JPH04320078A - Optical wavelength converter - Google Patents

Abstract

PURPOSE:To prevent a wavelength conversion efficiency from decreasing out of a state in which a linearly polarizing direction of a solid-state laser oscillation beam is formed at a predetermined angle to a crystal optical axis in an optical wavelength converter for converting the beam to a second harmonic wave by a biaxial crystal having a nonlinearly optical effect. CONSTITUTION:An Nd:YVO4 rod 16 of a solid-state laser medium is formed in a wedge shape. The rod 16 is moved in a direction in which a length of an optical path of a laser beam 11 is altered by a precise screw 24 to adjust its linearly polarizing direction.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to an optical wavelength conversion device for converting a fundamental wave to a second harmonic.
The present invention relates to an optical wavelength conversion device using a crystal of a nonlinear optical material that achieves type II phase matching with harmonics.

0002

[Prior Art] For example, as shown in Japanese Patent Application Laid-Open No. 189783/1983, a laser diode pumped solid-state laser, in which a solid-state laser rod 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, in order to obtain laser light with a shorter wavelength, a bulk single crystal of a nonlinear optical material is placed inside the resonator, and the solid-state laser oscillation beam is converted into a second harmonic. Wavelength conversion is also performed.

By the way, as the crystal of the above-mentioned nonlinear optical material, a biaxial crystal such as KTP is often used. J. Appl. Phys. Vol. 55,
p.65 (1984), Yao et al. describe in detail the phase matching method for KTP, which is a biaxial crystal. The phase matching method in the biaxial crystal described herein will be explained below. As shown in FIG. 4, θ is the angle between the light traveling direction and the optical axis Z of the crystal, and φ is the angle of the light traveling direction from the X axis in a plane including the optical axes X and Y. Here, the fundamental wave and the second wave when incident at an arbitrary angle.
Each crystal's refractive index for harmonics is

0004

[Math 1]

[0005] and the optical axis of the fundamental wave and the second harmonic is
, the refractive index of the crystal for the polarized light components in the Y and Z directions, respectively.

[0006]

[Math 2]

[0007] Next, kX = sin θ・cos φ kY = sin θ・sin φ kZ = cos θ
When

[0008]

[Math 3]

[0009]

[Math 4]

The solutions of (Equation 3) and (Equation 4) above serve as phase matching conditions.

[0011]

[Math 5]

##EQU3## The solutions of equations (3) and (4) are:

[0013]

[Math 6]

[0014]

[Math. 7]

(The double sign is + when i=1, - when i=2
).

[0016] Here,

[0017]

[Math. 8]

When the following conditions are satisfied, the fundamental wave and the second
Phase matching is achieved with the harmonics, which is referred to as type I phase matching. Also,

[0019]

[Math. 9]

Even when the following conditions are satisfied, phase matching is achieved between the fundamental wave and the second harmonic, and this is generally referred to as type II phase matching.

By the way, when type II phase matching is achieved using a biaxial crystal as described above, the fundamental wave incident on the crystal senses two refractive indices with respect to the crystal. For example, when using the nonlinear optical constant d24 of the crystal, that is, as shown in FIG.
A fundamental wave 11 that is linearly polarized in a direction tilted at 45 degrees toward the axis (that is, has a linearly polarized component in the Y-axis direction and a linearly polarized component in the Z-axis direction) is incident, and a second wave that is linearly polarized in the Y-axis direction is generated. When extracting harmonic 12, fundamental wave 11 has a refractive index

0
022]

[Math. 10]

In other words, the refractive index felt by the polarized light component in the Z-axis direction and the refractive index

[0024]

[Math. 11]

In other words, both the traveling direction of the light and the refractive index felt by the polarized light component in the Y' direction perpendicular to the Z axis are felt.

Note that when the crystal 10 is cut as shown in FIG. 5, strictly speaking, the fundamental wave 11 is linearly polarized in the Y' direction (direction tilted from the Y axis toward the X axis) and in the Z axis direction. Although the second harmonic wave 12 is extracted in a state polarized in the Y' direction, in practical terms, it may be considered as described above.

As described above, when the fundamental wave senses two refractive indices, the following phase difference Δ occurs between the polarized light components for each refractive index.

[0028]

[Math. 12]

When this phase difference Δ occurs, the linear polarization direction of the fundamental wave changes in accordance with the value of the phase difference Δ. When the linear polarization direction of the fundamental wave changes in this way, the angle of the fundamental wave polarization direction with respect to the optical axis of the nonlinear optical material crystal deviates from the predetermined angle that obtains the maximum wavelength conversion efficiency, and the output of the second harmonic wave decreases. become. The output fluctuation of the second harmonic generated in this way has periodicity, and this is due to the temperature dependence of each parameter in the above equation, as shown in Figure 6.
There are temperature dependent ones that appear as shown in Figure 7, and crystal length dependent ones that appear as shown in Figure 7.

