Anti-Reflection coated laser diodes in an ECDL system.*

We have been studying the effect of low-reflectivity coatings designed to improve the frequency control of an Extended Cavity Diode Laser (ECDL) system. The system includes a semiconductor laser diode chip that has two reflective facets. It also uses an external diffraction grating that reflects light back into the laser chip. The competition between reflected light from the external grating and the chip’s front facet to the back facet of the laser diode often leads to an unstable light frequency. This summer, we coated the front facet of the chip with a thin anti-reflection layer of SiO to observe fewer and more predictable frequency mode hops. Using a precision spectrometer, I measured the frequency of light and how it depends on the length of the extended cavity.  Through the measurement of the mode frequency spacing of lasers, I was able to classify observed mode hops in terms of either external or chip cavity mode hops. I analyzed the optical behavior of the coated lasers and looked for changes or improvement relative to the uncoated lasers. The results show a successful reduction in chip mode hopping and improved stability of the laser.