UV laser device

Abstract

The present invention provides an ultraviolet laser device that is compact, high-brightness, high-efficiency and long-life. The ultraviolet laser device comprises: an excitation semiconductor laser (1, 1A); an optical fiber laser medium (10) that receives excitation laser light from the excitation semiconductor laser and performs laser oscillation; and an external resonator (20) for wavelength conversion that converts the wavelength of the laser light oscillating in the optical fiber laser medium and outputs at least 0.1W of ultraviolet continuous wave from the external resonator for wavelength conversion.

Description

Technical Field

[0001] The invention relates to an ultraviolet laser device for outputting ultraviolet laser. Background Art

[0002] Conventionally, ultraviolet laser devices are used as laser light sources for 3D printers of optical shaping or ultraviolet laser markers, etc. Currently, He-Cd laser devices (oscillation wavelength 325nm) or YAG laser devices (oscillation wavelength 355nm) are used as ultraviolet laser devices.

[0003] Patent document 1 discloses the following ultraviolet laser device, which has a laser light source that generates fundamental wave light and a wavelength conversion crystal that converts the wavelength of at least one of the fundamental wave light and its higher harmonic light into ultraviolet laser light as incident light, wherein a reflecting surface that allows at least one of the fundamental wave light and its higher harmonic light and any one of the ultraviolet laser light to pass through and reflects the other is formed inside the wavelength conversion crystal in a direction inclined relative to the optical axis of at least one of the fundamental wave light and its higher harmonic light (claim 1).

[0004] Prior art literature

[0005] Patent Document 1: Japanese Patent Application Publication No. 2012-177806

[0006] Problems to be solved by the invention

[0007] He-Cd laser devices are large, and in order to obtain an output of 50mW, the device housing is, for example, about 146×197×1400mm in size, and its efficiency is less than 0.01%. In addition, due to the evaporation of cadmium raw materials, maintenance must be performed once a year due to the short life of the laser tube.

[0008] Since the oscillation wavelength of the YAG laser device is in the near-infrared region, in order to obtain ultraviolet light, it is necessary to perform wavelength conversion using nonlinear optical crystals at least twice. The number of wavelength conversions directly leads to an increase in the components and costs within the device, and becomes the main reason for the reduction in output stability and beam quality. For the above reasons, it is desired to realize a miniaturized, high-brightness, high-efficiency and long-life ultraviolet laser device.

[0009] The ultraviolet laser device of Patent Document 1 has a wavelength conversion crystal that converts the wavelength of incident light into a wavelength in the ultraviolet region, but the wavelength conversion crystal requires a reflection surface tilted relative to the optical axis inside, and the reflection surface is composed of multiple optical components, so the structure of the wavelength conversion crystal becomes complicated. Summary of the invention

[0010] In view of the above-mentioned problems of the prior art, the present invention aims to provide an ultraviolet laser device that is miniaturized, has high brightness, high efficiency and long life.

[0011] The present inventors have conducted intensive studies on the following matters in order to achieve the above-mentioned object, and have completed the present invention.

[0012] (1) To achieve high brightness, high output and high beam quality are required, and the use of fiber lasers is the best. However, in fiber lasers that use ordinary quartz glass as the base material and dope rare earths, the oscillation wavelength becomes near-infrared, and in order to become ultraviolet light, a second wavelength conversion is required, which reduces efficiency and beam quality.

[0013] (2) To improve efficiency, it is considered to use a fiber laser whose base material is a fluoride with an oscillation wavelength in the visible region. In a fluoride fiber laser, ultraviolet light can be obtained after a single wavelength conversion, which improves efficiency.

[0014] (3) In order to efficiently excite the fiber laser, it is preferable to combine the outputs of two semiconductor lasers, transmit the light using an optical fiber, and couple it to the fiber laser.

[0015] (4) When the temperature of the optical fiber itself rises with high output, the output saturation is further reduced. In contrast, by using a metal material with high thermal conductivity and a linear expansion coefficient close to that of the fiber (optical fiber) material for the housing or the ferrule, the heat of the fiber (optical fiber) is efficiently dissipated, the excitation output can be increased, and the saturation reduction of the output can be reduced.

