For the reason described above, I have added at the laser exit the
Edmund optics spatial filter (3)
with the pinhole removed and replaced by a second short focal range
objective especially designed for He-Ne laser. Both objectives have the same
focal length. The laser beam is focused between the objectives and can
be made slightly divergent by adjusting the position of the second lens.
It improves the focus of the beam which has a diameter of 2 to 5µ with
the 50X NA 0.85 microscope objective.
The beam splitter assembly (7)
and laser rejection filter (8)
are described in more details on next page. As seen on the
photograph above, this assembly is now part of the microscope column. To
do that it is absolutely necessary to use an infinity corrected
microscope's optics so accessories can be introduced between objective
and microscope head without disturbing the image. The height of the beam
splitter head is around 15 cm which is outside the specifications given
by Olympus. Even in those conditions, I did not notice any significant
The second flip mirror (10) is
equipped with a small neon bulb (9)
for reference and to the left by a photo eyepiece to refocus the light
onto spectrograph entrance slit. At the end of the beam splitter tube, a
removable cylinder (an astronomical accessory from Orion) gives access
to the entrance slit. With the help of an external lens
(20) and the
(19), the focused spot can be precisely centered on the
entrance slit by moving slightly the front of the spectrograph.
The spectrograph (12) used in the
present device is certainly not optimized but as I said before, I tried
to use available components as far as possible to demonstrate the
possibilities of Raman microscopy before optimizing all devices.
The Jobin-Yvon H20 used in this experimental arrangement is not a
spectrograph but a scanning monochromator with entrance and exit slit.
In normal use, the image of the entrance slit is formed in the plane of
the exit slit. I have removed the exit slit to record a band spectrum
and not a single wavelength in the monochromator mode. It was not
possible for mechanical reasons to place directly the CCD detector
(16) in the image plane of the
spectrograph. On the other hand, the direct imaging on a CCD detector
with such a spectrometer optics is not suitable because the image
provided by the concave grating is too large so only a small portion of
the spectrum can be recorded at a time ( about 35 nm) and the image
intensity is too faint for Raman spectroscopy. To overcome this problem,
a demagnification with an additional lens was necessary. I have
chosen to photograph the spectrometer image ( virtual object ) as
usually done in photo macrography that is with a bellow
(14b) and macro lens
(15) (Zuiko makro f2 for OM system).
The biggest drawback of this optics is the vignetting produced by both
bellows (14a and b). I'll describe
this limitation later when presenting the first results.
The CCD (16) detector is an astronomical camera cooled by Peletier
around 30°C below ambient temperature ( Atik 16HR equipped with a SONY
ICX285 CCD sensor 1392x1040 pixels ). I used this device first to make
some astronomical pictures of faint nebulae. As it allows to record
images of faint objects barely visible with a telescope in the light
polluted sky of Belgium, I decided to try it for Raman spectrography. I
have reported below an example image I have made with this camera with a
narrow band H alpha filter of such a faint object. (Rosette nebula).
With such a camera, it is possible to make binning of the pixels that is
to add the charge of some adjacent pixels directly on the CCD chip to
improve the signal to noise ratio. I'm using a 8x1 binning to add 8
pixels in a column. This is necessary again due to the low intensities
of Raman spectra.
After setting up the components, the optics must be aligned
starting from the laser output. Both objectives of the spatial filter
(3) must be aligned vertically with
the 2 screws provided so that optic axes are perfectly collinear. The dichroic beam splitter
(7) should make an
angle of 45° with the laser beam in order to minimize transmitted laser
intensity and thus maximize reflection. The position of the laser head
is then adjusted (1c, 1d, 1e) so the beam
is focused at the center of microscope stage
(6). The first step is to bring the laser beam at the center
of the field
with the microscope objective being removed. Then in succession,
microscopes objectives of increasing power can be used with an
adjustment of the laser beam at each step. Finally, the beam width is
adjusted with the spatial filter micrometric screw to minimize laser
With a properly aligned laser beam, the spectrometer can be tested with
a high Raman scatterer. For that purpose, I'm using a section of the mineral crocoite (a lead chromate) which gives a very high Raman effect, more
than sulfur. With this mineral, a Raman spectrum is collected in a few
tens of a second so the spectrometer alignment can be set in real time.