JPH04323627A - Optical wavelength conversion method - Google Patents

Abstract

PURPOSE:To improve the efficiency of the wavelength converting method which uses a light wavelength converting element type II. CONSTITUTION:A laser beam basic wave omega which is linear polarized light is made incident on the light wavelength converting element type II1 and a 1st mixed wave consisting of both the basic wave omega and a 2nd higher harmonic 2omega is projected; and this 1st mixed wave is passed through a Faraday rotary polarizing means 3, reflected by a total reflecting means 4, and passed through the rotary polarizing means 3 in the opposite direction from the direction to match the linear polarizing direction of the remaining basic wave omega in the 1st mixed wave with the converting direction of the light wavelength converting element type II. Then the linear polarizing direction of the 2nd higher harmonic 2omega in the 1st mixed wave is deviated from the converting direction of the light wavelength converting element II and the wave is made incident on the light wavelength converting element type II1 in the opposite direction from the direction and part of the remaining basic wave omega in the 1st mixed wave is converted into the 2nd higher harmonic 2omega.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] In laser processing using a laser beam, the shorter the wavelength, the smaller the focus and the more precise processing becomes possible, so the second harmonic of the laser beam is used as the laser beam with the shorter wavelength. be done.

The present invention relates to an optical wavelength conversion method for generating a second harmonic of a linearly polarized laser beam using an optical wavelength conversion element type II, and particularly relates to a highly efficient optical wavelength conversion method.

0003

2. Description of the Related Art A conventional optical wavelength conversion method will be explained with reference to the drawings. 2 and 3 are diagrams showing the configuration and operation of a conventional optical wavelength conversion device. In the optical wavelength conversion element type II, the wavelength conversion axis (phase matching direction) of the crystal is 22 in the figure.
A KTP crystal 21 installed so as to be used is used. KTP
The second harmonic conversion efficiency of the crystal 21 is determined by the azimuth relationship between the wavelength conversion axis 22 of the crystal and the polarization direction of the incident light. As shown in FIGS. 2 and 3, only the component whose polarization direction is polarization direction 25 (±π/4 with respect to the X axis of the wavelength conversion axis) of the incident laser beam (fundamental wave) 23 is transmitted to the KTP crystal 2
1, a part of it is converted to the second harmonic,
Light is emitted from the KTP crystal 21, and light with other polarization directions passes through. Therefore, this emitted laser beam 24
is a mixed wave of the fundamental wave and the second harmonic, and the fundamental wave has two polarization directions, one parallel to the X-axis and one perpendicular to it, and the polarization direction of the second harmonic is perpendicular to the X-axis (the polarization direction in the figure 26).

[0004] The conventional method of generating second harmonics using an optical wavelength conversion element type II, typified by a KTP crystal, is to pass a laser beam through the optical wavelength conversion element type II arranged as described above only once. Then, the laser beam emitted from the optical wavelength conversion element type II, in which the fundamental wave and the second harmonic are mixed, is converted into the fundamental wave and the second harmonic using a dichroic mirror (not shown) that reflects the fundamental wave and transmits the second harmonic. It is separated into second harmonics, and the second harmonics are extracted from there.

[0005]

[Problems to be Solved by the Invention] However, in the conventional method using the optical wavelength conversion element type II, the KTP crystal 21
In the emitted laser beam 24 that has passed through, there remains a large amount of the fundamental wave that has not been converted to the second harmonic. Conventionally, there is no technology to convert this remaining fundamental wave back into the second harmonic. There was a problem that the remaining fundamental wave was wasted.

The present invention solves the above problems and converts the fundamental wave remaining in the emitted laser beam after passing through the optical wavelength conversion element type II into a second harmonic by a method using the optical wavelength conversion element type II. The objective is to provide an optical wavelength conversion method that can be used without waste.

