Modes for Molecules: Lasers for Optical Diagnostics of Combustion

P. Ewart1, Y. Arita1, K. Bultitude1, K. Richard1 and P.J. Manson2

1Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU, UK.

p.ewart@physics.ox.ac.uk

2Department of Physics, University of Otago, Dunedin, New Zealand.

ABSTRACT The development of laser systems for application in combustion diagnostics is described with particular reference to their spectral mode structure. Following a brief review of “modeless” laser systems and their applications a description is given of the Diode-Seeded Modeless Laser (DSML) as a source of tunable, high power, pulsed, single longitudinal mode radiation suitable for high resolution spectroscopic diagnostics. Applications of the DSML to Degenerate Four Wave Mixing (DFWM) and Cavity Ring Down Spectroscopy (CRDS) of OH in flames are briefly described. Finally a novel spectroscopic technique, Multi-Mode Absorption Spectroscopy (MUMAS), is reported that potentially offers simultaneous high spectral resolution and wide spectral coverage for measurement of multiple molecular transitions or multiple molecular species for combustion diagnostics and environmental or chemical analysis. 1. INTRODUCTION

Lasers have proven to be of enormous benefit to optical diagnostics of combusting and non-combusting flows [1]. The properties of coherence and high intensity permit a wide range of spectroscopic techniques from simple absorption spectroscopy to methods involving high-order nonlinear optical processes. Different techniques may require radically different laser properties. Absorption spectroscopy can often be carried out using simple, inexpensive diode lasers that provide high spectral resolution on the order of 10–4 cm–1 at c.w. power levels in the mW range. Some types of nonlinear spectroscopy on the other hand, such as Coherent Anti-Stokes Raman Spectroscopy, CARS, may require pulsed powers in the MW range and spectral widths covering ~102 cm–1. Whatever the technique being used, however, the spectral content of the laser light is a key parameter. Experimentally, the laser bandwidth is often the limiting factor in spectral resolution which may then affect the accuracy and precision of measurements. Theoretical analysis also requires a field that can be represented accurately in order to derive quantitative results from the experimental data. Single-mode fields are inherently simpler to represent mathematically than a field composed of a finite number of fields with randomly varying phases and amplitudes. This paper reviews selected developments in laser systems for combustion diagnostics. The main focus will be on non-linear laser spectroscopy of molecules with particular attention paid to the laser mode structure. The paper begins however with a brief review of lasers with no mode structure, the so-called “Modeless Lasers”, and their

applications in combustion diagnostics. This technique relies on a laser with a wide spectral bandwidth. The major part of the paper considers the diode-seeded modeless laser DSML as a source of high power, pulsed, tunable, single-mode radiation for high resolution linear and nonlinear spectroscopy. Finally a new technique for spectroscopic diagnostics using multi-mode lasers is introduced, Multi-Mode Absorption Spectroscopy, MUMAS, that has the potential for high resolution and wide spectral coverage. 2. THE MODELESS LASER All conventional lasers and Optical Parmetric Oscillators OPO, operate by placing an amplifying medium inside a resonant cavity resulting in mode structure and consequent random noise on the spectrum. Since dye laser or OPO modes cannot be actively controlled on nanosecond time-scales one approach to the problem is to eliminate the modes by using a “modeless” laser [2]. The device uses traveling wave amplified spontaneous emission, ASE, and, since there is no resonant cavity, the output lacks any longitudinal mode structure. The broadband continuous spectrum and reduced mode-noise of a modeless laser makes it ideal for multiplex spectroscopy such as broadband CARS [3]. When used as the Stokes laser in combination with a single-mode pump laser the noise on CARS spectra of N2 has been shown to be substantially reduced compared to levels achieved using conventional lasers [4]. Temperature precision of the order of 1 – 2% is reported for single-shot N2 CARS in the range of 1000K. This allows cycle-by-cycle fluctuations in engine temperatures to be recorded that were previously obscured by noise from mode fluctuations of the Stokes laser [5]. Owing to the relatively wide separation of the Q-branch transitions in the Raman spectrum of H2, broadband CARS of H2 has previously been even more seriously limited by mode noise than in the case of N2 since there are many fewer transitions that may be averaged in the broadband spectrum. Using a modeless laser, however, accurate and precise single-shot temperature measurements have been recorded by H2 CARS in a CVD plasma [6]. 3. LASERS FOR HIGH RESOLUTION OPTICAL DIAGNOSTICS. Nonlinear spectroscopic techniques offer significant advantages over conventional methods in certain circumstances but place demands on the laser source that are often hard to meet using standard laser systems. In

