Measurements of the polarization dependence of the gain of strained multiple quantum well InGaAs-InP lasers

The polarization-dependent gain spectra of both tensile and compressive strain multiple-quantum-well (MQW) In/sub x/Ga/sub 1-x/As-InP lasers in a relatively large strain regime are presented. The results show that MQW lasers with tensile strain and an In concentration as low as 43% in the wells lase in a pure transverse magnetic (TM) mode rather than a transverse electric (TE) mode with a gain difference of 60-70 cm/sup -1/ at all the injection currents investigated. The peak gain for the TE mode is shifted toward shorter wavelengths from that of the TM mode, indicating that the emission is principally due to light hole-electron transition. The differential gain of the TM mode is about 1.5 times higher than that of the TE mode operation. Opposite phenomena were observed in the compressive strained MQW lasers.<>     

Analysis of the performance of 1.55-μm InGaAs-InP tensilestrained quantum-well lasers

We fabricated 1.55-μm tensile strained InGaAs quantum-well (QW) lasers into broad-area and ridge waveguide lasers, and their performance was analyzed and compared with compressive strained and lattice-matched QW lasers. It is seen that the limitation on the tensile strain to a value less than 0.7%, which is required to prevent the emission wavelength being shorter than 1.55 μm, imposes restrictions on the performance enhancement in several aspects. Broad-area InGaAs QW lasers with a tensile strain of 0.7% show a larger gain coefficient and smaller transparency current density per well than those with InGaAsP QW lasers with a compressive strain of 1.0%. However, the internal quantum efficiency is much smaller than that for compressive ones and the internal optical loss increases rapidly as the number of QW's increases. These are thought to be caused by a smaller conduction band offset and the onset of dislocation generation at the well-barrier interfaces with the number of QW's, respectively. Ridge waveguide lasers with two QW's with tensile strain of 0.7%, which is designed not to exceed the critical thickness for dislocation generation, show smaller modal gain coefficients and inferior temperature characteristics as compared to those with six 0.7% compressive strained QW's and those with three lattice matched InGaAs QW's. However, the modulation bandwidth is measured to be larger than that for one that is compressively strained. It is believed to originate from the small effective capture time of the carriers due to thicker wells     

Optical-confinement-factor dependencies of the K factor, differential gain, and nonlinear gain coefficient for 1.55 mu m InGaAs/InGaAsP MQW and strained-MQW lasers

Optical-confinement-factor Gamma dependencies of the K factor, differential gain, dg/dN, and nonlinear gain coefficient epsilon , for 1.55 mu m InGaAs/InGaAsP multiple-quantum-well (MQW) and compressively strained MQW lasers, were investigated experimentally. For both MQW and strained-MQW lasers, when Gamma is increased, the K factor is reduced, dg/dN is increased, but epsilon is almost constant. These results indicate that the Gamma dependence of the K factor mainly results from a change in dg/dN, and does not result from a change in epsilon . For the strained MQW lasers, the K factor, dg/dN, and epsilon are, respectively, half as large, twice as large, and the same as those for the MQW lasers, when both types of lasers have the same Gamma (=0.05). This suggests that the strained MQW lasers with a large Gamma have a small K factor and thus are preferable for achieving large modulation bandwidths.<>     

1.3-μm InAsP modulation-doped MQW lasers

The effect of both n-type and p-type modulation doping on multiple-quantum-well (MQW) laser performances was studied using gas-source molecular beam epitaxy (MBE) with the object of the further improvement of long-wavelength strained MQW lasers. The obtained threshold current density was as low as 250 A/cm² for 1200-μm-long devices in n-type modulation-doped MQW (MD-MQW) lasers. A very low CW threshold current of 0.9 mA was obtained in 1.3-μm InAsP n-type MD-MQW lasers at room temperature, which is the lowest ever reported for long-wavelength lasers using n-type modulation doping, and the lowest value for lasers grown by all kinds of MBE in the long-wavelength region. Both a reduction of the threshold current and the carrier lifetime in n-type MD MQW lasers caused the reduction of the turn-on delay time by about 30%. The 1.3-μm InAsP strained MQW lasers using n-type modulation doping with very low power consumption and small turn-on delay time are very attractive for laser array applications in high-density parallel optical interconnection systems. On the other hand, the differential gain was confirmed to increase by a factor of 1.34 for p-type MD MQW lasers (N_A=5×10¹⁸ cm ^-3) as compared with undoped MQW lasers, and the turn-on delay time was reduced by about 20% as compared with undoped MQW lasers. These results indicate that p-type modulation doping is suitable for high-speed lasers     

