JP2003304019A - Wavelength conversion laser device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a wavelength conversion laser device which can retrieve higher harmonic laser beams of a high output while a relative positional regulation to the beam is easy. <P>SOLUTION: Two QPM elements 4a, 4b are used as a wavelength conversion element 4, upper surfaces opposite to an upper part of a bulk crystal of parts having good uniformity of a period of a polarization inversion layer of each QPM element are disposed opposite to each other and brought into contact with each other, and substantially functioned as one QPM element. The element 4 is disposed in an optical resonator so that the close contact surface coincides with an optical axis C in the resonator having a reflecting layer 3a of a laser medium 3 and an output mirror 6. Thus, since the part having the good uniformity of the period of the inversion layer is enlarged about twice around the axis C, a beam alignment is facilitated, and even when the position is slightly deviated, high conversion efficiency is obtained and hence its output can be increased. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength conversion laser device for generating harmonic light from a fundamental wave light by using a wavelength conversion element and taking out the harmonic light to the outside. The present invention relates to a wavelength conversion laser device using a quasi-phase matching type wavelength conversion element that performs wavelength conversion by matching.

[0002]

2. Description of the Related Art In recent years, short-wavelength lasers such as green and blue have attracted attention in a wide range of fields such as interferometers, optical pickups for optical discs, printing devices, and research and development of laser devices that generate such laser light. Is being actively promoted in various places. As one of such short-wavelength laser devices, there is known a wavelength conversion laser device in which a wavelength conversion element is inserted in the optical path of a fundamental wave laser light to generate harmonic light, and the harmonic light is extracted to the outside. ing.

In this type of conventional wavelength conversion laser device,
A nonlinear optical crystal KN (KNb) is used as a wavelength conversion element.
O ₃ ) and KTP (KTiO ₄ ) have been used, but such crystals have problems that the temperature dependence of the wavelength is large and the crystals that can be used for the wavelength are limited. On the other hand, recently, quasi phase matching (QPM: Quasi-
Wavelength conversion elements that utilize phase-matching) are attracting attention.

Quasi-phase matching type wavelength conversion element (hereinafter referred to as "Q
"PM element") is LiNbO ₃And LiTaO₃Such
A periodic domain-inverted layer can be formed inside the bulk crystal of
And, for pseudo-phase matching with laser light of a predetermined wavelength.
To achieve the period of the polarization inversion layer.
By changing it, almost any wavelength can be supported.
Conversely speaking, the uniformity of the period of this domain inversion layer is not good.
If so, the wavelength conversion efficiency will be low and the wavelength will be extracted to the outside.
This can cause a significant decrease in the output of harmonic light.

[0005]

With respect to such a QPM element, various studies have been made on a method of forming a periodic domain-inverted layer inside a bulk crystal (for example,
"Blue-green solid-state laser using MgO-LiNbO ₃ domain-inverted bulk crystal and its application" (Laser research,
(March 1998))).

FIG. 4 is a diagram for explaining an example of a method of manufacturing a QPM element. As a method of forming the domain inversion layer,
For example, as shown in FIG. 4A, slit-shaped electrodes 40 are formed on the upper surface and one side surface of the bulk crystal 41 at a predetermined interval (same as the period of the domain inversion layer). As the method of forming the electrode 40, the same method as the semiconductor manufacturing process such as sputtering, patterning by photolithography, and etching can be used. Then, when a high voltage is applied to the electrode 40 formed as described above, as shown in FIG. 4B, the domain-inverted layer 42 is formed inside the bulk crystal 41 corresponding to the position of the electrode 40.

When the QPM element as described above is arranged in the optical resonator of the laser apparatus, since the oscillation beam diameter in the optical resonator is about several tens to 100 μm, the position adjusting work of the QPM element is included. Considering productivity, it is desirable that the period of the domain inversion layer 42 be uniform over a thickness of several hundreds of μm. However, in a QPM element for outputting a short-wavelength laser light such as blue, the period of the domain inversion layer 42 is as short as several μm, and therefore, in manufacturing the QPM element, the bulk crystal 41 having a thickness of 400 μm or more is used. It is very difficult to form the domain-inverted layer 42 having a uniform period from the upper part to the lower part. Therefore, in the conventional wavelength conversion laser device of this type, a part of the oscillation beam is irradiated inside the optical resonator to a part where the polarization inversion layer of the QPM element is not uniform. Since the non-uniform polarization inversion layer does not contribute to conversion of a desired wavelength, light loss occurs there, the conversion efficiency deteriorates, and it is difficult to sufficiently secure the output of harmonic light extracted to the outside. is there.

The present invention has been made in view of the above problems, and its main purpose is to eliminate the nonuniformity of the period of the polarization inversion layer of the QPM element and to output the wavelength conversion element. It is an object of the present invention to provide a wavelength conversion laser device capable of obtaining high-output harmonic light by reducing the loss of 1) as much as possible.

