An intracavity tripling laser

Abstract

The application relates to an intracavity third-harmonic laser, comprising a laser resonant cavity, wherein a laser generating unit, a modulation unit, a second-harmonic unit and a third-harmonic unit are sequentially arranged along an optical path in the laser resonant cavity; a dual-wavelength wave plate is arranged in the optical path between the second-harmonic unit and the third-harmonic unit, the dual-wavelength wave plate is configured as a rotatable structure, the dual-wavelength wave plate shows full-wave plate characteristics to fundamental light and shows half-wave plate characteristics to second-harmonic light, the polarization direction of the second-harmonic light can be continuously adjusted by rotating the angle of the dual-wavelength wave plate, the proportion of the second-harmonic light participating in the third-harmonic process is controlled, the photon number proportion of the fundamental light and the second-harmonic light reaches 1:1, and the best conversion efficiency requirement of the third-harmonic unit is met.

Description

Technical Field

[0001] This invention relates to the field of laser technology, and more particularly to an intracavity third-harmonic laser. Background Technology

[0002] Laser frequency conversion technology mainly includes nonlinear frequency conversion methods such as second harmonics, third harmonics, and optical parametric oscillations, which are widely used to generate laser sources with specific wavelengths. Among them, third harmonics technology is the process of generating a new frequency photon by the interaction of two photons of different frequencies in a nonlinear medium. This technology can obtain ultraviolet wavelength lasers of 300-400nm.

[0003] To achieve the highest third-harmonic conversion efficiency, the most common phase-matching method is type II phase matching, which combines two orthogonally polarized photons to generate a third-harmonic photon of the ordinary light. This matching method has a relatively large nonlinear coefficient, resulting in high conversion efficiency. To achieve the optimal third-harmonic efficiency, the fundamental and second-harmonic light need to have the same number of photons.

[0004] However, in practical applications, traditional intracavity third-harmonic lasers have significant drawbacks: due to their enclosed structure, it is difficult to directly observe the energy ratio of the fundamental frequency light and the second-harmonic laser within the cavity, making it impossible to adjust to the optimal ratio in real time to achieve the maximum third-harmonic efficiency. Existing solutions typically use second-harmonic crystals of different sizes and specifications to alter the second-harmonic efficiency, thereby indirectly controlling the intracavity laser ratio. This method requires preparing multiple second-harmonic crystals of different specifications for repeated experiments to determine the optimal configuration, which not only increases experimental costs but also prolongs the debugging cycle, significantly reducing R&D and production efficiency.

[0005] To address the aforementioned issues, there is an urgent need for a technical solution that can precisely adjust the ratio of fundamental frequency light to second harmonic light within the cavity in real time, in order to simplify the debugging process, reduce R&D costs, and achieve optimal third harmonic efficiency. Summary of the Invention

[0006] This invention discloses an intracavity third-harmonic laser to address the problems existing in the prior art.

[0007] This invention provides an intracavity third-harmonic laser, comprising a laser resonant cavity, wherein a laser generation unit, a modulation unit, a second-harmonic unit, and a third-harmonic unit are sequentially arranged along the optical path within the laser resonant cavity;

[0008] A dual-wavelength waveplate is provided in the optical path between the second and third harmonic units. The dual-wavelength waveplate is configured as a rotatable structure. The dual-wavelength waveplate exhibits full-wave plate characteristics for the fundamental frequency light and half-wave plate characteristics for the second harmonic light. By rotating its angle, the polarization direction of the second harmonic light can be continuously adjusted to control the proportion of second harmonic light participating in the third harmonic process, so that the ratio of photons of the fundamental frequency light to the second harmonic light reaches 1:1, thereby meeting the optimal conversion efficiency requirements of the third harmonic unit.

