P7 - Design of Diffractive Optical Elements for Gas Analyzing Photometers
- Event
- SENSOR+TEST Conferences 2011
2011-06-07 - 2011-06-09
Nürnberg - Band
- Proceedings IRS² 2011
- Chapter
- IP - Poster Session
- Author(s)
- C. Graulig, A. Grjasnow, R. Riesenberg - Institute of Photonic Technology, Jena (Germany)
- Pages
- 124 - 129
- DOI
- 10.5162/irs11/ip7
- ISBN
- 978-3-9810993-9-3
- Price
- free
Abstract
For infrared spectrophotometer it is interesting to combine functions like dispersion and focusing in one optical element, a diffractive optical element (DOE).
The state of the art of design techniques for DOEs focuses on the coherent illumination case, which corresponds to a monochromatic point source. In this case the function of the DOE can be analyzed for one wavelength e.g. using scalar wave field propagation based on the Rayleigh-Sommerfeld-Integral. Often infrared photometers use a thermal light source and it is essential to use the throughput generated by a spatially extended emitter. So the thermal source can not be limited spectrally and spatially to make it a coherent point source. The point wise propagation of all source points by the coherent propagation technique would be necessary. For the presented design of a photometer for CO2 detection the simulation would have taken tens of hours. The emitter area has to be sampled and the sampling depends on the system’s imaging properties. For a numerical aperture of 0.1 and a magnification of 1.75 a sampling of 20 μm was sufficient. This resulted in 2500 source points for the 1x1 mm² source area.
A new technique is presented to calculate the intensity distribution of a DOE that is illuminated by an extended spatially incoherent light source. Only one source point has to be propagated through the systems by means of scalar wave field propagation. The result will be the point spread function of the system. The propagations of the other source points are replaced by a geometric ray tracing step. This algorithm greatly reduces the computing time. In the example the time was below three minutes for one wavelength. This allows the timesaving development of DOEs for incoherent infrared light sources and might enable the online computation of phase masks for programmable spatial light modulators. Results are discussed for a DOE based CO2 photometer working at 4.26 μm with a dispersion of 33nm/mm. An efficiency of about 0.2 is predicted for a DOE with binary height profile.