Published in IET Optoelectronics Received on 9th November 2009 doi: 10.1049/iet-opt.2010.0019 ISSN 1751-8768 Effect of saturable index change on all-optical logic operations in a passive vertical cavity semiconductor saturable absorber P.K. Datta 1 R. Pradhan 2 L. Mishra 1 S. Saha 3 1 Department of Physics & Meteorology, IIT, Kharagpur 721302, India 2 Department of Physics, Midnapore College, Midnapore 721101, WB, India 3 Department of Physics, Vidyasagar University, Midnapore 721102, WB, India E-mail: pkdatta@phy.iitkgp.ernet.in Abstract: Semiconductor multiple quantum wells (QWs) have saturable absorption and saturable refractive index change properties. When it is embedded within a reflective Fabry– Perot cavity, the device shows forward and inverse saturation reflectivity for a radiation at cavity resonance wavelength with input intensity around the impedance matching point. Consideration of non-linear index change increases the extinction of forward saturation characteristics of the device. However its application in logic devices is very limited. Whereas, the inverse characteristics for which a number of logic devices can be proposed is severely affected in its extinction because of the consideration of non-linear index change. Here the authors optimise the parameters of vertical cavity semiconductor saturable absorber for the static and dynamic device performance of all-optical logic operations based on inverse saturation characteristics with both dispersive and absorptive saturation into consideration. The authors obtain an extinction of almost 77.5% with input intensity of the order of saturation intensity of a multiple QWs. The novelty of this work lies in the observation that the saturation of non-linear index of refraction has enormous effect on the characteristics of logic operations, which is not addressed in earlier reports. It is also observed that the detrimental effect of non-linear index saturation can be overcome by proper absorber parameters. 1 Introduction Ultrafast signal processing in all-optical networks require logical operations to be done in optical domain. All optical gates can be used to perform many functions in optical packet-switched network. There are number of ways to realise non-linear optical gate, such as, fibre-based non-linear optical loop mirror [1], semiconductor optical amplifier (SOA)-based interferometers [2], electro-absorption modulators [3], bistable lasers [4] and passive saturable absorbers [5]. These non-linear gates are either through cross-regeneration or self-regeneration. However Szoke et al. [6] in 1969 first proposed and realised two stable output states of a saturable absorber introduced inside a resonant cavity. The saturable non-linearity was too poor to use it in low-power communication devices. Non-linear optical gate devices based on optical Kerr non-linearity [7] got importance over the others in subsequent years. However recently, owing to the matured technology of growing semiconductor multiple quantum wells (QWs), there has been an enormous increase of saturation absorption non- linearity, fast response, low saturation energy, wide spectral bandwidth and low non-saturable loss. As compared to other types of optical gate devices, saturable absorber-based devices will have definite advantages such as: no need of any extra laser source, no wavelength shift, large non-linear optical effect, fully passive operating mode, small size and simple to operate. It should have properties like low switching energy and insertion loss, minimum contrast ratio of 5 dB, recovery time as low as 1.5 ps, polarisation insensitivity and compatible with wavelength division multiplexing (WDM) systems. All these requirements can be treated as follows: the response of the absorber gets enhanced and the switching energy between the stable states is reduced by embedding the saturable absorber inside an asymmetric Fabry– Perot micro-cavity and the device is called vertical cavity saturable gate (VCSG). The overall reflectivity of the device is very low at the impedance- matched (IM) condition, which can be effected at very low input power in a properly designed VCSG. It is polarisation insensitive because of its normal incidence mode of operation. The recovery time of semiconductor multiple QWs can be reduced significantly by introducing capture and recombination centres. Earlier, Fabry – Perot semiconductor saturable absorbers have played a great role in passive mode-locking of lasers for the generation of optical pulses in picosecond and femtosecond regimes [8, 9]. Later, all optical AND/NAND logic operations were reported by Ibrahim et al. [10]. Mikroulis et al. [11] realised 40 Gb/s AND logic gate using the semiconductor microring resonator. Applications of VCSGs in switching, different logical operations and wavelength conversions have been reported earlier by Porzi et al. [12–14]. Vertical cavity SOA-based AND operation IET Optoelectron., 2011, Vol. 5, Iss. 2, pp. 77–82 77 doi: 10.1049/iet-opt.2010.0019 & The Institution of Engineering and Technology 2011 www.ietdl.org