Therefore, in order to obtain the maximum second harmonic output, it is necessary to optimally control the crystal temperature or optimally adjust the crystal length. For example, U.S. Patent No. 4,91
The specification of No. 3,533 shows an example of an optical wavelength conversion device that adopts the former method, while the specification of Japanese Patent Application Laid-open No. 1-1527
No. 81 and No. 1-152782 disclose an example of an optical wavelength conversion device that adopts the latter method. In particular, in the above-mentioned Japanese Patent Application Laid-Open No. 1-152781, a crystal of a nonlinear optical material whose thickness gradually changes along the direction intersecting the fundamental wave optical path is used, while this crystal is moved in the above direction. Structures with varying crystal lengths are shown.

[0031]

[Problem to be Solved by the Invention] However, if the crystal length is arbitrarily set and the crystal temperature is controlled, the maximum second
Attempting to obtain harmonic output requires a large temperature adjustment stroke, which increases the size of the temperature control power supply and heat sink, leading to an increase in the size and cost of the optical wavelength conversion device.

On the other hand, if the temperature is adjusted so that the crystal temperature is constant and the length of each crystal is adjusted to the optimum value for that temperature, the tolerance of the crystal length is extremely small. Therefore, in reality, it is very difficult to obtain the maximum second harmonic output. Even if such a thing were possible, in this case it would be necessary to strictly measure and adjust the crystal length, which would significantly increase the cost of the optical wavelength conversion device.

Furthermore, in the structure shown in JP-A-1-152781, it is necessary to take a relatively large amount of movement of the nonlinear optical material crystal, so adjusting the linear polarization direction of the fundamental wave changes the position of the resonator mode. may shift.

The present invention has been made in view of the above-mentioned circumstances, and provides type II
Using a crystal of nonlinear optical material that achieves phase matching of
It is an object of the present invention to provide an optical wavelength conversion device that can obtain a high-output second harmonic, does not shift the position of a resonator mode, and can be formed compactly and inexpensively.

[0035]

[Means for Solving the Problems] The optical wavelength conversion device according to the present invention converts a laser beam as a fundamental wave obtained by pumping a solid laser medium into a nonlinear optical material, like the laser diode pumped solid-state laser described above. In an optical wavelength conversion device that enters a crystal, performs type II phase matching with this fundamental wave, and emits a second harmonic, ◆As a solid laser medium, it has birefringence and intersects with the optical path of the fundamental wave therein. While the thickness gradually changes along the direction, ◆ it is characterized by being provided with means for relatively moving the solid-state laser medium and the pumping source along the above-mentioned direction.

[0036]

[Operation and Effects of the Invention] When a solid laser medium having birefringence as described above is used, similar to the phase difference Δ generated in the crystal of the nonlinear optical material shown in equation (12) above,
A phase difference Δ' occurs within the laser medium. At this time, when the solid-state laser medium with the above shape is moved in the above direction with respect to the pumping source, the effective optical path length of this solid-state laser medium changes and the phase difference Δ' changes, so the basic The polarization direction of the wave also changes. Therefore, by appropriately adjusting the amount of this movement, it is possible to set the polarization direction of the fundamental wave to the nonlinear optical material crystal so as to obtain the maximum wavelength conversion efficiency. It becomes possible to obtain a laser beam.

Furthermore, since solid laser media generally have greater birefringence than nonlinear optical material crystals,
Even if the degree of change in the thickness (that is, the angle of inclination of the end face) is smaller, and even if the amount of movement is relatively small, the phase difference Δ' can be sufficiently changed. Therefore, in the device of the present invention, the polarization direction of the fundamental wave can be optimally set with a small adjustment amount that does not shift the position of the resonator mode.

The device of the present invention having the above structure does not require a large and highly accurate temperature control means, nor does it require precise measurement or adjustment of the crystal length, so it can be formed in a small size and at low cost.

Furthermore, in the device of the present invention, the amount of movement is adjusted to cause oscillation at an appropriate thickness of the solid laser medium, and the amount of birefringence within the resonator is adjusted, so that the fundamental wave becomes multimode. Even in the case of oscillation, generation of mode competition noise can be suppressed.