High-speed modulation of long-wavelength In1-xGaxAsyP1-y andIn1-x-yGaxAlyAs strained quantum-welllasers

In_1-xGa_xAs_1-yP_y quantum-well (QW) lasers with compressive strain and In_1-x-yGa_xAl_yAs QW lasers with two strain types (compressively strained and lattice matched) for 1.55-μm telecommunication applications are investigated both in the steady-state and high-speed microwave modulation schemes. Under steady-state electric bias, the gain and intrinsic loss are measured based on the well-known Hakki-Paoli method from below threshold to threshold. The photon lifetime is obtained from this measurement. A comprehensive theoretical gain model with realistic band structure, including valence band mixing and many-body effects, is then used to fit the experimentally obtained modal gain profiles and extract the carrier density and, therefore, the differential gain. In the high-speed microwave modulation scheme, the experimental modulation response curves are fitted by the theory and parameters such as the differential gain and K factor are obtained. The differential gain agrees very well with the value obtained from the steady-state direct optical gain measurement. The comparison of two material systems will be important to design high-bandwidth high-performance semiconductor lasers in order to meet requirements of 1.55-μm telecommunication applications     

Barrier composition dependence of differential gain and externaldifferential quantum efficiency in 1.3-μm strained-layermultiquantum-well lasers

Dependence of the differential gain and the external differential quantum efficiency on the composition of InGaAsP barrier layers were investigated for 1.3 μm InGaAsP-InGaAsP compressively strained layer (SL) multiquantum well (MQW) lasers. In this investigation, we compared between SL-MQW lasers and unstrained MQW lasers having the same well thicknesses and the same emitting wavelength in order to clarify the effect of strain for each barrier composition. As a result It has been found that the barrier composition has large influence on the differential gain and the external differential quantum efficiency in the SL-MQW lasers. Narrower band-gap barrier means little effect of strain on the differential gain due to the electron overflow from a well layer, while wider band-gap barrier means degradation in the differential gain and the external differential quantum efficiency due to the nonuniform injection of hole into a well layer. In this experiment, the barrier composition of 1.05 μm is suitable for 1.3 μm InGaAsP-InGaAsP SL-MQW lasers to realize large differential gain and high external differential quantum efficiency simultaneously     

Effect of in-phase and antiphase gain coupling on high-speedproperties of MQW DFB lasers

Effect of in-phase and antiphase gain-coupling on high-speed properties is studied for MQW DFB lasers with periodically truncated quantum-wells. The enhancement of modulation bandwidth due to antiphase gain-coupling is found significantly suppressed, and gain-coupled DFB lasers with high κL are preferred for large modulation bandwidth due to the presence of linear gain saturation in MQW lasers     

Long wavelength quantum well lasers: Synopsis of the RACE “AQUA” project: MOVPE/MBE/GSMBE for InGaAsP/InP and InGaAlAs/InP high speed MQW DFB lasers

The technological limits for ultra high speed devices are now rapidly expanding due to the use of quantum well (QW) materials. This new class of materials gives the opportunity of tailoring materials parameters by controlling geometries on an atomic scale. They look very promising as materials for lasers, detectors and transistors suitable even above 10 Gb/s. It will be demonstrated that state of the art MQW structures can be realized in both material systems, InGaAsP/InP and InGaAlAs/InP. Parallel lateral laser structures (e.g. SIBH, BRS and TBH) have been designed to take full benefit of QW technology. Ultimate reduction of parasitics, whilst using potential low cost fabrication technologies is the basis for achieving high bitrate (10 Gb/s) MQW lasers, even with the stronger damping in QW material. Using the DFB-SIBH laser structure 10 Gb/s large signal experiments are successfully performed with bulk, MQW and SLMQW lasers. Extremely low fall times of 44 ps are achieved. Additional MQW based improvements are observed such as: −3 times higher differential gain, increased output power (>110 mW), 2.5 times lower chirp (Δλ_−20dB = 0.40 nm at 10 Gb/s modulation), and 2 dB gain in power budget at 10 Gb/s digital transmission.     

