Tailored scaling allows the effectiveness of physical effects and mechanical stability to be enhanced. This is shown for micromachined 1.55μm vertical-resonator-based filters and VCSELs, capable of wide, continuous, and kink-free tuning by a single control parameter. Tuning is achieved by mechanically actuating one or several membranes in a vertical air-gap resonator including two highly reflective DBR mirrors. Electrostatically actuatable single-chip filters including InP/air-gap DBR's (3.5 periods) reveal a continuous tuning up to 14% of the absolute wavelength. Varying a reverse voltage (U=0 .. -3.2V) between the membranes (almost flat in the unactuated condition) a tuning range up to 142nm was obtained. Varying a reverse voltage (U=0 .. -28V) between the membranes (strained and curved in the unactuated condition) a tuning range up to 221nm was obtained. Optically pumped and continuously tunable 1.55μm VCSELs show 26nm spectral tuning range, 400μW maximum output power, and 57dBm SMSR. This two-chip VCSEL has a movable top mirror membrane, which is precisely designed to obtain a specific air-gap length and a tailored radius of curvature in order to efficiently support the fundamental optical mode of the plane-concave resonator. The curved top mirror DBR membrane consists of periodically alternating differently stressed silicon nitride and silicon dioxide multilayers. The lower InP-based part consists of the InP/GaInAsP bottom DBR and the GaInAsP active region.
Ultra-widely tunable microcavity devices implemented by surface micromachining are studied. We model, fabricate, and characterize 1.55-μm vertical-resonator-based optical filters and vertical cavity surface emitting lasers (VCSELs) capable of wide, monotonic, and kink-free tuning by a single control parameter. Our devices are comprised of single or multiple horizontal air gaps in the dielectric and InP-based material system. Distributed Bragg mirrors with multiple air gaps are implemented. Due to the high refractive index contrast between air (n = 1) and InP (n = 3.17), only three periods are sufficient to guarantee a reflectivity exceeding 99.8% and offer an enormous stop-band width exceeding 500 nm. Unlike InGaAsP/InP or dielectric mirrors, they ensure short penetration depth of the optical intensity field in the mirrors and low absorption values. Stress control of the suspended membrane layers is of utmost importance for the fabrication of these devices. By controlling the stress, we are able to fabricate InP membranes that are extremely thin (357 nm thick) and at the same time flat (radius of curvature above 5 mm). Micromechanical single parametric actuation is achieved by both thermal and electrostatic actuation. Filter devices with a record tuning more than 142 nm with 3.2 V are presented.
Surface-micromachined 1.55µm vertical-resonator-based devices, capable of wide, continuous, monotonic and kink-free tuning are designed, technologically implemented and characterized. Tuning is achieved by mechanically actuating one or several membranes in a vertical resonator including two ultra-highly reflective DBR mirrors. The tuning is controlled by a single parameter (actuation voltage). The two different layers composing the mirrors reveal a very strong refractive index contrast. Filters including InP/air-gap DBR's (3.5 periods) using GaInAs sacrificial layers reveal a continuous tuning of up to 9% of the absolute wavelength. Varying a reverse voltage (U=0 .. -3.2V) between the membranes, a tuning range up to 142nm was obtained by electrostatic actuation. The correlation of the wavelength and the applied voltage is accurately reproducible without any hysteresis. Theoretical model calculations are performed for symmetric and asymmetric device structures, varying layer thickness and compositions. Models of highly sophisticated color tuning can be found in nature, e.g. in tunable spectral light filtering by trogon and butterfly wings. Bionics transfers the principles of success of nature into natural science, engineering disciplines and applications (here filters and VCSELs for optical communication on the basis of WDM). Light interferes constructively and destructively with nano- and microstructures of appropriate shape, dimensions and materials, both in the artificial DBR structures fabricated in our labs as well as in the natural ones.
