A pump-enhanced optical parametric oscillator and active stabilization method and application

Abstract

The present invention relates to a pump-enhanced optical parametric oscillator and an active stabilization method and application, comprising an optical parametric oscillator module and a control module; the optical parametric oscillator module comprises a first input lens, a second input lens, a first cavity mirror, a second cavity mirror, a third cavity mirror, a fourth cavity mirror, a PPLN crystal, and a temperature-controlled furnace; the control module comprises piezoelectric ceramics, a photodiode, a phase-locked amplifier, a servo controller, and a high-voltage amplifier; the present invention employs an input mirror with a pump light reflectivity R=60%, which can effectively increase the range of high conversion efficiency and increase the range of high-efficiency output from 100 milliwatts to 1.8W to accommodate a variety of usage scenarios. The present invention employs active control technology using a low-frequency modulation and demodulation method, capable of stabilizing the intracavity pump light power over a wide range to achieve stable idler light output. At the same time, the optoelectronic modulator can be eliminated, and the performance requirements for the device can be reduced, which is more conducive to miniaturization and integration.

Description

Technical Field

[0001] The present invention relates to a pump-enhanced optical parametric oscillator and an active stabilization method and application, belonging to the technical field of lasers, in particular to the technical field of continuous light optical parametric oscillators. Background Art

[0002] Mid-infrared lasers (3-5μm) have broad application prospects in environmental monitoring, infrared remote sensing, lidar, military countermeasures, and other fields, and have long been a hot research area. Optical parametric oscillators (OPOs) are currently one of the key methods for generating lasers in the 3-5μm band, and continuous-wave optical parametric oscillators (CPOS) are an indispensable component. They utilize nonlinear crystals to convert short-wavelength pump light into longer-wavelength signal and idler light.

[0003] In continuous-wave optical parametric oscillators (OCOs), the corresponding pump threshold is generally high due to the characteristics of nonlinear crystals, and the conversion efficiency is low at low pump powers. Even using cavity mirrors that are highly reflective of signal light, the effect of lowering the threshold is very limited, and the conversion efficiency also decreases when the signal light oscillating power in the cavity is too high. Therefore, even low-power mid-infrared laser output requires a very high pump power, which is not conducive to the miniaturization and integration of lasers.

[0004] For continuous light optical parametric oscillators, how to effectively improve the conversion efficiency at low power is one of the key issues in achieving miniaturization and integration of lasers.

[0005] To improve the conversion efficiency of continuous-wave optical parametric oscillators (OPOs) at low power, a typical approach is pump enhancement technology. This involves using a lens with high reflectivity for pump light as the input mirror, typically with a reflectivity of R > 90%, and using a lens with high reflectivity for pump light as the other cavity mirrors. Pump light transmitted into the OPO cavity continuously oscillates within the cavity, coherently adding together, thereby increasing the pump power inside the cavity to a higher level than outside, effectively lowering the threshold. However, due to the generally narrow linewidth of the pump light, it is sensitive to cavity length. Even slight air disturbances can cause significant fluctuations in the cavity power. Therefore, Pound-Drever-Hall (PDH) technology is typically used to actively control the cavity length, maintaining a stable maximum pump oscillation power within the cavity.

[0006] However, the above existing solutions have the following disadvantages:

[0007] 1) Since the conversion of pump light increases the intracavity pump light loss, the intracavity pump light oscillation power is not proportional to the extracavity pump light power, resulting in a weakened pump enhancement effect. Therefore, the idler light output range with higher conversion efficiency is very small, generally corresponding to an output power of only about 100 mW.

[0008] 2) Currently, existing pump-enhanced lasers all use a method of constructing the crystal perpendicular to the pump. Although this facilitates the construction of the optical path, the reflection on the crystal surface and the Bragg reflection caused by the polarization period inside the crystal will cause the light beam in the cavity to propagate in the reverse direction, thereby causing the forward-propagating light beam to split in mode and reduce the pump enhancement effect, affecting the stability of the pump light oscillation power and conversion efficiency in the cavity.

[0009] 3) PDH technology requires the use of an electro-optic modulator to modulate a portion of the intracavity oscillating pump light with a reference signal at a frequency in the MHz range, and then use a phase shifter to shift the phase of another portion of the intracavity pump light intensity. The two are then mixed to obtain an error signal. However, the electro-optic modulator and phase shifter increase the size and complexity of the entire system. In addition, PDH technology has high performance requirements for the instrument, making it difficult to miniaturize and integrate.

[0010] In response to the shortcomings of the existing technology, the present invention provides a pump-enhanced optical parametric oscillator. The pump-enhanced optical parametric oscillator adopts a pump-enhanced structure, that is, the incident cavity mirror is coated with a film layer with a certain reflectivity to the pump light, and the remaining cavity mirrors are coated with a film layer with high reflectivity to the pump light. As a result, the pump light is coherently enhanced in the optical parametric oscillator cavity, making the pump oscillation power in the cavity several to dozens of times higher than the pump power outside the cavity, so as to achieve the effect of lowering the threshold and improving the conversion rate. Compared with the general pump-enhanced cavity that uses an input mirror coated with a film layer with a reflectivity of 90% or more to the pump light, the present invention uses an input mirror coated with a film layer with a lower reflectivity to the pump light, which effectively improves the high conversion efficiency range of the idler light output, so that this type of pump-enhanced optical parametric oscillator can meet more usage scenarios.

