cw assist light in the gain region or at transparency wavelength of a semiconductor optical amplifier (SOA) is introduced into an SOA-based clock recovery circuit to overcome the pattern effect. A numerical model considering intraband effects in the SOA, such as carrier heating and spectrum hole burning, is constructed to study the pattern effect on clock recovery. The pattern effect reductions for different injection powers and wavelengths of cw assist light and different bias currents of the SOA are simulated to optimize this system. In an experiment, high-quality clock pulses were recovered from a (231–1)-pseudorandom-bit sequence (PRBS) at 20 and 40 Gbit/s by using cw assist light. Simulation and experimental results show that a lower pattern effect can be effectively obtained by using a cw assist light with highest power near the highest-gain wavelength when the SOA works at a high current level. In this respect, cw assist light in the gain region is superior to light at the transparency wavelength. This scheme can be applied to SOA-based all-optical clock recovery and other all-optical signal processing for pattern effect reduction.
Future network will include wavelength division multiplexing (WDM) and optical time division multiplexing (OTDM) technologies. All-optical format conversion between their respective preferable data formats, non-return-to-zero (NRZ) and return-to-zero (RZ), may become an important technology. In this paper, 10Gbit/s all-optical NRZ-to-RZ conversion is demonstrated based on terahertz optical asymmetric demultiplexer (TOAD) using clock all-optically recovered from the NRZ signal for the first time. The clock component is enhanced in an SOA and the pseudo-return-to-zero (PRZ) signal is filtered. The PRZ signal is input into an injection mode-locked fiber ring laser for clock recovery. The recovered clock and the NRZ signal are input into TOAD as pump signal and probe signal, respectively, and format conversion is performed. The quality of the converted RZ signal is determined by that of the recovered clock and the NRZ signal, whereas hardly influenced by gain recovery time of the SOA. In the experimental demonstration, the obtained RZ signal has an extinction ratio of 8.7dB and low pattern dependency. After conversion, the spectrum broadens obviously and shows multimode structure with spectrum interval of 0.08nm, which matches with the bit rate 10Gbit/s. Furthermore, this format conversion method has some tolerance on the pattern dependency of the clock signal.
A numerical model considering carrier heating and spectrum hole burning effects is given to study the pattern effect of clock recovery based on injection mode-locked SOA fiber laser, which is applicable for ultrashort pulses with several picoseconds pulsewidth. The reduction of pattern effect in the recovered clock at 40GHz successfully demonstrated by using a CW assist light at the transparency wavelength or gain region to SOA. The obtained minimum recovery time is less than 10ps, amplitude fluctuation will reduce from 64% to 5% and timing jitter will reduce from more than 3ps to 0.5ps. Moreover, the dependence of pattern effect on the power and wavelength of CW assist light and the bias current of SOA is analyzed, which is a useful guide to optimize this system. In order to get lower pattern effect, the required injection power of CW light at transparency wavelength will be higher than that of a wavelength in gain region, the minimum pattern effect occurs at a wavelength around the gain maximum. In this sense, a CW assist light in gain region is superior to the assist light at transparency wavelength. This scheme is an attractive method for SOA-based clock recovery and other all-optical signal processing.
Frame clock is useful for packet processing such as header detection and payload demultiplexing. A novel all-optical frame clock recovery scheme based on "intensity reshaper" and mode-locked semiconductor fiber ring laser is demonstrated. The "intensity reshaper" including a polarization controller and a polarizer is the key element to realize frame clock recovery from equal-amplitude even-multiplexed OTDM signals. In theory, a mathematical expression is given to analyze the intensity of harmonic of clock-frequency component. The relative intensity of each clock-frequency component will change with the alterative angle caused by adjusting the PC in the "intensity reshaper", so the desirable clock-frequency component can be enhanced, which is helpful for clock recovery. Moreover, the intensity of harmonic of clock-frequency component is also related to the pulse amplitude, width and period in the multiplexed data. In experiment, 2.5GHz frame clock is extracted from even-multiplexed 4x2.5GHz and 8x2.5GHz OTDM signals respectively. At the same time, bit clock is also recovered by using this scheme. The extracted clock pulses have several desirable features such as low timing jitter, broad wavelength tuning range and polarization independence. This scheme simplifies signal generation and propagation in OTDM systems, which can be applied to clock recovery in high-speed OTDM network.
A theoretical model of clock recovery based on SOA fiber laser is presented, through which a complete numerical analysis about the clock characteristics at 40Gbit/s rate is given, which is an effective guide for experiment and necessary to optimize the system performance. The crucial parameters that determine the pulse width, energy and frequency chirp of the recovered clock pulses are investigated, including the pulse width and energy of the external data signals, the small signal gain and carrier lifetime of SOA, and the cavity loss. Injection mode-locked SOA fiber laser is not only suitable for bit clock recovery but also for frame clock recovery. A frame clock recovery scheme based on SOA is proposed, which requires the data packet to have a fixed header marker to generate a stronger frame clock frequency component. Simulation results show a high-quality frame clock can be obtained using this scheme. In experiment, bit clock recovery is realized at bit rates up to 20Gb/s using injection mode-locked SOA fiber laser. In addition, 2.5GHz frame clock is also extracted from 8x2.5Gb/s and 16x2.5Gb/s OTDM signals. The recovered clock pulses are wavelength tunable and very stable, which can be used for high-speed all-optical signal processing.
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