Optical self-switching effects in Mach-Zehnder interferometers

Future optical networks may require both wavelength conversion and bit shape regeneration in an all-optical domain. The possibility of pulse reshaping while providing wavelength conversion may support new demands over medium and large distances links (Kelly, 2001). Indeed, during propagation the optical data signal suffers deterioration due to the amplified spontaneous emission (ASE) from optical amplifiers, pulse distortion from intrinsic dispersion, crosstalk, and attenuation. All-optical regenerators may be important components for the restoration of these signals, providing complexity and cost reductions with the avoidance of optoelectronic conversions. The regeneration could be 2R (reamplification and reshaping) or 3R, which also provide retiming to solve jitter (Simon et al., 2008). Several 3R regenerators using the semiconductor optical amplifier (SOA) have been proposed, such as cascaded SOAs setups (Funabashi et al., 2006) or SOA based MachZehnder interferometers (MZI) (Fischer et al., 1999). However for small and medium distances systems, where the signal amplitude noise and distortions form the main problem and where jitter has fewer magnitudes, the simpler 2R processes can be adequate to keep signal quality (Simon et al., 1998). In addition, the SOA is a helpful device for both 2R-regeneration (Ohman et al., 2003) and wavelength conversion (Durhuus et al., 1996). Several techniques for 2R-regeneration based on SOAs have been proposed and tested, for example by using four-wave mixing (FWM) (Simos et al., 2004), cross-gain modulation (XGM) (Contestabile et al., 2005), integration within MZI (Wang et al., 2007), multimode interferometric SOA (Merlier et al., 2001), cross-phase modulation (XPM) with filtering (Chayet et al., 2004), and feed forward technique (Conforti et al., 1999). However, these techniques require complex designs and involve critical operation points, even the simplest ones based on XGM features. In addition, most of these techniques are not capable of wavelength conversion and regeneration simultaneously. Recently a regenerator based on cross-gain modulation was proposed using one SOA for wavelength conversion (in a counter-propagating mode) and another deeply saturated SOA (synchronized by an optical delay line) to achieve cross-gain compression (Contestabile 2005). This efficient approach has similarities with the all-optical feed-forward techniques. In addition, this regenerator could not done wavelength conversion if the wavelength of the

We experimentally demonstrate uncompensated 8-channel wavelength division multiplexing (WDM) and single channel transmission at 10.7 Gb/s over a 470 km hybrid fiber link with in-line semiconductor optical amplifiers (SOAs). Two different forms of the duobinary modulation format are investigated and compared. Maximum Likelihood Sequence Estimation (MLSE) receiver technology is found to significantly mitigate nonlinear effects from the SOAs and to enable the long transmission, especially for optical duobinary signals derived from differential phase shift keying (DPSK) signals directly detected after narrowband optical filter demodulation. The MLSE also helps to compensate for a non-optimal Fabry-Perot optical filter demodulator.

Semiconductor Optical Amplifier (SOA) is used for a 2.5Gbps Coarse Wavelength Division Multiplexing (CWDM) transmission in O to L-band (1271–1611 nm). Analysis of the proposed topology is conducted using Optisystem software. In this paper, discussions on the gain spectrum and the quality of signal of an optically amplified 18 channel system are provided. A wideband travelling wave SOA which operates as an inline amplifier resulted in wide 3 dB bandwidth of 200 nm. The overall quality of signal at the receiver is evaluated with the Q factor value. Better performance based on Q factor analysis is obtained at larger lengths of the first fiber span and a flat gain over 11 CWDM channels is obtained.

Four-wave mixing (FWM) in semiconductor optical amplifiers (SOAs) has several important features, such as, high speed and high FWM conversion efficiency as well as optical demultiplexing (DEMUX) (Mecozzi et al., 1995; Mecozzi & Mrk, 1997; Das et al., 2000). The are several applications of FWM in SOAs for all-optical devices, such as, wavelength converters (Vahala et al., 1996), optical samplers (Inoue & Kawaguchi, 1998), optical phase conjugators (Kikuchi & Matsumura, 1998) and optical multiplexers/demultiplexers (Kawanishi et al., 1997; Kawanishi et al., 1994; Morioka et al., 1996; Uchiyama et al., 1998; Tomkos et al., 1999; Kirita et al., 1998; Buxens et al., 2000) have been demonstrated for optical communication systems. When a pulse of a time-multiplexed signal train (for example, a probe pulse) and a pump pulse are injected simultaneously into an SOA, gain and refractive index in the SOA are modulated and an FWM signal is generated by the modulations. Thus, we can obtain a demultiplexed signal as an FWM signal at the output of SOA. All-optical demultiplexing has been experimentally demonstrated up to 200 Gbit/s (Morioka et al., 1996). Tomkos et al., (Tomkos et al., 1999) suggested a number of ways to improve the performance of the dual-pump demultiplexer at 40 Gbit/s as follows; adjustment of the input wavelengths at the peak gain wavelength of the SOA under saturation conditions, the use of higher pump power at the input of the device, or/and the use of pulsed pumps with short pulsewidths. For the higher bit-rate, the overlap of the input to the FWM signal pulses may appear both in the time and spectral domain. The pattern effect may also appear in the FWM signal due to the slow components of the optical nonlinearities in SOAs (Saleh & Habbab, 1990). These effects degrade the usefulness of the FWM in SOAs as a practical DEMUX device in optical network/ communication systems. Therefore, it is very important to analysis the optical DEMUX characteristics based on FWM in SOAs for the ultrafast multi-bit input optical pulses.

