Laser interferometric techniques have been used in the field of biomedical optics since the 1960s. One of the first applications was concerned with measuring the visual acuity of cataract patients.

A Fourier transform spectrometer is used to simultaneously measure distance, dispersion and spectrum. It is shown that short coherence interferometry has the potential to measure the three-dimensional distribution of the spatial structure of a sample with a resolution determined by the coherence length of the light source, absorption spectrum with a resolution of 1 cm?1 and a dispersion with a resolution of up to 10?5.

An optical coherence tomography (OCT) system to produce both longitudinal and transversal images of the in vivo human eye is presented. For the first time, OCT transversal images collected from the living eye at 50-?m depth steps show details unobtainable with the state-of-the-art scanning laser ophthalmoscope. Images of up to 3x3 mm are produced from the retina in less than a second. For images larger than 1.6x1.6 mm, a path modulation is introduced by the galvanometric scanning mirror and is used as an effective phase modulation method.

‘‘Coherence radar,’’ an optical 3-D sensor based on short coherence interferometry, is used to measure skin surface topology. This method is called optical coherence profilometry (OCP) and it may be a useful tool for medical diagnosis in dermatology because different medical conditions show distinct alterations of the skin surface. The measuring uncertainty is less than 2 mm. The measuring time is about 4 s. in vivo 3-D mapping of naked skin was performed without preparation. For clinical application, a fiber optical implementation was introduced. Spectral radar is an optical sensor for the acquisition of skin morphology based on OCT techniques. The scattering amplitude a(z) along one vertical axis from the surface into the bulk can be measured within one exposure. No reference arm scanning is necessary. The theory of the sensor, including the dynamic range, is discussed and in vivo measurements of human skin by a fiber optical implementation of the sensor are demonstrated.

A new method for evaluating tear film stability on the human eye is reported. The tear film distribution on the cornea is measured by the lateral shearing interference technique. The eye is kept open during approximately a 2-min recording, when blinking has to be prevented. Continuous recording and viewing of interferograms allows the changes in disturbances of the interference fringes to be registered during elapsed time. The changes in fringes are caused by the evaporation of tears from the ocular surface and appearance of the breakups. For precise and repetitive assessment of the tear film breakup time, a fast fourier transform (FFT) is applied to consecutive interferograms. Larger fringe disturbances result in wider Fourier spectra. The tear breakup time can be evaluated noninvasively by comparing the value of the second momentum of Fourier spectra calculated from the consecutive interferograms.

Two in-fiber Bragg grating (FBG) temperature sensor systems for medical applications are demonstrated: (1) an FBG flow-directed thermodilution catheter based on interferometric detection of wavelength shift that is used for cardiac monitoring; and (2) an FBG sensor system with a tunable Fabry-Perot filter for in vivo temperature profiling in nuclear magnetic resonance (NMR) machines. Preliminary results show that the FBG sensor is in good agreement with electrical sensors that are widely used in practice. A field test shows that the FBG sensor system is suitable for in situ temperature profiling in NMR machines for medical applications.

In the past 10 years, a dual beam version of partial coherence interferometry has been developed for measuring intraocular distances in vivo with a precision on the order of 0.3 to 3 ?m. Two improvements of this technology are described. A special diffractive optical element allows matching of the wavefronts of the divergent beam reflected at the cornea and the parallel beam reflected at the retina and collimated by the optic system of the eye. In this way, the power of the light oscillations of the interfering beams incident on the photodetector is increased and the signal-to-noise ratio of in vivo measurements to the human retina is improved by 20 to 25 dB. By using a synthesized light source consisting of two spectrally displaced superluminescent diodes with an effective bandwidth of 50 nm, and by compensating for the dispersive effects of the ocular media, it was possible to record the first optical coherence tomogram of the retina of a human eye in vivo with an axial resolution of ~6 to 7 ?m. This is a twofold improvement over the current technology.

The dual beam version of optical coherence topography can be used for noninvasive, high-resolution imaging of the human eye fundus, enabling in vivo visualization of retinal morphology as well as accurate quantification of the thickness profiles of its layers. Interferometric fundus signals—optical A-scans—and retinal tomograms of patients with glaucoma, diabetic retinopathy, and age-related macular degeneration are compared with those of healthy, normal subjects to elucidate the origin of the signal peaks detected and to investigate and interpret the retinal microstructures contained in the cross-sectional images.

CLEAN, an iterative point-deconvolution algorithm developed originally for use in radio astronomy, was investigated as a means of restoring optical coherence tomography (OCT) images of biological tissue. The CLEAN deconvolution kernel was derived from the theoretical point-spread function of an OCT scanner, which depends on the properties of the imaging system as well as the characteristics of the scattering properties of the medium. The kernel incorporates a modification based on an inverse Wiener filter that is designed to reduce ripple artifacts in images of densely packed scatterers. Evaluation of the performance of the CLEAN algorithm was carried out on a set of images acquired with a prototype scanner with built-in specklereduction hardware. The results show the ability of the algorithm to improve the resolution of features in coherence images of scattering phantoms and living tissue. In many cases, the restored images reveal tissue morphology not evident in the unprocessed images. CLEAN tolerates speckle noise well and its performance degrades gracefully as the number of unresolvable scatterers causing mutual interference increases. Ways of coping with the long processing time required for the restorations are outlined, along with possible improvements that would permit CLEAN to take advantage of both amplitude and phase information in partially coherent interference signals.

