In this work we report on an experimental investigation of the nonlinear dynamics of a 850 nm multitransverse mode
vertical-cavity surface-emitting laser (VCSEL) when subject to high-frequency current modulation. Different frequencies
and modulation amplitudes are applied to the VCSEL. Regular periodic dynamics - with periods equal to the modulation
period or twice the modulation period - and irregular pulsating dynamics are obtained. Different dynamical behaviors are
illustrated by using power time traces, radio-frequency spectra and bifurcation diagrams. Our results show that irregular
pulsating dynamics in multimode VCSELs subject to large-signal current modulatton can be obtained due to the
competition between different transverse modes.
We numerically investigate how polarization properties of vertical cavity surface emitting lasers (VCSELs) are affected by optical feedback from an extremely short external cavity. In order to do it we use a two modes rate equation model that accounts for infinite round trips in the external cavity. With it we perform maps of bistability finding out a modulation of the polarization switching currents when increasing the external cavity length with a period equal to half the wavelength of operation of the device. When the external mirror reflectivity is high enough there is a region within each period of modulation where the VCSEL polarization is stable at any injected current within the operation range. Moreover, using polarized feedback the map of bistability is channelled and the bistable region is almost suppressed. A preliminar study of the effects of temperature variations on the map of bistability is also carried out and presents polarized feedback as a more robust technic to obtain polarization stabilization. An experimentally obtained map of bistability supports the model showing very good agreement with it.
Vertical-cavity surface-emitting lasers (VCSELs) have now entered
the stage of implementation in optical devices and networks.
Besides the number of properties superior to EEL, VCSELs have the
drawback of less stabilized polarization. As any semiconductor
laser, VCSELs are sensitive to Optical Feedback (OF) due to
parasitic reflections inevitable in practical applications. Here
we experimentally and numerically investigate the spectral and
polarization properties of VCSELs subject to polarization
insensitive optical feedback from an extremely short external
cavity -ESEC (few microns). We observe that the total output power
and the wavelength of laser emission are sinusoidally, in-phase
modulated with the external cavity (EC) length and with a period
equal to half the wavelength of the VCSEL. Moreover the currents
at which the switches between the two linearly polarized (LP)
modes happen and the hysteresis width are also sinusoidally
modulated with the same period. When biasing the VCSEL at the
switching current polarization switching between two orthogonal
linearly polarized states happens at different EC lengths. This
polarization switching happens through a hysteresis when
decreasing and increasing the EC length. We have also develop a
two modes rate equation model that proves and explains the changes
of all VCSEL characteristics observed in the experimental part,
showing very good agreement with the experimental results.
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.