Pump enhanced optical parametric oscillator based on monolithic cavity PPLN crystal, vortex light generation method and application

Abstract

The invention relates to a pump enhanced optical parametric oscillator based on a monolithic cavity PPLN crystal, a vortex light generation method and application. The pump enhanced optical parametric oscillator comprises a pump laser, a matching lens, a half-wave plate, a polarization beam splitter, a quarter-wave plate, a radial polarization converter, the PPLN crystal, a temperature control furnace, a multi-dimensional adjusting frame and a spectroscope. The two end faces of the PPLN crystal are spherical surfaces, the end faces are coated with film layers, and the PPLN crystal serves as an optical parametric oscillation cavity, so that a complex cavity mirror system is omitted; the end face of the PPLN crystal is coated with a film, so that the effect of pumping enhancement is achieved, the threshold value is reduced, the conversion efficiency is improved, meanwhile, the wavelength tuning characteristic is considered, the thermal self-locking effect and the thermal lens effect are utilized, long-time wavelength stability and power stability are achieved, and a complex stable cavity system is omitted; when pump light is converted into vortex light through the S wave plate, orbital angular momentum can be selectively transmitted to idler frequency light or signal light, intermediate infrared or near-infrared vortex light output is achieved, and a miniaturized and portable vortex light generator and multi-field application can be achieved.

Description

Technical Field

[0001] This invention relates to a pump-enhanced optical parametric oscillator based on a monolithic cavity PPLN crystal and a method for generating vortex light, belonging to the technical field of continuous optical parametric oscillators. Background Technology

[0002] Mid-infrared lasers (wavelength range 3–5 μm) have become a key area of ​​scientific research due to their wide application potential in environmental monitoring, infrared remote sensing, lidar, and military countermeasures. In particular, continuous-wave (CW) lasers, with their wide tuning range and narrow linewidth, have further expanded the application value of mid-infrared lasers in these fields. Furthermore, mid-infrared optical vortex light carrying orbital angular momentum (OAM) exhibits unique advantages in chiral molecule detection, optical tweezers manipulation, and super-resolution microscopy.

[0003] Optical parametric oscillators (OPOs) are one of the most effective methods for generating continuous-wave mid-infrared lasers due to their high output power, narrow linewidth, wide wavelength tuning range, and excellent beam quality. However, existing narrow-linewidth OPO systems are typically complex in structure and rely on frequency stabilization systems to achieve stable single-frequency output, limiting their practicality and integration. Monolithic cavity OPOs, obtained by directly combining an OPO with a crystal, can effectively solve these problems. However, there are currently no reports of monolithic cavity OPOs generating high-power mid-infrared lasers.

[0004] Mid-infrared vortex light OPO systems are more complex and cannot be miniaturized or integrated. Currently, there are no reports of single-chip cavity OPOs generating mid-infrared vortex light. Summary of the Invention

[0005] To address the gaps in existing technologies, this invention provides a pump-enhanced optical parametric oscillator (OPO) based on a monolithic cavity PPLN crystal and a method for generating vortex light. A monolithic cavity pump-enhanced OPO is designed and constructed to generate W-level single-frequency mid-infrared laser light and 0.6W mid-infrared vortex light, with significantly improved stability.

[0006] This invention can improve the accuracy of chiral molecule detection, or be used as a compact mid-infrared optical frequency comb pump source, and can also meet the needs of other complex scenarios.

[0007] The technical solution of the present invention is as follows:

[0008] A pump-enhanced optical parametric oscillator based on a monolithic cavity PPLN crystal is provided, which includes, in sequence along the optical path, a pump laser, a matching lens group, a first half-wave plate, a first polarizing beam splitter, a second polarizing beam splitter, a second half-wave plate, a first quarter-wave plate, a radial polarization converter (S-wave plate), a second quarter-wave plate, a third half-wave plate, a PPLN crystal, and a beam splitter. The PPLN crystal is located in a temperature-controlled furnace, which controls the temperature of the PPLN crystal to a set temperature. The temperature-controlled furnace is placed on a multi-dimensional adjustment frame to fix the position of the PPLN crystal.

[0009] The pump laser is used to output continuous single-frequency pump light, the half-wave plate is used to adjust the polarization state, and the polarization beam splitter is used to adjust the direction of the optical path. The two polarization beam splitters rotate the direction of the optical path by 180°. The PPLN crystal itself forms an optical parametric oscillator cavity. The pump light is coherently enhanced inside the crystal and undergoes a nonlinear effect with the crystal to generate signal light and idler light.

