Semiconductor laser-pumped solid-state laser

Abstract

This semiconductor laser-pumped solid-state laser, equipped with an optical wavelength conversion element that converts the light emitted from two solid-state lasers into a sum frequency, achieves high output and stable operation. [Solution] In an LD-pumped solid laser 1 comprising a solid laser crystal 19 such as a Pr:YLF crystal, an LD 11 that emits excitation light L to excite the laser crystal 19, and a resonator that resonates the light emitted from the solid laser crystal 19, wavelength control means such as a narrow-band bandpass filter 13 is provided to make the oscillation wavelength of the excitation light L from the LD 11 substantially match the absorption peak wavelength of the solid laser crystal 19.

Description

Technical Field

[0001] The present invention relates to a semiconductor laser-excited solid laser, and more particularly to a solid laser that excites a solid laser crystal as a laser medium with excitation light emitted from a semiconductor laser and causes laser oscillation.

Background Art

[0002] For example, as shown in detail in Patent Document 1, a solid laser crystal such as a YLF crystal doped with Pr (hereinafter referred to as a Pr:YLF crystal) is excited by excitation light emitted from a semiconductor laser (laser diode; hereinafter referred to as LD), and light emitted from the excited solid laser crystal is resonated by a resonator. A LD-pumped solid laser is conventionally known. As the solid laser crystal, a YAG crystal doped with Nd (hereinafter referred to as an Nd:YAG crystal) shown in Patent Document 2 is also applicable. Further, converting the laser oscillation light obtained as described above to a desired light wavelength has also been conventionally performed. Patent Document 3 shows converting laser oscillation light of two wavelengths to a sum frequency of one wavelength. 3+ 3+ 3+ The Pr:YLF crystal can absorb light with wavelengths of 442 nm, 444 nm, 469 nm, and 479 nm and can perform laser oscillation at wavelengths of 479 to 720 nm, and is an extremely attractive laser crystal. However, since the Pr:YLF crystal has a narrow absorption wavelength width, when applied to a semiconductor laser-excited solid laser, the oscillation wavelength of the excitation LD must match the absorption wavelength, and furthermore, the oscillation wavelength width must be 0.5 to 1 nm or less for high-power and stable laser oscillation to be possible. More specifically and realistically explaining the absorption wavelength and oscillation wavelength, none of them are precisely determined, but they have waveforms that spread over a certain range. When specific numerical values are given for the wavelength, it means the peak wavelength in that waveform (which may also be the center wavelength of the waveform).

[0003]

[0004]

[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 5-10 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 LD 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 an oscillation wavelength that matches the absorption wavelength of the Pr:YLF crystal, taking these characteristics into account.

[0005] Non-patent document 1 shows 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 LD 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 offer high output, they suffer from poor spatial coherence due to their multi-mode transverse configuration. 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. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2001-36176 [Patent Document 2] Japanese Patent Publication No. 2001-36175 [Patent Document 3] Japanese Patent Publication No. 2006-66436 [Non-patent literature]

[0008] [Non-Patent Document 1] Applied Physics, Vol. 85, pp. 857-858 [Overview of the project] [Problems that the invention aims to solve]

[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. [Means for solving the problem]

[0010] The LD-pumped solid-state laser according to the present invention is Solid-state laser crystals such as Pr:YLF crystals, An LD that emits excitation light to excite the solid laser crystal, A resonator that resonates the light emitted from the solid laser crystal after it has been excited, In an LD-pumped solid-state laser having, The semiconductor laser-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 mentioned above into a sum frequency, A wavelength control means that causes the oscillation wavelength of the excitation light from the above-mentioned LD to substantially match the absorption peak wavelength of the solid laser crystal, This was established.

[0011] In the LD-pumped solid-state laser according to the present invention, An external resonator is provided to constitute the above wavelength control means. This external resonator has a transmission / reflection mirror that transmits the excitation light emitted from the semiconductor laser toward the solid laser crystal and reflects it toward the semiconductor laser. This transmissive and reflective mirror is preferably 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 periphery 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 periphery 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 periphery of each glass plate.

