Andrei B. Vakhtin

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.

Differential spectral interferometry: an imaging technique for biomedical applications

Differential spectral interferometry (DSI), a novel method of biomedical imaging that combines the high dynamic range of optical coherence tomography (OCT) with inherently parallel low-bandwidth image acquisition of spectral interferometry (SI), is described. DSI efficiently removes the deleterious dc background inherent in SI measurements while maintaining the parallel nature of SI. We demonstrate DSI on both synthetic and biological samples. Because DSI preserves the low-bandwidth, parallel nature of SI, it is competitive with OCT for biomedical applications in terms of image quality and acquisition rate.

Resolving the complex conjugate ambiguity in Fourier-domain OCT by harmonic lock-in detection of the spectral interferogram

A method of resolving the complex conjugate ambiguity in Fourier-domain OCT is described. The complex differential spectral interferogram is obtained by simultaneous acquisition of the first and second harmonics of the ac component of the phase-modulated interferogram. The harmonics represent the imaginary and real parts of the interferogram, respectively. The complex conjugate rejection ratio is found to be at least 45 dB and is limited by background noise.

Demonstration of complex-conjugate-resolved harmonic Fourier-domain optical coherence tomography imaging of biological samples

Complex-conjugate-resolved Fourier-domain optical coherence tomography, where the quadrature components of the interferogram are obtained by simultaneous acquisition of the first and second harmonics of the phase-modulated interferogram, is applied to multisurface test targets and biological samples. The method provides efficient suppression of the complex-conjugate, dc, and autocorrelation artifacts. A complex-conjugate rejection ratio as high as 70 dB is achieved.

Complex-conjugate-resolved imaging using two-harmonic FD-OCT

The two-harmonic FD-OCT method, where the quadrature components of the spectral interferogram are obtained by simultaneous acquisition of the first and second harmonics of the phase-modulated interferogram, is used for complex-conjugate- resolved imaging of biological samples. The method is implemented using sampling of the phase modulated interferogram with an integrating detector array followed by digital demodulation at the first and second harmonics. A complex conjugate rejection ratio as high as 70 dB is achieved.

Real-time video-rate harmonically detected Fourier domain optical coherence tomography

Real-time video-rate imaging using harmonically detected Fourier domain OCT is demonstrated using an 800 nm light source and a silicon line scan camera. At an imaging rate of 11.7 B-scans (1024 pixels × 256 pixels) per second, the measured complex conjugate artifact suppression is 30-35 dB, the sensitivity is 121 dB, and the dynamic range is about 60 dB.

Simultaneous acquisition of the real and imaginary components in Fourier domain optical coherence tomography using harmonic detection

Fourier domain optical coherence tomography (FD-OCT) is an interferometric imaging technique that allows imaging to depths of a few mm in scattering biological tissues with high resolution of the order of 1-10 μm. However, the usefulness of FD-OCT is limited by background and autocorrelation interference terms that reduce the sensitivity and by phase ambiguity that halves the useful imaging depth range. These limitations can be overcome by obtaining the full, complex spectral interferogram. Simultaneous detection of the imaginary and real terms is obtained by phase modulating the reference arm of the interferometer and detecting at the first and second harmonics. A mathematical derivation of harmonically detected FD-OCT and experimental measurements showing that phase ambiguity artifacts can be suppressed by up to 70 dB are presented. The method provides efficient suppression of the complex conjugate, dc, and autocorrelation artifacts and has low sensitivity to phase noise. Beyond the removal of artifacts, the ability to obtain the full, complex interferogram is key to the development of spectrally resolved FD-OCT which would add depth-resolved spectroscopic detail to the structural information.

Optical Breath Gas Sensor for Extravehicular Activity Application

The function of the infrared gas transducer used during extravehicular activity (EVA) in the current space suit is to measure and report the concentration of carbon dioxide (CO2) in the ventilation loop. The next generation Portable Life Support System (PLSS) requires next generation CO2 sensing technology with performance beyond that presently in use on the Shuttle/International Space Station extravehicular mobility unit (EMU). Accommodation within space suits demands that optical sensors meet stringent size, weight, and power requirements. A laser diode (LD) spectrometer based on wavelength modulation spectroscopy (WMS) is being developed for this purpose by Vista Photonics, Inc. Two prototype devices were delivered to NASA Johnson Space Center (JSC) in September 2011. The sensors incorporate a laser diode based CO2 channel that also includes an incidental water vapor (humidity) measurement and a separate oxygen (O2) channel using a vertical cavity surface emitting laser (VCSEL). Both prototypes are controlled digitally with a field-programmable gate array (FPGA)/microcontroller architecture. Based on the results of the initial instrument development, further prototype development and testing of instruments leveraging the lessons learned were desired. The present development extends and upgrades the earlier hardware to the Advanced PLSS 2.0 test article being constructed and tested at JSC. Various improvements to the electronics and gas sampling are being advanced by this project. The combination of low power electronics with the performance of a long wavelength laser spectrometer enables multi-gas sensors with significantly increased performance over that presently offered in the EMU. .

Real-Time Video-Rate Harmonically Detected Fourier Domain Optical Coherence Tomography

Real-time video-rate complex-conjugate-resolved subsurface imaging is demonstrated using harmonically detected Fourier domain OCT at 795 nm. Work toward implementing the method at 1300 nm in both broadband and swept-source configurations is presented.

COMBINATION OF FOURIER-DOMAIN OPTICAL COHERENCE TOMOGRAPHY AND PHOTO-STIMULATED LUMINESCENCE PIEZO-SPECTROSCOPY AS AN NDE TOOL FOR THERMAL BARRIER COATINGS

A combination of two optical methods — Fourier-domain optical coherence tomography (FD-OCT) and photo-stimulated luminescence piezo-spectroscopy (PLPS) is used as a non-destructive evaluation (NDE) tool for thermal barrier coatings (TBC). This research is focused on NDE of electron beam physical vapor deposition (EB-PVD) TBC’s. FD-OCT is an interferometric technique, which uses spectrally broadband visible or infrared light to obtain spectrally resolved interferograms of the light that is back-scattered from subsurface structures and defects (e.g., interfaces, cracks, voids) in optically translucent material. When the Fourier transform is applied to the interferogram, a depth-resolved image of the back-scattering sites is obtained. FD-OCT is shown to be a useful NDE tool that can profile the top coat-metal substrate interface and measure the top coat thickness. Also, it has the potential of assessing microcracking and spallation damage. PLPS provides quantitative information on stress in the thermally grown oxide (TGO) by measuring the spectral shifts in the laser-induced luminescence spectra of the Cr3+ ions present in the TGO. When combined, the PLPS and FD-OCT methods can provide a set of important input parameters for the TBC remaining life predicting model. Ultimately they will collect spatially resolved data on matching spatial domains. The two optical methods are applied to thermally cycled EB-PVD TBC samples. The experimental results are compared to destructive inspection data.Copyright