Ellipsometric approach to anti-reflection coatings of semiconductor laser amplifiers

摘要

The invention describes a real-time in situ ellipsometric monitoring and control system using an ellipsometer to control the averaged refractive index of the deposited film during the AR coating of semiconductor laser diode facets for laser amplifiers and superluminescent LED. The input and output window birefringences are taken into account and calibrated the windows mounted on the vacuum chamber to include the effects of the pressure and mounting stress. In addition to the conventional four-medium model which gives an averaged refractive index, an adaptive multilayer model which takes into account an increasing number of layers as the evaporation proceeded is developed to monitor the instantaneous changes of the refractive index. Each ellipsometric measurement lasts only 0.5s and provides two sets of refractive index and thickness data as derived by the two multilayer models. Both measured data are used for the refractive index control to achieve a good feedback response. This fast and sensitive measurement technique makes possible the feedback control of the refractive index in real time which in turn allows for better control of the deposition condition and also improves the reproducibility of the AR coating process. By combining the two measured refractive indices and using the weighted average as the control factor, precise control of the average refractive index within +/-0.01 can be achieved and traveling-wave semiconductor laser amplifiers and superluminescent LEDs with facet reflectivities of order 10-5 or less are obtained reproducibly for a single layer coating. Multilayer AR coatings can also be fabricated by using the invention technique with different combination of materials. The coatings can further be fine tuned by using wet etching or an ion gun mounted in the same deposition chamber for film thinning.