[0011] The present invention places the crystal at a certain tilt angle so that the reflected light cannot self-reproduce in the cavity, reducing the oscillation caused by coherent superposition, thereby avoiding the phenomenon of mode splitting caused by crystal end face reflection and Bragg reflection of the polarization period inside the crystal, and is more conducive to the stability of active control and the improvement of conversion efficiency.

[0012] The present invention provides a method and application for actively controlling the aforementioned pump-enhanced optical parametric oscillator and low-frequency modulation and demodulation method. The low-frequency modulation and demodulation method utilizes a kHz-level sinusoidal reference signal transmitted to a piezoelectric ceramic to scan the cavity length, obtaining an intracavity oscillating pump light intensity signal related to the cavity length, namely the cavity mode signal. This signal is then mixed and demodulated with the reference signal through a phase-locked amplifier to produce an error signal that is a derivative of the cavity mode signal. The zero point of the error signal corresponds to the maximum value of the cavity mode signal, and the error signal is an odd function near zero. The magnitude and direction of the cavity length change can be determined based on the magnitude of the error signal. A servo controller uses a PID algorithm to adjust the voltage bias output to the piezoelectric ceramic based on the error signal, thereby stabilizing the pump light power within the cavity.

[0013] The present invention can achieve high conversion efficiency output over a large range and long-term output power stability to cope with different usage scenarios, while taking into account miniaturization and integration.

[0014] The technical solution of the present invention is:

[0015] A pump-enhanced optical parametric oscillator, comprising an optical parametric oscillator module and a control module;

[0016] The optical parametric oscillator module includes a first incident lens, a second incident lens, a first cavity mirror, a second cavity mirror, a third cavity mirror, a fourth cavity mirror, a PPLN crystal, and a temperature-controlled furnace;

[0017] After passing through the first incident lens, the second incident lens, the first cavity mirror, the second cavity mirror, the third cavity mirror, and the fourth cavity mirror, the pump light converges to the center of the PPLN crystal, is reflected from the second cavity mirror to the third cavity mirror, and then is reflected from the fourth cavity mirror back to the first cavity mirror, and then circulates in the cavity to achieve self-reproduction.

[0018] The control module includes piezoelectric ceramics, a photodiode, a lock-in amplifier, a servo controller, and a high-voltage amplifier;

[0019] The photodiode is placed behind the third cavity mirror and connected to the input port of the lock-in amplifier. The output port of the lock-in amplifier is connected to the input port A of the servo controller. The reference signal output port of the lock-in amplifier is connected to the scanning signal input port of the servo controller. The output port of the servo controller is connected to the input port of the high-voltage amplifier. The output port of the high-voltage amplifier is connected to the piezoelectric ceramic.

[0020] The reference signal generated by a lock-in amplifier is used to directly modulate the piezoelectric ceramic drive voltage, that is, to modulate the cavity length of the optical parametric oscillator module. The cavity mode signal is then received by a photodiode, mixed with the reference signal through a lock-in amplifier, and low-pass filtered to demodulate an error signal that is a derivative of the cavity mode signal. The zero point of the error signal corresponds to the maximum point of the cavity mode signal, and the error signal is an odd function near the zero point. The size of the error signal determines the size and direction of the cavity length change that needs to be determined. The servo controller actively controls the piezoelectric ceramic drive voltage according to the PID algorithm, so that the oscillating pump light in the cavity always remains at the maximum power.

[0021] According to a preferred embodiment of the present invention, the photodiode receives the pump light intensity signal, converts it into an electrical signal, and inputs it into the signal input port of the lock-in amplifier;

[0022] The lock-in amplifier separates a specific carrier frequency signal from an interference environment and amplifies the signal. The lock-in amplifier demodulates the signal received from the photodiode to obtain an error signal, and outputs the error signal from an output port to an input port A of the servo controller.

[0023] The servo controller uses a control system that performs control based on the proportion, integration, and differentiation of the error generated by comparing the information collected from the real-time data of the controlled object with the given value. By analyzing the obtained error signal, the required output control signal is calculated, thereby actively controlling the target parameter to be stable.

[0024] The high-voltage amplifier amplifies the input voltage signal and outputs it, while setting a bias voltage.

[0025] Preferably, according to the present invention, the PPLN crystal is placed in a temperature-controlled furnace; the temperature-controlled furnace tunes the signal light and the idler light by adjusting the temperature.

[0026] Preferably, according to the present invention, the wavelength of the pump light is 1-1.1 μm, the wavelength of the generated signal light is 1.3-1.7 μm, and the corresponding idler light wavelength is 3-5 μm.

[0027] Further preferably, the wavelength of the pump light is 1.06 μm, the wavelength of the generated signal light is 1.5 μm, and the corresponding idler light wavelength is 3.8 μm.

[0028] Preferably, according to the present invention, both sides of the first incident lens and the second incident lens are coated with a pump light anti-reflection film, and the focal length of the pump light is 50-300 mm.

[0029] Further preferably, the focal length of the pump light is 100 mm.

