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The latest breakthroughs on the frontiers of semiconductor laser capabilities are presented. Achievements including the impressive advances in high-speed lasers with low pJ/bit energy consumption, high-power vertical external cavity surface emitting lasers (VECSELs), advances in Ill-nitrides, record-high temperature operation quantum dot lasers, the longest wavelength Type-I quantum well lasers to date, and the fascinating field of nanolasers with ultralow volume and threshold are all discussed.

In this paper, a technology comparison between monolithic passively mode-locked lasers (MLLs) fabricated from 1.24 μm InAs dots-in-a-Well (DWELL) and 1.25 μm InGaAs single quantum well (SQW) structures grown using elemental source molecular beam epitaxy (MBE) is presented. 5 GHz optical pulses with sub-picosecond RMS jitter, high pulse peak power (1W) and narrow pulse width (< 10 ps) are typical of these monolithic two-section InAs DWELL passive MLLs. An InGaAs single quantum well MLL with the 42% indium is shown to exhibit a superior high-temperature performance. Compared with the typical operating range of the InAs DWELL devices (

This paper examines and models the effect of temperature on the mode-locking capability 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 material parameters, measured on the actual device itself, namely, the modal gain and loss characteristics, and govern the limit describing the onset of mode-locking. 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 down 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 can be maintained. Excellent agreement between the measured and the modeled mode-locking stability over a wide temperature range is achieved for an 8-stack InAs/GaAs quantum dot mode-locked laser. This is an attractive tool to guide the design of monolithic passively mode-locked lasers for applications requiring broad temperature operation.

Different energy levels are generally assumed to be associated with ground state (GS) and excited state (ES) gain in semiconductor quantum dot lasers and amplifiers. We present calculations based on an 8 band k ·PHamiltonian which show that this is not the case. Two distinct absorption bands are calculated due to transitions between the GS electrons and all dot hole states. In addition to the peak due to transitions between GS electron and GS hole levels, a higher energy band is also calculated due to transitions between GS electrons and highly excited dot hole states. We demonstrate that the removal of electron–hole pairs through stimulated emission at the ES gain maximum can result in an amplitude reduction for a probe pulse tuned to the GS gain maximum, due to the existence of these higher energy transitions, and also due to the effects of two photon absorption. We also find that the instantaneous phase change at the ES gain maximum due to dot to dot interband transitions is small compared to the measured value. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The temperature dependence of the radiative and nonradiative components of the threshold current density of 1.3 mum InAs/GaAs quantum-dot lasers have been analyzed both experimentally and theoretically. It is shown that the weak temperature variation measured for the radiative current density arises because the optical matrix element for excited state transitions is significantly smaller than for the ground state transition. In contrast, nonradiative Auger recombination can have a similar probability for transitions involving excited states as for those involving ground state carriers. The sharp increase in the threshold current density at high temperatures follows the temperature variation of the cubed threshold carrier density confirming that Auger recombination is the dominant recombination mechanism in these devices at room temperature.

In this paper, performance of monolithic quantum dot passively mode-locked lasers over broad temperature excursions is characterized. It is shown that there is a linear dependence between absorber to gain length ratio and the characteristic temperature that a device transitions from ground-state to excited-state lasing when the saturable absorber is grounded. The pulse shape and optical spectrum characteristics are examined in detail around these transition regimes. Experimental operational maps have also been constructed showing the range of biasing conditions that produce stable mode-locking across a wide range of temperatures. A comparison is made between regions of mode-locking stability for two devices having the same absorber to gain length ratio, with varying ridge waveguide widths. Finally, gain and absorption characteristics are derived from measurements of amplified spontaneous emission, and a correlation between reduced values of unsaturated absorption and reduced time-bandwidth product is shown. Key features in the experimental operational maps and their respective significance on the operation and design of future devices is discussed.

A previously unreported correlation is presented between reduced values of unsaturated absorption at the laser gain peak and low measured time-bandwidth-product (TBP) values of optical pulses emitted from monolithic quantum dot passively mode-locked lasers.

Record performance over temperature is reported in a two-section InAs/GaAs quantum dot passively mode-locked laser. Stable pulses with less than 18 ps full-width-half-max are observed from 20–110 °C.

In this work we propose a high-speed hybrid optical-time-division-multiplexing (OTDM) and wavelength-division-multiplexing (WDM) system that seamlessly generates high bit-rate data (>200Gbit/s) from a low speed (5Gbit/s) quantum-dot mode locked laser pulse train. The high-speed output data can be generated using electro-optical micro-ring modulators that operate as low as (5Gbit/s). By utilizing time and wavelength domains, the proposed design is a promising solution for high-speed, compact and low-power consumption optical networks on chip.

