JP2002287193A - Wavelength conversion element, wavelength conversion device and laser device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a wavelength conversion element having high conversion efficiency while compensating for the walk off effect, a wavelength conversion device and a laser device. SOLUTION: A refractive index matching means 9 is disposed in contact with a second face 5 of a nonlinear optical crystal 2, and a third face 11 which forms a total reflection plane is disposed in contact with the refractive index matching means 9.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength conversion element, a wavelength conversion device, and a laser device used for generating second harmonic, sum frequency, difference frequency and optical parametric light from laser light.

[0002]

2. Description of the Related Art As a technique for obtaining tunable coherent light, a second harmonic generation (SH) using a laser as an excitation light source is known.
G), sum frequency generation (SFG), difference frequency generation (DFG),
A method using a non-linear optical effect such as optical parametric generation (OPG) is known.

In the nonlinear optical effect, the refractive index of a medium changes according to the intensity of the incident light (more precisely, the electric field intensity). By utilizing this effect, a device capable of changing the wavelength of light can be configured. An optical harmonic generation element that doubles the frequency of light (halves the wavelength), an optical parametric wavelength conversion element that generates a sum frequency or a difference frequency by mixing two types of light waves, or a different wavelength for incident light And an optical parametric oscillator that generates the light wave of

In the nonlinear optical effect, a total of three electric fields, that is, incident light and light after wavelength conversion, interact via a second-order nonlinear susceptibility inherent to a substance, thereby satisfying the energy conservation rule before and after wavelength conversion. Thus, the optical process of converting one photon into two (or one of two photons). This means that the photon energy is split (coupled),
Accordingly, the wavelength becomes longer (shorter). What has been devised so as to efficiently generate this nonlinear optical effect is a wavelength converter.

[0005] A birefringent crystal is generally used as a medium for effectively producing a nonlinear optical effect as a wavelength conversion device.

In this birefringent crystal medium, phase matching can be satisfied between lights having different wavelengths before and after wavelength conversion, and efficient wavelength conversion is realized. Conversely, the conversion wavelength can be selected by determining the phase matching condition.

In a wavelength conversion device utilizing the birefringence of an optical crystal such as a birefringent crystal, the wavelength conversion efficiency increases in proportion to the square of the length of the optical crystal. In a conventional wavelength conversion device, there are an example in which a crystal having a long crystal length is used and an example in which a plurality of crystals are arranged on the optical axis to increase the wavelength in order to improve the wavelength conversion efficiency. Further, JP-A-10-260
No. 438 and Utility Model No. 2502615,
A technology that coats a part of the incident surface and the reflective surface of the optical crystal with a total reflection optical thin film to form a waveguide structure that folds the laser light passing through the optical crystal in a zigzag shape and lengthens the optical path in the optical crystal. It has been disclosed.

[0008]

In a wavelength converter utilizing the nonlinear optical effect of an optical crystal, it is necessary that three photons can interact with each other, and the efficiency depends on the distance in the light propagation direction. Become. Therefore, in order to improve the conversion efficiency, the propagation distance in a nonlinear medium that is an optical crystal must be increased.

However, while utilizing the birefringence of a nonlinear medium can satisfy the phase matching condition, when the harmonic is extraordinary light, the pointing vector of the extraordinary light is that of ordinary light (fundamental wave). An off-axis phenomenon (walk-off effect) occurs. Therefore, there is a limit on the propagation distance (interaction length) at which three lights can overlap at the same time. The refractive index for ordinary light is constant irrespective of the traveling direction of light, but in the case of extraordinary light, it is a function of the angle between the traveling direction of light and the crystal axis.

For example, in an SHG using a KTP (potadium titanyl phosphate) crystal, when the propagation distance in the crystal becomes 1 cm or more, the deviation of the extraordinary light due to the walk-off effect becomes larger than the beam diameter and becomes larger than this. Even if an optical crystal is used, improvement in conversion efficiency cannot be expected.

In order to solve the limitation of the interaction length as described above, it is general to arrange a plurality of optical crystals in the direction of light propagation such that their optical axes are alternated.