[0030] According to a preferred embodiment of the present invention, the first cavity mirror is a concave mirror with a spherical radius of 50-200 mm, the outer side of which is coated with a high-transmittance film for pump light and idler light, and the inner side of which is coated with a film layer having a certain reflectivity (<90%) and transmittance for pump light, high reflectivity for signal light, and high transmittance for idler light;

[0031] The second cavity mirror is a concave mirror with a spherical radius of 50-200 mm, and the outer surface is coated with a film layer that is highly transparent to idler light and highly reflective to pump light and signal light;

[0032] The third cavity mirror is a plane mirror, the inner side of which is coated with a film layer that is highly reflective to pump light and signal light;

[0033] The fourth cavity mirror is a plane mirror, the inner side of which is coated with a film layer with high reflectivity to pump light and a reflectivity R of 90-99.8% to signal light.

[0034] Further preferably, the first cavity mirror is a concave mirror with a spherical radius of 100 mm, the outer side of which is coated with a high-transmittance film for pump light and idler light, and the inner side of which is coated with a film layer with a reflectivity R of 60% for pump light, high reflectivity for signal light, and high transmittance for idler light;

[0035] The second cavity mirror is a concave mirror with a spherical radius of 100 mm;

[0036] The fourth cavity mirror is a plane mirror, the inner side of which is plated with a film layer that is highly reflective to pump light and has a reflectivity R of 98.5% to signal light.

[0037] Preferably, according to the present invention, the PPLN crystal is a cuboid with a size of (20-100) mm*(1-5) mm*(0.5-5) mm, with both end faces plated with a film layer with high transmittance for pump light, signal light and idler light, and a polarization period of 26-35 μm.

[0038] Further preferably, the PPLN crystal is a cuboid with a size of 50 mm*3 mm*1 mm, a polarization period of 29.5 μm, and an incident angle of light on the PPLN crystal of 2.5°.

[0039] The active stabilization method of the pump-enhanced optical parametric oscillator comprises the following steps:

[0040] 1) The lock-in amplifier outputs a kHz-level low-frequency sinusoidal reference signal to the servo controller as a scanning signal. After being amplified by the high-voltage amplifier, it is transmitted to the piezoelectric ceramic to scan the cavity length. The photodetector after the third cavity mirror receives the pump light and obtains the cavity mode signal of the pump light.

[0041] 2) adjusting the amplitude of the scanning signal of the servo controller and adjusting the first cavity mirror, the second cavity mirror, the third cavity mirror, and the fourth cavity mirror to obtain a sharp and clear cavity mode signal;

[0042] 3) The cavity mode signal is processed by a lock-in amplifier, mixed with the sinusoidal reference signal output by the lock-in amplifier, and then low-pass filtered to obtain an error signal, which is transmitted to the servo controller; the phase difference between the reference signal and the cavity mode signal, as well as the offset of the error signal and the output offset of the high-voltage amplifier are adjusted so that the zero point of the error signal corresponds to the position of the maximum value of the cavity mode signal, and the slope near the zero point is large;

[0043] 4) Turn on the LFGL mode of the servo controller to stabilize the cavity mode signal and error signal, then reduce the amplitude of the scan signal. At the same time, adjust the scan signal bias and error signal bias so that the cavity mode signal gradually flattens and is at the maximum level, and the error signal fluctuates around 0.

[0044] 5) Turn on the Lock On mode of the servo controller to completely lock the optical parametric oscillator and stabilize the idler optical power.

[0045] Applications of the above-mentioned pump-enhanced optical parametric oscillator include applications in remote spectral detection.

[0046] The applications of the above-mentioned pump-enhanced optical parametric oscillator include applications in lidar.

[0047] The applications of the above-mentioned pump-enhanced optical parametric oscillator include applications in mid-infrared optical frequency combs.

[0048] Compared with the existing pump-enhanced optical parametric oscillator, the present invention has the following significant advantages:

[0049] 1. By employing an input mirror with a different pump light reflectivity than existing pump-enhanced optical parametric oscillators, namely a reflector with a pump light reflectivity of R = 60%, the present invention achieves a higher conversion efficiency than that of non-pump-enhanced optical parametric oscillators at higher powers (within an idler light output power of 1.8 W). This effectively increases the range of high conversion efficiency and raises the range of efficient idler light output from hundreds of milliwatts to 1.8 W, adapting to a variety of usage scenarios.

[0050] 2. The crystal of this invention forms a specific angle with the incident pump light, effectively preventing mode splitting and weakening of the pump enhancement effect caused by reflections from the crystal end faces and Bragg reflections from the internal polarization period. This increases the maximum stable power of the idler light from 1.5W to 1.8W, while also boosting the maximum conversion efficiency from 50% to 61%.

