U.S. patent application number 14/191682 was filed with the patent office on 2017-08-31 for optical coherence tomography imaging system and method.
This patent application is currently assigned to Thorlabs, Inc.. The applicant listed for this patent is Thorlabs, Inc.. Invention is credited to Scott Barry, Alex E. Cable, James Y. Jiang.
Application Number | 20170248405 14/191682 |
Document ID | / |
Family ID | 53881895 |
Filed Date | 2017-08-31 |
United States Patent
Application |
20170248405 |
Kind Code |
A9 |
Jiang; James Y. ; et
al. |
August 31, 2017 |
OPTICAL COHERENCE TOMOGRAPHY IMAGING SYSTEM AND METHOD
Abstract
An optical imaging system includes an optical radiation source
(410, 510), a frequency clock module outputting frequency clock
signals (420), an optical interferometer (430), a data acquisition
(DAQ) device (440) triggered by the frequency clock signals, and a
computer (450) to perform multi-dimensional optical imaging of the
samples. The frequency clock signals are processed by software or
hardware to produce a record containing frequency-time relationship
of the optical radiation source (410, 510) to externally clock the
sampling process of the DAQ device (440). The system may employ
over-sampling and various digital signal processing methods to
improve image quality. The system further includes multiple stages
of routers (1418, 1425) connecting the light source (1410) with a
plurality of interferometers (1420a-1420n) and a DAQ system (1450)
externally clocked by frequency clock signals to perform high-speed
multi-channel optical imaging of samples.
Inventors: |
Jiang; James Y.;
(Hackettstown, NJ) ; Barry; Scott; (Lafayette,
NJ) ; Cable; Alex E.; (Newton, NJ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Thorlabs, Inc. |
Newton |
NJ |
US |
|
|
Assignee: |
Thorlabs, Inc.
Newton
NJ
|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 20150241202 A1 |
August 27, 2015 |
|
|
Family ID: |
53881895 |
Appl. No.: |
14/191682 |
Filed: |
February 27, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13092414 |
Apr 22, 2011 |
8705047 |
|
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14191682 |
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12016484 |
Jan 18, 2008 |
7936462 |
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13092414 |
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60885874 |
Jan 19, 2007 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01B 9/02027 20130101;
G01B 9/02091 20130101; G01B 9/02069 20130101; G01B 9/02004
20130101; G01B 9/02083 20130101 |
International
Class: |
G01B 9/02 20060101
G01B009/02 |
Claims
1. A system for optical imaging of samples with multiple imaging
channels, by using multiple stages of routers to multiplex a
plurality of OCT imaging channels with one light source into one
system, the system acquiring image data from multiple points on a
sample or from multiple samples, and image data acquisition is
externally clocked by frequency clock signals of light sources used
in the system, the system comprising: an optical radiation source
outputting optical frequencies as a function of time; a frequency
clock module monitoring the output optical frequencies of the
optical radiation source, and outputting frequency clock signals; a
signal processing circuit processing the frequency clock signals to
produce a pulse train indicating a relationship of the output
optical frequencies of the source and time; a data acquisition
(DAQ) system adapted for an analog to digital conversion process
and externally clocked by the pulse train, wherein the pulse train
is connected to an external clock input of the DAQ system; a
plurality of optical interferometer modules, wherein the
interferometer modules are in connection with the optical radiation
source and the samples, and output interference signals containing
information about the sample, wherein the interference signals are
connected to input channels of the DAQ system; a first stage of
routers distributing the output power of the source to the
plurality of optical interferometer modules, wherein the plurality
of optical interferometer modules output interference signals
containing information about the sample; a second stage of routers
distributing the output of the plurality of optical interferometer
modules to the DAQ system; and a processor controlling the DAQ
system, processing the data output from the DAQ system and
constructing multiple dimensional images of the sample for every
imaging channel.
2. The system as claimed in claim 1, wherein the first router is an
optical switching device that switches the output of the optical
source among optical interferometer modules input ports, or the
first router is an M.times.N optical coupling device that couples
the output of the optical source to multiple optical interferometer
modules input ports.
3. The system as claimed in claim 1, wherein the second router is
an optical switching device that switches the optical signals from
multiple interferometers to multiple optical detectors, and
multiple optical detectors convert the optical signals to electric
signals input to the DAQ system.
4. The system as claimed in claim 1, wherein the second router is
an electric switching device that switches the OCT signals from
multiple interferometers, when every interferometer has its own
detector to convert the optical interference signals to electric
signals, to the DAQ system.
5. The system as claimed in claim 1, wherein the first and second
router can be the same optical switching device, wherein an optical
beam splitter or an optical circulator is used to separate the
optical input and output of the interferometers.
6. The system as claimed in claim 1, wherein the DAQ system
comprises multiple DAQ devices with communication capabilities
among the DAQ devices.
Description
[0001] This application is a continuation of U.S. application Ser.
No. 13/092,414, filed Apr. 22, 2011, which is a
continuation-in-part of U.S. application Ser. No. 12/016,484, filed
Jan. 18, 2008, which claims the benefit of U.S. Application
60/885,874, filed Jan. 19, 2007 now expired, the contents of each
of which are incorporated herein by reference.
INTRODUCTION
[0002] This application relates to a new OCT imaging system and
methods for improving the efficiency, speed, and quality of the
acquisition, generating, and display of one dimensional or
multi-dimensional OCT images.
[0003] Optical coherence tomography (OCT) is an emerging imaging
technology based on low-coherence interferometry that enables
non-invasive, cross-sectional imaging of a sample with micrometer
scale resolution. It has been demonstrated that Fourier domain OCT
(FD-OCT) techniques can significantly improve the sensitivity and
imaging speed of an OCT system. In FD-OCT systems, the interference
fringe signals are recorded as a function of optical frequency at
high-speed using a broadband light source and a spectrometer pair,
or a frequency swept source and a detector pair. After analyzing
the interference fringe signals, the depth-encoded reflectivity
profiles of the sample are retrieved and used to construct the OCT
images.
[0004] A frequency swept source has been demonstrated to have many
advantages for OCT imaging because it enables high efficiency
detection of back-reflected signals from the sample via a balanced
detection scheme. Such high signal collection efficiency is
essential for high-speed detection of very weak signals reflected
from deeper regions in a sample. However, a swept source based OCT
system has sonic drawbacks. First, because the scanning wavelength
of a high-speed tunable laser is usually not linear in optical
frequency space, recorded OCT data points must be recalibrated from
time domain to equally spaced data points in optical frequency. A
frequency clock module connected to the laser is typically used to
provide the frequency clock signals of the laser as the
recalibration reference. This recalibration process can be
time-consuming because it must be performed for each scan of the
laser corresponding to one axial line (A-scan) in the constructed
OCT image, which greatly limits the real-time imaging speed of
conventional OCT systems. Second, in the dynamic process of
actively tuning the laser frequency, the laser undergoes
significant changes in cavity conditions (i.e., cavity length,
average mode number, or number of modes), which cause both
intensity and phase instabilities and noise in the laser output.
