JP2008117914A - Solid-state laser device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To attain an output energy as high as several hundreds of mJ per pulse with a simple arrangement in a solid-state laser device which generates a laser beam of wavelengths in a 2.94 μm band. <P>SOLUTION: A solid-state laser device 1 comprises a solid-state laser rod 11 of an erbium-yttrium aluminum garnet (Er:YAG) crystal, a high reflective mirror 3 and an output mirror 4 forming a resonator arranged at both ends of the solid-state laser rod 11, and a semiconductor laser module 12 for illuminating the side surface of the solid-state laser rod 11 with exciting light Lp having a wavelength component included in an absorption wavelength band (0.96 μm band) when an energy level of Er<SP>3+</SP>transits from<SP>4</SP>I<SB>15/2</SB>to<SP>4</SP>I<SB>11/2</SB>. The peak wavelength of a laser beam L is included in a luminous wavelength band (2.94 μm band) when the energy level of Er<SP>3+</SP>is transits from<SP>4</SP>I<SB>11/2</SB>to<SP>4</SP>I<SB>13/2</SB>, and the peak wavelength of the exciting light Lp is longer than the maximum absorption peak wavelength included in the absorption wavelength band of the Er<SP>3+</SP>. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a solid-state laser device.

The laser light having a wavelength of 2.94 μm generated by exciting a YAG crystal doped with erbium ion (Er ³⁺ ) (hereinafter abbreviated as Er: YAG crystal) has the highest absorption coefficient in biological tissues containing moisture. Since it has a high wavelength band, it is applied to laser treatment devices that perform cutting of soft tissue and hard tissue. For example, in some applications such as tooth cutting, a relatively high peak power and a large energy of several hundred mJ per pulse are required. Therefore, an Er: YAG crystal is often excited by a flash lamp. It has been. However, in such a system, the life of the flash lamp that is an excitation light source is as short as about 10 million to 20 million pulses, and therefore it is necessary to frequently exchange the excitation light source. Therefore, it takes time for maintenance and the work efficiency is lowered.

  In order to deal with such problems, a system using a semiconductor laser having a longer lifetime as an excitation light source instead of a flash lamp has been considered. For example, Patent Document 1 discloses a solid-state laser device having a configuration in which excitation light from a semiconductor laser is irradiated onto an end face of an Er: YAG crystal via an optical fiber.

  Non-Patent Document 1 discloses a solid-state laser device having the configuration shown in FIG. This solid-state laser device 100 includes a laser crystal 101 made of a plate-like Er: YAG crystal having a thickness of about 1.5 mm, and excitation light on the side surface of the laser crystal 101 via an optical member (Transfer Optics) 102. A plurality of semiconductor laser arrays (Stacked Diode Lasers) 103 that emit Lpa and generate laser light La, high reflection mirrors (HR-Mirror) 104 and lasers disposed at one end and the other end of the optical path of the laser light La, respectively An optical extraction mirror (OC-Mirror) 105 is provided. A hole 101a is formed at the center of the laser crystal 101, and the laser crystal 101 is cooled by bringing a heat sink (not shown) into contact with the hole 101a and both end faces of the laser crystal 101.

  Non-Patent Document 2 discloses a solid-state laser device having the configuration shown in FIG. The solid-state laser device 110 irradiates the laser crystal 111 with excitation light Lpb via a laser crystal (Er: YAG Crystal) 111 made of an Er: YAG crystal and a coupling optical system (Coupling Optics) 112, and emits the laser light Lb. Semiconductor laser array (4-bar InGaAs Laser Diode array) 113 to be generated, high reflection mirror 114 and laser light extraction mirror (Output Coupler) arranged at one end and the other end of the optical path of laser light Lb, respectively 115. An etalon filter (Uncoated YAG Etalon) 116 for adjusting the oscillation wavelength is provided between the high reflection mirror 114 and the laser crystal 111.

A solid-state laser device using a rare earth element other than erbium is also considered. For example, the laser device described in Patent Document 2 includes a YAG crystal doped with Nd (Nd: YAG crystal), and converts the wavelength of laser light having a wavelength of 1.06 μm generated by exciting the crystal. Thus, a laser beam having a wavelength of 2.9 μm is obtained. Patent Document 3 discloses a configuration in which excitation light is irradiated from a plurality of laser diode arrays onto a side surface of a laser rod made of an Nd: YAG crystal.
Japanese Patent No. 3167844 JP 2001-352118 A US Pat. No. 5,742,634 C. Ziolek, et.al., “High-repetition-rate, high-average-power, diode-pumped 2.94-μm Er: YAGlaser”, Optics Letters, Vol.26, No.9, pp.599 (2001) CE Hamilton, et. Al., “1-W average power levels and tunability from a diode-pumped 2.94-μmEr: YAG oscillator”, Optics Letters, Vol. 19, No. 20, pp. 1627 (1994)

  However, with the configuration described in Patent Document 1, the amount of excitation light that can be applied to the Er: YAG crystal is small, and it is difficult to obtain a high-energy light pulse as described above. In addition, the configurations described in Non-Patent Documents 1 and 2 (see FIGS. 16 and 17) both have a resonator configuration using total internal reflection in an Er: YAG crystal, and are excited with high energy. Since it is structurally difficult to input light to the Er: YAG crystal, the energy per pulse is only 32 mJ / pulse and 7.1 mJ / pulse, respectively. In addition, it is conceivable to increase the output by superimposing a plurality of unit structures by using the structure described in FIG. 16 or FIG. 17 as a unit structure, but high-energy laser light of several hundred mJ per pulse is obtained. If so, a large number of unit structures are required, and the structure becomes complicated. Therefore, the loss of laser light in the resonator increases, and a significant decrease in laser light generation efficiency is expected.

