NIR and IR spectroscopy with MCT linear arrays

This paper will discuss recent results obtained when applying a photoconductive linear MCT array in a demonstration spectrometer designed for the NIR wavelength range from 1300 to 2500 nm. A new 128x1 element MCT sensor was developed specifically for spectroscopy, i.e. with “tall”, rectangular pixels in order to optimize both wavelength resolution and optical throughput. Also new read-out electronics was developed using multilayer LTCC (Low Temperature Co-Fired Ceramics) techniques, which is integrated into the package and realizes synchronous (“lock in”) detection for each of the 128 channels. Advantages of this current-detection scheme include compatibility with chopped light sources (insensitivity to ambient stray light) and elimination of read-out noise (affecting charge-detection amplifiers). The first test results reported here confirm spectrometer operation and present encouraging performance, even though the system is not yet optimized. The spectrometer is very fast, with minimum integration time of 1.2 ms, while photometric noise will reduce with longer integration times. There is no fundamental limit in the maximal length of the integration time. Testing with integration times of 1.2, 12, 120 and 1200 ms resulted in absorbance noise levels of approximately 2500, 330, 94 and 49 myyA units. Demonstration spectra were measured from lactose and copying paper samples. Thanks to high speed and parallel spectral recording of 128 wavelengths, MCT array technology appears highly potential for developing powerful on-line spectrometers for process analytical applications not only in the near infrared (NIR) but also for the lower mid-IR wavelengths, up to approximately 6 myym.