JPH04330425A - Light wavelength converter - Google Patents

Abstract

PURPOSE:To obtain the small-sized, inexpensive light wavelength conversion device which can obtain the largest 2nd harmonic output as to a light wavelength conversion device which projects a 2nd higher harmonic by making the basic wave incident on crystal of a nonlinear optical material and matching the phase of the basic wave with a type II. CONSTITUTION:Two crystal blocks 10 and 10' which are made of the same material and same length each other are used as the crystal of the nonlinear material and arranged so that the equal optical axes (e.g. Z axis) deviate from each other by 90 deg..

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] 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. [0003] Also, J. Appl. Phys. Vol.
．． 55, p65 (1984) by Yao et al.
The contents regarding the phase matching method of KTP, which is an axial crystal, are described in detail. 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 refractive index of the crystal for the fundamental wave and the second harmonic when incident at any angle is ◆000
4] [Equation 1] [0005], and the optical axis X of the fundamental wave and the second harmonic
The refractive index of the crystal for polarized light components in the , Y, and Z directions is expressed as follows. Next, kX = sin θ・cos φ kY = sin θ・sin φ kZ = cos θ
Then, ##EQU3## The solutions of the above (Equation 3) and (Equation 4) become the phase matching condition. B1 =-kX 2 (b1 +c1)-kY 2
(a1 + c1 ) - kZ 2 (a1 + b1 ) C1 = kX 2 b1 c1 + kY 2 a1
c1 +kZ 2a1 b1 B2 =-kX 2 (b2 +c2)-kY 2
(a2 + c2 ) - kZ 2 (a2 + b2 ) C2 = kX 2 b2 c2 + kY 2 a2
c2 +kZ 2a2 b2 [0011] [Formula 5] [0012] The solutions of equations (3) and (4) are as follows. The sign is + when i=1 and - when i=2.
). [0016] Here, when the condition ◆ [0017] [Equation 8] is satisfied, the fundamental wave and the second
Phase matching is achieved with the harmonics, which is referred to as type I phase matching. Also, when the condition ◆ [Equation 9] is satisfied, phase matching is achieved between the fundamental wave and the second harmonic, and this is generally referred to as type II phase matching. ing. 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 the harmonic wave 12, the fundamental wave 11 is: ◆ [Equation 10] In other words, the refractive index felt by the polarized light component in the Z-axis direction, and ◆ [Equation 11] In other words, the progress of the light Both the direction 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. ##EQU12## When this phase difference Δ occurs, a phenomenon of periodic fluctuation of the second harmonic output occurs. There are two types of dependence: one is temperature dependence, as shown in FIG. 6, which is derived from the temperature dependence of each parameter in the above equation, and the other is 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. Problem to be Solved by the Invention However, by setting the crystal length arbitrarily and controlling the crystal temperature, 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. [Means for Solving the Problems] The optical wavelength conversion device according to the present invention uses two crystals of the same nonlinear optical material, and after making them equal in length,
It is characterized in that the optical axes thereof are arranged in directions shifted by 90° from each other. [Operation] When the orientations of both the crystals are shown in accordance with the example of FIG. 5, they are as shown in FIG. That is, one crystal 10 and a crystal 10' having the same length L are arranged such that their Z axes are shifted by 90 degrees from each other, for example. In this case, the phase difference Δ in the crystal 10 is ◆ [Equation 13], and the phase difference Δ' in the other crystal 10' is as follows: [Equation 14] Therefore, the phase difference between the crystals 10 and 10' becomes Δ+Δ'=0. Note that even if the lengths L of the two crystals 10 and 10' are slightly different from each other, the phase difference (Δ+Δ
') approaches 0, the period of output fluctuation of the second harmonic with respect to temperature becomes very long, and even in that case, it is possible to obtain a second harmonic output close to the maximum. [Embodiments] The present invention will be explained in detail below based on embodiments shown in the drawings. FIG. 2 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 condensing lens 15 that focuses the laser beam 13 that is a diverging light, and a neodymium (N
d) is a doped solid state laser rod YV
O4 rod (hereinafter referred to as Nd:YVO4 rod) 16 and the front side of this Nd:YVO4 rod 16 (
resonator 17 arranged on the right side in the figure) and resonator 17 and N
d: YVO4 Consisting of KTP crystals 10 and 10' arranged between the rod 16. Each element 10 to 17 mentioned above
are 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 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). On the other hand, the surface 17a of the resonator 17 on the KTP crystal 10, 10' side is shaped to form part of a spherical surface, and the laser beam 11 with a wavelength of 1064 nm and the laser beam 13 with a wavelength of 809 nm are well reflected on that surface. ,
A coating 19 is applied that allows a second harmonic wave 12 having a wavelength of 532 nm, which will be described later, to be transmitted satisfactorily. 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 KTP crystals 10 and 10', which are nonlinear optical materials, and the wavelength becomes 1/1.
The wavelength is converted to the second harmonic 12 of 2 or 532 nm. Since the surface 17a of the resonator 17 is coated with the coating 19 as described above, almost only the second harmonic 12 is extracted from the resonator 17. As shown in detail in FIG. 1, the KTP crystal 10, which is a biaxial crystal, is cut with the YZ plane rotated by 24 degrees around the Z axis (the same goes for the KTP crystal 10').
The linear polarization direction of the laser beam 11 indicated by the arrow P and the Z axis are arranged at an angle of 45°. And the other KTP crystal 10' has its Z axis KT