Surface-micromachined 1.55μm vertical-resonator-based devices, capable of wide, continuous, monotonic and kink-free tuning are designed, technologically implemented and characterized. Tuning is achieved by mechanically actuating one or several membranes in a vertical resonator including two ultra-highly reflective DBR mirrors. The tuning is controlled by a single parameter (actuation voltage). The two different layers composing the mirrors reveal a very strong refractive index contrast. Filters including InP/air-gap DBR's (3.5 periods) using GaInAs sacrificial layers reveal a continuous tuning of >9% of the absolute wavelength. Varying a reverse voltage (U=0 .. -3.2V) between the membranes, a tuning range up to 142nm was obtained by electrostatic actuation. The correlation of the wavelength and the applied voltage is accurately reproducible without any hysteresis. Appropriate miniaturization is shown to increase the mechanical stability and the effectiveness of spectral tuning by electrostatic actuation since the relative significance of the fundamental physical forces can be varied considerably by appropriate scaling. Model calculations are performed for symmetric and asymmetric optical filter structures, varying layer thickness and compositions. Finally the filter results are used to design micromachined tunable air-gap VCSEL´s. Theoretical model calculations demonstrate very wide spectral tuning by micromachined actuation of air-gap VCSEL resonators.
Continuously tunable Fabry-Perot filters manufactured using multiple air-gap MOEMS technology are studied and presented. The InP/air-gap filters optimized for optical telecommunication systems using the third optical telecommunication window (1550nm) exhibit a wide tuning range of 142nm and an extremely wide stop-band of 550nm (1250nm-1800nm). The tuning is continuously adjustable requiring ultra-low actuation voltages between 0V (1599nm) and 3.2V (1457nm). The filters are based on a relatively simple vertical structure which is fabricated by few surface micro machining steps. No mirror alignment or subsequent micro mounting are necessary facilitating a compact batch process production.
A novel low cost technology for fabrication of micro-opto-electro-mechanical devices based on plasma enhanced chemical vapor deposition (PECVD) of dielectric materials is presented. Applying surface micromachining, we produce suspended dielectric membranes and cantilevers by involving a common photo resist as sacrificial layer. The intrinsic stress in the layers is adjusted using an interlacing of high (13.56MHz) and low (130kHz) plasma excitation frequencies in the PECVD. A diffraction image method and microstructures are used for the homogeneous stress evaluation. The stress of silicon nitride can be varied in a wide range between +850MPa compressive and −300MPa tensile and no dependence of the frequency on silicon dioxide intrinsic stress is noticed. Depending on lateral design and gradient stress variation, Fabry-Perot filter membranes with radius of curvature (ROC) between −1.7mm and 51mm as well as cavity lengths between 2.3μm and 13.5μm are implemented. Thus, convex, concave and plane membranes are produced. Furthermore, a thermally tuned air-gap Fabry-Perot filter with 8nm FWHM and a tunability of 15nm/mA is fabricated. Strategies of combining these filters with organic laser materials are developed. For this purpose, molecular glasses capable of amplified spontaneous emission (ASE) are chosen, e.g. the molecular glass 4-Spiro which shows an amplified spontaneous emission line at a low threshold of 3.2μJ/cm2 pump laser power density.
In technology and nature, tailored scaling represents a principle of success which allows the effectiveness of physical effects to be enhanced. For our optical microsystems, we state that appropriate miniaturization increases the mechanical stability and the effectiveness of spectral tuning by electrostatic and thermal actuation since the relative significance of the fundamental physical forces involved considerably changes with scaling. These basic physical principles are rigorously applied in micromachined 1.55μm vertical-resonator-based filters, capable of wide, monotonic and kink-free tuning by a single control parameter. Tuning is achieved by mechanical actuation of one or several air-gaps which are part of a vertical resonator including two ultra-highly reflective DBR mirrors of strong refractive index contrast: (I) Δn=2.17 for InP/air-gap DBR's (3.5 periods) using GaInAs sacrificial layers and (II)Δn=0.5 for Si3N4/SiO2 DBR’s (12 periods) with a polymer sacrificial layer to implement the air-cavity. In semiconductor multiple air-gap filters, a continuous tuning of >9% of the absolute wavelength is obtained. Varying the reverse voltage (U=0 .. 3.2V) between the membranes (electrostatic actuation), a tuning range up to 142nm was obtained. The correlation of the wavelength and the applied voltage is accurately reproducible without any hysteresis. The extremely wide tuning range and the very small voltage required are record values to the best of our knowledge. Principles of III/V semiconductor micromachining and the detailed technological fabrication process of our filters are focused.
