Solid-state laser

Abstract

To provide a solid state laser capable of efficiently generating laser light in a visible light range without requiring wavelength conversion.SOLUTION: A solid state laser 1 includes a base material crystal 10 (laser medium) doped with holmium ions, and an excitation light source 20 that injects excitation light having a wavelength that excites the electron energy level of the holmium ions from the ground level 5I8 to 5F5 into the base material crystal 10, and a laser beam is emitted as the electron energy level of the holmium ions returns from 5F5 to 5I8.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a solid-state laser capable of outputting laser light in the visible light range. [Background technology]

[0002] Solid-state lasers have the advantage of being able to generate high-quality laser light compared to semiconductor lasers, and have been used in various technical fields. Conventionally, as a solid-state laser, as shown in, for example, Patent Document 1, a generally known type uses a host crystal doped with neodymium ions, such as Nd:YVO4 or Nd:YAG, as a laser medium (laser crystal) and oscillates laser light in the 1 μm band (near-infrared region).

[0003] Also, as seen in Patent Document 2, for example, solid-state lasers that use a base crystal doped with yttrium ions, such as Yb:YAG or Yb:SYS, and oscillate laser light in the 1 μm band (near infrared region), solid-state lasers that use a base crystal doped with thulium ions, such as Tm:YLF, and oscillate laser light in the 2 μm band, and solid-state lasers that use a base crystal doped with holmium ions, such as Ho:YLF, and oscillate laser light in the 2 μm band, are also known. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2019-016639 A [Patent Document 2] JP 2007-242940 A Summary of the Invention [Problem to be solved by the invention]

[0005] As seen in Patent Documents 1 and 2, conventional solid-state lasers oscillate laser light in a wavelength range longer than the visible light range, and therefore, in order to obtain laser light in the visible light range from the laser light output by the solid-state laser, it is necessary to perform wavelength conversion of the laser light using a nonlinear optical crystal, etc. This makes it difficult to efficiently generate laser light in the visible light range, and also tends to lead to an increase in size and cost of the device for generating laser light in the visible light range.

[0006] Furthermore, for example, when generating ultraviolet laser light having a shorter wavelength than the visible light region, in addition to converting the wavelength of the laser light output from the solid-state laser into the visible light region, further wavelength conversion from the visible light region to the ultraviolet region is required, which leads to a further decrease in the efficiency of generating ultraviolet laser light and promotes the increase in size and cost of the ultraviolet laser light generating device.

[0007] As a solid-state laser capable of oscillating laser light in the visible light region, for example, a titanium:sapphire laser, which uses sapphire doped with titanium ions as a laser crystal, is generally known. However, in the titanium:sapphire laser, the pumping light is obtained by wavelength conversion of 1 μm band laser light obtained by a solid-state laser such as Nd:YAG into second harmonic laser light. For this reason, it is difficult to generate laser light in the visible light region with high efficiency, and it is likely to lead to an increase in size and cost of the pumping light source.

[0008] The present invention has been made in view of the above background, and has an object to provide a solid-state laser that can efficiently generate laser light in the visible light range without requiring wavelength conversion. [Means for solving the problem]

[0009] As a result of various experiments, the inventors of the present invention have found that, for a laser medium such as a host crystal doped with holmium ions, the energy level of the electrons of the holmium ions is 5 From I8 5When excitation light having a wavelength capable of exciting F5 (for example, excitation light having a wavelength component of 638 nm) is incident, the energy level of the excited electron becomes 5 From F5 5 They discovered that the laser medium returned to I8, and that accompanying this return, laser light with a wavelength in the visible light range (≒658 nm), i.e., laser light with a wavelength in the red range, was emitted from the laser medium.

[0010] As described in Patent Document 2, a base crystal doped with holmium ions has been conventionally known as a laser crystal capable of generating laser light in the 2 μm band, but its use as a laser crystal for generating laser light in the visible light range has not yet been known.

[0011] Therefore, the solid-state laser of the present invention is a laser medium to which holmium ions are added, and the energy level of the electrons of the holmium ions is set to the ground level. 5 From I8 5 and an excitation light source for irradiating the laser medium with excitation light having a wavelength for exciting F5, wherein the energy level of the electron of the holmium ion is 5 F5 to the above 5 The laser light generated in response to the direct return to I8 is configured to be emitted. At the same time, the excitation light has a central wavelength in the range of 630 to 660 [nm]. It is characterized by:

[0012] According to the solid-state laser of the present invention, the energy level of the electrons of the holmium ions is 5 From I8 5 Since the difference between the wavelength of the excitation light that can excite F5 (e.g., 638 nm) and the wavelength of the laser light emitted in response to the return of the energy level from that excitation (≒658 nm) is very small, the conversion efficiency from excitation light to laser light is extremely high.

