Measurement of gain saturation coefficients in strained-layermultiple quantum-well distributed feedback lasers

Gain saturation coefficients of unstrained- and strained-layer multiple-quantum-well lasers were measured experimentally. These coefficients were higher in lasers that had compressive strain in their active-layer wells: 2.45×10^-17 cm³ with unstrained wells and 12.6×10^-17 cm³ with strained wells. The higher gain saturation coefficient in lasers with strained active-layer wells is related to their higher linear TE mode gain coefficient. The linearity factor (K factor) between a laser's damping constant and the square of the laser's resonant frequency decreased slightly with the introduction of the strain in the laser's active layer wells. This factor, however, took the value of about 0.2×10^-9 s for each of these lasers     

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     

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     

Theoretical study of differential gain in strained quantum wellstructures

The differential gain expected in a strained quantum well (QW) and in a lattice-matched QW is discussed based on a theoretical treatment of the band structures where the band nonparabolicity is taken into account. The differential gain in strained QWs will be larger by about three to four times relative to lattice-matched QWs, and a maximum differential gain of 4-6×10^-15 cm² will be possible in strained QWs. The anisotropy of the subband nonparabolicity in lattice-matched QWs contributes to the larger difference between the two types of QWs. The calculated band-edge effective masses and the calculated laser properties, are compared to the available measurements, and some comments are given for realizing high-speed strained lasers     

Gain and linewidth enhancement factor in InAs quantum-dot laserdiodes

Amplified spontaneous emission measurements are investigated below threshold in InAs quantum-dot lasers emitting at 1.22 μm. The dot layer of the laser was grown in a strained quantum well (QW) on a GaAs substrate. Ground state gain is determined from cavity mode Fabry-Perot modulation. As the injection current increases, the gain rises super-linearly while changes in the index of refraction decrease. Below the onset of gain saturation, the linewidth enhancement factor is as small as 0.1, which is significantly lower than that reported for QW lasers     

Compressively strained 1.3 μm InAsP/InP and GaInAsP/InP multiplequantum well lasers for high-speed parallel data transmission systems

Turn-on delay times in the pulse response of compressively strained InAsP/InP double-quantum-well (DOW) lasers and GaInAsP/InP multiple-quantum-well (MQW) lasers emitting at 1.3 μm were investigated. DQW lasers with 200-μm cavity length and high-reflection coating achieved both a very low threshold current (1.8 mA) and a small turn-on delay time (200 ps), even under a biasless 30-mA pulse current. Compressively strained or lattice-matched GaInAsP MQW lasers and GaInAsP double-heterostructure (DH) lasers were also fabricated and compared. It was observed that the carrier lifetime was enhanced for InAsP DQW lasers and strained GaInAsP MQW lasers compared to the lattice-matched GaInAsP MQW lasers and conventional double-heterostructure lasers. To explain this increase in the carrier lifetime, the effect of the carrier transport on the carrier lifetime was studied. The additional power penalty due to the laser turn-on delay was simulated and is discussed     

Spontaneous Radiative Efficiency and Gain Characteristics of Strained-Layer InGaAs–GaAs Quantum-Well Lasers

The optical gain spectra, unamplified spontaneous emission spectra, and spontaneous radiative efficiency are extracted from the measurement of amplified spontaneous emission (ASE) on a single pass, segmented contact 0.98-mum-emitting aluminum-free InGaAs-InGaAsP-GaAs quantum-well (QW) laser diode. These measurements provide a baseline for which to compare higher strain InGaAs QW lasers emitting near 1.2 mum. The peak gain-current relationship is extracted from gain spectra and the peak gain parameter go is found to agree within 25% of the value extracted using conventional cavity length analysis for 0.98-mum-emitting devices. The spontaneous radiative current is extracted using the fundamental connection between gain and unamplified spontaneous emission, which in turn gives an estimate of the amount of nonradiative recombination in this material system. The spontaneous radiative efficiency, the ratio of spontaneous radiative current to total current, at room temperature of 0.98-mum-emitting InGaAs QW laser material is found to be in the range of 40%-54%, which is 2.5-3.5 times larger than that of highly strained InGaAs QW laser emitting near lambda = 1.2 mum. Whereas the gain parameter, g₀ = dg/d(ln j), was measured to be 1130 and 1585 cm^-1 for the 0.98-mum- and 1.2-mum-emitting materials, respectively. From the calculated below threshold current injection efficiency of 75%-85%, we deduce that the internal radiative efficiency of the QW material is ~ 20% higher than the ratio of internal radiative current to external injected current extracted directly from ASE measurements.     

