Reliability in InGaAsP/InP buried heterostructure 1.3 µm lasers

A study was conducted of aging-induced Sn whisker growth at the surface of Au-Sn bonding solder layers around a laser chip, as well as metallurgical reactions, especially in p-side down lasers, at the interface of the solder and laser crystal just below the active layer. These phenomena cause electrical shorts in InGaAsP/InP laser diodes, which occur suddenly in devices operated for long periods without any previous symptoms having appeared in their aging characteristics. To completely eliminate such failures, a novel assembling method in which chips were mounted p-side up on semiinsulating SiC submounts using Pb-Sn solder, was applied to InGaAsP/InP buried heterostructure (BH) lasers emitting at 1.3 μm. BH lasers assembled by this method do not suffer any shorting failure even after 8000 h operation under 60°C and 5 mW/facet output conditions. The small degradation rates obtained, for example, 3 percent/kh (median) at 60°C, certify the reliability of these improved lasers. As long as the stripe width of the active layer was optimized to be in the range of1.5-2.5 mum necessary for obtaining kink-free light output versus current properties, no detrimental changes in laser characteristics, including transverse and longitudinal modes and dynamic output response, were observed in aged lasers. In this paper, the long-term degradation modes observed are presented, and possible causes are discussed.     

Spectral properties of strongly coupled 1.5 µm DFB laser diodes

Spectral measurements of strongly coupled DFB lasers operating at 1.5 μm are presented. The magnitude of the coupling coefficientkin these devices was determined to be 80 cm^-1for lasers withlambda = 1.12 mum cladding layers and 160 cm^-1for devices withlambda = 1.3 mum cladding layers. These values forkare believed to be the largest reported for 1.5 μm DFB lasers. CW spectral linewidths as low as 10 MHz at 15 mW output power were obtained, and the linewidth was observed to vary approximately as the inverse of the device length cubed. Spectral measurements performed under 2 Gbit/s direct modulation exhibited a side mode suppression ratio of >38 dB. The effects of transient wavelength chirping were also investigated in detail and the maximum wavelength deviation was found to be ≃1.5 Å.     

Temperature characteristics of double-carrier-confinement (DCC) heterojunction InGaAsP(λ = 1.3 µm)/InP lasers

This paper reports on a detailed study of the oscillation characteristics of double-carrier-confinement (DCC) heterojunction InGaAsP(lambda = 1.3 mum)/InP lasers, with special emphasis on their temperature characteristics. In addition to conventional double-heterojunction lasers, these lasers have a p-InGaAsP second active layer sandwiched between the p-InP clad layers. The spectral characteristics below threshold were examined in order to verify electron leakage beyond the hetero-barrier of the p-InP thin clad layer, and to study the contribution of the second active layer to optical gain and laser action. Threshold temperature characteristics were also investigated through an analysis of the heterojunction energy band structure. The results indicate that emission from the second active layer is caused by electron leakage. In order to obtain high temperature stability for these lasers, it is essential that both the first and second active layers contribute to the optical gain spectrum and laser action.     

Growth and characterization of 1.3 µm CW GaInAsP/InP lasers by liquid-phase epitaxy

The variation of the threshold current density and its temperature dependence with acceptor concentration in GaInAsP/InP lasers emitting at 1.3 μm is described. Mechanisms responsible for the dependence are identified. A model is developed to predict the effect of the above dependence on the CW operation range of these devices. The validity of the model is verified experimentally.     

Characteristics of diffused-stripe InP/InGaAsP/InP lasers emitting around 1.55 µm

Fabrication and lasing characteristics of InGaAsP/InP lasers emitting around 1.55 μm are described. Zn-diffused stripe-geometry lasers with emission wavelengths in the1.53-1.60mum range were fabricated from InP/InGaAsP/InP DH epitaxial wafers prepared by a low temperature LPE technique. The dependence of the lasing charaeteristics on the stripe width was examined. The lowest threshold current (≃160 mA under CW operation at 27°C) was obtained for a laser with a 13 μm stripe. CW operation of the laser has been achieved at a heat-sink temperature as high as 53°C. For sufficiently narrow stripe widths (simeq6 mum), fundamental-transverse mode and single-longitudinal mode operation was obtained under CW operation. Moreover, the lasers have good high-frequency performance. The lasers showed excellent dynamic properties without waveform distortion under high-frequency (800 Mbits/s) large-signal pulse modulation. The full width at half maximum of the longitudinal mode envelope was approximately 30 Å at 800 Mbits/s.     