We demonstrate two short-coherence-length, rare-earth-doped fiber optical sources for performing optical coherence tomography (OCT) in human tissue. The first source is a stretched-pulse, mode-locked Er-doped fiber laser with a center wavelength of 1.55 µm, a power of 100 mW, and a bandwidth of 80 nm. The second is a Tm-doped silica fiber fluorescent source emitting up to 7 mW of power at 1.81 µm with a bandwidth of 80 nm. The OCT imaging depth of penetration in in vitro human aorta is compared using these sources and conventional 1.3-µm sources.

An ultraviolet to visible acousto-optic tunable filter was used to measure native fluorescence images from in vitro breast tissues at different wavelengths. Pseudocolor maps based on fluorescence images at two wavelengths were used to separate normal and abnormal regions in human breast tissues in vitro, providing diagnostic information.

Fiber-guided ablation of soft tissue with pulsed holmium and thulium lasers was investigated for intraluminal incisions. A bare fiber/tissue-contact application system with a nearly tangential irradiation geometry was first used in vitro on porcine ureter tissue. The efficiency and precision of the method was analyzed for different laser and application parameters. The ablation dynamics in water and tissue was investigated by fast flash photography. Uniform cuts could be achieved with 200- and 318-µm fibers using a free-running holmium laser with a pulse repetition rate of 10 Hz and an average power of up to 4 W. The depth of the cuts could be increased by using a thulium laser with the same laser parameters. By reducing the pulse duration by one order of magnitude, the quality of the incisions was made more irregular, the zone of thermomechanical damage increased, and the cuts became deeper owing to the growing influence of cavitation on shorter laser pulse durations. In a first clinical trial, 20 patients underwent holmium laser therapy to reopen ureteral strictures. Neither bleeding nor other adverse effects due to the laser treatment occurred, showing IR laser ureterotomy to be a suitable and promising minimally invasive technique.

An in vitro study of laser tissue welding mediated with a dye-enhanced protein patch was conducted. Fresh sections of porcine aorta were used for the experiments. Arteriotomies were treated using an indocyanine green dye-enhanced collagen patch activated by an 805-nm continuous-wave fiber-delivered diode laser. Temperature histories of the surface of the weld site were obtained using a hollow glass optical fiber-based two-color infrared thermometer. The experimental effort was complemented by simulations with the LATIS (LAser-TISsue) computer code, which uses coupled Monte Carlo, thermal transport, and mass transport models. Comparison of simulated and experimental thermal data indicated that evaporative cooling clamped the surface temperature of the weld site below 100 °C. For fluences of approximately 200 J/cm2, peak surface temperatures averaged 74°C and acute burst strengths consistently exceeded 0.14x106 dyn/cm (hoop tension). The combination of experimental and simulation results showed that the inclusion of water transport and evaporative losses in the computer code has a significant impact on the thermal distributions and hydration levels throughout the tissue volume. The solid-matrix protein patch provided a means of controllable energy delivery and yielded consistently strong welds.

The general problems of describing local thermal coagulation dynamics leading to the growth of necrosis that is limited by heat diffusion to surrounding live tissue is considered. It is demonstrated that in this case the typically used distributed model for thermal coagulation is based on a self-inconsistent approach, and a more rigorously justified free boundary model is derived. This free boundary model takes into account only the general properties of thermal coagulation and so provides a self-consistent description. It is shown that the two models, nevertheless, predict practically the same dynamics of necrosis growth because this growth is insensitive to the particular properties of heat transfer in the thin layer of partially damaged tissue. Necrosis growth is also simulated numerically under various physical conditions to verify the assumptions adopted.

A side-firing fiber device for arthroscopic Ho:YAG (?=2.12 µm) and Er:YAG (?=2.94 µm) laser applications was designed and constructed. The fiber delivery instrument consisted of a zirconium fluoride (ZrF4) fiber equipped with a coaxially mounted short end-piece of low OH? quartz fiber polished at an angle of 30 deg. The dynamics and depth of the vapor channel in water and the amplitude of pressure transients associated with the collapse of the vapor channel were measured for pulse energies up to 1 J (Ho:YAG) and 200 mJ for the Er:YAG laser (pulse duration t5400 µs), respectively. To assess the feasibility of the side-firing fiber delivery instrument, the ablation efficiency and laser-induced damage in poly(acylamide) and meniscal tissue were determined after Ho:YAG and Er:YAG laser ablation.