[0009] As a preferred technical solution, the laser generation unit includes a 0° total reflection mirror, a laser crystal, a pump source, and a mode compensation lens;

[0010] A 0° total reflection mirror is positioned at one end of the laser resonant cavity;

[0011] The laser crystal is located between the 0° total reflection mirror and the modulation unit;

[0012] The pump source is configured to side-pump the laser crystal;

[0013] A mode compensation lens is placed between the laser crystal and the modulation unit to compensate for the thermal lensing effect within the laser crystal and to adjust the oscillation mode.

[0014] As a preferred technical solution, a polarizer is also provided between the 0° total reflection mirror and the laser crystal. The polarizer is used to select the polarization mode to ensure that the fundamental frequency light with a single polarization direction is formed in the cavity.

[0015] As a preferred technical solution, the pump source includes a continuous wave semiconductor laser with an output power configuration in the range of 250W to 300W; the laser crystal is a neodymium-doped yttrium lithium fluorine crystal, and the crystal orientation and size of the neodymium-doped yttrium lithium fluorine crystal are matched with the pump wavelength and pumping mode of the pump source.

[0016] As a preferred technical solution, the modulation unit includes an acousto-optic Q-switcher, which is used to modulate the continuous light output by the laser generation unit to generate pulsed laser.

[0017] As a preferred technical solution, the acousto-optic Q-switcher is driven by a 41MHz radio frequency and configured to output a pulse repetition frequency with an adjustable range of 1kHz to 10kHz, enabling the laser to produce a stable Q-switched pulse laser output.

[0018] As a preferred technical solution, a first beam splitter is provided between the modulation unit and the second harmonic unit. The first beam splitter has high transmittance for fundamental frequency light and high reflectivity for harmonic light, and is used to reflect the harmonic light generated by the second harmonic unit to the third harmonic unit.

[0019] As a preferred technical solution, the second harmonic unit includes an LBO crystal using a type I phase-matching method, which is used to convert the fundamental frequency light into second harmonic light.

[0020] As a preferred technical solution, the third harmonic unit includes an LBO crystal using a type II phase matching method, which is used to mix the fundamental frequency light with the second harmonic light to generate the third harmonic light.

[0021] As a preferred technical solution, a second beam splitter is provided after the third harmonic unit. The second beam splitter has high reflectivity for the fundamental frequency light and high transmittance for the third harmonic light. It is used to reflect the fundamental frequency light back into the laser resonant cavity for recycling, while allowing the third harmonic light to be output.

[0022] Compared with the prior art, the technical solution adopted in this invention can achieve the following beneficial effects:

[0023] This invention provides an intracavity third-harmonic laser. Compared with traditional technologies, this invention uses a dual-wavelength waveplate between the second and third harmonic units. This dual-wavelength waveplate exhibits full-waveplate characteristics for the fundamental frequency light and half-waveplate characteristics for the second harmonic light. By simply rotating its angle, the ratio of the fundamental and second harmonic light participating in the third harmonic process can be precisely adjusted, eliminating the need to prepare multiple frequency-harmonic crystals of different specifications for experiments, greatly reducing testing time and cost. At the same time, this scheme makes the intracavity third harmonic effect, which is difficult to observe and adjust, precisely controllable through the dual-wavelength waveplate, so that the photon ratio of the fundamental and second harmonic light reaches 1:1, thus meeting the optimal conversion efficiency requirements of the third harmonic crystal. Attached Figure Description

[0024] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below, forming part of the present invention. The illustrative embodiments of the present invention and their descriptions explain the present invention and do not constitute an improper limitation of the present invention. In the accompanying drawings:

[0025] Figure 1 This is a schematic diagram of the structure of an intracavity third-harmonic laser in a preferred embodiment of the present invention.

[0026] Explanation of reference numerals in the attached figures:

[0027] 0° total reflection mirror 1, polarizer 2, laser crystal 3, pump source 4, mode compensation lens 5, modulation unit 6, first beam splitter 7, second frequency doubling unit 8, dual-wavelength waveplate 9, third frequency doubling unit 10, second beam splitter 11. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below in conjunction with specific embodiments and corresponding drawings. In the description of this invention, it should be noted that the term "or" is generally used to include the meaning of "and / or," unless otherwise expressly indicated.