[0010] The pump light is calibrated by the first polarizing beam splitter, then the second polarizing beam splitter further calibrates its direction, the second half-wave plate adjusts its polarization state, the first quarter-wave plate adjusts it to circularly polarized light, the S-wave plate transforms it into vortex light, the second quarter-wave plate converts it into linearly polarized light, and the third half-wave plate adjusts it to the required polarization state of the PPLN crystal before injecting it into the PPLN crystal. The PPLN crystal has special coatings at both ends, forming an optical parametric oscillation cavity. The pump light undergoes coherent enhancement within the crystal and exhibits a nonlinear effect, generating signal and idler light, which are emitted from the rear surface of the PPLN crystal. Finally, a beam splitter separates the signal and idler light for output. A temperature-controlled furnace and a multi-dimensional adjustment frame are responsible for fixing the crystal and controlling its temperature to the set temperature. The multi-dimensional adjustment frame then adjusts the crystal's position to achieve optical path matching.

[0011] According to a preferred embodiment of the present invention, the pump laser outputs pump light of 5-100W, wavelength of 400-2000nm, and linewidth of 0.1kHz-1GHz.

[0012] Further preferably, the pump laser outputs 35W of pump light with a wavelength of 1064nm and a linewidth of 10kHz.

[0013] According to a preferred embodiment of the present invention, the matching lens group comprises one or more convex or concave lenses with a focal length of 10-1000mm, which match the pump light spot to the PPLN to achieve self-reproducing oscillation.

[0014] A further preferred embodiment includes two convex lenses with a focal length of 100mm, designated as a first matching lens and a second matching lens. The first matching lens is located between the pump laser and a first half-wave plate, while the second matching lens is located between the first half-wave plate and a first polarizing beam splitter. The pump light emitted by the pump laser is focused by the first matching lens, its polarization state is adjusted by the first half-wave plate, and the spot size is adjusted by the second matching lens.

[0015] Preferably, the surface of the radial polarization converter (S-wave plate) is coated with a film layer that is highly transparent to pump light, and the microstructure at its center can convert linearly polarized light into radially / angularly polarized light and circularly polarized light into one or more order vortex light.

[0016] According to a preferred embodiment of the present invention, the two ends of the PPLN crystal are polished into spherical surfaces, the incident end face is coated with a film layer that partially reflects pump light, is highly reflective of signal light, and is highly transparent of idler light, and the exit end face is coated with a film layer that is highly reflective of pump light, has a high reflectivity of signal light, and is highly transparent of idler light.

[0017] Further preferably, the PPLN crystal has a polarization period of 5-50 μm, a spherical radius of 10-1000 mm at both end faces, a crystal length of 5-100 mm, and an incident end face coated with a film having a reflectivity of 20-80% in the 0.8-1.2 μm band, high reflectivity in the 1.2-2.0 μm band, and high transmittance in the 2-5 μm band. The exit end face is coated with a film having high reflectivity in the 0.8-1.2 μm band, a reflectivity of 80%-99.9% in the 1.2-2.0 μm band, and high transmittance in the 2-5 μm band. The high reflectivity mentioned herein refers to a reflectivity of 97% or higher, and the high transmittance refers to a transmittance of 97% or higher.

[0018] More preferably, the PPLN crystal has a polarization period of 30.5 μm, a spherical radius of 50 mm at the end face, a crystal length of 50 mm, and an incident end face coated with a film having a reflectivity of 50% in the 1.06 μm band, high reflectivity in the 1.46-1.66 μm band, and high transmittance in the 2.9-3.6 μm band. The exit end face is coated with a film having high reflectivity in the 1.06 μm band, a reflectivity of 98.5% in the 1.46-1.66 μm band, and high transmittance in the 2.9-3.6 μm band.

[0019] According to a preferred embodiment of the present invention, a temperature-controlled furnace fixes the PPLN crystal, surrounds its four sides, and controls the temperature of the PPLN crystal. The temperature control accuracy is 0.0001-0.01℃, and the range is -100 to +300℃.

[0020] Further preferred, the temperature control furnace has a temperature control accuracy of 0.001℃ and a range of 0-+150℃.

