Generation and control of bandwidth-limited, single-mode picosecond optical pulses by strong RF modulation of a distributed feedback InGaAsP diode laser

Abstract

Bandwidth-limited, single-mode ultrashort optical pulses with excellent coherency were generated for the first time from an InGaAsP DC-PBH distributed feedback (DFB) diode laser oscillating at 1.3 μm. It was achieved only by the laser diode itself through the most feasible and reliable method of strong RF modulation superimposed on the dc bias current. This method appears to be practically superior over the other techniques such as modelocking and injection locking because the simplest configuration with no requirement of any other optical elements and external cavity other than a laser diode as well as careful alignment and precise control. The measurement of second-order intensity auto- and cross-correlation demonstrated to be their traces completely free from noise spikes, with the contrast ratio of almost three. The time-bandwidth products of generated coherent picosecond pulses measured at different RF modulation frequencies and positive and negative dc bias current levels gave almost the values between 0.32 and 0.44 for Lorentzian pulse shape, and this fact proves definitely the bandwidth limited or coherent character. The pulsewidth was demonstrated to be continuously controllable over a range of approximately twice to three times depending upon the modulation frequency by changing the dc bias current from positive to negative. Also, the strong sinusoidal RF current superimposed on the negative dc bias current was found to be effective for high peak power operation for this InGaAsP DFB-DC-PBH diode laser. The theoretical analysis using the single-mode rate equations provides quantitatively the good agreement with the experimental results presented in this paper.