[0009]

[Means and effects for solving the problems] Generally, QP
In the M element, some bulk crystals (400 to 50
When the thickness is 0 μm or more), it is difficult to form the domain inversion layer. Further, even if the domain inversion layer is formed,
The uniformity of the period is high from the upper surface of the bulk crystal to a certain depth, and the disorder of the period becomes significant below that.

As an example, the QPM shown in FIG.
FIG. 3 shows a photograph of a cross section of the device which was cut vertically at the position P in FIG. Here, the bulk crystal thickness is about 4
Although it is 00 μm, the uniformity of the period of the domain inversion layer is good in the range of the depth from the upper surface of the bulk crystal to about 1/3 in the thickness direction, but the period disturbance is significant below that. Has become. Of course, the depth of the portion of the domain inversion layer where the uniformity of the period is good also depends on the method of forming the domain inversion layer, etc., but generally, the lower the manufacturing cost and the simpler the process, the more It tends to be difficult to ensure the uniformity of the period up to the deep part inside the bulk crystal.

In order to solve the above problems, the present invention provides an excitation light source for generating excitation light, a laser medium excited by the excitation light to emit fundamental wave light, and a fundamental wave light emitted from the laser medium. An optical resonator that amplifies while resonating
A wavelength conversion laser device that is inserted in the optical path of the optical resonator and that generates a harmonic wave from the fundamental wave light, wherein the wavelength conversion laser device extracts the harmonic light to the outside of the optical resonator. Is a plurality of quasi-phase-matching wavelength conversion crystals (that is, QPM elements) formed by forming periodic polarization inversion layers inside a bulk crystal, and the uniformity of the period of the polarization inversion layers of each wavelength conversion crystal is Is adhered to each other, and the contact surface or the vicinity thereof is arranged so as to substantially coincide with the optical axis in the optical resonator.

In the simplest mode of the present invention, two QPM elements are used, and a portion having good period uniformity in each wavelength conversion crystal (hereinafter referred to as "uniform period portion"), for example, a bulk crystal is used. The planes facing the upper part are made to face each other and are in close contact with each other, and substantially function as one QPM element.
By arranging the wavelength conversion element so that the contact surface or the vicinity thereof coincides with the optical axis in the optical resonator, the period uniformity of the polarization inversion layer is good around the optical axis. To do so. For example, assuming that the thickness of the uniform periodic portion is substantially the same in the two QPM elements, by combining these two QPM elements, the thickness of the uniform periodic portion is approximately doubled across the optical axis. It will be expanded to.

Therefore, according to the wavelength conversion laser device of the present invention, the uniform period portion of each quasi phase matching type wavelength conversion crystal is sufficient when the beam diameter, the beam irradiation alignment accuracy, and the like are taken into consideration. Even if the thickness is not large, when the wavelength conversion element is formed by using a plurality of the thicknesses, the uniform period portion can be substantially expanded so that the beam can surely hit the portion. Therefore, the loss of light is small, the wavelength conversion efficiency is improved, and the output of the harmonic light extracted to the outside can be increased even with the same optical resonator length. Further, by enlarging the portion having a good periodicity in this way, the beam alignment and the like can be facilitated, and the adjustment cost during production can be reduced.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the wavelength conversion laser device according to the present invention will be described below with reference to the drawings. FIG. 1 is a schematic configuration diagram of a wavelength conversion laser device according to this embodiment, and FIG. 2 is a schematic configuration diagram of a QPM element in this embodiment.

This device comprises a semiconductor laser 1 for generating pumping light, a lens 2 for collecting the pumping light from the semiconductor laser 1, and a stimulated emission of laser light containing fundamental wave light by being excited by the pumping light. Laser medium 3, a wavelength conversion element 4 that generates second harmonic light (here, simply referred to as "harmonic light") from the stimulated emission fundamental wave light, and selectively transmits light of a specific wavelength. The etalon 5 includes an etalon 5, an output mirror 6 that reflects light and partially transmits the light, and a thermo module 7 that controls the temperature of each optical member.

The operation of the apparatus having the above configuration will be briefly described. The excitation light emitted from the semiconductor laser 1 is condensed by the lens 2 and applied to the laser medium 3. A reflection layer 3a that transmits the excitation light efficiently and reflects the fundamental wave light and the harmonic light with a high reflectance is formed on the incident surface of the laser medium 3 for the excitation light. The mirror 6 constitutes an optical resonator. Therefore, the fundamental wave light that is stimulated and emitted from the laser medium 3 by the excitation light is oscillated and amplified in the optical resonator. The laser medium 3 is, for example, Nd: YAG for a blue emission wavelength of 946 nm and a green emission wavelength of 1064n.
Nd: YVO ₄ or the like is used for the oscillation of m.

Wavelength conversion element 4 inserted in the optical resonator
Due to the non-linear optical effect, light having a wavelength half the fundamental wave light, that is, harmonic light, is generated. Therefore, the fundamental wave light and the harmonic light are mainly mixed inside the optical resonator.
The fundamental wave light is reflected by the output mirror 6, while the harmonic light is transmitted through the output mirror 6. Therefore, as shown in FIG. 1, only the harmonic light is emitted from the output mirror 6 to the right. Although the etalon 5 is not an essential component for resonance, it has a function of selecting one of a plurality of modes generated during the oscillation here.

