Theoretical analysis of temperature sensitivity of differentialgain in 1.55-μm InGaAsP-InP quantum-well lasers

We study the temperature sensitivity of the differential gain in InGaAsP-InP strained-layer (SL) quantum-well (QW) lasers operating at a wavelength of 1.55 μm. Electrostatic deformation in conduction-band and valence-band profiles is taken into account by solving Poisson's equation and the effective-mass equations for conduction and valence bands in a self-consistent manner. We demonstrate that electrostatic deformation in both band profiles plays a significant role in determining the temperature sensitivity of the differential gain in 1.55-μm InGaAsP-InP SL QW lasers. The physical mechanism for limiting the differential gain at elevated temperatures is also discussed     

Theoretical study of the temperature dependence of 1.3-μmAlGaInAs-InP multiple-quantum-well lasers

The temperature dependence of the differential gain, carrier density, and transparency current density for 1.3-μm AlGaInAs-InP multiple-quantum-well lasers has been theoretically studied using the optical gain calculation from 250-380 K. The characteristic temperatures of the carrier density and differential gain at threshold are calculated to be 254 and 206 K, respectively. The Auger current density accounts for more than 50% of the total current density. The leakage current density exhibits the highest temperature sensitivity and becomes an essential part of the total current density at a high temperature. The calculated characteristic temperatures of the transparency and threshold current densities are 106 and 84 K, respectively, which agree well with the reported experimental results     

Role of p-doping profile and regrowth on the static characteristicsof 1.3-μm MQW InGaAsP-InP lasers: experiment and modeling

In this paper, we study both experimentally and theoretically how the change of the p-doping profile, particularly the p-i junction placement, affects the output characteristics of 1.3-μm InGaAsP-InP multiple-quantum-well (MQW) lasers. The relationship between the p-doping profile before and after regrowth is established, and the subsequent impact of changes in the p-i junction placement on the device output characteristics, is demonstrated. Device characteristics are simulated including carrier transport, capture of carriers into the quantum wells, the quantum mechanical calculation of the properties of the wells, and the solution for the optical mode and its population self-consistently as a function of diode bias. The simulations predict and the experiments confirm that an optimum p-i junction placement simultaneously maximizes external efficiency and minimizes threshold current. Tuning of the base epitaxial growth Zn profile allows one to fabricate MQW devices with a threshold current of approximately 80 A/cm ² per well for devices with nine QW's at room temperature or lasers with a characteristic temperature T₀=70 K within the temperature range of 20°C-80°C     

Design criteria of 1.3-μm multiple-quantum-well lasers forhigh-temperature operation

We present design criteria for high-temperature operation in 1.3-μm multiple-quantum-well (MQW) lasers from the viewpoint of the light output power penalty, i.e., the change in the light output power at a fixed drive current with increasing temperature. It is shown that not only the characteristic temperature (T₀) but also internal loss dependence on temperature (γ) and threshold current (I_th) are significant parameters for reducing the power penalty. We compare the high-temperature performance of InGaAsP-based and AlGaInAs-based MQW lasers and demonstrate that AlGaInAs-based lasers have more potential in terms of the power penalty. Furthermore, we also demonstrate that the power penalty can be reduced by introducing a buried-heterostructure (BH) structure into AlGaInAs-based lasers. From these results, we conclude that the AlGaInAs-based BH lasers are promising for high-temperature performance     

High-temperature single-mode operation of 1.3-μm strained MQWgain-coupled DFB lasers

Single-mode and high-power operation at temperatures up to 120°C has been achieved in 1.3-μm strained MQW gain-coupled DFB lasers. A stable lasing wavelength is maintained due to a large modal facet loss difference of the two Bragg modes, which is provided by the gain-coupling effect. A very low temperature dependence of the threshold current has been obtained by detuning the lasing wavelength to the long wavelength side of the material gain peak at room temperature, which effectively compensates the waveguide loss at higher temperatures. An infinite characteristic temperature T_o can be realized at certain ranges of temperature depending on the detuning value     

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     

Well-thickness dependence of high-temperature characteristics in1.3-μm AlGaInAs-InP strained-multiple-quantum-well lasers

Well-thickness dependence of temperature characteristics of 1.3-μm AlGaInAs-InP strained-multiple-quantum-well lasers was investigated. Higher characteristic temperatures of threshold current density were obtained for thicker wells up to 6 nm. Fabricated ridge-waveguide lasers with 6-nm-thick wells exhibited characteristic temperature of as high as 125 K. Relaxation-oscillation frequency reduced by only 13% between 25°C and 85°C     

