Quantum-enhanced laser phase noise filtering device and method

Abstract

The invention provides a quantum-enhanced laser phase noise filtering device and method, and belongs to the technical field of laser phase noise filtering. The technical problem to be solved is to provide the quantum-enhanced laser phase noise filtering device and method. According to the technical scheme, laser generated by a single-frequency laser sequentially passes through an acousto-optic modulator and a Faraday isolator and then is injected into a frequency doubling cavity; the frequency doubling cavity reflects fundamental frequency light in the received laser back to the Faraday isolator, and the fundamental frequency light is reflected to a mode cleaner locked in a semi-detuning state by a polarization beam splitter prism in the Faraday isolator and then is reflected to a second beam splitter by the mode cleaner; the second beam splitter divides the received optical signal into an inner ring feedback signal and an outer ring evaluation signal, and the divided inner ring signal is detected by the first photoelectric detector and then is output to the feedback controller for phase noise active control; the method is applied to laser phase noise filtering.

Description

Technical Field

[0001] The present invention provides a quantum-enhanced laser phase noise filtering device and method, belonging to the technical field of laser phase noise filtering. Background Art

[0002] At present, laser technology has been widely applied in the fields of medicine, communication, industry, military, etc. However, the laser noise generated during the generation and use of lasers limits the use of many optical measurement and sensing devices, seriously affecting the detection accuracy of the devices. To improve the optical detection sensitivity, it is necessary to suppress the laser noise.

[0003] Laser noise includes intensity noise and phase noise. Currently, there are various suppression schemes for these two types of noise. Feedback control is a commonly used means of laser noise suppression. By directly detecting the laser field with a photodetector and comparing it with a reference signal to generate an error signal, the laser intensity noise can be actively controlled. However, the inner-loop noise of the feedback loop cannot break through the shot noise limit. To address this problem, a scheme has also been developed to use a squeezed vacuum state optical field to stabilize the laser intensity noise to break through the shot noise limit. However, since the laser phase noise cannot be directly read out from the laser output, it is very difficult to suppress the noise through quantum enhancement of the phase. The currently commonly used phase noise suppression scheme is the PDH heterodyne detection technology. However, it will inevitably introduce quantum noise penalty during the demodulation process, resulting in a maximum quantum-enhanced noise suppression amplitude of only 3 dB for this scheme. Therefore, to achieve laser phase noise suppression that breaks through the shot noise limit, it is necessary to improve the corresponding laser phase noise suppression scheme. Summary of the Invention

[0004] In order to overcome the deficiencies in the prior art, the technical problem to be solved by the present invention is to provide a quantum-enhanced laser phase noise filtering device and method.

[0005] To solve the above technical problem, the technical solution adopted by the present invention is: a quantum-enhanced laser phase noise filtering device, including a single-frequency laser, and the laser generated by the single-frequency laser is sequentially injected into a frequency doubling cavity after passing through an acousto-optic modulator and a Faraday isolator;

[0006] The frequency doubling cavity reflects the fundamental frequency light in the received laser back to the Faraday isolator, and is reflected by the polarization beam splitting prism in the Faraday isolator to a mode cleaner locked in a half-detuned state, and then transmitted by the mode cleaner to a second beam splitter;

[0007] The second beam splitter divides the received optical signal into an inner-loop feedback signal and an outer-loop evaluation signal, where:

[0008] The divided inner-loop signal is detected by a first photodetector and then output to a feedback controller for active phase noise control;

[0009] The divided outer ring signal is output to a spectrum analyzer for phase noise evaluation after being detected by a second photodetector;

[0010] The frequency doubling cavity injects the frequency-doubled light generated by parametric up-conversion into an optical parametric oscillator, and the optical parametric oscillator performs parametric down-conversion to generate a squeezed vacuum state optical field. The squeezed vacuum state optical field is separated from the pump light by a dichroic mirror and then injected into the remaining port of a second beam splitter;

[0011] The optical signal after phase noise feedback control is divided by placing a first beam splitter in front of the mode cleaner for output use.

