JPH04318529A - Light wavelength converting device - Google Patents

Abstract

PURPOSE:To prevent the linear polarizing direction of a fundamental wave from deviating from a specific angle to the optical axis of the biaxial crystal, which is arranged in the resonator of a solid laser and has nonlinear optical effect, owing to the phase difference of the fundamental wave in the crystal to cause a decrease in wavelength conversion efficiency, in the light wavelength conversion device which converts a solid laser oscillation beam as the fundamental wave into a 2nd higher harmonic. CONSTITUTION:An Nd:YVO4 rod 16 as a solid laser medium is held on a rotary shaft 30, which is coupled with the rotary shaft 32 of an adjusting knob 33. The Nd:YVO4 rod 16 is rotated by operating the adjusting knob 33 to vary the optical path length of the laser beam 11 in the crystal.

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. 3, 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 the direction of arrow P tilted at 45° 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 the fundamental wave 11 is linearly polarized in the Y-axis direction. When extracting the second harmonic 12, the fundamental wave 11 has a refractive index

[0022]

[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. 4, strictly speaking, the fundamental wave 11 is linearly polarized in the Y' direction (direction tilted from the Y axis toward the X axis) and 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 5.
There are two types: one is temperature dependent, as shown in Figure 6, and the other is crystal length dependent, as shown in Figure 6.

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.

[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.

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 the maximum second harmonic output and can be formed compactly and inexpensively.

[0034]

[Means for Solving the Problems] The optical wavelength conversion device according to the present invention uses a laser as a fundamental wave obtained by pumping a solid-state laser medium arranged in a resonator, like the laser diode pumped solid-state laser described above. In an optical wavelength conversion device that makes a beam incident on a crystal of a nonlinear optical material, performs type II phase matching with this fundamental wave, and outputs a second harmonic, ◆A laser medium with birefringence is used as the solid laser medium. the solid-state laser medium is rotated about an axis that intersects the optical path of the fundamental wave therein, thereby changing the optical path length of the fundamental wave in the laser medium. It is something to do.

[0035]

[Operation and Effects of the Invention] When a solid laser medium with birefringence is used as described above, the phase difference Δ generated in the crystal of the nonlinear optical material shown in equation (12) above is similar to the phase difference Δ in the laser medium. A phase difference Δ' occurs. At this time, when this laser medium is rotated, the optical path length L' in the solid-state laser becomes
changes, and the phase difference Δ' changes. Therefore, by rotating the laser medium to change and adjust Δ' to compensate for the phase difference Δ that occurs within the crystal of the nonlinear optical material, the polarization direction of the fundamental wave with respect to the crystal of the nonlinear optical material can be changed.
It can be set so as to obtain the maximum wavelength conversion efficiency, and it becomes possible to obtain a high-intensity wavelength-converted short wavelength laser beam.

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 strict 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, by adjusting the amount of rotation and adjusting the phase difference Δ' in the solid laser medium, even when the fundamental wave is oscillating in multiple modes, mode competing noise can be suppressed. The occurrence can be suppressed.

[0038]

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 an embodiment of the present invention. As an example, this optical wavelength conversion device is incorporated into a laser diode pumped solid-state laser. This laser diode pumping solid-state laser includes a semiconductor laser (phased array laser) 14 that emits a laser beam 13 as pumping light, a collimator lens 15a that converts the laser beam 13, which is a diverging light, into a parallel beam, and this lens 15a. A condenser lens 15b that focuses the passed laser beam 13, and a YV which is a solid laser rod doped with neodymium (Nd) and has birefringence.
O4 rod (hereinafter referred to as Nd:YVO4 rod) 16 and the rear side of this Nd:YVO4 rod 16 (
The resonator mirror 18 arranged on the left side in the figure, and the Nd:Y
KTP crystal 10 placed on the front side of the VO4 rod 16
It consists of Each of the elements described above is mounted and integrated in a common housing (not shown). The temperature of the phased array laser 14 is 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 fundamental wavelength λ2 = 10 due to neodymium atoms being excited by the laser beam 13.
A linearly polarized laser beam 11 of 64 nm is emitted.