[0013] In the above configuration, there are not necessarily two sets of solid laser crystals, LDs, and resonators. In other words, some of them may be shared to generate two solid laser beams with 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 a solid-state laser pumped with an LD) 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. [Effects of the Invention]

[0015] According to the LD-excited solid-state laser of the present invention, by using the wavelength control means as described above, the oscillation wavelength of the excitation light by the LD can be made substantially coincide with the absorption peak wavelength of the solid-state laser crystal. Therefore, the yield of applicable LDs is dramatically improved. Since the oscillation wavelength of the excitation LD thus becomes stable with respect to the absorption wavelength of the solid-state laser crystal, the optical output of the solid-state laser also becomes stable. Further, even when the drive current value or the outside air temperature of the excitation LD changes and its oscillation wavelength fluctuates, the state where the oscillation wavelength is stable with respect to the absorption wavelength of the solid-state laser crystal is maintained. Therefore, the optical output of the LD-excited solid-state laser and its related performance continue to be stable.

Brief Description of the Drawings

[0016] [Figure 1] Schematic configuration diagram showing the LD-excited solid-state laser according to the first embodiment of the present invention [Figure 2] Figure showing the absorption characteristics of the solid-state laser crystal used in the solid-state laser of FIG. 1 [Figure 3] Figure showing another absorption characteristic of the solid-state laser crystal used in the solid-state laser of FIG. 1 [Figure 4] Schematic configuration diagram showing the LD-excited solid-state laser according to the second embodiment of the present invention [Figure 5] Schematic configuration diagram showing the LD-excited solid-state laser according to the third embodiment of the present invention [Figure 6] Schematic configuration diagram showing the LD-excited solid-state laser according to the fourth embodiment of the present invention [Figure 7] Schematic side view showing the state before assembly of the main part in the apparatus of FIG. 4 [Figure 8] Schematic side view showing the state after assembly of the main part shown in FIG. 7 [Figure 9] Schematic side view showing deposits formed on the transmission plane mirror of the LD-excited solid-state laser [Figure 10] Schematic plan view showing an example of the beam spot of the excitation light affected by deposits [Figure 11] Schematic diagram showing an example of the normal beam profile of the excitation light [Figure 12] Schematic diagram showing an example of the abnormal beam profile of the excitation light [Figure 13] A schematic side view showing the main parts of the apparatus of the fifth embodiment of the present invention before assembly. [Figure 14] Schematic side view showing the main components before assembly, as shown in Figure 13. [Figure 15] A schematic side view showing the main parts of the apparatus of the sixth embodiment of the present invention before assembly. [Figure 16] Schematic side view showing the assembled state of the main parts as shown in Figure 15. [Figure 17] A schematic side view showing the main parts of the apparatus of the seventh embodiment of the present invention before assembly. [Figure 18] Schematic side view showing the assembled state of the main parts as shown in Figure 17. [Modes for carrying out the invention]

[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 a 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. The LD-pumped solid-state laser 1 also 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 from bottom to top in the figure. In the diagram above, elements 11 to 19 arranged from bottom to top 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 is a laser oscillator that 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 through 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 above 698 nm light 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 BPF13s 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 BPF13, and in all combinations, the oscillation wavelength of the first excitation LD11 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 LD11 matched the absorption wavelength of the Pr:YLF crystal 19, and the oscillation full width at half maximum (FMAX) was narrower than the absorption width of the crystal 19, which is 1.0 nm. As a result, the energy of the first excitation LD11 could be absorbed by the Pr:YLF crystal 19 stably and efficiently. The oscillation wavelength and amplitude of the first excitation LD11 remained unchanged even when the ambient temperature changed or a long time elapsed after power was applied, allowing for stable oscillation. Therefore, it was no longer necessary to select good excitation LDs from a large number to resolve the problem of oscillation wavelength variations in the excitation LDs, and the lifespan of the LD-pumped solid-state laser 1 was also extended.