[0021] According to a preferred embodiment of the present invention, the multi-dimensional adjustment frame includes 1-6 adjustment dimensions.

[0022] Further preferred, the multi-dimensional adjustment frame includes 5 adjustment dimensions.

[0023] According to a preferred embodiment of the present invention, the two surfaces of the beam splitter are coated with a film layer that is highly reflective to signal light and highly transparent to idler light.

[0024] A further preferred embodiment has a beam splitter coated with a film that is highly reflective in the 1.46-1.66 μm band and highly transparent in the 2.9-3.6 μm band on both surfaces.

[0025] A tuning method for generating vortex light using a pump-enhanced optical parametric oscillator based on a monolithic cavity PPLN crystal includes the following steps:

[0026] 1) Adjusting the crystal temperature changes the phase matching conditions in the crystal, thereby changing the wavelength of the output idler light;

[0027] 2) Adjust the pump laser wavelength to change the output idler wavelength.

[0028] The present invention relates to the application of a pump-enhanced optical parametric oscillator based on a monolithic cavity PPLN crystal and a method for generating vortex light, including its application in the detection of chiral molecules. Based on the difference in the interaction between chiral molecules and vortex light in different directions, the pump-enhanced optical parametric oscillator based on a monolithic cavity PPLN crystal is used as a mid-infrared tunable laser source for the detection of chiral molecules.

[0029] The present invention relates to the application of the pump-enhanced optical parametric oscillator and vortex light generation method based on a monolithic cavity PPLN crystal, including its application as a compact mid-infrared optical frequency comb pump source.

[0030] This invention utilizes a PPLN crystal end-face coating to achieve pump enhancement, lowering the threshold and improving conversion efficiency. The conversion efficiency reaches 67% at low power, with a maximum output power of 1.4W. Simultaneously, it considers wavelength tuning characteristics, with an idler tuning range of 3015-3200nm. Due to the OPO cavity's isolation from external ring mirrors, and by utilizing thermal locking and thermal lensing effects, this invention achieves long-term wavelength and power stability, eliminating the need for a complex cavity stabilization system. When using an S-wave plate to convert the pump light into vortex light, orbital angular momentum can be selectively transferred to the idler or signal light, achieving mid-infrared or near-infrared vortex light output. This invention enables a miniaturized, portable vortex light generator, applicable to fields such as chiral molecule detection and compact Kerr frequency combs.

[0031] The beneficial effects of this invention are as follows:

[0032] Compared with existing mid-infrared monolithic cavity OPOs, the significant advantages of this invention are:

[0033] 1. The PPLN crystal in this invention uses a film with a reflectivity R=50% at a wavelength of 1.06μm on the incident end face and a film with a high reflectivity at a wavelength of 1.06μm on the output end to form pump enhancement, better meet impedance matching, effectively reduce the threshold of OPO, and ensure that the single-cavity OPO can operate efficiently in a wider range of pump light power, ultimately achieving watt-level output in the mid-infrared band.

[0034] 2. In the design and construction of the OPO cavity in this invention, the thermal lensing effect is taken into consideration. Under the combined effect of thermal self-locking and thermal lensing, it has very high power stability and wavelength stability.

[0035] Compared with existing mid-infrared vortex lasers based on OPO, the significant advantages of this invention are:

[0036] 1. Using PPLN crystal as the optical parametric oscillator, the vortex light OPO is located inside the crystal, isolating it from external interference. At the same time, by utilizing its thermal self-locking property, the vortex light OPO can be kept from mode jumping for a long time, improving frequency stability and power stability, and eliminating the need for a cavity stabilization system, making the laser more compact.

[0037] 2. Placing the vortex optical OPO inside the PPLN crystal eliminates the need for the original OPO cavity mirror, effectively saving overall space and achieving miniaturization and portability;

[0038] 3. By utilizing the thermal lensing effect to adjust the mode matching between the pump light and the signal light in the OPO cavity, the transfer of orbital angular momentum to the signal light or idler light can be controlled. Attached Figure Description

[0039] Figure 1 This is a schematic diagram of the overall structure of the pump-enhanced optical parametric oscillator based on a monolithic cavity PPLN crystal according to the present invention.

[0040] Figure 2 A schematic diagram showing the variation of output idler optical power with pump optical power;

[0041] Figure 3 This is a schematic diagram illustrating the variation of quantum conversion efficiency with pump light power.

