Optical parametric oscillator based on double compensation structure and twice pump energy extraction

Abstract

The application belongs to the technical field of laser, and proposes an optical parametric oscillator based on double compensation structure and secondary pump energy extraction, which comprises a pump light input unit, a four-mirror plane ring cavity unit, a parametric gain unit composed of four KTA crystals, a signal light polarization control unit, a dichroic mirror light splitting unit and a residual pump multiplexing and timing adjustment unit. The four crystals are divided into two paths, and each path has two crystals that complete walk-off compensation in pairs. Meanwhile, the two crystal pairs are orthogonal in walk-off direction through orientation transformation, exchange and average the transverse gain asymmetry caused by walk-off, realize the transverse cavity mode bias compensation, and effectively improve the parametric light space output characteristics. The residual pump light of the first path is separated and time-adjusted, and then guided to the second path crystal for secondary energy extraction, which significantly improves the conversion efficiency. The application realizes high average power, high beam quality and consistent two-axis divergence trend of laser output.

Description

Technical Field

[0001] This invention relates to the field of laser technology, and in particular to an optical parametric oscillator based on a double compensation structure and secondary pump energy extraction. Background Technology

[0002] The statements in this section are merely background information related to the present invention and do not necessarily constitute prior art.

[0003] The near-infrared band around 1.5µm and the mid-infrared band (3µm-5µm) both possess excellent atmospheric transmission characteristics. The former is commonly used for laser ranging and target detection, while the latter is frequently used for infrared imaging and active illumination, thus exhibiting complementary applications. Frequency conversion in nonlinear crystals via optical parametric oscillators (OPOs) and optical parametric amplifiers (OPAs) is a common approach to simultaneously obtain output in both bands, and it can support higher pulse energy output under high repetition rates. Furthermore, employing a critical phase-matching process allows for wavelength tuning, which is even more beneficial for multi-scenario and multi-task applications.

[0004] However, critical phase matching is often accompanied by significant spatial walk-off and the resulting spatial effects such as transverse gain anisotropy, which are more pronounced at high power operation. This leads to severe directional asymmetry in the output beam pattern and significant differences in the divergence trends in the two orthogonal directions. These adverse factors not only limit ranging and detection performance, reduce illumination and imaging quality, and decrease effective operating distance and energy utilization efficiency, but also seriously affect system docking and engineering tolerances.

[0005] The aforementioned biaxial asymmetry problem is more easily amplified during the phase matching process of birefringent crystal angle tuning, making it more difficult to simultaneously improve both "conversion efficiency and pulse energy" and "beam morphology and beam quality". This greatly limits its applicability as a light source and its reliability in application. Summary of the Invention

[0006] To address the problems of existing critical phase-matched high-power optical parametric oscillators (OPOs) operating under large-aperture, high-gain conditions, such as intracavity mode evolution bias, output beam spot morphology directional asymmetry, and excessive differences in divergence trends between the two orthogonal directions caused by spatial walk-off and lateral gain anisotropy, as well as the difficulty in simultaneously improving parametric conversion efficiency and spatial output characteristics, this invention provides an optical parametric oscillator based on a double compensation structure and secondary pump energy extraction. This exchange and averages the lateral gain asymmetry caused by walk-off, achieving two-axis mode balance, thereby improving the symmetry of the output beam spot morphology and enhancing the consistency of the two-axis divergence trends. While improving the aforementioned spatial output characteristics, the parametric conversion efficiency is improved by secondary energy extraction from the remaining pump, making it suitable for high repetition frequency and high-energy pulse operation.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] In a first aspect, the present invention provides an optical parametric oscillator based on a double compensation structure and secondary pump energy extraction.

[0009] An optical parametric oscillator based on a double compensation structure and secondary pump energy extraction includes:

[0010] Pump light input unit, used to provide pump light;

[0011] The ring resonant cavity unit consists of a coupling input mirror, a coupling output mirror, a first reflecting mirror, and a second reflecting mirror arranged sequentially along the optical path to form a four-mirror planar ring cavity oscillation optical path.

[0012] The parametric gain unit includes a first KTA crystal pair and a second KTA crystal pair disposed within the ring resonant cavity unit. Each crystal pair includes two KTA crystals. In the same crystal pair, one KTA crystal is rotated 180° relative to the other KTA crystal under the same planar orientation conditions around the optical axis of the pump light. At the same time, the second KTA crystal pair is rotated 180° relative to the first KTA crystal pair under the same planar orientation conditions, and then rotated 90° around the optical axis of the pump light.

[0013] The signal light polarization control unit includes a first signal light half-wave plate and a second signal light half-wave plate disposed on the optical path of the ring resonant cavity unit. The first signal light half-wave plate is located on the optical path between the coupling output mirror and the first reflector, and the second signal light half-wave plate is located on the optical path between the second reflector and the coupling input mirror, and is used to switch the polarization direction of the signal light.

[0014] The residual pump multiplexing and timing adjustment unit is used to guide the residual pump light transmitted through the first KTA crystal pair to the second KTA crystal pair for secondary energy extraction, and to perform pulse timing adjustment and polarization control.

[0015] In one implementation of the first aspect of the present invention, the pump light input unit includes a first pump light half-wave plate, a first polarizing beam splitter, a second pump light half-wave plate and a first pump light quarter-wave plate arranged sequentially along the optical path, wherein the transmission optical path of the first polarizing beam splitter points to the coupling input mirror.

[0016] In one implementation of the first aspect of the present invention, the coupling input mirror has high transmission of pump light, high reflection of signal light, and high transmission of idler light; the coupling output mirror has high transmission of pump light, partial transmission of signal light, and high transmission of idler light; and the spectral characteristics of the first and second reflecting mirrors are the same as those of the coupling input mirror.

[0017] In one implementation of the first aspect of the present invention, the first KTA crystal pair includes a first KTA crystal and a second KTA crystal, and the second KTA crystal pair includes a third KTA crystal and a fourth KTA crystal. All four crystals are potassium oxytitanate crystals with critical phase matching cut angles.

[0018] As a further limitation of the first aspect of the present invention, the first KTA crystal, the second KTA crystal, the third KTA crystal and the fourth KTA crystal are all placed with a preset skew angle.

