JPH04310929A - Optical second harmonic generating element - Google Patents

Abstract

PURPOSE:To realize highly efficient wavelength conversion by improving the power of a basic wave. CONSTITUTION:A nonlinear optical member has periodic domain inverted structure 2. Reflective coating members 1 are provided at both ends of the nonlinear optical member. A resonator consists of the nonlinear optical member and reflective coating members. The reflective coating members 1 provide high reflection to the basic wave. Consequently, the high-efficiency wavelength conversion is enabled by increasing the power of the basic wave.

Description

[Detailed description of the invention]

0001

TECHNICAL FIELD The present invention relates to an optical second harmonic generation element. For example, it is applied to optical disk memory light sources, laser printer light sources, semiconductor processes, and the like.

0002

[Prior Art] Publicly known documents describing the prior art related to the present invention include, for example, "Second harmonic generation by periodic domain inversion optical waveguide" (Hiromasa Ito, "Optics" Vol. 19)
No. 6 June 1990 p373-374). According to this document, second harmonic generation (second
harmonic generation:hereinafter referred to as SHG
In order to efficiently perform nonlinear interactions such as
Utilizing materials with large nonlinear coefficients, ■Increasing optical energy density by confining light waves, ■Achieving long interaction lengths, ■
It is said that it is essential that the superposition of interacting electromagnetic fields be large. In particular, in recent years, with the increase in the output power and performance of semiconductor lasers, it is expected that the wavelength of the semiconductor laser itself will be shortened by SHG, and wavelength conversion of semiconductor laser pumped solid-state lasers is also becoming important. It is stated that research into highly efficient wavelength conversion for light sources is underway.

In addition, in the explanation of the fabrication and operation of a periodic domain inversion optical waveguide, a domain inversion planar LiNbO3 waveguide with a domain period 2Λ=7.7 μm has a fundamental wave wavelength of 1.064 μm, an interaction length of 4.65 mm, Input 70m
Under the condition of w, normalized conversion efficiency ηnorm = 33%
Quasi phas matching of μm2/Wcm2
e matching: QPM) operation has been confirmed. In general, phase matching under energy conservation is important in nonlinear interactions between different wavelengths. If this condition is not satisfied, interference will occur between the nonlinear polarization waves generated by the interaction and the light waves, and high conversion efficiency will not be obtained. In addition, in second harmonic generation (SHG), the wavelength conversion efficiency is generally

0004

[Math 1]

[0005] However, P(ω), P(2ω): fundamental law, second harmonic power d
eff: effective nonlinear optical constant ε0 contributing to SHG
: Dielectric constant of vacuum c : Speed of light n(ω) n(2ω) : Refractive index d of fundamental wave and second harmonic
2/n3: Figure of merit Ln: Interaction length Δk: Represented by phase mismatch. As mentioned above, in order to increase conversion efficiency, ■ use materials with a large figure of merit, ■ ensure phase matching,
■It is necessary to increase the power density of the fundamental wave and ■lengthen the interaction length.

[0006]Using the domain inversion structure, the above
■ can be solved. For example, LiNbO is used as a nonlinear optical material.
3, it is possible to perform quasi-phase matching using the nonlinear optical coefficient d33, which cannot be used in the angular phase matching method. The value of this nonlinear optical coefficient d33 is d31 where angular phase matching can be achieved.
This leads to an improvement in the performance index. quasi phase matching
(g; QPM), the inversion structure must be precisely miniaturized in order to reduce the phase mismatch. However, with current microfabrication technology, it is extremely difficult to fabricate domain inversion on the order of micrometers with high precision over a long length.

[0007]

[Objective] The present invention was made in view of the above-mentioned circumstances, and it is an object of the present invention to provide an optical second harmonic generation element that can obtain highly efficient wavelength conversion by increasing the power of the fundamental wave. It was made for the purpose of

[0008]

[Structure] In order to achieve the above objects, the present invention provides (1)
a nonlinear optical member having a periodically domain-inverted structure;
and reflective coating members provided at both ends of the nonlinear optical member, and the reflective coating member has a high reflection with respect to the fundamental wave, and further, (2
) The nonlinear optical member has an optical waveguide type domain inversion structure, and (3) the nonlinear optical member has a bulk type domain inversion structure. Hereinafter, the present invention will be explained based on examples.

FIG. 1 is a configuration diagram for explaining an embodiment of the optical second harmonic generation device according to the present invention. In the figure, 1 is a mirror (reflective coating member), 2 is a periodic domain inversion structure, and 2Λ is the domain period and L is the resonator length. A resonator in which a resonant mirror includes a nonlinear material with a periodic domain inversion structure will be described. Here, LiNbO3
Let us take a Z-cut plate of a uniaxial crystal as an example. (2) It is assumed that the mirror 1 has a reflectance r and a transmittance t (r2+t2=1) for both the fundamental wave ω and the harmonic 2ω. ■The domain is the coherence lens length

0010

[Math 2] "However, λ (ω) is the fundamental wavelength."

