Stress compensation system for balancing thermal-induced birefringence of laser crystal, laser and method

Abstract

The application discloses a stress compensation method for balancing thermal-induced birefringence of a laser crystal, comprising the following steps: S1, fixing the crystal on a stress compensation device, and guiding a laser beam to pass through the geometric center of the crystal along the optical axis direction; the application further discloses a stress compensation system for balancing thermal-induced birefringence of a laser crystal, and the stress compensation method for balancing thermal-induced birefringence of a laser crystal, wherein the stress compensation system comprises a polarization measurement unit, a controller and a stress compensation device; the stress compensation device comprises a clamping table, the clamping table is provided with a first limiting surface and a second limiting surface which intersect and are arranged at a first set angle, and the first limiting surface and the second limiting surface combine to form a positioning structure; the polarization state of the laser beam can be detected in real time through the polarization measurement unit, and force is applied to the crystal through the stress compensation device, so that the internal stress of the crystal is more balanced, and the birefringence effect is reduced.

Description

Technical Field

[0001] This invention relates to the field of laser technology, and more specifically to a stress compensation system, laser, and method for balancing thermally induced birefringence of laser crystals. Background Technology

[0002] Solid-state lasers, especially high-power solid-state lasers, are widely used in industrial processing, medical applications, and scientific research. Yb:YAG (ytterbium-doped aluminum garnet) and other laser crystals are commonly used gain media. However, under high-power pumping, the laser crystal absorbs pump light, generating a large amount of waste heat, leading to a temperature gradient within the crystal. This non-uniform temperature distribution causes non-uniform thermal stress within the crystal.

[0003] Due to the photoelastic effect of crystals, thermal stress causes changes in the crystal's refractive index, and these changes are usually anisotropic, meaning that light with different polarization directions experiences different refractive indices. This is the phenomenon of thermal birefringence. Thermal birefringence causes depolarization loss in lasers, reduces the polarization purity of the laser output, deteriorates beam quality (e.g., increases the M² factor), limits the improvement of laser output power and efficiency, and may even damage optical components in some cases.

[0004] In the prior art, methods to mitigate thermally induced birefringence include: selecting crystal materials with high thermal conductivity and low thermo-optical and stress-optical coefficients; optimizing the crystal cutting direction; employing a specially designed pumping scheme to obtain a more uniform heat distribution; using special optical compensation elements (such as quartz rotators, Faraday rotators combined with waveplates, etc.) for intracavity or extracavity compensation; or using gain media with geometries such as slabs or thin-disks to improve heat dissipation.

[0005] However, thermally induced birefringence remains a significant problem for conventional lasers operating at high average power. Existing compensation schemes are often passive or fail to fully and flexibly adapt to dynamic thermal effects under different operating conditions. In particular, thermal stress-induced birefringence often exhibits an imbalance by having different intensities in two mutually perpendicular directions (typically radial and tangential).

[0006] Therefore, how to solve the problem that the laser crystal (such as Yb:YAG) described in the prior art generates unbalanced stress inside due to thermal effects when operating at high power, which in turn causes severe thermal birefringence and affects laser performance, has become the subject of this invention. Summary of the Invention

[0007] This invention provides a stress compensation system, laser, and method for balancing the thermally induced birefringence of laser crystals, aiming to solve the technical problems mentioned in the background art.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is: a stress compensation system for balancing the thermally induced birefringence of a laser crystal, comprising: S1, fixing the crystal on a stress compensation device and guiding the laser beam along the optical axis through the geometric center of the crystal; S2, acquiring the initial stress applied by the stress application device in this state when the laser beam passes through the crystal without depolarization; S3, detecting the polarization state of the laser beam in real time through a polarization measurement unit and generating corresponding detection information, and the controller obtaining the depolarization deviation value of the laser beam based on the detection information fed back by the polarization measurement unit and the set target value, and calculating the depolarization deviation value to obtain the compensation force to be applied; S4, applying a compensation force to the crystal through the stress compensation device, wherein the direction of the compensation force is perpendicular to the propagation direction of the laser beam, and the magnitude is calculated by the controller based on the depolarization deviation value; the set target value is the detection information when the laser beam has not depolarized.

[0009] In the above schemes, the crystals are mostly Yb:YAG (ytterbium-doped yttrium aluminum garnet). Providing initial stress ensures that the crystal's position is fixed and does not move arbitrarily. The compensation force to be applied is based on the initial stress, and obtaining the initial stress also eliminates the influence of stress when the laser beam has not depolarized; that is, it is necessary to avoid the initial stress preventing the laser beam from depolarizing.

[0010] In the above scheme, after the laser beam passes through the geometric center of the crystal along the optical axis, it heats up the crystal, causing a temperature gradient to form inside the crystal. This non-uniform temperature distribution causes non-uniform thermal stress inside the crystal. Thermal stress causes a change in the crystal's refractive index, and this change is usually anisotropic, meaning that light with different polarization directions experiences different refractive indices. This is thermal birefringence.

[0011] In the above scheme, the polarization state of the laser beam is detected in real time by a polarization measurement unit, and corresponding detection information is generated. The controller calculates the depolarization deviation value of the laser beam based on the detection information fed back by the polarization measurement unit and the set target value, and calculates the depolarization deviation value to determine the compensation force that needs to be applied.

[0012] The polarization measurement unit detects the power of the laser beam after reflection and projection through the beam splitter in real time. The higher the power of the reflected laser beam, the more severe the depolarization, i.e., the greater the birefringence.

[0013] The target value is the detection information when the laser beam does not undergo thermal birefringence, which is the debiasing deviation value when the laser beam does not undergo thermal birefringence.