The intensity noise and phase noise degrade detection sensitivity
and final image quality. This is a problem that cannot be
completely resolved by the balanced detection method.
SUMMARY OF THE INVENTION
[0005] This application discloses a method to improve OCT signals
processing and imaging speed: The frequency clock signals of the
source are processed to obtain a pulse train containing the
relation of the optical frequencies of the source and the time. The
pulse train is connected to the external clock signals input of the
data acquisition (DAQ) device. The DAQ converts the input OCT
interference signals at each pulse in the pulse train and transfer
the converted data points to computer memory. This operation mode
of the DAQ system advantageously relieves the data transfer load of
the computer data bus and simplifies OCT signal processing.
Embodiments of using both hardware and software methods to achieve
this goal are disclosed.
[0006] Also disclosed are methods to improve OCT imaging quality:
By over-sampling the OCT signals and frequency clock signals, and
applying various algorithms to digitally process the over-sampled
data points to improve signal quality and reduce the amount of data
points needed to be transferred to computer memory, the OCT image
quality can be significantly improved without compromising the
imaging speed.
[0007] Additionally, disclosed are methods of computer processors
controlling the overall operation of the imaging system to employ
parallel data acquisition and signal processing routines. Parallel
processing is important for real-time high-speed signal acquisition
and image construction because the computer is not idle while the
DAQ fills the data buffers.
[0008] A multiple-channel OCT imaging system that generates high
quality OCT images at high speed from multiple-channels
simultaneously is also disclosed. The system employs multiple
stages of routers to route (i) the optical output of the source to
illuminate a plurality of interferometers, and (ii) the optical or
electric output of the interferometers to the detectors and DAQ
system. In this manner, the imaging system can provide multiple OCT
imaging channels for a single or a plurality of samples.
[0009] A real-time video-rate OCT microscope using swept source is
demonstrated as an embodiment of the inventive system and
method.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] Embodiments of the invention are particularly pointed out
and distinctly claimed at the conclusion of the specification in
the claims. The foregoing and other objects, features and
advantages of exemplary embodiments of the invention will be
apparent from the following detailed description taken in
conjunction with the accompanying drawings.
[0011] FIG. 1 illustrates an exemplary embodiment of an optical
coherence imaging system with microscope sample interface;
[0012] FIG. 2 illustrates an exemplary temporal intensity profile
of a swept source showing a forward and a backward scan;
[0013] FIG. 3 illustrates an exemplary hardware signal processing
board schematic according to an embodiment of the invention;
[0014] FIG. 4 illustrates an exemplary optical imaging system
according to an embodiment of the invention;
[0015] FIG. 5 illustrates an exemplary optical imaging system
according to another embodiment of the invention;
[0016] FIGS. 6a and 6b illustrate exemplary embodiments of an
interferometer of an optical imaging system;
[0017] FIGS. 7a and 7b illustrate exemplary embodiments of a
frequency clock module of an optical imaging system;
[0018] FIGS. 8-12 each illustrate exemplary embodiments of optical
imaging systems according to the principles of the invention;
[0019] FIG. 13 illustrates, in block diagram form, an exemplary
method realized in accordance with the principles of the present
invention;
[0020] FIGS. 14-16 each illustrate exemplary embodiments of
multiple-channel optical imaging systems according to the
principles of the invention.
[0021] FIG. 17a shows the frequency clock, signals and the clock
pulse train generated from the piezo tuning VCSEL swept source with
coherence length longer than 20 mm.
[0022] FIG. 17b shows the frequency clock signals and the clock
pulse train from the piezo tuning Fabry Perot swept source with
coherence length about 8 mm.
[0023] FIG. 18a shows the OCT images from a tape sample for swept
source with coherence length longer than 20 mm.
[0024] FIG. 18b show the OCT images from a tape sample for swept
source with coherence length about 8 mm.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0025] This disclosure describes the best mode or modes of
practicing the invention as presently contemplated. This
description is not intended to be understood in a limiting sense,
but provides an example of the invention presented solely for
illustrative purposes by reference to the accompanying drawings to
advise one of ordinary skill in the art of the advantages and
construction of the invention, in the various views of the
drawings, like reference characters designate like or similar
parts.
1. Principle of Swept Source OCT
[0026] In swept source Fourier domain OCT, a light source outputs
optical frequencies as a function of time. The light is coupled to
a sample and a reference reflector using an optical interferometer.
Back reflected or back scattered light from different depths within
the sample is combined with light from the reference reflector to
form OCT interference fringe signals. After converted to electric
signals by a photodiode detector, the OCT signals are digitized by
a DAQ device to discrete digital data points. A Fourier transform
is applied to the data points to detect the echo time delay and
amplitude of the back reflected light from different depths within
the sample and to construct cross-sectional images of the
sample.
[0027] An interference signal detected by a single photodiode, as a
function of optical frequency, is expressed as:
I.sub.PD(.omega.)=I.sub.R(.omega.)+2 {square root over
(I.sub.R(.omega.)I.sub.S(.omega.))}
cos(.DELTA..phi.(.omega.))+I.sub.S(.omega.) (1)
where I.sub.R(.omega.) and I.sub.S(.omega.) are the optical
frequency dependent intensities reflected from the reference and
sample arms; .DELTA..phi.(.omega.) is the optical frequency
dependent phase difference between the reference and sample arms;
.omega.=.omega.(t) is the optical frequency sweep profile as a
function of time. The interference tern on the right hand side of
Eq. (1) is expressed as:
I int ( .omega. ) = 2 n I R ( .omega. ) I n ( .omega. ) cos [
.omega. c z n ] ( 2 ) ##EQU00001##
where I.sub.n(.omega.) is the intensity of light reflected from the
n-th layer in the sample and can be expressed as
I.sub.n(.omega.)=R.sub.n(.omega.)I.sub.SS(.omega.);
R.sub.n(.omega.) is the optical frequency dependent reflectivity
from the n-th layer; I.sub.SS(.omega.) is the original spectrum of
the swept source; and z.sub.n is the depth of the n-th layer. It is
supposed that the attenuation for the reference arm light is
uniform for all frequency components:
I.sub.R(.omega.)=.mu..sup.2I.sub.SS(.omega.), where .mu..sup.2 is
the attenuation coefficient,
[0028] When a balanced detection scheme is employed in the
interferometer, there is a 180-degree phase shift between the
interference signals that occur in the two interference channels
connected to the balanced detector. Since the output from the
balanced detector is the difference between the two input channels,
the subtraction of these two signals adds the second term but
subtracts first and third terms in Eq. (1). In the ideal case where
the two input channels of the balanced detector are perfectly
balanced, the output from the balanced detector is given by:
I BD = 4 n I R ( .omega. ) I n ( .omega. ) cos [ .omega. c z n ] =
4 .mu. I SS ( .omega. ) n R n ( .omega. ) cos [ .omega. c z n ] ( 3
) ##EQU00002##
[0029] Eq. 3 reveals the fact that the optical reflectivity
R.sub.n(.omega.) from the n-th layer z.sub.n is linearly encoded in
the frequency of the sinusoidal function
cos [ .omega. c z n ] . ##EQU00003##
The deeper that the reflection occurs (corresponding to a larger
z.sub.n value), the higher the frequency in the detected
interference signals. Applying a Fourier transform to the
interference fringe signals decomposes the mixed signals into
differentiate frequency components. The amplitude of the frequency
component is in proportional to the light reflectivity from a
certain depth. Therefore, a complete depth profile of a sample can
be measured by plotting the Fourier transformed amplitude as a
function of frequency. When the incident beam performs another one
dimensional or two dimensional scans across the sample surface, 2D
or 3D OCT image data of the sample can be acquired and
displayed.
