Light Trapping by Optical Path Increase

The simplest and the most obvious approach is to utilize some kind of high-reflection structure to change the path of the beam and return it back to the active 

A different situation is met if a high-reflection layer is deposited on the detector backside, as shown in Fig. 2.33b. The signal is completely reflected from it and passes a twice longer path through the active region before leaving the structure. This solution is very often implemented and represents almost a rule for practically all conventional thin photodetectors. In real situations, there are again multiple reflections due to refractive index mismatch. A calculation that takes into account these reflections has been presented in [11]. 

The next section analyzes different high-relleclion stractures convenient for cavity enhancement. It also considers strategies and conditions of their use. 

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