[0042] Figure 4 This is a schematic diagram showing the variation of signal optical power with pump optical power;

[0043] Figure 5 This is a schematic diagram of the signal light spectrum;

[0044] Figure 6 This is a schematic diagram of the idler frequency optical spectrum;

[0045] Figure 7 This is a schematic diagram of temperature tuning;

[0046] Figure 8 Schematic diagram for adjusting the temperature of the pump laser;

[0047] Figure 9 This is a schematic diagram of the signal light wavelength stability test;

[0048] Figure 10 This is a schematic diagram of the output power stability test.

[0049] Figure 11 This is a schematic diagram of idler beam quality measurement.

[0050] Figure 12 The diagram shows the beam and self-interference spot pattern in each state during vortex optical pumping.

[0051] Figure 13 This is a schematic diagram showing the variation of vortex optical power with pump optical power;

[0052] Figure 14 A schematic diagram for testing the beam quality of the output vortex idler light;

[0053] 101. Pump laser; 201. First matching lens; 202. Second matching lens; 301. First half-wave plate; 302. Second half-wave plate; 303. Third half-wave plate; 401. First polarizing beam splitter; 402. Second polarizing beam splitter; 501. First quarter-wave plate; 502. Second quarter-wave plate; 601. Radial polarization converter (S-wave plate); 701. PPLN crystal; 801. Temperature-controlled furnace and multi-dimensional adjustment frame; 901. Beam splitter. Detailed Implementation

[0054] The present invention will be further described below with reference to the embodiments and accompanying drawings, but is not limited thereto.

[0055] Example 1

[0056] A pump-enhanced optical parametric oscillator based on a monolithic cavity PPLN crystal, such as Figure 1 As shown, it includes 101, a pump laser; 201, a first matching lens; 301, a first half-wave plate; 202, a second matching lens; 401, a first polarizing beam splitter; 402, a second polarizing beam splitter; 302, a second half-wave plate; 501, a first quarter-wave plate; 601, a radial polarization converter (S-wave plate); 502, a second quarter-wave plate; 303, a third half-wave plate; 701, a PPLN crystal; 801, a temperature-controlled furnace and a multi-dimensional adjustment frame; and 901, a beam splitter.

[0057] The pump light emitted by the pump laser 101 passes through the first matching lens 201, its polarization state is adjusted by the first half-wave plate 301, and the spot size is adjusted by the second matching lens 202. The light path is then adjusted by the first polarizing beam splitter 401, followed by further adjustment by the second polarizing beam splitter 402. After the polarization state is adjusted by the second half-wave plate 302, the light is converted to circular polarization by the first quarter-wave plate 501. After passing through the S-wave plate 601, it becomes vortex light, and finally, it is converted to linear polarization by the second quarter-wave plate 502. The polarized light, adjusted to the required polarization state by the third half-wave plate 303, is injected into the PPLN crystal 701. It undergoes coherent enhancement within the crystal and exhibits a nonlinear effect, generating signal light and idler light. These are emitted from the rear surface of the PPLN crystal and finally separated and output by the beam splitter 901. The temperature-controlled furnace and multi-dimensional adjustment frame 801 serve to fix the crystal and control its temperature to the set temperature. The multi-dimensional adjustment frame then adjusts the crystal's position to achieve optical path matching.

[0058] The pump laser outputs 35W pump light with a wavelength of 1064nm and a linewidth of 10kHz. The matching lens assembly includes two convex lenses with a focal length of 100mm, designated as the first matching lens and the second matching lens. The first matching lens is located between the pump laser and the first half-wave plate, while the second matching lens is located between the first half-wave plate and the first polarizing beam splitter. The pump light emitted by the pump laser is focused by the first matching lens, its polarization state is adjusted by the first half-wave plate, and the spot size is adjusted by the second matching lens. This matches the pump light spot to the oscillating spot in the PPLN.

[0059] The S-wave plate 601 is coated with a 1.06μm high-transmittance film. The microstructure at its center can convert linearly polarized light into radially / angularly polarized light and circularly polarized light into first-order vortex light.

