Semiconductor laser-pumped solid-state laser

Abstract

[Problem] To achieve high output and stable operation in a semiconductor laser-pumped solid-state laser which is provided with an optical wavelength conversion element for converting two solid-state laser oscillation lights into sum frequency. [Solution] An LD-pumped solid-state laser (1) includes: a solid-state laser crystal (19) such as a Pr:YLF crystal; an LD (11) that emits excitation light (L) for pumping the laser crystal (19); and a resonator that resonates the light emitted from the solid-state laser crystal (19). The LD-pumped solid-state laser comprises a wavelength control means, such as a narrow bandpass filter (13), for substantially matching the oscillation wavelength of the excitation light (L) from the LD (11) with the absorption peak wavelength of the solid-state laser crystal (19).　

Description

Semiconductor laser-pumped solid-state laser

[0001] This invention relates to a semiconductor laser-pumped solid-state laser, more specifically, to a solid-state laser that generates laser oscillation by exciting a solid laser crystal, which serves as the laser medium, with excitation light emitted by a semiconductor laser.

[0002] For example, as detailed examples are shown in Patent Document 1, for example, Pr ３＋ Conventionally known solid-state lasers are in which a solid-state laser crystal, such as a YLF crystal doped with Pr (hereinafter referred to as Pr:YLF crystal), is excited by excitation light emitted from a semiconductor laser (laser diode; hereinafter referred to as LD), and the light emitted from the excited solid-state laser crystal is resonated by a resonator. As for the solid-state laser crystal, Nd(YLF) as shown in Patent Document 2 is a known example. ３＋ YAG crystals doped with Nd (hereinafter referred to as Nd:YAG crystals) can also be used. Furthermore, it has been conventional to convert the laser oscillation light obtained as described above into a desired optical wavelength, and Patent Document 3 shows how to convert laser oscillation light of two wavelengths into a sum frequency of one wavelength.

[0003] Pr:YLF crystals are extremely attractive laser crystals because they absorb light at wavelengths of 442 nm, 444 nm, 469 nm, and 479 nm, and can generate laser oscillations at wavelengths of 479 to 720 nm. However, because Pr:YLF crystals have a narrow absorption wavelength range, when applied to semiconductor laser-pumped solid-state lasers, the oscillation wavelength of the excitation LD must match the absorption wavelength, and the oscillation wavelength range must be 0.5 to 1 nm or less for high-power and stable laser oscillations to be achieved. To explain the absorption and oscillation wavelengths in more detail and practical terms, they are not fixed at pinpoint locations, but rather have waveforms that extend over a certain range. When specific numerical values ​​are given for wavelengths, they refer to the peak wavelength (which may also be the center wavelength of the waveform) in that waveform.

[0004] However, commercially available GaN-based LDs for excitation that can emit light at 400 nm, the absorption wavelength band of Pr:YLF crystals, have an oscillation wavelength variation of 5 to 10 nm. If an LD with an oscillation wavelength that matches the absorption wavelength band is selected, the yield becomes only a few percent. Considering that the unit price of an LD is originally around 100,000 yen, the cost of the LDs alone would exceed 1 million yen. Therefore, it is extremely difficult to carry out mass production on an industrial level. Furthermore, the oscillation wavelength width is wider than the absorption wavelength width of the Pr:YLF crystal, by about 2 nm, so even if the wavelengths match, there is a problem of reduced absorption efficiency. In addition, LDs have the characteristic that their oscillation wavelength fluctuates depending on the light output and temperature, so it is extremely difficult to mass-produce LDs with oscillation wavelengths that match the absorption wavelength of the Pr:YLF crystal, taking these characteristics into account.

[0005] Non-patent document 1 indicates that LDs are selected and used so that their oscillation wavelength matches the absorption wavelength of the Pr:YLF crystal. However, even when LDs are selected and used in this way, if the oscillation wavelength of the LDs fluctuates with temperature, it becomes impossible to achieve stable, high-power laser oscillation.

[0006] To match the oscillation wavelength of the excitation LD with the absorption wavelength of the Pr:YLF crystal, it is conceivable to use a broad-area LD with a relatively wide oscillation wavelength range. However, although broad-area LDs have high output power, they have the drawback of poor spatial coherence because they are multimode transverse modes. Therefore, even if an external resonator is formed to control the oscillation wavelength and fed back to the LD, optical losses increase, making it difficult to control the oscillation wavelength of the excitation LD to the desired value. This problem is also observed in LD-pumped solid-state lasers equipped with an optical wavelength conversion element that converts laser oscillation light of two wavelengths into a sum frequency of one wavelength, as mentioned above.

[0007] Japanese Patent Publication No. 2001-36176, Japanese Patent Publication No. 2001-36175, Japanese Patent Publication No. 2006-66436

[0008] "Applied Physics" journal, Vol. 85, pp. 857-858

[0009] The present invention has been made in view of the above circumstances, and aims to achieve high output and stable operation in an LD-pumped solid-state laser, which excites a solid-state laser crystal such as a Pr:YLF crystal with an LD, and in particular an LD-pumped solid-state laser having a configuration that converts solid-state laser oscillation light of two wavelengths into a sum frequency of one wavelength.

[0010] The LD-pumped solid-state laser according to the present invention comprises: a solid-state laser crystal such as a Pr:YLF crystal; an LD that emits excitation light to excite the solid-state laser crystal; and a resonator that resonates the light emitted from the excited solid-state laser crystal. The LD-pumped solid-state laser is configured to generate two solid-state laser oscillations with different frequencies; an optical wavelength conversion element that converts the two solid-state laser oscillations into a sum frequency; and a wavelength control means that substantially matches the oscillation wavelength of the excitation light from the LD to the absorption peak wavelength of the solid-state laser crystal.

[0011] In the LD-pumped solid-state laser according to the present invention, an external resonator is provided to constitute the wavelength control means, and this external resonator has a transmission-reflection mirror that transmits the excitation light emitted from the semiconductor laser toward the solid-state laser crystal and reflects it toward the semiconductor laser, and it is preferable that this transmission-reflection mirror is formed from two glass plates joined together.

[0012] In that case, the two glass plates can preferably be those joined together by optical contact, those joined by forming a metal plating near the edge of each glass plate and heating and welding the metal platings together, those joined by forming a metal plating and a metal plate overlapping the metal plating near the edge of each glass plate and heating and welding the metal plates together via the metal plating, or those joined by melting low-melting-point glass placed near the edge of each glass plate.

