Optical regeneration technique using an electro-absorption modulator (EAM) is reviewed. Simple 3R optical regeneration using an EAM was proposed and verified at 20 Gbit/s. The optical nonlinearities including cross-absorption modulation (XAM) and cross-phase modulation (XPM) induced in an EAM were quantitatively characterized by experiment. High bit-rate 2R type all-optical regeneration (wavelength conversion) at 100 Gbit/s was demonstrated by an EAM in conjunction with a delayed interferometer (DI) with required optical pulse energy of 1.5 pJ. It was verified that the operable bandwidth of the EAM-DI wavelength converter at 40 Gbit/s covered almost full range of C-band without tuning operation conditions.

This paper describes recent progress in research on wavelength converters that employ quasi-phase-matched LiNbO3 (QPM-LN) waveguides. The basic structure and operating principle of these devices are presented. The conversion efficiency in difference frequency generation (DFG), second harmonic generation (SHG) and an SHG/DFG cascade scheme are explained. Device fabrication technologies such as periodic poling, and those used for annealed proton-exchanged (APE) waveguides, and direct bonded waveguides are introduced. An APE waveguide is used to demonstrate the wavelength conversion of broadband (> 1 Tbit/s) WDM signals. The low penalty conversion of high-speed (40 Gbit/s) based WDM signals is also reported. Excellent resistance to photorefractive damage in a direct bonded waveguide is presented. This high level of resistance enabled highly efficient wavelength conversion. A new design concept is introduced for a multiple QPM device based on the continuous phase modulation of a periodically poled structure. This multiple QPM device enables the variable wavelength conversion of WDM signals. High-speed wavelength switching between ITU-T grid wavelengths using a finely tuned multiple QPM device is also reported. QPM-LN based wavelength converters have several advantages, including the ability to convert high-speed signals of 1 THz or greater, no signal-to-noise (S/N) ratio degradation, no modulation format dependence, and they are capable of the simultaneous conversion of broadband WDM channels. They will therefore be key devices in future photonic networks.

Ultrafast all-optical switching was experimentally demonstrated using four-wave mixing in an SOA. Two pump pulses with different wavelengths and timings were used for 12 switching. The cross-correlation measurements of FWM signals using a short reference pulse show the high-speed switching capability for wavelength routing in OTDM networks.

All optical regenerations or wavelength conversions using SOA-based polarization discriminated switch injected by an assist light were investigated. First of all, cross gain modulation (XGM) and cross phase modulation (XPM) in a SOA injected by an external assist light were quantitatively analyzed. A simple measurement technique of XGM and XPM was shown to confirm that the injection of assist light could reduce a gain recovery time with some sacrifice for XGM and XPM efficiency. All-optical 3R regeneration using two-stage SOA-based polarization discriminated switch at 40 Gbit/s and its tolerances for some degradation against intensity deviation and optical signal-to-noise ratio (OSNR) were also shown. Finally, regeneration capability was evaluated through a dispersion shifted fiber (DSF)-based re-circulating loop transmission experiment. Those results indicate that the SOA-based polarization discriminated switch is a promising candidate for all-optical regenerator from the practical point of view.

In this paper, we consider QoS-guaranteed wavelength allocation for WDM networks with limited-range wavelength conversion. In the wavelength allocation, the pre-determined number of wavelengths are allocated to each QoS class depending on the required loss probability. Moreover, we consider two wavelength selection rules and three combinations of the rules. We analyze the connection loss probability of each QoS class for a single link using continuous-time Markov chain. We also investigate the connection loss probability for a uni-directional ring network by simulation. In numerical examples, we compare connection loss probabilities for three combinations of selection rules and show how each combination of selection rules affects the connection loss probability of each QoS class. Furthermore, we show how wavelength conversion capability affects the connection loss probability. It is shown that the proposed allocation with appropriate wavelength selection rule is effective for QoS provisioning when the number of wavelengths is large. We also show the effective combination of wavelength selection rules for the case with small wavelength conversion capability.

Wavelength conversion using the cross-gain modulation (XGM) of amplified spontaneous emission (ASE) in a traveling-wave type semiconductor optical amplifier (TW-SOA) is theoretically studied. Taking into account the spatial and temporal variations of carrier density along the SOA length, output signal and converted ASE waveforms are analyzed. We also reveal the dependency of the signal and converted ASE waveforms on input signal power and repetition frequency, and confirm that numerical analyses well agree with the experimental results. Finally we qualitatively clarify the way to improve frequency response by simulating eye-diagrams for long SOAs and assist light pumping for the first time.