[0060] The PPLN crystal has spherical ends, a polarization period of 30.5 μm, a spherical radius of 50 mm, and a crystal length of 50 mm. The incident end face is coated with a film that provides 50% reflectivity at 1.06 μm, high reflectivity at 1.46-1.66 μm, and high transmittance at 2.9-3.6 μm. The exit end face is coated with a film that provides high reflectivity at 1.06 μm, 98.5% reflectivity at 1.46-1.66 μm, and high transmittance at 2.9-3.6 μm. High reflectivity is defined as 99.99% or higher, and high transmittance is defined as 99.99% or higher.

[0061] To achieve miniaturization and high stability, this invention uses the front and rear facets of the crystal as resonant cavity mirrors, making the PPLN crystal the cavity of the OPO. This eliminates the need for mirrors, saving space, while simultaneously isolating external environmental interference and improving the stability of the OPO. It also eliminates the need for a complex cavity stabilization system, thus achieving miniaturization. Furthermore, the front facet of the PPLN crystal is coated with a pump semi-transparent film, and the rear surface is coated with a highly reflective film for the pump light. This achieves pump enhancement that satisfies impedance matching, lowers the threshold, and improves conversion efficiency at low power, thereby further reducing the required pump power while ensuring output at high power, further achieving miniaturization. The stability of the OPO is significantly improved by relying on thermal self-locking and thermal lensing effects.

[0062] The tuning of OPO mainly relies on the temperature change of PPLN crystal and the change of pump light wavelength. In this invention, the tuning range of signal light wavelength is 1595-1645nm, and the tuning range of idler light is 3015-3200nm.

[0063] When the pump light is a vortex light, this OPO can transfer the orbital angular momentum of the pump light to the idler or signal light, achieving for the first time the generation of mid-infrared vortex light by a single-chip cavity OPO. Simultaneously, the transfer of orbital angular momentum to the signal or idler light can be controlled using the thermal lensing effect without replacing the cavity mirror.

[0064] Figure 2 This diagram illustrates the variation of output idler optical power with pump optical power. The horizontal axis represents the input pump power, and the vertical axis represents the output idler optical power. The threshold of a single-chip cavity OPO is approximately 2W. As the pump optical power increases, the idler optical power continuously increases, eventually reaching a maximum output power of 1.4W.

[0065] Figure 3 This diagram illustrates the quantum conversion efficiency as a function of pump power. The horizontal axis represents the input pump power, and the vertical axis represents the quantum conversion efficiency. The diagram shows that at a pump power of 6.6 W, the output power is 741 mW, with a quantum efficiency of 35%, representing the highest quantum efficiency corresponding to single-ended output power. However, since the monolithic cavity OPO is a standing-wave cavity, nonlinear effects occur in both directions, resulting in idler light generation in both directions. Measurements show that the reverse output power is 717 mW, the total output power is 1.458 W, and the quantum efficiency is 67%.

[0066] Figure 4 This diagram illustrates the variation of signal light power with pump light power. The horizontal axis represents the input pump power, and the vertical axis represents the output signal light power. As can be seen, along with the idler light output, the OPO simultaneously outputs high-power signal light, with a maximum power of 2.7W. Combined with the idler light, this can meet the needs of many applications, such as dual-wavelength detection.

[0067] Figure 5This is a schematic diagram of the signal light spectrum at W-level output. The horizontal axis represents wavelength, and the vertical axis represents the signal light intensity at that wavelength, in dBm. The diagram shows that the signal light operates in single-mode with a signal-to-noise ratio of 50dB. Simultaneously, the signal beamwidth was measured to be 228kHz using the zero-beat heterodyne method.

[0068] Figure 6 This is a schematic diagram of the idler light spectrum when outputting at the W level. The horizontal axis represents wavelength, and the vertical axis represents the idler light intensity at that wavelength, after normalization.

[0069] Figure 7 This is a schematic diagram of temperature tuning. The horizontal axis represents temperature, the left vertical axis represents the wavelength of the signal light, and the right vertical axis represents the wavelength of the idler light. The dots represent the measured wavelengths, and the lines represent the wavelength curves obtained from simulations. When the PPLN crystal temperature changes from 120℃ to 30℃, the signal light wavelength tuning range is 1645nm to 1595nm, the idler light wavelength tuning range is 3015nm to 3200nm, and the tuning range is 185nm.