The surface 18a of the resonator mirror 18 on the Nd:YVO4 rod 16 side is shaped as a part of a spherical surface, and the laser beam 11 with a wavelength of 1064 nm is well reflected on the surface (reflectance of 99.9). % or more), wavelength 809
A coating 20 is applied which allows the pumping laser beam 13 of nm wavelength to pass through it well (transmittance of 99% or more). On the other hand, the front end surface 10a of the KTP crystal 10 is also shaped to form a part of a spherical surface, and the front end surface 10a of the KTP crystal 10 has a shape that forms a part of a spherical surface.
The laser beam 13 with a wavelength of 1064 nm and the laser beam 11 with a wavelength of 1064 nm are well reflected, and the laser beam 11 with a wavelength of 1064 nm is well reflected.
A coating 19 is applied which allows good transmission of the second harmonic 12 of 32 nm. Therefore, the wavelength is 1064n
The laser beam 11 of m is confined between the surfaces 18a and 10a, causing laser oscillation.

This laser beam 11 is wavelength-converted by a KTP crystal 10, which is a nonlinear optical material, into a second harmonic wave 12 having a wavelength of 1/2, that is, 532 nm. K.T.
Since the end face 10a of the P crystal 10 is coated with the coating 19 as described above, almost only the second harmonic 12 is extracted from the KTP crystal 10.

As shown in detail in FIG. 2, the KTP crystal 10, which is a biaxial crystal, is cut on a plane obtained by rotating the YZ plane by 24° around the Z axis. In this configuration,
When the linear polarization direction of the laser beam 11 shown by the arrow P and the Z axis form an angle of 45°, the large nonlinear optical constant d24 is used, and the laser beam 11 as the fundamental wave and the second harmonic 12 are Good type II phase matching is achieved between the two, and the second harmonic 12 of maximum intensity is 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 will change according to the value, so if it is left as it is, the above-mentioned 45° angle will change. It is possible that this cannot be achieved. Hereinafter, the point of realizing this 45° angle will be explained.

The Nd:YVO4 rod 16 is fixed to a rotating shaft 30 extending substantially perpendicular to the optical path of the laser beam 11 therein, and this rotating shaft 30 is rotatably held on a holding table 31. An adjustment knob 33 is attached to the holding base 31 so as to be rotatable about a rotating shaft 32. The rotating shaft 32 is connected to the rotating shaft 30 via a reduction gear train (not shown). Therefore, by rotating the adjustment knob 33, the Nd:YVO4 rod 16 can be rotated about the rotation shaft 30.

When the Nd:YVO4 rod 16 rotates in this direction, the optical path length of the laser beam 11 within the Nd:YVO4 changes. When this optical path length changes,
The phase difference Δ' of the laser beam generated in Nd:YVO4 having birefringence changes, and Δ+Δ' combined with the phase difference Δ of the laser beam 11 due to the KTP crystal 10 changes, and the linear polarization thereof changes. Therefore, Nd
:YVO4 By rotating the rod 16 minutely, it is possible to realize the above-mentioned angle of 45° and obtain the second harmonic wave 12 with maximum intensity. Other solid laser media with birefringence include LNP, NAB, and NP.
P etc. may also be used.

[Brief explanation of the drawing]

FIG. 1: Side view of an apparatus according to an embodiment of the present invention.

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

[Fig. 3] Angle θ between the fundamental wave traveling direction and the optical axis Z inside the crystal related to the present invention, and an angle φ between the fundamental wave traveling direction and the optical axis X
Schematic diagram illustrating

[Figure 4] 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 5] Graph showing periodic fluctuations of second harmonic output depending on temperature changes

[Figure 6] Graph showing periodic fluctuations of second harmonic output depending on crystal length

[Explanation of symbols]

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 18 Resonator mirrors 30, 32 Rotating shaft 31 Holding base 33 Adjustment knob

Claims (1)

[Claims] Claim 1: A laser beam as a fundamental wave obtained by pumping a solid laser medium placed in a resonator is incident on a crystal of a nonlinear optical material, and type II phase matching is achieved with this fundamental wave. In an optical wavelength conversion device that emits a second harmonic, as the solid laser medium,
Means for changing the optical path length of the fundamental wave in the laser medium by using a laser medium having birefringence and rotating the solid-state laser medium around an axis that intersects the optical path of the fundamental wave therein. An optical wavelength conversion device characterized by being provided with.