitter prism in the first Faraday isolator, and after passing through the electro-optic amplitude modulator, it is incident on the beam splitter. Step 5: Use the adjustable beam splitter to attenuate the reflected beam of the beam splitter. The inner loop photodetector receives its signal and feeds it back to the electro-optic amplitude modulator through the first proportional integral derivative controller to perform active control. Step Six: Another laser beam output by the laser preparation module is modulated by the first acousto-optic modulator and the second acousto-optic modulator and then injected into the optical parametric oscillator as frequency-shifted light. The second photodetector is used to receive the reflection signal of the third Faraday isolator. The demodulation signal generated by the fourth signal source and the reflection signal of the third Faraday isolator received by the second photodetector are mixed and demodulated at the second mixer to generate an error signal. The third proportional integral derivative controller uses this error signal to control the first phase shifter in the pump optical path to lock the relative phase between the pump field and the squeezed field to π; Step Seven: The stable squeezed vacuum state optical field output by the optical parametric oscillator and the fundamental frequency reflected light after power stabilization are coupled at the beam splitter. The third photodetector is used to receive the reflection signal of the beam splitter. The demodulation signal generated by the fifth signal source and the reflection signal of the beam splitter received by the third photodetector are mixed and demodulated at the third mixer to extract the error signal. The fourth proportional integral derivative controller uses this error signal to control the second phase shifter to lock the relative phase between the squeezed vacuum state optical field and the fundamental frequency reflected light to; At this time, a stable and output bright low-frequency squeezed optical field is generated at the transmission end of the beam splitter.

[0014] The beneficial effects of this application compared with the prior art are as follows: This application adopts a bright low-frequency squeezed light generating device, which has a simple structure, is easy to operate, and has high practical value. By combining active and passive noise stabilization technologies, and integrating PDH locking and coherent control technologies, it can achieve stable bright low-frequency squeezed light output in a low-frequency wide frequency band, which has good practical value for the preparation of bright squeezed light sources. Brief Description of the Drawings

[0015] The following further describes this application with reference to the drawings: Figure 1 It is a schematic diagram of the device structure provided by the embodiment of this application; In the figure: 1 - Laser preparation module, 2 - First Faraday isolator, 3 - First dichroic mirror, 4 - Frequency doubling cavity, 5 - Electro-optic amplitude modulator, 6 - Half-wave plate, 7 - Polarizing beam splitting prism, 8 - Beam splitter, 9 - Adjustable beam splitter, 10 - Inner loop photodetector, 11 - First proportional integral derivative controller, 12 - Outer loop photodetector, 13 - Spectrum analyzer, 14 - Electro-optic phase modulator, 15 - First signal source, 16 - Second Faraday isolator, 17 - Second dichroic mirror, 18 - Optical parametric oscillator, 19 - First photodetector, 20 - First mixer, 21 - Second proportional integral derivative controller, 22 - First acousto-optic modulator, 23 - Second electro-optic modulator, 24 - Second signal source, 25 - Third signal source, 26 - Third Faraday isolator, 27 - Second photodetector, 28 - Fourth signal source, 29 - Second mixer, 30 - Third proportional integral derivative controller, 31 - First phase shifter, 32 - Third photodetector, 33 - Third mixer, 34 - Fifth signal source, 35 - Fourth proportional integral derivative controller, 36 - Second phase shifter. Detailed implementation mode

[0016] As Figure 1 shown, the present application provides a device for generating bright low-frequency squeezed light, including a laser preparation module 1, which contains a single-frequency laser and a mode cleaner. The free-running laser output by the single-frequency laser filters out the laser noise above the cavity linewidth of the mode cleaner in the laser preparation module 1, and improves the purity of the laser spatial mode and polarization. The laser output by the laser preparation module 1 is divided into two beams. One beam enters the frequency doubling cavity 4 to generate frequency-doubled light and passively suppresses the noise of the reflected fundamental frequency light. The fundamental frequency reflected light undergoes further noise suppression in the active feedback control loop. The frequency-doubled light is injected into the optical parametric oscillator 18 as pump light and uses coherent control technology to generate a stable amplitude-squeezed vacuum state light field. The amplitude-squeezed vacuum state light field and the fundamental frequency reflected light after passive and active noise suppression are coupled on the beam splitter 8 to generate bright low-frequency amplitude-squeezed light.