  Further, in the solid-state laser device described in Patent Document 2, the proven output energy is limited to 125 mJ per pulse due to a limitation due to optical damage in the wavelength conversion crystal. Patent Document 3 does not describe obtaining laser light with a wavelength of 2.94 μm.

  The present invention has been made in view of the above-described problems. In a solid-state laser device that generates laser light having a wavelength of 2.94 μm, high output energy of several hundred mJ per pulse is realized with a simple configuration. For the purpose.

In order to solve the above-described problems, a solid-state laser device according to the present invention includes a columnar solid-state laser rod of YAG crystal doped with erbium ions, a first mirror provided on one end of the solid-state laser rod, and a solid-state laser rod. A second mirror that is provided on the other end side and constitutes a resonator together with the first mirror to cause laser oscillation and emit laser light, and is provided around the solid laser rod. The energy level of erbium ions is ⁴ A semiconductor laser element that irradiates the side surface of the solid-state laser rod with excitation light having a wavelength component included in the absorption wavelength band in the case of transition from I _15/2 to ⁴ I _11/2 , and the laser light is pulsed , the peak wavelength of the laser beam is included in the emission wavelength band when the energy level of erbium ions is changed from ⁴ I _11/2 to ⁴ I _13/2, peak wavelength of the excitation light Is longer than the maximum absorption peak wavelength included in the absorption wavelength band of erbium ions.

  The solid-state laser device according to the present invention is provided with a columnar solid-state laser rod of YAG crystal to which erbium ions are added, a first mirror provided on one end side of the solid-state laser rod, and on the other end side of the solid-state laser rod, A resonator is formed together with the first mirror to cause laser oscillation, and is provided around the solid laser rod and the second mirror that emits laser light, and is included in the 0.96 μm band of the absorption wavelength band of erbium ions. A semiconductor laser element that irradiates the side surface of the solid-state laser rod with excitation light having a wavelength component, the laser light is pulsed, and the peak wavelength of the laser light is a 2.94 μm band of the emission wavelength band of erbium ions The peak wavelength of the excitation light is longer than the maximum absorption peak wavelength included in the 0.96 μm band of erbium ions. That.

  In each of the above-described solid-state laser devices, a resonator is configured by the solid laser rod of Er: YAG crystal, the first mirror, and the second mirror, and excitation light is irradiated from the semiconductor laser element to the solid-state laser rod. Thus, the light stimulated and emitted from the Er: YAG crystal is extracted as a laser beam.

Here, FIG. 18 is a diagram showing the structure of the energy level of the Er: YAG crystal. As shown in FIG. 18, Er: YAG crystal, by absorbing the excitation light near a wavelength of 0.96 .mu.m, the energy level changes from a ⁴ I _15/2 to ⁴ I _11/2, and excited state Become. Thereafter, when the energy level transitions from ⁴ I _11/2 to ⁴ I _13/2 , light having a wavelength of about _2.94 μm is emitted, and laser light is extracted by subsequent oscillation and stimulated emission. However, the stimulated emission cross section of Er: YAG crystal at the transition from ⁴ I _11/2 to ⁴ I _13/2 is as small as about 3 × 10 ⁻²⁰ cm ² , and this transition is ⁴ I _{11 /} The lifetime of the ₂ state is about 100 μs, whereas the lifetime of the ⁴ I _13/2 state is as long as several ms, so-called self-terminating characteristics. Therefore, in the Er: YAG crystal, the luminous efficiency in the vicinity of the wavelength of 2.94 μm with respect to the excitation light in the vicinity of the wavelength of 0.96 μm is essentially low.

However, when an Er: YAG crystal having a high concentration of erbium ions (Er ³⁺ ), for example 50 atomic percent, is irradiated with excitation light having a wavelength of about _{0.96 μm} , two ions in the ⁴ I _13/2 state are With one probability, one transitions to ⁴ I _9/2 and the other transitions to ⁴ I _15/2 (non-radiative cross relaxation process). Furthermore, there is a recycling process from the lower level to the upper level in which the ions that have transitioned to ⁴ I _9/2 transition to ⁴ I _11/2 . And it is thought that the probability that such an effect occurs is higher as the ion density of the ⁴ I _13/2 state is larger. Therefore, by exciting the Er: YAG crystal at a high density to increase the ion density in the ⁴ I _13/2 state, it compensates for the low intrinsic luminous efficiency, and a high output energy of several hundred mJ per pulse is obtained. It can also be obtained.

In order to increase the ion density of the ⁴ I _13/2 state by exciting the Er: YAG crystal at a high density, the _{inventors in} the solid laser rod viewed from the optical axis direction (longitudinal direction of the solid laser rod). We focused on the distribution of the excitation density at. That is, if a lot of excitation light is absorbed near the side surface of the solid-state laser rod, the excitation light cannot sufficiently reach the center line of the solid-state laser rod (that is, the optical axis of the resonator), and the excitation density near the optical axis. Becomes smaller. Therefore, the ion density in the ⁴ I _11/2 state and ⁴ I _13/2 state in the vicinity of the optical axis becomes small, and it becomes difficult to efficiently extract laser light of good quality. In the solid-state laser devices described in Non-Patent Documents 1 and 2 described above, 962 nm and 963 nm are selected as the center wavelengths of the excitation laser light, respectively, but the absorption of Er: YAG crystals with respect to light of these wavelengths is selected. Since the rate is extremely high, even if the side surface of the solid laser rod is irradiated with excitation light, it is almost absorbed near the side surface.

On the other hand, the excitation density should be a distribution (such as a Gaussian distribution) that decreases largely toward the side surface near the optical axis, or a substantially uniform distribution from the vicinity of the optical axis to the side surface. For example, the Er: YAG crystal can be excited with a high density as a whole, and the ion density in the ⁴ I _13/2 state can be increased, so that the laser beam can be extracted efficiently.