High-speed performance of partly gain-coupled 1.55-μmstrained-layer multiple-quantum-well DFB lasers

A partly-gain-coupled 1.55-μm distributed feedback (DFB) laser with a strained-layer multiple-quantum-well (MQW) active region with high relaxation oscillation frequency and maximum intrinsic bandwidth of 28 GHz is reported. An effective differential gain of 1.80×10^-15 cm² was achieved, which may be attributed to the strain effect in the MQW active region as well as the combination of the longitudinal gain/index coupling mechanism and fast lateral carrier injection from the cladding layers into the wells     

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     

1.3-μm GaInNAs-AlGaAs distributed feedback lasers

Room temperature continuous-wave operation of 1.3-μm single-mode GaInNAs-AlGaAs distributed feedback (DFB)-lasers has been realized. The laser structure has been grown by solid source molecular beam epitaxy (MBE) using an electron cyclotron resonance plasma source for nitrogen activation (ECR-MBE). Laterally to the laser ridge a metal grating is patterned in order to obtain DFB. The evanescent field of the laser mode couples to the grating resulting in single-mode DFB emission. The continuous wave threshold currents are around 120 mA for a cavity with 800-μm length and 2 μm width. Monomode emission with side-mode suppression ratios of nearly 40 dB have been obtained     

Linewidth enhancement factor of 1.3 mu m InGaAsP/InP strained-layer multiple-quantum-well DFB lasers

The linewidth enhancement factor alpha in a 1.3 mu m InGaAsP/InP strained-layer multiple-quantum-well (SL-MQW) distributed feedback (DFB) laser has been evaluated from the relation between the frequency and intensity modulation indexes, and the spontaneous emission spectra below threshold current. It is demonstrated that the measured alpha -parameter of a 1.3 mu m SL-MQW DFB laser is about two, and is much smaller than that in a conventional bulk DFB laser. From the resonance frequency dependence on the output power, it is concluded that this reduction of the alpha -parameter originates in the increased differential gain. The reduction of wavelength chirping, as a result the low alpha -parameter, was experimentally confirmed for the SL-MQW DFB laser.<>     

Optical feedback on linearity performance of 1.3-μm DFB andmultimode lasers under microwave intensity modulation

Under large signal modulation and quantitatively controlled optical feedback (from about -50 dB to -25 dB, from the end of ~2-m pigtails), the linearity performance of both a 1.3-μm distributed-feedback (DFB) and a multimode laser is examined. Under proper bias levels and modulation frequencies, both lasers exhibit good linearity even with strong optical feedback. The DFB laser exhibits good linearity because of its coherence reduction, resulting from modulation-induced frequency chirping     

Effect of p-doping profile on performance of strainedmulti-quantum-well InGaAsP-InP lasers

Leakage of electrons from the active region of InGaAsP-InP laser heterostructures with different profiles of acceptor doping was measured by a purely electrical technique together with the device threshold current. Comparison of the obtained results with modeling data and SIMS analysis shows that carrier leakage of electrons over the heterobarrier depends strongly on the profile of p-doping and level of injection. In the case of a structure with an undoped p-cladding/waveguide interface, the value of electron leakage current can reach 20% of the total pumping current at an injection current density of 10 kA/cm at 50°C. It is shown that carrier leakage in InGaAsP-InP multi-quantum-well lasers can be minimized and the device performance improved by utilizing a p-doped separate-confinement-heterostructure layer     

Aging-induced wavelength shifts in 1.5-μm DFB lasers

We report measured wavelength shifts of over one hundred 1.5-μm DFB lasers aged under three different conditions far a period corresponding to the system's lifetime (~25 years). The results show that the lasers aged at lower temperature (thus higher optical power) have wider spread of wavelength shifts than the lasers aged at higher temperature. No correlation was observed between the wavelength shifts and the aging rates or the aging-induced changes in the threshold currents. The aging-induced wavelength shifts were relatively small (±1 Å) for most lasers. However, about 10% of the lasers exhibit larger wavelength shifts of up to about ±4 Å     

Temperature performance of 1.3-/spl mu/m InGaAsP-InP lasers with different profile of p-doping

Temperature dependencies of the threshold current, device slope efficiency, and heterobarrier electron leakage current from the active region of InGaAsP-InP multiquantum-well (MQW) lasers with different profiles of acceptor doping were measured. We demonstrate that the temperature sensitivity of the device characteristics depends on the profile of p-doping, and that the variance in the temperature behavior of the threshold current and slope efficiency for lasers with different doping profiles cannot be explained by the change of the measured value of the leakage current with doping only. The entire experimental data can be qualitatively explained by suggesting that doping ran affect the value of electrostatic band profile deformation that affects temperature sensitivity of the output device characteristics. We show that doping of the p-cladding/SCH layer interface in InGaAsP-InP MQW lasers leads to improvement of the device temperature performance.     

