Postgrowth control of the quantum-well band edge for the monolithic integration of widely tunable lasers and electroabsorption modulators

We describe a quantum-well intermixing process for the monolithic integration of various devices, each with a unique band edge. The process involves a single ion implant followed by multiple etch and anneal cycles. We have applied this method to design and fabricate widely tunable sampled-grating distributed Bragg reflector lasers with integrated electroabsorption modulators. The devices employ three unique band edges, and demonstrate exceptional tuning, gain, and absorption characteristics.     

40-Gb/s Widely Tunable Transceivers

We present the first monolithic widely tunable 40-Gb/s transceivers. The devices integrate sampled grating distributed Bragg reflector (SG-DBR) lasers, quantum-well electroabsorption modulators (EAM), low-confinement semiconductor optical amplifiers (SOA), and uni-traveling carrier (UTC) photodiodes for state-of-the-art light generation, modulation, amplification, and detection. A relatively simple high-flexibility fabrication scheme combining quantum-well intermixing (QWI) and blanket metal-organic chemical vapor deposition (MOCVD) regrowth was used to integrate components with performance rivaling optimized discrete devices. The SG-DBR/EAM transmitters demonstrate 30 nm of tuning, 39-GHz bandwidth, low-drive voltage, and low power penalty 40-Gb/s transmission through 2.3 km of fiber. The SOA/UTC photodetector receivers provide 23-28 dB of gain, saturation powers up to 18.6 dBm, and -20.2 dBm of chip-coupled sensitivity at 40 Gb/s. By connecting the transmitters and receivers off-chip, we demonstrate 40-Gb/s wavelength conversion     

Monolithic integration via a universal damage enhanced quantum-wellintermixing technique

A novel technique for quantum-well intermixing is demonstrated, which has proven a reliable means for obtaining postgrowth shifts in the band edge of a wide range of III-V material systems. The technique relies upon the generation of point defects via plasma induced damage during the deposition of sputtered SiO₂, and provides a simple and reliable process for the fabrication of both wavelength tuned lasers and monolithically integrated devices. Wavelength tuned broad area oxide stripe lasers are demonstrated in InGaAs-InAlGaAs, InGaAs-InGaAsP, and GaInP-AlGaInP quantum well systems, and it is shown that low absorption losses are obtained after intermixing. Oxide stripe lasers with integrated slab waveguides have also enabled the production of a narrow single lobed far field (3°) pattern in both InGaAs-InAlGaAs, and GaInP-AlGaInP devices. Extended cavity ridge waveguide lasers operating at 1.5 μm are demonstrated with low loss (α=4.1 cm^-1) waveguides, and it is shown that this loss is limited only by free carrier absorption in waveguide cladding layers. In addition, the operation of intermixed multimode interference couplers is demonstrated, where four GaAs-AlGaAs laser amplifiers are monolithically integrated to produce high output powers of 180 mW in a single fundamental mode. The results illustrate that the technique can routinely be used to fabricate low-loss optical interconnects and offers a very promising route toward photonic integration     

Theoretical analysis of diffused quantum-well lasers and optical amplifiers

Diffused quantum-well (QW) distributed feedback (DFB) lasers and optical amplifiers will be theoretically analyzed in this paper. For DFB lasers, a design rule will be proposed and the validity of the design rule will be discussed with respect to changes in the injected carrier density. The range of grating period, which can be used in the design, is discussed. As a consequence, the maximum tuning range of the emission wavelength can be estimated without involving the time-consuming self-consistent simulation. The features of polarization independence of optical amplifiers achieved by using diffused QWs are also discussed. Our theoretical results successfully explain why polarization independence can achieve in the long-wavelength tail of the modal gain and absorption coefficient but not at photon energies above the transition edge. This explanation applies to other tensile-strained QWs for polarization-independent applications. The understanding is crucial for optimizing polarization-independent devices. To conclude, our analysis of the diffused QW optical devices demonstrates that QW intermixing technology is a practical candidate for not only realizing monolithic photonic integrated circuit, but also enhancing optical device performance.     

