We present an experimental and theoretical study of optical feedback in a semiconductor laser for the case of an extremely short external cavity (EC) configuration. When the length of the EC is changed both the output power and the voltage drop onto the laser are modulated with a period of half of the solitary laser wavelength. We also perform modulation experiments in which the EC length is modulated with amplitude corresponding to the half of the solitary laser wavelength and with different signal shapes. In this way we prove that by using optical feedback we are able to detect very small features. Such detection is of general interest from an application point of view, e.g. for optical data readouts, resulting in a reduced number of optical components. Optical feedback also affects the frequency of the laser light and results in a longitudinal mode hopping. With increasing the EC length we observe mode hops between neighboring solitary laser modes followed by large jumps at the EC frequency splitting. These large EC mode hops can be exploited for broad band frequency tuning of the emitted light. We also study the dependence of the amplitude and the period of the EC mode hops on the EC length. We reveal the existence of a cut-off EC length of a few micrometers for which the amplitude of the EC mode hops reaches a maximum and then strongly decreases. We give a theoretical explanation of our experimental findings based on Fabry-Perot resonant condition for coupled cavities.
Semiconductor lasers can be used simultaneously as optical sources
and optical sensors, as they are extremely sensitive to a small
amount of coherent optical feedback. We present a survey on experimental results on optical feedback in semiconductor lasers
and on different approaches to describe its effect on the laser
properties. We show that for long and moderate long external
cavities (hundreds of meters down to centimeters) the Lang-Kobayashi delay model, multiple delays and multimode delay rate equation models are in very good agreement with experiments on edge emitting lasers (EELs) and vertical-cavity surface-emitting lasers (VCSELs). We present examples of frequency and polarization mode hopping, periodic and quasiperiodic behavior, different routes to chaos, regular pulse packages, high frequency pulsations and stochastic and coherence resonance, that all have been experimentally and numerically demonstrated. Suitable models for studying laser diodes subject to optical feedback from extremely short external cavity, or ESEC (of the order of the wavelength) are the composite cavity and the multimode butt coupling models that either consider the field
amplitudes after multiple reflections in the external cavity (EC)
as stationary or treat the whole compound cavity at once. Numerical and experimental studies showed that optical feedback in ESEC leads to detectable change of the laser output power or the voltage drop over the laser for a small change of either the phase or the optical feedback strength. As an example, we discuss experimental and numerical results on spectral and polarization properties of VCSELs subject of insensitive optical feedback from ESEC. The wavelength and the current of polarization switching between the two linearly polarized fundamental modes of the VCSEL are periodically modulated with the external cavity length. High contrast polarization switching is thus possible for quarter-wavelength change of external cavity length. In the case of EEL we experimentally demonstrate that with changing the length of the EC the emitted power, the wavelength and the laser voltage are periodically modulated. We explain the longitudinal mode-hopping between the neighboring composite cavity modes followed by large jumps at the external cavity frequency splitting as a result of the spectral modulation of the effective losses of the composite cavity system.