High-compression-degree all-fiber vacuum compression state light source based on waveguide resonant cavity

Abstract

The invention provides a high-compression-degree all-fiber vacuum compression state light source based on a waveguide resonant cavity, and relates to the technical field of quantum optics and quantum information. After laser passes through an isolator and a beam splitter, most of the laser enters a phase modulator, and a small part of the laser serves as background light; after the phase of the laser is modulated by the phase modulator, the laser enters the frequency doubling medium to generate pump light; pumping light returned by the waveguide resonant cavity enters the detector; the detector detects the intensity of the pump light returned from the waveguide resonant cavity, and the narrow-band filter filters the pump light and outputs compressed light; a waveguide crystal in the waveguide resonant cavity depends on laser emitted by the two-end coated tuning laser; the servo control system obtains an error signal through demodulation of a demodulation signal sent by the signal generator and an electric signal output by the detector, the error signal enters a control module of the servo control system to obtain a signal required for controlling the wavelength of the laser, and the wavelength is adjusted in real time by controlling tunable parameters of the laser. And the locking of the laser wavelength and the waveguide resonant cavity is realized.

Description

Technical Field

[0001] The present invention proposes a high-compression all-fiber vacuum squeezed state light source based on a waveguide resonator, which relates to the fields of quantum optics and quantum information technology. Background Art

[0002] A squeezed state light source can redistribute the noise of orthogonal components without violating the Heisenberg uncertainty principle, such that the noise of one of the orthogonal components is significantly lower than the shot noise limit. As an important non-classical light source in quantum technology, it is widely used in the fields of quantum precision measurement, quantum information, quantum radar, and quantum computing. According to the number of photons, the squeezed state light source can be divided into a vacuum squeezed state and a bright squeezed state. The vacuum squeezed state acts on the vacuum, while the bright squeezed state acts on a coherent state with a certain number of photons. Among them, the vacuum squeezed state is currently the most maturely developed and successfully applied type of squeezed light source, especially showing excellent performance in the field of gravitational wave detection, far exceeding classical light sources.

[0003] Currently, the schemes for preparing a vacuum squeezed state light source are mainly divided into two categories: free space schemes and all-fiber schemes. Free space schemes perform excellently in terms of compression degree through technologies such as precise tuning and multi-channel locking control. For example, in 2016, the Max Planck Institute in Germany achieved an output of a 15 dB vacuum squeezed state. However, due to the complex system and strict requirements for the working environment, such schemes are not conducive to the application expansion and industrial development of the squeezed state light source. Thanks to the progress of waveguide technology and materials, all-fiber schemes have developed rapidly in recent years. In 2023, NTT Corporation in Japan reported an output of an 8.3 dB all-fiber vacuum squeezed state. Compared with free space schemes, all-fiber schemes have a simple structure and do not require complex mode matching, which is conducive to the miniaturization and popularization of the squeezed state light source. However, limited by the low non-linear conversion efficiency, the compression degree of its output optical field is still slightly inferior. Summary of the Invention

[0004] To solve the above technical problems, the present invention proposes a high-compression all-fiber vacuum squeezed state light source based on a waveguide resonator, which is characterized by including: a laser, an isolator, a beam splitter, a phase modulator, a frequency doubling medium, a circulator, a waveguide resonator, a narrowband filter, a signal generator, a servo control system, and a detector;

[0005] The laser emitted by the laser enters the beam splitter after passing through the isolator. After passing through the beam splitter, most of the laser enters the phase modulator, and a small part of the laser is used as the background light for subsequent detection;

[0006] The laser after being phase - modulated by the phase modulator enters the frequency - doubling medium to generate pump light; the pump light is input from the first port of the circulator, output from the second port, and enters the waveguide resonator; the waveguide crystal in the waveguide resonator relies on the laser emitted by the tunable laser with coatings on both ends; the narrow - band filter is an optical - fiber narrow - band filter corresponding to the wavelengths of the pump light and the signal light of the system, and is used to filter out the pump light and output squeezed light;

[0007] The pump light returning from the waveguide resonator is input from the second port of the circulator, output from the third port, and enters the detector;

[0008] The detector detects the intensity of the pump light returning from the waveguide resonator and generates an electrical signal to be input into the servo - control system;

[0009] The modulation signal output by the signal generator modulates the phase of the fundamental - frequency light through the phase modulator; the demodulation signal output by the signal generator demodulates the error signal required by the servo - control system;

[0010] The servo - control system demodulates the error signal through the demodulation signal sent by the signal generator and the electrical signal output by the detector. The error signal enters the control module of the servo - control system to obtain the signal required to control the wavelength of the laser, and the wavelength is adjusted in real - time by controlling the tunable parameters of the laser to achieve the locking of the laser wavelength and the waveguide resonator.

