Which 785 nm laser for Raman spectroscopy?

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. To understand more on how the choice of wavelength can affect the Raman spectroscopy measurements, see our post on How to choose lasers for Raman Spectroscopy. In this post, we introduce the different laser technologies available at 785 nm and discuss how the different technologies perform in relation to the critical performance specifications to consider when selecting lasers for Raman spectroscopy.

Laser performance parameters

There is a number of different kinds of lasers available at 785 nm. They offer different performance and cost characteristics, which means a careful selection can be important to find the best solution for a particular Raman spectroscopy set-up.

Critical performance parameters are:

1. Spectral bandwidth
   • This should be less than a few 10s of pm in order not to limit the resolution of the system. In some high resolutions applications a linewidth of much less than that can be required
2. Spectral purity (or side-mode suppression ration – SMSR):
   • Of the illumination source should be at least better than 60 – 70 dB in the spectral region where Raman peaks are to be detected
3. Wavelength stability:
   • Must be low, in the order of few pm, both over time and temperature in order not to limit the resolution of the system
4. Beam quality:
   • For high resolution imaging applications it is important with a high quality TEM00 transversally single-mode beam profile. (For probe-based systems multimode beams work, as long as they can be efficienly coupled into fibers of 50-200 µm core sizes.)

Technologies for lasers around 785 nm

There are a number of different lasers types available at 785 nm and commonly used for Raman Spectroscopy, each having their own strengths and weaknesses depending on the application:

1. 785 nm DFB or DBR single-mode diode lasers:

• These are single transversal mode semiconductor emitters with a DBR (distributed bragg reflector) structure integrated with the gain structure on the chip itself. They can be purchased as a single laser chip or packaged ready for use. They can offer very narrow linewidths and even single-frequency performance but are limited in output power to a few 10s of mW. SMSR is normally only 30-50 dB even several nm form the main peak, meaning that a spectral clean-up filter must be used to make them suitable for Raman spectroscopy. The compact size makes them suitable for small hand-held systems.

1. 785 nm frequency stabilized diode lasers:

• Frequency stabilized diode lasers are based on high power Fabry-Perot semiconductor diode lasers with an external grating structure (typically a VBG – Volume Bragg grating) for frequency-locking the emission to a narrow linewidth. The approach works with both single transversal (TEM00) and multi transversal mode emitters and results in laser emission with linewidths of a few 10s of pm down to single-frequency performance. Output powers range from just over 100 mW for single transversal mode lasers to Watt level lasers for multi mode. TEM00 lasers are suitable for high spatial resolution Raman microscopy applicfations whereas higher power multi-mode lasers are often fiber-coupled and suitable for probe-based process control applications. Like with the DBR lasers it is usually required to combine these lasers with a spectral clean-up filter to achieve sufficient SMSR for high quality Raman spectroscopy results. With the dichroic spectral clean-up filters it is possible to achieve a spectral purity of >60dB is at around 1-2 nm from the main peak, which is sufficient for detection of Raman shifts in the finger-print spectral region of 200-40000 cm-1. However, low-frequency Raman applications, in the region <200 cm-1, require a high side-mode suppression ratio (SMSR) a few 100 pm from the main peak, which is achieved with the Cobolt 08-NLD(M) ESP 785 nm lasers. These lasers have, thanks to a unique design, an enhanced spectral purity (ESP) and are very suitable for Raman spectroscopy with spectral shifts in the low-frequency or THz region. See Cobolt 08-01 Series.

3. 785 nm diode-pumped lasers

• New advanced laser technology has enabled developmen of diode-pumped lasers at 785 nm This type of lasers can generate up to >500 mW of output power in a perfect TEM00 beam, a spectral linewidth of <50kHz and a wavelength drift of a fraction of a pm over hours of operation. This diode-pumped laser also has a spectral purity of >80dB at <0.5 nm from the main peak thanks to the absence of amplified spontaneous emission (ASE), which altogether makes this 785 nm laser technology suitable for demanding high spectral and spatial resolution Raman spectroscopy applications. See Cobolt 05-01 Disco.