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 bet ween light emitted from the front laser facet and light emitted from t he rear laser facet that is reflected forward by the external mirror. The change in the relative phase of light emitted from each facet caus ed 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 detec ted 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 preve nt mode hops caused by drift, mechanical shock, or laser tuning; the t echnique permitted complete frequency tuning over the range of operati on 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.