[0021] The second excitation LD11 of the second excitation / oscillation system 32 also generates its own laser oscillation, 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 transmitting plane mirror 15, and then passes through the mirror 15. The excitation light L that has passed through the transmitting 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 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 also 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 LD11 can be made to nearly 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, dramatically improving the yield of applicable LD11s. Since the oscillation wavelength of the excitation LD11 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 LD11 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 using a narrowband BPF13 for wavelength control, a diffraction grating, a VBG (Volume Bragg Grating), or even a confocal optical system may be used as a means of wavelength control. As mentioned earlier, the narrowband BPF13 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 by using a diffraction grating or a VBG. However, when using a VBG, it is necessary to basically incident the excitation light perpendicularly to the grating, so it is difficult to set the oscillation wavelength precisely to the desired value. Also, when using a confocal optical system, the optical system is stable, so the optical output can be stabilized.

[0025] Returning to Figure 1, the detailed configuration of the LD-pumped solid-state laser 1 will now be explained. 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 configured such that their c-axis extends in the direction shown in the figure. Thus, the solid-state laser oscillation light L21 with a wavelength of 698 nm has an electric field oscillation direction in the vertical direction in the figure, and the solid-state laser oscillation light L22 with a wavelength of 607 nm has an electric field oscillation direction perpendicular to the plane of the paper in the figure, and these are incident on the polarization beam splitter 21. The polarization beam splitter 21 has a dielectric multilayer film 21a tilted at 45° with respect to the propagation direction of the solid-state laser oscillation light L21 and the propagation direction of the solid-state laser oscillation light L22. The solid-state laser oscillation light L21 is incident on this dielectric multilayer film 21a in a p-polarized state, and the solid-state laser oscillation light L22 is incident on it in an s-polarized state. For solid-state laser oscillation light L21 and L22 in such polarized 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 polarizing beam splitter 21, the solid-state laser oscillation light L21 and solid-state laser oscillation light L22, which were propagating in different directions, are combined and emitted in a common direction, i.e., to the right in Figure 1. These combined solid-state laser oscillation light L21 and solid-state laser oscillation light L22 are incident on a nonlinear optical material, a BBO crystal (β-BaB2O4 crystal) 22, and are wavelength-converted by the BBO crystal 22 to a sum frequency L23 with a wavelength of 325 nm under type II phase matching conditions. This sum frequency L23 passes through the concave mirror 20 and is emitted outside the LD-pumped solid-state laser 1 as usable light. The concave surface 20a of the concave mirror 20, i.e., the surface facing the planar mirror 18, is coated with an AR (anti-reflective) coating for light with a wavelength of 325 nm.

[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 as extraordinary light 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] Furthermore, 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 607 nm through stimulated emission. This light is reflected by the concave mirror 20 and incident on the planar mirror 51, where it resonates within the solid-state laser resonator composed of the planar mirror 51 and the planar mirror 18, thereby obtaining solid-state laser oscillation light L22 with a wavelength of 607 nm.

[0031] The concave mirror 20 is positioned at an angle to the optical axis of the planar mirror 18, and the planar mirror 51 is positioned so that the reflection axis of the concave mirror 20 coincides with its optical axis. A BBO (β-BaB2O4) crystal 52 is positioned between the concave mirror 20 and the planar mirror 51 so that their optical axes pass through it. The BBO crystal 52 is a nonlinear optical material that wavelength-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 type II phase matching conditions. This sum frequency L23 passes through the concave mirror 20 and is emitted out of the LD-pumped solid-state laser 2. The concave surface 20a of the concave mirror 20, that is, the surface facing the planar mirror 51, is coated with an AR (anti-reflective) coating for light with a wavelength of 325 nm.