In order to realize such an excitation density distribution, in each of the above-described solid-state laser devices, the peak wavelength of the excitation light is greater than the maximum absorption peak wavelength included in the Er ³⁺ absorption wavelength band (0.96 μm band). It is getting longer. As a result, the amount of excitation light absorbed near the side surface of the solid-state laser rod can be suppressed and the excitation light can sufficiently reach the vicinity of the center line (optical axis), so that the above-described excitation density distribution can be suitably obtained. it can. From the above, according to each solid-state laser device described above, the Er: YAG crystal can be excited with high density as a whole and the laser beam can be efficiently extracted, so that high output energy such as several hundred mJ per pulse can be obtained. It becomes possible to obtain the laser beam. In addition, the configuration can be made extremely simple as compared with a configuration in which a large output is obtained by using a plurality of conventional solid-state laser devices.

  Each of the above-described solid-state laser devices may be characterized in that the half-value width of the spectrum of the excitation light is 2 nm or more and 10 nm or less. The absorption spectrum of the Er: YAG crystal in the 0.96 μm band is composed of a plurality of peak waveforms in a very narrow band, such as a half width of 2 nm or less. The wavelength of the pumping light output from the semiconductor laser element varies depending on a temperature change or the like. However, if the half-value width of the pumping light spectrum is excessively narrow and the monochromaticity is high, the Er: YAG crystal is changed with the pumping light wavelength change. The absorptance of the excitation light changes greatly, and the excitation density distribution seen from the optical axis direction changes greatly. On the other hand, according to the solid-state laser device described above, the half-width of the excitation light spectrum is widened to 2 nm or more, so that the change in the excitation density distribution accompanying the change in the wavelength of the excitation light can be kept small, and the high-energy laser light Can be taken out stably. If the spectrum of the excitation light becomes excessively wide, the proportion of the excitation light amount that is not absorbed by the Er: YAG crystal increases. Therefore, the half width of the spectrum is preferably 10 nm or less.

  In addition, the first and second solid-state laser devices may be characterized in that the peak wavelength of the excitation light is 969 nm or more and 975 nm or less. According to studies conducted by the present inventors, the maximum absorption peak wavelength of the Er: YAG crystal is about 966 nm, and if the peak wavelength of the excitation light is 969 nm or more, a good excitation density distribution can be obtained. If the peak wavelength of the excitation light is 975 nm or less, the proportion of the excitation light amount that is not absorbed by the Er: YAG crystal is sufficiently low. Therefore, when the peak wavelength of the excitation light is included in the above range, the Er: YAG crystal can be excited with high density as a whole, and the laser light can be efficiently extracted.

  According to the solid-state laser device of the present invention, high output energy of several hundred mJ per pulse can be realized with a simple configuration in a solid-state laser device that generates laser light having a wavelength of 2.94 μm.

  Embodiments of a solid-state laser device according to the present invention will be described below in detail with reference to the accompanying drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted.

  FIG. 1 is a diagram showing a configuration of an embodiment of a solid-state laser device according to the present invention. The solid-state laser device 1 according to the present embodiment is a device for generating a pulsed laser beam L, and includes a pump module 2, a high reflection mirror (first mirror) 3, an output mirror (second mirror) 4, and a QCW. (Quasi Continuous Wave) A power source 5, a laser cooler 6, a controller 7, a current supply cable 8, and a cooling water supply pipe 9 are provided. The pump module 2 includes a so-called side excitation structure that excites a columnar laser crystal containing an active element from the side. The pump module 2 includes a solid-state laser rod 11 made of a laser crystal containing an active element, a plurality of semiconductor laser modules 12 for exciting the solid-state laser rod 11, a cooling pipe 13, a pair of reflecting mirrors 14 and 15, and each semiconductor laser. A plurality of cylindrical lenses 16 arranged corresponding to the module 12 and a housing 17 for housing them are provided.

The solid laser rod 11 is a member made of a YAG crystal to which erbium ions (Er ³⁺ ) are added as an active element, and functions as a laser medium that performs stimulated emission upon receiving excitation light. The solid laser rod 11 of the present embodiment is doped with Er ^{3+ at a} relatively high rate of 25 to 70 atomic percent, preferably 50 atomic percent. The solid laser rod 11 is formed in a cylindrical shape (rod shape) having a certain direction as a longitudinal direction, and the dimensions of the solid laser rod 11 are, for example, a diameter of 4 mm and a length of 115 mm.

  Further, the high-reflection mirror 3 is disposed on one end side in the longitudinal direction of the solid-state laser rod 11, and the output mirror 4 is disposed on the other end side with an interval from the end face of the solid-state laser rod 11. That is, the high reflection mirror 3, the solid laser rod 11, and the output mirror 4 are arranged side by side on the central axis along the longitudinal direction of the solid laser rod 11. This is the optical axis C of the resonator composed of the laser rod 11 and the output mirror 4. Laser oscillation occurs between the high reflection mirror 3 and the output mirror 4, and the laser light L is extracted from the output mirror 4. The reflectivity of the high reflection mirror 3 is approximately 100%, and the reflectivity of the output mirror 4 is, for example, 92%.

  When the energy per pulse of the laser beam L is high, the solid laser rod 11 has a certain diameter or more in order to avoid optical damage of both end surfaces of the solid laser rod 11 by the laser beam L. It is desirable. For example, when the output energy is several hundred mJ per pulse, the diameter of the solid laser rod 11 is desirably 3 mm or more.