The theory of strained-layer quantum-well lasers with bandgaprenormalization

A theoretical model for the strained-layer quantum-well laser is presented taking into account the valence-band mixing and the bandgap renormalization. Our theoretical approach for the electronic properties is based on the Luttinger-Kohn Hamiltonian, including the strains and the carrier-induced bandgap shifts using the Hartree-Fock approximation. The effects of the biaxial compressive and tensile strains on the gain, the output characteristics, the bandgap renormalization, and the modulation response of strained-layer quantum-well lasers are studied. We present new results incorporating the many-body effects in the form of the bandgap renormalization with the valence-band mixing and the multisubband effects. It is found that the bandgap renormalization depends strongly on the nature of strain applied to the quantum well. The differential gain that determines the upper frequency limit of the direct current modulation is calculated from the total derivative of the equigain surface with respect to the carrier- and the photon-densities near the threshold condition. Our approach to the differential gain yields reasonable agreement between theory and experiment for the 3-dB modulation bandwidth. Both InGaAs-AlGaAs and InGaAs-InP strained quantum-well systems are considered     

Reduction of linewidth enhancement factor in InGaAsP-InPmodulation-doped strained multiple-quantum-well lasers

The linewidth enhancement factor α in InGaAsP-InP modulation-doped strained multiple-quantum-well (MQW) lasers has been evaluated theoretically and experimentally. A reduction of the α-parameter due to modulation doping is demonstrated. A small α-parameter of less than 1 is obtained not at wavelength for the gain peak but within certain range of wavelength where the gain is positive. The smaller α-parameter in modulation-doped strained MQW lasers should result in performance improvements that are advantageous for optical communication system applications     

Reduction of noise figure in semiconductor laser amplifiers with Ga1-xInxAs/GaInAsP/InP strained quantum wellstructures

The noise characteristics of semiconductor laser amplifiers (SLAs) in the Ga_1-xIn_xAs/GaInAsP/InP strained quantum well (QW) system are theoretically calculated and analyzed using density-matrix theory and taking into account the effects of band mixing on both the valence subbands and the transition dipole moments. The numerical results show that a reduced noise figure can be obtained in both tensile and compressively strained QW structures due to the increase in differential gain and the decrease in transparent carrier density. From a comparison among compressively strained (x=0.70), unstrained (x=0.53), and tensile strained (x=0.40) QW SLAs at a fixed carrier density and optical confinement factor, it is found that the noise figure of the tensile strained QW reaches its lowest value of 3.4 dB at average input optical power of -20 dB     

Calculating the optical properties of multidimensionalheterostructures: Application to the modeling of quaternary quantum welllasers

A method for calculating the electronic states and optical properties of multidimensional semiconductor quantum structures is described. The method is applicable to heterostructures with confinement in any number of dimensions: e.g. bulk, quantum wells, quantum wires and quantum dots. It is applied here to model bulk and multiquantum well (MQW) InGaAsP active layer quaternary lasers. The band parameters of the quaternary system required for the modeling are interpolated from the available literature. We compare bulk versus MQW performance, the effects of compressive and tensile strain, room temperature versus high temperature operation and 1.3 versus 1.55 pm wavelength operation. Our model shows that: compressive strain improves MQW laser performance. MQW lasers have higher amplification per carrier and higher differential gain than bulk lasers, however, MQW performance is far from ideal because of occupation of non-lasing minibands. This results in higher carrier densities at threshold than in bulk lasers, and may nullify the advantage of MQW lasers over bulk devices for high temperature operation     

