80-GHz AlGaInAs/InP 1.55 µm colliding-pulse mode-locked laser with low divergence angle and timing jitter

摘要

Colliding-pulse mode-locked lasers (MLLs) offer many advantages compared to other types of MLLs as they provide relatively stable and better pulse-shaped mode-locking operation [1]. Colliding-pulse MLLs (CPM) with narrow far-field patterns are highly desirable for simple and high-yield passive optical alignment, as a low divergence angle improves the coupling efficiency and imposes less stringent alignment tolerances between the laser and a single-mode fiber (SMF). However, conventional epi-structure designs result in large beam divergence angles and highly asymmetric far-field patterns [2]. A great deal of work has been carried out over the past several years towards the development of spot-size converted lasers to improve the coupling efficiency with SMF. Many of the techniques, such as butt-coupling or lateral-taper-vertical-shift, require etch-and-regrowth over the active region, which is not readily applicable to AlGaInAs/InP-based lasers due to rapid oxidation of exposed Al-containing surfaces [3]. In addition, mode-locked lasers emitting optical pulse trains with low phase noise and timing jitter are also desirable for optical time-division-multiplexed (OTDM) communication systems and optical signal processing. The timing jitter of passive CPM operation can be reduced either by lowering the internal loss of the cavity or by increasing the saturation energy (Esat) of the gain section [4]. To obtain a high Esat in the gain section, one should try to use either a low optical confinement factor (Γ) or a large spot size (A), i.e. maximize A/Γ. Esat can also be improved by decreasing the differential gain (dg/dN), which can be achieved by reducing the number of quantum wells (QWs) in the gain medium.