Frequently Asked Questions
What’s an inexpensive way to split unpolarized light into two polarized parallel beams? We need the beams to be separated by 1.7 mm and we’re operating at 810 nm, 200 W peak power in a 20-ms pulse duration. Our beam is approximately 0.8 mm by 15 mm.
There are a few ways you can achieve what you want. If you had a smaller beam size and lower optical powers, you could use a birefringent crystal (such as YVO4 or calcite) and a half-waveplate. With the higher powers and larger beam size in your application, an expensive but effective design is to use a polarizing beamsplitter that splits the two beams into two orthogonal directions, then a right-angle prism to orient them parallel to each other. We suggest a simpler, more elegant design as shown below. It consists of a custom thin-film plate polarizer with a high-reflectivity coating on the back surface (see the figure). The thickness of the plate polarizer is specifically chosen so that the beams will be separated by 1.7 mm. One of the nice benefits of this design is that it uses only a single optical element. 
Beamsplitter Cubes
In the past, we’ve had problems where the optical properties of our dielectric coatings changed when we put them under vacuum or in a dry nitrogen environment. Their transmission or reflection curves spectrally shifted leaving us with less than optimal conditions. Why did they do this, and will your ion-beam-sputtered coatings do this also?
The problem you are seeing is due to humidity effects. Water molecules are absorbed into the coatings, causing the coatings to swell, thus changing their optical thickness and the optical properties of the dielectric thin films. This typically only occurs if your supplier is using an evaporation process (E-beam) for depositing your coatings. PPC uses ion-beam sputtering (IBS) to deposit our coatings. The IBS process actually forms covalent bonds resulting in a packing density so tight and uniform that the water cannot be absorbed. Thus, IBS films exhibit exceptional environmental stability, and you won’t see any shifting under vacuum or in a dry nitrogen environment. IBS films also offer several other advantages including: lower scatter and absorption losses, fewer pin-hole defects, high laser damage thresholds, chemical resistance, and superior adhesion.
I need a custom etalon, but don’t know how to order one so I get exactly what I need. What should I specify?
We get this type of question all the time. In order to properly specify an etalon, you need to tell us the operating wavelength range, operating temperature (and temperature tuning range, if applicable), angle of incidence (it is best to keep this to less than 0.5° to minimize beam walkoff effects), clear aperture, input beam size, and any other operating conditions specific to you application. In addition, you’ll need to let us know the free-spectral range or FSR (the distance between the transmission peaks) and the finesse (the ratio of the FSR to the FWHM of the transmission peak). The finesse can also be specified in terms of the FHWM, or because this is directly related to the reflectivity, you can tell us your desired mirror reflectivity. It will also be important to note if you need to center the transmission peak on a reference such as the ITU grid or an absorption line. We’ve added an etalon calculator to our website to help you design the key parameters of your etalon. Give us a call and we can discuss these parameters in more detail. Also, for more advanced information, see our Etalon Introduction and Etalon Advanced white papers.
Graph showing the transmitted intensities of etalons of various reflectivities as a function of frequency. The higher the reflectivity, the higher the finesse, and the narrower the FHWM. 
Etalons