Q-Switch-Laservorrichtung

Abstract

Q-switch laser device with: an end cap (6) which has a yttrium aluminium garnet (YAG) crystal not doped with a rare earth element; a laser medium (8) comprising a rare-earth-element-doped YAG; and a Q-switch (10), where the end cap (6), the laser medium (8) and the Q-switch (10) are arranged in this order on a straight line, a <100> -axis or a <110> -Axis of the undominated YAG of the end cap (6) and the doped YAG of the laser medium (8) extend along the straight line and the undominated YAG of the end cap (6) and the doped YAG of the laser medium (8) are bonded together.

Description

[0001] The present application discloses a Q-switched laser device (also known as a Q-switched laser device) that emits pulsed laser light upon optical excitation. More specifically, the application discloses a Q-switched laser device that uses a rare-earth element-doped yttrium aluminum garnet crystal as a laser medium. The yttrium aluminum garnet crystal is referred to herein as YAG.

[0002] A laser device with an oscillator system comprising a doped YAG is known. When excitation light is introduced into the oscillator system, the oscillator system emits laser light. The yttrium aluminum garnet crystal containing a rare earth element is called doped YAG. It is known to generate pulsed laser light by using a Q-switch in the oscillator system. A Q-switched laser device using a doped YAG as the laser medium is described in the article "High Average Power Diode End-Pumped Composite Nd:YAG Laser Passively Q-switched by Cr 4+ :YAG Saturable Absorber, Nicolaie Pavel, Jiro Saikawa, Sunao Kurimura and Takunori Taira, Jpn. J. Appl. Phys. Vol. 40, (2001) pp 1253-1259”.

[0003] There is a need to increase the peak power of a pulsed laser. To achieve this, it seems promising to increase the spatial density of the excitation light entering the laser medium. This means that the peak power of a pulsed laser is expected to increase if the excitation light is focused for entry into the laser medium. In fact, the laser power for a continuous-wave laser is increased by focusing the excitation light.

[0004] However, in the case of a Q-switched laser device, increasing the degree of focusing of the excitation light only increases the repetition rate of the pulsed laser, not the peak power. A solution to this problem is disclosed in JP 2003 158 325 A and US 6 950 449 B2. Rather than increasing the density of the excitation light, the beam diameter of the excitation light entering the laser medium is increased. This means that the total population inversion stored at the time the optical switch is opened is increased. More specifically, the excitation light is not focused before entering the laser medium, but rather the beam area of ​​the excitation light is enlarged. The increased beam diameter increases the total population inversion, which in turn increases the output energy of the pulsed laser light emitted by the Q-switch laser. This means that the peak power of the pulsed laser light is increased.

[0005] When the excitation light enters the laser medium, the laser medium is heated due to a quantum defect. Specifically, the laser medium is locally heated in the excitation light irradiation region and in the vicinity of this region on the excitation light entrance surface. The locally heated laser medium causes local thermal expansion of the laser medium, deformation of the excitation light entrance surface, and adverse effects on laser oscillation (hereinafter referred to as "thermal problem").

[0006] According to JP 2003 158 325 A and US 6 950 449 B2, in which the beam area of ​​the excitation light entering the laser medium is increased (hereinafter referred to as "increasing an excitation volume"), the excitation light power per unit area on the excitation light entrance surface of the laser medium is reduced, resulting in a reduction in the calorific value per unit area of ​​the laser medium.

[0007] The deformation on the excitation light entrance surface can be suppressed by placing a solid plate transparent to the excitation light and bonding it to the excitation light entrance surface of the laser medium (hereinafter referred to as the "end capping technique"). The end capping technique is also advantageous in solving the thermal problem.

[0008] Either continuous light or pulsed light can be used as the excitation light for exciting the Q-switched laser device. Using pulsed light as the excitation light reduces the time-averaged power of the excitation light entering the excitation light entrance surface of the laser medium, thereby alleviating the thermal problem. Meanwhile, the pulsed excitation light causes strong excitation in an instant, which can increase population inversion. When excitation is performed with pulsed light at a lower repetition rate, the time-averaged power of the excitation light is reduced, improving the characteristics of the Q-switched laser device while significantly reducing the thermal problem (hereinafter referred to as "low repetition rate excitation").

