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 gas-turbine test beds and the cylinder of a combustion engine.
©2013 Optical Society of America
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, high-repetition-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 . 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 wind-tunnel facilities when using PLIF and PIV [8,9]. An alternative has been suggested which 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 high-power fiber delivery because the cumulative thermal effects caused by high-PRR lasers can reduce the optical-damage threshold of the fiber . 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 . 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) .
In this study we developed a fiber-coupled, 10-kHz PIV system that employs a large-aperture, tapered step-index fiber to permit efficient delivery of high pulse energy for high-speed 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, large-core step-index fiber used in Ref . 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 high-PRR, 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) .
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-softened-section 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 . 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.
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 . 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.
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 . 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.
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].
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, high-power, 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 1- and 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.
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 beam-focusing 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 . 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 . 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.
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 fiber-coupled 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-mm-square 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 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 . To create a controlled transient event in the flame, a millisecond-time-scale high voltage was applied to disturb the flame.
To examine the impact of fiber delivery on the PIV measurements, we acquired velocity-vector 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.
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 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 velocity-vector 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 .
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 beam-delivery 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.
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).
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