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particular these techniques require high power, narrow linewidth, tunability and wide spectral coverage from the ultra-violet to the infra-red. These specifications are generally mutually exclusive. Only pulsed lasers can deliver the high peak power required for non-linear processes and such systems usually have a broad bandwidth. Conventional lasers may be constrained to single longitudinal mode, SLM, operation but allow only a very limited tuning range between mode-hops. A variety of designs for tunable laser oscillators have been devised to provide a SLM output [7]. The first systems to produce tunable high power SLM radiation amplified the output of a c.w. dye laser using multi-stage dye amplifiers pumped by a frequency doubled SLM Nd:YAG laser [8]. Such a system has been used in a range of high-resolution nonlinear spectroscopic studies [9]. Such systems are large, complex, expensive and have a small mode-hop free tuning range. The Optical Parametric Oscillator, OPO, has promised an all solid-state alternative but they have proved expensive to construct, difficult to operate reliably and again have a limited SLM tuning range. Optical parametric devices such the OPO and Optical Parametric Generator/Amplifier, OPG/OPA, have undergone a revival in recent years owing to improvements in nonlinear crystal materials with higher damage thresholds, pump sources with better spatial and spectral properties and the introduction of quasi-phase-matched materials such as periodically poled Lithium Niobate, PPLN. There is an extensive literature on OPO and OPG/OPA systems dating from their first introduction over 30 years ago [10]. Useful reviews of recent developments and combustion related applications are available [11,12]. Building on previous work, Orr and coworkers have introduced coherent sources based on OPO and OPG/OPA systems that are seeded with radiation from single-mode diode lasers to provide tunable, single-mode signal outputs [11]. These devices have been applied to detection of combustion relevant species using the high resolution spectroscopic techniques of Cavity Ring-Down Spectroscopy, CRDS, and Doppler-free two-photon Laser Induced Fluorescence, TP-LIF spectroscopy. Lucht and co-workers have adopted a similar approach for diagnostics of combustion species [12]. Pulsed Alexandrite lasers are commercially available and operate using active control of the Q-switching and oscillating mode to provide SLM output with high peak power. The high peak power available allows efficient frequency conversion to the infra-red and these devices have been used recently for two-photon Laser Induced Fluorescence and Polarization Spectroscopy of combustion relevant species [13]. The mode-hop free tuning range is however limited and the systems are relatively complex and expensive. 3.1 The Diode Seeded Modeless Laser In Oxford we have developed an alternative system that uses a modified modeless laser (see section 1) as a narrow-band amplifier of an injected seed [14]. The basic idea was extended to amplify a HeNe laser [15] and a single-mode diode laser [16]. The system, shown in figure 1 is seeded by typically 2 – 3 mW from an external cavity diode laser,

ECDL, and pumped by a frequency doubled Nd:YAG laser. A grazing incidence diffraction grating discriminates

Dye Cell

Strip MirrorOutput beam

Cut-off Prism

Diffraction Grating

Input beam from Diode Laser

Pump beams from Nd:YAG Laser

Tuning Prism

λ/2 plate

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3

4

Figure 1. Schematic of optical layout of DSML. against ASE in the output. Optical isolators prevent feedback from the modeless laser to the seeding diode laser. The output of the modeless laser is amplified in two longitudinally pumped dye amplifiers. This Diode-Seeded Modeless Laser, DSML, provides output in the range 630 – 690 nm with powers of up to 6 MW in 5 ns pulses (30 mJ energy per pulse) at 10 Hz repetition rate, with a Fourier-limited bandwidth of 0.006 cm–1 (165 MHz). The device has recently been refined by the addition of a simple servo-loop system to lock the bandpass of the modeless laser to the ECDL frequency so that effective mode-hop free tuning over 60 cm–1 has been achieved [17]. Figure 2 shows a section of the continuous single-mode scan over a 9 cm–1 section of the absorption spectrum of Iodine. The high spectral brightness is ideal for frequency conversion to the UV or mid-IR using second harmonic generation, SHG, or difference frequency generation, DFG. The advantages of the system are its simplicity, robustness, stability and reliability. There is no sensitive cavity mode-matching or critical phase-matching or expensive, damage-prone, nonlinear crystals required.