[0009] In JP 2003 158 325 A and US 6 950 449 B2, the thermal problem is suppressed by combining the increase of the excitation volume, the end-capping technique, and low repetition rate excitation. The disclosure "Intrinsic reduction of the depolarization loss in solid-state lasers by use of a (110)-cut Y 3 Al 5 O 12 crystal, Ichiro Shoji and Takunori Taira, Applied Physics letters. Vol. 80 Number 17, (2002) pp 3048-3050" and JP3585891 are described below. SUMMARY OF THE INVENTION

[0010] JP 2003 158 325 A and US 6 950 449 B2, which combine the increase of the excitation volume, the end-cap technique and the low repetition rate excitation, are very effective in increasing the peak power of the pulsed laser while suppressing the thermal problem, but there is the problem that the repetition rate of the excitation light needs to be lowered (more precisely, it should be about 100 Hz or less).

[0011] A mass imaging device is currently being developed to enable measurement of the distribution of substances in particulate matter such as PM2.5. Such an analyzer, for example, requires a pulsed laser with a repetition rate of approximately 1 kHz. Since the technique disclosed in JP 2003 158 325 A and US 6 950 449 B2 uses low repetition rate excitation, this goal cannot be achieved.

[0012] In the technique disclosed in JP 2003 158 325 A and US 6 950 449 B2, the repetition rate of the excitation light was increased to approximately 1 kHz. It was found that the increase in excitation volume and the end-capping technique could effectively eliminate the thermal problem without any obvious deterioration in the peak power of the pulse laser, even when the repetition rate of the excitation light was increased to approximately 1 kHz. However, a laser whose wavelength was converted using a wavelength converter showed a drastic deterioration in the peak power of the pulse laser after conversion. To emit the wavelength-converted pulsed laser light, the increase in excitation volume, the end-capping technique, and low repetition rate excitation must be used together. If low repetition rate excitation is not used, the wavelength conversion efficiency deteriorates drastically.Therefore, to enable a Q-switched laser device using a YAG to emit pulsed laser light for wavelength conversion, the repetition rate should be controlled to be about 100 Hz or less.

[0013] This application discloses a technique in which pulsed laser light does not require low repetition rate excitation for wavelength conversion. When the pulsed laser light is linearly polarized, the peak power of the pulsed laser can be maintained at a high level after wavelength conversion. This application discloses a technique for emitting pulsed, linearly polarized laser light directly from the YAG, which does not require low repetition rate excitation.

[0014] After omitting low-repetition-rate excitation, a certain level of pulsed laser intensity was maintained, but the degree of linear polarization of the pulsed laser light deteriorated. The reason for this was investigated. Since a certain level of intensity was maintained, it was assumed that the thermal problem was suppressed. However, the cause of the linear polarization deterioration remained unknown. As a result of the study, it was shown that increasing the excitation volume and the end-capping technique can suppress the thermal problem in the region along the optical axis of the laser medium and maintain a certain level of pulsed laser intensity even when low-repetition-rate excitation is not used.However, it was also shown that a degree of linear polarization of the laser light was deteriorated due to birefringence generated by a region of local heating formed in the bonding interface of the laser medium and the end cap. Specifically, stress arises between the doped YAG and the undoped YAG because the doped YAG generates heat and expands due to the quantum defect induced by excitation, while the undoped YAG itself does not generate heat. When the bonding occurs more at an atomic level, greater stress is generated at the interface. On the other hand, when there is no tight bonding at the interface, the stresses are reduced. However, in this case, the heat dissipation effect of the undoped YAG deteriorates.

[0015] Based on this, the idea was developed that if a new crystallographic axis is present, which causes less deterioration of polarization due to locally generated stresses, even though a strong bond is present, these problems can be immediately solved. Consequently, it was found that the deterioration of the laser light with respect to linear polarization was suppressed when the YAG was oriented in the direction of <100> -axis extending along the optical axis of the oscillator system of the laser device. When the <100> -axis of the YAG along the optical axis of the oscillator system of the laser device, the deterioration of the linear polarization of the laser light was suppressed.