[0013] In the above configuration, there are not necessarily two sets of solid laser crystal, LD, and resonator. In other words, some of them may be shared to generate two solid laser beams of different frequencies.

[0014] Specifically, as a means of controlling the wavelength described above, it is preferable to use a bandpass filter that is placed in a resonator that resonates the excitation light (which is generally different from the resonator for an LD-pumped solid-state laser) and narrows the wavelength of the resonating light. It is also preferable that the resonator that resonates the excitation light includes a diffraction grating that selects the wavelength of the resonating light. It is also preferable that this resonator includes a VBG (Volume Bragg Grating) that selects the wavelength of the resonating light. It is also preferable that the resonator includes a confocal optical system. Furthermore, it is preferable that a GaN-based LD is used as the excitation LD. And it is preferable that this excitation LD is a transverse multimode LD.

[0015] According to the LD-pumped solid-state laser of the present invention, the oscillation wavelength of the excitation light from the LD can be made to substantially match the absorption peak wavelength of the solid-state laser crystal by using the wavelength control means described above, thereby dramatically improving the yield of applicable LDs. As the oscillation wavelength of the excitation LD becomes stable relative to the absorption wavelength of the solid-state laser crystal, the optical output of the solid-state laser also becomes stable. Furthermore, even if the oscillation wavelength fluctuates due to changes in the drive current value of the excitation LD or the ambient temperature, the state in which the oscillation wavelength is stable relative to the absorption wavelength of the solid-state laser crystal is maintained, so the optical output of the LD-pumped solid-state laser and related performance remain stable.

[0016] Figure 1 shows a schematic configuration diagram of an LD-pumped solid-state laser, which is the first embodiment of the present invention. Figure 1 shows the absorption characteristics of the solid-state laser crystal used in the solid-state laser of Figure 1. Figure 2 shows another absorption characteristic of the solid-state laser crystal used in the solid-state laser of Figure 1. Figure 3 shows a schematic configuration diagram of an LD-pumped solid-state laser, which is the second embodiment of the present invention. Figure 4 shows a schematic configuration diagram of an LD-pumped solid-state laser, which is the third embodiment of the present invention. Figure 4 shows a schematic side view of the main parts of the apparatus in Figure 4 before assembly. Figure 7 shows a schematic side view of the main parts after assembly. Figure 4 shows a schematic side view of deposits occurring on the transmission plane mirror of the LD-pumped solid-state laser. Figure 5 shows a schematic plan view of an example of a beam spot of excitation light affected by deposits. Figure 6 shows a schematic side view of an example of a normal beam profile of excitation light. Figure 7 shows a schematic side view of an example of an abnormal beam profile of excitation light. Figure 7 shows a schematic side view of an apparatus in the fifth embodiment of the present invention before assembly. Figure 13 shows a schematic side view of the main parts before assembly. Figure 7 shows a schematic side view of an apparatus in the sixth embodiment of the present invention before assembly. Figure 8 shows a schematic side view of the main parts after assembly. Figure 9 shows a schematic side view of an apparatus in the seventh embodiment of the present invention before assembly. Figure 10 shows a schematic side view of the main parts after assembly.

[0017] Embodiments of the present invention will be described below with reference to the drawings. <First Embodiment> Figure 1 shows a schematic configuration of an LD-pumped solid-state laser 1, which is the first embodiment of the present invention. This LD-pumped solid-state laser 1 has a first excitation LD 11, a collimating lens 12, a narrow-band BPF (bandpass filter) 13, a focusing lens 14, a transmission plane mirror 15, a collimating lens 16, a focusing lens 17, a transmission plane mirror 18, and a rod-shaped Pr:YLF crystal 19, all arranged from left to right in the figure. The elements 11 to 19 arranged from right to left in the figure are collectively referred to as the first excitation / oscillation system 31. Furthermore, the LD-pumped solid-state laser 1 has a second excitation LD 11, a collimating lens 12, a narrow-band BPF (bandpass filter) 13, a focusing lens 14, a transmission plane mirror 15, a collimating lens 16, a focusing lens 17, a transmission plane mirror 18, and a rod-shaped Pr:YLF crystal 19, all arranged in a line from bottom to top in the figure. The elements 11 to 19 arranged in a line from bottom to top in the figure are collectively referred to as the second excitation / oscillation system 32.

[0018] The first excitation / oscillation system 31 will now be explained in detail. The first excitation LD 11 generates its own laser oscillation and emits excitation light L with a wavelength of 444 nm in a divergent state. This excitation light L travels to the right in the figure, is made into parallel light by the collimating lens 12, and this parallel light passes through the narrowband BPF 13, which acts as a wavelength control means, and is incident on the converging lens 14. This incident parallel light is focused by the converging lens 14 so as to converge on the front end face of the transmission plane mirror 15, and then passes through the mirror 15. The exit end face of the first excitation LD 11 and the exit end face of the transmission plane mirror 15 are coated to reflect a predetermined amount of excitation light L, and these end faces constitute a resonator that resonates the excitation light L. This is the same in the second excitation / oscillation system 32, which will be described later. The excitation light L that passes through the transmissive plane mirror 15 is incident on the collimating lens 16 and is made into parallel light. The parallel excitation light L is then focused by the converging lens 17 so as to converge within the Pr:YLF crystal 19 and then passes through the transmissive plane mirror 18. The Pr:YLF crystal 19 that receives the excitation light L is excited by the excitation light L and emits light with a wavelength of 698 nm by stimulated emission. This light passes through the dielectric multilayer film 21a of the polarizing beam splitter 21 and is reflected by the concave surface 20a of the concave mirror 20. In this way, the light with a wavelength of 698 nm resonates in the solid-state laser resonator composed of the plane mirror 18 and the concave mirror 20, and solid-state laser oscillation light L21 with a wavelength of 698 nm is obtained.

[0019] The above will be explained in more detail below, with some specific numerical examples. The reflectivity of the coating applied to the transparent planar mirror 15 was set to 20%, 35%, and 50%, and narrowband BPFs 13 with half-widths of 1 nm, 2 nm, and 5 nm were used. In this case, the oscillation wavelength was controlled by rotating the narrowband BPF 13, and in all combinations, the oscillation wavelength of the first excitation LD 11 could be matched to the absorption wavelength of the Pr:YLF crystal 19, which is 444 nm. Furthermore, the oscillation half-width of the excitation light L with an oscillation wavelength of 444 nm could be narrowed to 0.5 nm or less.

