Highly nonlinear GaSb-based saturable absorber mirrors

Abstract

ABSTRACT We present a gallium antimonide-based semiconductor saturable absorber mirror (SESAM) operating at 2 µm wavelength region. GaSb-based material system is the pref erred choice for fabricating su rface-normal devices operating beyond 2 µm because it enables the use of highly reflective semiconductor reflectors and quantum wells for wide wavelength range. For the purpose of generating short laser pules, the SESAM was carefully designed to attain a large modulation depth. The device was utilised successfully to passively Q-switch a 2 µm Tm 3+ -/Ho -doped fiber laser, demonstrating record-short Q-switch pulses of about 20 ns. Keywords: GaSb, antimonide, SESAM, Q-switching, mid-IR 1. INTRODUCTION Semiconductor saturable absorber technology has had a notable impact on reliable and practical ultrafast fiber lasers operating at 1 µm and 1.55 µm wavelengths [1-3]. These absorbers are fabricated using GaAs- or InP-based compound semiconductors for 1 µm and 1.55 µm wavelength regions, respectively. Such technology is well studied and commonly used nowadays. Recently there has been increased interest in devices operating at longer wavelengths, 2 µm and beyond. SESAMs for these wavelengths would, for example, allow the development of prac tical ultrafast Tm- and Ho-doped fiber lasers. Lasers operating at 2-3 µm are considered to be prom ising tools for monitoring greenhouse gases, remote sensing, free space communication and micro-surgery [4]. SESAMs are nonlinear optical elements th at impose intensity-dependent attenuation on a light beam incident upon them. Incident light of low intensity is absorbed, while high intensity light passes the saturable absorber with much less attenuation. This nonlinear behaviour is the key mechanism for passive Q-switching and modelocking of lasers [5]. As can be seen in Fig. 1, the SESAM structure consists of an absorbing layer placed inside a Fabry-Perot cavity formed between a bottom and top mirror. These mirrors are usually distributed Bragg reflectors (DBRs), but in some cases the top mirror is omitted since the semiconduc tor/air interface itself acts as a mirror, albeit with low reflectivity. Absorption in a SESAM is provided by a certain number of quantum well (QW) or quantum dot (QD) layers. When compared to other conventional saturable absorbers, a dvantages of SESAMs are that they can be custom-made for different wavelength ranges and their properties can be tailored for a specific purpose. For instance, to obtain stable, self-starting Q-switching with a SESAM, one key parameter, the modulation depth, 4 R , can be enhanced in many ways: by (i) increasing the number of QWs or QDs, (ii) placing the absorbing layers at the antinodes of the optical field inside the cavity, (iii) enhancing the finesse of the cavity by increasi ng the number of top DBR layers, and (iv) designing the cavity thickness for resonant operation in conjunction with the laser operating wavelength.