Before coating, the optics are cleaned in an automatic ultrasonic cleaning line, followed by a final manual visual inspection of each surface.
Numerous papers describe how to obtain coatings with very low absorption losses The methods include: standard e-beam gun evaporation, ion beam assisted e-beam gun evaporation (IAD) , ion beam sputtering (IBS) , plasma assisted reactive magnetron sputtering (PARMS) , sol-gel technique , atomic layer deposition , molecular beam epitaxy (MBE) , and anti-reflection nano-textures.
At Ophir Optics, the coatings are produced with standard e-beam gun evaporation and with IAD for the reason that these methods provide excellent results at moderate costs. Coating materials are HfO2, Ta2O5 and SiO2. The coating chambers are equipped with: high vacuum pump with Meissner trap, heating systems above and below the calotte in which the optics to be coated are inserted, gas inlet for reactive evaporation, ion source for cleaning and for IAD, e-beam guns, quartz crystal and optical systems for monitoring layer thickness. The coating chambers are in a cleanroom class 1000. Cleaning, inspection and packing of the optics are done in a class 100 environment.
A major problem in the production of coatings with low absorption is the uniformity of layer properties (thickness, refractive index, absorption) on all optics in a coating batch (throughout the whole calotte which has diameter 760mm). In order to ensure uniformity, all parameters of the evaporation process must be optimized carefully. Several optics from each coating batch, and from different locations in the calotte are tested.
The simplest AR coating, which (theoretically) enables zero residual reflectance, is the so-called "V-coating," named after the "V" shape of the reflectance curve (red curve in Fig. 2). It consists of one layer with a high refractive index (here HfO2) and one layer with a low refractive index (here SiO2). As the layer made of the relatively strongly absorbing high-index material is quite thin, total absorption of the coating is very low (see table 1).
High transmittance both for 1µm and for a laser pointer (typical wavelength 650nm) can also be achieved with a coating consisting of two layers, but only with increased thickness of the high-index layer and therefore higher absorption (blue curve in Fig.2, table 1). With such a coating, it is possible to achieve a residual reflectance below 0.1% for a single wavelength in the range 1030-1070 nm and below 0.2% for the entire range 1030-1070 nm.