In contrast to conventional optical systems, which are optimized for wavelength-independent imaging, hyperchromats aim for strongly wavelength-dependent focal lengths. In this contribution, the design parameters of hyperchromatic two-lens optical systems were derived that provide strong axial color splitting expressed by extremely low equivalent Abbe numbers. These systems have been investigated for compositions of either pure refractive or all diffractive lenses, as well as hybrid configurations thereof. First, lens doublets made of cemented elements are considered and the variables affecting the equivalent Abbe number of the system are investigated. In particular, the influence of the focal lengths of the individual lenses and the Abbe numbers of the selected lens materials are taken into account. The best parameter-sets were determined by paraxial numerical simulations for different cemented configurations. To ensure a simple implementation, especially to avoid exotic or potentially harmful materials, only readily available inorganic standard glasses were considered. In the next phase of this investigation an air gap was inserted between the two lenses, which is an additional influence parameter on the equivalent Abbe number. Following the paraxial considerations, selected two-lens configurations were transferred to the non-paraxial domain and refined using optical design software, also taking aberrations into account. To further reduce achievable equivalent Abbe numbers, an aspherical surface was introduced to compensate for spherical aberrations. Finally, for the refractive doublets an equivalent Abbe number of 2.4 was achieved, which corresponds to only 12% of the smallest Abbe number of the selected materials. This result was even surpassed by the hybrid hyperchromat, resulting in an extraordinary minimum equivalent Abbe number of -0.6 that is more than five times smaller than the Abbe number of diffractive lenses.
Filter-based spectral detectors convince with their simple concept, an extremely compact and robust design and the possibility to adapt the addressed spectral range and the resolution to the individual application requirements. Unfortunately, these filter-based sensors usually suffer from low detection efficiency. In this contribution we discuss and compare different methods that allow to substantially increase the detection efficiency of filter-based spectral sensors. An initial concept is based on a wavelength-dependent redistribution of the incident light before it reaches the individual filter elements of the array. This approach allows a substantial increase in detection efficiency, but requires additional dichroic elements in the beam path. An alternative approach uses a folded beam path architecture and completely waives additional dichroic elements. This approach is not only suitable for filter-based spectral sensors, but can also be transferred to increase the efficiency of hyperspectral imaging systems.
Echelle-inspired cross-grating spectrometers try to combine the high performance of classical Echelle spectrometers and the small footprint of compact line-grating spectrometers. Therefore, a cross-grating is used which is a superposition of two perpendicularly oriented line gratings in a single element. Highly resolved, but overlapping, diffractions orders are created by the main grating, which are separated by the cross-disperser. This powerful approach is connected to different challenges concerning the optical design, the fabrication of the cross-grating and implementation of the device. These challenges are addressed by a compact and rigid double-pass design, which utilizes the same refractive elements for collimation of the incoming beam and focusing of the diffracted light on the detector. This contribution gives an overview on the design and focusses on the implementation of the spectrometer. This includes on one hand the mounting of the cross-grating and the refractive elements in a rigid objective group and, on the other hand, the adjustment of the objective to the entrance fiber and the 2D detector. Furthermore, the implemented and calibrated instrument allows to conduct several validating experimental tests in order to proof the working principle. The spectrometer addresses a spectral range from 400 nm to 1100 nm and reaches a resolving power of 300 with an entrance pinhole diameter of 105 μm. An even higher resolving power of more than 1000 is reached with a reduced pinhole diameter of approximately 5 μm.
This contribution addresses an alternative lithographic technique for the tailored fabrication of rotationally symmetric meso- and microscale optical components. A variable ring-shaped light distribution is created by an axicon-pair based zoom-concept and can be used for the manufacturing of single optical components and array elements as well. First, design considerations of the basic axicon system and the achievable system characteristics are discussed. In particular, minimum and maximum ring diameter depending on axicon angle variations and displacement distance of employed axicons as well as potential deviations from the telecentricity condition are considered. Additionally, further aspects concerning the system implementation are presented, e.g. the achievable resolution which is dependent on the entrance pinhole. Finally, the performance of the system is presented by demonstrating the fabrication of exemplary meso- and microscale structures.
