JPH06110098A - Optical wavelength conversion device - Google Patents

Abstract

PURPOSE:To provide the optical wavelength conversion device which is wide in a permissible temp. range in attaining phase matching of a funcamental wave and a wavelength conversion wave by using plural crystals of nonlinear optical materials and disposing the respective crystals in the state of varying the directions of the crystal axes with the progressing direction of the fundamental wave from each other. CONSTITUTION:A laser beam 11 is made incident on the KNbO3 (KH) crystals 10A, 10B and is wavelength converted to a second harmonic wave. This second harmonic wave resonates between the light incident side end face of the Nd: YAG crystal and the mirror face of a resonance mirror and is partly emitted from the resonance mirror. The two KN crystals 10A, 10B are so cut that the arrangement angles are common but that the directions of the (b) axis with the progression direction of the laser beam 11 vary from each other. The temp. to wavelength conversion output characteristics of the respective crystals 10A, 10B vary in accordance with the directions of the crystal axes with the progressing direction of the fundamental wave and, therefore, the way of shifting the two characteristics is arbitrarily changed by adjusting the directions.

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 wave or the like through a crystal of a nonlinear optical material.

[0002]

2. Description of the Related Art Conventionally, nonlinear optical materials have been used to
Various attempts have been made to convert the wavelength of laser light into a second harmonic wave (shorter wavelength). As a light wavelength conversion element that performs wavelength conversion in this way, a bulk crystal type is often used.

[0003]

By the way, some of the above-mentioned nonlinear optical materials have an extremely narrow allowable temperature range for realizing phase matching between the fundamental wave and the wavelength-converted wave. For example, KNbO ₃ (hereinafter referred to as KN) has an allowable temperature range of 8 ° C. when used with a thickness of 0.5 mm.

When the optical wavelength conversion element having a narrow allowable temperature range is used, there arises a problem that the oscillation wavelength of the laser used to generate the fundamental wave is severely limited. Hereinafter, that point will be described with an example.

The above-mentioned KN crystal having an allowable temperature range of 8 ° C. is used for converting a YAG laser beam into a second harmonic in an LD (laser diode) pumping YAG laser. Here, the KN crystal temperature is 25
Assuming that the crystal is cut so that the fundamental wave and the second harmonic wave have the best phase matching at 0 ° C., it is necessary to control the temperature of this KN crystal to 25 ° C. ± 4 ° C. On the other hand, the LD for pumping needs to match the oscillation wavelength to 808.5 nm, which is the maximum absorption band of YAG, and this oscillation wavelength changes at a change rate of 0.3 nm / ° C. with respect to the LD temperature. So LD
When simultaneously controlling the temperature of KN and KN, it is necessary to prepare an LD having an oscillation wavelength of λ = 808.5 + {0.3 × (± 4)} = (808.5 ± 1.2) nm.

LDs generally produced by MOCVD
The lasing wavelength has a variation of about ± 3 nm within each lot. Therefore, the error of the oscillation wavelength is ± 1.
In order to prepare LDs within 2 nm, it is necessary to select LDs within each lot quite strictly, and as a result, L
This leads to a decrease in yield of D and an increase in cost.

In order to avoid such a situation, if the selection of LDs is loosened, in that case, LDs that do not match the absorption band of YAG are often used, and the output or emission efficiency of the YAG laser is lowered. Invite.

As mentioned above, the LD used as the pumping source
However, even when the laser beam emitted from the LD is directly incident on the optical wavelength conversion element as a fundamental wave, if the allowable temperature range of the optical wavelength conversion element is narrow,
Since the oscillation wavelength of the LD as the fundamental wave light source is severely limited, the same problem as described above occurs.

The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide an optical wavelength conversion device having a wide allowable temperature range for realizing phase matching between a fundamental wave and a wavelength conversion wave. It is intended.

[0010]

An optical wavelength conversion device according to the present invention is an optical wavelength conversion device for converting a wavelength of a fundamental wave into a crystal of a nonlinear optical material and converting the wavelength into a second harmonic or the like. The present invention is characterized in that a plurality of crystals are used and the crystals are arranged such that the directions of the crystal axes with respect to the traveling direction of the fundamental wave are different from each other.

