We report here on the first observation of hysteresis in the asymmetrical tuning curve of a semiconductor laser operating as a resonant-type amplifier. This bistable amplification resulting from a nonlinear resonance phenomenon is caused by the dependence of the refractive index on the injected carrier density.
Dispersive bistability arising from the carrier-dependence of the refractive index has been observed in semiconductor lasers operated just below threshold with a small injected optical signal. The minimum optical input required for switch-up is as low as 1 μW for InGaAsP amplifiers at 1.55 μm(1) and 1.3 μm(2) (cf 30 μW for GaAs devices(3)). As yet, reliable measurements of the dynamic response are not (4) available, although we predict recovery times to be on the order of carrier lifetimes, ie a few nanoseconds at room temperature. This contribution will review our experimental and theoretical work on 1.55 μm amplifier bistability and set it in the context of other semiconductor bistable optical devices.
We demonstrate optically bistable (light switching light) operation in a room-temperature InGaAsP Fabry-Perot amplifier. The amplifier is an InGaAsP/InP laser structure biased at ~95% of its lasing threshold. It is switched with <3 μW of optical power provided by the mechanically chopped output of a quasi-single-mode InGaAsP/InP laser. The experimental apparatus is depicted in Fig, 1. A change in the refractive Index in the amplifier of only 5 × 10−4 is sufficient to cause switching. Input and output pulses from the nonlinear amplifier are shown in Fig. 2 when the source frequency is initially detuned by 14 GHz from the closest Fabry-Perot mode of the amplifier. Smaller switching powers are required at lesser detunings. The physical mechanism of the intensity-dependent change of the refractive index is thought to be the change in carrier density resulting from stimulated recombination of free carriers.
We propose and report the first experimental demonstration of a new type of active optical bistable device composed of an optical cavity and a lasing medium with a pumping source of either optical or nonopticai means. The basic principle underlying the operation of the optical bistable laser (OBL) is based on the stable-unstable cavity configuration transition of an active optical resonator. Generally speaking, a laser system showing both pump- and laser-induced tensing (focusing or defocusing) effects may display such a hysteresis loop in its input-output characteristics. Optical bistability (OB) based on self-focusing of light in a medium having a nonlinear refractive index has recently been shown experimentally.1
Experimental results of optical polarization bistability in InGaAsP lasers are reported and the causes of polarization switching and nonlinear response in this bistability are discussed. The input light signal consisted of the transverse magnetic (TM) wave, while the semiconductor lasers operated in a fundamental transverse electric (TE) mode when the light input was not injected. The light output versus light input characteristics were dependent on the input wavelength. Switching times of up to a few hundred picoseconds were achieved for both switch-up and switch-down. It was found that the TE and TM outputs originate from the TE oscillation and the TM amplification, respectively. Calculations using the rate equations showed that the TM input induces the nonlinearity of the carrier density. These results indicate that this bistability is a type of dispersive bistability.>