[0017] In this embodiment, the frequency doubling cavity 4 is specifically a two-mirror cavity or a multi-mirror cavity, and at least one cavity mirror in the frequency doubling cavity 4 is provided with a piezoelectric ceramic for changing the cavity length. The optical parametric oscillator 18 has the same design as the above-mentioned frequency doubling cavity 4 except for the coating of the end mirror.

[0018] Specifically, of the two laser beams output from the laser preparation module 1, one beam passes through the first Faraday isolator 2 and the first dichroic mirror 3 and then enters the frequency doubling cavity 4. The frequency-doubled light generated by the frequency doubling cavity 4 passes through the first dichroic mirror 3 and then enters the electro-optical phase modulator 14. After passing through the electro-optical phase modulator 14, the frequency-doubled light passes through the second Faraday isolator 16 and the first phase shifter 31 as pump light and then passes through the second dichroic mirror 17 to enter the optical parametric oscillator 18; the pump reflected light of the optical parametric oscillator 18 passes through the second dichroic mirror 17 and the second Faraday isolator 16 and then enters the first photodetector 19; the first signal source 15 is electrically connected to the electro-optical phase modulator 14, and the second mixer 20 is electrically connected to the first signal source 15 and the second proportional-integral-differential controller 21, and the second proportional-integral-differential controller 21 is electrically connected to the piezoelectric ceramic in the optical parametric oscillator 18. The modulated signal generated by the first signal source 15 and the signal received by the first photodetector 19 are mixed and demodulated at the second mixer 20 to extract an error signal, and the second proportional integral differential controller 21 uses the error signal to control the piezoelectric ceramic in the optical parametric oscillator 18 to lock its cavity length.

[0019] The active feedback control loop includes an electro-optical amplitude modulator 5, an adjustable beam splitter 9, an inner-loop photodetector 10, and a first proportional-integral-differential controller 11. The fundamental frequency light reflected by the frequency-doubling cavity 4 is reflected and output by the first dichroic mirror 3 and the polarization beam splitting prism in the first Faraday isolator 2. After passing through the electro-optical amplitude modulator 5 and the second phase shifter 36, it is incident on the beam splitter 8. A half-wave plate 6 and a polarization beam splitting prism 7 can be used in combination between the electro-optical amplitude modulator 5 and the beam splitter 8 to change the laser power incident on the beam splitter 8. The reflected light beam from the beam splitter 8 enters the outer-loop photodetector 12 and the adjustable beam splitter 9, respectively. After passing through the adjustable beam splitter 9, the reflected light beam from the beam splitter 8 is incident on the inner-loop photodetector 10 and the third photodetector 32, respectively. The electro-optical amplitude modulator 5 is electrically connected to a first PID controller 11, which is electrically connected to an inner-loop photodetector 10. A third photodetector 32 is electrically connected to a third mixer 33, which is electrically connected to a fifth signal source 34 and a fourth PID controller 35. The fourth PID controller 35 is electrically connected to a second phase shifter 36. The outer-loop photodetector 12 is electrically connected to a spectrum analyzer 13.

[0020] The reflected light beam from the beam splitter 8 is attenuated using an adjustable beam splitter 9, and the inner ring photodetector 10 receives its signal and feeds it back to the electro-optical amplitude modulator 5 through a first proportional-integral-differential controller 11 to perform active control. By using the adjustable beam splitter 9 to attenuate the reflected light beam from the beam splitter 8, power saturation of the photodiode in the inner ring detector 10 can be avoided.