A method to drastically enhance detection efficiency of a linear variable filter (LVF) sensor across an extended and continuous wavelength range is presented. The efficiency is increased by a wavelength preselection concept, where the incoming light is divided into partial spectra to reduce otherwise unavoidable reflection losses of filter-based spectrometers. The simple but effective setup uses selected and successively arranged dichroic beamsplitters, which ensures an optimized compromise between efficiency enhancement and minimum increasing complexity. When connected to a two-dimensional camera and combined with a tilted LVF, this compact optical system allows the continuous recording of the full wavelength range between 450 and 850 nm with a resolution of ∼19 nm at 508.6 nm. An efficiency enhancement factor of up to 5.7 is achieved in comparison to a conventional LVF setup. The working principle was verified by measuring the reflection spectra of different natural and artificial green leaves. The proposed approach for increasing the efficiency can be miniaturized and applied to a broad range of other filter-based sensors.
Micro-opto-electro-mechanical systems (MOEMS) micromirror and shutter arrays have gained huge interest in research and applications. Our study starts with an overview on the technological achievements and experimental results of groups that have been working on this field. The main part of our study is revealing the MOEMS micromirror array technology for light steering via smart glazing for buildings. The mirror array is actuated electrostatically and integrated between the panes of insulation glazing. Depending on user activity as well as daytime and season requirements, the MOEMS micromirror arrays shall enable personalized light steering, energy saving, and reduction of CO2 emission. Technological fabrication of subfield addressing up to 64 fields inside the arrays is presented. Experimental characterization results such as actuation voltages, maximum and minimum transmission, contrast, and energy saving potential are reported. Using an industrial window module fabrication process, a laboratory demonstrator and a function demonstrator have been implemented. Rapid aging tests including vibrations, extreme temperatures, multiple temperature cycles, and long-term electrostatic actuation of micromirror structure were performed to evaluate reliability and lifetime. These results validate extrapolated lifetimes—in future applications as active windows—far beyond 40 years, as well as their robustness during transportation, installation, and against all vibration influences in buildings.
Optical measurement techniques are widely applied in high-resolution contour, topography and roughness measurement. In this context vertical scanning white-light interferometers and confocal microscopes have become mature instruments over the last decades. The accuracy of measurement results is highly related not only to the type and physical properties of the measuring instruments, but also to the measurement object itself. This contribution focuses on measurement effects occurring at edges and height steps using white-light interferometers of different numerical apertures. If the edge is perfectly perpendicular, batwing effects appear at height steps. These batwings show maximum height if the height-to-wavelength-ratio (HWR) is about one forth or three forth, and they disappear if the HWR value is about an integer multiple of one half. The wavelength that is relevant in this context is the effective wavelength, i.e. the center wavelength of the illuminating light multiplied by a correction factor known as the numerical aperture correction. However, in practice the edges are usually not perfectly perpendicular. In this case, the measurement results depend also on the derivative of the surface height function and they may differ from theory and the prediction according to the HWR value. Measurements of such steps show systematical effects depending on the lateral resolution of the instrument. In this context, a Linnik interferometer with a magnification of 100x and NA = 0.9 is used to characterize the three dimensional topography of more or less rectangular calibration specimens and quasi-perpendicular structures produced by the nanoimprint technology. The Linnik interferometer is equipped with LED light sources emitting at different wavelengths, so that the HWR value can be changed. This is possible since the high NA objective lenses show a rather limited depth of focus such that the temporal coherence gating may be replaced by focal gating in this particular instrument. In addition, measurement results are compared with those achieved by a Mirau interferometer of NA = 0.55. A commercial confocal microscope with NA of 0.95 serves as a reference instrument for further comparison. Numerical simulations considering diffraction effects are carried out in order to explain the experimental results obtained by the different white and colored light interferometers
We present a direct patterning method of dielectric materials via temporally shaped femtosecond laser pulses. A thinfilm waveguide with a 2D periodic pattern of photonic crystals with circular base elements is investigated. We use dielectrics since they are transparent especially in the visible spectral range, but also in UV and near infrared range. Thus, they are very suitable as optical filters in the very same spectral region. Since structuring of non-conductive dielectric materials suffers from charging, the implementation of laser processing as patterning method instead of conventional processing techniques like electron beam lithography or focused ion beams is a very attractive alternative. Despite a low refractive index contrast, we show by numerical results that normal incident of light to the plane of periodicity couples to a waveguide mode and can excite Fano resonances. That makes the device extremely interesting as narrow-band optical filter. Applications of optical filters in the visible and UV range require fabrication of photonic crystal structures in the sub-100 nm range. Temporally shaped femtosecond laser pulses are applied as a novel method for very high precision laser processing of wide band gap materials to create photonic crystal structures in dielectrics. Shaping temporally asymmetric pulse trains enable the production of structures well below the diffraction limit.1 We combine this process with deposition of a high refractive index layer to achieve the targeted resonant waveguide structure. Additionally, we focus on the rim formation arising by laser processing since this is an important issue for fabrication of photonic crystal arrays with small lattice constants.