[0020] Thus, the oscillation wavelength of the first excitation LD 11 matched the absorption wavelength of the Pr:YLF crystal 19, and the oscillation full width at half maximum (FWHM) was narrower than the absorption width of the crystal 19, which is 1.0 nm. As a result, the energy of the first excitation LD 11 could be absorbed by the Pr:YLF crystal 19 stably and efficiently. The oscillation wavelength and amplitude of the first excitation LD 11 remained unchanged even when the ambient temperature changed or a long time elapsed after power was applied, allowing for stable oscillation. Therefore, to resolve the problem of oscillation wavelength variations of the excitation LDs, it was no longer necessary to select good excitation LDs from a large number, and the lifespan of the LD-pumped solid-state laser 1 was also extended.

[0021] The second excitation LD 11 of the second excitation / oscillation system 32 also generates laser oscillations itself, emitting excitation light L with a wavelength of 444 nm in a divergent state. This excitation light L travels upward in the figure, is made into parallel light by the collimating lens 12, and this parallel light passes through the narrowband BPF 13, which acts as a wavelength control means, and is incident on the converging lens 14. This incident parallel light is focused by the converging lens 14 so as to converge at the front end face of the transmission plane mirror 15, and then passes through the mirror 15. The excitation light L that has passed through the transmission plane mirror 15 is incident on the collimating lens 16 and made into parallel light, and the now parallel excitation light L is focused by the converging lens 17 so as to converge within the Pr:YLF crystal 19, and then passes through the plane mirror 18. The Pr:YLF crystal 19 that has received the excitation light L is excited by the excitation light L and emits light with a wavelength of 607 nm by stimulated emission. This light is reflected by the dielectric multilayer film 21a of the polarizing beam splitter 21 so that it is bent at a right angle, and then reflected by the concave surface 20a of the concave mirror 20. In this way, the light with a wavelength of 607 nm resonates in the solid-state laser resonator composed of the planar mirror 18 and the concave mirror 20, and solid-state laser oscillation light L22 with a wavelength of 607 nm is obtained.

[0022] As described above, in the first excitation / oscillation system 31, the wavelength of the excitation light L is controlled by the narrowband BPF 13 so as to precisely match the absorption peak wavelength of the Pr:YLF crystal 19. Similarly, in the second excitation / oscillation system 32, the wavelength of the excitation light L is also controlled by the narrowband BPF 13 so as to precisely match the absorption peak wavelength of the Pr:YLF crystal 19. In this example, the absorption peak wavelength of the Pr:YLF crystal 19 in the first excitation / oscillation system 31 is 444 nm, as shown in the absorption characteristic diagram in Figure 2, while the absorption peak wavelength of the Pr:YLF crystal 19 in the second excitation / oscillation system 32 is 444 nm, as shown in the absorption characteristic diagram in Figure 3.

[0023] By performing the above wavelength control, the oscillation wavelength of the excitation light L from the excitation LD 11 can be made to approximately match the absorption peak wavelength of the Pr:YLF crystal 19 in both the first excitation / oscillation system 31 and the second excitation / oscillation system 32, thereby dramatically improving the yield of applicable LDs 11. Since the oscillation wavelength of the excitation LD 11 is stable with respect to the absorption wavelength of the Pr:YLF crystal 19, the optical output of the LD-pumped solid-state laser 1 also becomes stable. Furthermore, even if the oscillation wavelength of the excitation LD 11 fluctuates due to changes in the drive current value or ambient temperature, the state in which the oscillation wavelength is stable with respect to the absorption wavelength of the Pr:YLF crystal 19 is maintained, so the optical output of the LD-pumped solid-state laser 1 and related performance remain stable.

[0024] Alternatively, instead of providing a narrowband BPF 13 for wavelength control, a diffraction grating, a VBG (Volume Bragg Grating), or even a confocal optical system may be used as a wavelength control means. As mentioned earlier, the narrowband BPF 13 can control the oscillation wavelength and narrow the bandwidth of the excitation light by rotating it, but the same "wavelength control" and "narrowing" can be achieved when a diffraction grating or VBG is used. However, when a VBG is used, the excitation light must be incident basically perpendicularly to the grating, making it difficult to precisely set the oscillation wavelength to the desired value. Also, when a confocal optical system is used, the optical system is stable, so the optical output can be stabilized.

[0025] Returning to FIG. 1, the detailed configuration of the LD-pumped solid laser 1 will be described. The Pr:YLF crystal 19 of the first excitation / oscillation system 31 and the Pr:YLF crystal 19 of the second excitation / oscillation system 32 are each in a state where the c-axis extends in the direction shown in the figure. Therefore, the solid laser oscillation light L21 with a wavelength of 698 nm has the vibration direction of the electric field in the vertical direction in the figure, and the solid laser oscillation light L22 with a wavelength of 607 nm has the vibration direction of the electric field perpendicular to the plane of the paper in the figure, and they are incident on the polarization beam splitter 21 respectively. The polarization beam splitter 21 has a dielectric multilayer film 21a inclined at 45° with respect to the traveling direction of the solid laser oscillation light L21 and the traveling direction of the solid laser oscillation light L22. The solid laser oscillation light L21 is incident on the dielectric multilayer film 21a in a p-polarized state, and the solid laser oscillation light L22 is incident in an s-polarized state. For the solid laser oscillation lights L21 and L22 in such polarization states, the dielectric multilayer film 21a transmits the former with a transmittance of 99.5% or more and reflects the latter with a reflectance of 99.5% or more.

[0026] Thus, from the polarization beam splitter 21, the solid laser oscillation light L21 and the solid laser oscillation light L22 that were traveling in different directions are combined and emitted in a common direction, that is, traveling to the right in FIG. 1. These combined solid laser oscillation lights L21 and solid laser oscillation light L22 are incident on a BBO crystal (β-BaB ２ O ４ crystal) 22, which is a nonlinear optical material, and wavelength-converted to a sum frequency L23 with a wavelength of 325 nm under the type-II phase matching condition by the BBO crystal 22. This sum frequency L23 passes through the concave mirror 20 and is emitted outside the LD-pumped solid laser 1 as useful light. An AR (anti-reflection) coating for light with a wavelength of 325 nm is applied to the concave surface 20a of the concave mirror 20, that is, the surface facing the plane mirror 18.

