Threshold currents of 1.3-μm bulk, 1.55-μm bulk, and1.55-μm MQW DFB P-substrate partially inverted buried heterostructurelaser diodes

We investigate the threshold currents of 1.3-μm bulk, 1.55-μm bulk, and 1.55-μm multi-quantum-well (MQW) distributed feedback (DFB) P-substrate partially inverted buried heterostructure (BH) laser diodes experimentally and theoretically. In spite of the larger internal loss of the 1.55-μm bulk laser diodes, the threshold current of the 1.55-μm bulk DFB P-substrate partially inverted BH laser diode is almost the same as that of the 1.3-μm bulk DFB P-substrate partially inverted BH laser diode. The experimentally obtained average threshold current of the 1.3-μm bulk DFB P-substrate partially inverted BH laser diodes is 17 mA and that of the 1.55 μm bulk DFB P-substrate partially inverted BH laser diodes is 16 mA. The calculated threshold current of the 1.3-μm bulk DFB laser diode is 15.3 mA and that of the 1.55-μm bulk DFB laser diode is 18.3 mA, which nearly agree with the calculated values, respectively. We have fabricated two types of five-well 1.55-μm InGaAs-InGaAsP MQW DFB P-substrate partially inverted BH laser diodes. One has barriers whose bandgap energy corresponds to 1.3 μm, and the other has barriers of which bandgap energy corresponds to 1.15 μm. The calculated threshold current of the MQW DFB laser diode with the barriers (λ_g =1.3 μm) is 8.5 mA, which nearly agrees with the experimentally obtained value of 10 mA. However, the calculated threshold current of the MQW DFB laser diode with the barriers (λ_g=1.15 μm) is 7.9 mA which greatly disagrees with the experimentally obtained value of 19 mA, which suggests that the valence band discontinuity between the well and the barrier severely prevents the uniform distribution of the injected holes among five wells     

1.55-μm buried-heterostructure VCSELs with InGaAsP/lnP-GaAs/AlAsDBRs on a GaAs substrate

We demonstrate 1.55-μm buried-heterostructure (BH) vertical-cavity surface-emitting lasers (VCSELs) on a GaAs substrate. Thin-film wafer-fusion technology enables InP-based BH VCSELs to be fabricated on GaAs/AlAs distributed Bragg reflectors. Detailed investigations of the device resistance are also described. As a result of introducing BH and obtaining low device resistance, the threshold current density under CW operation shows the independence of mesa size due to a strong index guide and small noneffective current. A 5-μm VCSEL exhibits a record threshold current of 380 μA at 20°C. This VCSEL also operates with single transverse mode up to the maximum optical output power     

High performance of 1.55-μm buried-heterostructurevertical-cavity surface-emitting lasers

We report a record low threshold current of 1.55-μm vertical-cavity surface-emitting laser (VCSEL). Thin-film wafer-fusion technology enables InP-based buried heterostructure VCSELs to be fabricated on GaAs-AlAs distributed Bragg reflectors. Threshold current density is independent of mesa size, and a 5-μm VCSEL exhibits a threshold current as low as 380 μA at 20°C and a single transverse mode up to the maximum optical output power under continuous-wave operation     

Room temperature pulsed operation of 1.5 μm GaInAsP/InPvertical-cavity surface-emitting laser

Room-temperature pulsed operation of a GaInAsP/InP vertical-cavity surface-emitting laser diode (VCSELD) with an emission wavelength near 1.55 μm is reported. A double heterostructure with a 34-pair GaInAsP (λ_g=1.4 μm)/InP distributed Bragg reflector (DBR) was grown by metalorganic chemical vapor deposition (MOCVD). The measured reflectivity of the semiconductor DBR is over 97% and threshold current is 260 mA for a 40-μmφ device with a 0.88-μm-thick active layer. Threshold current density is as low as 21 kA/cm² at room temperature     

Wavelength dependence of characteristics of 1.2-1.55 μmInGaAsP/InP p-substrate buried crescent laser diodes

Laser diodes with the p-substrate buried-crescent structure have been fabricated for the 1.2-1.55-μm wavelength region. The dependence of laser characteristics on wavelength has been measured. Up to 70°C, the increasing rates of the threshold current with temperature are similar, while, above 70°C, a shorter-wavelength laser shows a larger increasing rate. At the same full width at half maximum of the far-field pattern perpendicular to the junction plane, the external differential quantum efficiency of the 1.55-μm laser diode is only 10% smaller than that of the 1.3-μm laser. The absorption loss coefficients in the active layer of the 1.2-, 1.3-, and 1.55-μm laser are estimated to be 26, 34, and 73 cm^-1, respectively     

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     

1.2-μm GaAsP/InGaAs strain compensated single-quantum-well diodelaser on GaAs using metal-organic chemical vapor deposition