[0011] In the preferred embodiment, the isolator is an optical - fiber isolator corresponding to the wavelength of the laser, and the beam splitter is a 99 / 1 ratio beam splitter corresponding to the wavelength of the laser.

[0012] In the preferred embodiment, the frequency - doubling medium is a periodically poled lithium niobate waveguide device; the waveguide material in the waveguide resonator is a periodically poled lithium niobate waveguide crystal.

[0013] In the preferred embodiment, the waveguide crystal in the waveguide resonator relies on the laser emitted by the tunable laser with coatings on both ends. The incident end face of the waveguide crystal is coated with a high - reflection film for the pump light and the fundamental - frequency light, and the reflectivity > 99%; the output end face of the waveguide crystal is coated with a partial - transmission film for the pump light and the fundamental - frequency light. The transmittance of the pump light is 5% - 20%, and the transmittance of the fundamental - frequency light is 5% - 20%.

[0014] In the preferred embodiment, the demodulation signal is a sine signal with the same frequency as the modulation signal. In the preferred embodiment, the laser is a 1550 - nm low - noise single - frequency tunable laser. In the preferred embodiment, the output light of the waveguide resonator contains the remaining 775 - nm pump light and 1550 - nm vacuum squeezed light. The remaining 775 - nm pump light is filtered out through a 1550 - nm narrow - band filter to achieve the output of the final 1550 - nm vacuum squeezed - state light source.

[0015] Compared with the prior art, the present invention has the following beneficial technical effects:

[0016] The present invention adopts a fully fiber-optic solution and, based on the design concept of a free-space standing-wave resonator, proposes a scheme for preparing a vacuum squeezed state based on a waveguide resonator. This scheme introduces a resonator structure with a higher optical power density in the resonator, which is beneficial to improving the nonlinear conversion efficiency. At the same time, different from the grating resonator and the mirror resonator, the resonator structure of this scheme is directly realized by coating a waveguide crystal, with fewer devices and less loss, which can further improve the nonlinear conversion efficiency and thus improve the theoretical squeezing degree of the squeezed-state light source. On this basis, the output of a fully fiber-optic vacuum squeezed-state light source has been successfully realized.

[0017] The high-compression fully fiber-optic vacuum squeezed-state light source based on a waveguide resonator of the present invention, compared with the mainstream free-space scheme, avoids complex mode matching problems, such as the matching between the pump light mode and the resonator cavity mode. Such mode matching work requires a lot of time for beam shaping and optical path adjustment. The structure is more compact, the anti-environmental interference ability is stronger, and it can be better compatible with the backend application system, thus accelerating the practical process of the squeezed-state optical field.

[0018] The high-compression fully fiber-optic vacuum squeezed-state light source based on a waveguide resonator of the present invention, compared with the fully fiber-optic compression light source scheme based on four-wave mixing, selects an optical parametric down-conversion scheme with higher nonlinear conversion efficiency, further improving the squeezing degree of the output optical field. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0020] Figure 1 It is a schematic structural diagram of the high-compression fully fiber-optic vacuum squeezed-state light source based on a waveguide resonator of the present invention.

[0021] Figure 2 It is a schematic comparison diagram of the waveguide resonator of the present invention and the existing grating waveguide resonator structure. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0022] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the following will clearly and completely describe the technical solutions in the embodiments of this application with reference to the accompanying drawings in the embodiments of this application. Apparently, the described embodiments are some, but not all, of the embodiments of this application. All other embodiments obtained by those of ordinary skill in the art based on the embodiments in this application without creative efforts shall fall within the scope of protection of this application.

[0023] As Figure 1 shown, a high-compression all-fiber vacuum squeezed state light source based on a waveguide resonator includes: a laser, an isolator, a beam splitter, a phase modulator, a frequency doubling medium, a circulator, a waveguide resonator, a narrowband filter, a signal generator, a servo control system, and a detector.

