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     

Low-damage dry-etched grating on an MQW active layer anddislocation-free InP regrowth for 1.55-μm complex-coupled DFB lasersfabrication

A grating dry-etched through the upper wells of a multiquantum-well active layer has been characterized before and after regrowth. TEM observation and carrier lifetime measurements have shown growth-free defects of the epitaxial layers. As a consequence, quasi-100% monomode oscillation on λ+1 mode has been achieved on complex-coupled distributed-feedback GaInAsP QW lasers fabricated with such grating     

Tolerant low-loss three-lens coupling system for 1.48-μmunstable-cavity lasers

A 1.48-μm unstable-cavity laser is coupled into single-mode fiber using three microlenses. Reproducible coupling of very high power is demonstrated with different types of lenses (plano-convex or biconvex, with different apertures). Over 550 mW in single-mode fiber (SMF) were reproducibly reached; to our knowledge, it is the highest power coupled into an SMF from a single semiconductor laser at this wavelength. Tolerance measurements on all of the coupling elements of a three-lens system are reported for the first time; an unexpected very large tolerance an the axial displacement of the second lens was measured. Results and interpretation with the aid of Gaussian and aberration simulations are also presented     

The effect of varying barrier height on the operationalcharacteristics of 1.3-μm strained-layer MQW lasers

This paper presents an empirical study of the effects that barrier layer composition has on the operational characteristics of 1.3-μm-wavelength InGaAsP-InP multiquantum-well (MQW) strained-layer ridge-waveguide lasers. A systematic empirical investigation of how this design choice affects practical device operation was undertaken by examining threshold current, efficiency, and modal gain as a function of temperature in five different laser structures. The results of these studies indicate that small barrier heights improve device performance, despite the loss of electronic confinement in the shallow conduction band quantum wells. Indeed, it appears that carrier uniformity in the MQW structure may be improved by carrier redistribution due to thermal or tunneling effects, which in turn enhances the operation of the low barrier height structures     

High power operation of GaInAsP/GaInAs MQW ridge lasers emitting at 1.48 mu m

Multi-quantum-well (MQW) ridge lasers have been produced with CW light outputs in excess of 100 mW at 500 mA. The wavelength of operation is 1480 nm and the lasers are suitable for pumping erbium-doped-fibre amplifiers. These are the highest power ridge lasers yet produced in the 1500 nm wavelength region.<>     

Design of the active structure of high-performance 1.55-μm In1-x-yGayAlxAs strained MQW lasers

We have used Harrison's model and the anisotropic parabolic approximation to calculate the band structure of In_1-x-yGa _yAl_xAs compressively strained quantum wells (QWs). To design 1.55 μm wavelength lasers, the relations between the well width, gain, and composition are presented. The well number and the cavity length are optimized to obtain a low threshold and high maximum operating temperature (550-560 K) QW laser. Several empirical formulas are presented for further applications     

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     

1.55-μm polarization-insensitive optical amplifier withstrain-balanced superlattice active layer

A polarization insensitive (sensitivity <1 dB) GaInAs-GaInAsP semiconductor optical amplifier has been realized at 1.55 μm. The active layer consists of a strain-balanced superlattice structure. Gain polarization insensitivity on a large bandwidth (60 nm) together with a 22.5-dB signal gain and a 11-dBm polarization-insensitive saturation output power are obtained     

2-μm GaInSb-AlGaAsSb distributed-feedback lasers

Room temperature continuous-wave operation of 2-μm single-mode InGaSb-AlGaAsSb distributed-feedback (DFB) lasers has been realized. The laser structure has been grown by solid source molecular beam epitaxy (MBE). Single-mode DFB emission is obtained by first-order Cr-Bragg gratings on both sides of the laser ridge. For a cavity with 900 μm length and 4 μm width, the threshold currents are around 20 mA and the continuous-wave output power is 10 mW at a drive current of 200 mA at 20°C. Monomode emission with sidemode suppression ratios of 31 dB has been obtained     

Bifurcation in 20-GHz gain-switched 1.55-μm MQW lasers and itscontrol by CW injection seeding

A gain-switched laser operating at a 20-GHz repetition rate exhibited chaos and period-doubling depending on the bias condition, which was the first observation for 1.55-μm strain-compensated InGaAlAs-InGaAsP multiple-quantum-well (MQW) lasers. Unlike the case previously reported for bulk lasers, the irregular behavior was set over wide bias current conditions for the MQW laser. We show that the instability is attributable to a small gain compression factor of the MQW laser and a high-repetition frequency of the gain-switching operation. Further, we report that the irregular behavior can be successfully suppressed by seeding the laser with external coherent-light injection. The numerical simulations of a set of rate equations using parameters extracted from small-signal modulation characteristics were carried out in order to clarify the conditions for the stabilization of the gain-switching operation. The scenario behind the inhibition of irregular behavior was due to the enhanced damping phenomena of a nonlinear oscillator induced by the injection seeding light. The influence of the α parameter on the optical pulse distortion under the external seeding and the parametric dependence for a single-period operation were investigated based on numerical simulation     