This paper provides an overview on efficient algorithms for multicasting in optical networks supported by Wavelength Division Multiplexing (WDM) with limited wavelength conversion. We classify the multicast problems according to off-line and on-line in both reliable and unreliable networks. In each problem class, we present efficient algorithms for multicast and multiple multicast and show their performance. We also present efficient schemes for dynamic multicast group membership updating. We conclude the paper by showing possible extension of the presented algorithms for QoS provision.

Using less wavelengths to serve more communication channels is one of the primary goals in the design of WDM networks. By installing wavelength converters at some nodes in a network, the number of wavelengths needed can be reduced. It has been observed that the more converters installed in a network, the less number of wavelengths is needed, given the same network load. In this paper, we study the relationship between the number of converters and the number of wavelengths needed in a system, and propose a suite of theories and results on how to place the minimal number of converters in the system so that the number of wavelengths W is at most a constant α times the maximal link load L (i.e., W α L), where α = 3/2 or 5/3. The results show a significant saving of converters in networks of both special topologies and general topology.

Wavelength converters are usually used for improving the performance of WDM optical networks. From the viewpoint of network economics and current technologies, the wavelength converters with a limited conversion range are necessary to be used sparsely in real applications. However, there have been little efforts for developing wavelength assignment algorithm that achieves a guaranteed high performance with either limited or sparse wavelength conversion. In this paper, we propose a new wavelength assignment algorithm that can be applied to both sparse and limited wavelength conversion. Through the results of simulation program, we show that the proposed algorithm outperforms other ones presented until now.

The effectiveness and possible applications of all-optical signal processing using highly-nonlinear dispersion-shifted fibers (HNL-DSFs) are described. Transparent and simultaneous processings of multi-channels WDM signal are key features of optical fiber processors. Simultaneous wavelength conversion of 3210 Gb/s WDM signal by four-wave mixing, all-optical 3R regeneration of 220 Gb/s WDM signal using nonlinear loop mirrors, and simultaneous recovery of 2020 GHz WDM optical clocks by supercontinuum were successfully demonstrated using HNL-DSFs, and possible applications of ultra-fast and multi-channel processing in future photonic networks are discussed.

The effectiveness and possible applications of all-optical signal processing using highly-nonlinear dispersion-shifted fibers (HNL-DSFs) are described. Transparent and simultaneous processings of multi-channels WDM signal are key features of optical fiber processors. Simultaneous wavelength conversion of 3210 Gb/s WDM signal by four-wave mixing, all-optical 3R regeneration of 220 Gb/s WDM signal using nonlinear loop mirrors, and simultaneous recovery of 2020 GHz WDM optical clocks by supercontinuum were successfully demonstrated using HNL-DSFs, and possible applications of ultra-fast and multi-channel processing in future photonic networks are discussed.

This paper describes a simple polarization-independent wavelength conversion method using degenerated four-wave mixing (FWM) in single-mode fibers pumped with cross-polarized high frequency, saw-tooth pulses from a single pump source. Successful polarization-independent wavelength conversion is experimentally confirmed with less than 12% and 5.6% variation using a gain-switched LD pumping and a mode-locked fiber laser pumping, respectively. We clarify that the interference effect between two orthogonal pump pulses must be taken into account to achieve a good polarization-insensitive operation, since even the small pulse edges bring about the large polarization fluctuations when they are interfered. Furthermore, it is reveal that the shorter pump pulse broadens its own spectrum due to the self-phase modulation in fibers, resulting in poor FWM efficiency. Finally, possibility of high-speed operation is discussed taking into account the pump pulse conditions.

We fabricated a 1.55-µm polarization insensitive Michelson interferometric wavelength converter (MI-WC). The MI-WC consists of a two-channel spot-size converter integrated semiconductor optical amplifier (SS-SOA) on a planar lightwave circuit (PLC) platform. Clear eye opening and no power penalty in the back-to-back condition were obtained at 10 Gb/s modulation. We also confirmed the polarization insensitive operation on the input signal. Moreover, for an application of the MI-WC to DWDM networks, we demonstrated the selective wavelength conversion of 2.5 G/s optical packets from Fabry-Perot laser diode (FP-LD) light to four ITU-T grid wavelengths. We confirmed the good feasibility of this technique for use in DWDM networks. The wavelength conversion we describe here is indispensable for future all-optical networks, in which optical signal sources without wavelength control will be used at user-end terminals.

This paper describes a simple polarization-independent wavelength conversion method using degenerated four-wave mixing (FWM) in single-mode fibers pumped with cross-polarized high frequency, saw-tooth pulses from a single pump source. Successful polarization-independent wavelength conversion is experimentally confirmed with less than 12% and 5.6% variation using a gain-switched LD pumping and a mode-locked fiber laser pumping, respectively. We clarify that the interference effect between two orthogonal pump pulses must be taken into account to achieve a good polarization-insensitive operation, since even the small pulse edges bring about the large polarization fluctuations when they are interfered. Furthermore, it is reveal that the shorter pump pulse broadens its own spectrum due to the self-phase modulation in fibers, resulting in poor FWM efficiency. Finally, possibility of high-speed operation is discussed taking into account the pump pulse conditions.