[0024] As Figure 2 shown in the upper figure in , the structural schematic diagram of the waveguide resonator of the present invention is shown. The design of the waveguide resonator is the core of the present invention. The difference between it and the traditional grating waveguide resonator and the mirror waveguide resonator lies in that: the waveguide resonator of the present invention is composed of only one device, the waveguide, and the resonance of the laser is realized by coating films on both ends of the waveguide crystal. According to the principle of external cavity enhancement, the power in the resonator will be greatly increased. Under the condition that the nonlinear crystal is fixed and the phase matching is achieved, the optical power density is positively correlated with the nonlinear conversion efficiency. In addition, this design not only simplifies the overall structure of the waveguide resonator, but also reduces the loss of the resonator, which is beneficial to the further improvement of the nonlinear conversion efficiency, and thus a squeezed state light source with a higher compression degree can be obtained.

[0025] Table 1 shows the differences between the waveguide resonator of the present invention and the traditional waveguide resonator in terms of component composition and advantage comparison.

[0026] Table 1 Differences between the traditional waveguide resonator and the waveguide resonator of the present invention

[0027]

[0028]

[0029] The complete squeezed state light source system involves an electrical locking loop. Therefore, in the present invention, in addition to the optical path part, the circuit part is also introduced. Since the waveguide resonator is realized by coating films on both ends of the crystal and it is not easy to dynamically adjust the cavity length, the locking loop composed of the phase modulator, the detector, the signal generator, and the servo system realizes the function of dynamically locking the wavelength to the waveguide resonator by controlling the wavelength of the laser.

[0030] Among them, the laser is a low-noise single-frequency tunable laser with linearly polarized output, which can be a fiber laser or a semiconductor laser.

[0031] Among them, the isolator is an optical fiber isolator corresponding to the wavelength of the laser, which is used to prevent the return light in the optical path from returning to the laser and damaging the laser or interfering with the entire system.

[0032] Among them, the beam splitter is a proportional beam splitter corresponding to the wavelength of the laser, such as a 99 / 1 beam splitter, and its purpose is to divide the fundamental frequency laser into two paths. One path (the majority) is used for frequency doubling to generate pump light, and the other path (a small portion) is used as the background light required for subsequent detection.

[0033] Among them, the phase modulator is an optical fiber phase modulator that can modulate the phase of the system laser, and is used to modulate the laser phase with a fixed frequency.

[0034] Among them, the frequency doubling medium can be an optical fiber medium or an optically fiber-coupled medium, and can be a common frequency doubling medium or a periodically poled frequency doubling medium. The higher the second-order nonlinear coefficient of the frequency doubling medium, the higher the pump light generated by frequency doubling, or the lower the fundamental frequency light power required to obtain the same pump light power.

[0035] Among them, the circulator is a circulator corresponding to the wavelength of the system pump light. The pump light is input from the first port 1 of the circulator and output from the second port 2. The light returned from the waveguide resonator is input from the second port 2 and output from the third port 3.

[0036] Among them, the waveguide material of the waveguide resonator is a periodically poled optical parametric down-conversion medium with high nonlinear conversion efficiency, such as PPLN. During the preparation process of the squeezed state light source, the nonlinear conversion efficiency directly affects the theoretical squeezing degree of the squeezed state light source generated in the nonlinear process. Therefore, in the free space scheme, it is preferably to adopt the structure of the resonator to improve the efficiency of nonlinear conversion. This scheme is also applicable to the all-fiber structure. The present invention innovatively proposes that the waveguide resonator can be realized by directly coating a film on the waveguide crystal. The incident end face of the crystal is coated with a high-reflection film for the pump light and the fundamental frequency light, and the reflectivity > 99%; the output end face is coated with a partial transmittance for the pump light and the fundamental frequency light, such as the transmittance of the pump light is 5% - 20%, and the transmittance of the fundamental frequency light is 5% - 20%.

[0037] The narrowband filter is an optical fiber narrowband filter corresponding to the wavelengths of the system pump light and the signal light, and is used to filter out the pump light so that only the signal light, that is, the squeezed light, is output.

[0038] Among them, the signal generator can output the modulation signal and the demodulation signal required by the system for the operation of the system locking loop. The modulation signal is generally a sinusoidal signal of dozens of MHz, such as 20 MHz, and the amplitude can be adjusted according to the specification parameters of the phase modulator. The demodulation signal is a sinusoidal signal with the same frequency as the modulation signal.