15753 15754 15755 15756 15757 15758 15759 15760

−0.2

0

0.2

Wavenumber / cm−1

−ln(

I/I0)

15771.46 15771.50 15771.54 15771.58 15771.6

0

0.4

Wavenumber / cm−1

−ln(I/I

0)

Figure 2. Continuous SLM scan of the DSML to record the absorption spectrum of I2 over 9cm–1. The lower figure shows a single absorption line. 3.2 Quantitative diagnostics with the DSML The frequency doubled output of the DSML has been used to record spectral lineshapes of Doppler-free DFWM signals in the A2Σ – X2Π (0,0) band of OH in a methane/oxygen flame [18]. Pressure broadening of the DFWM lineshape (see figure 3) was studied for the first time in a low pressure flame and power broadening effects were also measured and compared with theoretical predictions. The DSML has also been used for CRDS of flame OH where the SLM bandwidth resulted in simple single exponential decays of the signals over the whole

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line-width of the transitions probed [19]. The high spectral resolution obtained allowed accurate temperatures to be derived from the Doppler width of the deconvolved lineshapes.

-0.2 -0.1 ν0 0.1 0.2

Relative Wavenumber [cm-1] Figure 3. DFWM lineshape of P1(15) line of the A2Σ – X2Π (0,0) band of OH (methane/oxygen flame) recorded using the DSML. The solid line is a fit to the experimental points using non-perturbative theory of DFWM [18]. The DSML is also very suitable for high resolution Polarization Spectroscopy, PS. Figure 4 shows PS data obtained in I2 in the forward and backward (Doppler-free) geometries. The Doppler-free data shows the emergence of hyper-fine structure in the PS lineshape.

Figure 4. PS signals in I2 using DSML. The dotted curve is fit to data from forward geometry. The solid curve is fit to data from backward (Doppler-free) geometry. These experiments show the advantages of using single-mode lasers for accurate and precise quantitative measurements using both linear and nonlinear spectroscopic techniques.

Figure 5. Experimental absorption spectrum of CH4, using DSML and DFG, compared to theoretical spectrum (HITRAN data).

The DSML is currently being applied to spectroscopy of CH4 and CH3 in the mid-IR (see figure 5). The visible output at 630 nm is mixed with the single-mode Nd:YAG second harmonic at 532 nm to generate 3.3 μm radiation. 4. MULTI-MODE ABSORPTION SPECTROSCOPY, MUMAS. Tunable diode laser absorption spectroscopy is usually carried out using single-mode diode lasers. Owing to the limited tuning range usually only a single transition is probed. Vertical cavity surface emitting laser, VCSEL, diodes provide wider tuning range but are, at present, limited to a few wavelength regions [20]. Multiple transitions, or transitions in multiple species, usually require multiplexing multiple diode lasers – one for each transition and demultiplexing the signals from multiple detectors. An alternative technique is reported here that is capable of simultaneous high resolution and wide spectral coverage suitable for combustion and environmental diagnostics. Multi-Mode Absorption Spectroscopy, MUMAS, uses a multimode laser with a broad spectrum. A multi-mode diode laser is a convenient source covering up to 10 or 20 nm (>10,000 GHz) capable of spanning an entire electronic-rotation-vibration band of a diatomic molecule. In practice a more limited set of modes is selected using an interference filter. The wavelength of all the modes may be scanned simultaneously by current modulation at up to 10 kHz. The transmitted energy is detected by a differential amplifier system monitoring the incident and transmitted beams. At the time, during the wavelength scan, when one of the modes comes into resonance with an absorption line, a small dip in overall transmission is detected. The time variation of the transmitted intensity produces a “fingerprint” signature for a given laser and molecule which carries information on the molecular density, quantum state populations and temperature. High resolution spectral information on multiple transitions or multiple species may be obtained using a single diode laser and a single detector. This feature will be potentially significant in the mid-infra-red spectral region where species such as CO, CO2, NH3 have neighbouring absorption features. 4.1 MUMAS of molecular Oxygen Feasibility studies are reported showing that MUMAS fingerprint spectra can be readily obtained in molecular Oxygen on the weak A-band, b3Σg

− − X1Σg+, around 760

nm [21]. Clear MUMAS signals were obtained by differential absorption in a ~10 m path of oxygen at 1 bar.

Figure 6 MUMAS “fingerprint” showing multiple transitions in the A-band, b3Σg

− − X1Σg+ of O2. The

peak absorptions for 10 metre path are typically ~0.1%.

