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We propose a novel method of in-band estimation of optical signal-to-noise ratio (OSNR) using a digital coherent receiver, where OSNR is determined from second- and fourth-order statistical moments of equalized signals in any modulation format. Our proposed method is especially important in recently-developed Nyquist wavelength-division multiplexed (WDM) systems and/or reconfigurable optical-add/drop-multiplexed (ROADM) networks, because in these systems and networks, we cannot apply the conventional OSNR estimation method based on optical-spectrum measurements of the in-band signal and the out-of-band noise. Effectiveness of the proposed method is validated with computer simulations of Nyquist-WDM systems and ROADM networks using 25-Gbaud quadrature phase-shift keying (QPSK) and 16 quadrature-amplitude modulation (16-QAM) formats.

The recently-developed digital coherent receiver enables us to employ a variety of spectrally-efficient modulation formats such as M-ary phase-shift keying (PSK) and quadrature-amplitude modulation (QAM). Moreover, in the digital domain, we can equalize linear transmission impairments, which may stem from group-velocity dispersion (GVD) and polarization-mode dispersion (PMD) of fibers for transmission, because the phase information is preserved after coherent detection.This paper reviews the history of coherent optical communications, the principle of coherent detection, and the concept of the digital coherent receiver. After that, we discuss digital signal processing (DSP) for mitigating transmission impairments, coherent transmission characteristics of multi-level optical signals, and future prospects of coherent optical communications.

The realization of an entanglement distribution optical fiber network connecting multiple parties would permit implementation of many information security applications such as entanglement-based quantum key distribution and quantum secret sharing. However, due to material absorption and scattering in optical fiber, photons that are the carriers of quantum entanglement experience loss during propagation and the overall photon arrival rate can be very low in such a network. One way to increase photon arrival rate is to make full use of the available transmission bandwidth of optical fiber and this is achievable via wavelength-multiplexing. We review our recent work on wavelength-multiplexed entanglement distribution and discuss system design considerations from a telecommunication engineering perspective.

This paper describes optical signal processing based on optical phase modulation and subsequent optical filtering, which is applicable to 160-Gb/s optical time-division multiplexed (OTDM) subsystems. Ultrafast phase modulation of an optical signal is done by self-phase modulation (SPM) and cross-phase modulation (XPM) when an optical pulse passes through a nonlinear optical fiber. Such phase modulation induces the spectral shift of the optical signal. Various types of optical signal processing are realized simply by filtering out the spectral-shifted component. Using SPM-based pulse reshaping in a 500-m-long silica-based highly nonlinear fiber (HNLF), we demonstrate highly stable generation of a 10-GHz 2-ps optical pulse train tunable over the entire C band. A phase-locked loop (PLL) can suppress the slow phase drift of the output pulse train induced by fluctuations of the nonlinear fiber length, enabling the application of the pulse generator to a 160-Gb/s OTDM transmitter. Based on XPM in a 2-m-long photonic crystal fiber, optical time-division demultiplexing of 160-Gb/s optical signals is demonstrated. The long-term stability is drastically improved as compared with the device composed of a conventional silica-based HNLF, because the short fiber length reduces the phase fluctuation between the signal and control pulses. Instead of nonlinear fibers, an electrooptic modulator such as a (LN) modulator also performs the phase modulation in a more practical manner. We propose and demonstrate an optoelectronic time-division demultiplexing scheme for a 160-Gb/s OTDM signal, which consists of an LN phase modulator driven by a 40-GHz electrical clock and an optical bandpass filter (BPF). We also demonstrate base-clock recovery from a 160-Gb/s optical signal with an optoelectronic PLL. The phase comparator is simply composed of an LN phase modulator and an optical BPF, ensuring the stable and reliable operation in the 160-Gb/s receiver.

