JP2008124371A - Laser oscillation method and laser apparatus - Google Patents

Abstract

The present invention provides a laser oscillation method and a laser apparatus capable of high-energy pulse oscillation and capable of oscillation in a wavelength region longer than 1.5 μm.
As a laser medium, Tm is a material in which at least one of Tm and Ho is added as a laser active ion to a transparent YAG ceramic having a garnet structure and a chemical formula represented as Y ₃ Al ₅ O _12. Ho: YAG ceramic is used, and a laser medium made of Tm, Ho: YAG ceramic is excited by excitation light having a wavelength band of 750 nm to 820 nm to oscillate.
[Selection] Figure 2

Description

  The present invention relates to a laser oscillation method and a laser apparatus, and more particularly to a laser oscillation method and a laser apparatus using a laser medium that is excited by excitation light and oscillates.

  In recent years, as the number of users increases with the development of aircraft, the demand for aviation safety has increased rapidly.

  In particular, aircraft accidents due to turbulent airflow in fine weather have been reported recently, and it is an urgent task to prevent such aircraft accidents.

  However, turbulence in fine weather is not something that can not be detected visually, but it cannot be detected using radio wave radar, and even today with a computer-controlled navigation system in place, It is pointed out that it is difficult to avoid.

  For this reason, remote sensing (remote exploration), which is a technology for observing an observation target from a remote location using radio waves or light, is a technology that makes it possible to detect and avoid sudden phenomena such as turbulence in fine weather in advance. Technology development is strongly desired.

By the way, in consideration of emitting laser light into the atmosphere in remote sensing, use a laser device that oscillates in a wavelength region longer than 1.5 μm, which is a so-called eye-safe region, which is highly safe for human eyes. Is desirable.

  For this reason, development of a laser oscillation method and laser apparatus that can be used for remote sensing for ensuring aviation safety, can oscillate at high energy pulses, and can oscillate in a wavelength region longer than 1.5 μm is strongly desired. It was rare.

Note that the prior art that the applicant of the present application knows at the time of filing a patent application is not an invention related to a known literature invention, so there is no prior art document information to be described.

  The present invention has been made in view of the above-described demands, and an object of the present invention is to provide a laser oscillation method and laser capable of high-energy pulse oscillation and capable of oscillation in a wavelength region longer than 1.5 μm. The device is to be provided.

In order to achieve the above object, a laser oscillation method and a laser apparatus according to the present invention are used as laser active ions in a transparent YAG ceramic having a garnet structure and a chemical formula represented as Y ₃ Al ₅ O ₁₂ as a laser medium. A material (hereinafter referred to as “Tm, Ho: YAG ceramic”) to which at least one of Tm and Ho is added (hereinafter appropriately referred to as “dope”) is used. It is.

  The laser oscillation method and laser apparatus according to the present invention are configured to excite a laser medium made of the above Tm, Ho: YAG ceramic with excitation light having a wavelength band of 750 nm to 820 nm to generate laser oscillation.

  Here, the YAG ceramic is a solid-state laser made of a polycrystalline body that is formed by collecting single crystals having a size of 1 mm or less.

Moreover, about the addition concentration (doping rate) of Tm ion added to YAG ceramic,
0 ≦ Tm ≦ 20% (except Tm = 0 and Ho = 0)
It is preferable that the addition concentration (doping rate) of Ho ions added to the YAG ceramic is
0 ≦ Ho ≦ 10% (except Tm = 0 and Ho = 0)
It is preferable that

Here, Tm, Ho: YAG ceramic, which is a YAG ceramic doped with at least one of Tm and Ho, that is, a solid-state laser comprising a polycrystal used as an ion for doping at least one of Tm and Ho The Tm, Ho: YAG ceramic oscillates by Ho ions, and the Tm ions act as a sensitizer that effectively absorbs excitation light, and the wavelength of 2 μm is obtained by excitation of excitation light in the wavelength band of 750 nm to 820 nm. More specifically, a high-energy pulsed laser beam with a wavelength of 1.9 μm to 2.2 μm is oscillated.

However, when Ho = 0%, oscillation is caused by Tm ions. When Ho = 0%, Tm becomes an active ion. In this case, in FIG. 3, ³ H ₄ becomes the upper level of the laser, and oscillation in the 1.9 μm to 2.1 μm band is generated by the transition to ³ H ₆ where the distribution accumulated at this level is the lower level.

