The laser will be comprised of more or less cheap and easy sourceable components. This is an important requirement in order to make this development available to the maker community.
The only possible issue are the optics for the resonator. There are suggestions (see Sam’s FAQ) to use polished pieces of copper as a mirror and to drill a hole into the copper as an output coupler. However, using such approach it seems difficult to achieve high power outputs, as desired by the laser. Thus, specialized mirrors and output couplers need to be purchased.
A cheap (and in my research, the only) option is to rely on an import from China. On Ebay there some companies offering CO2 laser optics. It is important to look for resonator optics, i.e. output couplers and concave mirrors.
Thus, the discussion of the resonator design follows the available resonator optics.
- output couplers: On Ebay, there seems to be two different kind of material output coupler: germanium (Ge) or zinc selenite (ZnSe). Both materials are coated to obtain the offered reflectivity of about 70%. The output couplers are plane. One advantage of ZnSe compared to Ge is its transparency in the visible. A laser pointer can be used to align the resonator.
- mirrors: On Ebay available mirrors are concave with an curvature radius varying from 2m up to 10m. The substrate is silicon or glass, that is coated by gold or/and additionally dielectric coating. One should prefer to use the silicon mirrors for higher power application. Since, the coatings achieve no 100% reflectivity and, hence, some of the laser power is transmitted to the substrate. Glass absorbs at wavelength of 10.6 um and the glass mirrors may be damaged.The requirements on the curvature are discussed below.
The resonator geometry is spherical-plane, as dictated by the available components. Then, the resonator needs to be stable, in order for a laser mode to form within the laser. If the resonator is unstable, the losses within the laser might be to large and no lasing is achieved.
In order to obtain a stable spherical-plane resonator, the distance L between the mirror and the output couple has to smaller than the curvature radius R of the mirror, i.e. L<R.
This requirement is easily meet for our 50 cm long glass tubes separating the mirror and output couplers. A mirror with R=2m would form a stable resonator. But, what are the advantages, if there are any, by choosing larger radii?
The crucial parameter to be considered is the beam radius. It is important to note that the laser beam is by no means cylindrical. The beam diameter varies within the resonator and outside, as shown in the following figure. The beam is indicated in red and the outputcoupler and mirror in blue. In this case, the laser would propagate to the left.
Furthermore, the intensity of the beam is the highest at its centre and drops exponentially off to its sides. (At leased, this is the case for the simplest mode, the fundamental mode.)
The calculation of the mode properties are done in Mathematica (see pdf ). The pdf contains all formulas and references to literature. Some of the possible configurations that are possible with the available components are highlighted below. (left blue line: output coupler, right blue line: curved mirror with radius R, thick red lines: beam contour containing 86% of the laser power, thin red lines: beam contour containing 98% of the laser power, dotted lines: beam diameter on mirrors, dash-dotted line: inner bore diameter of discharge tube)
- L=0.5m, R=2m
- L=1m, R=2m
- L=0.5m, R=5m
- L=1m, R=5m
There is a trade-off between beam divergence, beam diameter, discharge volume and diffraction losses. For instance, in the case of L=0.5m, R=2m, the mode is well contained within the discharge tube. The diffraction loss will be small. However, the gas discharge volume is larger than the mode volume. Thus the gain is not optimum.
The other extreme is the case L=1m, R=5m. The 98% contour line does not fit in the discharge tube, thus the diffraction losses are larger than 2%. This limits the output power of the laser, as well.
We will use a ZnSe outputcoupler (70% reflectance) and a dielectric coated silicon substrate mirror with an radius of R=2m.