[0014] In the above scheme, the polarization state of the laser beam can be detected in real time through a polarization measurement unit, and then a stress compensation device can be used to apply force to the crystal to make the internal stress of the crystal more balanced, thereby reducing the birefringence effect.

[0015] Unlike existing technologies, the polarization measurement unit and stress compensation device in this invention can work together to detect and adjust in real time, thereby achieving dynamic compensation for the thermally induced birefringence effect. Simultaneously, through continuous analysis and feedback adjustment of the detection information by the controller, the polarization state of the laser beam is ensured to remain stable near the set target value, avoiding optical performance degradation caused by temperature gradients within the crystal. This solution not only improves the output stability of the laser system but also extends the lifespan of the laser crystal, making it suitable for applications in high-power lasers where high beam quality is required.

[0016] It should be noted that since the two implementation methods are divided into manual and automatic, over-adjustment can easily occur during control. Therefore, a further operating method is provided, which also includes step five:

[0017] The polarization state detected within a set time period is combined with the compensation force generated by the controller to construct a detection-force application-detection closed-loop system. Within this system, a stress adjustment area is provided for each compensation force to ensure that the adjustment range does not exceed the stress adjustment area during each adjustment.

[0018] At the same time, the stress adjustment area is divided equally, and the specific range of each adjustment is planned.

[0019] Traditional thermally induced birefringence compensation methods often employ static, open-loop stress application. This involves applying a fixed compensation stress after laser assembly or preheating. However, during actual operation, the laser's thermal load is a dynamically changing parameter. A fixed compensation stress cannot track this dynamic change, leading to a decrease in compensation effectiveness, a drift in the polarization state of the output laser, and a deterioration in the extinction ratio.

[0020] The technical solution adopted in this invention is to construct a closed-loop control system based on real-time feedback of polarization state.

[0021] The system continuously detects the polarization state of the output laser through a polarization measurement unit and feeds the detection information back to the controller. The controller compares the detection information with a preset target value (i.e., the signal value under ideal depolarization-free state), calculates the depolarization deviation value, and generates a dynamic compensation force control command accordingly. The stress compensation device receives the command and applies corresponding compensation stress to the laser crystal.

[0022] Furthermore, to improve the stability and reliability of the closed-loop system, this invention introduces a control strategy with a stress adjustment region. The controller sets an adjustment region (or tolerance band) for each compensation force. The controller only initiates calculation and outputs a new compensation force command when the deflection deviation exceeds the boundary of this adjustment region. This design effectively suppresses frequent system adjustments and oscillations, prevents malfunctions caused by measurement noise or minor fluctuations, and significantly reduces mechanical wear of the actuator while ensuring compensation accuracy, thereby improving the overall system lifespan and stability.

[0023] It should be noted that sometimes the compensation force is not an integer. When the compensation force has a decimal, such as 5.4 Newtons, the stress adjustment range is selected based on 5 Newtons according to the rounding standard.

[0024] Meanwhile, each time an adjustment is needed, the controller does not directly calculate a final target force, but intelligently plans which sub-range the adjustment should be applied to based on the current deflection deviation value. For example, for a large initial deviation, a larger adjustment step size (spanning multiple sub-ranges) can be planned for rapid convergence; when approaching the target, a minimum adjustment step size (a single sub-range) is planned for fine-tuning.

[0025] Ensuring adjustment stability: This mechanism ensures that the magnitude of each adjustment is strictly limited to a pre-planned, finite sub-range, fundamentally avoiding system oscillations caused by excessive single adjustments, making the compensation process smoother, faster, and more reliable.

[0026] By constructing a real-time feedback closed-loop control system, dynamic, online, and adaptive compensation for the thermally induced birefringence effect is achieved. This system can effectively track thermal load fluctuations caused by changes in factors such as laser power and ambient temperature, completely solving the problem that static compensation methods cannot adapt to dynamic processes, and ensuring that the laser output maintains a high polarization extinction ratio throughout the entire working period.

[0027] A further technical solution is that the specific operation of S3 is as follows:

[0028] S301. When the laser beam passes through the geometric center of the crystal along the optical axis, the controller runs the built-in control algorithm in real time.

[0029] S302. The laser beam is split into two parts, transmitted light I1 and reflected light I2, by a beam splitter. The polarization measurement unit detects the power of transmitted light I1 and reflected light I2 in real time, and inputs the power of transmitted light I1 and reflected light I2 detected by the polarization measurement unit to the controller. The controller calculates the polarization extinction ratio based on the power of transmitted light I1 and reflected light I2, and uses the extinction ratio as the currently detected depolarization data.

[0030] S303, The controller calculates the deflection deviation value based on the currently detected deflection data and the set target value;

[0031] S304. The controller processes the deflection deviation value according to the control algorithm and calculates the compensation force required to reduce the deflection deviation value.

[0032] The polarization extinction ratio is equal to the power of the transmitted light I1 divided by the power of the reflected light I2.

[0033] The above design enables the controller to precisely control the stress compensation device to apply corresponding compensation force to the crystal, thereby dynamically adjusting the stress distribution inside the crystal. Through the cooperation of the polarization measurement unit and the controller, continuous closed-loop feedback regulation is achieved (i.e., detection – calculation of depolarization deviation value – calculation of compensation force – application of force – detection), ensuring that the polarization state of the laser beam remains stable near the set target value, effectively suppressing beam quality degradation caused by thermal birefringence. This method features fast response speed and high control precision, making it suitable for applications in high-power laser systems with stringent requirements for polarization stability, significantly improving the output stability and beam quality consistency of the laser.

[0034] The algorithm can be a PID algorithm.

[0035] It is important to note that the polarization extinction ratio (PER) is a direct indicator of the amount of thermally induced birefringence. The polarization extinction ratio PER = I1 / I2. A higher PER value indicates less thermally induced birefringence.

