JP7624701B2 - Solid-state laser - Google Patents

Description

The present invention relates to a solid-state laser capable of outputting laser light in the visible light range.

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 shown in, for example, Patent Document 1, a solid-state laser generally known 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).

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 emit laser light in the 1 μm band (near infrared region), or solid-state lasers that use a base crystal doped with thulium ions, such as Tm:YLF, and emit laser light in the 2 μm band, or solid-state lasers that use a base crystal doped with holmium ions, such as Ho:YLF, and emit laser light in the 2 μm band, are also known.

JP 2019-016639 A JP 2007-242940 A

As seen in Patent Documents 1 and 2, conventional solid-state lasers emit laser light in a wavelength range longer than the visible light range, so in order to obtain visible light from the laser light output by the solid-state laser, it is necessary to convert the wavelength of the laser light using a nonlinear optical crystal or the like. This makes it difficult to efficiently generate visible light laser light, and also tends to lead to larger devices and higher costs for generating visible light laser light.

Furthermore, for example, when generating ultraviolet laser light, which has a shorter wavelength than the visible light range, in addition to converting the wavelength of the laser light output from the solid-state laser to the visible light range, further wavelength conversion from the visible light range to the ultraviolet range is required. This leads to a further decrease in the efficiency of generating ultraviolet laser light, and also leads to an increase in the size and cost of the device for generating ultraviolet laser light.

As a solid-state laser capable of oscillating laser light in the visible light range, for example, a titanium:sapphire laser, which uses sapphire doped with titanium ions as the laser crystal, is commonly known. However, in a titanium:sapphire laser, the excitation light is obtained by wavelength conversion of 1 μm band laser light obtained from 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 range with high efficiency, and this tends to lead to an increase in the size and cost of the excitation light source.

The present invention has been made in light of this background, and aims to provide a solid-state laser that can efficiently generate laser light in the visible light range without requiring wavelength conversion.

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 increased from the ground level ^5I8 _to
We have discovered _that when excitation light having a wavelength capable of exciting ^5F5 (for example, excitation light having a wavelength component of 638 nm) is irradiated, the energy level of the excited electrons returns from ^5F5 to ^5I8 _, and with 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, is emitted from the laser medium.

As seen in Patent Document 2, the base crystal doped with holmium ions has been known for some time 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.

Therefore, _the solid-state laser of the present invention comprises a laser medium doped with holmium ions, and an excitation light source that emits excitation light having a wavelength that excites the energy level of the electrons of the holmium ions from the ground level _of ^5I8 to ^5F5 into the laser medium, and is configured to emit laser light generated in response to the direct return of the energy level of the electrons of the holmium ions from _the ^5F5 to _{the 5I8} ^, and is characterized in that the excitation light has a central wavelength in the range of 630 to 660 nm .

In the solid- _state laser of the present invention, the difference between the wavelength of the excitation light capable of exciting the energy level of the electrons of the holmium ions from ^5I8 to ^5F5 (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, and therefore the efficiency of conversion from the excitation light to the laser light is extremely high.

Also, a known semiconductor laser or the like can be used as an excitation _light source having a wavelength that excites the electrons of the holmium ions from ^5I8 to ^5F5 . Then, a laser beam _in the visible light region can be directly emitted from the laser medium.

Therefore, the solid-state laser of the present invention can efficiently generate laser light in the visible light range without requiring wavelength conversion. Furthermore, it is possible to output laser light in the visible light range using various types of laser media.

The solid-state laser of the present invention preferably further comprises a resonator that amplifies and emits the laser light. This makes it possible to emit high-quality laser light in the visible light range with high output.

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.

FIG. 1 is a diagram illustrating a configuration of a solid-state laser according to an embodiment of the present invention. FIG. 2 is a diagram showing the transition of the energy levels of the electrons of a holmium ion. 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. FIG. 4 is a diagram showing the wavelength ranges required for exciting electrons of holmium ions in each of a number of types of host crystals (laser media). 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.

One embodiment of the present invention will be described below with reference to Figs. 1 to 5. With reference to Fig. 1, the 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 the laser light generated by the base crystal 10.

The base crystal 10 is a crystal doped with holmium (Ho) ions at a required doping ratio. As the material of the base crystal 10, for example, _YPO4 , _LaF3 , LaCl3, _KCaF3 , _BYF , GdAlO, _YVO4 , _YLiF4 (YLF), _Y3Al5O12 (YAG), _LuLiF4 (LLF), _{GdVO4 ,} etc. can be used. The appropriate doping ratio of holmium ions in the base crystal ₁₀ will be described later.