[0019] In one implementation of the first aspect of the present invention, a dichroic mirror beam splitting unit is further included, comprising a first dichroic mirror disposed on the transmission optical path of the coupling output mirror, a second dichroic mirror disposed on the transmission optical path of the first dichroic mirror, and a third dichroic mirror disposed on the transmission output optical path of the second KTA crystal pair.

[0020] In one implementation of the first aspect of the present invention, the residual pump multiplexing and timing adjustment unit includes a second polarizing beam splitter prism, a second pump light quarter-wave plate, a pump light zero-degree reflector, a collimating concave mirror, and a third pump light half-wave plate disposed on the reflected light path of the second dichroic mirror, and the pump light zero-degree reflector is mounted on a translation stage.

[0021] In one implementation of the first aspect of the present invention, a water-cooled heat sink is further included, wherein the first KTA crystal pair and the second KTA crystal pair are respectively mounted on the water-cooled heat sink.

[0022] In one implementation of the first aspect of the present invention, in the parametric gain unit, the two KTA crystals in the same crystal pair achieve spatial walk-off compensation through relative orientation configuration, so as to reduce beam deflection and nonlinear coupling mismatch caused by single crystal walk-off.

[0023] The equivalent walk-off direction of the second KTA crystal pair is orthogonal to the equivalent walk-off direction of the first KTA crystal pair, so that the walk-off-induced transverse gain imbalance is exchanged between the two orthogonal directions and averaged during the cavity oscillation, so as to achieve transverse cavity mode bias compensation and two-axis mode balance.

[0024] The residual pump multiplexing and timing adjustment unit adjusts the optical path through a translation stage, so that the time window of the residual pump pulse and the intracavity signal light in the second KTA crystal pair is matched, so as to realize the secondary energy extraction of pump energy that has not been consumed in the first process.

[0025] Secondly, the present invention provides an optical parametric oscillation method based on a double compensation structure and secondary pump energy extraction.

[0026] An optical parametric oscillation method based on a double compensation structure and secondary pump energy extraction, utilizing the optical parametric oscillator based on a double compensation structure and secondary pump energy extraction according to the first aspect of this invention, includes the following process:

[0027] The pump light is coupled into the first KTA crystal pair through the pump light input unit, so that the signal light in the ring resonant cavity unit establishes a single resonant oscillation.

[0028] The remaining pump light transmitted through the first KTA crystal pair is guided to the second KTA crystal pair for secondary energy extraction via the remaining pump multiplexing and timing adjustment unit.

[0029] The polarization switching necessary for phase matching between the two sets of crystal pairs in the cavity is achieved by using the first signal light half-wave plate and the second signal light half-wave plate.

[0030] Go-away compensation is achieved by using the reverse go-away configuration between the two crystals in the same crystal pair, and the lateral cavity mode offset compensation is achieved by using the orthogonal go-away direction configuration between the two crystal pairs.

[0031] The output signal light, residual pump light, and idler light are separated by a beam splitter.

[0032] Compared with the prior art, the beneficial effects of the present invention are:

[0033] This invention achieves biaxial gain balance through a unique dual-path configuration and crystal plane orientation transformation. Within the same KTA crystal pair, reversible orientations are used to achieve paired walk-off compensation. Simultaneously, the walk-off directions between the two crystal pairs are orthogonalized through overall crystal plane orientation transformation. Combined with the polarization switching of the intracavity signal light half-wave plate, the transverse gain imbalance experienced by the beam during oscillation within the cavity is continuously exchanged and averaged between the horizontal and vertical orthogonal directions, achieving transverse cavity mode bias compensation. This synergistic mechanism effectively counteracts the anisotropy caused by spatial walk-off under critical phase matching, significantly improving the directional asymmetry of the output beam spot, making the spatial distribution of the beam spot tend to be consistent in the two orthogonal directions, and completely solving the problem that traditional single-path walk-off compensation cannot eliminate beam spot ellipticization.

[0034] This invention successfully promotes the consistency of biaxial divergence. Utilizing two KTA crystal pairs orthogonally arranged in the walk-off direction, along with a first and second signal beam half-wave plate for switching the polarization of the oscillating signal beam, the beam's lateral divergence characteristics are symmetrically corrected in both the horizontal and vertical directions during nonlinear interaction. This design significantly reduces the intracavity gain anisotropy caused by the spatial walk-off effect, resulting in a higher beam quality factor (Mi) in the two orthogonal directions. 2 The differences are minimal. Actual measurements show that the divergence trends of the two axes tend to be consistent, and the normalized asymmetry of the two axes relative to the average diameter is less than 3%, which greatly improves the engineering applicability of the spatial output quality and meets the stringent requirements of high-precision applications for circularly symmetric beams.

[0035] This invention optimizes spatial characteristics while achieving a dual improvement in efficiency and power. By separating the first residual pump through a second dichroic mirror, the remaining pump is guided to a second KTA crystal pair for secondary energy extraction using an optical path consisting of a second polarizing beam splitter, a quarter-wave plate, and a zero-degree reflector mounted on a translation stage. In particular, by precisely adjusting the optical path through the translation stage, the time window of the remaining pump pulse and the signal light in the cavity is perfectly matched. This mechanism fully utilizes the pump energy that is not consumed in the first process, significantly improving the parametric conversion efficiency and greatly enhancing the system's average power and single-pulse energy output capability while maintaining high beam quality.

[0036] This invention demonstrates exceptional engineering feasibility and stability. Built upon a mature four-mirror planar ring cavity platform, it eliminates the need for complex three-dimensional image rotation constraint structures or massive optical component arrays. By dividing four KTA crystals into two paths and integrating them onto a water-cooled heat sink, combined with a segmented polarization management architecture, the system maintains high performance while boasting a compact structure and clear logic. The signal beam polarization control unit and the residual pump multiplexing and timing adjustment unit are all positioned in easily accessible locations, reducing the assembly and adjustment complexity of the precision optical system. This design not only facilitates engineering implementation but also improves long-term operational stability, providing a solid foundation for the mass production of high-power all-solid-state lasers.

