Fiber-Coupled Laser Add-on

One of the easiest upgrades to address the problem of laser bandwidth and power is to use an external fiber-coupled laser. The upgrade uses the same support as for the base spectrometer but with extra components as shown in Figure 1 with an FC/APC fiber. The NE520A is a neutral density filter that can be used as tooling during the alignment phase.

Installing a fiber-coupled laser has some technical consequences as it requires more optical parts. First, you need to collimate the light exiting the optical fiber using a lens (here a F110APC-532). Second, you need to filter the light itself. Indeed, as the laser light passes through the optical fiber it produces its own Raman signal. If you inject the signal as-is, you will have a mix of Raman from your sample and Raman from the optical fiber after it experienced normal scattering in your sample.

The filter used here is a FL0532-10 from Thorlabs with a 10 nm bandwidth centered on 532 nm. The response of the filter is given in Figure 3 in comparison to the 1 nm bandwidth version (FL0532-1) and the DMPL550 dichroic mirror used in the base spectrometer. The 10 nm filter is less half the price of the 1 nm filter but the blocking optical density is less (~10^4.5 compared to ~10⁶). Also, the 10 nm filter has 70% transmission at 532 nm whereas the 1 nm filter has only 40% transmission. In the balance, we also need to account for DLMP550 dichroic mirror which will let some of the light pass through instead of being reflected to the sample port.

Any FC/APC 532 nm laser is suitable. SMA and FC/PC of the collimators are also available but were not tested here.

The table below sumarizes lasers that have been used with this upgrade and that we can recommend: