Optical: systems and elements – Optical frequency converter – Dielectric optical waveguide type
Reexamination Certificate
1999-03-26
2002-03-12
Lee, John D. (Department: 2874)
Optical: systems and elements
Optical frequency converter
Dielectric optical waveguide type
C359S326000, C359S344000, C385S122000
Reexamination Certificate
active
06356382
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical wavelength converter for converting an input optical pulse signal having a first wavelength into an output optical pulse signal having a second wavelength which is different from said first wavelength.
2. Description of the Related Art
Such an optical wavelength converter can be preferably used as a resource in the wavelength division multiplex (WDM) optical communication system. In accordance with recent abrupt progress in digital communication systems, the development of the WDM optical communication system has been strongly required. This WDM optical communication system requires an optical wavelength converter in order to utilize channels in an efficient manner by channel switching. Such an optical wavelength converter has been known. For instance, a XGM type optical wavelength converter utilizing cross gain modulation has been proposed. In the XGM type optical wavelength converter, an intensity-modulated input optical signal having a wavelength &lgr;
1
and an optical signal having a wavelength &lgr;
2
and a constant amplitude are supplied to a semiconductor optical amplifier, and a polarity-inverted output optical signal having a wavelength &lgr;
2
is produced by utilizing a difference in gain for optical power impinging upon the semiconductor optical amplifier.
There has been further proposed a XPM optical type optical wavelength converter. A conventional type optical wavelength converter utilizes the principle of the Mach-Zehnder type interferometer.
In the XPM optical wavelength converter, an input side waveguide upon which an input optical signal having a wavelength &lgr;
1
is divided into two waveguides, a semiconductor optical amplifier is arranged in one of the waveguides, and these two waveguides are set to be in-phase for light having a wavelength &lgr;
2
to be modulated. When an input optical signal having a wavelength &lgr;
1
and an optical signal having a wavelength &lgr;
2
and a constant amplitude propagate, there is produced a phase difference of a half wavelength between the two waveguides due to the function of the input optical signal. By utilizing this phase difference, an inverted optical output having the wavelength &lgr;
2
is generated.
In the known XGM type optical wavelength converter, the output optical signal having a wavelength &lgr;
2
produced in response to the input optical signal having a wavelength &lgr;
1
has a rather small gain. Therefore, a zero level of the output optical signal deviates from a real zero. Consequently, the extinction ratio of this optical wavelength converter is small.
In the above mentioned XPM type optical wavelength converter, although it is possible to obtain a sufficiently large extinction ratio, since it reveals a periodical response, an extremely larger tolerance is required for the device length. Therefore, a through-put of the known XPM type optical wavelength converter.
Furthermore, the above mentioned known optical wavelength converter is relatively large in size. That is to say, the typical size of the known optical wavelength converter is not smaller than several to ten millimeters, and thus it is practically difficult to integrate it as a single chip.
SUMMARY OF THE INVENTION
The present invention has for its object to provide a novel and useful optical wavelength converter which can avoid or at least mitigate the above explained problems of the known optical wavelength converters and can have a large extinction ratio, can operate even in a digital fashion, and can be manufactured easily.
It is another object of the invention to provide an optical wavelength converter which can be small in size and can be integrated as a single chip.
According to the invention, an optical wavelength converter for converting an input optical pulse signal having a first wavelength into an output optical pulse signal having a second wavelength which is different from said first wavelength comprises:
a first waveguide constructed by an active waveguide and receiving the input optical signal having the first wavelength, a propagation constant of said first waveguide being changed in accordance with the input optical signal; and
a second waveguide arranged in parallel with said first waveguide to partially overlap with said first waveguide such that the first and second waveguides are optically coupled with each other to such an extent that evanescent light of light propagating along one of the first and second waveguides extends into the other waveguide;
wherein said first and second waveguides have a waveguide length L which is substantially equal to a coupling length at which a power transition of an optical signal propagating along the first waveguide into the second waveguide becomes maximum; and
said first and second waveguides are constructed such that a propagation constant difference &Dgr;&bgr; between the first and the second waveguides in a case that only an optical signal having the second wavelength propagates along said first waveguide is smaller than a propagation constant difference &Dgr;&bgr; between the first and the second waveguides when both the input optical signal having the first wavelength and the optical signal having the second wavelength propagate along the first waveguide.
According to the invention, said optically coupled first and second waveguides are arranged in parallel with each other and are constructed such that the waveguide length of these first and second waveguides becomes equal to the coupling length. Then, the two waveguides are coupled with each other and constitute an optical coupler in which a light wave propagating along one of the waveguides can be transferred or shifted into the other waveguide. In this case, a transition ratio of optical power is dependent not only upon the waveguide length, but also upon a difference in a propagation constant difference &Dgr;&bgr; between these two waveguides. When the propagation constant difference &Dgr;&bgr; decreases, the optical power transition ratio becomes high, and when the propagation constant difference &Dgr;&bgr; is increased, the optical power transition ratio becomes extremely small. Under a given condition, the optical power transition ratio can be substantially zero. Therefore, by controlling the propagation constant difference &Dgr;&bgr; between the two waveguides, it is possible to control the condition of the optical coupling between the waveguides in a digital fashion. The present invention is based on such a recognition and at least one of the two waveguides is constructed by the active waveguide in which the propagation constant is changed in accordance with the input optical signal. Therefore, by controlling the propagation constant of the active waveguide, the propagation constant difference &Dgr;&bgr; between the waveguides can be adjusted to control the optical coupling condition between the waveguides.
According to the invention, the active waveguide may be constructed by a semiconductor optical amplifier. In the semi-conductor optical amplifier, the refractive index of an active layer is changed in accordance with the amount of carriers which are injected into the active layer and are stored therein. When the optical power of the input optical signal is high, the amount of carriers which are consumed by the amplifying function becomes large and the amount of carriers stored in the active layer is decreased, and therefore the refractive index of the waveguide is relatively increased. Contrary to this, when the optical power of the input optical signal is low, the amount of consumed carriers is decreased and the amount of carriers stored in the active layer is relatively increased, and thus the refractive index is decreased. When the refractive index of the waveguide is changed, the propagation constant of the waveguide is also changed. According to the invention, such an active function of the semiconductor optical amplifier is utilized to control the propagation constant of the waveguide through the change in th
Ma Byong-Jin
Nakano Yoshiaki
Frank Robert J.
Lee John D.
The University of Tokyo
Venable
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