We demonstrate here 1.2-μm laser emission from a GaAsP-InGaAs strain compensated single-quantum-well (SQW) diode. This development enables the fabrication of vertical-cavity surface-emitting lasers for optical interconnection through Si wafers. Strain compensation and low temperature growth were used to extend the wavelength of emission to the longest yet achieved on a GaAs substrate in this materials system. The minimum threshold density achieved was 273.4 A/cm² at a cavity length of 610 μm. We have also demonstrated an 1.144-μm lasing wavelength in a 820-μm-long cavity on a GaAs substrate with a strained InGaAs-GaAs SQW laser for comparison using a low-temperature metal-organic chemical vapor deposition growth technique. The threshold current density for a 590-μm-long cavity under CW operation was 149.7 A/cm²     

Design and fabrication of low-threshold 1.55-μm graded-indexseparate-confinement heterostructure strained InGaAsPsingle-quantum-well laser diodes

This paper presents a guideline for designing an optimum low-threshold 1.55-μm graded-index (GRIN) separate confinement-heterostructure (SCH) strained InGaAsP single quantum-well (SQW) laser diode (LD). The guideline was formulated based on the results of numerical and experimental analysis. After calculating the sheet carrier density at the lasing threshold, the guideline was obtained by considering the tradeoff between carrier and optical confinements in the well: the GRIN layer energy gap should be varied parabolically from InP to InGaAsP having a band gap wavelength of 1.1 μm to inject a large number of carriers into the well, and the thickness of one side of the GRIN layer should be more than 300 nm to keep a strong optical confinement. The GRIN SQW LD designed using the guideline has a J_th as low as 98 A/cm² at a cavity length of 5 mm, which proves the guideline is effective for designing low-threshold 1.55-μm GRIN SQW LDs     

MOVPE growth of a monolithic VCSEL at 1.56 μm in theInGaAlAs-InAlAs system lattice matched to InP

The first demonstration of a one-step-growth vertical-cavity surface-emitting laser (VCSEL) at 1.56 μm by low-pressure metal-organic vapor phase epitaxy in the InGaAlAs (λ_gap=1.43 μm)-InAlAs system lattice matched to InP is presented. The VCSEL's threshold current density was 7.5 kA/cm² and pulsed lasing had been obtained up to +55°C for 45-μm diameter proton implanted devices. This material system represents a high potential for continuous-wave VCSELs at 1.55-μm wavelength using a simple approach for large-scale industrial production     

λ/4-shifted DFB laser/electroabsorption modulator integratedlight source for multigigabit transmission

High-speed, low-chirp, and low voltage driving characteristics of 1.55-μm λ/4-shifted distributed-feedback (DFB) laser/InGaAsP electroabsorption modulator integrated light sources are reported. By optimization of the composition and thickness of the modulator waveguide, the driving voltage for a 10-dB extinction ratio was reduced to 1.4-3 V, depending on the modulator length in the range of 240-125 μm. High-speed modulation up to 10-Gb/s NRZ modulation was achieved by the integrated device with a 125-μm modulator length. The linewidth enhancement factor of the integrated modulator was estimated to be 0.15-0.48     

Multiwavelength InGaAs/InGaAsP strained-layer MQW-laser array usingshadow-masked growth

A simple technique for fabricating multiwavelength laser arrays is presented. The lateral variations in bandgap (or emission wavelength) between the different lasers are obtained by the use of shadow-masked growth. The shadow masked growth results in variations in thickness (and to a lesser extent, in composition) over the substrate. In combination with a multiquantum well (MQW) active region, this gives the required bandgap variations. By varying the window width in the shadow mask between 10 μm and >500 μm it was possible to obtain a wavelength span of 130 nm centered around 1.55 μm. The strained-layer-ridge MQW Fabry-Perot lasers showed a constant threshold current (around 70 mA for an 11-μm×500-μm stripe)     

Optical WDM transceiver module using wavelength-selective couplerand WDM-PD for optical access networks

A new optical wavelet division multiplexing (WDM)-transceiver module has been designed and fabricated for optical access networks. Conventional 1.3/1.55-μm WDM-coupler and Y-branch were replaced by a new wavelength-selective coupler in order to reduce module size. A new WDM-photodiode (PD), which was photosensitive to 1.3 μm light and transparent to 1.55 μm light, and a 1.55-μm PD were arranged in series along the optical axis. An MQW-FP-LD was used as a 1.3-μm transmitter. Fundamental characteristics were measured and discussed. Evaluation results, P_out of 0 dBm, responsivity of 0.36 A/W at 1.31 μm and 0.74 A/W at 1.55 μm, and IMD2 of -76.2 dBc, imply that this WDM-transceiver module design is promising to application for optical access networks     