Focused ion-beam implantation induced thermal quantum-wellintermixing for monolithic optoelectronic device integration

By focused ion beam implantation induced thermal intermixing the bandgap of quantum-well layer structures can be selectively changed. This allows lateral bandgap engineering and gives a new degree of freedom for lateral structuring. The principle technological aspects like the dependence of the bandgap shift on implantation parameters and the spatial resolution are investigated and applied to the fabrication of photonic and optoelectronic devices. Lateral waveguiding in InP-based materials, the possibility of monolithic integration of bandgap shifted waveguide areas into active devices and the improvement of the lateral carrier confinement in ridge waveguide lasers are demonstrated. Due to the high spatial resolution, modulated bandgap gratings could be realized with periods down to 90 mn. These bandgap gratings were used to create gain-coupled distributed-feedback lasers in different material systems with well controlled single-mode emission     

Photonic integrated circuits fabricated using ion implantation

Intermixing the wells and barriers of quantum-well (QW) laser heterostructures generally results in an increase in the bandgap energy and is accompanied by changes in the refractive index. A technique, based on ion implantation-induced QW intermixing, has been developed to enhance the quantum-well intermixing (QWI) rate in selected areas of a wafer. Such processes offer the prospect of a powerful and simple fabrication route for the integration of discrete optoelectronic devices and for forming photonic integrated circuits     

Submilliampere threshold current InGaAs-GaAs-AlGaAs lasers andlaser arrays grown on nonplanar substrates

High performance buried heterostructure InGaAs-GaAs-AlGaAs quantum-well lasers and laser arrays with tight spatial confinement of the electrical current and the optical fields have been fabricated by metalorganic chemical vapor deposition. The lasers ace fabricated in a single growth step, using nonplanar substrates as a template for the active region definition. CW room temperature threshold currents, as low as 0.5 mA and 0.6 mA, are obtained for as-cleaved double and single quantum-well lasers, respectively. External quantum efficiencies exceeding 80% are obtained in the same devices. High-reflectivity facet-coated lasers have room temperature CW threshold currents as low as 0.145 mA with 10% external quantum efficiency. Lasers made by this technique have high yield and uniformity, and are suitable for low threshold array applications     

Influence of strain on lasing performances of Al-freestrained-layer Ga(In)As(P)-GaInAsP-GaInP quantum-well lasers emitting at0.78<λ<1.1 μm

The influence of strain on lasing performances of Al-free strained-layer Ga(In)As(P)-GaInAsP-GaInP quantum-well lasers is investigated for the first time over a large emission range of 0.78<λ<1.1 μm. GaAsP and InGaAs are used for tensile and compressive-strained quantum-well layers, respectively, while GaAs and GaInAsP lattice-matched to GaAs are applied for unstrained quantum wells. The laser structures were prepared by using gas-source molecular beam epitaxy, and broad-area and ridge waveguide Fabry-Perot laser diodes were fabricated. This study shows that applying both tensile and compressive strains in the quantum well reduces threshold current density for the Al-free strained-layer quantum-well lasers. However, it was found that the lattice relaxation set a limitation of maximum compressive strain (i.e., maximum lasing wavelength) for the compressive strained InGaAs lasers while the carrier confinement determined the acceptable maximum tensile strain (i.e., minimum lasing wavelength) and lasing performances for the tensile strained GaAsP lasers. Threshold current density as low as 164 A/cm² has been obtained for 1.4% compressive-strained InGaAs-GaInAsP-GaInP lasers having a 12-nm thick quantum well. However, excellent characteristics, such as low threshold current, high efficiency low internal loss, and high output power, have been achieved for the Al-free strained-layer quantum-well lasers     

Strained-layer InGaAs quantum-well heterostructure lasers

The incorporation of intentional strain in heterostructure lasers was almost unheard of a decade ago or so and considered a problem to be avoided. Advances in both epitaxial crystal growth technology and the understanding of the physics and reliability of these materials have led to a remarkable increase in the commercial use of strained-layer lasers. The industry has benefited from an increase in the available range of emission wavelengths from quantum-well diode lasers and dramatic improvement in their time-zero performance. In the paper, we review the characteristics of strained-layer InGaAs quantum-well heterostructure lasers that have resulted in the emergence of this important technology     

