JPH04318528A - Light wavelength converting device - Google Patents

Abstract

PURPOSE:To obtain high wavelength conversion efficiency at all times by holding crystal of a nonlinear optical material at phase matching temperature in the light wavelength conversion device which converts a fundamental wave into a 2nd higher harmonic, etc. CONSTITUTION:The temperature of KNbO3 crystal 10 is controlled by a Peltier element 31. Then a photodetector 34 detects the light intensity of the 2nd higher harmonic 12 and a feedback controller drives and controls the Peltier element 31 so that the light intensity is converged on a target value.

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 using a crystal of a nonlinear optical material.

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 the generation of second harmonics and the like by nonlinear optical materials. Specifically, an optical wavelength conversion element that performs wavelength conversion in this manner is described in, for example, "Fundamentals of Optoelectronics", A. Written by YARIV, translated by Kunio Tada and Takeshi Kamiya (Maruzen Co., Ltd.) p200-20
A bulk crystal type as shown in No. 4 is well known.

By the way, when using a crystal of the above-mentioned nonlinear optical material, it is often necessary to control the crystal temperature to a predetermined temperature within a relatively narrow tolerance range in order to phase match the fundamental wave and the wavelength-converted wave. . For example, in the case of KNbO3, which is suitably used to obtain wavelength-converted waves in the blue region, the above-mentioned allowable temperature range is about 0.3° C.cm. Furthermore, for LiNbO3, which is suitably used to obtain wavelength-converted waves in the green and blue regions, the above-mentioned allowable temperature range is about 0.6° C.cm. Others BNNB, KD
There are crystals of nonlinear optical materials such as P that have a narrow temperature tolerance range.

[0004] Conventionally, therefore, a Peltier element or the like has been used to control the temperature of a crystal of a nonlinear optical material to a predetermined temperature.

[0005]

[Problems to be Solved by the Invention] However, even if the crystal temperature is accurately set to the phase matching temperature T0, if phase matching is achieved and a wavelength converted wave is generated, this short wavelength converted wave is transferred to the nonlinear optical material crystal. The oscillation region absorbs the heat and the temperature rises, and the temperature of that part becomes higher than the phase matching temperature T0. In this case, especially when a crystal having a narrow temperature tolerance range for phase matching is used, such as the above-mentioned KNbO3, the wavelength conversion efficiency will be significantly reduced.

The present invention has been made in view of the above-mentioned circumstances, and is intended to prevent the oscillation region of a nonlinear optical material crystal from absorbing a wavelength-converted wave and deviate from the phase matching temperature. It is an object of the present invention to provide an optical wavelength conversion device that can obtain wavelength conversion efficiency.

[0007]

[Means for Solving the Problems] A first optical wavelength conversion device according to the present invention, as described above, makes a fundamental wave incident on a crystal of a nonlinear optical material, and converts the fundamental wave into a second harmonic or the like. In an optical wavelength conversion device, ◆ temperature control means for adjusting the temperature of the crystal of the nonlinear optical material, ◆ detection means for detecting the light intensity of the wavelength-converted wave, ◆ receiving the output of this detection means, the output indicates The apparatus is characterized in that a control means for controlling the driving of the temperature adjustment means so that the light intensity converges to a predetermined target value is provided.

Further, a second optical wavelength conversion device according to the present invention is an optical wavelength conversion device that converts a fundamental wave into a second harmonic or the like using a crystal of a nonlinear optical material in the same way as described above. , heated until the phase matching temperature T0 is reached, then T1 = T0 - ΔT (where Δ
The present invention is characterized in that a temperature adjusting means is provided for converging the temperature to a temperature T1 (T is the temperature rise due to wavelength conversion wave absorption in the oscillation region of the crystal).

[0009]

[Operation and Effects of the Invention] In the first optical wavelength conversion device having the above configuration, the crystal temperature is adjusted so that the optical intensity of the wavelength-converted wave actually emitted from the crystal of the nonlinear optical material converges to a predetermined target value. As a result, the crystal temperature is maintained at the phase matching temperature, and the wavelength conversion efficiency is kept high.

Furthermore, in the second optical wavelength converter having the above configuration, when the temperature of the crystal is initially raised to the phase matching temperature T0, the temperature distribution due to absorption of the wavelength converted wave changes until good phase matching is achieved. Since there is almost no oscillation, the temperature of the oscillation region eventually reaches T0, where a wavelength-converted wave is generated with high efficiency. If the crystal temperature is then converged to T1, the temperature of the oscillation region of the crystal is maintained exactly at T0, so that high-intensity wavelength-converted waves continue to be generated.

