Thanks to rapid technology advancements in recent years, Raman spectroscopy has become a routine, cost-efficient, and much appreciated analytical tool with applications in material science and in-line process control for pharmaceutical, food & beverage, chemical and agricultural industries. Improvements in laser technology, detectors (CCDs and InGaAs arrays), and spectral filters (VBG-based notch filters), along with developments of new schemes for signal generation and detection, have aided Raman instrument manufacturers in overcoming the challenge of weak signals which has accelerated instrument development and market growth.
In this post, we outline the important performance parameters to consider when selecting a laser for Raman spectroscopy.
Six important laser performance parameters
- Laser wavelength:
- The most commonly used wavelength in Raman spectroscopy is 785 nm. It offers the best balance between scattering efficiency, influence of fluorescence, detector efficiency and availability of cost-efficient and compact, high-quality laser sources. However, the use of visible lasers in the blue and green (in particular at 532 nm) is increasing. It also depends on the material under investigation.
- Spectral linewidth:
- This sets a limit to the spectral resolution of the recorded Raman signal (i.e. how small of a difference in Stokes shift can be detected). For most fixed-grating systems, the laser linewidth should be a few 10 pm or less in order to not limit the spectral resolution of the system. However, high resolution systems may require linewidths much less than that, sometimes even less than 1 MHz.
- Frequency stability:
- The laser line must stay very fixed in wavelength during recording of the spectrogram in order not to deteriorate spectral resolution. Typically, the laser should not drift more than a few pm over time and over a temperature range of several degrees C.
- Spectral purity:
- Detecting the Raman signal normally requires a spectral purity of >60dB from the laser source (ie how well side-modes to the main laser line are suppressed). For many cases, it is sufficient if the level of spectral purity is reached at around 1-2 nm from the main peak. However, low-frequency Raman applications require a high side-mode suppression ratio (SMSR) of a few 100 pm from the main peak.
- Beam quality
- In confocal Raman imaging applications, it is necessary to use diffraction limited TEM00 beams for optimum spatial resolution. However, for probe-based quantitative Raman analysis, the requirement is not as tight. It is normally sufficient with a beam quality that allows for efficient coupling into multi-mode fibres, e.g. with 50-100 μm core diameters.
- Output power and power stability
- Typical laser output powers range from around 10 mW in the UV, up to several 100 mW in the near-IR. The output power requirement is related to the wavelength, the type of material(s) that will be investigated, as well as the sampling frequencies and imaging speeds.The output power of the laser should not fluctuate with more than a few %, also in a varying ambient temperature.
The different types of laser technology which can meet all these requirements can be grouped into 4 categories:
- Diode-pumped lasers: SLM (single-longitudinal mode)
- Single-mode diode lasers: DFB (distributed feedback) or DBR (distributed Bragg reflection)
- VBG frequency stabilized diode lasers
- Tunable single frequency lasers
Learn more by reading our full article or watching our webinar on How to select a laser for Raman spectroscopy: