Large-aperture, tapered fiber–coupled, 10-kHz
particle-image velocimetry
Paul S. Hsu,1 Sukesh Roy,1,* Naibo Jiang,1 and James R. Gord2
2
1
Spectral Energies, LLC, 5100 Springfield Street, Suite 301, Dayton, Ohio 45431, USA
Air Force Research Laboratory, Aerospace Systems Directorate, Wright-Patterson Air Force Base, Ohio 45433,
USA
*
sroy@woh.rr.com
Abstract: We demonstrate the design and implementation of a fiber-optic
beam-delivery system using a large-aperture, tapered step-index fiber for
high-speed particle-image velocimetry (PIV) in turbulent combustion flows.
The tapered fiber in conjunction with a diffractive-optical-element (DOE)
fiber-optic coupler significantly increases the damage threshold of the fiber,
enabling fiber-optic beam delivery of sufficient nanosecond, 532-nm, laser
pulse energy for high-speed PIV measurements. The fiber successfully
transmits 1-kHz and 10-kHz laser pulses with energies of 5.3 mJ and 2 mJ,
respectively, for more than 25 min without any indication of damage. It is
experimentally demonstrated that the tapered fiber possesses the high
coupling efficiency (~80%) and moderate beam quality for PIV.
Additionally, the nearly uniform output-beam profile exiting the fiber is
ideal for PIV applications. Comparative PIV measurements are made using
a conventionally (bulk-optic) delivered light sheet, and a similar order of
measurement accuracy is obtained with and without fiber coupling.
Effective use of fiber-coupled, 10-kHz PIV is demonstrated for
instantaneous 2D velocity-field measurements in turbulent reacting flows.
Proof-of-concept measurements show significant promise for the
performance of fiber-coupled, high-speed PIV using a tapered optical fiber
in harsh laser-diagnostic environments such as those encountered in gasturbine test beds and the cylinder of a combustion engine.
©2013 Optical Society of America
OCIS codes: (060.2310) Fiber optics; (120.7250) Velocimetry; (120.1740) Combustion
diagnostics; (060.2270) Fiber characterization.
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1. Introduction
Particle image velocimetry (PIV) has proven to be a useful flow and combustion-diagnostics
tool for measuring the velocity of a flow field over a large area [1,2]. Recently, highrepetition-rate (1–10 kHz) PIV laser systems have been developed that enable time-series
measurements of high-frequency events such as thermo-diffusive instability and acoustic
instability in turbulent combustion flows [3–6]. This technique is often used together with
planar laser-induced fluorescence (PLIF) for simultaneous measurements of velocity and
species concentration in turbulent flames [7]. However, combustors and engine test facilities
that contain high-pressure/-temperature liquid, gas, or equally reactive materials are often
challenging to access, even with a simple optical method such as PIV. Additionally, the harsh
environments associated with these combustion facilities (i.e., dust particles, uncontrolled
humidity, vibration, and large thermal gradients) may restrict the operation of sensitive laser
systems. Recent works by Jiang et al. have shown that long, complicated optical paths (~15
m) can be employed to mitigate such problems that are encountered in hypersonic windtunnel facilities when using PLIF and PIV [8,9]. An alternative has been suggested which
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involves the use of a beam-delivery system based on a 3D-articulated light arm and bulk
optics (e.g., the LaserPulse Light Arm for PIV Model 610015manufactured by TSI Inc.).
However, this type of beam-delivery system has a limited working distance (~2 m), does not
provide sufficient flexibility and ability to access non-windowed test sections, and is
relatively expensive. In contrast, a fiber-based optical-beam-delivery approach not only
overcomes the aforementioned difficulties for a PIV system performing in harsh optical
environments but also provides sufficient flexibility and working distance.