[0007]

[Means for Solving the Problems] In order to solve the above problems, the optical wavelength conversion method of the present invention converts a linearly polarized laser beam fundamental wave into a second harmonic using an optical wavelength conversion element type II. In the optical wavelength conversion method, a fundamental wave whose linear polarization direction is aligned with the conversion direction of the optical wavelength conversion element type II is converted into optical wavelength by passing it through a half mirror that reflects this fundamental wave and transmits the second harmonic. incident on element type II,
A part of this fundamental wave is converted into a second harmonic, a first mixed wave in which the remaining fundamental wave and second harmonic are mixed is emitted, and this first mixed wave is passed through Faraday optical rotation means, By reflecting by the total reflection means and passing through the optical rotation means again from the opposite direction, the linear polarization direction of the remaining fundamental wave in the first mixed wave is changed to the conversion direction of the optical wavelength conversion element type II. At the same time, the linear polarization direction of the second harmonic in the first mixed wave is removed from the conversion direction of the optical wavelength conversion element type II, and the second harmonic is incident on the optical wavelength conversion element type II from the opposite direction. A part of the remaining fundamental wave in the first mixed wave is converted into a second harmonic, and this first
The second harmonic in the mixed wave is converted to the optical wavelength conversion element type II.
The optical wavelength conversion element type II generates a second mixed wave in which the second harmonic that was converted on the outward path and passed through on the return path, the second harmonic that was converted on the return path, and the remaining fundamental wave are mixed. is emitted, and this second mixed wave is separated into a fundamental wave and a second harmonic through the half mirror.

[0008]

[Operation] With the above-described configuration, the present invention provides a method of converting optical wavelength using an optical wavelength conversion element type II, in which the second harmonic is converted into the second harmonic remaining in the output laser beam after passing through the optical wavelength conversion element type II. The first mixed wave of the fundamental wave and the second harmonic that has not been detected is passed through the Faraday optical rotation means, then reflected by the total reflection means, and passed through the Faraday optical rotation means again from the opposite direction. The linear polarization direction of the remaining fundamental wave in the first mixed wave is adjusted to the conversion direction of the optical wavelength conversion element type II, and the linear polarization direction of the second harmonic in the first mixed wave is adjusted to the optical wavelength conversion. A part of the remaining fundamental wave in the first mixed wave is converted into a second harmonic by entering the optical wavelength conversion element type II from a direction opposite to the conversion direction of the element type II. At the same time, the second harmonic in the first mixed wave is passed through the optical wavelength conversion element type II,
The optical wavelength conversion element type II emits a second mixed wave in which the second harmonic that was converted on the outward path and passed through on the return path, the second harmonic that was converted on the return path, and the remaining fundamental wave are mixed. The second mixed wave is passed through a half mirror,
By separating the fundamental wave and the second harmonic, the conversion efficiency of the second harmonic to the energy of the incident laser beam can be increased without increasing the number of expensive optical wavelength conversion elements type II.

[0009]

[Embodiment] An embodiment of the present invention will be explained based on the drawings. FIG. 1 is a diagram showing the configuration and operation of an embodiment of the present invention when a KTP crystal is used as the optical wavelength conversion element type II.

In FIG. 1, an optical wavelength conversion element type II
The KTP crystal 1 used as a wavelength conversion axis (
A KTP crystal 1 installed so that the phase matching direction) is in the direction 2 in FIG. 1 is used. This optical wavelength conversion element type I
I1 indicates that only the component whose polarization direction is ±π/4 with respect to the X-axis of the wavelength conversion axis (phase matching direction) 2 in FIG. It is converted into a second harmonic and is emitted from the KTP crystal 1, while components in other polarization directions pass through the optical wavelength conversion element type II.

[0011] The π/8-rotated Faraday rotation photon used as a Faraday optical rotation means (can be used either clockwise or counterclockwise; the one shown is clockwise) rotates the polarization direction of the fundamental wave by π/8 to the right. . A total reflection mirror 4 used as a total reflection means totally reflects both the fundamental wave and the second harmonic when the laser beam is perpendicularly incident thereon. The dichroic mirror 5 used as a half mirror totally reflects the fundamental wave of the laser beam and transmits the second harmonic. Laser beam 6, 8, 10
, 12, 14, 15, 17, and 19 pass through and reflect each optical element in this order. Polarization direction 7, 9, 11, 13, 1
6 and 18 indicate the polarization direction of the laser beam at each point. ω represents the fundamental wave, and 2ω represents the second harmonic.

