A detailed study of the pulse characteristics emitted from a monolithic Quantum Dot (QD) passively Mode-Locked Laser (MLL) has been performed using a state-of-the-art Frequency Resolved Optical Gating (FROG) pulse measurement system. While traditionally the time-domain pulse characteristics of semiconductor MLLs have been studied using digital sampling oscilloscope or intensity autocorrelation techniques, the FROG measurements allow for simultaneous characterization of time and frequency, which has been shown to be necessary and sufficient for true determination of mode-locked stability. In this paper, FROG pulse measurements are presented on a two-section QD MLL operating over wide temperature excursions. The FROG measurement allows for extraction of the temporal and spectral intensity and phase profiles from which the Group Delay Dispersion (GDD) can be determined. The magnitude of the GDD is found to decrease from 16.1 to 3.5 ps/nm when the temperature is increased from 20 to 50 oC, mirroring the trend of pulse width reduction at elevated temperature, which has been shown to correlate strongly with reduced unsaturated absorption. The possibility to further optimize pulse generation via intra-cavity dispersion compensation in a novel three-section MLL design is also examined, and shows strong potential toward providing valuable insight into the optimal cavity designs and operating parameters for QD MLLs.
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.
The current modulation of a two-section semiconductor laser is first reviewed analytically using a well-known, closed-form,
modulation expression. A system of traveling-intensity equations is then used to investigate spatial effects in
these lasers including cavity layout and the role played by cavity length. The numerical simulations verify the accuracy
of the analytic expression for short cavities (low frequencies) but identify shortcomings as the cavity length
(modulation frequency) is increased. One notable difference is the presence of resonant peaks in the modulation response.
Although this effect has been addressed in the past, the arrangement of sections within the laser is shown
to play a prominent role in these monolithic devices for what we believe to be the first time. In the course of this
investigation the thirteen different ways a two-section semiconductor laser can be current modulated are identified and
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.
The linewidth enhancement factor (LEF) and nonlinear gain coefficient of an InAs/AlGaInAs quantum dot (QD) laser
are measured using an injection locking technique. The nonlinear gain coefficient was found by curve-fitting the
measured LEF as a linear function of the output power. The LEF of the InAs/AlGaInAs quantum dot laser was
measured to be 1.2 to 8.6 at output powers from 2 to 10.2 mW, leading to a corresponding nonlinear gain coefficient of
1.4 x 10<sup>-14 </sup>cm<sup>3</sup>. This value for the nonlinear gain coefficient is three orders of magnitude higher than the typical
quantum well nonlinear gain coefficient of 10<sup>-17</sup> cm<sup>3</sup>. Consequently we expect that the dynamics under optical injection
and external feedback of this type of quantum dot laser will be dramatically different than in quantum well lasers,
suggesting that a careful re-examination of the dynamics of this type of laser is needed.
The dynamical response of a quantum dot photonic integrated circuit formed with a combination of passive
and active gain cells is investigated. When these cells are appropriately biased and positioned within the
multi-section laser cavity, fundamental frequency and harmonic mode-locking at repetition rates from 7.2
GHz to 115 GHz are found. Carefully engineered multi-section configurations that include a passive waveguide
section significantly lower the pulse width up to 34% as well as increase the peak pulsed power by
49% in comparison to conventional two-section configurations that are formed on the identical device
under the same average power. In addition an ultra broad operation range with pulse widths below ten
picoseconds is obtained with a 3<sup>rd</sup>-harmonic mode-locking configuration. The fundamental design
principles for using QDs in mode-locked lasers are presented to explain the observed results and to describe
why QDs are particularly well-suited for reconfigurable laser devices.
The modulation characteristics of multi-section gain lever quantum dot lasers are investigated in this paper. A 20-dB
enhancement in the amplitude modulation efficiency is observed in a two-section quantum dot laser. Based on rate
equation analysis a novel modulation response equation is derived to describe the device dynamics. In addition the
dependence of the modulation efficiency enhancement and 3-dB bandwidth on the length of the modulation section is
discussed. A conservative estimate of the gain lever value of 33 is derived from the measured results.
The correlations between the photoluminescence (PL) wavelength, integrated intensity, peak intensity, and FWHM with laser diode performance such as the maximum gain, injection efficiency, and transparency current density are studied in this work. The primary outcome is that the variation in PL intensity within a wafer originates primarily from differences in the radiative and non-radiative recombination rates and not from dot density variation. PL generated from 980 nm wavelength pumping appears to give more consistent data in assessing the optical quality of quantum dots that emit in the 1300 nm from the ground state.