BIOS 2018

January 28th, 2018

A 16 MHz frequency-comb stretched-pulse mode-locked (SPML) laser using a continuously chirped fiber Bragg grating for subsampled OCT

Session 6: OCT New Technology I
Paper 10483-36
Tuesday 30 January 2018
10:30 AM - 12:00 PM

Abstract
In this work we demonstrate circular ranging by using a frequency comb source to subsample an extended depth field into a reduced measurement space. As a result, imaging at significant higher voxel rates and reduced data load become possible. Using a continuously chirped fiber Bragg grating we present an improved cavity layout compared to a previously reported laser design. Wavelength-stepping is demonstrated in a stretched-pulse active mode locking architecture using a single continuously chirped fiber Bragg grating, achieving a repetition rate of 16 MHz and a coherence length of 5 cm.

Fig. 1: [a] Experimental configuration of the CFBG-SPML laser. EOM, electro-optic modulator; CFBG, chirped fiber Bragg grating; FP, Fabry–Perot etalon; SOA, semiconductor optical amplifier; DCF, dispersion compensated fiber. [b] Electronic drive pulse provided to the EOM. [c] Round-trip times for cavities A, B, and AB. [d] EOM and SOA modulation signals [upper panel] were used to suppress light circulation in short cavities A and B, while allowing circulation in cavity AB [lower panel].



Definitive depolarization contrast for optical frequency domain imaging

Session 10: Polarization
Paper 10483-63
Wednesday 31 January
10:30 AM - 12:00 PM

Abstract
Depolarization of light provides intrinsic contrast for tissue heterogeneity. Different metrics have been used to characterize depolarization such as cross-polarization, degree-of-polarization (uniformity), (differential) depolarization index and entropy. Due to its complexity, the origin of depolarization can be manifold, making the current characterization of depolarization ambiguous and/or incomplete. In this work, we start with a general form of a mixed state to derive depolarization from the eigenvalues of a density matrix that describes polarization entropy. Furthermore, we highlight the dependency of depolarization on an incident polarization state by imaging the retinal pigment epithelium (RPE) and using gold nanorods as a depolarizing phantom. We present polar decomposition and eigenvalue decomposition as a definitive measure of sample depolarization that is independent of an input polarization state, sample depth and sample birefringence. Definitive depolarization is demonstrated by imaging GNRs in lymphatic vessels of the hind limb of mice and human RPE in vivo.

Fig. 2: Polar plots of GNR solution [0.4 nM] using catheter-based OFDI. [a] Depolarization contrast for two polarization states orthogonal in Stokes space. [b] Depolarization contrast for two polarization states orthogonal in Jones space. [c] Depolarization factors and differential depolarization factor using polar decomposition and two input states orthogonal in Jones space.