[0039] Among them, the servo control system demodulates the error signal from the demodulation signal sent by the signal generator and the electrical signal output by the detector. The error signal enters the control module of the servo control system, and through control algorithms such as the proportional integral derivative algorithm, the current or voltage signal required to control the wavelength of the laser is obtained. This signal adjusts the wavelength in real time by controlling the tunable parameters of the tunable laser, achieving the effect of dynamic matching with the waveguide resonator and realizing the locking of the laser wavelength and the waveguide resonator.

[0040] Among them, the detector is a photodetector corresponding to the pump light wavelength, which is used to detect the intensity of the pump light returning from the waveguide resonator.

[0041] Among them, the optical fibers are all polarization-maintaining optical fibers.

[0042] As Figure 1 shown, the black lines in the figure represent the optical path, that is, the optical fiber part, the arrows indicate the transmission direction of light, and the blue lines represent the circuit.

[0043] In the preferred embodiment, the all-fiber vacuum squeezed state light source based on the waveguide resonator includes: a 1550nm low-noise single-frequency tunable laser, a 1550nm isolator, a 1550nm 99 / 1 beam splitter, a 1550nm phase modulator, a frequency doubling medium, a 775nm circulator, a waveguide resonator, a 1550nm narrowband filter, a signal generator, a servo control system, and a 775nm detector.

[0044] The 1550 nm low-noise single-frequency tunable laser enters the 1550 nm 99 / 1 beam splitter after passing through the 1550 nm isolator. The function of the isolator is to prevent the light reflected by subsequent components from entering the laser and affecting the stability of the laser. After passing through the 1550 nm 99 / 1 beam splitter, most of the 1550 nm laser first enters the 1550 nm phase modulator and then enters the frequency doubling medium to generate 775 nm pump light. A small part of the 1550 nm laser is used as the background light for subsequent detection. The signal generator outputs a sine wave with a frequency of 20 MHz and is connected to the 1550 nm phase modulator to modulate the phase of the fundamental frequency light. The frequency doubling medium is a periodically poled lithium niobate (PPLN) waveguide device. The pump light enters the waveguide resonator through the 775 nm circulator. The waveguide material in the waveguide resonator is a PPLN crystal. A high reflection film for the pump light and the fundamental frequency light is deposited on the incident end face of the crystal, with a reflectivity > 99%; a partial transmission film for the pump light and the fundamental frequency light is deposited on the output end face, with a pump light transmittance of 10% and a fundamental frequency light transmittance of 8%. The pump light reflected by the waveguide resonator is input through the second port 2 of the circulator and output through the third port 3. The detector monitors the intensity of the reflected light in real time, and the generated electrical signal is input to the servo control system to demodulate and feedback to control the wavelength of the laser to be locked with the waveguide resonator. At this time, the output light of the waveguide resonator includes the remaining 775 nm pump light and 1550 nm vacuum squeezed light. The remaining 775 nm pump light can be filtered out through the 1550 nm narrowband filter to achieve the output of the final 1550 nm vacuum squeezed state light source.

[0045] The present invention adopts an all-fiber scheme and realizes the output of a high-compression vacuum squeezed state light source based on a waveguide resonator. Drawing on the design concept of a free-space standing wave resonator, a scheme for preparing a vacuum squeezed state based on a waveguide resonator is proposed. This scheme not only improves the nonlinear conversion efficiency but also successfully realizes the output of a high-compression vacuum squeezed state light source in all-fiber form.

[0046] Compared with the mainstream free-space scheme, the present invention avoids the complex mode matching problem, has a more compact structure, stronger anti-environmental interference ability, and can be better compatible with the backend application system, thus accelerating the practical application process of the squeezed state light field.

[0047] Compared with the all-fiber compressed light source scheme based on four-wave mixing, the present invention selects the optical parametric down-conversion scheme with higher nonlinear conversion efficiency, further improving the compression degree of the output light field.

