JPH041616A - Optical wavelength converting method - Google Patents

Abstract

PURPOSE:To obtain high wavelength converting efficiency and to easily obtain even a second harmonic wave of a blue region by adequately setting the incident direction and polarizing direction of the basic wave to be made incident on a specific crystal. CONSTITUTION:The single crystal of the nonlinear optical material (4- hydroxy-3-methoxy benzonitrile HMN) expressed by formula I is cut at the X-Y plane inclusive of the optical axis X and Y axis (a axis and b axis of the crystal axes) and is cut out in the Z axis direction (c axis of the crystal axis) to form the wavelength converting element 10 of a bulk single crystal type. A laser beam (basic wave) 15 is made incident on this wavelength converting element 10 by inclining the beam by an angle theta in to the Y axis side from the X axis in such a manner that the polarization direction is directed approximately parallel approximately with the (b) axis. The wavelength of the basic wave 15 is converted by the Z polarized light (c axis polarized light) to the second harmonic wave 15' of 1/2 the wavelength. The high wavelength converting efficiency is attained in this way and the efficient taking out of the wavelength conversion wave of the blue purple region is possible as well.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention converts at least one fundamental wave generated from a light source into a second harmonic of 1/2 wavelength, a sum frequency, etc. using an optical wavelength conversion element. The present invention relates to an optical wavelength conversion method for converting into .

(Prior Art) Various attempts have been made to convert the wavelength of laser light (shorten the wavelength) by utilizing second harmonic generation using nonlinear optical materials. Specifically, an optical wavelength conversion element that performs wavelength conversion in this manner is, for example, [Fundamentals of Optoelectronics JA, YARIV, translated by Kunio Tada and Takeshi Kamiya (Maruzen Co., Ltd.), pages 200 to 204]. The bulk crystal type is well known.

Conventionally, LiNb has been used as an inorganic material as a nonlinear optical material for forming the bulk crystal type optical wavelength conversion element as described above.
For O3, KTP, and organic materials, for example, JP-A-60-250
MNA (2-methyl-4-
NPP (N-(4-nitrophenyl)) described in J. Opt. Sac. Am.
-L-prolinol), NPAN (N-(4-nitrophenyl)-N-methylaminoacetonitrile), and the like are known. Compared to inorganic nonlinear optical materials such as LiNbO3 and KTP, organic nonlinear optical materials such as MNA and NPP have advantages such as high wavelength conversion efficiency and high dielectric breakdown threshold because of their large nonlinear optical constants. have.

(Problem to be Solved by the Invention) However, since the absorption edges of these organic nonlinear optical materials exist near 450 nm for MNA and 480 nm for NPP, it is difficult to generate second harmonics in the blue region. It has become.

The present invention was made in view of the above-mentioned circumstances, and provides high wavelength conversion efficiency.
It is an object of the present invention to provide an optical wavelength conversion method that can easily obtain harmonics.

(Means for Solving the Problems) The optical wavelength conversion method of the present invention uses a nonlinear optical material (4-hydroxy-3-methoxy Using a bulk single crystal of benzonitrile di (hereinafter referred to as HMN), a linearly polarized fundamental wave is incident on the bulk single crystal in a direction that makes an angle θ from the a-axis to the b-axis of the crystal. and the direction of polarization is parallel to the abbreviated axis, and the polarization direction of the wavelength-converted wave is substantially parallel to the C-axis to achieve TYPEI angle phase matching. That is.

(Function) Until now, the above-mentioned HMN has been studied to find out how to achieve good phase matching between the fundamental wave and the wavelength-converted wave when actually forming and using a bulk crystal type optical wavelength conversion element. It was unclear whether it would be possible.

In the method of the present invention, by determining the incident direction of the fundamental wave as described above, TY
Angular phase matching of PEI can now be achieved. This point will be explained below.

The bulk crystal structure of this HMN is shown in FIG.

The crystal of this HMN has an orthorhombic system, and the space group Fdd2 point group is mm2. Therefore, the tensor of nonlinear optical constants is: d33 is the nonlinear optical constant when the fundamental wave of X polarization is input and the second harmonic of X polarization is extracted, and d24 is the fundamental of Y and X polarization. Nonlinear optical constant when inputting a wave and extracting the second harmonic of Y polarization,
dl, is the second wave of X polarized light when X and the fundamental wave of X polarized light are incident.
This is a nonlinear optical constant when extracting harmonics. The magnitude of each nonlinear optical constant is shown in the table below.

becomes. Here, d3+ is the crystal axis a as shown in Figure 2.
, b, and c, light linearly polarized in the X direction (hereinafter referred to as "X polarized light".Y
, The same goes for Z. ) is the nonlinear optical constant when the fundamental wave is input as the fundamental wave and the second harmonic of the The values in the table are values determined by X-ray crystal structure analysis, and the unit is [Pm/V].

Also, since HMN is an orthorhombic and biaxial crystal,
The refractive index nx when the polarization plane of light is in the direction of the X-axis (the a-axis of the crystal axis), which is the optical axis, and the refractive index when the plane of polarization of light is in the direction of the Y-axis (the b-axis of the crystal axis), which is perpendicular to this X-axis. nY, and X, Y
It has a refractive index n2 in the Z-axis (crystal axis a-axis) direction perpendicular to both axes. These refractive indices nX s nY s
The value of nZ is ny>nz>nX from the results of actual fiPl and X-ray crystal structure analysis. -For example, for light with a wavelength of 550 nm, n
x-1,50, ny=1.75, nz-1,68, and the above relationship is satisfied.

Therefore, in this case, the fundamental wave is made incident in a direction intermediate between the X-axis direction and the Y-axis direction, and the refractive index ny'y, which is intermediate between the refractive index IR and nF, for the fundamental wave in that case is set as the second If the refractive index for harmonics is made to match n, angular phase matching can be achieved.

