At&t Bell Laboratories

Other

American Telephone & Telegraph Co., At&t Bell Laboratories

Mohammed N Islam

Linn F Mollenauer

James Power Gordon

Richard E Slusher

Timing restoration for a series of input pulses is performed optically in a transmission or switching system, using a nonlinear material with negligible walk-off that also receives an essentially orthogonally polarized series of reference pulses. In the nonlinear material, the input pulses are frequency shifted by the presence of the reference pulses. For a material with negligible walk-off, the frequency shift only occurs when the pulses partially overlap, but not when the pulses are coincident. The frequency shifted output from the nonlinear material is supplied to a dispersive delay line that translates the frequency shift into a time shift, such that the input pulses are retimed by the reference pulses. If the nonlinear material has a nonlinear index of refraction n.sub.2 &gt;0, then the dispersive delay line must have an anomalous Group Velocity Dispersion (GVD); on the other hand, if the nonlinear material has an index of refraction n.sub.2 &lt;0, then the dispersive delay line must have a normal GVD. The nonlinear and dispersive media may be separate sections or distributed in the same medium.

All-optical timing restoration

It has been recognized that AlGaAs-containing planar waveguides can be advantageously used in all-optical gates for wavelengths in the range 1.2-1.7 .mu.m. In particular, such waveguides can produce in radiation pulses of center wavelength, .lambda..sub.s a phase shift of magnitude .pi. or larger while, at the same time, causing attenuation of the pulse by less than 1/e, provided hc/.lambda..sub.x &lt;E.sub.g /2, where E.sub.g is the bandgap associated with the Al.sub.x Ga.sub.2-31 x As-contianing waveguide. In preferred embodiments .lambda..sub.s and/or x are selected such that [hc/(.lambda..sub.s -.DELTA..lambda.]&lt;E.sub.g /2, where .DELTA..lambda. is chosen such that 99% of the pulse energy is contained in the spectral region .lambda..sub.s .+-..DELTA..lambda..

Apparatus comprising an all-optical gate

Timing restrictions for coincidence of control and data signal pulses to a soliton-based optical logic device are significantly relaxed by predistorting a characteristic of data signal pulses input to the device. Predistortion in the form of normal dispersion is sufficient to cause each predistorted data signal pulse to change shape subsequently in the optical logic device and thereby permit effective interaction between the data signal pulses and the control pulses to achieve the desired result of the logic operation. The temporal separation between control and data signal pulses has been extended to cover a total range of up to four pulse widths without degrading the operation of the optical logic device.

Predistortion apparatus for optical logic device

An optical logic device based on the time-shift-keying architecture is described in which digital logic functions are realized by applying appropriate signal pulses to a nonlinear shift or 'chirp' element whose output is supplied to a dispersive element capable of supporting soliton propagation. In an optical fiber realization of the optical logic device, two orthogonally polarized pulses are supplied to the combination of a moderately birefringent fiber acting as the nonlinear chirp element and a polarization maintaining fiber acting as the soliton dispersive delay element having a anomolous group velocity dispersion at the signal wavelengths of interest. A nonlinear frequency shift is created in one of the pulses in the former element through cross-phase modulation and, in turn, the frequency shift is translated into a temporal shift of the affected pulse in the latter element. These devices operate at switching energies approaching 1pJ.

Optical logic device

Combinatorial logic devices are presented in which data signals interact with an enable signal in the device to move the enable signal outside its prescribed time slot when one or both of the data signals are present. Data signals are discarded within the device to avoid propagation through subsequent logic device stages. Such logic devices are particularly well suited to all-optical realizations in which soliton pulse signals are used. These devices exhibit high gain, cascadability and potentially large fanout capability.

Cascadable optical combinatorial logic gates

High incremental gain and ultrafast switching operation are achieved in a fiber interferometer switch by employing a control optical signal in one arm of the interferometer and a pump optical signal split between both arms of the interferometer wherein signal frequencies are in the anomalous group velocity dispersion region of only one arm of the fiber interferometer to cause modulation instability to occur. Net gain in excess of 10.sup.4 has been observed where net gain is the ratio of device output peak power to control signal input power. In a preferred embodiment of the invention, the interferometer is a balanced Mach-Zehnder interferometer utilizing a single mode, dispersion shifted, polarization-preserving optical fiber.

Modulation instability-based fiber interferometric switch

Combinatorial (Boolean) logic functions are provided by ultrafast optical logic devices which utilize soliton trapping between two optical signals propagating in a birefringent fiber. The logic devices are three terminal devices having orthogonally polarized soliton input signals and a single output signal. Optically filtering the output from the fiber permits the desired combinatorial logic operation to be performed on the input optical signals. Logic operations include AND, exclusive-OR, NOT, and NOR functions. In operation, the devices exhibit phase insensitivity, low switching energy, high contrast ratio between output logic levels, and cascadability. In one embodiment of the invention, a first optical signal and a second optical signal are optically coupled into the principal axes of a birefringent fiber. A Fabry Perot etalon centered at the center frequency of both the first and second signals is utilized to realize a exclusive-OR operation whereas centering the etalon on the frequency related to the spectral shift caused by soliton trapping realizes an AND operation.

Ultra-fast optical logic devices

The propagation speed of optical solitons in single mode optical fiber depends on the wavelength of the solitons. Thus, if solitons of different wavelengths are co-propagating, 'collisions' between pulses can be expected to result. It has been found that collisions between solitons do preserve the soliton character of the colliding pulses, even in the presence of perturbations of the type present in fiber communications systems, e.g., core size variations, distributed or lumped loss, and distributed gain. Thus, a wavelength division multiplexed soliton system is possible, and techniques and formulae for the design of such systems are disclosed. In preferred embodiments, fiber loss is periodically compensated by Raman gain. Typical amplification periods (using currently available silica-based fiber) are 30-50 km, typical pump powers are less than 100 mV, and rate-length products of the order of 3.multidot.10.sup.5 GHz km are possible. Multiplexed soliton systems have several advantages over prior art soliton systems. For instance, they permit attainment of very high transmission rates without the use of very high speed electronics. For instance, an exemplary 3000 km, 24 channel multiplexed soliton system has a total transmission rate of 106 GHz, i.e., 4.5 GHz/channel.

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Hazlet, NJ, United States of America

Mohammed N Islam