That is, the invention according to claim 1 of the present invention is a laser oscillation method in which a laser medium is excited by excitation light to generate laser oscillation, and has a garnet structure and a chemical formula is represented as Y ₃ Al ₅ O _12. Using a laser medium composed of Tm, Ho: YAG ceramic, which is a material obtained by adding at least one of Tm and Ho as laser active ions to transparent YAG ceramic, the wavelength is 1.9 μm to 2. A laser beam of 2 μm band is generated.

  Further, the invention described in claim 2 of the present invention is the invention described in claim 1 of the present invention, wherein the YAG ceramic is made of a polycrystal formed by assembling single crystals having a size of 1 mm or less. Is a solid-state laser.

The invention according to claim 3 of the present invention is the invention according to claim 1 or 2 of the present invention, wherein the addition concentration of Tm added to the YAG ceramic is:
0 ≦ Tm ≦ 20% (except Tm = 0 and Ho = 0)
It is intended to be.

Further, the invention according to claim 4 of the present invention is the invention according to any one of claims 1, 2, or 3 of the present invention, wherein the additive concentration of Ho added to the YAG ceramic is:
0 ≦ Ho ≦ 10% (except Tm = 0 and Ho = 0)
It is intended to be.

  The invention according to claim 5 of the present invention is the invention according to any one of claims 1, 2, 3 or 4 of the present invention, wherein the laser medium has a wavelength band of 750 nm to 820 nm. The laser oscillates by being excited by the excitation light.

  According to a sixth aspect of the present invention, in the invention according to any one of the first, second, third, fourth, and fifth aspects of the present invention, the laser medium is excited by a semiconductor laser. Excitation light is generated.

  Further, the invention according to claim 7 of the present invention is the invention according to any one of claims 1, 2, 3, 4, 5 or 6 of the present invention, wherein the excitation light is a predetermined value. The pulsed light has a repetition frequency.

According to an eighth aspect of the present invention, a laser medium is disposed in the resonator, and excitation light is incident on the laser medium to cause laser oscillation in the resonator, thereby causing the resonance. In the laser apparatus for emitting laser light from the vessel, the laser medium has at least one of Tm and Ho as laser active ions in a transparent YAG ceramic having a garnet structure and a chemical formula represented as Y ₃ Al ₅ O ₁₂ A laser medium having at least a region having a Tm, Ho: YAG ceramic as a material to which one of these is added, and a laser having a wavelength of 1.9 μm to 2.2 μm by exciting the laser medium with excitation light It is designed to generate light.

  The invention according to claim 9 of the present invention is the invention according to claim 8 of the present invention, wherein the YAG ceramic is made of a polycrystal obtained by assembling single crystals having a size of 1 mm or less. Is a solid-state laser.

Further, the invention according to claim 10 of the present invention is the invention according to any one of claims 8 or 9 of the present invention, wherein the concentration of Tm added to the YAG ceramic is:
0 ≦ Tm ≦ 20% (except Tm = 0 and Ho = 0)
It is intended to be.

The invention according to claim 11 of the present invention is the invention according to any one of claims 8, 9 or 10 of the present invention, wherein the additive concentration of Ho added to the YAG ceramic is:
0 ≦ Ho ≦ 10% (except Tm = 0 and Ho = 0)
It is intended to be.

  The invention according to claim 12 of the present invention is the invention according to any one of claims 8, 9, 10 or 11 of the present invention, wherein the laser medium comprises a rod-shaped body. A region of Tm, Ho: YAG ceramic is disposed in the central region along the axial direction of the rod-shaped body, and laser active ions are added to the regions on both sides of the central portion of the rod-shaped body. An area of YAG ceramic that is not present is arranged.

  According to a thirteenth aspect of the present invention, in the invention according to the twelfth aspect of the present invention, the laser medium includes laser active ions arranged on both sides of the central portion of the rod-shaped body. In the region of YAG ceramic to which no is added, it is supported in the resonator.

  The invention according to claim 14 of the present invention is the invention according to any one of claims 8, 9, 10, 11, 12, or 13 of the present invention, wherein the laser medium has a wavelength of 750 nm to 750 nm. The laser oscillates by being excited by excitation light having a wavelength band of 820 nm.

  The invention according to claim 15 of the present invention is the laser according to any one of claims 8, 9, 10, 11, 12, 13 or 14 of the present invention. Excitation light for exciting the medium is generated.

  The invention described in claim 16 of the present invention is the center of the present invention according to any one of claims 12 or 13, wherein the center of the rod-shaped body along the axial direction is formed by the semiconductor laser. Excitation light having a wavelength band of 750 nm to 820 nm is incident only on the Tm, Ho: YAG ceramic region disposed in the region of the part from the outer peripheral surface of the region.