FIG. 5 is a schematic explanatory view of a conventional walk-off effect compensation. Non-linear optical crystals 51 and 52, which are two optical crystals, are arranged such that optical axes 53 and 54 are symmetrical with each other. In this state, incident light 55 is incident from one end face of the nonlinear optical crystal 51. At this time, the ordinary light 55a of the incident light 55 is shown by a solid line, and the extraordinary light 55b is shown by a dotted line. The ordinary light 55a and the extraordinary light 55b have different refractive indexes, and the nonlinear optical crystal 51 and the nonlinear optical crystal 52 have a symmetric relationship between their optical axes 53 and 54. Is canceled by the effect of the walk-off effect generated inside the nonlinear optical crystal 52, and the nonlinear optical crystal 5
The walk-off effect is compensated as a whole for 1, 52.

However, in this case, since a plurality of nonlinear optical crystals are used, technical difficulties such as phase matching and wavelength angle adjustment through the plurality of nonlinear optical crystals occur. In addition, the necessity of such an adjusting mechanism and cooling means for each nonlinear optical crystal causes a problem that the number of components and the volume of the entire apparatus increase.

In the technique disclosed in Japanese Patent Application Laid-Open No. Hei 10-260438 and Japanese Utility Model No. 2502615, a laser beam passing through the optical crystal is folded in a zigzag shape to form a waveguide structure. Although a technique for lengthening the optical path is disclosed, there is no description about compensation for the walk-off effect.

The present invention has been made based on these circumstances, and an object of the present invention is to provide a wavelength conversion element, a wavelength conversion device, and a laser device with high conversion efficiency that compensate for a walk-off effect.

[0016]

According to the first aspect of the present invention, an incident surface on which light is incident while being inclined with respect to the formed opposing surface and an exit surface on which the incident light exits are formed. Having a second surface that is one of the opposing surfaces that reflects or transmits the light that has entered the interior from the incident surface according to the wavelength, and a first surface that forms a total reflection surface on the other of the opposing surfaces. A nonlinear optical crystal, a refractive index matching means provided in close contact with the second surface, and a third surface forming a total reflection surface provided in close contact with the refractive index matching means. This is a characteristic wavelength conversion element.

According to the second aspect of the present invention,
The said refractive index matching means is a wavelength conversion element characterized by filling the inside with a refractive index matching liquid.

Further, according to the means of the invention of claim 3,
The refractive index matching liquid has a function as a constant temperature liquid, and is a wavelength conversion element.

According to the means of the invention of claim 4,
It has an incident surface on which light is incident while being inclined with respect to the formed facing surface, and an exit surface from which the incident light is emitted, and reflects or reflects the light incident from the incident surface to the inside according to the wavelength. A non-linear optical crystal including a second surface that is one of the opposing surfaces and a first surface that forms a total reflection surface on the other of the opposing surfaces, and a refractive index provided in close contact with the second surface A wavelength conversion element having a matching means, and a third surface forming a total reflection surface provided in close contact with the refractive index adjustment means, and the incident surface and the emission surface of the wavelength conversion element. A wavelength conversion device characterized in that a resonator mirror is provided so as to face each other.

According to the fifth aspect of the present invention,
A laser device using the wavelength converter described above as a wavelength converter that converts the wavelength of light emitted from a laser oscillator.

[0021]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a schematic configuration diagram of an optical parametric generation (OPG) device which is a wavelength conversion device of the present invention.

The nonlinear optical element 1 used as a wavelength conversion element for laser light or the like uses KDP (potassium dihydrogen phosphate), KTP (potadium titanyl phosphate), ADP (ammonium dihydrogen phosphate), or the like. . In this nonlinear optical element 1, a nonlinear optical crystal 2 has an optical axis 3 formed along a plane parallel to the longitudinal direction of the crystal, and a first surface 4 (upper surface in FIG. 1) in the longitudinal direction of the crystal and the crystal. The second surface 5 (the lower side surface in FIG. 1) is cut out and formed in a shape symmetrical with respect to the center line 6.
A total reflection coating is applied to the first surface 4, and cut surfaces polished as chamfered optical surfaces are formed at both ends. One of the cut surfaces is an incident surface 7 and the other is an output surface 8. In addition, a dichroic mirror 5a is formed on the lower surface, which is the second surface 5. The refractive index matching means 9 is in close contact with the dichroic mirror 5a.
The refractive index matching means 9 is provided with a total reflection mirror 12 forming a third surface 11 in close contact therewith.

The refractive index matching means 9 adjusts the refractive index with respect to the wavelength to be passed, and serves as a refractive index matching liquid (not shown) enclosed therein. Use a solvent, alcohol, or the like. Further, since the refractive index has a temperature dependency, the refractive index matching means 9 needs a constant temperature means such as a cooling liquid to maintain a constant temperature state.
In this case, if the refractive index matching liquid is also used as the cooling liquid, the apparatus can be further simplified.

