An external cavity frequency doubling system for laser

Abstract

The external cavity frequency doubling system of the present invention includes a second harmonic generation crystal (1), a matching lens (2), a first wave plate (3), a dichroic mirror (4), a second wave plate (5), a polarization beam splitter (6), and a fourth harmonic generation crystal (8) arranged in sequence along the optical path transmission direction, and a fourth harmonic generation waste light collector (9) for collecting the waste light reflected by the polarization beam splitter (6). The stability of the above external cavity frequency doubling system of the laser has been greatly improved, achieving the advantages of reducing pollutants, controlling newly added pollutants to be removed, enhancing the overall machine life, eliminating the output of non-collinear reflected light, improving the beam quality of ultraviolet laser, and reducing the damage of devices, etc.

Description

Technical Field

[0001] The present invention belongs to the technical field of lasers, and particularly relates to an external cavity frequency doubling system for lasers. Background Art

[0002] End-pumped ultraviolet lasers are widely used in industries, medical treatment, military, aerospace, scientific research and other fields due to their advantages such as short wavelength, small spot size, high peak power, etc. Since both laser crystals and frequency conversion crystals have certain damage thresholds, especially ultraviolet frequency conversion crystals are prone to being damaged by ultraviolet light. During use, laser crystals are very easy to be damaged; the area of the crystal damaged by the ultraviolet beam only accounts for a very small part of the crystal cross-sectional area, but still the laser needs to be repaired or the crystal needs to be replaced, resulting in a great waste of manpower and material resources, shortening the overall service life of the ultraviolet laser and being unable to meet the requirement of long-term stable operation.

[0003] Please refer to Figure 1 and Figure 2 which shows a schematic diagram of the principle of a traditional frequency doubling device. When performing external cavity frequency doubling, the common crystal series connection method is used to obtain the ultraviolet output laser after multiple frequency doublings. In Figure 1 , taking the frequency doubling of 532nm to 266nm laser as an example (also applicable to other wavelengths), the laser output by the second harmonic generation crystal 1 is shaped in terms of spot size and divergence angle through the matching lens 2, and the wave plate 3 adjusts the matching phase, and then directly (or after filtering the fundamental frequency light through the dichroic mirror 4) enters the fourth harmonic generation crystal 8 to generate 266nm output. Coating the fourth harmonic generation crystal 8 with an antireflection film will shorten its service life, while if it is not coated, there will be 266nm laser reflected back. The single photon energy is large, and there are even photoexcitation effects and optical tweezer effects, which will damage the previous devices and there will be non-collinear stray light output; when using Figure 2 the optical path system (the fourth harmonic generation crystal 8 is not coated), it is found that the dichroic mirror 4 is the device that is damaged fastest. Even if it is replaced with a dichroic mirror that transmits 266nm and reflects 532nm (1064nm can be filtered out by other lenses), the dichroic mirror 4 is still the device with the shortest service life. The main reason for the analysis is that the damage resistance of the high-pass film system in the ultraviolet band in coated lenses is much higher than that of the low-pass film system. And the stray light is still pollutants that are uncertainly excited. If a 266nm reflection film system is coated on a certain device before the fourth harmonic generation in the original optical path, the reflected light is likely to hit the crystal or the surrounding metal and non-metal clamping parts, still causing damage and pollution, and the reflected light entering the output optical path will bring stray background, thus affecting the overall service life and beam quality of the whole machine. Summary of the Invention

[0004] In view of this, it is necessary to provide an external cavity frequency doubling system for lasers with a long service life and high beam quality in view of the defects existing in the prior art.

[0005] To solve the above problems, the present invention adopts the following technical solutions:

[0006] The present invention provides an external cavity frequency doubling system for a laser, comprising: a second harmonic generation crystal (1), a matching lens (2), a first wave plate (3), a dichroic sheet (4), a second wave plate (5), a polarization beam splitter (6), and a fourth harmonic generation crystal (8) sequentially arranged along the optical path transmission direction, and a fourth harmonic generation waste light collector (9) for collecting the waste light beam reflected by the polarization beam splitter (6); wherein:

[0007] The laser beam output by the second harmonic generation crystal (1) enters the matching lens (2), the matching lens (2) shapes the spot size and divergence angle of the incident laser beam, the shaped laser beam then enters the first wave plate (3), the first wave plate (1) adjusts the matching phase of the incident laser beam, the laser beam after phase adjustment is filtered by the dichroic sheet (4) to remove the fundamental frequency light and then enters the second wave plate (5), the second wave plate (5) adjusts the matching phase of the incident laser beam and then transmits through the polarization beam splitter (6) and enters the fourth harmonic generation crystal (8), the fourth harmonic generation crystal (8) outputs a part of the laser beam, and reflects a part of the waste light beam, and after being reflected by the polarization beam splitter (6), it is collected by the fourth harmonic generation waste light collector (9).

