200
C. W. Huck
8.2.2 Detectors
The silicon detectors are entry-level solutions that enable constructing low cost 1D
and 2D array sensors. Multiplied Si detector elements can be fitted with own filter
each, with each such element tuned toward measuring its own channel (i.e., wavelength region). Therefore, they are suitable for constructing very simple and inexpensive multichannel detectors. However, Si detectors yield inferior S/N parameter
and because of a cut-off at ca. 1000 nm (10,000 cm
−1 ) are limited to operate in
a narrow spectral region of visible/short-wave NIR (Vis/SW-NIR). Silicon-based
detectors offer practical advantages, e.g., low power consumption. There exist two
major types of such detectors, complementary metal–oxide–semiconductor (CMOS)
and charge-coupled device (CCD), with CMOS requiring lower power consumption
[6]. Charge-coupled device (CCD) is a silicon-based photon detector. When light
strikes the chip, it directly induces as a small electrical charge in each cell of the
photosensor. The cell is an analog circuitry, and the charge is amplified, converted
into a digital value, and the output registered. Commercial success and wide-spread
use of CMOS and CCD technology have brought down the price per unit of such
detectors. However, silicon photodetectors offer inferior sensitivity toward sensing
the NIR wavelengths. Therefore, for higher S/N, better performing detectors are
preferable. Here, indium–gallium–arsenide (InGaAs) detector may be considered
state of the art, with excellent sensitivity at wavelengths longer than ca. 1050 nm,
superior S/N and scan time. The detector noise varies with temperature, which has
been a problem in some earlier designs. Temperature stabilization by thermoelectric
cooling was proved to be helpful in this regard [9]. Temperature correction functions
have been also introduced in newer designs, e.g., MicroNIR 2200.
8.2.3 Wavelength Selectors
Wavelength selector can be considered the most critical element for the design of
a miniaturized spectrometer. There is a large variety of the available solutions in
this regard. Micro-electro-mechanical systems (MEMS; in combination with microoptics: micro-opto-electro-mechanical systems, MOEMS or optical MEMS) are insilicon microscaled mechanical devices manufactured similar to integrated circuitry.
This technology advanced together with the progress made in semiconductor industry
enabling the assembly of extremely miniaturized moving parts. MEMS technology
can be used to implement few different wavelength selector principles in microscale,
e.g., Hadamard mask, digital micromirror, Michelson and Fabry–Perot interferometers. Thus, grating-based monochromators for ‘dispersive-like’ and interferometers
for Fourier transform (FT) spectrometers can be manufactured. It has become fairly
popular solution for miniaturized NIR spectrometers with a number of MEMS-based
devices proposed in the last 20 years.
C. W. Huck
8.2.2 Detectors
The silicon detectors are entry-level solutions that enable constructing low cost 1D
and 2D array sensors. Multiplied Si detector elements can be fitted with own filter
each, with each such element tuned toward measuring its own channel (i.e., wavelength region). Therefore, they are suitable for constructing very simple and inexpensive multichannel detectors. However, Si detectors yield inferior S/N parameter
and because of a cut-off at ca. 1000 nm (10,000 cm
−1 ) are limited to operate in
a narrow spectral region of visible/short-wave NIR (Vis/SW-NIR). Silicon-based
detectors offer practical advantages, e.g., low power consumption. There exist two
major types of such detectors, complementary metal–oxide–semiconductor (CMOS)
and charge-coupled device (CCD), with CMOS requiring lower power consumption
[6]. Charge-coupled device (CCD) is a silicon-based photon detector. When light
strikes the chip, it directly induces as a small electrical charge in each cell of the
photosensor. The cell is an analog circuitry, and the charge is amplified, converted
into a digital value, and the output registered. Commercial success and wide-spread
use of CMOS and CCD technology have brought down the price per unit of such
detectors. However, silicon photodetectors offer inferior sensitivity toward sensing
the NIR wavelengths. Therefore, for higher S/N, better performing detectors are
preferable. Here, indium–gallium–arsenide (InGaAs) detector may be considered
state of the art, with excellent sensitivity at wavelengths longer than ca. 1050 nm,
superior S/N and scan time. The detector noise varies with temperature, which has
been a problem in some earlier designs. Temperature stabilization by thermoelectric
cooling was proved to be helpful in this regard [9]. Temperature correction functions
have been also introduced in newer designs, e.g., MicroNIR 2200.
8.2.3 Wavelength Selectors
Wavelength selector can be considered the most critical element for the design of
a miniaturized spectrometer. There is a large variety of the available solutions in
this regard. Micro-electro-mechanical systems (MEMS; in combination with microoptics: micro-opto-electro-mechanical systems, MOEMS or optical MEMS) are insilicon microscaled mechanical devices manufactured similar to integrated circuitry.
This technology advanced together with the progress made in semiconductor industry
enabling the assembly of extremely miniaturized moving parts. MEMS technology
can be used to implement few different wavelength selector principles in microscale,
e.g., Hadamard mask, digital micromirror, Michelson and Fabry–Perot interferometers. Thus, grating-based monochromators for ‘dispersive-like’ and interferometers
for Fourier transform (FT) spectrometers can be manufactured. It has become fairly
popular solution for miniaturized NIR spectrometers with a number of MEMS-based
devices proposed in the last 20 years.