Optical interconnects have been highlighted as a key technology to alleviate the shortcomings inherent to traditional inter-chip copper interconnects in terms of bandwidth, power consumption and reliability [1]. In particular, the monolithic two-section quantum dot passively mode-locked laser (QDMLL) has emerged as an impressive low noise source of pico-second optical pulses [2]. As well as its compact size and direct electrical pumping, the QDMLL emits at wavelengths compatible with Si-based waveguides and detectors. For use in optical interconnects, ideally, the transmitter will be situated close to the CPU cores and will therefore need to operate over a broad temperature range, with highs typically in the vicinity of 100 °C. It is therefore desirable to develop uncooled optical interconnects, firstly to reduce system size and complexity and secondly to eliminate the power-hungry cooling requirements associated with diode lasers. In this paper, we discuss our approach to developing uncooled, ultra low energy/bit QD MLL transmitters capable of providing high quality optical pulse trains suitable for multiplexing up to the 100s of Gbps level. Up until now an analytical method to guide the design of temperature resistant MLLs based on convenient, measureable material parameters has not existed.

In this paper, we investigate the dynamics of a nonlinear delay differential equation model for passive mode-locking in semiconductor lasers, when the delay model is seeded with parameters extracted from the gain and loss spectra of a quantum dot laser. The approach used relies on narrowing the parameter space of the model by constraining the values of most of the model parameters to values extracted from gain and loss measurements at threshold. The impact of the free parameters, namely, the linewidth enhancement factors that are not available from the gain and loss measurements, on the device output is then analyzed using the results of direct integration of the delay model. In addition to predicting experimentally observed trends such as pulse trimming with applied absorber bias, the simulation results offer insight into the range of values of the linewidth enhancement factors in the gain and absorber sections permissible for stable mode-locking near threshold. Further, the simulations show that this range of permissible values progressively decreases with increasing bias voltage on the absorber section. This is important for telecomm and datacom applications where such devices are sought as pulsed sources, as well as in military RF photonic applications, where mode-locked diode lasers are used as low noise clocks for sampling.

The unique optical properties of quantum dot semiconductors have brought about significant new capabilities in light-emitting devices fabricated from these materials. In particular, the performance of laser diodes based on epitaxial quantum dots grown on a GaAs substrate has been the focus of intense study for about 20 years. The advances for such lasers emitting at wavelengths longer than 1.2 μm are reviewed here. Four vital aspects are discussed, including the record-low-threshold currents and the impressive temperature stability these lasers have achieved, the extreme range of linewidth enhancement factors possible in these devices, the ultralow linewidth–power product and insensitivity to optical feedback of quantum dot distributed feedback lasers, and, finally, the progress made to date in achieving higher modulation bandwidths in directly modulated and injection-locked quantum dot lasers.

International audience; An approach to determine the carrier relaxation ratio for the gain and saturable absorber sections of a passively mode-locked quantum dot laser using measurable static laser parameters is presented.

A two-color laser operating in the excited states of a quantum dot is realized by asymmetric pumping and external feedback stabilization. A mode separation ranging from 1.3-THz to 3.6-THz is demonstrated.

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.

A passively mode-locked laser with a 2.1 GHz fundamental repetition rate is demonstrated using a 20-mm-long monolithic 2-section quantum dot laser. A pulse width of 17 ps and 280 mW peak power were obtained.

The optical matrix element for excited-states is significantly weaker than the ground-state leading to thermally stable radiative recombination. This is not so for non-radiative Auger recombination, causing a sharp increase in threshold current with temperature.

A delay differential equation-based model for passive mode locking in semiconductor lasers is shown to offer a powerful and versatile mathematical framework to simulate quantum-dot lasers, thereby offering an invaluable theoretical tool to study and comprehend the experimentally observed trends specific to such systems. To this end, mathematical relations are derived to transform physically measured quantities from the gain and loss spectra of the quantum-dot material into input parameters to seed the model. In the process, a novel approach toward extracting the carrier relaxation ratio for the device from the measured spectra, which enables a viable alternative to conventional pump-probe techniques, is presented. The simulation results not only support previously observed experimental results, but also offer invaluable insight into the device output dynamics and pulse characteristics that might not be readily understood using standard techniques such as autocorrelation and frequency-resolved optical gating.

In this paper, a detailed study is presented on a series of quantum dot (QD) passively mode-locked lasers (MLLs) with variable absorber to gain-section length ratios. The effect of temperature on the stability of pulses emitted from the QD ground state is primarily examined and compared to an analytical model that predicts regions of mode-locking stability for a given device layout. The model correctly predicts the temperatures of maximum operability in each device for a variety of absorber voltages. Prediction of the regimes of excited-state operation from the QDs is also included and experimentally verified. For the first time, the unsaturated absorption is identified as a key parameter that strongly influences the range of biasing conditions that produce stable mode-locked pulses. This dataset offers valuable insight into design of future MLL devices for maximum optical pulse quality over a large range of temperature and biasing conditions.

Broad temperature operation is demonstrated from 20 to 110 °C in a 5-GHz monolithic two-section InAs/GaAs quantum dot passively mode-locked laser with an optimized absorber to gain section length ratio of 0.11. Stable pulses of less than 19 ps full-width-half-maximum are measured over this entire temperature range. For a grounded absorber, mode-locking from the ground-state occurred over the range 20–92 °C, dual-mode lasing involving both ground and excited states from 93 to 98 °C and exclusively from the excited-state from 99 to 110 °C. The observed broad temperature operation agrees with theoretical analysis based on measured gain and absorption data that predicted improved temperature performance for a short absorber. The results are promising for the development of temperature-insensitive pulsed sources for uncooled applications such as data multiplexing and optical clocking.