Guest editors Nicholas Madamopoulos, Yizheng Zhu, and Jianzhong Zhang introduce the Special Section on Fiber-Optic Sensors for Environmental Health Monitoring.
We present a review of recent progress in quantitative performance assessment of optical interferometry. Specifically, a three-level framework is introduced for evaluating the sensitivity of optical pathlength measurement. Its application is demonstrated for whitelight interferometry and wavelength shifting interferometry. It shows that there is a performance gap, potentially significant, between currently achieved measurement sensitivity and the sensitivity limit of the system. The framework allows one to accurately assess hardware performance, identify sources of degradation, and devise optimization strategy. It may offer guidance to lower sensor cost and improve commercial competitiveness.
KEYWORDS: Phase interferometry, Phase imaging, Signal processing, Interferometry, Detection and tracking algorithms, Data modeling, Cameras, Estimation theory, Electrons, Signal to noise ratio, Imaging systems, Sensors
Sensitivity is a critical index to measure the temporal fluctuation of the retrieved optical pathlength in quantitative phase imaging system. However, an accurate and comprehensive analysis for sensitivity evaluation is still lacking in current literature. In particular, previous theoretical studies for fundamental sensitivity based on Gaussian noise models are not applicable to modern cameras and detectors, which are dominated by shot noise. In this paper, we derive two shot noiselimited theoretical sensitivities, Cramér-Rao bound and algorithmic sensitivity for wavelength shifting interferometry, which is a major category of on-axis interferometry techniques in quantitative phase imaging. Based on the derivations, we show that the shot noise-limited model permits accurate estimation of theoretical sensitivities directly from measured data. These results can provide important insights into fundamental constraints in system performance and can be used to guide system design and optimization. The same concepts can be generalized to other quantitative phase imaging techniques as well.
A dual-modality birefringence/phase imaging system is presented. The system features a crystal retarder that provides polarization mixing and generates two interferometric carrier waves in a single signal spectrum. The retardation and orientation of sample birefringence can then be measured simultaneously based on spectral multiplexing interferometry. Further, with the addition of a Nomarski prism, the same setup can be used for quantitative differential interference contrast (DIC) imaging. Sample phase can then be obtained with two-dimensional integration. In addition, birefringence-induced phase error can be corrected using the birefringence data. This dual-modality approach is analyzed theoretically with Jones calculus and validated experimentally with malaria-infected red blood cells. The system generates not only corrected DIC and phase images, but a birefringence map that highlights the distribution of hemozoin crystals.
Morphological assessment of spermatozoa is of critical importance for in vitro fertilization (IVF), especially intracytoplasmic sperm injection (ICSI)-based IVF. In ICSI, a single sperm cell is selected and injected into an egg to achieve fertilization. The quality of the sperm cell is found to be highly correlated to IVF success. Sperm morphology, such as shape, head birefringence and motility, among others, are typically evaluated under a microscope. Current observation relies on conventional techniques such as differential interference contrast microscopy and polarized light microscopy. Their qualitative nature, however, limits the ability to provide accurate quantitative analysis.
Here, we demonstrate quantitative morphological measurement of sperm cells using two types of spectral interferometric techniques, namely spectral modulation interferometry and spectral multiplexing interferometry. Both are based on spectral-domain low coherence interferometry, which is known for its exquisite phase determination ability. While spectral modulation interferometry encodes sample phase in a single spectrum, spectral multiplexing interferometry does so for sample birefringence. Therefore they are capable of highly sensitive phase and birefringence imaging. These features suit well in the imaging of live sperm cells, which are small, dynamic objects with only low to moderate levels of phase and birefringence contrast. We will introduce the operation of both techniques and demonstrate their application to measuring the phase and birefringence morphology of sperm cells.
Holographic phase microscopy has seen rapid growth in the past two decades. Numerous schemes have been proposed and commercial products are now available. Since most systems are laser based, speckle noise and other non-signal interference in the system have been problematic, limiting the technique’s phase sensitivity, image quality and the ability for accurate quantitative analysis. Low coherence source-based HPM have also been proposed to mitigate this issue, but often with increased system complexity and reduced implementation flexibility.