In the above structure, the wavelength conversion element 4 includes two QPM elements 4 as shown in FIGS. 2 (A) and 2 (B).
a and 4b are the upper surfaces facing the portion where the period inversion of the domain-inverted layer 42 is good (the above-mentioned uniform periodic portion), that is, the portion corresponding to the upper portion in FIG. 3 and FIG. 4B. In a state where they face each other, they are pasted together by using an adhesive 4c having a good translucency. Therefore, each QPM element 4
Assuming that the thickness of the uniform periodic portion in a and 4b is d, in the wavelength conversion element 4, the substantial thickness of the uniform periodic portion expands to 2d across the boundary surface. The wavelength conversion element 4 is arranged inside the optical resonator so that the optical axis C comes to the boundary surface of the wavelength conversion element 4 thus manufactured.

For example, in the example of the QPM element shown in FIG. 3, when one QPM element is used as a wavelength conversion element, a beam having a diameter of several tens of μm is 100 to 150 μm.
It is necessary to irradiate a uniform period part having a thickness of about m, and the position adjustment for that purpose is considerably strict, and even if adjustment is possible, the productivity does not increase. On the other hand, in the configuration of this embodiment, the thickness of the uniform periodic portion is 200 to 300 μm.
Since the beam is expanded to some extent and the allowable range of positional deviation with respect to the beam is expanded, the beam can be easily applied to this portion, and the productivity is significantly improved.

Therefore, in the wavelength conversion laser device of the present embodiment, since the fundamental wave light efficiently hits the portion where the period inversion of the polarization inversion layer of the wavelength conversion element 4 is good inside the optical resonator, a high wavelength is obtained. Conversion efficiency can be obtained, and higher-power harmonic light can be extracted to the outside. In addition, since the walk-off angle is generally zero in a QPM element,
If the element length (optical path length inside the element) can be increased, the conversion efficiency can be increased. However, this has the disadvantage that the optical resonator itself becomes large, but according to this configuration, in order to obtain the same conversion efficiency, the QPM
Since the element length of the element can be shortened, it is also advantageous for downsizing the optical resonator.

In FIG. 2B, the periods of the polarization inversion layers 42 of the upper and lower QPM elements 4a and 4b are perfectly matched, but in principle, the harmonics are present inside the wavelength conversion element 4. Since it suffices to perform phase matching between the portion where the wave light is generated and the optical axis C, it is not necessary to strictly match the upper and lower periods. In the above embodiment, the two QPM elements 4a and 4a
Although b is adhered with an adhesive, the same effect can be achieved without adhering as long as it is adhered as closely as possible. Furthermore, in the wavelength conversion element 4 having the above structure, the portion of the domain inversion layer 42 where the periodicity is not good is unnecessary. Therefore, as shown in FIG. 2C, this portion may be removed by polishing or the like.

In addition to the above, it is apparent that appropriate changes and modifications made within the scope of the present invention are also included in the scope of the claims of the present invention.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of a wavelength conversion laser device according to an embodiment of the present invention.

FIG. 2 is a schematic structural diagram of a QPM element according to an embodiment of the present invention.

FIG. 3 is a diagram showing a photograph of an actual observation of a plane obtained by vertically cutting the wavelength conversion crystal shown in FIG. 4B at a position P.

FIG. 4 is a diagram for explaining an example of a method of manufacturing a QPM element.

[Explanation of symbols]

1 ... Semiconductor laser 2 ... Lens 3 ... Laser medium 3a ... Reflective layer 4 ... Wavelength conversion element 4a, 4b ... QPM element 4c ... Adhesive 41 ... Bulk crystal 42 ... polarization inversion layer 5 ... Etalon 6 ... Output mirror 7 ... Thermo module

Claims (2)

[Claims] 1. A pumping light source that generates pumping light, a laser medium that is excited by the pumping light to emit fundamental wave light, and an optical resonator that amplifies the fundamental wave light emitted from the laser medium while resonating. A wavelength conversion laser device that is inserted in the optical path of the optical resonator and that generates a harmonic light from the fundamental wave light, and that extracts the harmonic light to the outside of the optical resonator. Is a part in which a plurality of quasi-phase-matching wavelength conversion crystals formed by forming periodic polarization inversion layers inside a bulk crystal are used, and the polarization inversion layers of the respective wavelength conversion crystals have good period uniformity. The wavelength conversion laser device is characterized in that the surfaces facing each other are in close contact with each other, and the contact surface or its vicinity is arranged so as to substantially coincide with the optical axis in the optical resonator. 2. The wavelength conversion element uses two of the wavelength conversion crystals, and the planes facing the portions of the polarization inversion layer of each wavelength conversion crystal where the period uniformity is good are brought into close contact with each other. The wavelength conversion laser device according to claim 1, wherein the wavelength conversion laser device is a laser.