The operation of the embodiment of the present invention is as follows. A linearly polarized laser beam (fundamental wave ω) 6 extracted by Pockels cell Q switch transmission (not shown) or the like is applied to the dichroic mirror 5 in the horizontal direction 45 with a polarization direction tilted by π/4 in the horizontal direction as shown in the polarization direction 7. incident at an angle of degrees.

The laser beam 8 reflected by the dichroic mirror 5 is aligned with the wavelength conversion axis (phase matching direction) of the crystal.
As shown in FIG. 1, the X-axis direction of 2 is incident on the KTP crystal 1, which is placed parallel to the linearly polarized laser beam (fundamental wave ω) 6. The polarization direction of the laser beam 8 is π in the horizontal direction, as shown in the polarization direction 9 in FIG.
The fundamental wave ω remains tilted by /4.

Laser beam 10 passing through KTP crystal 1
, a part of which is converted to the second harmonic 2ω (for the sake of explanation, it is assumed here that 50% is converted to the second harmonic 2ω). After conversion, the polarization direction of the mixed laser beam 10 of the remaining fundamental wave ω and the converted second harmonic 2ω is as shown in the polarization direction 11, and the remaining fundamental wave ω is divided into two horizontal and vertical straight lines. In the polarization direction, the second harmonic 2
ω is the vertical linear polarization direction. mixed laser beam 10
is incident on the Faraday rotation photon 3 which rotates clockwise by π/8, and is emitted as a laser beam 12.

The polarization direction of the laser beam 12 is such that the remaining fundamental wave ω becomes linearly polarized in directions -π/8 and 3π/8 from the horizontal direction, as shown in the polarization direction 13 in the figure.
The second harmonic 2ω becomes linearly polarized light rotated by π{2−(λ1/λ2)×2/8} from the horizontal direction. Here λ1 is the wavelength of the fundamental wave ω, λ2
is the wavelength of the second harmonic 2ω. The laser beam 12 is
The light enters the total reflection mirror 4 at an incident angle of 0 degrees, is totally reflected, and becomes a laser beam 14 whose traveling direction is opposite to the laser beam 12.

The polarization direction of the laser beam 14 remains as shown in the polarization direction 13, and it again enters the Faraday rotator 3 with π/8 rotation, and is emitted as a laser beam 15.

As shown in the polarization direction 16 in the figure, the polarization direction of the laser beam 15 is such that the fundamental wave ω is within ±π/
4, and the second harmonic 2ω becomes linearly polarized light rotated by π{2-(λ1 /λ2)×2/4} from the horizontal direction, but in reality, λ1 vs. λ2 is approximately 1/
2, as shown in FIG. The energy component of the remaining fundamental wave ω is the original linearly polarized laser beam (fundamental wave ω)6
It is about 50% of the total. The laser beam 15 is again
Passes through KTP crystal 1.

At this time, a portion of the remaining fundamental wave ω of the laser beam 15, which is linearly polarized light in two directions of ±π/4 from the horizontal direction, becomes the second harmonic 2ω due to the conversion efficiency of the KTP crystal 1. converted. Then, the second harmonic 2ω that has been converted earlier passes through the KTP crystal 1 as the linearly polarized light in the direction rotated from the horizontal direction by π{2-(λ1 /λ2)×2/4}. do. For the sake of explanation, if we assume that the conversion efficiency of the KTP crystal 1 to the second harmonic 2ω is proportional to the square of the energy density of the incident fundamental wave ω, then by passing through the KTP crystal 1 this second time, the original linearly polarized laser beam (fundamental wave ω)
6, approximately 12.5% of the fundamental wave ω is converted to the second harmonic 2ω.

After this conversion again, the remaining fundamental wave ω
The mixed laser beam 17 of
8, the fundamental wave ω becomes linearly polarized light in two polarization directions, horizontal and vertical, and its second harmonic 2ω
becomes two linearly polarized lights, one in the vertical direction and the other in the direction rotated from the horizontal direction by the amount of π{2-(λ1/λ2)×2/4}, as shown in the polarization direction 18 in FIG.

