Semiconductor optical amplifier-based nonlinear optical-loop mirror with feedback

The behavior of a semiconductor optical amplifier (SOA)-based nonlinear loop mirror with feedback has been investigated as a potential device for all-optical signal processing. In the feedback device, input signal pulses (ones) are injected into the loop, and amplified reflected pulses are fed back into the loop as switching pulses. The feedback device has two stable modes of operation - block mode, where alternating blocks of ones and zeros are observed, and spontaneous clock division mode, where halving of the input repetition rate is achieved. Improved models of the feedback device have been developed to study its performance in different operating conditions. The feedback device could be optimized to give a choice of either of the two stable modes by shifting the arrival time of the switching pulses at the SOA. Theoretically, it was found possible to operate the device at only tens of fJ switching pulse energies if the SOA is biased to produce very high gain in the presence of internal loss. The clock division regime arises from the combination of incomplete SOA gain recovery and memory of the startup sequence that is provided by the feedback. Clock division requires a sufficiently high differential phase shift per unit differential gain, which is related to the SOA linewidth enhancement factor.

Multichannel regeneration of dual quadrature signals

We demonstrate the first multi-wavelength regeneration of quadrature phase shift keyed (QPSK) formatted signals, showing a simultaneous Q2-factor improvement in excess of 3.8 dB for signals degraded by phase distortion

Dual-polarization multi-band OFDM signals for next generation core networks

Dual-polarization multi-band orthogonal frequency-division multiplexing (DP-MB-OFDM) signals are optimized at data rates up to 500 Gb/s in dispersion compensation fiber (DCF)-free long-haul links. We compare different polarization crosstalk compensation techniques and explore the DP-MB-OFDM transmission distance limitations. In addition, the impact of the OFDM subcarrier number and high-level modulation format on the transmission performance are also investigated. Finally, a comparison is made between DP-MB-OFDM and the industrial solution of single-carrier DP quaternary phase-shift keying (DP-QPSK) at 1000 km.

Experimental full duplex simultaneous transmission of LTE over a DWDM directly modulated RoF system

In this paper, we experimentally demonstrate the seamless integration of full duplex system frequency division duplex (FDD) long-term evolution (LTE) technology with radio over fiber (RoF) for eNodeB (eNB) coverage extension. LTE is composed of quadrature phase-shift keying (QPSK), 16-quadrature amplitude modulation (16-QAM) and 64-QAM, modulated onto orthogonal frequency division multiplexing (OFDM) and single-carrier-frequency division multiplexing for downlink (DL) and uplink (UL) transmissions, respectively. The RoF system is composed of dedicated directly modulated lasers for DL and UL with dense wavelength division multiplexing (DWDM) for instantaneous connections and for Rayleigh backscattering and nonlinear interference mitigation. DL and UL signals have varying carrier frequencies and are categorized as broad frequency spacing (BFS), intermediate frequency spacing (IFS), and narrow frequency spacing (NFS). The adjacent channel leakage ratio (ACLR) for DL and UL with 64-QAM are similar for all frequency spacings while cross talk is observed for NFS. For the best case scenario for DL and UL transmissions we achieve error vector magnitude (EVM) values of ~2.30%, ~2.33%, and ~2.39% for QPSK, 16-QAM, and 64-QAM, respectively, while for the worst case scenario with a NFS EVM is increased by 0.40% for all schemes. © 2009-2012 OSA.

Reach improvement of mode division multiplexed systems using fiber splices

This letter proposes the introduction of discrete modal crosstalk (XT) through fiber splices for the improvement of the distance reach (DR) of mode division multiplexed (MDM) transmission systems over few mode fibers (FMFs). The proposed method increases the DR, reducing the time spread of the FMFs' impulse response. The effectiveness of this method is assessed through simulation considering 3 × 136-Gbit/s MDM-coherently-detected polarization-multiplexed quadrature-phase-shift-keying ultralong haul transmission systems employing inherently low differential mode delay (DMD) FMFs or DMD compensated FMFs. A maximum DR increase factor of 1.9 is obtained for the optimum number of splices per span and optimum splice XT level. © 1989-2012 IEEE.

Nonlinear semi-analytical model for simulation of few-mode fiber transmission

In this letter, a nonlinear semi-analytical model (NSAM) for simulation of few-mode fiber transmission is proposed. The NSAM considers the mode mixing arising from the Kerr effect and waveguide imperfections. An analytical explanation of the model is presented, as well as simulation results for the transmission over a two mode fiber (TMF) of 112 Gb/s using coherently detected polarization multiplexed quadrature phase-shift-keying modulation. The simulations show that by transmitting over only one of the two modes on TMFs, long-haul transmission can be realized without increase of receiver complexity. For a 6000-km transmission link, a small modal dispersion penalty is observed in the linear domain, while a significant increase of the nonlinear threshold is observed due to the large core of TMF. © 2006 IEEE.

Optically tunable fiber grating transmission filters

We propose and demonstrate single- and multiple-passband fiber grating transmission filters that are remotely tunable by exploitation of the optical pump-induced thermal effects in Er Yb-codoped fiber sections. A repeatable, wavelength-independent induced phase shift of 0.1p mW is obtained without hysteresis and anisotropic effects. A transmission extinction ratio of .23 dB with a 3-mW change in pump power is achieved.

