Alexei Tikhomirov

A fiber laser hydrophone

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.

Experimental and theoretical characterization of the mode profile of single-mode DFB fiber lasers

The characterization of the spatial mode profile of distributed feedback fiber lasers (DFB-FLs) is discussed. A new approximate solution for the mode profile of single longitudinal mode DFB-FLs with nonuniform phase and grating profiles is derived. An enhanced high resolution heat perturbation method for mode shape characterization is described. Experimental mode shape profiles are compared with theoretical predictions with good agreement.

Pressure compensated distributed feedback fibre laser hydrophone

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.

A 16 channel fibre laser sensor array

A 16 element wavelength division multiplexed linear array of distributed feedback fibre lasers (DFB FL), pumped at 980 nm is reported. The array incorporates DFB FL technology specifically developed for optimal array multiplexing performance. This technology has important applications to all- optical underwater acoustic sensing. An overview of key technical issues in DFB FL multiplexing is also included.

Fundamental limits on low frequency cavity fluctuations in optical fibre lasers

We present new experimental measurements of cavity fluctuations in short cavity fibre lasers at frequecies below 100Hz. The measured strain spectral density is less than 1 picostrain/√Hz in a 0.6cm cavity at 5Hz corresponding to an equivalent length fluctuation of less than 10fm/√Hz. We believe these results are the most sensitive measurements of free running cavity fluctuations in opical fibre yet reported.

Fast Four Step Digital Demodulation for Multiplexed Fibre Laser Sensors

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.

How sensitive is the fibre laser strain 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.

DFB FL sensor multiplexing noise

Up to 50 dB frequency noise increase was demonstrated for multiplexed DFB FL. Laser noise limitations of acoustic sensitivity of DFB FL sensors are discussed and ways to reduce DFB FL sensitivity to external reflections proposed.

Demonstration of an advanced fibre laser hydrophone array in Gulf St Vincent

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.

Field demonstration of an eight-element fiber laser hydrophone array

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.