Further, resonator mirrors 13 and 14 are arranged in parallel with the entrance surface 7 and the exit surface 8, respectively, to form an optical resonator.

With these configurations, laser light from a laser oscillator (not shown) is used as the incident light 15 as the resonator mirror 1.
3, the light passes through the nonlinear optical element 1 and exits from the resonator mirror 14 as the outgoing light 16, and forms the optical axis 3 (in the direction parallel to the plane of the drawing) of the crystal with the incident light 15. The phase matching angle, which is the angle, is θ, and the incident light 15
Will be described as an ordinary light 10a and a harmonic as an extraordinary light 10b.

The incident light 15 passes through the resonator mirrors 13 and 14, enters the nonlinear optical element 1 from the entrance surface 7 of the nonlinear optical element 1, and travels inside the nonlinear optical element 1.
The ordinary light 10a of the incident light 15 has a wavelength that is reflected by the second surface 5, and is reflected by the second surface 5, and the reflected light is reflected by the first surface 4 of the nonlinear optical element 1, and again, the second surface 5, the light travels in a zigzag manner inside the nonlinear optical element 1 and is reflected from the emission surface 8 of the nonlinear optical element 1 by the resonator mirror 14 and turned back, passes through the opposite optical path, and enters the resonator mirror 13. When the light reaches a predetermined level by repeating the amplification operation, the light passes through the resonator mirror 14 and is output.

On the other hand, since the extraordinary light 10b has a wavelength that is not reflected by the second surface 5, the extraordinary light 10b enters the inside of the refractive index matching means 9 after passing through the second surface 5, is refracted and proceeds, and travels on the third surface 11. The light reaches a certain total reflection mirror 12 and is reflected. The position where the light is reflected by the total reflection mirror 12 is on the virtual normal 17, and is just on the same virtual normal 17 as the position where the ordinary light 10 a is reflected on the second surface 5. If it is necessary to adjust the phase matching at that time, the tilt adjustment of the total reflection mirror 12 may be performed.
Thereafter, the reflected extraordinary light 10b travels inside the refractive index matching means 9, refracts and enters the nonlinear optical element 1, travels there, reaches the first surface 4 of the nonlinear optical element 1, and is reflected. I do. The position where the ordinary light 10a is reflected on the first surface 4 coincides with the position where the ordinary light 10a is reflected on the first surface 4. Therefore, at the position of the first surface 4, the optical axes of the ordinary light 10a and the extraordinary light 10b coincide with each other, and no walk-off effect occurs between the ordinary light 10a and the extraordinary light 10b. Thereafter, the extraordinary light 10b travels in a zigzag manner along the same path inside the nonlinear optical element 1 and travels from the exit surface 8 to the resonator mirror 14b.
, Reflected by the resonator mirror 14 and turned back, passes through the opposite optical path, enters the resonator mirror 13 and is reflected. When the amplification operation is repeated and reaches a predetermined level, the light passes through the resonator mirror 14. Output.

As described above, the ordinary light 10a and the extraordinary light 10b
The walk-off effect of the optical path, which is generated after the light enters the first surface 4 and reaches the second surface 5, reaches the first surface 4 after being turned back at the second surface 5 or the third surface 11. By then, the direction is reversed by the inverted walk-off effect. Thereafter, the light path turned back again by the first surface 4 proceeds through a similar process, so that the walk-off effect can be repeatedly canceled and compensated.

Next, as an example of the present invention, a modified example of the above-described wavelength converter will be described. FIG. 2 is a schematic configuration diagram of a modified example of a parametric generation (OPG) device that is a wavelength conversion device of the present invention. The same reference numerals are given to the same functional portions as those in FIG. 1, and the description thereof is omitted.

In this case, the basic structure of the nonlinear optical element 1 and the wavelength converter using the same is the same as that of the above-described embodiment, but the ordinary light 10a and the extraordinary light 10b are different in wavelength from each other. And a corresponding refractive index matching liquid (not shown) of the refractive index matching means 9a is used.