Singlemode InAs/InP quantum dash distributed feedback lasers emitting in 1.9 μm range

Laterally coupled, complex distributed feedback lasers based on InAs/ InGaAs/InAlGaAs/InP quantum dash-in-a-well layers emitting in the 1.9 mum wavelength range were fabricated. Total CW powers above 25 mW at room temperature and sidemode suppression ratios of more than 35 dB are demonstrated, which makes the devices promising for gas sensing applications.     

Static characteristics of 1.5 - 1.6 µm GaInAsP/InP buried heterostructure butt-jointed built-in integrated lasers

Lasing characteristics of GaInAsP/InP buried heterostructure butt-jointed built-in distributed Bragg reflector integrated lasers (BH-BJB-DBR lasers) intended for dynamic single mode operation in the wavelength range of1.5-1.6 mum are given. In the first section, the coupling property between the active region and the butt-jointed external waveguide region is calculated to show the possibility of large fabrication tolerance. A coupling coefficient of more than 95 percent is estimated. Secondly, the GaInAsP/InP integrated lasers were fabricated and tested in the view of the static characteristics and the axial mode selection property. Lateral mode control was achieved by the use of a buried heterostructure, so that the axial and lateral modes were maintained to a fixed single mode. The lasers thus fabricated were operated in CW conditions at room temperature with a threshold current of about 100 mA. Single longitudinal mode operation was observed with a temperature dependence of about 0.13 nm/deg with the temperature range of more than50-65degC at around 0°C. Differential quantum efficiency as high as 13 percent/ facet was obtained for the laser with power output of more than 5 mW/facet. The output spectrum below threshold indicated the strong wavelength selectivity of the DBR region, and the net gain difference between the main DBR mode and the adjacent submode was measured to be about 6 cm^-1. No appreciable degradation and change of characteristics have been observed even after CW operation of more than 9770 h at 20°C.     

Low-threshold InGaAsP ridge waveguide lasers at 1.3 µm

The ridge waveguide configuration is shown to provide reliable low-threshold fundamental-transverse-mode lasers that are readily fabricated. Two variants are described: in the simple ridge laser, the 1.3 μm bandgap active layer is sandwiched between InP layers and in the cladded ridge, the active layer is surrounded by 1.1 μm bandgap InGa AsP. Thresholds as low as 34 mA and efficiencies as high as 66 percent are observed. Output power is linear to more than 12 mW. Several lasers have been operated at 30°C for over 1500 h without measurable degradation. Selected lasers exhibit stabilized longitudinal mode behavior over extended temperature and current ranges. The potential manufacturability of this device is its most attractive feature.     

Double heterostructure and multiquantum-well lasers at 1.5?1.7 ?m grown by atmospheric pressure MOVPE

The first successful growth and fabrication of GaInAs/InP MQW and CW GaInAsP DH lasers by atmospheric pressure MOVPE is reported. Room-temperature CW operation of DH lasers was obtained by using the ridge-waveguide laser design, and threshold currents as low as 53 mA were measured. MQW lasers, which also operated at room temperature, were fabricated, and emission spectra obtained from these devices showed a clear spectral narrowing compared with the DH lasers together with a wavelength shift, in good agreement with theoretical predictions.     

Spectral linewidth estimation of a 1.5 µm range InGaAsP/InP distributed feedback laser

Spectral linewidth of a 1.5 μm range distributed feedback buried heterostructure (DFB-BH) laser in CW operation is estimated theoretically and experimentally. Considering the equivalent mirror facet loss coefficient and the confinement factor in the active layer, etc., we modified the conventional formula for the spectral linewidth of single-mode semiconductor lasers and presented a formula for the linewidth of DFB lasers. Furthermore, power-dependent linewidth measurements of a 1.5 μm range InGaAsP/InP DFB-BH laser with a window region were carried out using Fabry-Perot interferometers. The linewidth was observed to increase linearly with inverse output power. The measured result was explained by the calculated result through the modified formula. The full width at half maximum was estimated to be 50 MHz at an output power of 1 mW.     

InGaAsP InP current confinement mesa substrate buried heterostructure laser diode fabricated by one-step liquid-phase epitaxy

New InGaAsP/InP buried heterostructure laser diodes fabricated by one-step liquid-phase epitaxy are described, in which the InGaAsP active region completely embedded in InP is grown on the top of the mesa stripe formed on the InP substrate while, simultaneously, current confinement structure is automatically formed on both sides of the mesa stripe. These current confinement mesa substrate buried heterostructure laser diodes (CCM-LD's) have current confinement structure which very effectively blocks unwanted leakage current bypassing the light emitting region which has enabled laser operation with a threshold current as low as 20 mA, 70-mW maximum CW output at room temperature, and 125°C maximum CW operation temperature. Life tests over 6000-h CW operation at 70°C and 5 mW/facet have confirmed good reliability of these devices.     