[0030] The sweep profile .omega.=.omega.(t) is determined by the
scanning mechanism (i.e., sinusoidal) of the wavelength tuning
component within the laser. The nonlinear nature of this tuning
curve requires that the resulting OCT signals be recalibrated from
equally spaced sample points in time to equally spaced sample
points in frequency. A frequency clock module is used to monitor
the output frequency of the laser, by generating an interference
fringe signals from a fixed delay in another optical
interferometer. This calibration process can be performed by
hardware or software to linearly map the acquired OCT interference
signal data points to optical frequency domain.
2. Methods and Systems
2.1. Optical Coherence Tomography Imaging System
[0031] FIG. 1 is a schematic of an embodiment of an optical
coherence tomography imaging system using a microscope (MS) 100 as
the sample interface. The light source (SS) 110 is, for example, a
rapidly swept external cavity laser with a wavelength sweep range
from 1250 nm to 1400 nm at scan frequency of 16 kHz (8 kHz forward
and 8 kHz backward for an total effective scan rate of 16 kHz). The
typical 3 dB spectral bandwidth is measured to be >110 nm, and
the typical average output power is 16 mW.
[0032] The main optical output of the laser is split by a 99:1
fiber coupler (FC) 120. One percent of the laser output is
connected to a Mach-Zehnder Interferometer (MZI) 130 as the
frequency clock module to produce frequency clock signals. The
frequency clock signals can be processed by a signal processing
board (SP) 140 to generate electrical pulses that are equally
spaced in optical frequency. The frequency clock signals can also
be processed by software, after digitization of the signals, to
generate a digital record containing the frequency-time
relationship of the laser. The frequency clock signal forms the
basis of the recalibration routine and will be described in detail
below. Another one percent of the laser output is tapped to record
the temporal intensity profile of the source.
[0033] The rest of the main laser output is routed to a fiber based
Michelson interferometer through another fiber coupler (FC). In the
reference arm of the interferometer, the beam exiting from the
fiber is collimated in collimator (C) 150 and reflected by a
stationary mirror (M) 160 back into the fiber. A manual or
electronically controlled variable attenuator (VA) 170 is used to
adjust the reference power to a proper level for better detection
sensitivity. In the sample arm, the fiber 180 is connected to a
microscope head, and the beam exiting from the fiber is also
collimated and directed by a XY scanner (SD) 190 toward the sample.
The sample is placed on an XY translation stage mounted on the
microscope. An infinity corrected long working distance objective
(OBJ) is used for focusing the beam onto the sample. The long
working distance of the objective provides a large clearance
(>20 mm) between the optics and the sample, which enables easy
handling of the sample. A 45.degree. incidence cold mirror is
inserted into the beam path to reflect the visible light from the
sample onto a CCD camera that records conventional video microscope
images of the sample. An aiming laser 200 centered at 632 nm (which
can be seen by a human eye), or at 780 nm (which can be detected by
the CCD camera), is coupled to the sample arm of the interferometer
to indicate the laser scanning position on the sample. A balanced
detector (BD) 210 (i.e., PDB140C, Thorlabs.RTM., Inc.) with 3 dB
cut-off frequency of 15 MHz, optimized for low DC-offset (<1 mV)
and high impedance gain (>180,000 V/A), is used to record the
interference fringe signals in the Michelson interferometer. The
interference fringe signals are connected to a signal processing
board (SP) 140. The signal processing board processes the output of
the MZI frequency clock 130 signals to generate a pulse train with
equal spacing in optical frequencies. A 14-bit digitizer is
configured in external clock mode to use the pulse train as trigger
signals to sample the OCT signals. The digitized data points are
equally spaced in optical frequency so no additional frequency
recalibration is needed. Fourier Transform is applied to the data
points and generates the depth-dependent reflectivity profile of
the sample. The computer also generates waveforms through an analog
output board to control the XY beam scanning in the microscope
head, to perform 2D or 3D OCT imaging of the sample. Also shown in
FIG. 1 is optical detector D.
2.2 Swept Source
[0034] The light source 110 may be a swept source used in the
imaging system is similar to that described in U.S. Patent
Publication No. 2006/0203859. Briefly, the swept source includes a
semiconductor gain chip with one partial-reflection coated facet
that serves as the output coupler of the laser, and another with
anti-reflection (AR) coated facet toward the intra-cavity. The beam
exiting from the AR coated facet of the chip is collimated by an
aspheric lens to illuminate a diffraction grating. The light
diffracted by the grating is collected by an achromatic doublet
lens and focused onto a highly-reflective mirror covered by a
10-.mu.m slit. The grating is mounted on a resonant galvanometer
scanner that rotates about its axis. When the grating is rotating,
the slit provides wavelength selection and feedback of the selected
wavelength into the cavity, thus enabling high-speed sweeping of
the output optical frequency of the laser. The measured temporal
intensity profile of the swept source for a forward and a backward
scan is shown in FIG. 2. The temporal intensity profile contains
noise which is from the residue interference effects within the
laser cavity such as the etalon effect of the semiconductor gain
chip. This interference signal can be used as the frequency clock
signal of the source since it is intrinsic in the laser output and
used to externally clock the acquisition of OCT signals.