Empirical formulas for design and optimization of 1.5 μmInGaAs/InGaAsP strained-quantum-well lasers

The effects of strain and number of quantum wells on optical gain, differential gain, and nonlinear gain coefficient in 1.55-μm InGaAs/InGaAsP strained-quantum-well lasers are theoretically investigated. Well-approximated empirical expressions are proposed to model these effects. Using these formulas, one can easily and accurately predict the performance of a laser diode for a given structure. Therefore, these empirical formulas are useful tools for design and optimization of strained quantum well lasers. As a general design guideline revealed from the empirical formulas, the threshold current is reduced with the compressive strain, and the modulation bandwidth is most efficiently increased with the number of wells     

Threshold current density in strained layerInxGa1-xAs-GaAs quantum-well heterostructurelasers

The authors consider the transparency carrier density in ideal and practical strained layer In_xGa_1-xAs-GaAs quantum-well heterostructure lasers. The transparency carrier density in practical structures is then related to transparency current density using realistic values for spontaneous recombination rates. These parameters are incorporated with representative structural parameters into a nonlinear model for gain in a quantum-well laser, in order to provide a complete model for the laser threshold current density in strained layer In_xGa_1-xAs-GaAs quantum-well heterostructure lasers. These results are then compared and contrasted with experimental laser results from several laboratories     

AlGaInAs/InP strained-layer quantum well lasers at 1.3 µm grown by solid source molecular beam epitaxy

Al_xGa_yIn_1−x−yAs/InP strained-layer multiple-quantum-well lasers emitting at 1.3 μm have been grown by solid source molecular beam epitaxy, and the performance characteristics have been studied. The lasers contain 4, 5, or 6 compressively strained quantum wells in the active region. They exhibit low transparency current densities, high gain coefficients, and high characteristic temperatures compared to conventional GaInAsP/InP quantum well lasers. The results show that desired lasing features can be achieved with relatively simple layer structures if the doping profiles and waveguide structures are properly designed and the material is grown to high structural perfection.     

高饱和电流14xx nm应变量子阱激光器的研制

报道了14xx nm应变量子阱(SQW)激光器管芯的研制成果。通过金属有机化学气相沉积(MOCVD)生长工艺生长14xx nm AlGaInAs/AlInAs/InP应变量子阱外延片,采用带有锥形增益区的脊型波导结构制作激光器管芯。生长好的外延片按照双沟脊型波导激光器制备工艺进行光刻、腐蚀,制作P面电极(溅射 TiPtAu)、减薄、制作N面电极(蒸发AuGeNi),然后将试验片解理成Bar;为获得高的单面输出功率,用电子回旋共振等离子体化学气相沉积(ECR)进行腔面镀膜,HR=90%,AR=5%;解理成的管芯P面朝下烧结到铜热沉上,TO3封装后在激光器综合测试仪进行测试。管芯功率达到440 mW以上,饱和电流3 A以上,峰值波长1430 nm,远场发散角为40°×14°。     

Analysis of differential gain in InGaAs-InGaAsP compressive andtensile strained quantum-well lasers and its application for estimationof high-speed modulation limit