[0027] When the BBO crystal 22 is used as a nonlinear optical material, if its cut angle θ = 52.1°, the BBO crystal 22 uses the solid-state laser oscillation light L21 with a wavelength of 698 nm as extraordinary light, the solid-state laser oscillation light L22 with a wavelength of 607 nm as ordinary light, and the sum frequency L23 with a wavelength of 325 nm as extraordinary light for wavelength conversion. On the other hand, if the cut angle θ = 57.3° of the BBO crystal 22, the solid-state laser oscillation light L21 with a wavelength of 698 nm as ordinary light, the solid-state laser oscillation light L22 with a wavelength of 607 nm as extraordinary light, and the sum frequency L23 with a wavelength of 325 nm for wavelength conversion.

[0028] In addition to the BBO crystal 22 mentioned above, CLBO crystals and the like can also be used as nonlinear optical materials. When a CLBO crystal is used, if its cut angle θ = 71.4°, the CLBO crystal performs wavelength conversion by treating the solid-state laser oscillation light L21 with a wavelength of 698 nm as extraordinary light, the solid-state laser oscillation light L22 with a wavelength of 607 nm as ordinary light, and the sum frequency L23 with a wavelength of 325 nm as extraordinary light.

[0029] ≪Second Embodiment≫ Next, a second embodiment of the present invention will be described with reference to Figure 4. Figure 4 shows a schematic configuration of an LD-pumped solid-state laser 2, which is the second embodiment. In this LD-pumped solid-state laser 2, excitation light L with a wavelength of 444 nm emitted in a divergent state from the excitation LD 11 is made into parallel light by the collimating lens 16, passes through the narrow-band BPF 13 as a wavelength control means, and is incident on the focusing lens 17. This incident parallel light is focused by the focusing lens 17 so as to converge within the Pr:YLF crystal 19 and then passes through the plane mirror 18. The Pr:YLF crystal 19, upon receiving the excitation light L, is excited by the excitation light L and emits light with a wavelength of 698 nm by stimulated emission. This light is reflected by the concave mirror 20 and incident on the plane mirror 51, resonates in the solid-state laser resonator composed of the plane mirror 51 and the plane mirror 18, and solid-state laser oscillation light L21 with a wavelength of 698 nm is obtained.

[0030] Further, the Pr:YLF crystal 19 that has received the excitation light L is excited by the excitation light L and emits light with a wavelength of 607 nm by stimulated emission. This light is reflected by the concave mirror 20 and enters the plane mirror 51, and resonates within the resonator for a solid-state laser composed of the plane mirror 51 and the plane mirror 18, and the solid-state laser oscillation light L22 with a wavelength of 607 nm can also be obtained.

[0031] The concave mirror 20 is disposed inclined with respect to the optical axis of the plane mirror 18, and the plane mirror 51 is disposed with the reflection axis and the optical axis of the concave mirror 20 aligned. And between the concave mirror 20 and the plane mirror 51, a BBO (β-BaB ２ O ４ ) crystal 52 is disposed. The BBO crystal 52 is a nonlinear optical material, and converts the incident solid-state laser oscillation light L21 with a wavelength of 698 nm and the solid-state laser oscillation light L22 with a wavelength of 607 nm into a sum frequency L23 with a wavelength of 325 nm under the type II phase matching condition. This sum frequency L23 passes through the concave mirror 20 and exits outside the LD-pumped solid-state laser 2. An AR (anti-reflection) coating for light with a wavelength of 325 nm is applied to the concave surface 20a of the concave mirror 20, that is, the surface facing the plane mirror 51.

[0032] As described above, in this embodiment, the solid-state laser oscillation light L21 and the solid-state laser oscillation light L22 traveling in different directions are combined by the BBO crystal 52 so as to travel in a common single direction, and then wavelength-converted into the sum frequency L23.

[0033] When the BBO crystal 52 is used as the nonlinear optical material as described above, when its cut angle θ = 52.1°, the BBO crystal 52 uses the solid-state laser oscillation light L21 with a wavelength of 698 nm as the extraordinary light, the solid-state laser oscillation light L22 with a wavelength of 607 nm as the ordinary light, and wavelength-converts the sum frequency L23 with a wavelength of 325 nm as the extraordinary light. And when the BBO crystal is used, when its cut angle θ = 57.3°, the solid-state laser oscillation light L21 with a wavelength of 698 nm is used as the ordinary light, the solid-state laser oscillation light L22 with a wavelength of 607 nm is used as the extraordinary light, and wavelength-converts the sum frequency L23 with a wavelength of 325 nm as the extraordinary light.

[0034] In addition to the BBO crystal mentioned above, CLBO crystals and the like can also be used as nonlinear optical materials. When a CLBO crystal is used, if its cut angle θ = 71.4°, the CLBO crystal uses the solid-state laser oscillation light L21 with a wavelength of 698 nm as extraordinary light, the solid-state laser oscillation light L22 with a wavelength of 607 nm as ordinary light, and the sum frequency L23 with a wavelength of 325 nm as extraordinary light to perform wavelength conversion.

[0035] In this embodiment, a narrow-band BPF 13 is provided between the collimating lens 16 and the focusing lens 17 as a wavelength control means. Therefore, in this embodiment as well, the excitation light L with a wavelength of 444 nm emitted from the excitation LD 11 in a divergent state is wavelength-controlled by this narrow-band BPF 13 so as to precisely match the absorption peak wavelength of the Pr:YLF crystal 19. Thus, in this embodiment as well as in the first embodiment, the yield of applicable LDs 11 is dramatically improved, the oscillation wavelength of the excitation LD 11 is stable with respect to the absorption wavelength of the Pr:YLF crystal 19, the optical output of the LD-pumped solid-state laser 1 is stabilized, and the optical output of the LD-pumped solid-state laser 1 and related performance remain stable even when the drive current value of the excitation LD 11 or the ambient temperature changes.

