explained in a
page of this description of the Raman microscope, the use of the
Jobin Yvon H20 spectrograph was not an ideal solution because of the
field curvature aberration and vignetting which prevent a wide spectral
range to be recorded at a time. To completely demonstrate that a Raman
microscope could be completely home made with commonly available
mechanical and optical parts, I decided to rebuild the spectrograph to
improve the performances of the instrument. The cost of the spectrograph
described below is approximately one half of a new H20 monochromator
with better results.
decided finally to build the so called classical spectrograph with a
reflection grating as described in the literature for amateur
astronomical spectroscopy (see
This spectrograph as described in the drawing below uses two lenses and
a plane grating as optical elements. The advantage of lenses is the cost
and the ease of use but the main drawback is of course the presence of
achromatic aberration even if achromatic lenses were used. This
aberration has a minimal effect in Raman spectroscopy for minerals as the wavelength
range is usually limited close to the laser emission line.
The laser beam coming from the microscope enter the spectrograph through
the inlet slit. The slit is situated at the focal point of the
collimator lens so that a parallel beam of light is diffracted from the
grating. The camera lens forms an image of the spectrum onto the CCD
detector. The f/value of the exit beam from the microscope has been
measured approximately at f/13 so the spectrograph optics has been
calculated with a f/value of f/12 (see sheet below). The collimator has a focal
length of 400 mm for a diameter of 50 mm. The grating size is 50X50 mm.
It means that the opening of the input optics of this spectrograph is much larger than
required ( f/value = f/8). This facilitates the adjustment of the whole
optic system and prevents vignetting.
The angle between
the collimator axis and
the camera optic axis has been chosen at about 38° which is a common
value found in the published literature. The grating is a
ruled grating with 600 grooves per millimeter blazed at 750 nm giving
high intensity for a red laser.
The grating position can be adjusted with a screw to allow the user to
examine different spectral regions. In practice with the helium-neon
laser, the grating is used at two different positions: low range from
to 2000 cm-1 (~ 630 - 730 nm) and CH , OH band range from 2500 to
4000 cm-1 (~ 750 - 850 nm).
length of the camera lens is lower than the one of the collimator to concentrate
the light for a better signal and to adjust the wavelength range on the
camera sensor to a suitable value (100 to 2000 cm-1) for
Raman spectroscopy. This focal length is a compromise between
the availability of the lens, f/value ,the cost of the lens, the intensity of
spectra and the spectral window. Finally, I have chosen a photographic lens
to improve the quality of the spectrum image. The first version of this
spectrograph was working with a 100 mm achromatic NIR lens from Thorlabs
but the quality of images on the CCD was rather poor so the photographic lens seems
to be preferable as imaging element for this kind of spectrograph design. On the other hand,
for the collimator, the 400 mm achromatic lens is suitable because this
lens is only imaging a small object (the entrance slit) located on its
focal point so aberrations are minimized.
The calculation of the parameters of
the complete spectrograph is based on the excellent article and
spreadsheet of Christian Buil about astronomical spectroscopy. The results are reproduced below.