[0029] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. Furthermore, in the description of this application, the terms "first," "second," etc., are used only for distinguishing descriptions and should not be construed as indicating or implying relative importance. Those skilled in the art will understand that, in order to achieve their respective functions and meet the practical application requirements of the laser, the specific shape / size / angle, etc., of each structure can be adaptively adjusted.

[0030] Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0031] To address the problems existing in the prior art, the present invention provides an intracavity third-harmonic laser in a preferred embodiment, referencing... Figure 1 The laser includes a laser resonant cavity, within which a laser generating unit, a modulation unit 6, a second harmonic unit 8, and a third harmonic unit 10 are sequentially arranged along the optical path. The laser generating unit generates fundamental frequency light and causes it to oscillate stably within the resonant cavity. The modulation unit 6 modulates the continuous fundamental frequency light to generate a pulsed laser with high peak power. The second harmonic unit 8 converts the fundamental frequency light into second harmonic light. The third harmonic unit 10 mixes the fundamental frequency light with the second harmonic light to generate third harmonic light output.

[0032] Preferably, a dual-wavelength waveplate 9 is provided in the optical path between the second harmonic unit 8 and the third harmonic unit 10. The dual-wavelength waveplate 9 is configured as a rotatable structure. The dual-wavelength waveplate 9 exhibits full-waveplate characteristics for the fundamental frequency light, maintaining the polarization state of the fundamental frequency light unchanged during rotation. It exhibits half-waveplate characteristics for the second harmonic light, and by rotating its angle, the polarization direction of the second harmonic light can be continuously adjusted to control the proportion of second harmonic light participating in the third harmonic process, so that the photon ratio of the fundamental frequency light to the second harmonic light reaches 1:1, thereby meeting the optimal conversion efficiency requirement of the third harmonic unit 10. In this embodiment, by setting the dual-wavelength waveplate 9, the adjustment of the intracavity photon ratio, which is difficult to observe directly, becomes controllable. Optimal third harmonic efficiency can be achieved without replacing multiple specifications of frequency doubling crystals, significantly improving the practicality and debugging efficiency of the system.

[0033] Some existing solutions employ waveplates capable of dual-wavelength operation. These waveplates exhibit quarter-waveplate characteristics for the fundamental frequency and half-waveplate characteristics for the second harmonic. Theoretically, the optical field characteristics can be adjusted by rotating the waveplate. However, their physical mechanism has a fundamental flaw, failing to meet the core requirement of Type II phase matching for intracavity third harmonic lasers. This is because Type II phase matching requires the polarization directions of the fundamental frequency light (o-light) and the second harmonic light (e-light) to be strictly orthogonal, i.e., perpendicular polarization states, to achieve maximum third harmonic conversion efficiency (requiring equal photon numbers). However, in existing waveplates, the quarter-waveplate characteristic disrupts polarization orthogonality. When the waveplate rotates, the fundamental frequency light becomes circularly polarized, while the polarization direction of the second harmonic light rotates. The fundamental frequency light cannot form a stable orthogonal polarization state with the second harmonic light, failing to meet the polarization conditions for Type II phase matching. Therefore, the ratio of the fundamental frequency light to the second harmonic light participating in the third harmonic process cannot be effectively adjusted.

[0034] Compared to existing waveplates, the dual-wavelength waveplate 9 in this embodiment is configured with full-waveplate characteristics for the fundamental frequency light and half-waveplate characteristics for the second harmonic light. During rotation, it does not change the linear polarization state of the fundamental frequency light, only altering the polarization direction of the second harmonic light, thereby precisely controlling the polarization orthogonality and energy ratio of the two beams to satisfy type II matching conditions. In a preferred embodiment, the dual-wavelength waveplate 9 employs a multilayer film structure, which can be fabricated by depositing a dual-wavelength antireflection film and a phase retardation film on the substrate surface. By precisely controlling the material composition and thickness of each dielectric film, it achieves full-waveplate characteristics with an optical path difference of 2π or an integer multiple of 2π for the fundamental frequency light wavelength, and simultaneously achieves half-waveplate characteristics with an optical path difference of π or an odd multiple of π for the second harmonic light wavelength. This multilayer film structure utilizes the dispersion characteristics of the material and the multilayer film interference principle to achieve independent phase modulation of two wavelengths of light on a single optical element.