The authors propose third-generation small and high-speed saturable absorbers to replace semiconductor-based saturable absorbers and optical-fiber-based nonlinear devices. The device is based on the carbon nanotube (CNT). This paper discusses the characteristics of CNT-based saturable absorbers. The present status of research on passively mode-locked fiber lasers using CNT-based saturable absorbers is reported. © 2007 Wiley Periodicals, Inc. Electron Comm Jpn Pt 2, 90(2): 17– 24, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecjb.20272

We experimentally demonstrate a practical benefit of our proposed continuous-wave (CW) supercontinuum (SC) generating ring laser scheme in terms of low-cost device implementation and then investigate its output intensity noise property, as a further study to our previous paper of Ref. [J.H. Lee, Y. Takushima, K. Kikuchi, Opt. Lett. 30 (2005) 2599]. First, we implement a practical and low-cost CW SC laser using a double clad Er/Yb codoped fiber and a highly nonlinear dispersion-shifted fiber (HNL-DSF). Unlike the scheme in Ref. [J.H. Lee, Y. Takushima, K. Kikuchi, Opt. Lett. 30 (2005) 2599] the gain medium of an Er/Yb fiber is clad-pumped by low quality multimode pump LDs to underline the fact that our proposed scheme does not require any high power single-mode laser pump. A SC of a bandwidth larger than 470 nm is readily achieved. Next, we measure relative-intensity-noise (RIN) of the generated SC and compare it with that of a pumped Er/Yb fiber ASE. The SC laser is found to have a much higher RIN than the ASE due to the nonlinear amplification of quantum fluctuations both in the seed light oscillation and in the Raman scattering process.

This paper describes a phase-diversity homodyne receiver that which can cope with multilevel modulation formats. The carrier phase drift is estimated with digital signal processing (DSP) on the homodyne-detected signal, entirely restoring the complex amplitude of the incoming signal. Our DSP-based phase-estimation scheme consists of a simple and demultiplexable architecture that allows the system to reach significantly higher performance than conventional optical delay detection. Since the whole optical signal information is preserved with our receiver, various kinds of postprocessing of the received signal become possible. For example, we can demultiplex wavelength-division/optical time-division multiplexed channels and compensate for group velocity dispersion of fibers as well as the nonlinear phase noise in the electrical domain. We also experimentally evaluate the performance of our receiver. Our offline bit-error rate experiments show the feasibility of transmitting polarization-multiplexed 40-Gb/s quadrature phase-shift keying signals over 200 km with channel spacing of 16 GHz, leading to spectral efficiency of 2.5 b/s/Hz

We design and demonstrate a high repetition-rate similariton generation system using normal dispersion fiber amplifiers (NDFA's). We numerically calculate the pulse evolution in NDFA's and clarify the condition to generate similariton pulses in a finite-length NDFA. Then we design the similariton generation system in consideration of the use of Erbium-doped fibers (EDF's) and show that a km-long fiber amplifier with low normal dispersion can generate a high repetition-rate similariton train from practical pico-second pulse sources. In the experiment, we demonstrate a 10-GHz similariton source using a 1.2-km-long EDF. For application to multi-wavelength light sources, we measure the bit-error rate of the spectrally sliced similariton, and show that it exhibits low-noise performance, which is attributed to the spectral flatness.

We report tunable optical devices based on fiber Bragg gratings (FBGs), whose filtering characteristics are controlled by strain distributions. These devices include a widely wavelength tunable filter, a tunable group-velocity dispersion (GVD) compensator, a tunable dispersion slope (DS) compensator, and a variable-bandwidth optical add/drop multiplexer (OADM), which will play important roles for next-generation reconfigurable optical networks.

In this paper, we present a high-speed wavelength-swept laser for application in a real-time optical device characterization system. The system is capable of simultaneous measurement of both the spectral amplitude and group-delay responses of the device-under-test, at a scan rate of 22 Hz over a wide wavelength range of 50 nm. This corresponds to a record sweep rate of >1000 nm/s.

We propose a novel method for introducing a chirp on a fiber Bragg grating. Applying a controllable nonlinear temperature gradient across a uniformly written Bragg grating chirps it nonlinearly to realize a tunable dispersion slope compensator for waveform distortion compensation. This scheme allows an adjustable, repeatable, stable, and precise nonlinear chirp onto the grating. The dispersion slope of the device was tuned from −37.06 to −199.45 ps/nm 2 , with observed GDR as low as ±5 ps.