The most suitable fibers for long-distance delivery of high-power, visible laser pulses are
step-index fused-silica fibers because of low bending and absorption losses [10,11]. In
previous studies several low-repetition-rate (~10 Hz), fiber-coupled PIV systems have been
developed using these fibers for detection of flow velocities [12–19]. The primary challenge
in the development of a fiber-coupled PIV system is delivery of sufficient laser pulse energy
(~10–30 mJ at 10 Hz) through the silica fiber for PIV particle illumination. The increase in
input pulse-repetition rate (PRR) further exacerbates the difficulties associated with highpower fiber delivery because the cumulative thermal effects caused by high-PRR lasers can
reduce the optical-damage threshold of the fiber [20]. To date the fiber bundle (i.e., a bundle
of fibers containing many small-core fibers) is considered to be an ideal candidate for PIV
beam delivery because it provides a higher damage threshold and better output beam quality
than standard, single, large-core step-index fibers [2,13,15,19]. However, such fibers have a
low laser-to-fiber coupling efficiency (<35%), which makes it difficult to deliver efficiently
the required energy from commercial PIV laser systems that have limited output energy (~4
mJ at 10-kHz repetition rate). To the best of our knowledge, because of the technical
challenges described above, all of the previous fiber-coupled PIV systems have been
demonstrated only at a low PRR of ~10 Hz. Such measurement speed, however, is much
lower than the required speed (~kHz) for time-series measurements of high-frequency events
in turbulent combustion flows. Recently, our group demonstrated fiber-coupled, 10-kHz
simultaneous OH-PLIF/PIV with a 600-μm-core, solarization-resistant step-index fiber [6].
However, the amount of pulse energy that can be delivered (~1.7 mJ/pulse) is lower than the
optimal energy required for high-repetition-rate PIV experiments (>2.5 mJ/pulse) [21].
In this study we developed a fiber-coupled, 10-kHz PIV system that employs a largeaperture, tapered step-index fiber to permit efficient delivery of high pulse energy for highspeed flow-velocity measurements. This fiber has delivery capability that is improved with
respect to pulse energy and coupling efficiency as compared with that of the standard, largecore step-index fiber used in Ref [6]. Particularly, such improvement was achieved without
sacrificing beam quality, which makes this fiber very suitable for fiber-coupled, high-speed
PIV applications. The fundamental transmission characteristics of high-PRR, 532-nm,
nanosecond (ns)-duration laser pulses were studied for the tapered fiber. The effects of highPRR, visible laser irradiation on fiber transmission are discussed.
2. Design and testing of large-aperture, tapered–fiber, high-power beam-delivery system
2.1 Design of large-aperture, tapered fiber
An ideal optical fiber for PIV beam delivery must meet two essential criteria: 1) transmission
of sufficient laser pulse energy for generation of a PIV signal with reasonable signal-to-noise
ratio (SNR) without causing fiber damage, 2) minimization of beam-profile distortion (i.e.,
with a smaller beam-quality factor M2). The transmission could be enhanced by increasing the
fiber core size (maximum transmission µ core area); however, this would result in
degradation of the beam quality M2 (i.e., the ability of the laser beam to be propagated and
focused). In general, the transmission capability of the ideal optical fiber is expected to be
equivalent to or greater than that of the typical silica fiber with a core diameter of ~1000 μm
[12,14] but with the beam quality equivalent to or better than that of the 600-μm-core fiber
(M2 ~90) [12].
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The geometry and length of the large-aperture, tapered step-index fiber are shown in Fig.
1. The core diameter of the fiber-entrance surface is 940 μm; therefore, high transmission is
expected. The fabricated fiber has an approximately 2:1 taper, with a tapered length of ~3 cm
at the distal (output) end (fiber tapering by Silicon Lightwave Technology Inc.). Such a taper
can be formed by heating a small section of a silica fiber and gently pulling the heat-softenedsection part. The large-aperture input end and tapered output end improve the coupling
efficiency and maximum power transmission and preserve moderate beam quality (lower M2
at fiber exit) for PIV applications. Furthermore, the delivered beam maintains low intensity in
the non-tapered region (99.5% of the fiber length) and only becomes high intensity in the
short tapered region; this fiber design minimizes nonlinear effects such as stimulated
Brillouin scattering (SBS) that can potentially damage the fiber [22]. Although the beam
quality can be improved by increasing the taper ratio, this would result in higher optical loss
in the tapered region. For maintaining the losses at a low level, the taper transition should be
very smooth (adiabatic tapering). In the present study, because of the limitation on the
tapering machine, the longest taper length achieved was ~3 cm, and the resultant optical loss
was ~0.2 dB (6%). The detailed fiber-transmission characteristics are discussed below.