[0035] In this embodiment, the specific technical parameters such as material composition, thickness parameters, surface treatment process, and coating specifications of the dual-wavelength waveplate 9 are no longer specifically limited. Those skilled in the art can adaptively select and optimize the dual-wavelength waveplate 9 according to the specific requirements of laser wavelength, power level, conversion efficiency, etc. in actual applications, combined with existing optical thin film design technology. The key to the dual-wavelength waveplate 9 is to achieve the full-waveplate characteristics for fundamental frequency light and the half-waveplate characteristics for second harmonic light. As long as this core functional requirement can be met, there are various technical paths for implementation, all of which fall within the protection scope of this invention.

[0036] In a preferred embodiment, the dual-wavelength waveplate 9 is mounted on a rotating base, which can achieve continuous and precise rotational adjustment within a range of 0 to 360° in the horizontal or vertical direction. The rotating base is preferably fixed in the laser resonant cavity by an optical bracket to ensure that the dual-wavelength waveplate 9 is perpendicular to the laser propagation direction, while maintaining good mechanical stability to avoid the influence of system vibration on optical performance.

[0037] Specifically, when the fundamental frequency light passes through the dual-wavelength waveplate 9, due to its full-waveplate characteristics, the polarization state of the fundamental frequency light remains unchanged regardless of the angle to which the dual-wavelength waveplate 9 is rotated. However, for the second harmonic light, due to the half-waveplate characteristics, rotating the waveplate by an angle θ will cause the polarization direction of the second harmonic light to rotate by an angle of 2θ. Through this differentiated polarization control characteristic, the polarization component of the second harmonic light entering the third harmonic crystal can be precisely controlled without affecting the polarization state of the fundamental frequency light. This allows for adjustment of the proportion of second harmonic light participating in the type II phase-matching third harmonic process, achieving optimal matching of the number of photons between the fundamental frequency light and the second harmonic light.

[0038] In a preferred embodiment, the laser generating unit is configured as a side-pumped flat-flat resonant cavity structure, which includes a 0° total reflection mirror 1, a polarizer 2, a laser crystal 3, a pump source 4, and a mode compensation lens 5. The 0° total reflection mirror 1 is located at one end of the laser resonant cavity and is used for total internal reflection of the fundamental frequency light. The polarizer 2 is located between the 0° total reflection mirror 1 and the laser crystal 3, which not only forces the laser to operate in a single polarization state but also suppresses stray oscillation modes, improving the laser beam quality and stability. Furthermore, the polarizer 2 has a wavelength selection function: when the polarization direction is parallel to the C-axis of the laser crystal 3, the output wavelength is 1047 nm; when the polarization direction is perpendicular to the C-axis of the laser crystal 3, the output wavelength is 1053 nm. In practical applications, this embodiment of the invention... A laser with a wavelength of 1053 nm is preferably used; the laser crystal 3 is located after the polarizer 2, and is preferably a neodymium-doped yttrium lithium fluorine (Nd:YLF) crystal, with the crystal orientation and size of the Nd:YLF crystal matching the pump wavelength and pumping mode of the pump source 4; the pump source 4 is configured to side-pump the laser crystal 3, and is preferably a continuous wave semiconductor laser with an output power in the range of 250~300W; the mode compensation lens 5 is disposed between the laser crystal 3 and the modulation unit 6, and is used to compensate for the thermal lensing effect in the laser crystal 3 and to adjust the oscillation mode.

[0039] Preferably, the continuous light output power of the laser generating unit is 70-80W.