[0016] The Q-switch laser device disclosed herein comprises an end cap, a laser medium, and a Q-switch arranged in this order on a straight line (i.e., on the optical axis of the laser device). The end cap comprises an undoped YAG, while the laser medium comprises a doped YAG. The YAG forming the end cap and the YAG forming the laser medium are arranged in such a way that the direction of <100> -axis of the same extends along the optical axis of the laser device. The YAG forming the end cap and the YAG forming the laser medium are bonded together.

[0017] Usually, the YAG is arranged so that the <111> -axis along the optical axis of the laser device, since a rod-like crystal extending along the <111> -axis, can be easily obtained, since the YAG is distributed along the <111> -axis grows. In the article “High Average Power Diode End-Pumped Composite Nd:YAG Laser Passively Q-switched by Cr 4+:YAG Saturable Absorber, Nicolaie Pavel, Jiro Saikawa, Sunao Kurimura and Takunori Taira, Jpn. J. Appl. Phys. Vol. 40, (2001) pp 1253-1259“, JP 2003 158 325 A and US 6 950 449 B2, the YAG is arranged so that the <111> -axis along the optical axis. When the excitation light entrance surface of the rod-like doped YAG, which extends in the direction of <111> By bonding the YAG crystal lattice extending along the YAG axis with the undoped YAG crystal lattice aligned with each other, thermal deformation on the excitation light entrance surface can be prevented by the end-cap effect. In conjunction with increasing the excitation volume, pulsed laser light with high peak power can be emitted even without simultaneous low-repetition-rate excitation. However, without low-repetition-rate excitation, the linear polarization of the pulsed laser light deteriorates.

[0018] On the other hand, linear polarization can be maintained without simultaneously using low repetition rate excitation if the YAG is arranged so that the <100> -axis extends along the optical axis of the laser device. The same can be achieved if the YAG is arranged so that the <110> -axis along the optical axis of the laser device. According to the laser device disclosed herein, pulsed laser light with high peak power can be emitted while maintaining linear polarization. A laser that achieves high peak power after wavelength conversion can be obtained.

[0019] A technique for arranging a YAG used for the laser medium in such a way that the <100> -axis of the same extends along the optical axis of the oscillator system, is described in the article “Intrinsic reduction of the depolarization loss in solid-state lasers by use of a (110)-cut Y 3 Al 5 O 12crystal, Ichiro Shoji and Takunori Taira, Applied Physics letters. Vol. 80 Number 17, (2002) pp 3048-3050" and in JP 3 585 891 B2. The technique relates to the emission of a continuous wave laser light in which the region along the optical axis of the YAG used as the laser medium is heated over the entire length of the laser medium. It is reported that the linear polarization can be achieved by arranging the YAG in the direction in which the <100> -axis along the optical axis can be maintained when the region along the optical axis of the laser medium is heated over its entire length. The technique disclosed herein has, although "arranging the <100> -axis and the optical axis so that they are parallel" has something in common with it, but the reason for the deterioration of linear polarization is different, which is why it is difficult to derive the concept disclosed herein from the prior art.Of course, the approaches to a phenomenon occurring in the region extending along the optical axis of the laser medium and a phenomenon occurring at the entrance surface (the bonding interface between an undoped YAG end cap and a doped YAG laser medium) of the excitation light are different. The inventors of the present application have published the article "Intrinsic reduction of the depolarization loss in solid-state lasers by use of a (110)-cut Y. 3 Al 5 O 12crystal, Ichiro Shoji and Takunori Taira, Applied Physics letters. Vol. 80 Number 17, (2002) pp. 3048-3050" and JP 3 585 891 B2. However, there are differences in the energy of the incoming excitation light, the extent of heating of the laser medium, and the range of heating of the laser medium between the continuous-wave laser oscillator device and the pulsed laser oscillator device. Until now, it was assumed that the continuous-wave laser technology described in the above-mentioned documents could not also be applied to Q-switch laser technology.

