Insights from industry

How Lasers are Used in Modern Raman Spectroscopy

In this interview, Håkan Karlsson, CEO and Co-founder of Cobolt AB,  talks to AZoM about using lasers in Raman Spectroscopy.  Håkan Karlsson also explains how in recent years, with rapid technology advancements, Raman Spectroscopy has become a routine, cost-efficient, and much appreciated analytical tool within material science.

Please tell us a bit about Cobolt’s contribution to Raman spectroscopy?

Cobolt AB is one of the leading suppliers of high performance diode-pumped lasers and laser diodes in the UV, visible and near-infrared spectral ranges.

Cobolt is recognized on the market for providing a very broad coverage of compact, high performance, continuous wave lasers with reliable single-frequency or narrow-linewidth performance. Thanks to the company’s proprietary HTCure™ manufacturing technology, the lasers are exceptionally robust and reliable being basically insensitive to environmental and mechanical shock.

It’s the inherent performance characteristics of the laser design along with the proprietary manufacturing technology which have led to Cobolt’s recognition as one of the leading laser suppliers to the Raman industry.

Cobolt was founded in 2000 and is head-quartered Solna, Sweden, where the company’s products are developed, designed, and manufactured in a modern and ISO 9001 certified clean-room factory.

The facility is specially designed for volume manufacturing of advanced lasers and laser systems. The company has direct sales offices in Sweden, Germany and the USA. Since December 2015, Cobolt AB is part of HÜBNER Photonics.

Cobolt 04-01, 05-01 and 08-01 Series of high performance CW lasers for Raman spectroscopy

How are lasers used in modern Raman Spectroscopy?

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 the pharmaceutical industry for instance, Raman instrumentation is nowadays used across the whole value chain: High-resolution Raman microscopes, which are capable of imaging at single-biomolecule level, aid researchers in the development of new drugs. Hand-held Raman instruments are used at pharmaceutical companies to inspect the purity of incoming raw material used in drug manufacturing.

Probe-based Raman systems are used to monitor the mixing of constituents in drug manufacturing. Raman microscopes are used to monitor the quality of compound distribution in produced medical pills and, finally, portable Raman instruments are used to detect and identify counterfeit drugs in the field.  

Examples of compound distribution in medical pill and label-free analysis of micro-organisms using Raman Spectroscopy.

The laser is the principle source for creating the weak photon-phonon interactions that result in the Raman signal. The required wavelength, power and performance specifications are dependent on the material being investigated, the desired resolution and the ultimate purpose for the system. In all cases, laser reliability is paramount.

Why is the choice of laser wavelength important for Raman Spectroscopy?

A number of different wavelengths are commonly used in Raman Spectroscopy, ranging from the UV, over the visible, and into the near IR. Choosing the best illumination wavelength for a given application is not always obvious. Many variables must be considered in order to optimize a Raman Spectroscopy experiment, many of which are connected to the wavelength selection.

To start with, the Raman signal is inherently very weak. It relies on the photon-phonon interaction in the sample material, which is typically a one-in-a-million event. In addition, the Raman scattering intensity is inversely proportional to the 4th order of the illumination wavelength, which means that illumination at longer wavelengths results in a decreased Raman signal.

The detection sensitivity is also dependent on the wavelength range. CCD Devices are commonly used for the Raman signal detection. The quantum efficiency of these CCD devices rolls off fairly quickly beyond 800 nm. For illumination beyond 800nm, it is possible to use InGaAs array devices, but those are associated with higher noise levels, lower sensitivity and higher cost.

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 trade-offs involved in choosing the optimum wavelength depends heavily on the specific application. It is therefore often desirable to have access to a selection of various laser wavelengths in the same Raman instrument, preferably with similar optical performance and similar user interfaces.

What performance characteristics should be considered when selecting a laser for Raman spectroscopy experiments?

In addition to wavelength, there are a number of important performance parameters that should be taken into account when choosing the best laser sources for a Raman spectrometer. Key performance requirements are the spectral linewidth, frequency stability, spectral purity, beam quality, output power and power stability, and optical isolation. In addition, the compactness, robustness, reliability, lifetime and cost structure should be considered.

Laser performance   parameters                                                      Considerations
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 oC.

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.

Isolation of laser beam

In confocal imaging set-ups in particularly, as the samples may generate optical feed-back which is very well-aligned with the excitation beam. Optical feed-back can induce power and noise instabilities and, in the worst case, cause permanent damage to the laser. It is normally preferred to have an optical isolator integrated directly in the laser source itself, because careful alignment is required to achieve high stability in the output after the isolator.

Finally, the compactness, robustness, reliability, lifetime and cost structure are very important parameters to be considered in the selection of the optimum illumination source for a Raman system. Raman instrumentation has progressed into becoming a standard analytical tool in many scientific and industrial applications. Users expect to run routine experiments or process monitoring measurements for years without the need for service or exchange of the laser source. In a growing number of cases, the instrument must also operate in harsh, industrial environments.

For these reasons, most Raman systems are nowadays equipped with solid-state based laser sources rather than the aforementioned gas lasers. Today, compact solid-state lasers with proven operation lifetimes of several 10,000 hours which meet the most advanced optical performance requirements are available at Cobolt in all wavelength ranges commonly used for Raman spectroscopy.