The semiconductor laser module 12, the wavelength components contained in the absorption wavelength band when the energy level of Er ³⁺ shown in FIG. 18 is changed from ⁴ I _15/2 to ⁴ I _11/2 (i.e. 0.96μm band) This is an excitation light source for irradiating the solid laser rod 11 with excitation light having The semiconductor laser module 12 is disposed around the solid-state laser rod 11 and irradiates the side surface of the solid-state laser rod 11 with excitation light via a cylindrical lens 16. In the present embodiment, a plurality of semiconductor laser modules 12 are arranged at equal intervals around the central axis (optical axis C) of the solid-state laser rod 11 and fixed to the housing 17.

Further, the excitation light emitted from the semiconductor laser module 12 is such that the excitation density inside the solid laser rod 11 as viewed from the central axis direction of the solid laser rod 11 becomes large near the central axis (that is, the excitation light is The wavelength is selected so that it reaches the central axis of the solid laser rod 11 sufficiently. Specifically, the peak wavelength of the excitation light emitted from the semiconductor laser module 12 is set longer than the maximum absorption peak wavelength included in the above-described absorption wavelength band (0.96 μm band) of Er ³⁺ , for example, 969 nm or more and 975 nm. The following is preferable. Moreover, it is preferable that the half value width of the spectrum of excitation light is 2 nm or more and 10 nm or less.

  The QCW power supply 5 is a component for supplying a drive current to the semiconductor laser module 12. The QCW power source 5 of the present embodiment is electrically connected to a plurality of semiconductor laser modules 12 via a current supply cable 8 so that each semiconductor laser module 12 emits excitation light having a predetermined peak wavelength. The rectangular pulsed drive current is supplied to each semiconductor laser module 12. The magnitude and pulse width of the drive current output from the QCW power supply 5 are controlled by the controller 7.

  The laser cooler 6 is a component for cooling the solid-state laser rod 11 and the semiconductor laser module 12 of the pump module 2. That is, the laser cooler 6 is connected to the pump module 2 via the cooling water supply pipe 9 and circulates the cooling water to the pump module 2 through the cooling water supply pipe 9. Inside the pump module 2, a cooling pipe 13 connected to the cooling water supply pipe 9 is disposed along the solid laser rod 11 and the housing 17. The cooling water passes through the cooling pipe 13. Simultaneously with cooling the solid laser rod 11, the semiconductor laser module 12 is cooled via the housing 17.

  Here, FIG. 2 is an enlarged view showing a main part of the pump module 2 shown in FIG. FIG. 3 is a sectional view taken along line III-III of the pump module 2 shown in FIG.

  The pump module 2 of the present embodiment has seven semiconductor laser modules 12 as shown in FIG. These semiconductor laser modules 12 are arranged in parallel at equal intervals around the central axis of the solid-state laser rod 11 (that is, the optical axis C shown in FIG. 2), and each light emitting surface 12a has a central axis (optical axis) of the solid-state laser rod 11. C). Such semiconductor laser modules 12 are preferably installed at equal intervals in a plurality of directions exceeding at least 3 on the side surface of the solid-state laser rod 11.

  As shown in FIG. 2, each semiconductor laser module 12 has a plurality of semiconductor laser arrays 121. Each semiconductor laser array 121 includes a plurality of semiconductor laser elements arranged one-dimensionally in a direction orthogonal to the central axis of the solid-state laser rod 11. The plurality of semiconductor laser arrays 121 are stacked along the central axis of the solid-state laser rod 11 so that the laser beam emission end face coincides with the light emission surface 12a. These semiconductor laser arrays 121 irradiate the excitation light Lp all at once toward the central axis of the solid-state laser rod 11.

  Each of the semiconductor laser arrays 121 has a peak power of, for example, 140 W, and a light emission width as viewed from the direction along the optical axis C of, for example, 10 mm. The semiconductor laser module 12 is formed by stacking 75 such semiconductor laser arrays 121 in the direction along the optical axis C, for example, at intervals of 0.4 mm. In this case, the emission width of the semiconductor laser module 12 viewed from the side of the solid-state laser rod 11 (that is, the direction intersecting the optical axis C) is 29.6 mm. Each semiconductor laser element included in the semiconductor laser array 121 has a beam divergence angle (first axis) projected on a plane perpendicular to the longitudinal direction of the semiconductor laser array 121 (that is, the direction orthogonal to the optical axis C). The direction is set so as to be larger than the beam divergence angle (slow axis) projected on the plane perpendicular to the optical axis C. As an example of the beam divergence angle, the divergence half angle in the fast axis direction of the semiconductor laser module 12 is, for example, 16 °, and the divergence half angle in the slow axis direction of the semiconductor laser module 12 is, for example, 3.5 °.

  A cooling pipe 13 is disposed around the solid laser rod 11. The cooling pipe 13 around the solid-state laser rod 11 is formed in a cylindrical shape with the optical axis C as the central axis, and the solid-state laser rod 11 is inserted through the central portion of the cylinder so that the cooling pipe 13 is solid. The laser rod 11 is covered. Cooling water flows through the gap between the cooling pipe 13 and the solid laser rod 11, thereby cooling the solid laser rod 11. The cooling pipe 13 is made of a material that is transparent with respect to the wavelength of the excitation light Lp. As such a material, for example, quartz is suitable. As a result, the excitation light Lp passes through the cooling pipe 13 and is irradiated onto the solid laser rod 11.

  The reflecting mirrors 14 and 15 limit the irradiation range of the excitation light Lp in the central axis direction of the solid-state laser rod 11. The reflecting mirrors 14 and 15 of the present embodiment have a disk shape with the optical axis C as the central axis, and a hole through which the solid laser rod 11 and the cooling pipe 13 are inserted is formed at the center. Has been. The reflecting mirror 14 is disposed near one end of the solid laser rod 11 in the central axis direction, and the reflecting mirror 15 is disposed near the other end. The surfaces facing each other in the reflecting mirrors 14 and 15 are light reflecting surfaces, and the portion of the excitation light Lp that extends toward both ends of the solid laser rod 11 is folded back by the reflecting mirror 14 or the reflecting mirror 15. The light Lp is focused on the solid laser rod 11 positioned between the reflecting mirrors 14 and 15.

