Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2002-02-13
2004-01-20
Schwartz, Jordan M. (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S245000, C359S247000
Reexamination Certificate
active
06680791
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to optical signal processing, and more particularly it relates to semiconductor devices that can function as optical switches in which one optical beam modulates another without the need to transform either beam to an electrical signal.
BACKGROUND ART
Communications systems are increasingly using optical fiber as the transmission medium for propagating optical beams that carry information. Optical fiber exhibits many advantages including low loss, immunity to interference, and an extremely large bandwidth. In wavelength-division-multiplexed (WDM) systems, multiple wavelengths of light are used to establish many communication channels in a single optical fiber. The use of a number of channels at different wavelengths increases information throughput and correspondingly augments system capacity. In a typical WDM system, information-bearing optical beams or optical signal beams at the selected channel wavelengths are mixed together or multiplexed with the aid of optical couplers and launched into the fiber. At the receiver the optical signal beams are separated or demultiplexed by optical filters.
It is often necessary to transfer optical signals between optical networks operating at different wavelengths, swap channels within the same network or perform other functions requiring a particular optical signal beam to be converted and transmitted at a different wavelength. For example, a transmission system may be set up to send all information at a first wavelength to a first destination, and all information at a second wavelength to a second destination. Changing the wavelength of the optical signal beam from the first to the second wavelength therefore switches the destination of the information borne by the optical signal beam. The process of changing the information from one signal light beam to another can also be used to regenerate the signal, that is, to improve the quality of the signal.
The transfer of an optical signal beam from one channel requires both a device that can convert signal wavelengths and system architecture, incorporating the device, which can be scaled to required capacities. The prior art describes several devices and systems for such purposes.
A WDM optical system is disclosed in U.S. Pat. No. 5,504,609 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 waveguide 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 of 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. 5,343,700 to Yoo. The converter acts as a nonlinear 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. The pump frequency determines the frequency shift according to the known rules of difference frequency generation (DFG). This 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 of DFG employed by the device of Yoo is used in a parametric wavelength interchanging cross-connect, described in U.S. Pat. No. 5,825,517 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. In other words, each wavelength converter in the cross-connect takes two input signals with wavelengths &lgr;
2
and &lgr;
2
, and produces two output signals of wavelengths &lgr;
2
and &lgr;
1
, transferring the information carried by input signal at wavelength &lgr;
1
to output signal at wavelength &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 wavelength conversion by means other than nonlinear optical frequency conversion have also been disclosed in the prior art. A number of these switches take advantage of the electroabsorption effect allowing some of these devices to operate on picosecond time scales. A high-speed electro-optical modulator is disclosed in U.S. Pat. No. 4,525,687 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 of light particles or photons having 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 the applied electric field, optical properties of the device can be changed at will. An optical signal beam consisting of photons whose photon energy is 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 electronically-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.
In U.S. Pat. No. 5,339,370 Sano et al. teach an optical modulator whose light absorptive layer changes its absorption as a function of voltage applied across the modulator. Sano et al. also teach the use of the optical modulator in an optical communication system. This type of optical modulator is responsive directly to an electrical control signal and is not designed to switch optical signals in response to other optical signals. In addition, it is not suitable for fast-switching WDM networks because of its low response speed. A related type of modulator employing a multiple quantum well in which absorption is changed by an applied voltage is taught by Dutta et al. in U.S. Pat. No. 5,608,566 entitled multi-directional electro-optic switch. This switch can be used to switch optical signals between waveguides but, as in the case of the modulator of Sano et al., it is not responsive to another optical signals and its response time is too slow to be used in fast-switching WDM networks.
In U.S. Pat. No.
Demir Hilmi Volkan
Miller David A. B.
Sabnis Vijit
Lumen Intellectual Property Services Inc.
Schwartz Jordan M.
Stultz Jessica
The Board of Trustees of the Leland Stanford Junior University
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