JPH04335587A - Laser diode pumping solid-state laser - Google Patents

Abstract

PURPOSE:To obtain wavelength-converted waves whose output is high and which are stable in a laser diode pumping solid-state laser which wavelength- converts a solid-state laser oscillation beam by using a crystal as a nonlinear optical material. CONSTITUTION:An external resonator for a semiconductor laser 14 is constituted of the following: the edge 16a of an Nd:YAG crystal 16 as a solid-state laser medium; and the edge 10a of a KNbO3 crystal 10 as a nonlinear optical material. One part of a laser beam 13 which has been resonated by the external resonator is fed back optically to the semiconductor laser 14.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to a laser diode-pumped solid-state laser, and more particularly, a crystal of a nonlinear optical material is arranged in a resonator to convert the wavelength of a solid-state laser oscillation beam (shorten the wavelength). The present invention relates to a laser diode pumped solid state laser.

0002

[Prior Art] For example, as shown in Japanese Patent Laid-Open No. 62-189783, a laser diode-pumped solid-state laser in which a solid-state laser medium doped with a rare earth element such as neodymium is pumped by a semiconductor laser (laser diode) has become known. ing. In this type of laser diode pumped solid-state laser, the solid-state laser oscillation beam is wavelength-converted within the resonator in order to obtain a laser beam with a shorter wavelength, as shown in Japanese Patent Application Laid-Open No. 2-77181, for example. Wavelength conversion of a solid-state laser oscillation beam into a second harmonic has also been carried out by disposing a bulk single crystal of a nonlinear optical material. Furthermore, for example, Applied Physics Letter
r) Vol. 52, No. 2,11 January
ry 1988, it has also been proposed to wavelength convert the solid state laser oscillation beam and the pumping light into a sum frequency by means of a bulk single crystal of a nonlinear optical material placed in the above-mentioned position.

In such laser diode-pumped solid-state lasers, it is desired that the pumping light emitted from the semiconductor laser be sufficiently absorbed by the solid-state laser medium to efficiently obtain a short wavelength laser beam. For example, Optics Letters
Letters) Vol. 16, No. 6/Ma
rch 15, 1991 p. As shown in No. 396, it has been proposed to provide an external resonator that resonates the pumping light in addition to the resonator that the semiconductor laser is originally equipped with, and to arrange a solid-state laser medium within this external resonator. . In conventional laser diode-pumped solid-state lasers equipped with this external cavity, an isolator is installed between the semiconductor laser and the external cavity to prevent the oscillation wavelength from becoming unstable due to light returning to the semiconductor laser. are placed.

0004

[Problem to be Solved by the Invention] However, in the above laser diode pumped solid-state laser, the resonator length of the external resonator fluctuates due to temperature changes, etc., and the resonant wavelength tends to deviate from the oscillation wavelength of the semiconductor laser. There's a problem. In this case, the amount of pumping light absorbed in the solid-state laser medium fluctuates, and the output of the wavelength-converted wave becomes unstable.

Furthermore, in the conventional laser diode-pumped solid-state laser having the above configuration, in order to obtain a wavelength-converted wave that can be well focused, a semiconductor laser with longitudinal and transverse single modes must be used. Therefore, it is not possible to use relatively high-power broad area lasers or phased array lasers as pumping sources.
There is a problem in that it is difficult to obtain a high-power wavelength-converted wave.

Furthermore, in the conventional laser diode-pumped solid-state laser having the above structure, an expensive isolator is used to stabilize the output, which increases the cost and makes it difficult to form an isolator with a large effective diameter. Therefore, the beam diameter of the semiconductor laser light is limited, and as a result, the semiconductor laser light, which is the pump light, is rejected by the isolator, so there is a problem that the semiconductor laser light cannot be inputted into the solid-state laser medium with high efficiency. there were.

The present invention has been made in view of the above-mentioned circumstances, and it stabilizes the output of the wavelength-converted wave, does not significantly increase the cost, and can be used as a pumping source with various high outputs. It is an object of the present invention to provide a laser diode pumping solid-state laser in which a semiconductor laser can also be used as appropriate and there is no restriction on the beam diameter of the semiconductor laser light serving as the pump light.

