[Laser Technology Sharing] A brief discussion on light source selection in Raman spectroscopy applications

Raman spectroscopy technology, as an advanced detection and analysis method, is playing an increasingly important role in many fields. This technology obtains relevant information about the measured substance by analyzing the scattered spectrum of a monochromatic light source illuminated on the sample surface, thereby achieving application purposes such as component analysis and detection imaging.

Therefore, a key factor in Raman spectroscopy detection technology is the monochromatic driving light source. The development of early Raman spectroscopy detection technology was limited by the monochromatic light sources that could be used. Even the discovery of the Raman phenomenon itself was based on the monochromatic light source created by sunlight and narrow-band filters.

Nowadays, thanks to the rapid development of laser technology, related research on Raman spectroscopy technology has also made great progress. This article will briefly introduce the characteristics of laser light sources commonly used in Raman spectroscopy applications, hoping to provide some reference for everyone.

Figure 1. Energy level diagram composed of Raman spectrum

Figure 2. Raman spectrum imaging of human esophageal tissue

1. Wavelength selection

It is currently known that the spectrum of light sources that can be used in Raman spectroscopy covers a very wide range, from UV, visible light to NIR, and specific application fields can be found. So how to choose the appropriate wavelength for a specific material?

Generally speaking, the Raman signal to be measured is very weak compared to the irradiation laser. Therefore, in order to obtain accurate measurement results, it is necessary to obtain high-brightness and high-resolution scattering signals.

First of all, the intensity of Raman scattering is inversely proportional to the fourth power of the irradiating laser wavelength. Therefore, unless there are special materials, it is not recommended to choose a long-wavelength laser as the driving light source.

However, short-wave driving light is not perfect. In addition to generating Raman signals after being excited by UV light, many materials are also accompanied by quite strong fluorescence emission, which submerges the already weak scattering signals and greatly reduces the detection accuracy of Raman signals.

In this case, some measures should be taken to filter out or avoid the effects of fluorescent radiation.

When the fluorescent radiation is insensitive to the driving light, a beam of light with the same conditions as the original driving light except for a slight difference in wavelength can be used as the reference light.

After irradiating the sample with two beams of light under the same conditions as possible, the two spectral results obtained are subtracted. This can eliminate most of the wavelength-insensitive fluorescence signals. This method is called: shifted excitation Raman difference spectroscopy (SERDS)

For some materials, the influence of fluorescence radiation can be avoided by correctly selecting the wavelength of the driving light. As shown in Figure 3, the driving light, Raman signal, and fluorescence radiation can be clearly distinguished according to the wavelength band. As long as the appropriate filter is selected, an effective Raman signal can be obtained.

Figure 3. Obtain high-resolution Raman signal using appropriate driving light source

2. Laser parameters

In addition to wavelength, there are a number of important performance parameters that should be considered when selecting the best laser source for Raman experiments. These key performance parameters include: spectral linewidth, frequency stability, spectral purity, beam quality, output power and power stability, and optical isolation.

1. Spectral linewidth:

The resolution of the recorded Raman spectra is mainly limited by the spectral linewidth (the spectral linewidth determines the accuracy with which the Stokes shift can be detected). For most fixed grating systems, in order not to limit the spectral resolution of the system, the laser linewidth should be tens of pm or shorter, while for some high-resolution systems the linewidth is even less than 1 MHz.

2. Frequency stability:

Generally, the laser output frequency may change with the passage of time and ambient temperature jitter. To avoid the reduction of the measurement resolution of the Raman spectrum, the frequency drift of the laser should be less than 10pm.

3. Spectral purity:

Detecting Raman signals usually requires the spectral purity of the laser source to be no less than 60dB (so that the influence of side mode components can be ignored). In most cases, spectral purity should meet the requirements within the range of 1 to 2 nm of the main peak. For low-frequency Raman applications, a higher side-mode suppression ratio (SMSR) is required, and the spectral purity needs to meet the requirements within a few hundred pm or less of the main peak.

4. Beam quality:

In confocal Raman imaging applications, the laser source needs to be a diffraction-limited TEM00 mode beam to obtain the best spatial resolution. For probe-based quantitative Raman analysis, this parameter requirement does not need to be too strict. The beam quality only needs to allow normal coupling into multimode optical fiber (core diameter 50-100μm).

5. Output power and power stability:

The laser output power is limited by the selection of wavelength, which is mainly based on the type of material, sampling frequency and imaging speed. The output power of laser light sources generally used for Raman spectroscopy experiments is around 10mW-100mW (depending on the wavelength), the power jitter should be less than 10%, and it should be as unaffected by the external ambient temperature as possible.

6. Beam isolation:

The design of laser return light isolation is particularly important in confocal imaging devices, because the sample can often reflect the excitation light back along its original path. This light return can cause noise and power instability, and even cause damage to the laser. Therefore, the laser should be equipped with an optical isolator at the output end to prevent the above phenomenon from occurring.

3. Commonly used laser types

1. Diode-pumped SLM lasers (DPL lasers)

This type of laser can cover the spectral output range from UV to NIR based on a built-in nonlinear frequency conversion module. The maximum power can reach up to W level (1064nm), can reach more than 100mW in the visible light band (660, 640, 561, 532, 515, 491, 473, 457nm), and can output 10-50mW (355nm) in the UV band.

Single longitudinal mode semiconductor-pumped solid-state lasers used for Raman spectroscopy applications can achieve beam quality close to the TEM00 mode. The wavelength is stable and almost drift-free, the linewidth is much less than 1MHz, and the spectral purity (SMSR) can reach more than 60dB in the main peak pm range.

2. Single-mode diode lasers

Single-mode semiconductor lasers used for Raman spectroscopy applications are very compact and relatively low-cost, but their parameters are slightly inferior to single-longitudinal mode semiconductor-pumped solid-state lasers. The commonly used output wavelengths are: 785, 830, 980, 1064 nm, the line width is in the MHz level, and the output power is also low. The spectral purity (SMSR) can reach about 50dB in the range of 100 pm of the main peak.

3. VBG frequency stabilized diode lasers based on volume Bragg grating technology

This type of laser integrates a volume Bragg grating device into a semiconductor laser, so that high-power multi-mode semiconductor lasers can also provide narrow linewidth output. When used with filters, the spectral purity (SMSR) of this type of laser can reach about 60-70dB within the 1-2nm range of the main peak.

References

[1] https://en.wikipedia.org/wiki/Raman_spectroscopy

[2] https://www.laserfocusworld.com/whitepapers/2018/04/have-i-selected-the-right-laser-for-raman.html