[0013] Also, the electrons of the holmium ion 5 From I8 5 A known semiconductor laser or the like can be used as an excitation light source having a wavelength for exciting F5. Laser light in the visible light range can be directly emitted from the laser medium.

[0014] Therefore, according to the solid-state laser of the present invention, laser light in the visible light range can be efficiently generated without the need for wavelength conversion. Furthermore, it is possible to output laser light in the visible light range using various types of laser media.

[0015] The solid-state laser of the present invention preferably further comprises a resonator for amplifying and emitting the laser light, thereby making it possible to emit high-quality laser light in the visible light range with high output.

[0016] In the solid-state laser of the present invention, the ratio of the holmium ions added to the laser medium is preferably 0.2 to 5 at %, and more preferably 0.5 to 1.5 at %. This makes it possible to appropriately output laser light in the visible light range from the laser medium. [Brief description of the drawings]

[0018] [Figure 1] FIG. 1 is a diagram illustrating a configuration of a solid-state laser according to an embodiment of the present invention. [Diagram 2] Figure 2 shows the transition of the energy levels of the electrons in a holmium ion. [Diagram 3] 3A to 3D are graphs illustrating the relationship between the proportion of holmium ions added to the base crystal (laser medium) and the power of the output laser light. [Figure 4] FIG. 4 is a diagram showing the wavelength range required for exciting electrons of holmium ions in each of several types of host crystals (laser media). [Diagram 5] FIG. 1 is a block diagram showing an example of a system for performing wavelength conversion of laser light generated by a solid-state laser according to an embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] An embodiment of the present invention will be described below with reference to Figures 1 to 5. With reference to Figure 1, a solid-state laser 1 of this embodiment includes a base crystal 10 as a laser crystal (laser medium), an excitation light source 20 that inputs excitation light to the base crystal 10, and a resonator 30 that amplifies and emits laser light generated in the base crystal 10.

[0020] The base crystal 10 is a crystal doped with holmium (Ho) ions at a required doping ratio. Examples of the material of the base crystal 10 include YPO4, LaF3, LaCl3, KCaF3, BYF, GdAlO, YVO4, YLiF4 (YLF), Y3Al5O 12 Crystals such as (YAG), LuLiF4 (LLF), GdVO4, etc. can be used. The appropriate ratio of holmium ions added to the base crystal 10 will be described later.

[0021] Here, the operating principle of the solid-state laser 1 of this embodiment, which uses the base crystal 10 as a laser crystal (laser medium), will be described with reference to Fig. 2. The holmium ions added to the base crystal 10 have an electron energy level of the ground level, as shown in Fig. 2. 5 I8 and its higher energy levels 5 F5 and F6. And these energy levels 5 I8, 5 F5 splits into multiple Stark levels due to the crystal field. 5 I8 and 5 When the energy difference between F1 and F5 is converted to the wavelength of light, the wavelength is in the visible light range (specifically, the red range). 5 I8 and 5 Between the two energy levels of F5 5 Other energy levels lower than F5 (specifically, 5 I7, 5 I6, 5 I5, 5 I4) also exists.

[0022] The energy level of the electrons of the holmium ions added to the base crystal 10 is 5 From I8 5 When excitation light having a wavelength (red wavelength) capable of exciting F5 is irradiated onto the base crystal 10, the energy level of the electron becomes 5 From I8 5 F5, and then the energy level of the excited electron is 5 From F5 5 The inventors of the present application have found through various experiments that the temperature returns to I8, and along with this, a phenomenon occurs in which a laser beam having a wavelength in the visible light range (red range) is emitted from the base crystal 10.

[0023] For example, when YLiF4 (YLF) or LuLiF4 (LLF) doped with holmium ions is used as the base crystal 10, it has been confirmed that when excitation light with a wavelength of 638 nm is incident on the base crystal 10, laser light in the visible light range (red range) with a wavelength of 658 nm is output from the base crystal 10.

[0024] In this way, by using the base crystal 10 doped with holmium ions as a laser crystal, the energy level of the electrons of the holmium ions can be 5 From I8 5 Laser light in the visible light range can be generated directly from excitation light having a wavelength capable of exciting F5.