[0012] The frequency doubling cavity is specifically a two-mirror cavity or a multi-mirror cavity structure, and at least one cavity mirror in the frequency doubling cavity is provided with a piezoelectric ceramic for changing the cavity length.

[0013] The first beam splitter and the second beam splitter are specifically 99:1 beam splitters, with a reflectivity R = 1% and a transmittance T = 99%.

[0014] Both sides of the dichroic mirror are coated. One side of the coating can transmit the fundamental frequency light and reflect the frequency-doubled light, and the other side of the coating can transmit both the fundamental frequency light and the frequency-doubled light.

[0015] A method for filtering by a quantum enhanced laser phase noise filtering device includes the following filtering control steps:

[0016] Step 1: Control the laser generated by a single-frequency laser to be injected into the frequency doubling cavity after passing through an acousto-optic modulator and a Faraday isolator. The acousto-optic modulator modulates the laser generated by the single-frequency laser, and the Faraday isolator is set to isolate the two reflection signals generated by the two mirror surfaces of the frequency doubling cavity;

[0017] Step 2: Control the phase-locked loop to lock the frequency doubling cavity. The frequency-doubled light generated by the frequency doubling cavity through the parametric up-conversion process is used as the pump light to be injected into the optical parametric oscillator. At the same time, the fundamental frequency light reflected back by the frequency doubling cavity is reflected by the polarization beam splitting prism in the Faraday isolator and then injected into a mode cleaner;

[0018] Step 3: Lock the mode cleaner in a half-detuned state through unilateral locking. Under the half-detuned condition, convert the phase noise of the laser beam into intensity noise, and detect the light intensity I d (t) The calculation formula is:

[0019]

[0020] In the formula I d (v 0 ) is the average value of the detection signal, δv(t) is the phase noise, is the conversion factor for converting phase noise to intensity noise, δI(t) is the intensity noise of the transmitted beam, and ν 0 is the laser frequency;

[0021] It can be determined from the above formula that the detected laser noise includes a phase component and an intensity component. First, a frequency doubling cavity should be used to suppress the intensity noise of the fundamental frequency reflected light to be injected into the half-detuned cavity;

[0022] Step 4: Use a second beam splitter to divide the optical signal after passing through the mode cleaner into an inner loop feedback signal and an outer loop evaluation signal, where:

[0023] The inner loop feedback signal is detected by a first photodetector, and the obtained signal is fed back to the upstream acousto-optic modulator through a feedback controller to actively control the phase noise of the fundamental frequency light;

[0024] The outer loop evaluation signal is detected by a second photodetector and then output to a spectrum analyzer for laser phase noise evaluation;

[0025] Step 5: Control the locking of the optical parametric oscillator to make the optical parametric oscillator operate below the threshold state, and generate a squeezed vacuum state optical field through the parametric down-conversion process;

[0026] Step 6: Separate the generated squeezed vacuum state optical field from the pump light through a dichroic mirror and inject it into the remaining port of the second beam splitter for quantum enhancement;

[0027] Control the phase-locked loop to lock the relative phase between the squeezed vacuum state and the main beam transmitted by the half-detuned cavity to 0. According to the data displayed on the spectrum analyzer, determine whether quantum enhanced laser phase noise suppression beyond the shot noise limit has been achieved.

[0028] The beneficial effects of the present invention compared with the prior art are as follows: The quantum enhanced laser phase noise filtering scheme provided by the present invention, compared with the existing laser phase noise suppression technology, does not require an electronic demodulation process, avoids the 3dB quantum noise penalty, has a large noise suppression amplitude, can achieve quantum enhanced noise suppression beyond the shot noise limit, has a large noise suppression bandwidth, and a wide range of applications; The present invention adopts a laser phase noise suppression device with squeezed injection quantum enhancement, which has a simple structure, is easy to operate, has high practical value, and has good practical value for improving the sensitivity of optical precision measurement and sensing equipment. Description of the Drawings