We derive a formula for retrieving the amplitude and phase of an optical pulse from its sonogram. When the transfer function of the frequency filter is known, the pulse amplitude and phase are completely retrieved from the sonogram without iterative calculations by using the derived formula. Based on this formula, we find that the width of a sampling pulse, which is used for cross-correlation measurement of the sonogram, must be much shorter than the width of the pulse under test, and that it is not necessarily possible to reconstruct the pulse when we use the pulse under test itself as a sampling pulse. Finally, we discuss possible schemes for sonogram characterization of optical pulses.

We prepared carbon nanotube (CNT)-incorporated sol–gel glass at a low temperature guaranteeing homogeneous CNT dispersion and intact nonlinearity of the CNTs for ultrafast optic devices. Significant defects and CNT agglomeration were avoided by optimizing the process steps, including stabilized CNT dispersion in dimethylformamide and the gelation of silica sol. After verifying undamaged nano-morphologies and optical properties of the enclosed CNTs via nonlinear absorption checks, we successfully demonstrated all-fiber, mode-locked laser modulating the intracavity absorption with the CNT-incorporated glass transferred onto a laser component. Resultant spectral width and repetition rate of the laser pulse output were 1.1 nm and 2.96 MHz, respectively.

We investigate the feasibility of the use of 1-m length of our fabricated bismuth oxide-based nonlinear fiber (Bi-NLF) for the implementation of a Mamyshev-type 2R regenerator that is based on self-phase-modulation-induced spectral broadening followed by spectral filtering, and evaluate the regeneration performance in both experimental and theoretical ways from a perspective of system application. Owing to the excellent dispersion and chi(3) nonlinearity characteristics of our 1-m-long Bi-NLF, a nonlinear S-shape power transfer curve is readily obtained. A Q-factor improvement of a noisy 10-Gb/s return-to-zero signal by 1.2 dB is achieved by use of the regenerator

We demonstrate coherent demodulation of optical multilevel (M-ary) phase-shift-keying (PSK) signals. Since the carrier phase is estimated accurately through digital signal processing after phase-diversity homodyne detection, the system performance is highly tolerant to the carrier phase noise. By off-line bit-error-rate measurements using distributed feedback semiconductor lasers with linewidths of 150 kHz as a transmitter and a local oscillator, it is shown that binary PSK (M=2), quadrature PSK (M=4), and eight-PSK (M=8) signals are successfully demodulated at the symbol rate of 10 Gsymbol/s

We demonstrate unrepeated optical transmission of 20-Gb/s quadrature phase-shift-keying (QPSK) signals over a 200-km-long standard single-mode fiber (SMF) without using any optical dispersion compensator. By employing optical homodyne detection, which can restore the entire information of the complex amplitude of the transmitted signal, group-velocity dispersion (GVD) of the SMF can be compensated electrically by a linear equalizer at the receiver. From off-line bit-error-rate measurements, we find that a simple transversal filter implemented in digital signal processing circuits after homodyne detection can effectively cancel the fiber GVD of up to 4000 ps/nm, enabling successful 20-Gb/s QPSK transmission.

We experimentally demonstrate that the use of only 1-m length of our fabricated Bismuth oxide-based nonlinear fiber readily allows for implementing a high-performance 10-Gb/s nonreturn-to-zero signal wavelength converter based on cross-phase-modulation-induced polarization rotation. No stimulated Brillouin scattering (SBS) suppression scheme was employed owing to a high SBS threshold of the fiber. Our wavelength converter is shown to provide a pattern-inverted or a noninverted output signal in the same configuration depending on the relative polarization directions between the probe and the polarizer. Error-free wavelength conversion over a 30-nm bandwidth is readily achieved for both cases, i.e., noninverted and inverted patterns.