[0021] Another laser beam output by the laser preparation module 1 is modulated by the first acousto-optic modulator 22 and the second acousto-optic modulator 23 and then injected into the optical parametric oscillator 18 as frequency-shifted light. Specifically, in this embodiment, another laser beam output by the laser preparation module 1 is injected into the optical parametric oscillator 18 after passing through the first acousto-optic modulator 22, the second acousto-optic modulator 23, and the third Faraday isolator 26. The third Faraday isolator 26 reflects the frequency-shifted light reflected back by the optical parametric oscillator 18 into the second photodetector 27. The second photodetector 27 is electrically connected to the second mixer 29. The second mixer 29 is also electrically connected to the fourth signal source 28 and the third proportional-integral-differential controller 30. The third proportional-integral-differential controller 30 is electrically connected to the first phase shifter 31. The first acousto-optic modulator 22 is electrically connected to the second signal source 24, and the second acousto-optic modulator 23 is electrically connected to the third signal source 25. A second photodetector 27 receives the reflected frequency-shifted optical signal. The demodulated signal generated by the fourth signal source 28 is mixed and demodulated with the reflected frequency-shifted optical signal at a second mixer 29 to generate an error signal. A third proportional-integral-differential controller 30 uses this error signal to control a first phase shifter 31 in the pump optical path, locking the relative phase between the pump field and the compression field to π. Coherent control technology is used to lock the relative phase between the pump field and the compression field, achieving compression angle control and outputting a stable compressed vacuum state optical field.

[0022] The pump reflected light from the optical parametric oscillator 18 passes through the second dichroic mirror 17 and enters the beam splitter 8. The stable compressed vacuum state light field output by the optical parametric oscillator 18 is coupled with the power-stabilized fundamental frequency reflected light at the beam splitter 8. A third photodetector 32 is used to receive the reflected signal from the beam splitter 8. The demodulated signal generated by the fifth signal source 34 is mixed and demodulated with the reflected signal at the third mixer 33 to extract an error signal. The fourth proportional-integral-differential controller 35 uses this error signal to control the second phase shifter 36 to lock the relative phase of the compressed vacuum state light field and the fundamental frequency reflected light to 0. At this time, a bright, stable low-frequency compressed light field is generated at the transmission end of the beam splitter 8, which is detected and analyzed using the outer ring photodetector 12 and spectrum analyzer 13.

[0023] The first dichroic mirror 3 and the second dichroic mirror 17 used in the present application are both coated on both sides. One side has high transmittance for fundamental frequency light and high reflection for doubled frequency light, while the other side has high transmittance for both fundamental frequency light and doubled frequency light.

[0024] The first photodetector 19, the second photodetector 27 and the third photodetector 32 used in this application are all resonant detectors. An LC resonant circuit is used inside the photodetector. The resonant circuit composed of the junction capacitance of the photodiode and the external inductor is used to amplify the signal, thereby improving the gain of the detector.

[0025] In the embodiments of the present application, the optimal fractional ratio of the beam splitter 8 used is 99:1. Other beam splitting ratios can also generate squeezing, and the squeezing degree is the largest when it is 99:1.

[0026] Embodiments of the present application Figure 1 In the specific optical path connection shown, the light guiding mirror used only plays the role of guiding light. As a specific implementation manner, the present application does not limit the setting position and quantity of the specific light guiding mirror, as long as the corresponding laser guiding can be achieved.

[0027] When the device works, first, the laser generated by the single-frequency laser is divided into two beams after passing through the laser preparation module 1. One beam enters the frequency doubling cavity 4 to generate frequency-doubled light and passively suppress the noise of the fundamental frequency light reflected by it. The fundamental frequency reflected light undergoes further noise suppression in the active feedback control loop. The frequency-doubled light is injected into the optical parametric oscillator 18 as pump light and uses coherent control technology to generate a stable amplitude-squeezed vacuum state light field. The amplitude-squeezed vacuum state light field and the fundamental frequency reflected light after passive and active noise suppression are coupled on the beam splitter 8 to generate bright low-frequency amplitude-squeezed light.