[0037] This invention achieves high-performance output while retaining excellent adaptability and tuning capabilities. All four KTA crystals employ critical phase-matching cut angles. While constructing a double walk-off compensation structure and a secondary pump energy extraction optical path, the freedom of wavelength tuning is not sacrificed. This invention can still change the phase-matching conditions by fine-tuning the crystal angle or temperature, thereby continuously adjusting the output wavelength of the signal light and idler light within a certain range. This flexibility allows users to obtain high beam quality and high average power lasers while adapting the wavelengths according to the specific needs of different application scenarios such as multispectral imaging and remote sensing, providing important foundational support for subsequent technology expansion and widespread application.

[0038] Advantages of additional aspects of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0039] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0040] Figure 1 A schematic diagram of a four-crystal ring cavity optical parametric oscillator structure based on a double compensation structure and secondary pump energy extraction, provided as an exemplary embodiment of the present invention;

[0041] Figure 2 A pump power variation curve of a pump light with a maximum average power of 153W and a center wavelength of 1064.4nm, input to an optical parametric oscillator under the control of a polarization beam splitter, provided as an exemplary embodiment of the present invention;

[0042] Figure 3 A schematic diagram of the orientation transformation of two-path four-block KTA crystal planes within an optical parametric oscillation cavity provided as an exemplary embodiment of the present invention;

[0043] Figure 4A curve showing the change of signal light output of an optical parametric oscillator with increasing pump light at a repetition frequency of 300 Hz, provided as an exemplary embodiment of the present invention;

[0044] Figure 5 The signal light spot at a maximum output of 28.1W is provided as an exemplary embodiment of the present invention;

[0045] Figure 6 As an exemplary embodiment of the present invention, the beam quality M is provided at a maximum pump power of 153W average power and a maximum signal light output of 28.1W. 2 Beam quality factor M 2 The values ​​are 4.1 in the horizontal direction and 4.2 in the vertical direction.

[0046] Figure 7 The spectral curve of the signal light with a maximum output of 28.1W at a maximum pump power of 153W average power is provided as an exemplary embodiment of the present invention, with a center wavelength of 1.49μm;

[0047] Figure 8 The pulse width provided for an exemplary embodiment of the present invention is 18ns when the maximum output signal light is 28.1W at a maximum pump power of 153W average power.

[0048] Among them, 1-1, first pump light half-wave plate; 1-2, second pump light half-wave plate; 1-3, third pump light half-wave plate; 2-1, first polarizing beam splitter; 2-2, second polarizing beam splitter; 3-1, first pump light quarter-wave plate; 3-2, second pump light quarter-wave plate; 4-1, coupling input mirror; 4-2, coupling output mirror; 4-3, first reflecting mirror; 4-4, second reflecting mirror; 5-1, first KTA crystal; 5-2, second KTA crystal; 5-3, third KTA crystal; 5-4, fourth KTA crystal; 6-1, first dichroic mirror; 6-2, second dichroic mirror; 6-3, third dichroic mirror; 7, pump light zero-degree reflecting mirror; 8, collimating concave mirror; 9-1, first signal light half-wave plate; 9-2, second signal light half-wave plate. Detailed Implementation

[0049] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0050] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0051] In a preferred embodiment of the present invention, such as Figure 1As shown, a four-crystal ring cavity optical parametric oscillator based on a double compensation structure and secondary pump energy extraction is proposed, specifically including:

[0052] First, a four-mirror planar ring cavity optical parametric oscillator structure with single-resonance signal light is constructed to realize parametric oscillation output and provide a basis for polarization management, segmented structure and mode control.

[0053] Secondly, four KTA nonlinear crystals with critical phase-matching cut angles are set in the annular cavity. The four crystals are arranged into two sets of crystal pairs to form a four-crystal dual compensation structure. The first compensation is paired walk-off compensation, that is, the two crystals in each set of crystal pairs achieve spatial walk-off compensation through relative orientation configuration to reduce beam offset and nonlinear coupling mismatch caused by single crystal walk-off. The second compensation is orthogonal walk-off direction, that is, the equivalent walk-off directions of the two sets of crystal pairs are set to be orthogonal to each other, so that the walk-off-induced transverse gain imbalance is exchanged between the two orthogonal directions, thereby achieving effective averaging during cavity oscillation and achieving the effect of transverse cavity mode offset compensation and two-axis mode balance.

[0054] Furthermore, during parametric oscillation, the remaining pump that was not fully consumed by one set of crystal pairs is separated and reused to pump another set of crystal pairs in the cavity, thereby further improving the parametric conversion efficiency and parametric optical power output capability.

[0055] Ultimately, through the synergistic mechanism of "two crystal walk-off compensation + two pairs of walk-off direction orthogonalization", the bias of walk-off-induced lateral anisotropy on the intracavity mode evolution is reduced, the symmetry of the output beam spot shape is improved, and the two-axis divergence trend and beam quality are made more consistent. Combined with the secondary energy extraction of the remaining pump, the parametric conversion efficiency and spatial output characteristics are simultaneously improved.

[0056] More specifically, the optical parametric oscillator based on a double compensation structure and secondary pump energy extraction in this implementation includes:

[0057] Pump light input unit, used to provide pump light for the optical parametric oscillator;

[0058] The ring resonator unit is a four-mirror planar ring cavity used to realize the optical parametric oscillation (OPO) process.

[0059] The parametric gain unit includes two channels with four KTA crystals, used to implement nonlinear frequency conversion;

[0060] The signal light polarization control unit is used to switch the polarization direction of the oscillating signal light in the resonant cavity;

[0061] The dichroic mirror beam splitter unit is used to separate pump light, signal light, and idler light.

[0062] The residual pump multiplexing and timing adjustment unit is used to guide the residual pump of the first channel to the second channel and perform pulse timing adjustment and polarization control.

[0063] In this embodiment, the optical parametric oscillation resonates only with the signal light, and does not resonate with the pump light and idler light.

[0064] In this implementation, the pump light input unit serves as the energy source entry point for the entire system. Its main function is to provide a stable and polarization-controllable pump light source and efficiently couple the pump light into the subsequent ring resonant cavity unit.

[0065] In this embodiment, the optical parametric oscillator is implemented through three processes: pump light input, optical parametric oscillation, and residual pump multiplexing. Specifically, the pump light input process includes: a pump light center wavelength of 1064.4 nm, a repetition frequency of 300 Hz, an average power of 153 W, a pulse width of 27 ns, a spot diameter of approximately 6 mm, and a beam quality M. 2 The values ​​are 1.61 in the horizontal direction and 1.60 in the vertical direction. Figure 2 The graph shows the pump power variation of a pump light with a center wavelength of 1064.4 nm and a maximum average power of 153 W input to an optical parametric oscillator under the control of a polarization-splitting switch. Figure 2 The curves show the smooth relationship between pump power and control element adjustment, verifying the system's controllability over a wide power range.