[0036] <Third Embodiment> Next, a third embodiment of the present invention will be described with reference to Figure 5. Figure 5 shows a schematic configuration of an LD-pumped solid-state laser 3, which is the third embodiment. This LD-pumped solid-state laser 3 is configured to combine solid-state laser oscillation light L21 with a wavelength of 698 nm generated in the first excitation / oscillation system 31 and solid-state laser oscillation light L22 with a wavelength of 607 nm generated in the second excitation / oscillation system 32 using a polarizing beam splitter 21. The first excitation / oscillation system 31 includes a first excitation system 61, a second excitation system 62, a third excitation system 63, and a fourth excitation system 64.

[0037] The first excitation / oscillation system 31 will now be described in detail. The first excitation system 61 of the first excitation / oscillation system 31 consists of an excitation LD 11, a collimating lens 12, a narrowband BPF 13, a focusing lens 14, a transmission plane mirror 15, a collimating lens 16, and a mirror 65, which are arranged sequentially from left to right along the optical axis extending horizontally in Figure 5. The second excitation system 62, on the other hand, consists of the same excitation LD 11, collimating lens 12, narrowband BPF 13, focusing lens 14, transmission plane mirror 15, and collimating lens 16 as described above, plus a wavelength multiplexing mirror 66. The third excitation system 63 consists of the same excitation LD 11, collimating lens 12, narrowband BPF 13, focusing lens 14, transmission plane mirror 15, and collimating lens 16 as described above (these elements are shared with the fourth excitation system 64, which will be described later), plus a wavelength multiplexing mirror 67. The fourth excitation system 64 is configured with the same excitation LD 11, collimating lens 12, and narrowband BPF 13 as described above, in addition to a polarizing beam splitter 91.

[0038] On the other hand, the second excitation / oscillating system 32 similarly has a first excitation system 61, a second excitation system 62, a third excitation system 63, and a fourth excitation system 64. The first excitation system 61 to the fourth excitation system 64 of the second excitation / oscillating system 32 will be described below. The first excitation system 61 consists of an excitation LD 11, a collimating lens 12, a narrowband BPF 13, a focusing lens 14, a transmission plane mirror 15, a collimating lens 16, and a mirror 65, which are arranged sequentially from bottom to top along the optical axis extending vertically in the figure. The second excitation system 62 consists of the same excitation LD 11, collimating lens 12, narrowband BPF 13, focusing lens 14, transmission plane mirror 15, and collimating lens 16 as above, plus a wavelength multiplexing mirror 66. The third excitation system 63 is configured with the same excitation LD 11, collimating lens 12, narrowband BPF 13, focusing lens 14, transmission plane mirror 15, and collimating lens 16 as described above (the above elements are shared with the fourth excitation system 64 described later), in addition to a wavelength multiplexing mirror 67. The fourth excitation system 64 is configured with the same excitation LD 11, collimating lens 12, and narrowband BPF 13 as described above, in addition to a polarizing beam splitter 92.

[0039] The operation of the first excitation / oscillation system 32 described above will now be explained. The excitation light L1 emitted by the first excitation system 61 of the first excitation / oscillation system 32 is reflected by the mirror 65 and then combined with the excitation light L emitted by the second excitation system 62 by the wavelength multiplexing mirror 66. After combination, the excitation light L emitted by the third excitation system 63 and the excitation light L emitted by the fourth excitation system 64 are combined by the wavelength multiplexing mirror 67. Note that the excitation light L emitted by the third excitation system 63 and the excitation light L emitted by the fourth excitation system 64 are combined by the polarizing beam splitter 91 before being incident on the wavelength multiplexing mirror 67. In this manner, the Pr:YLF crystal 19 of the first excitation / oscillation system 31 is incident on the Pr:YLF crystal 19, which is then combined from a total of four excitation systems, resulting in good excitation and the acquisition of high-intensity solid-state laser oscillation light L21.

[0040] The same applies to the second excitation / oscillation system 32. In other words, the Pr:YLF crystal 19 of the second excitation / oscillation system 32 is also incident on it with high-intensity excitation light L that has been emitted from a total of four excitation systems and then combined into one, so that excitation is performed well and high-intensity solid-state laser oscillation light L22 can be obtained.

[0041] The solid-state laser light L21 has a wavelength of 698 nm, and the solid-state laser light L22 has a wavelength of 607 nm. They propagate in directions orthogonal to each other and are incident on the polarizing beam splitter 21, where they are combined into a single beam. The combined solid-state laser light L21 and L22 are incident on the BBO crystal 22, where the BBO crystal 22 converts their wavelength to a sum frequency L23 with a wavelength of 325 nm. The sum frequency L23 passes through the concave mirror 20 and is emitted outside the LD-pumped solid-state laser 3 as usable light.

[0042] In this embodiment as well, in each of the first excitation system 61, second excitation system 62, third excitation system 63, and fourth excitation system 64 of the first excitation / oscillation system 31, a narrowband BPF 13 is provided to control the wavelength of the excitation light L so that it precisely matches the absorption peak wavelength of the Pr:YLF crystal 19. Similarly, in the second excitation / oscillation system 32, a narrowband BPF 13 is provided in each of the first excitation system 61, second excitation system 62, third excitation system 63, and fourth excitation system 64 so that the wavelength of the excitation light L precisely matches the absorption peak wavelength of the Pr:YLF crystal 19. Therefore, in this embodiment as well as in the first and second embodiments, the yield of applicable LDs 11 is dramatically improved, the oscillation wavelength of the excitation LD 11 is stable with respect to the absorption wavelength of the Pr:YLF crystal 19, thus stabilizing the optical output of the LD-excited solid-state laser 1, and the optical output of the LD-excited solid-state laser 1 and related performance remain stable even when the drive current value of the excitation LD 11 or the ambient temperature changes.

[0043] By using a narrowband BPF 13 with center wavelengths near the absorption lines of the Pr:YLF crystal 19 (wavelengths 442 nm, 469 nm, and 479 nm) and excitation LDs 11 that oscillate at each of these wavelengths, and by rotating the narrowband BPF 13, the wavelength of the excitation light L was controlled to precisely match the absorption peak wavelength of the Pr:YLF crystal 19. As a result, laser oscillations at 442 nm, 469 nm, and 479 nm were obtained, similar to those at 444 nm. Consequently, the Pr:YLF crystal 13 could be excited stably and efficiently at all wavelengths.

