Multiquantum-well Lasers Research Articles

Strained GaInAs-AlGaInAs 1.5-μm-wavelength multiquantum-well lasers loaded with GaInAs-AlInAs multiquantum barriers at the p-side optical confinement layer

Compressively strained GaInAs-AlGaInAs multiquantum-well (MQW) lasers (LD's) operating at 1.5-/spl mu/m-wavelength were fabricated with a multiquantum barrier (MQB) structure at the p-side of the optical confinement layer, using molecular beam epitaxial (MBE) growth technology. The effect of loading MQB is demonstrated from both laser performances and electro-luminescence (EL) spectra from LD's. As evidence of the suppression of carrier overflow by loading of MQB, it was confirmed from the laser performances that 1) the temperature dependence of slope efficiency was improved so that the efficiency was increased at high temperature and the maximum operating temperature was raised about 15/spl deg/C, 2) the temperature dependence of the K factor was improved, and the K factor was reduced more than 30% at high temperature. More direct evidence was observed in EL spectra from LD's. Reduction of about one magnitude was confirmed in the peak intensity of EL from the optical confinement layer. The rate of suppression of carrier overflow is discussed from above EL reduction rate.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Molecular-beam-epitaxy growth of strained Ga 1− x In xAs/AlInAs/InP and application to 1.55 μm multi-quantum-well lasers

Dependence of critical layer thickness ( h c ) and photoluminescence (PL) intensity on growth temperature ( T g ) was investigated for 1% compressively strained Ga 0.32In 0.68As/AlInAs on InP. We found, for the first time, a strong T g dependence of h c in this system with h c of 24 nm at 500°C and 14 nm at 530°C. The PL intensity increased with T g , reaching a maximum at 530°C. We fabricated 1.55 μm multi-quantum-well (MQW) lasers with 1% compressively strained wells under optimized growth conditions. Threshold current density as low as 1.08 kA/cm 2 at a cavity length of 800 μm was obtained. This threshold current density is one of the lowest values obtained for 1.55 μm AlGaInAs MQW lasers grown by molecular beam epitaxy (MBE).

Gain nonlinearity and its temperature dependence in bulk- and quantum-well quaternary lasers

Spectrally resolved measurements of unclamped spontaneous emission above threshold in 1.3 μm InGaAsP bulk double heterostructure and compressively strained multiquantum-well lasers are analyzed using steady state rate equations. Both types of structures are characterized by nonlinear gain coefficients in the range of (2.7-3.0)×10/sup -17/ cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> at 20/spl deg/C. The nonlinear gain coefficient decreases with increasing temperature due to thermionic emission of carriers out of the active region and decreasing differential gain. The former effect was found to be more pronounced in quantum-well lasers. Excellent agreement with the rate equation based model is obtained.

InGaAsP/InP strained MQW laser with integrated modesize converter using the shadow masked growthtechnique

The authors report the realisation of a planar buried-heterostructure (PBH) strained multiquantum-well (MQW) laser emitting at 1.52 µm with integrated vertical taper using the shadow masked growth (SMG) technique. The threshold current is 8 mA and the coupling loss to a cleaved singlemode fibre is only 3.3 dB.

Low-threshold pulsed operation of long-wavelengthlasers on Sifabricated by direct bonding

Low-threshold room-temperature pulsed operation of InGaAs/InGaAsP multiquantum-well (MQW) lasers (λ ≃ 1.55 µm) on Si substrates is demonstrated. These laser structures were first grown on InP substrates, then bonded at 700°C onto Si substrates with buffer layers. The mesa-stripe broad-area lasers have a threshold current density of 1.7 kA/cm2 (50 µm mesa width, 330 µm cavity length), which is comparable to the value for lasers on InP substrates.

1.75 µm strained InGaAs multiquantumwell laser grown on InAs 0.08 P 0.92 ternary substrate

The authors have demonstrated a strained InGaAs multiquantum well laser grown on an InAs0.08P0.92 ternary substrate. The epitaxial structure was grown using metal organic vapour phase epitaxy (MOVPE). The InAsP wafers were characterised using room temperature photoluminescence, x-ray diffraction and x-ray topography. Pulsed power levels of 60 mW/facet at λ = 1.75 µm were measured at T = 250 K. A characteristic temperature of T0 = 46 K was observed over a temperature range of 78–250 K.

