Due to their advantages, double monochromators are one of the most frequently requested instruments for optical spectroscopy.
Compared to single monochromators, they can provide greater dispersion and exhibit much more effectivestray light rejection, which translates into a better signal-to-noise ratio (sensitivity) for luminescence spectrometers . It appears to be critical especially in the case of samples that exhibit low quantum yield or strong scattering.
Single vs. dual monochromator
| FLS1000-S2S2 | FLS1000-D2D2 | |
|---|---|---|
| Monochromators | single excitation, single emission | Double excitation, double emission |
| Suppression of unwanted stray light | 1:105 | 1:1010 |
| Focal length |
325 nm |
2 x 325 nm |
| Sensitivity (Raman water signal) | > 20000:1 | > 30000:1 |
| Number of emission ports | 2 | 3 |
Parasitic signal suppression with dual monochromator
For monochromators, the termStray light refers to the unwanted signal that passes through the monochromator along with the desired (set) spectral band. Its main sources include:
- Scattering and reflection of light on optical and mechanical elements inside the monochromator
- Higher diffraction orders created by the diffraction grating
- Poor instrument design
Modern monochromators now have an automatic carousel with filters built in as standard to remove higher diffraction orders. They also use collimating and focusing mirrors and dispersive elements with high quality and low levels of scattering. In addition, the parasitic signal can be significantly suppressed by using a dual monochromator (the first monochromator selects a spectral band that is additionally "cleaned" in the second).
Example of measuring the level of unwanted stray light in practice
Using a PTFE plate, the level of parasitic stray light was measured on the FLS980 luminescence spectrometer. For comparison, the spectrometer was first used in a single excitation and emission monochromator configuration, then replaced by a dual monochromator spectrometer. PTFE does not exhibit fluorescence and diffusely scatters over a wide spectral range. It is therefore an almost ideal material for scattering studies.
Additive and subtractive arrangements
Dual monochromators can be designed to operate in an additive or subtractive arrangement.
In the additive configuration, the spectral band exiting the first monochromator is re-exposed to spectral decay within the second monochromator. In this mode, the behavior of the instrument is analogous to a single monochromator with twice the focal length.
If the monochromator is designed to operate in the subtractive mode, the spectral dispersion at the output is zero. This is achieved by ensuring that the second monochromator no longer decomposes the spectral band that is split at the middle slit between the two monochromators. However, it does exhibit dispersion in the opposite direction, thus offsetting the two contributions. The result is then a spectrally uniform output.
A typical example of the use of subtractive double monochromators is picosecond time-resolved spectroscopy. In conventional monochromators, the transmitted pulses are broadened due to the realistic dimensions of the diffraction grating. A beam on one side of the diffraction grating will travel a shorter path than a beam on the opposite side (i.e. one side of the optical beam will be delayed relative to the other). In a subtractive arrangement, a second monochromator acts as a compensator, on whose diffraction grating the inversion process occurs. This eliminates time delay and time dispersion.
Source of the report:
TN5/2013/07, Edinburgh Instruments, Comparison of Stray Light Performance for FLS980 Spectrometers with either Single or Double Monochromators www.edinst.com