2.3 Frequency Clock Module
[0035] In Fourier domain OCT, as required by Eq. (3), the OCT
signals must be re-sampled into linear frequency space, so adjacent
data points have an equal optical frequency interval. Fourier
transform can then be applied to accurately recover the
depth-dependent sample reflectivity information. The
photo-detectors detect the interference fringes as a function of
time. However, because the optical frequency sweep of the laser is
determined by its sweep mechanism, or through application of
external driving signals, the resulting sweep of the laser
frequency output is typically not linear in time and simple
sampling of the detector signal using a fixed time base results in
significantly degraded image quality. Therefore, a frequency
reference or a digital record containing the time-frequency
relationship of the laser must be established prior to the
recalibration process. The recalibration process maps the OCT data
points to equal spacing in optical frequency domain. Applying
Fourier transforms to the recalibrated OCT data points yields the
depth profiles of the samples.
[0036] A Mach-Zehnder Interferometer (MZI) is used as the frequency
clock module. The MZI can have a fixed delay between its two arms
or have a translation stage to control the path length difference d
between the two arms. Changing d changes the frequency of the
resulting MZI clock signal. Another balanced detection detector
(for example, a Thorlabs.RTM. PDB120C, 80 MHz) is used to record
the interference fringes of the MZI. The output of the MZI balanced
detector is a sinusoid wave similar to Eq. 3 and can be expressed
as below:
I MZI = 4 I SS ( .omega. ) cos [ .omega. c d ] ( 4 )
##EQU00004##
[0037] Although .omega.(t) is usually not linear in time, all the
maximas and zero-crossings (4 points per MZI fringe cycle) in a
signal measured as a function of time are equally spaced in
frequency. The Free-Spectral Range of the MZI clock is given
by:
FSR.sub.MZI=c/d (5)
The number of MZI fringe cycles per laser swept is given by:
N MZI_fringes = ( .omega. max - .omega. min ) d 2 .pi. c ( 6 )
##EQU00005##
where .omega..sub.max and .omega..sub.min are the maximum and
minimum angular frequencies of the swept source. For a wavelength
scanning range of 1240 nm to 1380 nm, when the delay d is set at 6
mm, the FSR is approximately 50 GHz and one scan of the laser
generates approximately 480 fringe cycles. If two data points are
taken per MZI cycle, a total number of .about.1000 points can be
generated as the frequency reference for sampling the OCT
interference fringe signals.
[0038] Since the MZI delay can be continuously adjustable, and the
number of data points per MZI cycle can be 2 or 4 or other numbers,
the MZI clock module is very flexible in generating the required
number of frequency reference data points tier swept source OCT
applications. Applying the balanced detection of the MZI clock
signal is a very effective method to remove the DC term in the
detected signal and double the contrast of the interference fringe
signals.
2.4 OCT Data Acquisition Externally Clocked by Frequency Clock
Signals
[0039] The frequency clock signals can be processed by software or
hardware to externally clock the acquisition of OCT signals. In
conventional SS-OCT systems, the OCT signals are recorded
simultaneously with the clock signals by a two channel high-speed
digitizer. A software algorithm analyzes the clock signals to build
a digital record containing the frequency-time relationship for
every laser scan. This digital record is then used to recalibrate
the acquired OCT signal into linear frequency space. This approach
requires sampling both the OCT data channel and frequency clock
signal channel. Since the laser can be scanning at a very high
speed, the data transfer load from the DAQ device to the computer
memory is very large, often exceeding the data transfer bandwidth
of the data bus (i.e., PCI bus). The data bus bandwidth limits the
maximum data that can be processed by the computer processor and
the OCT system imaging speed.
[0040] In view of the problems of the prior art, an embodiment of
the present invention includes an OCT system, which uses the
frequency clock signal to externally clock the DAQ device to sample
the OCT interference signals. This may advantageously reduce the
amount of data needed to be transferred from the DAQ device to the
computer memory. The triggering mechanism can be an electrical
pulse train input to the DAQ device, or a copy of digital record
residing in or uploaded to the DAQ board. The electrical pulse
train and the digital record are generated by hardware or software
processing of the frequency clock signal. Using this method, the
recalibration process of the OCT signals is done in the DAQ device
and only the data points required for OCT image construction are
transferred via the computer data bus. Certain experimental systems
showed a decrease of the data transfer load of the data bus by a
factor of at least 2-5. in addition, the recalibration process is
moved from the computer processor to the DAQ device which
alleviates the computation load of the computer processor to allow
the processor more duty cycle for performing other tasks like
Fourier transform, logarithm calculation and display of the
multi-dimensional image data.
[0041] This method is very useful for reducing the data transfer
load in high-speed OCT imaging systems, since the output frequency
of the laser is usually nonlinear in time. This method can be also
applied to other applications requiring, for example, high speed
analog to digital conversion, high density sampling of raw signals,
and a nonlinear time base. Uploading a digital record, which
contains the nonlinear time base information and can be dynamically
modified, to the DAQ device allows the DAQ device to use this
digital record to externally clock the data acquisition processes
for all signal channels input to the DAQ device. Only the data
points that are in a particular relationship with the digital
record are processed and transferred to memory devices, while other
redundant data points are discarded or not transferred. This method
requires less bandwidth and less time for transferring the data
points, shares some data processing load of the main processor,
thus advantageously improves the speed and efficiency for the
applications.
[0042] Another embodiment of the invention provides a hardware
method for accelerating the recalibration of OCT signals from time
to frequency space using a signal processing board. The signal
processing board processes the frequency clock signals to generate
clock pulses that indicate when the output optical frequencies of
the source have a linear relationship. The digitizer is configured
in external clock mode, and the clock pulses output from the signal
processing board are connected to the external clock input of the
digitizer to serve as the time base for the analog to digital
conversion of the OCT signal channel. In this mode, the OCT signals
are digitized into data points with equal spacing in the optical
frequency domain, ready for Fourier transform to generate the depth
profiles of the sample and construct OCT images.
[0043] FIG. 3 is an illustrative schematic of an example SS-OCT
signal processing board. The signal processing board 300 generates
pulse trains with the pulses indicate when the output optical
frequencies of the source have a linear relationship. The pulse
trains are connected to the external clock input of the DAQ card,
to serve as the time base to synchronize the data acquisition of
OCT signals. In the example of FIG. 3, fiber couplers 307 are used
to split the light from a swept source 305. A small portion
(.about.1%) of the laser output P.sub.REF from swept source 305 is
monitored by an on-board optical detector 310 (typically a
photodiode). The detection bandwidth is chosen to be higher than
the maximum frequencies generated by the MZI clock 315 to prevent
signal aliasing. The laser scanning frequency signal from the MZI
clock P.sub.CLK from the output of the balanced detector 1 316 is
divided by P.sub.REF at Divider 1 320. This division normalizes the
measurement data to the time dependent laser power curve. A 2.5-7.0
MHz band-pass filter 325 designed for a 20 kHz scanning laser is
used for bandpass filtering the frequency clock signal. The
bandpass filter rejects the low frequency and high frequency noises
on the clock signals and reduces the errors from the decision
circuit 330 in later stages of the signal processing board. The
decision circuit 330 is a fast voltage comparator with a reference
voltage setting at zero, so all the zero-crossings in the clock
signals are converted to digital pulses indicating the time when
the output optical frequencies of the source have a linear
relationship. If more valid pulses for sampling the OCT
interference fringes are required, a frequency doubling circuit is
used. A differentiator 335 generates the first order derivative of
the clock signal and converts all the peaks to zero-crossings.