  Further, the invention described in claim 17 of the present invention is the invention described in any one of claims 8, 9, 10, 11, 12, 13, 14, 15 or 16 of the present invention. The excitation light is pulsed light having a predetermined repetition frequency.

  Further, the invention according to claim 18 of the present invention is the invention according to any one of claims 8, 9, 10, 11, 12, 13, 14, 15, 16 or 17 of the present invention. Furthermore, a cooling means for cooling the laser medium is provided.

  According to the present invention, it is possible to provide a laser oscillation method and a laser apparatus that can oscillate at high energy pulses and can oscillate in a wavelength region longer than 1.5 μm.

  Hereinafter, an example of an embodiment of a laser oscillation method and a laser apparatus according to the present invention will be described in detail with reference to the accompanying drawings.

First, FIG. 1 shows a perspective configuration explanatory diagram of a laser medium used in a laser apparatus according to an example of an embodiment of the present invention.

  The laser medium 10 is made of a rod-shaped body of YAG ceramic having a cylindrical shape as a whole.

  The YAG ceramic rod constituting the laser medium 10 has a composition in which the composition is located on the first region 10a located at the center along the central axis direction (longitudinal direction) of the cylindrical shape and on both sides of the first region 10a. The second region 10b and the third region 10c are different.

  That is, in the first region 10a, at least one of Tm and Ho is doped into a YAG ceramic to form a Tm, Ho: YAG ceramic, while the second region 10b and the third region 10c are ion ions. Is an undoped YAG (Non-doped YAG) ceramic that is not doped with anything.

As for the Tm, Ho: YAG ceramic as the first region 10a, the concentration of ions added when Tm is
0 ≦ Tm ≦ 20%
Preferably,
3 ≦ Tm ≦ 6
And about Ho,
0 ≦ Ho ≦ 10%
Preferably,
0.3 ≦ Ho ≦ 0.4
And However, Tm = 0 and Ho = 0 are excluded.

  The laser medium 10 is made of a single YAG ceramic and is not formed by connecting the first region 10a, the second region 10b, and the third region 10c with an adhesive or the like.

  In ceramic, a substance in which a plurality of types of materials such as the laser medium 10 are bonded can be easily manufactured without using bonding.

FIG. 2 is a conceptual structural explanatory diagram of a laser apparatus 100 according to an example of an embodiment of the present invention that includes the laser medium 10 described above.

  The laser apparatus 100 includes an excitation chamber 102 in which a laser medium 10 is disposed, and a rear mirror 104 and an output mirror 106 that are disposed so as to face each other with the excitation chamber 102 interposed therebetween.

  That is, in this laser apparatus 100, the rear mirror 104 and the output mirror 106 constitute a linear laser resonator.

  As the rear mirror 104, a high reflection mirror for light having a wavelength of 1.9 μm to 2.2 μm was used. Further, as the output mirror 106, a partial reflection mirror that transmits a part of light with a wavelength of 1.9 μm to 2.2 μm and outputs laser light with a wavelength of 1.9 μm to 2.2 μm to the outside of the laser device 100. Using.

Here, the excitation chamber 102 will be described in more detail. In the excitation chamber 102, one end surface 10d (see FIG. 1) of the laser medium 10 is opposed to the rear mirror 104, and the other end of the laser medium 10 is disposed. The laser medium 10 is disposed so that the end face 10 e (see FIG. 1) faces the output mirror 106.

  Here, the laser medium 10 is supported in the excitation chamber 102 by engaging the second region 10b and the third region 10c with the excitation chamber 102, and is supported in the linear laser resonator. .

  The periphery of the laser medium 10 is surrounded by a flow tube (not shown), whereby the laser medium 10 is cooled with water to cool the laser medium 10.

  Further, the excitation chamber 102 is irradiated with laser light as excitation light toward the first region 10 a from the radial direction of the laser medium 10, that is, from the side surface of the laser medium 10 so as to be positioned on the outer periphery of the laser medium 10. A semiconductor laser (laser diode) 108 is disposed. The semiconductor laser 108 is arranged to irradiate laser light only to the first region 10a, and the second region 10b and the third region 10c are irradiated with laser light from the semiconductor laser 108. It will never be done.

  As such a semiconductor laser 108, a laser capable of emitting laser light having a wavelength band of 750 nm to 820 nm is used.

  The laser medium 10 and the semiconductor laser 108 are cooled by a chiller (not shown) of the same system.

