JPH0454210B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to an optical wavelength conversion element and a short wavelength laser light source used in the field of optical information processing using coherent light or the field of optical measurement and control.

Prior Art FIG. 6 shows a configuration diagram of a conventional optical wavelength conversion element. Below, harmonic generation (wavelength 0.42 μm) for the fundamental wave with a wavelength of 0.84 μm will be described in detail using a diagram. [T.Taniuchi and K.yamamoto,
“Second harmonic generation by Cherenkov
radiation in proton−exchanged LiNbO ₃
optical waveguide”, CLEO
'86, WR3, 1986, see]. When the fundamental wave P1 light from the semiconductor laser is incident on the entrance surface 5 of the buried optical waveguide 2 formed on the LiNbO ₃ substrate 1,
When the condition that the effective refractive index N1 of the waveguide mode of the fundamental wave is equal to the effective refractive index N2 of the harmonic is satisfied, the light of the harmonic P2 is efficiently radiated from the optical waveguide 2 into the LiNbO ₃ substrate 1. and operates as an optical wavelength conversion element.

Such conventional optical wavelength conversion elements have a buried optical waveguide as a basic component. A method of manufacturing this buried optical waveguide will be explained. Cr or Al is deposited on a LiNbO ₃ substrate 1, which is a ferroelectric substrate, and slits of several μm in width are made by photoprocessing and etching, and then heat treated in benzoic acid to form a high refractive index layer that will become the optical waveguide 2. (Refractive index difference ΔNe with the substrate was about 0.13). [JLJackel, CERice, and JJVeselka,
“Proton exchange for high−index
waveguides in LiNbO ₃ ”Appl.phys.Left. (Applied Physics Letter), Vo141,
No.7, pp607-608 (1982)] The optical wavelength conversion element produced by the benzoic acid treatment described above is a fundamental wave with a wavelength of 0.84 μm from a semiconductor laser.
For P1, the maximum conversion efficiency was shown at a waveguide thickness of 0.5 μm, and when the waveguide length was 6 mm and P1 = 40 mW, a harmonic of P2 = 0.4 mW was obtained. The conversion efficiency P1/P2 in this case is 1%.

Problems to be Solved by the Invention In an optical wavelength conversion element based on a semiconductor laser as described above, the output and longitudinal mode of the semiconductor laser fluctuate due to the return light from the entrance surface and output surface of the fundamental wave of the optical wavelength conversion element. However, there was a problem in that the output of the generated harmonics fluctuated by about ±20% and was not constant. In particular, the return light from the exit surface greatly contributed to longitudinal mode fluctuations. Therefore, it is at a practical level for short wavelength laser light sources.
It was difficult to stably obtain harmonics of 1 mW or more.

Means for Solving the Problems The present invention makes it possible to increase and stabilize harmonic output power by adding new ideas to the structure of an optical wavelength conversion element based on a semiconductor laser.

To this end, the optical wavelength conversion device of the present invention includes a semiconductor laser, a substrate having a nonlinear optical effect, and an optical waveguide formed on the substrate, and the fundamental wave emitted from the semiconductor laser is transmitted to one of the optical waveguides. A method is used in which harmonics are incident on the end face and propagated through the optical waveguide, and are radiated from the optical waveguide into the substrate, so that the semiconductor laser is harmonic-superimposed.

Furthermore, by forming a scatterer or an absorber at the output part of the optical waveguide, the fundamental wave from the semiconductor laser is scattered or absorbed at the output part of the optical waveguide.

Effect By superimposing a high frequency on the semiconductor laser that generates the fundamental wave by the above means, fluctuations due to returned light are less likely to occur, and furthermore, fluctuations in the output and longitudinal mode due to returned light at the emission part of the optical wavelength conversion element can be prevented. The resulting harmonic output can also be stabilized.

Embodiment As one of the embodiments, an optical wavelength conversion element used in the optical wavelength conversion device of the present invention will be described with reference to the drawings. FIG. 1 shows a configuration diagram of an embodiment of the optical wavelength conversion element of the present invention. In this example, as an optical wavelength conversion element.
This device uses a proton exchange optical waveguide fabricated on a LiNbO ₃ substrate 1. FIG. 1a is a perspective view of the optical wavelength conversion element, and FIG. 1b is a cross-sectional view taken along a plane parallel to the optical waveguide. In Figure 1, 1 is the LiNbO ₃ substrate of the +Z plate (the + side of the substrate cut perpendicular to the Z axis), 2 is the optical waveguide formed by proton exchange treatment in phosphoric acid, and 3 is the incident fundamental wave P1. Section 4 is a fundamental wave emission section. The fundamental wave P1 focused by the focus lens is transmitted to the LiNbO ₃ substrate 1 from the entrance surface 5 of the entrance section 3.
to go into. The pronto exchange optical waveguide 2 does not reach the entrance surface 5. Semiconductor laser light P1 entering from the incident part 3 of the fundamental wave P1 generates harmonics inside the optical waveguide 2.
It is converted into P2 and radiated into the LiNbO ₃ substrate 1.
At this time, most of the fundamental wave propagating without being converted into harmonics P2 is scattered at the output section 4. Therefore, since the fundamental wave does not return to the semiconductor laser, the output and longitudinal mode of the semiconductor laser become stable. Along with this, the harmonic output also becomes stable.