Another voltage comparator 337 receives the output from the
differentiator and generates a second set of digital pulses. The
two sets of digital clock pulses are phase shifted by 90 degrees
and are combined by XOR digital logic circuits 340 into frequency
doubled clock pulses. Depending on the XORs logic design, 2 or 4
pulses per MZI fringe cycle can be generated. The original or the
frequency doubled clock pulses are connected to the external clock
input of DAQ device 350 to externally clock the analog to digital
conversion process. In the case of 2 pulses per MZI fringe cycle,
the clock pulses have a typical frequency range from 4-14 MHz when
a swept source as described in section 2.2 is used; other sources
may have significantly different clock frequencies. This method can
be adapted to provide a higher number of clock pulses per MZI
fringe cycle if required. As shown in FIG. 3, a balanced detector
360 couples the interference fringe signals from the interferometer
365 to the OCT clock board 300.
2.5 Parallel Computing
[0044] In an example embodiment software configures the data
acquisition routine and signal processing routine in parallel. The
software controls the DAQ device to start the data acquisition
routine; and without waiting for the data acquisition routine to be
finished, the software starts the signal processing routine to
process the previously acquired data stored in memory; the software
then checks the data acquisition status after processing a certain
amount of previously acquired data. The flow chart shown in FIG. 13
illustrates the principles of the image construction routine.
2.6 Signal Enhancement by Over-Sampling OCT and Frequency Clock
Signals
[0045] Further disclosed is a method to enhance signal and image
quality in an OCT imaging system. The method includes over-sampling
the OCT signals and frequency clock signals at a sampling density
higher than required by Nyquist sampling theory and utilizing the
on-board processing power of the DAQ device or computer processor
to process the over-sampled data points according to the processed
frequency clock signals--a digital record that contains the
frequency-time relationship of the source. Various signal
processing algorithms can be applied to significantly improve
signal strength, image contrast, and image quality.
[0046] Over-sampling the OCT signals increases the amount of data
points that can be processed to produce images. Since many balanced
detectors convert the received photons into electric current, and a
trans-impedance gain module inside the balanced detector converts
the current into voltage output, for a continuous wave (CW) light
source like a swept source or a super luminescent diode, sampling
the OCT signals at a higher density means better photon detection
efficiency and better system sensitivity. The recalibration process
picks out the data points that are linearly spaced in optical
frequency domain and throws out other data points that are still
valid points representing the optical interference fringe signals.
By using, for example, specially designed algorithms to process the
over-sampled data points and salvaging the photons that are
discarded in conventional OCT signal processing methods, the OCT
signal strength can be increased, signal-to-noise can be enhanced,
and the final image quality can be significantly improved.
[0047] Over-sampling of the frequency clock signals produces more
data points to represent the raw signals, thus the frequency-time
relationship of the laser scans can be measured more precisely. An
accurate frequency-time relationship of the laser is critical for
recalibration of the raw OCT signals from time into optical
frequency space. As a result of over-sampling the frequency clock
signals, the resolution and signal-to-noise ratio of OCT images can
be significantly improved, and some imaging artifacts are reduced
or totally removed.
[0048] The algorithms that can be used for processing over-sampled
OCT signals include, for example: [0049] 1. Multiple data points
are averaged to be one data point according to a digital record
containing the frequency-time relationship of the source; Fourier
transform is applied to the averaged data points to construct a
depth profile of the sample. Alternatively, multiple sets of the
OCT data points are generated from the over-sampled OCT data points
according to multiple sets of digital records containing the
frequency-time relationship of the same source; Fourier transforms
are applied to each set of OCT data points. The outputs of Fourier
transforms of multiple sets of OCT data points are averaged into
one set to construct a depth profile of the sample. [0050] 2. The
over-sampled OCT data points are stored in memory and compared with
another set of over-sampled data points to improve signal to noise
ratio, enhance image resolution or contrast, or enhance phase,
polarization and spectrum information. The other over-sampled data
points used for the comparison can be acquired from previous scans
of the laser, or from other signal channels acquired simultaneously
with a current channel, or from a pre-calculated data set stored in
the memory device. [0051] 3. The over-sampled OCT data points are
averaged according to a digital record containing the
frequency-time relationship of the source. A Fourier transform of
the averaged OCT data points generates intensity and phase
information. The intensity information is averaged to construct a
depth profile of the sample. The phase information is averaged to
provide information about sample position and motion, or various
sample properties to the incident light conditions. The various
sample properties include, for example, optical birefringence,
absorption, fluorescence emission spectrum, optical harmonic
generation, and other linear or nonlinear optical properties of the
sample. [0052] 4. The over-sampled OCT data points are averaged
according to the processed frequency clock signals. A Fourier
transform of the averaged OCT data points generates intensity and
phase information. The intensity and phase information are compared
with another data set in the memory or acquired from another signal
channel simultaneously or non-simultaneously. The compared
intensity information is used to construct a depth profile of the
sample. The intensity information can be averaged and digitally
interpolated to improve the resolution in detecting of certain
reflection layers in the sample. The compared phase information is
averaged to provide highly sensitive information about, for
example, sample position, motion of particles in the sample, or
various sample properties under incident light conditions. The
various sample properties, for example, include optical
birefringence, absorption, fluorescence emission, optical harmonic
generation, and other linear of nonlinear optical properties of the
sample.
2.7 Multiple-Channel OCT Imaging System
[0053] Also disclosed herein is an OCT imaging system for acquiring
high quality image data from multiple samples simultaneously. The
system employs multiple stages of routers to multiplex a plurality
of OCT imaging channels with one light source. Each light source
has a frequency clock module to externally clock the DAQ device
acquisition of the imaging channel that is illuminated by this
light source. The DAQ device has multiple input channels for the
plurality of OCT imaging channels and the image data from multiple
channels is acquired simultaneously or by using time-multiplexing.
In this multiple-channel OCT imaging system, the DAQ process is
externally clocked by the processed frequency clock signals from
the frequency clock module serving each light source, which results
in very efficient data acquisition and processing, and very high
imaging speed. In this multiple-channel OCT imaging system, the
over-sampling methods of OCT signals and frequency clock signals
may also be applied to improve the OCT image.