[0036] The debiasing deviation value is an indicator that measures how much the current PER deviates from the target PER. The larger the deviation value, the more severe the thermally induced birefringence (i.e., the lower the PER value), the more the system debiases, and the greater the compensation force required to correct it.

[0037] The "depolarization deviation value" is not the "polarization extinction ratio (PER)" itself, but rather the difference between the "target PER" (i.e., the set target value, which is the depolarization deviation value when the laser beam does not undergo thermal birefringence) and the "actual measured PER". This deviation value is the input signal that the control system directly uses for calculation.

[0038] It is important to note that the polarization measurement unit can be equipped with two photodiodes, two photodetectors, a polarization beam analyzer, or an online polarization detection module, etc., to measure the power of transmitted light I1 and reflected light I2. It is also necessary to select a model with matching response wavelength, fast response speed, and good linearity.

[0039] The controller can be an embedded microcontroller or a Raspberry Pi. Its core task is to execute control algorithms (such as PID) in real time, calculate the accurate compensation force based on the input deflection deviation value, and display it.

[0040] In a further technical solution, the laser beam in S1 is configured as a pump laser beam with an output power of less than 300W. This is because a power output greater than 300W would cause excessive stress within the crystal, making it prone to damage. Therefore, a pump laser beam with an output power of less than 300W is chosen.

[0041] A further technical solution is to set the initial stress in S2 to 0-0.1 MPa. With the above design, structural damage to the crystal due to excessive preload stress can be avoided when the crystal is not in operation, while ensuring that the subsequent applied compensation force can adjust the internal stress distribution of the crystal within a reasonable range.

[0042] In a further technical solution, the direction of the compensation force in S4 is perpendicular to the direction of laser beam propagation, and the crystal cross-section is preferably quadrilateral. After the compensation force is applied, forces act on all four sides of the crystal, making the stress distribution inside the crystal more uniform. By adjusting the magnitude of the applied force, the birefringence effect caused by the temperature gradient can be effectively counteracted, thereby maintaining the stability of the polarization state of the laser beam.

[0043] This invention also provides a stress compensation system for balancing the thermally induced birefringence of a laser crystal and a stress compensation method for balancing the thermally induced birefringence of a laser crystal. The stress compensation system includes a polarization measurement unit, a controller, and a stress compensation device. The stress compensation device includes a clamping stage, and the clamping stage has a first limiting surface and a second limiting surface that intersect and are set at a first predetermined angle. The first limiting surface and the second limiting surface combine to form a positioning structure. The first limiting surface is configured to fit against a first surface on the crystal; the second limiting surface is configured to fit against a second surface on the crystal. The first surface and the second surface intersect at an angle equal to the first predetermined angle, such that the intersection of the first limiting surface and the second limiting surface abuts against the intersection of the first surface and the second surface. The clamping stage also has a third limiting surface and a fourth limiting surface. The third limiting surface intersects with the first limiting surface and the second limiting surface at a first predetermined angle. A first limiting surface is parallel and spaced apart, and a fourth limiting surface is parallel and spaced apart from the second limiting surface. The crystal also has a third surface and a fourth surface. The first surface, the second surface, the third surface, and the fourth surface are evenly distributed along the outer periphery of the crystal. The third surface is parallel to the first limiting surface and is positioned corresponding to the third limiting surface. The fourth surface is parallel to the second limiting surface and is positioned corresponding to the fourth limiting surface. The stress compensation device also includes a force applicator. A force applicator is positioned and connected between the third surface and the third limiting surface. The working end of the force applicator is configured to act on the third surface of the crystal and apply a vertical force. A force applicator is also positioned and connected between the fourth surface and the fourth limiting surface. The working end of the force applicator is configured to act on the fourth surface of the crystal and apply a vertical force.

[0044] With the above design, the force applicator only needs to apply compensating force to two of the crystal surfaces to make the stress distribution inside the crystal uniform.

[0045] Specifically, the crystal has a first surface, a second surface, a third surface, and a fourth surface. The first limiting surface is in close contact with the first surface of the crystal, and the second limiting surface is in close contact with the second surface of the crystal. This indicates that the first and second limiting surfaces together serve as the supporting surfaces of the crystal, effectively restricting its position.

[0046] The initial angle can be 90 degrees, 120 degrees, or other angles. As a preferred value, this application selects 90 degrees. Its geometric center can be quickly located.

[0047] When the compensating force is applied by the subsequent force applicator, only the third and fourth surfaces need to be compensated. The first and second limiting surfaces will then apply opposite forces to the first and second surfaces respectively, thereby ensuring that the stress distribution inside the crystal is uniform.

[0048] It is important to note that two or more stressors can be positioned at different locations around the laser crystal, such as in two mutually perpendicular directions, to independently apply and adjust stress in different directions. Furthermore, the direction of stress application is typically perpendicular to the propagation direction of the laser beam.

[0049] In a further technical solution, the stress compensation device further includes a layered protective structure; the layered protective structure includes a flexible heat-conducting block and a pressure head; along the force application direction of the force applicator's working end, the pressure head and the flexible heat-conducting block are sequentially attached and arranged, the pressure head abuts against the force applicator's working end, and the flexible heat-conducting block abuts against the crystal surface; wherein, the layered protective structure is provided between the third surface and the corresponding force applicator's working end; and the layered protective structure is also provided between the fourth surface and the corresponding force applicator's working end.

[0050] With the above design, the third and fourth surfaces of the crystal will not be damaged when the force applicator applies a compensating force. Specifically, since the layered protective structure is provided on both the third and fourth surfaces, and the working principle of the layered protective structure on the fourth surface is the same as that on the third surface, the following explanation will focus on the third surface.

