TE-Cooled CPS532
Using this upgrade requires using a TEC driver. Browse our tooling page for various compatible TEC drivers.
The base spectrometer comes with a Thorlabs CPS532 low-cost, 4.5 mW, 532 nm DPSS laser. By default, this laser has multiple longitudinal modes with a bandwidth that strongly depends on temperature as shown in Figure 11.
Figure 1 - Lasing modes as a function of temperature
In the ambient temperature range, we have found our laser to have a bandwidth of 0.4 nm but only 0.1 nm bandwidth at temperature higher than 30°C2. Although the results of Figure 1 are valid only for our specific laser unit, Thorlabs reports similar conclusions on a different, older, batch such that the results of Figure 1 can be considered typical for the CPS532.
Figure 2 shows an upgrade to heat the CPS532 lasers in a reproducible way. This upgrade must be combined with a TEC driver such as those proposed in our tooling section.
Figure 2 - TEC laser module for Thorlabs CPS532
Thermal control of the laser housing is done using Thorlabs TECD2S peltier element but any compatible peltier will do the job as well. The “hot” side of the peltier is cooled by natural convection using a heat sink RA-T2X-38E while the “cold” side is connected to the laser via a custom clamping mechanism that is insulated from natural convection using a 3D part that also keep both the hot and cold side clamped together via plastic screws. I’m putting “hot” and “cold” sides between quotes here because we will mainly use the TEC for heating and not for cooling so the heat sink will actually be the cold part.
Warning
Thermal paste should be applied between all parts to ensure proper thermal contact.
One advantage of using a TEC instead of a resistive heater element to heat the laser is that we can heat and cool with the TEC whereas a resistive element can only heat. This makes a big difference in terms of thermal control. Using a 10 kΩ thermistor, we can control the temperature of the assembly using a PID loop. I have tested this solution successfully at up to 1A driving currents between 15°C and 45°C and the temperature settles to a steady point pretty quickly.
The upgrade was tested with a 15 µm slit, at 40°C operating point, on isopropanol. The results are shown in FIgure 3.
Figure 3 - isopropanol spectrum using Thorlabs CPS532 set at 40°C
The resolution was evaluated using the FWHM of the peak near 800 cm-1. The result is shown in Figure 4 and is equal to 11 cm-1.
Figure 4 - FWHM of the peak near 800 cm-1