Optical Serial-to-Parallel Converter

This invention relates to optical components generally, that is, components which operate in the optical domain. Such components are designed for use in various systems which utilize optical fibers, such as optical communications systems, optical data processing systems and optical switching systems. Specifically, this invention pertains to an optical serial-to-parallel converter.

The development of optical fiber and optical semiconductor technologies in recent years has made possible various types of optical communications systems and optical switching systems. To utilize the full bandwidth and speed available in optical systems, it has become necessary to design and fabricate optical logic and switching components which eliminate the necessity for electrical-to-optical and optical-to-electrical conversions.

Electronic serial-to-parallel converters have played a major role in digital electronic systems for many decades. Numerous electronic systems and subsystems have been based on them. Optical serial-to-parallel converters, on the contrary, were relatively neglected because there were few optical systems that really needed serial-to-parallel converters. This is no longer true. The deployment of optical fibers for information transmission has necessitated a different approach to this problem. At present, the high bandwidth advantage of optical fibers is still not fully utilized because electronic switching systems are not fast enough for high speed and high throughput optical switching. To solve this problem, the switching functions must be performed in the optical domain, eliminating the optical-to-electrical and electrical-to-optical conversions. Despite the rapid development of optical fiber technology, optical serial-to-parallel converters have received only scant attention. It is presently anticipated that optical serial-to-parallel converters will play an important role in optical switching systems such as self-routing optical switching systems and optical ring switching systems. In these switching systems, the use of optical serial-to-parallel converters will simplify the design of the systems and improve their performance.

In existing optical time-division switching systems, time slot interchanges are accomplished by using the demultiplexing/storing/retrieving/multiplexing procedure. Examples may be found in R. A. Thompson and P. P. Giordano, "An experimental photonic time-slot interchanger using optical fibers as reentrant delay-line memories," J. Lightwave Technology, vol. LT-5, pp. 154-162, January 1987, and H. Goto, "Photonic time-division switching technology," Technical Digest, Topical Meeting on Photonic Switching, pp. 132-134, March 1987. In this prior art optical writing gates and bistable devices are employed for demultiplexing and storing, respectively. These two types of devices may be replaced by an optical serial-to-parallel converter which performs the same functions.

The principal object of the present invention is to provide an optical serial-to-parallel converter in which serial optical pulses are converted to parallel optical pulses without optical-to-electrical or electrical-to-optical conversion.

Another object of the invention is to provide an optical serial-to-parallel converter in which adjacent optical pulses are well isolated, causing no crosstalk among time slots.

Still another object of the present invention is to provide an optical serial-to-parallel converter which is simple in both structure and operation, but which has none of the shortcomings of prior combinations of optical devices or electronic equivalents when used in optical systems.

A further object of the invention is to provide an optical serial-to-parallel converter in which the speed of the optical serial-to-parallel converter may be adjusted according to the incoming optical data rate.

Still a further object of the invention is to provide an optical serial-to-parallel converter for which the control timing is simple.

Another object of the invention is to provide an optical serial-to-parallel converter which can be implemented using integrated optics technology.

This invention is an optical serial-to-parallel converter having a single input to receive serially a set of optical pulses and a plurality of outputs to present said set of optical pulses simultaneously at its output. Said optical serial-to-parallel converter is constructed from a plurality of cascaded optical single bit shift registers and 1.times.2 optical switches. In one aspect of the invention, a two-bit optical serial-to-parallel converter consists of two cascaded single bit optical shift registers and two 1.times.2 optical output switches. Each single bit optical shift register, in turn, consists of two cascaded optical memory cells, each memory cell having a 1.times.2 optical switch, a passive optical combiner, delay line fibers and an optical amplifier. The two optical switches in the single bit optical shift registers are controlled by a first clock and a second clock, running at the same rate, but out of phase with each other. When the first clock controlling both single bit optical shift registers is low, one optical pulse can enter the optical converter through its input and into the first optical memory cell. When the first clock switches to high and the second clock to low, the remaining optical pulses are shifted to the next optical memory cell. When the clocks again change to first clock low and second clock high, the optical pulse can exit the first single bit optical shift register or be switched to an output of the optical converter. This enter-shift-exit cycle repeats at the rate of the first and second clocks, until each single bit optical shift register in the optical converter contains one optical pulse. Each of the two single bit optical shift registers is connected to an optical switch, controlled by a third clock, at its output. In the straight-through state of the output optical switch the ouput of the first optical shift register is passes onto the next optical shift register. The clock at the output optical switch operates at the rate of the optical shift register clocks divided by the number of bits in the optical converter. As serial optical pulses enter the optical converter, they are shifted from the first optical shift register to the next until all optical shift registers are full. At this point, the optical output switches change to high and all optical pulses in the optical converter appear on its output simultaneously.