[0051] When the force-applying end of the actuator applies a compensating force toward the third surface, its actuating end comes into close contact with the pressure head, effectively preventing damage to the crystal surface. Simultaneously, the flexible heat-conducting block ensures good thermal contact and buffering effect.

[0052] There are many types of force applicators. The following is one embodiment. The force applicator includes two screws, all of which are inserted through the clamping platform. One screw passes through the third limiting surface and its axis is perpendicular to the third limiting surface. The other screw passes through the fourth limiting surface and its axis is perpendicular to the fourth limiting surface. One end of each screw serves as the working end and abuts against the crystal surface, while the other end serves as the free end.

[0053] It should be noted that the compensation force calculated by the controller in this implementation can be displayed on a screen. The screen is electrically connected to the controller.

[0054] With the above design, the operator can manually apply compensating force. Specifically, a constant torque wrench can be used to rotate the free end of the screw, as long as the torque does not exceed the preset torque of the constant torque wrench. Adjustments can then be made based on the data displayed in real time.

[0055] In this implementation, there is no need for complex controllers, sensors, or drive circuits, thus eliminating the problems of temperature drift, aging, or noise associated with electronic components.

[0056] Another embodiment of the force applicator is provided, namely, the second embodiment of the force applicator, wherein the force applicator includes a high-voltage driver and two piezoelectric ceramic actuators, wherein one of the piezoelectric ceramic actuators is positioned and connected between the third surface and the third limiting surface;

[0057] Another of the piezoelectric ceramic actuators is positioned between the fourth surface and the fourth limiting surface;

[0058] The high-voltage driver is electrically connected to the piezoelectric ceramic actuator. The high-voltage driver is configured to convert the electrical signal output by the controller into a high voltage required to drive the piezoelectric ceramic actuator, so that the piezoelectric ceramic actuator generates pressure and acts on the crystal.

[0059] With the above design, the applied compensation force can be adjusted in real time. Specifically, once the controller calculates the required compensation force, it will be converted into an electrical signal and output to the force applicator. Then, the high-voltage driver will convert the electrical signal output by the controller into the high voltage required to drive the piezoelectric ceramic actuator, so that the piezoelectric ceramic actuator generates pressure according to the driving high voltage and acts on the crystal.

[0060] Unlike the first implementation, this closed-loop system can automatically adapt to different pump power laser beams and ambient temperature changes, maintaining the best birefringence compensation effect, thereby obtaining highly stable and high-quality laser beam output.

[0061] It should be noted that in this implementation, after the compensation force is calculated, it is directly converted into an electrical signal and output to the force applicator, and then the high-voltage driver will respond.

[0062] A third embodiment of the force applicator is provided, wherein the force applicator further includes a heat-driven force application structure that completes the force application action through thermal expansion and contraction.

[0063] The terms "first," "second," etc., used in this article do not specifically refer to order or sequence, nor are they intended to limit this case; they are merely used to distinguish components or operations described using the same technical terms.

[0064] The terms "connection" or "positioning" as used in this article can refer to two or more components or devices making direct physical contact with each other, or making indirect physical contact with each other, or to two or more components or devices operating or moving with each other.

[0065] The terms “include,” “including,” and “have” used in this article are all open-ended, meaning they include but are not limited to.

[0066] Unless otherwise specified, the terms used herein generally have their ordinary meaning in the context of the art, the subject matter, and the specific context. Certain terms used to describe this case will be discussed below or elsewhere in this specification to provide additional guidance to those skilled in the art in describing the case.

[0067] The terms “front,” “back,” “up,” “down,” “left,” and “right” used in this article are directional terms. In this case, they are only used to describe the positional relationship between the structures and are not intended to limit the specific direction of the protection scheme or its actual implementation.

[0068] The working principle and advantages of this invention are as follows:

[0069] This invention can detect the polarization state of a laser beam in real time through a polarization measurement unit, and then apply force to the crystal through a stress compensation device to make the internal stress of the crystal more balanced, thereby reducing the birefringence effect.

[0070] Unlike existing technologies, the polarization measurement unit and stress compensation device in this invention can work together to detect and adjust in real time, thereby achieving dynamic compensation for the thermally induced birefringence effect. Simultaneously, through continuous analysis and feedback adjustment of the detection information by the controller, the polarization state of the laser beam is ensured to remain stable near the set target value, avoiding optical performance degradation caused by temperature gradients within the crystal. This solution not only improves the output stability of the laser system but also extends the lifespan of the laser crystal, making it suitable for applications in high-power lasers where high beam quality is required.

[0071] Meanwhile, the stress compensation device allows the force applicator to apply compensation force to only two surfaces of the crystal, thus ensuring a uniform stress distribution within the crystal.

[0072] Specifically, the crystal has a first surface, a second surface, a third surface, and a fourth surface. The first limiting surface is in close contact with the first surface of the crystal, and the second limiting surface is in close contact with the second surface of the crystal. This indicates that the first and second limiting surfaces together serve as the supporting surfaces of the crystal, effectively restricting its position.

[0073] When the compensating force is applied by the subsequent force applicator, only the third and fourth surfaces need to be compensated. The first and second limiting surfaces will then apply opposite forces to the first and second surfaces respectively, thereby ensuring that the stress distribution inside the crystal is uniform. Attached Figure Description

[0074] Appendix Figure 1 This is a schematic diagram of the force applicator in the first embodiment of the present invention;

[0075] Appendix Figure 2 This is a schematic diagram of the force applicator in the second embodiment of the present invention;

[0076] Appendix Figure 3 This is a schematic diagram illustrating the principle of the polarization measurement unit detecting transmitted light I1 and reflected light I2 in an embodiment of the present invention.

