JPH05265066A - Optical waveguide type wavelength conversion element - Google Patents

Abstract

PURPOSE:To provide the wavelength conversion element which is for LDs having high efficiency. CONSTITUTION:This optical waveguide type wavelength conversion element has a polymer medium layer which exhibits a quadratic optical waveguide response and causes the pseudo phase matching of an incident laser beam and a generated second harmonic wave as an optical waveguide part. This optical waveguide part forms a spacial periodic structure by subjecting the polymer precursors of at least one kind of polymerizable monomers exhibiting the quadratic nonlinear optical waveguide response to a polarization treatment and this polarization state is fixed by polymerizing the polymer precursors. Further, period DELTA of the spacial frequency structure is expressed by the equation, where A is the propagation constant of the basic mode at the wavelength of the second harmonic wave and B is the propagation constant of the basic mode at the wavelength of the laser wave.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an optical waveguide type wavelength conversion element.

[0002]

2. Description of the Related Art Organic nonlinear optical materials have been found to have higher performance than conventional materials, and have been attracting attention as materials for wavelength conversion elements that generate second harmonics. However, it is difficult to fabricate a device having high performance as it has poor workability and often has a crystal structure having a symmetric center and does not exhibit performance in a macro. In order to solve this problem, it is necessary to disperse molecules with high nonlinear optical performance in a polymer matrix and perform polarization treatment, or to perform polarization treatment on a polymer that incorporates a molecular structure with high nonlinear optical performance in the main chain or side chain. However, since the number of molecules that can be doped is limited, the performance is not so high and the performance is attenuated over time due to relaxation of polarization. Therefore, after a monomer having a high non-linear optical performance is subjected to a polarization treatment, it is polymerized to generate a material in which the performance is improved and the relaxation time of polarization is reduced [eg, D. Jungb
Auer, etc, Appl. Phys. Lett. 5
6 (26), p. 2610]. However, it is generally difficult to satisfy the phase-matched state when an LD wavelength conversion element having sufficient efficiency for practical use is made using an organic nonlinear optical material. It has not been achieved yet because of the fact that many materials do not have absorption at the wavelength of the second harmonic of the LD.

[0003]

An object of the present invention is to provide a highly efficient wavelength conversion element for an LD by solving the problems of the prior art as described above.

[0004]

According to the present invention, as an optical waveguide part, a second-order nonlinear optical response is exhibited, and a second laser light is generated with incident laser light.
In an optical waveguide type wavelength conversion element provided with a polymer medium layer having a spatial periodic structure for quasi-phase matching of harmonics, the optical waveguide part is a polymer precursor of a polymerizable monomer showing at least one kind of second-order nonlinear optical response. The body is polarized to form a spatial periodic structure, and the polarization state is fixed by polymerizing the polymer precursor, and the period Δ of the spatial periodic structure is represented by the following formula (I). It is characterized by being shown by.

[Equation 2] (However, A is the propagation constant of the fundamental mode at the wavelength of the second harmonic, and B is the propagation constant of the fundamental mode at the wavelength of the incident laser wave.) The basic configuration of the optical waveguide type wavelength conversion element of the present invention is: For example, as shown in FIGS. 1 to 5, an electrode, for example, a comb-shaped electrode 7, a cladding layer 2 and an optical waveguide portion 5 composed of the polymer medium layer are formed on a substrate 1. Usually, the efficiency η of the second harmonic generation is expressed by the following formula (II) in the case of a waveguide device, for example.

[Equation 3] μ ₀ Vacuum permeability ε ₀ Vacuum permittivity ω Fundamental wavefront frequency dx Effective SHG constant nx Effective refractive index of waveguide layer L ² Interaction length PF Input power D Waveguide width I Spatial coupling coefficient where sin ² (Δβ ・L / 2) / (Δβ · L / 2) ² is a term representing the phase matching property, which is 1 at the maximum when Δβ = 0,
When Δβ ≠ 0, η becomes extremely small. This Δβ = 0
The condition of is generally very strict because β has wavelength dependence, and phase matching between modes, Cherenkov phase matching, etc. have been proposed, but for reasons such as small film thickness tolerance and small spatial coupling coefficient, High efficiency has not been obtained. Here, for example, when the value of d changes for each length l defined by l = (2m−1) 2π / Δβ (m is a natural number), it is called quasi phase matching, which is as if phase matching is achieved. It is known to show such behavior. So, for example, when the sign of dx changes and the absolute values are equal, η is approximately

