Multiwavelength Digital Holography for Spectral Discrimination of Bacteria and Minerals

We have previously developed a digital holographic microscope with submicrometer resolution, a cubic mm scale sample volume, and video frame rates to search for microbial life in liquid environments.  In this task we will add multi-wavelength capability to this design to enable spectral imaging.  This additional capability will allow for better discrimination between abiotic and potentially biotic objects in the sample.  The interferometric nature of the instrument and our ability to separate the wavelengths by fringe angle makes it possible to do this without increasing the data volume, providing a naturally compressed data stream.

In the current single-wavelength implementation, the holographic information is encoded with fringes in a single, unique orientation of the detector plane. However, if for each new wavelength we add, we encode the new fringe in a unique geometry – then we have increased the capability without increasing complexity. The figure in the appendix illustrates the beam pattern for the following example: 405 nm generating fringes at 45 degrees, 488 nm generating fringes at -45 degrees, and 532 nm at 0 degrees). There is no change to the detector, or the image recording process; the only changes are to the 1) source optics and 2) sample chamber, and the 3) reconstruction process.  The object beam for each wavelength goes through the central sample volume, while the reference beams are kept apart.         

The goal of this work is to increase by almost an order of magnitude our knowledge of the spectral behavior our our samples – in both amplitude and phase. We note that within JPL as well as in the astrobiology instrument community, we are the only team working on digital holographic microscopy, and currently the only capability we have is monochromatic.        

Once we have done the engineering design, implementation and lab testing in years one and two, we plan to carry out our field tests in year three.  Specifically, we would like to continue testing our instrument in extreme environments here on Earth. This is less well defined at the moment, but our designs are currently compatible with extreme thermal and pressure environments. We plan to start in nearby locations with extreme environments (e.g., Mono Lake, Salton sea), and are also considering sub-glacial lakes, archaic glacial fields, and deep oceanic vents as prime examples where this new capability can be demonstrated.