[0077] In the above figures: 1. Clamping stage; 2. First limiting surface; 3. Second limiting surface; 4. Crystal; 5. First surface; 6. Second surface; 7. Third limiting surface; 8. Fourth limiting surface; 9. Third surface; 10. Fourth surface; 11. Force applicator; 12. Layered protective structure; 13. Flexible heat-conducting block; 14. Pressure head; 15. Screw; 16. Piezoelectric ceramic actuator; 17. Polarization measurement unit; 18. Controller; 19. High-voltage driver;

[0078] Q. Stress compensation device. Detailed Implementation

[0079] The present invention will be further described below with reference to the accompanying drawings and embodiments:

[0080] Example: The present invention will be clearly described below with illustrations and detailed description. Any person skilled in the art who understands the examples of the present invention can make changes and modifications based on the technology taught in the present invention without departing from the spirit and scope of the present invention.

[0081] The terminology used herein is for the purpose of describing specific embodiments only and is not intended to limit the scope of this work. Singular forms such as “a,” “this,” “this,” “the,” and “the” as used herein also include plural forms.

[0082] Reference Appendix Figure 3A stress compensation method for balancing thermally induced birefringence of a laser crystal includes: S1, fixing a crystal 4 on a stress compensation device Q and guiding a laser beam along the optical axis through the geometric center of the crystal 4; S2, acquiring the initial stress applied by the stress application device in this state when the laser beam passes through the crystal 4 without depolarization; S3, detecting the polarization state of the laser beam in real time through a polarization measurement unit 17 and generating corresponding detection information, and the controller 18 obtaining the depolarization deviation value of the laser beam based on the detection information fed back by the polarization measurement unit 17 and the set target value, and calculating the depolarization deviation value to obtain the compensation force to be applied; S4, applying a compensation force to the crystal 4 through the stress compensation device Q, wherein the direction of the compensation force is perpendicular to the propagation direction of the laser beam, and the magnitude is calculated by the controller 18 based on the depolarization deviation value; the set target value is the detection information when the laser beam has not depolarized.

[0083] In this embodiment, crystal 4 is mostly Yb:YAG (ytterbium-doped yttrium aluminum garnet). The provision of initial stress ensures that the position of crystal 4 will not move arbitrarily after it is fixed.

[0084] In this embodiment, after the laser beam passes through the geometric center of crystal 4 along the optical axis, it will cause crystal 4 to heat up, resulting in a temperature gradient inside crystal 4. This non-uniform temperature distribution will cause non-uniform thermal stress inside crystal 4. Thermal stress will cause the refractive index of crystal 4 to change, and this change is usually anisotropic, that is, light with different polarization directions experiences different refractive indices. This is thermal birefringence.

[0085] In this embodiment, the polarization state of the laser beam is detected in real time by the polarization measurement unit 17, and corresponding detection information is generated. The controller 18 calculates the depolarization deviation value of the laser beam based on the detection information fed back by the polarization measurement unit 17 and the set target value, and calculates the depolarization deviation value to determine the compensation force that needs to be applied.

[0086] The polarization measurement unit 17 is used to detect the power of the laser beam after reflection and projection by the beam splitter in real time. The higher the power of the reflected laser beam, the more severe the depolarization, that is, the greater the birefringence.

[0087] The target value is the detection information when the laser beam does not depolarize, that is, the depolarization deviation value when the laser beam does not undergo thermal birefringence.

[0088] In this invention, the polarization state of the laser beam can be detected in real time through the polarization measurement unit 17, and then a force can be applied to the crystal 4 through the stress compensation device Q to make the internal stress of the crystal 4 more balanced, thereby reducing the birefringence effect.

[0089] Unlike existing technologies, the polarization measurement unit 17 and stress compensation device Q in this invention can cooperate to detect and adjust in real time, thereby achieving dynamic compensation for the thermally induced birefringence effect. Simultaneously, the controller 18 continuously analyzes and provides feedback adjustments to the detected information, ensuring that the polarization state of the laser beam remains stable near the set target value, avoiding optical performance degradation caused by the internal temperature gradient of the crystal 4. This solution not only improves the output stability of the laser system but also extends the lifespan of the laser crystal 4, making it suitable for applications in high-power lasers where high beam quality is required.

[0090] Preferably, the specific operation of S3 is as follows:

[0091] S301. When the laser beam passes through the geometric center of the crystal 4 along the optical axis, the controller 18 runs the built-in control algorithm in real time.

[0092] S302. The laser beam is split into two parts, transmitted light I1 and reflected light I2, by a beam splitter. The polarization measurement unit 17 detects the power of the transmitted light I1 and the power of the reflected light I2 in real time, and inputs the power of the transmitted light I1 and the power of the reflected light I2 detected by the polarization measurement unit 17 to the controller 18. The controller 18 calculates the polarization extinction ratio based on the power of the transmitted light I1 and the power of the reflected light I2, and uses the extinction ratio as the depolarization data obtained at the moment of detection.

[0093] S303, Controller 18 calculates the deflection deviation value based on the currently detected deflection data and the set target value;

[0094] S304, the controller 18 processes the deflection deviation value according to the control algorithm and calculates the compensation force required to reduce the deflection deviation value;

[0095] The polarization extinction ratio is equal to the power of the transmitted light I1 divided by the power of the reflected light I2.

[0096] With the above design, the controller 18 can precisely control the stress compensation device Q to apply the corresponding compensation force to the crystal 4, thereby dynamically adjusting the stress distribution inside the crystal 4. Through the cooperation of the polarization measurement unit 17 and the controller 18, continuous closed-loop feedback regulation is achieved (i.e., detection – calculation of depolarization deviation value – calculation of compensation force – application of force – detection), ensuring that the polarization state of the laser beam remains stable near the set target value, effectively suppressing the beam quality degradation problem caused by thermal birefringence. This method has a fast response speed and high control precision, making it suitable for applications in high-power laser systems with stringent requirements for polarization stability, significantly improving the output stability and beam quality consistency of the laser.

