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Upgrading the Spectrometer

The base spectrometer was designed as an introductory solution to Raman spectroscopy. As you move forward into spectroscopy, you may have refined requirements in terms of spectral resolution, dynamic range, or sample dilutions.

In this section, you will find instructions on how to upgrade your spectrometer to fit those needs. However, since no users have the same requirements, there is no ultimate upgrade and you will have to choose the one that best fits. Some of the upgrades might be easier to implement but are more expensive, others might be cheaper but may require more technical skills or have other form of trade-offs.

Since the laser plays an important role in Raman spectroscopy, you will find reviews of the different lasers we have tested. We encourage users who test other lasers to contribute as well to grow this list.

Resolution

Spectral resolution is bound to three parameters in the spectrometer:

\[r(\lambda ) \approx \sqrt{\left({psf\space fwhm}^2+{slit\space size}^2+{laser\space fwhm}^2\right)}\space\left[nm\right]\]

where

  • PSF FWHM is the full-width at half-maximum of the PSF of the imaging system.
  • Slit Size is the size of the image of the slit on the sensor.
  • Laser FWHM is the bandwidth of the laser.

In the base spectrometer, the laser is the limiting factor of the equation with a bandwidth of ~0.5 nm. To improve resolution of the spectrometer, it is important to improve on the bandwidth of the laser first.

All lasers have some longitudinal modes distributions which depends on their current and temperature. For instance, the default Thorlabs CPS532 that I have1 has the modal distributions shown in Figure 1.

Illustrations of Laser Modes

Figure 1 - Modal distribution of the CPS532 laser at two different temperatures

At ambient temperature, the laser has three distinct modes spread by about 0.4 nm. But at a higher temperature, here 30°C, the laser modes distribution change to a much narrower bandwidth of only ~0.1 nm. Multimodes lasers will typically produces peaks dedoubling, as shown in Figure 2 with iso-propanol. To avoid peak dedoubling, the slit is increased to a size such that it smoothes out the multiple peaks but the overall consequence is to blur the spectra.

Effect of Laser Modes on Raman Spectra

Figure 2 - Effects of laser modes on a Raman spectrum

Some lasers, such as the DJ532-40 illustrated in Figure 3, have operating conditions where only one mode subsist. These lasers are called single longitudinal modes lasers.

Single Mode Laser

Figure 3 - Example of single-mode laser

Depending on your applications, we recommend the following laser bandwidth and slit size:

Application Laser Bandwidth Slit Size Notes
Liquid <0.15 nm 15 µm Limited by natural broadening of peaks
Gaz single mode 5 µm Limited by imaging system PSF and spectrometer dispersion

  1. Modal distribution will depend on every laser so it is important to characterize the laser you are using. ↩