Numerical investigation of the impact of reflectors on spectral performance of Raman fibre laser

Using a cavity mode model we study numerically the impact of bandwidth and spectral response profile of fibre Bragg gratings on four-wave-mixing-induced spectral broadening of radiation generated in 6 km and 22 km SMF-based Raman fibre lasers.

Optical fibre-based detection of DNA hybridization

A dual-peak LPFG (long-period fibre grating), inscribed in an optical fibre, has been employed to sense DNA hybridization in real time, over a 1 h period. One strand of the DNA was immobilized on the fibre, while the other was free in solution. After hybridization, the fibre was stripped and repeated detection of hybridization was achieved, so demonstrating reusability of the device. Neither strand of DNA was fluorescently or otherwise labelled. The present paper will provide an overview of our early-stage experimental data and methodology, examine the potential of fibre gratings for use as biosensors to monitor both nucleic acid and other biomolecular interactions and then give a summary of the theory and fabrication of fibre gratings from a biological standpoint. Finally, the potential of improving signal strength and possible future directions of fibre grating biosensors will be addressed.

Mode-locking in 25-km fibre laser

We demonstrate mode-locking and single-pulse generation in fibre laser with record-setting cavity length of 25 km. Substantial increase in the pulse round trip duration leads to ultra-low repetition rate of 8.097 kHz and pulse energy of 3.7 uJ.

Tilted fibre Bragg grating based 800nm WDM interrogation system for strain, temperature and refractive index sensing

We report the implementation of a low-cost high-resolution WDM interrogation system operating around 800nm region with bandwidth up to 60 nm and resolution of 13 pm by utilising a tilted fibre Bragg grating as an out-coupling device and a CCD-array detector. The system has been evaluated for interrogating fibre Bragg grating based strain, temperature sensors, giving sensitivities of 0.6 pm/µe and 5.6 pm/°C which are in good agreement with previously reported values. Furthermore, the system has been utilised to detect refractive index change of sample liquids, demonstrating a capability of measuring index change as small as 10-5.

Potential for broad range multiplexing of fibre Bragg gratings in polymer optical fibres and operation in the 700-nm wavelength range

Refractive index and structural characteristics of optical polymers are strongly influenced by the thermal history of the material. Polymer optical fibres (POF) are drawn under tension, resulting in axial orientation of the polymer molecular chains due to their susceptibility to align in the fibre direction. This change in orientation from the drawing process results in residual strain in the fibre and also affects the transparency and birefringence of the material (1-3). PMMA POF has failure strain as high as over 100%. POF has to be drawn under low tension to achieve this value. The drawing tension affects the magnitude of molecular alignment along the fibre axis, thus affecting the failure strain. The higher the tension the lower the failure stain will be. However, the properties of fibre drawn under high tension can approach that of fibre drawn under low tension by means of an annealing process. Annealing the fibre can generally optimise the performance of POF while keeping most advantages intact. Annealing procedures can reduce index difference throughout the bulk and also reduce residual stress that may cause fracture or distortion. POF can be annealed at temperatures approaching the glass transition temperature (Tg) of the polymer to produce FBG with a permanent blue Bragg wave-length shift at room temperature. At this elevated temperature segmental motion in the structure results in a lower viscosity. The material softens and the molecular chains relax from the axial orientation causing shrinking of the fibre. The large attenuation of typically 1dB/cm in the 1550nm spectral region of PMMA POF has limited FBG lengths to less than 10cm. The more expensive fluorinated polymers with lower absorption have had no success as FBG waveguides. Bragg grating have been inscribed onto various POF in the 800nm spectral region using a 30mW continuous wave 325nm helium cadmium laser, with a much reduced attenuation coefficient of 10dB/m (5). Fabricating multiplexed FBGs in the 800nm spectral region in TOPAS and PMMA POF consistently has lead to fabrication of multiplexed FBG in the 700nm spectral region by a method of prolonged annealing. The Bragg wavelength shift of gratings fabricated in PMMA fibre at 833nm and 867nm was monitored whilst the POF was thermally annealed at 80°C. Permanent shifts exceeding 80nm into the 700nm spectral region was attained by both gratings on the fibre. The large permanent shift creates the possibility of multiplexed Bragg sensors operating over a broad range. -------------------------------------------------------------------------------------------------------------------- 1. Pellerin C, Prud'homme RE, Pézolet M. Effect of thermal history on the molecular orientation in polystyrene/poly (vinyl methyl ether) blends. Polymer. 2003;44(11):3291-7. 2. Dvoránek L, Machová L, Šorm M, Pelzbauer Z, Švantner J, Kubánek V. Effects of drawing conditions on the properties of optical fibers made from polystyrene and poly (methyl methacrylate). Die Angewandte Makromolekulare Chemie. 1990;174(1):25-39. 3. Dugas J, Pierrejean I, Farenc J, Peichot JP. Birefringence and internal stress in polystyrene optical fibers. Applied optics. 1994;33(16):3545-8. 4. Jiang C, Kuzyk MG, Ding JL, Johns WE, Welker DJ. Fabrication and mechanical behavior of dye-doped polymer optical fiber. Journal of applied physics. 2002;92(1):4-12. 5. Johnson IP, Webb DJ, Kalli K, Yuan W, Stefani A, Nielsen K, et al., editors. Polymer PCF Bragg grating sensors based on poly (methyl methacrylate) and TOPAS cyclic olefin copolymer2011: SPIE.

