JPH04309931A - Channel waveguide type optical 2nd harmonic generation device - Google Patents

Abstract

PURPOSE:To obtain a Cherenkov radiation angle which is less than a specific value at good yield by forming plural channel waveguides which differ in line width on a common base body. CONSTITUTION:On the common substrate 1, the channel waveguides C (C1, C2, C3...) are formed to line widths of 2-10mum which are dispersed having 0.01-0.2mum differences. Then fundamental wave light is made incident on a waveguide selected among the waveguides C of plural channels and 2nd harmonic light can be guided out at <=1 deg. Cherenkov radiation angle. The channel waveguides C which are mutually varied in line width are formed on the common substrate 1 previously and the waveguide whose Cherenkov radiation angle is <=1 deg. is selected to perform 2nd harmonic generating operation.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Cerenkov radiation type channel waveguide type optical second harmonic generator.

0002

2. Description of the Related Art An optical second harmonic generator SHG generates second harmonic light of a frequency of 2ω when light of a frequency of ω is introduced, and the wavelength range is expanded by this SHG. Accordingly, it is possible to further expand the scope of use of lasers and to optimize the use of laser light in each technical field.

For example, by shortening the wavelength of laser light, it is possible to improve the recording density in optical recording/reproducing, magneto-optical recording/reproducing, etc. using laser light.

[0004] Cerenkov radiation type SHG, in which a linear optical waveguide is formed on a nonlinear single crystal substrate, the fundamental wave is passed through it, and the second harmonic is extracted from the substrate side as a radiation mode, is described in, for example, Applied Physics Letters (App.
Lied Physics Letters) 17,
477 (1970).

This SHG has a ZnS polycrystalline optical waveguide formed on a ZnO nonlinear single crystal substrate, and has a thickness of 1.06μ.
The second harmonic of 0.53 μm was obtained using a Nd:YAG laser of 0.53 μm. However, in this case, the propagation loss is large because the waveguide is polycrystalline, and the d
The constant is small and the efficiency is quite low.

[0006] Furthermore, in the SHG disclosed in Japanese Unexamined Patent Publication No. 189524/1983, a LiNbO3 (referred to as LN) substrate is used and an optical waveguide structure is formed by proton exchange of the substrate. In this case, the SHG efficiency η is as high as 1% or more.

The efficiency η of SHG is determined by the Cerenkov radiation SH
In the case of G, η for the nonlinear optical constant d of the nonlinear optical material used, the fundamental wave power density PW, and the interaction length L
There is a relationship ∝d2 ・PW ・L.

Therefore, in order to obtain high efficiency, it is necessary to use a material with a large d constant, to increase the fundamental wave power density PW, and to increase the interaction length L.

[0009] The value of the d constant differs even for one material depending on the geometric relationship between its crystal orientation and the polarization direction of the fundamental wave. In the case of LN, |d33|=34.4×10-12 (m/V )
The largest in

In addition, in the case of proton exchange, only the refractive index ne for extraordinary light can be increased;
Since the surface of a plate is roughened due to etching during proton exchange, LN as a nonlinear single crystal substrate can only be used as a Z plate (a substrate with a plate surface perpendicular to the z-axis in the direction along the c-axis). mode is available, which complicates the optical coupling with the semiconductor laser of the light source. Further, the refractive index difference Δn between the waveguide and the substrate is approximately 0.14 at maximum. For this reason, there is a limit to light confinement, that is, maximum efficiency. Furthermore, proton exchange is effective only for LN, and other nonlinear optical materials such as LiTaO3 (referred to as LT) or KTiOPO4 (referred to as KTP) cannot be used.

However, on the other hand, SH using an LN substrate
Since the Cherenkov radiation angle when the maximum efficiency is reached at G is a relatively large angle of about 16°, if the length L of the substrate in the light traveling direction is increased in order to increase the SHG efficiency η mentioned above, then This SHG light is reflected at the interface between the back surface of the substrate and the outside, such as air, and the radiation mode in the output becomes complicated, resulting in a crescent-shaped spot. This method requires measures such as increasing the SHG light, and has the disadvantage that the SHG light condensing optical system becomes complicated.

