Soeren Irmer

Tuning efficiency and linewidth of electrostatically actuated multiple air-gap filters

We investigated the tuning efficiency of electrostatically actuated multiple air-gap filters fabricated in InP for dense wavelength division multiplex applications by comparing measured tuning curves with the results of optical and mechanical simulations. These filters exhibit a record tuning range of 127 nm at 7.3 V tuning voltage. The filters were measured in reflection using standard single mode fiber. The subsequent analysis is based on a one-dimensional electromechanical and optical model providing a reasonable estimation for the pull-in voltage. Optical simulations show that the filter linewidth does not affect the tuning efficiency.

Potential of micromachined photonics: miniaturization, scaling, and applications in continuously tunable vertical air-cavity filters

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 DBRs (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.

Novel low-cost and simple fabrication technology for tunable dielectric active and passive optical air-gap devices

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.

Continuously tunable air gap micro-cavity devices for optical communication systems

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.

Ultrawide continuously tunable 1.55-μm vertical-air-cavity filters and VCSELs based on micromachined electrostatic actuation

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 DBRs (3.5 periods) and (II) (Delta) n=0.5 for Si3N4/SiO2 DBRs (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 VCSELs 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.

Continuously tunable InP-based multiple air-gap MOEMS filters with ultrawide tuning range

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.

Wide continuously tunable 1.55 μm vertical air-cavity wavelength selective elements for filters and VCSELs using micromachined actuation

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 DBRs (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.

Wide continuously tunable 1.55µm vertical air-cavity wavelength selective elements for filters and VCSELs using micromachined actuation

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 DBRs (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.

Wide continuously tunable 1.55μm vertical air-cavity wavelength selective elements for filters and VCSELs using micromachined actuation (Invited Paper)

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 DBRs (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.

142-nm electrostatically actauted tuning using surface-micromachined vertical air-cavity wavelength-selective elements for applications in 1.55μm VCSELs

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 DBRs (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.