Fizeau sensors constitute a large proportion of the fiber optic interferometric type sensors in use today. These include EFPI,
FFPI, certain MEMS devices and in-line fiber intrinsic dual-reflector type sensors. The vast majority of the published
literature covering these sensor types models them with a "2-beam" interferometer approximation, and implement
interrogation approaches considering the same. Analysis performed and results presented show that the 2-beam model is
not sufficient when reflection coefficients exceed 1% and traditional quadrature interrogation can result in linearity or
distortion errors roughly in directly proportion to the reflectivity coefficients of the Fizeau sensor. A 4-beam multi-path
interferometer model is developed and exercised to demonstrate this problem. Further this model shows that the "errors" in
comparison to an ideal 2-beam interferometer model are symmetric across the unit circle and suggests that linear
interrogation may be accomplished if orthonormal sample sets over the entire unit circle are used to replace the traditional
(simple) quadrature sampling. This is shown to be true in both modeling and lab evaluations. The resulting approach has
capabilities of remote, passive sensor operation, high frequency response, large, linear dynamic range and low noise. The
interrogation technique demonstrated involves a phase generated carrier with full fringe sampling and quadrature
determination which cancels the errors experienced from simple quadrature determination. Such an improvement enables
higher reflectivity, higher SNR, high-fidelity fiber Fizeau sensor designs. Applications include embedded sensors, line
sensors, or mechanically adapted for acoustic, pressure, vibration, acceleration or seismic sensing.
A high-speed, swept-laser interferometric interrogation approach is introduced. Dynamic measurements of weak Fabry-Perot or Fizeau type interferometers with gap ranges of 50 um to 1 mm up to 70 Ksamples per second are demonstrated and discussed. Displacement resolution is < 1 pm/rt-Hz. This has application with MEMS and FPPI type sensors.
A 1550 nm DWDM planar external cavity laser is demonstrated to provide low phase / frequency noise and narrow
linewidth. The cavity includes a semiconductor gain chip and a planar lightwave circuit waveguide with Bragg
grating, packaged in a 14-pin butterfly package. The laser shows linewidth < 30 kHz and phase/frequency noise
comparable with that of long cavity fiber lasers. Performance is suitable for various fiber optic sensing systems,
including interferometric sensing in Oil and Gas, military/security and other applications, currently served mostly by
costly and less reliable laser sources.
We demonstrate for an unpolarized Fiber Optic Gyroscope (FOG) with open-loop electronics, that, by applying more
source power and conserving optical power in the optical path, we can achieve improved Angle Random Walk (ARW)
performance without enlarging loop or put in multiple turns of fiber. The predicted trends are demonstrated by the
experiment in terms of bandwidth. Power-law dependency is shown within the accuracy of the instrumentation.
Previous studies have demonstrated that Polarization Sensitive Optical Coherence Tomography (PS-OCT) can be used
to image early dental caries. The primary objective of this study was to compare the measured reflectivity of natural
occlusal caries lesions with the relative mineral loss measured using digital microradiography. There was excellent
agreement between the increase in the integrated reflectivity in the perpendicular polarization axis of the PS-OCT
system and the increase in the integrated mineral loss or lesion severity for occlusal lesions. Therefore, PS-OCT is
ideally suited to image natural caries lesions in the important occlusal surfaces for the assessment of the lesion severity
and activity. A secondary objective was to compare the performance of a new autocorrelator-based PS-OCT system
employing a novel polarization-switching probe with our polarization-maintaining fiber based PS-OCT system, both
operating at 1310-nm. The new PS-OCT system produced clean images with no artifacts and achieved high
penetration depth. Yet a third objective was to determine if interproximal lesions can be imaged from the occlusal
surface (from above) since interproximal lesions may only be accessible in vivo from buccal or lingual surfaces or from
the occlusal surface. Simulated and natural interproximal caries lesions were imaged from the occlusal surfaces as long
as there was no intervening dentin.