Next, a method for manufacturing this optical wavelength conversion element will be explained using figures. FIG. 2 shows a manufacturing process diagram of the optical wavelength conversion element of the present invention. First, the LiNbO ₃ substrate 1 was etched by approximately 2 μm except for the entrance portion 3. Specifically, the upper part of the entrance part 3 is Ta
It was covered with a mask and etched by dry etching. Next, as shown in FIG. 1A, a protective mask 10 made of Ta was deposited on the LiNbO ₃ substrate 1 at a thickness of 300 A by electron beam evaporation. Next, as shown in Figure b, the thickness is
After patterning 0.5μm photoresist 11
The Ta protective mask 10 was dry etched using CF ₄ . Next, after removing the photoresist 11, the LiNbO ₃ substrate 1 is heat-treated (proton exchange treatment) at 230°C for 5 minutes in pyrophosphoric acid, which is a type of phosphoric acid, to form an optical waveguide 2 with a thickness of 0.37 μm, as shown in FIG. After that, the protective mask 10 is removed. Next, a surface perpendicular to the optical waveguide 2 is optically polished to form a fundamental wave entrance surface 5 and a harmonic output surface 6 shown in FIG. Thereafter, the fundamental wave output section 4, which forms the edge of the output surface 6, is roughened by rough polishing so that the fundamental wave is scattered. Finally, an optical wavelength conversion element can be manufactured by forming an antireflection film (AR coating) on the incident surface 5 for the fundamental wave and applying an AR coating on the output surface 6 for harmonic waves. AR coating 5' on this incident surface 5
This is to prevent light from returning to the semiconductor laser. The length of this fabricated element is 6 mm. Figure 1b
Semiconductor laser light (wavelength 0.84 μm) is used as the fundamental wave P1.
When the wave was guided through the input section 3, single mode propagation occurred, and a harmonic wave P2 with a wavelength of 0.42 μm was extracted from the output surface 6 to the outside of the substrate. The coupling efficiency between the semiconductor laser and the optical waveguide was 55%, which was significantly improved compared to the conventional one (40%). Furthermore, since the output surface 6 is AR-coated against harmonics, the output of harmonics can be effectively extracted and increased by 15%. 1mW harmonic (wavelength 0.42μm) with fundamental wave 40mW input
I got it. Furthermore, since the amount of reflected light could be significantly reduced, the semiconductor laser operated stably, with fluctuations in harmonic output being less than ±3%. Furthermore, by performing harmonic superposition on the semiconductor laser, it was possible to suppress the variation in harmonic output to less than ±1%.

Note that multimode propagation is not practical because the high frequency output is unstable compared to the fundamental wave. Also, 0.65
We confirmed harmonic generation by this optical wavelength conversion device using a fundamental wave with a wavelength of ~1.6 μm.

Next, embodiments of the optical wavelength conversion device of the present invention will be described with reference to the drawings. FIG. 3 shows a configuration diagram of a short wavelength laser light source. In FIG. 3, the semiconductor laser 21 is
The oscillation wavelength is 0.78μm, and a constant current is applied from the CW power supply 22 and a sine-shaped high frequency is applied from the harmonic superimposition circuit 23, and the fundamental wave has an average power of 50mW.
P1 is being emitted. This fundamental wave P1 is the lens 2
4, 25 and the half-wave plate 26, the light enters the optical wavelength conversion element 27 and a harmonic wave P2 is generated. The half-wave plate 26 includes the semiconductor laser 21 and the optical wavelength conversion element 27
This was inserted in order to match the polarization direction with the optical waveguide 2 formed in . In addition, the output part 4 of the optical waveguide 2
The file has been roughened so that the fundamental wave P1 is scattered. With this optical wavelength conversion device, harmonics of 1 mW were obtained and the conversion efficiency was 2%. Further, the stability of harmonics was ±1% or less. FIG. 4 shows the incident current waveform for the semiconductor laser. In this example, a 600MHz sine-shaped waveform is superimposed as shown in figure a, but the semiconductor laser can also be stabilized by using a square wave, a triangular wave, etc. as shown in figure b and c, and the waveform shape does not depend heavily on In addition, stable operation is possible with only harmonics as shown in d of the same figure. We also confirmed stable operation at a frequency of around 1MHz. Furthermore, in this embodiment, by using a substrate doped with MgO, optical damage can be prevented even with short wavelength light, and there is no harmonic output fluctuation.

