Section 16.6: Spectrophotometers
Spectrophotometers measure the transmission and refl ectivity of optical
components and are essential for characterizing the performance of
optical coatings (Figure 16.10). A typical spectrophotometer consists of a
broadband light source, a monochrometer, and a detector (Figure 16.11).
Light from the light source is sent into the monochrometer’s entrance
slit where it splits into its component wavelengths using a dispersive
element such as a diff raction grating or prism. The monochrometer’s
exit slit blocks all wavelengths except for a narrow band that passes
through the slit, and that narrow wavelength band illuminates the test
optic. Changing the angle of the diff raction grating or prism changes the
wavelengths that pass through the exit slit, allowing the test wavelength
band to be fi nely tuned. Light refl ected or transmitted through the test
optic is then directed onto a detector, determining the optic’s refl ectivity
or transmission at a given wavelength.
The light source must be incredibly stable and have adequate intensity
across a broad range of wavelengths to prevent false readings. Tungsten
halogen lamps are one of the most commonly used light sources for
spectrophotometers because of their long lifespan and ability to maintain
a constant brightness.8 Multiple light sources covering diff erent
wavelength ranges are often used if a very broad total range is required.
The smaller the width of the monochrometer’s slits, the higher the spectral
resolution of the spectrophotometer. However, reducing the width
of the slits also reduces the transmitted power and may increase the
reading acquisition time and amount of noise.1
A wide variety of detectors are used in spectrophotometers as diff erent
detectors are better suited for diff erent wavelength ranges. Photomultiplier
tubes (PMTs) and semiconductor photodiodes are common detectors
used for ultraviolet, visible, and infrared detection.8 PMTs utilize
a photoelectric surface to achieve unmatched sensitivity compared to
other detector types. When light is incident on the photoelectric surface,
photoelectrons are released and continue to release other secondary
electrons, which causes a high gain. The high sensitivity of PMTs is benefi
cial for low intensity light sources or when high levels of precision are
required. Semiconductor photodiodes such as avalanche photodiodes
are less expensive alternatives to PMTs; however, they have more noise
and a lower sensitivity than PMTs.
The smaller the width of the monochrometer’s slits, the higher the spectral
resolution of the spectrophotometer. However, reducing the width
of the slits also reduces the transmitted power and may increase the
reading acquisition time and amount of noise.1
While most spectrophotometers are designed for use in the ultraviolet,
visible, or infrared spectra, some spectrophotometers operate in more
demanding spectral regions such as the extreme ultraviolet (EUV) spectrum,
with wavelengths from 10 - 100nm. EUV spectrophotometers typically
use diff raction gratings with extremely small grating spacings to
eff ectively disperse the incident EUV radiation.
Section 16.7: Group Delay
Dispersion Measurement
In addition to measuring surface roughness, white light interferometers
are used to measure the group delay dispersion (GDD) of both refl ective
and transmissive optical components. GDD is critical to the performance
of ultrafast laser optics, as the short pulse duration of ultrafast lasers
leads to signifi cant chromatic dispersion in optical media. More information
on GDD and ultrafast optics can be found in Section 13: Ultrafast
Dispersion pages 40-41.
Interferograms reveal signals whenever the optical path lengths of the
two arms become identical, and the exact position at which this occurs
is wavelength dependent. This allows for the optical path length diff erence
between diff erent wavelengths to be precisely determined, revealing
the test optic’s GDD (Figure 16.12).
56 +44 (0) 1904 788600 | Edmund Optics®
193nm 45°
90
Unpol
80
S (%)
70
60
50
P (%)
40
30
20
10
0
160 170 180 190 200 210 220 230
R (%)
(nm)
Figure 16.10: Sample refl ectivity spectrum of a TECHSPEC® Excimer
Laser Mirror captured using a spectrophotometer
Light Source
Detector
Test Optic
Test Optic
Grating
Grating
Detector
Light Source
Figure 16.11: The test wavelength of a spectrophotometer can be fi nely
tuned by adjusting the angle of the diff raction grating or prism in the
monochrometer
GDD (fs²)
-300
-500
-700
-900
-1100
-1300
-1500
7° AOI 1030nm Highly-Dispersive Ultrafast Mirrors
1010 1020 1030 1040 1050 1060
(nm)
S-Polarization, 7°
Figure 16.12: Plot of GDD vs. wavelength for a highly-dispersive ultrafast
mirror obtained using white light interferometry