JPH11346019A - Laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To conduct a miniaturization of the whole laser even while light, which should not be made to output outside of the optical resonator of the laser, is effectively removed in the case where there is such the light in the interior of the resonator. SOLUTION: A laser is a solid laser of a structure, wherein a laser, particularly a semiconductor laser element is used as an excitation light source 1 and the laser has a laser crystal 2. At least the output side mirror 4 out of one pair of mirrors 3 and 4 of an optical resonator is formed as an independent mirror component. The mirror 4 is formed using a transparent member as its parent body 4b. A dielectric multilayer film 4a, which constitutes the resonator and reflects a laser oscillation beam, is provided on the surface, which is used as the inside of the resonator, of said parent body. Moreover, a dielectric multilayer film 4c, which transmits wavelength light Lx, which should be made to output from said optical resonator, and reflects wavelength light Ly, which should not be made to output from the resonator, is provided on the surface, which is used as the outside of the resonator, of the parent body 4b.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention belongs to the technical field of laser devices, and more particularly, to a device capable of wavelength-converting laser light in an optical resonator and outputting the converted light.

[0002]

2. Description of the Related Art A solid-state laser device using a semiconductor laser element as an excitation light source can be manufactured relatively easily as a small-sized one with an output of about several mW to several tens of mW. The basic configuration of such a solid-state laser device is as follows:
Laser light from a semiconductor laser element, which is an excitation light source, is incident on a laser crystal inside the optical resonator by an optical lens, and a specific wavelength light emitted from the crystal is laser-oscillated by the optical resonator, and the output of the optical resonator The light is emitted from the side mirror.

In the configuration of the optical resonator described above, an end face of a laser crystal is usually coated with an appropriate coating to form a mirror on one side for miniaturization. For further miniaturization, the pumping light from the semiconductor laser element is axially pumped along the optical axis of the optical resonator through the mirror (incident side mirror) to enter the laser crystal. On the other hand, in order to perform fine adjustment of the laser transverse mode, an independent mirror having an appropriate reflectivity to the laser oscillation light is used as one component for the other side mirror (output side mirror).

In such a configuration of the solid-state laser device,
Excessive pumping light that has not contributed to the pumping of the laser active medium may also be a problem because it is mixed with the output light and exits the optical resonator.

On the other hand, there is known an apparatus in which a wavelength conversion element such as an SHG element is arranged on an optical path in an optical resonator, a fundamental wave is oscillated by a laser in the optical resonator, and the fundamental wave is wavelength-converted and output. ing. Usually, in such an apparatus, it is preferable that a fundamental wave is not output because light resulting from wavelength conversion is required.

For this reason, a resonator is formed by setting the reflectivity of the output side mirror to the fundamental wave as high as possible and almost 100%. In this case, the electric field inside the resonator becomes very strong, so that a slight transmittance ( For example, even if the reflectivity of the mirror is 99.9%, 0.1%), some fundamental wave leaks out of the resonator.

In the above case, in order to remove extra pumping light and fundamental wave, a filter for cutting off the light is usually provided after the output side mirror as an independent component. As this filter, a color glass filter having various characteristics depending on the wavelength is used.

[0008]

However, in a configuration using a filter, the overall length of the laser device is lengthened by that amount, so that there has been a problem that the entire device cannot be sufficiently reduced in size. As a measure against this,
As described in JP-A-6-69573,
Attempts have been made to use the above color glass filter as a base material of an output mirror. However, in this case, since the refractive index distribution in the color glass is larger than that of a normal synthetic quartz base material, there is a problem that an in-plane distribution occurs in the transmittance when a dielectric multilayer mirror is deposited. Was.

An object of the present invention is to provide a laser device,
When there is light inside the optical resonator that should not be output to the outside of the optical resonator, it is necessary to reduce the size of the entire laser device while effectively removing such light.

[0010]

The light intensity modulation laser device according to the present invention has the following features. (1) Of the pair of mirrors of the optical resonator, at least the output side mirror is formed as an independent mirror component, and the output side mirror has a transparent member as a base, and the output side mirror has a surface inside the optical resonator of the base. Is provided with a dielectric multilayer film that reflects laser oscillation light from a laser active medium in order to form an optical resonator, and a light having a wavelength to be output from the optical resonator is provided on a surface outside the optical resonator. A dielectric multilayer film that transmits light and reflects light of a wavelength that should not be output.

(2) The laser device according to (1), wherein the light having a wavelength to be output from the optical resonator is laser oscillation light, and the light having a wavelength not to be output is excitation light.

(3) A wavelength conversion element for converting the wavelength of the laser oscillation light is provided between a pair of mirrors of the optical resonator, and the light of the wavelength to be output from the optical resonator is converted by the wavelength conversion element. The laser device according to the above (1), wherein the light having a wavelength which should not be outputted is laser oscillation light and excitation light.