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

[0033] When the BBO crystal 52 is used as a nonlinear optical material as described above, if 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 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. Furthermore, when the BBO crystal is used, if its cut angle θ = 57.3°, 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.

[0034] In addition to the BBO crystal mentioned above, CLBO crystals and other nonlinear optical materials can also be used. 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 for 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. 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. In this embodiment as well, similar to the first embodiment, the yield of applicable LD 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 a first excitation / oscillation system 31 and solid-state laser oscillation light L22 with a wavelength of 607 nm generated in a 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 be described in detail below. 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 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 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 LD11, collimating lens12, and narrowband BPF13 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 this 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 (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 polarization 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 mutually orthogonal directions 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 the 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 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 LD11 is dramatically improved, the oscillation wavelength of the excitation LD11 is stable with respect to the absorption wavelength of the Pr:YLF crystal 19, the optical output of the LD-excited solid-state laser 1 is stable, 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 LD11 or the ambient temperature changes.

[0043] By using a narrowband BPF13 with center wavelengths near the absorption lines of the Pr:YLF crystal 19 (442nm, 469nm, and 479nm) and excitation LD11s that oscillate at each of these wavelengths, and by rotating the narrowband BPF13, the wavelength of the excitation light L was controlled to precisely match the absorption peak wavelength of the Pr:YLF crystal 19. This allowed for laser oscillation at 442nm, 469nm, and 479nm, similar to the 444nm wavelength. As a result, 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, we will omit the explanation of configurations that are the same as those in Figure 1 and mainly explain the differences.

[0045] As shown in detail in Figure 7, the transmission-reflection mirror 115 is formed by joining two glass plates 115a and 115b, each being a parallel plate with a rectangular or circular shape (Figure 7 shows the state before joining). One surface of glass plate 115a is coated with an anti-reflective coating AR2, and the other surface is coated with a high-reflection coating HR. One surface and the other surface of another glass plate 115b are coated with anti-reflective coatings AR1 and AR2, respectively.

[0046] The above high-reflection coating film HR is, for example, HfO 2、 The film is composed of a multilayer structure made of Ta2O5, TiO2, etc., and the anti-reflective coating films AR1 and AR2 are made of, for example, an SiO2 film. The anti-reflective coating film AR1 is designed and manufactured so that its reflectivity is 0.5% or less after the glass plate bonding described below, and the anti-reflective coating film AR2 is designed and manufactured so that its reflectivity is 0.5% or less in air.

[0047] As described above, after depositing the anti-reflective coating films AR1 and AR2 and the high-reflectivity coating film HR, the surfaces of the anti-reflective coating film AR1 and the high-reflectivity coating film HR were activated by ozone treatment, plasma treatment, or ultraviolet light treatment. Then, the two glass plates 115a and 115b were joined by overlapping their surfaces (see Figure 8). The joining was performed in air or vacuum. In this case, the two glass plates 115a and 115b were joined by so-called optical contact. After joining, the reflectivity of the joint was measured using a laser beam for reflectivity measurement, and the reflectivity was 30%. This reflectivity measurement can also be performed using the excitation light L in the configuration of Figure 6, and in that case as well, the reflectivity was 30%. Furthermore, this reflectivity can also be measured using a spectrometer, which is commonly used.

[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 constituting the external resonator (shown by the solid double arrows indicating the resonator range 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 operation at a constant optical output for several hundred hours or more, or continuous operation 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, prolonged energization of the excitation LD11 will trigger a photochemical reaction 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 the above-mentioned deposit occurs 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. Moreover, the beam of the excitation light L1 also has an abnormal beam profile shape, so 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 edges. 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-reflection coating film HR is, for example, HfO 2、 The film is composed of a multilayer structure made of Ta2O5, TiO2, etc., and the anti-reflective coating films AR1 and AR2 are made of, for example, an SiO2 film. 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 depositing the coating films HR, AR1, and AR2 described above, 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 preferably carried out in an inert gas such as dry N2 with a dew point of -50°C or lower. In addition to dry N2, argon and other inert 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 thought to be 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 glass plates 115a and 115b are each coated with the anti-reflective coating film AR1.

