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Journal of Modern Optics

Temporal evolution of amplitude restoration and thresholding in an all-optical regenerative memory

Alistair J. Poustie, K. J. Blow, Anthony E. Kelly, Robert J. Manning

Published Jul 1, 1999• 5 citations
Publisher

Abstract

In this paper, we experimentally measure the temporal evolution of amplitude restoration and thresholding in an all-optical regenerative memory. The memory architecture comprises two coupled all-optical switching gates and different wavelength optical pulses are used for clock and data sources. The all-optical gates are nonlinear interferometers incorporating semiconductor optical amplifiers as the nonlinear optical material. The concatenated response of these nonlinear interferometers provides the required square top transmission function that allows the stored optical pulses to be equalized in amplitude. The edge of the transmission function forms the threshold for pulse storage in the memory. We input a variable amplitude data packet into the memory and record the temporal evolution of the pulses with a fast real-time digitizing oscilloscope. Complete amplitude restoration of the pulses occurs after ∼5 circulations of the optical fibre memory loop. This measurement is in good agreement with the theoretical prediction and indicates that the square top transmission response could also be achieved for data ‚on the fly‘ with a linear concatenation of only a few all-optical switching gates.

DOI
10.1080/09500349908231333

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In this paper, we experimentally measure the temporal evolution of amplitude restoration and thresholding in an all-optical regenerative memory. The memory architecture comprises two coupled all-optical switching gates and different wavelength optical pulses are used for clock and data sources. The all-optical gates are nonlinear interferometers incorporating semiconductor optical amplifiers as the nonlinear optical material. The concatenated response of these nonlinear interferometers provides the required square top transmission function that allows the stored optical pulses to be equalized in amplitude. The edge of the transmission function forms the threshold for pulse storage in the memory. We input a variable amplitude data packet into the memory and record the temporal evolution of the pulses with a fast real-time digitizing oscilloscope. Complete amplitude restoration of the pulses occurs after ∼5 circulations of the optical fibre memory loop. This measurement is in good agreement with the theoretical prediction and indicates that the square top transmission response could also be achieved for data ‚on the fly‘ with a linear concatenation of only a few all-optical switching gates.
We demonstrate an all-optical memory which can discriminate between input data pulses of differing amplitude. Only optical pulses with amplitudes above a predetermined threshold are stored and their amplitudes are equalised. We have demonstrated an all-optical regenerative memory with variable storage threshold and amplitude restoration. This functionality should be scaleable to high data rates approaching 100 Gbits and to low latency memory stores using integrated TOAD/SLALOM nonlinear optical switching devices.
The function of ‘memory’ is one of the key building blocks to implement advanced information processing in future ultrahigh-speed photonic networks. Previously, we have described an all-optical regenerative memory in which stable storage of data is obtained for several hours [1]. In addition, the memory architecture provides amplitude restoration for the stored pulses and the ability to set a threshold storage level [2]. Here we demonstrate an important extension to this design which not only allows optical pulses to be written to the memory but also permits optical pulses to be selectively read from storage. This additional capability greatly extends the usefulness of the memory as a sub-system in more complex processing operations [3]. To achieve the ‘read’ function we incorporate a third nonlinear optical switching element (TOAD/SLALOM [4,5]) to act as an all-optical modulator for the data stored in the regenerative memory.
The use of fibre-loop memories is being experimented for solving routing contention of data cells in photonic high bit rate ATM switching fabrics.1 The simplest fiber-loop memory looks like a ring laser consisting of a semiconductor laser amplifier (SOA), a span of fibre introducing a delay of the order of an ATM cell duration (10–6-sec) and a 50/50 directional coupler (Fig. 1). To compensate for the power tapping at the directional coupler while avoiding the onset of laser oscillations, the SOA is biased above the ring laser threshold only for a limited number of round trips, then the bias current is switched off. The memory is thus driven through the transition from a state ruled by spontaneous emission toward a state above lasing threshold, in which stimulated emission is predominant. Far enough from the beginning of the transition the intensity fluctuations display a maximum.2,3 The transition is completed after a time depending on the final state intensity.2 These features significantly affect the maximum storage time and noise behavior of the memory.
Ultrafast all-optical memory at 250 Gb/s using Mach-Zehnder interferometers (MZIs) with symmetrical quantum-dot semiconductor optical amplifiers (QDSOAs), acting as all-optical AND gate and regenerator in a loop configuration, is theoretically demonstrated for 500 loop circulations using a return-to-zero modulated format. The memory operation is examined and evaluated by the quality factor (QF), which is a more sensitive method of measuring memory quality. The obtained numerical results show that the proposed memory can be operated up to 250 Gb/s for 500 circulations with high QF.
We present an experimental demonstration of an all optical memory consisting of a single semiconductor optical amplifier as the active medium based on wave mixing. The circuit is a fiber ring consisting of a semiconductor amplifier, a bi-directional 2x2 fiber coupler, an isolator, a polarization controller, and a Faraday mirror. The output is observed on the spectrum analyzer, which consists of the peak wavelength of the SOA plus an additional signal generated through wave mixing, which is tunable within the band gap of the SOA. We shall demonstrate two modes of operation of the device, which will consist of a flip flop switch between three states, as well as a single data storage memory. The stable states consist of two modes, where the mode amplitude represents the state. Mode 1 corresponds to 1500 nanometers and mode 2 corresponds to 1530 nanometers. A contrast ratio between an “on” and “off” state is measured for each mode and the mode of operation is based on the polarization. We shall present the states for the circuit, the effect of drive current on the system, the effect of the SOA structure, wave mixing effects, and how to operate the device for both logic operation and data storage.