[0029] The following further describes the present invention with reference to the drawings:

[0030] Figure 1 is a schematic structural diagram of the filtering device of the present invention;

[0031] Figure 2 is the effect diagram of intensity noise suppression after passive filtering by the frequency doubling cavity in the embodiment of the present invention;

[0032] Figure 3 This is the effect diagram of phase noise suppression enhanced by quantum in the embodiments of the present invention;

[0033] The meanings of the serial numbers in the figure are as follows: 1 - single - frequency laser, 2 - acousto - optic modulator, 3 - Faraday isolator, 4 - frequency - doubling cavity, 5 - dichroic mirror, 6 - optical parametric oscillator, 7 - first beam splitter, 8 - mode cleaner, 9 - second beam splitter, 10 - first photodetector, 11 - second photodetector, 12 - feedback controller, 13 - spectrum analyzer. Detailed implementation manners

[0034] As Figure 1 shown, the present invention provides a quantum - enhanced laser phase noise filtering device. The filtering device has a simple structure, is easy to adjust, and has excellent noise suppression effect. The laser phase noise filtering scheme provided by the present invention is different from the PDH scheme, can avoid the 3dB quantum noise penalty introduced in the demodulation process, can achieve laser phase noise suppression that breaks through the inner - loop shot - noise limit, and significantly improves the performance of the laser in experiments.

[0035] The laser phase noise filtering device provided by the present invention includes a single - frequency laser 1, an acousto - optic modulator 2, a Faraday isolator 3, a frequency - doubling cavity 4, a dichroic mirror 5, an optical parametric oscillator 6, a first beam splitter 7, a mode cleaner 8, a second beam splitter 9, a first photodetector 10, a second photodetector 11, a feedback controller 12, and a spectrum analyzer 13. When the device works, first, the laser generated by the single - frequency laser 1 passes through the acousto - optic modulator 2 and the Faraday isolator 3 and then is injected into the frequency - doubling cavity 4. The fundamental - frequency light reflected back from the frequency - doubling cavity 4 is reflected by the polarization beam - splitting prism in the Faraday isolator 3 and then passes through a mode cleaner 8 locked in a half - detuned state. Then, it is split into an inner - loop feedback signal and an outer - loop evaluation signal by the second beam splitter 9. The inner - loop signal is detected by the first photodetector 10 and then fed back to the feedback controller 12 for active phase noise control, and the outer - loop signal is detected by the second photodetector 11 and then output to the spectrum analyzer 13 for phase noise evaluation; the frequency - doubling cavity 4 generates second - harmonic light through a parametric up - conversion process, and the second - harmonic light is injected into the optical parametric oscillator 6 for a parametric down - conversion process to generate a squeezed vacuum state optical field. The squeezed vacuum state optical field is separated from the pump light by the dichroic mirror 5 and then injected into the remaining port of the second beam splitter 9; finally, a first beam splitter 7 is placed in front of the mode cleaner 8 to split out the optical signal after phase noise feedback control for output use.

[0036] The above - mentioned frequency - doubling cavity is specifically 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.

[0037] The above - mentioned optical parametric oscillator and the above - mentioned frequency - doubling cavity have exactly the same design except for the coating of the end mirrors.

[0038] The above-mentioned first and second beam splitters are specifically 99:1 beam splitters.

[0039] Both sides of the above-mentioned dichroic mirror are coated. One side has high transmittance for the fundamental frequency light and high reflectivity for the second harmonic light, and the other side has high transmittance for both the fundamental frequency light and the second harmonic light.

[0040] Based on the above device, the present invention also provides a quantum-enhanced laser phase noise filtering method, which successively includes the following steps:

[0041] Step 1: Inject the laser generated by the single-frequency laser 1 into the second harmonic generation cavity 4 after passing through the acousto-optic modulator 2 and the Faraday isolator 3.

[0042] Among them, the acousto-optic modulator modulates the laser generated by the laser. The Faraday isolator can isolate the two reflection signals generated by the two mirrors of the second harmonic generation cavity, avoiding the reflection light from feeding back into the laser and causing damage to the laser.