Fig. 1. Geometry of a large-aperture, tapered silica fiber.
2.2 High-power, tapered-fiber beam-delivery system
A schematic diagram of the optical system for coupling high-power, high-PRR, 532-nm ns
laser beams through the tapered fiber is shown in Fig. 2. After the laser beam was passed
through two 0.25° diffractive optical elements (DOEs) (HOlO/OR, RD-203-Q-Y-A and RPC
Photonics, EDC-0.25), it was coupled into the fiber using an f = + 70-mm spherical lens. The
fiber was placed in a six-axis kinematic mount, which was attached to a 1D translational stage
that moved along the direction of the laser-beam propagation. The input end of the fiber was
positioned at the focal point of the lens such that the beam filled ~80% of the core area. The
use of DOEs not only smoothes the input-beam profile but also increases the number of
spatial modes existing in the beam [15]. This setup minimizes the formation of a hot spot that
can potentially damage the entrance surface of the fiber. It also prevents the occurrence of the
self-focusing effect within the fiber. The intensity cross section of the focused spot produced
at the fiber entrance surface is shown in Fig. 2.
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Fig. 2. Optical arrangement for conditioning the laser light and coupling it into the optical
fiber.
2.3 Transmission characteristics of the tapered-fiber beam-delivery system
The experimental setup and the method employed for the fiber-transmission test are the same
as those used for the fiber-transmission test described in Ref [23]. The fiber end surface had
been polished by the vendor, and no marks were observed under a microscope at 100x
magnification. In all of the fiber-transmission tests, the fibers were coiled at a bending radius
of ~50 cm. The indicator of fiber damage was a sudden increase in fiber attenuation (decrease
of transmission by 90%). To evaluate the capability of the tapered fiber to deliver high-power,
high-PRR laser pulses, we studied the laser-induced damage threshold (LIDT), the long-term
transmission behavior, and the beam quality.
2.3.1 LIDT
The lasers used for the LIDT study were a 10-Hz Nd:YAG laser (Spectra Physics, PRO 350)
and a high-speed kHz-repetition-rate PIV laser (Quantronix, Dual-Hawk). All of the fiber
damage was observed on the fiber entrance surface. As shown in Fig. 3, the use of the tapered
fiber in conjunction with DOEs can significantly enhance the LIDT of the fiber. The LIDT
(tested at 10 Hz) of the tapered-fiber beam-delivery system is approximately a factor of seven
higher than that of the conventional fiber-optic beam-delivery system (standard 550-μm-core
multimode step-index fiber (MSIF) with conventional coupling via bulk optics [11,23]).
Because the large-aperture, tapered fiber is capable of coupling the full pulse energy output
from the high-speed PIV laser (~7 mJ/pulse at 1 kHz, ~5.5 mJ/pulse at 5 kHz, ~3.3 mJ/pulse
at 10 kHz), the LIDT of the silica fiber was tested with a 550-μm-core MSIF. In our
experience the damage-threshold intensity of the two fibers should be very similar. When the
input intensity of the 10-Hz (8-ns-duration), 1-kHz (95-ns-duration), 5-kHz (112-ns-duration),
and 10-kHz (160-ns-duration) beams at the front surface of the silica fiber exceeded ~1
GW/cm2, ~24 MW/cm2, ~15 MW/cm2, and ~5 MW/cm2, respectively, fiber-surface
catastrophic damage was observed, and the transmission decreased by 90% or more . The
observed lower LIDT caused by the higher PRR pulses may be due to cumulative thermal
effects in the fiber. Also note that the increase in the pulse duration of the higher PRR pulses
may also decrease the LIDT (LIDT µ τ-0.5) [11,20].
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Fig. 3. Maximum output of 532-nm ns laser pulses from MSIFs (550-μm core) and tapered
MSIFs with the use of DOE and bulk-optics (conventional) couplers as a function of pulse
repetition rate.
The coupling efficiency (i.e., the ratio of input-beam energy to output-beam energy) of the
tapered fiber is ~80%, which is higher than that of the standard 550-μm-core MSIF (~70%).