[0070] Figure 8 This diagram illustrates temperature tuning of the pump laser. The horizontal axis represents the set temperature of the seed laser in the pump laser, and the horizontal axis also represents the wavelength of the output idler light. It can be seen that as the pump laser temperature increases, the wavelength of the idler light decreases. Figure 7 Due to the thermal lock-in effect, decreasing the crystal temperature intensifies the thermal effects within the crystal, making the OPO cavity resonance more stable and increasing the output idler light power, but also increasing the idler light wavelength. Conversely, increasing the pump laser temperature also induces the thermal lock-in effect, increasing the output power but decreasing the idler light wavelength. Therefore, efficient OPO output and the desired idler light wavelength can be achieved by adjusting the crystal and pump laser temperatures.

[0071] Figure 9 This is a schematic diagram of the signal light wavelength stability test. The horizontal axis represents the test time, and the vertical axis represents the output signal light wavelength. No mode hopping was observed in the signal light within 0.5 hours, mainly due to the high-precision temperature control of the crystal temperature control furnace and the thermal self-locking effect, which improved the wavelength stability.

[0072] Figure 10 This is a schematic diagram of the output power stability test. The horizontal axis represents the test time, and the vertical axis represents the output idle frequency optical power. The average power over one hour is 0.9W with a standard deviation of 0.74%, while the corresponding standard deviation of the pump light is 1.4%. This is mainly due to the high-precision temperature control of the crystal temperature control furnace and the thermal self-locking effect, which improves the stability of the output power and reduces the impact of pump light power fluctuations.

[0073] Figure 11This is a schematic diagram illustrating the measurement of idle frequency beam quality during Gaussian optical pumping. When operating at OPO W-level power, the beam quality factor M was measured. 2 The two directions are 1.35 and 1.34 respectively.

[0074] Figure 12 Figures show the beam patterns and self-interference spots of the vortex-pumped light at various states. The intersections of the interference spots are circled. Figure a shows the vortex-pumped light spot, exhibiting a typical "donut" structure. Figure b shows the self-interference spot of the vortex-pumped light, showing intersecting interference fringes, confirming it is a vortex light. Figure c shows the output vortex idler light spot, again exhibiting a typical "donut" structure. Figure d shows the self-interference spot of the vortex idler light, again showing intersecting interference fringes, confirming it is a vortex light. Figure e shows the Gaussian signal light spot, a typical Gaussian shape. Figure f shows the self-interference spot of the Gaussian signal light, showing no intersecting interference fringes, confirming it is Gaussian light. Figure g shows the Gaussian idler light spot, a typical Gaussian shape. Figure h shows the self-interference spot of the Gaussian idler light, showing no intersecting interference fringes, confirming it is Gaussian light. Figure i shows the output vortex signal light spot, where a typical "donut" structure can be observed. Figure j shows the self-interference spot of the vortex signal light, where intersecting interference fringes can be observed, proving that it is vortex light. With a fixed pump power, the output optical power can be adjusted by regulating the crystal temperature and pump light wavelength using the thermal locking effect. When the output optical power is low, the orbital angular momentum of the pump light is transferred to the idler light, resulting in the output vortex idler light and Gaussian signal light, as shown in figures c, d, e, and f. When the output optical power is increased, due to the thermal lensing effect, the pump light has a better self-reproducible state in the cavity and a higher overlap with the signal light mode. Therefore, the orbital angular momentum is transferred to the signal light, while the idler light becomes a Gaussian beam, as shown in figures g, h, i, and j.

[0075] Figure 13 This diagram illustrates the variation of vortex idler power with pump power. The squares represent the maximum vortex idler power under different pump power levels, and the dots represent the maximum vortex signal power under different pump power levels. By adjusting the thermal lensing effect, controllable transfer of orbital angular momentum can be achieved. The OPO can output >600mW of vortex idler power or >1.8W of vortex signal power.

[0076] Figure 14 This diagram illustrates the beam quality test for output vortex idler light. Vortex light differs from fundamental mode light in that its ideal beam quality factor M... 2 The order is +1. When the single-chip cavity OPO of this invention outputs the maximum vortex idler optical power, its beam quality factor M is measured. 2 The x and y axes are 2.46 and 2.61 respectively, indicating good beam quality.