We present the characterization of a dry-etching process for high-contrast TiO2/SiO2 distributed Bragg reflectors, by inductively coupled plasma reactive ion etching, focusing on the etch rate and the etch selectivity. Photoresists and metals as etch masks were investigated. An excellent etch profile using an indium tin oxide mask was obtained, with an etch rate of >80 nm/min at a pressure of 6 mTorr. The experiments were developed for structuring Fabry-Pérot filters for tunable optical sensor arrays.
The characteristics of photodiodes integrated on CMOS ASICs depend on wavelength of radiation, structure of the
photodiode itself and the parameters of the process of production. In this paper, the influence of the structure of
integrated CMOS photodiodes produced in a standard 0.5 μm mixed signal CMOS process on the sensitivity is
described. These photodiodes are used as image sensor elements arranged in an array for noncontact optoelectronic
measurements.
Models of integrated photodiodes distinguish the lateral and the vertical region of the photodiodes. The standard 0.5 μm
CMOS process offers three types of pn-junctions: n+/p-substrate, p+/n-well and n-well/p-substrate. Based on our
previous research and on the results from other authors the p+/n-well is chosen due to its better sensitivity and isolation
against other structures.
The local sensitivity is measured with a scanning setup by applying a diffraction limited spot spot of light on the surface
of the diodes. Independent of the wavelength of radiation the charge carriers are generated mainly in the lateral region
and not - as expected - in the vertical region. The maximum value of the local sensitivity is found in photodiodes with
subdivided p+ regions showing a distance of 1.5 μm between these regions in the space between these two adjacent p+
regions. This local sensitivity is three times smaller than that of a reference PIN photodiode. According to this result, the
new photodiodes will be constructed with optimized geometries.
All examined structures of this type of photodiodes show a maximal spectral sensitivity in the range of 650 nm - 700 nm.
We have investigated the etching characteristics of high-index-contrast TiO2/SiO2 DBR mirrors by inductively coupled
plasma reactive ion etching (ICP-RIE) with a focus on the etch rate and the etch selectivity by varying etch parameters
(gas flow rate, RF and ICP power, pressure and temperature). Chrome, aluminum and ITO (indium tin oxide) were
applied as etch masks. Various mixtures of SF6/Ar gas were used for the etch processes. An optimum etch profile was
obtained with an etch rate of approximately 80 nm/min at a pressure of 6 mTorr and a temperature of 20 °C. The
experimental results were applied to develop Fabry-Perot filters for tunable optical sensor arrays.
We present the potential and the benefits of actuable micromirror arrays for large area applications for daylight
deflection. The described micromirror arrays are intended to be implemented into windows of buildings and to provide
functionalities like daylight guiding into rooms, heat regulation and glare protection. Placed between two panes of a
window, these mirrors are maintenance-free and not subject to defilement. The use of micro system technology on large
areas requires very low cost processes and materials, as well as a concept with a minimum of process steps and a very
easy and reliable process control. We present theoretical and technological approaches and first technological results.
We present an alternative method of determining the oscillating state of a laser and demonstrate the suitability of relative
intensity noise (RIN) measurements for this purpose.