  The cylindrical lens (cylindrical lens) 16 is an optical member for converging the excitation light Lp emitted from the semiconductor laser module 12 toward the solid laser rod 11, and is arranged along the central axis of the solid laser rod 11. Yes. In the present embodiment, as shown in FIG. 3, seven cylindrical lenses 16 are disposed between the corresponding semiconductor laser module 12 and the solid-state laser rod 11, and each cylindrical lens 16 includes the solid-state laser rod 11. The excitation light Lp is converged in a plane orthogonal to the central axis of the.

In the solid-state laser device 1 having the above configuration, when a drive current is supplied from the QCW power source 5 to each semiconductor laser module 12, the pumping light Lp having a peak in the 0.96 μm band is transmitted from each semiconductor laser module 12 to the solid-state laser. The rod 11 is irradiated. Er included in the solid-state laser rod 11: YAG crystals by absorbing the excitation light Lp, the energy level as shown in FIG. 18 shifts from ⁴ I _15/2 to ⁴ I _11/2, Excited state. Thereafter, when the energy level transitions from ⁴ I _11/2 to ⁴ I _13/2 , light having a wavelength of about _2.94 μm is emitted, and this light resonates between the high reflection mirror 3 and the output mirror 4. The laser beam L is extracted by inducing stimulated emission.

  Here, the conditions for obtaining the laser beam L having a high output energy of several hundred mJ per pulse in the solid-state laser device 1 having the above configuration will be further described.

The stimulated emission cross section of Er: YAG crystal at the transition from ⁴ I _11/2 to ⁴ I _13/2 is as small as 3 × 10 ⁻²⁰ cm ² , and this transition is ⁴ I _11/2 While the lifetime of the state is about 100 μs, the lifetime of the ⁴ I _13/2 state is as long as several ms, so-called self-terminating characteristics. Therefore, in the Er: YAG crystal, the luminous efficiency in the vicinity of the wavelength of 2.94 μm with respect to the excitation light in the vicinity of the wavelength of 0.96 μm is essentially low.

However, when Er ³⁺ ions in a ⁴ I _13/2 state are accumulated at a high density in an Er: YAG crystal having a high concentration of erbium ions (Er ³⁺ ) as in the solid laser rod 11 of the present embodiment, ⁴ Two ions in the I _13/2 state interact with a certain probability, one transitions to ⁴ I _9/2 and the other transitions to ⁴ I _15/2 . Furthermore, there is a recycling process from the lower level to the upper level in which the ions that have transitioned to ⁴ I _9/2 transition to ⁴ I _11/2 . And it is thought that the probability that such an effect occurs is higher as the ion density of the ⁴ I _13/2 state is larger. Therefore, the Er: YAG crystal (solid laser rod 11) is excited at a high density to increase the ion density in the ⁴ I _13/2 state, thereby compensating for the inherent low luminous efficiency, and several hundred per pulse. It is also possible to obtain high output energy such as mJ.

Here, in order to increase the ion density of the ⁴ I _13/2 state by exciting the Er: YAG crystal at a high density, it was viewed from the direction along the optical axis C (the central axis direction of the solid-state laser rod 11). It is important that the excitation density in the solid laser rod 11 is large in the vicinity of the central axis of the solid laser rod 11. This is a proof that a sufficient amount of excitation light Lp has reached the vicinity of the central axis of the solid-state laser rod 11, and the excitation density inside the solid-state laser rod 11 is distributed in this manner, so that the optical axis The ion density in the ⁴ I _13/2 state in the vicinity of C is increased, and a high-quality laser beam L can be generated efficiently. In other words, a distribution in which the excitation density inside the solid laser rod 11 is reduced in the vicinity of the central axis of the solid laser rod 11 and decreases toward the side surface (such as a Gaussian distribution), or the solid laser rod If the distribution is almost uniform from around the central axis of 11 to the side, the Er: YAG crystal is excited with high density as a whole to increase the ion density of the ⁴ I _13/2 state, and the laser beam is made efficient. Can be taken out well.

In order to solve such a problem, in the solid-state laser device 1 according to the present embodiment, the peak wavelength of the excitation light Lp is greater than the maximum absorption peak wavelength included in the Er ³⁺ absorption wavelength band (0.96 μm band). It is getting longer. As a result, the absorption amount of the excitation light Lp in the vicinity of the side surface of the solid-state laser rod 11 can be suppressed, and the excitation light Lp can sufficiently reach the vicinity of the center line (optical axis C). Can be suitably obtained. Accordingly, the Er: YAG crystal of the solid laser rod 11 can be excited with a high density as a whole, and the laser light L can be extracted efficiently, so that a laser light with a high output energy of several hundred mJ per pulse can be obtained. It becomes possible. In addition, the configuration can be made extremely simple as compared with a configuration in which a plurality of conventional solid-state laser devices (for example, see FIGS. 16 and 17) is used to obtain a large output.

  The emission wavelength of a semiconductor laser element used in the semiconductor laser module 12 varies depending on the temperature, repetition frequency, pulse width, peak current value, etc. of the element itself. Causes a change in the above-described excitation density distribution. Therefore, it is not necessary that the excitation density distribution is good only at a specific wavelength of the excitation light Lp. Even if the emission wavelength of the semiconductor laser element changes within a certain range, the above-described excitation density distribution is not obtained. It is preferable that it can be maintained.