[0066] The pump light input unit includes a polarization beam splitter switch consisting of a first pump light half-wave plate 1-1 (1064nm HWP) and a first polarization beam splitter prism 2-1 (PBS), as well as a second pump light half-wave plate 1-2 (1064nm HWP) and a first pump light quarter-wave plate 3-1 (1064nm QWP) for adjusting the polarization state of the pump light. By rotating the first pump light half-wave plate 1-1, the angle scale can be accurately read and the pump power entering the system can be adjusted accordingly. The polarization beam splitter switch is used to control and adjust the pump power entering the parametric oscillation (OPO) system while maintaining a predetermined pump light power, so as to avoid changes in the spot size and collimation of the pump beam.

[0067] The original pump beam emitted from the laser is first incident on the first pump beam half-wave plate 1-1, which is fixedly mounted on a precision rotating adjustment frame, with its optical axis center coinciding with the center of the pump beam. After being adjusted by the first pump beam half-wave plate 1-1, the pump beam continues to propagate forward and is incident on the incident end face of the first polarizing beam splitter 2-1.

[0068] The pump light transmitted from the first polarizing beam splitter 2-1 propagates along a predetermined optical path, then passes sequentially through the second pump half-wave plate 1-2 and the first pump quarter-wave plate 3-1. The second pump half-wave plate 1-2, also mounted on a rotatable adjustment frame, is located between the transmission path exit of the first polarizing beam splitter 2-1 and the first pump quarter-wave plate 3-1. Its function is to adjust the polarization of the pump light to meet the specific requirements of subsequent nonlinear crystal phase matching. Following closely is the first pump quarter-wave plate 3-1, positioned after the second pump half-wave plate 1-2 and before the coupling input mirror 4-1 of the ring resonator unit. Its function is to further fine-tune the polarization state of the pump light to ensure its purity. After being processed by this series of polarization control elements, the pump light finally reaches the entrance of the ring resonator unit, namely the coupling input mirror 4-1.

[0069] The ring resonator unit is the core structure for realizing the optical parametric oscillation process. This embodiment adopts a four-mirror planar ring cavity structure, which consists of a coupling input mirror 4-1, a coupling output mirror 4-2, a first reflecting mirror 4-3, and a second reflecting mirror 4-4. These four reflecting mirrors are respectively mounted on independent precision multi-dimensional adjustment frames to ensure that the angle and position of the cavity mirrors can be precisely adjusted, thereby achieving the alignment and steady-state operation of the resonator. The geometric configuration of the four-mirror planar ring cavity includes two long arms and two short arms, with a cavity length of 260mm. The two long arms are both 85mm long, and the two short arms are both 45mm long. This specific ratio of long arms to short arms not only provides sufficient space for placing a longer nonlinear crystal component in the long arm position, but also reserves a suitable optical path length for installing signal light polarization control elements in the short arm position.

[0070] This embodiment provides strict definitions for the spectral characteristics and coating requirements of the four-sided cavity mirror to ensure a single-resonant operation mode for the signal light while the pump light and idler light operate in a non-resonant manner. The coupling input mirror 4-1, as the element for the pump light to enter the cavity, exhibits high transmittance for the pump light at 1064nm 45°, high reflectance for the signal light at 1.49μm 45°, and high transmittance for the idler light at 1.72μm 45°. The coupling output mirror 4-2, located on the other side of the optical path, exhibits high transmittance for the pump light at 1064nm 45°, 50% transmittance for the signal light at 1.49μm 45°, and high transmittance for the idler light at 1.72μm 45°. The first reflecting mirror 4-3 and the second reflecting mirror 4-4 are located at the other two vertices of the annular cavity, respectively, and their specifications are identical to those of the coupling input mirror 4-1. The design of these spectral characteristics ensures efficient pump light injection, with the signal light forming a resonant feedback within the cavity, while idler light and unconsumed pump light can freely pass through or escape from the cavity.

[0071] Inside the ring resonator unit, a parametric gain unit is arranged in the optical path. This is the key component for realizing nonlinear frequency conversion. The parametric gain unit consists of four KTA crystals with identical specifications, namely potassium oxytitanium arsenate (KTiOAsO4) crystals. These four crystals are labeled as KTA crystal 5-1, KTA crystal 5-2, KTA crystal 5-3, and KTA crystal 5-4, respectively. The four KTA crystals have identical specifications: θ=41.2°, φ=0, and the KTiOAsO4 crystal is cut in the xz plane. The crystal size is 10×10×20 mm. 3 The crystal surface is not coated. Among them, the four crystals are divided into two paths. The first KTA crystal 5-1 and the second KTA crystal 5-2 are the first KTA crystal pair, and the third KTA crystal 5-3 and the fourth KTA crystal 5-4 are the second KTA crystal pair.

[0072] like Figure 3 The diagram shows the orientation transformation of four KTA crystals in two paths within an optical parametric oscillator cavity. It clearly demonstrates the configuration of the second KTA crystal 5-2 in the first crystal pair, which is rotated 180° around the optical axis relative to the first KTA crystal 5-1 under the same orientation conditions. It also shows the configuration of the fourth KTA crystal 5-4 in the second crystal pair, which is rotated 180° around the optical axis relative to the third KTA crystal 5-3 under the same orientation conditions. Furthermore, it demonstrates the orthogonal departure direction configuration of the second crystal pair relative to the first crystal pair, which first rotates 180° around the normal of the plane containing the four-mirror annular cavity and then rotates 90° around the optical axis of the pump light under the same orientation conditions. This intuitively reveals the orientation transformation of the four crystals in the double compensation structure.