[0008] In some embodiments, the material of the dichroic sheet (4) is ultraviolet quartz or calcium fluoride.

[0009] In some embodiments, the surface of the polarization beam splitter (6) facing the fourth harmonic generation crystal (8) is coated with a film system with high reflectivity for s-polarized light and high transmittance for p-polarized light.

[0010] In some embodiments, the wavelength of the high reflectivity for s-polarized light includes but is not limited to 266 nm, and the wavelength of the high transmittance for p-polarized light includes but is not limited to 532 nm.

[0011] In some embodiments, the material of the fourth harmonic generation waste light collector (9) is aluminum material plated with gold or nickel.

[0012] In some embodiments, a window plate (7) is further arranged between the polarization beam splitter (6) and the fourth harmonic generation crystal (8).

[0013] The present application adopting the above technical solutions has the following effects:

[0014] The external cavity frequency doubling system provided by the present application, the laser beam output by the second harmonic generation crystal (1) enters the matching lens (2), the matching lens (2) shapes the spot size and divergence angle of the incident laser beam, and the shaped laser beam then enters the first wave plate (3). The first wave plate (1) adjusts the matching phase of the incident laser beam. After the phase adjustment, the laser beam is filtered by the dichroic mirror (4) to remove the fundamental frequency light and then enters the second wave plate (5). The second wave plate (5) adjusts the matching phase of the incident laser beam and then transmits through the polarization beam splitter (6) and enters the fourth harmonic generation crystal (8). The fourth harmonic generation crystal (8) outputs part of the laser beam, reflects part of the waste beam, and then is reflected by the polarization beam splitter (6) and collected by the fourth harmonic waste light collector (9). The external cavity frequency doubling system provided by the present application can achieve no coating on the output end face of the fourth harmonic generation crystal. Compared with the traditional external cavity frequency doubling laser system, the stability has been greatly improved, the pollutants have been reduced, the newly added pollutants removed have been controlled, the overall machine life has been improved, the output of non-collinear reflected light has been eliminated, the beam quality of the ultraviolet laser has been improved, and the damage of the device has been reduced, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required for use in the embodiments of the present invention or the description of the prior art. Obviously, the following described drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0016] Figure 1 It is a structural diagram of an external cavity frequency doubling system of a laser provided by an embodiment of the prior art.

[0017] Figure 2 It is a structural diagram of an external cavity frequency doubling system of a laser provided by another embodiment of the prior art.

[0018] Figure 3 It is a structural diagram of an external cavity frequency doubling system of a laser provided by an embodiment of the present invention.

[0019] Wherein: second harmonic generation crystal (1); matching lens (2); wave plates (3), 4, dichroic mirror (4), wave plate (5), polarization beam splitter (6), window plate (7); fourth harmonic generation crystal (8), fourth harmonic waste light collector (9). DETAILED DESCRIPTION OF THE EMBODIMENTS

[0020] Embodiments of the present invention will be described in detail below. Examples of the embodiments are shown in the accompanying drawings, where the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below by referring to the accompanying drawings are exemplary and are intended to explain the present invention, but should not be construed as limiting the present invention.

[0021] In the description of the present invention, it should be understood that the orientation or positional relationship indicated by the terms "upper", "lower", "horizontal", "inner", "outer", etc. is based on the orientation or positional relationship shown in the accompanying drawings, and is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus should not be construed as limiting the present invention.

[0022] In addition, the terms "first" and "second" are only used for descriptive purposes and should not be construed as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, the features defined with "first" and "second" may explicitly or implicitly include one or more of such features. In the description of the present invention, "a plurality" means two or more unless otherwise specifically defined.

[0023] In order to make the purpose, technical solutions and advantages of the present invention more clear and understandable, the present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments.

[0024] Please refer to Figure 3 , which is a schematic structural diagram of an external cavity frequency doubling system of a laser provided by an embodiment of the present application, including: a second harmonic generation crystal (1), a matching lens (2), a first wave plate (3), a dichroic sheet (4), a second wave plate (5), a polarization beam splitter (6), and a fourth harmonic generation crystal (8) arranged in sequence along the optical path transmission direction, and a fourth harmonic generation waste light collector (9) for collecting the waste light reflected by the polarization beam splitter (6).

[0025] The working mode of the above external cavity frequency doubling system of a laser is as follows:

[0026] The laser beam output by the second harmonic generation crystal (1) enters the matching lens (2). The matching lens (2) shapes the spot size and divergence angle of the incident laser beam. The shaped laser beam then enters the first wave plate (3). The first wave plate (1) adjusts the matching phase of the incident laser beam. After the phase adjustment, the laser beam enters the dichroic beam splitter (4), where the fundamental frequency light is filtered out, and then enters the second wave plate (5). The second wave plate (5) adjusts the matching phase of the incident laser beam and then transmits through the polarization beam splitter (6) and enters the fourth harmonic generation crystal (8). The fourth harmonic generation crystal (8) outputs a part of the laser beam and reflects a part of the waste beam. After being reflected by the polarization beam splitter (6), it is collected by the fourth harmonic waste light collector (9).