Threshold current analysis of compressive strain (0-1.8%) inlow-threshold, long-wavelength quantum well lasers

A comprehensive study of the effect of compressive strain on the threshold current performance of long-wavelength (1.5 μm) quantum-well (QW) lasers is presented. Model predictions of threshold currents in such devices identify QW thickness as a parameter that must be considered in optimizing laser performance when Auger currents are present. Experimental comparisons between strained and unstrained devices reveal strain-induced reductions in internal transparency current density per QW from 66 to 40 A/cm², an increase in peak differential modal gain from 0.12 to 0.23 cm/A, and evidence for the elimination of intervalence band absorption as compressive strain increases from 0 to 1.8%. However, most of these improvements arise in the first ~1% of compressive strain. To fabricate low-threshold 1.5-μm buried heterostructure (BH) devices in InP using the strained QW active regions an optimized design which shows that threshold current is at its lowest when the stripe width is approximately 0.6-0.7 μm is derived. Results for uncoated BH lasers are reported     

High-speed 1.5-μm compressively strained multi-quantum wellself-aligned constricted mesa DFB lasers

A great improvement in the high-speed characteristics for compressively strained multi-quantum-well (MQW) distributed-feedback (DFB) lasers with self-aligned constricted mesa structures is described. Negative wavelength detuning is an important factor in making possible the extraction of potential advantages for the compressively strained MQW DFB lasers. A 17-GHz bandwidth, which is the highest among the 1.5-μm MQW DFB lasers, is demonstrated. A wavelength chirp width of 0.42 nm at 10 Gb/s is obtained due to a reduced linewidth enhancement factor that has a magnitude of less than 2. Nonlinear damping K factor in a DFB laser with 45-nm negative detuning has drastically decreased to 0.13 ns, about half of that for unstrained MQW lasers. This is mainly due to an enhanced differential gain as large as 6.9×10 ^-12 m³/s. The estimated intrinsic maximum bandwidth is 68 GHz     

Effect of linear gain saturation on small-signal dynamics of MQWDFB lasers

The linear gain saturation effect is shown to be important in determining the dynamics of multiple-quantum-well (MQW) distributed-feedback (DFB) lasers. A more realistic logarithmic dependence of material gain on carrier density is assumed in a comprehensive MQW DFB laser model. It is found through simulation that because of the linear gain saturation, the interplay between modal gain and differential gain leads to an optimal κL for maximum small-signal modulation bandwidth in λ/4-shifted MQW DFB lasers     

Theoretical study on enhanced differential gain and extremelyreduced linewidth enhancement factor in quantum-well lasers

Low-chirp lasing operation in semiconductor lasers is addressed in a theoretical investigation of the possibility of reducing the linewidth enhancement factor (α factor) in quantum-well (QW) lasers to zero. It is shown that in reducing the α factor it is essential that lasing oscillation be around the peak of the differential gain spectrum, not in the vicinity of the gain peak. The condition for such lasing oscillation is analytically derived. The wavelength dependence of the material gain, the differential gain, and the α factor are calculated in detail taking into account the effects of compressive strain and band mixing on the valence subband structure. The effect of p-type modulation doping in compressively strained QWs is discussed. It is shown that the α factor, the anomalous dispersion part in the spectrum, crosses zero in the region of positive material gain, which makes is possible to attain virtual chirpless operation by detuning     

Novel design scheme for high-speed MQW lasers with enhanceddifferential gain and reduced carrier transport effect