Also in this case, the incident light 15 passes through the resonator mirrors 13 and 14, enters the nonlinear optical element 1 from the incident surface 7 of the nonlinear optical element 1, and travels inside the nonlinear optical element 1. However, since the extraordinary light 10b of the incident light 15 has a wavelength that is reflected by the second surface 5, it is reflected by the second surface 5,
The reflected light is reflected by the first surface 4 of the nonlinear optical element 1, reflected again by the second surface 5, travels in a zigzag manner inside the nonlinear optical element 1, and emerges from the exit surface 8 of the nonlinear optical element 1. Then, the light is reflected by the resonator mirror 14 and turned back, passes through the opposite optical path, enters the resonator mirror 13 and is reflected. When the amplification operation is repeated and reaches a predetermined level, the resonator mirror 14
And output.

On the other hand, since the ordinary light 10a has a wavelength that is not reflected by the second surface 5, after passing through the second surface 5, it enters the inside of the refractive index matching means 9 to be refracted and travels to the third surface 11. The light reaches the total reflection mirror 12 and is reflected. The position where the light is reflected by the total reflection mirror 12 is on the virtual normal 17, and is exactly on the same virtual normal 17 as the position where the extraordinary light 10 b is reflected on the second surface 5. Thereafter, the reflected ordinary light 10a travels inside the refractive index matching means 9, refracts and enters the nonlinear optical element 1, travels, reaches the first surface 4 of the nonlinear optical element 1, and is reflected. . The position reflected by the first surface 4 coincides with the position where the ordinary light 10a is reflected by the first surface 4. Therefore, at the position of the first surface 4, the optical axes of the extraordinary light 10 b and the ordinary light 10 a coincide with each other, and the extraordinary light 1
No walk-off effect occurs between 0b and the ordinary light 10a. Thereafter, the ordinary light 10a follows the same path inside the nonlinear optical element 1 and travels in a zigzag manner, reaches the resonator mirror 14 from the emission surface 8, is reflected by the resonator mirror 14, and is turned back. The light passes through the resonator mirror 13 and is reflected there. When the amplification operation is repeated and reaches a predetermined level, the light passes through the resonator mirror 14 and is output as emission light 14.

As described above, similarly to the above-described embodiment, the walk-off effect of the optical path generated after the ordinary light 10a and the extraordinary light 10b enter the first surface 4 and reach the second surface 5 is as follows. After being folded at the second surface 5 or the third surface 11, it is canceled by the walk-off effect whose direction has been inverted before reaching the first surface 4. Thereafter, the light path turned back again by the first surface 4 proceeds through a similar process, so that the walk-off effect can be repeatedly canceled and compensated.

As described above, in the wavelength converters described in the above embodiments, the optical paths of the ordinary light 10a and the extraordinary light 10b output from the nonlinear optical element 1 coincide with each other. As an oscillator (OPO), highly efficient wavelength conversion is possible.

Next, a laser device equipped with the above-described wavelength converter will be described.

FIG. 3 is a schematic view of a laser device equipped with the wavelength converter 20 of the present invention. As the wavelength converter, the one shown in FIG. 1 or the one shown in FIG. 2 can be used. Also, in FIG.
1 and 2 are denoted by the same reference numerals,
The description of each of them will be omitted.

A laser oscillator 21 is arranged on the optical axis on the back surface of the resonator mirror 13 on which the incident light 15 enters the wavelength converter. A commonly used laser oscillator can be used as the laser oscillator 21, for example, YA as a laser medium.
Using a G rod, the laser is excited by an exciting means (not shown) such as a laser diode or a lamp, and amplified by two resonator mirrors 22 and 23 to output laser light.
The light enters the wavelength conversion device 20 from the back surface of the resonator mirror 13 of the wavelength conversion device 20. After the light enters the wavelength conversion device 20, an output in which the optical paths of the ordinary light 10a and the extraordinary light 10b coincide with each other can be obtained as described in the wavelength conversion device described above.

FIG. 4 is a schematic view of a modification of the laser device. Also in this case, the wavelength converter shown in FIG. 1 or the one shown in FIG. 2 can be used. In FIG. 3, the same reference numerals are given to the same functional portions as those in FIGS. 1 and 2, and the description thereof is omitted.

In this case, the basic arrangement of each part is the same as that of the laser device shown in FIG.
The resonator mirror 14a on the output side of the laser oscillator 21 also functions as the resonator mirror on the output side of the laser oscillator 21. The laser light output from the laser oscillator 21 is obtained by the wavelength converter 20 so that an output in which the optical paths of the ordinary light 10a and the extraordinary light 10b match is obtained, and is output from the resonator mirror 14a.