[Equation 4] When 0 and a certain value dx are alternately taken for each l,

[Equation 5] The present inventor has derived that Here, in order to obtain particularly high efficiency, it is desirable that m = 1. However, although quasi-phase matching is currently being investigated in inorganic materials, if m = 1

[Equation 6] Therefore, when Ti and the like are diffused to form a domain-inverted structure, there is diffusion in the lateral direction with respect to the depth of the waveguide. Therefore, the domain-inverted structure is produced under the condition of m = 1. Things are very difficult. However, in the case of a polarized polymer, Δβ is small because the refractive index is generally low.
Even if = 1 is set, l does not become so small, and since the polarization treatment by the electrode can be performed, the polarization treatment up to the limit of the fine processing of the polarization can be performed. Therefore, the first-order quasi phase matching (m =
1) is possible. Further, in addition to m = 1, it is necessary for I to be large in order to increase η, and I becomes maximum when both the fundamental wave and the SH wave are in the fundamental mode.
Generally, Δβ becomes large between the basic modes. In the case of the polymer of the present invention, since the refractive index is generally smaller than that of an inorganic material, Δβ becomes small even when quasi phase matching is performed between the fundamental modes, and a feasible value of 1 can be obtained. From the above, for the waveguide type wavelength conversion element that uses the polymer to perform quasi phase matching, the period Δ is

[Equation 7] Is effective. Also, the currently obtained LD
As for the above, the upper limit of the output is generally up to 40 mW, and the output of several mW is practically required, so the efficiency η is up to 10%
Degree is necessary.

Generally, an optical waveguide having a film thickness of 2 to 3 μm is often used with respect to the wavelength of an LD, and in the case of a polymer in particular, it is difficult to manufacture an extremely thin film from the film thickness manufacturing conditions.
At this time, when m = 1 and there is no lateral confinement even if the quasi-phase matching is performed between the fundamental modes, that is, when D is on the order of several mm which is the width when the LD is collimated, η˜1
0 ^-3, which means that theoretically practically required efficiency cannot be obtained (however, dx is the maximum reported value of 50 for polymers).
pm / V was used and 10 mm was used for L. ).
Actually, since there is a loss of coupling between the LD and the waveguide, attenuation of the fundamental wave, etc., the efficiency becomes even lower, and it is very difficult to obtain practical efficiency without lateral confinement. Is. Therefore, it is essential that the optical waveguide type wavelength conversion element using the polymer has a lateral confinement effect. In this case, as an element structure which can be expected to have a lateral confinement effect, there are elements such as those shown in FIGS. Further, in particular, a polymer subjected to polarization treatment generally has a larger refractive index in the polarization direction than in a non-polarization region. For example, an element as shown in FIG. 4 has a lateral confinement effect with respect to the TM mode. The structure shown in FIG.

The formula (II) is a formula when it is approximated that there is no absorption in the fundamental wave and the SH wave. In the case where there is absorption, particularly in the case where there is absorption in the optical waveguide layer, the efficiency is remarkably reduced. To obtain efficiency, the polymer should have a fundamental wave and SH
It is essential that there is no absorption in the wavelength range of waves, 380-1000 nm. Therefore, it is necessary that the polymer forming the polymer medium layer is composed of a highly non-linear monomer and does not have absorption in the above wavelength range. As such a polymer, it has been discovered that an ester (1) of alcohol and methacrylic acid having at least one double bond and a copolymer of methyl methacrylate (2) and ketone (3) are preferable.
Methacrylic acid ester and methyl methacrylate show photopolymerizability as shown in the following formula (V),

