| The characteristics of the external cavity laser diode are investigated. Emphasis is placed on the short external cavity configuration. The general governing model equations which include the multiple reflection effect are derived. It is shown that the coupling coefficient becomes a complex function of the external cavity length and the near field beam size of the laser diode due to the diffraction loss of an open resonator. The validity of this open resonator model is proven experimentally by the asymmetry in optical power versus external cavity length characteristic curve. For analysis of the ultra short external cavity laser, it is necessary to include this complex coupling coefficient. As an example, the flying optical disk head is studied using this model. In some cases, especially when the external cavity length is longer than several micrometers, the external cavity may be simply regarded as a lossy resonator. With this constant loss per roundtrip approximation, the feedback operator can be reduced to the closed form. The stationary and transient solutions are investigated analytically and numerically. The single mode condition, and dynamical stability condition are derived. Also, the simple expression for the relaxation oscillation frequency of the short external cavity laser is given for the first time. With our model, it can not be expected to find the strange attractor for the constant biased autonomous operation. However, the chaotic behavior in the current modulated nonautonomous operation is investigated. It seems that the route to chaos in this system is not limited to the period-doubling route, which has been believed as the only route in the modulated solitary laser diode so far. Generally, it is suggested that the operating external cavity length should be close to the constructive interference position, as it will give superior properties. Some experimental results applying the short external cavity laser as a laser diode sensor are given. |