It is arranged in a direction shifted by 90 degrees from the Z axis of the P crystal 10. Further, the crystal length L of the KTP crystal 10' is made equal to the crystal length L of the KTP crystal 10. For the laser beam 11, K
By arranging the TP crystals 10 and 10', type II phase matching is achieved between the laser beam 11 as the fundamental wave and the second harmonic 12. Also KTP crystal 10 and K
By arranging the TP crystals 10' as described above, the phase differences Δ and Δ' generated in the laser beam 11 are canceled out, making it possible to obtain the second harmonic 12 with the maximum output. The reason for this is as described in detail above. In order to make the crystal lengths L of the KTP crystals 10 and 10' exactly equal, for example, as shown in FIG. After polishing the crystals 10a and 10b so that they are exactly parallel to each other (for example, within a parallelism error of 10 to 20 seconds), the two crystals 10 and 10' are separated and the polished surfaces are aligned as shown in Figure (2). Both crystals 10, 10 are in close contact with each other.
' should be fixed. Furthermore, as shown in FIG. 9(1), the two surfaces 10a and 10b of one KTP crystal 10A are polished in parallel in the same manner as above, and then the crystal is polished on the surface 10B passing through both polished surfaces 10a and 10b. Cut 10A into two KTP crystals 1
0 and 10' are obtained, and then both crystals 10 and 10' are fixed so that the polished surfaces are in close contact with each other as shown in FIG. 2 (2). Next, a second embodiment of the present invention will be described with reference to FIG. The optical wavelength conversion device of this embodiment is also incorporated into a laser diode pumped solid-state laser. In this example, Nd:YVO4
The rod 16 and the KTP crystals 10, 10' are fixed on a heat sink 20 made of copper or the like, and this heat sink 20 is fixed on a TE cooler 21 made of a Peltier element or the like. Then, the TE cooler 21 is driven by a drive circuit (not shown), and the Nd:YVO4 rod 16
and KTP crystals 10, 10' are always maintained at a predetermined temperature. The phased array laser 14 is made of Nd:
It is brought into close contact with one end surface 16a of the YVO4 rod 16, and is also tightly fixed on the heat sink 20. This phased array laser 14 has a wavelength λ1 =809
A device that emits a laser beam 13 of nm wavelength is used. The Nd:YVO4 rod 16 emits a laser beam 11 with a wavelength λ2 = 1064 nm when the neodymium atoms are excited by the laser beam 13. [0051] Nd: YVO4 One end surface 16 of the rod 16
A is provided with a coating 22 that allows the laser beam 11 with a wavelength of 1064 nm to be well reflected and the pumping laser beam 13 with a wavelength of 809 nm to be well transmitted. On the other hand, on one end surface 10c of the KTP crystal 10',
A coating 23 is applied which provides good reflection of the laser beam 11 with a wavelength of 1064 nm and good transmission of the second harmonic 12 with a wavelength of 532 nm. Further, the other end surface 16b of the Nd:YVO4 rod 16 is provided with a coating 24 that allows the laser beam 11 to pass through it well and reflects the pumping laser beam 13 and the second harmonic 12 well. Therefore, the laser beam 11 with a wavelength of 1064 nm is confined between the surfaces 16a and 10c, causing laser oscillation. This laser beam 11 is incident on the KTP crystals 10, 10' and has a wavelength of λ3.
The wavelength is converted to the second harmonic 12 of =532 nm. K
Since the coating 23 as described above is applied to one end surface 10c of the TP crystal 10', this second harmonic 1
2 is efficiently emitted from the KTP crystal 10'. In this example as well, two KTP crystals 1
0 and 10' have the same length and are arranged with the same optical axes deviating from each other by 90°. Therefore, in this case as well, it is possible to obtain the second harmonic 12 with the maximum output, as in the first embodiment. Note that in the two embodiments described above, K
Although a linearly polarized fundamental wave is made incident on the TP crystals 10 and 10', the present invention can also be applied to an optical wavelength conversion device that converts an unpolarized fundamental wave into a second harmonic. , and has the same effect. Furthermore, the present invention is similarly applicable to optical wavelength conversion devices that use crystals of nonlinear optical materials other than KTP. Effects of the Invention As explained in detail above, the optical wavelength conversion device of the present invention can obtain the second harmonic of maximum intensity with a simple configuration using a combination of two crystals of nonlinear optical materials. It becomes. 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, in the device of the present invention, it is only necessary to form the crystals of two nonlinear optical materials to have the same length, and there is no need to set the length of the crystals to a strictly predetermined value. There is no need for measurement or adjustment, and from this point of view it can be formed at low cost.

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing the arrangement relationship of crystals of two nonlinear optical materials in a device according to a first embodiment of the present invention.

[Fig. 2] Side view 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 periodic fluctuations in second harmonic output

[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] Schematic diagram showing a method for producing a nonlinear optical material crystal used in the present invention

FIG. 9 is a schematic diagram showing another example of the method for producing the nonlinear optical material crystal used in the present invention.

[Explanation of symbols]

10, 10' KTP crystal 11 Laser beam (fundamental wave) 12 2nd harmonic 16 Nd:YVO4 rod

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, two crystals made of the same material are used as the crystals, the lengths of which are equal to each other, and the same optical axes are arranged in directions shifted by 90 degrees from each other. An optical wavelength conversion device characterized by: