Ultra-high speed optical logic devices are needed to continue growth of such emerging technologies as optical computing and optical switching. All-optical, cascadable soliton logic devices have recently been demonstrated in birefringent optical fiber to provide several picojoule switching energy and a fan-out of six. See, for example, Opt. Lett., Vol. 15, pp. 417 et seq. (1990). Logic outputs for this family of logic devices are presented according to a time-shift-keying criterion. That is, a logical "1" corresponds to the occurrence of a control pulse within a desired time slot or sampling interval, whereas a logical "0" corresponds to the substantial absence of the pulse during the desired time slot or sampling interval. Logic operations are performed by slowing or "time-shifting" the control pulse through interactions with a signal pulse within the birefringent optical fiber. The interactions produce a soliton dragging effect.
Solitons are nonlinear optical pulses which propagate in optical fiber without dispersing provided the soliton wavelength corresponds to anomalous group velocity dispersion within the birefringent fiber. Birefringence is a material property which causes two different polarization states to propagate at different velocities because the material has a different refractive index for each polarization state, namely, an ordinary and extraordinary indices of refraction. Solitons having different polarization states can travel at different group velocities due to birefringence of the fiber. When one pulse propagates past another pulse, a condition called "walk-off", each pulse induces a frequency shift of the center frequency of the pulse. This frequency shift is referred to as a "chirp". Frequency shifts of the pulse are translated into time shifts via fiber group velocity dispersion in the remaining length of fiber.
For the optical logic devices described above, the control pulse is introduced on the fast axis of the birefringent fiber while the data signal pulse is introduced along the slow axis of the fiber. It has been required that the interaction of the pulses during walk-off be asymmetric so that a net chirp occurs. Symmetry of the interaction causes the chirp induced during the first half of the interaction (when pulses walk toward each other) to be exactly cancelled by the chirp induced during the second half of the interaction (when pulses walk off from each other). Asymmetry is achieved by having substantially coincident or overlapping control and data signal pulses so that the pulses appear only to "walk off" from each other. To achieve the necessary degree of interaction in the fiber, an overlap or coincidence of pulses input to the device on the order of one pulse width has been suggested. Pulse widths of several hundred femtoseconds have been demonstrated with this family of logic devices.
For such ultra-high speed device operation, an undue amount of care for pulse timing is necessary to assure that control and data pulses properly coincide at the input of an optical logic device situated within a large array of such devices. In turn, this means that each signal path must be treated as a critical timing path. Fiber lengths must be accurately trimmed to meet the stringent pulse timing requirements.