Comparison of passive mode-locked laser diodes with colliding and anti-colliding designs containing a DQW with a broad gain spectrum

Abstract

Compact, cost and energy efficient diode lasers which generate short infrared pulses in the picosecond (ps) and sub-ps ranges are ideal pulse sources for many applications like e.g. THz-TDS scanning systems [1]. Using a passively driven diode laser with a colliding-pulse mode-locking (CPM) contact layout pulses below 10 ps can be generated [2]. This design contains a cavity centred saturable absorber (SA) surrounded by two gain sections. An improvement of the laser performance has been predicted with an anti-colliding pulse mode-locking design (ACPM), where the SA is placed close to the low-reflectivity front facet and the gain section to the high reflectivity rear facet [3]. Here we compare experimentally the behaviour of 6 mm long CPM and 3 mm long ACPM ridge waveguide lasers in dependence on several absorber lengths and different driving conditions. Both lasers emit pulses with a repetition rate of ∼ 13 GHz, have a 5 μm wide ridge and facet reflectivities of R r = R f = 0.3 (CPM) and R r = 0.98 and R f = 0.3 (ACPM). The gain medium is a double quantum well (QW) were the Incontent of the In x Ga 1−x As y P 1-y QWs differs by Δχ = 0.08 to broaden the gain spectrum to support more longitudinal modes and to enhance the mode-locking performance [4]. A forward bias applied to the gain sections leads to a modal gain of about 19 cm−1 at an injection current density of 2.4 kA/cm2 at the laser wavelengths. The optimal absorber length is 200 μm for both designs, leading to short pulses with high peak power. The SA is driven by a negative bias with an optimum voltage of Vs A = −0.5 V for the ACPM design with an unsaturated absorption of α = −43 cm−1 at λ = 876 nm and Vs A −1.0 V for the CPM design with α = −31 cm−1 at λ = 879 nm (see Fig. 1 a).