Low-energy ion-implantation-induced quantum-well intermixing

In this paper, we present the attractive characteristics of low-energy ion-implantation-induced quantum-well intermixing of InP-based heterostructures. We demonstrate that this method can fulfil a list of requirements related to the fabrication of complex optoelectronic devices with a spatial control of the bandgap profile. First, we have fabricated high-quality discrete blueshifted laser diodes to verify the capability of low-energy ion implantation for the controlled modification of bandgap profiles in the absence of thermal shift. Based on this result, intracavity electroabsorption modulators monolithically integrated with laser devices were fabricated, for the first time, using this postgrowth technique. We have also fabricated monolithic six-channel multiple-wavelength laser diode chips using a novel one-step ion implantation masking process. Finally, we also present the results obtained with very low-energy (below 20 keV) ion implantation for the development of one-dimensional and zero-dimensional quantum confined structures.     

Microring-Resonator-Based Widely Tunable Lasers

We describe widely tunable lasers incorporating microring resonators. By using a double-ring-resonator-coupled filter, a wide wavelength tuning range is achieved with a low tuning current. A double-ring-resonator-coupled tunable laser has series-connected microring resonators in the laser cavity. A tuning range of 50 nm is achieved with injection current of less than 5.2 mA. This low injection current reduces the frequency drift caused by thermal transients to less than 5 GHz. We also describe an integrated filtered feedback tunable laser consisting of a Fabry–Perot (FP) laser and integrated filtered feedback sections. In this device, the filter section is removed from the laser cavity. The device exhibits a 24-nm tuning range with a side-mode suppression ratio of 50 dB. The frequency drift is less than 1 GHz because the longitudinal mode of the laser is mainly determined by the FP laser section. We have also developed a filter-free widely tunable wavelength converter by monolithically integrating a tunable laser and a wavelength converter based on a semiconductor optical amplifier. Wavelength conversion for a 10-Gb/s nonreturn-to-zero signal is also demonstrated, tunable over a 40-nm range.      

III-V semiconductor heterojunction devices grown by metalorganicchemical vapor deposition

Several important early developments in the metalorganic chemical vapor deposition technology relate to the demonstration of high-performance AlGaAs-GaAs injection lasers and solar cells in the late 1970s. It has been nearly 24 years since the first semiconductor injection lasers grown by metalorganic chemical vapor deposition (MOCVD) were made and nearly 22 years since the first continuous-wave quantum-well injection lasers were made by this process. In this past 20 odd years, MOCVD has been developed for the production of AlGaAs, InAlGaP, InGaAsP, InAlGaN, and a variety of other compound semiconductor materials. It is now the dominant technology for the production of light-emitting diodes, injection lasers, solar cells, photodetectors, and heterojunction bipolar transistors and a variety of other solid-state devices. The paper reviews some of the early developments in this technology     

Double-pass fiber Raman laser-a powerful and widely tunable in theultraviolet, visible, and near-infrared fiber Raman laser for biomedicalinvestigations

We present the operating principle, general features, and advantages of a novel concept for the development of powerful and widely tunable fiber Raman lasers (FRL's), based on the high-power pumping of large-core low-loss multimode fibers using a simple double-pass laser arrangement with Littrow-prism-tuned emission. Basic experimental results obtained with various modification double-pass FRL schemes, which provide the generation of both discretely tunable emission in the UV/blue (360-493 nm) at λ_p=355 nm and continuously tunable emission in the visible/near-IR (0.54-1.01 μm) at λ _p=532 nm, are reported. The double-pass FRL schemes permit powerful and widely tunable spectral components to be generated, due to the double passing of nonlinear-converted laser emission through the optical fiber, maximum cavity feedback, the use of multimode fibers, and high-power pumping     

Modulation of the second-order nonlinear tensor components inmultiple-quantum-well structures

In this paper, we present experimental results which demonstrate that quantum-well intermixing techniques can be used to modulate the magnitude of the second-order nonlinear coefficient χ⁽²⁾. Impurity-free vacancy disordering with SiO₂ and Ga₂O₃ caps was used to modulate the position of the band edge and hence, the magnitude of χ_eff⁽²⁾ . Using a coupled quantum-well structure we were able to demonstrate modulation of the d₃₃ tensor components associated with the asymmetric structure and of the d₁₄ component associated with the bulk crystal structure     