[0011]

When the crystals of a plurality of nonlinear optical materials are arranged as described above, the temperature range in which the fundamental wave and the wavelength-converted wave are phase-matched is different for each crystal. FIG. 3 shows an example of this. Curves a and b in the figure show characteristic examples of the temperature-wavelength converted wave (second harmonic) output P for each of the two KN crystals provided. In this case, the phase matching range is 1 / of the maximum output Pmax.
It is assumed that the second harmonic having an output of 2 or more is obtained, and the temperature width (allowable temperature range) for achieving this phase matching is 8 ° C. for each crystal. As described above, if the temperature ranges in which the two crystals are phase-matched are deviated from each other, the allowable temperature range of the entire optical wavelength conversion device becomes wider than 8 ° C.
The curve c in FIG. 3 is the output of the synthesized second harmonic.

The allowable temperature range expanded as described above is maximum (that is, twice the allowable temperature range of one crystal when the respective characteristics of the two crystals are deviated as shown in FIG. 3). , 16 ° C in the above example). The temperature-wavelength converted wave output characteristic of each crystal changes according to the direction of the crystal axis with respect to the fundamental wave traveling direction. Therefore, by adjusting the direction, it is possible to arbitrarily change the shift between the two characteristics. Therefore, if it is not necessary to expand the allowable temperature range so much, and on the other hand, there is a demand to obtain a higher output second harmonic, the two characteristics may be shifted as shown in FIG. 4, for example.

Although the case where two crystals of the nonlinear optical material are used has been described above, in the present invention, three or more crystals of the nonlinear optical material are provided to realize the phase matching between the fundamental wave and the wavelength conversion wave. It is possible to further expand the above allowable temperature range.

As described in detail above, in the optical wavelength conversion device of the present invention, the fundamental wave and the wavelength-converted wave are arranged by arranging a plurality of crystals in a state in which the directions of crystal axes with respect to the traveling direction of the fundamental wave are different from each other. It is possible to set a wide allowable temperature range for achieving the phase matching with. Therefore, according to this device, the oscillation wavelength of the laser used to generate the fundamental wave (including the one used as the pumping source of the solid-state laser as described above and the fundamental wave light source itself) is limited. Is alleviated, laser yield is improved and cost is reduced.

If the allowable temperature range for achieving the phase matching between the fundamental wave and the wavelength-converted wave is wide as described above, the LD for generating the fundamental wave is deteriorated and the oscillation wavelength is increased. When it fluctuates, it is possible to return the oscillation wavelength to a desired value by changing the set temperature for temperature control.

Further, in the optical wavelength conversion device of the present invention, a plurality of crystals are arranged such that the directions of the crystal axes with respect to the traveling direction of the fundamental wave are different from each other, so that the phase matching between the fundamental wave and the wavelength converted wave is achieved. The permissible range of the fundamental wave wavelength for the realization becomes wider.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below based on the embodiments shown in the drawings. FIG. 1 shows a laser diode pumped solid-state laser having an optical wavelength converter according to an embodiment of the present invention. This laser diode pumped solid-state laser includes a semiconductor laser 14 that emits a laser beam 13 as pumping light, a condenser lens 15 such as a SELFOC lens that focuses the laser beam 13 that is divergent light, and neodymium (Nd) -doped. YAG crystal (hereinafter referred to as NAG) which is a solid-state laser medium
16) and this Nd: YAG crystal.
Two KN crystals arranged on the front side of 16 (right side in the figure)
It is composed of 10A and 10B and a resonator mirror 17 arranged further to the front side thereof.

The above-mentioned elements are fixed to a common mount 18 so as to be integrated, and the temperature is adjusted to a predetermined temperature by a Peltier element 19 and a temperature adjusting circuit (not shown). 20 in the figure is a heat sink.

The semiconductor laser 14 has a fundamental wavelength λ ₁
A laser beam that emits a laser beam 13 of 808 nm is used. The Nd: YAG crystal 16 uses this laser beam 13
When the neodymium atom is excited by the laser beam, a laser beam 11 having a wavelength λ ₂ = 946 nm is emitted.

The light incident side end face 16a of the Nd: YAG crystal 16 favorably reflects the laser beam 11 having a wavelength of 946 nm and the second harmonic 12 having a wavelength λ ₃ = 473 nm which will be described later, and pumps the light having a wavelength of 808 nm. The laser beam 13 for use is provided with a coating that allows it to pass through well. On the other hand, on the mirror surface 17a of the resonator mirror 17, the laser beams 11, 13
Is reflected, while a coating that partially transmits the second harmonic 12 is applied. Therefore wavelength 946 n
The m laser beam 11 is confined between the surfaces 16a and 17a to cause laser oscillation. This laser beam 11 is incident on the KN crystals 10A and 10B and has a wavelength of 1 /
The wavelength is converted into the second harmonic wave 12 of 2, that is, 473 nm. The second harmonic 12 also resonates between the surfaces 16a and 17a, and a part of the second harmonic 12 is emitted from the resonator mirror 17.

