Interferometric sensor interrogation using an arrayed waveguide grating

We experimentally investigate the use of an arrayed waveguide grating (AWG) to interrogate interferometric sensors. A single broad-band light source is used to illuminate the system. Reflected spectral information is directed to an AWG with integral photodetectors providing 40 electrical outputs. We show that using the dual-wavelength technique we can measure the length of a Fabry-Perot cavity by determining the optical phase changes of the scanned interferometric pattern, which produced a maximum unambiguous range of 1440 mum with an active sensor and a maximum unambiguous range of 300 mum with the introduction of a second processing interferometer, which allows the sensor to be passive.

Optical in-fiber Bragg grating sensor systems for medical applications

Two in-fiber Bragg grating (FBG) temperature sensor systems for medical applications are demonstrated: (1) an FBG flow-directed thermodilution catheter based on interferometric detection of wavelength shift that is used for cardiac monitoring; and (2) an FBG sensor system with a tunable Fabry-Perot filter for in vivo temperature profiling in nuclear magnetic resonance (NMR) machines. Preliminary results show that the FBG sensor is in good agreement with electrical sensors that are widely used in practice. A field test shows that the FBG sensor system is suitable for in situ temperature profiling in NMR machines for medical applications.

The application of fibre Bragg grating networks as strain sensors and as phased array antenna true time delay elements

The fabrication of in-fibre Bragg gratings, and the application of arrays of such gratings as strain sensors and as true time delay elements for the control of phased array antennas is reported. Chirped period Bragg gratings were produced using the fibre deformation fabrication technique, with chirps of between 2.9nm and 17.3nm achieved. Arrays of 5mm and 2mm long uniform period Bragg gratings were fabricated using the inscription method, for use as true time delay elements,dissimilar wavefronts and their spectral characteristics recorded. The uniform period grating arrays were used to create minimum time delays of 9.09ps, 19.02ps and 31ps; making them suitable for controlling phased array antennas operating at RF frequencies of up to 3GHz, with 10° phase resolution. Four 4mm long chirped gratings were produced using the dissimilar wavefronts fabrication method, having chirps of 7nm, 12nm, 20nm and 30nm, and were used to create time delays of between 0.3ps and 59ps. Hence they are suitable for controlling phased array antennas at RF frequencies of up to 48GHz. The application of in fibre Bragg gratings as strain sensors within smart structure materials was investigated, with their sensitivity to applied strain and compression measured for both embedded and surface mounted uniform period and fibre Fabry-Perot filter gratings. A fibre Bragg grating sensor demultiplexing scheme based on a liquid crystal filled Fabry-Perot etalon tuneable transmission filter was proposed, successfully constructed and fully characterised. Three characteristics of the LCFP etalon were found to pose operational limitations to its application in a Bragg grating sensor system; most significantly, the resonance peak wavelength was highly (-2,77nm/°C) temperature dependent. Several methods for minimising this temperature sensitivity were investigated, but enjoyed only limited success. It was therefore concluded that this type (E7 filled) of LCFP etalon is unsuitable for use as a Bragg grating sensor demultiplexing element.

Sensing properties of germanate and tellurite glass optical fibres

Strain and thermal sensitivities of germanate and tellurite glass fibres were measured using a fibre Fabry-Perot (FFP) interferometer and fibre Bragg gratings (FBG). The strain phase sensitivity for germanate and tellurite fibre were 5900×103 rad/m and 5600×103 rad/m respectively at a central wavelength of 1540nm using FFP interferometer, which is consistent with the value of 1.22pm/µepsilon obtained for a germanate fibre FBG. The Young's modulus for germanate and tellurite fibre were also measured to be 58GPa and 37GPa. The thermal responses of germanate fibre were examined as 24.71 and 16.80 pm/°C at 1540nm and 1033nm wavelength using the FBG.