We investigate the cross-gain modulation (XGM) in a semiconductor optical amplifier (SOA) fiber ring loop theoretically and experimentally. Based on the XGM, a wavelength conversion is designed. We experimentally carry out an SOA wavelength conversion, which converts a 20 MHz sinusoidal modulated input optical signal at a wavelength of 1317 nm to an output optical signal at a wavelength of 1314.3 nm. In this type of wavelength conversion, a tunable optical filter is utilized to select the target wavelength of the output signal from the fiber ring loop, and this target wavelength signal is accumulated and amplified as the resonance of the fiber ring loop. The introduction of the tunable optical filter into the fiber ring loop system makes it very easy to obtain any desired target wavelength from this XGM wavelength conversion system; this remarkable property means that the SOA wavelength conversions have broader applications in optical networking and beyond. Higher frequency modulation XGM wavelength conversion may be obtained by scaling down the length of the fiber ring loop.

In-line and reflective semiconductor optical amplifiers (SOA and RSOA) can be effectively utilized as external optical intensity modulators. However the limited modulation bandwidth affects the application possibilities, because next generation access networks desire high trans- mission speed. Bandwidth enhancement can be achieved by means of optical equalization thanks to the chirping char- acteristics of semiconductor optical amplifier. The limited modulation transfer function can be combined with the converted phase modulation transfer function. Investiga- tion of optical filter with optimal slope is a crucial chal- lenge in this approach. The presented simulations describe the slope dependence of the modulation band. The model can take into account the different type of realizable opti- cal filters. However there is a trade-off between modula- tion efficiency and modulation bandwidth due to the extra insertion loss of the set-off optical filter. In the paper theo- retical and simulation investigations are presented with the result of improved modulation bandwidth.

The optimum condition of all optical signal regeneration (2R) was analysed in a cross-phase modulation (XPM) wavelength converter based on a Mach–Zehnder interferometer (MZI) using semiconductor optical amplifiers (SOAs). Since the input and probe signal power affect the transfer function of an interferometer, the optimum operation conditions are needed to improve the pulse shape of the converted signal. A serially cascaded dual MZI–XPM wavelength converter scheme was proposed to realise the wavelength conversion and optical signal regeneration at the same time. The signal reshaping was achieved by controlling the gain and phase conditions of conventional SOAs in an interferometric decision circuit. To obtain the operation of a nonlinear optical gate (NLOG) using a decision circuit, the two SOAs should have different length and injection current. Based on the simulations of pulse shape in time domain and bit error rate, we have confirmed that this scheme greatly improves the optical signal quality.

Fiber transmission systems with semiconductor optical amplifiers without in-line filtering

A nonlinear phase shift is introduced to a Mach-Zehnder interferometer (MZI), and we present a scheme for enhancing the phase sensitivity. In our scheme, one input port of a standard MZI is injected with a coherent state and the other input port is injected with one mode of a two-mode squeezed-vacuum state. The final interference output of the MZI is detected with the method of active correlation output readout. Based on the optimal splitting ratio of beam splitters, the phase sensitivity can beat the standard quantum limit and approach the quantum Cram\'{e}r-Rao bound. The effects of photon loss on phase sensitivity are discussed. Our scheme can also provide some estimates for units of $\chi^{(3)}$, due to the relation between the nonlinear phase shift and the susceptibility $\chi^{(3)}$ of the Kerr medium.

Summary form only given. In optical burst switching (OBS) networks, a short switching time is very important for handling fine grain packets such as TCP ACK packets, which are only 40 bytes in length. At data rate of 10 Gbps, such a data burst will occupy a period of only 32 ns. Therefore OXC (optical cross connector) with fast switching speed, for example < 100 ns, are generally required for OBS networks. There are many methods to configure an OXC. Tunable optical filters may used to build large scale OXC. In an optical filter, the tuning ability is usually achieved by introducing extra phase shift via the electro optic, thermo optic, or piezoelectric effect, etc. Up to date, the most commonly used tunable filter structures are interferometric such as Fabry-Perot (FP) and Mach-Zehnder (MZ) interferometers. They are usually poor in the performance of passband flatness and the crosstalk, and require robust electronics to stabilize the central wavelength. Hence, they are rarely practically used as switching elements in the dense wavelength division multiplexing (DWDM) optical networks. In recent years, digitally tunable optical filter (DTOF) has received increasingly attention. Structures based on the arrayed waveguide grating (AWG) and on-off switches have been reported. DTOF based on AWG may require precision thermal control for AWG usually suffers the problem of temperature dependence. In this talk, we present a novel approach to realize the DTOF (digitally tunable optical filter) based on DWDM thin film filters (TTF) and semiconductor optical amplifier (SOA). Apart from the outstanding feature being consistent with the wavelength comb suggested by international telecommunication union-telecommunication (ITU-T), the proposed DTOF is very stable in temperature performance. Furthermore it requires less SOAs in comparison with the reported structures.