[0051] 3. The present invention adopts active control technology using a low-frequency modulation and demodulation method. Compared with the commonly used PDH control technology, the low-frequency modulation and demodulation method can obtain a clearer error signal by increasing the amplitude of the scanning signal at high power, actively controlling the stability of the intracavity oscillating pump light power and achieving a wide range of stable idler light output. At the same time, the error signal is obtained from the cavity mode signal obtained by scanning the cavity length with the scanning signal, which is equivalent to directly modulating the optical field intensity of the intracavity oscillating pump light. Therefore, the electro-optical modulator and phase shifter can be eliminated, simplifying the entire control system. Moreover, since electro-optical modulators and phase shifters are not required, and the interference of the pump-enhanced optical parametric oscillator is generally low-frequency interference, a kHz-level scanning signal can be used, which reduces the performance requirements of each component and is more conducive to miniaturization and integration. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Figure 1 Schematic diagram of the overall structure of the pump-enhanced optical parametric oscillator of the present invention;

[0053] Figure 2 Schematic diagram of the relationship between idler optical power and pump optical power;

[0054] Figure 3 This is a schematic diagram of the idler optical power stability test;

[0055] Figure 4 Schematic diagram of the idler light maximum power stability test after tilting the crystal.

[0056] 101. First incident lens, 102. Second incident lens, 201. First cavity mirror, 202. Second cavity mirror, 203. Third cavity mirror, 204. Fourth cavity mirror, 301. PPLN crystal, 401. Temperature-controlled furnace, 501. Piezoelectric ceramic, 601. Photodiode, 701. Lock-in amplifier, 801. Servo controller, 901. High-voltage amplifier. DETAILED DESCRIPTION

[0057] The present invention will be further described in detail below with reference to the accompanying drawings and examples, but is not limited thereto.

[0058] Example 1

[0059] A pump-enhanced optical parametric oscillator, such as Figure 1 As shown, it includes an optical parametric oscillator module and a control module;

[0060] The optical parametric oscillator module includes a first incident lens 101, a second incident lens 102, a first cavity mirror 201, a second cavity mirror 202, a third cavity mirror 203, a fourth cavity mirror 204, a PPLN crystal 301, and a temperature-controlled furnace 401;

[0061] After passing through the first incident lens 101, the second incident lens 102, the first cavity mirror 201, the second cavity mirror 202, the third cavity mirror 203, and the fourth cavity mirror 204, the pump light converges to the center of the PPLN crystal 301, is reflected from the second cavity mirror 202 to the third cavity mirror 203, and is reflected from the fourth cavity mirror 204 back to the first cavity mirror 201, where it circulates within the cavity, achieving self-reproduction.

[0062] The control module includes piezoelectric ceramics 501, photodiodes 601, lock-in amplifiers 701, servo controllers 801, and high-voltage amplifiers 901;

[0063] The photodiode 601 is placed behind the third cavity mirror 203 and connected to the input port of the lock-in amplifier 701. The output port of the lock-in amplifier 701 is connected to the input port A of the servo controller 801. The reference signal output port of the lock-in amplifier 701 is connected to the scanning signal input port of the servo controller 801. The output port of the servo controller 801 is connected to the input port of the high-voltage amplifier 901. The output port of the high-voltage amplifier 901 is connected to the piezoelectric ceramic 501.

[0064] The reference signal generated by the phase-locked amplifier 701 is used to directly modulate the driving voltage of the piezoelectric ceramic 501, that is, to modulate the cavity length of the optical parametric oscillator module. The cavity mode signal is then received by the photodiode 601, mixed with the reference signal through the phase-locked amplifier 701, and low-pass filtered to demodulate an error signal that is a derivative of the cavity mode signal. The zero point of the error signal corresponds to the maximum point of the cavity mode signal, and the error signal is an odd function near the zero point. The size and direction of the cavity length that needs to be changed are determined based on the size of the error signal. The servo controller 801 actively controls the driving voltage of the piezoelectric ceramic 501, thereby keeping the pump light in the cavity stable. The photodiode 601 receives the pump light intensity signal, converts it into an electrical signal, and inputs it into the signal input port of the phase-locked amplifier 701; the phase-locked amplifier 701 separates and amplifies a specific carrier frequency signal (generally a high-frequency modulated signal) from an extremely noisy environment (the signal-to-noise ratio can be as low as -60dB or even lower). The phase-locked amplifier 701 demodulates the signal received from the photodiode 601 to obtain an error signal, and outputs the error signal from the output port to the input port A of the servo controller 801; the phase-locked amplifier 701 provides a 500Hz, 0.15V sine wave; the servo controller 801 adopts a control system that controls the proportional, integral, and differential (PID control) of the error generated by comparing the information collected from the real-time data of the controlled object with the given value. By analyzing the obtained error signal, the required output control signal is calculated, thereby actively controlling the target parameter to be stable; the model of this control system is New Focus LB1005; servo controller 801 adjusts the output signal based on the error signal to stabilize the error signal and outputs the signal to the input port of high-voltage amplifier 901. High-voltage amplifier 901 amplifies the input voltage signal and outputs it, while also setting the bias voltage. High-voltage amplifier 901 (Pintech HA-400) amplifies the output signal of servo controller 801 and inputs it to piezoelectric ceramic 501, maintaining the maximum power in the cavity for maximum conversion efficiency.

[0065] Example 2

[0066] The pump-enhanced optical parametric oscillator according to embodiment 1 is different in that:

[0067] The PPLN crystal 301 is placed in a temperature-controlled oven 401 . The temperature accuracy of the temperature-controlled oven 401 is 0.01° C. The signal light and the idler light can be tuned by adjusting the temperature.