  Note that the generation efficiency of the laser light L is easily affected by the change in the excitation density distribution due to the change in the wavelength of the excitation light Lp. This is a phenomenon peculiar to the side-pumped solid-state laser device as in this embodiment. is there. That is, in the end face excitation method as described in Patent Document 1 described above, the excitation density distribution changes mainly in the direction parallel to the optical axis. Therefore, the influence of the change in the excitation density distribution on the laser oscillation is relatively small. In addition, even when the resonator is configured by total internal reflection in the laser medium as described in Non-Patent Documents 1 and 2, there is an inversion action of the image viewed from the optical axis during total reflection, and the excitation density distribution The effect of this change on laser oscillation is still small. Thus, particularly in the side excitation method, it is important to suppress the change in the excitation density distribution due to the change in the wavelength of the excitation light.

Here, FIG. 4, Er: YAG crystal (diameter 5 mm, thickness 1 mm, Er ³⁺ concentration: 50 atomic percent) in, Er ³⁺ is excited from ⁴ I _15/2 state to the ⁴ I _11/2 state This is the result of actually examining the absorption spectrum in the 0.96 μm wavelength band. As can be seen from FIG. 4, in the 0.96 μm band of Er: YAG crystal, the absorption spectrum is very steep and has a plurality of absorption peaks. Further, it can be seen that the maximum absorption peak wavelength in the 0.96 μm band of the Er: YAG crystal, that is, the peak wavelength of the absorption peak having the maximum absorption coefficient among the plurality of absorption peaks shown in FIG. 4 is about 966 nm.

The excitation light Lp incident from the side surface of the solid laser rod 11 is attenuated while being absorbed by Er ³⁺ inside the solid laser rod 11. If the emission spectrum of the excitation light Lp is monochromatic or the absorption spectrum of the Er: YAG crystal is flat, the intensity of the excitation light when it reaches the side surface of the solid laser rod 11 is I ₀ , and the inside of the solid laser rod 11 The penetration light intensity I (L) at the depth L, where L is the penetration depth into the liquid crystal and L is the absorption coefficient, is

It is expressed. However, as shown in FIG. 4, the absorption spectrum of the Er: YAG crystal is steep and the spectrum of the excitation light Lp, which is the laser light from the semiconductor laser module 12, also has a certain width, so that the actual excitation The light Lp does not have such attenuation characteristics. That is, the peak wavelength of the excitation light Lp is λ, the shape function f (λ) of the spectrum of the excitation light Lp and the absorption spectrum α (λ) of the Er: YAG crystal are introduced, and the actual excitation light intensity I (L )

It is expressed.

  FIG. 5 is a diagram showing the correlation between the penetration depth L and the excitation light intensity I (L) when the spectrum of the excitation light Lp calculated based on the equation (2) has a shape as shown in FIG. It is. FIG. 6 shows a case where the half-value width is 1 nm (graph G5), a half-value width is 2 nm (graph G6), a half-value width is 3 nm (graph G7), and a half-value width is 4 nm. The spectrum of the excitation light Lp in (Graph G8) is shown, and the peak wavelength in each case is 969 nm. FIG. 5 shows the excitation light intensity I (L) when there is one excitation light source (semiconductor laser module 12), and the graphs G1 to G4 correspond to the graphs G5 to G8 in FIG. Yes. FIG. 5 shows that the attenuation characteristic of the excitation light Lp also depends on the half width of the excitation light Lp.

  FIG. 7 shows the peak of the excitation light Lp determined from the shape of the excitation density distribution calculated based on Equation (2) in the solid-state laser device 1 including the plurality of semiconductor laser modules 12 as in this embodiment. It is a graph which shows the suitable value of a wavelength. Each peak wavelength shown in FIG. 7 is longer than the maximum absorption peak wavelength (about 966 nm, see FIG. 4) of the Er: YAG crystal in the 0.96 μm band, and any of them is a suitable wavelength as the excitation light Lp. FIG. 7 also shows the excitation light absorption rate of the solid-state laser rod 11 corresponding to each peak wavelength.

  8A, FIG. 8B, FIG. 9A, and FIG. 9B are examples in which the peak wavelengths of the excitation light Lp are 968 nm, 969 nm, 970 nm, and 974 nm, respectively. It is a three-dimensional graph which shows the calculation result of excitation density distribution. 8A, 8B, 9A, and 9B, the horizontal axis (X axis and Y axis) is a plane coordinate in a cross section orthogonal to the central axis of the solid-state laser rod 11. Yes, the coordinates of the central axis are (X, Y) = (0, 0). The vertical axis represents the excitation density.

As shown in FIG. 8A, when the peak wavelength of the excitation light Lp is 968 nm, the excitation density is higher in the vicinity of the side surface than in the vicinity of the central axis of the solid laser rod 11. On the other hand, as shown in FIGS. 8B, 9A, and 9B, when the peak wavelength of the excitation light Lp is 969 nm, 970 nm, or 974 nm, the solid-state laser rod 11 It can be seen that the excitation density is high in the vicinity of the central axis, and excitation near the central axis is sufficiently performed. The excitation density distribution when the peak wavelength of the excitation light Lp was 970 nm to 973 nm was almost the same as that at 970 nm. Further, the excitation density distribution when the peak wavelength of the excitation light Lp is 974 nm to 976 nm was almost the same as that at 974 nm. For this reason, the peak wavelength of the excitation light Lp is preferably 969 nm or more, whereby a sufficient amount of excitation light Lp reaches the vicinity of the central axis of the solid-state laser rod 11 and ⁴ I _13/2 near the optical axis C. It turns out that the ion density of a state becomes large and can generate | occur | produce the high quality laser beam L efficiently.