Acoustic emission detection using optical fibre sensors for aerospace applications

Structural Health Monitoring (SHM) ensures the structural health and safety of critical structures covering a wide range of application areas. This thesis presents novel, low-cost and good-performance fibre Bragg grating (FBG) based systems for detection of Acoustic Emission (AE) in aircraft structures, which is a part of SHM. Importantly a key aim, during the design of these systems, was to produce systems that were sufficiently small to install in an aircraft for lifetime monitoring. Two important techniques for monitoring high frequency AE that were developed as a part of this research were, Quadrature recombination technique and Active tracking technique. Active tracking technique was used extensively and was further developed to overcome the limitations that were observed while testing it at several test facilities and with different optical fibre sensors. This system was able to eliminate any low frequency spectrum shift due to environmental perturbation and keeps the sensor always working at optimum operation point. This is highly desirable in harsh industrial and operationally active environments. Experimental work carried out in the laboratory has proved that such systems can be used for high frequency detection and have capability to detect up to 600 kHz. However, the range of frequency depends upon the requirement and design of the interrogation system as the system can be altered accordingly for different applications. Several optical fibre configurations for wavelength detection were designed during the course of this work along with industrial partners. Fibre Bragg grating Fabry-Perot (FBG-FP) sensors have shown higher sensitivity and usability than the uniform FBGs to be used with such system. This was shown experimentally. The author is certain that further research will lead to development of a commercially marketable product and the use of active tracking systems can be extended in areas of healthcare, civil infrastructure monitoring etc. where it can be deployed. Finally, the AE detection system has been developed to aerospace requirements and was tested at NDT & Testing Technology test facility based at Airbus, Filton, UK on A350 testing panels.

Grating based devices in polymer optical fibre

We describe recent research into devices based on fibre Bragg gratings in polymer optical fibre. Firstly, we report on the inscription of gratings in a variety of microstructured polymer optical fibre: single mode, few moded and multimoded, as well as fibre doped with trans-4-stilbenmethanol. Secondly, we describe research into an electrically tuneable filter using a metallic coating on a polymer fibre Bragg grating. Finally we present initial results from attempts to produce more complex grating structures in polymer fibre: a Fabry-Perot cavity and a phase-shifted grating.

Optical in-fiber Bragg grating sensor systems for medical applications

Two in-fiber Bragg grating (FBG) temperature sensor systems for medical applications are demonstrated: (1) an FBG flow-directed thermodilution catheter based on interferometric detection of wavelength shift that is used for cardiac monitoring; and (2) an FBG sensor system with a tunable Fabry-Perot filter for in vivo temperature profiling in nuclear magnetic resonance (NMR) machines. Preliminary results show that the FBG sensor is in good agreement with electrical sensors that are widely used in practice. A field test shows that the FBG sensor system is suitable for in situ temperature profiling in NMR machines for medical applications.

Photonic processing for wideband cancellation and spectral discrimination of RF signals

Photonic signal processing is used to implement common mode signal cancellation across a very wide bandwidth utilising phase modulation of radio frequency (RF) signals onto a narrow linewidth laser carrier. RF spectra were observed using narrow-band, tunable optical filtering using a scanning Fabry Perot etalon. Thus functions conventionally performed using digital signal processing techniques in the electronic domain have been replaced by analog techniques in the photonic domain. This technique was able to observe simultaneous cancellation of signals across a bandwidth of 1400 MHz, limited only by the free spectral range of the etalon. © 2013 David M. Benton.

Enhanced self-coherent OFDM by the use of injection locked laser

An injection-locked laser based pilot carrier enhancement technique is proposed for self-coherent OFDM. An improvement of 2.2 dB is observed when compared to Fabry-Perot filter based self-coherent OFDM. © 2012 OSA.