3. Additional System and Method Embodiments
[0054] FIG. 4 is an embodiment of a system for performing optical
imaging of a sample. This system includes an optical radiation
source shown as a swept source 410. The swept source 410 outputs
its optical frequencies as a function of time. A frequency clock
module 420 monitors the output optical frequency of the swept
source 410 and outputs frequency clock signals. The system also
includes an optical interferometer 430 which receives an output
from the swept source 410 and an optical detector to detect the
interference fringe signals from the interferometer and convert
them to analog electrical signals. The optical detector may be an
embedded module in the optical interferometer 430 or external to an
optical interferometer (see FIG. 1 optical detector D). Although
the optical interference happens inside an optical interferometer
with or without the optical detector. The system further includes a
DAQ device 440 to convert the analog electric signals to digital
data points. The data acquisition is externally clocked by the
frequency clock signals. A computer controls 450 the DAQ system,
processing of the digital signals, and construction of the depth
profiles, and multi-dimensional images 460 of the sample.
[0055] FIG. 5 is an embodiment of a system for performing optical
imaging of a sample. The optical radiation source 510 has a
broadband spectral output and outputs all optical frequencies
simultaneously. The broadband source 510 illuminates an
interferometer 530 and produces OCT interference fringe signals in
optical frequency domain. A digital record is used as the frequency
clock signal from the frequency clock module 520 to externally
clock the data acquisition of OCT signals from a spectrometer. A
computer 550 controls the DAQ system 440, processing of the digital
signals, and construction of the depth profiles, and
multi-dimensional images 560 of the sample. The digital record is
generated from an optical wavelength meter or an optical
spectrometer as the frequency clock module 520 of the light
source.
[0056] As shown in FIG. 6a, a Michelson interferometer may include
an optical path leading to a reference optical reflector 610, an
optical path leading to a sample 620 to be imaged, and an optical
path where the light from the sample and the reflector interfere to
produce interference fringe signals. In addition as shown in FIG.
6b, the optical path leading to the reference reflector 630 and
optical path leading to the sample 640 can share a same optical
path, forming a common path interferometer. The reference reflector
in either the Michelson type or common path type or any other type
interferometers is a fixed single reflection surface, or has
multiple reflection surfaces from known depths, or the position of
the reflector is programmable.
[0057] The frequency clock modules shown in FIGS. 7a and 7b may use
a fixed or variable delay optical interferometer to monitor the
output frequency of the source. Thus, FIG. 7a, for example, shows a
variable delay Mach-Zehnder interferometer (MZI), while FIG. 7b
depicts a fixed delay MZI frequency clock. The MZI has a balanced
output to suppress the common mode signals and enhance the
interference fringe signals. Other embodiments of the frequency
clock module include an optical interferometer with at least one
known delay that is fixed or variable, a wavelength meter or an
optical spectrometer, or the light source and the optical
interferometer used in the same imaging system that have some
residue interference effects within them.
[0058] In another alternative embodiment of an optical imaging
system, as illustrated in FIG. 8, the frequency clock signals from
the frequency clock module 815 are processed by an electronic
signal processing board 810 to produce a pulse train 820 indicating
a relationship between an output frequency of the source and time.
As shown in FIG. 8, the signal processing board 810 processes the
frequency clock signals to produce a pulse train 820. The pulses in
the pulse train 820 indicate when the output optical frequencies of
the source have a linear relationship. The pulse train 820 is
connected to the external clock input of the DAQ device 840 to
externally clock the DAQ system 840 to sample the OCT signals from
the interferometer 830.
[0059] FIG. 9 is an embodiment of a system for performing optical
imaging of a sample. In this embodiment the DAQ device 910
processes the frequency clock signals to generate a digital record
920 containing the frequency-time relationship information of the
source. The digital record is updated after every source sweep or
after a certain number of source sweeps, and is used as a reference
to control the sampling of the OCT signals coming from the
interferometer 930.
[0060] FIG. 10 is an embodiment of a system for performing optical
imaging of a sample. In this embodiment the digital record 1010
containing the frequency-time relationship of the source is
generated by the software processing, for example in computer 1050,
of the frequency clock signals after the signals are digitized by
the DAQ device 1040 and transferred to computer memory. The digital
record is uploaded to the DAQ device 1040. Using the uploaded
digital record as a reference, the DAQ device 1040 acquires the OCT
signals and selectively transfers the sampled data points to PC
memory or disk files which are accessible to the software. The DAQ
device 1040 is configured to use a fixed frequency sampling clock
that is either internal or external to the device 1040.
[0061] In another embodiment of the imaging system as shown in FIG.
11, the digital record 1110 containing the frequency-time
relationship of the source is saved in a memory device 1120 either
as in the computer memory or as in a disk file. The digital record
1110 is loaded to the DAQ device 1130 and stored in an on-board
memory buffer of the DAQ device 1130, or in other programmable data
or code buffers of the DAQ device 1130. The DAQ device 1130
acquires the OCT signals and selectively transfers the data points
to computer memory 1140 for further processing.
[0062] In another alternative embodiment of the imaging system as
shown in FIG. 12, a DAQ device 1210, memory devices 1220 and
processors 1230 are embedded in one compact module or integrated
into one board to form an embedded computing system 1200, or a
single board computer system 1200. The DAQ device 1210 also can be
a stand-alone device in communication with the embedded computing
system 1200 or is a plug-in device installed inside a computer as a
standard computer system configuration. The computing system 1200
has a number of memory devices 1220 that are accessible to both the
DAQ device 1210 and the processor 1230 which is controlled by
software. The software controls the data acquisition process of the
DAQ device 1210 which is externally clocked by the frequency clock
signals of the light source 1250. The software processes the data
points transferred from the DAQ device 1210 to memory and generates
multi-dimensional OCT images 1260. The software also provides an
operational interface for a user to control the imaging system.
[0063] In an embodiment illustrative of a method according to the
principles of the invention as shown in FIG. 13, the data
acquisition routine 1310 and signal processing routine 1320 are
configured in parallel. For example, a DAQ device starts the data
acquisition routine 1310; and, without waiting for the data
acquisition routine 1310 to be finished, the signal processing
routine 1320, to process the previously acquired data stored in
memory, is started. The status of both routines 1310 and 1320 is
checked. As shown in block 1330, when the routines 1310 and 1320
are finished the method proceeds to constructing a new OCT image
1300. The method may be executed by hardware, software or a
combination thereof. The software may be embodied in a computer
readable medium, which, when executed by a processor or control
system, causes the method to be executed.
