Abstract

We report the generation of subpicosecond optical pulses at 0.3-THz repetition rate using an induced modulational instability (MI) of lightwave propagation in a single-mode fiber.^1–4 Ml occurs in many nonlinear systems and is a process in which the amplitude and phase modulations of a wave grow due to an interplay of noniinearity and anomalous dispersion. Hasegawa² first proposed the use of induced Ml for generating optical pulse trains by introducing a periodic modulation on a lightwave. The pulse train formation can be understood in both time and frequency domains. In the time domain, Ml deepens the modulation depth of input lightwaye to a sinusoidallike shape. Subsequently, the peaks of the sine wave undergo compression by high-order soliton compression⁵ to form well-separated pulses. In the frequency domain, parametric four-photon mixing due to Ml excites and amplifies many various order Stokes and anti-Stokes components. The interference among them gives rise to a pulse train similar to that in a mode-locked laser. In the experiments, we used a 1.319-μm Nd:YAG laser as the carrier (or pump) and introduced the periodic modulation by beating the YAG frequency with that from an external-grating-cavity InGaAsP laser diode, whose wavelength was tuned to the Stokes or anti-Stokes side of the pump by ~0.3 THz. 3-W (peak) power of the 100-ps YAG pulses and 0.5 mW of the cw diode beam were coupled into a 1-km fiber whose zero-dispersion wavelength (1.275 μm) is shorter than 1.319 μm to provide the anomalous dispersion. We monitored the output by a second-harmonic autocorrelator. Figure 1 shows the autocorrelation trace for the output pulse train (lower half). The pulse train disappeared (see upper fiat trace) when the diode was blocked. The period of the pulse train is ~3 ps and the pulse duration (FWHM) is ~0.5 ps, obtained by dividing the autocorrelation width by 1.5. The spectrum of this pulse train exhibited several discrete frequency peaks on both sides of the carrier as expected. The dc component in the autocorrelation comes from the wings of the mode-locked pulse, which did not develop pulsations. Figure 2 shows the pulse trains for two different diode wavelengths. It clearly illustrates the tunability in rep-rate, which is equal to the frequency detuning between two lasers. The carrier wavelength of the pulse train could also be tunable by using a tunable pump source. We have used Q-switched pulses (0.75 μs) for the pump and obtained a similar induced Ml spectrum. We are extending the experiment for using a cw pump. In conclusion, we have generated optical solitonlike pulse trains at 0.3 THz. Such pulse trains may be useful, for example, in communications or in ultrahigh-speed computations.

© 1986 Optical Society of America