202 CHINESE OPTICS LETTERS / Vol. 8, No. 2 / February 10, 2010
All-fiber ring-cavity based frequency swept laser source for
frequency domain OCT
Minghui Chen (²²²¨¨¨)
1
, Zhihua Ding (¶¶¶uuu)
1∗
, Lei Xu (MMM [[[)
1
, Tong Wu (ÇÇÇ ÕÕÕ)
1
,
Chuan Wang ( AAA)
1
, Guohua Shi (¤¤¤IIIuuu)
2
, and Yudong Zhang (ÜÜÜÀÀÀ)
2
1
State Key Lab of Modern Optical Instrumentation, Zhejiang University, Hangzhou 310027, China
2
Institute of Optics and Electronics, Chinese Academy of Sciences, Chengdu 610209, China
∗
E-mail: zh ding@zju.edu.cn
Received April 9, 2009
We develop a high-speed tunable, quasi-continuous-wave laser source for frequency domain (FD) optical
coherence tomography (OCT). The laser resonance is realized within a unidirectional all-fiber ring cavity
consisting of a fiber coupler, two fib er isolators, a semiconductor optical amplifier (SOA), and a fiber Fabry-
Perot tunable filter (FFP-TF) for frequency tuning. Light output from the coupler is further amplified
and spectral shaped by a booster SOA terminated at both ends with two isolators. The developed laser
source provides up to 8000 sweeps per second over a full-width wavelength tuning range of 120 nm at
center wavelength of 1320 nm with an average power of 9 mW, yielding an axial resolution of 13.6 µm in
air and a maximum sensitivity of about 112 dB for OCT imaging. The instantaneous linewidth is about
0.08 nm, enabling OCT imaging over an axial range of 3.4 mm in air. For optimization consideration based
on this custom-built swept laser, experimental study on imaging quality relevant parameters of the swept
laser with sine and ramp driving waveforms to the FFP-TF is conducted, and investigation of the swept
laser on the cavity length is done. Implementing the laser source in our established swept source based
OCT (SS-OCT) system, real-time structural imaging of biological tissue is demonstrated.
OCIS co des: 170.4500, 230.6080, 140.3460.
doi: 10.3788/COL20100802.0202.
Optical coherence tomography (OCT) which is a non-
invasive, non-contact imaging modality enables cross sec-
tional imaging of tissues with micron scale resolution
[1]
.
Frequency domain OCT (FD-OCT), different from time
domain OCT (TD-OCT), obtains full depth profile by
Fourier transform of the interference spectra with a static
reference arm
[2−4]
. FD-OCT can be classified by spec-
tral domain OCT (SD-OCT) and swept source OCT
(SS-OCT). In a SD-OCT system, a broadband light
source is used and the interference spectral components
are measured in a spectrometer by a high-speed charge-
coupled device (CCD). Alternatively, in a SS-OCT sys-
tem, a high-speed tunable, continuous-wave (CW) laser
source is used and the interference spectral signal is de-
tected by a single detector. Compared with SD-OCT,
SS-OCT has the advantages of point detection and nar-
row instantaneous linewidth without crosstalk, resulting
in a large imaging depth range. Reliable swept laser
source is the key component and crucial technology in
the development of SS-OCT, especially around 1300- and
1000-nm wavelength ranges where low cost CCDs are not
available
[5−8]
. Therefore, the development of high-speed
swept laser source causes great attention and becomes
one of the hot areas in biomedical imaging in recent
years.
Since Chinn et al. obtained the first SS-OCT images
of glass cover slips using a grating-tuned external cav-
ity super luminescent light emitting diode (LED) with
a peak gain at 840 nm
[9]
, different swept laser sources
for SS-OCT have been demonstrated. Elements such
as acousto-optic tunable filter (AOTF), rotating gal-
vanometer mirror or p olygon mirror with diffraction
grating filter, and fiber Fabry-Perot tunable filter (FFP-
TF) have been used for frequency tuning
[10−13]
. Huber
et al. developed a Fourier domain mode-locked (FDML)
swept laser based on a FFP-TF providing a sweeping
rate up to several hundred kilohertzs
[14−16]
. The FDML
swept source is superior in achieving scanning speed up
to several hundred kilohertzs. However, such mechanism
requires very long fiber ring-cavity length up to several
tens of kilometers, and fiber dispersion and temperature-
dependent ring-cavity length variations must be care-
fully managed. Therefore, the short ring-cavity length
swept source structure first proposed by Huber et al. is
adopted in this letter
[13]
. Such short ring-cavity length
swept source with moderate scanning speed is robust,
easy to maintain and miniaturize, making it suitable for
point-of-care applications. We develope a homemade
all-fiber short ring-cavity length swept laser without
any free space beam paths based on a fiber coupled
semiconductor optical amplifier (SOA), a FFP-TF, and
an additional fiber coupled booster SOA for spectrum
modification and power enhancement. Implementing
this swept laser source in our established SS-OCT sys-
tem, real-time structural imaging of biological tissue is
demonstrated. Custom-built swept laser source instead
of a commercial one interfacing with OCT system offers
more degrees of freedom in the optimization of imaging
quality and system. For optimization consideration, ex-
perimental study on imaging quality relevant parameters
of the swept laser with sine and ramp driving waveforms
to the FFP-TF is conducted, and investigation of the
swept laser on the cavity length is done.
Figure 1 illustrates the schematic diagram of the all-
fiber ring-cavity based frequency swept laser. The ring-
cavity consists of two isolators, a fiber coupled SOA, a
1671-7694/20140/020202-04
c
° 2010 Chinese Optics Letters