JP2727259B2 - Optical wavelength converter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical wavelength conversion device for converting a fundamental wave into a second harmonic, and more particularly, to an optical wavelength conversion device which is disposed in a resonator of a laser diode pumped solid-state laser. The present invention relates to an optical wavelength conversion device capable of achieving type II phase matching with harmonics.

[0002]

2. Description of the Related Art As disclosed in, for example, Japanese Patent Application Laid-Open No. Sho 62-189783, a laser diode pumped solid laser in which a solid laser rod doped with a rare earth such as neodymium is pumped by a semiconductor laser (laser diode) is known. ing. In this type of laser-diode-pumped solid-state laser, a bulk single crystal of a nonlinear optical material is disposed in the resonator to obtain a laser beam of a shorter wavelength, and the solid-state laser oscillation beam is converted to a second harmonic. Wavelength conversion is also performed.

By the way, a biaxial crystal such as KTP is often used as a crystal of the nonlinear optical material. J. Appl. Phys. Vol. 55, p65 (1984)
Describes in detail the method of phase matching of KTP, which is a biaxial crystal, by Yao et al. Hereinafter, the phase matching method in the biaxial crystal described here will be described. As shown in FIG. 4, θ is the angle between the traveling direction of light and the optical axis Z of the crystal, and φ is the angle of the traveling direction of light from the X axis on a plane including the optical axes X and Y.
Here, the fundamental wave and the second wave when incident at an arbitrary angle
The index of refraction of the crystal for each harmonic

[0004]

(Equation 1)

[0005] The refractive indices of the crystal with respect to the polarization components of the fundamental wave and the second harmonic in the optical axes X, Y and Z are respectively

[0006]

(Equation 2)

[0007] Next, when k _X = sin θ · cos φ k _Y = sin θ · sin φ k _Z = cos θ,

[0008]

(Equation 3)

[0009]

(Equation 4)

The solutions of (Equation 3) and (Equation 4) are phase matching conditions.

[0011]

(Equation 5)

Then, the solutions of the equations (3) and (4) are

[0013]

(Equation 6)

[0014]

(Equation 7)

(The double sign is + when i = 1 and − when i = 2).

Here,

[0017]

(Equation 8)

When the following condition 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,

[0019]

(Equation 9)

When the following condition is satisfied, phase matching is performed between the fundamental wave and the second harmonic, which is generally called type II phase matching.

When the type II phase matching is performed using the above-described biaxial crystal, the fundamental wave to be incident on the crystal has two refractive indexes with respect to the crystal. For example, when using the nonlinear optical constant d ₂₄ of a crystal,
That is, as shown in FIG. 5, a fundamental wave linearly polarized in a direction inclined by 45 ° from the optical axis Y of the crystal 10 to the Z axis side (that is, having a linear polarization component in the Y axis direction and a linear polarization component in the Z axis direction) as shown in FIG. 2nd harmonic linearly polarized in the Y-axis direction with 11 incident
When taking out 12, fundamental wave 11 is refractive index

[0022]

(Equation 10)

That is, the refractive index felt by the polarization component in the Z-axis direction and the refractive index

[0024]

[Equation 11]

Both the traveling direction of the condensed light and the refractive index felt by the polarized light component in the Y 'direction perpendicular to the Z axis are felt.

When the crystal 10 is cut as shown in FIG. 5, strictly speaking, the fundamental wave 11 is linearly polarized in the Y 'direction (the direction inclined from the Y axis to the X axis) and the Z axis direction. In this state, the second harmonic 12 is extracted in a state of being polarized in the Y 'direction. However, in practice, the second harmonic 12 may be considered as described above.

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

[0028]

(Equation 12)

When this phase difference Δ occurs, the direction of linear polarization of the fundamental wave changes according to the value of the phase difference Δ. When the linear polarization direction of the fundamental wave changes in this manner, the angle of the polarization direction of the fundamental wave with respect to the optical axis of the nonlinear optical material crystal deviates from a predetermined angle at which the maximum wavelength conversion efficiency is obtained, and the second
The light intensity of the harmonics will decrease.

Therefore, in order to obtain the maximum second harmonic output, it is necessary to optimally control the crystal temperature or adjust the crystal length optimally. For example, U.S. Pat.
Japanese Patent Application Laid-Open No. 1-152781 and 1-15 describe an example of an optical wavelength conversion device that adopts the former method.
Japanese Patent Publication No. 2782 discloses an example of an optical wavelength conversion device employing the latter method.

[0031]

However, the crystal length is set arbitrarily, and the maximum second length is controlled by controlling the crystal temperature.
In order to obtain a harmonic output, a large temperature control stroke is required, so that the temperature control power supply and the heat sink are increased in size, resulting in an increase in the size and cost of the optical wavelength converter.

On the other hand, when the temperature is adjusted so that the crystal temperature becomes constant, and the length of each crystal is adjusted to an optimum value for the temperature to cope with the temperature, the tolerance of the crystal length is extremely small. Therefore, it is actually very difficult to obtain the maximum second harmonic output. And even if such a thing is possible, in this case, the work of strict measurement and adjustment of the crystal length is required, and the cost of the optical wavelength converter is greatly increased.