Here, we demonstrate a swept-source HPM technique, which acquires on-axis holograms while continuously scanning the laser through a range of wavelengths. This technique is capable of identifying interference from various sources and effectively isolating sample interference, therefore minimizing unwanted signals and achieving high spatial and temporal sensitivity across the entire field of view. The ability of acquiring spectral interferogram for each pixel also make it possible to implement spectral shaping, which can further suppress interference side-lobes and improve sensitivity. Additionally, when coupled with a spectral modulation technique, such interference spectrum will permit spectroscopic measurement of phase-related properties of the sample. We will introduce the principle of the system, discuss its theoretical sensitivity bound, and present its application to phase imaging of live cells.
A spectral multiplexing interferometry (SXI) method is presented for integrated birefringence and phase gradient measurement on label-free biological specimens. With SXI, the retardation and orientation of sample birefringence are simultaneously encoded onto two separate spectral carrier waves, generated by a crystal retarder oriented at a specific angle. Thus sufficient information for birefringence determination can be obtained from a single interference spectrum, eliminating the need for multiple acquisitions with mechanical rotation or electrical modulation. In addition, with the insertion of a Nomarski prism, the setup can then acquire quantitative differential interference contrast images. Red blood cells infected by malaria parasites are imaged for birefringence retardation as well as phase gradient. The results demonstrate that the SXI approach can achieve both quantitative phase imaging and birefringence imaging with a single, high-sensitivity system.
Digital holographic phase microscopy is a well-established quantitative phase imaging technique. However, interference artifacts from inside the system, typically induced by elements whose optical thickness are within the source coherence length, limit the imaging quality as well as sensitivity. In this paper, a swept laser source based technique is presented. Spectra acquired at a number of wavelengths, after Fourier Transform, can be used to identify the sources of the interference artifacts. With proper tuning of the optical pathlength difference between sample and reference arms, it is possible to avoid these artifacts and achieve sensitivity below 0.3nm. Performance of the proposed technique is examined in live cell imaging.
A spectral-domain differential interference contrast (SD-DIC) microscopy system is presented for quantitative imaging of both reflective and transparent samples. The spectral-domain interferometry, combined with the common-path DIC geometry, provides a shot noise-limited sensitivity of 14.3pm in optical pathlength gradient measurement. The optical resolution of the system was characterized using images of a USAF resolution target. Fused silica microspheres were imaged to demonstrate the reconstruction of two-dimensional optical pathlength topography from measured gradient fields. The exquisite sensitivity of the system showed potential in quantitative imaging of sub-diffraction limit objects such as gold nanoparticles.
Angle-resolved low coherence interferometry (a/LCI) is an optical biopsy technique that allows for depth-resolved, label-free measurement of the average size and optical density of cell nuclei in epithelial tissue to assess the tissue health. a/LCI has previously been used clinically to identify the presence of dysplasia in Barrett's Esophagus patients undergoing routine surveillance. We present the results of a pilot, ex vivo study of tissues from 27 patients undergoing partial colonic resection surgery, conducted to evaluate the ability of a/LCI to identify dysplasia. Performance was determined by comparing the nuclear morphology measurements with pathological assessment of co-located physical biopsies. A statistically significant correlation between increased average nuclear size, reduced nuclear density, and the presence of dysplasia was noted at the basal layer of the epithelium, at a depth of 200 to 300 μm beneath the tissue surface. Using a decision line determined from a receiver operating characteristic, a/LCI was able to separate dysplastic from healthy tissues with a sensitivity of 92.9% (13/14), a specificity of 83.6% (56/67), and an overall accuracy of 85.2% (69/81). The study illustrates the extension of the a/LCI technique to the detection of intestinal dysplasia, and demonstrates the need for future in vivo studies.
We present Fourier domain low coherence interferometry (fLCI) applied to the detection of preneoplastic
changes in the colon using the ex-vivo azoxymethane (AOM) rat carcinogenesis model. fLCI measures
depth resolved spectral oscillations, also known as local oscillations, resulting from coherent fields induced
by the scattering of cell nuclei. The depth resolution of fLCI permits nuclear morphology measurements
within thick tissues, making the technique sensitive to the earliest stages of precancerous development. To achieve depth resolved spectroscopic analysis, we use the dual window method, which obtains simultaneously high spectral and depth resolution and yields access to the local oscillations. The results show highly statistically significant differences between the AOM-treated and control group samples. Further, the results suggest that fLCI may be used to detect the field effect of carcinogenesis, in addition to identifying specific areas where more advanced neoplastic development has occurred.