[0044] ≪Fourth Embodiment≫ Next, a fourth embodiment of the present invention will be described with reference to Figure 6. Figure 6 shows a schematic configuration of the LD-pumped solid-state laser 4, which is the fourth embodiment. Compared to the LD-pumped solid-state laser 1 of the first embodiment shown in Figure 1, this LD-pumped solid-state laser 4 differs in that a transmission-reflection mirror 115 is provided instead of a transmission-plane mirror 15, and other aspects are basically the same as the configuration in Figure 1. Therefore, in the following, the explanation of the configuration which is the same as the configuration in Figure 1 will be omitted, and the differences will be explained mainly.

[0045] As shown in detail in Fig. 7, the transmissive reflection mirror 115 is formed by joining two glass plates 115a and 115b, each of which is a parallel flat plate and has a rectangular or circular shape (Fig. 7 shows the state before joining). An antireflection coating (antireflection coating) AR2 is applied to one surface of the glass plate 115a, and a high-reflection coating HR is applied to the other surface. Antireflection coatings AR1 and AR2 are applied to one surface and the other surface of the other glass plate 115b, respectively.

[0046] The above high-reflection coating HR is composed of, for example, a multilayer film made of HfO ２、 Ta ２ O ５ , TiO ２ etc., and the antireflection coatings AR1 and AR2 are composed of, for example, SiO ２ films. The antireflection coating AR1 is designed and film-formed so that the reflectance becomes 0.5% or less after the following glass plate joining, and the antireflection coating AR2 is designed and film-formed so that the reflectance becomes 0.5% or less in the air.

[0047] After forming the antireflection coatings AR1 and AR2 and the high-reflection coating HR as described above, the surfaces of the antireflection coating AR1 and the high-reflection coating HR are activated by ozone treatment, plasma treatment or ultraviolet light treatment, and then the two glass plates 115a and 115b are joined by overlapping their surfaces (see Fig. 8). The joining was performed in air or in vacuum. In this case, the two glass plates 115a and 115b are joined by so-called optical contact. When the reflectance of the joint was examined using a laser beam for reflectance measurement after joining, the reflectance was 30%. This reflectance measurement can also be performed using the excitation light L in the configuration of Fig. 6, and in such a case, the reflectance was also 30%. Furthermore, this reflectance can also be measured using a commonly used spectroscope.

[0048] The two glass plates 115a and 115b are joined together and arranged as a transmission-reflection mirror 115 as shown in Figure 16 to form an LD-pumped solid-state laser 4 according to the fourth embodiment. In this configuration, the excitation light L is focused by the focusing lens 14 so as to converge at the joint surface of the two glass plates 115a and 115b. 70% of the excitation light L passes through the transmission-reflection mirror 115, and the remaining 30% is reflected at the joint surface (more specifically by the high-reflection coating film HR) and returns to the excitation LD 11 (see Figure 8).

[0049] In this embodiment, the portion where the focused excitation light L converges is sandwiched between the two glass plates 115a and 115b, so that the beam profile of the excitation light L at the convergence position can be maintained in a normal state. This beam profile will be described in detail below.

[0050] First, as shown in the configuration in Figure 1, when the excitation light L is focused on one surface of the transmission plane mirror 15 and energized for a long period of time, the beam profile will change abnormally over time. This tendency is particularly strong when a confocal optical system is present and the oscillation wavelength is also controlled, as in the configuration in Figure 1. In other words, on the mirror surface that constitutes the external resonator (the resonator range is shown by the solid double arrow in Figure 1), the beam spot of the focused excitation light L is small, resulting in a high optical power density. A high optical power density makes the beam profile more prone to becoming abnormal. Here, long-term energization refers to driving at a constant optical output for several hundred hours or more, or continuous driving for several thousand hours or more.

[0051] For example, if the excitation light L is a short-wavelength blue to violet light with a high output of several hundred mW to over 1 W, and the excitation LD 11 is energized for a long time, a photochemical reaction will occur in the focused portion of the excitation light L. As a result, organic matter floating in the surrounding atmosphere will gradually accumulate on the mirror surface. Due to the influence of this accumulation, the beam profile of the excitation light L focused on the mirror surface will gradually change from a normal Gaussian beam shape and become abnormal.

[0052] Here, in the configuration shown in Figure 1, if deposits as described above occur on one surface of the transmission plane mirror 15, an example of the schematic side shape of the deposit T is shown in Figure 9, and an example of the schematic planar shape of the beam spot BS affected by this deposit T is shown in Figure 10. Furthermore, the Gaussian beam shape, which is the normal beam profile of the excitation light L, is shown in Figure 11, and an example of an abnormal beam profile is shown in Figure 12. The abnormal beam profile shape changes to various disordered shapes, not limited to the profile shape in Figure 12, depending on the operating environment conditions. Because it is so different from the Gaussian beam shape used in normal optical design, the reflected light does not return to the excitation LD 11 normally, and wavelength control becomes insufficient. Furthermore, since the beam of the excitation light L1 also has an abnormal beam profile shape, the beam cannot be focused to the design value within the YLF crystal 19, resulting in a malfunction in which normal laser oscillation cannot be achieved. Note that the beam profiles in Figures 11 and 12 are represented by position on the horizontal axis and light intensity on the vertical axis.

[0053] In this embodiment, as shown in Figure 6, the position where the focused excitation light L converges is sandwiched between the two glass plates 115a and 115b, thus preventing the accumulation of organic matter floating in the surrounding atmosphere around the glass plates 115a and 115b. Therefore, the beam profile of the excitation light L is not abnormalized due to the influence of deposits, and a normal Gaussian beam shape is maintained.

[0054] <Fifth Embodiment> Next, a fifth embodiment of the present invention will be described. Compared to the LD-pumped solid-state laser 4 of the fourth embodiment shown in Figure 6, the LD-pumped solid-state laser of this fifth embodiment differs in the configuration of the transmission-reflection mirror 115 (and therefore in its manufacturing method), but in other respects, it is basically the same as the configuration in Figure 6. Therefore, the following will mainly describe the differences from the configuration in Figure 6, but this is also true for the sixth and seventh embodiments which will be described later.