The 80% coupling efficiency is achieved under the condition of a 6% power penalty due to
fiber tapering. The high coupling efficiency of the tapered fiber results from the large
entrance aperture that enables coupling of higher-order modes. Such high coupling efficiency
and high power transmission make the tapered fiber an ideal candidate for efficient, highpower, PIV beam delivery. The tapered-fiber beam-delivery system is capable of delivering
>2.5 mJ (at 10 kHz) of pulse energy through a 6-m-long fiber. Such energy in our experience
is sufficient to form a 10-cm-tall laser sheet for performing PIV in reacting flows with a good
SNR.
2.3.2 Long-term transmission
Figure 4(a) displays the typical long-term transmission of the 6-m-long tapered fiber with 1and 10-kHz pulses for different transmission pulse energies. For both cases the transmission
was maintained at about ~95% of the original value. Figure 4(b) shows that the tapered fiber
is able to transmit stable dual laser pulses (E ~2.7 mJ/pulse) that are separated by a very short
time interval (∆t ~2 μs). Thus, the designed tapered-fiber beam-delivery system can be used
for PIV measurements in high-speed flows.
Fig. 4. (a) Long-time transmission for a 6-m-long tapered fiber. Energies of 5.3 mJ (solid line)
and 2 mJ (dashed line) represent the initial energy of 1- and 10-kHz pulses, respectively, that
are output from the fiber. (b) Fiber transmission of dual laser pulses at different pulseseparation time intervals, Δt. The pulse energy for each 1-kHz laser beam is ~2.7 mJ (total 5.4
mJ).
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2.3.3 Output-beam quality
The quality of the fiber-transmitted laser beam is important to the fiber-coupled PIV system
because the quality of the delivered beam must be such that the light can be focused into a
thin laser sheet of sufficient extent to fill the area of interest. Typically, 600-μm or smaller
core size fibers (NA of 0.22) are capable of providing moderate beam quality for PIV
measurements [13,15]. Figure 5 shows that under the same optical arrangement, the beam
output from the large-aperture, tapered fiber (entrance 940 μm and exit 550 μm) is capable of
forming a thinner sheet than that from the 940-μm-core fiber. A similar order of beamfocusing ability was obtained for the tapered fiber and 550-μm-core fiber (M2 ~90). The taper
of the fiber can decrease the core size and, hence, effectively reduce the number of modes that
propagate through the fiber, leading to improved beam quality at the fiber exit [24,25]. The
beam quality can be further improved by means of a higher tapering ratio [25]. By increasing
the tapering ratio of the current fiber to 6:1 (core size ~150 μm), the estimated beam quality
M2 can be improved to ~20, which is ideal for PIV applications [15]. The increase in tapering
ratio will result in an increase in optical loss, but this can be minimized by making the taper
transition very smooth (adiabatic tapering) [24,25]. Also, a nearly top-hat beam profile was
acquired using the tapered fiber, as shown in the inset of Fig. 5. Such a beam profile is highly
desirable for PIV applications that require homogenous laser sheets for uniform illumination
of the tracer particles. Recently, Yalin et al. proposed to use large-clad fibers to improve the
beam quality of the fiber output [26,27]. We are in the process of exploring large-clad fibers
in conjunction with fiber tapering technique for generating high-output beam quality (low M2)
to improve the spatial resolution of the PIV measurements.
Fig. 5. Thickness of light sheets generated by 550-μm-core MSIF, 940-μm-core MSIF, and
tapered MSIF as a function of working distance. Shown in the inset is the beam- output profile
from the tapered fiber.
3. Fiber-coupled, 10-kHz PIV measurements
The experimental apparatus for the fiber-coupled, high-speed PIV system is shown in Fig. 6.
The 10-kHz, 160-ns-duration, 532-nm laser pulses were generated by frequency doubling the
output of a diode-pumped Nd:YAG laser (Quantronix, Dual-Hawk). The separation time
between the two PIV pulses was 20 μs. The laser-to-fiber coupling employed for the fibercoupled PIV system is the same as that used for the fiber-transmission test discussed in Sect.