Bionics transfers the principles of success of nature into natural science, engineering disciplines and applications. Often generation and detection of different spectral colors play key roles in communication in both, nature and technology. The latter one refers e.g. to dense wavelength division multiplex optical communication systems. This paper shows interesting parallels in tunable spectral light filtering by butterfly wings and by tunable optical filters used in optical communication systems. In both cases light interferes constructively and destructively with nano- and microstructures of appropriate shape, dimensions and materials. In this paper methodology is strongly emphasized. We demonstrate that tailored scaling allows the effectiveness of physical effects to be enhanced in nature and technology. These principles are rigorously applied in micromachined 1.55μm vertical-resonator-based filters, capable of wide, continuous, monotonic and kink-free tuning by a single control parameter. Tuning is achieved by mechanically actuating one or several membranes embedded by air-gaps in a vertical resonator including two ultra-highly reflective DBR mirrors. The layers of mirrors reveal a very strong refractive index contrast. Filters including InP/air-gap DBR's (3.5 periods) using GaInAs sacrificial layers reveal a continuous tuning of >9% of the absolute wavelength. Varying a reverse voltage (U=0 .. -3.2V) between the membranes, a tuning range up to 142nm was obtained due to electrostatic actuation. Appropriate miniaturization is shown to increase the mechanical stability and the effectiveness of spectral tuning by electrostatic actuation since the relative significance of the fundamental physical forces can be shifted considerably by appropriate scaling.
We present ultra-widely tunable micro-cavity devices realized by micro-opto-electro-mechanical system (MOEMS) technology. We modeled, fabricated and characterized 1.55μm micromachined optical filter and VCSEL devices capable of wide, monotonic and kink-free tuning by a single control parameter. Our vertical cavity devices comprise single or multiple horizontal air-gaps in the dielectric and InP-based material system. Distributed Bragg mirrors with multiple air-gaps are implemented. Due to the high refractive index contrast between air (n=1) and InP (n=3.17) only 3 periods are sufficient to guarantee a reflectivity exceeding 99.8% and offer an enormous stop-band width exceeding 500nm. Unlike InGaAsP/InP or dielectric mirrors they ensure short penetration depth of the optical intensity field in the mirrors and low absorption values. Stress control of the suspended membrane layers is of outmost importance for the fabrication of MOEMS devices. By controlling the stress we are able to fabricate InP membranes which are extremely thin (357nm thickness) and at the same time flat (radius of curvature above 5mm). Micromechanical single parametric actuation is achieved by both, thermal and electrostatic actuation. Filter devices with a record tuning over 127nm with 7.3V are presented.
We study 1.55micrometers filter and VCSEL devices capable of wide and continuous tuning based on a single control parameter. Ultra-high reflective DBR mirrors are realized with a low number of DBR periods using high refractive index contrast: (I) (Delta) n=2.17 for InP/airgap DBR's (3.5 periods) and (II) (Delta) n=0.5 for Si3N4/SiO2 DBR's (12 periods) with a polymer sacrificial layer to implement the air-cavity. Corresponding fabrication technologies are presented in detail. In both cases spectral tuning (>100nm, theoretically) is obtained by micomachined actuation of the included air-cavity. Large stopband widths and very large tuning efficiencies are obtained by model calculations. For VCSEL's a trade-off between lasing efficiency and tuning efficiency is obtained. Experimental results show very good optical properties: high mirror reflectance and clear single-line filter transmission. The first tunable dielectric filter based on polymer sacrificial layers is presented: (Delta) (lambda) /(Delta) U= -7nm/V at 1mA. The potential of the airgap concept: the filter transmission or the laser emission wavelength can be continuously tuned over more than 100nm, thus, the whole spectral gain profile can be addressed by a single control parameter.
