JPH04318527A - 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 biaxial crystal owing to the phase difference of the fundamental wave in the biaxial crystal having nonlinear optical effect to cause a decrease in wavelength conversion efficiency, in the light wavelength conversion device which converts the fundamental wave into a 2nd higher harmonic by the crystal. CONSTITUTION:The KTP crystal 10 is held on a rotary shaft 30, which is coupled with the rotary shaft 32 of an adjusting knob 33. The KTP crystal 10 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

2. Description of the Related Art Conventionally, various attempts have been made to convert the wavelength of laser light (shorten the wavelength) by utilizing second harmonic generation using nonlinear optical materials. Specifically, as an optical wavelength conversion element that performs wavelength conversion in this way, for example, "
"Fundamentals of optoelectronics" A. Bulk crystal types such as those shown on pages 200 to 204 of YARIV, translated by Kunio Tada and Takeshi Kamiya (Maruzen Co., Ltd.) are well known.

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 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. 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]

Note that the crystal length L is the effective length, that is, the optical path length of the fundamental wave in the crystal. When this phase difference Δ occurs, the linear polarization direction of the fundamental wave changes depending on 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 includes the temperature dependence that appears as shown in Figure 6 due to the temperature dependence of each parameter in the above equation. and crystal length dependence as shown in FIG.

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] As described above, the optical wavelength conversion device according to the present invention makes a fundamental wave incident on a crystal of a nonlinear optical material, performs type II phase matching with this fundamental wave, and converts the second harmonic into a second harmonic. In an optical wavelength conversion device that emits ◆
The present invention is characterized in that means is provided for rotating the crystal about an axis in a direction that intersects the optical path of the fundamental wave therein, thereby changing the optical path length of the fundamental wave in the crystal.

[0035]

[Operation] The above fundamental wave optical path length is
), and if the value of L changes, the phase difference Δ changes. When the value of the phase difference Δ changes in this way, the polarization direction of the fundamental wave changes accordingly. Therefore, by rotating the nonlinear optical material crystal as described above,
The polarization direction of the fundamental wave can be adjusted to obtain maximum wavelength conversion efficiency.

[0036]

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. 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 condensing lens 15b that focuses the laser beam 13 that has passed, a YVO4 rod (hereinafter referred to as Nd:YVO4 rod) 16, which is a solid laser rod doped with neodymium (Nd), and the front of the Nd:YVO4 rod 16. Side (right side in the figure)
a resonator mirror 17 arranged in the resonator mirror 17;
and the Nd:YVO4 rod 16.
It consists of crystal 10. 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 wavelength λ2 = 1064 due to neodymium atoms being excited by the laser beam 13.
A linearly polarized 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 20 is applied that allows a 9 nm pumping laser beam 13 to pass through it well (transmittance of 99% or more). On the other hand, the surface 17a of the resonator mirror 17 on the KTP crystal 10 side is shaped to form a part of a spherical surface, and the laser beam 13 with a wavelength of 809 nm and the laser beam 13 with a wavelength of 1
A coating 19 is applied which allows a laser beam 11 of 0.064 nm to be well reflected, and a second harmonic wave 12 of a wavelength of 532 nm, which will be described later, to be well transmitted. Therefore, the laser beam 11 with a wavelength of 1064 nm is confined between the surfaces 16a and 17a, causing laser oscillation.

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.

As shown in detail in FIG. 2, in the KTP crystal 10, which is a biaxial crystal, the incident direction of the laser beam 11, which is the fundamental wave, and the X axis typically form an angle of φ=24°. Further, the incident direction and the Z axis are arranged so as to form an angle of θ=90°. In this configuration, when the linear polarization direction of the laser beam 11 shown by arrow P and the Z axis form an angle of 45°, a large nonlinear optical constant d2
4 is used, and the laser beam 1 as the fundamental wave
Good type II phase matching is achieved between the second harmonic 12 and the second harmonic 12, and the second harmonic 12 of maximum intensity is obtained.

However, if the above-mentioned phase difference Δ is generated in the laser beam 11 by 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 KTP crystal 10 is fixed to a rotating shaft 30 extending parallel to its Z axis, 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 KTP crystal 10 can be rotated about the rotating shaft 30.