[0040]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be explained in detail below based on embodiments shown in the drawings. FIG. 1 shows an optical wavelength conversion device according to a first embodiment of the present invention. The laser diode pumped solid-state laser with this optical wavelength conversion device is
A semiconductor laser (phased array laser) 14 that emits a laser beam 13 as pumping light, a collimator lens 15a that converts the diverging laser beam 13 into parallel light, and a collimator lens 15a that focuses the laser beam 13 that has passed through this lens 15a. A condensing lens 15b, a YVO4 rod (hereinafter referred to as Nd:YVO4 rod) 16, which is a solid laser rod doped with neodymium (Nd), and this Nd:YVO4 rod 1.
Resonator mirror 17 arranged on the front side of 6 (right side in the figure)
And this resonator mirror 17 and Nd:YVO4 rod 1
6 and a KTP crystal 10 disposed between them. Each of the elements described above is mounted and integrated in a common housing (not shown). Furthermore, phased array laser 1
4 is temperature-controlled to a predetermined temperature by a Peltier element and a temperature control circuit (not shown).

As this phased array laser 14, one that emits a laser beam 13 having a wavelength λ1 = 809 nm is used. On the other hand, the Nd:YVO4 rod 16 has a wavelength λ2 = 1064 due to neodymium atoms being excited by the laser beam 13.
A laser beam 11 of nm wavelength is emitted.

A laser beam 11 with a wavelength of 1064 nm is attached to the light incident side end surface 16a of the Nd:YVO4 rod 16.
is reflected well (reflectance of 99.9% or more), and the wavelength is 80%.
A coating 18 is applied that allows a 9 nm pumping laser beam 13 to pass through it well (transmittance of 99% or more). In addition, a laser beam 1 with a wavelength of 1064 nm is provided on the light output side end surface 16b of the Nd:YVO4 rod 16.
A non-reflective coating 9 that allows good transmission of light (transmittance of 99.9% or more) is applied. On the other hand, the resonator mirror 17
The surface 17a on the side of the KTP crystal 10 is formed into a part of a spherical surface, and a laser beam 11 with a wavelength of 1064 nm and a laser beam 13 with a wavelength of 809 nm are emitted on the surface.
A coating 19 is applied which allows good reflection of the wavelength 532 nm and good transmission of the second harmonic 12 having a wavelength of 532 nm, which will be described later. Therefore, the laser beam 11 with a wavelength of 1064 nm is confined between the surfaces 16a and 17a, causing laser oscillation. Although not particularly shown, the end face of the KTP crystal 10 on the Nd:YVO4 rod 16 side is also coated with a coating similar to the anti-reflection coating 9 described above.

This laser beam 11 is incident on a KTP crystal 10, which is a nonlinear optical material, and is converted into a second harmonic wave 12 having a wavelength of 1/2, that is, 532 nm. Since the surface 17a of the resonator mirror 17 is coated with the coating 19 as described above, almost only the second harmonic 12 is extracted from the resonator mirror 17.

[0044] Nd:YVO4 rod 1 with birefringence
6 is formed into a wedge shape whose thickness gradually changes along the direction intersecting the optical path of the laser beam 11 (vertical direction in the figure). Further, the holding member 21 that holds it has a plurality of guide rods 2 extending vertically in the figure.
2 is inserted. The lower ends of these guide rods 22 are fixed to a fixed base 23, and the holding member 21 is movable in the vertical direction along the guide rods 22. A precision screw 24 is rotatably held on the fixed base 23, and the tip of the precision screw 24 is screwed into the holding member 21. Therefore, when the precision screw 24 is rotated, the holding member 21 is screwed forward and backward in the vertical direction, and the Nd:YVO
4. The rod 16 moves up and down.

As shown in detail in FIG. 2, the KTP crystal 10, which is a biaxial crystal, is cut by rotating the YZ plane by 24° around the Z axis. In this configuration, when the linear polarization direction of the laser beam 11 indicated by the arrow P and the Z axis form an angle of 45°, a large nonlinear optical constant d2 is obtained.
4 is used, and the laser beam 1 as the fundamental wave
Good type II phase matching is achieved between the first and second harmonics 12, and the second harmonics 12 of maximum intensity are obtained.

However, if the above-mentioned phase difference Δ occurs in the laser beam 11 due to the KTP crystal 10, the linear polarization direction of the laser beam 11 changes according to the value, so that the above-mentioned 45° angle is not achieved. There may be things that cannot be done. Therefore, when the above-mentioned precision screw 24 is rotated clockwise or counterclockwise to move the Nd:YVO4 rod 16 up and down by minute amounts, the optical path length of the laser beam 11 in the rod 16 changes, and the Since the phase difference Δ' changes, the linear polarization direction changes. By finely adjusting the linear polarization direction of the laser beam 11 in this way, a state can be obtained in which the linear polarization direction forms an angle of 45° with respect to the Z axis, and at that time, the second harmonic 12 with the maximum intensity can be obtained. Can be done.