[0016] (5) By using the nonlinear optical crystal with a high-order harmonic generation function in the external resonator as a fundamental wave resonator reflector, it becomes alignment-free and the output stability is improved.

[0017] In order to achieve at least one of the above-mentioned purposes, an ultraviolet laser device reflecting one aspect of the present invention comprises: an excitation semiconductor laser; an optical fiber laser medium, which is incident with the excitation laser from the excitation semiconductor laser and performs laser oscillation; an external resonator for wavelength conversion, which converts the wavelength of the laser oscillating in the optical fiber laser medium, and outputs at least 0.1 W of continuous wave in the ultraviolet region from the external resonator for wavelength conversion.

[0018] According to the ultraviolet laser device, the optical fiber laser medium performs laser oscillation through the excitation laser from the excitation semiconductor laser, and the wavelength of the oscillating laser is converted by an external resonator for wavelength conversion, thereby outputting at least 0.1W of continuous wave in the ultraviolet region, achieving miniaturization, and being able to realize a high-brightness, high-efficiency, and long-life ultraviolet laser device.

[0019] In the ultraviolet laser device, the oscillation wavelength of the excitation semiconductor laser is preferably 445±5 nm, and the optical fiber laser medium is composed of a fluoride material doped with praseodymium (Pr). Thus, ultraviolet laser light can be obtained by a single wavelength conversion, thereby improving efficiency.

[0020] In addition, the external resonator for wavelength conversion preferably includes: a nonlinear optical crystal that also serves as a harmonic generation function for the wavelength conversion and an optical conjugate reflector, a plurality of high reflectivity reflectors, and a feedback mechanism for phase control. By making the nonlinear optical crystal also serve as an optical conjugate reflector, it becomes a free state, and alignment adjustment of the optical axis in the device is unnecessary or easy, and the output stability of the laser is improved. In addition, the feedback mechanism for phase control improves the output stability of the laser relative to environmental changes such as external temperature.

[0021] In addition, it is preferable to provide an optical fiber for transmitting light between the excitation semiconductor laser and the fiber laser medium. Thus, light can be efficiently transmitted from the excitation semiconductor laser to the fiber laser medium.

[0022] In addition, it is preferred that: a ferrule inserted into the end of the fiber laser medium, a housing that accommodates the fiber laser medium and is supported by the ferrule, the housing and the ferrule are respectively made of a metal material having a thermal expansion coefficient similar to that of the material of the fiber laser medium. Thus, the heat of the fiber laser medium is efficiently dissipated, the excitation output can be increased, the saturation reduction of the output can be reduced, and a high-efficiency output can be achieved.

[0023] The ultraviolet laser device can generate ultraviolet laser with a wavelength below 320nm.

[0024] Effects of the Invention

[0025] According to the present invention, it is possible to realize a miniaturized, high-brightness, high-efficiency, and long-life ultraviolet laser device. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is an optical path diagram showing a schematic structure of the ultraviolet laser device according to this embodiment.

[0027] Figure 2 It is a rough representation of storage Figure 1 Front view (a) and side view (b) of the shell of the fluoride optical fiber.

[0028] Figure 3 It means installed in Figure 2 (a) Diagram of the ferrule at the end of a fluoride optical fiber.

[0029] Figure 4 This embodiment shows Figure 1A graph showing the input-output characteristics of the ultraviolet laser device. DETAILED DESCRIPTION

[0030] Hereinafter, a mode for carrying out the present invention will be described using the drawings. Figure 1 It is an optical path diagram showing a schematic structure of the ultraviolet laser device according to this embodiment. Figure 2 It is a rough representation of the storage Figure 1 Front view (a) and side view (b) of the optical fiber housing. Figure 3 It means installed in Figure 2 (a) Diagram of the ferrule at the end of an optical fiber.

[0031] like Figure 1 As shown, the ultraviolet laser device 40 of this embodiment comprises: a fluoride fiber 10 that oscillates the fundamental wave laser of visible light; an excitation semiconductor laser 1, 1A that generates laser light (wavelength 441nm) that excites the fluoride fiber 10; and an external resonator 20 that changes the wavelength (637nm) of the fundamental wave laser light to a wavelength in the ultraviolet region, and outputs ultraviolet laser light with an oscillation wavelength of 318nm. According to this embodiment, a miniaturized, high-brightness, high-efficiency, and long-life ultraviolet laser device can be realized.