The experiments were carried out using a two-section DFB laser. Optical and RIN spectra have been recorded and
correlated subsequently. The variation of the maxima of the RIN spectra have been evaluated with respect to intensity and
position in the frequency domain. Varying the frequency, a distinct transition in the above mentioned parameters can be
observed, wich can be correlated clearly to the mode degeneracy at the transition and a dominating oscillating mode below
and above. This delivers a conclusive means of determining the lasing state from RIN spectra.
InP based tunable optical MEMS devices, such as Fabry-Perot filters, VCSELs, photodiodes, consist of two distributed
Bragg reflectors (DBRs) and a cavity. Tuning of the filter wavelength is achieved by electrostatic actuation of the two
DBRs which are p-doped and n-doped, respectively, and reversely biased. The cavity and the DBRs consist of a stress
compensated InP/airgap structure which is fabricated by sacrificial layer removal, using FeCl3 wet etching of InGaAs
layers. In this work we investigated the influence of p-and n-type conductivity on the etching process. We observed that
the sacrificial layer etch rate of n-InGaAs is 7 times slower than the p-InGaAs. This influences the stress in the n-DBR
section of the tunable device. Based on these results novel wavelength tunable optical devices with multiple InP
membrane/airgap structures will be designed.
KEYWORDS: Micromirrors, Mirrors, Solar cells, Photovoltaics, Aluminum, Solar concentrators, Sun, Microopto electromechanical systems, Finite element methods, 3D modeling
Micro-opto-electromechanical systems (MOEMS) based on micromechanical mirrors can be used as key elements for
light guiding, steering and concentration. We propose micromechanical mirror arrays for light concentration on
photovoltaic modules. The semiconductor materials for solar cells are the most expensive components of a photovoltaic
system. One of the ways to reduce cost is to use light concentration by focusing sunlight onto small solar cell areas using
optical components such as lens systems. The whole system requires an external rotation mechanism to track the suns
position. As an alternative, we propose and implemented micromachined mirror arrays to concentrate light. This allows
precise dynamic light steering onto the solar cell module. These micromirror arrays can be electrostatically tuned to track
the sun position or the maximum of the brightness distribution in the sky. The micromirror arrays are located in a sealed
environment and, therefore, insensitive to external influences, such as atmosphere and wind. The advantages of the
micromachined mirrors based concentrator photovoltaic systems are dynamic light steering onto solar cells, mass
production compatibility, long lifetime and low cost. The concept of the micromachined mirror arrays will be presented.
Static thermal analyses for the earlier concept and the enhanced quasi-monolithic integration technology (QMIT) are performed in detail. The effects of several parameters such as the properties of the materials involved and different geometries in all possible structures are described. Simulation results confirm a very low thermal resistance for the enhanced QMIT structure and highlight its superiority to the earlier concept of QMIT structures. This leads to a longer lifetime, a higher reliability and a better performance of the packaging.
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.
In this paper, application of scanning probe microscopy (SPM) and nanometer surface profiler of DEKTAK for determination of thermal stress in standard structure of QMIT is described. A three dimension finite element (3DFE) thermal stress simulator, a scanning probe microscopy measurements and nanometer surface profiler accompanied with a Peltier element (PE) have been used to determine the thermal stress distribution in the standard structure of QMIT. In this method by measuring and mapping the surface profile of Si-wafer around the embedded devices using SPM and DEKTAK the induced thermal stress is determined. Effects of different parameters such as baking temperature, power dissipation of the embedded GaAs-FET, geometry and elastic properties of thermal conductive epoxy have been described in details. Remarkable agreement between calculated and measured displacements created by thermal stress was found.