Angle-resolved scattering measurements have shown promise as a method of detecting neoplasia and
analyzing cellular structure. Recently we have developed new systems for interferometric measurement of
two-dimensional, depth resolved scattered fields with excellent depth resolution and polarization sensitivity.
We present inverse analysis of oriented ensembles of micro-spheroidal phantoms and cells showing sub-wavelength
accuracy in size and shape determination, and additionally precise estimates of scatterer
orientation. Finally we show that inverse fits provided are essentially free of multiple solutions over a wide
range of possible scatterer sizes and shapes.
We present a novel Fourier-domain angle-resolved low-coherence interferometry (a /LCI) fiber probe designed for in vivo clinical application in gastrointestinal endoscopy. The a/LCI technique measures the depth-resolved angular scattering distribution to determine the size distribution and optical density of cell nuclei for assessing the health of epithelial tissues. Clinical application is enabled by an endoscopic fiber-optic probe that employs a 2.3-m-long coherent fiber bundle and is compatible with the standard 2.8-mm-diam biopsy channel of a gastroscope. The probe allows for real-time data acquisition by collecting the scattering from multiple angles in parallel, enabled by the Fourier domain approach. The performance of the probe is characterized through measurement of critical parameters. The depth-resolved sizing capability of the system is demonstrated using single- and double-layer microsphere phantoms with subwavelength sizing precision and accuracy achieved. Initial results from a clinical feasibility test are also presented to show in vivo application in the human esophagus.
We introduce a new interferometric setup for single-exposure wide-field holographic phase imaging of highly dynamic biological samples. In this setup, the interferometric signal originates from a specially designed reflective interferometric chamber (InCh), creating an off-axis interferogram on the output plane of the system. The setup only requires the InCh and a simple reflection-mode two lens imaging system, without the need for additional optical elements such as gratings in the beam path. In addition, due to the close-to-common-path geometry of the setup, phase noise is greatly reduced. We experimentally compare the inherent phase stability of the system in ambient conditions to that of a conventional interferometer. We also demonstrate use of this system for wide-field quantitative phase imaging of two different highly dynamic, optically transparent biological samples: beating myocardial cells and moving unicellular microorganisms.
Improved methods for detecting dysplasia, or pre-cancerous growth are a current clinical need, particularly in the
esophagus. The currently accepted method of random biopsy and histological analysis provides only a limited
examination of tissue in question while being coupled with a long time delay for diagnosis. Light scattering
spectroscopy, in contrast, allows for inspection of the cellular structure and organization of tissue in vivo.
Fourier-domain angle-resolved low-coherence interferometry (a/LCI) is a novel light scattering spectroscopy
technique that provides quantitative depth-resolved morphological measurements of the size and optical density of
the examined cell nuclei, which are characteristic biomarkers of dysplasia. Previously, clinical viability of the a/LCI
system was demonstrated through analysis of ex vivo human esophageal tissue in Barrett's esophagus patients using
a portable a/LCI, as was the development of a clinical a/LCI system. Data indicating the feasibility of the technique
in other organ sites (colon, oral cavity) will be presented.
We present an adaptation of the a/LCI system that will be used to investigate the presence of dysplasia in vivo in
Barrett's esophagus patients.
We propose a fiber-optic system for Fourier-domain angle-resolved low coherence interferometry. The system is
based on singlemode fiber couplers and employs a scanning fiber to collect the angular scattering distribution of the
sample. The measured distribution is then fitted to Mie theory to estimate the size of the scatterers. Depth resolution
is achieved with Fourier-domain low coherence Mach-Zehnder interferometry. The sample arm of the interferometer
illuminates the specimen with polarized light and collects back-scattered light from the sample. The probe's optical
performance is evaluated and its depth-resolved sizing capability is demonstrated with subwavelength accuracy
using a two-layer microsphere phantom.