The light emitted from these laser devices can be used for processing a laser marker or the like, or for measuring a distance measuring device or the like. For example, 1 μm to 1.5 μm
In this conversion, eye-safe light that does not adversely affect the eyes can be obtained and used for these applications.

In each of the above embodiments, the case of the fundamental wave and the harmonic wave of the laser beam has been described. However, it goes without saying that the present invention can be similarly applied to the sum frequency and the difference frequency.

As described above, according to the present invention,
Using one nonlinear optical crystal, the optical path is turned between the first surface, the second surface, and the third surface, and the process is repeated to walk off the ordinary light 10a and the extraordinary light 10b in a single optical crystal. By compensating for the effect, an interaction length exceeding the length of the optical crystal can be made possible, and the wavelength conversion efficiency has been dramatically improved.

Further, since the number of parts to be increased thereby is small, not only the size can be reduced, but also the cost is extremely advantageous. As a result, in the walk-off effect compensating means using a plurality of nonlinear optical crystals, which has been a conventional problem, the number of parts increases due to the mounting of the nonlinear optical crystals and their cooling, and the accompanying increase in the size of the device. Has been resolved. If the refractive index matching liquid used in the refractive index matching means is shared with the cooling liquid, the apparatus can be further simplified.

Conventionally, the technically difficult problem of adjusting the same phase matching and wavelength angle tuning for each nonlinear optical crystal is that in the case of the present invention, the phase matching is adjusted by adjusting the tilt of the total reflection mirror. Alone can adjust the entire optical system, and the wavelength angle tuning can be extremely easily adjusted because the entire optical system can be adjusted only by adjusting the incident angle of the human light.

[0046]

According to the present invention, it is possible to obtain a highly efficient wavelength conversion element and a wavelength conversion device by compensating for a walk-off effect between ordinary light and extraordinary light in an optical crystal, and a laser device using the same. it can.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a wavelength conversion device of the present invention.

FIG. 2 is a schematic configuration diagram of a modified example of the wavelength conversion device of the present invention.

FIG. 3 is a schematic configuration diagram of a laser device of the present invention.

FIG. 4 is a schematic configuration diagram of a modified example of the laser device of the present invention.

FIG. 5 is a schematic configuration diagram of a conventional wavelength converter.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Nonlinear optical element, 2 ... Nonlinear optical crystal, 3 ... Optical axis, 4 ... First surface, 5 ... Second surface, 7 ... Incident surface, 8 ... Emission surface, 9 ... Refractive index matching means, 10a ... Ordinary light, 10b extraordinary light, 11 third surface, 12 total reflection mirror, 13 resonator mirror, 14 resonator mirror, 14a resonator mirror,
15: incident light, 16: outgoing light, 20: wavelength converter, 2
1: laser oscillator, 22: resonator mirror, 23: resonator mirror

Claims (5)

[Claims] 1. An incident surface on which light is incident obliquely with respect to a formed opposing surface, and an exit surface on which the incident light exits, and the light incident on the inside from the incident surface has a wavelength. A non-linear optical crystal including a second surface that is one of the opposing surfaces that reflects or transmits according to the first surface and a total reflection surface that is formed on the other of the opposing surfaces, in close contact with the second surface A wavelength conversion element comprising: a refractive index matching unit provided; and a third surface that forms a total reflection surface and is provided in close contact with the refractive index matching unit. 2. The wavelength conversion element according to claim 1, wherein said refractive index matching means is filled with a refractive index matching liquid. 3. The wavelength conversion element according to claim 2, wherein the refractive index matching liquid has a function as a constant temperature liquid. 4. An incident surface on which light is incident obliquely with respect to the formed opposing surface, and an exit surface on which the incident light exits, and the light incident from the incident surface to the inside has a wavelength. A non-linear optical crystal including a second surface that is one of the opposing surfaces that reflects or transmits according to the first surface and a total reflection surface that is formed on the other of the opposing surfaces, in close contact with the second surface A wavelength conversion element, comprising: a refractive index matching unit provided; and a third surface forming a total reflection surface provided in close contact with the refractive index matching unit. A wavelength conversion device comprising: a resonator mirror facing a surface and the emission surface. 5. A laser device using the wavelength converter according to claim 4 as a wavelength converter for converting the wavelength of light emitted from a laser oscillator.