Stress relaxation in InGaAsP/InP heterostructures for 1064-nm laser radiation converters

Technical Physics Letters - Specific features of mechanical-stress relaxation in InGaAsP/InP heterostructures for 1064 nm laser radiation converters have been studied. It is established that stress...     

Interferometer for optical coherence tomography

We describe a new interferometer setup for optical coherence tomography (OCT). The interferometer is based on a fiber arrangement similar to Young's two-pinhole interference experiment with spatial coherent and temporal incoherent light. Depth gating is achieved detection of the interference signal on a linear CCD array. Therefore no reference optical delay scanning is needed. The interference signal, the modulation of the signal, the axial resolution, and the depth range are derived theoretically and compared with experiments. The dynamic range of the setup is compared with OCT sensors in the time domain. To our knowledge, the first images of porcine brain and heart tissue and human skin are presented.     

Low loss high mesa optical waveguides based on InGaAsP/InP heterostructures

Low loss high mesa optical waveguides were fabricated on InGaAsP/InP heterostructures by utilizing inductively-coupled-plasma reactive ion etching (ICP-RIE) and electron beam lithography technique. The fabrication process was optimized by measuring sidewall roughness of deep-etched waveguides. Atomic force microscope loaded with carbon nanotude was used to obtain three-dimensional image of the etched sidewall of waveguides. The obtained statistical information such as rms roughness and correlation length was used to theoretically calculate scattering loss of waveguides. Several waveguides with different number of sharp bends and the length were fabricated and their propagation losses were measured by modified Fabry-Perot method. The measured propagation losses were compared with theoretically calculated losses.     

Two-section InGaAsP/InP Fabry-Perot laser with a 12 nm tuning range

Wavelength tuning over a 12 nm range is obtained for a two-section InGaAsP/InP Fabry-Perot laser (λ=1.55 μm). The method used to vary the gain profile of the laser allows one to predict the range of possible wavelength tuning. Pis’ma Zh. Tekh. Fiz. 23, 10–15 (March 26, 1997)     

Nonparaxial vector-field modeling of optical coherence tomography and interferometric synthetic aperture microscopy

A large-aperture, electromagnetic model for coherent microscopy is presented and the inverse scattering problem is solved. Approximations to the model are developed for near-focus and far-from-focus operations. These approximations result in an image-reconstruction algorithm consistent with interferometric synthetic aperture microscopy (ISAM): this validates ISAM processing of optical-coherence-tomography and optical-coherence-microscopy data in a vectorial setting. Numerical simulations confirm that diffraction-limited resolution can be achieved outside the focal plane and that depth of focus is limited only by measurement noise and/or detector dynamic range. Furthermore, the model presented is suitable for the quantitative study of polarimetric coherent microscopy systems operating within the first Born approximation.     

Inverse scattering for optical coherence tomography

Inverse scattering theory for optical coherence tomography (OCT) is developed. The results are used to produce algorithms to resolve three-dimensional object structure, taking into account the finite beam width, diffraction, and defocusing effects. The resolution normally achieved only in the focal plane of the OCT system is shown to be available for all illuminated depths in the object without moving the focal plane. Spatially invariant resolution is verified with numerical simulations and indicates an improvement of the high-resolution cross-sectional imaging capabilities of OCT.     

Evaluation of selective laser sintering processes by optical coherence tomography

Selective laser sintering (SLS) enables the fast, flexible and cost-efficient production of parts directly from 3D CAD data. Unlike more established machine tools, there is a marked lack of process monitoring and feedback control of key process variables. In-situ analysis techniques permit the emergence of repair techniques, in-process optimization of production parameters, and will also serve to save time and material. In this study, optical coherence tomography (OCT) is used for the first time to evaluate components produced by SLS. Using a Polyamide-PA2200, surface defects are analyzed and the limiting factors associated with the measurement technique are quantified. OCT is shown to be a useful technique for evaluating surface irregularities alongside sub-surface defects that have resulted from poor sintering or non-homogeneous powder spreading. We demonstrate detection and quantification of surface defects such as cracks, pores and voids on a ~ 30 μm scale. Furthermore, we show that this technique can resolve ‘built-in’ fine features within a 200 to 400 μm depth below the surface, covering typical layer thicknesses used by this process. This capability paves the way for real-time monitoring of the SLS process for assurance, or even dynamic correction of defects during the build.     