[0027] In some embodiments, the surface of the polarization beam splitter (6) facing the fourth harmonic generation crystal (8) is coated with a film system with high reflectivity for s-polarized light and high transmittance for p-polarized light.

[0028] Specifically, the wavelength of the high reflectivity for s-polarized light is 266 nm, and the wavelength of the high transmittance for p-polarized light is 532 nm. It can be understood that the wavelength of the high reflectivity for s-polarized light is not limited to 266 nm, and the wavelength of the high transmittance for p-polarized light is not limited to 532 nm, and can be adjusted according to the situation.

[0029] In some embodiments, the surface of the fourth harmonic generation crystal 8 is not coated, avoiding the shortening of its service life due to the antireflection coating in the prior art.

[0030] In some embodiments, the fourth harmonic waste light collector (9) is made of aluminum material plated with gold or nickel.

[0031] It can be understood that the fourth harmonic waste light collector 9 is made of a material that is not prone to the photoelectric effect and is easy to dissipate heat, such as aluminum material plated with gold or nickel, and is used as an electrode to collect charged particles and is led out of the laser chamber through a gas path.

[0032] In some embodiments, a window plate (7) is further provided between the polarization beam splitter (6) and the fourth harmonic generation crystal (8).

[0033] It can be understood that for the external cavity frequency doubling system provided by the present invention, a dichroic sheet 4 is inserted into the optical path, and then a polarization beam splitter 6 is incident at an incident angle of 45° (or Brewster angle). Among them, the surface facing the fourth harmonic generation crystal needs to be coated with a film system with high reflectivity for s light and high transmittance for p light, and the other surface is not coated. Since the fourth harmonic generation crystal 8 does not need to be coated, the service life shortening caused by the coating method in the prior art is avoided. At the same time, the reflected waste light is collected by the fourth harmonic generation waste light collector 9, and the pollutants accumulated by the optical tweezers effect are guided through the gas path to the fourth harmonic generation waste light collector 9, avoiding optical damage and optical pollution. Compared with the traditional external cavity frequency doubling laser system, its stability has been greatly improved, realizing the reduction of pollutants, controlling the newly added pollutants removed, improving the overall machine life, eliminating the output of non-collinear reflected light, improving the beam quality of ultraviolet laser, and reducing the damage of devices and other advantages.

[0034] The above are only embodiments of the present application and are not used to limit the present application. For those skilled in the art, various changes and modifications can be made to the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present application shall be included within the scope of the claims of the present application.

Claims

1. A laser extracavity frequency doubling system, characterized in that, Including: A second harmonic generation crystal (1), a matching lens (2), a first wave plate (3), a dichroic beam splitter (4), a second wave plate (5), a polarization beam splitter (6) and a fourth harmonic generation crystal (8) arranged in sequence along the optical path transmission direction, and a fourth harmonic waste light collector (9) for collecting the waste light beam reflected by the polarization beam splitter (6); wherein: The laser beam output by the second harmonic generation crystal (1) enters the matching lens (2), the matching lens (2) shapes the spot size and divergence angle of the incident laser beam, and the shaped laser beam then enters the first wave plate (3). The first wave plate (3) adjusts the matching phase of the incident laser beam. After the phase adjustment, the laser beam enters the second wave plate (5) after the fundamental frequency light is filtered by the dichroic beam splitter (4). The second wave plate (5) adjusts the matching phase of the incident laser beam and then transmits through the polarization beam splitter (6) at an incident angle of 45° and enters the fourth harmonic generation crystal (8). The fourth harmonic generation crystal (8) outputs a part of the laser beam and reflects a part of the waste light beam, and then is reflected by the polarization beam splitter (6) and collected by the fourth harmonic waste light collector (9); The surface of the polarization beam splitter (6) facing the fourth harmonic generation crystal (8) is coated with a film system with high reflectivity for s-polarized light and high transmittance for p-polarized light, and the other surface is not coated; The material of the fourth harmonic waste light collector (9) is aluminum coated with gold or nickel; The output end face of the fourth harmonic generation crystal is not coated.

2. The laser extracavity frequency doubling system according to claim 1, characterized in that, The material of the dichroic beam splitter (4) is ultraviolet quartz or calcium fluoride; 3. The laser extracavity frequency doubling system according to claim 1, characterized in that, The wavelength of the high reflectivity for s-polarized light includes 266 nm, and the wavelength of the high transmittance for p-polarized light includes 532 nm.

4. The laser extracavity frequency doubling system according to claim 1, characterized in that, It further includes a window plate (7) arranged between the polarization beam splitter (6) and the fourth harmonic generation crystal (8).

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