[0008]

[Means for Solving the Problems] A laser diode-pumped solid-state laser according to the present invention is a laser-diode-pumped solid-state laser in which the wavelength of a solid-state laser oscillation beam is converted by a crystal of a nonlinear optical material arranged in a resonator, as described above. , characterized in that it is provided with an external resonator that resonates the pumping light emitted from the semiconductor laser, and has a structure in which a part of the pumping light that resonates with the external resonator is optically fed back to the semiconductor laser. It is.

[0009]

[Operation and Effects of the Invention] When the pumping light resonated in the external resonator is optically fed back to the semiconductor laser as described above, so-called frequency locking is achieved and the longitudinal mode of the semiconductor laser is unified. In this case, the amount of pumping light absorbed by the solid-state laser medium becomes stable, and the output of the wavelength-converted wave becomes stable.

Furthermore, even if a semiconductor laser with multi-mode longitudinal or transverse modes is used as a pumping source, the longitudinal modes can be unified by optical feedback as described above, so in order to stabilize the output of the wavelength-converted wave, There are no limitations on the semiconductor lasers that can be used. If so, it would be possible to obtain a high-power wavelength-converted wave using a high-power broad area laser or phased array laser as a pumping source.

Furthermore, in the above configuration, since no special optical element such as an isolator is used for stabilizing the oscillation wavelength, the cost for stabilizing the oscillation wavelength does not increase significantly, and the pump light There is no limit to the beam diameter of the semiconductor laser light.

In a particularly preferred embodiment of the present invention, a solid laser medium and a crystal of a nonlinear optical material are integrated, and their end surfaces are used to construct the external resonator. In such a configuration, an effect can be obtained in that the resonator length of the external resonator does not easily change even if the ambient temperature changes.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be explained in detail below based on embodiments shown in the drawings.

<First Embodiment> FIG. 1 shows a laser diode pumped solid-state laser according to a first embodiment of the present invention. This laser diode pumping solid-state laser uses laser beam 1 as pumping light.
3, and a collimator lens 1 that converts the diverging laser beam 13 into parallel light.
5a, a condenser lens 15b that focuses the collimated laser beam 13, and a YAG crystal (hereinafter referred to as Nd) which is a solid laser medium doped with neodymium (Nd).
d:YAG crystal) 16, and a KNbO3 crystal 10, which is a nonlinear optical material, fixed to the front side (lower side in the figure) of this Nd:YAG crystal 16. The Nd:YAG crystal 16 of this example has a thickness of 1 mm, and the Nd:YAG crystal 16 and the KNbO3 crystal 10 are
It is formed into a hemispherical shape with a radius of 2.5 mm. These crystals 10 and 16 are such that the laser beam 13 is Nd:
The light is arranged so as to be obliquely incident on the YAG crystal 16.

[0015] Each of the above-mentioned elements is mounted and integrated in a common housing (not shown). Note that the semiconductor laser 14 is fixed to a copper block 17, and its temperature is controlled to a predetermined temperature by a Peltier element and a temperature control circuit (not shown).

The semiconductor laser 14 has a fundamental wavelength λ
A device that emits a laser beam 13 with a wavelength of 1 = 808 nm is used. The Nd:YAG crystal 16 emits a laser beam 11 with a wavelength λ2 = 946 nm when the neodymium atoms are excited by the laser beam 13.

