Pulse shepherding (defined below) can be exploited to impress synchronized amplitude-modulation pulses, with durations of the order of picoseconds, onto beams of light with different wavelengths that propagate together along a single-mode nonlinear optical fiber. This is significant because synchronized picosecond pulses on wavelength-division-multiplexing (WDM) light beams will be essential for the operation of future ultrafast bit-parallel data-communication systems. In comparison with the customary use of an array of mode-locked lasers to generate synchronized picosecond pulses on WDM beams, the pulse-shepherding approach is simpler and more economical.

Pulse shepherding is a nonlinear signal-propagation phenomenon attributable to cross phase modulation, which is an interaction between copropagating light beams that arises from a nonlinearity in the response of the fiber-optic material. In particular, the cross phase modulation arises from the intensity dependence of the index of refraction.

These Oscilloscope Traces were obtained in experiments in which shepherd pulses of 60-ps duration and peak power somewhat greater than 200 mW were launched into a 20-km-long single-mode, dispersion-shifted optical fiber along with 33-mW CW laser beams.

Pulse shepherding is so named because it involves the use of one pulse (denoted the shepherd pulse) to "herd" together a number of other pulses that propagate along with it. In the present application, the shepherd pulse is also used to generate the other copropagating pulses. The shepherd pulse - in this case, a high-power picosecond pulse - is launched into a single-mode optical fiber, along with a number of low-power continuous-wave (CW) beams. The high-power shepherd pulse and the copropagating low-power beams all have different wavelengths.

At the input end of the optical fiber, the low-power beams are unmodulated. However, as the beams propagate along the fiber, the cross phase modulation causes the low-power beams to become increasingly amplitude-modulated by pulses that travel along with the shepherd pulse. For each low-power beam, the amplitude modulation is either a brightening pulse or a darkening pulse, according to whether the coefficient of group-velocity dispersion of the optical fiber at the beam wavelength is negative or positive, respectively (see figure). The magnitude of the modulation of each beam at the output end of the fiber depends on the magnitude of the dispersion coefficient, the length of the fiber, the coefficient of the nonlinearity that affects the index of refraction, the coefficient of absorption in the fiber at the given wavelength, "walk-off" among the various pulses, and other factors.

This work was done by Larry Bergman, Cavour Yeh, John Michael Morookian, and Steve Monacos of Caltech for NASA's Jet Propulsion Laboratory.


Photonics Tech Briefs Magazine

This article first appeared in the March, 2000 issue of Photonics Tech Briefs Magazine.

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