ABSTRACT

Tunable external cavity diode lasers (ECDLs) are useful in many applications, including atomic and molecular laser spectroscopy, laser cooling, atomic clocks, environmental sensing, phase-shifting interferometry, coherent optical telecommunications, and exciting new fields such as optical microresonators and quantum computing. Aside from tunability, these applications require continuous-wave, single-mode, and narrow-linewidth laser sources. Semiconductor diode lasers are attractive because of their low cost, although not an ideal choice since they typically operate with several longitudinal modes lasing simultaneously, leading to low coherence and therefore large linewidths. However, controlled optical feedback into the laser diode with external optics can extract highly coherent light and narrow linewidths from a semiconductor-based laser. Figure 22.1 shows a simple ECDL schematic. The laser cavity is now external to the diode itself and defined by the back facet of the laser diode and output coupler. By putting an antireflection (AR) coating on the front facet of the laser diode, the diode becomes purely a gain element. This is essential to the performance of tunable ECDLs because it suppresses mode competition and self-lasing because of internal diode reflections.