The present invention has been made in view of the above-described circumstances, and uses a crystal of a nonlinear optical material that achieves type II phase matching between a fundamental wave and a second harmonic, thereby maximizing the efficiency. It is an object of the present invention to provide an optical wavelength converter that can obtain a second harmonic output and that can be formed small and inexpensively.

[0034]

The optical wavelength converter according to the present invention is arranged in a resonator of a laser diode-pumped solid-state laser as described above, and converts an incident solid-state laser oscillation beam as a fundamental wave into a type II laser. In addition to the crystal of the nonlinear optical material that converts the phase into the second harmonic, the crystal of the nonlinear optical material is disposed inside the resonator without including the crystal, and the wavelength of the solid-state laser oscillation beam is increased. And a wavelength selection element for adjusting the wavelength is provided.

[0035]

In the above configuration, when the oscillation wavelength of the solid-state laser is changed by the wavelength selection element, the fundamental wavelength λ in the above equation (12) is changed, and the refractive index is accordingly changed.

[0036]

(Equation 13)

Changes, the value of the phase difference Δ changes. Then, the polarization direction of the fundamental wave changes accordingly. Therefore, if the oscillation wavelength is appropriately adjusted by the wavelength selection element, the polarization direction of the fundamental wave with respect to the nonlinear optical material crystal can be set so as to obtain the maximum wavelength conversion efficiency, and the wavelength conversion of high intensity is performed. It is possible to obtain a short wavelength laser beam.

The apparatus of the present invention having the above configuration does not require a large-sized and highly accurate temperature control means and does not require strict measurement and adjustment of the crystal length, so that it can be formed small and inexpensively.

Further, in the above configuration, the solid-state laser oscillates in a single longitudinal mode by the action of the wavelength selection element, thereby preventing generation of noise due to mode competition in the wavelength-converted short-wavelength laser beam.

[0040]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail based on embodiments shown in the drawings. FIG. 1 shows an optical wavelength converter according to a first embodiment of the present invention. A laser diode pumped solid-state laser having this optical wavelength conversion device,
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 that is divergent light into parallel light, and a laser beam 13 that has passed through the lens 15a is focused. The condenser lens 15b and neodymium (N
d) a solid-state laser rod doped with YV
O ₄ rod (hereinafter referred to as Nd: YVO ₄ rod) 16
A resonator mirror 17 disposed on the front side (the right side in the figure) of the Nd: YVO ₄ rod 16;
KTP crystal arranged between Nd: YVO ₄ rod 16
An etalon plate 20 is provided between the KTP crystal 10 and the Nd: YVO ₄ rod 16. The components described above are mounted and integrated on a common housing (not shown). The temperature of the phased array laser 14 is adjusted to a predetermined temperature by a Peltier device (not shown) and a temperature adjustment circuit.

As the phased array laser 14, a laser emitting a laser beam 13 having a wavelength λ ₁ = 809 nm is used. On the other hand, the Nd: YVO ₄ rod 16
When the neodymium atoms are excited by the laser beam 13, a laser beam 11 having a fundamental wavelength λ ₂ = 1064 nm is emitted.

Nd: YVO ₄ rod 16 light incident side end face 16
The coating 18a is coated with a laser beam 11 having a wavelength of 1064 nm, which reflects well (a reflectance of 99.9% or more), and a laser beam 13 for pumping, having a wavelength of 809 nm, which transmits well (a transmittance of 99% or more). I have. Also Nd: YVO
_An anti-reflection coating 9 that allows the laser beam 11 having a wavelength of 1064 nm to pass therethrough (having a transmittance of 99.9% or more) is applied to the light emitting side end face 16b of the _four rods 16. On the other hand, the surface 17a of the resonator mirror 17 on the side of the KTP crystal 10 has a shape which forms a part of a spherical surface, and the laser beam 11 having a wavelength of 1064 nm and the laser beam 13 having a wavelength of 809 nm are reflected well on the surface, and The second harmonic 12 with a wavelength of 532 nm
Has a coating 19 that allows good transmission. Therefore, the laser beam 11 having a wavelength of 1064 nm
It is trapped between 16a and 17a and causes laser oscillation. The anti-reflection coating is not particularly applied to both end surfaces of the etalon plate 20. Although not shown, the end face of the KTP crystal 10 on the etalon plate 20 side is coated with the same coating as the antireflection coating 9.

This laser beam 11 is incident on the KTP crystal 10 which is a nonlinear optical material, and has a wavelength of す な わ ち
The wavelength is converted to a second harmonic 12 of 2 nm. Resonator mirror
Since the coating 19 as described above is applied to the surface 17 a of the mirror 17, almost only the second harmonic 12 is extracted from the resonator mirror 17.