In the above configuration, according to the laser device 100, Tm, Ho: YAG ceramic, which is a YAG ceramic doped with at least one of Tm and Ho, that is, ions doped with at least one of Tm and Ho. The first region 10a made of Tm, Ho: YAG ceramic, which is a solid laser made of the polycrystalline material used, oscillates with Ho ions, and the Tm ions act as a sensitizer that effectively absorbs excitation light, High-energy pulsed laser light with a wavelength of 1.9 μm to 2.2 μm and a wavelength of 2 μm is oscillated by excitation of excitation light with a wavelength of 750 nm to 820 nm irradiated by the semiconductor laser 108.

Here, trivalent Tm and Ho become active ions for laser oscillation in the wavelength band of 2 μm, and are oscillated by Ho ions, and Tm acts as a sensitizer that effectively absorbs excitation light.

However, when Ho = 0%, oscillation is caused by Tm ions. When Ho = 0%, Tm becomes an active ion. In this case, in FIG. 3, ³ H ₄ becomes the upper level of the laser, and oscillation in the 1.9 μm to 2.1 μm band is generated by the transition to ³ H ₆ where the distribution accumulated at this level is the lower level.

Although FIG. 3 energy level diagram of the Tm-Ho-based laser is shown, in the laser apparatus 100, as described above, ³ H ₆ of 780nm near the Tm - tailored to the absorption of ³ F ₄ transition The laser diode 108 is used to excite Tm. ^The distribution accumulated in ³ F ₄ transitions to ³ H ₄ , but cross relaxation occurs with a certain probability, and at the same time, adjacent Tm ions are excited from the ground level to ³ H ₄ , and the distribution is accumulated in ³ H _4. . Energy is transferred from the ³ H ₄ level of Tm to the ⁵ I ₇ level of Ho. This ⁵ I ₇ level is the upper level of the laser and is 2.1 μm due to the transition to the lower level of ⁵ I ₈ . This causes oscillation in the vicinity.

  Ho can be directly pumped with light of 1.9 μm. However, in the laser device 100, the wavelength near 780 nm to 820 nm is used as the pump wavelength near Tm of 780 nm, where a low-cost and high-power semiconductor laser is available. A semiconductor laser 108 that emits laser light in a wavelength band was used.

Next, an experiment conducted by the inventor of the present application using the laser device 100 described above will be described.

In addition, the inventor of the present application as a preliminary experiment of the experiment using the laser device 100,
(1) Tm, Ho: YA G ceramic (hereinafter referred to as “Sample 1”) having a doping ratio of 6% Tm and 0.4% Ho.
(2) Tm, Ho: YAG ceramic (hereinafter referred to as “sample 2”) having a doping ratio of 3% Tm and 0.3% Ho.
(3) YAG single crystal (Tm, Ho: YAG single crystal) doped with 6% Tm and 0.4% Ho (hereinafter referred to as “sample 3”)
The three types of samples were subjected to an experiment to acquire their optical characteristics.

  First, in order to obtain spectral characteristics, Sample 1, Sample 2 and Sample 3 whose surfaces are 2 mm thick are prepared, and their absorption spectra are measured using a spectrophotometer (Shimadzu Corporation UVPC-3100). did.

  In addition, the data on the fluorescence spectrum and lifetime are obtained by chopping and exciting a CW-Ti: sapphire laser that oscillates in the vicinity of the Tm absorption peak of 785 nm, and the intensity of the fluorescence is measured by a photodiode. Observed at.

  FIG. 4 shows an absorption spectrum near 780 nm according to the above-described experiment. In sample 1 and sample 3 having substantially the same doping concentration, the peak position and absorption coefficient of the absorption spectrum are almost the same.

  Compared to Sample 1 and Sample 3, Sample 2 in which the doping concentration of Tm ions was halved had a half absorption coefficient, but the spectrum shape was similar to Sample 1 and Sample 3.

  That is, the spectrum of sample 1 which is Tm, Ho: YAG ceramic is a spectrum which is not much different from that of sample 3 which is Tm, Ho: YAG single crystal, and it can be said that the ceramic is equivalent to single crystal in terms of absorption. .

  FIG. 5 shows an emission spectrum in the vicinity of 2000 nm, which is a trace obtained by using an InGaAs photodiode to acquire fluorescence that has passed through a spectrometer, and does not indicate absolute fluorescence intensity. Is possible.

From FIG. 5, it can be seen that the peak corresponding to the transition of ⁵ I ₇ → ⁵ I _{8 of} Ho exists around 2088 nm.

  In the experiment relating to FIG. 5 above, no significant difference in spectral trace shape could be found between Sample 1, Sample 2 and Sample 3, for which absolute intensity could not be evaluated.

  Here, FIG. 6 shows a relaxation trace of emission at 2088 nm. This shows temporal relaxation of fluorescence at 2088 nm, which is one of the emission peaks.

