New spectrometer design to
extend the wavelength range in the near infrared up to 1700nm.
The small Jobin Yvon H20 monochromator used in the
first version cannot be easily modified to change its wavelength
range so I have completely redesigned my microscope spectrometer. The
new monochromator is a 450mm focal length Cerny Turner design. The
optical components are coming from an old Perkin Elmer atomic absorption
spectrophotometer. A double grating back to back mounting is used. The
first grating is a Perkin Elmer 1440 groves per millimeter, it covers
the range 400-1100 nm. The second grating has been bought from Edmund
Optics, it is a 600 grooves / mm, it covers the range from 600 nm to
more than 2000 nm. The grating can be selected by the computer using an
auxiliary motor which turn the assembly by an angle of 180°. The grating
position is fined tuned by a stepper motor, computer controlled,
actuating the sine bar mechanism.
The light coming out of the monochromator passes through a home made
Nicol prism and is thus polarized. In this design, the Nicol prism
replaces the polaroid filters due to the fact that infrared light above
800 nm is no more polarized by conventional microscope polarizer. A
first solenoid order filter can be introduced into the light pass. This
orange filter rejects the second order of the first grating. The exit
lens focuses the light on the microscope stage. A motorized filter wheel
can select one of three additional filters into the light pass. A blue filter is
used between 400 and 500 nm to reject stray light because the
sensitivity in this wavelength region is rather low. A cyan filter is
used as an optical attenuator between 500 and 800 nm for the first
grating. The third filter
is a Kodak 87C filter used as a second order rejection for the 600 gr/mm
grating above 900 nm.
Two different detectors must be used to cover the wide wavelength
range. The first one was
already described, it uses a silicon photodiode. A second detector
has been build with a InGaAs photodiode for the range 600 to 1700 nm.
This detector has a 2 stages pre-amplifier with adjustable gain. The
power supply is external to both detectors.
The signal measurement is performed by an Escort digital multimeter and
is sent to the computer via a RS232 interface.
The complete spectrometer is represented
2: Light source enclosed in an aluminum
chimney and cooled down by a ventilator.
3: grating motion mechanism actuated by stepper motor not
shown. 4: motor used to change
the grating actuated by a relay computer controlled.
5: Filter wheel.
6: electronics to control filter
wheel, solenoid filter and motor changing the gratings and power
supplies. 7: power supply for
detectors pre-amplifiers. 8:
visible detector mounted on the microscope.
9: NIR: detector not mounted.
10: digital multimeter. 11:
White light source for microscope. The red line indicates
the light path between monochromator exit and microscope.