Frequency stabilization of 1.55 μm DFB laser diode usingvibrational-rotational absorption of 13C2H2 molecules

The vibrational-rotational absorption of ¹³C₂H₂ molecules (VRAMs) in the 1.52-1.55 μm region was investigated in detail. On the basis of this investigation, frequency stabilization of 1.55-μm distributed-feedback (DFB) laser diodes was demonstrated. Frequency stability to within 2 MHz peak/peak fluctuation was achieved at the 1.54949-μm wavelength. In addition, frequency stabilizations in the wavelength regions of 1.53 μm and 1.54 μm were also carried out by using the strong absorption lines of ¹³C₂H₂ VRAM in these wavelength regions. The spectral width of the frequency-stabilized DFB laser diode was found to be 25 MHz. The absolute frequency was found to be stabilized at least to within 25 MHz by the measurement of beat spectrum     

1.55-μm picosecond all-optical switching by using intersubbandabsorption in InGaAs-AlAs-AlAsSb coupled quantum wells

We report on the first all-optical switching operation of intersubband absorption at an optical communication wavelength (~1.55 μm). The 1.55-μm intersubband absorption was achieved by InGaAs-AlAs-AlAsSb coupled quantum wells. A switching operation on an ultrafast signal (equivalent to 1 THz) was successfully demonstrated with a control pulse energy as low as 27 pJ     

5 Gb/s optical soliton transmission experiment using Ramanamplification for fiber-loss compensation

Optical soliton transmission of 5 Gb/s over a 23-km amplification spacing using a gain-switched 1.55-μm distributed feedback laser diode and Ti:LiNbO₃ intensity modulator is discussed. An Er ⁺-doped fiber amplifier, pumped by 1.45- and 1.48-μm laser diodes, is employed for achieving intense optical pulses. Transmission fiber-loss is completely compensated for by Raman amplification using by 1.45- and 1.48-μm laser-diode pumping. A bit error rate (BER) of 2×10^-10 has been obtained     

Measurements and modeling of reflective bistability in 1.55-μmlaser diode amplifiers

We present experimental measurements of bistable characteristics for the case of a 1.55-μm symmetric Fabry-Perot laser amplifier with a particularly rich variety of behavior for reflected signals. Three main forms of hysteresis loops are observed on reflection. The hysteresis loop changes from anticlockwise to clockwise hysteresis in the optical output-input characteristic when the drive current or initial phase detuning is changed. A “butterfly” hysteresis loop is observed experimentally at an optical input power as low as 300 μW for a drive current equal to 94% of the lasing threshold. The dependence of the reflective optical bistability on drive current, initial phase detuning, and injected optical pewter has been investigated for the first time in an uncoated 1.55-μm amplifier. Measurements of the device voltage shifts on switching reveal hysteresis loops which match those in the corresponding optical powers and which indicate the amount of gain saturation and nonlinear refraction responsible for bistability. An analytical model is used to describe the observed reflective bistability and to give insight into the detailed physical mechanisms responsible     

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     

Reduced timing jitter of two-section 1.55-μm laser diodes undergain-/loss-switching regime at multigigahertz rates

Picosecond pulses at multigigahertz frequencies with low timing-jitter have been generated from two-section 1.55-μm semiconductor lasers. A pulse jitter of ~1 ps was measured at a 5-GHz repetition rate with the laser diodes operated in gain-switching regime. More than 2x lower values were obtained in the loss-switching regime. A timing jitter model has been adapted to treat two-section laser diodes in the two regimes. The superiority of the loss-switching regime is explained by faster laser dynamics around threshold. A good agreement is obtained between calculations and timing jitter measurements     

Static and dynamic characteristics of DFB lasers with longitudinalnonuniformity

Static and dynamic characteristics of 1.55-μm distributed-feedback (DFB) lasers with longitudinal nonuniformity in optical intensity and carrier density are discussed. The nonuniformity is manifested in soft light-current threshold and varying differential quantum efficiency, blue-shifted static tuning, and free-carrier induced FM response     

Predistortion of electroabsorption modulators for analog CATVsystems at 1.55 μm

A predistortion circuit with adjustable amounts of third- and fifth-order predistortion is studied both theoretically and experimentally. The circuit is used to cancel the third- and fifth-order intermodulation distortion of a 1.55-μm integrated electroabsorption modulator/DFB laser. The CSO obtained is -61 dBc and the CTB is reduced 22.6 dB to -65 dBc. This performance is maintained after fiber amplification and propagation through 13 km of nondispersion shifted fiber due to the modulator's low chirp. Dithering of the DFB laser's injection current is employed to increase the stimulated Brillouin scattering (SBS) threshold to +13.4 dBm