[0028] Based on the above device, the present application also proposes a method for generating bright low-frequency squeezed light, which successively includes the following steps: Step 1: Use the laser preparation module 1 to perform preliminary noise suppression on the initial laser.

[0029] The laser preparation module 1 includes a single-frequency laser and a mode cleaner. The free-running laser output by the single-frequency laser filters out the laser noise above the cavity linewidth of the mode cleaner in the laser preparation module and improves the purity of the laser spatial mode and polarization.

[0030] Step 2: Divide the laser output by the laser preparation module 1 into two beams. Inject one beam of laser into the frequency doubling cavity 4. The generated frequency-doubled light passes through the electro-optic phase modulator 14 and is then injected into the optical parametric oscillator 18 as pump light.

[0031] In this device, the frequency doubling cavity 4 not only provides pump light for the parametric down-conversion process of the optical parametric oscillator 18 to generate an amplitude-squeezed vacuum state light field, but also can suppress the intensity noise of the fundamental frequency reflected light to a level close to the shot noise limit in the kHz-MHz frequency band for subsequent use.

[0032] Step 3: Use the first photodetector 19 to receive the pump reflected light of the optical parametric oscillator 18. The modulation signal generated by the first signal source 15 and the signal received by the first photodetector 19 are mixed and demodulated at the first mixer 20 to extract the error signal. Use the second proportional integral derivative controller 21 to use this error signal to control the piezoelectric ceramic in the optical parametric oscillator 18 to lock its cavity length.

[0033] Step 4: Reflect and output the fundamental frequency light reflected by the second harmonic cavity 4 through the polarization beam splitting prism in the first Faraday isolator 2, and after passing through the electro-optic amplitude modulator 5, it is incident on the beam splitter 8.

[0034] Between the electro-optic amplitude modulator 5 and the beam splitter 8, a combination of a half-wave plate 6 and a polarization beam splitting prism 7 can be used to change the laser power incident on the beam splitter 8.

[0035] Step 5: Use the tunable beam splitter 9 to attenuate the reflected beam of the beam splitter 8. The inner loop photodetector 10 receives its signal and feeds it back to the electro-optic amplitude modulator 5 through the first proportional integral derivative controller 11 to perform active control.

[0036] Use the tunable beam splitter 9 to attenuate the reflected beam of the beam splitter 8 to avoid power saturation of the photodiode in the inner loop detector 10.

[0037] Step 6: Another beam of laser output by the laser preparation module 1 is modulated by the first acousto-optic modulator 22 and the second acousto-optic modulator 23 and then injected into the optical parametric oscillator 18 as frequency-shifted light. The second photodetector 27 receives the reflected signal. The demodulation signal generated by the fourth signal source 28 and the reflected signal are mixed and demodulated at the second mixer 29 to generate an error signal. The third proportional integral derivative controller 30 uses this error signal to control the first phase shifter 31 in the pump optical path to lock the relative phase between the pump field and the squeezed field to π.

[0038] Use coherent control technology to lock the relative phase between the pump field and the squeezed field, realize squeezed angle control and output a stable squeezed vacuum state optical field.

[0039] Step 7: Couple the stable squeezed vacuum state optical field output by the optical parametric oscillator 18 and the fundamental frequency reflected light with stable power at the 99:1 beam splitter 8. Use the third photodetector 32 to receive the reflected signal of the beam splitter 8. The demodulation signal generated by the fifth signal source 34 and the reflected signal are mixed and demodulated at the third mixer 33 to extract the error signal. The fourth proportional integral derivative controller 35 uses this error signal to control the second phase shifter 36 to lock the relative phase between the squeezed vacuum state optical field and the fundamental frequency reflected light to 0. At this time, a stable and output bright low-frequency squeezed optical field is generated at the transmission end of the beam splitter 8, and it is detected and analyzed using the outer loop photodetector 12 and the spectrum analyzer 13.

