Cross-gain Modulation In Semiconductor Optical Amplifier Research Articles

Optical Content Addressable Memory Matchline for 2-bit Address Look-Up at 10 Gb/s

We demonstrate the first experimental evaluation of a 10Gb/s optical matchline (ML) memory architecture that designates successfully whether an incoming 2-bit optical address is identical to the optical address stored in an optical content addressable memory (CAM) line. The optical CAM line is formed by two optical CAM cells, with every cell comprising a monolithically integrated InP flip-flop and an SOA-MZI-based XOR gate, while the ML is completed by a subsequent wavelength multiplexing element and a semiconductor optical amplifier-cross gain modulation gate. Error-free operation at 10Gb/s has been obtained for 2-bit optical addresses and for both content addressing and write memory functionalities.

XPM- and XGM-Based Optical RAM Memories: Frequency and Time Domain Theoretical Analysis

We demonstrate a frequency and time domain analysis for optical random access memory (RAM) cells that rely on semiconductor optical amplifier (SOA)-based switches but employ different switching mechanisms. The first RAM cell utilizes SOA cross gain modulation (XGM) switches both for the access gate as well as latching mechanism, whereas the second RAM cell configuration utilizes SOA-Mach-Zehnder interferometer cross phase modulation (XPM) switches. The frequency domain analysis exploits first-order perturbation theory approximations towards deriving the RAM cell frequency response, which is shown to exhibit in both RAM cell layouts a comb like resonant behavior. The free spectral range is dictated by the coupling length between the coupled switches that form the latching element, whereas the finesse depends on the temporal response of the switching mechanism employed. The qualitative speed and signal quality results obtained in the frequency domain are confirmed by a respective time-domain analysis carried out for both RAM cell layouts, using an experimentally validated time-domain SOA simulation model that relies on the transfer matrix method. Performance analysis in the time domain reveals in addition important quantitative RAM output signal measures like the extinction ratio and its dependence on the coupling length and the operational speed, as well as the input power dynamic range for successful RAM operation. Our holistic frequency- and time-domain analysis framework provides an in-depth understanding of performance-critical design parameters and their relationship to expected RAM cell performance characteristics. This is then utilized for a one-by-one system level comparison between the two RAM cell layouts in terms of readout extinction ratio, maximum speed, footprint, and power consumption concluding that the SOA-XGM-based RAM cell offers certain advantages when operational speeds not higher than 10 Gb/s are targeted, and the SOA-XPM-based RAM cell setup dominating when higher RAM serial speeds even up to 40 GHz are targeted.

Optical RAM and Flip-Flops Using Bit-Input Wavelength Diversity and SOA-XGM Switches

In this paper, we demonstrate a novel RAM cell based only on three traveling waveguide semiconductor optical amplifier-cross gain modulation (SOA-XGM) switches. The RAM cell features wavelength diversity in the incoming bit signals and provides Read/Write operation capability with true random access exclusively in the optical domain. Two of the SOA-XGM switches are coupled together through an 70/30 coupler to form an asynchronous flip-flop, which serves as the memory unit. Random access to the memory unit is granted by a third SOA-ON/OFF switch and all three SOAs together form the proposed RAM cell. Proof-of-principle operation is experimentally demonstrated at 8 Mb/s using commercial fiber-pigtailed components. The distinctive simplicity of the proposed RAM cell architecture suggests reduced footprint. The proposed flip-flop layout holds all the credentials for reaching multi-Gb/s operational speeds, if photonic integration technologies are employed to obtain wavelength-scale waveguides and ultrashort coupling lengths. This is numerically confirmed for 10 Gb/s using a simulation model based on the transfer matrix method and a wideband steady-state material gain coefficient.

SOA-EAM frequency up/down-converters for 60-GHz bi-directional radio-on-fiber systems

We investigate a frequency up/down-converter based on a single cascaded semiconductor optical amplifier (SOA)-electroabsorption modulator (EAM) configuration for bi-directional 60-GHz-band radio-on-fiber (RoF) system applications. SOA cross-gain modulation and photodetection in EAM are used for frequency up-conversion, and EAM nonlinearity is used for frequency down-conversion. In our scheme, both 60-GHz local-oscillator (LO) signals and IF signals are optically transmitted from a central station to base stations. We characterize the dependence of frequency up/down-conversion efficiencies on EAM bias and optical LO power. For frequency up-conversion, maximum conversion gain of approximately 8 dB was obtained and, for frequency down-conversion, more than approximately 18-dB conversion loss was measured. Utilizing this frequency up/down converter, we demonstrate a 60-GHz bi-directional RoF link. Optically transmitted downlink 10-Mb/s quadrature phase-shift keying (QPSK) data at 100-MHz IF are frequency up-converted to the 60-GHz band at the base station, and uplink 10-Mb/s QPSK data in the 60-GHz band are frequency down-converted to 150-MHz IF and transmitted to the central station. In addition, the dependence of error vector magnitudes on IF signal power and wavelength is investigated.

Optical routing characteristics of wavelength switched probe light from super-structure-grating DBR laser and cross-gain modulation in SOA

Optical routing characteristics of the optical routing systems consisting of a semiconductor optical amplifier (SOA) as a wavelength converter, a super-structure-grating DBR laser as a probe light source, and an arrayed-waveguide-grating as an output routing component are investigated. Wavelength conversion signals switched between two ITU-T grids of the AWG are observed. Routing time of less than 5 ns and the emergence of clear eye opening of less than 2 ns have been achieved.

All-optical actively modelocked fibre ring laser based on cross-gain modulation in SOA

A novel actively modelocked fibre ring laser based on cross-gain modulation in a semiconductor optical amplifier (SOA) employed as both the gain medium and modelocking element has been demonstrated. Stable uniform pulse trains with pulse-widths about 21 ps at 5 GHz repetition frequency were obtained. The entire laser was operated in the optical domain with a large tuning range that could probably span the whole gain region of the gain medium employed.

Performance analysis of wavelength converters based on cross-gain modulation in semiconductor-optical amplifiers

An analytical theory describing all-optical wavelength converters based on cross-gain modulation (XGM) in semiconductor-optical amplifiers is derived. Our theory consists of two parts: a large-signal analysis yielding the transmission function for the signal, and a small-signal analysis in order to describe the transformation of the signal and probe intensity noise. Both the large-signal as well as the small-signal theory reveal similar performance for the co- and the counterpropagating injection scheme for bit rates up to 2.5 Gb/s. This is confirmed by computer simulations. Consequently, the counterpropagating configuration is preferable because the implementation is simpler and conversion to the same wavelength is possible. In order to increase the conversion efficiency it is better to reduce the average signal power than to increase the probe power, which additionally reduces the output power range. However, there is a tradeoff between conversion efficiency and output extinction ratio. According to the small-signal analysis, the relative-intensity noise (RIN) due to the probe and due to the amplified spontaneous emission is negligible. Moreover, the converted signal has a lower RIN than the input signal.