In this paper, the optical amplifier noise accumulation in long-distance optical transmission systems is analyzed under the influence of the parametric process caused by the Kerr effect and group-velocity dispersion. By considering the input signal as a continuous-wave (CW) carrier and the optical amplifier noise as the small modulation, the general theory on the evolution of the optical amplifier noise along the system is proposed. This theory enables us to treat all of the noise enhancement effects, the so called parametric instability, discussed so far in a unified manner. The validation of the theory is finally confirmed by using extensive computer simulations.

We explore the ultimate potential offered by state-of-the-art Bismuth oxide-based optical fiber technology in a high-speed optical phase-locked loop-based clock recovery subsystem for an optical time-division multiplexed (OTDM) signal, in which the use of optical fiber-based devices has been considered to be inappropriate due to its tight requirement of short optical loop length. Here, we experimentally demonstrate the implementation of a compact all-fiber-based OTDM receiver incorporating both clock recovery and demultiplexing functions by use of short lengths of Bismuth oxide-based nonlinear fiber and erbium-doped Bismuth oxide fiber. Successful clock recovery and subsequent error-free demultiplexing are readily achieved at a data rate of 80 Gb/s. The clock recovery subsystem is also shown to be operable at 160 Gb/s.

We experimentally demonstrate a compact and tunable four-wave-mixing-based wavelength converter using a Bi/sub 2/O/sub 3/-based nonlinear fiber (Bi-NLF). An only 40-cm-long Bi-NLF is used as a nonlinear optical medium for wavelength conversion of a 40-Gb/s nonreturn-to-zero (NRZ) signal with no additional stimulated Brillouin scattering (SBS) suppression scheme. The Bi-NLF used in this experiment has an extremely high SBS threshold owing to both its short length and relatively low Brillouin gain. The 40-cm Bi-NLF is fusion-spliced to standard single-mode fibers and its nonlinearity is measured to be /spl sim/1100 W/sup -1//spl middot/km/sup -1/. Error-free wavelength conversion over a 10-nm bandwidth at a 40-Gb/s NRZ data rate is readily achieved with a pure continuous-wave pump.

Polarization-independent all-optical wavelength conversion is demonstrated by using cross-phase modulation in a twisted fiber and subsequent optical filtering. By twisting a 1-km-long fiber at a rate of 15 turns/m and aligning the probe light to circular polarization, we reduce the polarization sensitivity successfully from 3.5 to 0.3 dB. Error-free operation with only 1-dB penalty is realized at 40 Gb/s with the input signal polarization scrambled. The demonstrated scheme also offers a novel function of restoring the degree-of-polarization of the signal light.

All-optical signal regeneration at 160 Gb/s is demonstrated with a nonlinear optical loop-mirror (NOLM). We introduce an optical bias controller into the conventional NOLM composed of a polarization-maintaining fiber, which can suppress degradation of the extinction ratio due to cross-phase modulation between counterpropagating lights; accordingly, full-swing operation is achieved at the bit rate of 160 Gb/s, enabling all-optical signal regeneration.

We propose and demonstrate a novel scheme of the all-optical, high-speed add-drop multiplexer (ADM) for the optical time-division multiplexed signal. The ADM employs a single nonlinear optical loop mirror (NOLM) composed of an optical fiber. The key technical point of this scheme is the simultaneous use of the reflection mode (add function) and the transmission mode (drop function) of the NOLM, which results in significant simplification of the overall ADM configuration. Experimental results show that simultaneous add-drop operation at 80 Gb/s can be successfully achieved without bit errors.

We propose and demonstrate a novel fiber-based all-optical regenerator, where the wavelength shift induced by cross-phase modulation is employed for level discrimination. Regeneration of a 10-Gb/s signal using the proposed scheme improves the Q-factor by 1.8 dB. The scheme is stable and robust against changes in environmental conditions.