  The slope is sluggish at the beginning of relaxation because it is excited by the chopped light, but here again, Sample 1 and Sample 3 having the same ion concentration are 9.8 ms, and Sample 2 has a low ion concentration. Was 10 ms.

  This light emission corresponds to a laser transition of Ho ions, but since any sample has a low Ho ion concentration, there is no significant difference in the light emission lifetime.

Next, the dimensions of the laser device 100 used in the experiment will be described as follows.

Laser cavity length L1: 200mm
Diameter D of laser medium 10: 3 mm
Total length L2 of the laser medium 10: 70 mm
Length L3 of the first region 10a: 10mm
Length L4 of second region 10b: 30mm
Length L5 of the third region 10c: 30 mm
In addition, the Tm, Ho: YAG ceramic Tm and Ho addition concentrations (doping rate) in the first region 10a were set to 6% for Tm and 0.4% for Ho as in the case of Sample 1.

  Both end faces 10d and 10e of the laser medium 10 were subjected to laser grade optical polishing and non-reflective coating at 2100 nm, and the outer peripheral surface of the first region 10a in the laser medium 10, that is, the side face was subjected to sand blasting.

  Further, the semiconductor laser 108 was driven by a power source that performs a QCW operation with a center wavelength of 783 nm, a line width of 3.5 nm, a maximum peak current of 80 A, and a pulse width of 0.5 ms. The maximum output energy of the semiconductor laser 108 at this time was 900 mJ / pulse.

  As the rear mirror 104, a highly reflective mirror having a reflectance of 99.9% or more with respect to light having a wavelength of 2100 nm was used.

  On the other hand, as the output mirror 106, a partial reflection mirror having a reflectance of 95% with respect to light having a wavelength of 2100 nm and a partial reflection mirror having a reflectance of 90% with respect to light having a wavelength of 2100 nm are prepared. Each was used.

  The pulse energy of the laser beam output from the output mirror 106 of the laser medium 10 is measured using a joule meter, and the oscillation wavelength is a fiber-coupled compact spectrophotometer (Ocean Optics, NIR256-2.5). ).

In the experiment, laser oscillation was performed at a flow tube water temperature of 20 ° C. under the condition that no control was performed, and when a partially reflecting mirror having a reflectance of 95% with respect to light having a wavelength of 2100 nm was used as the output mirror 106. Is easily oscillated at a main oscillation wavelength of 2120 nm (see FIG. 7), and when a partial reflection mirror having a reflectance of 90% with respect to light having a wavelength of 2100 nm is used as the output mirror 106 The laser easily oscillated at the main oscillation wavelength of 2088 nm (see FIG. 8).
FIG. 9 shows the output energy (Output energy) with respect to the excitation energy (Pump energy) when a partial reflection mirror having a reflectance of 95% with respect to light having a wavelength of 2100 nm is used as the output mirror 106.

  FIG. 10 shows the output energy with respect to the excitation energy when a partial reflection mirror having a reflectance of 90% with respect to light having a wavelength of 2100 nm is used as the output mirror 106.

  9 and 10, □ indicates the result at a repetition frequency of 1 Hz, ○ indicates the result at a repetition frequency of 5 Hz, ■ indicates the result at a repetition frequency of 10 Hz, and ● indicates the result at a repetition frequency of 20 Hz. .

  As shown in FIGS. 9 and 10, the output energy per pulse was lower when the repetition frequency was higher.

  Note that at the same repetition frequency, when the output mirror 106 is a partially reflecting mirror having a reflectivity of 95% with respect to light having a wavelength of 2100 nm, the output mirror 106 has reflectivity with respect to light having a wavelength of 2100 nm. The output energy is higher than that in the case of using the 90% partial reflection mirror.

  However, even when a partial reflection mirror having a reflectance of 90% with respect to light having a wavelength of 2100 nm was used as the output mirror 106, the output energy was 41 mJ at 1 Hz at the maximum excitation energy of 860 mJ, and 30 mJ at 10 Hz.

Moreover, when the experiment was conducted while changing the water temperature, the output energy increased by lowering the water temperature.

  FIG. 11 shows an experimental result of an experiment in which the water temperature (cooling water temperature) is changed. In this experiment, only a partial reflector having a reflectance of 90% with respect to light having a wavelength of 2100 nm is output. Used as a mirror 106.

  In FIG. 11, ■ indicates the result at a repetition frequency of 1 Hz, and □ indicates the result at a repetition frequency of 10 Hz. The excitation energy was 860 mJ / pulse.

