October 10, 2009 / Vol. 7, No. 10 / CHINES E OPTICS LETTERS 941
Swept source optical coherence tomography based on
non-uniform discrete fourier transform
Tong Wu (
ÇÇÇ
ÕÕÕ
), Zhihua Ding (
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)
∗
, Kai Wang (
ppp
), and Chuan Wang (
AAA
)
State Key Lab of modern Optical Instrumentation, Zhejiang University, Hang Zhou 310027, China
∗
E-mail: zh ding@zju.edu.cn
Received October 8, 2008
A high-speed high-sensitivity swept source optical coherence tomography (SSOCT) system using a high
speed swept laser source is developed. Non-uniform discrete fourier transform (NDFT) method is intro-
duced in the SSOCT system for data processing. Frequency calibration method based on a Mach-Zender
interferometer (MZI) and conventional data interpolation method is also adopted in the system for com-
parison. Optical coherence tomography (OCT) images from SSOCT based on the NDFT method, the MZI
method, and the interpolation method are illustrated. The ax ial resolution of the SSOCT based on the
NDFT method is comparable to that of the SSOCT system using MZI calibration method and conventional
data interpolation method. The SSOCT system based on the NDFT method can achieve higher signal
intensity than that of the system based on the MZI calibration method and conventional data interpolation
method because of the better utilization of the p ower of source.
OCIS codes: 170.3800, 110.4500, 140.3600.
doi: 10.3788/COL20090710.0941.
Optical coherence tomography (OCT)
[1]
is a noninva -
sive and noncontact ima ging modality that can pro-
vide micrometer scale cros s sec tio nal images of tissue
microstructure
[2,3]
. Rec e ntly, applications of Fourier do-
main methods to OCT have attracted much attention
because of its s ignificant improvements in detection sen-
sitivity and imaging s peed
[4,5]
. A variation of Fourier do-
main OCT called swept source OCT (SSOCT) measures
the backscattered sample information using a la ser source
whose frequency is rapidly swept with time
[6]
. Cr oss sec-
tional images of biology tissue is obtained by Fourier
transform of the interference fringe signals.
In order to implement discrete Fourier transform to the
discrete sampled data, fast Fourier transform (FFT) al-
gorithm has b een widely applied in SSOCT system. One
requirement of using FFT is that the sampled data must
be equa lly spaced in the frequency do main (k-space), oth-
erwise, ima ge quality in terms of resolution, sensitivity,
and absolute measures would be degraded. However,
directly sampled raw data in a SSOCT system is not
equally spaced in k-space because of frequency nonlin-
earity verse time of its swept laser source.
In order to reach the best image q uality, calibrating
the raw data from non-uniformly spaced one to equally
spaced one in k-space is nec essary. In recent literatures,
several a pproaches for frequency calibration fo r SSOCT
have be e n demonstrated. One of them is the simulta-
neous frequency monitoring method using a fiber Fabry-
Perot (FFP) interferometer
[7]
to calibrate the interfer-
ence data, in which a portion of the light source is in-
troduced into the FFP to generate series of peak signals.
Via simultaneously detecting and storing the OCT in-
terference s ignal and FFP signal, calibration parameters
can be calculated using the output signal of FFP, and
then the linearly frequency spaced da ta can be acquired
using the calibration parameters. This method requires a
high-end analog/digital (A/D) converter and large mem-
ory space to store the two sig nals. The frequency even
clock method
[8]
is similar to the simultaneous fre quency
monitoring method, but the output from an FFP or a
fiber Bragg grating (FBG) is mo nitored by a photo detec-
tor and converted to a transistor-trans istor logic (TTL)
pulse train, which is used as a sampling clock signal of
an A/D converter which detects the interference signal.
This method does no t store the FFP/ FBG output in the
memory, but still requires expensive FFP/FBG. Data in-
terpolation method is a conventional software method to
calibrate the interference spectrum signal. T he method
does not need FFP/FBG, and in this method, the de-
tector signal is sampled in constant time intervals and
nonlinearly in k-space, and then the interference signal is
interpolated to get data equally spaced in k-space
[9]
. Hu-
ber et al. demonstrated a fast calibration and rescaling
algorithm by using a FFP and a nearest neighbor check
algorithm
[10]
. This method is capable of high-sp e e d cali-
bration because of the elaborated algorithm, but a FFP
device is still r equired.
As mentioned above, the hardware calibration meth-
ods based on FFP/FBG require expensive FFP/FBG
and additional storing space, and the software calibra-
tion method has a drawback of interpolation inaccuracy.
Therefore in this letter, we introduce a non-uniform dis-
crete fourier transform (NDFT) method and apply it pro-
cess raw data of OCT interference sig nal without any ad-
ditional expensive calibration device.
Firstly we briefly review the definition of Fourier trans-
form in the continuous domain and discrete domain. The
Fourie r transform of a time signal f(t) in the continuous
domain is defined by
F (ω) =
Z
+∞
−∞
f(t)e
−jωt
dt, (1)
where ω = 2πf and f is the temporal frequency. The ex-
tension of Eq. (1) to the discrete domain is called discrete
1671-7694/2009/100941-04
c
2009 Chinese Optics Letters