We demonstrate a simple scheme for the all-optical time-division demultiplexer, featuring a polarization-independent optical gate and a peak-holding optical phase-locked loop (PLL) for clock recovery. A fiber four-wave mixer (FWM) is employed as an optical gate. The polarization insensitive operation is achieved by the polarization diversity scheme, where only a short piece of polarization-maintaining fiber is attached to the input port of a conventional fiber FWM. By using the peak-holding optical PLL, the clock pulse is locked to the peak position of the incoming signal pulse. In such a case, a single optical gate acts as the gate switch for time-division demultiplexing as well as the phase detector for phase locking; therefore, the system configuration is greatly simplified. The all-optical time-division demultiplexer we develop enables us to demultiplex a 160-Gb/s signal into 10-Gb/s tributary channels with polarization sensitivity less than 0.5 dB.

Highly efficient tunable wavelength conversion based on nondegenerate fiber four-wave mixing is demonstrated. We apply simultaneous phase modulation to the two pump waves so that stimulated Brillouin scattering is suppressed without affecting the generated idler wave. The signal light is converted from an arbitrary wavelength to another arbitrary one within the entire C-band with a conversion efficiency higher than -10 dB.

Cross-phase modulation between counterpropagating lights degrades the extinction ratio of nonlinear optical-loop-mirror (NOLM) switches. To suppress such effect, we propose an NOLM with an optical-bias controller that can give a proper amount of optical bias between lights propagating through an NOLM in opposite directions. Implementing such optical bias controller in a conventional NOLM composed of a polarization-maintaining fiber, we demonstrate, for the first time, wavelength conversion of a nonreturn-to-zero signal under the full-swing condition with the maximum extinction ratio.

We report the design and fabrication of a tunable dispersion-slope compensating module based on a concatenation of a linearly strain-chirped fiber Bragg grating (FBG) and a nonlinearly strain-chirped FBG. By bending a substrate in an S shape, on which each FBG is mounted, we can adjust only the dispersion slope while maintaining the group-velocity dispersion as well as the center wavelength. The dispersion slope can be tuned up to -58.4 ps/nm/sup 2/ in this module for a bandwidth larger than 1.65 nm.

Fiber Bragg grating (FBG)-based bandpass filters, while possessing close to ideal sharp rolloff characteristics, can suffer from a significant amount of in-band dispersion. Results concerning the compensation of the in-band dispersion of a typical 100-GHz FBG using two thin-film all-pass filters, each composed of two coupled-cavities packaged in a compact configuration, are presented. The total peak insertion loss of the compensation package is less than 2.5 dB.

We propose and demonstrate a simple scheme of a highly efficient polarization-independent wavelength converter utilizing the two-pump fiber optical parametric amplification. First, binary phase-shift-keying modulation is applied to two pump waves, which allows us to suppress stimulated Brillouin scattering without broadening the idler spectrum. Then, polarization states of the two pumps are aligned orthogonally to realize the wavelength conversion which is insensitive to the signal polarization. Using this method, we achieve polarization-independent wavelength conversion with an efficiency larger than 0 dB over a 30-nm signal bandwidth.

We develop a practical subpicosecond pulse generator consisting of an electroabsorption modulator (EAM) and a double-stage pulse compressor. The pulse compressor is carefully designed so that the timing jitter and the pedestal level of compressed pulses are not increased during the pulse compression process. A 10-GHz 20-ps pulse from an EAM is compressed to a 400-fs pulse.

We demonstrate the in-service dispersion monitor in a 16 /spl times/ 10.6 Gb/s 960-km wavelength-division-multiplexing transmission system. We use the optical frequency method for the dispersion monitor, and modify it so as to be able to detect the sign of the dispersion without the loss of simplicity. The accuracy of the dispersion monitoring and the influence on the bit-error-rate performance are experimentally evaluated.

We demonstrate a silica-based tunable fiber Bragg grating filter with a wavelength tuning range over 90 nm. A bend-tuning technique in a simple tuning package is employed to obtain a wide wavelength tuning range from 1634 to 1544 nm, covering the entire L-band and a half of the C-band. The polarization-mode dispersion and polarization-dependent loss of the device show little variations over the whole operating region.

We demonstrate a dispersion compensation device based on thermal chirping of a glued fiber Bragg grating. The device can change the group velocity dispersion (GVD) while maintaining a center wavelength or change the center wavelength while preserving a GVD value. The GVD can be tuned from -122.5 to -57 ps/nm with a center wavelength at 1552.9 nm. On the other hand, the center wavelength can be shifted by 2.2 nm with a GVD value around -105 ps/nm.