[0097] The algorithm can be a PID algorithm.

[0098] It is important to note that the polarization extinction ratio (PER) is a direct indicator of the amount of thermally induced birefringence. The polarization extinction ratio PER = I1 / I2. A higher PER value indicates less thermally induced birefringence.

[0099] The debiasing deviation value is an indicator that measures how much the current PER deviates from the target PER. The larger the deviation value, the more severe the thermally induced birefringence (i.e., the lower the PER value), the more the system debiases, and the greater the compensation force required to correct it.

[0100] The "depolarization deviation value" is not the "polarization extinction ratio (PER)" itself, but rather the difference between the "target PER" and the "actual measured PER". This deviation value is the input signal that the control system directly uses for calculation.

[0101] It should be noted that the polarization measurement unit 17 can be equipped with two photodiodes, two photodetectors, a polarization beam analyzer, or an online polarization detection module, etc., to measure the power of transmitted light I1 and reflected light I2. A model with matching response wavelength, fast response speed, and good linearity should be selected.

[0102] The controller 18 can be an embedded microcontroller 18 or a Raspberry Pi, etc. Its core task is to execute control algorithms (such as PID) in real time, calculate the accurate compensation force based on the input deflection deviation value and display it.

[0103] Preferably, the laser beam in S1 is configured as a pump laser beam with an output power of less than 300W. This is because a power output greater than 300W would cause excessive stress inside the crystal 4, making it prone to damage. Therefore, a pump laser beam with an output power of less than 300W is selected.

[0104] Preferably, the initial stress in S2 is 0-0.1 MPa. With the above design, structural damage to crystal 4 due to excessive preload stress can be avoided when the crystal 4 is not in operation, while ensuring that the subsequent applied compensation force can adjust the stress distribution inside crystal 4 within a reasonable range.

[0105] Preferably, the direction of the compensating force in S4 is perpendicular to the direction of laser beam propagation, and the cross-section of crystal 4 is preferably quadrilateral. After the compensating force is applied, all four sides of crystal 4 are subjected to force, making the stress distribution inside crystal 4 more uniform. By adjusting the magnitude of the applied force, the birefringence effect caused by the temperature gradient can be effectively counteracted, thereby maintaining the stability of the polarization state of the laser beam.

[0106] See appendix Figure 1 - Figure 2As shown, the present invention also provides a stress compensation system for balancing the thermally induced birefringence of a laser crystal and a stress compensation method for balancing the thermally induced birefringence of a laser crystal. The stress compensation system includes a polarization measurement unit 17, a controller 18, and a stress compensation device Q. The stress compensation device Q includes a clamping stage 1, which has a first limiting surface 2 and a second limiting surface 3 intersecting and arranged at a first predetermined angle. The first limiting surface 2 and the second limiting surface 3 combine to form a positioning structure. The first limiting surface 2 is configured to fit against a first surface 5 on the crystal 4. The second limiting surface 3 is configured to fit against a second surface 6 on the crystal 4. The first surface 5 and the second surface 6 intersect at an angle equal to the first predetermined angle, such that the intersection of the first limiting surface 2 and the second limiting surface 3 abuts against the intersection of the first surface 5 and the second surface 6. The clamping stage 1 also has a third limiting surface 7 and a fourth limiting surface 8. The third limiting surface 7 intersects with the first limiting surface 2. The fourth limiting surface 8 is parallel and spaced apart from the second limiting surface 3; the crystal 4 also has a third surface 9 and a fourth surface 10, the first surface 5, the second surface 6, the third surface 9 and the fourth surface 10 are evenly distributed along the outer periphery of the crystal 4, the third surface 9 is parallel to the first limiting surface 2 and is disposed corresponding to the third limiting surface 7, the fourth surface 10 is parallel to the second limiting surface 3 and is disposed corresponding to the fourth limiting surface 8; the stress compensation device Q also includes a force applicator 11; a force applicator 11 is positioned and connected between the third surface 9 and the third limiting surface 7, the working end of the force applicator 11 is configured to act on the third surface 9 of the crystal 4 and apply a vertical force; a force applicator 11 is also positioned and connected between the fourth surface 10 and the fourth limiting surface 8, the working end of the force applicator 11 is configured to act on the fourth surface 10 of the crystal 4 and apply a vertical force.

[0107] With the above design, the force applicator 11 can apply a compensating force to only two surfaces of the crystal 4 to make the internal stress distribution of the crystal 4 uniform.

[0108] Specifically, the crystal 4 has a first surface 5, a second surface 6, a third surface 9, and a fourth surface 10. The first limiting surface 2 is in close contact with the first surface 5 of the crystal 4, and the second limiting surface 3 is in close contact with the second surface 6 of the crystal 4. This indicates that the first limiting surface 2 and the second limiting surface 3 together serve as the supporting surfaces of the crystal 4, effectively restricting the position of the crystal 4.

[0109] When the compensating force is applied by the subsequent force applicator 11, only the compensating force needs to be applied to the third surface 9 and the fourth surface 10. The first limiting surface 2 and the second limiting surface 3 will then apply the opposite force to the first surface 5 and the second surface 6 respectively, thereby ensuring that the stress distribution inside the crystal 4 is uniform.

[0110] It should be noted that two or more force applicators 11 can be set at different positions around the laser crystal 4, for example, in two mutually perpendicular directions, so as to independently apply and adjust the stress in different directions. And the direction of stress application is usually perpendicular to the propagation direction of the laser beam.