[0011]

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 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 YA
G rod (hereinafter referred to as Nd:YAG rod) 16 and the front side of this Nd:YAG rod 16 (right side in the figure)
It consists of KNbO3 crystals 10 arranged in. 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). This phased array laser 14 has a wavelength λ1
A device that emits a laser beam 13 of =810 nm is used. On the other hand, the Nd:YAG rod 16 emits a laser beam 11 with a wavelength λ2 = 946 nm due to neodymium atoms being excited by the laser beam 13 described above.

Light incidence side surface 16 of Nd:YAG rod 16
In a, the laser beam 11 with a wavelength of 946 nm is well reflected (reflectance of 99.9% or more), and the laser beam 11 with a wavelength of 810 nm is well reflected.
A coating 18 is applied to transmit the pumping laser beam 13 (transmittance of 99% or more). On the other hand, the front end surface 10a of the KNbO3 crystal 10 is shaped as a part of a spherical surface, and the surface has a wavelength 81
0 nm laser beam 13 and wavelength 946 nm
A coating 19 is applied which allows a laser beam 11 to be reflected well and a second harmonic wave 12 having a wavelength of 473 nm, which will be described later, to be well transmitted. Therefore wavelength 9
The 46 nm laser beam 11 is directed to the above-mentioned surface 16a,
10a, causing laser oscillation.

This laser beam 11 is wavelength-converted by a KNbO3 crystal 10, which is a nonlinear optical material, into a second harmonic wave 12 having a wavelength of 1/2, that is, 473 nm. K
Since the surface 10a of the NbO3 crystal is coated with the coating 19 as described above, this KNbO3 crystal 1
0, almost only the second harmonic 12 is extracted.

As shown in detail in FIG. 4, the KNbO3 crystal 10 has a Y'Z plane rotated by 30° around the Z axis.
It is cut on the side. In this configuration, when the linear polarization direction of the laser beam 11 shown by the arrow P matches the Y' axis, a large nonlinear optical constant d32 is used, and the laser beam 11 as the fundamental wave and the second harmonic Good type I phase matching is achieved with the wave 12, and the second harmonic 12 of maximum intensity (the linear polarization direction is in the direction of arrow Q, that is, the Z-axis direction) is obtained. Note that the optical axes Y and Z are the b-axis and c-axis, respectively, and the X-axis is the a-axis in terms of crystal axes.

Furthermore, the oscillation region of the KNbO3 crystal 10 is
In order to achieve the above phase matching, the temperature T0 = 20.0°
Must be maintained at C. And as mentioned above,
The phase matching temperature tolerance range for KNbO3 is 0.3 °C.
cm, and in this example the crystal length is 0.5 cm, so the temperature of the oscillation region of the KNbO3 crystal 10 is 20 cm.
．． Must be maintained at 0±0.3 °C. Hereinafter, the point of realizing this temperature of 20.0±0.3°C will be explained.

[0016] The KNbO3 crystal 10 is a copper block 30.
The entire copper block 30 is held within the Peltier element 3.
The temperature is controlled by 1. This Peltier element 31 is driven and controlled by a feedback controller 32. On the other hand, on the front side of the KNbO3 crystal 10, there is a half mirror 3 that partially reflects the second harmonic wave 12.
3 is placed. The light intensity of the second harmonic 12 reflected by the half mirror 33 is detected by a photodetector 34 such as a photodiode. The output signal S of the photodetector 34 is input to the feedback controller 32.

The feedback controller 32 drives and controls the Peltier element 31 so that the light intensity of the second harmonic 12 indicated by the output signal S of the photodetector 34 is maintained at the maximum value P0 obtained with this configuration. That is, when the Peltier element 31 is turned on at time t0 as shown in (1) of FIG. 2, it is driven so that the temperature rises as it is. At this time, the KNbO3 crystal 1 is passed through the copper block 30.
0 is uniformly heated, no significant temperature distribution occurs within the KNbO3 crystal 10, and therefore this KNbO3 crystal 10
3. The temperature of the oscillation region 10a of the crystal 10 (see (1) in FIG. 3) also rises to approximately T0 as shown in (3) in FIG. When the temperature of the oscillation region 10a rises in this way,
When the temperature reaches T2, the second harmonic 12 begins to be generated. And at time t1, KNbO3
When the oscillation region 10a of the crystal 10 reaches the temperature T0,
The light intensity of the second harmonic 12 reaches its maximum value P0.

