222
A. Ikehata
similar to LCTF, in the NIR region. The wavelength switching speed depends only
on the dwell time of the ultrasound. The higher the speed of sound, the faster the
wavelength switching speed, but the diffraction efficiency (the ratio of diffracted light
intensity to incident light intensity) is inversely proportional to the square of speed,
v. As a result, the use of a slow speed of sound is useful for AOTFs. The acoustic
velocity of a longitudinal wave along the surface of (001) surface in a TeO 2 crystal is
4260 m s
−1 , and the transverse wave along (110) surface is very slow at 616 m s
−1 .
Consequently, AOTFs with a TeO 2 crystal maintain their diffraction efficiency by
use of transverse waves, and achieve a wavelength switching speed within 10 ms.
When used with a thermal radiation source, the refractive index of the AOTF medium
will change by warming, making it impossible to select the correct wavelength. For
this reason, it is desirable to use a cold filter or apply an electrical correction method
[10].
9.1.4 Detector
Since it is difficult to cover the entire NIR region with a single detector, it is necessary
to select from the detectors shown below, depending on the wavelength band of
interest [11].
9.1.4.1 Silicone Photo Diode (Si)
Photovoltaic silicon (Si) photodiodes can be used in the NIR region close to visible
light (700–1100 nm: often called short NIR). Si photodiodes are suitable for quantitative analysis because of their wide dynamic range. The short NIR region is especially suitable for samples with high water content, so Si photodiodes are often used
in sugar-content meters of fruits. Since Si photodiodes respond to visible light, it is
necessary to use a wavelength cut-off filter to avoid indoor lighting and sunlight.
9.1.4.2 Indium Gallium Arsenide (InGaAs)
Indium gallium arsenide (InGaAs) is a photovoltaic device with a pn junction. The
sensitivity band varies depending on the composition ratio of In and Ga. The larger
the In:Ga ratio, the more sensitive it is to longer wavelengths. The standard sensitivity
range is 900–1600 nm, but it is possible to extend this to 2600 nm. The pn junction
type has a response speed of GHz, and high-speed measurement is therefore possible.
A. Ikehata
similar to LCTF, in the NIR region. The wavelength switching speed depends only
on the dwell time of the ultrasound. The higher the speed of sound, the faster the
wavelength switching speed, but the diffraction efficiency (the ratio of diffracted light
intensity to incident light intensity) is inversely proportional to the square of speed,
v. As a result, the use of a slow speed of sound is useful for AOTFs. The acoustic
velocity of a longitudinal wave along the surface of (001) surface in a TeO 2 crystal is
4260 m s
−1 , and the transverse wave along (110) surface is very slow at 616 m s
−1 .
Consequently, AOTFs with a TeO 2 crystal maintain their diffraction efficiency by
use of transverse waves, and achieve a wavelength switching speed within 10 ms.
When used with a thermal radiation source, the refractive index of the AOTF medium
will change by warming, making it impossible to select the correct wavelength. For
this reason, it is desirable to use a cold filter or apply an electrical correction method
[10].
9.1.4 Detector
Since it is difficult to cover the entire NIR region with a single detector, it is necessary
to select from the detectors shown below, depending on the wavelength band of
interest [11].
9.1.4.1 Silicone Photo Diode (Si)
Photovoltaic silicon (Si) photodiodes can be used in the NIR region close to visible
light (700–1100 nm: often called short NIR). Si photodiodes are suitable for quantitative analysis because of their wide dynamic range. The short NIR region is especially suitable for samples with high water content, so Si photodiodes are often used
in sugar-content meters of fruits. Since Si photodiodes respond to visible light, it is
necessary to use a wavelength cut-off filter to avoid indoor lighting and sunlight.
9.1.4.2 Indium Gallium Arsenide (InGaAs)
Indium gallium arsenide (InGaAs) is a photovoltaic device with a pn junction. The
sensitivity band varies depending on the composition ratio of In and Ga. The larger
the In:Ga ratio, the more sensitive it is to longer wavelengths. The standard sensitivity
range is 900–1600 nm, but it is possible to extend this to 2600 nm. The pn junction
type has a response speed of GHz, and high-speed measurement is therefore possible.