[0077] Example 2

[0078] A pump-enhanced optical parametric oscillator based on a monolithic cavity PPLN crystal is disclosed, with the structure described in Example 1. The difference is that the pump laser 101 outputs continuous single-frequency pump light; the pump laser 101 outputs 5W pump light with a wavelength of 400nm and a linewidth of 0.1kHz. The PPLN crystal has a polarization period of 5μm, spherical radii of 10mm at both ends, and a crystal length of 5mm. The incident end face is coated with a film having a reflectivity of 20% in the 0.8μm band, high reflectivity in the 1.2μm band, and high transmittance in the 2μm band. The output end face is coated with a film having high reflectivity in the 0.8μm band, a reflectivity of 80% in the 1.2μm band, and high transmittance in the 2μm band.

[0079] Example 3

[0080] A pump-enhanced optical parametric oscillator based on a monolithic cavity PPLN crystal is disclosed, with the structure described in Example 1. The difference is that the pump laser 101 outputs continuous single-frequency pump light; the pump laser 101 outputs 100W pump light with a wavelength of 2000nm and a linewidth of 1GHz. The PPLN crystal has a polarization period of 50μm, spherical radii of 1000mm at both ends, and a crystal length of 100mm. The incident end face is coated with a film that has 80% reflectivity at 1.2μm, high reflectivity at 2.0μm, and high transmittance at 5μm. The output end face is coated with a film that has high reflectivity at 1.2μm, 99.9% reflectivity at 2.0μm, and high transmittance at 5μm.

[0081] Example 4

[0082] A tuning method for generating vortex light using any of the pump-enhanced optical parametric oscillators based on monolithic cavity PPLN crystals in Examples 1-3 includes the following steps:

[0083] 1) Adjusting the crystal temperature changes the phase matching conditions in the crystal, thereby changing the wavelength of the output idler light;

[0084] 2) Adjust the pump laser wavelength to change the output idler wavelength.

[0085] Example 5

[0086] The application of the pump-enhanced optical parametric oscillator and vortex light generation method based on a monolithic cavity PPLN crystal as described in any of Examples 1-3 includes applications in chiral molecule detection.

[0087] Mid-infrared vortex light contains the phase of the vortex and has chirality. When vortex light with different chirities interacts with chiral molecules, the effect varies greatly. Chiral molecules often contain chemical bonds such as OH / NH / CH, so they have strong absorption of light at 3-5 μm. Therefore, they are very sensitive in detecting and distinguishing molecules and their chirality.

[0088] Example 6

[0089] The application of the pump-enhanced optical parametric oscillator and vortex light generation method based on a monolithic cavity PPLN crystal described in any of Examples 1-3 includes its use as a pump source for a compact Kerr frequency comb.

[0090] Mid-infrared Kerr frequency combs generally require watt-level mid-infrared single-frequency pump light, while portable PPLN-based monolithic pump-enhanced optical parametric oscillators well meet the requirements. At the same time, since Kerr frequency combs have fewer components, they are suitable for miniaturization. Combined with this invention, a portable Kerr frequency comb source can be realized, which greatly improves spectral detection efficiency and enriches application scenarios.

Claims

1. A pump-enhanced optical parametric oscillator based on a monolithic cavity PPLN crystal, characterized in that, The following components are arranged sequentially along the optical path: a pump laser, a matching lens group, a first half-wave plate, a first polarizing beam splitter, a second polarizing beam splitter, a second half-wave plate, a first quarter-wave plate, a radial polarization converter, a second quarter-wave plate, a third half-wave plate, a PPLN crystal, and a beam splitter. The PPLN crystal is located inside a temperature-controlled furnace, which controls the temperature of the PPLN crystal to a set temperature. The temperature-controlled furnace is placed on a multi-dimensional adjustment frame to fix the position of the PPLN crystal. The pump laser is used to output continuous single-frequency pump light, the half-wave plate is used to adjust the polarization state, and the polarization beam splitter is used to adjust the direction of the optical path. The two polarization beam splitters rotate the direction of the optical path by 180°. The PPLN crystal itself forms an optical parametric oscillator cavity. The pump light is coherently enhanced inside the crystal and undergoes a nonlinear effect with the crystal to generate signal light and idler light.

2. The pump-enhanced optical parametric oscillator based on a monolithic cavity PPLN crystal according to claim 1, characterized in that, The pump laser outputs 5-100W of pump light with a wavelength of 400-2000nm and a linewidth of 0.1kHz-1GHz; Further preferably, the pump laser outputs 35W of pump light with a wavelength of 1064nm and a linewidth of 10kHz.