[0111] Preferably, the stress compensation device Q further includes a layered protective structure 12; the layered protective structure 12 includes a flexible heat-conducting block 13 and a pressure head 14; along the force application direction of the force-applying end of the force-applying device 11, the pressure head 14 and the flexible heat-conducting block 13 are sequentially attached and arranged, the pressure head 14 abuts against the force-applying end of the force-applying device 11, and the flexible heat-conducting block 13 abuts against the surface of the crystal 4; wherein, the layered protective structure 12 is provided between the third surface 9 and the corresponding force-applying end of the force-applying device 11; and the layered protective structure 12 is also provided between the fourth surface 10 and the corresponding force-applying end of the force-applying device 11.

[0112] With the above design, the third surface 9 and the fourth surface 10 of the crystal 4 will not be damaged when the force applicator 11 applies the compensating force. Specifically, since the layered protective structure 12 is provided on both the third surface 9 and the fourth surface 10, and the working principle of the layered protective structure 12 on the fourth surface 10 is the same as that on the third surface 9, the following description will focus on the third surface 9.

[0113] When the actuating end of the force applicator 11 applies a compensating force toward the third surface 9, its actuating end comes into close contact with the pressure head 14, effectively preventing damage to the surface of the crystal 4. At the same time, the flexible heat-conducting block 13 ensures good thermal contact and buffering effect.

[0114] There are many types of force applicators 11. The following is one embodiment. The force applicator 11 includes two screws 15. All the screws 15 are inserted into the clamping platform 1. One screw 15 passes through the third limiting surface 7 and its axis is perpendicular to the third limiting surface 7. The other screw 15 passes through the fourth limiting surface 8 and its axis is perpendicular to the fourth limiting surface 8. One end of the screw 15 serves as the working end and abuts against the surface of the crystal 4. The other end serves as the free end.

[0115] It should be noted that the compensation force calculated by the controller 18 in this embodiment can be displayed on the screen.

[0116] With the above design, the operator can manually apply compensating force. Specifically, a constant torque wrench can be used to rotate the free end of screw 15, as long as the torque does not exceed the preset torque of the constant torque wrench. Adjustments can then be made based on the data displayed in real time.

[0117] In this implementation, there is no need for complex controllers 18, sensors, or drive circuits, so there are no issues with temperature drift, aging, or noise of electronic components.

[0118] Another embodiment of the force applicator 11 is provided, namely, a second embodiment of the force applicator 11, wherein the force applicator 11 includes a high-voltage driver 19 and two piezoelectric ceramic actuators 16, one of which is positioned between the third surface 9 and the third limiting surface 7; the other is positioned between the fourth surface 10 and the fourth limiting surface 8; the high-voltage driver 19 is electrically connected to the piezoelectric ceramic actuator 16, and the high-voltage driver 19 is configured to convert the electrical signal output by the controller 18 into a high voltage required to drive the piezoelectric ceramic actuator 16, so that the piezoelectric ceramic actuator 16 generates pressure and acts on the crystal 4.

[0119] With the above design, the applied compensation force can be adjusted in real time. Specifically, once the controller 18 calculates the required compensation force, it will be converted into an electrical signal and output to the force applicator 11. Then, the high-voltage driver 19 will convert the electrical signal output by the controller 18 into the high voltage required to drive the piezoelectric ceramic actuator 16, so that the piezoelectric ceramic actuator 16 generates pressure and acts on the crystal 4.

[0120] Unlike the first implementation, this closed-loop system can automatically adapt to different pump power laser beams and ambient temperature changes, maintaining the best birefringence compensation effect, thereby obtaining highly stable and high-quality laser beam output.

[0121] It should be noted that in this implementation, after the compensation force is calculated, it is directly converted into an electrical signal and output to the force applicator 11, and then the high-voltage driver 19 will respond.

[0122] A third embodiment of the force applicator 11 is provided, wherein the force applicator 11 further includes a heat-driven force application structure that completes the force application action through thermal expansion and contraction.

[0123] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A stress compensation method for balancing thermally induced birefringence in laser crystals, characterized in that: include S1. Fix the crystal (4) on the stress compensation device (Q) and guide the laser beam along the optical axis through the geometric center of the crystal (4); S2. When the laser beam passes through the crystal (4) and no depolarization occurs, obtain the initial stress applied by the stress application device in this state; S3. The polarization state of the laser beam is detected in real time by the polarization measurement unit (17) and corresponding detection information is generated. The controller (18) obtains the depolarization deviation value of the laser beam based on the detection information fed back by the polarization measurement unit (17) and the set target value, and calculates the depolarization deviation value to obtain the compensation force that needs to be applied. The set target value is the detection information when the laser beam does not depolarize; S4. A compensation force is applied to the crystal (4) through the stress compensation device (Q). The direction of the compensation force is perpendicular to the propagation direction of the laser beam, and the magnitude is calculated by the controller (18) based on the deflection deviation value. S5. Summarize the polarization state detected within the set time period and the compensation force generated by the controller to construct a detection-force application-detection closed-loop system. Within this system, provide a stress adjustment area for each compensation force and divide the stress adjustment area equally to plan the specific range of each adjustment.

2. The stress compensation method for balancing thermally induced birefringence of laser crystals according to claim 1, characterized in that: The specific operation of S3 is as follows: S301. When the laser beam passes through the geometric center of the crystal (4) along the optical axis, the controller (18) runs the built-in control algorithm in real time. S302. The laser beam is split into two parts, transmitted light I1 and reflected light I2, by a beam splitter. The polarization measurement unit (17) detects the power of the transmitted light I1 and the power of the reflected light I2 in real time. The power of the transmitted light I1 and the power of the reflected light I2 detected by the polarization measurement unit (17) are input to the controller (18). The controller (18) calculates the polarization extinction ratio based on the power of the transmitted light I1 and the power of the reflected light I2, and uses the extinction ratio as the depolarization data obtained at the moment of detection. S303, The controller (18) calculates the deflection deviation value based on the deflection data obtained from the current detection and the set target value; S304. The controller (18) processes the deflection deviation value according to the control algorithm and calculates the compensation force required to reduce the deflection deviation value. The polarization extinction ratio is equal to the power of the transmitted light I1 divided by the power of the reflected light I2.

