We demonstrate a grating recording method using an excimer KrF UV laser for batch imprinting of distributed feedback (DFB) fiber lasers (FL) with complex profiles and high grating strength (up to K<4 cm-1) for underwater acoustic sensing applications. The specially developed FPGA-controlled stepping algorithm allows for the recording of long (up to 52mm) DFB laser structures with step error compensation, chirping and apodisation for the FL side-lobe suppression at arbitrarily high exposures. Batch recording involving simultaneous exposure of a number of fibers placed in the recording zone, to achieve time and cost reduction during manufacturing is demonstrated. A number of different gratings in a passive fiber as well as DFB FL in Er-doped active fiber have been fabricated and studied. Further ways to improve the recording process are proposed.
We demonstrate fabrication and characterizations of intrinsically magneto-sensitive fiber with potential applications as a high-efficiency remote magnetic field sensing platform. The fibre was fabricated using lead-silicate glass and the rod-intube fibre drawing technique. The thin glass rod of ~1 mm diameter was first coated with nitrogen-vacancy (NV) centreenriched diamond particles of ~1 μm diameter, and subsequently inserted into the glass outer tube. This rod-in-tube assembly was drawn down to fibre, with the diamond particles distributed at the fused interface between rod and tube. We experimentally coupled 532 nm continuous-wave laser into a 30-cm-length fibre piece from the fibre endface, and examined the photoluminescence (PL) properties of the fibre from both the side of the fibre and the output end of the fibre. PL mapping results showed that the glass-embedded NV emitters showed bright and photostable fluorescence, demonstrating characteristic NV centre zero phonon line emission. Moreover, the mapping result obtained at the output end of fibre indicated that the transmitted NV fluorescence was coupled into the propagation modes of the fibre. By using optically detected magnetic resonance (ODMR) from the NV ensemble along the fibre, we demonstrate detection of local magnetic fields via longitudinal excitation and side collection. Based on the current light transmission and collection configuration, the hybrid diamond-glass optical fibre sensor demonstrated a shot noise-limited DC magnetic field sensitivity of 3.7 μT/√Hz at room temperature. Our results open the possibility of robust, field-deployable fibre optical magnetometry.
We present fundamental limits on the strain resolution of phase sensitive distributed acoustic sensing based on considerations of thermal noise. These limits suggest that, despite ongoing advances, distributed sensors are unlikely to approach the sensing performance of traditional geophones and hydrophones.
We have developed an 8-element fibre laser seabed array demonstrating state-of-the art performance characteristics for a
fibre laser sensing system. The system employs sea-state-zero sensitivity hydrophones with a flat acoustic response over
a bandwidth exceeding 5kHz and very low inertial sensitivity. The system contains no outboard electronics and few metal
components making it extremely light, compact, and low complexity. The array may be deployed up to 4 km from a land
or sea based platform to a depth of up to 80m. Power delivery and telemetry for all 8 sensors is achieved via a single 2mm
diameter optical fibre cable weighing less than 5kg per km. We report here results of the first field trials of this system.
We have developed an 8-element fibre laser seabed array demonstrating state-of-the art performance characteristics for a fibre laser sensing system and highlighting the advantage this technology provides in the underwater sensing domain. The system employs sea-state-zero sensitivity hydrophones with a flat acoustic response over a bandwidth exceeding 5kHz and very low inertial sensitivity. The system contains no outboard electronics and few metal components making it extremely light, compact, and low complexity. The array may be deployed up to 4 km from a land or sea based platform to a depth of up to 80m. Power delivery and telemetry for all 8 sensors is achieved via a single 2mm diameter optical fibre cable weighing less than 5kg per km. We report here results of the first field trials of this system.
An analysis of the effect of weak feedback on Bragg grating fibre laser output parameters is presented and a simple sensor
arrangement based on monitoring intensity fluctuations of a DFB fibre laser with a weak external reflector is considered.
Calculations suggest that it may be possible, under certain conditions, to resolve changes to the length of the external cavity
at close to the thermal noise limit. This introduces the possibility of achieving strain resolution in the sub-picostrain regime
in the low frequency domain where resolution of conventional fibre laser sensors is limited by 1/f noise.
We report on the first field demonstrations of a depth compensated, light weight, compact fibre laser hydrophone array
for a rapidly deployable seabed SONAR system. The demonstration was held in Jervis Bay off the east coast of
Australia. Using SONAR processing techniques, we were able to detect, characterise, and coherently process multiple
The fiber laser strain sensor promises a dramatic performance benefit impacting many high
performance sensor applications. This would lead to smaller, lower cost sensors operating over a broader
frequency range. This paper explains why.
An acoustic vector sensor based on distributed feedback fibre laser technology is reported. The sensor is designed as an
accelerometer that will sense acceleration induced by the difference in acoustic pressure across the sensor. Experimental
measurements demonstrate that the sensor has excellent response characteristics and sufficient cross-axis discrimination
to be a useful acoustic vector sensor.
It is known that the strain resolution of fibre laser sensors is determined by frequency noise of the laser sensor element.
We compare the frequency noise of a number of distributed feedback fibre laser sensors and find that all exhibit similar
dominant 1/f noise below 10kHz. Examination of current theories of thermodynamic noise and 1/f noise in fibre lasers
suggests that these lasers may be close to the theoretical limits of strain sensitivity.
A pressure compensated distributed feedback fibre laser hydrophone is described. Pressure compensation is achieved by
incorporating an air-filled bladder and acoustic filter into the hydrophone making it insensitive to hydrostatic pressures to
depths of up to 50m.
We demonstrate an electro-optic phase modulator based demodulation system for acoustic frequency modulated
optical signals. The system is free from polarisation fading and is capable of retrieving high frequency signals,
ultimately limited by the time of flight of the interferometer itself. Digital interrogation method is based on the Carré
algorithm and does not require fine tuning of the introduced phase and is suitable for simultaneous phase retrieval at
multiple wavelengths without the need for correction transfer filters.
A new fibre optic hydrophone based on a distributed feedback fibre laser (DFB FL) sensor is described. The sensor is designed to achieve ocean noise limited pressure sensitivity. Unlike previous fibre laser acoustic sensors, this device is acceleration insensitive making it less susceptible to vibrational noise. Experimental results for one implementation of the sensor are presented.