In a second aspect of the invention, a multibit optical serial-to-parallel converter is constructed by cascading a larger number of single bit optical shift registers together. For an N bit optical word to be converted from serial-to-parallel optical pulses, N optical shift registers are required, having N output optical switches controlled by N clocks operating at the clock rate of the optical shift register clocks divided by N. There is no change in the operational principles.

The above and other objects, features and advantages of this invention are illustrated in the following detailed description, in which reference is made to the accompanying drawings.

FIG. 1 is a diagrammatic illustration of the preferred embodiment of a three-bit optical serial-to-parallel converter according to the present invention; and

FIG. 2 is a timing control diagram of the three-bit optical serial-to-parallel converter of FIG. 1.

FIG. 1 is a diagrammatic illustration of the preferred embodiment of a two-bit optical serial-to-parallel converter 10 constructed in accordance with the principles of the present invention. Two-bit optical serial-to-parallel converter 10 has a single input 12 to receive serially a set of two optical pulses and two output ports 14, 16 to output said optical pulses simultaneously in parallel. Two-bit optical converter 10 consists of two cascaded single bit optical shift registers 20, 30 each single bit optical shift register combining two cascaded single bit optical memory cells 22, 24, 32, 34. The optical memory cell described and illustrated herein has been disclosed in a copending U.S. patent application entitled OPTICAL MEMORY CELL, Ser. No. 280,396, filed Dec. 5, 1988 by the inventor of this application and assigned to the same assignee as this application. The optical shift register described and illustrated herein has been disclosed in a copending U.S. patent application entitled OPTICAL SHIFT REGISTER, Ser. No. 280,045 filed Dec. 5, 1988 by the inventor of this application and assigned to the same assignee as this application. These two related applications are incorporated herein by reference, and specifically the drawings and the descriptions of the preferred embodiments are incorporated herein by reference with respect to the structure and operation of said single bit optical memory cell and of said single bit optical shift register. Optical memory cells 22, 32, are controlled by a first clock 21, while optical memory cells 24, 34 are controlled by a second clock 23, and said memory cells include 1.times.2 optical switches 25, 26, 35, 36 and optional optical amplifiers 27, 29, 37, 39. Single bit optical shift registers 20, 30 are cascaded to form optical converter 10 by adding to each a third optical output switch 42, 43 controlled by a third clock 41. Each output optical switch 42, 43 is connected to the output of its respective optical shift register in optical converter 10. The cross-over outputs of optical switches 42, 43 constitute the output ports 14, 16 of optical converter 10.

All of the optical switches are in the cross-over state when their controlling clocks are high. They are in the straight-through state when their controlling clocks are low. These clocks may be optical or electrical, depending on the types of optical switches used. If the optical switches are LiNbO.sub.3 -based directional couplers or cross-couplers, the clocks are electrical. If the optical switches are pure optical switches, then the clocks are optical. As shown in FIG. 2, clocks 21 and 23 run at the same rate, but out of phase with each other. The rate of clock 41 is given by the rate of clock 23 divided by N, where N is the number of bits or optical pulses in optical converter 10. In the example described and herein illustrated in FIG. 2, clock 41 runs at one-half the rate of clock 23 for a two-bit optical converter. The duty cycles of all clocks in optical converter 10 are less than fifty per cent. This prevents adjacent optical pulses from being mixed together. The optical amplifiers 27, 29, 37, 39 are used to compensate for the optical losses caused by the various optical components in optical converter 10. These optical amplifiers could be discrete components or distributed optical fiber amplifiers.

