Optical waveguides – With optical coupler – Switch
Reexamination Certificate
1999-11-05
2002-09-03
Lee, John D. (Department: 2874)
Optical waveguides
With optical coupler
Switch
C385S002000, C385S008000, C359S248000
Reexamination Certificate
active
06445839
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to nonlinear semiconductor optical devices. More particularly, it relates to an ultrafast optically controlled optical switch and crossbar architecture for use in wavelength-division-multiplexed systems.
BACKGROUND ART
Communications systems are increasingly using optical fiber as the transmission medium, because of its low loss, immunity to interference, and extremely large bandwidth. In wavelength-division-multiplexed (WDM) systems, multiple wavelengths are used to allow many communication channels on a single optical fiber, allowing for much greater information transmission and network capacity. Modulated light beams are mixed into the fiber using optical couplers, and demultiplexed at the receiver end by optical filters. It is often necessary to transfer signals between optical networks operating at different wavelengths, and therefore transfer a particular optical signal from one channel to another. Switching information between channels requires the ability to change a particular signal of information from one wavelength to another. Switching of this manner requires both a device that can convert signal wavelengths and a system architecture, incorporating the device, that can be scaled to required capacities.
A WDM optical system is disclosed in U.S. Pat. No. 5,504,609, issued to Alexander et al. This system includes complex remodulators for transferring a signal from an input wavelength to an output wavelength. Each remodulator contains a photodiode or similar means for converting an optical input signal to an electrical signal, which is then amplified, filtered, and amplified again. The resultant electrical signal is used to modulate an optical source by exploiting the electro-optical effect in a waveguiding medium to create an amplitude-modulated output signal. The combination of electronic and optical elements required in the system of Alexander et al. greatly limit the net throughput in the system, and do not effectively take advantage of the increased bandwidth provided by the optical fiber. The remodulators also dissipate large amounts of power and make large arrays of switches impractical.
An all-optical wavelength converter is provided in U.S. Pat. No. 5,434,700, issued to Yoo. The device acts as a nonlinear optical mixer to combine an input signal with a pump signal to generate an output signal of a different wavelength. Specifically, the output frequency is the difference between the pump frequency and the input frequency. As is explicitly stated in the description, the pump frequency determines the frequency shift, and therefore the device cannot be used to convert multiple input channels to multiple output channels selectively. Instead, a separate device is required to convert between each input frequency and output frequency, requiring a set of parallel converters operating between neighboring WDM networks. Of course, this system cannot practically be scaled to WDM systems containing large numbers of channels. Furthermore, systems based on these techniques dissipate large amounts of power and are therefore not feasible for large-scale systems.
The technique employed by the device of Yoo, difference frequency generation, is used in a parametric wavelength interchanging cross-connect, described in U.S. Pat. No. 5,825,517, issued to Antoniades et al. The cross-connect of Antoniades et al. combines 2×2 spatial optical switches with the wavelength converters of Yoo to allow arbitrary switching of signals among the channels of the WDM network. By selecting particular wavelengths of pump sources, the wavelength converters can be made to interchange signals between two channels in a single device. That is, each wavelength converter in the cross-connect takes two input signals with wavelengths &lgr;
1
and &lgr;
2
, and produces two output signals of wavelengths &lgr;
2
and &lgr;
1
, transferring the information carried in input signal &lgr;
1
to output signal &lgr;
2
, and vice versa. Switching between systems with more than two channels requires complicated networks of 2×2 spatial switches and wavelength converters. Because each wavelength converter is limited to a few predetermined frequencies, arbitrary switching requires a series of wavelength converters, each of which has a different pump frequency. In addition, the cross-connect of Antoniades et al. uses only a single set of WDM wavelengths for both input and output signals, and does not allow for truly arbitrary switching.
Optical switches for modulating optical signals have been disclosed in the prior art. These switches take advantage of the electroabsorption effect in devices that operate on picosecond time scales. A high-speed electro-optical modulator is disclosed in U.S. Pat. No. 4,525,687, issued to Chemla et al. This semiconductor device contains a multiple quantum well structure across which an electric field is applied. The applied electric field increases absorption for photon energies just below the band gap by the quantum-confined Stark effect (QCSE). As the electric field is increased further, the band edge shifts to lower photon energies. By carefully controlling an applied voltage, and therefore electric field, optical properties of the device can be changed at will. An optical signal with photon energy just below the band gap of the quantum well structure is absorbed or transmitted with just a small change in the applied voltage. Because this device is an electrically-controlled optical modulator, it cannot be used alone to provide the wavelength conversion required in WDM systems. The desired result can only be produced by combining this device with a photodetector for generating the required electrical signal in response to the optical signal. As with the system of Alexander et al., the combination is complicated, incurs high power dissipation, cannot operate at the required switching speeds, and is not easily integrated into arrays.
There is still a need, therefore, for a wavelength converting switch that can be used in an architecture that allows for a complete cross-connect, in which the signal from any input wavelength can be used to modulate the output at any wavelength, with multiple different wavelengths of data in and out of the cross-connect system.
OBJECTS AND ADVANTAGES
Accordingly, it is a primary object of the present invention to provide an optical wavelength-converting cross-connect that allows for arbitrary switching of information between input and output signals.
It is a further object of the invention to provide a high-speed optically controlled optical switch for transferring signal information between an input light beam and an output light beam.
It is an additional object of the invention to provide a wavelength-converting switch that requires very low electrical and optical power inputs by exploiting the quantum-controlled Stark effect (QCSE).
It is another object of the present invention to provide an optical switch with controllable picosecond switching time scales.
It is an additional object of the present invention to provide an optical device that is easily integrated with the required electronics.
It is a further object of the present invention to provide an ultrafast gated photodetector that produces a signal requiring minimal processing.
SUMMARY
These objects and advantages are attained by a semiconductor device for modulating an optical power light beam at a first wavelength with an optical signal light beam at a second wavelength. The device consists of two diodes: a detector diode, containing a detector absorbing layer for absorbing the optical signal beam; and a modulator diode, containing a modulator absorbing layer for absorbing the optical power beam. The modulator absorbing layer has an electric field-dependent absorption coefficient; the two diodes are in sufficient electrical communication that this coefficient is altered by absorption of the optical signal beam by the detector diode. Altering the coefficient modulates absorption of the optical power beam, and therefore transfers
Lumen Intellectual Property Services
Song Sarah U
The Board of Trustees of the Leland Stanford Junior University
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