This paper proceeds from basic research on carrier dynamics to applications in high-speed laser devices. Different retardation mechanisms are studied experimentally and theoretically providing input for the design of high-speed laser devices. Optically detected carrier dynamics in III/V semiconductor quantum well (QW) heterostructures perpendicular to the interfaces is studied. Photoluminescence emissions originating from different semiconductor layers are recorded time-resolved to probe the carrier dynamics between these layers. High spatial and temporal resolution is obtained experimentally, partly even in the nm and sub-ps ranges, respectively. Retardation effects are separated and studied experimentally and theoretically by corresponding model calculations. A material comparison shows that GaInAsP is beneficial due to considerable advantages in technological implementation processes and AlGaInAs is superior from a physical point of view enabling higher modulation band-widths due to larger conduction band discontinuities. The equalization of the carrier densities in the individual wells is found to be mainly retarded by hole thermionic emission. Hole transport in the p-sided confinement layer and electron capture from the p-sided confinement layer is also found to be also a limiting factor. These results are used to optimized AlGaInAs/InP lasers with asymmetric confinement layers. The p-sided confinement layer is reduced on the costs of the n- sided confinement layer to obtain (1) a faster hole transport across the p-sided confinement layer and (2) to accelerate the capture of electrons from the p-sided confinement layers being uncaptured during the transfer across the QWs. In our experiments a modulation bandwidth of 26 GHz is obtained. Even higher values are found in corresponding theoretical model calculations demonstrating an interesting development potential.
Our studies on the thermal cross talk of laser arrays with integrated thin film heaters involve theoretical as well as experimental investigations. Comparing the effects of intentional geometrical variations of the device design, we found that the relative thermal crosstalk depends critically on the distance between the active area and the film heater. The most striking result is that a minimization of the thermal resistivity of the device does not always lead to a reduced thermal crosstalk. We demonstrate that an additional heat barrier close to the active region and/or an improved heat transfer between the submount and the heat sink may reduce the relative thermal crosstalk, a result completely unexpected from intuitive considerations. Model calculations showed that the device yield can be increased by 20 percent improving the heat transfer between the submount and the heat sink.
Optically detected carrier dynamics in III/V semiconductor quantum well (QW) heterostructures perpendicular to the interfaces is studied. Photoluminescence emission originating form different semiconductor layers are recorded time-resolved to probe the carrier dynamics between these layers. High spatial and temporal resolution is obtained experimentally, partly even in the nm and sub-ps ranges, respectively. Using several specially tailored semiconductor heterostructures enable the following individual dynamic effects to be studied and separated: transport in extended unquantized layers, capture into the QWs, relaxation in the QWs, tunneling between the QWs and thermal re-emission from the QWs. These basic physical effects have to be studied and understood to design and implement modern high-speed semiconductor laser devices. AlGaInAs and GaInAsP heterostructures are compared with respect to interwell transfer efficiencies and problems in technological implementation. This paper proceeds from basic research to applications in high-speed laser devices.
In this article we will generalize and analyze different self-aligned techniques for different stripe lasers, for example: ridge waveguide lasers, buried heterostructure stripe lasers, mushroom-stripe lasers, etc. Finally, a novel self-aligned processing technique for fabricating buried mushroom-stripe lasers is proposed and it has been utilized successfully in fabricating bent waveguide mushroom-stripe MQW DFB lasers.
Carrier transport and carrier capture were reported to markedly influence the carrier and photon dynamics in quantum-well semiconductor lasers and to limit the modulation bandwidth. Recently, model calculations of various degrees of complexity have isolated special aspects of the problem. We given an extended overview and report on our theoretical and experimental results on 1.55 micrometer AlGaInAs/InP lasers with strongly asymmetric transversal waveguide structures. The self-consistent solution of the Poisson and continuity equations is based on measured carrier mobilities and not only limited to the confinement region. The confinement factor is pointed out to be important when comparing different asymmetric structures. The use of optimized asymmetric structures is demonstrated theoretically and experimentally to enable a distinct improvement in modulation bandwidth and to counteract the limiting physical processes such as carrier transport and carrier capture-escape. Finally, the influence of the shape of the longitudinal carrier and photon density profiles on the modulation behavior is studied. We found that a better homogenization of these profiles for transversally optimized structures may slightly increase the bandwidth. This is further confirmed experimentally by comparing lasers of different profiles applying chirped DFB gratings implemented by bent waveguides.