(4) The surface of the base outside the optical resonator is a surface that intersects a plane perpendicular to the central axis of the pair of mirrors at an angle greater than 0 degree and 45 degrees or less. )
The laser device as described.

Hereinafter, the light having the wavelength to be output will be referred to as “output light”, and the light having the wavelength not to be output will be referred to as “non-output light”.

[0015]

FIG. 1 shows a configuration example of a laser device according to the present invention. In the example of FIG.
Is used as an excitation light source, and a solid-state laser device in which a laser active medium (laser crystal) 2 is sandwiched by an optical resonator including a pair of mirrors 3 and 4. The description of the optical system is omitted. Excitation light L1 from the excitation light source 1 enters the laser crystal 2 from one side on the optical axis of the optical resonator through the mirror 3 on the incident side, and the laser oscillation light L2 oscillated by the optical resonator is output light Lx Out of the output side mirror 4. The problem in the example of FIG. 1 is that the excitation light L1 passes through the laser crystal 2 without being completely used,
That is, an attempt is made to exit from the output side mirror 4 as the non-output light Ly. FIG. 7 shows an example of a mode for preventing this.

In the present invention, as shown in FIG. 1, at least the output side mirror 4 of the pair of mirrors 3 and 4 of the optical resonator is formed as an independent mirror part. The output side mirror 4 has a structure in which a base member 4b is made of a transparent member, and dielectric multilayer films 4a and 4c for giving a reflection function to the base member 4b. A dielectric multilayer film 4a for reflecting light of a predetermined wavelength and causing laser oscillation is provided on a surface inside the optical resonator of the base 4b so as to form an optical resonator. The reflectivity of the dielectric multilayer film 4a is set to an appropriate value so that the laser oscillation light L2 is output. On the other hand, on the surface outside the optical resonator, the dielectric multilayer film 4c that transmits the output light Lx (= L2) and reflects the non-output light Ly (= L1)
Is provided.

With the above configuration, the non-output light L
It is possible to prevent y from going out of the optical resonator on the outer surface of the optical resonator. This configuration does not require a separate component such as a filter, so that the entire laser device is more compact. Further, as a base material of the output mirror, a commercially available high-quality base material such as synthetic quartz or BK7 can be used as it is, as they have no refractive index distribution and their values are accurately known. The design and fabrication of the mirror are also easy.

As shown in FIG. 1, the basic configuration of the laser device is preferably a solid laser device using a semiconductor laser element as an excitation light source. As a laser active medium,
YAG crystal doped with Nd or Tm, or Nd or E
A laser active medium for a general solid-state laser device such as a YVO ₄ crystal doped with r can be used.

The wavelength of the excitation light from the semiconductor laser element is determined by the combination with the laser active medium used to obtain a desired oscillation wavelength. For example, when using a laser active medium doped with a rare earth element (or directly formed as a compound), an excitation light having a wavelength appropriate for the type of the rare earth element should be selected. In particular, a combination using a Nd-doped YVO ₄ crystal as a laser active medium and a semiconductor laser element that emits a laser beam having a wavelength of about 810 nm as an excitation light source is preferable because an inexpensive and compact solid-state laser device can be formed.

In order to reduce the size of the laser, it is preferable to apply a coating to the end face of the laser crystal to provide a reflection function, and use this as a mirror on one side of a pair of mirrors of the optical resonator. When the excitation light is made incident on the laser crystal through the mirror, this mirror becomes the incident side mirror.

As described above, the output side mirror is formed as an independent mirror component, has a structure in which a transparent member is used as a base, and a dielectric multilayer film for providing a reflection function is provided thereon. Of the matrix, the surface inside the optical resonator is
A reflection surface for the optical resonator, and a surface outside the optical resonator is a reflection surface for blocking non-output light. Hereinafter, the dielectric multilayer film on the surface inside the optical resonator of the base is referred to as the “inner film”.
Also, the dielectric multilayer film on the surface outside the optical cavity of the base material will be referred to as “outer film”.

In order to stabilize laser oscillation, the shape of the transparent member serving as a base should be such that the surface inside the optical resonator (reflection surface for the optical resonator) is a curved surface having an appropriate curvature. it can. The material of the transparent member may be a material that is transparent to light in a wide wavelength range including the wavelength of laser oscillation light, and does not require a special amount of transmission characteristics only at a specific wavelength as in color glass. . This is because a reflection function of blocking emission of non-output light is separately provided by the dielectric multilayer film. The term “transparent” as used herein means that it is not intended to absorb a specific wavelength. Specific materials for the transparent member include, for example, synthetic quartz, BK7
And the like.

The inner film has a certain degree of transmittance for the output light in order to extract the output light to the outside. This transmittance may be about 90 to 99% as in the case of a normal optical resonator.

