270M
Rapid Scanning Imaging Spectrograph
8 Optical Interfacing Considerations:
8.1
Grating Selection:
Once the spectrometer itself is chosen, a key variable
in
determining the working
specifications
of
the system is the grating.
Groove density affects resolution
or
bandpass
or
coverage on an array detector. The
grating can often be chosen
to
enhance sensitivity over a range
of
wavelengths of
interest. There are gratings available to allow the 270M
to
be used from 185 nm to
50
pm
and beyond. In many cases resolution or throughput can
be
enhanced by
choosing a higher groove density. Or you may change the spectral coverage on your
array by selecting a different groove density than you are now using.
The blaze wavelength of a Ruled grating is its wavelength of peak efficiency. generally
the efficiency will stay stronger farther toward longer wavelengths than shorter.
Although Ruled gratings can be highly efficient, beware of the inevitable grating ghosts
that can reduce signal to noise ratios. This
is
not apparent from efficiency data. The
effect can be quite distracting when using a single grating system in conjunction with a
laser. Even worse, if the signal of interest is weak compared to the laser, as in Raman
spectroscopy, for example. The ghosts are caused by minute mechanically induced
errors in groove spacing. Even using state-of-the-art dual interferometer controlled
ruling engines cannot eliminate this problem entirely. Generally, the ghosts are most
severe in high groove density gratings, Ghost intensity increases as the square
of
groove density.
Most systems using Low density gratings for broad coverage
on
an array,
or
blazed
for the Infrared
do
not suffer significantly from ghosts. Ruled gratings remain the best
choice for these cases.
Standard Holographic gratings are created by optical holographic techniques, and as
a result, have
no
spacing error, hence no ghosts. They generally exhibit broader
response, at the expense
of
high peak efficiency. There are Holographic gratings
optimized for wavelengths from the UV
to
the near
IR.
The newer Blazed Holographic gratings provide the high efficiency comparable to that
of
a ruled grating without the ghosts. For many cases in the UV and Visible, these are
now
the best gratings available.
The growing list
of
available gratings is
too
extensive
to
describe adequately in this
manual. The gratings are also
too
important
to
system performance
to
make quick,
uninformed selections.
Revised
November 24,
1992
29
Leica Microsystems, Inc.'s Ex. 1033
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