Coronagraphic Notch Filter for Raman Spectroscopy

NASA's Jet Propulsion Laboratory, Pasadena, California

Sunday, August 01 2004

Design could be optimized for attenuating pump light and transmitting Raman-scattered light.

A modified coronagraph has been proposed as a prototype of improved notch filters in Raman spectrometers. Corona-graphic notch filters could offer alternatives to both (1) the large and expensive double or triple monochromators in older Raman spectrometers and (2) holographic notch filters, which are less expensive but are subject to environmental degradation as well as to limitations of geometry and spectral range.

Measurement of a Raman spectrum is an exercise in measuring and resolving faint spectral lines close to a bright peak: In Raman spectroscopy, a monochromatic beam of light (the pump beam) excites a sample of material that one seeks to analyze. The pump beam generates a small flux of scattered light at wavelengths slightly greater than that of the pump beam. The shift in wavelength of the scattered light from the pump wavelength is known in the art as the Stokes shift. Typically, the flux of scattered light is of the order of 10x that of the pump beam and the Stokes shift lies in the wave-number range of 100 to 3,000 cm -1. A notch filter can be used to suppress the pump-beam spectral peak while passing the nearby faint Raman spectral lines.

The basic principles of design and operation of a coronagraph offer an opportunity for engineering the spectral transmittance of the optics in a Raman spectrometer. A classical coronagraph may be understood as two imaging systems placed end to end, such that the first system forms an intermediate real image of a nominally infinitely distant object and the second system forms a final real image of the intermediate real image. If the light incident on the first telescope is collimated, then the intermediate image is a point-spread function (PSF). If an appropriately tailored occulting spot (e.g.,a Gaussian-apodized spot with maximum absorption on axis) is placed on the intermediate image plane, then the instrument inhibits transmission of light from an on-axis source. However, the PSFs of off-axis light sources are formed off axis — that is, away from the occulting spot — so that they become refocused onto the final image plane.

A properly designed coronagraph utilizes the diffraction from the intermediate occulting spot. In the exit-pupil plane, this diffraction forms a well-defined ring image in the vicinity of the geometric image of the exit pupil. By placing an aperture stop sized to block the passage of the diffracted light (such an aperture is known in the art as a Lyot stop) in the exit-pupil plane, it is possible, in principle, to obtain an extremely high rejection ratio. While coronagraphs are not new, recent developments make it possible to enhance performance. One such development is that of the ability to write arbitrary absorption patterns on occulting spots at submicron resolution by use of electron-beam lithography. Another such development is that of superpolished optics.