[0032] In order to achieve high output, the lasers of the two semiconductor lasers 1 and 1A are collimated by the lenses 2 and 2A in the long axis direction and the lenses 3 and 3A in the short axis direction, and the two lasers are polarized and synthesized by the polarization beam splitter 5 to form a single beam of excitation laser. The excitation laser is focused on the quartz fiber 7 by the aspheric focusing lens 6, and after being emitted from the quartz fiber 7, it is collimated by the lens 8, and focused on the cladding of the fluoride fiber 10 by the focusing lens 9 and input, thereby forming a structure for transmitting the excitation laser to the fluoride fiber 10. With this structure, it is possible to reduce the light transmission loss, and more than 80% of the lasers of the semiconductor lasers 1 and 1A can be input to the fluoride fiber 10. A wavelength plate 14 is arranged on the semiconductor laser 1A side. In addition, the lenses 2, 2A, 3, 3A can also be a combination of a suitable collimating lens and an anamorphic prism pair. In addition, the polarization beam splitter 5 can also be a suitable polarization filter.

[0033] The excitation laser is transmitted to the fluoride optical fiber 10 through the quartz optical fiber 7. In the transmission of the excitation laser, spatial coupling without passing through the optical fiber can also be used, but as described above, by passing through the optical fiber, the excitation beam pattern can be made uniform, and the problem of damage caused by the excitation can be reduced. Even if the excitation light is used as spatial light, the beam uniformization method using a random phase plate or the like is essentially the same.

[0034] As fluoride optical fibers, there are optical fibers with ZBLAN, ZrF4 and YF3 as main components, but optical fibers with AlF3 as main component are fluoride glass and seek to improve weather resistance (especially water resistance), so the fluoride optical fiber 10 is preferably an optical fiber with AlF3 as main component. In order to miniaturize the device, a gain fiber to which a high concentration of active elements (rare earth elements, transition elements, etc.) or rare earth ions (Yb, Nd, Er, Pr, Dy, Ce, Tb, etc.) is added is used as the fluoride optical fiber 10, so that sufficient fundamental wave output can be obtained with a fiber length of less than 30 cm. In addition, an anti-reflection film that transmits the excitation laser and a high-reflection film that reflects the fundamental wave laser are formed on the end face on the input side of the fluoride optical fiber 10, and an anti-reflection film that transmits the fundamental wave laser and a high-reflection film that reflects the excitation laser are formed on the end face on the output side. As materials for these anti-reflection films and high-reflection films, for example, TiO2, Ta2O5, Al2O3, SiO2, MgF2, and HfO2 can be used.

[0035] In addition, in order to prevent the saturation of the fundamental wave output caused by the temperature rise of the fluoride optical fiber 10 due to the excitation, Figure 2 (a), (b) and Figure 3 As shown, metal ferrules 31 and 32 are installed at both ends of the fluoride optical fiber 10, and a metal with high heat dissipation effect is used in the housing 30 of the fluoride optical fiber 10 to suppress the temperature rise of the optical fiber. This structure is the structure proposed by the inventor in Japanese Patent Application No. 2019-138108.

[0036] That is, Figure 2 As shown in (a) and (b), the ferrules 31 and 32 are bonded to both ends of the fluoride optical fiber 10, and the ferrules 31 and 32 and the fluoride optical fiber 10 are housed in the housing 30 in a state where the ferrules 31 and 32 are in contact with the side plates 30c and 30d on both sides of the housing 30 using a thermal conductive paste to minimize thermal resistance. The housing 30 is Figure 2 The structure of (a) and (b) is divided into two parts, and when the upper part 30a and the lower part 30b are connected, the rings 31 and 32 are located in the semicircular cutouts formed in the side plates 30c and 30d and are sandwiched and supported. Figure 2 , Figure 3 As shown, the fluoride optical fiber 10 is inserted into the through holes of the ferrules 31 and 32 using an adhesive.

