Interference fringes in the far field of short-external-cavity InGaAsP diode lasers: a method for longitudinal mode control.

Interference fringes in the far field of short-external-cavity InGaAsP diode lasers are discussed and explained. The interference occurs between light emitted from the front laser facet and light emitted from the rear laser facet that is reflected forward by the external mirror. The change in the relative phase of light emitted from each facet caused by a change in the longitudinal mode results in a shift of position of a portion of the fringes in the far field. This shift can be detected and processed to produce a discrimination signal that is used in a feedback control system to detect or prevent longitudinal mode hops. The control system has a response time as short as 15 ms and can prevent mode hops caused by drift, mechanical shock, or laser tuning; the technique permitted complete frequency tuning over the range of operation of the laser. The behavior of the control system was analyzed with the help of a model that can be used to optimize the control system.