George W. Hopkins

In-vivo NIR diffuse-reflectance tissue spectroscopy of human subjects

There are few published works (Marbach 1993,1995) in the field of tissue spectroscopy for glucose self-monitoring, in the wavelength range from 1.0 microns to 2.5 microns, and the error described is too large for clinical use or for home monitoring. Measurements (Arnold 1990, Marbach 1993) of aqueous solutions of glucose, with concentrations in the physiological range, have been promising, including measurements (McNulty 1998) in our laboratory.

Semi-imaging light pipe for collecting weakly scattered light

A simple reflective light pipe, formed from a cylindrical tube with an external reflective coating and a small central aperture, can be a highly efficient optical element for collecting light from molecular scattering processes along the path of a laser beam. When the laser beam is co-linear with the axis of the light pipe, scattered light from any location along the interaction region (near the pipe axis) re-images repeatedly to another location along the axis of the pipe. This semi-imaging property of the light pipe permits a large fraction of the total scattered light to re- image along the entire length of the interaction region. If one observes through the small central aperture, scattered light from the single segment of the laser beam in view appears to come from all the locations along the interaction length, as well as from the single segment. In this manner, one can have the advantage of collecting scattered light from a small segment (and thus onto a small detector), while observing an effective interaction length that is many times longer than the segment. Measurements from practical light pipes confirm effective gains of about 10X with light pipes a few centimeters long (Effective gain is defined as the ratio of light collected with the light pipe divided by the light collected from a direct image of the beam using the collection optics).

Cylindrical light pipes for collecting light scattered from a Gaussian beam

An investigation to enhance the efficiency of Raman- scattered light showed that cylindrical light pipes can significantly increase light collection from a Gaussian beam. Further, the enhanced signal from the light pipe retains the image of the laser beam, permitting the use of smaller detectors and resulting in a favorable signal-to- noise ratios. This investigation focussed on real-time measurements of gaseous media in a laser buildup cavity; however, the imaging properties of the light pipe apply to all measurements of molecular scattering. The light pipe matched the constraints of our measurement system: spectral separation and detection with an optical spectrograph, the need to reduce background light, the need to minimize cost, and stimulation by a laser beam in an optical cavity. After initial experiments collecting light from the ends of light pipes, we developed light pipes with a window on the cylindrical surface. Light emitted from these windows is much more intense than the direct image of the laser beam (typically 10X for light pipes 50 - 100 mm long), and the signal retains the image of the beam. Computer ray tracing modeled this side collection using Monte Carlo techniques, which are discussed in detail. We fabricated and tested light pipes using several different coatings.

Refractive elements in spectrographs

A spectrograph design was needed for an application in medial optics. Requirements for resolution were modest, cost was to be minimized, and every photon was precious. The initial design used a concave, aberration-corrected, holographically-recorded, diffraction grating. This design cost too much and collected too few of the precious photons. Several design forms, which use refractive optics with a planar diffraction grating, were investigated. The design that was chosen and built uses two aspheric glass lenses, one for collimating and one for focusing, together with a planar diffraction grating.

Projection Lenses for a C-Size Light-Valve Display

We studied two types of projectors for a liquid-crystal-display breadboard and designed and used an on-axis f/4 lens.