[0043] Step 2: Lock the second harmonic generation cavity 4 through a phase-locked loop. The second harmonic light generated by the second harmonic generation cavity 4 through the parametric up-conversion process is injected into the optical parametric oscillator 6 as the pump light. At the same time, the fundamental frequency light reflected back by the second harmonic generation cavity 4 is reflected by the polarization beam splitter prism in the Faraday isolator 3 and then injected into a mode cleaner 8.

[0044] In this device, the second harmonic generation cavity not only provides the pump light for the parametric down-conversion process of the optical parametric oscillator to generate the squeezed vacuum state optical 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.

[0045] Step 3: Use the single-side locking technique to lock the mode cleaner 8 in a half-detuned state.

[0046] The mode cleaner is an impedance-matched cavity, which can convert the phase noise of the laser beam into intensity noise under the half-detuned condition. The detected optical intensity I d (t) can be deduced as:

[0047]

[0048] In the formula, I d (v 0 ) is the average value of the detection signal, δv(t) is the phase noise, is the conversion factor for converting phase noise into intensity noise, δI(t) is the intensity noise of the transmitted light beam, and v 0 is the laser frequency.

[0049] As can be seen from the above formula, the detected laser noise under the condition of complete conversion includes the combined contributions of the phase component and the intensity component. Only when the intensity noise is extremely low can the phase noise be independently extracted for feedback control. Therefore, before injecting squeezing for quantum enhancement, it is necessary to first use a frequency doubling cavity to suppress the intensity noise of the fundamental frequency reflected light to be injected into the half-detuned cavity to the lowest possible level.

[0050] Step 4: Use the second beam splitter 9 to divide the light beam passing through the mode cleaner 8 into two parts: inner-loop feedback and outer-loop evaluation. The inner-loop feedback part is detected by the first photodetector 10, and the obtained signal is fed back to the upstream acousto-optic modulator 2 through the feedback controller 12 to actively control the phase noise of the fundamental frequency light. The outer-loop evaluation part is detected by the second photodetector 11 and then output to the spectrum analyzer 13 for laser phase noise evaluation.

[0051] Step 5: Lock the optical parametric oscillator 6 so that the optical parametric oscillator 6 operates below the threshold, and a squeezed vacuum state optical field is generated through the parametric down-conversion process.

[0052] Step 6: Separate the squeezed vacuum state optical field from the pump light through the dichroic mirror 5 and inject it into the remaining port of the second beam splitter 9 for quantum enhancement. The relative phase between the squeezed vacuum state and the main beam transmitted by the half-detuned cavity is locked to 0 through the phase-locked loop. At this time, according to the data display of the spectrum analyzer 13, the present device has achieved quantum-enhanced laser phase noise suppression that breaks through the shot noise limit.

[0053] The active control of the relative phase in the present invention is completed by means of a frequency-shifted light having the same phase as the squeezed state optical field. The dichroic mirror is a mirror with high reflectivity for fundamental frequency light and high transmittance for second harmonic light. The second beam splitter is a 99:1 beam splitter with a reflectivity R = 1% and a transmittance T = 99%.

[0054] Place the first beam splitter 7 in front of the mode cleaner 8 to separate a large part of the fundamental frequency reflected light that has undergone phase noise feedback control for practical applications; this design also saves an additional mode cleaner required for measuring the phase noise of the outer-loop laser beam.