[0068] The pump light wavelength is 1-1.1μm, the generated signal light wavelength is 1.3-1.7μm, and the corresponding idler light wavelength is 3-5μm.

[0069] The first and second incident lenses 101, 102 are coated with pump light anti-reflection coatings on both sides, with a focal length of 50-300mm for the pump light. The fiber laser output port is fixed in front of the first incident lens 101, so that the pump light passes through the first and second incident lenses 101, 102, and then enters the first cavity mirror 201. After passing through the first cavity mirror 201, it converges at the center of the PPLN crystal 301, with a beam waist radius of 36μm.

[0070] The first cavity mirror 201 is a concave mirror with a spherical radius of 50-200mm. The outer side is coated with a high-transmittance film for pump light and idler light, and the inner side is coated with a film with a reflectivity R of 40-90% and a transmittance for pump light, high reflectivity for signal light, and high transmittance for idler light. The reflectivity for pump light is determined by the required maximum efficiency output power and high-efficiency output range as well as the loss in the cavity. The second cavity mirror 202 is a concave mirror with a spherical radius of 50-200mm. The outer side is coated with a film with high transmittance for idler light and high reflectivity for pump light and signal light. The third cavity mirror 203 is a plane mirror, the inner side is coated with a film with high reflectivity for pump light and signal light. The fourth cavity mirror 204 is a plane mirror, the inner side is coated with a film with high reflectivity for pump light and a reflectivity R of 90-99.8% for signal light.

[0071] The PPLN crystal 301 is a rectangular parallelepiped measuring (20-100) mm by (1-5) mm by (0.5-5) mm. Its two end faces (2 mm by 1 mm) are coated with a coating that provides high transmittance for pump light, signal light, and idler light. The polarization period is 26-35 μm. The polarization period is determined by the wavelength of the pump light used and the wavelength of the idler light to be generated. The crystal is positioned at a specific angle of incidence with respect to the pump light, which is determined by parameters such as the crystal size and the polarization period.

[0072] Example 3

[0073] The pump-enhanced optical parametric oscillator according to embodiment 2 is different in that:

[0074] The pump light is a fiber laser with a wavelength of 1.06μm and an output power greater than 15W; the generated signal light has a wavelength of 1.5μm, and the corresponding idler light has a wavelength of 3.8μm.

[0075] The focal length of the pump light is 100 mm. The fiber laser output port is fixed in front of the first incident lens 101, so that the pump light passes through the first incident lens 101 and the second incident lens 102 and is incident on the first cavity mirror 201. After passing through the first cavity mirror 201, it converges at the center of the PPLN crystal 301, with a beam waist radius of 36 μm.

[0076] The first cavity mirror 201 is a concave mirror with a spherical radius of 100 mm. Its outer surface is coated with a high-transmittance coating for pump and idler light, and its inner surface is coated with a coating with a reflectivity R of 60% for pump light, high reflectivity for signal light, and high transmittance for idler light. The first cavity mirror 201 is positioned at a 5° angle to the light beam. The second cavity mirror 202 is a concave mirror with a spherical radius of 100 mm. It is positioned at a 5° angle to the light beam and 52 mm from the end face of the PPLN crystal 301. The third cavity mirror 203 is positioned at a 5° angle to the light beam. The distance between the third cavity mirror 203 and the second cavity mirror 202 is 143 mm, with the center of the cavity mirror coinciding with the center of the light beam. The fourth cavity mirror 204 is a plane mirror, its inner surface coated with a coating with a high reflectivity R of 98.5% for signal light. The fourth cavity mirror 204 is positioned at a 5° angle to the light beam and 128 mm from the third cavity mirror 203. The fourth cavity mirror 204 reflects the light beam to the first cavity mirror 201 and overlaps with the incident light.

[0077] The PPLN crystal 301 is a rectangular parallelepiped with dimensions of 50 mm*3 mm*1 mm and a polarization period of 29.5 μm. The distance between the end face of the PPLN crystal 301 and the inner surface of the first cavity mirror 201 is 52 mm. The incident angle of the light on the crystal is 2.5°.

[0078] The control module can stabilize the pump light at the peak of coherent enhancement, so that the optical parametric oscillator module can output stably and efficiently. Figure 2 Schematic diagram of the relationship between idler optical power and pump optical power; Figure 2 In the figure, square dots represent vertical incidence, round dots represent oblique incidence, and triangles represent the efficiency of oblique incidence, corresponding to the values ​​on the right ordinate. Figure 3 Schematic diagram of the idler optical power stability test when the light is incident vertically. Figure 3 The idler power averaged 1.3W over a one-hour period, with a standard deviation of 1.9%. The corresponding pump power averaged 12.2W, with a mean square error of 2.2%. The idler remained stable even with fluctuating pump power, with a smaller relative standard deviation than the pump power, demonstrating the excellent stability achieved by low-frequency modulation and demodulation technology. Figure 4 This is a test chart of the maximum idler light power stability when the pump light is incident on the crystal at an angle. In the figure, the average idler light power is 1.8W, with a standard deviation of 1.9%. The pump light power is 13W, with a standard deviation of 1.2%.