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A typical MUMAS signature of O2 is shown in figure 6. Good signal-to-noise ratio was obtained even without noise reduction or cavity enhancement techniques. In principle the S/N ratio could be further improved by frequency or amplitude modulation techniques. Application of the technique to other combustion species such as CO and H2O in the infra-red is planned. The technique is generic to any multi-mode laser and, in principle, systems other than diode lasers could be used. CONCLUSIONS We have shown how the variety of laser techniques used in combustion diagnostics calls for a variety of laser systems having spectral properties suited to the particular application. Modeless lasers provide an effectively continuous spectrum that reduces the effects of mode noise on spectra obtained by multiplex or broadband CARS or DFWM. The modeless laser, in turn, is the basis of the DSML that produces high power, single mode radiation that yields improved accuracy and precision in linear and nonlinear high resolution spectroscopy. Finally, we have shown that even multi-mode lasers can be used for high resolution spectroscopy over a wide spectral range by the technique of MUMAS. ACKNOWLEDGMENTS We acknowledge the collaborative contributions to the work on CRDS by Alex Schocker and Dr Andreas Brockhinke from the University of Bielefeld, Germany. Financial support of the work has been provided by the Engineering and Physical Sciences Research Council, UK, Royal Academy of Engineering, UK, the British Council / German Academic Exchange Service and British Gas plc. REFERENCES [1] A.C. Eckbreth, (1996) Laser diagnostics for Combustion, Temperature and Species, 2nd Ed. Gordon and Breach, New York. [2] P. Ewart, (1985) A modeless variable bandwidth tuneable laser, Optics Communications 55, 124. [3] P. Snowdon, S.M. Skippon and P. Ewart, (1991) Improved precision of single-shot temperature measurements by broadband CARS by use of a modeless laser, Applied Optics 30, 1008. [4] D.R. Snelling, R.A. Sawchuk, T. Parameswaran, (1994) Noise in single-shot broadband coherent anti-Stokes Raman spectroscopy that employs a modeless dye laser, Applied Optics 33, 8295. [5] P. Ewart, R.B. Williams, E.P. Lim and C.R. Stone, (2001) Comparison of in-cylinder coherent anti-Stokes Raman scattering temperature measurements with predictions from an engine simulation, International Journal of Engine Research 2, 149. [6] C.F. Kaminski and P. Ewart, (1997) Multiplex coherent anti-Stokes Raman spectroscopy of H2 using a modeless laser, Applied Optics 36, 731.

[7] F. J. Duarte Ed. (1995) Tunable Lasers Handbook Academic Press, San Diego. [8] M.M. Salour, (1977) Powerful dye laser oscillator/amplifier system for high resolution and coherent pulse spectroscopy, Optics Communications 22, 202. [9] R.L. Farrow and R.P. Lucht, (1986) High resolution measurements of saturated coherent anti-Stokes Raman scattering lineshapes, Optics Letters 11, 374. [10] R.L. Byer and R.L. Herbst, (1977) in Nonlinear Infrared Generation Ed. Y.-R. Shen p. 81, Springer. [11] G.W. Baxter, M.A. Payne, B.D.W. Austin, C.A. Halloway, J.G. Haub, Y. He, A.P. Milce, J.F. Nibler and B.J. Orr, (2000) Spectroscopic diagnostics of chemical processes: applications of tunable optical parametric oscillators, Applied Physics B 71, 651. [12] W.D. Kulatilaka, T.N. Anderson, T.L. Bougher and R.P. Lucht, (2005) Development of injection-seeded, pulsed optical parametric generator/oscillator systems for high resolution spectroscopy, Applied Physics B 80, 669. [13] Z.T. Alwahabi, Z.S. Li, J. Zetterberg, and M. Alden, (2004) Infrared polarization spectroscopy of CO2 at atmospheric pressure Optics Communications 233, 373. [14] P. Ewart and D.R. Meacher, (1989) A novel, widely tunable, single mode pulsed dye laser Optics Communications 71, 197. [15] J.F. Black and J.J.Valentini (1994) Compact, high-gain pulsed dye amplifier for weak cw sources Appl. Optics 33, 3861. [16] M.J. New and P. Ewart, (1995) High power single-mode radiation by narrowband amplification of a diode laser, Optics Communications, 123, 139. [17] K. Richard, P. Manson and P. Ewart, to be published. [18] K. Bultitude, R. Stevens and P. Ewart, (2004) High-resolution degenerate four-wave-mixing spectroscopy of OH in a flame with a novel single-mode tunable laser Applied Physics B 79, 767. [19] A. Schocker, A. Brockhinke, K. Bultitude and P. Ewart, (2003) Cavity ring-down measurements in flames using a single-mode tunable laser system Applied Physics B 77, 101. [20] Sanders, S.T., Wang, J., Jeffries, J.B. and Hanson, R.K. (2001) Diode-laser absorption sensor for line-of-sight gas temperature distributions, Applied Optics, 40, 4404. [21] Y. Arita and P. Ewart, to be published.

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