Time resolved bit error rate analysis of a fast switching tunable laser for use in optically switched networks

We investigate the use of different direct detection modulation formats in a wavelength switched optical network. We find the minimum time it takes a tunable sampled grating distributed Bragg reflector laser to recover after switching from one wavelength channel to another for different modulation formats. The recovery time is investigated utilizing a field programmable gate array which operates as a time resolved bit error rate detector. The detector offers 93 ps resolution operating at 10.7 Gb/s and allows for all the data received to contribute to the measurement, allowing low bit error rates to be measured at high speed. The recovery times for 10.7 Gb/s non-return-to-zero on–off keyed modulation, 10.7 Gb/s differentially phase shift keyed signal and 21.4 Gb/s differentially quadrature phase shift keyed formats can be as low as 4 ns, 7 ns and 40 ns, respectively. The time resolved phase noise associated with laser settling is simultaneously measured for 21.4 Gb/s differentially quadrature phase shift keyed data and it shows that the phase noise coupled with frequency error is the primary limitation on transmitting immediately after a laser switching event.

Bit-error rate performance of 20 Gbit/s WDM RZ-DPSK non-slope matched submarine transmission systems

Applying direct error counting, we assess the performance of 20 Gbit/s wavelength-division multiplexing return-to-zero differential phase-shift keying (RZ-DPSK) transmission at 0.4 bit/(s Hz) spectral efficiency for application on installed non-zero dispersion-shifted fibre based transoceanic submarine systems. The impact of the pulse duty cycle on the system performance is investigated and the reliability of the existing theoretical approaches to the BER estimation for the RZ-DPSK format is discussed.

Environmentally stable high-power soliton fiber lasers that use chirped fiber Bragg gratings

Environmentally stable high-power erbium fiber soliton lasers are constructed by Kerr or carrier-type mode locking. We obtain high-energy pulses by using relatively short fiber lengths and providing large amounts of negative dispersion with chirped fiber Bragg gratings. The pulse energies and widths generated with both types of soliton laser are found to scale with the square root of the cavity dispersion. Kerr mode locking requires pulses with an approximately three times higher nonlinear phase shift in the cavity than carrier mode locking, which leads to the generation of slightly shorter pulses with as much as seven times higher pulse energies at the mode-locking threshold.

Performance analysis of 20Gb/s RZ-DPSK non-slope matched transoceanic submarine links

Direct computation of the bit-error rate (BER) and laboratory experiments are used to assess the performance of a non-slope matched transoceanic submarine transmission link operating at 20Gb/s channel rate and employing return-to-zero differential-phase shift keying (RZ-DPSK) signal modulation. Using this system as an example, we compare the accuracies of the existing theoretical approaches to the BER estimation for the RZ-DPSK format. © 2007 Optical Society of America.

Linear binocular combination of responses to contrast modulation:contrast-weighted summation in first- and second-order vision

Binocular combination for first-order (luminancedefined) stimuli has been widely studied, but we know rather little about this binocular process for spatial modulations of contrast (second-order stimuli). We used phase-matching and amplitude-matching tasks to assess binocular combination of second-order phase and modulation depth simultaneously. With fixed modulation in one eye, we found that binocularly perceived phase was shifted, and perceived amplitude increased almost linearly as modulation depth in the other eye increased. At larger disparities, the phase shift was larger and the amplitude change was smaller. The degree of interocular correlation of the carriers had no influence. These results can be explained by an initial extraction of the contrast envelopes before binocular combination (consistent with the lack of dependence on carrier correlation) followed by a weighted linear summation of second-order modulations in which the weights (gains) for each eye are driven by the first-order carrier contrasts as previously found for first-order binocular combination. Perceived modulation depth fell markedly with increasing phase disparity unlike previous findings that perceived first-order contrast was almost independent of phase disparity. We present a simple revision to a widely used interocular gain-control theory that unifies first- and second-order binocular summation with a single principle-contrast-weighted summation-and we further elaborate the model for first-order combination. Conclusion: Second-order combination is controlled by first-order contrast.

Highly sensitive liquid-level sensor based on dual-wavelength double-ring fiber laser assisted by beat frequency interrogation

A highly sensitive liquid-level sensor based on dual-wavelength single-longitudinal-mode fiber laser is proposed and demonstrated. The laser is formed by exploiting two parallel arranged phase-shift fiber Bragg gratings (ps-FBGs), acting as ultra-narrow bandwidth filters, into a doublering resonators. By beating the dual-wavelength lasing output, a stable microwave signal with frequency stability better than 5 MHz is obtained. The generated beat frequency varies with the change of dual-wavelength spacing. Based on this characteristic, with one ps-FBG serving as the sensing element and the other one acting as the reference element, a highly sensitive liquid level sensor is realized by monitoring the beat frequency shift of the laser. The sensor head is directly bonded to a float which can transfer buoyancy into axial strain on the fiber without introducing other elastic elements. The experimental results show that an ultra-high liquidlevel sensitivity of 2.12 × 107 MHz/m within the measurement range of 1.5 mm is achieved. The sensor presents multiple merits including ultra-high sensitivity, thermal insensitive, good reliability and stability. © 2012 Optical Society of America.

Comments on multiple oscillatory solutions in systems with time-delay feedback

A complex Ginzburg-Landau equation subjected to local and global time-delay feedback terms is considered. In particular, multiple oscillatory solutions and their properties are studied. We present novel results regarding the disappearance of limit cycle solutions, derive analytical criteria for frequency degeneration, amplitude degeneration, frequency extrema. Furthermore, we discuss the influence of the phase shift parameter and show analytically that the stabilization of the steady state and the decay of all oscillations (amplitude death) cannot happen for global feedback only. Finally, we explain the onset of traveling wave patterns close to the regime of amplitude death.

Optimisation of random DFB Raman laser amplifier

We numerically optimise 2nd-order random DFB Raman laser amplifiers for transmission for the first time. Optical signal to noise ratio, nonlinear phase shift, signal power variation and the impact of the reflectivity of FBG are investigated in the links from 10 - 120 km.