The efficiency of optical communication system operation is limited by the low operation rate and relatively complicated realization of electronic components. The transparent and fast optical signal processing becomes very important for the modern optical communication networks in order to overcome the electronic bottleneck. In this work, we consider two examples of all-optical signal processing based on the unique nonlinear optical characteristics of semiconductor optical amplifiers (SOAs). We developed a theoretical model of all-optical logic gates based on quantum dot SOAs (QD-SOAs), which are the most promising candidates for up to 100 Gb/s optical networks. We also proposed a novel method of the all-optical automatic gain control (AGC) of erbium doped fiber amplifiers (EDFA) based on the application of semiconductor optical amplifiers (SOAs).

Semiconductor optical amplifiers are important for wide range of applications including optical networks, optical tomography and optical logic systems. For many of these applications particularly for optical networks and optical logic high speed performance of the SOA is important. The speed of operation of SOA is limited by the gain and phase recovery times in the SOA. We have demonstrated higher speed operation (i) for SOAs with a carrier reservoir layer, (ii) for SOAs with a multi-quantum well modulation doped active region, and, (iii) for SOAs with a quantum dot (QD) active region. The multi-quantum well SOA has been integrated with InGaAsP/InP based waveguides to build Mach- Zehnder interferometers (MZI). XOR optical logic has been demonstrated at 80 Gb/s using these SOA-MZI structures. XOR operation has been analyzed by solving the rate equation of the SOA, for SOAs with both regular and QD active region. Mach-Zehnder interferometers fabricated using SOA with quantum dot active region (QD-SOA) can be used for XOR operation at 250 Gb/s. Pseudo random bit stream (PRBS) generation using both regular and QD-SOA have been studied and their performance modeled. The model shows QD-SOA based devices can be used to produce PRBS generators that operate near 250 Gb/s.

The semiconductor optical amplifiers (SOAs) are employed in all optical networking and all optical signal processing due to the excellent nonlinearity and high speed. The gain recovery time is the key parameter to describe the response speed of the SOA. The relationship between the gain dynamics and a few operation parameters is obtained in this article. A few simple formula and some simulations are demonstrated, from which, a few methods to improve the response speed of the SOA can be concluded as following, lengthening the active area, or lessening the cross area, increasing the injection current, increasing the probe power, operating with a CW holding beam.

We investigate the effect of intraband carrier dynamics on a nonlinear phase shift induced in a semiconductor optical amplifier (SOA) in terms of its applicability to the Symmetric Mach–Zehnder (SMZ) all-optical switch. Nonlinear phase shifts in an SOA and a passive semiconductor waveguide are compared under control-pulse durations ranging from 3.2 to 0.4 ps. The results show that femtosecond switching with higher efficiency is still possible by using the SOA. We experimentally achieve femtosecond (670 fs), femtojoule (140 fJ) switching with the SOA-based SMZ all-optical switch.

In this paper, we propose to use a semiconductor optical amplifier (SOA) in the optical network unit (ONU) to improve the loss budget in time and wavelength division multiplexed-passive optical network (TWDM-PON) systems. The SOA boosts the upstream signal to increase the output power of the electro-absorption modulated laser (EML) and simultaneously pre-amplifies the downstream signal for sensitivity improvement. The penalty caused by cross gain modulation (XGM) effect is negligible due to the low extinction ratio (ER) of upstream signal and the large wavelength difference between upstream and downstream links. In order to achieve a higher output power, the SOA is driven into its saturation region, where the self-phase modulation (SPM) effect converts the intensity into phase information and realizes on-off-keying (OOK) to phase-shifted-keying (PSK) format conversion. In this way, the pattern effect is eliminated, which releases the requirement of gain-clamping on SOA. To further improve the loss budget of upstream link, an Erbium doped fiber amplifier (EDFA) is used in the optical line terminal (OLT) to pre-amplify the received signal. For the downstream direction, directly modulated laser (DML) is used as the laser source. Taking advantage of its carrier-less characteristic, directly modulated signal shows high tolerance to fiber nonlinearity, which could support a downstream launch power as high as + 16 dBm per channel. In addition, the signal is pre-amplified by the SOA in ONU before being detected, so the sensitivity limitation for downstream link is also removed. As a result, a truly passive symmetric 40-Gb/s TWDM-PON was demonstrated, achieving a link loss budget of 51 dB.