Clay K. Kirkendall

Overview of high performance fibre-optic sensing

An overview of fibre-optic interferometry based sensing is given, particularly as it applies to high-performance sensing applications. The operation of a fibre-optic interferometer as a sensor is reviewed. The sensitivity limitations of a fibre-optic sensor are derived, and the system impact of multiplexing many sensors together is explored. A review of the development of the fibre-optic acoustic transducer is presented, as well as system applications and future trends in fibre-optic interferometric sensing.

Distributed Feedback Fiber Laser Strain Sensors

The distributed feedback (DFB) fiber laser strain sensor has demonstrated strain resolution comparable to that obtained from high-performance fiber-optic interferometry. This manuscript describes the characteristics and performance of this fiber laser strain sensor and discusses the technological developments necessary to obtain comparable performance from a multiplexed array of laser sensors. The design of the Bragg grating and doped fiber are discussed, where possible providing simplified equations to quantify the relevant design parameters. Techniques based on fiber-optic interferometry to decode the wavelength shifts of the laser are presented and potential noise sources are described. Measurements conducted on a test laser demonstrate the capability of the DFB fiber laser to resolve effective length changes to less than 0.76 fm/Hz1/2 at 2 kHz. The accuracy of the strain measurement, calculated by subtracting the output of two lasers subjected to the same strain, is found to be less than 1%. Issues relating to multiplexing lasers, such as pump power depletion and optical feedback, are described along with methods to maximize the number of lasers serially multiplexed on a single fiber. Finally, the strain transduction mechanism and methods to mount the laser sensor are described. It is shown that for certain applications, the DFB fiber laser sensor provides significant performance benefits when compared with remotely interrogated fiber-optic interferometric sensing techniques.

Large-scale remotely interrogated arrays of fiber-optic interferometric sensors for underwater acoustic applications

The fiber-optic interferometric acoustic sensor array has established itself as a potential alternative to the conventional sonar array based on electroceramic transducers. In this paper, we discuss all the aspects of a large-scale fiber-optic interferometric sensor array. We review the basic operating principles of the fiber-optic interferometric sensor, signal processing, and multiplexing techniques, we present results from a noise model for a full size system, and we determine the benefit of incorporating a remotely-pumped optical amplifier in the array. As a practical example we describe the design and construction of a prototype array with 96 hydrophones incorporating a remotely pumped erbium-doped fiber amplifier, called the fiber-optic bottom mounted array, which is based on a dense wavelength division and time division multiplexed architecture. These arrays have applications in military sonar and seismic surveying.

Intensity noise characteristics of erbium-doped distributed-feedback fiber lasers

We present an experimental and theoretical investigation into the low-frequency intensity noise characteristics of erbium-doped distributed feedback (DFB) fiber lasers. The intensity noise characteristics of six nonidentical erbium-doped DFB fiber lasers are presented along with the characteristics of the grating and doped fibers. An analytical model has been used to predict the intensity noise generated in a linear fiber laser and explain the observed noise characteristics. Overall we find good agreement between our analytical model and observations. In particular, we find the intensity noise at frequencies close to the relaxation oscillation frequency significantly elevated due to excess noise from either spontaneous emission or cavity loss modulation. These results can be used to optimize the fiber laser design for sensor applications.

Overview of high performance fiber optic sensing

Summary form only given. For the purposes here, a high performance sensing application is defined as one that simultaneously requires high sensitivity (1-100 f/spl epsiv/), wide bandwidth (1-100 kHz), and large dynamic range (>120 dB). High performance fiber optic sensing applications include tactical and surveillance grade underwater acoustics, high sensitivity acceleration, and acoustic and seismic sensing for oil exploration. Traditionally, high performance fiber optic sensor systems have relied on interferometric measurement techniques to extract the strain information from the fiber optic transducer. Distributed Bragg reflector (DBR) fiber laser based systems with interferometric readout approach the sensitivity of interferometric approaches, but have seen little use outside the laboratory The performance of DFB fiber laser sensors can rival interferometric approaches, but there are questions about the multiplexing the lasers and the packaging requirements for stable operation. While interferometric based approaches still offer the highest performance levels with proven multiplexing gain, DFB fiber laser sensors could fill some application requirements in the near future.