On the other hand, for example, Applied Physics Letters
rs) 50, 1216 (1987) uses a KTP nonlinear single crystal substrate,
An optical waveguide is formed on this KTP substrate by an ion exchange method, and SHG using the mode dispersion of the waveguide is possible.

This KTP single crystal has a fairly large nonlinear constant d33 of d33=13.7×10-12 (mV), and is transparent in the blue wavelength region, and its refractive index is, for example, nx at a wavelength of 0.84 μm. =1.7
48, ny = 1.755, nz = 1.840 and LN
It is quite small compared to , LT, etc.

However, the SHG efficiency when actually formed by this ion exchange method is extremely sensitive to parameters such as the wavelength of the fundamental wave and the depth of the optical waveguide, making it impractical. Therefore, it has been desired to put the KTP nonlinear single crystal substrate, which is a material with a small refractive index n as described above and can be expected to have a good optical confinement effect in the optical waveguide formed thereon, into SHG.

[0015] We have solved the problem of propagation loss when using a polycrystalline material as a waveguide as described above, the problem of being able to use only the TM mode, and the limit of maximum efficiency due to the inability to obtain a sufficient refractive index Δn. In addition, we aim to put the optical waveguide device using the above-mentioned KTP substrate into practical use, especially in SHG.
In order to obtain highly efficient light output and improve characteristics, the present applicant previously published in Japanese Patent Application Laid-Open No. 63-265786,
Cerenkov radiation type SHG in which an amorphous optical waveguide in which Ta2O5 or Ta2O5 is doped with TiO2 is formed on a KTP nonlinear optical single crystal substrate.
proposed.

[0016]

[Problem to be Solved by the Invention] Also, in the application of Japanese Patent Application No. 2-201731 filed by the present applicant,
A linear waveguide made of Ta2O5 is formed on a KTP substrate, and a Cherenkov radiation type SHG with a radiation angle of 1° or less is constructed by changing the optical waveguide dimensions, that is, its width and thickness, and the material selection of the cladding layer. I suggested getting an SHG.

However, in this type of SHG device,
In order to realize a channel waveguide with a single line width, the thickness of the Ta2O5 film of the waveguide must be within about 2 Å, the refractive index must be within 10-4, and the refractive index of the cladding layer must be within 10. -3 or less, which places severe restrictions on the material properties, dimensions and shape, and therefore on manufacturing accuracy. In particular, thickness control is difficult in actual mass production, and it is extremely difficult to manufacture devices with uniform thickness for all devices.

The present invention is directed to, for example, reducing the accuracy of the film thickness of the optical waveguide in the channel waveguide type optical second harmonic generator which suppresses the radiation angle to within 1°.
That is, it is possible to relax the manufacturing conditions and to stably obtain an optimal SHG element of this type.

[0019]

[Means for solving the problem] In the present invention, FIG.
2 shows an enlarged plan view of an example, and FIG. 2 shows a cross-sectional view taken along line A-A in FIG.
A plurality of channel waveguides C selected to have line widths dispersing with a difference of 0.01 μm to 0.2 μm from each other within the range of
(C1, C2, C3,...) are provided.

Fundamental wave light is introduced into a selected waveguide of the plurality of channel waveguides C thus provided, and second harmonic light is derived with a Cerenkov radiation angle of 1° or less. Make things possible.

In other words, by introducing the fundamental wave light into each channel waveguide C, a channel waveguide that is most suitable for leading out the second harmonic light with its Cerenkov radiation angle of 1° or less can be selected. Then, a channel waveguide type SHG is configured using the selected channel waveguide.

[0022]

[Operation] As described above, according to the present invention, a plurality of channel waveguides C having mutually different line widths are formed on a common substrate in advance, so that the optimum Cherenkov radiation angle is 1° or less. Select a waveguide with SH
Perform the G motion.