Here, the operating principle of the solid-state laser 1 of this embodiment using 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 the energy levels of their electrons, ⁵ I _8, which is the ground level, and ⁵ F ₅ , which is the higher energy level, as shown in FIG. 2. These energy levels ⁵ I ₈ and ⁵ F ₅ are each split into a plurality of Stark levels due to the crystal field. In addition, when the energy difference between ⁵ I ₈ and ⁵ F ₅ is converted to the wavelength of light, the wavelength is the wavelength of visible light (specifically, the red region). Although not shown, there are other energy levels (specifically, ⁵ I ₇ , ⁵ I ₆ , ⁵ I ⁵ , ⁵ I ₄ ) lower than ⁵ _{F 5} between the two energy levels 5 I ₈ and ⁵ F ₅ .

The energy level of the electrons of the holmium ions added to the base crystal 10 is changed from ⁵ I ₈ to
The inventors of ^{the present application} have found through various experiments that when excitation light having a wavelength (red wavelength) capable of exciting the electrons to _5F5 is incident on the base _{crystal 10} , the energy level of the electrons is excited from ^5I8 to ^5F5 , and then _the energy level of the excited electrons returns from ^5F5 to ^5I8 , and accordingly, laser light having a wavelength in the visible light range (red range ₎ is emitted from the base crystal 10.

For example, when YLiF ₄ (YLF) or LuLiF ₄ (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.

In this way, by using the base crystal 10 doped with holmium ions as a laser crystal, it is possible to directly generate laser light in the visible light range from excitation light having a wavelength _that can excite the energy level of the electrons of the holmium _ions from ^5I8 to ^5F5 .

In this case, the wavelength of the generated laser light is slightly longer than that of the excitation light, so the quantum efficiency (energy conversion efficiency 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 YLiF ₄ (YLF) or LuLiF ₄ (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%.

In addition, since the wavelength of the excitation light capable of exciting the energy level of the electrons of the holmium ion from ^5I8 to ^5F5 is in the red region, the _excitation light can be generated by a known semiconductor laser or the _like without requiring wavelength conversion.

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 to generate 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.

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) with a wavelength in the red range, and a focusing optical system 22 that focuses the excitation light output from the semiconductor laser 21. The focusing optical system 22 can be composed of a known optical lens, etc.

The semiconductor laser 21 changes the energy level of the electrons of the holmium ions in the base crystal 10
It is a semiconductor laser that outputs excitation light in the red region having a _{predetermined wavelength or a central wavelength in the vicinity thereof that can be excited to 5I8} ^to _5F5 ^.

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 638 nm.

As such, a GaAs-based semiconductor laser or an AlGaInP-based semiconductor laser can be used as the semiconductor laser 21 capable of outputting excitation light having a wavelength in the red range. The appropriate range of the central wavelength of the excitation light will be described later.

The resonator 30 includes a total reflection mirror 31 that totally reflects the laser light of a specific wavelength emitted from the base crystal 10, and an output mirror 32 that transmits part of the laser light and reflects the remainder. 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.

Here, the solid-state laser 1 of this embodiment is, for example, an end-pumped type 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.

The resonator 30 has a mode radius and other settings so that the laser light output from the base crystal 10 in response to the incidence of the excitation 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 excitation light.

In addition, the output mirror 32 is configured to reflect the laser light amplified in the cavity 33 at a predetermined reflectance of less than 100%, for example, within a range of 80 to 99%, in order to transmit (emit) a portion 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 can 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 oscillate laser light continuously, the reflectance of the output mirror 32 can be set to a higher reflectance, and when the solid-state laser 1 is configured to oscillate high-power pulsed laser light, the reflectance of the output mirror 32 can be set to a lower reflectance.

The resonator 30 may have any known structure as long as it can amplify and emit laser light of 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 from the side of the resonator 30 on the base crystal 10 placed in the cavity 33 of the resonator 30.

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 changed from ^5I8 to
By injecting excitation light having _{a wavelength (red wavelength) capable of exciting 5F5} ^into the base crystal 10, it is possible to directly output high-quality visible light (red) laser light having good coherency and other properties from the base crystal 10 with high efficiency, without the need for wavelength conversion.

In addition, excitation light having a wavelength capable of exciting the energy level of the electrons of holmium ions from ^5I8 to ^5F5 can be generated using a semiconductor laser 21 or the _like without the need for _wavelength conversion.
Therefore, it is possible to provide a solid-state laser 1 that is small in size, has a simple configuration, and is inexpensive, and that can efficiently output high-quality laser light in the visible light range (red range).

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

The conditions for this analysis are as follows:
Wavelength of excitation light: 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: 658 nm
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.

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

Figure 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 the output mirror 32 of the resonator 30 is 97% and the power of the excitation light is 1 W, 2 W, 3 W, 4 W, and 5 W.

Figure 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 the output mirror 32 of the resonator 30 is 90% and the power of the excitation light is 1 W, 2 W, 3 W, 4 W, and 5 W.

