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LOW-COHERENCE OPTICAL PROBE FOR NON-CONTACT
DETECTION OF PHOTOTHERMAL AND PHOTOACOUSTIC
PHENOMENA IN BIOMATERIALS
Sergey A. Telenkov, Digant P. Dave and Thomas E. Milner
Department of Biomedical Engineering, University of Texas at Austin,
Austin, TX 78712
ABSTRACT. Laser-induced mechanical deformations in materials can be detected without contact
using various optical methods. We have developed a low-coherence optical probe to monitor
thermal elastic deformations and acoustic transients in materials exposed to laser excitation. Our
approach utilizes principles from low-coherence interferometry with phase-sensitive detection of the
coherence function of backscattered light from two spatially separated sites in a test material. High
spatial resolution and sensitivity of the optical probe may be used to identify subsurface lightabsorbing structures in turbid media and determine optical properties non-invasively. The lowcoherence optical sensor may prove useful for non-contact studies of tissue-like materials in
biomedical engineering.
INTRODUCTION
Optical low-coherence tomography (OCT) is frequently utilized for non-contact
imaging of subsurface structures in biological materials [1]. This technique has proven
valuable as a non-invasive imaging modality for studies of microstructures in biological
tissue where strong scattering of incident light poses significant difficulties for traditional
optical methods. A typical OCT setup utilizes a broadband light source and a Michelson
interferometer with variable path length in the reference arm as well as an apparatus for
lateral scanning to produce cross-sectional images of test specimens. The short
coherence length of the broadband light source (typically less than 20 jam) provides high
depth resolution in recorded images. Specific implementation of the OCT setup allows
one to detect variations of refractive index [2], birefringence [3] or Doppler shift of
scattered light due to flowing blood [4].
Objective of the present study is to demonstrate that low-coherence interferometry
can be applied not only to visualize subsurface microstructure but also observe
photothermal and photoacoustic phenomena in tissue exposed to laser radiation.
Inasmuch as laser sources are often utilized in clinical applications, non-invasive
monitoring of laser-tissue interaction and resulting effects of laser exposure has important
practical implications. Localized temperature increase is the most evident effect of laser
absorption and often used in clinical applications to alter tissue properties non-invasively
[5]. Thermoelastic deformation is a response of biological tissue to localized temperature
increase, which results in thermal expansion of the heated area. Since laser heating can
occur very fast, such localized photothermal deformations produce acoustic waves
propagating throughout the specimen. Detection of acoustic transients induced by laser
CP657, Review of Quantitative Nondestructive Evaluation Vol. 22, ed. by D. O. Thompson and D. E. Chimenti
© 2003 American Institute of Physics 0-7354-0117-9/03/S20.00
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,SLD Source
LiNbO3 EOM
Reference Path
T
I*
-<->
Sample Path
L
n
i •
L<
Polarization
Channels^ ^
Sample
FIGURE 1. Experimental apparatus for non-contact detection of photothermal and photoacoustic
phenomena in tissue using a low-coherence dual-channel optical probe.
absorption may also be useful for analysis of laser-tissue interaction. Specific optical
properties of biological tissue including strong scattering and rough surfaces pose
significant difficulties for application of conventional optical methods for detection of
photothermal and photoacoustic phenomena.
We believe that low-coherence
interferometry may provide an attractive alternative to traditional optical and contact
detection techniques providing truly non-invasive visualization of photothermal and
photoelastic phenomena in tissue.
EXPERIMENTAL APPARATUS
Our low-coherence optical probe utilizes a polarization-maintaining (PM) fiberbased dual-channel Michelson interferometer capable of differential phase measurement
from two spatially separated probe beams backscattered from the sample (Figure 1). A
broadband light source (SLD) with the central wavelength X0 — 1.3 Jim and coherence
length of 14 jim is coupled into a PM fiber-optic interferometer and output of the sample
arm is focused on the test material. A Wollaston (W) prism in the sample arm allows one
to separate two polarization channels in space and a galvanometer mirror (G) provides
lateral scanning of both beams parallel to the sample surface. This dual-channel
differential configuration significantly improves interferometer stability due to effective
cancellation of common-mode low frequency environmental vibrations. Additionally,
differential phase of the interference fringes in two coherence functions can be used to
determine relative displacement in a light-scattering media caused by thermoelastic
deformation following laser excitation. In our experiments, two different Wollaston
prisms were used in the sample arm to provide beam separations of 4.5 and 0.5 mm. The
relatively large separation distance (4.5 mm) allows identification of a propagating
acoustic wavefront by detecting relative displacement between two channels. However,
increased beam separation results in increased noise level due to non-synchronous
variations of optical pathlength between the two channels.