The etalon plate 20 as a wavelength selection element is formed in a wedge shape. Holding member holding it
A plurality of guide rods 22 extending in the vertical direction in the figure are inserted through 21. The lower ends of these guide rods 22 are fixed to a fixed base 23, and the holding members 21
It can move up and down along. A precision screw 24 is rotatably held on the fixed base 23, and the tip of the precision screw 24 is screwed to the holding member 21. Therefore, when the precision screw 24 is rotated, the holding member 21 advances and retreats in the vertical direction, and the etalon plate 20 moves up and down. Since the etalon plate 20 is inserted in the optical path of the laser beam 11, the oscillation wavelength of the laser beam 11 is selected to a predetermined value according to the thickness of the etalon plate 20.

As shown in detail in FIG. 5, the KTP crystal 10, which is a biaxial crystal, is cut by 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 indicated by the arrow P and the Z axis make an angle of 45 °, a large nonlinear optical constant d ₂₄ is used, and the laser beam 11 as a fundamental wave is used. And the second harmonic 12, good type II phase matching is obtained, and the second harmonic 12 having the maximum intensity is obtained.

However, when 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 changes according to the value, so that the angle of 45 ° is realized. Something you can't do can happen.
Therefore, when the precision screw 24 is rotated clockwise or counterclockwise to move the etalon plate 20 by a small amount in the vertical direction, the optical path length of the laser beam 11 in the etalon plate 20 changes. The value of the oscillation wavelength changes very little. When the wavelength of the laser beam 11 changes, the value of the phase difference Δ changes as described above, and therefore the direction of the linearly polarized light also changes. By finely adjusting the linear polarization direction of the laser beam 11 in this manner, a state is obtained in which the linear polarization direction forms an angle of 45 ° with the Z axis as described above. You can get 12. Note that the wedge-shaped etalon plate 20 may be formed such that the other light passage surface forms an angle of, for example, about 1 'with respect to one light passage surface.

Further, by providing the etalon plate 20 as described above, the laser diode pumped solid-state laser oscillates in a single longitudinal mode. Therefore, in the laser diode pumped solid-state laser, longitudinal mode competition does not occur, and generation of noise due to the competition can be prevented.

Next, a second embodiment of the present invention will be described with reference to FIG. In FIG. 2, the same elements as those in FIG. 1 are denoted by the same reference numerals, and a duplicate description thereof will be omitted (the same applies hereinafter).

The etalon plate 30 in the second embodiment is
Although the two light passing surfaces are parallel planes, they are held by a holding member 31 so as to be rotatable about a rotation shaft 32. An adjusting knob 33 is provided on the holding member 31 as a rotating shaft.
It is mounted to rotate around 34. This rotation axis
34 is connected to the rotating shaft 32 via a reduction gear train (not shown). Therefore, when the adjustment knob 33 is rotated, the etalon plate 30 rotates about the rotation shaft 32. When the etalon plate 30 is rotated in this manner, the passing length of the laser beam 11 there changes, so that the linear polarization direction of the laser beam 11 can be adjusted also in this case as in the first embodiment.

Next, a third embodiment of the present invention will be described with reference to FIG. In this embodiment, Nd: YV
The O ₄ rod 16 is formed in a wedge shape. The above-mentioned anti-reflection coating 9 (see FIGS. 1 and 2) is not applied to the light emitting side end face 16b. N of this configuration
d: In YVO ₄ rod 16, since a part of the laser beam 11 (e.g. about 20%) is returned to the light incident surface 16a side is reflected by the light exit end face 16b, so that a standing wave is generated, The rod 16 also functions as a wavelength selection element capable of adjusting a selected wavelength. This Nd: YVO ₄ rod 16
Attached to a holding member 21 similar to that of the device of FIG.
By rotating the precision screw 24, it can be moved up and down in the figure. Also in this configuration, Nd: YV
By moving the O ₄ rod 16 up and down, the oscillation wavelength of the laser beam 11 can be changed and its linear polarization direction can be adjusted.

[Brief description of the drawings]

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

FIG. 2 is a side view of an apparatus 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 is a schematic diagram illustrating an angle θ between a fundamental wave traveling direction and an optical axis Z and an angle φ between a fundamental wave traveling direction and an optical axis X inside a crystal related to the present invention.

FIG. 5 is a schematic diagram for explaining the relationship between the optical axis of a nonlinear optical material and the direction of linear polarization of a fundamental wave.

[Explanation of symbols]

9, 18, 19 Coating 10 KTP crystal 11 Laser beam (fundamental wave) 12 Second harmonic 16 Nd: YVO ₄ rod 17 Resonator mirror 20, 30 Etalon plate 21, 31 Holding member 22 Guide rod 23 Fixing member 24 Precision screw 32, 34 Rotary axis 33 Adjustment knob

Claims (1)

(57) [Claims] A crystal of a nonlinear optical material which is arranged in a resonator of a laser diode pumped solid-state laser and converts an incident solid-state laser oscillation beam as a fundamental wave into a second harmonic by taking phase II phase matching. inner If, Oite within said resonator, a crystal of the nonlinear optical material
It is disposed in a state free of parts, solid laser oscillator beam adjustably optical wavelength converter and the wavelength selection element is provided for selecting a wavelength of.