There has been a demand for development of small-size, light-weight and high-performance optical devices as portable electronic apparatuses are becoming practical. Conventionally, as representative optical devices, bulk type optical devices and light waveguide type optical devices have been known. The bulk type optical devices are optical devices into which a variety of optical components are incorporated. The light waveguide type optical devices are optical devices which are used for communication, and the like, and are formed on a substrate.
A bulk type optical device is described in Japanese Laid-Open Publication No. 6-20297. An optical pickup 52 which is such a bulk type optical device is shown in FIG. 15. In the optical pickup 52, a plurality of optical system members, which constitute the optical system of the optical pickup 52, are fixed at a housing 51 whose upper side is open. A semiconductor laser 53 as a light source and a collimator lens 54 for causing laser light (light beam) to be parallel light are provided at a predetermined position outside the housing 51. A spot lens 58 for detecting a signal and an attachment substrate 59 are provided inside the housing 51. On the attachment substrate 59, a Wollaston prism 55 which is a type of polarizing prism, a composite prism 56 for refracting laser light emitted from the above-described semiconductor laser 53, and a mirror 57 are mounted. Note that the above-described optical system members 55 through 57 are made of optical glass, for example. The Wollaston prism 55, the composite prism 56, and the mirror 57 are adhered and fixed at predetermined positions on the upper side of the attachment substrate 59, which is made of the same composition as that of the optical system members, using an adhesive for optical glass. The above-described optical system members 55 through 57 can be fixed with high precision to the attachment substrate 59 by adjusting their mount positions and angles using amounting jig (not shown). The upper and lower surfaces of the above-described attachment substrate 59 are flat planes parallel to each other. The spot lens 58 is housed in a lens holder 58a which is fixed at the inside surface of a side of the housing 51.
Optical components (a lens, a mirror, a prism, a diffraction grating, etc.) in a bulk type optical device are produced by a process such as polishing or molding. The size of the optical components is usually of the order of several millimeters. Therefore, there is a limit on the miniaturization of an optical device comprising these components.
If the size of each optical component is further reduced, the cost of the component is increased and the mount precision is made stringent. A bulk type optical device is produced by arranging and assembling optical components which have been separately produced. In this case, the greater the number of optical components, the greater the number of steps. Further, it is difficult to perform positioning with high precision.
To solve the above-described problem with bulk type optical devices, light waveguide type optical devices have been proposed.
The following stringent conditions are required for light waveguide type optical devices: (1) a small loss of light; (2) ease of production; (3) controllability of the difference in refractive index between a core layer and a cladding layer; (4) excellent heat resistance; and (5) a small difference in birefringence.
A quartz-based light waveguide requires a process at a high temperature of 1000° C. or more in the production thereof. For this reason, there are some problems, such as a substrate on which a light waveguide is formed is limited, a light waveguide cannot be formed after optical components have been mounted on the substrate, and the like.
In contrast, a plastic light waveguide, which can be formed at a low temperature, has been recently proposed. However, the plastic light waveguide has drawbacks, such as poor heat resistance and the like. A low birefringent polyimide, which is the most heat resistant plastic and which is clear and colorless, has been proposed as an exemplary material for light waveguide type optical components in Japanese Laid-Open Publication No. 8-134211. Such a polyimide light waveguide is generally produced by the following method. A polyimide precursor solution is applied to a semiconductor substrate made of silicon, gallium arsenide, or the like, or an optical substrate made of quartz, glass, or the like by spin coating or printing, followed by heating to evaporate a solvent, and curing, thereby preparing a polyimide thin film. Further, a polyimide waveguide is obtained via optional processes, such as wet etching, dry etching, laser ablation, and the like. With the above-described steps, a polyimide thin film having a thickness of several μm or so can be advantageously produced.
Generally, in a light waveguide, light is confined within a core having a size as small as several μm, thereby various optical functions are achieved. A light waveguide contributes to integration and miniaturization of an optical system. A plurality of light waveguide type optical components can be simultaneously produced by a process, such as photolithography, etching, and the like, and positioning precision for the light waveguide type optical components is high.
However, the core size of a light waveguide type optical component is as small as several μm. Therefore, a technique for aligning the light waveguide with other optical components on the order of micro meters is required for coupling light with the core. Thus, a reduction in assembling cost is required. For this reason, current light waveguide type optical components are mainly limited to applications in which a single mode optical fiber.
Moreover, in light waveguide type optical devices, nonuniformity in core size and/or light loss due to scattering at core interface is great and it is therefore difficult to mass-produce optical devices having low loss and consistent performance.
Further, in light waveguide type optical devices, even if a core and cladding are made of isotropic materials, birefringence easily develops due to distortion in the shape of the waveguide core or variations in the refractive index of the cladding. As a result, difference in propagation time between polarized light beams occurs, which creates distortion in a signal. It is therefore difficult to use light waveguide type optical devices as light transmission devices.
The present invention is provided to solve the above-described problems by means of technical philosophy which is absolutely different from the above-described bulk type and waveguide type optical devices. The objective of the present invention is to provide a small-sized and light-weight bulk type optical component.