[0017] Light incident side end face 16 of Nd:YAG crystal 16
In a, the laser beam 11 with a wavelength of 946 nm is well reflected (reflectance of 99.9% or more), and the laser beam 11 with a wavelength of 808 nm is well reflected.
The pumping laser beam 13 of m is 92% reflected, and the second harmonic 12 of wavelength λ3 = 473 nm, which will be described later, is
is coated with a coating 18 that allows good transmission (transmittance of 99% or more). On the other hand, the spherical end surface 10a of the KNbO3 crystal 10 is exposed to the laser beams 11, 13
A coating 19 is applied to reflect all of the second harmonic wave 12 having a wavelength of 473 nm. Therefore, the laser beam 11 with a wavelength of 946 nm is confined between the surfaces 10a, 16a, and 10a with a V-shaped optical path, causing laser oscillation. This laser beam 11 enters the KNbO3 crystal 10 and has a wavelength of 1
/2, that is, the wavelength is converted to the second harmonic 12 of 473 nm. This second harmonic wave 12 is emitted from the Nd:YAG rod end surface 16a on which the coating 18 described above is applied.

As explained above, in this example, Nd:YA
The G crystal 16 and the KNbO3 crystal 10 constitute a resonator for a solid-state laser. In addition, the Nd:YAG crystal 16 and the KNbO3 crystal 10 are connected to the semiconductor laser 14.
It also acts as an external resonator. That is, wavelength 808
nm laser beam 13 also hits the surfaces 10a, 16
A and 10a form a V-shaped optical path and resonate. By doing this, even if the Nd:YAG crystal 16 is formed as thin as 1 mm, the laser beam 13 is sufficiently absorbed there, and the high-intensity laser beam 11 is
begins to be emitted.

Since the coating 18 on the surface 16a is supposed to reflect 92% of the laser beam 13 with a wavelength of 808 nm, a part of the laser beam 13 resonated in the external resonator is transmitted to the semiconductor laser 14. Feedback will be provided. This causes the semiconductor laser 14 to be frequency locked and oscillate in a single longitudinal mode. If this happens, the amount of absorption of the laser beam 13 into the Nd:YAG crystal 16 will be stabilized as described above, and as a result, the output of the second harmonic 12 will be stabilized.

In this embodiment, a single mode laser is used as the semiconductor laser 14, and its output is 100 m.
When W, the second harmonic 12 of 10 mW can be obtained. In addition, since the longitudinal mode of the semiconductor laser 14 is unified as described above, even when considering the focusing ability of the second harmonic 12, a high-output broadband signal that is originally multi-mode in both longitudinal and transverse modes can be used. An area laser or a phased array laser can be used, and by doing so, a higher output second harmonic 12 can be obtained.

<Second Embodiment> FIG. 2 shows a laser diode pumped solid state laser according to a second embodiment of the present invention. Note that in FIG. 2, the same elements as those in FIG. In this second embodiment, the end face 16a of the Nd:YAG crystal 16 reflects well the laser beam 11 with a wavelength of 946 nm, and reflects the laser beam 13 with a wavelength of 808 nm and the second harmonic 12 with a wavelength of 473 nm, respectively. It has a coating 20 that is 92% reflective. The device of this second embodiment differs from the device of the first embodiment only in this coating 20.

In this example, coating 2 as described above is used.
0, the second harmonic 12 also has a surface 10
A, 16a, and 10a form a V-shaped optical path and resonate. Thereby, in this second embodiment, wavelength conversion can be performed with higher efficiency than in the first embodiment. For example, when a single mode laser with an output of 50 mW is used as the semiconductor laser 14, the second harmonic 1 with an output of 10 mW
You can get 2.

<Third Embodiment> FIG. 3 shows a laser diode pumped solid state laser according to a third embodiment of the present invention. In this third embodiment, a laser beam 13 that has been collimated by a collimator lens 15a is reflected by a mirror 30 and enters a condenser lens 15b. Mirror 30 can be moved in the direction of arrow A by PZT element 31. Laser beam 13 with wavelength λ1 = 808 nm
is condensed by the condensing lens 15b, and Nd:YA
The light is incident on the G crystal 16. A KNbO3 crystal 10 is integrated into this Nd:YAG crystal 16. Nd:Y
The AG crystal 16 is pumped by a laser beam 13 and receives a laser beam 1 of wavelength λ2 = 946 nm.
1, and this laser beam 11 emits a second harmonic of wavelength λ3 = 473 nm by the KNbO3 crystal 10.
The wavelength is converted to 2.