We have fabricated InGaAs/InP PIN diodes with a coplanar waveguide design. The diodes have been integrated in a two- chip wavelength-selective Fabry-Perot filter for the use as wavelength-selective PIN receivers for dense wavelength division multiplex systems.
Rolf Riemenschneider, Joachim Peerlings, Joachim Pfeiffer, A. Dehe, Andreas Vogt, Peter Meissner, Hans Hartnagel, N. Chitica, Juergen Daleiden-, Klaus Streubel, Harald Kuenzel, W. Goertz
Monolithically-integrated wavelength-selective receivers are needed for dense wavelength-division multiplex transmission in the 1.55 micrometers wavelength regime. We present a novel concept for tunable optical Fabry-Perot filters with long resonant cavities of about 30 micrometers . III-V semiconductor technology compatible to PIN detector integration is applied to fabricate bulk-micromachined movable membrane Bragg mirrors. The initial membrane curvature as well as the actuation-induced bending are analyzed using a white light interferometer. Continuous filter tuning achieved by thermal or electrostatical actuation is analyzed in the optical as well as in the mechanical regime. Opto-mechanical constraints of the realized filters are discussed in view of novel epitaxial demands and optimum design for micro-opto- electro-mechanical receiver systems.
John Ralston, Eric Larkins, K. Eisele, S. Weisser, Susann Buerkner, A. Schoenfelder, Juergen Daleiden, Konrad Czotscher, Ignacio Esquivias, Joachim Fleissner, R. Sah, Martin Maier, Willy Benz, Josef Rosenzweig
Optimized molecular-beam epitaxial growth of pseudomorphic MQWs, the application of abrupt and spatially-localized carbon doping, and the development of short-cavity coplanar ridge-waveguide structures with high-quality chemically-assisted ion-beam etched facets have been combined to fabricate GaAs-based MQW lasers which have achieved damping-limited direct modulation bandwidths exceeding 40 GHz. More detailed measurements indicate intrinsic modulation bandwidths exceeding 60 GHz for devices with p-doped active regions. The reduced linewidth enhancement factor, (alpha) , observed in these lasers also indicates their suitability for low-chirp high-speed direct modulation. The laser design has been further incorporated into a complete technological process for the monolithic integration of GaAs MQW lasers and HEMT-based laser-driver circuits capable of operation in data rates up to 20 Gb/s. Using the impurity-free interdiffusion process, large shifts in the lasing wavelength have been achieved with no strain relaxation and while maintaining the high-speed modulation properties of the pseudo-morphic InGaAs/GaAs MQW lasers, demonstrating the feasibility of fabricating high-speed multi-wavelength laser arrays.
We describe short-cavity In0.35Ga0.65As/GaAs multiple quantum well (MQW) lasers with undoped and p-doped active regions. The epilayer structure consists of four 5.7 nm QWs separated by 20.1 nm barriers in a GaAs core. The cladding layers consist of Al0.8Ga0.2As. In the case of p-doped devices a 4.5 nm carbon (C)-doped region (2.5 multiplied by 1019 cm-3) was inserted above each QW, separated by a 3.1 nm GaAs spacer, resulting in a modulation-doped core region. Using a CAIBE process, short-cavity ridge-waveguide lasers are fabricated in a triple-mesa geometry suitable for on-wafer probing. The best device (6 multiplied by 130 micrometers squared) with an undoped active region attained a damping-limited direct modulation bandwidth exceeding 40 GHz at a cw bias current of 160 mA. In contrast, the p-doped devices, demonstrating a maximum bandwidth of 37 GHz, are still limited by power dissipation. (alpha) -factors as low as 1.4 and 1.5 for undoped and p-doped devices, respectively, are extracted from measurements of the sub- threshold gain spectra. In addition, we demonstrate eye diagrams at 25 Gbit/s (limited by the pulse pattern generator) for these laser diodes. A complete characterization of dc and rf properties of these lasers is presented.
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