Page 239 - Photonics Essentials an introduction with experiments

Measurements in Photonics

Measurements in Photonics 233

grooves per millimeter and the second is the blaze wavelength. The
number of grooves per millimeter gives an indication of the possible
wavelength resolution of the grating. All other things being constant,
a grating having 1200 grooves per millimeter will have a higher reso-
lution than a grating having 600 grooves per millimeter.
The blaze wavelength is the wavelength for which the grating has
the highest diffraction efficiency. This can be determined from the an-
gle of the grooves relative to the grating surface. A specification of a
grating typically used in the characterization of GaAs lasers has a
blaze wavelength of 600 nm and 1200 grooves/mm.
The output spectrum of the grating depends on the wavelength. The
output spectrum of a light bulb depends on wavelength. When you use
a light bulb and a monochromator to create a tunable source of light,
the wavelength dependence of the output will be a combination of the
output of the light bulb and the grating in the monochromator. Most
measurements in optoelectronics are concerned with relative response
of a component as a function of wavelength. If the absolute optical pow-
er is required, a careful calibration of the light source and monochro-
mator must be made over the entire optical range of interest.
Typical transmission spectra of some gratings are shown in Fig.
10.6. These graphs are not a substitute for your own calibration, if
needed. They are useful to help in understanding that there are peaks
and valleys in the optical spectrum that are due to features of every
grating.
The grating shown in Figure 10.6a has a blaze wavelength of 300
nm, so it is designed to be used in the blue to ultraviolet part of the
spectrum. Note that if you were to use this grating to make a meas-
urement near 1200 nm, you would have to deal with a huge peak in
the transmission of the grating. You might mistake this transmission
artifact for something real.

10.5 Mirrors
The mirrors that we use for everyday applications are sheets of glass
coated with metal. The reflecting surface is protected by the glass
from possible damage during use, such as scratching, etc. There are
two reflections from such a mirror. One comes from the metal surface
and the other, which is somewhat weaker, comes from the front sur-
face of the glass. The mirrors used in optics experiments, including
the mirrors inside a monochromator, are front-surface mirrors. That
is, the metal, usually aluminum is coated on the front surface of the
glass. This procedure eliminates the second reflection. Front coating
comes at the price of having an exposed metal surface that is soft, eas-
ily scratched, and difficult to clean. To avoid leaving your fingerprints

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