Single‐shot thermometry using fiber‐based picosecond coherent anti‐stokes Raman scattering (CARS) spectroscopy

Abstract

Coherent anti‐Stokes Raman scattering (CARS)‐spectroscopy‐based thermometry techniques developed for laboratory flames face a stiff challenge when implemented in practical combustion environments such as engine test facilities. In addition to limited optical access, the harsh environments associated with these test facilities (i.e. uncontrolled humidity, vibration, and large thermal gradients) often restrict the operation of sensitive laser systems. To circumvent this problem, we have developed a fiber‐based, picosecond (ps) CARS system employing long‐length (up to 6 m), multimode silica fibers that permits the laser system to be isolated from the high heat and vibration of the test hardware. Proof‐of‐principle, single‐laser‐shot temperature measurements using a 3‐m‐long fiber system are demonstrated in atmospheric pressure, near‐adiabatic, laboratory H2–air flames. Limitations that are imposed by long‐length multimode‐fiber beam delivery for CARS thermometry are identified as nonlinear effects during beam propagation along the fiber, depolarization of the laser beam, and degradation of the beam quality at the output of the fiber. These effects are investigated in detail, and potential improvements are suggested. The current study shows promise for implementation of fiber‐based ps‐CARS spectroscopy in harsh environments such as those encountered in combustors and afterburners in practical gas‐turbine‐engine test facilities. Copyright © 2013 John Wiley & Sons, Ltd.

100-ps-pulse-duration, 100-J burst-mode laser for kHz–MHz flow diagnostics

“…Because of quadratic and higher order dependences on excitation-beam power density, laser sources with sub-ns pulse duration are usually required for nonlinear spectroscopic measurements with high signal-to-noise ratio, especially for acquiring spatially and temporally resolved one-or two-dimensional data. Furthermore, sub-ns pulses have also been shown to improve the peak power available for nonlinear spectroscopic approaches when fiberbased laser delivery is utilized [5]. Fiber coupling is potentially important for propulsion devices with limited optical access and for measurements in harsh operating conditions.…”

“…The availability of a high-energy, sub-ns burst-mode laser would allow sufficient energy for pumping frequency-conversion devices and be suitable for fiber-based delivery. For example, it was shown that 100 ps pulses are optimal for fiber coupling in the case of single-shot CARS thermometry [5]. Although the 100 ps pulse duration is optimal, there is a lack of high-energy ps burst-mode lasers, and commercially available systems have repetition rates below 50 Hz.…”

“…With regard to fiber delivery, the use of ns pulses for nonlinear spectroscopic techniques is also problematic because of the low damage threshold of the fibers compared to the pulse energy required for nonlinear spectroscopy in the ns regime [5]. The availability of a high-energy, sub-ns burst-mode laser would allow sufficient energy for pumping frequency-conversion devices and be suitable for fiber-based delivery.…”

“…Measurements of temperature, velocity, and chemical-species concentrations at high bandwidth (typically 10 kHz-1 MHz) are required for providing better insight into processes, such as supersonic injection, fuel-air mixing, ignition, flame holding, turbulenceflame interactions, nonequilibrium chemistry, thermoacoustic instabilities, and shock/boundary-layer interactions. Thus far, high-bandwidth measurements of such processes have been investigated primarily with linear spectroscopic techniques based on nanosecond (ns) laser architectures as first demonstrated by Wu and Miles [2] and recently reviewed by Thurow et al [3].The availability of commercial, turn-key picosecond (ps) and femtosecond (fs) laser systems has aided the advancement of nonlinear optical spectroscopic techniques such as coherent anti-Stokes Raman scattering (CARS) [4,5], two-photon planar laser-induced fluorescence (TPLIF) [6], and Raman-excited laser-induced electronic fluorescence (RELIEF) [7]. Because of quadratic and higher order dependences on excitation-beam power density, laser sources with sub-ns pulse duration are usually required for nonlinear spectroscopic measurements with high signal-to-noise ratio, especially for acquiring spatially and temporally resolved one-or two-dimensional data.…”

“…The availability of commercial, turn-key picosecond (ps) and femtosecond (fs) laser systems has aided the advancement of nonlinear optical spectroscopic techniques such as coherent anti-Stokes Raman scattering (CARS) [4,5], two-photon planar laser-induced fluorescence (TPLIF) [6], and Raman-excited laser-induced electronic fluorescence (RELIEF) [7]. Because of quadratic and higher order dependences on excitation-beam power density, laser sources with sub-ns pulse duration are usually required for nonlinear spectroscopic measurements with high signal-to-noise ratio, especially for acquiring spatially and temporally resolved one-or two-dimensional data.…”