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C. W. Huck
Hadamard transform resolves the spectrum from the collected wavelengths [13].
Hadamard principle allows to employ a cost-effective single-pixel photodetector.
For example, in microPHAZIR, NIR beam emitted from a low-power tungsten bulb
is focused on a fixed grating that serves as the dispersive element. The keys of the
MOEMS chip are actuated successively, reflecting the selected wavelengths onto a
single-pixel InGaAs detector. This design enabled a rapid scanning capability (<10 s),
good S/N, and a reasonable optical resolution of 11 nm. However, the device operates
in a rather narrow wavelength region of 1596–2396 nm (6267–4173 cm
−1 ) [14, 15].
In early 2010s, the success of microPHAZIR leads to anticipation that MEMS
spectrometers would rapidly dominate the market of miniaturized NIR sensors based
on Hadamard principle. However, the subsequent progress was much less dynamic
[6]. The limitations resulting from the size of the optics created issues with repeatability of operation and the ability of a MEMS comb actuator to drive the moving
mirror; these factors outweigh the advantages of this technology, given its price.
Nonetheless, commercial success of some handheld FT-IR (mid-IR) devices even
paved the path for the appearance of FT-NIR spectrometers. NIR light sources are
brighter, and detectors have a higher specific detectivity than those used in IR spectrometers; hence, mirror size is less of a constraint. For example, Thermo Fisher
Scientific successfully scaled down the interferometer design, with a voice-coil
and piston-bearing scheme, and a moving mirror of 1.2 cm diameter. A MEMSbased Michelson interferometer was commercialized by NeoSpectra, the division of
Si-Ware Systems, with their FT-NIR miniaturized instrument (Fig. 8.6a).
The problem of maintaining a stable operation of MEMS element and the optical
throughput of such devices is under constant development. Recent examples of
refined designs include NIRONE sensors from spectral engines (Fig. 8.6d) and
nanoFTIR NIR spectrometer from Hefei SouthNest Technology (Fig. 8.6f). The
nanoFTIR NIR is a very recent sensor that uses a MEMS Michelson interferometer,
in which in order to improve its light throughput efficiency, a large mirror in relation
to the area of MEMS chip was implemented. Further, the spectrometer operates over
the entire NIR wavelength region (800–2600 nm; 12,500–3846 cm
−1 ), which is a
notable improvement over early MEMS-based sensors (Table 8.1). This is accompanied by a relatively high spectral resolution (6 nm), good S/N, and rapid scanning.
Noteworthy, the design achieved significantly more compact dimensions (14.3 × 4.9
× 2.8 cm; weight 220 g) than any previous MEMS-based FT-NIR spectrometers.
Fabry–Perot interferometers are very suitable to serve as wavelength selectors
in miniaturized spectrometers. The key element in such interferometer is Fabry–
Perot filter consisting of two mirrors, either planar or curved, facing each other and
separated by a distance d. Two variants exist, an etalon with fixed d, and the other with
variable d. Interference condition in a Fabry–Perot interferometer is achieved through
the standing wave effect between the two mirrors and division of a polychromatic light
into several narrow wavelength bands. Important for miniaturized spectrometers,
MEMS technology can be used to fabricate a fully programmable optical filter based
on Fabry–Perot interferometer in microscale. This solution is implemented, e.g.,
NIRONE sensor series (Fig. 8.6d).
C. W. Huck
Hadamard transform resolves the spectrum from the collected wavelengths [13].
Hadamard principle allows to employ a cost-effective single-pixel photodetector.
For example, in microPHAZIR, NIR beam emitted from a low-power tungsten bulb
is focused on a fixed grating that serves as the dispersive element. The keys of the
MOEMS chip are actuated successively, reflecting the selected wavelengths onto a
single-pixel InGaAs detector. This design enabled a rapid scanning capability (<10 s),
good S/N, and a reasonable optical resolution of 11 nm. However, the device operates
in a rather narrow wavelength region of 1596–2396 nm (6267–4173 cm
−1 ) [14, 15].
In early 2010s, the success of microPHAZIR leads to anticipation that MEMS
spectrometers would rapidly dominate the market of miniaturized NIR sensors based
on Hadamard principle. However, the subsequent progress was much less dynamic
[6]. The limitations resulting from the size of the optics created issues with repeatability of operation and the ability of a MEMS comb actuator to drive the moving
mirror; these factors outweigh the advantages of this technology, given its price.
Nonetheless, commercial success of some handheld FT-IR (mid-IR) devices even
paved the path for the appearance of FT-NIR spectrometers. NIR light sources are
brighter, and detectors have a higher specific detectivity than those used in IR spectrometers; hence, mirror size is less of a constraint. For example, Thermo Fisher
Scientific successfully scaled down the interferometer design, with a voice-coil
and piston-bearing scheme, and a moving mirror of 1.2 cm diameter. A MEMSbased Michelson interferometer was commercialized by NeoSpectra, the division of
Si-Ware Systems, with their FT-NIR miniaturized instrument (Fig. 8.6a).
The problem of maintaining a stable operation of MEMS element and the optical
throughput of such devices is under constant development. Recent examples of
refined designs include NIRONE sensors from spectral engines (Fig. 8.6d) and
nanoFTIR NIR spectrometer from Hefei SouthNest Technology (Fig. 8.6f). The
nanoFTIR NIR is a very recent sensor that uses a MEMS Michelson interferometer,
in which in order to improve its light throughput efficiency, a large mirror in relation
to the area of MEMS chip was implemented. Further, the spectrometer operates over
the entire NIR wavelength region (800–2600 nm; 12,500–3846 cm
−1 ), which is a
notable improvement over early MEMS-based sensors (Table 8.1). This is accompanied by a relatively high spectral resolution (6 nm), good S/N, and rapid scanning.
Noteworthy, the design achieved significantly more compact dimensions (14.3 × 4.9
× 2.8 cm; weight 220 g) than any previous MEMS-based FT-NIR spectrometers.
Fabry–Perot interferometers are very suitable to serve as wavelength selectors
in miniaturized spectrometers. The key element in such interferometer is Fabry–
Perot filter consisting of two mirrors, either planar or curved, facing each other and
separated by a distance d. Two variants exist, an etalon with fixed d, and the other with
variable d. Interference condition in a Fabry–Perot interferometer is achieved through
the standing wave effect between the two mirrors and division of a polychromatic light
into several narrow wavelength bands. Important for miniaturized spectrometers,
MEMS technology can be used to fabricate a fully programmable optical filter based
on Fabry–Perot interferometer in microscale. This solution is implemented, e.g.,
NIRONE sensor series (Fig. 8.6d).