Spatial frequency modulation for imaging with incoherent light sources

Abstract

This thesis discusses the initial execution of a microscope that achieves imaging by implementing the technique SPatIal Frequency modulation for Imaging (SPIFI). SPIFI is a robust imaging technique even in the presence of scattering because a single element detector is used in place of a camera. A line geometry is used to illuminate the sample and each place along the line is modulated at a specific temporal frequency. In this way the location information is encoded in the illumination light. After passing through the sample the light reaches the single element detector, which produces a time varying signal. The Fast Fourier Transform (FFT) of the signal gives the spectrum of modulation frequencies and their respective amplitudes. Since each of those frequencies corresponds to a position on the sample, the spectrum of the signal can be used to produce an image. Scanning the line across the sample and stitching the data together generates an entire image. Prior work has demonstrated a coherent version of SPIFI using a spinning disk to modulate the laser line-focus with linearly varying frequencies across its spatial extent. In this work, light emitting diodes (LEDs) are used as the light source and are electronically modulated, eliminating the need for a spinning reticle. There are a number of advantages to using LEDs over a laser. They are inexpensive compared to pump lasers that cost thousands of dollars before they are even implemented in an expensive femtosecond laser oscillator. The cost of an LED array with supporting electronics costs only around hundred dollars. Also, laser light is coherent and typically collimated making it easily focused by the lens of the eye to an intense spot on the retina, which can cause permanent damage depending on the energy of the beam. LED light is incoherent and highly divergent, making it much safer and the implementation of these microscopes in K-12 schools more realistic. Other imaging techniques, such as confocal microscopy, have been able to image through scattering media by physically rejecting out of focus photons. While video-rate (30 frames/s) confocal microscopy has been demonstrated, the pathway to video-rate SPIFI in terms of complexity, is quite straightforward and imminently achievable.