Generation of triangular-shaped optical pulses in normally dispersive fibre

We determine through numerical modelling the conditions for the generation of triangular-shaped optical pulses in a nonlinear, normally dispersive (ND) fibre and experimentally demonstrate triangular pulse formation in conventional ND fibre.

Nonlinear fibre-based photonic technologies

Editorial

Generation of triangular pulses in normally dispersive fibre

In this scheme, nonlinearity and dispersion in the NDF lead to various reshaping processes of an initial, conventional pulse according to the chirping value and power level at the input of the fibre. In particular, we have observed that triangular-shaped pulses can be generated for sufficiently high energies and a positive initial chirp parameter. In our experiments, 2.8 ps-FWHM, transform-limited pulses generated from a mode-locked fibre laser source at a repetition rate of 1.25 GHz were pre-chirped by propagating the pulses through different lengths of standard mono-mode fibre. The chirped pulses were then amplified to different power levels before being launched into a 2.3 km section of True Wave fibre (TWF). The corresponding numerically calculated pulse temporal intensity profile and numerical and experimental second-harmonic generation frequency-resolved optical gating (SHG FROG) spectrograms were also derived. In conclusion, we have presented numerical modelling results which show the system design parameters required for the generation of triangular-shaped pulses in a nonlinear NDF, and experimentally demonstrated triangular pulse shaping in conventional NDF.

Dispersion and nonlinearity-management in mode-locked fibre lasers

In this work we extend theory of dispersion-managed (DM) solitons to dissipative systems with the main focus on applications in mode-locked lasers. In general, pulses in mode-locked fibre lasers experience both nonlinear and dispersion management per cavity round trip. In stretched-pulse lasers, this concept was utilized to obtain high energy pulses. Here we model the pulse propagation in a mode-locked fibre laser with a distributed nonlinear and DM Ginzburg-Landau type equation. We extend existing results on DM solitons and investigate the impact on soliton properties of dissipative perturbations that occur due to the effects of gain amplification, saturable absorption, and loss. In conclusion, in contrast to standard DM solitons in Hamiltonian systems, dissipative DM solitons do exist at high map strengths, thus opening a way for the generation of stable, short pulses with high energy.

All-optical nonlinear fibre signal processing

All-optical technologies for data processing and signal manipulation are expected to play a major role in future optical communications. Nonlinear phenomena occurring in optical fibre have many attractive features and great, but not yet fully exploited potential in optical signal processing. Here, we overview our recent results and advances in developing novel photonic techniques and approaches to all-optical processing based on fibre nonlinearities. Amongst other topics, we will discuss phase-preserving optical 2R regeneration, the possibility of using parabolic/flat-top pulses for optical signal processing and regeneration, and nonlinear optical pulse shaping. A method for passive nonlinear pulse shaping based on pulse pre-chirping and propagation in a normally dispersive fibre will be presented. The approach provides a simple way of generating various temporal waveforms of fundamental and practical interest. Particular emphasis will be given to the formation and characterization of pulses with a triangular intensity profile. A new technique of doubling/copying optical pulses in both the frequency and time domains using triangular-shaped pulses will be also introduced.

Pulse shaping in mode-locked ring-cavity fibre lasers

By means of extensive numerical modelling we have demonstrated the possibility of nonlinear pulse shaping in a mode-locked fibre laser using control of the intra-cavity propagation dynamics by adjustment of the normal net dispersion and integrated gain. Beside self-similar mode-locking, the existence of a novel type of pulse shaping regime that produces pulses with a triangular temporal intensity profile and a linear frequency chirp has been observed.

Dissipative dispersion managed solitons in mode-locked fibre lasers

We extend theory of dispersion-managed solitons to dissipative systems with a focus on mode-locked fibre lasers. Dissipative structures exist at high map strengths, and different pulse evolutions are observed depending on the net cavity dispersion.