Colourless adaptively modulated optical OFDM transmitters using SOAs as intensity modulators

Detailed numerical investigations are undertaken of wavelength reused bidirectional transmission of adaptively modulated optical OFDM (AMOOFDM) signals over a single SMF in a colorless WDM-PON incorporating a semiconductor optical amplifier (SOA) intensity modulator and a reflective SOA (RSOA) intensity modulator in the optical line termination and optical network unit, respectively. A comprehensive theoretical model describing the performance of such network scenarios is, for the first time, developed, taking into account dynamic optical characteristics of SOA and RSOA intensity modulators as well as the effects of Rayleigh backscattering (RB) and residual downstream signal-induced crosstalk. The developed model is rigorously verified experimentally in RSOA-based real-time end-to-end OOFDM systems at 7.5 Gb/s. It is shown that the RB noise and crosstalk effects are dominant factors limiting the maximum achievable downstream and upstream transmission performance. Under optimum SOA and RSOA operating conditions as well as practical downstream and upstream optical launch powers, 10 Gb/s downstream and 6 Gb/s upstream over 40 km SMF transmissions of conventional double sideband AMOOFDM signals are feasible without utilizing in-line optical amplification and chromatic dispersion compensation. In particular, the aforementioned transmission performance can be improved to 23 Gb/s downstream and 8 Gb/s upstream over 40 km SMFs when single sideband subcarrier modulation is adopted in the downstream systems.

Band-gap shrinkage is an important effect in semiconductor lasers and optical amplifiers. In the former it leads to an increase in the lasing wavelength and in the latter an increase in the gain peak wavelength as the bias current is increased. The most common model used for carrier-density dependent band-gap shrinkage is a cube root dependency on carrier density, which is strictly only true for high carrier densities and low temperatures. This simple model, involves a material constant which is treated as a fitting parameter. Strained InGaAsP material is commonly used to fabricate polarization insensitive semiconductor optical amplifiers (SOAs). Most mathematical models for SOAs use the cube root bandgap shrinkage model. However, because SOAs are often operated over a wide range of drive currents and input optical powers leading to large variations in carrier density along the amplifier length, for improved model accuracy it is preferable to use band-gap shrinkage calculated from knowledge of the material bandstructure. In this letter the carrier density dependent band-gap shrinkage for strained InGaAsP is calculated by using detailed non-parabolic conduction and valence band models. The shrinkage dependency on temperature and both tensile and compressive strain is investigated and compared to the cube root model, for which it shows significant deviation. A simple power model, showing an almost square-root dependency, is derived for carrier densities in the range usually encountered in InGaAsP laser diodes and SOAs.

During the last few years, large-scale efforts towards realizing high-photonic inte‐ gration densities have put SOAs in the spotlight once again. Hence, the need to de‐ velop a complete framework for SOA-induced signal distortion to accurately evaluate a system’s performance has now become evident. To cope with this de‐ mand, we present a detailed theoretical and experimental investigation of the deter‐ ministic timing jitter and the pulse peak power equalization of SOA-amplified intensity-modulated optical pulses. The deterministic timing jitter model relies on the pulse mean arrival time estimation and its analytic formula reveals an approxi‐ mate linear relationship between the deterministic timing jitter and the logarithmic values of intensity modulation when the SOA gain recovery time is faster than the pulse period. The theoretical analysis also arrives at an analytic expression for the intensity modulation reduction (IMR), which clearly elucidates the pulse peak pow‐ er equalization mechanism of SOA. The IMR analysis shows that the output intensi‐ ty modulation depth is linearly related to the respective input modulation depth of the optical pulses when the gain recovery time is faster than the pulse period. This novel theoretical platform provides a qualitative and quantitative insight into the SOA performance in case of intensity-modulated 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.

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

Summary form only given. An overview is presented of the progress in design and characteristics of semiconductor optical amplifiers (SOA) for 1310, 1400 and 1550 nm operation in fiber optical transmission systems. The technology and performance of SOAs will be discussed with respect to optical gain, noise figure, saturation output power and polarization (in-) dependent behavior. The use of both strained quantum wells and bulk tensile material will be discussed in view of obtaining polarization independent behavior. Gain clamping in SOAs has been shown to improve the linearity performance especially for analogue signal transmission. The SOA technology has been demonstrated for optical pre-amplification, in-line application and optical booster applications. DWDM experiments using SOAs in up to 20 Gbit/s systems in short- and medium-distance transmission will be highlighted. Also, SOAs have been used in optical demultiplexing from 40 Gbit/s down to 10 Gbit/s RZ signals using its phase properties and in integrated photonic devices like multiwavelength lasers.

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.

Fiber optic amplifier is a field of new growing interest owing to its application to the optical metro and access networks for amplifying a weak optical signal. In order to design short-fiber-length and broadband optical amplifier, a new amplified fiber with semiconductor inner cladding has been presented. The semiconductor optical amplifier (SOA) is usually pumped by electric currents, but an optical pump is used to the semiconductor inner cladding fiber amplifier in our work. The rate equation is derived in the optical pumped amplifier, and the properties of the amplifier are analyzed. Comparison with conventional optical pumped amplifier, the semiconductor inner cladding fiber amplifier can be pumped by a wide-band optical pumped power, and high pump efficiency can be designed.