[0020] A laser device disclosed herein can emit pulsed laser light that provides a desired degree of linear polarization and high peak power even after wavelength conversion without using low repetition rate excitation. Therefore, it becomes possible to obtain a pulsed laser (a high repetition rate pulsed laser with linear polarization) required for realizing a next-generation mass imaging device. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 schematically shows an exemplary structure of a Q-switch laser device. Fig. Figure 2 shows the relationship between the excitation light repetition rate and the pulse laser energy. Fig. Figure 3 shows the relationship between the excitation light repetition rate and the degree of linear polarization of the pulse laser. Fig. 4 schematically shows another exemplary structure of a Q-switch laser device. Fig. 5 schematically shows another exemplary structure of a Q-switch laser device. Fig. 6 schematically shows another exemplary structure of a Q-switch laser device. Fig. 7 schematically shows another exemplary structure of a Q-switch laser device. Fig. 8 schematically shows another exemplary structure of a Q-switch laser device. Fig. 9 schematically shows another exemplary structure of a Q-switch laser device. Fig. 10 schematically shows another exemplary structure of a Q-switch laser device. DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] Some features of the exemplary embodiments described are given below: (Feature 1) A YAG containing Nd is used for the laser medium. (Feature 2) An undoped (no rare earth element containing) YAG is used for the end cap. (Feature 3) A YAG that Cr 4+ is used for the Q-switch. The Q-switch acts as a passive Q-switch for utilizing a saturable absorber. (Feature 4) The wavelength is converted from 1064 nm to 532 nm by an LBO crystal. (Feature 5) The wavelength is converted from 532 nm to 266 nm by a BBO crystal.

[0022] In Fig. In Figure 1, reference numeral 2 represents a semiconductor laser device that emits an excitation light 4. In the experiment, a pulsed excitation light 4a was obtained by applying a pulsed current 2a to the semiconductor laser device 2. For the excitation light 4, the wavelength was set to 808 nm, the power to 100 W, and the pulse width to 120 µs. The repetition rate was varied in the range between 100 Hz and 1 kHz and used experimentally. Reference numeral 6 represents an undoped (containing no rare earth element) YAG, which serves as an end cap. The YAG is formed, for example, in the shape of a disk with a diameter of 5 mm and a thickness of 1 mm. In the YAG 6, the <100> -axis in the direction of the thickness of the disc.

[0023] Reference numeral 8 represents a YAG containing 1.1 at.% Nd and serving as a laser medium. The YAG 8 is formed, for example, in the form of a disk with a diameter of 5 mm and a thickness of 4 mm. In the YAG 8, the <100> -axis in the direction of the thickness of the disc.

[0024] The YAG 6 is bonded to an end face of the Nd:YAG 8 on the semiconductor laser device 2 side. The undoped YAG 6 is transparent to an excitation light 4 with a wavelength of 808 nm. The excitation light 4 enters the end face of the Nd:YAG 8 on the semiconductor laser device 2 side. The increase in the excitation volume is applied to the excitation light 4.

[0025] The reference numeral 10 represents a Cr 4+ containing YAG with an initial transmission of 40%. The Cr 4+ :YAG 10 is a saturable absorber and acts as a passive Q-switch.

[0026] Reference numeral 12 represents an output coupler having a specular surface film formed thereon. The YAG 6 also has a specular surface film formed thereon. The specular surface films of the output coupler 12 and the YAG 6 form an oscillator system in which an end cap (YAG 6), a laser medium (Nd:YAG 8), and a passive Q-switch (Cr 4+ :YAG 10). The end cap 6, the laser medium 8 and the passive Q-switch 10 are arranged on a straight line perpendicular to the two mirror surface films. The YAG 6 and the Nd:YAG 8 are arranged so that the <100> -axis of the same extends along the straight line. For the Cr 4+:YAG 10, the relationship between the crystallographic axis and the optical axis is not further restricted. For example, the <111> -axis parallel to the optical axis, or the <100> -axis or the <110> -axis can be parallel to the optical axis.

[0027] When an excitation light 4 enters the previously described laser device, a pulsed laser light 14 is output from the output coupler 12. The wavelength of the pulsed laser light 14 is 1064 nm, and the repetition rate is equivalent to that of the excitation light 4. The pulse width (half-width) of the pulsed laser light 14 was 600 ps.

