Time domain all-optical signal processing at a RZ optical receiver

We propose a new all-optical signal processing technique to enhance the performance of a return-to-zero optical receiver, which is based on nonlinear temporal pulse broadening and flattening in a normal dispersion fiber and subsequent slicing of the pulse temporal waveform. The potential of the method is demonstrated by application to timing jitter-and noise-limited transmission at 40 Gbit/s. © 2005 Optical Society of America.

Nonlinearly generated advanced pulse waveforms for optical signal processing

Nonlinear phenomena occurring in optical fibres have many attractive features and great, but not yet fully explored potential in signal processing. Here, we review recent progress on the use of fibre nonlinearities for the generation and shaping of optical pulses, and on the applications of advanced pulse waveforms in all-optical signal processing. Among other topics, we will discuss ultrahigh repetition-rate pulse sources, the generation of parabolic-shaped pulses in active and passive fibres, the generation of pulses with triangular temporal profiles, and coherent supercontinuum sources. The signal processing applications will span optical regeneration, linear distortion compensation, optical decision at the receiver in optical communication systems, spectral and temporal signal doubling, and frequency conversion. © 2012 IEEE.

Novel approaches in nonlinear optical fibre-based signal processing

All-optical data processing is expected to play a major role in future optical communications. Nonlinear effects in optical fibres have many attractive features and a great, not yet fully explored potential in optical signal processing. Here, we overview our recent advances in developing novel techniques and approaches to all-optical processing based on optical fibre nonlinearities.

Optical chemsensors utilizing long-period fiber gratings UV-inscribed in D-fiber with enhanced sensitivity through cladding etching

A novel implementation of an optical chemsensor device is reported based on long-period fiber grating structures ultraviolet-inscribed in D-fiber, with sensitivity enhancement by cladding etching. The results of a comparative study using D-fiber devices and similar structures in standard optical fiber reveal that the D-fiber devices offer substantially greater sensitivity both with and without etching. Based on a calibrated response to changes in refractive index, the grating devices have been used to measure the concentrations of aqueous sugar solutions, demonstrating the potential capability to detect concentration changes as small as 0.2%.

Optical chemsensor based on etched tilted Bragg grating structures in multimode fiber

We present a simple optical chemsensor device based on tilted Bragg grating structures ultraviolet-inscribed in conventional multimode fiber and sensitized by a hydrofluoric (HF)-etching treatment. The transition behaviors of fiber Bragg gratings (FBGs) from normal to tilted structures and their spectral evolution under HF-etching have been studied. The etched devices have been used to measure the concentrations of sugar solution, showing a potential capability of detecting concentration changes as small as 0.5%, which is an order of magnitude lower than that of previously reported FBG sensors in single-mode fiber.

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.

Two-dimensional optical power distribution of side-out-coupled radiation from tilted FBGs in multimode fibre

Two-dimensional optical power distribution of the out-coupled radiation from UV-inscribed tilted fibre Bragg gratings (TFBGs) in multimode fibre (MMF) has been side-detected with high spatial/spectral resolution, showing a near-identical radiation mode profile to that measured from the fibre-end detection method. A comparative investigation of the radiation characteristics of TFBGs fabricated in singlemode fibre (SMF) and MMF clearly indicates that the radiation out-coupling is stronger and spatially more confined in MMF. The unique spatial-to-spectral encoding property of the coupling mechanism offers potential application in low-cost WDM devices.

Implementation of optical chemsensors based on HF-etched fibre Bragg grating structures

High-sensitivity optical chemsensors have been implemented by exploiting fibre Bragg grating structures UV-inscribed in D-shape, single-mode and multimode fibres and post-sensitized by hydrofluoric acid (HF) etching treatment. We have demonstrated that the Bragg grating structures which are intrinsically insensitive to chemicals can be sensitized by effective etching. All etched devices possess refractive index sensing capability that offers an encoding function to chemical concentrations. Most etched devices have been used to measure the concentrations of sugar solutions, showing a potential capability of detecting concentration changes as small as 0.1–0.5%.

Two-dimensional optical power distribution of side-out-coupled radiation from tilted FBGs in multimode fibre

Two-dimensional optical power distribution of the out-coupled radiation from UV-inscribed tilted fibre Bragg gratings (TFBG) in multimode fibre (MMF) has been side-detected with high spatial spectral resolution, showing a near-identical radiation mode profile to that measured from the fibre-end detection method. A comparative investigation of the radiation characteristics of TFBGs fabricated in singlemode fibre (SMF) and MMF clearly indicates that the radiation out-coupling is stronger and spatially more confined in MMF. The unique spatial-to-spectral encoding property of the coupling mechanism offers potential application in low-cost WDM devices.