[0040] As Figure 1As shown, in the embodiments of the present application, the above device is adopted to generate bright low-frequency squeezed light. Specifically, it is necessary to control the output power of the 1550 nm continuous single-frequency fiber laser (NKT, Koheras BASIKX15) with an output power of 1 W in the laser preparation module 1. The output free-running laser is first transmitted to the locked mode cleaner to improve the purity of the laser spatial fundamental mode and polarization, and filter the laser amplitude noise and phase noise above the cavity linewidth of the mode cleaner. Then, the laser output by the laser preparation module 1 is split into two beams. A beam of laser with a power of approximately 500 mW is injected into the frequency doubling cavity 4 with a conversion efficiency of 70%. The frequency doubling cavity 4 is composed of a meniscus concave mirror and a nonlinear crystal, and the cavity linewidth is 68 MHz; the nonlinear crystal is a periodically poled potassium titanyl phosphate (PPKTP) crystal with a size of 1*2*10 mm. The convex curvature radius of the nonlinear crystal is 12 mm, and the coating is HR1550 nm / 775 nm; the plane coating of the nonlinear crystal is AR1550 nm / 775 nm; the curvature radius of the meniscus concave mirror is 30 mm, which also serves as the input and output mirror of the frequency doubling cavity 4; the transmittance of the output face of the meniscus concave mirror to the 1550 nm laser is 12%, and it is antireflective to the 775 nm laser; the input face coating of the meniscus concave mirror is AR1550 nm / 775 nm; the air gap length between the meniscus concave mirror and the nonlinear crystal is 27 mm, corresponding to the eigenmode radius at the 42 μm fundamental wave. The 775 nm second-harmonic light generated by the parametric up-conversion effect of the frequency doubling cavity 4 is modulated by the electro-optic phase modulator 14 to generate a pair of 118 MHz sidebands and then used as the pump light of the optical parametric oscillator 18 to prepare the squeezed vacuum state. The optical parametric oscillator 18 has the same design as the above frequency doubling cavity 4 except for the coating of the meniscus concave mirror; the reflectivity of the concave mirror output face (meniscus concave mirror) of the optical parametric oscillator 18 at 1550 nm is 84.3%, and the reflectivity at 775 nm is 97.8%. The 1550 nm fundamental frequency light reflected from the frequency doubling cavity 4 is used as the passively stabilized laser, and approximately 100 mW of the fundamental frequency light is reflected and output from the polarization beam splitter prism in the first Faraday isolator 2. The fundamental frequency reflected light is incident on the beam splitter 8 after passing through the electro-optic amplitude modulator 5 and serves as the light source for active stable sensing and outer loop applications. Between the electro-optic amplitude modulator 5 and the beam splitter 8, a combination of a half-wave plate 6 and a polarization beam splitter prism 7 is used to change the laser power incident on the beam splitter 8. The reflected beam of the beam splitter 8 is used as the inner loop detection beam, and approximately 1 mW of the transmitted beam enters the outer loop photodetector 12 to characterize the amplitude noise. Before the inner loop photodetector 10, an adjustable beam splitter 9 is used to attenuate the laser to 10 mW to avoid the power saturation of the photodiode in the inner loop photodetector 10. The signal detected by the inner loop photodetector 10 is fed back to the electro-optic amplitude modulator 5 through the first proportional integral derivative controller 11 to perform active control.The two photodetectors used in the inner and outer rings are both Newport 2053 models, with a saturation power of 10 mW, an electronic noise of -168 dB / Hz, and a response frequency range of 1 kHz to 3 MHz. In this embodiment, the photodiodes in the original photodetectors were replaced with high-quantum-efficiency photodiodes customized by Laser Components GmbH of Germany. A proportional-integral-derivative controller with a 2 MHz bandwidth (Vescent model D2-125) provides high-gain feedback control to achieve the desired noise suppression between 1 kHz and 1 MHz. A 99:1 beam splitter is used in this embodiment.