A physical model for mode-locking process of colliding pulse mode-locked multiquantum-well semiconductor laser

In this paper, we first present a physical model for mode-locking process of the colliding pulse mode-locked multiquantum-well laser diode (CPM-MQW-LD). The absorption of saturable absorber on pulse leading edge and the scattering of the transient grating generated in absorption and gain regions on pulse trailing edge are considered. As an example, the mode-locking process of a multiquantum-well structure laser and the effect of transient grating on pulse width and peak power are studied. >

Metalorganic molecular beam epitaxy of strained [formula omitted] multi-quantum-wells for 1.3 μm wavelength laser diodes

This paper describes the growth of both InAsP single layers and InAsPInGaAsP multi-quantum-well (MQW) structures by metalorganic molecular beam epitaxy (MOMBE). The AsP ratio in the InAsyP1−y films is proportional to the ratio of the AsH3PH3 supply sources. The well-number dependence of the MQWs is characterized by X-ray analysis, photoluminescence (PL), and transmission electron microscopy, revealing that the critical thickness of InAs0.5P0.5 is approximately 70 nm at 520°C. The PL spectrum of an MQW with 8 nm thick InAsP well layers has a full width at half maximum (FWHM) of 4.1 meV at 4 K. The MQW lasers have a threshold current density of 0.74 kA/cm2 with a cavity length of 300 μm. The maximum operating temperature is as high as 145°C for a 10-well MQW laser with cleaved facets.

Imaging InGaAsP quantum-well lasers using near-field scanning optical microscopy

The application of near-field scanning optical microscopy (NSOM) to the characterization of InGaAsP multiquantum-well lasers is reported. Collection mode images are collected at varying drive currents from well below to well above the threshold current. The high resolution of NSOM (∼λ/20) provides a detailed mapping of the laser output from the active region as well as additional output from the surrounding mesa. Spectral analysis of the image shows that the extra emission is due to InP electroluminescence. In addition to the emission characteristics of the laser it is also possible to detect local heating of the laser facet via thermal expansion. Topographical images are achieved simultaneously with NSOM images by digitizing the feedback signal which maintains a constant tip-surface gap. It is shown that these data have direct implications on device performance and problems associated with carrier leakage and nonradiative defects.

Investigation of tensile-strained InGaAlP multiquantum-well active regions by photoluminescence measurements

The use of a tensile-strained multiquantum-well (MQW) active region in 630-nm-band InGaAlP MQW laser diodes is investigated through photoluminescence (PL) measurements. The critical conditions for lattice relaxation of strained MQW layers are also discussed. Emissions due to electron–heavy-hole recombination and electron–light-hole recombination are observed in the PL spectrum. A large energy difference (about 38 meV) is found between the two peaks, and radiative recombination is dominant in the MQW structure. These are thought to decrease the threshold current of tensile-strained MQW laser diodes. The MQW active region is thought to be extremely close to the critical conditions for lattice relaxation, but no adverse effects on optical properties are observed. It is concluded that such a tensile-strained MQW active region has advantages for use as the active region of 630-nm-band laser diodes.

Low-threshold-current 670 nm multiquantum-welllaser diodes

Low-threshold-current (20 mA) 670 nm multiquantum-well laser diodes (LDs) for bar-code readers (BCRs) and laser pointers have been developed. A marked reduction in power consumption (40% lower than conventional bulk LDs) and a maximum CW temperature of 110°C have been realised simultaneously. Estimated lifetimes of more than 10000 h, which is sufficient for both BCRs and laser pointers, were obtained at 5 mW, 60°C, with operating currents of less than 40 mA.

