A complication of Fourier domain optical coherence tomography (OCT) methods, such as spectral domain and swept
source OCT, is the complex conjugate ambiguity due to inverse Fourier transform of real-valued data. As a result, the
image is symmetric to the zero plane, and only half of the theoretical imaging depth range is used to avoid overlapping
"mirror images" that confuse the image. We have previously demonstrated harmonic detection in a video-rate spectral
domain OCT system using a high speed line scan camera. Harmonic detection removes the complex conjugate
ambiguity by providing the real and imaginary components of the spectral interferogram. In this work, we show that
harmonic detection is easily applied to swept source OCT to remove the complex conjugate ambiguity while maintaining the imaging performance of the original swept source instrument. Harmonic detection swept source OCT allows simultaneous experimental determination of the real and imaginary components of each spectral interferogram without the need to measure consecutive A-scans at differing phase. This harmonic detection swept source optical coherence tomography system exhibits 110 dB sensitivity, up to 55 dB dynamic range, ≥ 50 dB complex conjugate rejection, and operates at the full 16 kHz sweep rate of the swept source laser for real-time video rate imaging.
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
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.
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.
Acetone is a good marker of metabolic stress as it is the most volatile and rapidly equilibrated of the ketone bodies
produced by human metabolism. If the body utilizes predominately fat to meet its energy requirements, blood and
breath acetone concentrations will increase. Elevated concentrations of breath acetone can indicate a normal response to
caloric imbalances in the diet, or a diseased state such as untreated diabetes. This paper describes a novel method of
acetone detection that uses a gas-solid chemical reaction of acetone with a hydroxylamine hydrochloride (HA) to
produce an easily detectable chemical species, HCl. Breath samples are passed through a reactor filled with solid HA
and the amount of HCl gas released is measured by sensitive near infrared diode laser spectroscopy. The breath acetone
instrument described is compact, low power and portable.