End faces 16a, 16 of Nd:YAG crystal 16
Coatings 32 and 33 are applied to the KNbO3 crystal 10, respectively, and coatings 34 and 35 are applied to the end faces 10a and 10b of the KNbO3 crystal 10, respectively. The wavelength λ1 = of these coatings 32, 33, 34, 35
808 nm, λ2 = 946 nm, λ3 = 473
The characteristics with respect to nm are as follows. Note that AR is non-reflective (transmittance of 99% or more), and HR is highly reflective (reflectance of 99% or more).
9.9% or more).

[0025]λ1=8
08 nm λ2 =946 nm λ3
=473 nm coating 32
AR HR
HR coating 33 AR
AR AR
Coating 34 AR
A.R.
AR coating 35
AR HR
AR Since the coatings 32, 33, 34, and 35 are applied as described above, the laser beam 13, which is the pumping light, is incident on the Nd:YAG crystal 16 well, and the KNbO3
The light is emitted from the crystal 10 forward (to the right in the figure). The laser beam 11 as a fundamental wave resonates between the end face 16a and the end face 10b. Also, the second harmonic 12 is KNb
O3 is emitted from the crystal 10 to the front side.

In this embodiment, unlike the first and second embodiments, the external resonator for the semiconductor laser 14 is
It is formed separately from the Nd:YAG crystal 16 and the KNbO3 crystal 10. That is, this external resonator has N
d: Resonator mirror 3 arranged on the rear side of the YAG crystal 16
6, a resonator mirror 37 disposed on the front side of the KNbO3 crystal 10, and a resonator mirror 38 that reflects the light reflected by the resonator mirror 37 toward the resonator mirror 36. ing. The wavelength λ1 of these resonator mirrors 36, 37, 38 = 808
nm, λ2 = 946 nm, λ3 = 473 nm
The characteristics for are as follows. λ1 =8
08 nm λ2 =946 nm λ3
=473 nm Resonator mirror 36 8
5% reflection −
- Resonator mirror 37 HR
AR AR
Resonator mirror 38 HR
−
- Laser beam 13
resonates in the external resonator with the above configuration, so Nd:
It can be sufficiently absorbed into the YAG crystal 16. Also, a part of the resonant laser beam 13 is reflected by the end face 16a of the Nd:YAG crystal 16, although the reflectance is extremely low.
, and is optically fed back to the semiconductor laser 14 . Thereby, the frequency of the semiconductor laser 14 is locked in this case as well, and its longitudinal mode is unified.

Note that the semiconductor laser 14, Nd:YAG
When the mount holding the crystal 16 and the KNbO3 crystal 10 thermally expands or contracts due to temperature changes,
Two cleavage planes of semiconductor laser 14 and Nd:YAG
The optical path length between the end faces 16a of the crystal 16 can vary. In order to deal with this, a dichroic mirror 29 that reflects only the laser beam 13 is arranged in front of the resonator mirror 37, and the amount of light of the laser beam 13 reflected here is detected by a photodetector 39 such as a photodiode. be done. A light amount signal S1 output from this photodetector 39 is input to a feedback circuit 40. The feedback circuit 40 sends a drive signal S2 corresponding to the light amount signal S1 to the P
The signal is input to the ZT element 31 and the mirror 30 is moved in the direction of arrow A. Thereby, the above-mentioned optical path length is always maintained at a length that allows the maximum output laser beam 13 to be obtained.

On the other hand, in the first and second embodiments, the Nd:YAG crystal 16 and the KNbO3
Since the semiconductor laser 14 is integrated with the crystal 10 and their end faces constitute the external resonator of the semiconductor laser 14, the optical path length variation as described above is unlikely to occur.

The coating 35 of this embodiment is AR (non-reflective) for the second harmonic wave 12 with a wavelength of 473 nm, but instead is AR (non-reflective) for the second harmonic wave 12 (for example, 5%). By applying a coating that makes the second harmonic 12 resonate between the surfaces 16a and 10b,
Even higher wavelength conversion efficiency can be achieved.