What types of lasers are commonly used for Raman spectroscopy and how do you decide which one is best for your application?

Solid-state based CW laser sources that are commonly used for Raman spectroscopy can be grouped into three categories:

  1. Diode-pumped SLM (single-longitudinal mode) lasers
  2. Single-mode diode lasers; DFB (distributed feedback) or DBR (distributed Bragg reflection)
  3. VBG frequency stabilized diode lasers

These laser technologies cover different wavelengths regions and have significant differences in optical performance.

Diode-pumped SLM lasers (DPL lasers) combined with built-in nonlinear optical frequency conversion are readily available in compact formats from the UV to the near-IR. Up to Watt power levels are achievable at 1064nm in the near IR. In the visible range, a large number of lines in the blue-green-red region (660, 640, 561, 532, 515, 491, 473, 457 nm) are available and with output powers on the scale of several hundred mW. Lower power levels are achievable in the UV, such as 10 to 50 mW at 355nm.

These lasers provide excellent TEM00 beams, very precise wavelengths with low drift and a single-frequency linewidth of typically far less than 1 MHz. These lasers also offer very high level of spectral purity with typically well >60dB SMSR up to within pm of the main peak. There may be occurrence of low level emissions at neighbouring laser lines, but they are several nanometers shifted from the main peak and therefore easily eliminated by integrating a dielectric band-pass filter.

Typically TEM00 beam profile and the very clean spectral characteristics of a Cobolt 532 nm DPL SLM laser.

Spectral linewidth characterisation and wavelength & power stability of a Cobolt 532 nm DPL SLM laser.

Single-mode diode lasers provide very compact and cost efficient illumination sources with single-frequency linewidth (<1 MHz) and single-transversal mode beam quality. A number of wavelengths are available in the red to near IR, with output powers up to a few 100 mW and MHz linewidth. The most commonly used wavelengths are 785, 830, 980 and 1064 nm. Side-band emission limits the SMSR of these lasers to around 50 dB, which is normally achieved at a few 100 pm away from the main peak.

The third group of laser sources for Raman is VBG frequency stabilized diode lasers. In these lasers a narrow linewidth Volume Bragg Grating (VBG) element is used with a diode laser emitter to achieve narrow-line emission at wavelengths that are not available as DFB or DBR sources. It is also possible to achieve narrow linewidth emission at higher power levels by frequency locking multi-transversal mode diode lasers.

Spectral linewidth stability in varying temperature and spectral characteristics with (yellow) and without (pink) integrated bandpass filter from a 500 mW Cobolt 08-NLD laser at 785 nm.

Careful thermo-mechanical control and high precision alignment inside of the laser is required to achieve high stability in the output wavelength and linewidth, especially with varying ambient temperatures. Linewidths range from single-frequency emission to a few 10 pm, depending on wavelength and output power. As with other diode lasers, the SMSR is limited to 40-50 dB close to the main peak. However, this can be improved to 60-70 dB at 1-2 nm away from the main peak by integrating a filter.

What are the features and benefits of Cobolt’s laser technologies, and how do they serve Raman Spectroscopy applications?

Cobolt serve the Raman industry with both Diode-pumped SLM lasers (DPL) and VBG-stabilized diode lasers. Both laser types require extremely high level of precision in the mounting and fixation of separate optical elements. Therefore, reliable performance from such lasers can only be achieved if they are assembled with advanced optical manufacturing technology, and with proven resilience to thermal and mechanical stress.

All of Cobolt’s DPL lasers and VBG-stabilized diode lasers are manufactured with the company’s proprietary HTCure™ Technology, which relies on high temperature baking of miniaturized optics high-precision mounted into thermo-mechanically stable and hermetically-sealed packages. This approach has proven to provide highly reliable lasers that can tolerate repeated thermal shocks over 100oC and mechanical shocks over 60G.

The Cobolt 08-01 Series of compact SLM and narrow-linewidth lasers is specifically designed for Raman spectroscopy applications. The same product platform hosts Diode-pumped SLM lasers (08-DPL) and VBG-stabilized diode lasers (08-NLD) in compact and hemertically-sealed packages.

The modules have fully integrated electronics and optional optical isolators. The output beam is collimated free-space or coupled into a multi-mode fiber (typically 50-100 µm core). Available wavelengths are 405, 457, 473, 515, 532, 561, 660, 1064 and 785 nm.

Cobolt 08-01 Series of compact SLM and narrow-linewidth lasers for Raman spectrosocpy.

About Håkan Karlsson

Håkan Karlsson is CEO and Co-founder of Cobolt AB. He has a PhD in Physics from The Royal Institute of Technology in Stockholm, Sweden.

Disclaimer: The views expressed here are those of the interviewee and do not necessarily represent the views of AZoM.com Limited T/A AZoNetwork the owner and operator of this website. This disclaimer forms part of the Terms and conditions of use of this website.

Mychealla Rice

Written by

Mychealla Rice

Mychealla graduated from Northumbria University in Newcastle with a 2:1 in Journalism with English Literature. Mychealla is a keen traveller, spending time in Australia, Thailand and Italy. Mychealla plans to see more of Europe in the future. Mychealla’s interests include photography and music. In her spare time, she likes to go shopping and visit family and friends back in Ireland.

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