  Under the condition that the water temperature was lowered to 16 ° C., an output energy of 48 mJ was obtained at a repetition frequency of 1 Hz, and an output energy of 43 mJ was obtained even at 10 Hz.

In the case of excitation with a semiconductor laser, the excitation efficiency of the laser medium is much higher than that of lamp excitation, and the problem is how to remove local heat generation. Since the laser medium 10 in the laser device 100 is made of YAG ceramic, Tm of the first region 10a, Ho: YAG ceramic, Tm of the second region 10b, and the third region 10c are used without using bonding. And an additive-free YAG ceramic to which no Ho is added are joined, and the second region 10b and the third region 10c can function as heat sinks to dissipate heat.

  In addition, in the case of a quasi-three level laser such as a 2 μm band oscillation in the Tm and Ho systems, the second region 10b and the third region 10c function as a heat sink as described above and receive excitation light. Therefore, the second region 10b and the second region 10b, which are portions where the pumping light cannot be incident without arranging the semiconductor laser 108 to be attached to the excitation chamber 102, are arranged only in the portion where the gain is generated (first region 10a). By not doping ions in the three regions 10c, there is a function of avoiding reabsorption loss that becomes a problem in the three-level system and contributing to more efficient oscillation.

Here, in general, a quasi-three-level solid-state laser is sensitive to changes in performance with respect to the temperature of the medium. This is because the energy difference between the laser lower level and the ground level is small compared to the 1.06 micron band oscillation in a four-level Nd laser, so the lower level has a distribution even at temperatures around room temperature. This is because the number of distributions is easily changed with a slight change in temperature.

  As a result, the formation of the inversion distribution shows a strong temperature dependence, and as a result, the gain decreases as the temperature of the medium increases.

  Therefore, the result obtained in the above-described experiment, that is, the output energy per pulse decreases as the repetition frequency of the pulse increases, the average power input to the laser medium 10 increases by increasing the repetition, This is because the gain has decreased due to an increase in the temperature of the laser medium 10.

  On the other hand, it is recognized that the reason why the efficiency increased when the water temperature was lowered was that the temperature of the semiconductor laser 108 changed in addition to the effect of cooling the laser medium 10 described above.

  When the wavelength dependency of the semiconductor laser 108 on the cooling water temperature was obtained, it was 0.17 nm / ° C. in the actual working state, and the longer the wavelength, the longer the oscillation.

  Since the semiconductor laser 108 oscillated at a central wavelength of 783 nm at a water temperature of 20 ° C., it was expected that the absorption efficiency would be improved for both the upper and lower temperatures because it was in the gap of the absorption spectrum when viewed only at the central wavelength. In order to estimate the change in absorption efficiency with respect to temperature change, the overlap between the spectrum of the semiconductor laser 108 and the absorption spectrum was calculated, and the relative change in absorption power with respect to the center wavelength of the semiconductor laser 108 was examined. Note that experimental values were used for the absorption spectrum of the semiconductor laser 108.

Assuming that the oscillation spectrum of the semiconductor laser 108 is a Lorentz type, when the center wavelength is λc and the spectrum width is Δλ, it is expressed by the following equation. The spectrum width is set to 3.5 nm as an actual measurement value.

The overlap between this spectrum and the absorption coefficient α (λ) obtained from the experiment is

It is. Here, I is an effective medium thickness.

  FIG. 12 shows the calculation result. In FIG. 12, the vertical axis is normalized by a peak at 781 nm.

  From the result curve shown in FIG. 12, it was found that when a semiconductor laser having a relatively wide spectrum width is used as an excitation light source, the effective absorption coefficient hardly changes in the vicinity of 782 nm.

  The permissible width of the center wavelength at which absorption power of 90% or more of 781 nm at which the maximum absorption can be obtained is obtained ranges from 779 nm to 786 nm, and the temperature of the semiconductor laser corresponds to 0 ° C. to 35 ° C. Therefore, the influence of the temperature change of the semiconductor laser on the laser performance is moderate.

  This result also concludes that the improvement in laser performance with decreasing cooling water temperature is not due to absorption peak matching, but due to the improvement in heat distribution described above.

  Tm, Ho: YAG ceramic has a steep absorption spectrum and a narrow spectrum width, but it can be said that precise control of temperature is not necessary in a practical laser having a wide spectrum width of a semiconductor laser.

  This means that even a laser using Tm, Ho: YAG ceramic as a laser medium does not vary greatly in quality, and the laser device 100 including the laser medium 10 using Tm, Ho: YAG ceramic is extremely different. It is practical.