[0079] Example 4

[0080] The active stabilization method of the pump-enhanced optical parametric oscillator described in any one of embodiments 1-3 comprises the following steps:

[0081] 1) The lock-in amplifier 701 outputs a 500 Hz, 0.15 V sinusoidal signal to the servo controller 801 as a scanning signal. After being amplified by the high-voltage amplifier 901, it is transmitted to the piezoelectric ceramic 501 to modulate the cavity length. The photodetector after the third cavity mirror 203 receives the pump light and obtains the cavity mode signal of the pump light.

[0082] 2) Adjust the amplitude of the scanning signal of the servo controller 801 so that the amplitude of the scanning signal is greater than 5V (the larger the better), and adjust the first cavity mirror 201, the second cavity mirror 202, the third cavity mirror 203, and the fourth cavity mirror 204 to obtain a sharp and clear cavity mode signal;

[0083] 3) The cavity mode signal is processed by the lock-in amplifier 701 and mixed with the sinusoidal signal output by the lock-in amplifier 701 to obtain an error signal, which is then transmitted to the servo controller 801. The phase difference between the sinusoidal signal and the cavity mode signal, as well as the offset of the error signal and the output offset of the high-voltage amplifier 901 are adjusted so that the error signal corresponds to the position of the maximum value of the cavity mode signal, passes through the zero point, and has a large slope near the zero point.

[0084] 4) Turn on the LFGL mode of the servo controller 801 to stabilize the cavity mode signal and the error signal, then reduce the amplitude of the scanning signal, and adjust the scanning signal bias and the error signal bias at the same time, so that the cavity mode signal gradually becomes flat and at the maximum level, and the error signal fluctuates around 0;

[0085] 5) Turn on the Lock On mode of the servo controller 801 to completely lock the optical parametric oscillator and stabilize the idler optical power.

[0086] The overall technical principles of the present invention are as follows:

[0087] The present invention employs a pump light wavelength of 1.06 μm. Second-order nonlinear optical interaction occurs within the PPLN crystal 301, typically generating longer-wavelength signal light and idler light. When phase matching conditions are met, the signal light wavelength is 1.5 μm and the idler wavelength is 3.8 μm. To lower the threshold, the signal light oscillation within the cavity is enhanced, reducing the output. As the pump light power increases, the signal light oscillation power within the cavity rapidly increases, resulting in a reverse conversion phenomenon that suppresses the generation of idler light. Even so, the threshold of the optical parametric oscillator remains high. Pump enhancement technology, however, can coherently enhance the pump light within the cavity, increasing the pump power within the cavity to several times that of the pump light outside the cavity. Furthermore, when the pump light power is slightly above the threshold, the power of the pump light entering the cavity increases due to a sudden increase in cavity losses, significantly improving the conversion efficiency of the pump light within the cavity. The first cavity mirror 201 of the present invention is coated with a film with a reflectivity of 60% for pump light, which can reduce the threshold of the optical parametric oscillator by 8 times while ensuring efficient conversion at low pump power. The optical parametric oscillator designed by the present invention is a four-mirror butterfly cavity, which includes four cavity mirrors. Each cavity mirror can control two-dimensional rotation to adjust the direction of the reflected light beam. After passing through the first cavity mirror 201, the pump light converges at the center of the PPLN crystal 301 with a beam waist radius of 36μm. Combined with pump enhancement technology, the sufficiently small beam waist radius is to make the threshold of the pump light sufficiently low while the conversion efficiency is sufficiently high. The incident angle of the light and the crystal is 2.5°. The results show that about 3W of pump light can output about 0.5W of idler light, and the quantum conversion efficiency is greater than 61%. By adjusting the orientation and position of the four cavity mirrors, the pump light, after passing through them sequentially, is refocused onto the center of the PPLN crystal 301. The first cavity mirror 201 and the second cavity mirror 202 are concave mirrors, providing a converging effect. After the pump light circulates once within the cavity, its waist radius and position remain the same as at the initial stage: the waist radius remains at 36 μm and the position remains at the center of the crystal, achieving self-regeneration. A temperature-controlled oven 401 controls the temperature of the PPLN crystal 301. This temperature affects the polarization period and, consequently, the wavelengths of the generated signal and idler light. Stabilizing the temperature of the PPLN crystal 301 ensures output wavelength stability. Furthermore, the oven 401 can tune the idler wavelength by varying its temperature. The temperature control accuracy of temperature-controlled furnace 401 is 0.01°C. When tuned within the range of 20 to 70°C, the idler wavelength can be tuned from 3765 to 3830nm. When the cavity mode is unlocked, the idler tuning coefficient is -1.2nm / °C, while when the cavity mode is locked, the idler tuning coefficient is -5.8nm / °C. Fine wavelength tuning can be achieved by first adjusting the temperature to near the desired wavelength without locking the cavity mode, and then finely adjusting the temperature after locking to obtain the desired wavelength.