Next, the optical wavelength conversion device of the present invention is converted to the wavelength of the fundamental wave.
A structure composed of a 1.3 μm semiconductor laser, a 0.8 μm wavelength semiconductor laser, and an optical wavelength conversion element will be described. In Figure 5, 31 is the wavelength of 1.3μm
32 is a semiconductor laser with a wavelength of 0.86 μm, and 27 is an optical wavelength conversion element. In this embodiment, each semiconductor laser is directly coupled to an optical wavelength conversion element. In other words, coupling is performed without using lenses. The fundamental waves P1 and P3 propagated through the input waveguides 33 and 34 are multiplexed at the Y branch 35, enter the optical waveguide 2, and are converted here into the sum frequency P2 (wavelength
0.52μm). The fundamental waves P1 and P3 that have propagated through the optical waveguide 2 and reached the end thereof are scattered by the output section 4, and the semiconductor laser has no returning light and operates stably. The emission section 4 was processed using a CO ₂ laser so that the fundamental wave was scattered. Semiconductor laser 3
By superimposing a high frequency on each of 1 and 32, the sum frequency P2 was obtained more stably. In this way, this configuration is also effective for generating sum frequencies.

Next, an example in which the optical wavelength conversion device of the present invention is applied to reading an optical disk will be described. The harmonics obtained by this short wavelength laser light source are beam-shaped using a shaping lens so that the diverging light side becomes parallel light, so that both sides are parallel light. After this collimated harmonic passes through a polarizing beam splitter,
The light is focused by a focusing lens and onto an optical disk.
Tie the 0.6μm spots. This reflected signal passes through the polarizing beam splitter again and then enters the photoreceiver. A harmonic of 1.4 mW was emitted using a semiconductor laser with a wavelength of 0.84 μm and an output of 60 mW.

In this way, by using the optical wavelength conversion device of the present invention, it is possible to narrow down the spot to half compared to the conventional optical disc reading system using a 0.8 μm band semiconductor laser, and the recording density of the optical disc can be reduced. It can be improved by 4 times.

In the embodiment, the fundamental wave was scattered at the emission part, but it is also effective to absorb the fundamental wave by applying an absorber to the emission part. In addition, although LiNbO ₃ with a large nonlinear optical constant was used in the example,
In addition, any substrate having a large nonlinear optical constant such as a ferroelectric material such as LiTaO ₃ or KNbO ₃ , an organic material such as MNA, or a compound semiconductor such as ZnS can be used.

Effects of the Invention As described above, according to the optical wavelength conversion device of the present invention, stable high frequency power can be obtained by applying high frequency to a semiconductor laser that generates a fundamental wave that is incident on the optical wavelength conversion element. Furthermore, by preventing the return light from the optical wavelength conversion element, the output and longitudinal mode of the semiconductor laser do not change, and stable high-output harmonics can be generated.

[Brief explanation of the drawing]

Figures 1 a and b are perspective views and cross-sectional views of the optical wavelength conversion element of the present invention, Figures 2 a to c are manufacturing process diagrams of the optical wavelength conversion element of the present invention, and Figure 3 is an optical wavelength conversion element of the present invention. The configuration diagram of the device and FIGS. 4a to 4d are current waveform diagrams applied by high frequency superimposition. FIG. 5 is a configuration diagram of an optical wavelength conversion device according to an embodiment of the present invention, and FIG. 6 a, b
These are a plan view and a cross-sectional view of a conventional optical wavelength conversion element. 1... LiNbO ₃ substrate, 2... Optical waveguide, 3...
Incident part, 4... Output part, 21... Semiconductor laser.

Claims (1)

[Claims] 1. A semiconductor laser, a substrate having a nonlinear optical effect, and an optical waveguide formed on the substrate, wherein a fundamental wave emitted from the semiconductor laser enters from one end surface of the optical waveguide. An optical wavelength conversion device, wherein the semiconductor laser propagates through the optical waveguide and radiates harmonics from the optical waveguide into the substrate, and the semiconductor laser is harmonic-superimposed. 2. A patent characterized in that the other end surface of the optical waveguide is provided with a scatterer or an absorber, the fundamental wave is scattered or absorbed by the scatterer or the absorber, and the fundamental wave is prevented from returning to the semiconductor laser. An optical wavelength conversion device according to claim 1. 3 LiNb _x as a substrate with nonlinear optical effect
The optical wavelength conversion device according to claim 1, characterized in that a Ta _1-x O ₃ (0≦X≦1) substrate is used. 4 LiNb _x Ta _1-x O ₃ doped with MgO as a substrate with nonlinear optical effect (0≦X≦1)
The optical wavelength conversion device according to claim 1, characterized in that a substrate is used. 5. The optical wavelength conversion device according to claim 1, characterized in that a pronto exchange optical waveguide is used as the optical waveguide. 6. The optical wavelength conversion device according to claim 1, wherein the fundamental wave from the semiconductor laser is directly incident on the optical waveguide. 7. The optical wavelength conversion device according to claim 1, wherein a fundamental wave is incident on a pronto exchange optical waveguide formed on a substrate having a nonlinear optical effect through a part of the substrate. 8. The optical wavelength conversion device according to claim 1, characterized in that an antireflection film for the fundamental wave is formed at the entrance portion of the fundamental wave.