We fabricated a 1.55-µm polarization insensitive Michelson interferometric wavelength converter (MI-WC). The MI-WC consists of a two-channel spot-size converter integrated semiconductor optical amplifier (SS-SOA) on a planar lightwave circuit (PLC) platform. Clear eye opening and no power penalty in the back-to-back condition were obtained at 10 Gb/s modulation. We also confirmed the polarization insensitive operation on the input signal. Moreover, for an application of the MI-WC to DWDM networks, we demonstrated the selective wavelength conversion of 2.5 G/s optical packets from Fabry-Perot laser diode (FP-LD) light to four ITU-T grid wavelengths. We confirmed the good feasibility of this technique for use in DWDM networks. The wavelength conversion we describe here is indispensable for future all-optical networks, in which optical signal sources without wavelength control will be used at user-end terminals.

In this paper, two distinct optical network design approaches, namely mesh and multi-ring, for survivable WDM networks are investigated. The main objective is to compare these two design approaches in terms of network costs so that their merits in practical environments can be identified. In the mesh network design, a new mathematical model based on integer liner programming (ILP) and a heuristic algorithm are presented for achieving a minimal cost network design. In the multi-ring network design, a heuristic algorithm that can be applied to large network problems is proposed. The influence of wavelength conversion and the number of wavelengths multiplexed in a fiber on system designs are also discussed. Based on the simulation results, the redundancy quantities required for full protection in multi-ring approach are significantly larger in comparison to the minimal cost mesh counterpart.

Fiber four-wave mixing (FWM) based parametric wavelength conversion experiment is demonstrated. Over 91nm multi-channel simultaneous conversion is achieved. The bandwidth is to our knowledge, the broadest value of the published results. We shall argue that the method to realize the broadband wavelength conversion. Efficiency and/or bandwidth of the wavelength conversion is degraded mainly by the following obstacles, (a) inhomogeneity of the chromatic dispersion distribution along the fiber, (b) mismatch of the states of polarization (SOP) between pump and signals and (c) bandwidth limitation from coherence length. We discuss that an extremely short high-nonlinear fiber should overcome the above three obstacles. Furthermore we comment on the higher-order dispersion and also the influence of the stimulated Brillouin scattering (SBS). High-nonlinearity dispersion-shifted fiber (HNL-DSF) is a promising solution to generate the FWM efficiently in spite of the short length usage. We develop and fabricate HNL-DSF by the vapor-phase axial deposition method. Nonlinear coefficient of the fiber is 13.8 W-1km-1. We measure the conversion efficiency spectra of the four HNL-DSFs with different lengths. Length of each fiber is 24.5 km, 1.2 km, 200 m and 100 m respectively. It is shown that conversion bandwidth increases monotonically as the fiber length decreases. The result apparently proves the advantage of the extremely short fiber.

Simultaneous wavelength conversion of multi-WDM channels is expected to be a key technique in near-future networks. In this paper, 4-channel wavelength conversion using four-wave mixing (FWM) in a hybrid wavelength selector is successfully demonstrated. The wavelength selector consists of two four-channel spot-size-converter-integrated semiconductor optical amplifier (SS-SOA) gate arrays on a planar-lightwave-circuit (PLC) platform and two PLC-arrayed-waveguide-gratings (AWGs). As the wavelength selector has an individual SS-SOA for the wavelength conversion of each channel, there is negligible interference between channels. Four WDM channels with an 2.5 Gb/s modulation were converted from 1555 to 1575 nm. Clear eye openings and only a small power penalty of less than 0.5 dB were observed. The receiver sensitivity was -31 dBm at a bit error rate (BER) of 10-9.

We present three-wave mixing devices useful for signal processing functions in WDM and TDM systems, including wavelength conversion, spectral inversion, and gated mixing. These mixers exhibit extremely wide bandwidth, low noise, high efficiency, and format transparency.

We have proposed and demonstrated a novel optical regenerator architecture employing electroabsorption modulators as wavelength converters. The employment of EA modulators is advantageous for high-speed operation and flexibility in the bit-rate for the pulse regeneration. In addition, the EA modulator-wavelength-converter acts also as a photo diode for clock extraction. Compensation of the optical SNR and Q-factor has been demonstrated, even in cascaded noise load. Furthermore, against dispersion loading, we have confirmed that waveform recovery and Q-factor improvement is obtained by midway insertion of the optical regenerator. The proposed architecture will offer a wide-band-electronics-free optical regenerator in multi-tens of gigabit per second WDM networks.