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

More specifically, the laser light power on the vertical axis of the graphs in Figures 3A to 3D is the power of the laser light output from the output mirror 32.

As can be seen in Figures 3A to 3D, the power of the output laser light varies with the holmium ion doping ratio (hereinafter simply referred to as the 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 its peak value is a doping ratio 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 a doping ratio within the range of 0.5 to 1.5 [at%].

However, at each power of excitation light, at a Ho doping ratio lower than the value of the Ho doping ratio at which the power of the laser light reaches its peak value, the power of the laser light decreases relatively rapidly as the Ho doping ratio decreases, but at a Ho doping ratio higher than the value of the Ho doping ratio at which the power of the laser light reaches its peak value, the power of the laser light decreases relatively slowly as the Ho doping ratio increases. Also, 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 reaches its peak value, by irradiating excitation light of an appropriate power onto the base crystal 10, it is possible to output laser light in the visible light range (red range).

Based on these findings, it is believed that if the Ho doping ratio of the base crystal 10 is within the range of 0.2 to 5 at% then it is possible to sufficiently output laser light in the visible light range (red range).

Next, the appropriate range of the wavelength of the excitation light will be described with reference to Fig. 4. Fig. 4 is a diagram showing, by horizontal lines, the range of wavelengths capable of exciting the energy level of the electrons of _holmium ions from ^5I8 to ^5F5 (the range obtained by converting the energy difference between ^5I8 and _5F5 into wavelengths ⁾ for each of a number _of representative types of host crystals that can be used as the host crystal ₁₀ .

Although not shown in Fig. 4, among the types of base crystal 10 described above, the wavelength ranges of the excitation light for LLF ( _LuLiF4 ) and _GdVO4 are approximately the same as those for YLF and _YVO4 . In this way, the wavelength ranges of the excitation light for each of the base crystals having similar compositions are approximately the same.

Here, in each type of base crystal, the wavelength at which the absorption of the excitation light is at its peak is often on the short wavelength side of the wavelength range in which excitation can be achieved from ^5I8 to ^5F5 _{. Considering this and the wavelength range for each type of base crystal shown in Fig. 5, it is considered that the range of the central wavelength of the excitation light is preferably, for example, 630 to 660 nm in order to excite the energy level of the electrons of the holmium ions from 5I8 to 5F5 for various types} ^{of base} crystals 10.

The solid-state laser 1 described above can generate high-quality laser light in the visible light range (red range) with high efficiency, and can be used for a variety of 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).

In addition, as shown in FIG. 5, the red laser light (hereinafter sometimes referred to as fundamental laser light) generated by the solid-state laser 1 can be wavelength-converted into second harmonic laser light (laser light with half the wavelength of the fundamental laser light) using a nonlinear optical crystal 41, thereby obtaining ultraviolet laser light with a single 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, it is possible to obtain ultraviolet laser light with a wavelength of 329 nm as the second harmonic laser light. This ultraviolet laser light (second harmonic laser light) can be used in fields such as fluorescence sensing, airborne particle sensing, and micromachining.

Furthermore, by wavelength-converting the fundamental and second harmonic laser beams into third harmonic laser beams (laser beams with 1/3 the wavelength of the fundamental laser beams) using a nonlinear optical crystal 42 as shown in FIG. 5, it is possible to obtain shorter wavelength ultraviolet laser beams. For example, from a fundamental laser beam with a wavelength of 658 nm, it is possible to obtain ultraviolet laser beams with a wavelength of 219 nm as the third harmonic laser beam. This ultraviolet laser beam (third harmonic laser beam) can be used, for example, in the medical field such as LASIK, as a substitute for an excimer laser, and for sterilization of equipment and spaces.

In the above-described solid-state laser 1, the resonator 30 is provided, but for example, a solid-state laser without the resonator 30 can be used as an optical amplifier. In addition, the laser medium (the 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. As the laser medium in the solid-state laser of the ^present invention, any crystal of a ^material can be used as long as it can be doped with _holmium ions, can receive excitation light capable of exciting the electrons of _holmium in from _5I8 to ^5F5 , and can emit laser light generated in response to the return of _the electrons from 5F5 to 5I8. Furthermore, glass, ceramics, and fibers can be used as the laser medium to which holmium ions are added.

1...Solid-state laser, 10...Base crystal (laser medium), 20...Excitation light source, 30...Resonator.

Claims (4)

A solid-state laser comprising: a laser medium doped with holmium ions; and _an excitation light source that emits excitation light having a wavelength that excites an energy level of an electron of the holmium ion from the ground level of ^5I8 to _5F5 into the ^laser medium, the solid-state laser being configured to emit laser light generated in response to the direct return of the energy level of the electron of the holmium ion from _the ^5F5 to _{the 5I8} ^, the excitation light having a central wavelength in the range of 630 to 660 nm . 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. 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 %. 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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