A new low-cost design for TDM interferometric interrogation of multi-channel fiber sensor arrays is presented. Interrogation elements comprise all-SMF components, SOA based optical switching for high optical extinction, and a new, highly efficient demodulation approach.
A new high-performance, low-cost design for OCT imaging is presented. It involves an all-SMF autocorrelator topology. An imaging instrument complete with a scanning probe was constructed and evaluated confirming high performance imaging capability.
We report a design and implementation of an Optical Coherence Tomography (OCT) system based on new topology-a probe with partial reflection from a fiber tip, connected with an all-fiber autocorrelator with Faraday mirrors. The system is made of communication fiber SMF-28 and doesn’t need any electronic means (like polarization diversity receiver) to compensate for static and dynamic polarization distortions, associated with birefringence of a flexible fiberoptic probe. Because of the system topology, it no longer requires a full optical length matching arm (reference arm) and is therefore insensitive to the probe length and wave dispersion properties.
The system is implementing time-domain scanning with Doppler detection, with two piezofiber delay elements, one for AC in-depth scanning, and another for DC adjustment of coherence gate scanning range. It uses a 6 mW, 55 nm bandwidth superluminescent diode with 1300 nm central wavelength, and has 15 μm in-depth (free space) and 25 μm lateral resolution, 0.7 frames per second acquisition rate. It has a catheter-based, en face, 8 Fr diameter universal probe, suitable for endoscopic imaging. Simplicity and cost effectiveness of the new topology result in the creation of a commercially available, FDA cleared system for medical OCT imaging.
Theoretical and experimental optimization of the system, including optimization of probe fiber tip "reference" reflection coefficient, has been performed. Special waveform is applied to the AC piezofiber delay line, resulting in good stability of the scanning velocity and instantaneous Doppler frequency over a 70% duty cycle, which enables use of narrow bandpass signal filtering such that signal to noise performance is optimized.
A low-cost design approach for Time Division Multiplexed (TDM) fiber-optic interferometric interrogation of multi-channel sensor arrays is presented. This paper describes the evolutionary design process of the subject design. First, the requisite elements of interferometric interrogation are defined for a single channel sensor. The concept is then extended to multi-channel sensor interrogation implementing a TDM multiplex scheme where “traditional” design elements are utilized. The cost of the traditional TDM interrogator is investigated and concluded to be too high for entry into many markets. A new design approach is presented which significantly reduces the cost for TDM interrogation. This new approach, in accordance with the cost objectives, shows promise to bring this technology to within the threshold of commercial acceptance for a wide range of distributed fiber sensing applications.
Traditional white-light scanning interferometers utilize bulk optic components mounted on a mechanical scanning mechanism. Many emerging applications for these interferometers require fiber optic probes. By design, such fiber compatible instruments are expensive and are limited to slow scan rates. A new “all-fiber” design approach is presented, which reduces the cost of the design and enables higher scan rates.
A novel fiber optic vibrometer has been built and characterized. The vibrometer's optical design is based on the Sagnac interferometer. Due to the nature of the Sagnac's optical configuration, the optical phase shift induced by the surface being probed is differentiated, and therefore results in a measured optical phase shift that is directly proportional to the velocity of surface. The path matched Sagnac design eliminates the need for a coherent light source used in laser doppler vibrometers and offers great design flexibility for tuning the vibrometer's frequency response and dynamic range. A fiber optic Sagnac vibrometer was built and operated with a modulation frequency of 100 kHz. The vibrometer's dynamic range exceeds 70 dB with a maximum velocity of greater than 10 m/s and a noise floor of less than 1 micron/s/(root)Hz. Experimental results demonstrating the vibrometers performance will be presented.
