In the past two decades, optical coherence tomography (OCT) has been established as an adjunct diagnostic technique for noninvasive, high-resolution, cross-sectional imaging in a variety of medical fields. The rapid development of ultrabroad bandwidth light sources has recently enabled a significant improvement in OCT imaging resolution, demonstrating the potential of OCT to accomplish its original goal of performing noninvasive optical biopsies, i.e., the in vivo visualization of microstructural morphology in situ, which had previously only been possible with histopathology. In addition, these novel light sources might also enable the use of spectroscopic OCT, an extension of ultrahigh-resolution OCT, for enhancing image contrast as well as detecting spatially resolved functional, biochemical tissue information. State-of-the-art-light sources that now permit ultrahigh-resolution OCT covering the whole wavelength region from 500 to 1600 nm are reviewed and fundamental limitations of OCT image resolution are discussed. Ex vivo ultrahigh-resolution OCT tomograms are compared with histological results; first clinical in vivo ultrahigh-resolution OCT and preliminary spectroscopic OCT results are presented and their impact for future clinical and research applications is discussed.

Ultrahigh Resolution Optical Coherence Tomography

A robust interferometric fiber-optic current sensor with inherent temperature compensation of the Faraday effect is presented. Sensor configurations based on Sagnac and polarization-rotated reflection interferometers are considered. The sensing fiber is residing and thermally annealed in a coiled capillary of fused silica. The capillary is embedded in silicone within a ring-shaped housing. It is theoretically and experimentally shown that the temperature dependence of the birefringent fiber-optic phase retarders of the interferometers can be employed to balance the temperature dependence of the Faraday effect (0.7/spl times/10/sup -4///spl deg/C). Insensitivity of the sensor to temperature within 0.2% is demonstrated between -35/spl deg/C and 85/spl deg/C. The influence of the phase retarders on the linearity of the sensor is also addressed. Furthermore, the sensitivity to vibration of the two configurations at frequencies up to 500 Hz and accelerations up to 10 g is compared. High immunity of the reflective sensor to mechanical perturbations is verified.

Temperature and vibration insensitive fiber-optic current sensor

Purpose. To demonstrate the feasibility of measuring the anterior structures of the human eye by partial coherence interferometry and to determine its precision for eyes under normal and cycloplegic conditions. Methods. The dual-beam version of partial coherence interferometry, a recendy developed noninvasive optical ranging technique, enables high resolution measurements of several intraocular distances with unprecedented precision. A modified, more sensitive scanning version of this technique was used to assess the central and peripheral corneal thickness, the anterior chamber depth, and the lens thickness of 20 healthy, emmetropic to moderately myopic eyes. Furthermore the anterior structures of three eyes were measured under cycloplegia (1% cyclopentolate) to investigate the influence on the precision of this technique after suppression of residual accommodations. Results. The mean geometric precision (standard deviation) of the measurement of the central corneal thickness was 0.29 fxm (range, 0.22 //m to 0.38 /um) and 0.43 /um (range, 0.27 /v,m to 0.56 /urn) for the peripheral corneal thickness at a distance 2 mm from its apex. The precision for measuring the anterior chamber depth and the lens thickness for fixation at infinity was 8.7 fj,m (range, 3.9 ^m to 16.8 fj.m) and 8.9 yum (range, 2.9 fim to 14.4 /urn) for noncycloplegic eyes and 1.9 fj,m (range, 1.7 /urn to 2 ^m) and 1.4 fini (range, 0.7 fim to 1.8 /um) for cycloplegic eyes, respectively. Conclusions. The dual-beam partial coherence interferometry enables fast, noninvasive, submicrometer precision biometry of the anterior segment of the eye. The precision of determining the anterior chamber depth and the lens thickness is more than one order of magnitude better than that of the currently used ultrasound and optical techniques, and it can be improved by a factor of 5 by using cycloplegia. Invest Ophthalmol Vis Sci. 1997; 38:13041313. 1 he precise biometry of several intraocular distances is fundamental to many applications in modern ophthalmology. Exact measurement of corneal thickness is needed in many physiological and clinical studies and is helpful in diagnosis of several corneal disorders, including corneal degenerations, endothelial dysfunc

Submicrometer precision biometry of the anterior segment of the human eye.