[0088] The low-frequency modulation and demodulation technology employed in this invention utilizes a low-frequency kHz-range sinusoidal signal to drive the piezoelectric ceramic 501, modulating the optical parametric oscillator cavity length. The photodiode 601 receives the weak pump light intensity signal transmitted from the third cavity mirror 203 and converts it into a voltage signal proportional to the intracavity pump light oscillation power, i.e., the cavity mode signal. The cavity mode signal is input into a lock-in amplifier 701 and mixed with a reference signal provided by the amplifier to demodulate an error signal. The error signal is an odd function with zero crossings, where the zero point corresponds to the maximum value of the cavity mode signal. The error signal is received by a servo controller 801. The servo controller 801 adjusts the bias voltage of the output signal based on the difference between the error signal and the zero point. The signal is then amplified by a high-voltage amplifier 901 and then drives the piezoelectric ceramic 501. This ensures that the cavity mode signal is always at its maximum value, i.e., the pump light power oscillating within the cavity is maximized, thus achieving efficient nonlinear conversion. Compared to the commonly used PDH control technology, the low-frequency modulation and demodulation method can generate a clearer error signal at high power by increasing the amplitude of the sweep signal. This allows for active control of the stable power of the intracavity oscillating pump light, achieving stable idler light output over a wide range. Furthermore, the error signal is derived from the optical intensity signal obtained by scanning the cavity length with the sweep signal, which is equivalent to directly modulating the optical field intensity of the intracavity oscillating pump light, thus eliminating the need for an electro-optical modulator. Furthermore, since no electro-optical modulator is required, and since the interference from the pump-enhanced optical parametric oscillator is generally low-frequency, a kHz-level sweep signal can be used, reducing the performance requirements for each component and facilitating miniaturization and integration.

[0089] The optical parametric oscillator can output idler light of up to 1.8W at a pump power of 13W, and has a quantum conversion efficiency of up to 61% at a pump power of 3W.

[0090] Example 5

[0091] The application of the pump-enhanced optical parametric oscillator described in any one of Examples 1-3 includes application in remote spectral detection.

[0092] The pump-enhanced optical parametric oscillator can output up to 1.8W of idler light at a wavelength of 3.8μm, with a signal light width of 345kHz (measured by the fiber heterodyne method when the idler light output power is 1.3W). The calculated idler light width is less than 360kHz.

[0093] In terms of remote spectral detection, the narrow-linewidth single-frequency mid-infrared pump-enhanced optical parametric oscillator mentioned above has a stronger echo signal intensity, higher resolution, and very low pump light power requirement. It can be miniaturized and integrated to adapt to more demand scenarios.

[0094] Example 6

[0095] The application of the pump-enhanced optical parametric oscillator described in any one of Examples 1-3 includes application in laser radar.

[0096] In terms of lidar, mid-infrared lasers can significantly increase the detection range of differential absorption radar, while also combining miniaturization and integration to improve the performance and practicality of mid-infrared lidar.

[0097] Example 7

[0098] The application of the pump-enhanced optical parametric oscillator described in any one of Examples 1-3 includes application in mid-infrared optical frequency comb.

[0099] In terms of optical frequency combs, narrow-linewidth, high-energy mid-infrared lasers can use the whispering gallery effect to excite mid-infrared optical frequency combs in microcavities made of special materials, such as magnesium fluoride microcavities. Moreover, the pump-enhanced optical parametric oscillator (PAO) meets the wavelength tunability required for optical frequency combs, making it an excellent pump source for generating high-quality mid-infrared optical frequency combs.

Claims

1. A pump-enhanced optical parametric oscillator, characterized in that: It includes an optical parametric oscillator module and a control module; The optical parametric oscillator module includes a first incident lens, a second incident lens, a first cavity mirror, a second cavity mirror, a third cavity mirror, a fourth cavity mirror, a PPLN crystal, and a temperature-controlled furnace; After passing through the first incident lens, the second incident lens, the first cavity mirror, the second cavity mirror, the third cavity mirror, and the fourth cavity mirror, the pump light converges to the center of the PPLN crystal, is reflected from the second cavity mirror to the third cavity mirror, and then is reflected from the fourth cavity mirror back to the first cavity mirror, and then circulates in the cavity to achieve self-reproduction. The control module includes piezoelectric ceramics, a photodiode, a lock-in amplifier, a servo controller, and a high-voltage amplifier; The photodiode is placed behind the third cavity mirror and connected to the input port of the lock-in amplifier. The output port of the lock-in amplifier is connected to the input port A of the servo controller. The reference signal output port of the lock-in amplifier is connected to the scanning signal input port of the servo controller. The output port of the servo controller is connected to the input port of the high-voltage amplifier. The output port of the high-voltage amplifier is connected to the piezoelectric ceramic. The reference signal generated by a lock-in amplifier is used to directly modulate the piezoelectric ceramic drive voltage, that is, to modulate the cavity length of the optical parametric oscillator module. The cavity mode signal is then received by a photodiode, mixed with the reference signal through a lock-in amplifier, and low-pass filtered to demodulate an error signal that is a derivative of the cavity mode signal. The zero point of the error signal corresponds to the maximum point of the cavity mode signal, and the error signal is an odd function near the zero point. The size of the error signal determines the size and direction of the cavity length change that needs to be determined. The servo controller actively controls the piezoelectric ceramic drive voltage according to the PID algorithm, so that the oscillating pump light in the cavity always remains at the maximum power.

