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OPTICS LETTERS / Vol. 36, No. 10 / May 15, 2011
Laser-induced fluorescence detection of hydroxyl (OH) radical by femtosecond excitation Hans U. Stauffer,1 Waruna D. Kulatilaka,1 James R. Gord,2 and Sukesh Roy1,* 1
2
Spectral Energies, LLC, 5100 Springfield Street, Suite 301, Dayton, Ohio 45431, USA Air Force Research Laboratory, Propulsion Directorate, Wright-Patterson AFB, Ohio 45433, USA *Corresponding author:
[email protected] Received February 8, 2011; revised April 1, 2011; accepted April 14, 2011; posted April 14, 2011 (Doc. ID 142420); published May 6, 2011
The development of a laser-induced fluorescence detection scheme for probing combustion-relevant species using a high-repetition-rate ultrafast laser is described. A femtosecond laser system with a 1 kHz repetition rate is used to induce fluorescence, following two-photon excitation (TPE), from hydroxyl (OH) radicals that are present in premixed laminar flames. The experimental P TPE and one-photon fluorescence spectra resulting from broadband excitation into the ð0; 0Þ band of the OH A2 þ −X 2 Π system are compared to simulated spectra. Additionally, the effects of non-transform-limited femtosecond pulses on TPE efficiency is investigated. © 2011 Optical Society of America OCIS codes: 300.6410, 300.2530, 320.7150, 320.7090, 280.1740, 320.5540.
Laser diagnostic measurements have become the standard for noninvasive characterization of complex combusting flows. In particular, laser-induced fluorescence (LIF) provides a straightforward single-input-beam diagnostic approach for determination of both temperature and species number density [1]. Traditionally, nanosecond-duration pulsed lasers have been used as excitation sources for LIF detection. However, these laser sources typically operate at repetition rates of 10–50 Hz, whereas the turbulent nature of many complex reactive-flow environments—particularly those associated with propulsion via fuel combustion—requires the continued development of high-repetition-rate optical probes to allow time-series measurements of highfrequency events. Recent technological advances have resulted in commercially available amplified ultrafast laser systems that operate at repetition rates of 1–10 kHz, and much progress has been made toward the development of single-shot diagnostic techniques using broadband femtosecond laser sources. For example, single-shot coherent anti-Stokes Raman scattering probes have been developed using ultrafast lasers for high-repetition-rate thermometry of stable gas-phase species, such as N2 in reacting-flow environments [2,3]. Nevertheless, the extreme environments associated with many real-world combusting flows are often challenging to access with optical methods, necessitating the use of simplified optical geometries [4]. Furthermore, the quadratic signallevel dependence of nonlinear optical probes on number density makes them prohibitively difficult to generalize to minor species detection. These considerations have led us to begin development of an LIF optical probe of important minor species within combusting flows using a high-repetition-rate femtosecond laser system. In fact, Laurendeau and coworkers have developed the picosecond time-resolved LIF (PITLIF) technique into a powerful tool to make highrepetition-rate measurements of minor species. For example, time-series measurements of the hydroxyl radical, OH, using PITLIF have been carried out at repetition rates of 10 kHz and greater [5]. Developing analogous 0146-9592/11/101776-03$15.00/0
LIF detection schemes using subpicosecond pulses holds the added promise of time-gated detection on time scales that are faster than typical collisional lifetimes, even in high-pressure environments. Such quenching effects in the presence of myriad combustion intermediates make the modeling of an LIF signal initiated by ns-duration sources difficult. Future directions of this approach include the development of a subpicosecond time-resolved LIF probe for single-shot detection of important species in complex combusting flows with 10 kHz repetition rate and the extension of this method to two-dimensional measurements via planar LIF. Described here, therefore, is the LIF detection of OH radicals within reacting flows P following TPE into the ð0; 0Þ band of the OH A2 þ −X 2 Π transition using a high-repetition-rate femtosecond pulsed laser source. This work particularly addresses several unique characteristics of subpicosecond pulsed sources—including large peak intensities and broad bandwidth—that provide both benefits and disadvantages for LIF detection. For example, typical OH LIF schemes involve P singlephoton excitation to the v ¼ 1 state of the A2 þ excited electronic state (generally using excitation wavelengths near 285 nm), which allows observation of redshifted emission in the ð1; 1Þ and ð0; 0Þ bands (λ ∼ 310 nm) that is readily filtered from scattered excitation-laser light [6]. In contrast, the use of broadband ultrafast pulses with wavelengths near 620 nm allows direct excitation in the ð0; 0Þ band of the OH A–X transition via farfrom-resonance TPE (i.e., no intermediary electronic state is resonant with single-photon 620 nm excitation) without concern for excitation-laser scatter. This TPE scheme is experimentally feasible by virtue of the strong peak intensities associated with ultrafast laser pulses, allowing exploitation of the stronger ð0; 0Þ transitions. Although two-photon absorption (TPA) cross sections are typically small relative to one-photon transitions [7], broadband femtosecond pulses provide a multiplexed source of two-photon excitation frequencies. In particular, for a given two-photon optical transition energy, ωTPE , multiple pairs of frequencies contained within © 2011 Optical Society of America
May 15, 2011 / Vol. 36, No. 10 / OPTICS LETTERS
the optical pulse, detuned by Δ relative to ωTPE =2, contribute simultaneously. The experimental apparatus consists of a Ti:sapphire oscillator/amplifier system (Spectra Physics Solstice) operating at a 1 kHz repetition rate. Approximately 1:1 mJ=pulse of the 800 nm output is used to pump an optical parametric amplifier (OPA) (Spectra Physics Topas), which produces tunable light from 600–670 nm [bandwidth: 12–16 nm full width at half-maximum (FWHM) over this tuning range]. Pulse characterization and shaping, used in some of the experiments described below, are facilitated by the use of a MIIPS Box 640 (Biophotonics Solutions, Inc.) pulse shaper [8]. The LIF excitation beam passes through an f ¼ þ300 mm spherical lens that focuses the beam within the sample, which consists of a near-adiabatic, laminar, premixed fuel—air flame that is produced by a Hencken calibration burner; either ethylene (C2 H4 ) or hydrogen (H2 ) is used as the fuel. Following bandpass filtration (Semrock FF01320/40), fluorescence collection optics are used to image the signal emanating from the focal volume of the excitation beam into the detector. For dispersed-fluorescence experiments, a 0:25 m spectrometer coupled to an intensified charge-coupled device is used to collect the signal. For all other experiments, a photomultiplier tube (Hamamatsu R9110) is employed to collect the total emission. Dispersed-fluorescence experiments were first carried out following TPE by a broadband subpicosecond pulse centered at 622 nm. During these measurements a 15 nsduration gate was used to discriminate against steadystate background OH fluorescence that was emanating from the reacting flow. Moreover, broadband background fluorescence signal resulting from a multiphoton laser-induced breakdown (LIB) process was observed at high intensities. Since this latter background emission was difficult to separate from the ð0; 0Þ emission band of OH, care was taken to eliminate LIB background by reducing the excitation energy to