[0025] In this case, the wavelength of the generated laser light is slightly longer than that of the excitation light, so the quantum efficiency (efficiency of energy conversion from excitation photons to laser photons) is extremely high. For example, when excitation light with a wavelength of 638 nm is incident on the base crystal 10 made of YLiF4 (YLF) or LuLiF4 (LLF) doped with holmium ions, the wavelength of the generated laser light is 658 nm, so the quantum efficiency is 638 / 658 = 0.97, and the quantum defect (energy loss rate) is only 3%.

[0026] The energy levels of the electrons in the holmium ion are 5 From I8 5Since the wavelength of the excitation light capable of exciting F5 is in the red region, the excitation light can be generated by a known semiconductor laser or the like without requiring wavelength conversion.

[0027] Therefore, by using the base crystal 10 doped with holmium ions as a laser crystal, it is possible to generate laser light in the visible light range with high efficiency without requiring wavelength conversion of the laser light generated in the base crystal 10 or wavelength conversion for generating the excitation light incident on the base crystal 10. The above are the basic technical matters regarding the operation of the solid-state laser 1 of this embodiment.

[0028] The excitation light source 20 of the solid-state laser 1 of this embodiment includes a semiconductor laser 21 that outputs excitation light (laser light) having a wavelength in the red region, and a focusing optical system 22 that focuses the excitation light output from the semiconductor laser 21. The focusing optical system 22 can be configured of a known optical lens or the like.

[0029] The semiconductor laser 21 changes the energy level of the electrons of the holmium ions in the base crystal 10 to 5 From I8 5 It is a semiconductor laser that outputs excitation light in the red region having a central wavelength at or near a predetermined wavelength that can excite F5.

[0030] For example, when the base crystal 10 is a laser crystal made of YLiF4 (YLF) or LuLiF4 (LLF) doped with holmium ions, the semiconductor laser 21 is configured to output excitation light (laser light) having a central wavelength at or near a wavelength of 638 nm.

[0031] In this way, as the semiconductor laser 21 capable of outputting pumping light having a wavelength in the red region, for example, a GaAs-based semiconductor laser or an AlGaInP-based semiconductor laser can be used. The appropriate range of the central wavelength of the pumping light will be described later.

[0032] The resonator 30 comprises a total reflection mirror 31 that totally reflects the laser light of a predetermined wavelength emitted from the base crystal 10, and an output mirror 32 that transmits part of the laser light and reflects the remainder, and the base crystal 10 is disposed in a cavity 33, which is an internal space formed between the total reflection mirror 31 and the output mirror 32.

[0033] Here, the solid-state laser 1 of this embodiment is, for example, an end-pumped laser, and the excitation light source 20, the resonator 30, and the base crystal 10 are arranged so that the excitation light output from the focusing optical system 22 of the excitation light source 20 passes through the total reflection mirror 31 and enters the base crystal 10 in the cavity 33 of the resonator 30. In this case, the total reflection mirror 31 is configured to totally reflect light having a wavelength that matches the wavelength of the laser light output from the base crystal 10, and to transmit light having a wavelength that matches the central wavelength of the excitation light and wavelengths nearby it with high transmittance.

[0034] Moreover, the resonator 30 has a mode radius and the like set so that the laser light output from the base crystal 10 in response to the incidence of the pumping light can be amplified while being reflected between the total reflection mirror 31 and the output mirror 32 within the cavity 33. For example, the mode radius of the resonator 30 is set to match the beam radius of the pumping light.

[0035] Moreover, the output mirror 32 is configured to reflect the laser light amplified in the cavity 33 at a predetermined reflectance less than 100%, for example, at a predetermined reflectance within a range of 80 to 99%, in order to transmit (emit) a part of the laser light from the cavity 33 to the outside. However, the reflectance of the laser light at the output mirror 32 is not limited to the range exemplified above, and may be set to various reflectances depending on the application, specifications, etc. of the solid-state laser 1. For example, when the solid-state laser 1 is configured to continuously oscillate a laser light, the reflectance of the output mirror 32 may be set to a higher reflectance, and when the solid-state laser 1 is configured to oscillate a high-output pulsed laser light, the reflectance of the output mirror 32 may be set to a lower reflectance.

[0036] The resonator 30 may have any known structure as long as it can amplify and emit the laser light having the wavelength output from the base crystal 10. The solid-state laser 1 is not limited to an end-pumped type, and may be configured as a side-pumped type in which pumping light is incident on the side of the resonator 30 to the base crystal 10 placed in the cavity 33 of the resonator 30.

