Lensless digital holographic microscopy as a means to search for life in the solar system

To search for extant life on distant solar system bodies such as the ocean-bearing moons Europa and Enceladus, or on the ice caps of Mars, a compact and robust microscope capable of carrying out rapid volume searches is of great interest. One technique well suited to 3-d volume searches is digital holographic microscopy, in which a coherent reference beam is combined with the light scattered by the particulates and/or microbes present in a sample volume to produce an interference pattern (the hologram) on a detector array. As the detected hologram encodes both the amplitude and phase of the scattered field on the detector plane, the electric field at any plane within the sample volume can be reconstructed by numerically propagating the field information contained in the measured hologram, thereby eliminating the need for any mechanical focusing elements. In fact, digital holographic microscopy has several advantages, including instantaneous 3-d volume imaging, high spatial resolution, large depth of field, large field of view, and intrinsic data compression due to the encoding of the entire volume into a 2-d hologram. Moreover, rapid readout can also enable the tracking of particle motions, which can aid in discriminating between inert particulates and living cells. With no need for mechanical focusing, digital holographic microscopes allow for robust and stable designs. In particular, very promising off-axis digital holographic microscope (DHM) designs include both the “common-mode” DHM, and the “lensless” DHM. A common mode DHM has recently performed very well in the field, with reliable performance and sub-micron resolution. On the other hand, the lensless DHM configuration holds the promise of minimal instrument volume and mass, as the optical system can consist of no more than a pair of coherent high-numerical-aperture single-mode light sources that expand through a sample volume to a detector array. Even this simple configuration has been recently shown to be capable of providing a resolution of just under a micron. Moreover, in some lensless DHM configurations, very large fields of view can also be provided. This paper describes the imaging capabilities of lensless digital holographic microscope configurations, both in theory and in practice.