Methods for the optimization of a/LCI for clinical use are presented. First, the use of the T-matrix light
scattering model to simulate scattering from spheroidal particles is presented as a more appropriate simulation of
cell nuclei scattering than the previously used Mie theory. In addition, the use of a broadband light source with a
bandwidth greater than 50nm similar to those utilized in OCT applications is demonstrated. Accurate sizing of
scatterers in tissue phantoms containing stretched and unstretched polystyrene microspheres along with
measurements of unstretched polystyrene microspheres in solution are presented, demonstrating advances in system
performance and design. In addition, preliminary human in vivo esophageal tissue data are presented.
Improved methods for detecting dysplasia, or pre-cancerous growth are a current clinical need, particularly in
the esophagus. The currently accepted method of random biopsy and histological analysis provides only a limited
examination of tissue in question while being coupled with a long time delay for diagnosis. Optical scattering
spectroscopy, in contrast, allows for inspection of the cellular structure and organization of tissue in vivo. Fourierdomain
angle-resolved low-coherence interferometry (a/LCI) is a novel scattering spectroscopy technique that provides
quantitative depth-resolved morphological measurements of the size and optical density of the examined cell nuclei,
which are characteristic biomarkers of dysplasia. Previously, the clinical viability of the a/LCI system was demonstrated
by analysis of ex vivo human esophageal tissue in Barrett's esophagus patients using a portable a/LCI system. We
present an adaptation of the portable a/LCI instrument that can be used in the accessory channel of a gastroscope,
allowing for in vivo measurements to be taken. Modifications to the previous generation system include the use of an
improved imaging spectrometer allowing for subsecond acquisition times and the redesign of the delivery fiber and
imaging optics in order to fit in the accessory channel of a gastroscope. Accurate sizing of polystyrene microspheres and
other preliminary results are presented, demonstrating promise as a clinically viable tool.
This paper presents the development of a sapphire-based fiber-optic sensing system for temperature monitoring in harsh
environment, including sensor and system design, implementation, laboratory tests and field demonstration. The sensor
is built with single-crystal sapphire fiber and sapphire wafer. As the sensing element, the wafer constitutes an extrinsic
Fabry-Perot interferometer (EFPI) by its two surfaces. Its optical thickness has significant thermal dependence and
provides temperature information through white light interferometry. The sensors were tested to 1600°C with 0.2% full
scale accuracy and 0.5°C resolution. They were further demonstrated in industrial environment. A complete sensing
system was developed around the sensor for temperature monitoring in a coal gasifier at the Tampa Electric Company's
Polk Power Station. It consists of three major components: 1) Sensors and their packaging which were installed in the
coal gasifier, 2) Optical interrogation unit for detection and transmission of sensor signal, and 3) Processing and control
unit for signal demodulation. The system continuously operated and delivered temperature readings for seven months.
We present the quasi-distributed temperature measurement results in a selective catalytic reduction unit of a power plant by using a frequency-division-multiplexing optical fiber measurement system with eight intrinsic Fabry-Perot interferometric fiber sensors along a single fiber. The sensor was constructed by splicing a section of multimode fiber between single mode fibers. A high resolution swept laser interrogator was used to measure the spectrogram of the reflected light from the sensors, which contains multiple frequency components in wave number domain corresponding to sensors with different cavity lengths. The temperatures were measured by estimating the optical path length of each Fabry-Perot interferometer. Field test results show that the proposed technology can potentially be used in applications of multi-point high temperature sensing.
Pressure sensors are the key elements for industrial monitoring and control systems to lower equipment maintenance cost, improve fuel economy, reduce atmospheric pollution, and provide a safer workplace. However, the testing environment is usually harsh. For example, inside the turbine engine, temperatures might exceed 600°C and pressures might exceed 100psi (690kPa), where most current available sensors cannot survive. Moreover, due to the restricted space for installation, miniature size of the sensor is highly desirable. To meet these requirements, a novel type of all fused silica optic fiber tip pressure sensor with a 125μm diameter was developed. It is a diaphragm based pressure sensor in which a Fabry-Perot interferometer is constructed by the end face of an optical fiber and the surface of a diaphragm connected by a short piece of hollow fiber. The FP cavity length and the interference pattern will change according to ambient pressure variation. Its main improvement with respect to previously developed optical sensors, such as those utilizing techniques of wet etching, anodic bonding and sol-gel bonding, is the fact that no chemical method is needed during the cavity fabrication. Its dynamic pressure performance was verified in a turbine engine field test, demonstrating not only that it can safely and reliably function near the fan of a turbine engine for more than two hours, but also that its performance is consistent with that of a commercial Kulite sensor.