That is, since the fundamental wave light is introduced only into this selected channel waveguide and the second harmonic light is extracted,
Even if there are variations in film thickness during the manufacturing process, the SH with the desired Cerenkov radiation angle of 1° or less
It is possible to obtain G. Therefore, to put it the other way around, it is possible to tolerate variations in precision during manufacturing and to manufacture a stable target SHG with a high yield.

[0024]

[Example] As shown in FIG. 1 is a schematic enlarged plan view of an example of the device of the present invention, and as shown in FIG. 2 is a sectional view taken along line A-A in FIG. , Ta2 O5
Alternatively, a plurality of channel type optical waveguides C (C1,
C2, C3...) are formed. In this case, Ta2O
TiO2 doping for 5 is, for example, Ti and Ta
The ratio of Ti to the sum of Ti/(Ti+Ta) is 0≦
Ti/(Ti+Ta)≦60 atomic %.

[0025] Then, within the range of 2 to 10 μm, preferably 3 to 5 μm, the line width of these plurality of channels C is set at a constant width of 0.01 to 0.2 μm.
．． Multiple lines with different line widths for each constant width of 1 to 0.2 μm, for example, 21 lines (only 7 lines are shown in the figure)
channel waveguides C are arranged in parallel.

Then, a cladding layer 2 made of, for example, SiO2 or a composite oxide of SiO2 and Ta2O5, which has a smaller refractive index than the channel waveguide C, is deposited to cover the entire channel waveguide C.

[0027] These plural channel waveguides C
At one end, a channel waveguide is selected in which the fundamental wave from each fundamental wave light source is sequentially introduced, and the second harmonic light (SHG light) can lead out the SHG light with high SHG efficiency at a radiation angle of 1° or less. Configure the SHG device using

Example 1 For example, the thickness is 0.7 μm, the width is 1.5 mm, and the thickness is 4 mm.
On a P single-crystal substrate 1, 21 channel waveguides C1, C2, C3, .

In this case, these channels C (C1 to
C21) has a line width of 0.1 μm within the range of 3 to 5 μm.
It is formed by changing m increments.

The formation of this channel waveguide can be carried out using, for example, K
CV is applied over the entire surface of the substrate made of a-plate of TP single crystal substrate.
An amorphous thin film of Ta2O5 was formed by the D (chemical vapor deposition) method to a target thickness of 2100 Å, and selective etching by photolithography was performed to form the line widths of each other as described above. 21 different parallel channel waveguides C1 to C21 are formed. This CVD of Ta2 O5 can use, for example, tantalum pentaethoxide Ta (OC2 H5) as a raw material.

Then, a cladding layer of SiO2 is formed to a thickness of, for example, 1 to 2 cm by CVD or sputtering, covering the entire surface of the channel waveguide C thus formed.
It is deposited to a thickness of .mu.m, that is, a thickness that is at least capable of functioning as a cladding, and a thickness that does not cause thermal distortion.

For example, this substrate is then cut into pieces to obtain a plurality of SHG devices each having the above-mentioned dimensions.

[0033] The SHG device obtained in this Example 1 is as follows:
By using semiconductor laser light with a fundamental wavelength of 0.83 μm, it is possible to derive light with a second harmonic of 0.415 μm. In this case, the refractive index n for fundamental wave light
1, the refractive index for the second harmonic was n1 = 1.46, and the refractive index for the second harmonic was n2 = 1.47. Moreover, according to the SHG device of this Example 1, about 8390 Å to 8290 Å
We were able to obtain a highly efficient SHG device with a Cherenkov angle of 1° or less for a fundamental wave with a wavelength of .

