Square-wave oscillations in a semiconductor ring laser subject to counter-directional delayed mutual feedback.

Square-wave (SW) switching of the lasing direction in a semiconductor ring laser (SRL) is investigated using counter-directional mutual feedback. The SRL is electrically biased to a regime that supports lasing in either counter-clockwise (CCW) or clockwise (CW) direction. The CCW and CW modes are then counter-directionally coupled by optical feedback, where the CCW-to-CW and CW-to-CCW feedback are delayed by τ1 and τ2, respectively. The mutual feedback invokes SW oscillations of the CCW and CW emission intensities with a period of T≈τ1+τ2. When τ1=τ2, symmetric SWs with a duty cycle of 50% are obtained, where the switching time and electrical linewidth of the SWs can be reduced to, respectively, 1.4 ns and 1.1 kHz by strengthening the feedback. When τ1≠τ2, asymmetric SWs are obtained with a tunable duty cycle of τ1/(τ1+τ2). High-order symmetric SWs with a period of T=(τ1+τ2)/n can also be observed for some integer n. Symmetric SWs of order n=13 with a period of T=10.3 ns are observed experimentally.

Optical gain in 1.3-µm electrically driven dilute nitride VCSOAs.

We report the observation of room-temperature optical gain at 1.3 µm in electrically driven dilute nitride vertical cavity semiconductor optical amplifiers. The gain is calculated with respect to injected power for samples with and without a confinement aperture. At lower injected powers, a gain of almost 10 dB is observed in both samples. At injection powers over 5 nW, the gain is observed to decrease. For nearly all investigated power levels, the sample with confinement aperture gives slightly higher gain.

Fast saturable absorption and 10 GHz wavelength conversion in Al-quaternary multiple quantum wells.

We measured the absorption recovery times in reverse biased AlInGaAs multiple quantum well material designed to emit at around 1.5 µm wavelength. Absorption recovery times as low as 2.5 ps were found at -4V bias, with values below 5 ps consistently found for biases above 3 V. The short absorption recovery times obtained under reverse bias were confirmed by using cross-absorption modulation in the material to demonstrate wavelength conversion of a 10 GHz pulse train, showing both up and down conversion of the incident pulses.