Enhanced relaxation oscillation frequency of 1.3 /spl mu/m strained-layer multiquantum well lasers

Dependence of relaxation oscillation frequency (f/sub r/) on the bandgap wavelength of InGaAsP barrier layers (/spl lambda//sub g//sup b/) and number of quantum wells (N/sub w/) were investigated for the first time, for 1.3 /spl mu/m InGaAsP/InGaAsP compressively strained multiquantum well (MQW) lasers. 1.3 times higher f/sub r/ was confirmed for strained-layer MQW lasers with large N/sub w/ (N/sub w//spl ges/7) and wide bandgap barrier layers (/spl lambda//sub g//sup b/=1.05 /spl mu/m) at the same injection level, compared with unstrained MQW lasers having the same well thicknesses and the same emitting wavelength. This enhancement mainly results from increased differential gain due to strain effects separated from the quantum-size effect. >

Carrier transport effects in long-wavelength multiquantum-well lasers under large-signal modulation

Significant differences are observed between long-wavelength multiquantum-well (MQW) and bulk distributed-feedback (DFB) diode lasers in terms of their gain-switched dynamic performance. Through careful comparison of experimental and theoretical results, carrier transport is found to be particularly important in determining gain-switched operation. >

Long-wavelength InGaAsP/InP multiquantum well distributed feedback and distributed Bragg reflector lasers grown by chemical beam epitaxy

We demonstrated the successful operation of long-wavelength InGaAsP low threshold-current index-coupled and gain-coupled DFB lasers grown by chemical beam epitaxy (CBE). For index-coupled DFB lasers, buried-heterostructure six-QW DFB lasers (250 /spl mu/m long and as-cleaved) operated at 1.55 /spl mu/m with CW threshold currents 10-15 mA and slope efficiencies up to 0.35 mW/mA (both facets). A side-mode suppression ratio (SMSR) as high as 49 dB was obtained. The lasers operated in the same range even at high temperatures (70/spl deg/C checked). For gain-coupled DFB lasers, gain-coupling is accomplished by using a InGaAsP quaternary grating or quantum-well grating that absorbs the DFB emission. The use of a quantum-well grating, in particular, greatly facilitates the reproducible regrowth (defect-free) over grating and the control of the coupling coefficient. CW threshold currents mere in the range of 10-15 mA for 250-/spl mu/m and 13-18 mA for 250-/spl mu/m and 500-/spl mu/m cavities, respectively. Slope efficiencies were high, /spl sim/0.4 mW/mA (both facets). SMSR was as high as 52 dB and remained in the same DFB mode with SMSR staying /spl sim/50 dB throughout the entire current range. Linewidth/spl times/power products of 1.9-4.0 mere measured with minimum linewidths of 1.8-2.2 MHz. No detectable chirp was measured under 2.5 Gb/s modulation. Unlike index-coupled DFB lasers in which mode partition events decrease slowly even when biased above threshold, these lasers have mode partition events shut off sharply as bias approaches threshold (/spl gsim/0.95 I/sub th/). A very small dispersion penalty of 1.0 dB was measured at 10/sup -11/ BER in transmission experiments using these lasers as sources at 1.7 Gb/s over an amplified fiber system of 230 km. No self-pulsation was observed in these gain-coupled DFB lasers. Gain-switching at 4 GHz with a 100% optical modulation depth and a FWHM pulse width of 23 ps was achieved with these gain-coupled DFB lasers. The peak power was /spl sim/72 mW and the FWHM bandwidth was 0.14 nm. We also fabricated InGaAs/InGaAsP multiquantum-well DBR lasers by CBE. Taking advantage of uniform thickness growth and proper design of weak and long gratings, a record high SMSR of 58.5 dB was obtained. >

High quality compressively strained InP-based GaInAs(P)/GaInAsP multi-quantum well laser structures grown by metalorganic vapor phase epitaxy

The growth and characterization of strained GaInAs and GaInAsP multiquantum well (MQW) laser structures has been investigated. The use of triple axis x-ray diffraction (XRD), in addition to the conventional high resolution double crystal XRD, yields further information about the structural integrity of these complex structures. Buried heterostructure lasers with eight GaInAsP wells (+1.0% strain, 80A thick) exhibit a record low threshold current of 3.1 mA. By using a constant As/P ratio in the wells and barriers of an MQW laser structure, we have shown that the structure can be annealed at 700°C for 1 h with only a minimal shift (-4 nm) in the photoluminescence emission wavelength. Conventional lattice-matched GaInAs/GaInAsP MQW structures exhibit a shift of-46 nm under the same conditions.