[0055] As Figure 2 and Figure 3As shown, in an embodiment of the present invention, the above filtering device is adopted to perform quantum-enhanced laser phase noise filtering. Specifically, the 1550 nm laser with a power of 2 W generated by the single-frequency laser 1 is injected into the frequency doubling cavity 4 after passing through the acousto-optic modulator 2 and the Faraday isolator 3. The frequency shift of the acousto-optic modulator 2 is 110 MHz, and the diffraction efficiency is 50%. The frequency doubling cavity 4 is locked through a phase-locked loop, and the incident power and temperature are adjusted to make the conversion efficiency of the frequency doubling process 70%. The frequency doubling cavity consists 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 front end face of the nonlinear crystal is 12 mm, and the coating is HR1550 nm / 775 nm, serving as the input mirror of the frequency doubling cavity. The rear end face of the nonlinear crystal is a plane, and the coating is AR1550 nm / 775 nm. The curvature radius of the meniscus concave mirror is 30 mm, acting as the output mirror of the frequency doubling cavity. The transmittance of the output mirror for 1550 nm is 12%, and it is antireflective for 775 nm. The air gap length between the concave mirror and the nonlinear crystal is 27 mm, corresponding to the eigenmode radius at 42 μm of the fundamental wave. The 775 nm frequency-doubled light with a power of 10 mW generated by the frequency doubling cavity 4 through the parametric up-conversion process is injected into the optical parametric oscillator 6 as the pump light, making the optical parametric oscillator operate below its oscillation threshold (20 mW). The optical parametric oscillator has the same design as the above frequency doubling cavity except for the different coating of the meniscus concave mirror. The reflectivity of the concave mirror of the optical parametric oscillator at 1550 nm is 84.3%, and the reflectivity at 775 nm is 97.8%.

[0056] The laser exports the fundamental frequency light with a power of 100 mW reflected back by the frequency doubling cavity 4 through the polarization beam splitting prism in the Faraday isolator 3, and uses the first beam splitter 7 to divide it into two parts. One part of about 90 mW is used as a stable laser output for practical applications, and the other part of about 10 mW is injected into a near impedance matching mode cleaner 8 with a linewidth of 35 MHz. The mode cleaner 8 is locked in a half-detuned state using the single-side locking technique to convert the phase noise of the laser beam into intensity noise. The second beam splitter 9 is used to divide the light beam passing through the mode cleaner 8 into two parts: inner loop feedback and outer loop evaluation. The inner loop feedback part is detected by the first photodetector 10, and the obtained signal is fed back to the upstream acousto-optic modulator 2 through the feedback controller 12 for active control of the phase noise of the fundamental frequency light. The outer loop evaluation part is detected by the second photodetector 11 and then output to the spectrum analyzer 13 for laser phase noise evaluation. The optical parametric oscillator 6 is locked through a phase-locked loop, and a squeezed vacuum state optical field at 1550 nm is generated in the parametric down-conversion process. The squeezed vacuum state optical field is separated from the pump light by the dichroic mirror 5 and then injected into the remaining port of the second beam splitter 9 for quantum enhancement. With the help of the frequency-shifted light having the same phase as the squeezed state optical field and through the phase-locked loop, the relative phase between the squeezed vacuum state and the main light beam transmitted by the half-detuned cavity is locked to 0. At this time, according to the data displayed by the spectrum analyzer 13, at 10 kHz, compared with the classical scheme, the phase noise enhanced by quantum can be reduced by up to 4.5 dB exceeding the shot noise limit.

[0057] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention 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 for 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 invention.

Claims

1. A quantum enhanced laser phase noise filtering device, comprising a single frequency laser (1), characterized in that: The laser light generated by the single-frequency laser (1) is injected into the frequency doubling cavity (4) after passing through the acousto-optic modulator (2) and the Faraday isolator (3) in sequence; The frequency doubling cavity (4) reflects the fundamental frequency light in the received laser back to the Faraday isolator (3), and the fundamental frequency light is reflected by the polarization beam splitting prism in the Faraday isolator (3) to a mode cleaner (8) locked in a semi-detuned state, and then transmitted by the mode cleaner (8) to a second beam splitter (9); The second beam splitter (9) splits the received optical signal into an inner loop feedback signal and an outer loop evaluation signal, wherein: The separated inner loop signal is detected by a first photodetector (10) and then output to a feedback controller (12) for active phase noise control; The separated outer loop signal is detected by a second photodetector (11) and then output to a spectrum analyzer (13) for phase noise evaluation; The frequency doubling cavity (4) injects the frequency doubling light generated by parametric up-conversion into the optical parametric oscillator (6), and the optical parametric oscillator (6) performs parametric down-conversion to generate a compressed vacuum state light field, and the compressed vacuum state light field is separated from the pump light by a dichroic mirror (5) and then injected into the remaining port of the second beam splitter (9); By placing a first beam splitter (7) before the mode cleaner (8), the optical signal after phase noise feedback control is split for output and use.