[0064] In the embodiment shown in FIG. 14, a system provides
optical images of samples 1440a-1440n using multiple imaging
channels. This system includes an optical radiation source 1410,
such as a swept source. The system also includes a frequency clock
module 1415 monitoring the output optical frequency of the source
1410. A plurality of optical interferometer modules 1420a-1420n are
also provided. A first router 1418 distributes the output power of
the optical source 1410 to the plurality of interferometers
1420a-1420n and a second router 1425 distributes outputs of the
plurality of interferometers 1420a-1420n to a data acquisition
(DAQ) system 1450. The DAQ system 1450 converts the OCT signals
from the plurality of interferometers 1420a-1420n to OCT data
points, triggered by the frequency clock signals. Over-sampling of
the OCT signals improves the signal quality and OCT image quality.
The software executed by computer 1455 processes the OCT data
points and generates multi-dimensional OCT images 1460 for multiple
imaging channels. The software also provides an operation interface
1460 for the user to control the imaging system. If the DAQ system
1450 has a sufficient number of sampling channels for the all the
interferometers 1420a-1420n, the second router 1425 is not
required. The computer 1455 also controls the probe control 1465
and sample probes 1430a-1430n.
[0065] The system of FIG. 14 may further includes an optical
radiation source, a frequency clock module both having similar
properties as discussed above with regard to FIG. 5 (broadband
source 510 and frequency clock module 520). The first stage of
routers 1418 of FIG. 14 may be an optical switching device that
switches the output of the optical source among input ports of the
plurality of interferometers 1420a-1420n, or an M.times.N optical
coupling device that couples the output of the optical source to
multiple interferometer input ports. The plurality of optical
interferometers 1420a-1420n of FIG. 14 is composed of multiple
optical interferometers. Each optical interferometer may include a
reference optical reflector, an optical path leading to the
reflector, an optical path leading to a sample to be imaged which
is labeled as sample probe 1430a-1430n in this figure, and an
optical path where the light from the sample and the reflector
interfere to produce interference fringe signals. Each optical
interferometer has similar properties as discussed above with
regard to FIG. 5.
[0066] In the embodiment shown in FIG. 15, the second router 1530
is an optical switching device that switches the optical
interference fringe signals from multiple interferometers
1420a-1420n to multiple optical detectors 1550, and the multiple
optical detectors 1550 convert the optical signals to electric
signals, and the electric signals are connected to the
multi-channel DAQ device 1540. Alternatively, the second router
1530 can be an electric switching device that switches the OCT
signals from multiple interferometers, when every interferometer
has its own detector to convert the optical interference signals to
electric signals, to the multi-channel DAQ device. In an example
alternative embodiment illustrated in FIG. 16, the first and second
routers of FIG. 15 can be the same optical switching device
illustrated as first router 1610 of FIG. 16. An optical beam
splitter or an optical circulator 1620 is used to direct the
optical output of the interferometers to the detector 1630 to
convert the optical signals to electric signals. The electric
signals are connected to the multi-channel DAQ device 1650.
[0067] In another embodiment of the system depicted in FIG. 14, the
DAQ system is composed of multiple DAQ devices with communication
capabilities among the DAQ devices. Each DAQ device has similar
properties as discussed above with regard to FIG. 5. The data from
different imaging channels are either time encoded or data
acquisition channel encoded. As described above a computer system
and DAQ system may be a combination of software and hardware,
integral or separate.
[0068] In a further embodiment of the system of FIG. 14, the data
from different imaging channels are time encoded or DAQ channel
encoded. The time encoded information allows the data of any
channel be recovered when the switching devices in the first and
second routers are activated to enable that particular channel. The
DAQ channel encoded information allows the data of any channel be
recovered from a known hardware connection method.
[0069] The embodiment shown in FIG. 14 is demonstrated for a
multi-channel imaging system based on one optical radiation source.
This configuration can be easily expanded to support multiple
optical radiation sources of same center wavelength or different
center wavelength in one system, with each optical radiation source
requires its own frequency clock module.
4. External Clocking Versus Internal Clocking
[0070] A data acquisition device can be considered as a functional
module that requires certain input and output signals to work. The
input signals include the analog signals from multiple channels
which are the signals to sample, the trigger signals which tell
when to start the data conversion process, and the clock signals
which is the time-base used to determine the rate to sample the
data for all channels simultaneously. The output signals are the
converted digital signals representing the input analog
signals.
[0071] Typically, a data acquisition device uses an internal sample
clock to determine the rate to sample all analog input channels.
The internal sample clock is usually generated by an on-board clock
circuit such as a crystal oscillator. The crystal oscillator
generates fixed frequency pulse train signals with equal time
spacing between adjacent pulses. When using this fixed frequency
pulse train as the sample clock for the analog to digital
conversion process, the converted digital data points are also
equally spaced in time.
[0072] However, there are occasions when using an external clock
source is advantageous, especially when sampling a signal where an
irregular sampling period is necessary. It is important to
configure the system so that the incoming signal to be sampled is
accompanied by an external pulse train. The external pulse train
acts as the external sample clock. The clock pulses have irregular
time periods indicating the time to perform the analog to digital
conversion process. Each pulse of this signal is used to sample
data for all channels simultaneously.
[0073] In swept source OCT imaging system, the Fourier domain OCT
algorithm requires the sampled data points of OCT interference
fringe signals to be equally spaced in optical frequency domain.
However, due to many different frequency tuning mechanisms used by
swept sources, the output frequencies of the swept sources are
usually not linear in time. When using an internal clock to sample
the OCT interference fringe signals, the sampled data points are
linear in time but are not linear in optical frequency. Therefore,
post signal processing steps such as numerical remapping process is
needed to convert the sampled data points from time to optical
frequency domain. Please note the optical frequency has a linear
relationship with the wave number.
[0074] Current technology (e.g., US Patent Application Publication
2005/0171438) uses numerical remapping process to convert the
sampled data points from time to optical frequency domain. However,
in order to cancel the distortion originating from the
nonlinearities in the wave number function, the data has to be
numerically remapped from uniform time to uniform wave number space
based on the wave number function which is determined by a spectra
calibration process. It is clear that in the current system and
method, the acquired data can be nonlinear in wave number and need
the numerical remapping process. In order to perform the
compensation, current technology requires the measurement of the
laser output frequency function k(t) which is the relationship of
laser output frequencies as a function of time. The numerical
remapping process is done by some sort of algorithm to convert the
data from time to optical frequency domain.