[0073] In this implementation, the first KTA crystal pair is positioned on one long arm of the four-mirror planar annular cavity. Specifically, after the pump light enters the cavity from the coupling input mirror 4-1, it first strikes the incident end face of the first KTA crystal 5-1, passes through the first KTA crystal 5-1, and then strikes the incident end face of the second KTA crystal 5-2. A crucial orientation relationship exists in this crystal pair arrangement: the second KTA crystal 5-2 has a reversed orientation relative to the first KTA crystal 5-1, forming a reversed orientation walk-off compensation structure. When the pump light and signal light propagate in the first KTA crystal 5-1, spatial walk-off occurs due to birefringence, causing lateral beam shift and nonlinear coupling mismatch. When the beam enters the reversed-orientation second KTA crystal 5-2, its walk-off direction is opposite to that in the first crystal, thus compensating for the walk-off effect generated by the first crystal in the second crystal. This significantly reduces the beam shift and nonlinear coupling mismatch caused by single-crystal walk-off, achieving walk-off compensation. The second KTA crystal pair is positioned on the other long arm of the four-mirror planar annular cavity, opposite to the first KTA crystal pair. The planar orientations of the third KTA crystal 5-3 and the fourth KTA crystal 5-4 are exactly the same as those of the first KTA crystal 5-1 and the second KTA crystal 5-2, respectively. That is, the third KTA crystal 5-3 and the fourth KTA crystal 5-4 also form a walk-off compensation structure with mutually opposite orientations, achieving walk-off compensation. The two crystal pairs independently achieve walk-off compensation, realizing the first layer of compensation. The paired KTA crystals in each path achieve spatial walk-off compensation through relative orientation configuration, thereby reducing beam deflection and nonlinear coupling mismatch caused by the single crystal spatial walk-off effect.

[0074] While the two crystal pairs independently compensate for walk-off, the second KTA crystal pair also exhibits a more crucial orthogonal orientation relative to the first KTA crystal pair in terms of walk-off direction. Specifically, the third KTA crystal 5-3 and the fourth KTA crystal 5-4 in the second KTA crystal pair, respectively, are placed relative to the first KTA crystal 5-1 and the second KTA crystal 5-2 in the first KTA crystal pair under the same planar axis placement conditions, i.e., the same paired walk-off compensation structure. Through an orientation transformation involving a 180° rotation around the normal of the plane containing the four-mirror annular cavity and a 90° rotation around the optical axis of the pump light, the walk-off directions of the second paired walk-off compensation KTA crystals are orthogonal to the walk-off directions of the first paired walk-off compensation KTA crystal pair. After this series of complex orientation transformations, the equivalent walk-off directions of the second paired walk-off compensation KTA crystal pair are orthogonal to the equivalent walk-off directions of the first paired walk-off compensation KTA crystal pair. This orthogonal arrangement achieves a second layer of compensation, namely, lateral cavity mode offset compensation. The physical significance lies in exchanging the walk-off-induced lateral gain imbalance between two orthogonal directions (e.g., horizontal and vertical). This allows the beam to experience walk-off effects in different directions during intracavity oscillation. These effects are effectively canceled out by time and spatial averaging, thus achieving lateral cavity mode bias compensation and two-axis mode balance. This significantly improves the symmetry of the output beam spot shape and makes the two-axis divergence trend more consistent with the beam quality. While each of the two KTA crystals performs paired walk-off compensation, the mutual orthogonality of the walk-off directions further achieves double compensation.

[0075] To ensure the thermal stability of the four KTA crystals under high-power pumping, this embodiment incorporates water-cooled heat sinks at the two long arms of the four-mirror annular cavity. The two pairs of KTA crystals with separate walk-off compensation are mounted on these water-cooled heat sinks and subjected to constant temperature control. These heat sinks are temperature-controlled via an external circulating cooling system to dissipate heat that may be generated by the nonlinear crystals under strong light, preventing deterioration caused by thermal lensing and thermally induced birefringence, as well as signal wavelength drift due to temperature changes. Furthermore, to suppress parasitic oscillations and Fabry-Perot interference effects, all pairs of walk-off compensated KTA crystals are placed with a predetermined small tilt angle. That is, the crystal end face normal is not completely aligned with the optical axis, but rather has a small tilt angle. This avoids the formation of a standard Fabry-Perot resonator between the crystal end faces, thereby eliminating unnecessary interference fringes and self-excited oscillations.

[0076] A signal light polarization control unit is provided on the short-arm optical path connecting the first KTA crystal pair and the second KTA crystal pair. This unit includes a first signal light half-wave plate 9-1 and a second signal light half-wave plate 9-2. Specifically, the first signal light half-wave plate 9-1 (1.49μm HWP) and the second signal light half-wave plate 9-2 (1.49μm HWP) are respectively positioned on the two short arms of the four-mirror annular cavity. The first signal light half-wave plate 9-1 is used to rotate the polarization of the 1.49μm signal light oscillating within the cavity by 90° before it enters the second KTA crystal pair. The second signal light half-wave plate 9-2 is used to rotate the polarization of the 1.49μm signal light after it has passed through the second KTA crystal pair by 90° in the opposite direction before it enters the first KTA crystal pair, thereby achieving the polarization switching required for phase matching between the two orthogonal KTA crystals. The first signal light half-wave plate 9-1 is installed on one of the short arms of the four-mirror annular cavity, specifically located on the optical path between the coupling output mirror 4-2 and the first reflecting mirror 4-3. Since the walk-off direction of the second KTA crystal pair is orthogonal to that of the first, the polarization direction of the signal light required for phase matching is also orthogonal to that of the first. Therefore, the polarization of the signal light must be rotated by 90° by the first signal light half-wave plate 9-1 to satisfy the phase matching condition of the second KTA crystal pair, allowing the signal light to effectively gain gain in the second crystal. The second signal light half-wave plate 9-2 is installed on the other short arm of the four-mirror annular cavity, specifically in the optical path between the second reflecting mirror 4-4 and the coupling input mirror 4-1. The function of the second signal light half-wave plate 9-2 is to rotate the polarization of the signal light emitted from the second KTA crystal pair 90° in the opposite direction, restoring it to its initial polarization state, so that it can satisfy the phase matching condition of the first crystal when it is incident on the first KTA crystal pair again. Through the coordinated work of these two half-wave plates, the polarization switching required for phase matching between the two orthogonally walk-off KTA crystals is achieved, ensuring continuous oscillation and amplification of the signal light throughout the annular cavity.