[0040] In a preferred embodiment, the modulation unit 6 is configured as an acousto-optic Q-switcher, which modulates the continuous light output from the laser generating unit to generate pulsed laser. Preferably, the acousto-optic Q-switcher is driven by a 41MHz radio frequency and configured to output a pulse repetition frequency adjustable from 1kHz to 10kHz, enabling the laser to generate stable Q-switched pulsed laser output.

[0041] Specifically, the upper level lifetime of the neodymium-doped yttrium lithium fluorine crystal is approximately 500 μs, and the output Q-switched laser power is 35~40 W under 1 kHz pulse modulation. It should be noted that this power value was measured during fundamental frequency tuning with the output mirror transmittance at 30%, meaning that the second beam splitter 11 is positioned at T=30% of the output mirror. During frequency doubling tuning, the output mirror at this position needs to be replaced with a dichroic mirror.

[0042] In a preferred embodiment, the second harmonic unit 8 includes an LBO crystal employing a type I phase-matching method for converting fundamental frequency light into second harmonic light. Specifically, this crystal has a high damage threshold, a wide phase-matching temperature bandwidth, and a large effective nonlinear coefficient, enabling it to exhibit excellent performance in high-power fundamental frequency light frequency conversion applications. The type I phase-matching method means that the incident fundamental frequency light has the same polarization direction, and the polarization direction of the generated second harmonic light is perpendicular to the fundamental frequency light. This configuration can fully utilize the nonlinear characteristics of the LBO crystal to achieve efficient frequency multiplication.

[0043] Preferably, a first beam splitter 7 is provided between the modulation unit 6 and the second harmonic unit 8. The first beam splitter 7 has high transmittance for the fundamental frequency light and high reflectivity for the harmonic light, so that the fundamental frequency light in the resonant cavity can efficiently pass through the first beam splitter 7 into the second harmonic unit 8, while the generated second harmonic light is reflected to the third harmonic unit 10, forming an optical unidirectional channel, which significantly improves the extraction efficiency and utilization rate of the second harmonic light.

[0044] Specifically, when a high-peak-power Q-switched fundamental frequency pulse light passes through the frequency-second harmonic unit 8, based on the second-order nonlinear effect, some fundamental frequency photons are converted into frequency-second harmonic photons with doubled energy. Due to the high power density and good beam quality within the cavity, coupled with precise phase matching conditions, the frequency-second harmonic process can achieve high conversion efficiency. At a repetition frequency of 1 kHz, the frequency-second harmonic unit 8 can stably output approximately 30 W of frequency-second harmonic light.

[0045] In a preferred embodiment, the third harmonic unit 10 includes an LBO crystal employing a type II phase matching method. Through precise control of the crystal orientation, the fundamental frequency light and the second harmonic light are combined to generate third harmonic light.

[0046] Preferably, the third harmonic unit 10 can convert the incident fundamental frequency light and second harmonic light into third harmonic ultraviolet light with an efficiency of about 50%, and finally output a 1kHz third harmonic pulsed laser of about 15W.

[0047] In a preferred embodiment, a second beam splitter 11 is provided after the third harmonic unit 10. The second beam splitter 11 has high reflectivity for the fundamental frequency light and high transmittance for the third harmonic light, ensuring that the unconverted fundamental frequency light can be reflected back into the resonant cavity to continue participating in the oscillation process, while the generated third harmonic light is efficiently transmitted through the beam splitter for output.

[0048] Specifically, the second beam splitter 11 not only has wavelength selection function, but also functions as an output coupler. Its position and tilt angle are adaptively adjusted to ensure that the fundamental frequency light reflected back into the cavity can return precisely along the original optical path, maintaining the stable oscillation of the resonant cavity. At the same time, the third harmonic light transmitted out has good beam quality and stability, and can be directly used for downstream applications.

[0049] Compared with existing technologies, the embodiments of the present invention solve the problem of the difficulty in accurately controlling the ratio of fundamental frequency light to second harmonic light in traditional third harmonic systems through the innovative application of dual-wavelength waveplate 9, thereby improving the third harmonic conversion efficiency, simplifying the system structure, and enhancing stability and reliability, providing a superior technical solution for the practical application of high-power ultraviolet lasers.

