Laser frequency fluctuations limit the ultimate resolution in interferometric fiber sensors. In this work, we demonstrate
an interferometric sensor insensitive to the effects of frequency change in an interrogating laser. A system is
characterized showing a minimum of 4.5 orders of magnitude frequency change reduction, and a demonstrated
broadband improvement of up to 1.5 orders of magnitude for signals between 100 mHz and 1 Hz. Using this technique a
resolution of less than a nanostrain/rtHz was achieved for a broad range of frequencies.
The resolution of fiber optic interferometry sensors is often limited by frequency noise in the laser. For this reason, prestabilization
techniques have been used to reduce laser frequency fluctuations and improve signal resolution. However,
for multi-element systems this becomes cumbersome and difficult to implement. In this paper, we demonstrate the use of
digitally-enhanced interferometry for the interrogation of a multi-element sensing system. Over 50 dB of cross-talk
rejection was found, with displacement resolutions of ~ 100 pm. Furthermore, using this technique, sub-frequency noise
displacement resolution was obtained without the need for high performance sensors.
Steady developments in cost and reliability in fiber optic sensors have seen an increase of their deployment in numerous
monitoring and detection applications. In high-end applications, greater resolution is required, especially in systems
where the environment is quiet, but the signal is weak. In order to meet these requirements the most dominant noise
source, laser frequency noise, must be reduced. In this paper we present a quasi-static strain sensing referenced to a
molecular frequency reference. A DFB CW diode laser is locked to a fiber Fabry-Perot sensor, transferring the detected
signals onto the laser frequency and suppressing laser frequency noise. The laser frequency is then read off using an
H13C14N absorption line. Phase modulation spectroscopy is used to both lock the laser to the sensor and read off the
signals detected by the sensor. The technique is capable of resolving signals below 1 nanostrain from 20 mHz, reaching a
white noise floor of 10 picostrain at several Hz.
We report on the development of a high-bandwidth, 3-axis accelerometer using π-shifted fibre Bragg gratings. The
gratings are inserted into a cantilever system and interrogated by three separate telecom-grade distributed-feedback diode
lasers. These lasers are frequency locked to the sensors using a radio-frequency Pound-Drever-Hall technique. This
scheme reduced the effect of laser's amplitude noise and allowed for an acceleration dynamic range of at least 20 g. An
operation bandwidth of 500 Hz is demonstrated so far with sensitivities between 20 and 50 μg/√Hz.
We present the latest results from our multiplexed fiber optic Fabry-Perot acoustic sensor array using modulated lasers. It
offers the possibility of a robust, reliable and easy to deploy system, meeting the demands of geophysical survey.
We propose active feedback frequency locking of a single longitudinal mode fiber laser to passive, high Q fiber
ring cavities, for filtering excess relative intensity noise, thereby producing shot-noise limited light source for
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 -
We propose and demonstrate a Mach-Zehnder-Sagnac hybrid interferometer for precision sensing. This configuration facilitates immunity from Rayleigh backscatter, polarization wander and scale factor drift in high
performance fiber gyroscopes.
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.
One of the main factors limiting high performance remote fiber sensing systems is the Rayleigh backscatter
associated with a long length of optical delivery fiber. Rayleigh backscatter introduces amplitude and phase
noise during interferometric signal extraction, resulting in degradation of system sensitivity. This noise source
increases with the length of optical fiber used in the architecture, and thus traditionally sets the lower limit on
signal strength and the total remote sensing distance. We present the latest results for a 100 km remote fiber
dynamic strain sensing system, where a radio-frequency (RF) modulated laser is used to interrogate a fiber Fabry-
Perot sensor. The signal extraction is derived interferometrically from the differential phase between the carrier
and its RF sidebands. We demonstrate unprecedented remote sensitivity performance by complete mitigation of
the debilitating effects associated with Rayleigh backscatter in the 100 km of optical delivery fiber. We show that
optimization of the laser modulation depth, as well as fiber Fabry-Perot design both facilitate a large signal-to-noise
ratio. This maximized signal-to-noise ratio enables the complete suppression of the noise associated with
Rayleigh backscatter. The result is a long-distance remote fiber sensing system that is limited only by the laser
frequency noise. This remote sensitivity is an important breakthrough for a range of applications, such as sea
floor acoustic sensing arrays, deep sea hydrophone arrays, and remote surveillance arrays.
We use a radio-frequency (RF) diode laser modulation technique to interrogate a fiber Fabry-Perot (FFP), and demonstrate unprecedented remote sensitivity performance for measuring fiber dynamic strain. We present results for its experimental demonstration in a 5 km remote strain sensing system, where we have attained sub-picostrain/√Hz resolution in an acoustic signal band from 100 Hz to 100 kHz, with better than 300 femtostrain/√Hz sensitivity above 300 Hz. This is unprecedented in sensitivity and broadband performance, unparalleled over such a long interrogation distance. Strain signals are extracted interferometrically from the differential phase between the carrier and its RF sidebands. This elegant architecture is immune to intensity noise in the laser, as well as ambient acoustic and mechanical perturbations in the remote delivery fiber. The excellent frequency discrimination by the FFP also facilitates a superior signal-to-noise ratio, to effectively overcome the random phase noise due to Rayleigh backscatter in the long length of fiber. Furthermore, the interrogation length can be well beyond the coherence length of the laser source. We show that this performance is limited only by the frequency noise of the diode laser source, as all systemic noise sources in the delivery fiber are effectively transparent to the sensing architecture. This remote sensitivity is a seminal demonstration for a range of applications, such as sea floor acoustic sensing arrays, deep sea hydrophone arrays, and remote surveillance. We will discuss upscaling of this single element experiment to multi-element sensing arrays.
We demonstrate and compare two similar pico-strain sensing techniques by laser frequency locking to a passive Bragg grating Fabry-Perot resonator. One technique uses auxiliary phase modulation while the other employs current modulation of the diode laser source. The former is based on the Pound-Drever-Hall locking technique, while the latter is its variant, as current modulation introduces both amplitude and frequency modulation. The two modulation schemes utilize radio-frequency sidebands to derive error signals from the complex optical response of the fiber Bragg resonator. Experimental results are presented that demonstrate when the laser is locked, these methods detect differential phase shift between the optical carrier and the sidebands, due to minute fiber optical path displacements.
We demonstrate, for the first time, a sensing architecture capable of detecting broadband dynamic strain beyond picostrain
resolution, with signal frequencies extending from 100 Hz to beyond 100 kHz. The system uses a pre-stabilized
external cavity diode laser to interrogate a passive fiber Bragg resonator, using the Pound-Drever-Hall frequency locking
technique. The low-loss resonator comprises of a Bragg grating pair written in standard SMF-28 fiber.
The Australian Consortium for Gravitational Astronomy has built a High Optical Power Test Facility north of Perth, Western Australia. Current experiments in collaboration with LIGO are testing thermal lensing compensation, and suspension control on an 80m baseline suspended optical cavity. Future experiments will test radiation pressure instabilities and optical spring in a high power optical cavity with ~200kW circulating power. Once issues of operation and control have been resolved, the facility will go on to assess the noise performance of the high optical power technology through operation of an advanced interferometer with sapphire tests masses, and high performance suspension and isolation systems. The facility combines research and development undertaken by all consortium members, which latest results are presented.