[0077] After the signal light establishes a stable single-resonant oscillation within the ring cavity, a portion of the signal light will be transmitted and output through the coupling output mirror 4-2. Simultaneously, the remaining unconsumed pump light and the generated idler light will also be transmitted. To separate and process these beams of different wavelengths, a dichroic mirror beam splitter unit is installed in the transmission path of the coupling output mirror 4-2. The dichroic mirror beam splitter unit includes: a first dichroic mirror 6-1, which has high reflectivity (45°) for the signal light and high transmittance (45°) for both the pump and idler light; a second dichroic mirror 6-2, which has high reflectivity (45°) for the pump light and high transmittance (45°) for the idler light; and a third dichroic mirror 6-3, which has high reflectivity (45°) for the pump light and high transmittance (45°) for the idler light. The first dichroic mirror 6-1 is placed directly in the transmission path of the coupling output mirror 4-2. Its coating is designed to have high reflectivity for the signal light (1.49μm at 45°) and high transmittance for the pump light (1064nm) and idler light (3.72μm at 45°), thus separating the 1.49μm signal light output from the planar ring cavity. Therefore, when the mixed beam emitted from the coupling output mirror 4-2 is incident on the first dichroic mirror 6-1, the 1.49μm signal light is reflected off its original path and separated as the useful output beam of the system; while the pump light and idler light continue to propagate along their original directions through the first dichroic mirror 6-1.

[0078] The pump light and idler light passing through the first dichroic mirror 6-1 are then incident on the second dichroic mirror 6-2. The second dichroic mirror 6-2 has high reflectivity for the 1064nm pump light at 45° and high transmittance for the 3.72μm idler light at 45°, and is used to separate the remaining pump light transmitted from the first path. It should be noted that the pump light passing through the first dichroic mirror 6-1 mainly comes from the unconsumed remaining pump light of the first KTA crystal pair. When this light is incident on the second dichroic mirror 6-2, the remaining 1064nm pump light is reflected and enters the optical path of the remaining pump multiplexing and timing adjustment unit; while the 3.72μm idler light (mainly generated by the first crystal) passes through the second dichroic mirror 6-2 and is separated as part of the idler light output.

[0079] In the transmission output path of the second KTA crystal pair, a third dichroic mirror 6-3 is used to separate the pump light and idler light transmitted from the second path. The coating characteristics of the third dichroic mirror 6-3 are the same as those of the second dichroic mirror 6-2, i.e., high reflectivity for pump light with a wavelength of 1064 nm at a 45° incident angle, and high transmittance for idler light with a wavelength of 3.72 μm at a 45° incident angle. The function of the third dichroic mirror 6-3 is to separate the pump light remaining after secondary energy extraction from the second KTA crystal pair from the idler light generated in that path. The pump light reflected by the third dichroic mirror 6-3 is usually treated as waste light or absorbed, while the transmitted idler light is combined with or collected separately from the idler light transmitted from the second dichroic mirror 6-2, together forming the system's idler light output. Through the coordinated operation of the first dichroic mirror 6-1, the second dichroic mirror 6-2, and the third dichroic mirror 6-3, the system successfully and completely separates the signal light, the remaining pump light, and the idler light in space, providing convenience for subsequent processing and applications.

[0080] The following section focuses on the detailed structure and workflow of the residual pump multiplexing and timing adjustment unit, which is key to improving conversion efficiency in this invention. The residual pump multiplexing and timing adjustment unit includes a second polarizing beam splitter 2-2, a second pump beam quarter-wave plate 3-2, a pump beam zero-degree reflector 7, a collimating concave mirror 8, and a third pump beam half-wave plate 1-3. The main task of this unit is to guide, polarize, and control the timing of the first path of residual pump light separated by the second dichroic mirror 6-2, enabling it to be effectively incident on the second KTA crystal pair for pumping. The residual pump light reflected from the second dichroic mirror 6-2 passes through the second polarizing beam splitter 2-2 and then incident on the second pump beam quarter-wave plate 3-2 (1064nm QWP). Then, the second pump light passes through the second pump light quarter-wave plate 3-2, and after being reflected by the pump light zero-degree reflector 7, it passes through the second pump light quarter-wave plate 3-2 again, and then enters the second polarization beam splitter 2-2 again, where it is refracted and reflected and guided to the second KTA crystal pair.

[0081] The optical axis of the second pump light quarter-wave plate 3-2 forms a 45° angle with the polarization direction of the incident horizontally polarized light. After the pump light passes through this plate, its polarization state changes from linearly polarized to circularly polarized. Next, the beam is incident on the pump light zero-degree reflector 7. The pump light zero-degree reflector 7 is a high-reflectivity plane mirror, unique in that it is mounted on a high-precision translation stage, capable of translating along the optical path to change the optical path length of the remaining pump, thereby adjusting the pulse timing of the remaining pump of the incident second KTA crystal pair, matching the effective gain time window of the remaining pump light with the cavity oscillation signal light in the second KTA crystal pair. After being reflected by the pump light zero-degree reflector 7, the pump light returns along its original path and passes through the second pump light quarter-wave plate 3-2 again. Because it passes through for the second time and the optical path is reversed, the circularly polarized light, after passing through the quarter-wave plate again, will have its polarization state converted to linearly polarized light orthogonal to its incident state, and will be deflected by the second polarization beam splitter 2-2.

[0082] Before the residual pump light is guided to the second KTA crystal pair, a collimating concave mirror 8 and a third pump half-wave plate 1-3 (1064nm HWP) are sequentially placed in the optical path to optimize the beam size and phase matching conditions. These collimating concave mirror 8 is used to collimate and adjust the polarization of the residual pump light, which may shrink after round-trip transmission, ensuring that its spot size and divergence angle are suitable for incident on the second crystal. The third pump half-wave plate 1-3 is used to finely adjust the polarization direction of the residual pump light, making it strictly conform to the phase matching requirements of the second KTA crystal pair. After collimation and polarization adjustment, the residual pump light is finally incident on the second KTA crystal pair (i.e., the third KTA crystal 5-3 and the fourth KTA crystal 5-4). The residual pump light transmitted from the first path is guided to the second KTA crystal pair, thereby realizing secondary energy extraction of the residual pump light and improving the parametric conversion efficiency of the nonlinear frequency conversion process.

