Anti-reflection coatings for semiconductor lasers
View Patent ↗The present invention concerns an anti-reflection coating for semiconductor lasers, in particular a coating on the laser facet with advantageous properties resulting in improved reliability and reduced probability of specific breakdowns, especially so-called catastrophic optical damages (CODs). It is a quarter-wave coating with a predetermined reflectivity, preferably between 0 and 10% and consists of or comprises SiN x :H. It is preferably applied by a Plasma-Enhanced Chemical Vapor Deposition (PE-CVD) process whose process parameters are controlled such that a desired optical thickness and refractive index of the coating are achieved. The PE-CVD process may be controlled to result in an Si/N ratio between about 0.5 and 1.5 and/or to produce a coating of essentially amorphous SiN x :H whose density approaches the density of crystalline Si 3 N 4 .
1. A semiconductor laser emitting at a given wavelength with a coating on its emitting facet, wherein
said coating comprises an essentially amorphous SiN x :H layer, x being a real number, with a predetermined thickness and a predetermined refractive index,
said thickness being determined by said laser's wavelength,
said refractive index being essentially determined by the Si/N ratio in said SiN x :H layer,
the Si/N ratio of the SiN x :H layer is selected between approximately 0.3 and approximately 1.5.
2. The semiconductor laser according to claim 1 , wherein
the refractive index of the SiN x :H layer is selected in relation to the refractive index of the laser facet.
3. The semiconductor laser according to claim 1 , wherein
the thickness, in particular optical thickness, of the coating is selected to be one quarter of the laser's wavelength.
4. The semiconductor laser according to claim 1 , wherein
the refractive index of the coating is tuned during the manufacturing process of the SiN x :H layer, essentially by controlling its Si/N ratio and/or its microstructure.
5. The semiconductor laser according to claim 1 , wherein
the coating is a multi-layer coating including at least one essentially amorphous SiN x :H layer.
6. The semiconductor laser according to claim 1 , wherein
the coating consists of or comprises an essentially homogeneous SiN x :H layer.
7. The semiconductor laser according to claim 6 , wherein
the ratio of Si to N of the SiN x :H layer is tuned to effect a refractive index of the coating close to √{square root over (n eff )}, wherein n eff is the effective refractive index of the laser facet.
8. The semiconductor laser according to claim 6 , wherein
the refractive index of the SiNx:H layer is tuned to achieve a refractive index of the coating between approximately 1.6 and approximately 2.4.
9. The semiconductor laser according to claim 6 , wherein
the refractive index of the SiNx:H layer is tuned to achieve a refractive index of the coating between 1.79 and 2.24.
10. The semiconductor laser according to claim 6 , wherein
the SiN x :H layer is located adjacent the laser facet and its refractive index is tuned to effect a reflectivity at the laser facet of approximately zero.
11. The semiconductor laser according to claim 6 , wherein
the coating constitutes a phase-shifting QW coating.
12. The semiconductor laser according to claim 6 , wherein
the Si/N ratio of the SiN x :H layer is selected between approximately 0.3 and approximately 1.5.
13. The semiconductor laser according to claim 6 , wherein
the optical thickness of the SiN x :H layer is selected to be one quarter of the laser's wavelength.
14. A coating on a facet of a semiconductor laser emitting at a given wavelength, said laser having an external cavity, wherein
said coating comprises or consists of an essentially amorphous SiN x :H layer, wherein x is a real number,
said SiN x :H layer having
a thickness determined by said laser's wavelength,
a refractive index essentially determined by the Si/N ratio in said SiNx:H layer, and
an Si/N ratio selected between approximately 0.3 and approximately 1.5.
15. The coating according to claim 14 , wherein
the refractive index of the SiN x :H layer is further determined by the microstructure of said layer.
16. The coating according to claim 15 , wherein
the Si/N ratio and/or the microstructure of the SiN x :H layer is selected to produce a refractive index of said coating close to √{square root over (n eff )}, n eff being the effective refractive index of the laser facet.
17. The coating according to claim 14 , wherein
the optical thickness of the SiN x :H layer is selected to be one quarter of the laser's wavelength.