Semiconductor lasers using diffused quantum-well structures

We assess the relative merits and prospects of using diffused quantum-well (QW) structures in semiconductor lasers. First, different techniques to achieve interdiffusion are analyzed and compared. Second, recent development of semiconductor lasers using interdiffusion technique is also discussed. Third, the optical properties of diffused QWs are studied. In addition, novel design of diffused QWs structures to maintain stable single-mode operation in semiconductor lasers is proposed. Finally, brief discussion and conclusion are given     

Widely tunable polarization-stable fiber lasers

Four different widely tunable polarization-stable lasers, each based on nonpolarization maintaining erbium-doped fiber are presented. We experimentally examine the effect of output coupling on laser performance, and compare the output power measurements with theoretical predictions. We also discuss the relative costs and construction factors of the four lasers     

Monolithic tunable diode lasers

After over two decades of exploration, tunable diode lasers are beginning to find significant applications, driven largely by the huge demand for bandwidth that is guiding many developments in the optical fiber communication business today. In the paper, some of the history and key developments that have led to the technologies available today are reviewed from the perspective of the author. After discussion of some of the early work, the focus shifts to widely tunable diode lasers, which would appear to be key enablers for future dense wavelength-division multiplexing and optical switching and networking systems. The distinguishing characteristics of the current technological alternatives are summarized     

High-performance 1200-nm InGaAs and 1300-nm InGaAsN quantum-well lasers by metalorganic chemical vapor deposition

In this paper, we present the characteristics of high-performance strain-compensated MOCVD-grown 1200-nm InGaAs and 1300-nm InGaAsN quantum-well (QW) lasers using AsH/sub 3/ and U-Dimethylhydrazine as the group V precursors. The design of the InGaAsN QW active region utilizes an In-content of approximately 40%, which requires only approximately 0.5% N-content to realize emission wavelengths up to 1315-nm. Threshold current densities of only 65-90 A/cm/sup 2/ were realized for InGaAs QW lasers, with emission wavelength of 1170-1233 nm. Room-temperature threshold and transparency current densities of 210 and 75-80 A/cm/sup 2/, respectively, have been realized for InGaAsN QW lasers with emission wavelength of 1300-nm. Despite the utilization of the highly-strained InGaAsN QW, double-QW lasers have been realized with excellent lasing performance.     

Modeling of strained quantum-well lasers with spin-orbit coupling

A complete model with the spin-orbit coupling for strained quantum-well lasers is presented. Explicit formulas for the momentum-matrix elements are given. The improvement in the threshold current density of tensile strained quantum-well lasers, as compared with that of the unstrained quantum well, is shown to result from the enhanced momentum matrix. The differential gain and the linewidth enhancement factor are calculated. The theoretical results show a smaller linewidth enhancement factor for compressively and tensile strained quantum wells than that of the unstrained structure, as has been experimentally observed. The temperature behavior of both the radiative component and the Anger component of the threshold current density is shown. Due to a decrease of gain and differential gain with increasing temperature, the threshold carrier density in unstrained quantum wells is increased with a large increment of the Auger recombination current at high temperature. For strained quantum wells, this increment is moderate because of the smaller threshold carrier density     

High efficiency widely tunable SGDBR lasers for improved direct modulation performance

We report on two novel approaches to improve the differential quantum efficiency (DQE) of widely tunable 1.55-/spl mu/m lasers: the bipolar cascade sampled grating distributed Bragg reflector (BC-SGDBR) laser and the gain-levered SGDBR (GL-SGDBR) laser. Each is fabricated on a robust InGaAsP/InP photonic integrated circuit platform. The lasers demonstrate improved direct modulation performance over conventional SGDBR lasers. The BC-SGDBR laser was also monolithically integrated with a semiconductor optical amplifier and photodetector receiver in order to perform wavelength conversion. Error free wavelength conversion at 2.5 Gb/s and improvements in conversion efficiency are demonstrated.