Theoretical design of single-layer antireflection coatings on laser facets

Single-layer antireflection coating films are used to transform semiconductor injection lasers into different kinds of active devices such as superluminescent diodes or optical amplifiers. In this paper, optimum film parameters (thickness h and index of refraction n) are established for a wide range of InGaAsP lasers emitting at 1.3 μm. Optimum film parameters are different for the TE and TM polarizations. The minimum theoretical power reflectivity is higher for the TM polarization than the minumum reflectivity for the TE polarization by a factor of about three. Both are very low, on the order of 10
⁻⁶
. Tolerances in film parameters for a power reflectivity R ≤ 10
⁻³
(which is acceptable for most practical applications) are calculated for a typical laser having a spot size a = 0.5 μm. The tolerances are Δh ≅ ±50Å and Δn/n ≅ ±3 percent for the TE polarization and Δh ≅ ±43Å and Δn/n ≅ ±2.4 percent for the TM polarization. Processing of high-quality antireflection coating films on InGaAsP devices is possible according to these tolerances by using sputtered SI
3
N
4
, which allows a very slow deposition rate (on the order of 75Å/min) and the tailoring of the film index by adjusting the nitrogen pressure in the plasma.