The differential gain a and the linewidth enhancement factor /spl alpha/ of 1.5-/spl mu/m strained quantum well active layers mere measured as functions of the carrier density and the wavelength. Both a and /spl alpha/ of compressively strained multiple-quantum-well (CS-MQW) active layers reveal a carrier-density dependence which is stronger than that of unstrained multiple quantum-well (MQW) and tensile-strained single-quantum-well (TS-SQW) active layers. The improvement in these parameters is achieved only when the carrier density is low. On the other hand, in the case of TS-SQW active layers, the carrier density dependence is much smaller, and the differential gain is improved significantly. >

We propose an all-optical polarization-insensitive time-division demultiplexer using a nonlinear optical-loop mirror (NOLM) with a pair of short polarization maintaining (PM) fibers. This configuration enables us to realize polarization-diversity operation of a conventional NOLM consisting of a non-PM fiber. When a signal pulse is launched on a PM fiber placed at the input port of the NOLM, the pulse is temporally separated into two pulses with orthogonal linear polarizations. Each polarization component is switched independently by the NOLM, and recombined by another piece of a PM fiber placed at the output port of the NOLM. We experimentally demonstrate that a 40-Gb/s signal is demultiplexed into 10-Gb/s channels independently of. signal polarizations.

The authors propose a novel method of designing all-optical ultrafast gate switches that employ the cascade of the second harmonic generation and difference frequency mixing in quasi-phase matched LiNbO/sub 3/ devices. In this design method, the device length is maximized under the condition that crosstalk of the switch is maintained below a certain allowable value at a given bit rate. Following the design method, the authors find that the switching efficiency can significantly be enhanced by compensation for the walkoff between the fundamental and second harmonic pulses.

In all-optical gate switches that employ the cascaded second-order nonlinear effect in quasi-phase-matched (QPM) LiNbO/sub 3/ devices, walkoff between the fundamental and second harmonic pulses is very large. The authors experimentally show that crosstalk of the switch induced by such walkoff limits the switching speed, but that the switching speed can significantly be enhanced by walkoff compensation. Using a 20-mm-long QPM LiNbO/sub 3/ waveguide device, the authors switch one of twin pulses separated by 6.25 ps without crosstalk, showing the possibility of switching a 160-Gb/s signal.

The authors design and demonstrate a widely tunable wavelength filter, which is realized by lateral bending of a fiber Bragg grating mounted on a substrate. The 40-nm tunable range of the device covers the entire C-band of erbium-doped fiber amplifiers with very small changes in a spectral width and a reflectivity across the operation bandwidth. Furthermore, employment of a hybrid material as a substrate alleviates the stringent requirement for the bending radius. With this hybrid substrate structure, a tuning range as wide as 50 nm is achieved.

In this letter, we demonstrate an adjustable dispersion slope compensator for waveform distortion compensation based on a nonlinearly strain-chirped fiber Bragg grating. The dispersion slope of the device can be tuned up to -18.9 ps/nm/sup 2/ with a bandwidth >2.4 nm. After transmission through a 120-km-long dispersion-shifted fiber, the resulting waveform distortion for a 2.65-ps pulse is successfully compensated by using this device.

Up to 27-nm continuous tuning is demonstrated from Er/Yb all-fiber distributed-feedback (DFB) lasers using a simple tuning technique for axial extension and compression. The demonstrated devices operate with powers up to 10 dBm and remain operating in single mode over the full tuning range. Our results represents the broadest tuning-range previously reported in any DFB laser configuration and demonstrate that uniform compression tuning of long Bragg gratings is possible with high reliability.

