Optical Low-coherence Reflectometry Research Articles

Rapid and Precise In Vivo Measurement of Human Corneal Thickness with Optical Low-Coherence Reflectometry in Normal Human Eyes

An optical low-coherence reflectometer is used for rapid noncontact measurements of the human corneal thickness in vivo. Thickness measurements on ten volunteers show a standard deviation of 3.4 μm. The experiments reveal that the optical reflectometer benefits from a 2.5 fold enhancement of the measurement precision and a 2.8 fold reduction in measurement time compared to a standard clinical ultrasonic pachometer. © 1998 Society of Photo-Optical Instrumentation Engineers.

A generalized approach to optical low-coherence reflectometry including spectral filtering effects

In the interpretation of optical low-coherence reflectometry measurements, the reflectivity of the device under test is in general supposed to be with a slow dependency on optical wavelength. However, recent research aims at investigating strongly wavelength-dependent devices, such as fiber Bragg gratings and semiconductor lasers. In this paper, a general theory including spectral filtering effects is developed. It appears as a generalization of previously reported results only valid under special conditions.

Noise in optical low-coherence reflectometry

It is theoretically and experimentally shown that Fresnel end reflection at a waveguide under test can degrade the sensitivity of an optical low-coherence reflectometer (OLCR) with a balanced detection scheme. Optical mixing of the local oscillator (LO) light and the end reflection produces beat noise whose current noise spectral density is represented by 2(1+P/sup 2/)<I/sub 1/><I/sub 2/>//spl delta//spl nu/, where P and /spl delta//spl nu/ are the degree of polarization and the effective linewidth of the light, respectively, and <I/sub 2/> and <I/sub 2/> are the total mean photocurrents of the LO light and the end reflection at the balanced mixer, respectively. The balanced detection technique suppresses the intensity noise of the light and the beat noise becomes the dominant source of sensitivity degradation. The minimum detectable reflectivity is derived which includes the effect of sensitivity degradation caused by beat noise.

Analysis of distributed feedback lasers using optical low-coherence reflectometry

The analysis of distributed feedback (DFB) lasers by optical low-coherence reflectometry (OLCR) shows the appearance of additional more or less periodic minima in the reflectogram. These do not reflect local minima of the reflectance, but are rather due to multipath interference effects caused by spectral filtering of the white light source by the laser under test. It is shown that the coupling coefficient of the laser can be determined from the period of these minima.

-78-dB shot-noise limited optical low-coherence reflectometry at 42-m/s scan speed

We report, to our knowledge, the fastest longitudinal scans in optical low-coherence reflectometry published in literature. The use of a rotating glass cube as an optical path length variator results in a longitudinal scan speed of 42 m/s at a longitudinal scan repetition rate of 1240 Hz. The high-speed reflectometer is operated at the shot-noise limit corresponding to a sensitivity of -78 dB. The scalability of the scanning method with respect to scan speed, scan repetitiveness and scan length is demonstrated by the use of differently sized glass cubes.

Leakage current measurement in multielectrode lasers using optical low-coherence reflectometry

In multielectrode lasers, the interelectrode spacing is often reduced in order to avoid the saturable absorber effect in this region, which could cause instabilities of the laser. This leads to an interelectrode leakage current between adjacent electrodes. Measuring this leakage current by conventional means is very difficult and often inaccurate, in particular, when the part of the leakage current reinjected into an active region has to be determined. Based on optical low-coherence reflectometry, a method is presented which allows the measurement of this leakage current with an error smaller than 10%. Additionally, the effective group indexes of the laser waveguide and its bulk material will be determined.

Rapid and scalable scans at 21 m/s in optical low-coherence reflectometry

An optical low-coherence reflectometer is presented that uses a fiber Michelson interferometer with a rotating cube to generate rapid depth scans at a high repetition rate. A folded optical path geometry allows the reference arm to scale up the scan range, scan speed, and scan repetitiveness. Thickness measurements with a repetitiveness of 384 Hz and a longitudinal scanning speed of 21 m/s in air over a range of ~3 mm are demonstrated.