[0028] Fig. Figure 2 shows the relationship between the excitation light repetition rate (equivalent to the repetition rate of the pulsed laser light) and the pulse laser energy (mJ). The pulse laser energy is not significantly reduced even when the excitation light repetition rate is increased. Even if the low repetition rate excitation is abandoned and excitation is performed at 1 kHz, a pulse energy of approximately 1.42 mJ (2.4 MW for peak power) is obtained.

[0029] Fig. Figure 3 shows the relationship between an excitation light repetition rate and the degree of linear polarization of the pulsed laser light 14. The vertical axis indicates an increasing deterioration of the linear polarization. Curve 32 shows the relationship when the <111> -axis and the optical axis are arranged parallel to each other. It shows that the pulsed laser light 14 is deteriorated in linear polarization when the excitation light repetition rate is increased. On the other hand, curve 34 shows the relationship when the <100> -axis and the optical axis are arranged parallel to each other. It shows that the linear polarization of the pulsed laser light 14 is maintained even when the excitation light repetition rate is increased. If the <100> -axis and the optical axis are arranged parallel to each other, a pulse laser can be obtained which delivers high intensity light after wavelength conversion.Although not shown, if the <110> -axis of the YAG and the optical axis are arranged parallel to each other, a curve similar to curve 34 can also be obtained. Even if the <110> -axis of the YAG and the optical axis are arranged parallel to each other, a pulse laser can be obtained which delivers light with a high intensity after wavelength conversion.

[0030] Fig. Figure 10 shows a device in which a laser light with a wavelength of 1064 nm output from the Q-switch laser device is converted into a laser light with a wavelength of 532 nm by an LBO crystal, and the laser light with the wavelength of 532 nm is converted into a laser light with a wavelength of 266 nm by a BBO crystal. As shown in Fig. 3, when the <100> -axis or the <110> -axis of the YAG is arranged parallel to the optical axis, a high linear polarization is obtained, so that a laser light with a high intensity is obtained after wavelength conversion, wherein the peak power for the laser light with the wavelength of 532 nm obtained by means of the LBO crystal was 1.1 MW, and the peak power for the laser light with the wavelength of 266 nm obtained by means of the BBO crystal was 0.511 MW.

[0031] The present invention is not limited to the examples given above, and other variations and modifications may be made which are also intended to fall within the scope of the present invention.

[0032] In the example above, the end cap, laser medium, and Q-switch were arranged in this order, but they can also be arranged in the order end cap, laser medium, end cap, Q-switch, or end cap, laser medium, end cap, Q-switch, end cap. Fig. 4 to 9 schematically show a structure of other exemplary Q-switch laser devices. A passive Q-switch 10 is used, but an externally controllable Q-switch may also be used. Although Nd was used as a rare earth element added to the laser medium, a rare earth element other than Nd may be used.

[0033] The components described herein, independently or jointly, have a technical benefit that is not limited to the combinations specified in the claims. The components described herein will solve multiple problems simultaneously and already have a technical benefit if they solve one of these problems.

[0034] It is explicitly emphasized that all features disclosed in the description and / or the claims are to be considered separate and independent of each other for the purpose of the original disclosure as well as for the purpose of limiting the claimed invention, regardless of the feature combinations in the embodiments and / or the claims. It is explicitly stated that all range specifications or specifications of groups of units disclose every possible intermediate value or subgroup of units for the purpose of the original disclosure as well as for the purpose of limiting the claimed invention, in particular also as a limit of a range specification.

Claims

[1] Q-switched laser device with: an end cap (6) comprising an yttrium aluminum garnet (YAG) crystal not doped with a rare earth element; a laser medium (8) comprising a YAG doped with a rare earth element; and a Q-switch (10), in which the end cap (6), the laser medium (8) and the Q-switch (10) are arranged in this order on a straight line, a <100> -axis or a <110> -Axis of the undoped YAG of the end cap (6) and the doped YAG of the laser medium (8) extend along the straight line and the undoped YAG of the end cap (6) and the doped YAG of the laser medium (8) are bonded together.

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