Optical waveguides – Planar optical waveguide – Thin film optical waveguide
Reexamination Certificate
2000-06-05
2002-04-16
Spyrou, Cassandra (Department: 2872)
Optical waveguides
Planar optical waveguide
Thin film optical waveguide
C359S344000, C385S132000, C385S014000
Reexamination Certificate
active
06374029
ABSTRACT:
BACKGROUND OF THE INVENTION
1) Field of the Invention
The present invention relates to an optical device which can be preferably used as a resource in the wavelength division multiplex (WDM) optical communication system.
2) Description of the Related Art
In accordance with a recent abrupt progress in the digital communication, the development of the WDM optical communication system has been strongly required. This WDM optical communication system requires an optical wave converter in order to utilize a limited number of channels in an efficient manner by channel switching. In conventional wavelength converters, XGM type wavelength converters utilizing cross gain modulation and XPM type wavelength converters utilizing cross phase modulation are known.
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 an optical power impinging upon the semiconductor optical amplifier.
The XPM type wavelength converter utilizes the principle of Mach-Zehnder interferometer. In this type of device, an input side of a waveguide upon which an input optical signal having a wavelength &lgr;
2
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 &pgr;/2 between the two waveguides due to the function of the input signal. By utilizing this phase difference, an inverted optical output having a wavelength &lgr;
2
is generated.
Since the known XGM type wavelength converter utilizes the saturated gain of the optical amplifier, the extinction ratio of this optical waveguide converter is small.
In addition, it has inherent drawback that only the inverted optical output signal is produced and non-inverted output signal could not be produced.
In the XPM type optical wavelength converter, although it is possible to obtain a sufficiently large extinction ratio, since it reveals a periodical response, an extremely severer tolerance is required for a device length. Therefore, a through-put of the known XPM type optical wavelength converter is substantially reduced.
Furthermore, the above mentioned known optical wavelength converter is relative large in size. That is to say, the typical size of the known optical 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
It is therefore an object of the present invention to realize a novel and useful optical device which can overcome the aforementioned drawbacks and can have a large extinction ratio.
It is another object of the present invention to provide an optical device which can produce an non-inverted output signal and can operate in the digital manner.
It is still another object of the present invention to provide an optical device which can be manufactured by a relatively simple process and which can operate as a wavelength converter or a waveform shaper.
According to the invention, an optical device for converting an input optical signal into an optically amplified output optical signal comprising:
a semiconductor substrate having mutually opposing first and second surfaces;
a waveguide structure comprising a plurality of semiconductor layers formed on said first surface of the semiconductor substrate and having an incident surface upon which an input optical signal is made incident and an exit surface opposed to said incident surface, said incident and exit surfaces being perpendicular to the semiconductor layers;
a first electrode formed on said second surface of the semiconductor substrate;
a second electrode formed on the top of said waveguide structure such that the second electrode is opposed to said first electrode; and
a DC bias source connected across said first and second electrodes such that carriers are injected into said waveguide structure for amplifying said input optical signal and an amplified output optical signal is emitted from said exit surface;
wherein said semiconductor layers of the waveguide structure are composed of semiconductor materials whose refractive indices vary according to an amount of carriers injected from said first and second electrodes and stored therein;
said first and second electrodes are formed such that a carrier injection region into which carriers are injected through the electrodes and a non-carrier-injection region into which carriers are not substantially injected are formed adjacent to each other in the waveguide structure; and
said waveguide structure is constructed such that, in a carrier injection operation state, when an input optical signal of a first power level propagates through the waveguide structure, a refractive index of the carrier injection region becomes higher than that of the non-carrier-injection region and the carrier injection region constitutes an optical waveguide which guides input light wave from said incident surface to said exit surface, and when an input optical signal of a second power level lower than the first power level propagates through the waveguide structure, a refractive index of the carrier injection region is kept lower than that of the non-carrier-injection regions and the input optical signal is emitted through said non-carrier-injection.
The present invention positively utilizes the optical amplification effect and the free carrier plasma effect in which a refractive index of a semiconductor material is in inverse-proportion to a concentration of carriers injected in the material and stored therein. The first and second electrodes are arranged such that the carrier injection region into which carriers are injected through the electrodes and the non-carrier-injection regions into which carriers are not substantially injected are formed adjacent to each other in the waveguide structure. When mass carriers are injected into the carrier-injection region through the electrodes, these mass carriers are stored in this region and the refractive index of this region is reduced relatively lower than that of the adjacent non-carrier-injection region. However, when the light wave propagates through the waveguide structure, the carriers stored in the carrier-injection region are consumed and the carrier concentration of the waveguide is decreased. The decrease in carrier concentration causes in turn a relative increase in the refractive index of this region higher than that of the adjacent non-carrier-injection regions. In addition, since an amount of consumed carriers substantially corresponds to a power level of the propagating light wave, the propagation of a light wave having a higher power level causes a further decrease in the carrier concentration of the carrier-injection region and thus the refractive index of this region to be significantly higher than that of the adjacent non-carrier-injection regions, thereby further enhancing the optical confinement effect. This results in that there is formed in the waveguide structure a propagation path for the optical signal with a higher refractive index than that of the surrounding medium. Once such a propagation path is formed, the optical confinement effect and the induced emission generated in the waveguide are further enhanced, which induces a state of positive feedback. Therefore, this type of waveguide is referred as an “optically induced waveguide”.
Conversely, when a signal light having a low power level propagates through the waveguide structure, since the amount of the carriers consumed by the optical amplifying function is rather small, a mass of carriers are still remained
Ma Byong-Jin
Nakano Yoshiaki
Frank Robert J.
Maxie Christopher S.
The University of Tokyo
Venable
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