This paper examines and models the effect of temperature on the mode-locking stability of monolithic two-section
InAs/GaAs quantum dot passively mode-locked lasers. A set of equations based on an analytic net-gain modulation
phasor approach is used to model the observed mode-locking stability of these devices over temperature. The equations
used rely solely on static device parameters, measured on the actual device itself, namely, the modal gain and loss
characteristics and describe the hard limit where mode-locking exists. Employment of the measured gain and loss
characteristics of the gain material over temperature, wavelength and current injection in the model provides a physical
insight as to why the mode-locking shuts at elevated temperatures. Moreover, the model enables a temperature-dependent
prediction of the range of cavity geometries (absorber to gain length ratios) where mode-locking exists.
Excellent agreement between the measured and the modeled mode-locking stability over a wide temperature range is
achieved for an 8-stack InAs/GaAs mode-locked laser. This is an extremely attractive tool to guide the design of
monolithic passively mode-locked lasers for applications requiring broad temperature operation.
Interest in quantum dot mode-locked lasers (QD MLLs) has grown in recent years since their first demonstration in 2001
as applications for optical time domain multiplexing, arbitrary waveform generation, and optical clocking are
anticipated. Ultrafast pulses below 1 ps have been reported from QD MLLs using intensity autocorrelation techniques,
but so far detailed characterization examining the pulse shape, duration, chirp, and degree of coherence spiking in these
lasers has not been carried out. We describe the first direct frequency-resolved optical gating (FROG) measurements on
a QD MLL operating at a repetition rate of 5 GHz.
Higher-order harmonic repetition rate generation in quantum dot mode-locked lasers (QDMLLs) was realized using a
double interval technique. Using this approach, a wider operation range and improved mode-locking performance was
demonstrated for generating the 6th harmonic of the fundamental repetition rate. Without changing the layout of the
device, mode-locking at a repetition rate of 60 GHz, which corresponds to the 10th harmonic of the fundamental
frequency of the QDMLL, was achieved which cannot be realized utilizing the single absorber technique.
Monolithic InAs quantum dash 1.58-micron passively mode-locked lasers grown on an InP substrate are reported. A
repetition rate of up to 18.5 GHz has been realized. The dashes-in-a-well (DWELL) active region consists of 5 stacks of
InAs quantum dashes embedded in compressively strained Al<sub>0.20</sub>Ga<sub>0.16</sub>In<sub>0.64</sub>As quantum wells separated by 30-nm
undoped tensile-strained Al<sub>0.28</sub>Ga<sub>0.22</sub>In<sub>0.50</sub>As spacers on both sides of the DWELL. 4 micron-wide ridge waveguides with
cavity lengths in the range of 2.3 to 4 mm were fabricated with multiple electrically-isolated anode contacts. The modal
gain and loss spectra of the InAs active region were then measured through the improved segmented contact method, and
the characteristics that make InAs quantum dash materials system desirable for semiconductor mode-locked lasers were
identified. The segmented waveguides were then reconfigured into mode-locked lasers by wire bonding the segments
together to form separate gain and absorber regions utilizing the same DWELL active region. A highly reflective coating
(95%) was applied to the mirror facet next to the absorber while the other facet was cleaved. To assist in the cavity
design and to determine the relative length of the absorber and gain sections, a model for the cavity geometry of the twosection
passively mode-locked lasers was studied and is based on a microwave photonics perspective. A new set of
theoretical equations was used to find the optimal device layout using the measured modal gain and loss characteristics
as input data.
The dramatic variation in the linewidth enhancement factor (α<sub><i>Η</i></sub>-factor) that has been reported for quantum dot lasers
makes them an interesting subject for optical feedback studies. A low α<sub><i>Η</i></sub>-factor combined with a high damping factor is
especially interesting because it should increase the tolerance to optical feedback in these devices and may offer
potential advantages for direct modulation. In the particular case of QD lasers, the carrier density is not clearly clamped
at threshold. The lasing wavelength can switch from the ground state (GS) to the excited state (ES) as the current
injection increases meaning that a carrier accumulation occurs in the ES even though lasing in the GS is still occurring.
The filling of the ES inevitably enhances the α<sub><i>Η</i></sub>-factor of the GS above threshold as experimentally and numerically
shown. Consequently, this strong variation of the GS α<sub><i>Η</i></sub>-factor in comparison to QW devices, should theoretically
produce a significant variation in the onset of coherence collapse due to feedback. This coherence collapse regime, in
which the laser is subject to instabilities, is incompatible with data transmission because of the induced high bit-error
rate. One method to investigate the tolerance to optical feedback is to compare experiment with the theoretical work
introduced by Petermann. It will be presented that under specific conditions, i.e., in the case of a strong enhancement in
the α<sub><i>Η</i></sub>-factor, the feedback sensitivity of the laser can vary by as much as 10dB within the same device.