Research

En Face Homodyne Optical Coherence Tomography using a 3x3 Fiber-optic Coupler

This OCT scheme is based on the use of a 3x3 fiber-optic coupler to realize a homodyne optical coherence tomography (OCT) system for en face imaging of highly scattering tissues and turbid media. The homodyne OCT setup exploits the inherent non-trivial phase shifts between different output ports of a 3x3 fiber-optic coupler, which enables simultaneous and independent acquisition of the sample phase and amplitude information.

Figure 1. (a) Schematic of homodyne en face optical coherence tomography system. (b) Schematically demonstrates triangular relationship between 3´3 coupler coefficients and interferometric phase shifts between the coupler arms.

Figure 2. (a) Photograph of a stage 52 Xenopus Laevis showing locations where en face images were acquired using homodyne OCT setup. Shown are the en face OCT images of (b) cornea and ciliary body in the eye, (c) heart, and (d) gill structures at depth of 600 mm into the sample. Figures (e)-(g) show en face images of the gill region [shown in Fig. 2 (d)] at 40 mm depth intervals into the sample. Each en face OCT image is ~790mm × 900mm.

This homodyne approach has a distinct advantage over heterodyne approaches in that we do not need to phase modulate either the reference or sample beam to create a heterodyne modulation. This implies that the pixel acquisition rate will not be limited by the heterodyne modulation frequency. The system design is simple, easy to implement, highly sensitive, and allows for high-speed en face OCT imaging of highly scattering tissues and turbid media.

Applications that can benefit from this simple imaging system include en face imaging of epithelial tissue layers of free surfaces of body for early detection and staging of near-surface microscopic pre-cancerous lesions. We note that the proposed en face homodyne OCT system can be very easily integrated into a standard confocal microscope; hence, leading to homodyne optical coherence microscopy (OCM) that will benefit from all the advantages of homodyne detection (ease of implementation), confocal microscopy (superior axial gating) as well as low coherence interferometry (high SNR as well as better depth penetration by virtue of sharper coherence gate rejection).

Reference:
Zahid Yaqoob, Jeff Fingler, Xin Heng, and Changhuei Yang. 'Homodyne en face optical coherence tomography,' Optics Letters 31, 1815-1817 (2006). (pdf)