A Raman spectroscopy system uses a laser to illuminate a target sample and a spectrometer to detect inelastic light scattering. The scattered light is displaced in wavelength from the laser emission because of contact with sample molecules.
The wavelength spectrum of the measured light then serves as a chemical fingerprint of the target sample. Increased light wavelengths result from energy loss during this interaction (e.g., Stokes-shifted), whereas decreased light wavelengths result from energy gain (e.g., anti-Stokes-shifted).
The signal-to-noise ratio (SNR) is an important statistic in a Raman spectroscopy system because it measures the quality of the Raman signal in comparison to the background noise. With a higher SNR, Raman peaks may be measured more precisely, allowing for more accurate peak positions, intensities, and ratios.
To maximize the SNR of a Raman spectroscopy system, the laser emission should have a narrower linewidth than the detector resolution. IPS' wavelength-stabilized external-cavity lasers are intended to have a small linewidth or spectral bandwidth.
A VBG serves as a wavelength-selective element, reflecting a restricted range of wavelengths back into the laser cavity while transmitting the remainder.
Without a laser line filter, a low-level broadband emission occurs due to amplified spontaneous emission (ASE) induced by band-to-band semiconductor recombination. This low-level light can increase detected noise, reducing the overall system SNR.
Laser line filters can be added to many IPS laser diodes and modules to prevent ASE while increasing SNR. They help isolate the intended excitation wavelength while eliminating background noise and undesired spectral components such as side modes.
Reducing these side modes yields a greater Side Mode Suppression Ratio (SMSR), which is advantageous in applications requiring spectral purity, such as optical communications, spectroscopy, or other precision measurement systems. SMSR is one component to consider while developing a system with optimal SNR.
Two examples of the benefits of implementing inbuilt laser line filters within the laser diode / module are provided below. Both examples are appropriate for Raman spectroscopy systems investigating longer-wavelength Stoke-shifted Raman light.
Adding a second laser line filter improves SMSR in the spectral area near the laser emission line, allowing for better measurement of low wavenumber Raman emission (e.g., < 100 cm-1).
The first example [Fig. 1] shows the suppression of long-wavelength emission in a 638 nm single spatial mode laser diode. This semiconductor laser diode has a greater intrinsic ASE (SMSR ~ 45 dB) due to a conventional anti-reflection (AR) coating on the front facet of the laser.
However, adding one or two laser line filters lowers the SMSR to more than 50 dB and more than 60 dB, respectively. In this example, the SMSR is lowered to more than 60 dB at 640 nm, corresponding to a Raman shift of 49 cm-1.
The second example [Fig. 2] depicts the suppression of long-wavelength emission in a 785 nm single spatial mode laser diode.
The semiconductor laser diode has a low-AR coating on the front facet, resulting in a lower intrinsic ASE (SMSR ~ 50 dB). Including one or two laser line filters lowers the SMSR to more than 60 dB and more than 70 dB, respectively.
In this example, the SMSR is lowered to more than 70 decibels at 787 nm, corresponding to a Raman shift of 32 cm-1.
The following are the various IPS laser/module possibilities, as well as the ability to add a single or dual laser line filter:
Source: m-oem
| Laser Line Filter |
Laser / Module Type |
| Single |
TO-Can, Butterfly, Stub, U-Type, M-Type, L-Type, Dual Laser, Dual Wavelength |
| Dual |
D-Type, Tethered Head, Integrated Raman Probe |
Figure 1. 638 nm laser emission intensity vs. wavelength with 0, 1, and 2 laser line filters used to suppress ASE. Image Credit: m-oem
Figure 2. 785 nm laser emission intensity vs. wavelength with 0, 1, and 2 laser line filters used to suppress ASE. Image Credit: m-oem
This information has been sourced, reviewed and adapted from materials provided by m-oem.
For more information on this source, please visit m-oem.