Advanced techniques for the improvement of optical transmission systems

This thesis presents the experimental investigation into two novel techniques which can be incorporated into current optical systems. These techniques have the capability to improve the performance of transmission and the recovery of the transmitted signal at the receiver. The experimental objectives are described and the results for each technique are presented in two sections: The first experimental section is on work related to Ultra-long Raman Fibre lasers (ULRFLs). The fibre lasers have become an important research topic in recent years due to the significant improvement they give over lumped Raman amplification and their potential use in the development of system with large bandwidths and very low losses. The experiments involved the use of ASK and DPSK modulation types over a distance of 240km and DPSK over a distance of 320km. These results are compared to the current state of-the-art and against other types of ultra-long transmission amplification techniques. The second technique investigated involves asymmetrical, or offset, filtering. This technique is important because it deals with the strong filtering regimes that are a part of optical systems and networks in modern high-speed communications. It allows the improvement of the received signal by offsetting the central frequency of a filter after the output of a Delay Line Interferometer (DLI), which induces significant improvement in BER and/or Qvalues at the receiver and therefore an increase in signal quality. The experimental results are then concluded against the objectives of the experimental work and potential future work discussed.

Dual-polarization multi-band optical OFDM transmission and transceiver limitations for up to 500 Gb/s uncompensated long-haul links

A number of critical issues for dual-polarization single- and multi-band optical orthogonal-frequency division multiplexing (DPSB/ MB-OFDM) signals are analyzed in dispersion compensation fiber (DCF)-free long-haul links. For the first time, different DP crosstalk removal techniques are compared, the maximum transmission-reach is investigated, and the impact of subcarrier number and high-level modulation formats are explored thoroughly. It is shown, for a bit-error-rate (BER) of 10-3, 2000 km of quaternary phase-shift keying (QPSK) DP-MBOFDM transmission is feasible. At high launched optical powers (LOP), maximum-likelihood decoding can extend the LOP of 40 Gb/s QPSK DPSB- OFDM at 2000 km by 1.5 dB compared to zero-forcing. For a 100 Gb/s DP-MB-OFDM system, a high number of subcarriers contribute to improved BER but at the cost of digital signal processing computational complexity, whilst by adapting the cyclic prefix length the BER can be improved for a low number of subcarriers. In addition, when 16-quadrature amplitude modulation (16QAM) is employed the digital-toanalogue/ analogue-to-digital converter (DAC/ADC) bandwidth is relaxed with a degraded BER; while the 'circular' 8QAM is slightly superior to its 'rectangular' form. Finally, the transmission of wavelength-division multiplexing DP-MB-OFDM and single-carrier DP-QPSK is experimentally compared for up to 500 Gb/s showing great potential and similar performance at 1000 km DCF-free G.652 line. © 2014 Optical Society of America.

An optical fiber Bragg grating tactile sensor

Tactile sensors are needed for many emerging robotic and telepresence applications such as keyhole surgery and robot operation in unstructured environments. We have proposed and demonstrated a tactile sensor consisting of a fibre Bragg grating embedded in a polymer "finger". When the sensor is placed in contact with a surface and translated tangentially across it measurements on the changes in the reflectivity spectrum of the grating provide a measurement of the spatial distribution of forces perpendicular to the surface and thus, through the elasticity of the polymer material, to the surface roughness. Using a sensor fabricated from a Poly Siloxane polymer (Methyl Vinyl Silicone rubber) spherical cap 50 mm in diameter, 6 mm deep with an embedded 10 mm long Bragg grating we have characterised the first and second moment of the grating spectral response when scanned across triangular and semicircular periodic structures both with a modulation depth of 1 mm and a period of 2 mm. The results clearly distinguish the periodicity of the surface structure and the differences between the two different surface profiles. For the triangular structure a central wavelength modulation of 4 pm is observed and includes a fourth harmonic component, the spectral width is modulated by 25 pm. Although crude in comparison to human senses these results clearly shown the potential of such a sensor for tactile imaging and we expect that with further development in optimising both the grating and polymer "finger" properties a much increased sensitivity and spatial resolution is achievable.

Femtosecond laser microstructuring through optical fibre end faces:inscription of surface gratings and sub-surface splitters

We present the results of femtosecond laser microstructuring of optical fibres by direct access of the fibre end face, both at the surface and several hundred microns into the fibre, to realise one-and two-dimensional grating structures and optical fibre splitters, respectively. We show the versatility of this simple but effective inscription method, where we demonstrate classic multiple slit diffraction patterns and show the potential for coarse wavelength division multiplexing for sensor signals. A key advantage for the fibre splitter is that the inscription method avoids the use of oil immersion that compensate for the fibre curvature in the standard side writing method. © 2012 SPIE.

Implementation of chirped fibre Bragg gratings (CFBG) as optical wear sensors

This study discusses the fabrication and implementation of chirped fiber Bragg grating (CFBG) as optical wear sensors. Such a sensor has potential applications in monitoring grinding and milling machines through a safety device to check the wear on a car brake. The CFBG wear sensor has advantages over existing wear sensing techniques as it does not require electrical current to be passed through the sensor itself. This makes it eligible for application in combustible environments such as in the oil and gas industry.

Optical biochemical sensors based on long-period fibre gratings UV-inscribed in D-fibre with enhanced sensitivity by HF etching process

We report a new concept of biochemical sensor device based on long-period grating structures UV-inscribed in D-fiber. The surrounding-medium refractive index sensitivity of the devices has been enhanced significantly by a hydrofluoric acid etching process. The devices have been used to measure the sugar concentrations showing clearly an encoding relation between the chemical concentration and the grating spectral response, demonstrating their capability for potential biochemical sensing applications.