  Further, as shown in FIG. 7, if the peak wavelength of the pumping light Lp is 975 nm or less, the pumping light absorptance of the solid laser rod 11 is generally high (80% or more). The percentage not absorbed by the water is sufficiently low. Since it is extremely important that the Er: YAG crystal has a high excitation light absorptivity for laser oscillation with high efficiency, the peak wavelength of the excitation light Lp is preferably 975 nm or less.

  Further, as shown in FIG. 4, the absorption spectrum of the Er: YAG crystal is composed of a plurality of peak waveforms in a very narrow band, for example, having a half-value width of 2 nm or less. Accordingly, if the half width of the spectrum of the pumping light Lp is narrow and the monochromaticity is high, the absorption rate of the Er: YAG crystal greatly changes due to the wavelength change of the pumping light Lp caused by the temperature change of the semiconductor laser module 12. It becomes difficult to maintain a good excitation density distribution when the peak wavelength of the light Lp changes over a wide range. On the other hand, by setting the half-value width of the spectrum of the excitation light Lp (that is, the emission spectrum of the semiconductor laser module 12) to 2 nm or more, even if the peak wavelength of the excitation light Lp is changed, the change in the excitation density distribution is changed. The laser beam L with high output energy can be stably extracted.

  Also, by setting the half width of the spectrum of the excitation light Lp to 2 nm or more, even if the peak wavelength of the excitation light Lp changes as described above, the change in the excitation density distribution can be suppressed, so that the peak wavelength of the excitation light Lp is Even in the case where the wavelength fluctuates over a wide wavelength range from 969 nm to 975 nm, the laser light L with high output energy can be suitably maintained. As a general characteristic of a semiconductor laser element, every time the element temperature rises by 1 ° C., the peak wavelength of the output light (excitation light) increases by about 0.33 nm. It is possible to suppress the change in the emission wavelength caused by it. However, if the allowable range of the peak wavelength is about 6 nm (969 nm to 975 nm) as described above, the change in the emission wavelength caused by the change in the drive current or the like can be kept in this range. It is possible to save the trouble of changing the cooling temperature of the semiconductor laser element each time it changes. Furthermore, variations in the emission peak wavelength due to manufacturing errors of the semiconductor laser module can be sufficiently absorbed.

  Further, when the spectrum of the excitation light Lp becomes excessively wide, the proportion of the total light quantity of the excitation light Lp that is not absorbed by the Er: YAG crystal increases. Therefore, the half width of the spectrum of the excitation light Lp is preferably 10 nm or less.

  According to the solid-state laser device 1 of the present embodiment, it is possible to obtain laser light with high output energy of several hundreds mJ per pulse as already described. Therefore, this solid-state laser device is used for, for example, a laser treatment apparatus. As a result, frequent replacement of the excitation light source is not required, and maintenance work can be saved and work efficiency can be improved. That is, the lifetime of the semiconductor laser element is 10 times longer than that of the flash lamp, so that the maintenance interval can be increased to 10 times or more by using a pump laser as the semiconductor laser element (semiconductor laser module 12) instead of the flash lamp. It becomes.

(Example)
Next, the result of actually making a prototype of the solid-state laser device 1 having the above configuration will be described as an example. In this example, the half width of the emission spectrum of the semiconductor laser module 12 was 3 nm. In addition, the peak wavelength of the excitation light Lp output from the seven semiconductor laser modules 12 used varied with a width of 1.1 nm. The peak wavelength of the excitation light Lp was adjusted by changing the cooling temperature of the semiconductor laser module 12.

  FIGS. 10-13 is a graph which shows the measurement result of the excitation density distribution in a present Example. 10 to 13, the horizontal axis represents the distance from the central axis of the solid laser rod 11, and the vertical axis represents the fluorescence intensity corresponding to the excitation density. Further, the peak wavelength of the excitation light Lp at the time of data measurement was adjusted as follows. That is, 968.0 nm to 969.1 nm in FIG. 10, 968.7 nm to 969.8 nm in FIG. 11, 969.3 nm to 970.4 nm in FIG. 12, 972.0 nm to 973.1 nm in FIG. .

  As shown in FIG. 10, when the peak wavelength of the pumping light Lp is 968.0 nm to 969.1 nm, the pumping density distribution is almost flat over the entire solid laser rod 11, but near the side surface of the solid laser rod 11. The excitation density increased slightly. Even with such an excitation density distribution, the laser beam L can be extracted efficiently, but it is more desirable to further increase the excitation density in the vicinity of the central axis of the solid laser rod 11.

  As shown in FIG. 11, when the peak wavelength of the pumping light Lp is 968.7 nm to 969.8 nm, the pumping light Lp sufficiently reaches the vicinity of the central axis of the solid laser rod 11, and the pumping density near the central axis is It became bigger. Moreover, as shown in FIG.12 and FIG.13, it was the same when the peak wavelength of excitation light Lp was 969.3 nm-970.4 nm and 972.0 nm-973.1 nm. That is, it was shown that if the peak wavelength of the pumping light Lp is set within these ranges, the Er: YAG crystal of the solid-state laser rod 11 can be pumped with high density as a whole, and the laser light L can be extracted efficiently. .

  Next, the results of measuring the intensity of the laser beam L extracted from the prototype solid-state laser device 1 will be described. 14 and 15 are graphs showing the relationship between the intensity of the excitation light Lp and the intensity of the laser light L in the solid-state laser device 1. FIG. 14 shows a case where the repetition frequency of the laser light L is 10 Hz and the pulse width (irradiation time per pulse) of the excitation light Lp is 200 μs. As shown in FIG. An output of 940 mJ could be obtained. FIG. 15 shows a case where the repetition frequency of the laser light L is 40 Hz and the pulse width of the excitation light Lp is 100 μs. As shown in the figure, the maximum output per pulse is 309 mJ and the average output is 12 W. Could get.

