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These multilayer dielectric coatings perform specialized beam-splitting functions. Probably the most demanding application is in laser cavity output couplers.
1. Laser line PR coatings
The PR coating on a laser cavity output coupling mirror, for example, allows the maximum amount of laser light to escape the laser cavity while simultaneously retaining enough light inside the laser cavity to sustain lasing.
One of the critical aspects of the PR coating is the precision allowable tolerance on the PR splitting ratio. For example, an engineer may design a laser cavity requiring an output coupler with a 40% transmission and 60% reflection. However, the coating must be fabricated with a standard manufacturing tolerance of ±4%. That means that the delivered production parts may have a PR splitting ratio ranging from 36%T/64%R to 44%T/56%R. The standard tolerances listed in the table below can be tightened-up, of course, at extra cost.
Standard reflectance at chosen wavelength:
Fig. 1-1 PR coating R=50% ± 5% at 532 nm, AOI=0°.
Fig. 1-2 PR coating R=98% ± 1% at 1064 nm, AOI=0°.
2. Broadband partial reflective coatings
Similarly, PR coatings are available for broad-band applications. A typical application is a beam-splitter used in visible imaging systems, where the light from an image may be split 70/30, with 70% of the light going to the viewer's eye, and the remaining 30% going to a video camera.
Fig. 2-1 PR coating at 470-520 nm for S- and P-polarization, AOI=45°.
Fig. 2-2 PR coating R=80% +/- 4% at 1500-1800 nm, AOI=0°.