[0083] In this process, the high-precision translation stage mounted on the pump light zero-degree reflector 7 plays a crucial role. By translating the pump light zero-degree reflector 7 along the optical path, the optical path length of the remaining pump light in the multiplexed optical path can be precisely changed. Since the pump light is a pulsed laser, the change in optical path directly corresponds to a time delay. The purpose of adjusting the optical path is to adjust the pulse timing of the remaining pump light incident on the second KTA crystal pair, ensuring that the remaining pump light pulse perfectly coincides in time with the signal light pulse oscillating in the ring cavity and propagating to the second KTA crystal pair. Only when the effective gain time window of the two matches in the second KTA crystal pair can the remaining pump light be effectively multiplexed and secondary energy extraction achieved. If the timing does not match, the remaining pump light will not be able to participate in the oscillation process, resulting in energy waste. Therefore, by adjusting the translation stage, precise synchronization between the remaining pump light and the signal light in the cavity is achieved, maximizing the efficiency of secondary energy extraction.

[0084] In specific implementation, this optical parametric oscillator resonates only with the signal light, and does not resonate with the pump light and idler light. More preferably, the center wavelength of the pump light is 1064.4 nm, the center wavelength of the signal light is 1.49 μm, and the center wavelength of the idler light is 3.72 μm. More preferably, the optical parametric oscillator outputs a 1.49 μm signal light and a 3.72 μm idler light. Figure 4 and Figure 5 The graph shows the change in signal light output of the optical parametric oscillator with increasing pump light at a repetition frequency of 300 Hz, and the signal light spot at the maximum output of 28.1 W. The slope efficiency fitting value for the signal light output is 39%. Figure 4 The curve clearly shows the linear growth trend of signal light output power with increasing pump power. Figure 5 This demonstrates that at the maximum output of 28.1W, the light spot exhibits a horizontally symmetrical distribution with approximately 1:1 horizontal to vertical axis lengths, and the normalized asymmetry of the two axes relative to the average diameter is less than 3%. At a repetition frequency of 300Hz, the optical parametric oscillation (OPO) threshold is approximately 81W. At the maximum pump power of 153W, the average output power of the OPO signal light is 28.1W, corresponding to a single-pulse energy of 94mJ. The pump-to-signal light conversion efficiency is 18.4%, while the total output power of the idler light is 7W, resulting in a total parametric conversion efficiency of 22.9%.

[0085] Figure 6 As an exemplary embodiment of the present invention, the beam quality M is provided at a maximum pump power of 153W average power and a maximum signal light output of 28.1W. 2 Beam quality factor M 2 With values ​​of 4.1 in the horizontal direction and 4.2 in the vertical direction, the test results show that the difference in beam quality factor in the two orthogonal directions is less than 3%, indicating that the lateral divergence of the output beam at this operating point is close to the consistency of the two axes, which strongly proves the improvement effect of the double compensation structure on the spatial characteristics of the beam.

[0086] Figure 7 The spectral curve provided as an exemplary embodiment of the present invention shows the signal light output at a maximum pump power of 28.1W with an average power of 153W. The center wavelength is 1.49μm. It can be seen that the spectral curve has clear peaks, a narrow half-maximum width at half-maximum (FWHM), and the center wavelength is stable around 1.49μm. At an average signal light output power of 28.1W, the measured center wavelength of the signal light spectral curve is 1490.5nm. According to the energy conservation relationship of optical parametric oscillation (OPO), under the condition that the pump light center wavelength is 1064.4nm, the corresponding idler center wavelength is 3723.3nm.

[0087] Figure 8 The pulse width provided as an exemplary embodiment of the present invention, at a maximum pump power of 153W with a maximum signal light output of 28.1W, is 18ns. As shown in the figure, the pulse waveform is complete, and the pulse width is stable at around 18ns, exhibiting good temporal characteristics. This result verifies the effectiveness of timing adjustment during the extraction of residual pump secondary energy, ensuring good temporal matching between the pump light and the signal light.

[0088] This embodiment also emphasizes engineering feasibility. Based on a planar ring cavity platform, this invention eliminates the need for complex three-dimensional image rotation constraint structures, providing a solid foundation for polarization management and segmented architecture, facilitating engineering assembly, adjustment, and long-term stable operation. Furthermore, this invention retains wavelength tuning capability under critical phase-matching conditions. By fine-tuning the crystal angle or temperature, it provides a basis for subsequent wavelength tuning and application adaptation of signal and idler light, meeting the needs of various application scenarios.

[0089] Through the detailed description above, it can be seen that this invention is not only innovative in its theoretical design, proposing a synergistic mechanism of "two-crystal walk-off compensation + two pairs of orthogonalization of walk-off directions" and a strategy for extracting secondary energy from the remaining pump, but also provides a clear, complete, and operable solution in its specific implementation structure. From the introduction and polarization control of the pump light, to the parametric oscillation, walk-off compensation, and transverse cavity mode bias compensation within the ring cavity, and then to the separation of the output light and the recycling of the remaining pump, each step has a clear mechanism and rigorous logic.

[0090] The preferred parameters mentioned in this embodiment, such as crystal size, cutting angle, cavity length, lens transmittance and reflectance, and pump source parameters, are all experimentally verified best-practice data, representing the current optimal implementation of the invention. Of course, those skilled in the art should understand that these specific values ​​are not limitations on the invention. Appropriate adjustments or substitutions to the parameters, such as using other types of nonlinear crystals, changing the cavity length ratio, or adjusting the pump wavelength, without departing from the core concept of the invention, are all within the scope of protection of this invention, as long as the core principles of the double-compensation structure and secondary pump energy extraction are still followed. However, for this specific embodiment, all the above data and structural descriptions constitute the most complete and specific technical presentation of the invention, fully demonstrating its enormous potential and practical value in improving the performance of optical parametric oscillators.