In other words 2ω　　　　- nz = nXv - (1).

Here, if the angle between the traveling direction of light on the X-7 plane and the optical axis X (assumed to be the inclination from the X axis to the Y axis side) is θ, then the following equation is obtained. Therefore, by setting an arbitrary angle θ at an arbitrary wavelength, TYPEI angle phase matching becomes possible.

Furthermore, non-critical phase matching is possible at θ-0°.

Furthermore, even if the traveling direction of the fundamental wave is tilted by an angle φ from the Z-axis, phase matching is possible at an angle θ′ different from the angle θ.

Further, the transmission spectrum of a bulk crystal of HMN (1 mm thick) is shown in FIG. 4, and as shown in the figure, this HMN does not absorb much light with a wavelength of around 350 nm.

Therefore, according to an optical wavelength conversion element using this HMN as a bulk crystal, it is possible to efficiently generate wavelength converted waves in the blue-violet region.

(Example) Hereinafter, the present invention will be described in detail based on an example shown in the drawings.

FIG. 1 shows an apparatus for converting a fundamental wave into a second harmonic of 1/2 wavelength by the method of the invention. First, a method for producing a bulk single-crystal type optical wavelength conversion element IO will be described. Element 10 can be made by the usual Bridgman method.

First, HMN in a molten state is poured into a suitable mold and then rapidly cooled to polycrystallize the HMN. Then this HM
By gradually withdrawing N from the inside of the furnace maintained at a temperature higher than its melting point of 88°C (for example, 91°C) to the outside of the furnace maintained at a temperature lower than the melting point, molten HMN is removed from the furnace. Single crystallize from the drawer part. As a result, 50
HMN is formed which is in a single crystal state over a long range of mm or more and has a uniform crystal orientation, and the optical wavelength conversion element IO can be made sufficiently long. As is well known, the wavelength conversion efficiency of this type of optical wavelength conversion element is proportional to the square of the length of the element, so the longer the optical wavelength conversion element is, the higher its practical value becomes.

The HMN single crystal formed as described above is cut along the X-Y plane that includes the optical axes A bulk single-crystal type optical wavelength conversion element 10 was formed by cutting out a piece with a thickness of 2 mm.

A fundamental wave 15 is incident on this optical wavelength conversion element IO as shown in FIG. In this example, a YAG laser 11 is used as a light source that generates a fundamental wave, and a laser beam (fundamental wave) 15 with a wavelength of 11064n emitted from the YAG laser 11 is incident on the optical wavelength conversion element 10. At this time, the fundamental wave 15 is incident with its direction of incidence tilted at an angle θ from the X-axis to the Y-axis side.

The fundamental wave 15 incident on the optical wavelength conversion element 10 made of HMN in this way has a wavelength of 1/2, or 532 nm.
It is converted into the second harmonic 15' of Z-polarized light (C-axis light). Therefore, a beam in which the second harmonic 15' and the fundamental wave 15 are mixed is emitted from the optical wavelength conversion element 10. This output beam has a second harmonic of 532 nm.
5° allows good transmission, while the fundamental wave 1 of 11064n
5 is passed through a bandpass filter 13 that absorbs the second
Only harmonic 15' is extracted. This second harmonic 15
' was confirmed to be Z-polarized light.

Therefore, in this example, the above-mentioned equation (1) is satisfied and phase matching between the fundamental wave 15 and the second harmonic wave 15' is achieved. As described above, the high value d32 is used as the nonlinear optical constant.

As a comparative example, a bulk single crystal of LiNbO3 with a thickness of 2 mm was formed, and a YAG laser beam was incident as a fundamental wave using the same apparatus as shown in FIG. 1 to generate a second harmonic.

The second harmonic 15° (
The light intensity of the second harmonic wave (which passed through the bandpass filter 13) was measured, and the light intensity of the second harmonic in the above example was higher than that of the comparative example by more than an order of magnitude.

(Effects of the Invention) As explained in detail above, in the optical wavelength conversion method of the present invention, by appropriately setting the incident direction and polarization direction of the fundamental wave to be incident on the HMN crystal, the fundamental wave and the second harmonic are According to this method, since the angular phase matching of H
Since the high nonlinear optical constant of MN can actually be utilized in a bulk crystal type nonlinear optical material, extremely high wavelength conversion efficiency can be achieved. Also, HMN is 330n
Since it has an absorption edge on the shorter wavelength side than m',
According to the method of the present invention using this bulk single crystal of HMN, it is also possible to efficiently extract the second harmonic in the blue-violet region.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing an example of an apparatus for carrying out the method of the present invention, FIG. 2 is a diagram of the bulk crystal structure of HMN used in the present invention, and FIG. 3 is a diagram of the fundamental wave in the apparatus of FIG. 1. FIG. 4 is a graph showing the transmission spectrum of the HMN. 10... Optical wavelength conversion element 11... YAG laser - 12... Collimating lens 15... Fundamental wave +5'... Second harmonic one

Claims (1)

[Claims] At least one linearly polarized fundamental wave is incident on a bulk single crystal of a nonlinear optical material represented by the following molecular formula ▲ There are mathematical formulas, chemical formulas, tables, etc. ▼ and the wavelength of the fundamental wave is converted. In the optical wavelength conversion method, the fundamental wave is incident on the nonlinear optical material in a direction forming an angle θ from the a-axis to the b-axis of the crystal, and the polarization direction is substantially parallel to the b-axis. An optical wavelength conversion method characterized in that angular phase matching of TYPE I is achieved by making the wavelength-converted wave incident such that the polarization direction of the wavelength-converted wave is substantially parallel to the c-axis.