When the KTP crystal 10 rotates in this direction,
The optical path length L of the laser beam 11 there changes. When the optical path length L changes in this way, the phase difference Δ of the laser beam 11 due to the KTP crystal 10 changes, and the linear polarization direction thereof changes. Therefore, by rotating the KTP crystal 10 in small increments, it is possible to realize the above-mentioned angle of 45° and obtain the second harmonic wave 12 with maximum intensity.

In this embodiment, the KTP crystal 10 is rotated in the direction in which the angle φ changes, but the KTP crystal 10 is rotated in the direction in which the angle θ changes, or in the direction in which both the angles φ and θ change. may be rotated. Angle φ and θ
It is particularly preferable to rotate the KTP crystal 10 in a direction in which both of .DELTA. and .DELTA.

Further, the amount of rotation of the KTP crystal 10 needs to be adjusted within the range of angles φ and θ that allow phase matching between the laser beam 11 and the second harmonic 12. The standard angle is θ
=90°, φ=24°, the phase matching angle tolerance range Δθ, Δφ is L1/2 ・Δθ=60mrad ・cm1/2 L・
Δφ=17 mrad·cm (However, the unit of L is cm). Therefore, |Δθ|≦60/L1/2 (mrad)|Δφ|≦
θ within the range satisfying 17/L (mrad)
, φ may be adjusted. As a specific example, standardly L=0
．． When a 5 cm 2 KTP crystal 10 is used, adjustment is possible within the range of |Δθ|≦85 mrad |Δφ|≦34 mrad.

In this example, KTP crystal 1
The laser beam 1 is directed obliquely to the light incident end surface 10a of
1 is incident, and the laser beam 11 and the second harmonic 12 are emitted obliquely from the light emitting end face 10b.
The phase difference Δ can be changed significantly with a small crystal rotation angle.

Next, a second embodiment of the present invention will be described with reference to FIG. Note that in FIG. 3, elements that are equivalent to those already described are given the same numbers, and a redundant explanation of them will be omitted.

In this embodiment, two KTP crystals 10, 10 are provided, and as in the first embodiment, they each rotate axes of rotation 30, 30 by a rotation mechanism connected to the operation of the adjustment knob 33. Rotate around the center. In this rotation mechanism, the KTP crystals 10, 10, the rotation shaft 30,
It is configured to rotate while maintaining a symmetrical relationship with respect to a plane H extending perpendicularly to the optical axes of the lenses 15a and 15b in the middle of the lenses 15a and 15b.

If the KTP crystals 10, 10 are rotated as described above, the second radiation emitted from the KTP crystal 10 on the right side of the figure
The optical path of the harmonic wave 12 remains constant regardless of the crystal rotation angle. With this configuration, the second harmonic 12 can always be incident at a constant angle on the device that uses it.

[0050]

As described above in detail, the optical wavelength conversion device of the present invention is capable of obtaining the second harmonic of maximum intensity by a simple configuration in which a crystal of a nonlinear optical material is rotated. As described above, since the present device does not require a large and highly accurate temperature control means, it can be made compact and inexpensive. Furthermore, since the device of the present invention does not require strictly setting the length of the crystal of the nonlinear optical material to a predetermined value, there is no need to strictly measure or adjust the crystal length, and from this point of view, it can be formed at low cost.

Furthermore, in the optical wavelength conversion device of the present invention, by selecting an appropriate rotation angle of the crystal of the nonlinear optical material, even when the fundamental wave is oscillated in multiple modes, the occurrence of mode competition noise can be prevented. can be suppressed.

[Brief explanation of drawings]

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

[Fig. 2] A 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 is a schematic diagram illustrating the angle θ between the fundamental wave traveling direction and the optical axis Z and the angle φ between the fundamental wave traveling direction and the optical axis X inside the crystal related to the present invention.

[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

[Explanation of symbols]

10 KTP crystal 11 Laser beam (fundamental wave) 12 Second harmonic 16 Nd:YVO4 rod 17 Resonator mirror 30, 32 Rotation shaft 31 Holding base 33 Adjustment knob

Claims (1)

[Claims] Claim 1: A fundamental wave is made incident on a crystal of a nonlinear optical material, type II phase matching is performed with this fundamental wave, and a second
In an optical wavelength conversion device that emits harmonics, means is provided for rotating the crystal around an axis that intersects the optical path of the fundamental wave therein, thereby changing the optical path length of the fundamental wave in the crystal. Features of optical wavelength conversion device.