3. The pump-enhanced optical parametric oscillator based on a monolithic cavity PPLN crystal according to claim 1, characterized in that, The matching lens group includes one or more convex or concave lenses with a focal length of 10-1000mm; more preferably, the matching lens group includes two convex lenses with a focal length of 100mm, namely a first matching lens and a second matching lens, the first matching lens being located between the pump laser and the first half-wave plate, and the second matching lens being located between the first half-wave plate and the first polarizing beam splitter.

4. The pump-enhanced optical parametric oscillator based on a monolithic cavity PPLN crystal according to claim 1, characterized in that, The radial polarization converter is coated with a film that is highly transparent to pump light, which converts linearly polarized light into radially / angularly polarized light and circularly polarized light into first- or multi-order vortex light.

5. The pump-enhanced optical parametric oscillator based on a monolithic cavity PPLN crystal according to claim 1, characterized in that, The PPLN crystal has spherical ends. The incident end is coated with a film that partially reflects the pump light, is highly reflective of the signal light, and is highly transparent of the idler light. The exit end is coated with a film that is highly reflective of the pump light, has a high reflectivity of the signal light, and is highly transparent of the idler light. Preferably, the PPLN crystal has a polarization period of 5-50 μm, a spherical radius of 10-1000 mm at both end faces, a crystal length of 5-100 mm, and an incident end face coated with a film layer with a reflectivity of 20-80% in the 0.8-1.2 μm band, high reflectivity in the 1.2-2.0 μm band, and high transmittance in the 2-5 μm band. The exit end face is coated with a film layer with high reflectivity in the 0.8-1.2 μm band, a reflectivity of 80%-99.9% in the 1.2-2.0 μm band, and high transmittance in the 2-5 μm band. The high reflectivity is defined as a reflectivity of 97% or higher, and the high transmittance is defined as a transmittance of 97% or higher. More preferably, the PPLN crystal has a polarization period of 30.5 μm, a spherical radius of 50 mm at the end face, a crystal length of 50 mm, and an incident end face coated with a film having a reflectivity of 50% in the 1.06 μm band, high reflectivity in the 1.46-1.66 μm band, and high transmittance in the 2.9-3.6 μm band. The exit end face is coated with a film having high reflectivity in the 1.06 μm band, a reflectivity of 98.5% in the 1.46-1.66 μm band, and high transmittance in the 2.9-3.6 μm band.

6. The pump-enhanced optical parametric oscillator based on a monolithic cavity PPLN crystal according to claim 1, characterized in that, The temperature control accuracy of the temperature control furnace is 0.0001-0.01℃, and the range is -100 to +300℃; the multi-dimensional adjustment frame includes 1-6 adjustment dimensions; Further preferred features include a temperature control accuracy of 0.001℃ and a range of 0-+150℃ for the temperature control furnace; and a multi-dimensional adjustment frame with 5 adjustment dimensions.

7. The pump-enhanced optical parametric oscillator based on a monolithic cavity PPLN crystal according to claim 1, characterized in that, The two surfaces of the beam splitter are coated with a film that is highly reflective to signal light and highly transparent to idler light; A further preferred embodiment has a beam splitter coated with a film that is highly reflective in the 1.46-1.66 μm band and highly transparent in the 2.9-3.6 μm band on both surfaces.

8. A tuning method for generating vortex light using the pump-enhanced optical parametric oscillator based on a monolithic cavity PPLN crystal as described in claim 1, characterized in that, The steps include the following: 1) Adjusting the crystal temperature changes the phase matching conditions in the crystal, thereby changing the wavelength of the output idler light; 2) Adjust the pump laser wavelength to change the output idler wavelength.

9. An application of the pump-enhanced optical parametric oscillator based on a monolithic cavity PPLN crystal as described in claim 1 in the detection of chiral molecules, characterized in that, Based on the difference in the interaction between chiral molecules and circularly polarized light, a pump-enhanced optical parametric oscillator based on a monolithic cavity PPLN crystal is used as a mid-infrared tunable laser source to detect chiral molecules.

10. An application of the pump-enhanced optical parametric oscillator based on a monolithic cavity PPLN crystal as described in claim 1 in compact Kerr optical frequency combs, characterized in that, The pump-enhanced optical parametric oscillator based on a monolithic cavity PPLN crystal serves as a compact mid-infrared optical frequency comb pump source.

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