Abstract

Photonic data transmission in novel telecommunication applications demands picosecond-short optical pulses at a multi-GHz repetition-rate (RR) with low pulse-to-pulse timing jitter (TJ). While nowadays mostly wavelengths around 1.55 μm are used, ytterbium-doped fiber amplifiers allow for data transmission over long distances at wavelength of around 1050 nm. Exemplary photonic transmission in this wavelength region has been demonstrated by a mode-locked (ML) multi quantum-well (MQW) semiconductor laser subject to an external cavity delivering a pulse width (PW) of 22 ps at a RR of 10 GHz, a harmonic of the external cavity (EC) frequency, yielding an integrated TJ of 120 fs (100 Hz to 100 MHz) [1]. Initial investigations on a monolithic MQW semiconductor laser emitting at 1070 nm have recently been presented [2]. In this work, we present a monolithic passively ML InGaAs MQW ridge waveguide laser that generates a stable train of picosecond-short optical pulses at a wavelength of 1080 nm with a RR of 40 GHz and a very low TJ of 55 fs. The optical pulses are Gaussian shaped and their width can be as short as 4.6 ps. Beam profile analysis confirms a Gaussian beam shape with a beam aspect ratio of 0.71 allowing for an efficient single-mode fiber coupling. The laser is a 1 mm long multi-section structure with an absorber-to-gain-section length ratio of 0.1 (Fig. 1). The facets are as cleaved. Pulse properties are analysed in a wide range of laser gain injection current (GC) and absorber reverse bias voltage (RBV). The PW are obtained from Gaussian fits to the Autocorrelator (AC) time traces. The TJ can be calculated from the RR and the RR linewidth (Δf) by TJ_ptp = (Δf / (2π RR³))^0.5 [3]. Beam profile analysis is performed by near-field imaging analysis. By using a 60x microscope objective and a CCD camera, a homogeneous Gaussian shaped profile is confirmed, as depicted on the right in Fig. 1. An aspect ratio of 0.71 is obtained by Gaussian fits to the horizontal and the vertical axes, indicating an excellent beam profile for efficient single-mode fiber-coupling. The TJ for two selected absorber reverse biases voltages of 1.8 V and 2.0 V and for increasing GC are depicted in Fig. 2. The lowest TJ amounts to 55 fs at an RBV of 1.8 V and a GC of 410 mA. The corresponding RR line with a Lorentzian fit (-3-dB linewidth: 1.43 MHz) and the corresponding AC trace with a Gaussian fit yield a deconvoluted PW of 4.6 ps, as depicted as insets in Fig. 2. The color-coded TJ performance for a selection of driving parameters is depicted in Fig. 3. The region indicated by blue colour coding at high GC and low RBV depicts TJ values below 100 fs.

© 2017 IEEE