[0050] The embodiments of the present invention have been described above with reference to the accompanying drawings. However, the present invention is not limited to the specific embodiments described above. The specific embodiments described above are merely illustrative and not restrictive. Those skilled in the art can make many other forms under the guidance of the present invention without departing from the spirit and scope of the claims, and all of these forms are within the protection scope of the present invention.

Claims

1. An intracavity third-harmonic laser, characterized in that, The system includes a laser resonant cavity, in which a laser generation unit, a modulation unit, a second harmonic unit, and a third harmonic unit are sequentially arranged along the optical path. The third harmonic unit includes an LBO crystal using type II phase matching, which is used to mix the fundamental frequency light with the second harmonic light to generate third harmonic light. A dual-wavelength waveplate is provided in the optical path between the second and third harmonic units. The dual-wavelength waveplate is configured as a rotatable structure. The dual-wavelength waveplate exhibits full-waveplate characteristics for the fundamental frequency light and half-waveplate characteristics for the second harmonic light. By rotating its angle, the polarization direction of the second harmonic light can be continuously adjusted, so that the fundamental frequency light and the second harmonic light form a stable orthogonal polarization state. The energy ratio of the second harmonic light participating in the type II phase-matched third harmonic process can be controlled, so that the photon ratio of the fundamental frequency light to the second harmonic light reaches 1:1, thereby meeting the optimal conversion efficiency requirement of the third harmonic unit.

2. The intracavity third-harmonic laser according to claim 1, characterized in that, The laser generation unit includes a 0° total reflection mirror, a laser crystal, a pump source, and a mode compensation lens. The 0° total reflection mirror is disposed at one end of the laser resonant cavity; The laser crystal is located between the 0° total reflection mirror and the modulation unit; The pump source is configured to side-pump the laser crystal; The mode compensation lens is disposed between the laser crystal and the modulation unit to compensate for the thermal lensing effect within the laser crystal and to adjust the oscillation mode.

3. The intracavity third-harmonic laser according to claim 2, characterized in that, A polarizer is also provided between the 0° total reflection mirror and the laser crystal. The polarizer is used to select the polarization mode to ensure that the fundamental frequency light with a single polarization direction is formed in the cavity.

4. The intracavity third-harmonic laser according to claim 2, characterized in that, The pump source includes a continuous-wave semiconductor laser with an output power configured in the range of 250W to 300W; the laser crystal is a neodymium-doped yttrium lithium fluorine crystal, and the crystal orientation and size of the neodymium-doped yttrium lithium fluorine crystal are matched with the pump wavelength and pumping mode of the pump source.

5. The intracavity third-harmonic laser according to claim 1, characterized in that, The modulation unit includes an acousto-optic Q-switcher, which is used to modulate the continuous light output by the laser generation unit to generate pulsed laser.

6. The intracavity third-harmonic laser according to claim 5, characterized in that, The acousto-optic Q-switcher is driven by a 41MHz radio frequency and configured to output a pulse repetition frequency with an adjustable range of 1kHz to 10kHz, enabling the laser to produce a stable Q-switched pulse laser output.

7. The intracavity third-harmonic laser according to claim 1, characterized in that, A first beam splitter is provided between the modulation unit and the second harmonic unit. The first beam splitter has high transmittance for fundamental frequency light and high reflectivity for harmonic light, and is used to reflect the harmonic light generated by the second harmonic unit to the third harmonic unit.

8. The intracavity third-harmonic laser according to claim 1, characterized in that, The second harmonic unit includes an LBO crystal employing a type I phase-matching method, used to convert fundamental frequency light into second harmonic light.

9. The intracavity third-harmonic laser according to claim 1, characterized in that, A second beam splitter is provided after the third harmonic unit. The second beam splitter has high reflectivity for the fundamental frequency light and high transmittance for the third harmonic light. It is used to reflect the fundamental frequency light back into the laser resonant cavity for recycling, while allowing the third harmonic light to be output.

Images