1.3-μm n-type modulation-doped AlGaInAs/AlGaInAsstrain-compensated multiple-quantum-well laser diodes

In this paper, we report the fabrication and characterization of 1.3-μm AlGaInAs/AlGaInAs laser diodes (LDs) with an n-type modulation-doped strain-compensated multiple-quantum-well (MD-SC-MQW) active region and a linearly graded index separate confinement heterostructure. The barrier in the MD-SC-MQW active region contains the 28 Å Si-doped modulation-doped region and two 29 Å surrounding undoped regions that serve to prevent the overflow of Si doping atoms into the wells. We investigate the threshold current density, infinite current density, differential quantum efficiency, internal quantum efficiency, internal optical loss, threshold gain (for the cavity length of 300 μm), and transparency current density as a function of doping concentration in the n-type AlGaInAs barrier for the 1.3-μm MD-SC-MQW LDs. The theoretical and experimental results show that the optimum doping concentration of doped barriers is 5×10 ¹⁸ cm^-3. With this optimum condition, the 3.5-μm ridge-striped LDs without facet coating will exhibit a lower threshold current and a higher differential quantum efficiency of 18 mA and 52.3% under the CW operation as compared to those of 22 mA and 43% for the undoped active region, respectively. In addition, a high characteristic temperature of 70 K, a low slope efficiency drop of -1.3 dB between 20 and 70°C, and a wavelength swing of 0.4 nm/°C for the LDs operated at 60 mA and 8 mW can be obtained in the LDs with doped barriers     

High-reliability 1.3-μm InP-based uncooled lasers in nonhermeticpackages

We report the first uncooled nonhermetic 1.3-μm InP-based communication lasers that have reliability comparable to their hermetically packaged counterparts for possible applications in fiber in the loop and cable TV. The development of reliable nonhermetic semiconductor lasers would not only lead to the elimination of the costs specifically associated with hermetic packaging but also lead the way for possible revolutionary low-cost optoelectronic packaging technologies. We have used Fabry-Perot capped mesa buried-heterostructure (CMBH) uncooled lasers with both bulk and MQW active regions grown on n-type InP substrates by VPE and MOCVD. We find that the proper dielectric facet passivation is the key to obtain high reliability in a nonhermetic environment. The passivation protects the laser from the ambient and maintains the proper facet reflectivity to achieve desired laser characteristics. The SiO facet passivation formed by molecular beam deposition (MBD) has resulted in lasers with lifetimes well in excess of the reliability goal of 3,000 hours of operation at 85°C/90% RH/30 mA aging condition. Based on extrapolations derived experimentally, we calculate a 15-year-average device hazard rate of <300 FITs (as against the desired 1,500 FITs) for the combination of thermal-and humidity-induced degradation at an ambient condition of 45°C/50% RH. For comparison, the average hazard rate at 45°C and 15 years of service is approximately 250 FITs for hermetic lasers of similar construction. A comparison of the thermal-only degradation (hermetic) to the thermal plus humidity-induced degradation (nonhermetic) indicates that the reliability of these nonhermetic lasers is controlled by thermal degradation only and not by moisture-induced degradation. In addition to device passivation for a nonhermetic environment, MBD-SiO maintains the optical, electrical, and mechanical properties needed for high-performance laser systems     

High-temperature characteristics of 1.3-μm InAsP-InAlGaAs ridgewaveguide lasers

High-temperature characteristics of InAsP-InAlGaAs strained multiquantum-well (MQW) lasers with a large conduction band discontinuity (ΔE_c) are demonstrated. The InAsP-InAlGaAs MQW ridge waveguide lasers with narrow stripes exhibited a characteristic temperature as high as 143 K in the range from 25°C to 85°C. This material system is promising for developing a cooling-system-free 1.3-μm laser     

Temperature dependence of lasing characteristics forlong-wavelength (1.3-μm) GaAs-based quantum-dot lasers

Data are presented on the temperature dependence of 1.3-μm wavelength quantum-dot (QD) lasers. A low-threshold current density of 90 A/cm² is achieved at room temperature using high reflectivity coatings. Despite the low-threshold current density, lasing at the higher temperatures is limited by nonradiative recombination with a rapid increase in threshold current occurring above ~225 K. Our results suggest that very low threshold current density (⩽20 A/cm ²) can be achieved at room temperature from 1.3-μm QD lasers, once nonradiative recombination is eliminated     

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