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     

1.3 µm high-power BH laser on p-InP substrates

In our fabrication of a 1.3 μm band high-power BH laser on a p-type InP substrate, 79 mW CW laser output was obtained, and the spectrum width was 10 nm at 50 mW; it also obtained a high-power pulse output of more than 200 mW at 30 ns pulse width. It shows high-speed pulse response at 2 Gbits/s. These CW and pulse lasing characteristics are reported in this paper, and we also show the output and threshold current distribution of about 1000 samples from six wafers. This high-power laser is very useful for light sources of measuring instruments.     

Zn-diffused, stripe-geometry, double-heterostructure GaInAsP/InP diode lasers

Deep Zn diffusion from a ZnP2source has been used to fabricate stripe-geometry double-heterostructure GaInAsP/InP diode lasers with emission wavelengths in the1.2-1.3 mum range. These devices exhibit good electrical and optical confinement. For sufficiently narrow stripe widths (<10 mum), emission is usually single mode and linear. CW outputs up to ∼8 mW per facet have been observed. In initial life tests, two Zn-diffused lasers have operated CW at room temperature for over 5000 h without appreciable degradation.     

High temperature CW operation of p-substrate buried crescent laser diode emitting at 1.3 ?m

A 1.3 ?m InGaAsP/InP buried crescent laser diode has been fabricated on p-InP substrate. The laser diode has a low threshold current, as low as 10 mA. It operates at the output power of 5 mW under CW condition at temperatures higher nary aging test at 70°C with a constant light output of 5 mW, the lasers have been operating stably for more than 1000 h.     

Semi-insulating InP grown by low pressure MOCVD

Fe-doped semi-insulating InP layers have been successfully grown in a vertical flow, low pressure metalorganic chemical vapor deposition (LPMOCVD) system, and used as current blocking layers in buried crescent (BC) laser structures emitting at 1.51 μm. Triethylindium ((C₂H₂)₃In), phosphine (PH₃) and iron pentacarbonyl (Fe(CO)₅) were used as the reactant gases. Process variables have been identified which produce high resistivity (10⁷ to 10₈ Ω-cm) InP having featureless surface morphology, and layer thickness and doping uniformity. There is optical and x-ray diffraction evidence for the presence of an unidentified In-Fe-P second phase associated with markedly degraded surface morphology under nonoptimized growth conditions. Early BC lasers incorporating LPMOCVD grown, Fe-doped InP blocking layers have operated CW with threshold currents as low as 12 m A and optical output > 18 mW.     

1.5 ?m region InP/GaInAsP buried heterostructure lasers on semi-insulating substrates

InP/GaInAsP buried heterostructure (BH) lasers for the 1.5 ?m region have been fabricated on semi-insulating InP substrates. The threshold current of the lasers is as low as 38 mA under CW operation at 25°C, which is nearly the same as for BH lasers fabricated on n-type InP substrates.     

MOVPE growth of AlGaAs/GaInP diode lasers

GaAs-based diode lasers for emission wavelengths between 800 nm and 1060 nm with AlGaAs-cladding and GaInP-waveguide layers were grown by MOVPE. For wavelengths above 940 nm broad area devices with InGaAs QWs show state-of-the-art threshold current densities. Ridge-waveguide lasers fabricated by selective etching achieve 200 mW CW monomode output powers. (In)GaAsP QW-based diode lasers with an emitting wavelengths around 800 nm suffer from problems at the upper GaInP/AlGaAs interface. Asymmetric structures with a lower AlGaAs/GaInP and an upper AlGaAs/AlGaAs waveguide not only avoid this interface but also offer better carrier confinement. Such structures show very high slope efficiencies and a high T₀. Maximum output powers of 7 W CW are obtained from 4 mm long devices.     

1.3-μm AlGaInAs buried-heterostructure lasers

1.3-μm AlGaInAs-InP strained multiple-quantumwell (MQW) buried-heterostructure (BH) lasers have been successfully fabricated. InP current blocking layers could be smoothly regrown using the simple HF pretreatment, although the etched active region includes Al-containing layers. The threshold current I_th was typically 11 mA for as-cleaved 350-μm-long devices, which is about 30% lower than that of the ridge laser counterparts. A maximum continuous-wave operating temperature as high as 155°C was achieved. For the 200-μm-long device with the high-reflective-coated rear-facet, I_th was as low as 7.5 mA and characteristic temperature T₀ was 80 K. The BH lasers also provided more circular far-field patterns and lower thermal resistances than for ridge lasers