Proposes and demonstrates a novel method of suppressing spectral broadening of the idler wave, which is generated in fiber four-wave mixing (FWM) pumped by a continuous wave. In this scheme, binary-phase-shift-keying (BPSK) modulation of the pump wave broadens the pump spectrum so that the Brillouin threshold for the pump power is increased, whereas it never affects the spectrum of the idler wave. By the 2-Gb/s BPSK modulation of the pump wave, we achieve a FWM efficiency as high as 10 dB, maintaining a side-band suppression ratio of the idler wave larger than 25 dB.

Conventional dispersion-slope compensation filters suffer from a fundamental tradeoff between the amount of dispersion-slope compensation and the compensation bandwidth. A novel multireflection device, composed of two coupled cavity filters in a parallel configuration is proposed that has an adjustable dispersion-slope independent of the compensation bandwidth. The device provides an adjustable dispersion-slope value between -6.7 and -35.7 ps/sup 3/ over a bandwidth of 3 nm. The center wavelength is adjustable over a 10-nm range.

We derive, for the first time, a simple formula for calculating the signal-to-noise ratio (SNR) in the transmission system where all-optical nonlinear interferometric switches are periodically inserted as all-optical regenerators. This formula predicts that even after the signal traveling over an infinite distance, the SNR can be maintained at a certain value, as long as the optical amplifier noise is smaller than a critical level.

We experimentally investigate the validity of the Gaussian approximation model of intraband crosstalk from multiple channels, which can degrade system performance significantly in wavelength-division multiplexed (WDM) networks. A recirculating fiber loop is employed for the first time to generate multiple crosstalk elements up to 20. The power penalties predicted by the Gaussian approximation model match well with the experimental results as the number of crosstalk elements is increased. This is because the probability distribution of the signal power mixed with intraband crosstalk approaches the Gaussian distribution with growing number of crosstalk elements in accordance with the central limit theorem.

Orthogonally pumped four-wave mixing is known as an effective technique for polarization-insensitive optical phase conjugation in optical fibers. However, for phase conjugation of broad-band optical signals, a wide pump wavelength separation is a necessity in order to avoid crosstalks. This can result in an increase in polarization sensitivity of the scheme. As a solution to this problem, we propose a filter-free scheme that automatically rejects unwanted four-wave mixing products, such that broad-band optical phase conjugation can be achieved with a narrow pump wavelength separation for improved polarization insensitivity.

We present a design guideline for ultrafast wavelength conversion using a nonlinear optical loop mirror (NOLM) composed of a dispersion-shifted fiber (DSF). By using the guideline, the wavelength allocation, the loop length, and the signal peak pourer are optimized for given values of the nonlinear coefficient and dispersion slope of the DSF. We actually construct a NOLM wavelength converter composed of a 50-m-long highly nonlinear DSF, and demonstrate 26-nm wavelength conversion of 500-fs pulse trains with a relatively low peak power of 4 W.

In optical phase conjugation systems, the third-order dispersion of fibers almost linearly accumulates along the transmission distance, and the distortion induced from the third-order dispersion can be perfectly compensated by using a linear third-order dispersion compensator placed at any point of the system. We demonstrate by numerical simulations that 100-Gb/s single-channel transmission can be achieved over a 10000-km distance in midway optical phase conjugation transmission systems by compensating the third-order dispersion.

Aiming at the application of supercontinuum (SC) generated from a fiber with a normal group-velocity dispersion (GVD) to multiwavelength transmitters, we theoretically study the enhancement of optical-amplifier noise in the SC-generation and spectrum-slicing processes. We find that significant enhancement of noise is not observed in these processes. The bit error rate measurement actually shows that the power penalty induced by these processes is less than 1 dB in all spectrum-sliced channels. These results assure low noise of the multiwavelength transmitter using spectrum-sliced SC generated in a normal GVD fiber.

The amplitude noise characteristics of femtosecond optical pulses generated from a synchronously pumped all-fiber Raman soliton laser with high-Q cavity are investigated under different operation states of the laser. By suppressing the soliton self-frequency shift (SSFS) effect in the high-Q fiber cavity, real femtosecond soliton oscillation can be obtained in the laser. Low-noise 400 fs optical pulses with a white amplitude noise level of -120 dBc/Hz have been generated from the laser operating in such an SSFS-free state. >