M. N. Islam

Soliton trapping in birefringent optical fibers.

We show experimentally the trapping of orthogonally polarized solitons in birefringent optical fibers with polarization dispersions as high as 90 psec/km. Solitons along two axes of a fiber compensate for the polarization dispersion by shifting their frequencies, and we observe frequency splitting up to 1.03 THz for a polarization dispersion of 80 psec/km. For a 20-m fiber the energy required to compensate for the polarization dispersion is ~84 pJ, and for a 76m fiber the energy required reduces to ~64 pJ.

Soliton switching in a fiber nonlinear loop mirror

We observe all-optical, ultrafast switching at an energy of 55 pJ in a fiber nonlinear-optical loop mirror. The entire 310-fsec soliton waveform switches with 90% peak transmission. The soliton self-frequency shift lowers the peak transmission and splits the output pulse at higher powers, in agreement with numerical simulations.

Ultrafast all-optical logic gates based on soliton trapping in fibers.

Ultrafast all-optical soliton-trapping logic gates, including an inverter, exclusive OR, and AND, are experimentally demonstrated in birefringent fibers. The soliton-trapping logic gates are three terminal devices with orthogonally polarized inputs, phase-insensitive nonlinear operation, and switching energies of ~42 pJ. Using a 0.2-THz bandpass filter, the contrast ratio for the exclusive-OR gate is ~8:1, but the output pulses are ~10 times broader than the input pulse width. By widening the filter bandpass to 0.58 THz, an inverter is demonstrated with an ~4:1 contrast ratio and output pulses that can propagate as solitons in a fiber. Numerical simulations show that the output from the inverter can be cascaded to other trapping gates.

All-optical cascadable NOR gate with gain

An ultrafast, all-optical, three-terminal NOR gate based on soliton dragging in fibers is demonstrated with a gain of 4.5 and a switching energy of 30 pJ. Cascadability is proved with a tandem of two NOR gates that are configured as inverters. Time shifts from soliton dragging between two orthogonally polarized pulses in a fiber can be used for logic operations in a clocked system. Larger-than-predicted time shifts are observed because of self- and cross-Raman amplification effects.

Color center lasers passively mode locked by quantum wells

Using multiple-quantum-well (MQW) saturable absorbers, a NaCl color center was passively mode locked to produce 275-fs transform-limited, pedestal-free pulses with a peak power as high as 3.7 kW. The pulses are tunable from lambda =1.59 to 1.7 mu m by choosing MQWs with different bandgaps. The output pulses from the laser were shortened to 25 fs using the technique of soliton compression in a fiber. The steady-state operation of the laser requires the combination of a fast saturable absorber and gain saturation. >

Broad bandwidths from frequency-shifting solitons in fibers.

By pumping a fiber in the anomalous group-velocity-dispersion regime with a color-center laser we generate pulses with tau greater, similar 100 fsec covering a wide spectral range of 1.55 microm < lambda < 1.85 microm. Cross-correlation measurements show the lack of correlation between different parts of the spectrum. Computer simulations show that modulation instability and the soliton self-frequency-shift effect initiate a multisoliton collision process that results in narrow, high-intensity, fundamental solitons. The observed broad spectra correspond to an ensemble average over these solitons, which start from noise and frequency shift by different amounts.

Modulation instability-based fiber interferometer switch near 1.5 μm.

We demonstrate a modulation-instability-based fiber interferometer switch, an ultrafast all-optical fiber switch operating near 1.5-microm wavelength with more than 40dB of small-signal gain. Switching is accomplished.by seeding the modulation instability in one arm of a Mach-Zehnder interferometer, thus destroying its balance. Computer simulations, which include the effects of Raman self-frequency shifts, suggest that as much as 74% of the power input to the interferometer can be transferred to its (initially nulled) output arm when cw pumps are used. Even with an 80% loss at the output analyzer, we have gated 184 mW of power from a color-center laser using only 4.4 microW from a semiconductor laser.

Soliton intensity-dependent polarization rotation

We show experimentally and numerically that intensity-dependent polarization rotation does not lead to pulseshape distortions for fundamental solitons because the soliton rotates as a unit. We demonstrate an optical limiter based on nonlinear polarization rotation in a standard, single-mode fiber followed by a polarizer. The output pulse shape from the limiter resembles that of the input, although the pulse width may be different.

All-optical time-domain chirp switches

We describe a novel architecture for an all-optical time-domain chirp switch in which digital logic is based on timeshift keying. This architecture is a generalization of fiber soliton-dragging logic gates that have a switching energy approaching 1 pJ. By using solitons we separate the nonlinear chirping from the time shifting and, consequently, reduce the required phase shift during the nonlinear interaction. We discuss the scaling laws for energy and latency versus pulse width and show that the chirp switches have low switching energies for high-bit-rate applications.

Billiard-ball soliton interaction gates.

We demonstrate a cascadable, Boolean complete, conservative-logic interaction gate that is based on elastic collisions between temporal solitons in optical fibers. The two identical-frequency and polarization inputs are initially separated by 4.5 pulse widths, and they interact in a 7.5 walk-off length of polarization-maintaining fiber. The group velocity of one of the inputs is altered by passing the pulse through a beam splitter with a wavelength-dependent reflection coefficient. Although details within the interaction region depend on the phase between the two inputs, after the pulses separate the result is phase independent. We find that each of two 17-pJ solitons is displaced after interaction by 3.5 pulse widths so as to increase the pulses separation.