[Chemical 1] Here, R has the following structure, for example. _{-CH 2 -CH = CH 2, -CH} 2 -CH = CH-CH 3, It is believed that the ketone reacts with the double bond of R to form an oxetane ring, and therefore the excitation energy of the ketone is lower than the excitation energy of C = C and the electronic transition from the n orbital to the π excitation orbital is dominant. Some are desirable, and examples thereof include benzophenone and 3-benzoylbenzophenone. Since a molecular skeleton with high nonlinear optical performance preferably has a donor group and an acceptor group in the skeleton having π electrons, these phenyl ketones have a donor group,
Alternatively, it is desirable that the acceptor group be substituted. However, in the case where a group having a strong donor property and a strong acceptor property are simultaneously present at positions having a strong resonance effect (for example, p-
The skeleton of nitroaniline) has a very long absorption wavelength, so it is better to avoid these. As a polymer satisfying the above requirements, a copolymer of an amine having at least two unsubstituted amino groups and an epoxide having at least two epoxy rings is used.
A molecular skeleton having an amino group generally exhibits non-linear optical performance, and in particular, substituted or unsubstituted aniline and aminostilbene having π electrons have high performance for the above-mentioned reason. Therefore, the amine of the present invention is preferably a substituted or unsubstituted aromatic amine, and since the amino group has a strong donor property, the substituent has a weak acceptor property such that absorption does not have a long wavelength, or a donor property. desirable. Further, the epoxide may further contain a non-linear optically active molecular skeleton. As the substrate 1, since it is necessary to load and process electrodes, a material such as glass which is resistant to heat and acid is used. A clad layer 3 having a refractive index lower than that of the polymer medium layer 6 is provided on the substrate 1 provided with the electrode 2. As a material forming the clad layer 3, a material having a lower refractive index and better transparency than the polymer layer is preferable, and therefore a fluorine-based polymer or the like can be used. The clad layer 3 is formed by applying the above material in a solution state by a coating method such as spin coating. The polymer layer is preferably a polymer of epoxide and amine because it has high non-linear performance and good thermal stability. The epoxide is, for example, N, N- (diclycidyl) -4-nitroanilineaniline (NNDN) represented by the following formula (I), and the amine is, for example, N- (2-amino) represented by the following formula (II). Phenyl) -4
-Nitroaniline (NAN).

[Chemical 2]

[Chemical 3]

The polarization process can be performed as follows. (1) A polymerizable monomer is heated to be a precursor, and an electric charge generated by corona discharge is charged on the precursor,
Applying a periodic electric field using electrodes such as a comb-shaped electrode,
A treatment method of fixing the polarization by polymerizing the precursor in a state where the polarization region is periodically generated by the electric field, or (2) laminating a cladding layer having a lower refractive index than the polymer medium layer on the precursor. Then, a polarized region is generated by a method of applying a periodic electric field by an electrode provided on the clad layer,
There is a method of polymerizing the precursor in this state and fixing the polarization. Furthermore, as shown in FIG. 5, (3) two sets of electrodes, for example, comb-shaped electrode layers, provided with an insulating layer on the substrate,
Forming an optical waveguide by the precursor layer on the insulating layer,
Alternatively, after further laminating a clad layer having a lower refractive index than the polymer medium layer on the precursor layer, the precursor is charged with electric charges by corona discharge, and the two electrodes and the charges generated by corona discharge are charged. Alternatively, an electric field is applied by another electrode provided on the clad layer, and the precursor is polymerized in a state where the polarization region is inverted and periodically generated, and the polarization is fixed. May be. Although two sets of electrodes, for example, comb-shaped electrodes are used in FIG. 5, two or more sets may be used. Also,
As the insulating layer that insulates these two or more sets of electrodes, polymers can be spin-coated, or a material such as SiO ² can be prepared by a known method such as vapor deposition, sputtering, or CVD. The present invention will be described below with reference to examples, but the present invention is not limited to the examples.

[0008]

【Example】

Example 1 As a polymerizable monomer component constituting the polymer medium layer,
2-Propenyl methacrylate, methyl methacrylate and 4-nitrobenzophenone were used. 2,2'-azobisisobutyronitrile was used as a polymerization initiator for the methacrylic acid ester. 4% by weight of a copolymer of methacrylate and the benzophenone in a weight ratio of about 1: 1.
A toluene solution was prepared, filtered, and then spin-coated on the substrate shown in FIG. As the clad layer, a copolymer of vinylidene fluoride and tetrachloroethylene was dissolved in acetone, applied by spin coating, and dried by heating.
Then, a film having a thickness of 2.0 μm was formed and treated at 80 ° C. for 40 minutes. Next, the optical waveguide layer is heated to a temperature not lower than the glass transition point while applying an electric field by a known corona poling method, and at the same time, as shown in FIG.
It was irradiated for a minute, and returned to room temperature while applying an electric field to perform polarization treatment. Benzophenone escaped from the portion to which no ultraviolet ray was applied, and the film thickness was reduced. Using a titanium sapphire laser as the light source, the wavelength of the titanium sapphire laser,
It was confirmed that there was no absorption at half the wavelength.

[Equation 8] At this time, the fundamental wave is 841 nm and the SH wave is 421.5 nm.
The refractive indices of the optical waveguide portion and the cladding portion in the film thickness direction are as follows. 841 nm 421.5 nm Air 1.00 1.00 Optical waveguide part 1.544 1.575 (TM mode) Vinylidene fluoride clad 1.424 1.438 Therefore, at this time, the effective refractive index for the fundamental mode is

[Equation 9] Therefore, the period of polarization processing for quasi phase matching is Δ
= 2π / Δβ = 10.8 μm, and the electrode pitch is set to 10.8 μm in advance, so primary (m = 1) quasi phase matching is possible. When a 94 mW titanium sapphire laser was loosely bound and coupled into the optical waveguide by the prism coupling method, it was 421.5 n.
m blue light was confirmed.