2. A quantum enhanced laser phase noise filter device according to claim 1, characterized in that: The frequency doubling cavity (4) is specifically a two-mirror cavity or a multi-mirror cavity structure, and at least one cavity mirror in the frequency doubling cavity (4) is provided with a piezoelectric ceramic for changing the cavity length.

3. The quantum enhanced laser phase noise filter device according to claim 1, characterized in that: The first beam splitter (7) and the second beam splitter (9) are specifically 99:1 beam splitters, with a reflectivity R=1% and a transmittance T=99%.

4. The quantum enhanced laser phase noise filter device according to claim 1, characterized in that: The two sides of the dichroic mirror (5) are coated, one side of the coating can transmit fundamental frequency light and reflect double frequency light, and the other side of the coating can transmit fundamental frequency light and double frequency light.

5. The method for filtering by a quantum enhanced laser phase noise filter device according to claim 1, characterized in that: The filtering control steps include the following: Step 1: Control the laser light generated by the single-frequency laser (1) to pass through the acousto-optic modulator (2) and the Faraday isolator (3) and then be injected into the frequency doubling cavity (4), the acousto-optic modulator (2) modulates the laser light generated by the single-frequency laser (1), and the Faraday isolator (3) is set to isolate two reflected signals generated by two mirrors of the frequency doubling cavity (4); Step 2: Control the phase-locked loop to lock the frequency doubling cavity (4), and inject the frequency doubling light generated by the frequency doubling cavity (4) through the parametric up-conversion process into the optical parametric oscillator (6) as pump light. At the same time, the fundamental frequency light reflected back from the frequency doubling cavity (4) is reflected by the polarization beam splitter prism in the Faraday isolator (3) and then injected into a mode cleaner (8); Step 3: Lock the mode cleaner (8) in a semi-detuned state by unilateral locking, convert the phase noise of the laser beam into intensity noise under the semi-detuned condition, and detect the light intensity I at the output end. d The calculation formula of (t) is: Where I d (ν0) is the average value of the detection signal, δv(t) is the phase noise, is the conversion factor from phase noise to intensity noise, δI(t) is the intensity noise of the transmitted beam, and ν0 is the laser frequency; It is determined from the above formula that the detected laser noise includes a phase component and an intensity component. The intensity noise of the fundamental frequency reflected light to be injected into the semi-detuned cavity should be suppressed by using a frequency doubling cavity (4); Step 4: Use a second beam splitter (9) to split the optical signal after passing through the mode cleaner (8) into an inner loop feedback signal and an outer loop evaluation signal, wherein: The inner loop feedback signal is detected by a first photodetector (10), and the obtained signal is fed back to an upstream acousto-optic modulator (2) through a feedback controller (12) to actively control the phase noise of the fundamental frequency light; The outer loop evaluation signal is detected by the second photodetector (11) and then output to the spectrum analyzer (13) for laser phase noise evaluation; Step 5: Controlling the optical parametric oscillator (6) to lock, so that the optical parametric oscillator (6) operates in a state below the threshold, and generating a compressed vacuum state light field through a parametric down-conversion process; Step 6: The generated compressed vacuum state light field is separated from the pump light by a dichroic mirror (5) and then injected into the remaining port of the second beam splitter (9) for quantum enhancement; The phase-locked loop is controlled to lock the relative phase between the compressed vacuum state and the main light beam transmitted by the semi-detuned cavity to 0, and the data displayed by the spectrum analyzer (13) is used to determine whether the quantum enhanced laser phase noise suppression that breaks through the shot noise limit is achieved.

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