Wavelength Conversion and 2R-Regeneration in Simple Schemes with Semiconductor Optical Amplifiers

Summary form only given. The large scale deployment of all-optical wavelength-division multiplexing (WDM) networks will require signal regeneration due to signal impairments occurring in transmission spans and optical switching nodes/cross-connects. A 2R regenerative interface for non-return to zero (NRZ) signals based on semiconductor optical amplifiers (SOAs) has been proposed by Chiaroni et al. (1997), with clear demonstration of regenerative properties in the amplitude domain. In this paper, a 3R regenerative interface based on the same core structure is described. The core of these interfaces is composed of the association of a wavelength converter (WC) under cross-gain modulation (XGM-WC) and a wavelength converter under cross-phase modulation (XPM-WC), both operating in polarity-inverting mode. Basically, the role of the first WC is to convert power fluctuations into extinction ratio variations and to provide the second WC with a constant input power. The second converter is in turn mainly used to regenerate the extinction ratio thanks to its nonlinear transfer function. This interface in addition limits the accumulation of amplified spontaneous emission (ASE) noise.

Summary form only. The regeneration of data signals is a requirement of future WDM networking schemes. As data streams propagate through a network, they inevitably suffers degradation, chiefly through chromatic dispersion and accumulated noise from erbium doped fibre amplifiers. Some form of regeneration is thus required to combat this degradation. Indeed all-optical 3R (retiming, reshaping and re-amplifying) regeneration has previously been demonstrated in optical loop mirrors and other interferometric arrangements. Here however, we demonstrate for the first time simultaneous all optical 3R regeneration and format conversion in a single, simple integrated device consisting of a semiconductor optical amplifier (SOA) monolithically integrated with a distributed feedback (DFB) laser. A data signal transmitted down 101 km of standard fibre is regenerated all-optically in this laser/amplifier device by gain saturation, converting the data from an NRZ to RZ format. The highly nonlinear transfer function of the SOA/DFB achieves the reshaping function while gain switching the DFB with an extracted clock achieves the retiming.

We analyze the all-optical wavelength converter (AOWC)-based on cross-gain modulation (XGM) in a single-port-coupled (SPC) semiconductor optical amplifier (SOA). A comprehensive dynamic model is developed by considering longitudinal variations of the carrier density, the residual rear-facet reflectivity of the SOA and the wide-band spontaneous noise emission. The numerical simulations for the novel wavelength conversion at 10 Gbit/s are presented based on the model. The extinction ratio (ER), conversion efficiency and pattern effect of the SPC-SOA-based wavelength converters are investigated, respectively. Compared with the traditional scheme of the double-portcoupled (DPC) SOA, the SPC-SOA scheme has better performance. We have obtained that the ER is higher than 10 dB with the pump wavelength turned over 15nm from experiments. The experimental results are in agreement with the simulation results.

All-optical wavelength conversion is demonstrated by using a spectrum-sliced amplified spontaneous emission (ASE) in the semiconductor optical amplifier (SOA) as the probe light and a delay interference configuration. Characteristics of the ASE output in terms of its spectral dependences under the co-propagating injection by a signal light are analyzed theoretically. Analytical expressions for the ASE output in terms of power and phase changes in the wavelength conversion are deduced. An experimental system based on this scheme is built and its operation principles are introduced. System's performances are investigated and several experimental results on measurements of bit error ratios, extinction ratios under different wavelengths and time waveforms as well as eye diagrams are presented.

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.

