Short-wavelength transmission-loss suppression in fibre Bragg gratings

Fibre Bragg Gratings (FBGs) are known to suffer from short-wavelength, transmission losses due to resonant coupling into backward-propagating cladding modes. Figure 1 shows a typical transmission spectrum of a 10cm standard FBG. The cladding mode losses increase with grating reflectivity and could eventually impose severe limitations in the use of FBGs. The problem can be quite acute in the case that FBG wavelength-multiplexing is required. So far, several attempts have been made to eliminate the short-wavelength, transmission losses and improve grating performance. In all cases, the resonant coupling of the forward-propagating core mode to the backward-propagating cladding modes is minimised by reducing the coupling strength. In this paper, we report on a novel method for reducing cladding-mode transmission losses in standard FBGs. We show that short-wavelength, transmission losses can be practically eliminated by damping the resonant excitation of the cladding modes. The damping is achieved by properly introducing a substantial propagation loss into the cladding modes. For maximum effect, the core mode should experience no extra propagation losses. By applying a thin lossy layer on the fibre cladding surface, a reduction of cladding-mode-losses of about 12dB was achieved

Fibre DFB lasers in a 4x10 Gbit/s WDM link with a single sinc-sampled fibre grating dispersion compensator

WDM transmission and dispersion compensation at 40 Gbit/s over 200 km standard fibre is demonstrated on a 100 GHz grid using four high power single-polarisation single-sided output DFB fibre laser based transmitters and a single 4 channel WDM chirped fibre Bragg grating dispersion compensator

Noise characteristics of erbium-doped fibre amplifier pumped at 980nm

We show that quantum-limited noise operation of an erbium-doped fibre amplifier is possible when pumped at 980nm. A noise figure of 2.9±0.4dB was measured for pump powers of only 5.8mW, consistent with a population inversion factor close to unity

Long continuously chirped fibre Bragg gratings for compensation of linear- and 3rd order-dispersion

For the first time long broadband chirped fibre Bragg gratings with a dispersion profile designed to compensate 3rd order-dispersion are presented. These results demonstrate how the increased demands for dispersion compensation at very high bit-rates can be met using chirped fibre Bragg grating

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

Second-generation erbium-doped fibre amplifiers for advanced optical communications

The erbium doped fibre amplifier (EDFA) [1] has seen rapid development and is now established as a key component in future optical networks. To date, EDFA development has been driven by specific requirements, primarily the single-channel, point-to-point fibre links. There are, however, many more applications which may require, for example, either broadband or narrowband amplifiers and signal-processing functions. In addition, non-telecommunication applications can benefit enormously by the use of EDFAs

Properties of dysprosium doped GaLaS fibre amplifiers operating at 1.3µm

In light of the recent progress on the fabrication of GaLaS fibres we have modelled the performance of dysprosium doped GaLaS fibre amplifiers operating at 1.3µm. Based on experimental data, we find the incorporation of a co-dopant (terbium) in the fibre core significantly shortens the optimum amplifier length from >30m to ~3m. Such a device may be practical given the fibre losses currently achieved in GaLaS fibres

10Gbit/s transmission over 700km of standard single mode fibre with a 10cm chirped fibre grating compensator and duobinary transmitter

The advent of erbium-doped fibre amplifiers makes optical fibre transmission in the 1.55µm wavelength window very attractive. However, with the large amounts of standard non-dispersion shifted fibres (NDSF) already installed, high bit rate transmission is restricted by the large dispersion of these fibres at 1.55µm, unless compensating techniques are used. A number of approaches have been put forward to address this issue, such as dispersion compensating fibre, mid-point spectral inversion, dispersion-supported transmission, solitons, and chirped fibre Bragg gratings. Of these, fibre gratings are attractive as they are passive, linear devices, highly dispersive yet compact and relatively easy to fabricate in large numbers. In recent years, progress in the use of fibre grating-based compensation for 10 Gbit/s transmission has been rapid, with distances reported from 160km, 220km, 270km, 400km and most recently to 540 km. In this work, we demonstrate that 10 Gbit/s transmission up to 700 km of NDSF is achievable with a single 10 cm long chirped fibre grating in combination with a reduced bandwidth phase-alternating duobinary transmitter

Interferometric fibre grating characterisation

One of the most important fibre-optic devices to have emerged in the recent years is the fibre grating. It finds applications in DFB and DBR fibre lasers, dispersion compensation and fibre sensors. Full and accurate amplitude and phase (dispersion) characterization of this device is therefore needed. We have demonstrated such a system based on an interferometric technique (figure 1). The signal arm of a fibre Michelson interferometer is phase-modulated with a saw-tooth function to generate an electric signal at the photodetector which carries the optical phase and amplitude information of the reflective fibre device under test (DUT). The amplitude response of the interferometer is directly proportional to the field reflection coefficient, whereas the measured relative phase is related to the time delay response of the device. The set-up is fully automated and uses a Hewlett Packard tunable laser source (1470-1560nm) with wavelength accuracy are high resonance piezoelectric ceramic cylinders which are wrapped round with some turns of fibre

All fibre 2x2 polarisation insensitive switch

We report the development of a polarisation insensitive, low loss, broad band all fibre 2×2 acousto-optic switch based on twisting the waist of a null taper coupler. The best polarisation insensitivity observed so far is 0.2dB