Reflectivity, laser facets

Fig. 11. Schematic of edge-emitting laser diodes where the arrows represent the direction of laser emission and U represents the active region (a) standard stmcture with cleaved facets for mirrors and (b) distributed feedback (DFB) laser that employs coherent reflection from a grating to generate optical...

As in any other laser, the lasing threshold in a semiconductor laser diode is reached when the gain of the active material overcomes the losses of the laser cavity. These losses have two basic origins, namely the finite reflectivity of the mirrors mid distributed losses due to scattering and parasitic absorption in the active medium. In contrast to other lasers, the mirrors in typical semiconductor lasers are simply formed by cleaved or etched crystal facets. Therefore, the reflectivity (Fresnel reflectivity) is rather low, about 20% in the case of the nitrides. 

The way a diamond is cut is one of the 4 Cs that gemologists use to determine a diamond s value. If diamond is the hardest mineral on Earth, then how is it possible to cut a diamond Diamond cutters use other diamonds and lasers to create facets that reflect and refract light. The more precisely the cuts are made, the greater the gems brilliance. If a diamond cut is too shallow or too deep, light escapes from the diamond without traveling back to the eye, resulting in a lackluster appearance. 

By the use of collimated laser light divided into double beams, one can measure the reflectance at two spots on one single facet, as shown schematically in Fig. 2.30 [97]. 

Laser chip facet coating reflectivity should be between 80—8... 

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