The outer film has a transmittance as high as possible for output light as long as the object of the present invention is achieved.
The non-output light has as large a reflectivity as possible.
Preferably, the transmittance of the output light is about 90% to 100%, and the reflectance of the non-output light is about 0.01% to 0.5%.

Next, as an effective embodiment of the present invention, a mode of a small solid-state laser device capable of emitting visible light will be described. FIG. 2 shows an example of a specific mode, which is a solid-state laser device using a semiconductor laser element 1 that emits infrared light as an excitation light source. A pair of mirrors 3 and 4 of the optical resonator
(Actually, between the laser crystal 2 and the output-side mirror 4), there is further provided a wavelength conversion element 5 for converting the wavelength of the laser oscillation light L2 into visible light using the fundamental wave, The wavelength-converted visible light is output as output light Lx through the output side mirror 4. In this example, only the wavelength-converted visible light is the output light Lx, and the fundamental wave (laser oscillation light) and the excitation light are the non-output light Ly.

Examples of the wavelength conversion element include an element (SHG element) for converting the wavelength by generation of the second harmonic, and an element (OPO element) for converting the wavelength by optical parametric oscillation. The wavelength conversion by these elements is not limited to the conversion to visible light as in the example of FIG. As the SHG element or OPO element, KTiO
A PO ₄ crystal, a BaB ₂ O ₄ crystal, a LiNbO ₃ crystal (PPLN) in which periodic polarization inversion is formed, and a LiTaO ₃ crystal (PPLT) in which polarization is inverted similarly can be used. Z-cut LiNbO with periodic polarization inversion
An element using _three crystals is preferable because it has high conversion efficiency by quasi-phase matching.

As shown in FIG. 2, by inserting the SHG element 5 in the optical resonator, a high-efficiency, high-output visible light laser device can be easily manufactured. At this time, by forming the dielectric multilayer film 4a of the output side mirror so as to have a reflectance of almost 100% with respect to the oscillation wavelength that becomes the fundamental wave L2,
The confinement of the fundamental wave can be increased to increase the oscillation intensity of the fundamental wave, increase the strength of the SHG, and prevent extra fundamental waves from coming out. However,
Actually, a mirror having a reflectivity of 100% cannot be manufactured, so a mirror having a reflectivity of about 99.9% is used. On the other hand, the energy of the fundamental wave confined in the optical resonator becomes considerably high, and even if it is 0.1% or less, the fundamental wave transmitted through the output mirror becomes energy that cannot be ignored. Therefore, a dielectric multilayer film 4c for exclusively removing the leaked fundamental wave as the non-output light Ly is provided outside the base 4b.

With this configuration, the leakage of the fundamental wave is reflected by the outer film as the non-output light Ly, and only the SHG light is output as it is as the output light Lx, and a compact visible light laser device without an independent filter member is obtained. . The reflectance of the fundamental wave on the outer film is the same as that on the inner surface.
If it is set to about 9%, the output of the fundamental wave finally leaking to the outside will be about 0.0001% of the internal electric field strength, which is a low level that does not cause any problem in practical use.

When an OPO element is used as a wavelength conversion element, a signal wave and an idler wave are generated, and there is a case where only one of these lights is desired to be used as output light. For example, there is a case where only a signal wave is used as output light and an idler wave is used as non-output light. Further, the fundamental wave and the pump light remaining without being converted by the OPO element are also not output. As in the case of FIG. 2, by providing a dielectric multilayer film for exclusive use to remove these non-output lights outside the base, a compact OPO laser device without an independent filter member can be obtained.

The transmission and reflection characteristics of the outer film of the output-side mirror may be any as long as the output light can be transmitted and the non-output light can be reflected. There is no limitation on the number of layers or the structure of the dielectric multilayer film. For example, in the configuration of FIG.
In order to output only the second harmonic as the fundamental wave 1064 nm and the second harmonic 532 nm, a 532 nm band-pass filter type transmission characteristic can be used.

As a result of the non-output light reflected by the outer film of the output-side mirror returning to the direction inside the optical resonator, a problem may occur in the characteristics of the solid-state laser device itself. For example, when the excitation light reflected by the outer film returns to the semiconductor laser device that is the excitation light source and destabilizes the characteristics of laser oscillation, or when the optical resonator structure for the fundamental wave becomes double and causes interference. And the like.

In the present invention, such a phenomenon caused by the apparatus of the present invention is set as a problem to be further solved, and a configuration for solving the problem is proposed. That is, as shown in FIG. 1, the face Fa of the base 4b on the outside of the optical resonator intersects the face Fb perpendicular to the central axis m of the pair of mirrors of the optical resonator at an appropriate angle θ. It is a configuration to form. With this configuration, the non-output light is prevented from being inverted by 180 degrees and returning to the original direction, and problems such as destabilizing the characteristics of laser oscillation are solved. The angle θ is not limited, but is preferably greater than 0 degrees and 45 degrees or less.

