We show that the carrier capture from the optical confinement layer into quantum dots (QDs) can strongly limit the modulation bandwidth ω−3 dB of a QD laser. As a function of the cross-section σn of carrier capture into a QD, ω−3 dB asymptotically approaches its highest value when σn→∞ (the case of instantaneous capture). With reducing σn, ω−3 dB decreases and becomes zero at a certain nonvanishing σnmin. The use of multiple-layers with QDs significantly improves the laser modulation response—ω−3 dB is considerably higher in a multilayer structure as compared to a single-layer structure at the same dc current.

A tunable microwave generation is demonstrated by monolithically integrated photonic chip, which combines a mirror section and two distributed feedback (DFB) lasers. The focus ion beam technology was employed to achieve high-quality air/semiconductor slab for the mirror. Two DFB lasers (named slave and master lasers, respectively) can operate in a single fundamental mode when they are individually pumped. A microwave signal with the differential frequency between the two lasers can be obtained through optical heterodyning. A tunable microwave signal up to 25.5 GHz can be generated by adjusting the injected currents of the two DFB lasers. A four-wave mixing spectrum was measured and the linewidth of the generated radio frequency signal is 1.9 MHz.

We consider direct and indirect (excited-state-mediated) capture of carriers from the waveguide region into the lasing ground state in quantum dots (QDs) and calculate the modulation response of a QD laser. We show that, when only indirect capture is involved, the excited-to-ground-state relaxation delay strongly limits the ground-state modulation bandwidth of the laser—at the longest tolerable relaxation time, the bandwidth becomes zero. When direct capture is also involved, the effect of excited-to-ground-state relaxation is less significant and the modulation bandwidth is considerably higher.

Articles you may be interested inEffect of excited states on the ground-state modulation bandwidth in quantum dot lasers Appl. Phys. Lett. 102, 191102 (2013) We calculate the light-current characteristic (LCC) of a quantum dot (QD) laser under the conditions of both direct and indirect capture of carriers from the optical confinement layer into the lasing ground state in QDs. We show that direct capture is a dominant process determining the ground-state LCC. Only when direct capture is slow, the role of indirect capture (capture into the QD excited state and subsequent intradot relaxation to the ground state) becomes important. V C 2014 AIP Publishing LLC.

We show that the internal optical loss, which increases with free-carrier density in the waveguide region, considerably reduces the modulation bandwidth ω−3 dB of a quantum dot laser. At a certain optimum value j0opt of the dc component of the injection current density, the maximum bandwidth ω-3dBmax is attained and the modulation response function becomes as flat as possible. With internal loss cross-section σint increasing and approaching its maximum tolerable value, ω-3dBmax decreases and becomes zero. As with j0opt, there also exists the optimum cavity length, at which ω−3 dB is highest; the larger is σint, the longer is the optimum cavity.