Monolithically integrated 4x4 SOA switch fabricated using quantum well intermixing

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ABSTRACT Monolithically-integrated semiconductor optical amplifiers (SOAs) have the potential for enabling high-speed and low-crosstalk optical switches in reconfigur able optical add-drop multiplexers (ROADMs). Using integrated 4x4 switches as the building blocks for large-scale ROADMs, instead of 2x2 switches, will reduce alignment issues and assembly steps during manufacturing. The switch is based on SOAs, quantum well intermixed (QWI) passive 1x4 MMI splitters/combiners, and total internal reflection mirrors. We present the results of the 4x4 switch design, for a switch of 5.3 mm x 3.5 mm in size, with estimated total excess on-chip losses of 23 dB. Keywords: Quantum-well intermixing, optical switch, monolithic integration 1. INTRODUCTION The development of next-generation fiber-optic networks will require fast and flexible routing of high volume traffic through many optical channels. High-speed multiple-channel optical switches are a necessary enabler. Numerous approaches have been proposed , including Mach-Zehnders [1], total-internal reflection [2], and ring resonators [3]. Switches based on semiconductor optical amplifiers (SOAs), however, offer minimal insertion loss due to gain compensation in forward-bias, have low crosstalk due to high attenuation in reverse-bias, and have high speeds due to nanometer-scale switching speeds [4,5]. SOA-based switches can be realized in disparate monolithic integration modalities, such vertical coupling [6], selective-area regrowth [7], or quantum well intermixing (QWI) [8]. We implement a 4x4 optical switch using QWI, as the process has demonstrated good results for other monolithically-integrated devices, such as lasers [9,10]. Various implementations of QWI exist, including ion-implantation [11], impurity-free vacancy disordering [12], sputtered silica-induced intermixing [13], pulsed laser irradiation induced intermixing [14], and inductively-coupled plasma-enhancement [15]. We use the inductively-coupled plasma process, but our design it is not restricted by it. In this paper, we discuss the design considerations of our 4x4 switch. Most earlier work has focused on 2x2 switches, with SOA-based optical switches up to 2x2 achieved using QWI [8]. Integrated 4x4 SOA switches with a low polarization dependence and insertion loss have been demonstrated using regrown passive waveguides [7]. Using integrated 4x4 switches as building blocks for large-scale ROADMs in place of 2x2 switches reduces alignment issues and the number of assembly steps in manufacturing. It also potentially reduces the number of separate temperature control devices that may be required. An integrated 4x4 switch, however, has intrinsically higher splitting/combining losses (12 dB) than a 2x2 switch (6 dB). However, to implement a 4x4 switch using a nonblocking configuration such as a Clos network [16] requires 6 discrete 2x2 switches, with three 2x2 switches in each path, for total splitting/combining losses of 18 dB per channel. The fully integrated switch can provide lower losses without even considering the additional coupling losses that would occur between discrete switching elements. To keep the integrated switch loss to a minimum, it is necessary to keep excess losses as low as possible. To realize such for our non-blocking integrated 4x4 switch, we report the detailed design of our multi-quantum well (MQW) active region and our 1x4 splitters/combiners.