Linear feedback shift registers (LFSRs) are essential components of communications systems such as pseudo random bit sequence (PRBS) generators, convolutional encoders, scramblers and randomizers [1, 2]. Fig.1 (left) shows a LFSR scheme in which the XOR gate is operated among two tapped bits i and i+k within the shift register. The result bit is fed to the shift register at the last position (n) and the sequence in the register is 1-bit shifted as well. The experimental setup for demonstrating an all-optical LFSR is shown in Fig.1 (middle). The main components of the scheme are an optical buffer [3], a bit selecting circuit, and a XOR gate. The data vector (A) is generated by modulating the continuous wave (CW) λ s with a 10Gbps non-return-to-zero (NRZ) electrical pattern whose period is T P =33.3168µs. Only 5 bits are used as the initial sequence for the shift register. The travelling time of the packet in the buffer is T L =1.85 µs, meaning that up to 18 round trips can be accommodated by the pattern's period T P but one of those should be preserved for erasing the buffer before it is fetched a new sequence. The initial sequence enters the buffer as well as propagates directly to the bit selecting circuit. Every round trip, a new sequence as a replica of the previous one plus the feedback bit comes out. At the last round trip, a pump signal inserted into the buffer through the circulator will strongly reduce the gain of the semiconductor optical amplifier (SOA). Both the circulating packet and the accumulating in-band amplified spontaneous emission (ASE) noise will be suppressed. The buffer is now completely cleaned for a new cycle. The first and the last bits of the 1-bit shifted sequence are selected for the XOR operation. Referring to Fig.1 (left), i=1 and k=4, n=5. With this setup, the bit selecting circuit requires 2 clock signals (C) for tap the proper bits from the incoming sequence. The sampling process is done by exploiting four-wave-mixing (FWM) in a 250m-long highly nonlinear fiber (HNLF). Since at every round trip the sequence must be 1-bit shifted, the sampling clocks must be accordingly delayed for 1 bit, resulting the clock period of T C =T L +T B where T B =100ps is the bit time. The outputs of the bit selecting circuit are fed as pump signals to port a and d of a SOA Mach Zehnder interferometer (MZI). They are synchronized with a clock at λ s used as the probe signal (F) at port b. The output of the XOR gate is then fed back into the buffer. The feedback bit should be precisely allocated at the end of the packet circulating inside the buffer, meaning that the feedback time should be T F =T L +T B +T seq in which T seq is the original sequence duration and equal to 500ps in our setup. Fig.1 (right) depicts the results at the 10th round trip for two different input initial sequences (10001) and (10011) taken at point (B). The equalization between bits in the sequence and the quality of the pulses is strongly limited by the accumulating ASE noise inside the buffer, the long fiber-based structure and the bit's polarization. The fiber-based setup also causes misalignment between the clocks, spoils the essential synchronism and degrades the performance of the FWM and the XOR operation. One possible solution for solving all of these problems and hence improving the performance is the integration approach. The bit selection could be implemented exploiting nonlinear effects into an SOA or a periodic-poled lithium niobate (PPLN) instead of using a long HLNF. Moreover the extinction ratio of the pulses at the SOA-MZI XOR output (<13.5dB) could be improved, e.g. with a saturable absorber at its output, thus increasing the number of allowed round trips into the shift register and reducing the noise on the final bit sequence.

Techniques for all optical wavelength conversion and regeneration using semiconductor optical amplifiers are reviewed. In SOA based optical signal processing two (or more) signals are launched into the SOA where they interact due to the nonlinear properties of the active region in gain saturation, allowing the amplitude and phase of one optical signal to be controlled by another optical signal. Depending on the input conditions and topology, this can result in cross gain modulation, cross phase modulation or four wave mixing.

In this paper, we propose and analyze a wavelength conversion using an cross-gain modulation (XGM) with two semiconductor optical amplifiers and CW-holding beam for improvement of wavelength conversion speed and frequency chirp characteristics. Wavelength conversion characteristics of the cascaded XGM with a CW-holding beam are investigated in time domain. From the simulation, we have observed that the proposed scheme provides a non-inverted efficient wavelength conversion and frequency chirp reduction at higher bit rate operation.

We study the impact of Amplified Spontaneous Emission (ASE) noise on a Semiconductor Optical Amplifier (SOA)-based optical pulse delay discriminator and SOA-based distance ranger. Our experiments show that ASE reduces the sensitivity of these SOA-based devices and we confirm this finding by carrying out extensive simulations by modeling the ASE response of SOAs. The simulation results, obtained by numerical integration of these equations in MATLAB^TM using the NIMROD^TM portal, are in qualitative agreement with experimental results.

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.