[0037] By forming the ferrules 31 and 32 from metals having high thermal conductivity (e.g., copper, aluminum, stainless steel, nickel, and alloys thereof), it is possible to achieve (1) efficient heat conduction from the end of the optical fiber; (2) protection of the optical fiber end which is thin and mechanically weak and easily breakable; (3) facilitation of the polishing of the optical fiber end face; and (4) facilitation of forming a dielectric coating film on the optical fiber end face.

[0038] In addition, the housing 30 is made of a metal material (e.g., copper, aluminum, stainless steel, nickel, and alloys thereof) that is a high thermal conductivity material, and can efficiently conduct the heat generated by the fluoride optical fiber 10 from the ferrules 31 and 32 that are in contact with the side plates 30c and 30d of the housing 30 in a manner that minimizes thermal resistance, thereby dissipating the heat. Therefore, the housing 30 also functions as a heat sink. The heat dissipated from the housing 30 is discharged through a Peltier element or the like provided in the housing 30, thereby effectively dissipating the heat accumulated in the fluoride optical fiber 10.

[0039] In addition, the shell 30 and the ferrules 31, 32 are made of a material having a thermal expansion coefficient that is consistent with or similar to that of the raw material of the fluoride optical fiber 10, thereby preventing damage to the optical fiber end face or the dielectric multilayer coating surface caused by the difference in thermal expansion coefficients resulting from heat accumulation during high-output operation of the device.

[0040] As described above, the materials constituting the housing 30 and the ferrules 31 and 32 have a thermal expansion coefficient similar to that of the raw material of the fluoride optical fiber 10 and have high thermal conductivity, thereby being able to avoid problems caused by heat accumulation of the optical fiber.

[0041] A lens 11 is disposed on the output side of the fluoride fiber 10 to collimate the fundamental wave laser output from the fluoride fiber 10, and the fundamental wave laser is incident on a matching lens 14 disposed in front of the external resonator 20 via reflection mirrors 12 and 13, and then incident on the external resonator 20. In order to make the fundamental wave laser coincide with the mode in the external resonator 20, the matching lens 14 is formed by combining lenses.

[0042] The external resonator 20 is a resonator for wavelength conversion, which is composed of reflectors 15 to 18, and has a nonlinear optical crystal 19 disposed between the reflectors 17 and 18 and having a harmonic generation function for wavelength conversion. The nonlinear optical crystal 19 also serves as an optical conjugate reflector, and can stabilize the wavelength conversion output to a higher level than when a fundamental wave resonator and an external resonator are used without using a fundamental wave emission reflector, thereby obtaining a high-output ultraviolet laser. The nonlinear optical crystal 19 does not require a reflective surface inside, as in Patent Document 1, and has a simple structure. In addition, the reflectors 15 to 18 have a high reflectivity of more than 99.9% with respect to the wavelength of the fundamental wave laser.

[0043] The nonlinear optical crystal 19 functions as a phase conjugate mirror, and the phase conjugate wave generated in the nonlinear optical crystal 19 travels in the opposite direction to the laser light incident on the nonlinear optical crystal 19, and returns to the fluoride optical fiber 10 as the laser medium. Therefore, the nonlinear optical crystal 19 and the fluoride optical fiber 10, which serve as phase conjugate mirrors, automatically and passively face each other, and laser resonance is performed to achieve laser oscillation. In this way, the fundamental laser light of the laser oscillation is locked passively in wavelength and phase, and no complicated alignment adjustment of the optical axis is required, thereby improving the output stability. In addition, the second harmonic is generated by the nonlinear optical crystal 19, and is emitted to the outside from the reflector 18 along the optical path 21, thereby outputting laser light in the ultraviolet region based on the second harmonic. In this way, high-output laser light in the visible and ultraviolet regions can be obtained simply and stably. This structure is the structure previously proposed by the inventor in Japanese Patent Application No. 2019-127329.

