Tips & tricks
Sometimes it is possible to improve the performance of a laser system by changing the setup, such as working inversely with the wavelengths or changing the polarization (if possible).
Here are some recommendations.
Use a small AOI.
You can achieve the sharpest edge for a filter coating and the broadest HR-range and highest reflection with an angle of incidence AOI = 0°.
The example shows four different short pass filters with the same type of design, centered at the same short edge but optimized for different AOIs.
Use the main wavelength in reflection.
Due to the natural variations in refractive index and thickness of each layer in a multi-layer coating normally the tolerances for the HR-range will be much better than for HT-areas. So it is easier to produce high reflection than high transmission.
The example shows the margin of error for a standard coating HR 808 nm /0° made by EBE.
Note: A higher precision can be achieved by using IAD or sputter techniques.
Avoid high transmission around λ/2.
It can be critical to achieve high transmission around half of the center wavelength of a mirror coating. Here a so called λ/2-peak can appear.
The depth of this peak varies from 0% to 100% depending on the thickness ratio of the used coating materials (see diagram showing 3 different ratios), angle of incidence, wavelength and condition of the coating machine.
With a special coating technique it is possible to minimize this peak.
Better use a Long Pass than a Short Pass Filter as ...
– the coating will be thinner (and hence probably cheaper).
– potentially you can use coating material with lower absorption and/or higher LIDT.
– you will have no effects from the λ/2-peak.
For example the SP (HT 355 nm HR 808 nm /0°) will have some absorption at 355 nm, the LP coated with a different material will not.
Use a Polarizer with the Brewster angle.
The best extinction ratio for a polarizer can be achieved by using the Brewster angle. For example, on Fused Silica (against air) choose AOI close to 55.4°.
The often requested Polarizer working with AOI = 45° has a smaller operating range and thus lower tolerances, see illustration right.
Additionally you do not need an AR coating on the rear surface as by using the Brewster angle the reflection of an uncoated surface is zero, but Rp ~0.6% with AOI = 45° (at 1064 nm, on FS against air).