[0037] The solid-state laser 1 of this embodiment is configured as described above. With this solid-state laser 1, the energy level of the electrons of the holmium ions added to the base crystal 10 is 5 From I8 5 By injecting excitation light having a wavelength (red wavelength) capable of exciting F5 into the base crystal 10, high-quality visible light range (red range) laser light with good coherency, etc., can be output directly from the base crystal 10 with high efficiency without the need for wavelength conversion.

[0038] The energy levels of the electrons in the holmium ion are 5 From I8 5 The pump light having a wavelength capable of exciting F5 can be generated using a semiconductor laser 21 or the like without the need for wavelength conversion. Therefore, it is possible to provide a small-sized, simply configured, and inexpensive solid-state laser 1 capable of efficiently outputting high-quality laser light in the visible light range (red range).

[0039] Next, an appropriate doping ratio of holmium ions to the base crystal 10 will be described with reference to Fig. 3A to Fig. 3D. In order to find an appropriate doping ratio of holmium ions, the inventors of the present application analyzed how the power of the laser light output from the base crystal 10 changes with respect to the doping ratio of holmium ions, for example, by using a known rate equation model.

[0040] The conditions for this analysis are as follows: Excitation light wavelength: 638 nm Excitation light power: 1~5W Beam radius of the excitation light incident on the base crystal 10: 100 μm Mode radius of resonator 30: 100 μm Length of the base crystal 10 in the optical axis direction of the excitation light beam: 2 mm Wavelength of output laser light: 658nm Reflectivity of output mirror 32 of resonator 30: 99%, 97%, 90%, 80% The total reflection mirror 31 of the resonator 30 has a reflection characteristic of total reflection with respect to the wavelength (658 nm) of the laser light, and has a transmission characteristic of high transmission with respect to the wavelength (638 nm) of the excitation light.

[0041] FIG. 3A is a graph showing the characteristics of the change in laser light versus the doping ratio [at %] of holmium ions when the reflectivity of output mirror 32 of resonator 30 is 99% and the power of the excitation light is 1 W, 2 W, 3 W, 4 W, and 5 W.

[0042] FIG. 3B is a graph showing the characteristics of the change in laser light power versus the doping ratio [at %] of holmium ions when the reflectivity of output mirror 32 of resonator 30 is 97% and the power of the excitation light is 1 W, 2 W, 3 W, 4 W, and 5 W.

[0043] FIG. 3C is a graph showing the characteristics of the change in laser light power versus the doping ratio [at %] of holmium ions when the reflectivity of output mirror 32 of resonator 30 is 90% and the power of the excitation light is 1 W, 2 W, 3 W, 4 W, and 5 W, respectively.

[0044] FIG. 3D is a graph showing the characteristics of the change in laser light power versus the doping ratio [at %] of holmium ions when the reflectivity of output mirror 32 of resonator 30 is 80% and the power of the excitation light is 1 W, 2 W, 3 W, 4 W, and 5 W, respectively.

[0045] 3A to 3D, the vertical axis of the graph indicates the power of the laser light output from output mirror 32. More specifically, the vertical axis of the graph indicates the power of the laser light output from output mirror 32.

[0046] As shown in Figures 3A to 3D, the power of the output laser light varies with respect to the doping ratio of holmium ions (hereinafter, simply referred to as Ho doping ratio) so as to have a peak value (maximum value) when the Ho doping ratio is at a certain value. In this case, the Ho doping ratio at which the power of the laser light reaches a peak value is within the range of approximately 0.5 to 1.5 [at%]. Therefore, it is preferable that the Ho doping ratio of the base crystal 10 is within the range of 0.5 to 1.5 [at%].

[0047] However, at each power of the excitation light, at a Ho doping ratio lower than the value of the Ho doping ratio at which the power of the laser light is at its peak, the power of the laser light decreases relatively rapidly with the decrease in the Ho doping ratio, but at a Ho doping ratio higher than the value of the Ho doping ratio at which the power of the laser light is at its peak, the power of the laser light decreases relatively slowly with the increase in the Ho doping ratio. Even if the Ho doping ratio of the base crystal 10 deviates to some extent from the value of the Ho doping ratio at which the power of the laser light is at its peak, by applying excitation light of an appropriate power to the base crystal 10, it is possible to output laser light in the visible light region (red region).

[0048] From these facts, it is considered that if the Ho doping ratio in the base crystal 10 is within the range of 0.2 to 5 [at %], laser light in the visible light region (red region) can be sufficiently output.