We present the principle, fabrication, and characterization of a novel wavefront splitting intrinsic Fabry-Perot fiber temperature sensor. The sensor is made by splicing a section of fused silica tubing to the tip of a single-mode fiber. The completed sensor has the same diameter as the fiber and the sensor length is less than 0.5 mm.
Pressure measurements at various locations of a gas turbine engine are highly desirable to improve the operational performance and reliability. However, measurement of dynamic pressure (1psi (6.9kPa) variation superimposed on the static bias) in the operating environment of the engine, where temperatures might exceed 600°C and pressures might exceed 100psi (690kPa), is a great challenge to currently available sensors. To meet these requirements, a novel type of fiber optic engine pressure sensor has been developed. This pressure sensor functions as a diaphragm-based extrinsic Fabry-Perot interferometric (EFPI) sensor. The structure of the sensor head, composed entirely of fused silica, allows a much higher operating temperature to be achieved in conjunction with a low temperature dependence. The sensor head and the fiber tail have been packaged in a metal fitting connected to a piece of metal extension tubing, which improves the mechanical strength of the sensor and facilitates easy sensor installation. The sensor exhibited very good performance in an engine field test, demonstrating not only that the sensors' package is robust enough for engine operation, but also that its performance is consistent with that of a commercial Kulite sensor.
In this paper, we present a miniature fiber optic pressure sensor. The sensor is extrinsic Fabry-Perot interferometer (EFPI) based with its FP cavity directly fabricated on the tip of the fiber by fusion splicing and chemical etching. The processes are simple, with no other materials but silica fibers involved. The sensor has the same dimension as the fiber itself, only 125μm in diameter. The length of the FP cavity and the interference pattern will change in response to ambient pressure variation. The signal is demodulated by tracing the spectrum shift. Sensitivity can be adjusted for different applications with low or high pressure range. Prototype sensors were fabricated and tested for static response. Dynamic measurements were performed in a turbine engine. Theoretical and experimental analysis of the sensor response are also presented.
Sapphire (single crystal alumina) has superior optical and mechanical properties. With a very high melting point of about 2050°C, sapphire fiber is an excellent candidate in optical fiber sensing area for high-temperature measurements. This paper presents a new type of sapphire-fiber-based extrinsic Fabry-Perot interferometric (EFPI) temperature sensor. The spectral interference pattern is generated by a sapphire diaphragm placed in front of the sapphire fiber. The sensing element is interrogated by a white-light source. Temperature is demodulated from the spectral change of interference pattern. Prototype sensor is tested at high temperature up to 1545°C. Both theoretical and experimental analysis are presented. Preliminary data shows the sensor is very promising for measuring ultra-high temperature.
In this paper, deep microstructures on fused silica material, which are useful for fabrication of the fiber optic sensors, were obtained by using a wet chemical etching process. The etching solutions and the masking materials used for developing deep structure are described in this paper. The etch rate of a fused silica diaphragm in room temperature ranged from 46nm per minute to 83nm per minute with different concentrations of Buffered Hydrogen Fluoride (BHF). The etch depth of one step etching was 25μm with the surface roughness less than 20nm (peak-to-peak value). The optical reflectance from the deep etched surface was 4%, which is the same as a well-cleaved fiber end face. This result made the visibility of interference fringes from the single mode fiber optic sensors to be as high as 96%. Furthermore, two-step structures on the fused silica diaphragms with the total depth greater than 35μm are demonstrated. To the best
knowledge of the authors, this is the deepest structure produced by wet etching process on fused silica material. Fiber optic pressure sensors based on deep etched diaphragms were fabricated and tested. Fabrication of microstructures on the fiber end faces by using this process is therefore possible.
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