Infrared Laser Illustrates Another Way to Achieve Gain

The standard way to build a laser is to produce a population inversion between an upper and a lower energy level. But laser gain is proportional not only to the population difference between two states but also to the oscillator strength, which depends on both the energy difference between the upper and lower states and the probability of a transition between them. By focusing on these often‐overlooked factors, a group at Bell Laboratories, Lucent Technologies, has made a semiconductor infrared laser with a unique property: It is electrically tunable over a 6% range.