Optimisation of DSF and SOA based phase conjugators by incorporating noise-suppressing fibre gratings

We compare the performance of dispersion-shifted-fibre (DSF) and semiconductor-optical-amplifier (SOA) based phase conjugators for a 10 Gb/s non-return-to-zero system with respect to conversion efficiency, noise figure and distortion. Fibre gratings are used for signal extraction and amplified spontaneous emission (ASE) suppression, allowing closer wavelength spacing and reducing the conjugation noise figure by up to 12 dB. Despite the higher SOA conversion efficiency, both conjugators give similar noise figures with ASE suppression. However, the DSF based conjugator has the advantage of distortion tolerance at higher input power. Introduction: Optical phase conjugation has attracted much recent research attention due to its potential application for group-velocity-dispersion and self-phase-modulation compensation in mid-point spectral inversion (MPSI) systems, and also for coherent wavelength conversion in optical switching and routing. The two most promising optical phase conjugation techniques are four-wave mixing (FWM) in either dispersion-shifted fibre (DSF), or semiconductor optical amplifiers (SOA). A DSF based conjugator requires phase matching close to its zero dispersion wavelength for efficient four-wave mixing. This restricts its wavelength flexibility compared to an SOA based conjugator which offers a much wider conversion bandwidth. Furthermore, the low FWM conversion efficiency in passive DSF seems to make the SOA a preferred phase conjugating medium. However, in a practical communication system, conjugation optical signal-to-noise ratio (SNR) is more important than conversion efficiency. The noise at the conjugate wavelength is usually dominated by the amplified spontaneous emission (ASE) noise from the pump and signal. The reduction of this noise has been demonstrated in an SOA based conjugator (i) by bandpass filtering of the pump and/or signal waves before mixing, and (ii) by the insertion of a notch filter at the conjugate wavelength before the conjugator. In this letter, SOA and DSF based conjugators are compared by investigating the conversion efficiency, noise and eye opening in a 10 Gb/s non-return-to-zero (NRZ) externally-modulated system, using an identical filtering network. We report for the first time the use of fibre gratings for efficient ASE noise filtering and conjugate signal extraction. The performance enhancement using these noise-suppressing gratings is also investigated

Transmission of 10 ps pulses over 101.5km of standard fibre using Midpoint Spectral Inversion

Linear transmission of 10 ps pulses over 101.5 km of standard fibre has been achieved by inverting the spectrum of the pulses at the midpoint of the link. The pulse source is a sliding frequency laser which generates transform limited pulses with 10 ps full width half maximum duration at a wavelength of 1532 nm. After 50 km of standard singlemode fibre with 16 ps/nm.km dispersion the pulses have broadened to 200 ps due to dispersion. The pulses are combined with a pump signal at 1535 nm, amplified and transmitted through 13 km of dispersion shifted fibre with dispersion zero at 1535 nm. The phase conjugate signal at 1538 nm, generated by four wave mixing, is filtered, amplified and transmitted through a second span of 51.5 km fibre which compresses the pulses back to its original pulse width. Limitations from self phase modulation in the transmission fibre have been observed. Ideally, spectral inversion should also compensate for self phase modulation if the power levels in the two spans are symmetrical. These effects has been studied with numerical simulation showing that the effect of self phase modulation is difficult to eliminate due to the asymmetrical fibre loss. Experimental results substantiate these findings and will be presented at the conference

Transmission of <10ps pulses over 318km standard fiber using midspan spectral inversion

We report experimental results on transmission of 5.7ps input pulses over 318km standard fibre. The minimum output pulsewidth was 9.5ps. In line filters, third order chromatic dispersion, and polarisation mode dispersion are the dominant broadening effects

Over 70nm wideband tuning of fiber Bragg gratings using a compressive bending technique

We demonstrate, for the first time, a silica-based fiber Bragg grating (FBG) with a wavelength tuning range over 70 nm, using compressive bending given by a simple tuning package. The wide tuning range of the device (from 1614 nm to 1544 nm), covers the entire L-band and a half of the C-band. Its polarization-mode dispersion (PMD) and polarization-dependent loss (PDL) remain low over the full operating regime

Greater than 90nm continuously wavelength-tunable fibre Bragg gratings

We report silica-based fibre Bragg grating filters with wavelength tuning-spans well over 90nm using a simple set-and-forget beam bending tuning-package. The key operational parameters of the filters are maintained over the entire tuning-window

Bragg grating package for simple broad-range tuning

A tunable package for fibre Bragg gratings is proposed and demonstrated. Reliable and stable axial compression is achieved using this with up to 29nm of continuous tuning with a precise and repeatable tuning algorithm

Linear and non-linear dispersion compensation of short pulses using midspan spectral inversion

Fibre nonlinearity and dispersion may limit the repeater spacing in high speed fibreoptic transmission systems. We report experimental and numerical results on the nonlinear propagation of short optical pulses over standard non-dispersion shifted fibre links which are dispersion compensated using midspan spectral inversion. It has been found that an increased transmission nonlinearity can be tolerated if the spectral inversion is moved away from the midpoint of the span. Full recovery of the initial pulsewidth is experimentally demonstrated in the presence of nonlinearities. A system design based on these results is presented, allowing high transmitted power and long repeater spacing

A novel polarisation independent phase conjugator/wavelength converter utilising inline fibre distributed feedback lasers

We propose and demonstrate a novel technique for optical wavelength conversion and phase conjugation by fibre four-wave mixing (FWM) using inline fibre distributed-feedback (DFB) lasers

Polarisation independent all-fibre phase conjugator incorporating inline fibre DFB lasers

We propose and demonstrate a novel technique for optical wavelength conversion and phase conjugation by fibre four-wave mixing (FWM) using inline fibre distributed-feedback (DFB) lasers as orthogonally polarized pump sources. This technique features polarisation independent operation without the need for externally injected pump light. Polarisation dependency as low as 0.5dB has been achieved using this technique