[0041] Using PDH technology, the cavity length of the optical parametric oscillator (OPO) cavity 18 is locked, allowing the 21mW threshold power OPO 18 to operate below threshold, at dual resonance at wavelengths of 1550nm and 775nm. A compressed vacuum state is generated through parametric down-conversion. When the pump power is controlled at 16mW, the compression degree is 11dB. Coherent control technology is used to stabilize the compression angle to produce stable amplitude-compressed light. Another 1mW 1550nm laser beam output by the laser preparation module 1 serves as frequency-shifted light. This beam is modulated by the first and second AOMs 22 and 23, driven by +100MHz and -80MHz sinusoidal signals generated by the second and third signal sources 24 and 25, respectively, resulting in a 20MHz frequency shift. The error signal is demodulated at 40MHz and fed back to the first phase shifter 31 in the pump optical path to lock the relative phase between the pump field and the compression field to π. At this point, the OPO 18 can output a stable compressed vacuum state light field.

[0042] The squeezed vacuum state light field is coupled to the passively and actively stabilized fundamental frequency reflected light at the 99:1 beam splitter 8, and the relative phase between the two is locked to zero through coherent control technology. Simultaneously, the pump power of the optical parametric oscillator 18 is reduced to half the threshold power to prevent the coherent control loop from becoming unstable due to increased anti-compression of the squeezed vacuum state. Due to losses, the effective squeezed vacuum state compression at the 99:1 beam splitter 8 is ultimately 8.6 dB. At this point, bright low-frequency squeezed light with a compression of 5.5 dB and a power of 1 mW can be generated at the 99:1 beam splitter 8 in the frequency range of 2 k to 1 MHz. This light is detected and analyzed using an outer ring photodetector 12 and a spectrum analyzer 13.

[0043] The Faraday isolator used in this application prevents light reflected from components in the optical path, such as cavity mirrors, from being reflected back along its original path and potentially damaging the laser. This "isolation" works by redirecting light reflected from the cavity mirror behind the isolator through a polarization beam splitter prism inside the Faraday isolator. This application utilizes the light reflected from the cavity mirror (which contains cavity information) for feedback control and locking.

[0044] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, rather than limiting them; although the present application has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. An apparatus for generating bright low-frequency squeezed light, characterized in that: It includes a laser preparation module (1), a frequency doubling cavity (4), an optical parametric oscillator (18) and a beam splitter (8). The laser preparation module (1) outputs two beams of laser. One beam of laser enters the frequency doubling cavity (4) to generate frequency-doubled light and passively suppress the noise of the reflected fundamental frequency light. The fundamental frequency reflected light undergoes further noise suppression in the active feedback control loop. The frequency-doubled light is injected into the optical parametric oscillator (18) as pump light. The other beam of laser is used as frequency-shifted light and injected into the optical parametric oscillator (18) after acousto-optic modulation. The coherent control technology is used to make the optical parametric oscillator (18) generate a stable amplitude-squeezed vacuum state optical field. The amplitude-squeezed vacuum state optical field and the fundamental frequency reflected light after passive and active noise suppression are coupled on the beam splitter (8) to generate bright low-frequency amplitude-squeezed light.

2. The generating device of bright low-frequency compressed light according to claim 1, characterized in that: A first Faraday isolator (2) and a first dichroic mirror (3) are arranged between the laser preparation module (1) and the frequency doubling cavity (4). An electro-optic phase modulator, a second Faraday isolator (16), a first phase shifter (31) and a second dichroic mirror (17) are arranged between the frequency doubling cavity (4) and the optical parametric oscillator (18). One beam of laser output by the laser preparation module (1) enters the frequency doubling cavity (4) after passing through the first Faraday isolator (2) and the first dichroic mirror (3). The frequency-doubled light generated by the frequency doubling cavity (4) enters the electro-optic phase modulator (14) after passing through the first dichroic mirror (3). The frequency-doubled light enters the optical parametric oscillator (18) as pump light after passing through the electro-optic phase modulator (14), the second Faraday isolator (16), the first phase shifter (31) and the second dichroic mirror (17). The polarization beam splitting prism in the second Faraday isolator (16) injects the reflected light of the optical parametric oscillator (18) into the first photodetector (19). The first photodetector (19) is electrically connected to the second mixer (20). The second mixer (20) is electrically connected to a first signal source (15) and a second proportional integral derivative controller (21). The second proportional integral derivative controller (21) is electrically connected to the piezoelectric ceramic in the optical parametric oscillator (18).

