Measurement of the nonlinear index of silica-core and dispersion-shifted fibers.

We have accurately measured the nonlinear refractive index, n(2), of the glass found in silica-core and dispersion-shifted optical fibers. We find that at 1.319 microm the n(2) of silica-core and dispersion-shifted fibers is 2.36 x 10(-16) and 2.52 x 10(-16) cm(2)/W (+/-5%), respectively. We also estimate that the n(2) of germania is three times the n(2) of pure silica. Because the wavelength dependence of n(2) between 1.3 and 1.5 microm is known to be small, our measured values should also be valid at 1.55 microm.

Observation of resonant enhancement of photoinduced second-harmonic generation in Tm-doped aluminosilicate glass fibers.

We report a 10(3) increase in efficiency of second-harmonic generation in Tm(3+)-doped aluminosilicate glass fibers when the wavelength of the fundamental beam is tuned to the maximum of the (3)H(6) ? (3)F(4) absorption of Tm(3+) at 785 nm. This result shows the role of resonance enhancement in the nonlinear optical process that is responsible for the formation of the X((2)) grating.

Experimental study of second-harmonic-generation saturation in fibers by phase modulation between the fundamental and second-harmonic writing beams.

We study saturation of photoinduced second-harmonic generation in a Ge-doped silica fiber by modulating the relative phase between the fundamental and second-harmonic writing beams. We shift the phase by pi and present measurements that identify saturation that is due to site depletion and saturation that is due to a backfield.

Seeded second-harmonic generation in optical fibers: the effect of phase fluctuations.

Phase fluctuations between the infrared and green beams during seeding of second-harmonic generation (SHG) in optical fibers lead to a drastic reduction of the initial SHG intensity and to a strong time dependence of the SHG after seeding. Experiments using mode-locked lasers with a stable phase relation between the infrared and green writing beams lead to reliable, reproducible results. Under these conditions we do not observe a significant difference in SH efficiency between fibers with Ge concentrations of 3 and 14 mol. %.

Numerical study of the Raman effect and its impact on soliton-dragging logic gates.

Recent experiments in birefringent optical fibers in which a signal pulse in one polarization is used to switch a control pulse in another polarization are affected by the Raman self-frequency shift. These experiments are modeled numerically, with the experimentally measured Raman profiles used as input to the simulations. Both parallel and perpendicular Raman gain are kept. The effect of keeping the full Raman response rather than just an often-used linear approximation is discussed. The experimental results are in good agreement with theory, although some discrepancies exist. The possibility that these discrepancies could be due to errors in the measurements of the low-frequency portion of the perpendicular Raman gain is examined and ruled out. Other possible sources of this discrepancy are then discussed.

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.

Preparation of long-coherence-length second-harmonicgenerating optical fibers by using mode-locked pulses.

We have prepared an optical fiber that produces efficient (0.24%) second-harmonic conversion by using modelocked 1.064-microm laser pulses along with a harmonic seeding beam. The effective coherence length increased to 35 cm as compared with only a few centimeters in fibers prepared with mode-locked Q-switched pulses. We show that the input bandwidth of the pump pulse imposes a fundamental limit on the effective coherence length and that this length can be severely reduced by competing nonlinear effects.

Self-organized phase-matched harmonic generation in optical fibers.

We generated second-harmonic light from a single-mode fiber after a few minutes of seeding with 532-nm harmonic light along with 1.064-microm pump light. This experiment supports a mechanism for photoinducing an alternating second-order nonlinearity in the fiber.

Cross-phase modulation in optical fibers.

When pulse pairs copropagate in a fiber, each pulse chirps the other through cross-phase modulation (XPM). In particular, XPM causes spectral broadening of signal pulses during Raman or parametric optical pulse amplification. We have studied XPM experimentally in the fiber Raman amplification soliton laser. We show that experimentally observed spectra can be explained by a simple model that concentrates on the phase change as a function of pulse walk-off.

Amplifier/compressor fiber Raman lasers.

