Dual-Channel Spectrometer-Based System

The dual-channel spectrometer-based LTS apparatus, Figure 2, was previously reported in Tanner et al. and is briefly described here. The apparatus employs a single broadband multi-wavelength light source, a fiber optic splitter, a collimating lens system for each channel, a collection fiber optic for each channel, and two grating spectrometers each coupled to a charge-coupled-device (CCD) detector. The theoretical and mathematical principles involved are similar to those of the scanning laser-based system; however, this new apparatus is capable of recording the data for a sample and its corresponding reference at all wavelengths simultaneously in real-time (~200 ms detector integration time).

Figure 2. Dual-channel multi-wavelength spectrometer system

The multi-wavelength light source contains both deuterium and tungsten halogen bulbs, which produce useful optical power over the wavelength range of 210-2000 nm. The light output is coupled to a bifurcated optical fiber, which consists of a pair of UV-resistant multimode fibers bound together on the input end and separated into two optical paths at their output ends. The two pathways are used to simultaneously measure light transmitted through both the sample and its reference. Along each path, the light output of the fiber is collimated with a lens into a beam, passes through a cuvette, is refocused with a lens into another fiber, and couples to a grating spectrometer with a CCD detector with a wavelength range of 200-1100 nm and resolution of <1 nm.

The collection efficiency is given by the geometrical parameters of the lens system and corresponds to a collection solid angle for scattered light of ~2.2 x 10^-4 relative to the intensity of the zero-angle transmitted beam. Thus, this apparatus similarly suppresses scattered light by ~4 orders of magnitude. A light-tight box surrounding the optical components is closed during data acquisition and prevents ambient light from entering the light-collection system. In addition, a shutter (not shown) is used to temporarily block the light source and the spectrometers are simultaneously triggered in order to measure dark counts, which are subtracted from each sample and reference spectrum.

First, data is collected simultaneously with the sample in path A and reference in path B. Second, data is collected with the sample in path B and reference in path A, again simultaneously. This combination of measurements serves two purposes. The first is to remove noise in the spectra due to light source variations in time, and the second is to remove any steady-state systematic differences in the transmission of light through the two different pathways. Two dark-count spectra and four transmission spectra, all gathered in a short period of time (few seconds), are passed to a computer. After dark-count subtraction, the four spectra described in Eq. comprise the raw data for further processing.