Non-invasive determination of port wine stain anatomy and physiology for optimal laser treatment strategies

The treatment of port wine stains (PWSs) using a flashlamp-pumped pulsed dye laser is often performed using virtually identical irradiation parameters. Although encouraging clinical results have been reported, we propose that lasers will only reach their full potential provided treatment parameters match individual PWS anatomy and physiology. The purpose of this paper is to review the progress made on the technical development and clinical implementation of (i) infrared tomography (IRT), optical reflectance spectroscopy (ORS) and optical low-coherence reflectometry (OLCR) to obtain in vivo diagnostic data on individual PWS anatomy and physiology and (ii) models of light and heat propagation, predicting irreversible vascular injury in human skin, to select optimal laser wavelength, pulse duration, spot size and radiant exposure for complete PWS blanching in the fewest possible treatment sessions. Although non-invasive optical sensing techniques may provide significant diagnostic data, development of a realistic model will require a better understanding of relevant mechanisms for irreversible vascular injury.

Low-Coherence Reflectometry Through Scattering Media with Degenerate Four-Wave Mixing in a Thin Nonlinear Optical Layer

We have demonstrated the coherence filtering operation for optical low-coherence reflectometry via degenerate four-wave mixing (DFWM) in a thin nonlinear optical layer. To perform proof-of-principle DFWM experiments, we use a thin dye solution with a thermally-induced optical nonlinearity. In contrast to previous works, which used c.w. light-emitting diodes or Fourier-transform limited ultra-short laser pulses, we have used broad-band nanosecond laser pulses for depth resolved measurements through scattering media.

Optical low-coherence reflectometry to enhance monte Carlo modeling of skin

Optical low-coherence reflectometry and confocal microscopy images were taken of the rat dorsal skin flap window model. Blood vessel depths and diameters measured with the two techniques, and preparation thickness determined from reflectometry images, are in reasonable agreement with measurements from histologic sections. Blood vessels appear as areas of low signal when constant-depth reflectometry images are taken at a depth near the center of a vessel, whereas they appear bright when taken close to the blood–dermis boundary. Doppler shift plus increased light absorption in blood, and the blood–dermis index of refraction mismatch, are discussed as possible causes of the dark- and bright-appearing vessels, respectively. One reflectometry image was used to generate an input grid for a novel Monte Carlo analysis program that is capable of determining the light distribution and heat generation @J/m3# within complex blood vessel geometries. The feasibility of imaging skin blood vessel accurately with optical low-coherence tomography, and using the acquired knowledge of blood vessels structure to create more realistic Monte Carlo analyses is demonstrated by the results of the study.

Tunable narrow-band light source using two optical circulators

We report a tunable narrow-band light source with a spectral width of 0.26/spl plusmn/0.04 nm, a 0-dBm output power, and a reduced background spectrum in the 1.5-μm band, which we have developed for optical low coherence reflectometry. The Rayleigh backscatter signal fluctuations were reduced to within /spl plusmn/0.4 dB by using this source. The source will also be used in a system for measuring optical waveguide circuits. This is because the coherence length is about 1 cm and, by using a fiber-optic depolarizer, the degree of polarization is less than 0.016.

Constant high-speed optical low-coherence reflectometry over 0.12 m scan range

A constant high-speed optical low-coherence reflectometer is presented. The use of both the reference and sample arms as delay lines results in highly linear optical path length changes. Scan speeds of up to 96 m/s are maintained over a range of typically 100 mm with a speed variation of only 0.1%.

Three-dimensional imaging of protein crystals with the use of optical low coherence reflectometry

Three-dimensional imaging of protein crystals with the use of optical low coherence reflectometry

Coupled-mode propagation in multicore fibers characterized by optical low-coherence reflectometry

A fiber-optical low-coherence ref lectometer has been used to probe a multicore fiber locally at a wavelength of 1.3 microm. This technique allows one to determine the group index of refraction of the modes in the multicore fiber with high accuracy. Light propagation that is due to noncoherent coupling of energy from one fiber core to adjacent cores through cladding modes can be distinguished quantitatively from light propagating in coherently coupled modes. Intercore coupling constants in the range of 0.6-2 mm(-1) have been evaluated for the coupled modes.