[0047] The amount of birefringence of Nd:YVO4 is ne
-no = 0.2079, and the amount of birefringence nZ of KTP
-nY =0.0853, it is significantly larger. Therefore, compared to the case where the end face of the KTP crystal 10 is formed obliquely and then moved, the Nd:YVO4
The slope of the end surface 16b of the rod 16 may be relatively gentle;
Further, the amount of movement thereof is also relatively small. Therefore, in this device, the polarization direction of the laser beam 11 can be optimally set with a small adjustment amount that does not shift the position of the resonator mode.

Next, referring to FIG. 3, a second embodiment of the present invention will be described. Note that in FIG. 3, elements that are equivalent to those in FIG. 1 are given the same numbers, and redundant explanations thereof will be omitted (the same applies hereinafter).

In this second embodiment, a phased array laser 14 is placed close to the Nd:YVO4 rod 16, and the laser beam 13 emitted therefrom is
is directly incident on this rod 16. Also KTP crystal 10
A coating 19 similar to that of the device in FIG. 1 is applied to the front end surface 10b of the KTP crystal 10 and the Nd:
The YVO4 rod 16 constitutes a resonator. The light incident side end face 10a of the KTP crystal 10 is cut obliquely, and a wedge-shaped Nd:YVO4 is formed on this end face 10a.
A rod 16 is joined. And KTP crystal 10
is attached to a holding member 21 similar to that of the device in FIG. 1, and by rotating a precision screw 24,
It can move in the vertical direction in the figure together with the YVO4 rod 16. Even in this configuration, the linear polarization direction can be adjusted by moving the Nd:YVO4 rod 16 up and down.

Next, a third embodiment of the present invention will be described with reference to FIG. The device of this third embodiment is different from the device of FIG. 1 in the resonator structure on the second harmonic output side. That is, in this device, the front end surface 10b of the KTP crystal 10 is shaped to form part of a spherical surface, and a coating 19 similar to that of the device shown in FIG. 1 is applied to the surface. Compared to the device shown in FIG. 1, the device of this embodiment can reduce the number of parts by one, so it is advantageous in achieving reductions in size, weight, and cost.

In the two embodiments described above, the phased array laser 14, which is the pumping source, is fixed and the Nd:YVO4 rod 16 is moved; however, on the contrary, the Nd:YVO4 rod 16 and phased array laser 1.
4 (in the devices of FIGS. 1 and 8, the lenses 15a and 15b as well) may be moved.

Further, the solid laser medium used in the present invention is Nd:YVO4 in the above-described embodiments.
The present invention is not limited to this, and other known materials (for example, direct compound laser crystals such as LNP, NAB, and NPP) may be used.

[Brief explanation of the drawing]

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

[Fig. 2] A schematic perspective view showing the main parts of the device of the first embodiment.

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

FIG. 4: Angle θ between the fundamental wave traveling direction and the optical axis Z inside the crystal related to the present invention,
A schematic diagram illustrating the angle φ between the fundamental wave traveling direction and the optical axis X.

[Figure 5] Schematic diagram for explaining the relationship between the optical axis of a nonlinear optical material and the linear polarization direction of the fundamental wave

[Figure 6] Graph showing periodic fluctuations of second harmonic output depending on temperature changes

[Figure 7] Graph showing periodic fluctuations of second harmonic output depending on crystal length

FIG. 8 is a side view of a device according to a third embodiment of the present invention.

[Explanation of symbols]

9, 18, 19 Coating 10 KTP crystal 11 Laser beam (fundamental wave) 12 Second harmonic 13 Laser beam (pumping light) 14
Phased array laser 16 Nd:YV
O4 Rod 17 Resonator mirror 21 Holding member 22 Guide rod 23 Fixing member 24 Precision screw

Claims (1)

[Claims] Claim 1: A laser beam as a fundamental wave obtained by pumping a solid laser medium is incident on a crystal of a nonlinear optical material, type II phase matching is performed with this fundamental wave, and a second harmonic is emitted. In the optical wavelength conversion device, the solid laser medium is birefringent and has a thickness that gradually changes along the direction intersecting the fundamental wave optical path. An optical wavelength conversion device characterized in that the device is provided with means for relatively moving the .