Observation of Q-switching and modelocking in two-section InAs/InGaAsP/InP (100) quantum dot lasers at 1.53µm

Modelocked lasers based on InAs/GaAs quantum dots (QDs) and quantum dash material in the 1.2 to 1.3 µm region have recently been attracting attention because of their wide operating ranges for passive modelocking and low tendency to Q-switching behaviour as discussed by e.g. Viktorov et al [1]. In this paper we report on the observation of laser dynamics observed for the first time in twosection ridge waveguide Fabry-Pérot InAs/InP QD lasers operating in the important 1.55 µm telecom region. Extensive regions of Q-switching behaviour and a particular form of passive modelocking are observed. The QD laser structure is grown on n-type (100) InP substrates by metal-organic vapor-phase epitaxy (MOVPE) [2]. The layerstack is compatible with a butt-joint active-passive integration process for possible further integration. The two-section devices are operated by forward biasing the longer gain section, creating a semiconductor optical amplifier (SOA) and by reversely biasing the shorter gain section, creating a saturable absorber (SA). Here we present results on two specific devices from a series of fabricated devices: one 7 mm long laser with a 350 µm long SA and one 9 mm long device with a 270 µm SA. The output from the devices is characterised using a 50 GHz photodiode and electrical spectrum analyser (ESA), a high resolution optical spectrum analyser, an autocorrelator and a 6 GHz bandwidth real time oscilloscope. CW and large Q-switching operating regions are observed with the 7 mm device. An example of a series of data obtained from the ESA is shown in Fig. 1 below. With increasing SOA current and a fixed SA voltage of -3 V we observe CW operation at the lower currents above threshold, and three different Q-switching regimes at higher currents. This is confirmed by observation on the oscilloscope. With the 9 mm long device we observe in the ESA spectra only the well defined peaks at the roundtrip- frequency of 4.67 GHz and its harmonics at SA voltages between approximately 0V and -1 V (Fig. 2). It turns out that the laser is producing approximately 100 ps long pulses that are highly chirped. By spectrally filtering the output (1.2 nm), 6 to 11 ps pulses were observed on the autocorrelator without background signal. We were able to demonstrate that the full bandwidth of the output is coherent and have determined the chirp of the long pulses. We attribute this dynamical behaviour to the particular inhomogeneous gain and absorption properties of our QDs compared to quantum dash material at 1.5 µm and QDs at 1.3 µm. These issues are currently under investigation. [1]