It is assumed that the period is inverted in accordance with the period of . The electric field e within the resonator is considered to be a superposition of forward and backward waves of the fundamental wave (ω1, ω2) and harmonic (ω3), respectively. When the phases of the forward wave and the backward wave are perfectly matched, the electric field e is

0012

[Math 3]

It can be expressed as: Now, for the sake of simplicity, if we ignore the absorption loss in the resonator, Maxwell's equation becomes

[0014]

[Math 4]

It can be expressed as follows. Here, ω3=ω1+ω2(
ω1=ω2), k32=ω32μ0ε3, and further,

0
016]

[Math 5]

Substituting the following condition into equation (2), dE3(x
)/dx, we get

[0018]

[Math 6]

##EQU1## is obtained. The harmonic electric field E3 from the resonator length L is

[0020]

[Math 7]

[0021] Therefore, the harmonic power is

[
0022

[Math. 8]

[0023] Furthermore, we will consider a region where the attenuation of the power of the fundamental wave due to the generation of harmonics can be sufficiently ignored. If the electric field incident on the resonator is E0, then

[0024]

[Math. 9]

Therefore, equation (3) can be modified as follows.

[0026]

[Math. 10]

[0027] The harmonic power can be found from the above equations (4), (5), and (6). As a specific example, Li is added to a nonlinear optical crystal.
The second harmonic (SH) power inside the resonator was numerically calculated for a bulk-type domain-inverted resonator made of a Z-cut plate of NbO3. It is assumed that the half period Λ of the domain is equal to the coherence length of LiNbO3, and five periods exist within the optical resonator. The refractive index for the fundamental wave ω harmonic 2ω was set as n(ω) = 2.1544 and n(2ω) = 2.2325, respectively, and the wavelength of the fundamental wave was calculated using λ(ω) = 1.064 μm. For comparison, when no optical resonator is configured, that is,
The SH power without the reflective film was also calculated numerically. Note that the reflectance is r=0.9, so the power reflectance R≒80
%. The fact that the domain has 5 periods and the power reflectance is about 80% is just one example for numerical calculations. This calculation assumes a plane wave approximation. There is no problem in bulk. Also, in optical waveguides,
It goes without saying that basically the same effect can be obtained by performing the following conversion. β (propagation constant)←k N (equivalent refractive index)←n

FIG. 2 is a diagram showing an optical waveguide type, and FIG. 3 is a diagram showing a bulk type. In the figure, 3 is a reflective film, 4 is an optical waveguide with domain inversion structure, 5 is a reflective mirror (reflective film), and 6 is a reflective film.
is a bulk domain inversion structure. Figure 4 shows the numerical calculation results of the SH optical power. The SH optical power in the resonator type immediately before the output mirror M2 is x = 10 in the non-resonator type.
This is approximately 30 times the SH optical power at the position of Lc (Lc: coherence length). Since R=81%, the power transmittance is about 20%, so from the output mirror, 30×0
．． 2=6 times the power of the SH light is transmitted.

[0029]

[Effects] As is clear from the above description, the present invention has the following effects. (1) In claim 1, by configuring the optical waveguide with a domain inversion structure using a resonator, the power of the fundamental wave can be increased and highly efficient wavelength conversion can be achieved. (2) In claim 2, the resonator is configured by bulk-type domain inversion, which increases the power of the fundamental wave, provides high efficiency, and is easier to manufacture than a bulk configuration.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram for explaining one embodiment of an optical second harmonic generation element according to the present invention.

FIG. 2 is a diagram showing an optical waveguide type optical second harmonic generation element.

FIG. 3 is a diagram showing a bulk type optical second harmonic generation element.

FIG. 4 is a diagram showing numerical calculation results of SH optical power.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1... Mirror (reflective coating member), 2... Periodic domain inversion structure, 2Λ... Domain period, L... Resonator length.

Claims (3)

[Claims] 1. A resonator comprising a nonlinear optical member having a periodic domain inversion structure and reflective coating members provided at both ends of the nonlinear optical member, the reflective coating member being An optical second harmonic generation element characterized by high reflection. 2. The optical second harmonic generation device according to claim 1, wherein the nonlinear optical member has an optical waveguide type domain inversion structure. 3. The optical second harmonic generating element according to claim 1, wherein the nonlinear optical member has a bulk type domain inversion structure.