Carrier capture time and its effect on the efficiency of quantum-well lasers

Carrier capture time of the quantum well, which is an important parameter in the laser operation, was estimated for separate-confinement-heterostructure single-quantum-well (SCH-SQW) lasers by measuring the spontaneous emission from the optical confinement layers, which increases with current even above the laser threshold due to finite capture time. By fitting theoretical analysis to the measurement, hole capture time was found to be the dominant factor for the spontaneous emission increase, and was estimated to be 0.2-0.3 ps for GaInAs/GaInAsP/InP step- and graded-refractive-index-(GRIN-) SCH-SQW lasers, independent of the optical confinement structures. The same measurement was done for multiquantum-well lasers, and it was found that transport across the barrier was also responsible for the spontaneous emission increase and inhomogeneous injection into each well. The effect of the hole capture time and the transport time on the threshold current and the quantum efficiency was analyzed for high-power operation, considering the absorption loss by the carriers in the optical confinement layers. GRIN-SCH structure is shown to keep high differential efficiency in high-power operation in comparison with step-SCH structures, because the carrier density in the confinement layer is small and increases very little above threshold.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Ray optics determination of the DFB coupling coefficient in separate confinement and multiquantum-well laser structures

A ray optics technique is used to derive a closed-form expression for the first-order backward coupling coefficient /spl kappa/ in a generic five-layer separate confinement distributed feedback laser configuration with sinusoidal, triangular, or rectangular gratings. The expression is applicable to multiquantum-well structures as well as strong coupling configurations, and can be evaluated without using any numerical techniques. For the weak perturbation configurations normally encountered in such devices, the ray optics /spl kappa/ values are in good agreement with those obtained using the standard overlap integral technique.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

High-reliability GaAs/AlGaAs multiquantum well lasers grown at a low temperature (375 °C)

We report on high-reliability GaAs/AlGaAs multiquantum well (MQW) lasers grown at a low temperature (375 °C) by molecular beam epitaxy (MBE). Typically, a threshold current (Ith) of 26 mA and a differential quantum efficiency of 62% were obtained during a continuous wave (cw) operation at room temperature. During the life test, a stable operation was observed beyond 5000 h under a 20-mW cw operation at 70 °C. We also observed an improvement of laser characteristics in low-temperature-grown MQW lasers during a cw operation at room temperature. The threshold current of our MQW lasers was reduced from 26 to 18 mA, and this suggests that the point defects in the low-temperature-grown MQW laser were decreased.

High temperature (77 °C) operation of 634 nm InGaAlP multiquantum-well laser diodes with tensile-strained quantum wells

634 nm InGaAlP multiquantum-well (MQW) laser diodes, with tensile-strained InGaP well layers, have been investigated. A threshold current as low as 59 mA was achieved, at 20 °C, under continuous wave (cw) operation. These MQW laser diodes were able to operate up to temperatures as high as 77 °C under cw conditions. This maximum cw operation temperature is the highest ever reported for 634 nm laser diodes. These MQW laser diodes have shown stable cw operation over 2000 h at an output power of 3 mW, at an ambient temperature of 50 °C.

A small-signal frequency response model with electron and hole transports in multiquantum-well lasers

The authors propose a small-signal frequency response model for high-speed multi-quantum-well (MQW) lasers. Electrons and holes are treated independently to deal with the high-frequency range faster than the carrier thermionic emission from the well, where the ambipolar approximation is unsuitable. The electron current modulates the stimulated emission even in the high-frequency range beyond the hole transport limit. Therefore, the present model predicts a larger response than the conventional ambipolar model in the high-frequency range, and it gives a better explanation of the experimental results for high-speed MQW lasers.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>