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FIGURE 2. Interference fringe intensity signal in one channel of the interferometer at modulation
frequency of 50 kHz. a) Signal from air-sample interface without optical excitation, b) Interference fringe
intensity signal in presence of surface acoustic wave.
The reference arm of the Michelson interferometer includes a rapid delay line for
depth scanning and a LiNbOs electro-optic phase modulator to establish an interference
fringe earner frequency. In experiments reported here, the scanning delay line is used to
identify a sample-air interface or a subsurface scattering layer while the LiNbO3 electrooptic phase modulator introduces ± n phase modulation at a 50 kHz interference fringe
carrier frequency. The interference fringe intensity signal in one channel recorded from
the air-sample interface without laser excitation is shown in Figure 2a. The nearly
harmonic waveform becomes significantly distorted (Figure 2b) when an acoustic wave
travels across the probing point.
Two effects are evident: 1) the Doppler frequency shift of the interference fringe
signal is proportional to the surface displacement velocity; and 2) the amplitude
modulation due to optical pathlength change of the coherence envelope. Although our
technique is similar to heterodyne interferometry described in the literature [6], our
system utilizes a low-coherence optical source to identify specific scattering objects and
heterogeneous samples such as biological tissue may be investigated. Figure 2b shows
that phase of the interference fringe intensity contains information about surface acoustic
waves and appropriate phase demodulation processing is required to extract the acoustic
waveform. We use a lock-in type synchronous processing algorithm to compute phase as
a function of time in each channel and subtract channels from another to determine
differential phase A9(t). Relative displacement in two channels is determined using the
computed differential phase (A0):
Ah = (V4rc)-A0
(1)
where XQ = 1.3 jim is central wavelength of the broadband source.
We demonstrate two applications of the low-coherence optical probe: 1) detection
of photoacoustic transients excited by absorption of pulsed laser radiation; and 2)
detection of thermal deformations produced by absorption of periodically modulated laser
radiation. Homogeneous and multilayered agarose-based phantoms are used in the
experiments to simulate optical properties of tissue. Additionally, to simulate subsurface
chromophores in tissue we use a dye-stained gel (Brilliant Blue R dye, Sigma-Aldrich
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Corp.) with peak absorption at X=585 nm. The multilayered gel samples are excited by a
flash-lamp pumped pulsed dye laser (FLPDL) (Candela Corp.) with pulse duration T=500
|is while for homogeneous clear gels a Ho:YAG laser with pulse duration T=300 (is is
used.
To study periodic laser heating and resulting photothermal deformations we excite
a colored polymer sample of polymethyl methacrylate (PMMA) by a modulated CW
radiation at A,=532 nm. The laser modulation frequency of 10 Hz is used to induce
material heating while probing beams being scanned over the heating area with frequency
of 1 Hz.
RESULTS AND DISCUSSION
Surface acoustic waves (SAW) in gel phantoms can be generated by absorption of
pulsed laser radiation in the test sample at a wavelength within the material absorption
band. High water content of the gel samples allows use of a Ho:YAG laser at wavelength
^k-2.1 (im (water absorption coefficient a = 5.9 mm"^?]). Photoexcitation relaxation
results in a localized temperature increase and the resulting thermoelastic expansion
creates an inhomogeneous deformation that propagates away from the excitation region.
The surface component of the acoustic field propagates along the air-sample interface and
can be detected as a surface displacement with characteristic time consistent with
duration of laser excitation. In order to detect the surface acoustic wave, backscattered
light from the air-gel interface is coupled into the interferometer and the LiNbOa electrooptic phase modulator generates interference fringes in both polarization channels. Two
probe beams were positioned 4.5 mm apart using a 20° Wollaston prism and 5 mm away
from the laser excitation spot (Figure 3a). In this setup, lateral scanning is disabled to
probe the sample surface at a fixed position. Following pulsed laser excitation, the
interference fringe intensity signal is recorded and phase of the signal in both channels is
computed. Relative surface displacement is determined from the computed differential
phase (A(|)) and equation (1). A typical phase-sensitive measurement is shown in Figure
3b. Arrows in Figure 3b indicate arrival of the SAW at the probing beams. One can see
, = 2.1 jim
T = 300 us
WL
L
Gel
FIGURE 3. Detection of surface acoustic waves in a gel specimen generated by a pulsed Ho:YAG laser, a)
Excitation schematics; b) Surface displacement computed from differential phase measurements (arrows
indicate arrival of acoustic front at the probing beams).