When the KNbO3 crystal 10 begins to generate the second harmonic wave 12, its oscillation region 10a absorbs the second harmonic wave 12 with a short wavelength, and the temperature rises. In this way, when the temperature of the oscillation region 10a becomes higher than T0, the light intensity of the second harmonic 12 becomes lower than P0. When the light intensity of the second harmonic 12 indicated by the output signal S of the photodetector 34 is lower than P0, the feedback controller 32 first drives the Peltier element 31 to lower the temperature by a small predetermined amount, If the optical intensity of the second harmonic 12 increases as a result of this operation, this operation is continued. Conversely, if the optical intensity of the second harmonic 12 is further reduced by this operation, the feedback controller 32 changes the direction of control and drives the Peltier element 31 to raise the temperature by a small predetermined amount.

When the light intensity of the second harmonic 12 decreases below P0 due to the above-mentioned absorption of the second harmonic, when the Peltier element 31 is driven to reduce the temperature by a small predetermined amount, the second harmonic This operation continues as the light intensity of harmonic 12 increases. When this operation causes the temperature of the oscillation region 10a to fall below T0, and therefore the light intensity of the second harmonic 12 to fall below P0, the feedback controller 3 performs the control operation as described above.
2, the Peltier element 31 is now driven and controlled to increase its temperature, and the temperature of the oscillation region 10a is returned to T0.

By performing the above control, the optical intensity of the second harmonic 12 is maintained at P0 as shown in (2) of FIG. 2, and the temperature of the Peltier element 31 is maintained as shown in (1) of FIG. As a result, the temperature of the oscillation region 10a is maintained at T0 =20.0±0.3°C. Figure 3 shows the KNbO3 crystal 1 at this time.
The temperature distribution at 0 is shown.

Next, a second embodiment of the present invention will be described with reference to FIG. The optical wavelength conversion device shown in FIG. 5 differs from the device shown in FIG. 1 in that the half-mirror 33 and photodetector 34 are removed, and a programmable controller 40 is used instead of the feedback controller 32.

This programmable controller 40 controls the Peltier element 31 so that the temperature changes according to the characteristics shown in FIG. 6(1). In other words, the Peltier element 31 is controlled so that its temperature overshoots the temperature T1 in the time region indicated by H in the figure, reaches the maximum temperature T0, and then converges to the temperature T1. These temperatures T
0 and T1 are the same as those described in the first embodiment, and T1 = T0 - ΔT.

By controlling the temperature of the Peltier element 31 in this way, the oscillation region 1 of the KNbO3 crystal 10
0a is heated to temperature T0, and the second harmonic 12
When this occurs, the temperature of the oscillation region 10a increases due to its absorption. The above ΔT is the temperature rise due to absorption of the second harmonic in this oscillation region 10a. In this way, the temperature of the oscillation region 10a is controlled to T0 as shown in (3) of FIG.
), it is maintained at P0.

[Brief explanation of drawings]

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

[Figure 2]
A graph showing the change characteristics of the Peltier element temperature, the light intensity of the second harmonic, and the oscillation region temperature of KNbO3 in the device of the first embodiment.

[Fig. 3] A schematic diagram showing the temperature distribution in the KNbO3 crystal of the first embodiment device.

FIG. 4 is a schematic diagram for explaining the relationship between the optical axis of the KNbO3 crystal and the linear polarization direction of the fundamental wave in the device of the first embodiment.

FIG. 5 is a schematic side view of a device according to a second embodiment of the present invention.

[Figure 6]
A graph showing the change characteristics of the Peltier element temperature, the light intensity of the second harmonic, and the oscillation region temperature of KNbO3 in the second embodiment device.

[Explanation of symbols]

10 KNbO3 crystal 11 Laser beam (fundamental wave) 12 Second harmonic 16 Nd:YAG rod 30 Copper block 31 Peltier element 32 Feedback controller 33
Half mirror 34 Photodetector

Claims (2)

[Claims] 1. An optical wavelength conversion device that converts wavelength by making a fundamental wave incident on a crystal of a nonlinear optical material, comprising: a temperature adjustment means for adjusting the temperature of the crystal; a detection means for detecting the light intensity of the wavelength-converted wave; , a control means for receiving the output of the detection means and controlling driving of the temperature adjustment means so that the light intensity indicated by the output converges to a predetermined target value. Device. 2. In an optical wavelength conversion device that converts wavelength by inputting a fundamental wave into a crystal of a nonlinear optical material, the crystal is heated until it reaches a phase matching temperature T0, and then T1 = T0 - ΔT (where T1 = T0 - ΔT). ΔT is the temperature T1 (the temperature rise due to absorption of wavelength converted waves in the oscillation region of the crystal)
What is claimed is: 1. An optical wavelength conversion device characterized by being provided with a temperature control means for converging the temperature.