1. Origin of the Invention
The invention described herein was made in the performance of work under a NASA contract, and is subject to the provisions of Public Law 96-517 (35 USC 202) in which the Contractor has elected not to retain title.
2. Technical Field
The invention is related to wave-division multiplexing optical communication systems using transmitters and receivers consisting of optical integrated circuits and optical fiber communication channels.
3. Background Art
In wave-division multiplexed optical communication systems, many different optical wavelength carriers provide independent communication channels in a single optical fiber. Future computation and communication systems place ever-increasing demands upon communication link bandwidth. It is widely recognized that optical fibers offer vastly higher bandwidth than conventional coaxial communications and that a single optical channel in a fiber waveguide uses a microscopically small fraction of the available bandwidth of the fiber (typically a few GHz out of several tens of THz). And it is also widely recognized that by sending several channels at different optical wavelengths down the fiber (i.e., wavelength division multiplexing, or WDM), this bandwidth may be more efficiently utilized.
If a single optical link transmits 2 Gbs (giga-bits per second), then an optical fiber contains the bandwidth to support upwards of ten thousand such links. However, current WDM implementations typically support only between two and four such links. This disparity is primarily due to two reasons. The first reason is that most WDM systems must be assembled from discrete transmitter and receiver devices (laser diodes and detector diodes). With this approach, the complexity and alignment requirements scale linearly with the number of channels while additional constraints (e.g., constraints on frequency stability) are added. The integrated devices reported thus far in the art use Y-branch couplers for beam combining, which yield a 50% loss with each factor of 2 increase in number of channels. The second reason is that currently available components for multiplexing (dichroic filters and holographic couplers) cannot discriminate among large numbers of closely-spaced channels. Channels must be spaced sufficiently far apart (e.g., from 3 to 5 nanometers) that only a small number of channels can fit within the roughly 50 nanometer gain bandwidth of a given laser active medium.
Single-chip implementations of WDM transmitters also do not exploit even the limits of current technology. For example, with a typical 5 mW laser diode dissipating 10 mW of power, a chip of power dissipation of 1 W/chip would still allow 100 channels on the chip. For a WDM receiver chip, the limits are even higher, since the power dissipated by a detector is negligible.
A natural application for high-density WDM (.gtoreq.8 channels) is in links within or between computers. A typical parallel computer link consists of 32 or 64 channels, yet conventional communication links are serial. Use of a parallel WDM link would not only increase the channel capacity by up to a factor of 32, but would actually simplify the electronics at each end by eliminating the serial/parallel conversion bottleneck, reducing latency in the network. In addition, more advanced protocols can make use of WDM to implement control signals separate from a data stream. A "hot potato" computer architecture including an "optical switch node" has been proposed by John Sauer at the National Testbed Facility (NTF) at the University of Colorado that requires high-density WDM (nine channels or greater) fiber links between nodes. This system is designed from the ground up to incorporate WDM fiber communications, but therefore requires high density wavelength division multiplexing (HDWDM) technology.
The NTF "optical switch node" can be broken down into several functional blocks: it requires wavelength multiplexing/demultiplexing, detection and generation of 1.3 micron radiation, level restoration of 1.55 micron radiation (possibly) to compensate for insertion loss, and some electronic control logic and drive. These requirements are representative of the needs of any HDWDM system. All of these functional blocks could be realized with off-the-shelf optics and electronics. Laser diodes and detector diodes at 1.55 microns are commercially available, as are high-speed amplifiers and detectors. The WDM could be accomplished by microoptics (i.e., separately micropositioned lens, diffraction grating and laser array). However, such a "discrete" approach would likely be impractical due to the large size of the system and the large number of precise alignments that would be required among the optical components.