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characteristic inversion of the profile when the surface acoustic wave reaches the second
probe beam. The time delay required for the SAW to travel 4.5 mm (beam separation
distance) gives a straightforward method to estimate SAW phase velocity. In an agarose
gel specimen, SAW velocity is 40.9 cm/s. These experiments demonstrate that our lowcoherence optical probe may be used to identify a scattering surface and detect acoustic
transients excited by absorption of pulsed laser radiation.
Acoustic waves generated due to photoelastic effect in subsurface chromophores
can be detected from surface displacement measurements. For these experiments we
utilized a layered gel specimen with a subsurface layer stained by Brilliant Blue R dye,
positioned 0.8 mm below the air-gel interface and 1 mm thick (Figure 4a).
Photoexcitation of the specimen is carried out using a flash-lamp pumped pulsed dye
laser at the wavelength of 585 nm which corresponds to an absorption peak of the
staining dye. This phantom simulates subsurface blood vessels heated by pulsed laser
radiation. The interferometer probe beam is backscattered from the air-gel interface and
coupled back into the interferometer to produce an interference fringe signal. The surface
displacement due to an acoustic wave excited in the subsurface layer is detected in one
polarization channel (Figure 4b). The first wavefront is followed by gradually decreasing
oscillations due to multiple reflections of the acoustic wave in the superficial layer.
Periodically modulated laser radiation can be used to produce a temperature
increase, which is often referred to as a thermal wave [8]. The oscillating temperature
field is characterized by an exponentially decreasing amplitude with characteristic
thermal diffusion length LD=(D/7ifm)1/2, where D is thermal diffusivity of a specimen and
f m is the modulation frequency of the laser radiation. In materials with low thermal
diffusivity or at high fm, the thermal wave is confined to the excitation region while at
greater distances only a DC component of the temperature field is present. We
demonstrate that laser-induced stationary photothermal deformations can be detected by
our low-coherence optical probe with lateral scanning. A small angle Wollaston prism
(2°) spatially separates orthogonal polarization channels (separation distance: d = 500
|im) and galvanometer mirror (G) provides lateral scanning at the frequency f s = IHz
(Figure 5a). In our experiments we use light-absorbing polymethyl methacrylate
Ch1
A, = 585 nm
= 500 us
FIGURE 4. Detection of photoexcited acoustic waves in a subsurface chromophore layer, a) Excitation
and detection setup; b) Surface displacement due to acoustic wave detected by one polarization channel.
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= 532nm
f m =10Hz
FIGURE 5. Photothermal deformation of the PMMA surface measured by our low-coherence optical
probe, a) Excitation and detection setup; b) Surface displacement [h(x)] probed across the excitation beam.
(PMMA) samples and CW radiation of the second harmonic of a Nd:YAG laser (A, = 532
nm). Incident radiation is modulated at f m = 10 Hz and focused by a cylindrical lens (CL)
on the surface into a line shape 1mm wide and 10 mm long. The laser-induced thermal
"bump" on the surface is measured by scanning interferometer probe beams across the
line of laser excitation. When beam separation distance d is much less than size of the
"bump" then differential phase (Ac))) is proportional to derivative of the surface profile
h(x), i.e.
A6 = [4rc/A,](dh/dx)-d
(2)
Measuring differential phase and computing the integral of A6 allows reconstruction of
the surface profile [h(x)]. Figure 5b shows the surface profile obtained from differential
phase measurements at incident laser power P = 40 mW. Our data indicate that lateral
extent of the laser-induced surface deformation in PMMA samples significantly exceeds
the area of optical excitation.
CONCLUSION
We have demonstrated application of a phase-sensitive low-coherence optical
probe for non-contact detection of laser-induced photothermal and photoelastic
deformations in materials simulating physical properties of biological tissue. This
technique can be used to identify light scattering structures in test specimens to measure
photothermal deformations and propagating acoustic waves induced by absorption of
laser radiation. Additional experiments are required to fully realize the quantitative
imaging capabilities of our system. Such experiments can provide depth information to
visualize photothermal deformations in subsurface layers of tissue. Measurements of
subsurface deformations may be used to determine laser-induced temperature increase in
tissue as well as to determine thermal and elastic properties of tissue constituents.
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ACKNOWLEDGEMENTS
This work was supported by a grant from the National Institutes of Health, the
National Center for Research Resources (RR 14069).
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