Depth determination of chromophores in human skin by pulsed photothermal radiometry

We report on the application of pulsed photothermal radiometry (PPTR) to determine the depth of in-vitro and in-vivo subsurface chromophores in biological materials. Measurements provided by PPTR in combination with a nonnegative constrained conjugate-gradient algorithm are used to determine the initial temperature distribution in a biological material immediately following pulsed laser irradiation. Within the experimental error, chromophore depths (50-450 µm) in 55 in-vitro collagen phantoms determined by PPTR and optical low-coherence reflectometry are equivalent. The depths of port-wine-stain blood vessels determined by PPTR correlate very well with their locations found by computer-assisted microscopic observation of histologic sections. The mean blood-vessel depth deduced from PPTR and histologic observation is statistically indistinguishable (p > 0.94).

Narrow-band light source with acoustooptic tunable filter for optical low-coherence reflectometry

An electrically-tunable narrow-band light source with no moving parts is constructed for jaggedness-free optical low coherence reflectometry. The source incorporates a double-path acoustooptic tunable bandpass filter module whose two passbands automatically agree to within 0.2 nm during wavelength tuning from 1529-1568 nm. The source emits narrow-band light with an FWHM of 1.8/spl plusmn/0.1 nm, a power of >0 dBm and a suppressed background spectrum. The entire wavelength range can be swept electrically within 100 ms by using a variable RF source.

Group refractive index measurement of dry and hydrated type I collagen films using optical low-coherence reflectometry

The group refractive index and physical thickness of dry and hydrated type I collagen films are measured using optical low-coherence reflectometry. The average value for the group refractive index of dry and fully hydrated collagen films at lo = 850 nm is, respectively, 1.53 6 0.02, and 1.43 6 0.02. The physical thickness of type I collagen films nearly doubles when going from the dry (56 mm) to hydrated (118 mm) state.

A three-dimensional modular adaptable grid numerical model for light propagation during laser irradiation of skin tissue

Information regarding energy deposition during laser irradiation of structurally complex biological tissue is needed to understand and improve the results of clinical procedures. A modular adaptive geometry numerical model capable of simulating the propagation of laser light in a wide variety of multiple component tissues has been developed and tested. A material grid array is generated by assigning a value representing a tissue type to each of a large number of small voxels. The grid array is used to indicate optical properties in an existing variable step size, weighted-photon Monte Carlo algorithm that has been modified to account for voxels-to-voxels changes in optical properties. To test the model, simple geometric shapes and optical low coherence reflectometry images of rat skin have been used to create material grids consisting of epidermis, dermis, and blood. The model assumes 1-J/cm/sup 2/ irradiation of the tissue samples with a 1.0-mm diameter uniform beam at 585 nm. Computed results show good quantitative and qualitative agreement with published data. Various effects due to shading and scattering, similar to those suggested in the literature, are noted. This model provides a way to achieve more realistic representation of anatomical geometry as compared to other models, and produces accurate results.

High power superluminescent diodes for 1.3 [micro sign]m wavelengths

Superluminescent diodes for optical low coherence reflectometry in the 1.3 µm range are reported with a singlemode fibre coupled output power of 8 mW. The high gain amplifier structure with 10° tilted SiO/Al2O3 coated facets ensures this high power, broad spectral emission over 30 nm. The integrated absorber, together with the tilted and coated facet, attenuates the light reflected back from the absorber facet into the gain region by up to –149 dB. Internal reflection induced nonuniformities at maximum output power remain below 2 × 10-2.

Measurements of return loss on OEIC structures, using optical low-coherence reflectometry

Optical low-coherence reflectometry (OLCR) has been applied for the first time to the ovaluation of optoelectronic integrated circuits (OEICs). Benefits are demonstrated for analysis and measurement of integrated structures. In particular reflection strengths of various integrated devices and interfaces on real fibred OEICs have been quantified.