In nondispersive media, the minimum distance that can be resolved by partial coherence interferometry (PCI) and optical coherence tomography (OCT) is inversely proportional to the source spectral bandwidth. Dispersion tends to increase the signal width and to degrade the resolution. We analyze the situation for PCI ranging and OCT imaging of ocular structures. It can be shown that for each ocular segment an optimum source bandwidth yielding optimum resolution exists. If the resolution is to be improved beyond this point, the group dispersion of the ocular media has to be compensated. With the use of a dispersion compensating element, and employing a broadband superluminescent diode, we demonstrate a resolution of 5 μm in the retina of both a model eye and a human eye in vivo. This is an improvement by a factor of 2-3 as compared to currently used instruments. © 1999 Society of Photo-Optical Instrumentation Engineers.

https://www.spiedigitallibrary.org/journals/Journal-of-Biomedical-Optics/volume-4/issue-01/0000/Dispersion-effects-in-partial-coherence-interferometry--implications-for-intraocular/10.1117/1.429900.pdf

Dispersion effects in partial coherence interferometry: implications for intraocular ranging

This paper reviews recent developments in the field of current measurement which employ a wide range of optical and fiber optic techniques. Depending on the current sensing mechanisms involved and the sensing materials used, optical current sensors (OCSs) may be categorized into four main groups: (i) OCSs employing optical fiber as their sensing elements, (ii) OCSs using bulk glass to sense the current, (iii) OCSs using electro‐optic hybrid sensing devices, and (iv) OCSs using magnetic field sensing devices. The operational modes of a variety of OCSs have been grouped and discussed, and several examples given. It can be seen that as a result of an intensive and wide ranging research effort using various approaches, substantial progress in the differing aspects of the optical current sensing techniques considered, such as the sensing material used, the configurations of sensing elements introduced, and the detection schemes adapted, has been achieved during the past years. An overall view of the field shows it to be as an active and exciting research area, highlighting several recently introduced and novel sensing materials and configurations which provide impressive results in this field of instrumentation. Examples of applications for the electric power industry are discussed more extensively.

Recent progress in optical current sensing techniques

Low-coherence-length light from laser diode sources has applications in extending the useful range of interferometric fiber optic sensors. The characteristics of two commercial low-coherence laser diodes were investigated and compared with theoretical models to determine the operational characteristics of the devices. Reasonable trends in the comparison were seen.

Characteristics of laser diodes for interferometric use.

Two very simple electro‐optic hybrid schemes which convert the output from a conventional current transformer to an optical signal are presented. These systems may be used to indicate the presence of and/or measure the magnitude of large currents at very high potential. The first scheme would be very useful for indicating whether the conductor was carrying current or for making measurements within an accuracy of ∼10% at extremely high potentials. Whilst the second scheme, with a demonstrated minimum detectable signal of 0.32 A/√Hz, provides a simple and low‐cost way for accurate current measurement in high potential locations. The optical signals from the measurement scheme can be transferred for subsequent demodulation via multimode fibers hence greatly reducing the electrical insulation required for safe operation of the current transformer thereby offering a significant cost saving in its manufacture.

Two low-cost robust electro-optic hybrid current sensors capable of operation at extremely high potential

With the increasing interest in the use of optical-fibre interferometer measurement systems, the importance of the incorporation of low-coherence light sources is becoming of particular significance In this work, the optical characteristics of different types of such sources in several interferometers are reviewed and examples are given Their advantages in terms of increased unambiguous range, immunity to perturbation, high resolution and dynamic range, are discussed

Fibre-optic interferometric systems using low-coherence light sources

Optic fiber sensors (OFS) offer a number of unique advantages for safety monitoring in explosive and hazardous environment such as underground coal mines. This paper reports on latest development of fiber optic sensors for monitoring methane and other gases, detection of seismic events and application systems for hazard monitoring and early warning of coal mine combustion, roof integrity and seismic activities. The application potential and future trend of OFS research for coal mine hazard detection and prevention are also discussed in the paper.

Y.N. Ning

Papers

Recent progress in optical current sensing techniques

Characteristics of laser diodes for interferometric use.

Two low-cost robust electro-optic hybrid current sensors capable of operation at extremely high potential

Fibre-optic interferometric systems using low-coherence light sources

Advances of optical fiber sensors for coal mine safety monitoring applications