H. Shahid

Direct modulation of excited state quantum dot lasers

The use of the excited state quantum dot lasers for high speed direct modulation is proposed and demonstrated. A direct comparison of lasers utilizing the ground state and excited state from the same laser material reveals a factor of two increase in the K-factor limited bandwidth. This is attributed to an increase in the saturated gain and reduced carrier scattering time of the excited state compared to the ground state.

Negative differential gain due to many body effects in self-assembled quantum dot lasers

The gain spectrum of a quantum dot laser operating at 1300 nm is studied at high carrier densities, corresponding to dot occupancies of ∼8 e-h pairs per quantum dot. A reduction in peak gain with increasing carrier density is observed, attributed to the saturation of peak gain, yet the continuous increase in dephasing acting to broaden the individual quantum dot transitions.

Toward 1550-nm GaAs-Based Lasers Using InAs/GaAs Quantum Dot Bilayers

By choice of appropriate growth conditions and optimization of the strain interactions between two closely stacked InAs/GaAs quantum dot (QD) layers, the emission wavelength of the QDs can be significantly extended, giving room-temperature emission from highly uniform QD ensembles in excess of 1500 nm. These QD bilayers are incorporated into edge-emitting laser structures and room-temperature ground-state lasing at 1420 nm and electroluminescence at 1515 nm are observed. Under high-bias conditions, asymmetric broadening of peaks in the laser gain spectra are observed, extending positive net modal gain from the devices to beyond 1500 nm, and the origin of this broadening is discussed.

Excited State Bilayer Quantum Dot Lasers at 1.3 µm

We report the realization of excited state bilayer quantum dot (QD) lasers in the 1.31 µm region. The higher saturated gain and lower scattering time of the excited states of the ensemble of QDs offers the opportunity for high modulation bandwidths. Gain measurements for these structures are discussed and compared to conventional QD laser structures. The extension of QD ground state operating wavelengths to 1.45 µm spanning the O- and E-band is also demonstrated.

Gain spectrum measurement using the segmented contact method with an integrated optical amplifier

The measurement of optical gain utilising a segmented contact and integrated optical amplifier is reported. We show that in a direct comparison of methods, the use of the integrated amplifier allows the gain spectrum to be deduced over wider spectral ranges and to lower carrier densities, as compared to the conventional segmented contact technique.

Negative differential gain in 1.3um quantum dot lasers: comparison of self-heating and free carrier effects

A comparative study of the gain spectra of quantum-dot lasers at high carrier densities is reported. The gain spectra of quantum dot lasers under a constant junction temperature (obtained by the use of Fabry-Perot modes as a temperature gauge) and under constant heat-sink temperatures are measured. Negative differential gain, observed for the ensemble of quantum dot ground-states is shown to be mainly due to free carrier effects, where increasing dephasing effects, combined with saturated gain, result in spectral broadening and a reduction in the peak gain.

Comparison of gain measurement techniques for 1.3μm quantum dot lasers

This paper reports on the direct comparison of Hakki-Paoli and segmented contact method gain spectra measurement techniques for InAs/GaAs 1.3μm quantum dot laser material. This was made possible by realizing a single-mode segmented contact device structure. The differences in required apparatus, ease of measurements, signal to noise ratio, and relative advantages and disadvantages of these two complementary techniques are discussed under continuous wave operation. As expected, at current densities (prior to possible self heating effects) the techniques are shown to give essentially identical results. However, the segmented contact method is demonstrated to be more accurate for internal loss measurements. A method to remove self heating effects in the Hakki-Paoli measurements is described and allows the gain and spontaneous emission measurements to be performed at carrier densities as high as 5.5kA/cm2 with a constant cavity temperature.

Gain and absorption characteristics of bilayer quantum dot lasers beyond 1.3 μm

In this paper we report on the multi-section gain and absorption analysis of strain engineered molecular beam epitaxy (MBE) grown GaAs and InGaAs capped bilayers. The InGaAs capped bilayer quantum dot (QD) lasers extends the room temperature lasing wavelength to 1.45 μm. The spectral measurement of gain demonstrates that net modal gain is achieved beyond 1.5 μm at room temperature. Analysis of the temperature and current density dependence gain characteristics of a GaAs capped bilayer sample indicate that the temperature sensitivity of threshold current around room temperature is due to phonon assisted thermal escape of carriers from the QDs.

Excited State Bilayer Quantum Dot Lasers at 1.3μm

We report the realization of excited state bilayer quantum dot (QD) lasers in the 1.31 m region. The higher saturated gain and lower scattering time of the excited states of the ensemble of QDs offers the opportunity for high modulation bandwidths. Gain measurements for these structures are discussed and compared to conventional QD laser structures. The extension of QD ground state operating wavelengths to 1.45 m spanning the Oand E-band is also demonstrated. # 2011 The Japan Society of Applied Physics

A Platform for GaAs Opto-electronic Integrated Circuits Based on GaAs/AlGaAs Regrowth Upon Patterned InGaP

We describe how novel approaches for fabrication of GaAs-based self-aligned lasers, superluminescent diodes and distributed feedback lasers provide the foundations for a toolkit for GaAs-based opto-electronic integrated circuits.