Instantaneous quadrature low-coherence interferometry with 3x3 fiber-optic couplers

We describe fiber-based quadrature low-coherence interferometers that exploit the inherent phase shifts of 3 x 3 and higher-order fiber-optic couplers. We present a framework based on conservation of energy to account for the interferometric shifts in 3 x 3 interferometers, and we demonstrate that the resulting interferometers provide the entire complex interferometric signal instantaneously in homodyne and heterodyne systems. In heterodyne detection we demonstrate the capability for extraction of the magnitude and sign of Doppler shifts from the complex data. In homodyne detection we show the detection of subwavelength sample motion. N x N (N> 2) low-coherence interferometer topologies will be useful in Doppler optical coherence tomography (OCT), optical coherence microscopy, Fourier-domain OCT, optical frequency domain reflectometry, and phase-referenced interferometry

Instantaneous complex conjugate resolved spectral domain and swept-source OCT using 3x3 fiber couplers

We report that the complex conjugate artifact in Fourier domain optical coherence tomography approaches (including spectral domain and swept source OCT) may be resolved by the use of novel interferometer designs based on 3x3 and higher order fiber couplers. Interferometers built from NxN (N>2) truly fused fiber couplers provide simultaneous access to non-complementary phase components of the complex interferometric signal. These phase components may be converted to quadrature components by trigonometric manipulation, then inverse Fourier transformed to obtain A-scans and images with resolved complex conjugate artifact. We demonstrate instantaneous complex conjugate resolved Fourier domain OCT using 3x3 couplers in both spectral domain and swept source implementations. Complex conjugate artifact suppression by factors of ~20dB and ~25dB are demonstrated for spectral domain and swept source implementations, respectively

Sensitivity advantage of swept source and Fourier domain optical coherence tomography

We present theoretical and experimental results which demonstrate the superior sensitivity of swept source (SS) and Fourier domain (FD) optical coherence tomography (OCT) techniques over the conventional time domain (TD) approach. We show that SS- and FD-OCT have equivalent expressions for system signal-to-noise ratio which result in a typical sensitivity advantage of 20-30dB over TD-OCT. Experimental verification is provided using two novel spectral discrimination (SD) OCT systems: a differential fiber-based 800nm FD-OCT system which employs deep-well photodiode arrays, and a differential 1300nm SS-OCT system based on a swept laser with an 87nm tuning range

Protein-based molecular contrast optical coherence tomography with phytochrome as the contrast agent

We report the use of phytochrome A (phyA), a plant protein that can reversibly switch between two states with different absorption maxima (at 660 and 730 nm), as a contrast agent for molecular contrast optical coherence tomography (MCOCT). Our MCOCT scheme builds up a difference image revealing the distribution of phyA within a target sample from pairs of consecutive OCT A-scans acquired at a probe wavelength of 750 nm, both with and without additional illumination of the target sample with 660-nm light. We demonstrate molecular imaging with this new MCOCT modality in a target sample containing a mixture of 0.2% Intralipid and 83 µM of phyA

Spectral triangulation molecular contrast optical coherence tomography with indocyanine green as the contrast agent

We report a new molecular contrast optical coherence tomography (MCOCT) implementation that profiles the contrast agent distribution in a sample by measuring the agent's spectral differential absorption. The method, spectra triangulation MCOCT, can effectively suppress contributions from spectrally dependent scatterings from the sample without a priori knowledge of the scattering properties. We demonstrate molecular imaging with this new MCOCT modality by mapping the distribution of indocyanine green, a FDA-approved infrared red dye, within a stage 54 Xenopus laevis

Instantaneous complex spectral domain OCT using 3x3 fiber couplers

We report that the complex conjugate ambiguity in spectral domain OCT approaches (including swept source OCT and Fourier-domain OCT) may be removed by the use of novel interferometer designs based on NxN couplers. An interferometer based on a 3x3 truly fused fiber coupler with equal splitting ratios provides simultaneous access to components of the complex interferometric signal separated by 120o. These phase components may be converted to quadrature components by use of a simple trigonometric operation, and then inverse Fourier transformed to obtain A-scans and images free of complex conjugate artifact. We demonstrate instantaneous complex spectral-domain OCT using a novel Fourier-domain OCT system employing photodiode arrays, and will also report on a similar system design for instantaneous complex swept-source OCT

Betaproteobacteria growth and nitrification rates during long-term natural dissolved organic matter decomposition experiments

As a first attempt to relate net nitrification rates with bacterial community structure in the coastal embayment of the Ría de Vigo (NW Spain), a set of 6 long-term (lasting 53 to 74 d) dissolved organic matter decomposition experiments were conducted in the dark and at a constant temperature of 15°C with surface seawater collected in January, February, April, and June 2008. Net nitrification rates were estimated from nitrate concentration changes. Evolution of bacterial community composition was followed using catalyzed reporter deposition-fluorescent in situ hybridization (CARD-FISH) and specific probes for 6 relevant bacterial groups, including Beta- and Gammaproteobacteria. The growth rates of Betaproteobacteria showed a significant linear positive relationship with nitrification rates that explained 82% of the observed variability, which strongly suggests that the main nitrifying microorganisms during the incubations belonged to the Beta subclass of Proteobacteria. A positive relationship was found between net nitrification rates and both ammonium (r2 = 0.92) and phosphate (r2 = 0.97) concentrations, which suggests a tight link among the nitrogen and phosphorus cycles, likely as a result of the role of phosphate as the bacterial growth-limiting nutrient.Peer reviewe