Interferometric and fibre Bragg grating sensor interrogation using an arrayed waveguide grating

We investigate the use of an arrayed waveguide grating (AWG) to interrogate both fibre Bragg grating (FBG) and interferometric sensors. A broadband light source is used to illuminate both the FBG and interferometric sensors. Reflected spectral information is directed to an AWG with integral photodetectors providing 40 electrical outputs. To interrogate interferometric sensors we investigated the dual wavelength technique to measure the distance of a Fabry-Perot cavity, which produced a maximum unambiguous range of 1440μm with an active sensor. Three methods are described to interrogate FBG sensors. The first technique makes use of the reflected light intensity in an AWG channel passband from a narrow bandwidth grating, giving a usable range of 500με and a dynamic strain resolution of 96nε/√Hz at 30Hz. The second approach utilises wide gratings larger than the channel spacing of the AWG; by monitoring the intensity present in corresponding AWG channels an improved range of 1890με was achieved. The third method improves the dynamic range by utilising a heterodyne approach based on interferometric wavelength shift detection providing a dynamic strain resolution of 17nε/√Hz at 30Hz.

Noise suppression in a harmonically mode-locked fibre ring laser

A simple and efficient method to stabilise harmonically mode-locked fibre ring laser is proposed. In this method, a linear optical filter and a nonlinear Fabry-Perot filter are introduced into the laser cavity. Stable harmonically mode-locked pulses with supermode noise suppression ratio more than 55dB was demonstrated.

Sensing properties of germanate and tellurite glass optical fibres

Strain and thermal sensitivities of germanate and tellurite glass fibres were measured using a fibre Fabry-Perot (FFP) interferometer and fibre Bragg gratings (FBG). The strain phase sensitivity for germanate and tellurite fibre were 5900×103 rad/m and 5600×103 rad/m respectively at a central wavelength of 1540nm using FFP interferometer, which is consistent with the value of 1.22pm/µepsilon obtained for a germanate fibre FBG. The Young's modulus for germanate and tellurite fibre were also measured to be 58GPa and 37GPa. The thermal responses of germanate fibre were examined as 24.71 and 16.80 pm/°C at 1540nm and 1033nm wavelength using the FBG.

Wavelength-division and spatial multiplexing using tandem interferometers for Bragg grating sensor networks

We present a new method for the interrogation of large arrays of Bragg grating sensors. Eight gratings operating between the wavelengths of 1533 and 1555 nm have been demultiplexed. An unbalanced Mach—Zehnder interferometer illuminated by a single low-coherence source provides a high-phase-resolution output for each sensor, the outputs of which are sequentially selected in wavelength by a tunable Fabry-Perot interferometer. The minimum detectable strain measured was 90 ne-vHz at 7 Hz for a wavelength of 1535 nm.

Optimisation of polymer optical fibre based interferometric sensors

A numerical model for studying the performance of polymer optical fibre-based interferometric sensors is presented. The strain sensitivity of Fabry-Perot and two-beam interferometric sensors is investigated by varying the physical and optical properties corresponding to frequently used wavelengths. The developed model was used to identify the regimes in which these devices offer enhanced performance over their silica counterparts when used for stress sensing. © (2015) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Evaluation of 100G DP-QPSK long-haul transmission performance using second order co-pumped Raman laser based amplification

We present, for the first time, a detailed investigation of the impact of second order co-propagating Raman pumping on long-haul 100G WDM DP-QPSK coherent transmission of up to 7082 km using Raman fibre laser based configurations. Signal power and noise distributions along the fibre for each pumping scheme were characterised both numerically and experimentally. Based on these pumping schemes, the Q factor penalties versus co-pump power ratios were experimentally measured and quantified. A significant Q factor penalty of up to 4.15 dB was observed after 1666 km using symmetric bidirectional pumping, compared with counter-pumping only. Our results show that whilst using co-pumping minimises the intra-cavity signal power variation and amplification noise, the Q factor penalty with co-pumping was too great for any advantage to be seen. The relative intensity noise (RIN) characteristics of the induced fibre laser and the output signal, and the intra-cavity RF spectra of the fibre laser are also presented. We attribute the Q factor degradation to RIN induced penalty due to RIN being transferred from the first order fibre laser and second order co-pump to the signal. More importantly, there were two different fibre lasing regimes contributing to the amplification. It was random distributed feedback lasing when using counter-pumping only and conventional Fabry-Perot cavity lasing when using all bidirectional pumping schemes. This also results in significantly different performances due to different laser cavity lengths for these two classes of laser.