[0055] In this embodiment, two glass plates 115a and 115b are joined by welding metals positioned near the inner circumference of their periphery. Figure 13 schematically shows the state before the metals M are welded, and Figure 14 schematically shows the state after welding. As shown in these figures, a high-reflectivity coating film HR is applied to the inner circumference of the annular metal M on one surface of glass plate 115a, and an anti-reflective coating film AR2 is applied to the inner circumference of the annular metal M on one surface of the other glass plate 115b. The other surfaces of glass plates 115a and 115b are each coated with an anti-reflective coating film AR1.

[0056] The above high-reflectance coating film HR is, for example, HfO ２、 Ta ２ O ５ , TiO ２ It is composed of a multilayer film made of the like, and the anti-reflective coating films AR1 and AR2 are for example SiO ２ It is composed of films. The anti-reflective coating film AR1 is designed and manufactured to have a reflectivity of 0.5% or less in air, and the anti-reflective coating film AR2 is designed and manufactured to have a reflectivity of 0.5% or less in an inert gas.

[0057] After forming the above coating films HR, AR1, and AR2, NiCr or Cr is deposited as a base layer on the peripheral areas (a predetermined range from the periphery to the center) of the glass plates 115a and 115b, and then AuSn (80% Au by weight) is plated on top of it until it reaches a thickness of 10 μm. In Figures 13 and 14, this plated portion is shown as metal M.

[0058] When glass plates 115a and 115b are stacked so that the plated portions overlap, and heated to a temperature above the melting point of AuSn (280°C), the AuSn melts and the glass plates 115a and 115b are joined together (as shown in Figure 14). This joining by melting is performed on dry N2 with a dew point temperature of -50°C or lower. ２ It is preferable to carry out the procedure in an inert gas such as dry nitrogen. ２ In addition, argon and other gases can also be used.

[0059] The transmission-reflection mirror 115, consisting of the glass plates 115a and 115b described above, was applied to the configuration shown in Figure 1 to obtain an LD-pumped solid-state laser according to the sixth embodiment. In this sixth embodiment as well, the beam profile at the convergence position of the excitation light L is maintained normally. This is because, in this embodiment as well, the convergence portion of the excitation light L is sandwiched between the glass plates 115a and 115b, and in this particular case, the two glass plates are joined via welded metal.

[0060] In other words, in this case, the area sandwiched between the two glass plates 115a and 115b is highly airtight, and even if something is present there, it is only an inert gas. Therefore, organic matter cannot penetrate this area, nor can deposits (see Figure 9) caused by organic matter be formed. Consequently, the beam profile of the excitation light L is not abnormally altered by long-term energization due to the influence of deposits, and is maintained normally.

[0061] <<Sixth Embodiment>> Next, a sixth embodiment of the present invention will be described. In this sixth embodiment, as in the fifth embodiment, two glass plates 115a and 115b are joined by welding the metal near their peripheral edges together, but the method of producing the joining metal differs from that of the fifth embodiment. The production of this joining metal will be described below.

[0062] Figure 15 schematically shows the state before the joining metal M described above is welded, and Figure 16 schematically shows the state after welding. As shown in these figures, a high-reflectivity coating film HR is applied to the inner circumference of the annular metal M on one surface of the glass plate 115a, and an anti-reflective coating film AR1 is applied to the inner circumference of the annular metal M on one surface of the other glass plate 115b. The other surfaces of the glass plates 115a and 115b are each coated with an anti-reflective coating film AR1.

[0063] The above high-reflectance coating film HR is, for example, HfO ２、 Ta ２ O ５ , TiO ２ It is composed of a multilayer film made of the like, and the anti-reflective coating films AR1 and AR2 are for example SiO ２It is composed of films. The anti-reflective coating film AR1 is designed and manufactured to have a reflectivity of 0.5% or less in air, and the anti-reflective coating film AR2 is designed and manufactured to have a reflectivity of 0.5% or less in an inert gas.

[0064] After forming the above coating films HR, AR1, and AR2, NiCr or Cr is deposited as a base layer on the peripheral areas (a predetermined range from the periphery to the center) of the glass plates 115a and 115b, and then AuSn (80% Au by weight) is plated on top of that until it reaches a thickness of 10 μm. Note that this plating process is omitted in Figures 15 and 16.

[0065] After the above plating is applied, when heated to a temperature above the melting point of AuSn (280°C or higher), the AuSn melts and the glass plate 115a and the metal M are fixed together, and the glass plate 115b and the metal M are fixed together (the state shown in Figure 15). Next, with the glass plates 115a and 115b stacked on top of each other so that the portions of the metal M overlap, when the metal M is further heated to a temperature above the melting point of AuSn (280°C or higher), the glass plates 115a and 115b are joined together via the molten metal M (the state shown in Figure 16).

[0066] The above bonding is for dry N with a dew point temperature of -50°C or lower. ２ It is preferable to carry out the procedure in an inert gas such as dry nitrogen. ２ In addition, argon and other gases can also be used. In such cases, a gap formed between the glass plate 115a and the glass plate 115b will be created by an airtightly sealed inert gas.

[0067] After the glass plate 115a is bonded to the metal M and the glass plate 115b is bonded to the metal M as shown in Figure 15, the melting point of AuSn is elevated because Au has diffused into the AuSn. Therefore, when joining glass plates 115a and 115b as shown in Figure 16, the melting of AuSn and the subsequent delamination of the previously established bond are prevented. Alternatively, this delamination can be prevented by using AuSn with different composition ratios (for example, the ratio of Au to Sn) and different melting points for the bonding and bonding processes.

[0068] <<Seventh Embodiment>> Next, a seventh embodiment of the present invention will be described. In this seventh embodiment, two glass plates 115a and 115b are joined using low-melting-point glass. This joining will be described below with reference to Figures 17 and 18.

[0069] Figure 17 schematically shows the state of glass plates 115a and 115b before joining, and Figure 18 schematically shows the state after welding. As shown in these figures, a high-reflectivity coating film HR is applied to the inner circumference of the annular low-melting-point glass G on one surface of glass plate 115a, and an anti-reflective coating film AR1 is applied to the inner circumference of the annular low-melting-point glass G on one surface of the other glass plate 115b. The other surfaces of glass plates 115a and 115b are each coated with an anti-reflective coating film AR1.