  From these measurement results, it was shown that high output energy such as several hundred mJ per pulse can be obtained by the solid-state laser device 1 according to the embodiment.

  The solid-state laser device according to the present invention is not limited to the above-described embodiment, and various other modifications are possible. For example, in the above embodiment, the solid laser rod 11 has a columnar shape, but the shape of the solid laser rod in the present invention is not limited to this, and may be a polygonal column shape such as a hexagonal column. . Further, in the above embodiment, seven sets of the semiconductor laser module 12 and the cylindrical lens 16 are provided, and these are arranged at equal intervals around the central axis of the solid laser rod 11, but the form, number and arrangement of the semiconductor laser elements are arranged. Is optional. Further, instead of the cylindrical lens, other optical components (such as a trapezoidal prism) that limit the irradiation range of the excitation light may be used.

Moreover, the various conditions (Er ³⁺ concentration, diameter, length, etc. of Er: YAG crystal) of the solid-state laser rod 11 in the above embodiment are only one preferred example, and can be changed without departing from the structure of the present invention. It is possible to configure a solid-state laser device that can obtain the same effect by combining these conditions.

  In addition to the configuration of the above embodiment, a giant pulse can be taken out by further incorporating a Q switch element between the first mirror (high reflection mirror 3) and the second mirror (output mirror 4).

It is a figure which shows the structure of one Embodiment of the solid-state laser apparatus by this invention. It is an enlarged view which shows the principal part of the pump module shown in FIG. It is sectional drawing along the III-III line of the pump module shown in FIG. It is the result of actually examining the absorption spectrum for the excitation light in the _0.96 μm band when Er ³⁺ is excited from the ⁴ I _15/2 state to the ⁴ I _11/2 state in the Er: YAG crystal. It is a figure which shows the correlation with the penetration depth and excitation light intensity when the spectrum of excitation light is a shape as shown in FIG. It is a figure which shows the example of the spectrum shape of excitation light. 5 is a chart showing suitable values for the peak wavelength of pumping light determined from the shape of pumping density distribution in a solid-state laser device including a plurality of semiconductor laser modules. It is a three-dimensional graph which shows an example of the calculation result of excitation density distribution in each case where the peak wavelength of excitation light is (a) 968 nm and (b) 969 nm. It is a three-dimensional graph which shows an example of the calculation result of excitation density distribution in each case where the peak wavelength of excitation light is (a) 970 nm and (b) 974 nm. It is a graph which shows the measurement result of the excitation density distribution in an Example. It is a graph which shows the measurement result of the excitation density distribution in an Example. It is a graph which shows the measurement result of the excitation density distribution in an Example. It is a graph which shows the measurement result of the excitation density distribution in an Example. It is a graph which shows the relationship between the intensity | strength of the excitation light in a solid-state laser apparatus, and the intensity | strength of a laser beam. It is a graph which shows the relationship between the intensity | strength of the excitation light in a solid-state laser apparatus, and the intensity | strength of a laser beam. It is a figure which shows the structure of the solid-state laser apparatus disclosed by the nonpatent literature 1. FIG. It is a figure which shows the structure of the solid-state laser apparatus disclosed by the nonpatent literature 2. FIG. It is a figure which shows the structure of the energy level of Er: YAG crystal.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Solid laser apparatus, 2 ... Pump module, 3 ... High reflection mirror, 4 ... Output mirror, 5 ... Power supply, 6 ... Laser cooler, 7 ... Controller, 8 ... Current supply cable, 9 ... Cooling water supply pipe DESCRIPTION OF SYMBOLS 11 ... Solid laser rod, 12 ... Semiconductor laser module, 13 ... Cooling piping, 14, 15 ... Reflector, 16 ... Cylindrical lens, 17 ... Housing.

Claims (4)

A columnar solid-state laser rod of YAG crystal doped with erbium ions;
A first mirror provided on one end side of the solid-state laser rod;
A second mirror provided on the other end of the solid-state laser rod, constituting a resonator together with the first mirror, causing laser oscillation, and emitting laser light;
The solid provided around the laser rod, said solid-state laser excitation light having a wavelength component contained in the absorption wavelength band when the energy level of the erbium ion transitions from ⁴ I _15/2 to ⁴ I _11/2 A semiconductor laser element for irradiating the side surface of the rod,
The laser light is pulse-like, the peak wavelength of the laser beam is included in the emission wavelength band when the energy level of the erbium ion transitions from ⁴ I _11/2 to ⁴ I _13/2,
A solid-state laser device, wherein a peak wavelength of the excitation light is longer than a maximum absorption peak wavelength included in the absorption wavelength band of the erbium ion. A columnar solid-state laser rod of YAG crystal doped with erbium ions;
A first mirror provided on one end side of the solid-state laser rod;
A second mirror provided on the other end of the solid-state laser rod, constituting a resonator together with the first mirror, causing laser oscillation, and emitting laser light;
A semiconductor laser element provided around the solid-state laser rod and irradiating a side surface of the solid-state laser rod with excitation light having a wavelength component included in a 0.96 μm band of the absorption wavelength band of the erbium ions;
The laser beam is pulsed, and the peak wavelength of the laser beam is included in the 2.94 μm band of the emission wavelength band of the erbium ions,
A solid-state laser device, wherein a peak wavelength of the excitation light is longer than a maximum absorption peak wavelength included in the 0.96 μm band of the erbium ions.   3. The solid-state laser device according to claim 1, wherein a half width of a spectrum of the excitation light is 2 nm or more and 10 nm or less.   The solid-state laser device according to claim 1, wherein a peak wavelength of the excitation light is 969 nm or more and 975 nm or less.