To elucidate the operation of the two-bit optical serial-to-parallel converter 10, we assume that it is empty at the beginning. At time t=t.sub.0, clock 21 is high, clocks 23 and 41 are low, optical switches 25, 35 are in the cross-over state and the other optical switches are in the straight-through state. The optical serial-to-parallel converter 10 is empty at this moment. At t=t.sub.1 clock 21 is low, switch 25 is in the straight-through state, and an optical pulse A enters memory cell 22 through input port 12. At t=t.sub.2, clock 21 is high, clock 23 is low, switch 25 is in the cross-over state, and optical pulse A shifts to memory cell 24. At t=t.sub.3, clock 21 and clock 41 are low, clock 23 is high, switch 42 is in the straight-through state, and optical pulse A shifts from memory cell 24 in shift register 20 to memory cell 32 in shift register 30 through optical switch 42 under the control of clock 41. At the same time, optical pulse B enters converter 10 through input 12 to memory cell 22. At t=t.sub.4, optical pulse A shifts to memory cell 34 and optical pulse B shifts to memory cell 24. At t=t.sub.5, optical pulses A and B exit from outputs 14 and 16 respectively via optical switches 42, 43 under the control of clock 41. At the same time, optical pulse C enters memory cell 22 through input port 12. Thus, two serial bits A and B are converted to parallel bits and the next conversion cycle begins. The conversion cycle repeats at the rate of clock 41. It should be pointed out that receiving an optical pulse at input port 12 and outputting optical pulses at output ports 14, 16 need not occur at exactly the same time. Memory cell 22 may accept an optical pulse through input port 12 at any time so long as clock 21 is low and stays low long enough for the entire optical pulse to get into memory cell 22. The shifts from memory cell 22 to memory cell 24 and from memory cell 32 to memory cell 34 are synchronous with clock 21, since they are controlled by clock 21. Similarly, the shift from memory cell 24 to memory cell 32 is synchronous with clock 23, and optical pulse outputting through optical output switches 42, 43 is synchronous with clock 41. The clock rates are adjustable so that the rate of clock 21 can accommodate various input data ranges. Practically, clock 21 should always run synchronously with the incoming data rate.

While FIG. 1 shows only a two-bit optical serial-to-parallel converter, it should be obvious to one skilled in the art to extend the bit length of two-bit optical serial-to-parallel converter 10. Multi-bit optical serial-to-parallel converters can be constructed by cascading a number of single bit optical shift registers together. The principles of operation do not change at all. For N serial bits or optical pulses to be converted to parallel bits or optical pulses, N single bit optical shift registers and N output optical switches are required. Only three clocks are required, two for the single bit optical shift registers and one for the optical output switches. The output clock needs a clock rate equal to the single bit optical shift register clock rate divided by N.

A variety of 1.times.2 optical switches may be used in optical serial-to-parallel converter 10 and its component optical memory cells. At the present time, electro-optic switches, such as LiNbO.sub.3 -based directional couplers and cross-couplers, are good candidates for this application because their technologies are relatively mature compared with those of pure optical switching elements. A pure optical switching element, such as a nonlinear optical coupler, as described in Friberg et al., "An ultrafast all-optical switch," Photonic Switching, Springer Series in Electronics and Photonics 25, pp. 92-94, 1988, will be a better element to use when it becomes available and practical.

The concept, structure and operational principle of the optical serial-to-parallel converter described herein are independent of the technology used to implement it. The optical serial-to-parallel converter 10 of the present invention can be implemented not only using discrete components, as shown in FIG. 1, but also using integrated optics technology, or other advanced technologies.

A variety of photonic systems, subsystems and/or components can be implemented which are based on and/or utilize the optical serial-to-parallel converter of the present invention, such as optical time slot interchangers. The optical serial-to-parallel converter is a needed building block for photonic switching. It enables a variety of optical switching architectures which were not considered feasible before. For example, in existing optical time-division switching architectures, the time slot interchanges are accomplished by using the demultiplexing/storing/retrieving/multiplexing procedure, as discussed supra in the Background of the Invention. New optical switching architectures equipped with optical serial-to-parallel converters will no longer need the demultiplexing process.

The novel optical serial-to-parallel converter of the present invention offers many features and advantages over prior art devices, most of which were not optical devices. Serial optical pulses are converted to parallel optical pulses without optical-to-electrical or electrical-to-optical conversion. The adjacent optical pulses are well isolated, causing no crosstalk among time slots. This optical serial-to-parallel converter can be implemented using integrated optics technology, principally because the physical length of the optic fiber is short. The speed of the optical serial-to-parallel converter may be adjusted according to the incoming optical data rate. And timing control of this novel optical serial-to-parallel converter is relatively easy and simple.