Fast Broadband Swept Source for Optical Coherence Tomography A. H. Cordes, G. Vilela de Faria and J. P. von der Weid Center for Telecommunications Studies Pontifícia Universidade Católica do Rio de Janeiro Rio de Janeiro, Brazil vdweid@opto.cetuc.puc-rio.br Abstract— A fast broadband swept source for optical coherence tomography at 1,300 nm is presented. Axial resolutions as high as 3 μm were achieved over a measuring range up to 0.5 mm. Keywords-component; optical coherence tomography; interfererometry; reflectometry; I. INTRODUCTION Optical coherence tomography (OCT) is a powerful tool for the non-contact, micrometer scale imaging of materials [1]. Its application to biological samples and in-vivo imaging in medicine bring this technique to the focus of a largely widespread research effort. Originally developed as a reflectometric technique for the inspection of optical devices, [2], [3] the technique was based on the measurement of the interference fringes of backscattered light originated from a low coherence source, launched on a Michelson interferometer. The location of the coherence spike as the reference mirror is scanned locates each individual reflection from the test arm. The axial resolution of the technique is given by the coherence length of the source, the broader the spectrum, the sharper the coherence spike. Once the axial reflectogram (or A-scan) is obtained, two dimensional (B-scans) or three dimensional (C- scans) images are obtained by coupling the reflectometer to a transversal positioning device. Driven by biomedical applications, especially the time required for data acquisition, many different experimental schemes were developed. Fast scanning methods were proposed using diffraction gratings or moving mirrors on galvanometric positioning devices [4]. A different approach to the original scheme was the Optical Frequency Domain Reflectometry (OFDR), in which a high coherence optical source has its frequency swept over a certain spectral range and the low frequency beat signal coming from the beat of delayed replicas of the signal at the detector is measured with a Fast Fourier Transform (FFT) spectrometer [5], [6]. This scheme has the advantage of having no moving parts. Again, as in the original scheme, the resolution is given by the total width of the spectral range covered by the source. Seeking for spectral broadness, pulsed supercontinuum optical sources were proposed for OCT applications, either using galvanometric scanners [7] or the OFDR approach [8- 11]. Most of these sources cover the near infrared 800 nm range or the 1550 nm one. In this paper we present a broad, fast scanning optical source for OCT applications in the 1300 nm spectral range. II. EXPERIMENTAL SET-UP A. The OFDR scheme and optical source spectrum Figure 1 displays the basic scheme used for the OFDR measurement. The all-fiber interferometer is made of a fiber optical coupler, with a fixed mirror at the reference arm, and the other butt-coupled to the sample to be measured. An index- matching liquid is used in this coupling to avoid excess phase noise coming from the Fresnel reflection at the sample arm end. Both arms were cut to the same length, to avoid ambiguities on the reflection peaks and additional phase noise. A 1-GHz photodetector and RF amplifier were used to convert the light signal into an electric signal acquired with a digital oscilloscope also with a 500 MHz bandwidth. The corresponding acquired data were averaged over a small number of sweeps (8) to increase the signal to noise ratio without smearing out the interference fringes due to phase changes along the measurement. After data acquisition from the scope the signal was processed in LabView in a PC computer. Figure 1. OFDR scheme for gernerating an A-scan in OCT This work receive support from the Brazilian Agencies FAPERJ and CNPq. Photodetector Optical source Optical 3 dB coupler Fixed mirror Sample Time Frequency Time Frequency FFT Frequency 978-1-4244-5357-3/09/$26.00©2009IEEE 148