Steven E. Staines

In-line fiber-optic components using Langmuir–Blodgett films

Langmuir-Blodgett (LB) films of omega-tricosenoic acid are deposited onto side-polished optical fibers, resulting in wavelength-selective elements. The wavelength response is shown to be strongly dependent on film thickness, giving a shift in central wavelength with film thickness of Deltalambda(0)/Deltad = 2.4, in agreement with a simple phasematching model. It is demonstrated that an alternative polishing technique, requiring no substrates or epoxy, allows more repeatable LB deposition and produces devices with similar characteristics. This is the only technique that will allow deposition of more complex electro-optic LB film-forming molecules.

Railway track component condition monitoring using optical fibre Bragg grating sensors

The use of optical fibre Bragg grating (FBG) strain sensors to monitor the condition of safety critical rail components is investigated. Fishplates, switchblades and stretcher bars on the Stagecoach Supertram tramway in Sheffield in the UK have been instrumented with arrays of FBG sensors. The dynamic strain signatures induced by the passage of a tram over the instrumented components have been analysed to identify features indicative of changes in the condition of the components.

Wavelength stabilisation of a DFB laser diode using measurement of junction voltage

Laser diode wavelength stability is vital for applications such as spectroscopy and data communication, and the emitted wavelength is a function of temperature. In a conventional system, the laser diode temperature is controlled using a Peltier element with a temperature-sensing thermistor, the latter placed at a short distance from the laser diode chip. Despite the use of good thermal design and a case, a change in ambient temperature may cause a change to internal thermal gradients, resulting in a systematic error in the laser diode wavelength. In this paper we describe a novel system to measure the temperature of the laser diode junction via measurement of the junction voltage. The method has been applied to a 1651 nm DFB laser diode for use in tunable diode laser spectroscopy (TDLS) of methane. The wavelength stability of both thermistor- and voltage- control systems are compared over a period of 30 minutes and with different ambient temperatures. Over 30 min at constant ambient temperature, thermistor control provided a precision of ± 0.4 pm (40 MHz) and junction voltage control gave a similar ± 0.6 pm (70 MHz). For an ambient temperature change of 20°C, conventional thermistor control suffered a wavelength change of 76 pm (8.4 GHz), whereas junction voltage control reduced this to 0.6 pm (70 MHz), at or below the level of long-term wavelength precision.

Polarization-maintaining fiber Bragg grating interrogation system for multi-axis strain sensing

Fiber Bragg gratings (FBGs) written into polarizations maintaining (PM) fibers have been employed for multi-axis strain and temperature sensing. In this paper we report the development of a FBG interrogation system that is capable of detecting independently the two orthogonally polarized signals reflected from a PM FBG. The interrogation system imposes no limitation on the measurable strain range. This relaxes the requirements on the bandwidth of the grating spectrum, allowing the use of a shorter PM FBG (larger bandwidth) for sensing applications where a higher spatial resolution is required. In particular, this system is useful in the cases where the spectra of the reflected polarized signals are broadened or split, resulting from strain-induced structural changes of the PM fibre, or caused by the application of a non-uniform strain on the FBG.

Bragg-grating-stabilized external cavity lasers for gas sensing using tunable diode laser spectroscopy

Conventional singlemode semiconductor DFB and VCSEL lasers used in high resolution spectroscopy are often required to operate at specific, custom wavelengths, such as those associated with gas absorption lines. We present the results of work to develop alternative sources in the 1550nm and 1650nm regions, the latter coinciding with an absorption line of methane. Custom wavelength Bragg gratings have been used to stabilize the output of external cavity lasers implemented in both optical fiber and planar silica-on-silicon integrated circuits, using commercially available semiconductor gain chips, to give laser output at 1648 and 1649 nm, respectively. Thermal expansion or mechanical strain of the Bragg grating offers a suitable wavelength tuning mechanism. Results are presented including the wavelength tuning range, output power, relative intensity noise (RIN), side-mode suppression and linewidth of devices for application in high resolution gas spectroscopy. The different methods of writing Bragg gratings in optical fiber and planar silica-on-silicon allow a high degree of flexibility in the choice of emission wavelength.

A technique for depositing non-centrosymmetric Langmuir-Blodgett films onto optical fibres

A novel technique for depositing non-centrosymmetric Langmuir-Blodgett films onto side-polished, single-mode optical fibres has been developed. The main feature is the ability to transfer the monolayer only on the up-stroke and thus, for suitable film-forming materials, to fabricate non-centrosymmetric Z-type structures in which the molecular dipoles are aligned. Strong second-harmonic generation has been observed from a multilayer film of E-N-octadecyl-4-[2-(4-dibutylaminophenyl)ethenyl]quinolinium octadecylsulphate deposited onto a single-mode optical fibre.

Toward track component condition monitoring using optical fibre Bragg grating sensors

Optical fibre Bragg grating sensors have been field-trialed for the monitoring of dynamic loading of fish-plates, stretcher bars and switchblades on a tram network, with the aim of developing a condition monitoring system. This paper provides preliminary data showing the ability to identify changes in track/component condition.

A 1.65 µm region external cavity laser diode using an InP gain chip and a fibre Bragg grating

We present the construction of an external cavity laser (ECL) diode using an InP semiconductor gain chip and a fibre Bragg grating (FBG), designed to have an emission wavelength that coincides with an absorption line of methane, to be used for spectroscopic gas sensing. The FBG was employed as a wavelength selective and feedback element, which will potentially provide the laser with wavelength tuning capability. Narrow linewidth lasing output of less than 5 MHz was achieved. To our knowledge, this is the first FBG based ECL in the 1.65 µm region.

Passive and active in-line fiber components using Langmuir-Blodgett films on monomode optical fiber

Efficient low loss components that are compatible with optical fibers are essential for the successful implementation of many single mode fiber optic sensor systems. In this paper we investigate the use of Langmuir-Blodgett (LB) films as waveguiding overlays evanescently coupled to side polished optical fibers which result in useful in-line wavelength selective elements. We demonstrate the use of two LB film- forming materials: firstly a simple fatty acid is used for a passive channel dropping device and secondly it is shown that hundreds of layers of a more complex electro-optic molecule may be successfully deposited onto fibers prepared using an alternative polishing technique, which does not require extensive surface preparation.

Passive and active in-line fiber components using high index overlays on monomode optical fiber

As single mode fiber optic sensors become more complex there is an increasing need for efficient low loss components that are compatible with optical fibers. In this paper we investigate two methods of constructing in-line fiber optic channel dropping filters with bandwidths in the range 7 nm to 40 nm and insertion losses <EQ 0.2 dB. Both devices exhibit a degree of thermal tunability.