We show that the chirp from cross-phase modulation (XPM) dominates the operation of fiber Raman lasers (FRL's). Thus a FRL in the anomalous group-velocity regime is best described as a XPM-chirped Raman amplifier followed by a linear fiber compressor. While the output of such a laser is generally a narrow pulse with a broad pedestal, we show both experimentally and by computer simulation that negligible background is achievable.

Polishing-induced birefringence in single-mode fibers.

Polishing a flat surface on a fiber changes the birefringence at the core or introduces birefringence in a circular fiber. In this paper, a circularly symmetric fiber is studied both experimentally and by a simple model in which the birefringence arises from the application of forces along the polished surface. The polishing-induced birefringence first rises to a maximum and then falls off as the polished surface approaches the core.

Experimental demonstration of soliton propagation in long fibers: loss compensated by Raman gain.

With sufficient Raman gain to compensate exactly for net fiber energy loss, we have demonstrated distortionless propagation of 10-psec FWHM fundamental (N = 1) soliton pulses (lambda = 1.56 microm) over a 10-km length of single-mode fiber. The implications of this experiment for development of an all-optical, high-bit-rate, long-distance telecommunications system are discussed briefly.

Optical wave breaking of pulses in nonlinear optical fibers.

A new effect appears in femtosecond optical pulse compression, using single-mode fibers, that we describe as optical wave breaking. In the fiber, frequency-shifted light in the leading and trailing edges of a pulse overtakes unshifted light in the pulse tails. Mixing of these overlapping frequency components generates sidelobes on the pulse spectrum. The effect often leads to computational instabilities, but careful numerical simulations, including fiber loss, give results in excellent agreement with experiment.

Polarization-selective 3-dB fiber directional coupler.

Measurements on a 3-dB polarization-selective fiber directional coupler demonstrate that the polarization selectivity arises from different mismatches for the propagation constants of the two polarizations.

The soliton laser.

By incorporating a length of single-mode, polarization-preserving fiber into the feedback loop of a mode-locked color-center laser (lambda~1.4-1.6 microm), we have created a device that we call the soliton laser. Pulse width (2.0 to 0.21psec obtained to date) is determined by fiber length, in accordance with N = 2 soliton behavior. Production of <50-fsec-wide pulses is indicated for compression in an additional, external fiber.

In-line fiber-polarization-rocking rotator and filter.

An in-line polarization rotator has been built into a single-mode birefringent fiber. The rotator utilizes periodic twists of the fiber's principal axes, which were formed by rocking the preform as the fiber was drawn. The polarization conversion between the principal axes is wavelength dependent, with a bandwidth inversely proportional to the number of twist periods. The bandwidth of the present rotator was 4.8 nm for 100% conversion in a fiber length of 170 cm.

Observation of pulse restoration at the soliton period in optical fibers.

Restoration in both shape and spectrum of a (train of) 6.4-psec optical pulses has been observed at the soliton period in a single-mode fiber. The source was an F(2)(+) color-center laser at 1.55microm, and the fiber was 1.3 km long, which was one soliton period for this pulse width and wavelength. As predicted by the nonlinear Schrödinger equation, pulse restoration occurs despite initial spectral broadening from self-phase modulation and temporal compression as a result of negative group-velocity dispersion acting on the chirped pulse.

Extreme picosecond pulse narrowing by means of soliton effect in single-mode optical fibers.

We report the narrowing of pulses, initially 7 psec FWHM, to widths as small as 0.26 psec by various lengths, short relative to the soliton period, of single-mode, low-loss optical fiber. Since the ~1.5-microm wavelength lies in the region of negative group-velocity dispersion ( partial differentialv(g)/ partial differentiallambda < 0), no auxiliary dispersive element was required to complete the narrowing. Optimum compression was obtained for peak-pulse input powers of several hundred watts, corresponding to relatively high (N > 10) soliton number. We show these results to be in at least semiquantitative agreement with prediction based on the nonlinear Schrödinger equation.