2. The laser beam was coupled into the 6-m-long tapered fiber, and the energy of each pulse,
as measured at the fiber output, was ~2.5 mJ. The output of the fiber was collimated by an f =
+ 50-mm spherical lens and focused onto a probe volume using an f = + 100-mm, 50.8-mmsquare cylindrical lens, which generated a laser sheet that was ~30 mm tall with a thickness of
~1 mm at the probe volume. Collection of the scattered light from the seed particles was
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performed using a dual-frame CMOS camera (Photron, SA5), coupled with an 85-mm f/1.8
lens. A 3-nm narrow-bandpass filter centered at 532 nm (Semrock, LL01-532-50) was
employed to eliminate unwanted signals originating from background sources and flame
emission. The image pairs were processed using LaVision DaVis v8.03 commercial PIV
software. As a simple demonstration, we used the delivery system to obtain PIV images of a
laboratory-based propane–air flame that was seeded with 1-μm Al2O3 particles. The flame
employed for the PIV studies was a premixed propane–air flame with an equivalence ratio φ =
1.06 that was stabilized over a 30-mm-diameter home-built burner having an adiabatic flame
temperature of ~2000 K. The detailed features of the burner are described in Ref [4]. To
create a controlled transient event in the flame, a millisecond-time-scale high voltage was
applied to disturb the flame.
Fig. 6. Schematic of fiber-coupled, high-speed PIV system.
To examine the impact of fiber delivery on the PIV measurements, we acquired velocityvector images of a steady flow (i.e., no applied voltage) with the tapered-fiber-delivered laser
sheet and with a free-space laser sheet that had very similar properties. Figure 7 shows that a
similar order of measurement accuracy was obtained with and without fiber coupling. The
slight velocity-profile difference for the two cases may be the result of clogging of the burner
by seed particles, which affects the flow-velocity patterns.
Fig. 7. Sample image showing the PIV correlation obtained with each delivery system in a
steady, premixed propane–air flow. Data collected with the fiber-delivered system are shown
in (a) and those collected with the directly delivered system are shown in (b).
To create a turbulent flame, we added an anode ~13 mm above the burner surface
(cathode) and applied a high DC voltage of ~2kV at a frequency of ~15 Hz to disturb the
flame. The strong electric field alters the ionic structure of the propane–air flame, which
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results in a phase transition from a stable, laminar flame to a highly unstable flame as well as
a modification of the flame speed. Partial sequences of 10-kHz PIV images and velocityvector maps for the electric-field-induced turbulent flames are shown in Fig. 8. Areas near the
flame top show a small “hole” where no cross-correlation data exist. This hole in the PIV data
is the result of the anode having blocked the seed particles. The acquired PIV data exhibit
time-dependent velocity profiles that are very similar to those previously observed from the
direct-beam measurements reported in Ref [4].
Fig. 8. Partial, instantaneous velocity-vector maps acquired from an atmospheric-pressure,
turbulent propane–air flame on a burner being pulsated by an applied DC voltage of + 2 kV at
a frequency of ~15 Hz.
4. Conclusions
Fiber-coupled, 10-kHz PIV imaging that employs a large-aperture, tapered step-index fiber
has been demonstrated in turbulent reacting flows. A similar order of measurement accuracy
was obtained with and without fiber coupling. The tapered fiber is capable of reliably and
efficiently delivering the laser energy at a kHz PRR required for performing high-speed PIV
measurements. The maximum energy that can be transmitted by the tapered-fiber beamdelivery system is greater than that possible with a conventional fiber-optic beam-delivery
system, and the quality of the delivered light sheet is superior to that obtained from a single
large-core fiber of power-handling capacity equivalent to that of the tapered fiber. This
achievement together with future developments, such as an image fiber bundle for PIV image
collection, will constitute a major step in the transition of the PIV diagnostic tool from
research laboratories to reacting-flow facilities of practical interest.
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Acknowledgments
The authors gratefully acknowledge useful discussions with Mr. Jacob Schmidt of Spectral
Energies, LLC. Funding for this research was provided by the Air Force Research Laboratory
under Contract No. FA8650-12-C-2200 and by the Air Force Office of Scientific Research
(Dr. Chiping Li, Program Manager).
#182045 - $15.00 USD
(C) 2013 OSA
Received 19 Dec 2012; revised 25 Jan 2013; accepted 27 Jan 2013; published 5 Feb 2013
11 February 2013 / Vol. 21, No. 3 / OPTICS EXPRESS 3626