3. The generating device for bright low-frequency compressed light according to claim 2, wherein: The active feedback control loop includes an electro-optic amplitude modulator (5), an adjustable beam splitter (9), an inner-loop photodetector (10), and a first proportional-integral-derivative controller (11). The fundamental-frequency light reflected by the frequency-doubling cavity (4) is output after being reflected by the polarization beam splitter prism in the first dichroic mirror (3) and the first Faraday isolator (2), and is incident on the beam splitter (8) after passing through the electro-optic amplitude modulator (5) and the second phase shifter (36). The reflected light beams of the beam splitter (8) enter the outer-loop photodetector (12) and the adjustable beam splitter (9) respectively. The reflected light beams of the beam splitter (8) are incident on the inner-loop photodetector (10) and the third photodetector (32) respectively after passing through the adjustable beam splitter (9). The electro-optic amplitude modulator (5) is electrically connected to the first proportional-integral-derivative controller (11), the first proportional-integral-derivative controller (11) is electrically connected to the inner-loop photodetector (10), the third photodetector (32) is electrically connected to the third mixer (33), the third mixer (33) is electrically connected to the fifth signal source (34) and the fourth proportional-integral-derivative controller (35), the fourth proportional-integral-derivative controller (35) is electrically connected to the second phase shifter (36), and the outer-loop photodetector (12) is electrically connected to the spectrum analyzer (13).

4. The generating device of bright low-frequency compressed light according to claim 3, characterized in that: Another laser beam output by the laser preparation module (1) is reflected to the third Faraday isolator (26) after passing through the first acousto-optic modulator (22) and the second acousto-optic modulator (23). The laser output by the third Faraday isolator (26) is injected into the optical parametric oscillator (18). The polarization beam splitter prism in the third Faraday isolator (26) injects the laser reflected by the optical parametric oscillator (18) into the second photodetector (27). The second photodetector (27) is electrically connected to the second mixer (29). The second mixer (29) is also electrically connected to the fourth signal source (28) and the third proportional-integral-derivative controller (30). The third proportional-integral-derivative controller (30) is electrically connected to the first phase shifter (31). The first acousto-optic modulator (22) is electrically connected to the second signal source (24), and the second acousto-optic modulator (23) is electrically connected to the third signal source (25).

5. A generating device for bright low-frequency compressed light according to any one of claims 1-4, characterized in that: The laser preparation module (1) includes a single-frequency laser and a mode cleaner. The free-running laser output by the single-frequency laser filters out the laser noise above the cavity linewidth of the mode cleaner in the laser preparation module (1).

6. A generating device for bright low-frequency compressed light according to any one of claims 1-4, characterized in that: The frequency-doubling cavity (4) adopts a two-mirror cavity or a multi-mirror cavity, and at least one cavity mirror in the frequency-doubling cavity (4) is provided with a piezoelectric ceramic for changing the cavity length; The frequency-doubling cavity (4) is composed of a meniscus concave mirror and a nonlinear crystal. The front end face of the nonlinear crystal is a plane, and the rear end face is a convex surface; the meniscus concave mirror serves as both the input and output mirror of the frequency-doubling cavity (4); and the convex surface coating of the nonlinear crystal is highly reflective to both the fundamental-frequency light and the frequency-doubled light, the plane coating of the nonlinear crystal is an antireflection coating, the output surface coating of the meniscus concave mirror is partially reflective to the fundamental-frequency light and antireflective to the frequency-doubled light, and the input surface coating of the meniscus concave mirror is antireflective to both the fundamental-frequency light and the frequency-doubled light.

