Cavity length vs. inverse of slope efficiency technique is most widely used to extract the injection efficiency in
semiconductor lasers which assumes that all the carriers occupy single energy level in the laser active region. However,
QD lasers contain multiple higher lying energy levels in addition to the ground level and have significant carrier capture
times which results in the occupation of these higher energy levels. In addition to the multiple energy levels, the density
of states of each energy level is inhomogeneously broadened, which leads to the broadening of the gain spectrum as a
whole. Inhomogeneous broadening is a result of the random size distribution of QDs grown by the self-assembled
growth technique. In this work, we present the results of an above threshold multi-level rate equation model developed
to understand the effect of inhomogeneous broadening on the measured low injection efficiencies of InAs-InGaAs based
quantum-dot (QD) lasers operating at 1.3 μm.
In this paper we present a novel implementation of high bandwidth constant modulation current circuit to the traditional
small signal optical response technique used to determine the differential carrier lifetime of a semiconductor laser. This
circuit is designed for the voltage to current conversion and to deliver a constant modulation current to the laser diode.
The circuit rectifies parasitic effects of high value surface mount resistor at high frequencies used in the impedance
independent optical technique and also has lower crosstalk. The application of this circuit can be generalized where the
requirement arises for a high bandwidth constant modulation current circuit.
In this paper we report on carrier lifetime measurements performed on 1.3 μm p-doped InAs quantum-dot lasers. The carrier lifetimes were determined by fitting the measured sub-threshold optical modulation response to a single pole response function, and then correcting this time constant for the diode junction capacitance to obtain the carrier lifetime. The sub-threshold frequency response curves did indeed show a single pole behavior at all the bias currents and, as expected, the extracted carrier lifetimes monotonically decrease with increasing bias currents. The differential carrier lifetime versus bias current data was then fitted, using a simple single carrier level rate equation analysis, to determine the recombination coefficients. Using this simplified analysis, the values of the recombination coefficients are found to be: A = 1.0 x 107 /s, B = 2.5 x 10-11 cm3/s, and C = 1.1 x 10-29 cm6/s at room temperature. Since, the carriers are distributed among the dots in a complicated manner that depends on bias, the lifetimes and recombination coefficients extracted using the single carrier level analysis are the effective or average values. Thus we have also built a multi-level rate equation model including the capture and escape times between various QD and wetting layer states. The multi-level rate equation model yields intrinsic recombination coefficients of AQD = 5.5 x 107 /s, BQD = 6.5 x 10-11 cm3/s, CQD = 5.6 x 10-29 cm6/s. Regardless of the model used the dominant contribution to the threshold current is found to be Auger recombination which accounts for approximately 80 % of the threshold current in our 1.3 μm p-doped QD lasers.
Carrier lifetime measurements are a powerful tool to understand and quantify the recombination mechanisms in semiconductor lasers. In this work we report the results of carrier lifetime measurements performed on 1.3 μm p-doped InAs Quantum-Dot lasers at room temperature using the small-signal modulation technique. The carrier lifetime at a particular bias current is determined by fitting the measured optical frequency response curves to the calculated response derived from sub-threshold carrier and photon rate equations. Calculated optical response curves are dominated by a single pole regardless of whether a single or multiple carrier level rate equation analysis is used. We also measure a single pole optical response, throughout the entire range of bias currents, thus allowing us to extract the differential carrier lifetime. The recombination coefficients are extracted by simultaneously fitting the variation of differential carrier lifetime with bias current to equations relating the current and carrier lifetime to the recombination coefficients and carrier density. Specifically we find a cubic (or Auger) recombination coefficient of 1.2 x 10-29 cm6/s and 5.6 x 10-29 cm6/s in the single and multi carrier level rate equations respectively, while the bimolecular (radiative) coefficients are 1.8 x 10-11 cm6/s and 6.5 x 10-11 cm6/s, and the monomolecular (defect) coefficients are 2.9x107 /s and 5.5x107 /s. Regardless of the analysis used we find that the vast majority, approximately 80%, of the current at threshold is due to the cubic recombination process which is traditionally assumed to be Auger recombination.
In this work we study the role of free carriers and excitons on the characteristics of 1.3 μm InAs/InGaAs quantum dot lasers. The study is carried out theoretically by building a mathematical model to calculate the threshold current in the laser and the charateristic temperature, T0. In order to determine the role of free carrier and excitons on the laser characteristics the model allows for different carrier distribution assumptions to be used, and we look at three cases; all free carriers, all excitons, and both free carriers and excitons in the dots. Our model results show that if we allow either free carriers or excitons to exist but not both, the calculated threshold current and T0 do not match with the experimental values. Thus we conclude that both free and bound carriers must exist and develop a method for modeling this case. We use a modified form of the Saha equation to calculate the ratio of free carriers to excitons and modify the material gain to account for this ratio. This model results in a threshold current density of approximately 39 A/cm2 and a T0 of 83 K, both of which are in excellent agreement with experimental results.
A novel method to measure the optical modulation response of laser diodes that uses as the modulation source the output of a femtosecond optical parametric oscillator (OPO) is described. The femtosecond OPO generates a train of ~ 150 fs pulses tunable between 1.03 and 1.35 μm with an average power of 12 mW at a repetition rate of 81 MHz. With such a narrow pulse a rich frequency spectrum of flat intensity distribution that easily surpasses the 2000 GHz 3 dB-bandwidth is obtained. To perform modulation response measurements the OPO is selectively tuned to modulate the carrier population in either the well or separate confinement region of the laser diode. Modulation traces obtained with this method in 1.3 μm InAsP lasers are presented and compared with those obtained from electrical modulation at the same operating conditions.
In this paper we present results of small signal response measurements of 1.3mm strained InAsP buried heterostructure multiple quantum well lasers obtained using optical and electrical excitation. Direct modulation of the carrier population in the quantum wells with a femtosecond pulse from an Optical Parametric Oscillator yields frequency response traces with modulation bandwidths of ~ 6 GHz at biases of 1.5 and 1.8+ threshold. These results contrast with those obtained with electrical excitation for which modulation bandwidths of ~ 3 GHz are obtained at the same DC bias conditions. Analysis of the modulation traces obtained with optical excitation show that in these lasers, transport processes play a dominant role in the frequency response.
In this paper we analyze the frequency response of 1.3+m highly strained InAsP/InGaAsP MQW lasers under small signal conditions. We show that in these lasers, electrical parasitics limit the high frequency response. These parasitics which are inherent to the laser structure, show an inductance-like behavior as determined from impedance measurements. We further show that the effect of the parasitic inductance in the laser modulation response can be significantly reduced by modifying the laser driving circuit.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.