PLIF AND IMAGE CHEMILUMINESCENCE
4.4 Laser, Delivery Optics and Imaging System .1 Laser Light Source .1 Laser Light Source
4.4.2 Delivery and Sheet-Forming Optics
UV light is directed out of the OPO by a right-angle prism followed by a Pellin-Broca prism which not only produces a second 90º turn but also provides wavelength selectivity. The second turn directs the beam down a long structure called “the bridge” and toward the test section.
The bridge contains all the optics associated with forming the laser sheet. Figure 4-2 shows a photograph of the bridge while Figure 4-3 provides a drawing of the layout of the optical elements.
Figure 4-2: Photograph of “the bridge” with orange acrylic optical guard installed. The bridge contains the laser pulse measurement photodiode, as well as apertures and sheet- forming optics. Beam path is from right to left.
Figure 4-3: Layout of optical elements on the bridge. Beam path is from right to left. Beam splitters BS302 and BS303 are on magnetic kinematic mounts and are removable. Note that the drawing is not to scale.
In the layout above, the beam path is from right to left. The beam exiting the harmonic separator of the OPO passes through aperture HA301. This is a white PTFE disk 25.4 mm in diameter and 6 mm thick, containing a 7.5 mm round hole through the center. The disk is mounted in a Thor Labs FMP1 fixed optic mount. (The same type of mount is also used for beam-splitter BS301, apertures HA302 and HA303, and beam stops S301 and S302.) A section of black poster- board is used to continue the stop beyond the edges of the mount. The poster-board section is rectangular, being 200 mm wide and 280 mm tall and containing a hole coincident with the location
of HA301. The hard-aperture along with its poster-board accessory allows the desired UV laser beam to pass while blocking all other harmonics separated by the Pellin-Broca prism.
After leaving the aperture, the UV beam passes through beam-splitter BS301. This beam- splitter is simply an uncoated fused-silica window (Edmund Optics P/N 43-424) rotated near the Brewster angle. A small amount of the laser energy is reflected from the surface and into the photodiode assembly. The remaining light passes through the optic. The photodiode energy detector is a custom built assembly fabricated around a fast photodiode purchased from Thor Labs (P/N FDS010). A 10 mm diameter, ground fused silica optical diffuser (Edmund Optics P/N 49- 155) is mounted on the assembly just in front of the photodiode to help spatially balance the light being measured. The photodiode is properly reverse biased in order to achieve the specified 1 ns response time and is terminated to a 50 Ohm output. A 50 Ohm coaxial cable feeds this signal directly back to the AFC-100 instrument. The AFC-100 not only uses this signal for measurement of laser shot timing, but also to measure and record the energy of each shot.
After leaving BS301, the UV beam travels down the bridge where it may encounter two optional beam-splitters, BS302 and BS303. These elements are mounted on internally-threaded, fixed optics holders (Thor Labs P/N LMR1), which are attached to magnetically-secured kinematic bases (Thor Labs P/N SB1). As such, they can be installed and removed as needed. These beam- splitters are used to strongly attenuate the laser sheet passing into the test section without changing its lateral position or intensity distribution. This is required when taking measurements of the laser sheet intensity profile for image post-processing. (Further details of this procedure are discussed in section 4.4.4) The beam-splitters are manufactured by CVI Laser Optics (P/Ns UVDA-266- 1.00-10 and UVDA-266-1.50-10). Under normal PLIF imaging conditions, BS302 and BS303 are removed.
Finally, the laser enters the sheet-forming section. The beam first passes through aperture HA302. This aperture is not circular, but instead is a horizontal slot with full-radius ends. The
element is fabricated from a white PTFE disk, 25.4 mm in diameter and 6.0 mm thick. The machined slot height is 7.0 mm and the length along the centerline is 19.5 mm. This crops any out- of-position rays in the vertical direction. After clearing the HA302, the beam passes through L301, a plano-concave cylindrical lens with focal length -500.0 mm (Thor Labs P/N LK4989-UV). This lens is mounted using a kinematic platform mount (Thor Labs P/N’s CH1A and KM100B) and is oriented to expand the beam in the vertical direction.
The vertically expanding beam is then passed through another slot aperture, HA303. This aperture is identical to HA302 with the exception that the slot is oriented vertically, blocking any out-of-position rays in the horizontal direction. After clearing HA303, the beam passes through L302, a plano-convex cylindrical lens with focal length +500.0 mm (Thor Labs P/N LJ4147-UV).
The lens is mounted similarly to L302, but with a slightly larger mount (Thor Labs P/N’s CH2B and KM100B). It is oriented to compress the beam in the horizontal direction, forming a thin sheet.
After leaving the bridge, and just prior to entering the test section, a final pair of independently adjustable aperture plates are used to trim the laser sheet to the correct height. The plates are adjusted such that the top of the laser sheet slightly grazes the bottom of the burner stagnation body, and the bottom of the laser sheet clears the top of the burner nozzle.