Efficient fiber Bragg grating and fiber fabry-Pe/spl acute/rot sensor multiplexing scheme using a broadband pulsed mode-locked laser

A pulsed broadband mode-locked laser (MLL) combined with interferometric interrogation is shown to yield an efficient means of multiplexing a large number of fiber Bragg grating (FBG) or fiber Fabry-Pe/spl acute/rot (FFP) strain sensors with high performance. System configurations utilizing time division multiplexing (TDM) permit high resolution, accuracy, and bandwidth strain measurements along with high sensor densities. Strain resolutions of 23-60 n/spl epsiv//Hz/sup 1/2/ at frequencies up to 800 Hz (expandable to 139 kHz) and a differential strain-measurement accuracy of /spl plusmn/1 /spl mu//spl epsiv/ are demonstrated. Interrogation of a low-finesse FFP sensor is also demonstrated, from which a strain resolution of 2 n/spl epsiv//Hz/sup 1/2/ and strain-measurement accuracy of /spl plusmn/31 n/spl epsiv/ are achieved. The system has the capability of interrogating well in excess of 50 sensors per fiber depending on crosstalk requirements. A discussion on sensor spacing, bandwidth, dynamic range, and measurement accuracy is also given.

Multichannel fiber-optic magnetometer system for undersea measurements

We have designed, fabricated, and operated an undersea array of eight fiber-optic vector magnetometers. Each magnetometer consists of three magnetostrictive transducers aligned on orthogonal axes and incorporated in a single Michelson interferometer. During undersea operation, each interferometer exhibited less than 1 /spl mu/rad//spl radic/ Hz phase noise, and the self-noise of each magnetic transducer was less than 0.2 nT//spl radic/ Hz at 0.1 Hz. We discuss the design and performance of the optical system including noise mechanisms. We present the results of magnetic measurements of the geomagnetic field and the magnetic tracking of ships. >

High-resolution distributed-feedback fiber laser dc magnetometer based on the Lorentzian force

A low-frequency magnetic field sensor, based on a current-carrying beam driven by the Lorentzian force, is described. The amplitude of the oscillation is measured by a distributed-feedback fiber laser strain sensor attached to the beam. The transduction mechanism of the sensor is derived analytically using conventional beam theory, which is shown to accurately predict the responsivity of a prototype sensor. Excellent linearity and negligible hysteresis are demonstrated. Noise sources in the fiber laser strain sensor are described and thermo-mechanical noise in the transducer is estimated. The prototype sensor achieves a magnetic field resolution of 5 nT Hz for 25 mA of current, which is shown to be close to the predicted thermo-mechanical noise limit of the sensor. The current is supplied optically through a separate optical fiber yielding an electrically passive sensor head.

Suppression of double Rayleigh scattering-induced excess noise in remotely interrogated fiber-optic interferometric sensors

This letter describes experimental observations of excess noise due to coherent double Rayleigh scattering in the input fiber of a remotely interrogated fiber-optic interferometric sensor. This noise source is generally only observable when a high coherence length laser is used to interrogate the sensor and the fiber length connecting the sensor is in excess of /spl sim/10 km. We present a simplified model to explain how this noise source affects the sensor resolution and demonstrate a method based on laser source modulation to reduce this noise by /spl sim/20 dB in a fiber-optic Michelson interferometric sensor with a 25-km input fiber, at frequencies less than 10 kHz.

Achieving narrow linewidth low-phase noise external cavity semiconductor lasers through the reduction of 1/f noise

The presence of laser phase noise (or frequency jitter) limits the resolution of a variety of interferometric sensors ranging from fiber optic acoustic sensors to gravitational wave detectors. At low frequencies, 0 to 100 kHz, the laser phase noise in semiconductor and diode pumped solid-state lasers is dominated by 1/f noise, the source of which is not well understood. We report on phase noise measurements for external cavity semiconductor lasers (ECSLs) utilizing a fiber Bragg grating in a compact butterfly package design produced by K2 Optronics. The results show that the phase noise is dominated by 1/f noise for low frequencies (10 to 100 kHz) transitioning to a white noise due to spontaneous emission for f > 100 kHz. We observed a factor of 40 improvement in the magnitude of the 1/f phase noise as compared to previously published results for a Hitachi HLP 1400 830 nm diode laser. The magnitude of the low frequency phase noise ranges from 100 to 10 microradians per meter per root Hz for frequencies ranging from 10 Hz to 2 kHz. These results are within a factor of 10 for phase noise measurements of the more expensive Lightwave Electronics Nd:YAG laser and a variety of Er-doped fiber lasers in this frequency range. For nominally similar ECSLs, experimental results indicate that the phase noise increases for lasers with larger leakage currents. Linewidth measurement results showed a Schawlow-Townes inverse power dependence for output powers up to 33 mWatts with the observed onset of a linewidth floor of 30 kHz. The RIN of the ECSLs varied from -120 to -155 dB Vrms per root Hz for frequencies ranging from 10 to 500 kHz. These RIN results are roughly equal to those observed for the Nd:YAG laser for frequencies less that 100 kHz. In summary, such low phase noise and RIN results make such ECSLs suitable for all but the most sensitive fiber optic sensing applications where the frequency range of interest is below 1 MHz.