1.95-μm strained InGaAs-InGaAsP-InP distributed-feedbackquantum-well lasers

Distributed-feedback emission from strained InGaAs-InGaAsP-InP quantum well lasers has been examined over a temperature range of 130 K to 300 K. Continuous single-mode output from 190 to 300 K with a side-mode-suppression ratio of about 10 dB was observed. The wavelength was 1.95 μm at 273 K and tuned at a rate of 0.13 nm/K. The current-tuning rate was 0.0043 nm/mA (-340 MHz/mA) at 273 and 283 K     

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     

1.3-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     

Observation of reduced nonradiative current in 1.3-μmAlGaInAs-InP strained MQW lasers

We investigated experimentally the temperature dependence of the threshold current in 1.3-μm AlGaInAs-InP strained multiple-quantum-well lasers. We find that radiative recombination constitutes almost 100% of the threshold current up to 220 K and remains more than 70% even at 300 K. This results in a high characteristic temperature T₀     

Intracavity 1.549-μm pumped 1.634-μm Er:YAG lasers at 300 K

A novel method of generating 1.634-μm laser action from Er:YAG crystals pumped intracavity by an Er:glass laser emitting at 1.549 μm is described. Operation of the Er:glass laser at 1.549 μm (red shifted from the standard 1.532 μm, but with comparable output) at 500 K was obtained using mirrors with tailored spectral reflectivities. Several Er:YAG crystals ranging in concentration from 0.3% to 2% and in length from 1 cm to 8 cm were lased in the intracavity pumping arrangement. All the Er:YAG crystals lased in the ⁴I_13/2 :Y1(6544 cm^-1)-⁴I_15/2:Z6(424 cm ^-1) 1.634-μm transition at 300 K     

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     

Low-loss 1.3-μm MQW electroabsorption modulators forhigh-linearity analog optical links

Low-loss InAsP-GaInP multiquantum-well electroabsorption waveguide modulators have been developed for transmitting microwaves as subcarriers over optical fibers. The fiber-to-fiber insertion loss is only 5 dB at 1.32-μm wavelength. The electrooptic slope efficiency of an 185-μm-long 11-GHz bandwidth device is equivalent to a Mach-Zehnder modulator with a V_π of 2.2 V. The linearity performance was characterized for a test link without any form of amplification. A RF-to-RF link efficiency of -25.5 dB, noise figure of 27 dB and suboctave spurious-free dynamic range of 114 dB.Hz^4/5 have been achieved with 16 mW input optical carrier power. The measured 3-dB electrical bandwidth exceeds 20 GHz for a 90-μm-long device     

Submilliampere-threshold 1.5-μm strained-layer multiple quantumwell lasers

The effect of high-reflection facet coatings on strained-layer multiple-quantum-well lasers was studied and submilliampere-threshold lasers were made in the 1.5-μm wavelength region with a short cavity and high-reflection-coated facets. As a result of the compressive strain, the threshold current density is loss-limited instead of transparency-limited. By the use of the step-graded-index separate confinement heterostructure to reduce the waveguide loss, a threshold current density of 550 A/cm² was measured on 30-μm wide broad area lasers with 1-mm long cavity     

Linearization of 1.3-μm MQW electroabsorption modulators usingan all-optical frequency-insensitive technique

A novel all-optical linearization technique has been developed for discrete multiquantum-well electroabsorption modulators by exploiting the wavelength dependence of the quantum-confined Stark effect. The correcting signal for a particular order of nonlinearity is generated by the same modulator at a detuned optical wavelength, canceling the undesired order of nonlinear distortion via coherent RF recombination at the photodetector. This dual-wavelength approach is simple and insensitive to the microwave subcarrier frequency. A dramatic 30-dB reduction in the third-order nonlinear distortion at 10 GHz and 8-dB increase in the suboctave link dynamic range were achieved experimentally     

Pulsed electrical operation of 1.5-μm vertical-cavitysurface-emitting lasers

Vertical cavity surface emitting lasers operating in the 1.3- and 1.5-μm wavelength ranges are highly attractive for telecommunications applications. However, they are far less well-developed than devices operating at shorter wavelengths. Pulsed electrically-injected lasing at 1.5 μm, at temperatures up to 240 K, is demonstrated in a vertical-cavity surface-emitting laser with one epitaxial and one dielectric reflector. This is an encouraging result in the development of practical sources for optical fiber communications systems