Example 2 N, N- (diglycidyl) -4-chloroaniline (NN
DC), N- (2-aminophenyl) -4-chloroaniline (NAC) was added to [D. Jungbauer et al.
pl. Phys. Lett. 56 (26). P261
[0] to prepare a polymer precursor, and apply the same to the substrate having the same electrode and clad layer as in Example 1 to form a film having a thickness of 2.0 μm. , Corona polling was done. However, at this time, the electrode that generates corona electrons has a very sharp tip (R: ~
30 μm) A needle is swept by approaching the polymer surface at a distance of 50 μm to about 100 μm.
Corona poling was performed by charging an electric charge only to a certain portion. Then, by heating, the optical waveguide layer was solidified and a lateral confinement structure as shown in FIG. 4 could be manufactured. The absorption of the polymer at this time is

[Equation 10] Since it was sufficiently transparent to the wavelength of titanium sapphire laser and the wavelength of SHG, titanium sapphire laser was used as a light source. At this time, the refractive index values are as follows: 841 nm 421.5 nm air 1.00 1.00 optical waveguide part 1.712 1.785 vinylidene fluoride clad 1.424 1.438 effective refractive index of fundamental mode Is

[Equation 11] Therefore, Δ = 2π / Δβ = 10.4 μm. By setting the electrode pitch to 10.4 μm, the primary (m
= 1) Quasi phase matching can be achieved. The same performance as in Example 1 was confirmed for the optical waveguide thus manufactured.

[0010]

【effect】

(1) Since it is an element using a high-performance and stable organic nonlinear optical polymer with first-order quasi-phase matching, highly efficient wavelength conversion is possible. (2) Since the device has the effect of confining in the film thickness direction and the lateral direction, the incident power density is improved, so that the conversion efficiency is improved. (3) Since the polymer constituting the device is made of a highly non-linear monomer and has good transparency in the wavelength range of 380 to 1000 nm, the conversion efficiency is improved.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a basic configuration of one embodiment of an optical waveguide type wavelength conversion element of the present invention having a structure having a confinement effect in a film thickness direction and a lateral direction.

FIG. 2 is a schematic view showing a basic configuration of another embodiment of the optical waveguide type wavelength conversion element of the present invention having a structure having a confinement effect in the film thickness direction and the lateral direction.

FIG. 3 is a schematic diagram showing a basic configuration of another embodiment of the optical waveguide type wavelength conversion element of the present invention having a structure having a confinement effect in the film thickness direction and the lateral direction.

FIG. 4 is a schematic view showing a basic configuration of another embodiment of the optical waveguide type wavelength conversion element of the present invention having a structure having a confinement effect in the film thickness direction and the lateral direction.

FIG. 5 shows an example of a substrate used in the present invention.

FIG. 6 is a diagram showing an example of a polarization treatment method adopted in the present invention.

[Explanation of symbols]

 1 substrate 2 clad layer 3 non-polarized part 4 polarization width 5 optical waveguide part 6 high refractive index part 7 comb-shaped electrode 8 photomask 9 ultraviolet light

Claims (3)

[Claims] 1. An optical waveguide type wavelength conversion element having, as an optical waveguide section, a polymer medium layer exhibiting a second-order nonlinear optical response and quasi-phase-matching an incident laser beam with a generated second harmonic, The optical waveguide section polarizes a polymer precursor of at least one kind of polymerizable monomer exhibiting a second-order non-linear optical response to form a spatially periodic structure, and polymerizes the polarization state with the polymer precursor. The optical waveguide type wavelength conversion element is characterized in that it is fixed by the above, and the period Δ of the spatial periodic structure is represented by the following formula (I). [Equation 1] (However, A is the propagation constant of the fundamental mode at the wavelength of the second harmonic, and B is the propagation constant of the fundamental mode at the wavelength of the laser wave.) 2. The optical waveguide type wavelength conversion element according to claim 1, wherein the optical waveguide portion has a structure having a confinement effect in a film thickness direction and a lateral direction. 3. The polymer constituting the polymer medium layer is composed of at least one kind of highly non-linear monomer and has no absorption for a wavelength in the range of 380 to 1000 nm. The optical waveguide type wavelength conversion element according to claim 1.