A novel design approach for a highly reliable buried intrusion detection sensor is described. The design involves the use of a low cost depolarized Sagnac fiber interferometer with a 'sensing loop' consisting of a delay line and buried fiber segment. The intrusion sensor is configured for an 'all fiber' remote deployment where active components (source, receiver, demodulator) are located separately and connected to the sensor through an insensitive fiber tether. A robust and cost effective buried sensor 'mat' design was developed. This design enabled high sensitivity as well as ease of deployment. Sensors were built and evaluated. Test results indicate an effective design.
Fiber optic sensors have the potential to be used in the very hostile environments necessary for advanced aerospace platforms. This paper reviews some of the key issues associated with the implementation of distributed fiber optic sensors in harsh environments and outlines baseline system designs.
A novel design approach for a highly reliable buried intrusion detection sensor is described. The design involves the use of a low cost depolarized Sagnac fiber interferometer with a 'sensing loop' consisting of a delay line and buried fiber segment. The intrusion sensor is configured for an 'all fiber' remote deployment where active components (source, receiver, demodulator) are located separately and connected to the sensor through an insensitive fiber tether. A robust and cost effective buried sensor 'mat' design was developed. This design enabled high sensitivity as well as ease of deployment. Sensors were built and evaluated. Test results indicate an effective design.
A novel low cost interferometric displacement sensor has been developed which tracks distance from the tip of a fiber optic probe. A unique interrogation technique is used which produces a 32-bit phase word, giving the system a dynamic range greater than 109. Therefore, a displacement resolution of less than 0.01 nm can be achieved with a full range of 6 mm. The measurement range can be extended beyond 10 m by simply adjusting the digital fringe counter and sacrificing resolution yet maintaining the greater than 109 dynamic range. Demodulation rates of 40 kHz have been achieved which facilitates dynamic measurements. Results from an application to hard disk (HD) profilometry are presented.
A twin-bridge fiber optic calorimeter has been built and is currently being tested at Los Alamos National Laboratory (LANL). The intrinsic optical phase shift induced by changes in temperature is measured in both a reference canister and a sample canister. This system incorporates two Michelson interferometers each with an optical path mismatch of 1.6 km. A digital demodulation scheme is used which produces a 32 bit phase word which tracks up to 500,000 fringes with a resolution of 10-4 fringe, giving the system a dynamic range greater than 109. Both interferometers are demodulated simultaneously at a rate of 83 kHz. The phase difference between the reference and sample interferometers is proportional to the temperature difference between the canisters and therefore correlates to the power produced by the sample in question. The optical system performance will be described along with preliminary calorimetric measurement results.
This paper outlines recent progress made by Optiphase Inc.in the development of low-cost, high performance interferometric demodulators applied to multi-channel interrogation. The paper's focus is on the Optiphase digital demodulation concept applied first for single channel operation and then to multi-channel operation. This paper also presents single and multi-channel demodulation test data taken from hardware developed for these applications.
An Erbium amplified spontaneous emission (ASE) source is described and test results are presented. The ASE output power, spectrum, coherence, relative intensity noise (RIN), and polarization were measured for three different Erbium doped fibers of various lengths. Output powers up to 15 mW were achieved with 70 mW of 980 nm pump power. Measurement of the ASE coherence resulted in 0.5 mm at the -30 dB level while the RIN was measured to be < -125 dB/rt- Hz. The coherence and noise characteristics of this source match specifications for a low noise fiber optic gyroscope and since the source design is simple, a low cost Erbium ASE source can be built.
The design and test result of an ultra miniature interferometric fiber optic rate gyro (IFOG) are reported. A unique IFOG gyro has been fabricated and tested. The gyro implements low cost components packaged in a volume less than 2.3 cubic inches including electronics. Test results verify that operational performance requirements over the temperature range of -57 to +71 C are met in the design. Key results include: electronic power consumption of 2.3 Watts, noise < 0.16 deg/sec/rt-Hz, scale factor of 32.9 mV/deg.sec, activation time of < 300 msec, threshold and resolution of 0.01 deg/sec, scale factor linearity error < 0.14 percent, and bias < 0.1 deg/sec. The gyro also survived a vibration test at 26.5 Grms. The gyro design is presented and accompanied with test data showing general conformance to the design's operational performance requirements.
