Neon Calibration Sources
During the alignment process, a calibration of the wavelength axis of the spectrometer is required. Due to the wavelength range used in Raman spectrometers (typ. 550-700 nm), good candidates are neon calibration sources which produce the spectrum of Figure 1.
Figure 1 - Neon Spectrum (credit: Astrosurf)
Stabilized neon calibration sources can be purchased from different suppliers, and we have successfuly used the following
| Supplier | Reference | Comments |
|---|---|---|
| Ocean Optics | NE-2 | SMA-coupled |
| Thorlabs | CSL1 | SMA-coupled, USB power supply |
These lamps works straight out of the box with the recommended 600 µm fibers but they are relatively expensive. To help those who are looking for more affordable alternatives, we propose the non-stabilized neon lamp of Figure 2. We used it successfuly in routine operations for non-critical work.
Figure 2 - Low-cost neon calibration source
The system of Figure 2 is directly connected to the main supply (110-220V) through a resistor. The value of the resistor should be matched to the current rating of your specific neon bulb used with some margin. Insulate the system to your best efforts to avoid shorts. In Figure 2, we used Kapton tape as insulator and hot glue to maintain the neon bulb in place.
DANGER OF DEATH
This system is not considered safe to operate by students or third parties. Use it at your own risk. Never handle the system when it is connected to the main supply.
Neon bulbs can be obtained from various sources, including e-bay at extremely low prices (~1€/pcs). When shopping for neon bulbs, beware of buying an actual neon lamp as some items sold under the name "neon" are in fact yellowish LEDs which are not suitable as calibration source. Searching for "neon lamp" at RS, Mouser or Digikey should give you a proper match. Check for something similar to the one of Figure 2 consisting of a glass bulb with two separated electrodes. Ideally, the datasheet should mention operating voltages above 50 Volts, which is the break up voltage of neon, and recommended currents in the range 20-50 mA.