Optical: systems and elements – Optical frequency converter – Harmonic generator
Reexamination Certificate
2000-07-13
2002-04-09
Lee, John D. (Department: 2874)
Optical: systems and elements
Optical frequency converter
Harmonic generator
C385S122000
Reexamination Certificate
active
06369937
ABSTRACT:
BACKGROUND
1. Field of the Invention
The present invention relates to a method and apparatus for performing optical serial-to-parallel data conversion. Amongst other things, the present invention utilizes second-harmonic generation to perform high-speed optical time-domain demultiplexing, optical code recognition and serial-to-parallel data conversion.
2. Background
Fiber-optic communication technology is being developed and commercialized at virtually unprecedented rates. It can be shown that, for transmission links on the order of kilometers, commercially available fibers can support data rates exceeding 100 Gb/s on a single optical carrier in the 1.3-1.6 micron wavelength range without resorting to operation at exactly the zero-dispersion wavelength. However, at these data rates, electronic components have difficulty generating, interpreting and/or switching the data. That is, the data rate conversion and manipulation has yet to catch up to the data transmission capabilities.
In view of the limitations of electronics, it is evident that to implement high-speed communication systems it is necessary to have a high-speed optical signal processing technology to compliment the high-speed optical-transmission technology. In addition, new system concepts which are compatible with the higher data rates and with the optical hardware should be developed.
One such concept being used to develop hardware to mate electrical and optical signal processing is the utilization of second-harmonic generation (SHG) and waveguide SHG in particular. Investigators Normandin and Stegeman have authored a number of papers detailing the occurrence of waveguide SHG. See for example: R. Normandin and G. I. Stegeman, “Non-Degenerate Four-Wave Mixing in Integrated Optics,”
Optics Letters,
Vol. 4, No. 2, February 1979; “Picosecond Signal Processing with Planar, Nonlinear Integrated Optics,”
Applied Physics Letters,
Vol. 36, No. 4, Feb. 15, 1980; P. J. Vella, R. Normandin, and G. I. Stegeman, “Enhanced Second-Harmonic Generation by Counter-Propagating Guided Optical Waves,”
Applied Physics Letters,
Vol. 38, No. 10, May 15, 1981; and, R. Normandin, S. Létoumeau, F. Chatenoud, and R. L. Williams, “Monolithic, Surface-Emitting, Semiconductor Visible Lasers and Spectrometers for WDM Fiber Communication Systems,”
IEEE Journal of Quantum Electronics,
Vol. 27, No. 6, June 1991. Normandin and Stegeman demonstrated waveguide SHG in strongly nonlinear optical materials by inserting an optical pulse at one end of the waveguide and another optical pulse at the other end of the waveguide. When the two injected fundamental signals collided, they produced a second-harmonic wave that propagated perpendicular to the waveguide surface. A serial-to-parallel converter device that utilizes SHG for the purposes of data communication is disclosed in U.S. Pat. No. 5,172,258 (the '258 patent).
However, it is desirable to increase the efficiency of the second-harmonic conversion inside the waveguide of the serial-to-parallel converter of the '258 patent in order to make the converter compatible with the small pulse energies required for greater than 100 Gb/s fiber-optic transmission. Additionally, a problem associated with nonlinear optical waveguides, such as those used for SHG, is the existence of two-photon absorption, which reduces the conversion efficiency by absorbing photons at the fundamental wavelength. The effects of two-photon absorption increase as the photon energy of light provided to the input of the waveguide approaches and passes through the center of the band gap of the material utilized for the waveguide. Finally, input power is often lost due to less than optimum coupling between a fiber optic element communicating an input optical serial data pulse stream to the waveguide; see, for example, V. Vusirikala, S. S. Saini, R. E. Bartolo, M. Dagenais, and D. R. Stone, “Compact Mode Expanders Using Resonant Coupling Between a Tapered Active Region and an Underlying Coupling Waveguide,”
IEEE Photonics Technology Letters,
Vol. 10, No. 2, February 1998.
Therefore, there is a need in the industry for a serial-to-parallel converter capable of optically converting a serial optical digital input signal into a set of parallel optical digital signals which addresses these and other related, and unrelated, problems.
SUMMARY OF THE INVENTION
The present invention allows the use of high-speed optical communication lines by efficiently converting a serial optical digital signal into a set of parallel optical digital signals. In one aspect, the present invention uses waveguide SHG to convert high-data-rate optical signals to lower data rates that are compatible with conventional high-speed electronic signal processing. The use of waveguide SHG also allows asynchronous operation, thereby greatly reducing circuit complexity as compared to conventional electronic methods of time demultiplexing or code recognition because clock recovery and synchronization can be done at the lower, demultiplexed data rate rather than at the greater than 100 Gb/s multiplexed rate.
According to a preferred embodiment of the present invention, second-harmonic photons are generated in the channel of a waveguide of an optical serial-to-parallel converter when each data pulse in an input serial optical data pulse stream collides with a single counter-propagating timing pulse. As currently conceived, half of the energy of the input optical stream is in the timing pulse, with the remainder divided equally among the data pulses. When the timing pulse reflects at the mirrored end of the waveguide, the timing pulse effectively counter-propagates through itself. This produces a comparatively large second-harmonic signal that is distinguishable from the reflections of the data pulses, and thus may be used as a trigger for other operations such as clock recovery and synchronization.
The collision between the timing pulse and each individual data pulse in the serial optical data pulse stream occurs at a predetermined, unique location in the waveguide. The SHG radiation generated by each collision travels in a direction which is perpendicular to the waveguide's longitudinal axis and parallel to the SHG radiation produced by other collisions. By placing a fiber optic element of a fiber optic array or a photodetector of a photodetector array above the location of each collision (i.e., at the waveguide's output), the serial-to-parallel converter derives a plurality of parallel output channels. In the case of fiber optic elements, the parallel output channels include a plurality of parallel optical data pulse streams. In the case of photodetectors, the parallel output channels include a plurality of parallel electrical signals.
In the preferred embodiment, a first reflector is positioned at a location substantially opposed to the waveguide output at the bottom of the waveguide's channel. The first reflector is, preferably, aligned to reflect a portion of the second-harmonic radiation propagating away from the waveguide output. A second reflector is interposed at a location between the first reflector and the waveguide output. Preferably, the second reflector is located on the exit surface of the waveguide at the waveguide's output. The second reflector selectively reflects a portion of the second-harmonic radiation propagating toward the waveguide output. Cooperating together to define a vertical resonant cavity in the waveguide that is resonant at the wavelength of the second-harmonic radiation, the first and second reflectors direct at least a portion of the plurality of photons of the second-harmonic radiation from each timing pulse-data pulse collision through the resonant cavity more than once during the time at which the collision occurs, thereby causing the generation of additional second-harmonic light. By generating additional second-harmonic light from each timing pulse-data pulse collision, the efficiency of the optical serial-to-parallel converter is increased. For the approximately 1 ps pulses required for greater than 1
Ulmer Todd G.
Verber Carl M.
Deveau Todd
Georgia Tech Research Corp.
Lee John D.
Rahll Jerry T
Schneider Ryan A.
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