Here, the above two KN crystals 10A and 10B are
As shown in detail in FIG. 2, the arrangement angles are common, but the laser beams 11 are cut so that the directions of the b-axis with respect to the traveling direction are different from each other. In this case, KN crystal 10A,
The angles formed by the b-axis of 10B with respect to the traveling direction of the laser beam 11 are θ ₁ = 28.6 ° and θ ₂ = 29.7 °, respectively. The thickness of both KN crystals 10A and 10B is 0.5 m.
m.

KN crystals 10A and 10B thus formed
The characteristics of the temperature vs. the second harmonic output P are shown by curves a and b in FIG. 3, respectively. That is, one KN crystal
For 10 A, the temperature at which the laser beam 11 and the second harmonic 12 have the best phase matching, that is, the phase matching temperature is 10
℃, the allowable temperature range to achieve this phase matching is 8 ℃
(Between 6 ° C and 14 ° C), the other KN crystal 10B has a phase matching temperature of 18 ° C and an allowable temperature range of 8 ° C (14 ° C to 14 ° C).
22 ℃).

Therefore, considering the entire optical wavelength conversion device, the allowable temperature range for phase matching is 16 ° C. (6 ° C. to 22 ° C.).
Between ℃). Therefore, in this case, as described above, Y
Assuming that the maximum absorption band of AG is 808.5 nm and the change rate of the oscillation wavelength of the semiconductor laser 14 is 0.3 nm / ° C, the oscillation wavelength λ ₁ of the semiconductor laser 14 is λ ₁ = 808.5 ± (0.3 × 16/2 ) = (808.5 ± 2.4) nm within a relatively wide range.

In that case, as described above, the yield when selecting and using the semiconductor laser 14 for pumping from the semiconductor laser having the variation of the oscillation wavelength within each lot of about ± 3 nm is improved to 80% or more. , That leads to cost reduction of the semiconductor laser 14.

As in the above embodiment, two K's are used.
In addition to changing the cut angles of the N crystals 10A and 10B from each other, the two KN crystals 10A and 10C have the same cut angle θ with respect to the b axis as shown in FIG. The action and effect can be obtained.

When the two KN crystals 10A and 10D are arranged with their cut angles θ changed, as shown in FIG. 6, the crystal axes are arranged so as to have a relationship of correcting the walk angle. It is also possible to make the cross section of the wavelength conversion beam emitted from them close to a circle.

Although the embodiments using the KN crystal as the nonlinear optical material have been described above, the present invention can be similarly applied to the case of using other nonlinear optical materials.

[Brief description of drawings]

FIG. 1 is a plan view of an LD pumping solid-state laser including an optical wavelength conversion device according to an embodiment of the present invention.

FIG. 2 is a plan view showing a main part of the optical wavelength conversion device.

FIG. 3 is a graph showing temperature-wavelength converted wave output characteristics of two light wavelength conversion elements of the light wavelength conversion device.

FIG. 4 is a graph showing another example of temperature-wavelength converted wave output characteristics of two optical wavelength conversion elements.

FIG. 5 is a plan view showing an essential part of another embodiment of the present invention.

FIG. 6 is a plan view showing a main part of a further different embodiment of the present invention.

[Explanation of symbols]

 10A, 10B, 10C, 10D KN crystal 11 Laser beam (fundamental wave) 12 Second harmonic wave 13 Laser beam (pumping light) 14 Semiconductor laser 16 Nd: YAG crystal 17 Resonator mirror

Claims (3)

[Claims] 1. An optical wavelength comprising a plurality of optical wavelength conversion elements made of a crystal of a non-linear optical material for wavelength-converting an incident fundamental wave, the crystal axes of which are different from each other with respect to the traveling direction of the fundamental wave. Converter. 2. The light wavelength conversion device according to claim 1, wherein as the plurality of light wavelength conversion elements, those having different cut angles with respect to a crystal axis are used. 3. The plurality of light wavelength conversion elements having a common cut angle with respect to a crystal axis are used, and the arrangement angles of these light wavelength conversion elements are different from each other. The optical wavelength conversion device described.