[0075] An embodiment of the invention provides the method that uses
an external circuit board to process the frequency clock signals of
the swept source, and produce a pulse train indicating the
relationship of the output optical frequencies of the source and
time. The pulse train is connected to the external clock input of
the DAQ device. The analog to digital data conversion happens
exactly at the time that each pulse occurs. Depending on the
circuit board design, we can choose either the rising edge or the
falling edge of the pulses to externally clock the data acquisition
of all analog input channels simultaneously. Because the output
frequency and time relationship of the source is contained in the
irregular spacing of the pulses in the pulse train, the swept
source output frequencies are linear at each rising edge or falling
edge of the pulses in the pulse train. Therefore the sampled data
are equally spaced in optical frequency domain automatically. There
is no need to perform numerical remapping of the data from uniform
time to uniform wave number space. There is also no need to measure
the laser output frequency function k(t) which is required for the
remapping process. The circuit board can work with different types
of swept sources without measuring their individual k(t)
functions.
[0076] Therefore, in contrast to the existing approach, the
disclosed method in this disclosure does not require the remapping
process because the acquired data are equally spaced in optical
frequency domain automatically; and it does not require the
measurement of the function k(t) of the swept sources either.
5. Long Coherent Length Swept Source
[0077] In one embodiment, this method is used for high speed, long
range OCT imaging, in which a special swept source that supports
long coherence length at high sweep speed is employed. This method
requires the swept source to support >20 mm long coherence
length, >100 nm tuning range at sweep rate higher than 100,000
A-scans per second.
[0078] Long coherence length is an important specification in OCT
imaging applications utilizing the external clocking method. The
relationship between the coherence length and the average
instantaneous line width of the swept source is given by
L c = 2 ln 2 .pi. .lamda. 2 .DELTA..lamda. , ##EQU00006##
where .lamda. is the center wavelength of the swept source and
.DELTA..lamda. is the instantaneous line width of the source
averaged over the whole sweep. In experiment, the coherence length
of the swept source is measured as the path length difference in
the interferometer arms where the interference fringe contrast
drops to 50% of the fringe contrast at zero path length different
of the interferometer. Fourier domain OCT theory requires the OCT
fringe signals to be sampled in optical frequency domain with even
frequency spacing of the data points. The denser the sampling
frequency (the smaller frequency spacing of sampled data points)
the larger the depth range can be measured and displayed on the OCT
images. Long coherence length of the swept source allows good
interference fringe contrast at long delay of the interferometer.
Good interference fringe contrast at long delays allows the
electronics circuit to process the signals and generate the very
accurate pulse train containing enough number of pulses to enable
dense sampling of OCT fringe signals.
[0079] This disclosure describes the coherence length impact on OCT
image quality when applying the "external clocking" method on two
swept sources of different coherence length specs. The two swept
sources are running at the same speed of 100,000 A-scans/second.
The first swept source is a piezo tuning VCSEL swept source with
coherence length measured longer than 20 mm, and the second swept
source is piezo tuning Fabry-Perot swept source with coherence
length measured to be about 8 mm. Both sources are connected to a
25 GHz frequency spacing Mach-Zehnder interferometer to generate
the frequency clock signals. The frequency clock signals are
processed by an electronics circuit to generate the digital pulse
trains indicating the relationship of output frequency and time of
the sources. The pulse train is connected to the external clock
input channel of the DAQ device to externally clock the data
acquisition of OCT signals. The acquired OCT data is processed by
computer software to generate OCT images. Except for the two
different swept sources used, all the other modules used in this
experiment are the same.
[0080] FIG. 17a shows the frequency clock signals and the clock
pulse train generated from the piezo tuning VCSEL swept source with
coherence length longer than 20 mm, The 25 GHz frequency clock
signals 1720 (green trace) has very good fringe contrast. The
circuit board processes these fringe signals to generate the clock
pulse train 1710 (yellow trace) with two pulses per fringe cycle
precisely. When connecting this pulse train to the external clock
input of the DAQ card we obtain very good OCT images from a tape
sample like shown in FIG. 18a.
[0081] FIG. 17b shows the frequency clock signals and the clock
pulse train from the piezo tuning Fabry Perot swept source with
coherence length about 8 mm. The 25 GHz frequency clock signals
1740 (green trace) have limited fringe contrast and large noise
because of the limited 8 mm coherence length of the laser. The
circuit board processes these fringe signals to generate the clock
pulse train 1730 (yellow trace) with missing clock pulses where the
fringe contrast reduces and noise becomes large. When connecting
this pulse train to the external clock input of the DAQ card we
obtain noisy OCT images from the same tape sample shown in FIG.
18b. Note the total imaging depth in the sample is also smaller
compare to FIG. 18a.
[0082] The above are some unique advantages of using external clock
module to externally clock OCT data acquisition with a special
swept source that supports long coherence length at high sweep
speed.
[0083] In an embodiment of the present invention, some or all of
the method components are implemented as a computer executable
code. Such a computer executable code contains a plurality of
computer instructions that result in the execution of the tasks
disclosed herein. Such computer executable code may be available as
source code or in object code, and may be further comprised as part
of for example, a portable memory device or downloaded from the
Internet, or embodied on a program storage unit or computer
readable medium. The principles of the present invention may be
implemented as a combination of hardware and software and because
some of the constituent system components and methods depicted in
the accompanying drawings may be implemented in software, the
actual connections between the system components or the process
function blocks may differ depending upon the manner in which the
present invention is programmed.
[0084] The computer executable code may be uploaded to, and
executed by, a machine comprising any suitable architecture.
Preferably, the machine is implemented on a computer platform
having hardware such as one or more central processing units
("CPU"), a random access memory ("RAM"), and input/output
interfaces. The computer platform may also include an operating
system and microinstruction code. The various processes and
functions described herein may be either part of the
microinstruction code or part of the application program, or any
combination thereof, which may be executed by a CPU, whether or not
such computer or processor is explicitly shown. In addition,
various other peripheral units may be connected to the computer
platform such as an additional data storage unit and a printing
unit.
[0085] The functions of the various elements shown in the figures
may be provided through the use of dedicated hardware as well as
hardware capable of executing appropriate software. Other hardware,
conventional and/or custom, may also be included. When provided by
a processor, the functions may be provided by a single dedicated
processor, by a single shared processor, by a multi-threaded single
processor, by a single processor with multiple processing cores, or
by a plurality of individual processors, some of which may be
shared. Explicit use of the term "processor" or "controller" should
not be construed to refer exclusively to hardware capable of
executing software, and may implicitly include, without limitation,
digital signal processor hardware, ROM, RAM, and non-volatile
storage.
[0086] All examples and conditional language recited herein are
intended for pedagogical purposes to aid the reader in
understanding the principles of the invention and the concepts
contributed by the inventor to furthering the art, and are to be
construed as being without limitation to such specifically recited
examples and conditions. Moreover, all statements herein reciting
principles, aspects, and embodiments of the invention, as well as
specific examples thereof, are intended to encompass both
structural and functional equivalents thereof. Additionally, it is
intended that such equivalents include both currently known
equivalents as well as equivalents developed in the future, i.e.,
any elements developed that perform the same function, regardless
of structure.
* * * * *