[0049] Next, the appropriate range of the wavelength of the excitation light will be described with reference to Fig. 4. Fig. 4 shows the energy levels of electrons of holmium ions for each of several representative types of host crystals that can be used as the host crystal 10. 5 From I8 5 The range of wavelengths that can excite F5 ( 5 I8 and 5 F5) is shown by a horizontal line segment.

[0050] Although not shown in Fig. 4, the wavelength ranges of the excitation light for LLF (LuLiF4) and GdVO4 among the above-mentioned types of base crystal 10 are almost the same as those for YLF and YVO4, respectively. The wavelength ranges of the excitation light for each of the base crystals having similar compositions are almost the same.

[0051] Here, the wavelength at which the amount of excitation light absorbed peaks in each type of base crystal is: 5 From I8 5 In many cases, the wavelength is on the short wavelength side of the wavelength range that can excite F5. Considering this and the wavelength range for each type of host crystal shown in FIG. 5, it is possible to set the energy level of the electrons of the holmium ion for various types of host crystals 10. 5 From I8 5 In exciting F5, it is considered that the range of central wavelength of the excitation light is preferably, for example, 630 to 660 nm.

[0052] The above-described solid-state laser 1 can generate high-quality laser light in the visible light range (red range) with high efficiency, and can be used for various purposes. For example, the red range laser light generated by the solid-state laser 1 can be used in fields such as vegetation sensing, biosensing, and LIDAR (Light Detection and Ranging).

[0053] Moreover, by converting the wavelength of red laser light (hereinafter sometimes referred to as fundamental laser light) generated by the solid-state laser 1 into second harmonic laser light (laser light with half the wavelength of the fundamental laser light) using a nonlinear optical crystal 41 as shown in Fig. 5, ultraviolet laser light can be obtained by one wavelength conversion. For example, from the fundamental laser light with a wavelength of 658 nm that can be generated by the solid-state laser 1, ultraviolet laser light with a wavelength of 329 nm can be obtained as the second harmonic laser light. Then, the ultraviolet laser light (second harmonic laser light) can be used in fields such as fluorescence sensing, airborne particle sensing, and micromachining.

[0054] Furthermore, as shown in Fig. 5, the fundamental and second harmonic laser beams can be wavelength-converted into third harmonic laser beams (laser beams with 1 / 3 wavelength of the fundamental laser beams) using a nonlinear optical crystal 42, thereby obtaining shorter wavelength ultraviolet laser beams. For example, ultraviolet laser beams with a wavelength of 219 nm can be obtained as the third harmonic laser beams from a fundamental laser beam with a wavelength of 658 nm. The ultraviolet laser beams (third harmonic laser beams) can be used in the medical field, such as LASIK, as a substitute for excimer lasers, and for sterilization of devices and spaces.

[0055] Although the solid-state laser 1 described above includes the resonator 30, for example, a solid-state laser not including the resonator 30 can also be used as an optical amplifier. Furthermore, the laser medium (laser medium to which holmium ions are added) in the solid-state laser of the present invention is not limited to the above-mentioned type of base crystal 10. The laser medium in the solid-state laser of the present invention can be one to which holmium ions can be added and which can absorb electrons of holmium ions. 5 From I8 5 F5 can be excited by excitation light, and 5 From F5 5 Any crystal can be used that can emit laser light generated in response to the return of electrons to I8. Furthermore, glass, ceramics, and fibers can be used as a laser medium to which holmium ions are added. [Explanation of symbols]

[0056] 1...solid-state laser, 10...host crystal (laser medium), 20...excitation light source, 30...resonator.

Claims

1. A laser medium to which holmium ions are added, and the energy level of the electrons of the holmium ions is set to the ground level. ５ I ８ from ５ F ５ and an excitation light source for irradiating the laser medium with excitation light having a wavelength for exciting the laser medium to the wavelength of the holmium ion. ５ F ５ From the above ５ I ８ a solid-state laser configured to emit laser light generated in response to a direct return to a laser diode, the excitation light having a central wavelength in the range of 630 to 660 nm.

2. 2. The solid-state laser according to claim 1, 4. A solid-state laser further comprising a resonator that amplifies and emits the laser light.

3. 3. The solid-state laser according to claim 1, A solid-state laser characterized in that the proportion of holmium ions added to the laser medium is 0.2 to 5 at %.

4. 3. The solid-state laser according to claim 1, A solid-state laser characterized in that the proportion of holmium ions added to the laser medium is 0.5 to 1.5 at %.

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