3. The stress compensation method for balancing thermally induced birefringence of laser crystals according to claim 1, characterized in that: In S1, the laser beam is configured as a pump laser beam with an output power of less than 300W.

4. The stress compensation method for balancing thermally induced birefringence of laser crystals according to claim 1, characterized in that: The initial stress in S2 is 0-0.1 MPa.

5. A stress compensation system for balancing thermally induced birefringence of laser crystals, characterized in that: The stress compensation system for balancing thermally induced birefringence of laser crystals according to any one of claims 1-4 includes a polarization measurement unit (17), a controller (18), and a stress compensation device (Q). The stress compensation device (Q) includes a clamping stage (1). The clamping stage (1) has a first limiting surface (2) and a second limiting surface (3) that intersect and are set at a first set angle. The first limiting surface (2) and the second limiting surface (3) are combined to form a positioning structure. The first limiting surface (2) is configured to be attached to the first surface (5) on the crystal (4); The second limiting surface (3) is configured to fit against the second surface (6) on the crystal (4), and the first surface (5) and the second surface (6) intersect at an angle equal to the first set angle, so that the intersection of the first limiting surface (2) and the second limiting surface (3) abuts against the intersection of the first surface (5) and the second surface (6). The clamping platform (1) also has a third limiting surface (7) and a fourth limiting surface (8). The third limiting surface (7) is parallel to and spaced apart from the first limiting surface (2), and the fourth limiting surface (8) is parallel to and spaced apart from the second limiting surface (3). The crystal (4) also has a third surface (9) and a fourth surface (10). The first surface (5), the second surface (6), the third surface (9) and the fourth surface (10) are evenly distributed along the outer periphery of the crystal (4). The third surface (9) is parallel to the first limiting surface (2) and is disposed corresponding to the third limiting surface (7). The fourth surface (10) is parallel to the second limiting surface (3) and is disposed corresponding to the fourth limiting surface (8). The stress compensation device (Q) also includes a force applicator (11). A force applicator (11) is positioned and connected between the third surface (9) and the third limiting surface (7), and the working end of the force applicator (11) is configured to act on the third surface (9) and apply a vertical force; A force applicator (11) is also positioned and connected between the fourth surface (10) and the fourth limiting surface (8). The working end of the force applicator (11) is configured to act on the fourth surface (10) and apply a vertical force. The polarization state of the laser beam is detected in real time by the polarization measurement unit (17) and a corresponding detection signal is generated. The controller (18) obtains the depolarization deviation value of the laser beam based on the detection signal fed back by the polarization measurement unit (17) and the set target value, and calculates the depolarization deviation value to obtain the compensation force that needs to be applied. The compensation force is applied to the crystal (4) by the stress compensation device (Q). The direction of the compensation force is perpendicular to the propagation direction of the laser beam, and the magnitude is calculated by the controller (18) based on the depolarization deviation value.

6. The stress compensation system for balancing thermally induced birefringence of laser crystals according to claim 5, characterized in that: The stress compensation device (Q) also includes a layered protective structure (12). The layered protective structure (12) includes a flexible heat-conducting block (13) and a pressure head (14). Along the force application direction of the force application end of the force applicator (11), the pressure head (14) and the flexible heat-conducting block (13) are sequentially attached to each other, the pressure head (14) abuts against the force application end of the force applicator (11), and the flexible heat-conducting block (13) abuts against the surface of the crystal (4). The layered protective structure (12) is provided between the third surface (9) and the corresponding force-applying device (11) action end. Furthermore, the layered protective structure (12) is also provided between the fourth surface (10) and the corresponding force-applying device (11) action end.

7. The stress compensation system for balancing thermally induced birefringence of laser crystals according to claim 6, characterized in that: The force applicator (11) includes two screws (15), all of which are inserted into the clamping table (1); One screw (15) passes through the third limiting surface (7) and its axis is perpendicular to the third limiting surface (7); the other screw (15) passes through the fourth limiting surface (8) and its axis is perpendicular to the fourth limiting surface (8). One end of the screw (15) serves as the working end and abuts against the surface of the crystal (4), while the other end serves as the free end.

8. The stress compensation system for balancing thermally induced birefringence of laser crystals according to claim 6, characterized in that: The force applicator (11) includes a high-voltage driver (19) and two piezoelectric ceramic actuators (16), one of which is positioned between the third surface (9) and the third limiting surface (7). Another of the piezoelectric ceramic actuators (16) is positioned between the fourth surface (10) and the fourth limiting surface (8); The high-voltage driver (19) is electrically connected to the piezoelectric ceramic actuator (16), and the high-voltage driver (19) is configured to convert the electrical signal output by the controller (18) into the high voltage required to drive the piezoelectric ceramic actuator (16) so that the piezoelectric ceramic actuator (16) generates pressure and acts on the crystal (4).

9. The stress compensation system for balancing thermally induced birefringence of laser crystals according to claim 6, characterized in that: The force applicator (11) also includes a heat-driven force application structure that performs the force application action through thermal expansion and contraction.

10. A laser, characterized in that: The laser has a stress compensation system for balancing the thermally induced birefringence of the laser crystal as described in any one of claims 5 to 9.

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