A simplified model that furnishes an intuitive insight for the change in quantum-well (QW) laser gain due to QW strain and quantum confinement is presented. Differential gain for InGaAs-InGaAsP compressive and tensile strained multi-quantum-well (MQW) lasers is studied using the model. The comparison between the calculated and experimental results for lattice-matched and compressive strained MQW lasers shows that this model also gives quantitatively reasonable results. It is found that the variance-band barrier height strongly affects the differential gain, especially for compressively strained MQW lasers. The tensile strained MQW lasers are found to have quite high differential gain, due to the large dipole matrix element for the electron-light-hole transition, in spite of the large valence-band state density. Furthermore, a great improvement in the differential gain is expected by modulation p doping in the tensile strained MQW lasers. The ultimate modulation bandwidth for such lasers is studied using the above results     

Gain saturation coefficients of strained-layer multiple quantum-well distributed feedback lasers

The gain saturation coefficients were measured for strained and unstrained multiple quantum-well distributed feedback (MQW-DFB) lasers. The gain saturation coefficient depends on the deviation of the laser's transverse-magnetic (TM) mode gain peak wavelength from its transverse-electric (TE) mode gain peak wavelength delta lambda , which is related to the strain on the active-layer wells. The gain saturation coefficient epsilon increased with increasing compressed strain on the active-layer wells. The coefficient epsilon of the unstrained MQW DFB laser with a wavelength deviation delta lambda of -350 AA was 2.45*10/sup -17/ cm/sup 3/, and epsilon increased up to 12.6*10/sup -17/ cm/sup 3/ in the SL-MQW DFB laser with a wavelength difference delta lambda of -890 AA.<>     

1.4-/spl mu/m InGaAsP-InP strained multiple-quantum-well laser for broad-wavelength tunability

InGaAsP-InP strained multiple-quantum-well (MQW) lasers for extended wavelength tunability in external cavity operation were designed, fabricated, and tested. The active layer was a strain compensated structure consisting of three 3.2/spl plusmn/0.3 nm and three 6.4/spl plusmn/0.3 nm 1.0% compressive strained wells and five 10.3/spl plusmn/0.3 nm 0.45% tensile strained barrier layers. A 2-/spl mu/m-wide ridge waveguide laser of length 250 /spl mu/m, when used in a grating external cavity and with no coatings to alter the reflectivity of the facets, was observed to operate over a range >110 nm. The lasers were designed for applications in trace gas and liquid detection with the goal to maximize the tunable range when operated in external cavities and with no facet coatings.     

Dynamic, polarization, and transverse mode characteristics ofvertical cavity surface emitting lasers

The dynamic, polarization, and transverse mode characteristics of strained InGaAs-GaAs quantum well vertical cavity surface emitting lasers (VCSELs) emitting at 0.98 μm are investigated. The dynamic behavior of VCSELs with high and low operating voltages and series resistances is compared. A large wavelength chirp in the lasing spectrum was observed for the lasers with high voltage/resistance, even under low-duty-cycle pulse operation. This is thought to be due to resistive heating close to the laser junction. It is observed that the transverse mode structure of VCSELs and their dependence on laser dimensions and drive current are highly analogous to those of edge emitting lasers, whereas the polarization characteristics of the two types of lasers are significantly different     

Pure strain effect on differential gain of strained InGaAsP/InPquantum-well lasers

The effect of pure strain on the differential gain of strained InGaAsP/InP quantum-well lasers (QWLs) is analyzed on the basis of the valence band structures calculated by k×p theory. By using an InGaAsP quaternary compound as an active layer, it becomes possible to study the relationship between the differential gain and strain (both tensile and compressive) when both the quantum-well thickness and the emission wavelength are kept constant. It is shown that the tensile strain not only reduces the density of states in the valence band but also increases the energy spacings between the first two valence subbands. It is concluded that tensile strain has a more pronounced impact on the improvement of differential gain in InP-based, strained QWLs as compared with compressive strain     

Anomalous in-plane polarization dependence of optical gain in compressively strained GaInAsP-InP quantum-wire lasers