The US Army Missile Command, Research, Development and Engineering Center has been conducting research and development activities on solid-state inertial components for next generation weapon systems for the past decade. The fiber optic gyroscope (FOG), which is on an upward trend as evidenced by the increase in patents and users, warrants a closer examination to address the technology limitations associated with ultra-miniature FOG components that are designed to operate in adverse environments. Several FOG testbeds are constructed for the characterization and evaluation of the optical components operating under a series of test conditions. Analytical models are developed to aid in explaining the experimental results. The data are used in assessing the size limitations for the optical components and design approaches that tend to degrade gyroscope performance. Some remarks are mad on the effects of bending-induced birefringence in single mode fiber on the performance of depolarized FOGs. It is found that reducing the inner diameter of the sensor coil below one inch has a detrimental effect on the gyroscope performance. Furthermore, the inherent fiber crossovers in the conventional precision wound coils tend to degrade the performance of the performance of the depolarized gyroscope. The findings of our investigation will aid FOG designers in optimizing FOG optical components for applications requiring small or restricted volume for various performance regimes.
Progress on the development of an ultra miniature fiber optic rate gyro is reported. A unique IFOG design implementing low cost components is presented where package volume is less than two cubic inches including electronics. This design is capable of meeting operational performance requirements over the temperature range of -57 to +71 C with electronic power limited to 3 watts. The IFOG circuit consists of an ELED source at 1.3 micrometers , a non-PM source/receiver multiplex circuit using a standard fused biconic tapered coupler, an annealed proton exchange integrated optic 'gyro chip', and a volume efficient 'racetrack' PM sensing coil configuration. Gyro electronics have been reduced to a single MCM board. A discussion of the design parameters, component requirements, and test data are presented.
This paper outlines recent progress made by Optiphase Inc. in the development of low cost fiber optic interferometric sensors. The paper's focus is on components under development, specific to interferometric fiber sensors (IFS), which aren't commercially available through normal telecommunications distribution channels.
A thorough investigation of the temperature induced performance errors of IFOG configured fiber gyros was performed. The investigation was focused on low-cost optical components which are capable of being assembled into a gyro package volume of less than two cubic inches including electronics. Testing performed at the optical component level includes diode sources, receivers, wound coils, and integrated optic assemblies. Testing also included the fabrication of IFOG circuits and performing gyro bias and noise measurements. The test results indicate that high performance rate sensing is possible given the use of low cost optical components.
This paper reviews the development progress of fiber optic interferometric acoustic sensors at OPTIPHASE, INC. The primary development objective is to evolve fiber optic interferometric sensor technologies which are cost compatible in their respective application methods. This mandates that only low cost approaches be considered. The sensor technology chosen involves extrinsic Fabry Perot devices configured as low finesse sensors which are compatible with low-cost dual wavelength interferometric demodulation techniques (also developed by OPTIPHASE). This paper addresses both single element and multi-element sensor systems.
KEYWORDS: Demodulation, Interferometry, Sensors, Analog electronics, Multiplexing, Digital signal processing, Signal processing, Fiber optics sensors, Clocks, Interferometers
This paper describes a digital approach to interferometric demodulation having capabilities of high sample rates and low self noise. Using this approach, a single demodulator can be used to demodulate many interferometric sensors where data is presented to the demodulator in a time domain multiplexed format. This paper presents the basic demodulation algorithm, theoretical performance predictions, and experimental data.
This paper critically analyzes a new open loop interferometric demodulation technique which utilizes digital processing approaches to achieve a direct optical phase measurement. This approach is compared to traditional FM discrimination approaches which are currently used for interferometric demodulation. The comparisons include intrinsic performance capabilities and measurement errors resulting from imperfect interferometric sensor conditions.
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