Coherent Anti-Stokes Raman scattering with reflective optics Richard R. Antcliff and Olin Jarrett, Jr.

Richard Antcliff is with Systems Research Laboratories, Research Applications Division, Dayton, Ohio 43440; Olin Jarrett, Jr. is with NASA High-Speed Aerodynamics Di­vision, Hampton, Virginia 23665. Received 20 January 1983. Coherent Anti-Stokes Raman spectroscopy (CARS) has

received considerable attention during the last several years as a possible technique for remote combustion diagnostics because of its applicability to practical combustion systems, e.g., furnaces, internal combustors, and combustion tunnels.1-5

Small scale flames studied with this technique have shown that it can provide time resolved (~10-nsec) and spatially resolved (~0.1-mm3) measurements of temperature and major flame species concentration simultaneously without appreciably perturbing the flame.

This Letter describes the implementation of a new optical configuration6 in which the lens normally employed to focus and cross the input laser beams is replaced by a spherical mirror. A large readily fabricated mirror allows large crossing angles to be obtained with one focusing device. With this arrangement, the laser generation and collection equipment can be located on one side of the sample region. A mirror has been used in CARS studies7 but only to retroreflect a single laser beam.

The experimental arrangement employed is a modification of a standard BOXCARS arrangement described previous­ly.2-3 The output of a doubled Nd: YAG laser is used to pump a broadband dye probe laser and provide the two input pump beams for CARS generation. The pump beam separation from the center line is 1.27 cm; the probe beam is 1.59 cm from the center line. The unique feature of the system is illustrated

1954 APPLIED OPTICS / Vol. 22, No. 13 / 1 July 1983

Fig. 1. Experimental arrangement, plan and side view.

in Fig. 1. A spherical mirror replaces the standard high quality input lens used to focus and cross the laser beams into the sample region. Two different mirrors were evaluated in these studies, a 25-cm (10-in.) diam, 1-m focal length and a 15-cm (6-in.) diam., 40-cm focal length mirror. Both mirrors were aluminum coated with a silicon monoxide overcoat. The CARS signal generated was recollimated, spatially separated with a flint prism, and directed into a 1-m monochromator SIT-vidicon detection system.

The initial experiments with the 1-m focal length mirror indicated that CARS signals could be generated with this setup, but the signal levels were very low. It was speculated that this was due to the long focal length which reduced the laser energy in the sample region. A 40-cm focal length mirror was used to approximate the focusing characteristics of an existing BOXCARS lens system at the NASA Langley facility. Comparison of the mirror system signal intensity with that of the lens system was then possible. The signals obtained from ambient nitrogen with the 40-cm focal length mirror were considerably larger than signals generated with the 1-m focal length mirror and were in fact the same order of magnitude or larger than the signals obtained with a conventional lens system of similar focal length. The interaction length, mea­sured by translating a glass cover slip through the focal region, was also found to be the same (<1 mm).

To assess the applicability of this technique to temperature measurement in a flame, premixed hydrogen-air combustion was set up on a single-slot (0.406-mm × 5-cm) burner. With the burner oriented as shown in Fig. 1, CARS measurements were obtained 1 cm above the burner surface. When the burner was oriented perpendicular to the laser beams, the CARS signal became very erratic. This effect has been shown to be caused by turbulence induced beam steering when the defocused beams pass through the flame.8 This drawback can be overcome by clever routing of the input beams, which may be aided by using the focusing mirror as a steering mirror. Such beam steering through small angles was demonstrated by focusing the CARS focal point below the plane of the probe beams. Large angle steering was not attempted but should be possible.

The feasibility of using a mirror based CARS system as an alternative to the lens based system has been shown. Ad­vantages of this setup include: (1) large crossing angles can be obtained with one focusing device; (2) cost of a large mirror will be less than for a large high quality lens; (3) generation and collection equipment can be located on one side of the sample region; and (4) the probe beams can be steered without

the use of an additional optical element. The major drawback of this technique is the turbulence effects on the laser beams. This will seriously limit the direct application of this technique to small laboratory flames. Trying to apply this technique to combustion environments will require routing the input beams around the combustion zone as is done with conven­tional lens systems. However, one set of optical components to cross, focus, and turn the laser beams will be combined.

References 1. A. C. Eckbreth, Combust. Flame 39, 133 (1980). 2. L. P. Goss, G. L. Switzer, and P. W. Schreiber, at AIAA Fifteenth

Thermophysics Conference (1980). 3. A. C. Eckbreth, Appl. Phys. Lett. 32, 421 (1978). 4. J. A. Shirley, R. J. Hall, and A. C. Eckbreth, Opt. Lett. 5, 380

(1980). 5. Y. Prior, Appl. Opt. 19, 1741 (1980). 6. This configuration was disclosed in U.S. Patent 4,277,760 (7 July

1981). 7. A. Compaan and S. Chandra, Opt. Lett. 4, 170 (1979). 8. L. P. Goss and P. W. Schreiber, Submitted to Opt. Lett.

1 July 1983 / Vol. 22, No. 13 / APPLIED OPTICS 1955