[0044] In addition, if Figure 1 As shown, the external resonator 20 is provided with a feedback mechanism for phase control, and the feedback mechanism for phase control has a piezoelectric element 22, a light receiving unit 23, and a control unit 24. A part of the reflected light from the reflector 18 passes through the reflector 15, and the light receiving unit 23 receives the part of the transmitted light. Under the condition that the resonator length of the ring resonator 20 is a multiple of the wavelength of the fundamental wave, the light receiving unit 23 measures the difference of polarized light of the fundamental wave in the ring resonator 20 using the Pound-Drever-Hall method or the Hansch-Couillaud method, and based on the measurement result, the control unit 24 sends an electrical signal to the piezoelectric element 22, and the piezoelectric element 22 drives the reflector 16 to control the resonator length to be constant, thereby stabilizing the wavelength conversion output with respect to changes in the environment such as the external temperature.

[0045] (Example)

[0046] Next, the following examples are given to illustrate Figure 1 to Figure 3 The ultraviolet laser device of this embodiment mode is described above, but the present invention is not limited to this embodiment.

[0047] The two semiconductor lasers 1 and 1A for excitation of the oscillator section constituting the ultraviolet laser device of this embodiment are mounted on a copper block with an oscillation wavelength of 441 nm and a rated output of 3.5 W. In addition, in order to stabilize the output and the excitation wavelength, the semiconductor lasers 1 and 1A are temperature-controlled by a Peltier element.

[0048] The excitation laser is transmitted to the fluoride fiber 10 through the quartz fiber 7 with a core diameter of 20 μm. The core diameter of the quartz fiber 7 can be in the range of 15 to 50 μm, and can be 15 μm, 30 μm, 40 μm, or 50 μm, as long as it can be adjusted to the cladding NA in the fluoride fiber 10.

[0049] The fluoride optical fiber 10 is composed of an optical fiber with a total length of 17 cm doped with 3000 ppm of Pr (praseodymium). In order to stabilize the output, the temperature of the housing 30 of the fluoride optical fiber 10 is controlled at a constant temperature of 25° C. by a Peltier element. In addition, the Pr doping concentration and the total length of the optical fiber are sometimes adjusted according to the requirements of the overall design of the device, and the doping concentration is adjusted to between 1000 and 5000 ppm, and the optical fiber length is adjusted to within the range of 10 to 30 cm, especially adjusted to 10 cm, 15 cm, 17 cm, 20 cm, 25 cm, and 30 cm.

[0050] The fluoride optical fiber 10 has metal ferrules 31 and 32 bonded to both ends, the end faces are polished, and anti-reflection films and high-reflection films are deposited. In addition, in order to reduce the heat storage caused by the excitation light in the fluoride optical fiber 10, the housing 30 that houses the fluoride optical fiber 10 is made of copper, and the ferrules 31 and 32 of the fluoride optical fiber 10 are made of nickel. The thermal conductivity of the fluoride optical fiber 10 is 0.898 W / mK, while that of nickel is 91 W / mK and that of copper is 403 W / mK. In addition, regarding the linear expansion coefficient, the fluoride optical fiber is 18.6×10 －6 / K, nickel is 13.4×10 －6 / K, copper is 16.5×10 －6 / K.

[0051] The external resonator 20 has a bow-tie type ring resonator composed of four reflectors 15 to 18, and a wavelength conversion crystal (nonlinear optical crystal) 19 composed of BBO (β-BaB2O4). In addition, the reflectors 15 to 18 are respectively mounted on reflector brackets, each of which has an adjustment mechanism for adjusting the XY direction (angle adjustment), and the wavelength conversion crystal 19 is mounted on the bracket, and the bracket has an adjustment mechanism for adjusting the position and rotation (left and right, up and down).

[0052] According to the ultraviolet laser device of this embodiment, the size of the oscillating part is 175 mm in width, 360 mm in depth, and 106 mm in height, which can be miniaturized. Figure 4 As shown in the input-output characteristics, two 442nm semiconductor lasers (LDs) rated at 3.5W can be used to output continuous wave ultraviolet laser light of 0.1W or more at an oscillation wavelength of 318nm.

[0053] The above describes the methods and embodiments for implementing the present invention, but the present invention is not limited thereto, and various modifications can be made within the scope of the technical concept of the present invention. The scope of the present invention is indicated by the claims described below. For example, Figure 1 The structure of the ultraviolet laser device is an example, and other structures are also possible.