As described above, in the laser apparatus 100, when the Tm, Ho: YAG ceramic, which is the first region of the laser medium 10, is excited by the semiconductor laser 108 that performs QCW oscillation, laser oscillation is obtained in the wavelength band of 2 μm. .

  That is, a maximum output energy of 48 mJ or more was obtained at 860 mJ excitation by the semiconductor laser 108 and a cooling water temperature of 16 ° C.

  The light-to-light conversion efficiency at this time achieved 5% or more, indicating that the effective absorption power of the semiconductor laser 108 does not strongly depend on the shift of the center wavelength of the semiconductor laser 108 in the vicinity of 783 nm. .

Although the attachment of the experimental results is omitted, an experiment similar to the above was performed using the laser medium 10 in which the concentration of ions in the first region 10 was the same as that of the sample 2. As a result, a sample with a high concentration of doped ions was obtained. Using the same laser medium 10 as 1 showed higher laser performance.

The embodiment described above may be modified as shown in (1) to (4) described below.

  (1) In the above-described embodiment, the result of the experiment using the semiconductor laser 108 having the center wavelength of 783 nm when exciting the laser medium 10 is shown. However, the excitation wavelength of the laser medium 10 is not limited to this. Of course, a wavelength band of 750 nm to 820 nm, preferably a wavelength band of 780 nm to 790 nm may be used.

(2) In the above-described embodiment, the results of the experiment in which the addition concentration of Tm and Ho of the Tm, Ho: YAG ceramic in the first region 10a is 6% and Ho is 0.4% are shown. Of course, the addition concentration of Tm and Ho is not limited to this. For example, the same Tm as Sample 2 may be 3% and Ho may be 0.3%. That is, the addition concentration of Tm and Ho is
0 ≦ Tm ≦ 20%
0 ≦ Ho ≦ 10%
It may be set as appropriate within the range of

  (3) In the above-described embodiment, the dimensions of the laser device 100 and the laser medium 10 are shown by specific numerical values, but these numerical values are only examples, and the laser device 100 and the laser medium 10 The dimension is not limited to this, and may be an appropriate dimension according to design conditions.

  (4) In the above-described embodiment, the laser medium 10 is composed of a rod-like body having a cylindrical shape as a whole. However, the shape of the laser medium 10 is not limited to this, and depends on the design conditions. And may be configured in an appropriate shape.

  (5) The above-described embodiment and the modifications shown in (1) to (4) may be used in appropriate combination.

  The present invention is widely used in the field of remote sensing and the like, and is an elemental technology necessary for future aviation safety.

FIG. 1 is an explanatory perspective view of a laser medium used in a laser apparatus according to an example of an embodiment of the present invention. FIG. 2 is an explanatory diagram of a conceptual configuration of a laser apparatus according to an example of the embodiment of the present invention including the laser medium shown in FIG. FIG. 3 is an energy level diagram of a Tm-Ho laser. FIG. 4 shows absorption spectra in the vicinity of a wavelength of 780 nm of the Tm, Ho: YAG ceramic and the Tm, Ho: YAG single crystal. FIG. 5 is an emission spectrum trace of a Tm, Ho: YAG ceramic and a Tm, Ho: YAG single crystal near a wavelength of 2000 nm. FIG. 6 is a relaxation trace of light emission at a wavelength of 2088 nm of Tm, Ho: YAG ceramic and Tm, Ho: YAG single crystal. FIG. 7 shows a spectrum of laser light output by a laser device when a partial reflection mirror having a reflectivity of 95% is used for light having a wavelength of 2100 nm as an output mirror. FIG. 8 shows a spectrum of laser light output by a laser device when a partial reflection mirror having a reflectance of 90% is used for light having a wavelength of 2100 nm as an output mirror. FIG. 9 is a graph showing the output energy with respect to the excitation energy in the laser device when a partial reflection mirror having a reflectance of 95% with respect to light having a wavelength of 2100 nm is used as the output mirror. FIG. 10 is a graph showing the output energy with respect to the excitation energy in the laser device when a partial reflection mirror having a reflectance of 90% with respect to light having a wavelength of 2100 nm is used as the output mirror. FIG. 11 is a graph showing the maximum output energy obtained for each cooling water temperature when the cooling water temperature is changed. FIG. 12 is a graph showing a calculation result of the relative absorption power with respect to the center wavelength of the excitation light of the semiconductor laser, and assumes a Lorentz type having a spectral width of 3.5 nm as the spectrum of the semiconductor laser.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Laser medium 10a 1st area | region 10b 2nd area | region 10c 3rd area | region 10d End surface 10e End surface 100 Laser apparatus 102 Excitation chamber 104 Rear mirror 106 Output mirror 108 Semiconductor laser (laser diode)