A dual-modality probe utilizing intravascular ultrasound and optical coherence tomography for intravascular imaging applications

We have developed a dual-modality biomedical imaging probe utilizing intravascular ultrasound (IVUS) and optical coherence tomography (OCT). It consists of an OCT probe, a miniature ultrasonic transducer and a fixed mirror. The mirror was mounted at the head of the hybrid probe 45° relative to the light and the ultrasound beams to change their propagation directions. The probe was designed to be able to cover a larger area in blood vessel by IVUS and then visualize a specific point at a much finer image resolution using OCT. To demonstrate both its feasibility and potential clinical applications, we used this ultrasound-guide OCT probe to image a rabbit aorta in vitro. The results offer convincing evidence that the complementary natures of these two modalities may yield beneficial results that could not have otherwise been obtained.     

Characterisation of scaffold architecture and tendons using optical coherence tomography and polarisation-sensitive optical coherence tomography

Scaffolds play an important role in the generation of functional tissues using tissue-engineering techniques. To generate highly organised tissue, scaffolds must have specific internal and external architectures. Here, optical coherence tomography (OCT) is exploited to characterise the architectures of various scaffolds, in particular scaffolds which have been fabricated to support the formation of uniaxially orientated collagen bundle for use in tendon tissue engineering. In parallel, a polarisation-sensitive OCT (PSOCT) has been built to assess the collagen fibre organisation in human tendon and monitor the growth of engineering tendon constructs online and non-destructively. The impact of mechanical stimuli on the modulation of tendon tissue formation and organisation was also assessed. It is shown that conventional OCT is capable of characterising scaffold architecture and the pore size, porosity or microchannel dimension can be determined quantitatively and qualitatively. PSOCT generated birefringence images of human tendon and demonstrated that low birefringence images, associated with fewer microstructural variations, correlated to the presence of scar tissue or degenerated tissue; whereas the tissue-engineered tendon exhibited lower degree of birefringence.     

Sub-micrometre distance measurements with a broadly tunable short-external-cavity InGaAsP/InP diode laser

Measurement of distance using a custom-designed, broadly tunable InGaAsP/lnP short-external-cavity diode laser is described. A tuning range of over 100 nm was achieved with the custom-designed laser in a diffractive optical element short external cavity. This tuning range made it possible to achieve a sub-micrometre resolution in measurement of distance with a single laser source for an interferometer. A non-linear, least squares fitting method was used to extract the displacement from the raw data. This fitting method showed a potential for extraction of accurate displacement in the presence of noise.     

Common-path interferometer for frequency-domain optical coherence tomography

A Michelson-type spectral interferometer that uses a common beam path for the reference and the sample arms is described. This optical arrangement is more compact and stable than the more commonly used dual-arm interferometer and is well suited for frequency-domain optical coherence tomography of biological samples. With a 16-bit CCD camera, the instrument has sufficient dynamic range and resolution for imaging to depths of 2 mm in scattering biological materials. Images obtained with this spectral interferometer are presented, including cross-sectional images in a Xenopus laevis tadpole.     

Long-optical-path scanning mechanism for optical coherence tomography

A new scanning mechanism for changing long optical paths is proposed. This mechanism consists of corner reflectors arranged equally upon a disk and an outer mirror. Rotating the 120-mm disk causes a long-optical-path change in each reflector with a near linearity of more than 40 mm. An optical coherence tomography system is described that confirms the usefulness of the proposed mechanism. Its operating characteristics and accuracy are evaluated by analysis and experiment. The deviation of the optical-path change is less than 1.52% at a reflector rotation angle of +/-10 degrees. A high-speed lock-in amplifier is utilized for fundamental measurements of glass samples.     

Imaging of hard- and soft-tissue structure in the oral cavity by optical coherence tomography

We have developed a prototype optical coherence tomography (OCT) system for the imaging of hard and soft tissue in the oral cavity. High-resolution images of in vitro porcine periodontal tissues have been obtained with this system. The images clearly show the enamel-cementum and the gingiva-tooth interfaces, indicating OCT is a potentially useful technique for diagnosis of periodontal diseases. To our knowledge, this is the first application of OCT for imaging biologic hard tissue.     