Techniques for multiplexing and demultiplexing between a single optical fiber comprising the multiplexed channel and plural optical fibers comprising the plural demultiplexed channels are known in the art. For example, multiplexing/demultiplexing with birefringent elements is disclosed in U.S. Pat. Nos. 4,744,075 and 4,745.991. Multiplexing/demultiplexing using optical bandpass filters (such as a resonant cavity) is disclosed in U.S. Pat. Nos. 4,707,064 and 5,111,519. Multiplexing/demultiplexing with interference filters is disclosed in U.S. Pat. Nos. 4,474,424 and 4,630,255 and 4,735,478. Multiplexing/demultiplexing using a prism is disclosed in U.S. Pat. No. 4,335,933. U.S. Pat. No. 4,740,951 teaches a complex sequence of cascaded gratings to demultiplex plural optical signals. U.S. Pat. Nos. 4,756,587 and 4,989,937 and 4,690,489 discloses optical coupling between adjacent waveguides to achieve a demultiplexing function. A similar technique is disclosed in U.S. Pat. No. 4,900,118. The foregoing techniques are limited by their discrete components to a small number of wavelengths in the multiplexed channel.
One way of overcoming such a limitation is to employ diffraction gratings to perform the multiplexing and demultiplexing functions. One problem with such an approach is that the input and output beams (the multiplexed and demultiplexed beams) are reflected by the grating at 180 degree angles, and are very closely spaced, a significant disadvantage due to the potential for cross-talk between the two beams. This is shown in U.S. Pat. Nos. 4,111,524 and 4,993,796. Such close spacing of the multiplexed and demultiplexed channels makes fabrication awkward and increases the likelihood of cross-talk. One way of overcoming this latter difficulty is to employ a curved diffraction grating which reflects the incoming signal at right angles. Such an approach is disclosed in U.S. Pat. No. 4,822,127. The problem with the latter technique is that the various wavelengths of the incoming single-channel fibers must be detected by an array comprising plural array sequences corresponding to the plurality of incoming signals to be detected. Such a structure is awkward for implementing WDM with a large number of channels.
The foregoing limitation is overcome in a technique in which a diffraction grating is combined with a lens, as disclosed in U.S. Pat. Nos. 4,777,663 and 4,839,884 and 4,367,040 and 4,739,501. The advantage of the lens and grating combination is that the plural optical fibers of the demultiplexed channels may interface directly with the single optical fiber of the multiplexed channel through the grating and lens combination. The problem with the lens and grating combination is that the lens is a large discrete component. Moreover, the diffraction grating itself is typically a discrete component. A related technique is disclosed in U.S. Pat. No. 5,107,359 employing two discrete components, namely either two diffraction gratings or a grating and a specular reflection surface.
Accordingly, none of the foregoing techniques are suitable for integrated circuit implementation and are therefore relatively large expensive devices incapable of exploiting the advantages of integrated circuits in WDM discussed above.
It is therefore one object of the invention to perform WDM multiplexing and demultiplexing functions in a WDM optical integrated circuit without discrete components such as a lens or specular reflection surface.
It is a related object of the invention to perform optical multiplexing in a WDM transmitter integrated circuit having orthogonal facets, one facet connected to the multiplexed channel optical fiber and plural optical sources or semiconductor laser diodes being near the other facet.
It is a further related object of the invention to provide optical demultiplexing in a WDM receiver integrated circuit having orthogonal facets, one facet connected to the multiplexed channel optical fiber and plural optical detectors or photodetector diodes being near the other facet.
The present invention described below recognizes that the NTF optical switch node and other high-rate communications architectures (e.g., supercomputing links and concurrent processors) are natural candidates for monolithic integration, which, among other benefits, would dramatically reduce the number of components, connections and alignments that must be made among a node's components. While lasers, detectors and electronics have all seen significant effort in discrete device fabrication as well as integration, relatively less effort has been focused on high-density WDM components. Thus, HDWDM components form an enabling technology for the much larger field of WDM opto-electronic integrated circuits (OEIC's).
Accordingly, it is a further object of the invention to provide vertical-diffraction-grating (VDG) integrated optical components for high-density WDM circuits. Such VDG components are formed in an integrated circuit, in contrast to the discrete waveguides employed in the prior art referenced above.
It is a related object to use such a VDG to implement a high density WDM transmitter by integrating the VDG with an array of lasers (laser diodes) with discrete, different wavelengths.
It is another related object of the invention to integrate a VDG coupler with an array of p-i-n edge-coupled diodes to make a single-chip receiver.
It is yet another related object of the invention is to provide an entire NTF optical switch node on a single monolithic substrate or integrated circuit chip.
A further object of the invention is to employ VDG's fabricated with compound semiconductor waveguides of the type including InP for 1.3 micron and 1.55 micron operation.