1. Field of the Invention
The present invention relates to a wavelength converter for use in a wavelength division multiplex (WDM) optical communication, and more particularly to a wavelength converter for use in high-speed WDM optical communication.
2. Description of the Prior Arts
In recent years, WDM optical communication has begun to be developed as a mass optical communication system. In this WDM optical communication, each wavelength in a signal light whose wavelength is multiplexed is allocated to each communication channel. In order to transmit or receive a signal between channels, it is necessary to convert the wavelength of an original signal string to the wavelength corresponding to the specified channel. Conventionally, this signal wavelength conversion has been performed by converting optical signals once to electric signals and then converting the electric signals to optical. signals having a different wavelength again.
On the other hand, recently research has been performed on a device designed to convert wavelengths of optical signals directly without converting them to electric signals. There have been proposed interaction-type devices such as a symmetrical Mach-Zender type device and a polarization-discrimination type device.
The symmetrical Mach Zender type wavelength converter has been conventionally reported as an optical switch ("Applied Physics Letters," by S. Nakamura et al., vol. 65, pp. 2445), which has also a feature of a wavelength converter since it outputs signal pulses having wavelengths different from those of input signal pulses. This symmetrical Mach-Zender type wavelength converter (hereinafter, a wavelength converter of a conventional example 1) has a configuration in which two waveguides 24 and 25 causing changes of nonlinear refractive indices (hereinafter, nonlinear waveguides) are arranged in respective arms of a Mach Zender interferometer as shown in FIG. 1A. If continuous wave (CW) light having a wavelength .lambda.2 output from a CW light source 21 is entered from a CW light input port 22, it is branched in a branch section 23 and then each light beam is introduced into the nonlinear waveguide 24 or 25. On the other hand, from a signal input port 26, an original signal of an input signal pulse having a wavelength .lambda.1 is entered. The input signal pulse is branched in a branch section 27, and both are transmitted through an optical path 28 or 29 and introduced to the nonlinear waveguide 24 or 25, so as to change a refractive index of the nonlinear waveguide 24 or 25 for a fixed period of time. Assuming that .DELTA.t is a delay time generated between input signal pulses transmitted through two optical paths due to an optical path difference between the optical paths 28 and 29, if the pulse width .DELTA.T of an input signal pulse is sufficiently shorter than .DELTA.t, entering the input signal pulse into the signal input port 26 at time t1, for example, outputs an output signal pulse having a wavelength .lambda.2 with the first transition at the time t1 and the last transition at time t1+.DELTA.t. In this manner, the wavelength converter of the conventional example 1 converts the wavelength .lambda.1 to the wavelength .lambda.2. As a wavelength converter which is the same as the above except that an optical delay section composed of two optical paths 28 and 29 is arranged differently from that in FIG. 1A, there is a converter which is disclosed in Japanese Non-examined Patent Publication No. 7-199240 to Nakamura.
Alternatively, a polarization-discrimination, type wavelength converter as shown in FIG. 1B (hereinafter, a wavelength converter of a conventional example 2) is also conventionally reported as an optical switch ("Applied Physics Letters," by T. Tajima et al., vol. 67, no. 25, pp. 3709-3711, 1995 and "IEEE Photonics Technology Letters," by N. S. Patel et al., vol. 8, pp. 1695-1697, 1996). This optical switch serves as a wavelength converter like the above-mentioned symmetrical Mach Zender type converter. It differs from the above wavelength converter of the conventional example 1 in that a specifically polarized light component is delayed by .DELTA.t using a polarization-discrimination delay circuit 43 and then advanced by .DELTA.t using a polarization-discrimination delay circuit 47 to resume the previous state. After an input signal pulse having a wavelength .lambda.1 is entered into the signal input port 45 at time t1 by these actions, it becomes possible to perform a wavelength conversion in which output signal pulses at time t1 to time t1+.DELTA.t can be output from a signal output port 48 comparable to the wavelength converter of the conventional example 1 by using only a single nonlinear waveguide 44. For example, assuming that the polarization-discrimination delay circuit 43 is used to perform a polarization-discrimination and a delay for an input light of an S polarized light and that the polarization-discrimination delay circuit 47 is used to perform a polarization-discrimination and a delay for an input light of a P polarized light, the operations are as follows: if a CW light having a wavelength .lambda.2 is entered from a CW light source 41, the polarization-discrimination delay circuit 43 performs a delay by .DELTA.t only for S polarized light components of the CW light and then the components are introduced to a nonlinear waveguide 44, while an input signal pulse having a wavelength .lambda.1 is entered with a pulse width .DELTA.T which is efficiently shorter than .DELTA.t into a signal input port 45 and introduced to the nonlinear waveguide 44 via a combiner section 46 so as to change a refractive index of the nonlinear waveguide 44. When the CW light passes through the nonlinear waveguide 44, only the P polarized light components are given a delay by .DELTA.t, in other words, the S polarized light components are advanced by .DELTA.t, by which the above output signal pulses are output from the signal output port 48.
The above-mentioned wavelength converters of the conventional example 1 and the conventional example 2, however, have problems as described below, respectively.
The wavelength converter of the conventional example 1 has a problem that a configuration is complicated due to two nonlinear waveguides 24 and 25 arranged therein. In addition, it is necessary to control a balance strictly between refractive indices (therefore a phase of a transmitting signal pulse) and changes of the nonlinear refractive indices of the two nonlinear waveguides 24 and 25 each, other. If waveguides are operated for a long time using semiconductor materials for the waveguides, for example, the refractive indices of the semiconductor waveguides change little by little. Such a slight change difference between refractive indices of the two nonlinear waveguides significantly deranges operating conditions in the wave-length converter of the conventional example 1, and therefore a long-time continuous change of the refractive indices becomes a significant problem and a precise control circuit is required in order to adjust a difference between the refractive indices of the two nonlinear waveguides each other.
On the other hand, the wavelength converter of the conventional example 2 includes only a single nonlinear waveguide 44, and therefore it has a simple configuration and easy controls in comparison with the conventional example 1. The wavelength converter of the conventional example 2, however, has problems that it is required to arrange elements used for polarization-discrimination in the polarization-discrimination delay circuits 43 and 47 in the configuration and that it is necessary to set an axis of each polarization-discrimination delay circuit according to a polarization status of an input CW light.