Study of Optical Imaging by Reflection through Random Media
Physics, Dr. Mellema
About My Research:
Why can’t we “see” a melanoma beginning to grow beneath the surface of a person’s skin? Skin is a highly-scattering medium – most of the light that strikes the skin scatters in random directions. Imagine suspending a small mirror inside a glass of milk. Although the mirror still reflects light, shining light on the milk will not reveal the mirror to our eyes. Most of the light is simply scattered by the fat globules suspended in the milk. But, if we use a special property (the “coherence”) of laser light, we can hope to identify and measure the tiny fraction of (“ballistic”) light that is not scattered, but travels directly to the mirror and then passes directly out of the glass. Doing so would allow us to create an image of the mirror inside the milk. This research is intended to produce a “reflectometer”, a modified interferometer that can accomplish this task. A laser beam is split into two, with one beam reflecting off the sample being studied. The other beam travels, unmodified, over a path of exactly the same length. When the beams are recombined and passed to a light detector, an “interference” pattern reveals the presence of ballistic light.
This research is intended to produce an optical heterodyning circuit (a modified Mach-Zender interferometer) for imaging reflective sample objects embedded in a highly-scattering medium. One beam of the interferometer is reflected from the sample object, and the other beam is frequency shifted by 80 MHz using an acousto-optic modulator. When the path lengths of the two beams are perfectly matched, the instrument produces a beating interference pattern. A high-speed detector coupled to a lock-in amplifier measures the intensity distribution of the reflected light and, by scanning across the illuminated region, the sample object is imaged. In the current research, efforts have been made to assemble mechanical translation stages, all of the optical components, and data-acquisition instruments, and to integrate the system control to be fully programmable through Visual Basic. A visible, long-coherence-length diode laser is used for testing the stability of the system. In the test run, a mirror is used as the sample, and the scan shows a complete reflection pattern as expected. In the later stages, a more powerful, short-coherence-length infrared laser will be used to image samples inside a highly-scattering medium, from which only a tiny fraction of the incident light is reflected.