Operating Principle

Before we present the detailed assembly procedure of the setup, it is important to review together the different parts that compose the spectrometer. A base spectrometer with a standard liquid cuvette is shown in Figure 1.

The operating principle is as follow:

The laser module emits green collimated light that bounces on the steering mirror and reaches the dichroic mirror. The dichroic mirror has a two-fold effect: it reflects green and blue light but transmits yellow and red light. This is why a dichroic mirror is also sometimes called a dichroic beamsplitter, because it splits light into a reflected and transmitted component. Since the laser is pure green, it is almost integrally reflected into the cuvette holder (efficiency of reflection is about 98-99% in the base spectrometer).

The cuvette holder is an assembly that comprises a lens group dedicated to focus the light onto the sample as a very small spot of a few micrometers diameter. This is where the Raman effect occurs: a very small fraction of the green light from the laser is transformed into red or yellow light through a process called inelastic scattering. This effect is very small, only 1:10⁶ to 1:10¹⁰ of the photons are converted - depending on how prone the molecule is to the Raman effect. The exact color (wavelength) of the light re-emitted will bring information on the composition of the liquid analyzed.

Scattered light (both Raman and non-Raman) will be re-emitted in roughly all directions and a part of it will be caught by the lens group of the cuvette holder and will collimate again. The optical configuration used in OpenRAMAN is called a back-scattering configuration. Other configurations exist but this one is extremely robust in terms of alignment. The scattered light will be separated one more time by the dichroic mirror and the Raman components, along with a remaining 1-2% of green scattered light will pass through the mirror. Note that this ~1% is still much larger than the 1:10⁶ to 1:10¹⁰ we previously discussed and we have to get rid of it using an edge-pass filter which cuts down the remaining laser light by a factor of 1:10⁷. As both the dichroic mirror and edge-pass filter will slightly shift the beam laterally because they are thick pieces of glass, we introduce a compensation window to bring the signal symmetric to the optical axis before it hits the lenses group.

The lenses group will focus light on a thin slit of 50 µm × 2 mm and re-collimates the light exiting from the slit to feed it to the grating and imaging lens. The size of the slit controls the resolution of the spectrometer. The base version of the spectrometer uses a 50 µm slit but upgraded versions can use a slit as small as 15 µm.

The light will then be split by the diffraction grating into its various colors, just like a prism would do but in reflection. Light is then caught by the imaging lens and imaged on the camera as many replicas of the slits, one for each wavelength of the Raman signal.

It is important that you familiarize yourself with these terms before you proceed to the assembly. If you would like to know more about how the spectrometer works, please consult our technical articles.