7. The generating device of bright low-frequency compressed light according to claim 6, wherein: The structure of the optical parametric oscillator (18) is the same as that of the frequency doubling cavity (4), and the coating on the nonlinear crystal of the optical parametric oscillator (18) is the same as that of the frequency doubling cavity (4). The coating on the meniscus concave mirror of the optical parametric oscillator (18) reflects both the fundamental frequency light and the frequency doubled light.

8. The generating device of bright low-frequency compressed light according to claim 3, wherein: The beam splitter (8) is a 99:1 beam splitter. A combination of a half-wave plate (6) and a polarization beam splitting prism (7) that can change the laser power incident on the 99:1 beam splitter is also provided between the electro-optic amplitude modulator (5) and the 99:1 beam splitter.

9. The generating device of bright low-frequency compressed light according to claim 2, wherein: Both sides of the first dichroic mirror (3) and the second dichroic mirror (17) are coated. One side has high transmittance for the fundamental frequency light and high reflectance for the frequency doubled light, and the other side has high transmittance for both the fundamental frequency light and the frequency doubled light.

10. A method for generating bright low-frequency squeezed light, characterized in that: Using the bright low-frequency squeezed light generating device according to any one of claims 1-9, comprising the following steps: Step 1: Use the laser preparation module (1) to perform preliminary noise suppression on the initial laser. Step 2: Divide the laser output by the laser preparation module (1) into two beams. Inject one of the laser beams into the frequency doubling cavity (4). The generated frequency doubled light is used as the pump light and injected into the optical parametric oscillator (18) after passing through the electro-optic phase modulator (14). Step 3: Use the first photodetector (19) to receive the pump reflected light of the optical parametric oscillator (18). The modulation signal generated by the first signal source (15) and the signal received by the first photodetector (19) are mixed and demodulated at the first mixer (20) to extract the error signal. Use the second proportional integral derivative controller (21) to use this error signal to control the piezoelectric ceramic in the optical parametric oscillator (18) to lock its cavity length. Step 4: Reflect and output the fundamental frequency light reflected by the frequency doubling cavity (4) through the polarization beam splitting prism in the first Faraday isolator (2), and after passing through the electro-optic amplitude modulator (5), it is incident on the beam splitter (8). Step 5: Use the adjustable beam splitter (9) to attenuate the reflected beam of the beam splitter (8). The inner ring photodetector (10) receives its signal and feeds it back to the electro-optic amplitude modulator (5) through the first proportional integral derivative controller (11) for active control. Step 6: The other beam of laser output by the laser preparation module (1) is used as the frequency shifted light and injected into the optical parametric oscillator (18) after being modulated by the first acousto-optic modulator (22) and the second acousto-optic modulator (23). Use the second photodetector (27) to receive the reflected signal of the third Faraday isolator (26). The demodulation signal generated by the fourth signal source (28) and the reflected signal of the third Faraday isolator (26) received by the second photodetector (27) are mixed and demodulated at the second mixer (29) to generate an error signal. The third proportional integral derivative controller (30) uses this error signal to control the first phase shifter (31) in the pump optical path to lock the relative phase between the pump field and the squeezed field to π. Step 7: Couple the stable squeezed vacuum state optical field output by the optical parametric oscillator (18) with the fundamental frequency reflected light after power stabilization at the beam splitter (8). Use the third photodetector (32) to receive the reflected signal of the beam splitter (8). The demodulation signal generated by the fifth signal source (34) and the reflected signal of the beam splitter (8) received by the third photodetector (32) are mixed and demodulated at the third mixer (33) to extract the error signal. The fourth proportional integral derivative controller (35) uses this error signal to control the second phase shifter (36) to lock the relative phase of the squeezed vacuum state optical field and the fundamental frequency reflected light to 0. At this time, a stable output bright low-frequency squeezed optical field is generated at the transmission end of the beam splitter (8).

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