Ian C. M. Littler

High-resolution absolute frequency referenced fiber optic sensor for quasi-static strain sensing.

We present a quasi-static fiber optic strain sensing system capable of resolving signals below nanostrain from 20 mHz. A telecom-grade distributed feedback CW diode laser is locked to a fiber Fabry-Perot sensor, transferring the detected signals onto the laser. An H(13)C(14)N absorption line is then used as a frequency reference to extract accurate low-frequency strain signals from the locked system.

Subpicometer length measurement using heterodyne laser interferometry and all-digital rf phase meters

We present an all-digital phase meter for precision length measurements using heterodyne laser interferometry. Our phase meter has a phase sensitivity of 3 μrad/√Hz at signal frequencies of 1 Hz and above. We test the performance of our phase meter in an optical heterodyne interferometric configuration, using an active Sagnac interferometer test bed that is flexible and low noise. We demonstrate more than 70 dB of laser frequency noise suppression to achieve an optical phase sensitivity of 5 μrad/√Hz and a corresponding displacement sensitivity of 0.5 pm/√Hz at signal frequencies above 10 Hz. In addition, we demonstrate the ability of our phase meter to follow full fringe signals accurately at 100 Hz and to track large signal excursions in excess of 10(5) fringes without cycle slipping. Finally, we demonstrate a cyclic error of ≤1 pm/√Hz, above 10 Hz.

Pico-strain multiplexed fiber optic sensor array operating down to infra-sonic frequencies

An integrated sensor system is presented which displays passive long range operation to 100 km at pico-strain (pepsilon) sensitivity to low frequencies (4 Hz) in wavelength division multiplexed operation with negligible cross-talk (better than -75 dB). This has been achieved by pre-stabilizing and multiplexing all interrogation lasers for the sensor array to a single optical frequency reference. This single frequency reference allows each laser to be locked to an arbitrary wavelength and independently tuned, while maintaining suppression of laser frequency noise. With appropriate packaging, such a multiplexed strain sensing system can form the core of a low frequency accelerometer or hydrophone array.

Using active resonator impedance matching for shot-noise limited, cavity enhanced amplitude modulated laser absorption spectroscopy

We introduce a closed-loop feedback technique to actively control the coupling condition of an optical cavity, by employing amplitude modulation of the interrogating laser. We show that active impedance matching of the cavity facilitates optimal shot-noise sensing performance in a cavity enhanced system, while its control error signal can be used for intra-cavity absorption or loss signal extraction. We present the first demonstration of this technique with a fiber ring cavity, and achieved shot-noise limited loss sensitivity. We also briefly discuss further use of impedance matching control as a tool for other applications.

Digital Laser Frequency Stabilization Using an Optical Cavity

Pound-Drever-Hall locking is a high performance technique for laser frequency stabilization. Radio frequency optical modulation combined with electronic demodulation provides a feedback signal to lock a lasers frequency to a cavity resonance. Although the demodulation and feedback system are typically implemented using analog electronics, digital implementations are now possible thanks to recent advances in digital signal processing. Aside from flexibility, digital systems have improved performance at low frequencies where electronic noise may be a problem. In this paper we analyze several noise sources that appear in a digital Pound-Drever-Hall locking loop and estimate their effect on the performance of the stabilization system. Furthermore, we implement a digital Pound-Drever-Hall scheme and characterize the feedback performance by beating two lasers locked to a single cavity. A relative frequency noise floor of 0.2 Hz/¿(Hz) above 3 Hz was measured, giving an upper bound on the noise performance of the digital system.

A Stabilized Fiber Laser for High-Resolution Low-Frequency Strain Sensing

We frequency stabilize a fiber laser for use in low-frequency sensing applications. Using a radio frequency locking technique, an Erbium-doped single longitudinal mode fiber laser is stabilized to a Mach-Zehnder interferometer. The low-frequency fiber laser noise was suppressed by more than 1.5 orders of magnitude at frequencies below 300 Hz reaching a minimum of 2 Hz/radicHz between 60 and 250 Hz. The corresponding strain sensitivities are 2 pepsiv/radicHz at 1 Hz and 15 fepsiv/radicHz from 60 to 250 Hz.

Quasi-static fiber strain sensing with absolute frequency referencing

We present a highly sensitive detection system for quasi-static strain, employing radio-frequency modulation interferometry and absolute frequency referencing, demonstrating a few tens of pε/√Hz sensitivity between 1 - 6 Hz.

Multiplexed fiber optic acoustic sensors in a 120 km loop using RF modulation

The interrogation, via optical fiber, of fiber Fabre Perot interferometers using laser based radio frequency modulation techniques, can provide ultra-sensitive acoustic sensing over very long distances. The benefits over other fiber optical acoustic sensing schemes include; immunity to laser polarization, coherence and intensity noise as well as reduced susceptibility to Rayleigh back scattering. Well defined error signals can be extracted at up to 120 km away. We report on the first multiplexed system, based on RF modulation interrogation techniques, in a 100 km fiber loop. We examine the achievable channel density as well as potential limits to strain sensitivity, such as inter-channel crosstalk, in a multiplexed RF modulated sensor system. The light-weight, small cross-section, intrinsic reliability, sensitivity and remote operation of the fiber sensor array based on RF techniques, enable new applications in hostile environments. The technique is free of electronics in the array part of the system, with all the electronic processing and control located remotely. There are no optical amplifiers or pump lasers - the technique is entirely passive. With appropriate packaging, an array of either hydrophones or geophones may be created with applications in security and defense as well as in geological survey.

Optical-Fiber Accelerometer Array: Nano-g Infrasonic Operation in a Passive 100 km Loop

In this paper, an all-optical accelerometer array system is presented, ideally suited for permanent seismic seabed arrays and passive surveillance. A directly measured accelerometer resolution of better than 60 ng/¿Hz (-145 dB/ reg/¿Hz) down to 10 Hz is achieved for all channels in a completely passive 100 km fiber loop. The detection bandwidth is chosen to be from 10 to 300 Hz, with a closed-loop dynamic range of 120 dB. The crosstalk between channels spaced by 100 GHz is better than -64 dB.

Passive nano-g fiber-accelerometer array over 100 km

A fiber accelerometer array is presented with an unprecedented breakthrough combination of high acceleration resolution after 100 km of fiber, in a bandwidth down to the infrasonic, with high multiplexing density and low crosstalk. The demonstrated resolution is better than 60 ng/√Hz for all channels down to 10 Hz, even after the 100 km length of fiber. Moreover, the system can accommodate 80 channels per fiber in wavelength division multiplexed operation with better than -64 dB crosstalk. The dynamic range is 120 dB in a 300 Hz bandwidth.