LPE growth of InP/InGaAsP/InP heterostructures and separate preparation of high-temperature solutions

We describe a process for separate preparation of high-temperature solutions, which includes a prolonged preannealing (synthesis) at elevated temperatures. This process enables liquid phase epitaxy (LPE) of InGaAsP with highly reproducible physical properties (layer thickness, photoluminescence wavelength, and lattice parameter) and electrical characteristics (current-power curve, dark current, output power, emission wavelength, and threshold and working currents). Using this process, InP/InGaAsP/InP heterostructures can be grown by LPE in a wide temperature range (480–680°C) with highly reproducible parameters, for both photodetectors and light-emitting devices. An original LPE process is proposed which takes advantage of the separate preparation and pouring of high-temperature solutions with the use of graphite equipment.     

Inverse scattering for frequency-scanned full-field optical coherence tomography

Full-field optical coherence tomography (OCT) is able to image an entire en face plane of scatterers simultaneously, but typically the focus is scanned through the volume to acquire three-dimensional structure. By solving the inverse scattering problem for full-field OCT, we show it is possible to computationally reconstruct a three-dimensional volume while the focus is fixed at one plane inside the sample. While a low-numerical-aperture (NA) OCT system can tolerate defocus because the depth of field is large, for high NA it is critical to correct for defocus. By deriving a solution to the inverse scattering problem for full-field OCT, we propose and simulate an algorithm that recovers object structure both inside and outside the depth of field, so that even for high NA the focus can be fixed at a particular plane within the sample without compromising resolution away from the focal plane.     

Inverse scattering for rotationally scanned optical coherence tomography

Optical coherence tomography of luminal structures, such as for intravascular or gastrointestinal imaging, is performed by using a fiber-optic catheter as a beam-delivery probe. The interrogating beam is scanned angularly by rotating the fiber around a fixed central axis. Because the beam is focused only at a fixed distance from the center of the fiber, only scatterers near this distance are resolved. We present a solution of the inverse scattering problem that provides an estimate of the susceptibility of the sample for an angularly scanned Gaussian beam focused at a fixed distance from the origin. This solution provides quantitatively meaningful reconstructions while also extending the volume of the sample that is resolvable by the instrument.     

Multiscan time-domain optical coherence tomography for retina imaging

A versatile time-domain optical coherence tomography system is presented that can generate cross-sectional images by using either transverse priority or depth priority scanning. This is made possible by using a transmissive scanning delay line compatible with balance detection operating at a speed similar to that of the transverse scanner used to scan the beam across the target. In vivo images from the retina are generated and shown using the same system switched to either transverse or depth priority scanning regime, by using the scanning delay line either in slow or fast scanning modes, respectively. A comparative analysis of different scanning regimes depending on image size to fit different areas to be imaged is presented. Safety thresholds due to the different continuous irradiation time per transverse pixel in different scanning regimes are also considered. We present the maximum exposure level for a variety of scanning procedures, employing either A scanning (depth priority) or T scanning (transverse priority) when generating cross-sectional images, en face images, or collecting 3D volumes.     

Polarization-sensitive optical coherence tomography for imaging human atherosclerosis

Polarization-sensitive optical coherence tomography (PS-OCT) combines the advantages of OCT with image contrast enhancement, which is based on its ability to detect phase retardation and the fast-axis angle. Both PS-OCT images and histopathology have demonstrated similar features that allowed differentiation of atherosclerotic structures (i.e., plaques) from normal tissue. Moreover, the picrosirius polarization method was used to confirm PS-OCT assessment of collagen in the fibrous cap of atherosclerotic plaques, and high-frequency (40 MHz) ultrasound images were used to identify calcium in the vessel wall. Our preliminary ex vivo investigation of human aortic specimens indicated that PS-OCT might help to identify atherosclerotic lesions.     

Temporal coherence and time-frequency distributions in spectroscopic optical coherence tomography

Traditional analysis of spectroscopic optical coherence tomography (SOCT) signals is limited by an uncertainty relationship between time (depth) and frequency (wavelength). The use of a bilinear time-frequency distribution for analysis, such as those that compose Cohen's class of functions, may provide a way to avoid this limitation. Here we present the relationship between traditional SOCT analysis and the relevant Cohen class functions: the Wigner and Choi-Williams distributions. While cross terms that arise in these bilinear time-frequency distributions have been viewed as an artifact, here we identify these terms with temporal coherence, which contains significant information about the signal through phase relationships. The utility of time-frequency distributions is illustrated through analysis of calculated signals.