Optical: systems and elements – Optical frequency converter – Dielectric optical waveguide type
Reexamination Certificate
2001-02-12
2002-12-24
Lee, John D. (Department: 2874)
Optical: systems and elements
Optical frequency converter
Dielectric optical waveguide type
C385S122000
Reexamination Certificate
active
06498675
ABSTRACT:
BACKGROUND OF THE INVENTION
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.
1
A. If continuous wave (CW) light having a wavelength &lgr;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 &lgr;1 is entered. The input signal pulse is branched in a branch section
27
, and both are is 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 &Dgr;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 &Dgr;T of an input signal pulse is sufficiently shorter than &Dgr;t, entering the input signal pulse into the signal input port
26
at time t
1
, for example, outputs an output signal pulse having a wavelength &lgr;2 with the first transition at the time t
1
and the last transition at time t
1
+&Dgr;t. In this manner, the wavelength converter of the conventional example 1 converts the wavelength &lgr;1 to the wavelength &lgr;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 &Dgr;t using a polarization-discrimination delay circuit
43
and then advanced by &Dgr;t using a polarization-discrimination delay circuit
47
to resume the previous state. After an input signal pulse having a wavelength &lgr;1 is entered into the signal input port
45
at time t
1
by these actions, it becomes possible to perform a wavelength conversion in which output signal pulses at time t
1
to time t
1
+&Dgr;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 &lgr;2 is entered from a CW light source
41
, the polarization-discrimination delay circuit
43
performs a delay by &Dgr;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 &lgr;1 is entered with a pulse width &Dgr;T which is efficiently shorter than &Dgr;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 &Dgr;t, in other words, the S polarized light components are advanced by &Dgr;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 wavelength 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.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a wavelength converter which has a simple configuration without a need for elements used for polarization-discrimination and does not have any dependency o
Tajima Kazuhito
Ueno Yoshiyasu
Dickstein Shapiro Morin & Oshinsky LLP.
Lee John D.
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