[0048] In the above embodiments, it can be implemented in whole or in part by software, hardware, firmware, or any combination thereof. When implemented using software, it can be implemented in whole or in part in the form of a computer program product. The computer program product includes one or more computer instructions. When the computer program instructions are loaded and executed on a computer, the processes or functions described in the embodiments of the present application are generated in whole or in part. The computer can be a general-purpose computer, a special-purpose computer, a computer network, or other programmable devices. The computer instructions can be stored in a computer-readable storage medium or transmitted through the computer-readable storage medium. The computer-readable storage medium can be any available medium that can be accessed by a computer or a data storage device such as a server or a data center that integrates one or more available media. The available medium can be a magnetic medium (e.g., a floppy disk, a hard disk, a magnetic tape), an optical medium (e.g., a DVD), or a semiconductor medium (e.g., a solid state disk (SSD)), etc.

[0049] As described above, the above are only specific embodiments of the present application, but the protection scope of the present application is not limited thereto. Any person skilled in the art within the technical scope disclosed in the present application can easily think of various equivalent modifications or substitutions, and these modifications or substitutions should all be covered within the protection scope of the present application. Therefore, the protection scope of the present application shall be subject to the protection scope of the claims.

Claims

1. A high-compression all-fiber vacuum compressed state light source based on a waveguide resonant cavity, characterized in that: include: Lasers, isolators, beam splitters, phase modulators, frequency doubling media, circulators, waveguide resonators, narrowband filters, signal generators, servo control systems and detectors; The laser emitted by the laser enters the beam splitter after passing through the isolator. After passing through the beam splitter, most of the laser enters the phase modulator, and a small part of the laser is used as background light for subsequent detection; The laser light after phase modulation by the phase modulator enters the frequency doubling medium to generate pump light; the pump light is input from the first port of the circulator, output from the second port, and enters the waveguide resonant cavity; The waveguide crystal in the waveguide resonant cavity relies on the coating at both ends to tune the laser emitted by the laser; the narrowband filter is an optical fiber narrowband filter corresponding to the wavelength of the system pump light and signal light, which is used to filter out the pump light and output compressed light; The pump light returned from the waveguide resonant cavity is input from the second port of the circulator, output from the third port, and enters the detector; The detector detects the intensity of the pump light returned from the waveguide resonant cavity and generates an electrical signal which is input into the servo control system; The modulation signal output by the signal generator modulates the phase of the fundamental frequency light through the phase modulator; the demodulation signal output by the signal generator demodulates the error signal required by the servo control system; The servo control system demodulates the error signal obtained by the demodulation signal sent by the signal generator and the electrical signal output by the detector. The error signal enters the control module of the servo control system to obtain the signal required to control the wavelength of the laser. The wavelength is adjusted in real time by controlling the tunable parameters of the laser to achieve locking of the laser wavelength and the waveguide resonant cavity.

2. The high-compression all-fiber vacuum compressed state light source based on a waveguide resonant cavity according to claim 1, characterized in that: The isolator is an optical fiber isolator corresponding to the laser wavelength, and the beam splitter is a 99 / 1 ratio beam splitter corresponding to the laser wavelength.

3. The high-compression all-fiber vacuum compressed state light source based on a waveguide resonant cavity according to claim 1, characterized in that: The frequency doubling medium is a periodically polarized lithium niobate waveguide device; the waveguide material in the waveguide resonant cavity is a periodically polarized lithium niobate waveguide crystal.

4. The high-compression all-fiber vacuum compressed state light source based on a waveguide resonant cavity according to claim 1, characterized in that: The waveguide crystal in the waveguide resonant cavity relies on the coating at both ends to tune the laser emitted by the laser. The incident end face of the waveguide crystal is coated with a high-reflective film for pump light and fundamental frequency light, with a reflectivity of >99%; the output end face of the waveguide crystal is coated with a partially transparent film for pump light and fundamental frequency light, with a pump light transmittance of 5%-20% and a fundamental frequency light transmittance of 5%-20%.

5. The high-compression all-fiber vacuum compressed state light source based on a waveguide resonant cavity according to claim 1, characterized in that: The demodulated signal is a sinusoidal signal with the same frequency as the modulated signal.

6. The high-compression all-fiber vacuum compressed state light source based on a waveguide resonant cavity according to claim 1, characterized in that: The laser is a 1550nm low-noise single-frequency tunable laser.

7. The high-compression all-fiber vacuum compressed state light source based on a waveguide resonant cavity according to claim 1, characterized in that: The output light of the waveguide resonant cavity includes the remaining 775nm pump light and the 1550nm vacuum compression light. The remaining 775nm pump light is filtered out by a 1550nm narrowband filter to achieve the output of the final 1550nm vacuum compression state light source.

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