The development of semiconductor optical amplifiers (SOAs) happened soon after the invention of the semiconductor laser. A SOA is very similar to a semiconductor laser without (or with negligible) optical facet feedback. An incoming signal injected into the SOA propagates along its optical waveguide and is amplified by stimulated emission. The optical gain is achieved by inverting the carrier population in the active region via electrical pumping. During the 1990s due to the development of the erbium doped optical amplifier (EDFA) the popularity of the SOA as a linear amplifier declined as the EDFAs provided higher gain without the detrimental nonlinearities associated with an SOA.

We proposed a novel extended semiconductor optical amplifier (SOA) model for applications in femtosecond pulse transmission, in which the various ultra-fast nonlinear effects, such as gain dispersion, group velocity dispersion (GVD) were taken into account. We use the extended SOA model to analyze the refractive index in response to variation of the input current and the length of SOA. In addition, the impact of carrier density pulsation and carrier heating on the refractive index dynamics of SOA was discussed respectively. The refraction index dynamics that occur in bulk InGaAsP SOA in response to femtosecond pulse (Full width at half maximum is 200fs) are studied numerically. We use the alpha factors (α-factors) to calculate the phase shift from the gain variations, which are related to the refractive index changes. We have considered the effects of carrier heating (CH), and spectrum hole burning (SHB) to analyze the impact of intra-band carrier dynamics on nonlinear refraction index. We also consider the impact of the length of SOA and operation condition on the refraction dynamics.

A time-differential (TD) Brillouin optical correlation domain analysis (BOCDA) sensor system was applied to measure the Brillouin gain spectrum of a 1 km long sensing optical fiber. The optical delay line used in all BOCDA measurement systems was eliminated in the TD-BOCDA system by using a bit-delayed modulation relationship between the probe and pump lightwaves. These lightwaves were phase modulated using 216-1 pseudo-random binary sequence codes at 5 Gbps. A 2 cm dispersion-shifted fiber placed at the end of the 1 km optical fiber was distinctly identified by the Brillouin frequency extracted from the Brillouin gain spectrum measurement. To investigate the measurement stability of the TD-BOCDA system, experiments were conducted under two different pumping conditions. A semiconductor optical amplifier (SOA) and an intensity modulator (MOD) were compared for the pump chopper used in the TD-BOCDA system to detect the extinction ratio of the pump and the resulting noise in the Brillouin gain measurement. The stability of the Brillouin frequency measurement from the Brillouin gain spectrum in the TD-BOCDA system was investigated by increasing the average value of the measurement using either the SOA or MOD. The repeated-measurement deviation of the system with the SOA was only half of the deviation observed in the system with the MOD. The performance of TD-BOCDA is equivalent to or better than that of conventional BOCDAs in terms of measurement reliability. Moreover, TD-BOCDA is free from the drawbacks of traditional BOCDA, which uses time-delayed fibers and varies the bit rates.

Semiconductor optical amplifier (SOA) is widely used for power amplification in O-band, particularly for passive optical networks (PONs) which can greatly benefit its advantages of simple structure, low power consumption and integrability with photonics circuits. However, the annoying nonlinear pattern effect degrades system performance when the SOA is needed as a pre-amplifier in PONs. Conventional solutions for pattern effect mitigation are either based on optical filtering or gain clamping. They are not simple or sufficiently flexible for practical deployment. Neural network (NN) has been demonstrated for impairment compensation in optical communications thanks to its powerful nonlinear fitting ability. In this paper, for the first time, NN-based equalizer is proposed to mitigate the SOA pattern effect for 50G PON with intensity modulation and direct detection. The experimental results confirm that the NN-based equalizer can effectively mitigate the SOA nonlinear pattern effect and significantly improve the dynamic range of receiver, achieving 29-dB power budget with the FEC limit at 1e-2. Moreover, the well-trained NN model in the receiver side can be directly placed at the transmitter in the optical line terminal to pre-equalize the signal for transmission so as to simplify digital signal processing in the optical network unit.

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.

Summary form only given. Summary form only given. Amplified spontaneous emission (ASE) noise is a major limit to the performance of modern day optical transmission systems. A way to alleviate system impairments due to ASE is the insertion of bandpass filters along the line. Soliton pulses have shown excellent stability to in-line narrow-band filtering, and also to sliding of the filter center frequency along the line. The dynamics of soliton signals in the presence of both fixed and sliding filters has been the subject of detailed studies. However, little attention has been given to the dynamics of the ASE growth in the presence of in-line filters. The analysis of the ASE noise growth in a chain of N amplifiers in the presence of in-line filters is the subject of this work. This analysis has been extended to the case of semiconductor optical amplifiers (SOA), where the noise growth, combined with the fast saturation of the amplifier gain, are more effective in degrading system performance. The analytical results have been compared with numerical simulations. Good agreement has been obtained for in-line erbium amplifiers, also considering a high inversion factor. For SOAs, on the other hand, the more complex amplifier dynamics makes the theory quantitatively less accurate, as expected. Even in this case, however, the analytical formulas can be taken as useful guidelines for the design of optical systems.

A semiconductor optical amplifier (SOA) is a versatile optical component with multiple functions, e.g., optical amplification, detection, and modulation.1 SOAs with these functions have been applied to transmission systems as full-duplex transceivers-’ and as transceivers with optical gain.3 In this paper we demonstrate still another application of SOAs as an important network element—an add—drop multiplexer (ADM). The ADM function of an SOA was implemented in a digitally modulated subcarrier-multiplexed system with the general configuration shown in Fig. 1(a). This dual-bus system can be used for local- area-network interconnections or linear SONET digital-loop- carrier ADM applications.