1. Field of the Invention
This invention relates to an optical wavelength conversion device or the so-called SHG (secondary harmonic generator), and more particularly to a waveguide type SHG, and a process for manufacturing a waveguide therefor.
More specifically, the invention concerns a waveguide type optical wavelength conversion device for producing a secondary harmonic wave by Cerenkov radiation which has an optical waveguide formed on a substrate of a nonlinear optical material and constituted by a first waveguide passage for confining a fundamental wave and a second waveguide passage for confining the secondary harmonic wave, the fundamental wave being converted into the secondary harmonic wave efficiently by the first waveguide and the nonlinear optical effect of the substrate and the resulting secondary harmonic wave is caused to propagate toward the end face of the second waveguide passage to emit therefrom a second harmonic radiation beam of a circular or elliptic shape.
Further, the invention concerns a process for manufacturing a ridge type optical waveguide with a ridge having a ridge angle of approximately 90.degree. and smooth surfaces on the lateral sides, by selection of suitable kinds of masking photoresist and metal or a suitable kind of etching gas which permits reductions in light propagation loss and has a high degree of integration.
2. Description of the Prior Art
Previously there has been proposed an optical wavelength conversion device (hereinafter referred to simply as "SHG device" for brevity) constructed with a waveguide utilizing Cerenkov radiation, which is arranged as shown in FIG. 5.
This SHG device has an optical waveguide 2 formed on the surface of an LiNbO.sub.3 single crystal substrate 1 with an ion exchange method using benzoic acid for confining a fundamental wave f into one end of the waveguide 2 and taking out the secondary harmonic wave s from the substrate 1. Namely, with this SHG device, a laser beam (a fundamental wave) f which is incident on an end face of the waveguide 2 is confined therein. (Guided Mode) At this time, the secondary harmonic wave (SH light) s which has one half of the wavelength of the fundamental wave f is generated by the nonlinear optical effect of lithium niobate (LiNbO.sub.3) which constitutes the substrate and the high energy density of the fundamental wave f.
By selecting a suitable thickness for the optical waveguide 2, the thus-generated SH light s is radiated in the depthwise direction of the substrate 1 with a certain .alpha. angle (Cerenkov angle). (Radiation Mode)
With these steps the fundamental wave is converted into the secondary harmonic wave (SH light) by the above-mentioned prior art device.
Heretofore, there have also been known optical waveguides of different types including the so-called ridge type waveguide which has a narrow strip-like ridge formed on a substrate as shown in FIG. 6.
The ridge type waveguide has a laminated structure composed of a first substrate 11 with a refractive index n.sub.1 and a second substrate 12 with a refractive index n.sub.2. In this instance, the refractive indices of the first and second substrates and 12 satisfy the condition of n.sub.2 &gt;n.sub.1. According to this construction, as light propagates, it is confined by the differences in the refractive indexes between the first and second substrates 11 and 12 in a direction vertical to the substrate and by the difference in the refractive indexes between the ridge 12a and air in a direction inward of the substrate plane. In FIG. 6, the reference character .alpha. indicates an angle (hereinafter referred to as the "ridge angle") which is formed by the lateral side surface of the ridge 12a and the horizontal face of the substrate.
A ridge type waveguide as shown in FIG. 6 can be formed by a selective growth or etching process. For instance, techniques of forming a ridge by a fine etching process were presented at the 1986 General National Meeting of the Society of Electronic Communications (Lecture No. 868). According to this process, for example, Ti is first deposited on a substrate of lithium niobate (LiNbO.sub.3) to form a metal layer thereon, and then a layer of photoresist which has a tradename of "AZ-1350J" is selectively formed on the metal layer by photo lithography, followed by patterning of the metal layer by wet etching using the photoresist layer as a mask. Thereafter, the photoresist layer is removed, and the metal layer which remains as a result of the patterning is used as a mask to form a ridge on the substrate by electronic cyclotron resonance-reactive ion etching (ECR-RIE) using C.sub.3 F8 as an etching gas.
The above-mentioned SHG device construction, however, has a drawback in that the beam of the SH light s is emitted in a crescent shape in the section as shown in FIG. 5, instead of in a circular shape which is desirable. Namely, taking a light source for a high density optical recording medium or a laser printer as an example of an application of the SHG device, the light source is required to be able to emit a beam of a circular or elliptic shape rather than a beam of crescent shape which would lower the efficiency of utilization.
In this connection, it may be conceivable to reform the emitted SH light beam from a crescent shape into a circular or elliptic shape, but it has been found extremely difficult to reform into a circular shape the SH light which tends to sink into the deep portions depthwise of the substrate.
In view of the above-mentioned drawbacks or problems inherent to the prior art techniques, it is an object of the present invention to provide an optical wavelength conversion device with a ridge type waveguide of improved construction, which is capable of generating a beam of substantially circular or elliptic shape.
With a conventional ridge type waveguide as shown in FIG. 6, in order to obtain high output, it is necessary to make the ridge angle .alpha. as close to 90.degree. as possible and to form smooth surfaces on the lateral sides of the ridge by the ECR-RIE process. These requirements have to be met because a smaller ridge angle will result in a lower efficiency in confining light within the ridge, and rough side surfaces of the ridge will increase the light propagation loss by scattering. In this regard, there is a tendency that the increase of propagation loss due to rough side surfaces becomes larger as compared to ridges which have higher grade of fineness. However, by making the ridge angle .alpha. close to 90.degree., it becomes possible not only to improve the just-mentioned tendency but also to attain a higher integration of optical IC devices with ridge type waveguides.
Nevertheless, the ridge angle which can be achieved by the prior art techniques is 70.degree.-80.degree. at most, which is insufficient for efficiently confining light. Besides, the suppression of surface roughening is not enough. Especially in case of the techniques which use the so-called lift-off process, the side surfaces of a formed metal layer is susceptible to bruises or blemishes which will be reflected by roughening of the side surfaces of a ridge when the metal layer is subsequently used as a mask in a ridge-forming process.
Further, the ratio of the etch rate (i.e., the selectivity ratio of etching) of the crystal substrate to that of the mask metal is approximately as small as 2-3, which is not necessarily sufficient in terms of the economy, reliability and productivity of the process.
Under these circumstances, the present invention also has an object for the provision of a process for producing a ridge type waveguide with a ridge angle .alpha. approximating 90.degree. and with smooth surfaces on the lateral sides of the ridge.