This mixed laser beam 17 is incident on the dichroic mirror 5 at an angle of 45 degrees in the horizontal direction. This mixed laser beam 1 is formed by a dichroic mirror 5.
The fundamental wave ω of 7 is totally reflected, and only the second harmonic 2ω is transmitted through the dichroic mirror 5 and is emitted as a laser beam 19.

The linear polarization direction of the second harmonic 2ω of the laser beam 19 remains the second harmonic 2ω of the polarization direction 18 in FIG.

The optical wavelength conversion device of the present invention uses only one KTP crystal 1 with a conversion efficiency of 50%, and converts 62.5% of the energy of the original linearly polarized laser beam (fundamental wave ω) 6 into the first crystal. It can be converted into second harmonic 2ω. However, this calculation is based on the reflection at the absorption/transmission interface in the optical element,
It is also assumed that there is no second harmonic walk-off phenomenon in the KTP crystal.

The optical wavelength conversion device of the present invention is not limited to the above-mentioned embodiments, and various embodiments are possible.

For example, the optical wavelength conversion element type II is K
Materials other than TP crystals may also be used. Faraday optical rotation means
The Faraday rotation photon is not limited to a rotation of π/8, but may be a rotation of an odd number of times of π/8 as long as the polarization direction of the fundamental wave can be matched to the conversion direction of the optical wavelength conversion element type II. The angle of incidence on the half mirror is not limited to 45 degrees.

[0025]

[Effects of the Invention] The present invention has the above configuration and operation.
In the conventional technology, the fundamental wave ω remaining in the emitted laser beam after passing through the optical wavelength conversion element type II, which could not be converted into the second harmonic 2ω, is converted into one optical wavelength conversion element type I.
In order to pass I again and convert it to the second harmonic 2ω, without increasing the number of optical wavelength conversion elements type II,
Second harmonic 2 relative to the energy of the original incident laser beam
The conversion efficiency to ω can be increased. Therefore, when the optical wavelength conversion method of the present invention is used, it is possible to perform precise processing efficiently in laser processing using a laser beam with a shorter wavelength and higher energy than conventional methods.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the configuration and operation of an embodiment of the present invention.

FIG. 2 is a diagram showing the configuration and operation of a conventional example.

FIG. 3 is a diagram showing the configuration and operation of a conventional example.

[Explanation of symbols]

ω Fundamental wave 2 ω Second harmonic 1 KTP crystal (light wavelength conversion element type II) 3
π/8 rotating Faraday optical rotator (Faraday optical rotation means) 4 Total reflection mirror (total reflection means)

Claims (1)

[Claims] Claim 1. In an optical wavelength conversion method of converting a linearly polarized laser beam fundamental wave into a second harmonic using an optical wavelength conversion element type II, the linear polarization direction is aligned with the conversion direction of the optical wavelength conversion element type II. The fundamental wave is passed through a half mirror that reflects this fundamental wave and transmits the second harmonic.
The light enters the optical wavelength conversion element type II, converts a part of this fundamental wave into a second harmonic, and outputs a first mixed wave in which the remaining fundamental wave and second harmonic are mixed. The mixed wave is passed through the Faraday optical rotation means, reflected by the total reflection means, and passed through the optical rotation means again from the opposite direction, thereby changing the linear polarization direction of the remaining fundamental wave in the first mixed wave to the above-described direction. In addition to matching the conversion direction of the optical wavelength conversion element type II, the linear polarization direction of the second harmonic in this first mixed wave is removed from the conversion direction of the optical wavelength conversion element type II, so that the optical wavelength conversion element type II is incident from the opposite direction to the above, converts a part of the remaining fundamental wave in this first mixed wave into a second harmonic, and converts the second harmonic in this first mixed wave into the optical wavelength conversion. Let element type II pass through,
The optical wavelength conversion element type II emits a second mixed wave in which the second harmonic that was converted on the outward path and passed through on the return path, the second harmonic that was converted on the return path, and the remaining fundamental wave are mixed. An optical wavelength conversion method characterized in that the second mixed wave is separated into a fundamental wave and a second harmonic through the half mirror.