Fourth Embodiment FIG. 4 shows a laser diode pumped solid state laser according to a fourth embodiment of the present invention. The fourth embodiment and the fifth embodiment described below convert the wavelength of pumping light and solid-state laser oscillation beam into a sum frequency. As shown in the figure, in this fourth embodiment device, the third
Example device Nd:YAG crystal 16 and KNbO3
Instead of crystal 10, NYAB (NdX Y1-X Al
3 (BO3)4 x=0.04 to 0.08) crystal 41 is disposed. This NYAB crystal 41 is
Generally Self-Frequency-Double
It is a type of crystal called gCrystal, and it is a solid laser medium and also has an optical wavelength conversion function.

On the other hand, the semiconductor laser 14 has a wavelength λ
A device that emits a laser beam 13 with a wavelength of 1 = 804 nm is used. The NYAB crystal 41 is pumped by this laser beam 13 and the wavelength λ2 = 1
A laser beam 11 of 062 nm is emitted, and the sum frequency 42 of this laser beam 11 and the laser beam 13, that is, the wavelength λ3 (1/λ1 +1/λ2
=1/λ3)=458 nm laser beam is emitted.

Both end faces 41a and 41b of NYAB crystal 41
are coated with coatings 43 and 44, respectively. Further, the external resonator for the semiconductor laser 14 is configured as a ring resonator by resonator mirrors 36, 37, and 38. These coatings 43, 44 and the resonator mirror 3
6, 37, 38, wavelength λ1 = 804 nm, λ2
= 1062 nm and λ3 = 458 nm, the characteristics are as follows. λ1 =8
04 nm λ2 = 1062 nm λ3
=458 nm coating 43
AR HR
HR coating 44 AR
HR AR
Resonator mirror 36 85% reflection
−
- resonator mirror 37
HR AR
A.R.
Resonator mirror 38 HR
−
- coating 43 as described above;
, 44, the laser beam 13, which is the pumping light, enters the NYAB crystal 41 well and is emitted from there forward (to the right in the figure). The laser beam 11, which is one of the fundamental waves, is transmitted to the end faces 41a and 41.
resonates with b. Also, the sum frequency 42 is NYAB crystal 4
It emits from 1 to the front side. And laser beam 13
resonates in the external resonator with the above configuration, so NYA
It can be sufficiently absorbed by the B crystal 41.

A part of the resonant laser beam 13 is reflected by the end face 41a of the NYAB crystal 41 and is optically fed back to the semiconductor laser 14, although the reflectance is extremely low. Therefore, in this case as well, the semiconductor laser 1
4 is frequency locked and its longitudinal mode is unified.

Fifth Embodiment FIG. 5 shows a laser diode pumped solid state laser according to a fifth embodiment of the present invention. This fifth embodiment device uses a YVO4 crystal (hereinafter referred to as Nd:Y
50 (referred to as a VO4 crystal) and a KTP crystal 51 which is a nonlinear optical material.

The semiconductor laser 14 has a wavelength λ1
A device that emits a laser beam 13 of =809 nm is used. The Nd:YVO4 crystal 50 is pumped by this laser beam 13 and the wavelength λ2 =
While emitting a laser beam 11 of 1064 nm,
The sum frequency 42 of this laser beam 11 and the laser beam 13, that is, the wavelength λ3 (1/λ1 +1/λ
2 = 1/λ3 ) = 460 nm laser beam is emitted.