In-plane polarization anisotropy of optical gain in compressively strained GaInAsP-InP quantum wire (Q-wire) lasers including elastic strain relaxation induced band mixing is studied. The interaction between two-dimensional (2-D) quantum confinement and elastic strain relaxation effects is found to be complex depending qualitatively also on the wire width. Additional valence band mixing due to strain relaxation has a strong influence on the polarization dependence of optical gain. In the absence of elastic strain relaxation, gain is the maximum for tranverse electric (TE) polarization with the electric field parallel to the wire axis (TE/sub /spl par//), in agreement with the existing theory. On the other hand, when strain relaxation is strong, contrary to the existing theory, valence band mixing causes the gain to be the maximum in TE polarization with the electric field normal to the wire axis (TE/sub /spl perp//). Moreover, Q-wire lasers without suppression of strain relaxation are more likely to exhibit ground-state lasing for TE/sub /spl perp// polarization. These results suggest that in the presence of strong strain relaxation, a laser cavity parallel to the wire axis would provide higher gain. Therefore, the appropriate orientation of the laser cavity in strained GaInAsP-InP Q-wire lasers should be decided after carefully studying the polarization dependence of gain. Our calculation also shows that strong strain relaxation causes in-plane polarization anisotropy to show complex, nonmonotonic dependence on the wire width. Consequently, in such structures, in-plane polarization anisotropy may not be regarded as a direct measure of 2-D confinement effects.     

高功率980nm垂直腔面发射激光器的温度特性

应变补偿量子阱结构因带宽大、增益高和波长漂移速度低等特点而成为近年来研究的热点.首次介绍了国内980 nm 高功率InGaAs/GaAsP应变补偿量子阱结构的垂直腔面发射激光器(VCSEL) 变温实验,测得脉冲条件下600 μm直径的器件在10-100℃温度范围内发射波长漂移速度为0.05 nm/K,阈值电流随温度变化呈现先缓慢下降后迅速上升的特性.结合VCSEL反射谱、PL谱和增益峰值波长漂移速度,对器件阈值电流特性进行了合理的分析和解释.连续工作状态下,测试得到器件峰值功率为1 W,根据波长与耗散功率的实验曲线及热阻计算公式,可估算出垂直腔面发射激光器热阻值为10 K/W.     

Optimization of long wavelength InGaAsP strained quantum-welllasers

A theoretical study of InGaAsP-InGaAsP multiple quantum-well lasers emitting at 1.55 μm has been carried out to investigate the variation of threshold current density and differential gain with strain, well width and well number. We show that the greatest scope for exploiting this quaternary alloy in laser structures is through the use of compressive wells with unstrained or tensile barriers. We consider structures with a fixed compressive strain of 1% but variable well width, and also with fixed well width but variable strain from 0% to 1.75%. For structures with 1% compressive wells and unstrained barriers we find that the optimum structure for lowest threshold current density with sizable differential gain consists of six 35-Å quantum wells. We find also that there is little benefit to having compressive strains greater than 1.2%. In addition we examine zero-net-strain (ZNS) structures with compressive wells and tensile barriers. We show how the conduction band offset can be significantly increased and valence band offset reduced in such structures. Our gain calculations suggest that the large modification in band offset can decrease the threshold current density compared to similar devices with unstrained barriers     

High-performance 1.5 μm wavelength InGaAs-InGaAsP strainedquantum well lasers and amplifiers

Improved performance of 1.5-μm wavelength lasers and laser amplifiers using strained In_xGa_1-xAs-InGaAsP quantum well devices is reported. The device structures fabricated to study the effects of strained quantum wells on their performance are described. These devices showed TM mode gain, demonstrating the strain-induced heavy-hole-light hole reversal in the valence band. Lasers using these tensile strained quantum wells show higher and narrower gain spectra and laser amplifiers have a higher differential gain compared to compressively strained quantum well devices. Consequently, the tensile strained quantum well lasers show the smallest linewidth enhancement factor α=1.5 (compression α=2.5) and the lowest K-factor of 0.22 ns (compression K=0.58 ns), resulting in an estimated intrinsic 3 dB modulation bandwidth of 40 GHz (compression 15 GHz)