We investigate the evolution of short-duration pulses injected into laser diodes biased above threshold with the use of spectrally and temporally resolved experimental and numerical methods. We show that stable transients may be formed as a result of spatially re-distributing the cavity energy. By controlling the phase of injected pulses with respect to the diode cavity radiation we show through simulation that it is possible to directly generate and control stable streams of pulses.

IntroductionThe interaction of short optical pulses with laser cavity modes is important in, for example, formation of mode-locked pulse trains, optical clock recovery, and external optical feedback [1]. In a dispersive nonlinear medium such as a semiconductor, a treatment of the optical field is required which goes beyond discrete modal frequencies. A description of coherent interactions requires resolution of the fast oscillating carrier wave on the femtosecond timescale, while the return to equilibrium may take place over nanoseconds due to a long cavity lifetime or relaxation oscillation period. In inhomogeneous structures such as multi-section or DFB lasers, sub-wavelength to millimeter spatial scales must also be described.The spatio-temporal dynamics of the electric field may be calculated from Maxwell's equations using a finite-difference time-domain (FDTD) method. Coupling a Lorentzian resonance allows an approximate model of the optical gain [2]. Realistic microscopic models of the semiconductor gain based on the semiconductor Bloch equations [3] are at present too computationally intensive for simulation over such a range of temporal scales.We consider a class of recent experiments in which a short optical pulse is injected into a semiconductor laser diode, allowing a study of the pulse-cavity interactions on time-scales shorter than the cavity roundtrip time. In addition to the expected pulse broadening and relaxation oscillations, new phenomena such as stable, long-lived 'dark pulses' were observed [4,5]. Numerical MethodsWe have coupled an FDTD calculation of the electric field with multiple Lorentzian resonances which approximate the spectral dependence of the semiconductor gain. Figure 1 shows that four resonances provide a good fit to the semiconductor gain spectrum [6]. We found that spatially resolving the carrier lifetime and incorporating carrier diffusion was essential in achieving a physically realistic number of lasing modes. Results and discussionDuring the propagation of a short pulse through a population-inverted semiconductor a region of depleted gain is left behind the injected pulse. For a laser under CW operation this region of depleted gain can evolve into a long lived 'dark pulse'. Figure 2 shows the simulated output from the laser facet following the injection of an optical pulse with a central frequency of 193 THz, resonant with the CW emission wavelength. The initial perturbation induces relaxation oscillations (period ~ 50 ps) which persist for ~400 ps. A bright pulse is emitted from the facet every ~7 ps, after each cavity round-trip. The amplitude of successive bright pulses decays on the timescale of a few hundred ps. A dark pulse forms after the first few round trips, and is the dominant feature after ~200 ps. On a longer time scale, this feature oscillates (with ~500 ps period) between a predominantly bright and dark pulse superimposed on the CW emission. All of this behavior is in good qualitative agreement with experimental results [4]. Inset (A) of Figure 2 shows the ...