Fiber Optic Tutorials

This is a continuation from the previous tutorial - modulation response of semiconductor lasers.   We have considered the emission characteristics of a semiconductor laser operating in isolation. In practice, however, a small portion of the emitted light is inevitably fed back into the laser cavity owing to parasitic reflections that may occur at a surface outside the cavity. In optical communication systems, such unintentional reflections may occur at the near end or the far end of the fiber link. It is often observed that even a relatively small amount of external optical feedback can significantly affect the performance of...

This is continuation from the previous tutorial - noise characteristics of semiconductor lasers.   One of the important advantages of semiconductor lasers is that they can be directly modulated; i.e., one can readily obtain short optical pulses useful for optical communications by modulating the device current. The modulation. response of semiconductor lasers have been studied from the early days. Because of an intrinsic resonance of the device it was found that the response peaks at the relaxation-oscillation frequency \(\Omega_\text{R}\), and the modulation efficiency sharply drops for modulation frequencies \(\omega_\text{m}\) greater than \(\Omega_\text{R}\). Experimentally, however, the resonance peak is often less...

This is a continuation from the previous tutorial - transient response of semiconductor lasers.   In the preceding deterministic description, laser power and frequency were assumed to remain constant in time once the steady state had been reached. In reality, however, laser output exhibits intensity as well as phase fluctuations. At the most fundamental level, the origin of these fluctuations lies in the quantum nature of the lasing process itself. A proper description therefore requires a quantum-mechanical formulation of the rate equations. In general, intensity noise reaches its peak in the vicinity of the laser threshold and then decreases rapidly...

This is a continuation from the previous tutorial - steady-state characteristics of semiconductor lasers.   The previous tutorial considered device behavior under steady-state conditions. When a semiconductor laser is turned on by changing the current \(I\), a relatively long time (~ 10 ns) elapses before the steady state is reached. In the transient regime, the power distribution among various longitudinal modes varies periodically as the laser goes through relaxation oscillations. In particular, a semiconductor laser whose CW mode spectrum is predominantly single-mode exhibits poor side-mode suppression under dynamic conditions. An understanding of the transient response is especially important for optical...

This is a continuation from the previous tutorial - rate equations of semiconductor lasers.   The rate equations of the previous tutorial can be used to obtain the steady-state response of a semiconductor laser at a fixed value of the current \(I\). The steady-state solution is obtained by setting all time derivatives to zero and is applicable for continuous-wave (CW) operation after transients have died out. The steady-state or CW solution is also applicable under pulsed operation provided the duration of the current pulse is much longer than the laser response time, which is a few nanoseconds. Two steady-state features...