[0033]

EXAMPLE An example of actually manufacturing a solid-state laser device having the structure shown in FIG. 2 will be described. The excitation light source 1 has a wavelength of 810 nm,
A semiconductor laser element having a continuous (CW) oscillation and a maximum output of 150 mW is used.
The wavelength of the fundamental wave is set to 1064 nm using a VO ₄ crystal,
The SHG element 5 has a 5 mm thick KTi in the direction of the central axis m.
Using an OPO ₄ crystal, the second harmonic (532 nm green light) was used as output light Lx, and leakage of the fundamental wave of 1064 nm was used as non-output light Ly.

The temperature of the semiconductor laser device was adjusted to 25.0 ° C. using a Peltier device. In addition, the configuration is such that the excitation light L1 from the semiconductor laser element is incident on the laser crystal 2 using an optical lens (not shown).

The YVO ₄ crystal is a parallel-plane material having an Nd concentration of 1.0 at% and a thickness of 1 mm in the direction of the central axis m. A high-reflection coating was applied to the reflected light at 1064 nm to obtain an incident-side mirror 3. The opposite end face was coated with an anti-reflection coating for 1064 nm.

The output side mirror 4 is made of synthetic quartz as the material of the transparent member to be the base 4b. As the base, a plane concave / convex substrate having an outer flat surface with a radius of curvature of 50 mm on the inner surface of the optical resonator is used. As shown in FIG.
Was polished so as to intersect with the plane Fb perpendicular to the central axis m at an angle of 3 degrees.

The inner film 4a of the output side mirror has the fundamental wave 10
About 99.9% reflectivity for 64 nm, second harmonic 5
A dielectric multilayer film having a transmittance of about 95% for 32 nm was formed by coating.

Similarly to the inner side, the fundamental wave 1064 leaking through the inner side membrane 4a is also applied to the outer side membrane 4c of the output side mirror.
99.9% reflectivity to nm, second harmonic 532n
A dielectric multilayer film having a transmittance of about 95% with respect to m was formed by coating.

When the solid-state laser device was operated as the above configuration, the second harmonic 532 nm as the output light Lx was obtained.
The light has an output of 10 mW and is a non-output light Ly of 1064.
The nm light was below the detection limit of the optical power meter.

The total length of the device from the semiconductor laser element to the outermost end of the output side mirror is 25 mm, which is 10 m longer than the conventional configuration in which non-output light is cut using a filter.
m shorter and a compact device.

COMPARATIVE EXAMPLE The solid-state laser device of Embodiment 1 was constructed in exactly the same manner as in Example 1, except that the outer film 4c of the output-side mirror was not formed.
When operated, the second harmonic 532 nm which is the output light
The light output was 12 mW, but at the same time 1
064 nm light was also output at about 5 mW.

[0042]

As described above, the laser device according to the present invention makes it possible to make the entire laser device more compact while effectively eliminating non-output light. Further, by appropriately tilting the outer surface of the output mirror, various problems that non-output light reflected by the outer film of the output mirror returns in the direction inside the optical resonator can be avoided.

[Brief description of the drawings]

FIG. 1 is a diagram schematically showing a configuration of a main part of a laser device according to the present invention.

FIG. 2 schematically shows another embodiment of the laser device according to the present invention.

[Explanation of symbols]

 Reference Signs List 1 excitation light source 2 laser active medium 3 one mirror of optical resonator 4 output mirror of optical resonator 4a inner film 4b base 4c outer film L1 excitation light L2 fundamental wave Lx output light Ly non-output light

Claims (4)

[Claims] At least an output-side mirror of a pair of mirrors of an optical resonator is formed as an independent mirror component, and the output-side mirror has a transparent member as a base, and the inside of the base is inside the optical resonator. The surface is provided with a dielectric multilayer film that reflects laser oscillation light from a laser active medium to form an optical resonator, and the surface outside the optical resonator has a wavelength to be output from the optical resonator. 1. A laser device comprising a dielectric multilayer film that transmits light of a wavelength and reflects light of a wavelength that should not be output. 2. The laser device according to claim 1, wherein the light having a wavelength to be output from the optical resonator is laser oscillation light, and the light having a wavelength not to be output is excitation light. 3. A wavelength conversion element for wavelength-converting laser oscillation light is provided between a pair of mirrors of the optical resonator, and light of a wavelength to be output from the optical resonator is wavelength-converted by the wavelength conversion element. 2. The laser device according to claim 1, wherein the light having a wavelength which should not be output is laser oscillation light and excitation light. 4. A surface of the base outside the optical resonator is 1
2. The laser device according to claim 1, wherein the surface intersects a plane perpendicular to the central axis of the pair of mirrors at an angle greater than 0 degree and 45 degrees or less.