2. The pump-enhanced optical parametric oscillator according to claim 1, characterized in that: The photodiode receives the pump light intensity signal, converts it into an electrical signal, and inputs it into the signal input port of the lock-in amplifier; The lock-in amplifier separates a specific carrier frequency signal from an interference environment and amplifies the signal. The lock-in amplifier demodulates the signal received from the photodiode to obtain an error signal, and outputs the error signal from an output port to an input port A of the servo controller. The servo controller uses a control system that performs control based on the proportion, integration, and differentiation of the error generated by comparing the information collected from the real-time data of the controlled object with the given value. By analyzing the obtained error signal, the required output control signal is calculated, thereby actively controlling the target parameter to be stable. The high-voltage amplifier amplifies the input voltage signal and outputs it, while setting a bias voltage.

3. The pump-enhanced optical parametric oscillator according to claim 1, characterized in that: The PPLN crystal is placed in a temperature-controlled furnace, which tunes the signal and idler light by adjusting the temperature.

4. The pump-enhanced optical parametric oscillator according to claim 1, characterized in that: The pump light wavelength is 1-1.1μm, the generated signal light wavelength is 1.3-1.7μm, and the corresponding idler light wavelength is 3-5μm; Further preferably, the wavelength of the pump light is 1.06 μm, the wavelength of the generated signal light is 1.5 μm, and the corresponding idler light wavelength is 3.8 μm.

5. The pump-enhanced optical parametric oscillator according to claim 1, characterized in that: Both sides of the first incident lens and the second incident lens are coated with pump light anti-reflection film, and the focal length of the pump light is 50-300mm; Further preferably, the focal length of the pump light is 100 mm.

6. The pump-enhanced optical parametric oscillator according to claim 1, characterized in that: The first cavity mirror is a concave mirror with a spherical radius of 50-200 mm, the outer side of which is coated with a high-transmittance film for pump light and idler light, and the inner side of which is coated with a film with a reflectivity R of 40-90% for pump light, high reflectivity for signal light, and high transmittance for idler light; The second cavity mirror is a concave mirror with a spherical radius of 50-200 mm. The outer side is coated with a high-transmittance film for pump light and idler light, and the inner side is coated with a film layer with reflectivity and transmittance for pump light, high reflectivity for signal light, and high transmittance for idler light. The third cavity mirror is a plane mirror, the inner side of which is coated with a film layer that is highly reflective to pump light and signal light; The fourth cavity mirror is a plane mirror, the inner side of which is coated with a film layer with high reflectivity for pump light and a reflectivity R of 90-99.8% for signal light; Further preferably, the first cavity mirror is a concave mirror with a spherical radius of 100 mm, the outer side of which is coated with a high-transmittance film for pump light and idler light, and the inner side of which is coated with a film layer with a reflectivity R of 60% for pump light, high reflectivity for signal light, and high transmittance for idler light; The second cavity mirror is a concave mirror with a spherical radius of 100 mm; The fourth cavity mirror is a plane mirror, the inner side of which is coated with a film layer that is highly reflective to pump light and has a reflectivity R of 98.5% to signal light.

7. The pump-enhanced optical parametric oscillator according to claim 1, characterized in that: The PPLN crystal is a rectangular parallelepiped with a size of (20-100) mm*(1-5) mm*(0.5-5) mm. Both end faces are coated with a film with high transmittance for pump light, signal light, and idler light. The polarization period is 26-35 μm. Further preferably, the PPLN crystal is a cuboid with a size of 50 mm*3 mm*1 mm, a polarization period of 29.5 μm, and an incident angle of light on the PPLN crystal of 2.5°.

8. The active stabilization method of a pump-enhanced optical parametric oscillator according to any one of claims 1 to 7, characterized in that: The steps are as follows: 1) The lock-in amplifier outputs a sinusoidal signal to the servo controller as a scanning signal, which is amplified by the high-voltage amplifier and then transmitted to the piezoelectric ceramic to modulate the cavity length; 2) adjusting the amplitude of the scanning signal of the servo controller, adjusting the first cavity mirror, the second cavity mirror, the third cavity mirror, and the fourth cavity mirror to obtain a cavity mode signal; 3) The cavity mode signal is processed by a lock-in amplifier and mixed with the sinusoidal signal output by the lock-in amplifier to obtain an error signal, which is then transmitted to the servo controller. The phase difference between the sinusoidal signal and the cavity mode signal, as well as the offset of the error signal and the output offset of the high-voltage amplifier are adjusted so that the error signal corresponds to the position of the maximum value of the cavity mode signal, passes through zero, and has a large slope near the zero point. 4) Turn on the LFGL mode of the servo controller to stabilize the cavity mode signal and error signal, then reduce the amplitude of the scan signal. At the same time, adjust the scan signal bias and error signal bias so that the cavity mode signal gradually flattens and is at the maximum level, and the error signal fluctuates around 0. 5) Turn on the Lock On mode of the servo controller to completely lock the optical parametric oscillator and stabilize the idler optical power.

9. Application of the pump-enhanced optical parametric oscillator according to any one of claims 1 to 7, including application in remote spectral detection.

10. Applications of the pump-enhanced optical parametric oscillator according to any one of claims 1 to 7, including applications in laser radar and mid-infrared optical frequency comb.

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