Development of modern optical fiber networks puts an increasing demand on the optical hardware. All-optical signal processing components enable the highest switching rates and allow all-optical regeneration of pulse streams without converting the optical signal into electrical current. The subject of our research is self-switching in a Mach-Zehnder interferometer (MZI) and its applications in optical telecommunication networks. In this device, light injected in one of the input ports is unequally distributed over the two interferometer arms. Due to the intensity dependent refractive index in the interferometer arms, there can be a nonlinear phase shift induced between the optical signals of unequal intensities. The two signals therefore interfere destructively or constructively depending on the input power: in this way we obtain nonlinear self-switching. Two mechanisms of nonlinear phase shifting were considered: active, based on semiconductor optical amplifiers (SOAs), and passive, based on quantum dots (QDs). Interferometers of two types were developed: 2-to-2 (two input ports and two output ports) and 2-to-1 (two input ports and one output port). The 2-to-2 SOA-MZIs based on self-switching have been investigated for two applications. One of them is the pattern effect compensator. If SOAs are employed for all-optical signal amplification, e.g. in optical access networks, an unwanted pulse distortion (known also as the pattern effect) takes place, as a result of the SOA gain saturation. Our component allows pattern-free amplification of the optical signals at bitrates up to 20 Gb/s. At 10 Gb/s it shows an extended input power range (up to 7 dB improvement) and comparable gain, which makes it suitable to be used as an optical amplifier. Another application of the 2-to-2 SOA-MZI is a 2R-regenerator. Optical amplifiers used in long distance optical links add noise to the optical signal, causing signal degradation. The signal can be regenerated by passing it through an optical gate with a nonlinear transfer function. The 2-to-2 SOA-MZI has such a nonlinear transfer function. The regeneration capabilities were demonstrated at 2.5 Gb/s by an improvement of the receiver sensitivity of about 2.5 dB. The dynamic characterization of the chips was carried out in a close cooperation with the research group COM at the Technical University of Denmark within the ePIXnet "network of excellence". The 2-to-1 MZI based on self-switching can be used as a low-loss optical combiner. An essential function in optical fiber networks is the combining of optical signals. A serious disadvantage of the conventional type of combiners used in the networks is that they let only half of the power (3 dB) through. In order to compensate for this loss, passive optical combiners are often used in combination with an in-line SOA. The first realization of the low-loss optical combiner uses SOAs as active phase shifters. Such an active low-loss combiner shows an improvement of transmission of over 2 dB compared to a conventional combiner with an in-line SOA. It is therefore expected that the output optical signal-to-noise ratio of the self-switching SOA-MZI is more than 2 dB better than that of a conventional combiner with an in-line SOA. While for the pattern effect compensator and the 2R-regenerator SOAs are used not only for inducing the nonlinear phase shift, but also for sufficient amplification, for the low-loss optical combiner the preferred nonlinear effects should be passive: combination of the signal is a passive function. Therefore, the second realization is based on a novel material, quantum dots. QDs provide improved all-optical nonlinearities resulting in a very small switching energy and large refractive index changes. Such a passive 2-to-1 QD-MZI based on self-switching showed an improvement up to 1.7 dB with respect to a conventional combiner. These improvements have a huge effect on e.g. the power budget in passive optical networks, where a large number of splitting stages are required. The Mach-Zehnder interferometers were realized in the InP/InGaAsP semiconductor material system, which is perfectly suitable for the integration of the photonic integrated circuits for the telecommunication applications. In order to realize both active components (such as e.g. SOAs) and passive components (such as e.g. waveguides, couplers), an active-passive integration technique was applied. This integration was realized in a close cooperation between JDS Uniphase Eindhoven and the COBRA Research Institute. It employs a three-step metal-organic vapor-phase epitaxy regrowth process. The quantum dot material was grown within the COBRA Research Institute. Our MZIs use a ridge waveguide design, for which a reactive ion etching process was developed in the COBRA cleanroom. As a result of photonic integration our integrated Mach-Zehnder interferometers have very small dimensions: less than a square millimeter.

2R regeneration is proposed with an integrated Mach-Zehnder interferometer containing a monomodal semiconductor optical amplifier (SOA) and an SOA with a 1 /spl times/ 1 multimode interference (MMI) coupler-SOA in the arms. Simulations and measurement of the static transfer function show a digital shape with a high extinction ratio improvement.

We develop a complete large-signal dynamic model of semiconductor optical amplifier (SOA)-based wavelength converters using cross-gain modulation (XGM) and cross-phase modulation (XPM). The large-signal dynamic model has been implemented using the modified transfer matrix method (TMM). This model takes into account longitudinal variations of carrier-induced refractive index, n-parameter, internal reflection, photon density, and amplified spontaneous emission noise at each small section. Therefore, we can accurately estimate frequency chirping, optical pulse patterns, and the extinction ratio as well as the dynamic characteristics of SOA's. Frequency chirping and the extinction ratio have been calculated for various parameters such as signal power and wavelength, CW power and wavelength, and facet reflectivity. Modulation bandwidth, defined by the 3-dB drop as the eye opening ratio (EOR) is investigated with current density, confinement factor, and cavity length.

We have developed a theory of high-speed operation of quantum dot (QD) semiconductor optical amplifiers (SOAs), and showed that pattern-effect-free amplification of pulse trains, cross gain modulation (XGM) and cross phase modulation (XPM) can take place in QD SOA in the regime with maximum gain. Formulas, which relate the maximum bit-rate for the pattern-effect-free operation and the average SOA output power to the SOA pump current density, were derived. XGM without pattern effect can be realized in the regime with maximum gain due to spectral hole burning effects. Possibility of ultrafast frequency conversion and demultiplexing of data pulse streams through this nonlinearity is illustrated. Expression for the nonlinear refractive index η nl due to spectral hole burning in QD structure was obtained. The value of η nl in QD SOAs can be by 4-5 orders larger than η nl in silica; and efficient ultrafast XPM without pattern effects can be carried out in QD SOA through this nonlinearity. In whole, usage of the regime with maximum gain in QD SOAs can lead to development of new generation of high-speed devices for ultrafast optical processing and communications.

We present a novel wavelength conversion scheme that exploits Cross Gain Modulation (XGM) in Semiconductor Optical Amplifiers (SOAs) without overall pattern effects. This technique uses two signals with reversed intensity modulation and different wavelength to exploit propagation and gain compression dynamics in gain- saturated SOAs at almost constant overall input power. The resulting Cross Gain Compression (XGC) mechanism between co-propagating waves can lead to all-optical waveform reshaping. By using this technique we experimentally demonstrate enhanced wavelength conversion by XGM at 10 Gb/s.