[0070] The above high-reflectance coating film HR is, for example, HfO ２、 Ta ２ O ５ , TiO ２ It is composed of a multilayer film made of the like, and the anti-reflective coating film AR1 is for example SiO ２ It is composed of films. The high-reflectivity coating film HR is designed and manufactured to have a reflectivity of 30% in an inert gas, while the anti-reflective coating film AR1 is designed and manufactured to have a reflectivity of 0.5% or less in an inert gas.

[0071] After forming the above coating film HR, the low-melting-point glass G is melted, and the glass plates 115a and 115b are joined together (the state shown in Figure 18). The low-melting-point glass G is lead-free glass Ta ２ O ５ (Tantalum pentoxide) and MoO ３ A mixture of powdered glass (molybdenum trioxide) was used and applied to the outer periphery of each glass plate 115a and 115b. The applied area was then heated to its melting point of 380°C or higher to melt and join the low-melting-point glass Gs together. In this way, glass plate 115a and glass plate 115b are joined, but a gap sealed with an inert gas is formed between them.

[0072] The aforementioned powdered glass mixture includes PbO·B ２ O ３ ・SiO２ A mixture consisting of the above can also be used. In that case, the above heating is performed at 500°C, which is above the melting point of the mixture. When the low-melting-point glass G is melted in this way and the glass plates 115a and 115b are joined to form an LD-excited solid laser 1 as shown in Figure 1, the beam profile of the Gaussian beam shape at the convergence position of the excitation light L was able to be maintained as normal, similar to the embodiments described above. Therefore, in this case as well, it is considered that the generation of deposits T due to long-term current application as shown in Figure 9 is suppressed.

[0073] Although embodiments of the present invention have been described above, it goes without saying that the LD-pumped solid-state laser according to the present invention is not limited to these embodiments. Furthermore, the LD-pumped solid-state laser according to the present invention is not limited to these embodiments, but can be particularly suitably used in, for example, devices for measuring the photoluminescence of semiconductors, flow cytometry devices, devices for analyzing genomics such as DNA sequencers, exposure devices for performing mask writing, direct writing, beam interference, etc., semiconductor wafer inspection devices, semiconductor mask inspection devices, laser processing devices, etc.

[0074] Furthermore, the specific nm value to which the excitation wavelength of the semiconductor laser is controlled to approximate the absorption peak wavelength of the solid laser crystal is not limited to the values ​​(nm) mentioned in each embodiment described above, but can be set appropriately according to the absorption peak wavelength of the target solid laser crystal. For example, if one wants to obtain an LD-pumped solid laser having an oscillation wavelength similar to that of a He-Ne laser, which is a type of gas laser, then it is natural to control the wavelength to approximately match the absorption peak wavelength of the solid laser crystal used therein. In addition, although the above describes an embodiment in which a transmission-reflection mirror 115 is applied in the first excitation / oscillation system 31 of Figure 6, a similar transmission-reflection mirror 115 may also be applied to the second excitation / oscillation system 32.

[0075] 1, 2, 3 LD-pumped solid-state laser 11 Excitation LD 12 Remating lens 13 Narrowband BPF 14 Focusing lens 15 Transmitting plane mirror 16 Collimating lens 17 Focusing lens 18, 51 Planar mirror 19 Pr:YLF crystal 20 Concave mirror 20a Concave surface of concave mirror 21, 91, 92 Polarizing beam splitter 21a Dielectric multilayer film 22 BBO crystal 31 First excitation / oscillation system 32 Second excitation / oscillation system 52 BBO crystal 61 First excitation system 62 Second excitation system 63 Third excitation system 64 Fourth excitation system 65 Mirror 66, 67 Wavelength multiplexing mirror

Claims

1. A semiconductor laser-pumped solid laser comprising: a solid laser crystal; a semiconductor laser that emits excitation light to excite the solid laser crystal; and a resonator that resonates the light emitted from the excited solid laser crystal, wherein the semiconductor laser-pumped solid laser is configured to generate two solid laser oscillations of different frequencies, and is provided with: an optical wavelength conversion element that converts the two solid laser oscillations into a sum frequency; and a wavelength control means that substantially matches the oscillation wavelength of the excitation light from the semiconductor laser to the absorption peak wavelength of the solid laser crystal.

2. The semiconductor laser-pumped solid-state laser according to claim 1, wherein the wavelength control means is disposed in a resonator that resonates the excitation light and comprises a bandpass filter that narrows the wavelength of the resonating light.

3. The semiconductor laser-pumped solid-state laser according to claim 1 or 2, wherein the resonator for resonating the excitation light further includes a diffraction grating for selecting the wavelength of the light to be resonated.

4. The semiconductor laser-pumped solid-state laser according to claim 1 or 2, wherein the resonator for resonating the excitation light further includes a VBG (Volume Bragg Grating) for selecting the wavelength of the light to be resonated.

5. The semiconductor laser-pumped solid-state laser according to claim 1 or 2, wherein the resonator for resonating the excitation light further includes a confocal optical system.

6. A semiconductor laser-pumped solid-state laser according to claim 1 or 2, wherein an external resonator is provided to constitute the wavelength control means, and the external resonator has a transmission-reflection mirror that transmits excitation light emitted from the semiconductor laser toward the solid-state laser crystal and reflects it toward the semiconductor laser, and the transmission-reflection mirror is formed from two glass plates joined together.

7. The semiconductor laser-pumped solid-state laser according to claim 6, wherein the two glass plates are joined to each other by optical contact.

8. The semiconductor laser-pumped solid-state laser according to claim 6, wherein the two glass plates are joined together by forming a metal plating near the periphery of each glass plate and heating and welding the metal platings together.

9. The semiconductor laser-pumped solid-state laser according to claim 6, wherein the two glass plates are joined together by forming a metal plating near the periphery of each glass plate and a metal plate overlapping the metal plating, and then heating and welding the metal plates to each other via the metal plating.

10. The semiconductor laser-pumped solid-state laser according to claim 6, wherein the two glass plates are joined together by melting low-melting-point glass placed near the periphery of each glass plate.

11. The solid laser crystal is Pr ３＋ A semiconductor laser-pumped solid-state laser according to claim 1 or 2, which is doped with [a specific substance].

12. The semiconductor laser-pumped solid-state laser according to claim 1 or 2, wherein the semiconductor laser is a GaN-based semiconductor laser.

13. The semiconductor laser-pumped solid-state laser according to claim 1 or 2, wherein the semiconductor laser is a transverse multimode semiconductor laser.

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