[0091] In summary, the pump light of this invention originates from the source, undergoes a series of controls and modulations, and is injected into the ring cavity. Inside the cavity, four crystals are arranged according to a specific geometric layout and planar orientation relationship, working with a half-wave plate to complete a complex polarization switching and parametric conversion process. After exiting the cavity, a dichroic mirror group separates light of different wavelengths. The separated remaining pump light is "recovered" and "reused" through a complex circuit including a mirror, a wave plate, and a translation stage. The paired KTA crystals in each path achieve spatial walk-off compensation through relative orientation configuration to reduce beam offset and nonlinear coupling mismatch caused by single-crystal spatial walk-off effect. The walk-off directions of the two KTA crystals are orthogonal to each other, thereby achieving transverse cavity mode bias compensation, reducing intracavity transverse gain imbalance induced by spatial walk-off under critical phase matching, improving the transverse symmetry of the output beam, and reducing the beam quality difference in the two orthogonal directions. The secondary energy extraction of the remaining pump light improves the parametric conversion efficiency of the nonlinear frequency conversion process.

[0092] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the mechanisms and strategies of the present invention should be included within the scope of protection of the present invention.

Claims

1. An optical parametric oscillator based on a double compensation structure and secondary pump energy extraction, characterized in that, include: A pump light input unit is used to provide pump light; the pump light input unit includes a first pump light half-wave plate, a first polarizing beam splitter, a second pump light half-wave plate and a first pump light quarter-wave plate arranged sequentially along the optical path, and the transmission optical path of the first polarizing beam splitter is directed to the coupling input mirror; The ring resonant cavity unit consists of a coupling input mirror, a coupling output mirror, a first reflecting mirror, and a second reflecting mirror arranged sequentially along the optical path to form a four-mirror planar ring cavity oscillation optical path. The parametric gain unit includes a first KTA crystal pair and a second KTA crystal pair disposed within the ring resonant cavity unit. Each crystal pair includes two KTA crystals. In the same crystal pair, one KTA crystal is rotated 180° relative to the other KTA crystal under the same planar orientation conditions around the optical axis of the pump light. At the same time, the second KTA crystal pair is rotated 180° relative to the first KTA crystal pair under the same planar orientation conditions, and then rotated 90° around the optical axis of the pump light. The signal light polarization control unit includes a first signal light half-wave plate and a second signal light half-wave plate disposed on the optical path of the ring resonant cavity unit. The first signal light half-wave plate is located on the optical path between the coupling output mirror and the first reflector, and the second signal light half-wave plate is located on the optical path between the second reflector and the coupling input mirror, for signal light polarization switching. The residual pump multiplexing and timing adjustment unit is used to guide the residual pump light transmitted through the first KTA crystal pair to the second KTA crystal pair for secondary energy extraction, and to perform pulse timing adjustment and polarization control.

2. The optical parametric oscillator based on a double compensation structure and secondary pump energy extraction as described in claim 1, characterized in that, The input coupling mirror has high transmission of pump light, high reflection of signal light, and high transmission of idler light. The output coupling mirror has high transmission of pump light, partial transmission of signal light, and high transmission of idler light. The spectral characteristics of the first and second reflecting mirrors are the same as those of the input coupling mirror.

3. The optical parametric oscillator based on a double compensation structure and secondary pump energy extraction as described in claim 1, characterized in that, The first KTA crystal pair includes a first KTA crystal and a second KTA crystal, and the second KTA crystal pair includes a third KTA crystal and a fourth KTA crystal. All four crystals are potassium oxytitanate crystals with critical phase-matching cut angles.

4. The optical parametric oscillator based on a double compensation structure and secondary pump energy extraction as described in claim 3, characterized in that, The first KTA crystal, the second KTA crystal, the third KTA crystal, and the fourth KTA crystal are all placed with a preset tilt angle.

5. The optical parametric oscillator based on a double compensation structure and secondary pump energy extraction as described in claim 1, characterized in that, It also includes a dichroic mirror beam splitting unit, comprising a first dichroic mirror disposed on the transmission optical path of the coupling output mirror, a second dichroic mirror disposed on the transmission optical path of the first dichroic mirror, and a third dichroic mirror disposed on the transmission output optical path of the second KTA crystal pair.

6. The optical parametric oscillator based on a double compensation structure and secondary pump energy extraction as described in claim 1, characterized in that, The remaining pump multiplexing and timing adjustment unit includes a second polarizing beam splitter prism, a second pump light quarter-wave plate, a pump light zero-degree reflector, a collimating concave mirror, and a third pump light half-wave plate, all disposed on the reflected light path of the second dichroic mirror. The pump light zero-degree reflector is mounted on a translation stage.

7. The optical parametric oscillator based on a double compensation structure and secondary pump energy extraction as described in claim 1, characterized in that, It also includes a water-cooled heat sink, with the first KTA crystal pair and the second KTA crystal pair respectively installed on the water-cooled heat sink.

8. The optical parametric oscillator based on a double compensation structure and secondary pump energy extraction as described in claim 1, characterized in that, In the parametric gain unit, the two KTA crystals in the same crystal pair achieve spatial walk-off compensation through relative orientation configuration, so as to reduce beam deflection and nonlinear coupling mismatch caused by single crystal walk-off. The equivalent walk-off direction of the second KTA crystal pair is orthogonal to the equivalent walk-off direction of the first KTA crystal pair, so that the walk-off-induced transverse gain imbalance is exchanged between the two orthogonal directions and averaged during the cavity oscillation, so as to achieve transverse cavity mode bias compensation and two-axis mode balance. The residual pump multiplexing and timing adjustment unit adjusts the optical path through a translation stage, so that the time window of the residual pump pulse and the intracavity signal light in the second KTA crystal pair is matched, so as to realize the secondary energy extraction of pump energy that has not been consumed in the first process.

9. An optical parametric oscillation method based on a double compensation structure and secondary pump energy extraction, characterized in that, Using the optical parametric oscillator based on a double compensation structure and secondary pump energy extraction as described in any one of claims 1-8, The process includes the following: The pump light is coupled into the first KTA crystal pair through the pump light input unit, so that the signal light in the ring resonant cavity unit establishes a single resonant oscillation. The remaining pump light transmitted through the first KTA crystal pair is guided to the second KTA crystal pair for secondary energy extraction via the remaining pump multiplexing and timing adjustment unit. The polarization switching necessary for phase matching between the two sets of crystal pairs in the cavity is achieved by using the first signal light half-wave plate and the second signal light half-wave plate. Go-away compensation is achieved by using the reverse go-away configuration between the two crystals in the same crystal pair, and the lateral cavity mode offset compensation is achieved by using the orthogonal go-away direction configuration between the two crystal pairs. The output signal light, residual pump light, and idler light are separated by a beam splitter.

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