FIG. 3 shows the KTP as in Example 1.
In a SHG waveguide with an interaction length of 6 mm, where a Ta2 O5 channel waveguide is formed on a substrate and a SiO2 cladding layer is coated on the channel waveguide, a radiation angle of 1° or less and a high efficiency, that is, a basic power of 100 mW. S against the wave
This figure shows the result of measuring the relationship between the fundamental wavelength and linewidth, which can exhibit an HG efficiency of 2% or more, using the thickness of the channel waveguide made of Ta2O5 as a parameter. In this case, ▲ means that the thickness of Ta2O5 is 2090 Å,
★ is 2100 Å, ◆ is 2100 Å, △ is 2
The case of 120 Å is shown. As is clear from this figure, the wider the line width, the wider the range in which high SHG efficiency of 2% or more can be obtained, so it is desirable to select the line width within the range of about 3 to 5 μm. I understand.

According to FIG. 3, for example, the line width is 3.
At 6 μm, the fundamental wave wavelength is 8300 at a film thickness of 2100 Å.
A highly efficient blue SHG with a radiation angle of 1° or less has been obtained in the vicinity of Å. However, if for some reason the film thickness becomes thinner, for example to 2090 Å, it will not be possible to obtain SHG for such a fundamental wavelength with a line width of 3.6 μm, and in this case, with a line width of 5 μm, By creating a channel, highly efficient SHG can be obtained for the fundamental wave wavelength of 8260 to 8280 Å. That is 3 to 5
By fabricating multiple channel waveguides C whose line widths vary by 0.1 μm, for example, within the line width range of μm, even if the film thickness changes by 2100 ± 10 Å, it is possible to By selecting and using a line width of , it is possible to obtain a highly efficient SHG. In other words, the tolerance of the Ta2O5 optical waveguide film thickness is significantly improved, and the accuracy of film thickness control is relaxed.

As the line width approaches 2 μm, as can be seen from FIG. 3, restrictions on the absolute value of the line width become necessary to obtain a SHG with high efficiency and a radiation angle of 1° or less. If the line width becomes 0.05 μm or less, it becomes impractical, and if the line width exceeds 10 μm, the efficiency will decrease or the stability of the transverse mode of the fundamental waveguide mode will deteriorate, so the line width should be 2 to 10 μm. Preferably 3
~5μm, preferably 0.01~0.2μm, preferably 0.01~0.2μm.
It is better to provide waveguides C that vary in increments of 1 to 0.2 μm.

Example 2 In Example 1 described above, the cladding layer 2 made of SiO2 was used, but as the cladding layer 2, (S
iO2 )x (Ta2O5)1-x (x is approximately 0
．． 925) can also be used. In this case, the fundamental wave wavelength is 0.82μm, and the second harmonic wavelength is 0.41μm.
The refractive indices n1 and n2 of m are n1 = 1.57,
n2 = 1.59. According to this Example 2, a highly efficient SHG with a Cerenkov angle of 1° or less was obtained for a fundamental wave wavelength of about 8290 Å to 8190 Å.

[0038]

Effects of the Invention As described above, in the SHG device according to the present invention, a plurality of channel waveguides C having different line widths are formed,
A waveguide with an appropriate linewidth was selected from these channel waveguides C, and a fundamental wave was introduced into it to obtain a second harmonic, so a highly efficient SH with a radiation angle of 1° or less
It is possible to reliably obtain a G device, for example, with sufficient tolerance for variations in the film thickness of its waveguide from the designed value, and it is possible to improve yield and, therefore, improve mass production.

[Brief explanation of drawings]

FIG. 1 is a plan view of a channel waveguide type optical second harmonic generator according to the present invention.

FIG. 2 is a sectional view taken along line AA in FIG. 1;

FIG. 3 is a measurement curve diagram of the relationship between fundamental wave wavelength and linewidth of high-efficiency SHG in an SHG waveguide.

[Explanation of symbols]

1 Board 2 Clad layer C Channel waveguide

Claims (1)

[Claims] 1. A plurality of channel waveguides are provided on a common substrate, and the channel waveguides are selected to have line widths dispersing within a range of 2 μm to 10 μm and with a difference of 0.01 μm to 0.2 μm from each other, A channel waveguide type optical second harmonic generator, characterized in that fundamental wave light is introduced into a selected channel waveguide, and second harmonic light is derived with a Cerenkov radiation angle of 1° or less.