Claims (18)

In a laser oscillation method in which a laser medium is excited by excitation light and oscillated,
Transparent YAG (Yttrium Aluminum Garnet) ceramics having a garnet structure and represented by the chemical formula Y ₃ Al ₅ O ₁₂ are laser-active ions such as Tm (Thulium) and Ho (Holmium). ) And a laser medium composed of Tm, Ho: YAG ceramic, which is a material to which at least one of them is added, generates laser light having a wavelength of 1.9 μm to 2.2 μm. Laser oscillation method. The laser oscillation method according to claim 1,
The laser oscillation method, wherein the YAG ceramic is a solid-state laser made of a polycrystal formed by assembling single crystals having a size of 1 mm or less. In the laser oscillation method of any one of Claim 1 or 2,
The additive concentration of Tm added to the YAG ceramic is:
0 ≦ Tm ≦ 20% (except Tm = 0 and Ho = 0)
A laser oscillation method characterized by that. In the laser oscillation method according to any one of claims 1, 2, and 3,
The additive concentration of Ho added to the YAG ceramic is:
0 ≦ Ho ≦ 10% (except Tm = 0 and Ho = 0)
A laser oscillation method characterized by that. In the laser oscillation method according to any one of claims 1, 2, 3, or 4,
A laser oscillation method, wherein the laser medium is excited by excitation light having a wavelength band of 750 nm to 820 nm to oscillate. In the laser oscillation method according to any one of claims 1, 2, 3, 4 or 5,
A laser oscillation method, characterized in that excitation light for exciting the laser medium is generated by a semiconductor laser. In the laser oscillation method according to any one of claims 1, 2, 3, 4, 5 or 6,
The laser oscillation method, wherein the excitation light is pulsed light having a predetermined repetition frequency. In a laser apparatus in which a laser medium is disposed in a resonator, laser oscillation is generated in the resonator by making excitation light incident on the laser medium, and laser light is emitted from the resonator.
The laser medium has a garnet structure and is a material obtained by adding at least one of Tm and Ho as a laser active ion to a transparent YAG ceramic having a chemical formula represented as Y ₃ Al ₅ O ₁₂ : Tm, Ho: A laser medium having at least a region having a YAG ceramic;
A laser apparatus, wherein the laser medium is excited by excitation light to generate laser light having a wavelength band of 1.9 μm to 2.2 μm. The laser apparatus according to claim 8, wherein
The YAG ceramic is a solid-state laser made of a polycrystal formed by assembling single crystals having a size of 1 mm or less. The laser apparatus according to any one of claims 8 and 9,
The additive concentration of Tm added to the YAG ceramic is:
0 ≦ Tm ≦ 20% (except Tm = 0 and Ho = 0)
A laser device characterized by The laser device according to any one of claims 8, 9 or 10,
The additive concentration of Ho added to the YAG ceramic is:
0 ≦ Ho ≦ 10% (except Tm = 0 and Ho = 0)
A laser device characterized by The laser device according to any one of claims 8, 9, 10 or 11,
The laser medium is composed of a rod-shaped body, and a region of Tm, Ho: YAG ceramic is disposed in a central region along the axial direction of the rod-shaped body, and the central portion of the rod-shaped body is arranged. A laser device characterized in that a region of YAG ceramic to which no laser active ions are added is arranged in the regions on both sides. The laser device according to claim 12, wherein
The laser apparatus is characterized in that the laser medium is supported in the resonator in a region of YAG ceramic which is arranged on both sides of the central portion of the rod-like body and to which no laser active ions are added. The laser apparatus according to any one of claims 8, 9, 10, 11, 12, or 13.
A laser apparatus, wherein the laser medium is excited by excitation light having a wavelength band of 750 nm to 820 nm to oscillate. The laser apparatus according to any one of claims 8, 9, 10, 11, 12, 13, or 14.
A laser apparatus that generates excitation light for exciting the laser medium with a semiconductor laser. The laser apparatus according to any one of claims 12 and 13,
With a semiconductor laser, excitation light having a wavelength band of 750 nm to 820 nm from the outer peripheral surface of the Tm, Ho: YAG ceramic region disposed only in the central region along the axial direction of the rod-shaped body. A laser device characterized by being incident. The laser apparatus according to any one of claims 8, 9, 10, 11, 12, 13, 14, 15, or 16.
The laser device, wherein the excitation light is pulsed light having a predetermined repetition frequency. The laser device according to any one of claims 8, 9, 10, 11, 12, 13, 14, 15, 16 or 17,
A laser device comprising cooling means for cooling the laser medium.