[0036] Both end faces 50a of Nd:YVO4 crystal 50
, 50b are coated with coatings 52 and 53, respectively. Both end faces 51a, 51 of the KTP crystal 51
b are coated with coatings 54 and 55, respectively. Further, the external resonator for the semiconductor laser 14 is configured as a ring resonator by resonator mirrors 36, 37, and 38. These coatings 52, 53, 5
4, 55 and resonator mirrors 36, 37, 38, wavelengths λ1 = 809 nm, λ2 = 1064 nm, λ3
The characteristics for =460 nm are as follows. λ1
=809 nm λ2 =1064 nm
λ3 = 460 nm coating 52
AR HR
HR Coating 53 AR
AR AR
Coating 54
AR AR
AR coating 55 AR
HR AR
Resonator mirror 36 85% reflection
−
- resonator mirror 37
HR AR
A.R.
Resonator mirror 38 HR
− −
Coatings 52, 53 as described above
, 54, and 55, the laser beam 13 serving as the pumping light can be effectively applied to the Nd:YVO4 crystal 5.
0 and forward from the KTP crystal 51 (to the right in the figure)
emitted to. The laser beam 11, which is one of the fundamental waves, resonates between the end faces 50a and 51b. Further, the sum frequency wave 42 is emitted from the KTP crystal 51 to the front side. Since the laser beam 13 resonates in the external resonator having the above configuration, it can be sufficiently absorbed by the Nd:YVO4 crystal 50.

A part of the resonant laser beam 13 is reflected by the end face 50a of the Nd:YVO4 crystal 50, although the reflectance is extremely low, and is optically fed back to the semiconductor laser 14. Thereby, the frequency of the semiconductor laser 14 is locked in this case as well, and its longitudinal mode is unified.

It goes without saying that the external resonator structure shown in FIG. 1 or 2 can be applied even when wavelength-converting the pumping light and the solid-state laser oscillation beam into a sum frequency. Furthermore, in the present invention, it is also possible to employ a ring resonator structure as shown in FIG. In the apparatus shown in FIG. 6, as an example, a Nd:YVO4 crystal 50 and a KTP crystal 51 are integrated, and the KTP crystal 50 is
An external resonator for the semiconductor laser 14 is configured by the rear end surface 51a, front end surface 51b, and upper end surface 51c of the semiconductor laser 14.

Furthermore, the solid-state laser medium used in the present invention is not limited to the crystals in the embodiments described above, but may include other known materials, such as LNP.
, Nd:YLF, etc. can be used as appropriate. Furthermore, the nonlinear optical materials are not limited to the materials in the above embodiments, but may include other known materials, such as BNNB, LiIO3.
, Urea, (3,5-dimethyl-1-(4-nitrophenyl)pyrazole shown in JP-A No. 2-77181, (3,5-dimethyl-1-( 4-nitrophenyl)-1,2,4-triazole, etc. can be used as appropriate.

[Brief explanation of drawings]

FIG. 1 is a side view of a device according to a first embodiment of the present invention.

FIG. 2 is a side view of a device according to a second embodiment of the present invention.

FIG. 3 is a side view of a device according to a third embodiment of the present invention.

FIG. 4: Side view of a device according to a fourth embodiment of the present invention.

FIG. 5 is a side view of a device according to a fifth embodiment of the present invention.

FIG. 6 is a side view of a device according to a sixth embodiment of the present invention.

[Explanation of symbols]

10 KNbO3 crystal 11 Laser beam (fundamental wave) 12 Second harmonic 13 Laser beam (pumping light) 14
Semiconductor laser 16 Nd:YAG crystal 18, 19, 20, 32, 33, 34, 35, 43, 4
4, 52, 53, 54, 55 Coating 36, 37, 38 Resonator mirror 41 N
YAB crystal 50 Nd:YVO4 crystal 51 KTP crystal

Claims (2)

[Claims] [Claim 1] Laser diode pumping, in which a solid-state laser medium doped with rare earth elements such as neodymium is pumped by a semiconductor laser, and the wavelength of the obtained solid-state laser oscillation beam is converted by a crystal of a nonlinear optical material placed in a resonator. The solid-state laser is characterized in that it is provided with an external resonator that resonates the pumping light emitted from the semiconductor laser, and has a structure that optically feeds back a part of the pumping light that resonates in the external resonator to the semiconductor laser. Laser diode pumping solid state laser. 2. The laser diode pumping solid state according to claim 1, wherein the solid laser medium and a crystal of a nonlinear optical material are integrated, and the external resonator is formed using an end face thereof. laser.