1. Field of the Invention
The present invention relates to a semiconductor optical device having device regions and diffraction gratings and more particularly, to a semiconductor optical device having device regions on one side of a semiconductor substrate and diffraction gratings on the opposite side of the substrate, which can provide various functions such as optical switching and optical broadcasting.
2. Description of the Related Art
An optical switch has an advantage because it can realize faster switching (such a speed as Gbit/sec) than an electrical switch. Conventionally, directional coupler type optical switches made of dielectric material such as LiNbO.sub.3 and a compound semiconductor such as AlGaAs and InGaAsP systems have been researched and developed. However, the conventional optical switches have a problem that the length thereof is too large and many channels cannot be provided.
FIG. 1 shows a conventional directional coupler type optical switch, which is disclosed in the Digest of Photonic Switching Topical Meeting, Mar. 6-8, 1991, Salt Lake City, Utah, USA, pp. 193-196.
The switch shown in FIG. 1 has a length L of 4.2 mm. A 4.times.4 matrix switch system using these switches has been reported, however, the total length of the switch system becomes 15 to 20 mm, so that it is difficult to compose a large-scale optical switch array using the 100 to 1000 optical switches shown in FIG. 1. Besides, the difficulty of the large scale integration of the switches is due to one-dimensional (1 D) inputs thereof.
A directional coupler type optical switch generally has a problem that optical coupling is not easy and an optical loss is easy to occur. The switch shown in FIG. 1, which is not of a directional coupler type, has an optical waveguide (i-GaAs guide layer) in which light propagates. The cross-section thereof is rectangular whose thickness is 0.26 .mu.m, so that the switch in FIG. 1 also has a problem that optical coupling is not easy to be done.
Next, it has been expected that an optical device can realize a broadcast function, which means a function that one signal is distributed simultaneously to a plurality of devices or systems. A conventional semiconductor optical device which can realize the broadcast function is shown in FIG. 2. The device in FIG. 2 is disclosed in the Digest of International Topical Meeting on Optical Computing, Apr. 8-12, 1990, Kobe, JAPAN, pp 164-166.
The conventional optical device in FIG. 2 has diffraction gratings which act as a lens, a reflecting mirror, a beam splitter and the like on a substrate made of glass or quartz. In this device, light which has exited from a micro laser formed on the substrate enters in a planar optical circuit made of glass and is made curved by the gratings in the circuit and thereafter, enters in a light receiver formed on the substrate. In the device, a 1.times.1 optical coupling is made; however, if appropriate gratings are used, for example, the broadcast function can be realized.
The planar optical circuit shown in FIG. 2 has an advantage that the substrate having a large area is easy to be acquired and the substrate is transparent to light generally produced by semiconductor lasers of AlGaAs and InGaAsP systems. However, first, there is a problem that the planar optical circuit and the semiconductor optical device are required to be fabricated separately and joined to each other in alignment, which is not easy.
Second, there is another problem that processing of a glass or quartz plate with high precision and minuteness is difficult to be realized.