The present invention relates to an optical switch and, more particularly, to an optical switch used in optical fiber communications, optical information processing, and other applications.
It is considered that constructing an all-optical system (i.e., using neither optoelectrical conversion nor electro-optical conversion in transmission paths, in multiplexing/demultiplexing circuits, or in logic circuits) is necessary to increase the speeds of optical transmission systems and of optical information-processing systems. To enable this, optical switches capable of operating at high speeds are required. Conventionally, a method of switching light by an electrical signal (electrooptical control) has been adopted in optical switches. In recent years, however, a method of switching light by means of light (all-optical control) has attracted attention as a method expected to provide higher speeds. Especially, in an optical transmission system, if an ultrafast all-optical switch can be employed in an optical demultiplexer, then a great breakthrough will be made in realizing a large-capacity time-division multiplexing system.
The performance required in making all-optical switches practical is not limited to the aforementioned high speeds. Rather, various other kinds of performance such as low switching energy, high-repetition operations, and compactness are needed. Especially, with respect to switching energy, this energy must be within the range of optical pulse energies achieved by a semiconductor laser, fiber amplifier, or semiconductor laser amplifier.
A first problem produced in realizing these kinds of performance is that the figure of merits of nonlinear optical effects on which all-optical switches are based, given by .chi..sup.(3) /.tau..alpha., are generally almost constant. In this formula, the .chi..sup.(3) is the magnitude of a nonlinearity, .tau. is the response time, and the .alpha. is a signal loss. That is, it is considered that a nonlinear optical effect satisfying a large nonlinearity and a high speed simultaneously is difficult to obtain. Nonlinear optical effects can be roughly classified into the nonresonantly excited type and the resonantly excited type. The nonresonantly excited type is expected to provide high speeds but it produces small nonlinearities. That is, it is considered that nonlinear optical effects relying on non-resonant excitation with a practical level of switching energy are difficult. In contrast, in the resonantly excited type, carriers excited in a nonlinear optical medium relax slowly and present problems in realizing high-speed operation. However, the nonlinearity is large, which is a large advantage in practical applications. Accordingly, various methods for solving slow relaxations and achieving high-speed operation have been proposed. A conventional all-optical switch utilizing a highly efficient, resonantly excited nonlinear optical effect is next given as an example.
Japanese Patent Unexamined Publication No. 20510/1995 discloses an all-optical switch having a nonlinear optical waveguide using a semiconductor medium whose nonlinear refractive index is varied by absorption of controlling light. The construction of this optical switch is shown in FIG. 1, where a Mach-Zehnder interferometer is constructed, using 3-dB couplers 23, 24 comprising fibers. An optical signal is entered through an optical signal input port 27 and divided into parts by the 3-dB coupler 23 and interfered by the 3-dB coupler 24. The phase difference between two interfering light waves determines which of optical signal output ports 30 and 31 delivers an optical output signal. Controlling light pulses are entered into controlling light input ports 28 and 29 with a given time difference T, pass through wavelength-selecting couplers 25 and 26, respectively, and then enter nonlinear optical waveguides 21 and 22, respectively. First, a controlling light pulse enters the nonlinear optical waveguide 21, varying its refractive index. An optical signal passing through this waveguide undergoes a nonlinear phase shift. It is assumed that an optical signal is delivered from the optical signal output port 30 under the initial condition. The nonlinear phase shift in the nonlinear optical waveguide 21 causes the optical signal to exit from the optical signal output port 31. The refractive index change in the nonlinear optical waveguide 21 is produced by excitation of carriers by resonant controlling light. Therefore, the refractive index change rises very quickly while following the controlling light pulse, but the relaxation time is long. Consequently, under this condition, it takes a long time to return to the initial state. However, after a lapse of the time T, a controlling light pulse is entered into the nonlinear optical waveguide 22, thus inducing a change in the nonlinear refractive index. An optical signal passed through this waveguide undergoes a nonlinear phase shift. This cancels out the effect of the refractive index change remaining in the nonlinear optical waveguide 21. Therefore, the enabled port is switched back to the optical signal output port 30. In this way, ultrahigh speed switching of optical signals is possible.
The problem with the above-described prior art technique lies in the fact that the optical circuitry is made complex and bulky. At branching portions and bending portions of waveguides, optical signal losses take place. Concomitantly, the branch angle is suppressed or the radius of curvature is increased, which in turn makes the optical circuitry larger.