Swept Laser Source Based on Acousto-optic Tunable Filter
Minghui CHEN
1, *
, Hao LI
1
, Rong CHEN
2
1University of Shanghai for Science and Technology, Shanghai Institute for Minimally Invasive
Therapy, No. 516 Jungong Road, Yangpu District, Shanghai 200093, China
2 Key Laboratory of OptoElectronic Science and Technology for Medicine of Ministry of Education,
Fujian Provincial Key Laboratory for Photonics Technology, Fujian Normal University, Fuzhou
350007, P. R. China
*
Corresponding author: cmhui43@gmail.com
Abstract: The design and development of the swept laser for optical coherence tomography is presented.
It is manifested by a semiconductor optical amplifier, a fiber coupler, two fiber isolators, a semiconductor
optical amplifier (SOA) and an acousto-optic tunable filter (AOTF) for frequency tuning within a
unidirectional all-fiber ring cavity. Light output from the coupler is further amplified and spectral shaped
by a booster SOA terminated at both ends with two isolators. The total loss in ring cavity is 8.2 dB. The
gain SOA provides fiber-to-fiber small signal gain of 22.2 dB with saturation output power of 9.0 dBm. The
developed laser source provides up to 100 kHz over a full-width wavelength tuning range of 140 nm at
center wavelength of 1308 nm with an average power of 8 mW, yielding an axial resolution of 5.4 µm in air
for OCT imaging. Theoretically, the measurement principle and the feasibility of the system are analyzed.
Implementing the laser source in swept source based OCT (SS-OCT) system, real-time structural imaging of
biological tissue is realized.
Key words: optical coherence tomography; frequency swept; laser source; tune; tissue
1. Introduction
Swept source optical coherence tomography (SS-OCT) is a Fourier domain technique that measures the backscattered
light from an object due to incident light from a laser source whose frequency is rapidly swept in time
[1-2]
. The imaging
quality of SS-OCT depends to a great extent on the swept laser source, which is expected to be high speed linear scan,
broad tunable range, narrow instantaneous line-width, high output power
[3-4]
. In swept laser source, this consists of
two principal components-gain medium and tunable filter.
The light from a broadband gain medium is spectrally
filtered by a tunable narrowband optical band-pass filter and coupled back to the gain medium. The tunable filter is
swept so that the transmission frequency varies in time and sufficient time is needed to allow lasing in the transmission
bandwidth to build up from spontaneous emission inside the cavity. Early studies were performed with bulk optic designs
using a Cr:Forsterite laser operating at 1250 nm range and a grating-tuned external cavity super luminescent LED with a
peak gain at 840 nm at 10 Hz and 2 kHz sweep rate respectively
[5-6]
. The fiber Fabry-Perot tunable filter was employed
as frequency swept band-pass in short cavity and Fourier domain mode-locked long cavity laser source
[7-9]
. Hybrid bulk
optic model using a diffraction grating with a galvanometer scanning mirror and a polygon rotating mirror for
wavelength selection were adopt in laser design
[10-12]
. Techniques for improving the scan speed of the swept source have
been demonstrated
[13-14]
.
and Fiber Optic Sensors, edited by J. Czarske, S. Zhang, D. Sampson, W. Wang, Y. Liao, Proc. of SPIE Vol. 9297,
Proc. of SPIE Vol. 9297 92970Q-1