[0063] The above high-reflection coating film HR is, for example, HfO 2、The film is composed of a multilayer structure made of Ta2O5, TiO2, etc., and the anti-reflective coating films AR1 and AR2 are made of, for example, an SiO2 film. 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 depositing the coating films HR, AR1, and AR2 described above, 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), 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), the glass plates 115a and 115b are joined together via the molten metal M (the state shown in Figure 16).

[0066] The above joining is preferably carried out in an inert gas such as dry N2 with a dew point temperature of -50°C or lower. In addition to dry N2, argon and other gases can also be used as the inert gas. In such a case, a hermetically sealed void of inert gas will be formed between the glass plate 115a and the glass plate 115b.

[0067] After the glass plate 115a and metal M are bonded together, and the glass plate 115b and metal M are bonded together, 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 (e.g., the ratio of Au to Sn) and contrasting 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-reflection coating film HR is, for example, HfO 2、 The coatings are composed of multilayer films made of Ta2O5, TiO2, etc., and the anti-reflective coating film AR1 is composed of, for example, an SiO2 film. 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 coating film HR described above, the low-melting-point glass G is melted, and the glass plates 115a and 115b are joined together (as shown in Figure 18). As the low-melting-point glass G, a mixture of powdered glass of lead-free glass Ta2O5 (tellurium dioxide) and MoO3 (molybdenum trioxide) is used. This mixture is applied to the outer periphery of each glass plate 115a and 115b, and the applied portion is heated to its melting point = 380°C or higher to melt and join the low-melting-point glass G together. In this way, glass plate 115a and glass plate 115b are joined together, but a gap sealed with inert gas is formed between them.

[0072] In addition, a mixture consisting of PbO·B2O3·SiO2 can be used as the powdered glass mixture described above. In that case, the 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-pumped solid-state 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-state 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-state laser crystal. For example, if one wants to obtain an LD-pumped solid-state 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-state 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 in Figure 6, a similar transmission-reflection mirror 115 may also be applied to the second excitation / oscillation system 32. [Explanation of symbols]

[0075] 1, 2, 3 LD-pumped solid-state lasers 11 Excitation LD 12 Remade Lenses 13 Narrowband BPF 14. Converging lens 15. Transparent Planar Mirror 16 Collimating lenses 17. Converging lens 18, 51 Flat mirror 19 Pr:YLF crystal 20 Concave mirrors 20a Concave surface of concave mirror 21, 91, 92 Polarizing Beam Splitter 21a Dielectric multilayer film 22 BBO crystals 31 First excitation / oscillating system 32 Second excitation / oscillation system 52 BBO crystals 61 First Excitation System 62 Second Excitation System 63 Third Excitation System 64 Fourth Excitation System 65 Mirror 66, 67 wavelength multiplex mirror

Claims

1. Solid-state laser crystals and, A semiconductor laser that emits excitation light to excite the solid laser crystal, A resonator that resonates the light emitted from the solid laser crystal after it has been excited, In a semiconductor laser-pumped solid-state laser having, The semiconductor laser-pumped solid-state laser is configured to generate two solid-state laser oscillations with different frequencies. A wavelength conversion element that converts the two solid-state laser oscillations into a sum frequency, A wavelength control means that causes the oscillation wavelength of the excitation light from the semiconductor laser to substantially match the absorption peak wavelength of the solid laser crystal, A semiconductor laser-pumped solid-state laser equipped with a [specific feature / feature].

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. An external resonator is provided to constitute the wavelength control means. This external resonator has a transmission / reflection mirror that transmits the excitation light emitted from the semiconductor laser toward the solid laser crystal and reflects it toward the semiconductor laser. The semiconductor laser-pumped solid-state laser according to claim 1 or 2, wherein 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 aforementioned 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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