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Technical description of the µ-Raman system


Design specifications

Laser wavelength: 532 nm
Resolution target: 4 cm-1
     corresponds to: 0.11 nm
Range: 1500 cm-1


Raman.tif


Microscope

The microscope chosen for the Raman project is a Leitz Orthoplan of the last generation. It is equipped with a rebuild LAF incident light illuminator which houses the halogen lamp for sample positioning under incident light and a manual laser shutter. A Leitz NPL 50x/0.85 P or a Plan L 50x/0.60 long working distance objective is used for the actual measurements.

Laser

The laser system used is a Coherent Compass 315M-20 diode pumped solid-state laser. It runs 20 mW at 523 nm in TEM00 and single longitudinal mode with a beam diameter of 0.32 mm. This laser was chosen because of its high quality, extreme stability and reliability. The laser is controlled by a control panel which also allows shut-down of the switch mode power supply and control of the lamp for incident light illumination. The laser head is mounted on a Leitz micromanipulator for beam adjustment, basically a xyz-stage allowing an additional angle adjustment. The laser runs through a Thorlabs BE05M-A beam expander which increases the beam diameter from 0.32 mm to 1.6 mm. The larger beam diameter allows a better z-resolution of the raman spectrum and decreases the energy density on the optical components.

Laser


Longpass filter

The high frequency stability of the laser allows the use of the ultrasteep razor edge filter LP03-532RE-25 manufactured by Semrock to block the laser wavelength from the spectroscope. The filter data is impressive:

Transmission Band: Tavg > 93% for 535.4 - 1200 nm
Edge Wavelength: 533.3 nm
Transition Width: 2.7 nm / 90 cm-1
Edge Steepness: 1.1 nm / 37.5 cm-1


Typical spectrum for the LP03-532RE-25 filter in the transition region:

Semrock performance


Spectroscope

The Lhires III (Littrow High Resolution) spectroscope is originally intended for the use with telescopes in astronomy. The Lhires III has a build-in neon glow lamp for calibration which is powered by a Mean Well GS15B-3P1J (12V, 1.25 A) switch mode power supply. The Lhires was chosen due to its high resolution and the possibility to change the gratings quickly as they come in modules. Additionally the slit is etched in a mirror which can be observed with an ocular or a second camera through the guiding port, a very useful help in positioning the laser spot in the slit. The slit mirror has four different slit widths. The current set-up has a 35 µm slit and a 1200 l/mm grating. The slit holder was modified to allow a small amout of side movement for alignment. Unlike all of the small desktop spectrometers, the grating position can be changed and adjusted so that the range of 1500 1/cm with a 1200 l/mm grating can be placed over the entire spectrum.

.....Clipboard01.tif

(modified after: http://www.astrosurf.com/thizy/lhires3/index-en.html)

Relay lens system

The slit is deep inside the spectroscope as it is build for f10 telescopes. The adaption is designed to get as most light of the microscopic picture into the slit as possible. It reduces the focal length of the telan lens with a 19 mm achromatic doublet, then uses a 75 mm achromatic doublet pair to transfer the reduced image to the slit. The transfer optics is countains two of the Semrock LP03-532RE-25 filters (blue).

.....Adapter w filters


The simulation with Qioptic’s WinLens shows vignetting with the 19 mm lens and a 10 mm aperture of the telan lens. In reality no negative effects were observed, likely the raman scattering is concentrated in a smaller spot. The adapter uses parts from Thorlabs SM1 series. The adapter assembly is mounted at the trinocular head with a c-mount to SM1 adapter.

Camera

The camera used is a Atik 460EX monochrome CCD with a cooled sensor. The camera is powered by a Mean Well GS15B-3P1J (12V, 1.25 A) switch mode power supply.

Camera data

Sensor Type:

Sony ICX694
Resolution 2750 x 2200 pixels
Pixel Size: 4.54 µM x 4.54 µM
ADC: 16bit
Readout Noise: 5e- typical value
Interface: Mini-USB 2.0 High Speed
Maximum Exposure Length: Unlimited
Minimum Exposure Length: 1/1000 s
Cooling: Thermoelectric, max ΔT=-25°C


Lightpaths through the system

Raman 3 text.tif

Raman 2_text.tif