L.A. Everall

Bragg grating fast tunable filter for wavelength division multiplexing

A Bragg grating fast tunable filter prototype working over a linear tuning range of 45 nm with a maximum tuning speed of 21 nm/ms has been realized. The tunable filter system is based on two piezoelectric stack actuators moving a mechanical device thus compressing an apodized fiber Bragg grating. The filter allows both traction and compression and can work in transmission and in reflection. It is designed to work with a channel spacing of 100 GHz according to the ITU specifications for wavelength division multiplexing systems.

All-fibre optical sensing system: Bragg grating sensor interrogated by a long-period grating

We report here a simple, low-cost all-fibre grating strain sensing interrogation technique offering high static-strain resolution and large dynamic range . A key interrogating component of this technique is a long-period grating which acts as an edge filter converting strain-induced wavelength variation into optical power measurement. The transmission profile of the long-period grating is shown to be nearly linear over a sufficiently wide range, yielding a linear relationship between the resultant intensity and applied strain, and a high resolution achieved by standard lock-in techniques.

Fiber Bragg grating fabrication for dispersion slope compensation

Group-delay dispersion slope causes signal deterioration in long-distance high-data-rate communication systems. We report the fabrication of high-quality apodized and chirped fiber gratings with dispersion slopes as high as -1800 ps/nm/sup -2/ over a 0.7-mn bandwidth-sufficient to compensate for the dispersion slope of >25000-km length of standard fiber at a wavelength of 1550 nm. We believe this is the first time that fiber Bragg gratings have been specifically designed and fabricated for dispersion slope compensation. The fabrication technique uses a standard unchirped phase mask and two scans of the inscribing UV beam.

Fabrication and characterization of bandpass filters based on concatenated chirped fiber gratings

Practical, low-loss ( 30 dB out-of-band rejection over /spl sim/25 mm spectral widths. The approach adopted is based on the concatenation of two or more tailored broadband chirped gratings fabricated with low short-wavelength loss to maximize the rejection and stopband width in a filter with /spl sim/1 nm passband. The effect of thermal aging for similar complex structures is considered in the context of three different fiber types: standard telecommunication, high germania (Ge)-doped and boron/germania (B/Ge)-codoped. It is shown that while hydrogenated B/Ge-codoped fiber exhibits excellent photosensitivity, the resulting decrease in guiding strength and thermal stability may make germania-doped fiber the more attractive choice.

Fabrication of multipassband moiré resonators in fibers by the dual-phase-mask exposure method

We report on the fabrication of in-fiber moiré filters by dual exposure of a nondedicated chirped phase mask. This simple technique produces broadband filters whose structure depends only on an intermediate stretch between two identical UV exposures. We demonstrate moiré filters with as many as four narrow passbands within a 2-nm stopband.

Optical sensor interrogation with a blazed fiber Bragg grating and a charge-coupled device linear array

We present what is to our knowledge the first comprehensive investigation of the use of blazed fiber Bragg gratings (BFBGs) to interrogate wavelength division multiplexed (WDM) in-fiber optical sensor arrays. We show that the light outcoupled from the core of these BFBGs is radiated with sufficient optical power that it may be detected with a low-cost charge-coupled device (CCD) array. We present thorough system performance analysis that shows sufficient spectral-spatial resolution to decode sensors with a WDM separation of 75 rhom, signal-to-noise ratio greater than 45-dB bandwidth of 70 nm, and drift of only 0.1 rhom. We show the system to be polarization-state insensitive, making the BFBG-CCD spectral analysis technique a practical, extremely low-cost, alternative to traditional tunable filter approaches.

Large-dynamic-range and high-resolution from a strain sensing system using long-period grating interrogating FBG strain sensor

We have demonstrated both large-dynamic range (8100 /spl mu//spl epsiv/) and high-resolution (0.5 /spl mu//spl epsiv/) from a strain sensing system using a long-period grating interrogating a fibre Bragg grating sensor. The sensor demonstrates both large dynamic sensing range and high resolution.

A high-performance miniaturized time division multiplexed sensor system for remote structural health monitoring

We report for the first time the design, implementation and commercial application of a hand-held optical time division multiplexed, distributed fibre Bragg grating sensor system. A unique combination of state-of-the art electronic and optical components enables system miniaturization whilst maintaining exceptional performance. Supporting more than 100 low-cost sensors per channel, the battery-powered system operates remotely via a wireless GSM link, making it ideal for real-time structural health monitoring in harsh environments. Driven by highly configurable timing electronics, an off-the-shelf telecommunications semiconductor optical amplifier performs combined amplification and gating. This novel optical configuration boasts a spatial resolution of less than 20cm and an optical signal to noise ratio of better than 30dB, yet utilizes sensors with reflectivity of only a few percent and does not require RF speed signal processing devices. This paper highlights the performance and cost advantages of a system that utilizes TDM-style mass manufactured commodity FBGs. Created in continual lengths, these sensors reduce stock inventory, eradicate application-specific array design and simplify system installation and expansion. System analysis from commercial installations in oil exploration, wind energy and vibration measurement will be presented, with results showing kilohertz interrogation speed and microstrain resolution.

Fast and widely tunable Bragg grating reflection filter

A fiber Bragg grating filter device linearly tunable over 45 nm is presented. The device has a maximum tuning speed of 19 nm/ms with a wavelength setting time below 1.5 ms.

Apodised fibre Bragg grating design subject to length constraints

We investigate apodisation profiles of fibre Bragg gratings to determine key factors in filter design, using a novel apodisation technique. This highlights some practical fabrication limitations and provides important information concerning trade-offs between sidelobe suppression and bandwidth.