[0054] In addition, as a composition Figure 2 , Figure 3 The material of the ferrules 31, 32 and the housing 30 may be any material as long as the thermal expansion coefficient is close to that of the optical fiber, for example, a material with a high thermal conductivity of more than 90 W / mK. Other materials may also be used.

[0055] in addition, Figure 1 The nonlinear optical crystal 19 may be an optical material other than BBO (β-BaB2O4), for example, LBO (LiB3O5) or CLBO (CsLiB6O 10 ).

[0056] Industrial Applicability

[0057] According to the present invention, a miniaturized, high-brightness, high-efficiency, and long-life ultraviolet laser device can be realized, thereby providing an ultraviolet laser device that is more convenient to use than conventional He-Cd laser devices or YAG laser devices and has reduced costs.

[0058] Explanation of symbols

[0059] 1.1A semiconductor laser

[0060] 5 Polarization beam splitter

[0061] 7 Quartz Fiber

[0062] 10 Fluoride optical fiber

[0063] 14 Matching lens

[0064] 15~18 Reflectors

[0065] 19 Nonlinear optical crystals (wavelength conversion crystals)

[0066] 20 External resonator

[0067] 30 Shell

[0068] 31, 32 ring

[0069] 40 UV laser device

Claims

1. An ultraviolet laser device, comprising: Semiconductor laser for excitation; an optical fiber laser medium into which the pumping laser light from the pumping semiconductor laser is incident and performs laser oscillation; an external resonator for wavelength conversion, which converts the wavelength of the laser light oscillating in the optical fiber laser medium, The wavelength conversion external resonator outputs at least 0.1 W of ultraviolet continuous wave, The external resonator for wavelength conversion includes: a nonlinear optical crystal that serves as a harmonic generation function for wavelength conversion and a phase conjugate mirror, a plurality of high reflectivity mirrors, and a feedback mechanism for phase control.

2. An ultraviolet laser device, comprising: Semiconductor laser for excitation; an optical fiber laser medium into which the pumping laser light from the pumping semiconductor laser is incident and performs laser oscillation; An external resonator for wavelength conversion converts the wavelength of laser light oscillating in the optical fiber laser medium, wherein: The external resonator for wavelength conversion comprises: a nonlinear optical crystal that has a harmonic generation function for wavelength conversion and a phase conjugate reflector, a plurality of high reflectivity reflectors, and a feedback mechanism for phase control. The nonlinear optical crystal in the external resonator for wavelength conversion allows the laser to be incident and acts as the phase conjugate mirror. The incident laser is reflected by the phase conjugate mirror and travels in the reverse optical path until it returns to the end face of the optical fiber laser medium where the excitation laser is input and the laser is reflected. As a result, the optical fiber laser medium and the phase conjugate mirror automatically face each other for laser oscillation, and higher harmonics of the laser oscillation are generated in the nonlinear optical crystal, and at least 0.1 W of continuous waves in the ultraviolet region are output from the external resonator for wavelength conversion.

3. The ultraviolet laser device according to claim 1 or 2, wherein: The oscillation wavelength of the excitation semiconductor laser is 445±5nm, The optical fiber laser medium is composed of a fluoride material doped with praseodymium (Pr).

4. The ultraviolet laser device according to claim 1 or 2, wherein: The nonlinear optical crystal is BBO (β-BaB2O4), LBO (LiB3O5) or CLBO (CsLiB6O 10 ).

5. The ultraviolet laser device according to claim 1 or 2, wherein: An optical fiber is provided for transmitting light between the pumping semiconductor laser and the fiber laser medium.

6. The ultraviolet laser device according to claim 1 or 2, wherein: It comprises: a ferrule inserted into the end of the optical fiber laser medium, and a shell that accommodates the optical fiber laser medium and is supported by the ferrule, wherein the shell and the ferrule are respectively made of a metal material having a thermal expansion coefficient similar to that of the material of the optical fiber laser medium.

7. The ultraviolet laser device according to claim 1 or 2, wherein: Generates ultraviolet laser with a wavelength below 320nm.

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