We present a theoretical analysis exploring the optimum design of high-speed multiple-quantum-well (MQW) lasers for 1.55-μm operation. Various combinations of well and barrier materials are examined for lattice-matched, strained-layered (SL), and strain-compensated (SC) MQW lasers with InGaAsP and InGaAlAs barriers. The gain characteristics are investigated for these MQW lasers with various barrier bandgap wavelengths and are used to evaluate the modulation characteristics based on the carrier dynamics model which includes a set of Poisson, continuity, and rate equations. The importance of band engineering aimed at simultaneously reducing the carrier transport effect and enhancing the differential gain is described. It is shown that SC-MQW lasers with InGaAlAs barriers have an advantage in reducing the density of states in the valence band by reducing the overlap integral between the heavy- and light-hole wave functions, which effect has previously been discarded as a minor correction in designing conventional InGaAsP-based MQW lasers. Furthermore, the hole transport rate across the barriers can be drastically reduced in SC-MQW lasers due to the reduced effective barrier height for the holes. Based on this novel design scheme, a 3-dB bandwidth approaching 70 GHz is expected for 20-well SC-MQW lasers with InGaAlAs barriers as a result of both the large differential gain and reduced transport effect     

Theoretical investigation of gain enhancements in strainedIn0.35Ga0.65As/GaAs MQW lasers via p-doping

We present a systematic theoretical investigation of the influence of p-doping on the gain characteristics of strained In_0.35Ga _0.65As/GaAs multiple-quantum-well (MQW) lasers, and compare the results with those obtained experimentally from devices with record 30 GHz modulation bandwidths. Experimentally, the combination of p-doping and strain has been found to lead to only a small increase in the differential gain, ∂g/∂n, but a large decrease in the non-linear gain coefficient, ϵ; this behaviour has been theoretically accounted for by a doping-induced decrease in the intraband relaxation time, τ_in. The theoretical investigations reveal that the assumption of a constant intraband relaxation time is not sufficient to describe the role of p-doping in the above devices, and highlight the importance of utilizing an appropriate lineshape function for the modelling of high speed laser modulation behaviour     

Theory of non-Markovian gain in strained-layer quantum-well laserswith many-body effects

A non-Markovian model for the optical gain of strained-layer quantum-well lasers is developed taking into account the valence-band mixing, strain effects, many-body effects, and the non-Markovian relaxation using the time-convolutionless reduced-density operator formalism given in previous papers for an arbitrary driven system coupled to a stochastic reservoir. Many-body effects are taken into account within the time-dependent Hartree-Fock approximation and the valence-band structure is calculated from the 6×6 Luttinger-Kohn Hamiltonian. The optical gain with Coulomb (or excitonic) enhancement is derived by integrating the equation of motion for the interband polarization. It is shown that the vertex function for the interband polarization can be obtained exactly without relying on the Pade approximation. As a numerical example, an In_xGa_1-xAs-InP quantum well (QW) is chosen for its wide application in optical communication systems. It is predicted that the Coulomb enhancement of gain is pronounced in the cases of compressive and unstrained QWs while it is negligible in the case of tensile strained QW     

High-performance λ=1.3 μm InGaAsP-InP strained-layerquantum well lasers

Compressively and tensile strained InGaAsP-InP MQW Fabry-Perot and distributed feedback lasers emitting at 1.3-μm wavelength are reported. For both signs of the strain, improved device performance over bulk InGaAsP and lattice-matched InGaAsP-InP MQW lasers was observed. Tensile strained MQW lasers show TM polarized emission, and with one facet high reflectivity (HR) coated the threshold currents are 6.4 and 12 mA at 20 and 60°C, respectively. At 100°C, over 20-mW output power is obtained from 250-μm-cavity length lasers, and HR-coated lasers show minimum thresholds as low as 6.8 mA. Compressively strained InGaAsP-InP MQW lasers show improved differential efficiencies, CW threshold currents as low as 1.3 and 2.5 mA for HR-coated single- and multiple quantum well active layers, respectively, and record CW output powers as high as 380 mW for HR-AR coated devices. For both signs of the strain, strain-compensation applied by oppositely strained barrier and separate confinement layers, results in higher intensity, narrower-linewidth photoluminescence emissions, and reduced threshold currents. Furthermore, the strain compensation is shown to be effective for improving the reliability of strained MQW structures with the quantum wells grown near the critical thickness. Linewidth enhancement factors as low as 2 at the lasing wavelength were measured for both types of strain. Distributed feedback lasers employing either compressively or tensile strained InGaAsP-InP MQW active layers both emit single-mode output powers of over 80 mW and show narrow linewidths of 500 kHz