Bent waveguides are superimposed on homogeneous distributed feedback (DFB) grating fields to generate chirped DFB gratings. This efficient and high-resolution method enables the fabrication of arbitrarily axially chirped DFB gratings by appropriately bent waveguides which reveal differences between neighboring effective DFB pitches in the sub-interatomic range. The bending functions are generated by high-resolution lithography on a set of masks which can be frequently reused for a low-cost production process. The design principles are described defining specific unit cells which are reproduced as often as necessary to obtain a full coverage of the wafer. The method is demonstrated for semiconductor lasers including DFB gratings.
The strong influence of facet properties on wavelength shift and wavelength chirp is studied in uncoated and coated distributed feedback (DFB) lasers. A detailed comparison is performed between various experimental laser data (spectra, electronic and thermal wavelength tunability, relative intensity noise, linewidth) and the results of model calculations combining rate equations and the transfer matrix method. From experimental data of different lasers, a set of physical DFB laser parameters is determined. We succeeded in describing all the experimental data of different lasers by the same parameter set. By use of this set and a large signal analysis we found that the wavelength chirp and the wavelength shift resulting from electronic effects including spatial hole burning varies considerably for different end facet phases (EFPs) and facet coatings, but otherwise identical DFB laser geometry.
Injection-locked directly current modulated semiconductor laser transmitters are theoretically investigated with respect to transmission performance. By large signal simulation of laser, standard single-mode fiber propagation and direct detection optically pre-amplified PIN receiver, transmission distances of 80-120 km at 10 Gb/s and 40-60 km at 15 Gb/s have been evaluated with a bit-error-rate < 10-9 with reasonable power penalty. Exploiting fiber nonlinearities with higher power launched into the fiber is demonstrated to increase the transmission distance by about 40%. Additionally the dynamics of the residual chirp of the laser is shown to act favorably on transmission performance. Guidelines for operation conditions of injection-locked lasers depending on detuning between laser and slave laser and injected power are given. Gain switching including optical feedback and the gain- levering effect have been investigated with respect to pulse production for optical time domain multiplexing. A new method for short pulse generation is presented. It is based on single frequency CW light injection into an unmodulated single mode laser under nonstable locking- conditions. Repetition frequencies larger than 150 GHz can be achieved. By soliton generation in a dispersion shifted fiber pulse widths of less than 3 ps FWHM with a squared hyperbolic cosecans shape can be generated.
Continuously and arbitrarily chirped distributed feedback (DFB) gratings of ultrahigh spatial precision for photonic components are implemented using bent waveguides on homogeneous grating fields. Choosing special bending functions, individual chirping functions and distributed phase shifts (PSs) are generated. Thus, additional degrees of freedom are obtained to tailor and improve specific device performances. This paper focuses on bending function design with respect to PS region extension, PS amount, bending radii, maximum tilt angles, threshold gain, photon density profile homogenization and side mode suppression ratio (SMSR). Continuously distributed PSs were implemented in DFB lasers revealing in the experiment reduced photon pile-ups, higher single axial mode stability, higher SMSR and higher yield than conventional abruptly phase-shifted (PS'ed) lasers. Second, multiple-section DFB lasers are implemented showing 5.5 nm wavelength tuning.
We report on the unexpected large influence of facet properties on wavelength shift and wavelength chirp in uncoated and coated distributed feedback (DFB) lasers. A detailed comparison is performed between various experimental laser data (spectra, electronic and thermal wavelength tunability, relative intensity noise, linewidth) and the results of transfer matrix model calculations. From experimental data of different lasers, a set of physical DFB laser parameters was determined. We succeeded in describing all the experimental data of different lasers by the same parameter set. By use of this set we found that the wavelength chirp and the wavelength shift due to electronic effects including spatial hole burning varies considerably for different end facet phases and facet coatings, but otherwise identical DFB lasers.
We have studied wavelength tuning in three-section distributed feedback (DFB) laser diodes under pulsed and continuous biasing. In these devices the side sections are connected to each other enabling a tuning operation with two injection electrodes. The technological processes for the device fabrication are presented, emphasizing the preparation of the separation grooves. Under pulsed bias conditions we found a maximum modejump-free tuning range of 2.5 nm. The duty cycle was chosen appropriately in this case in order to isolate the nonthermal tuning effects. Under continuous biasing a maximum of total tuning range of 3.7 nm was measured in modejump-free operations. This value represents the interplay of all the wavelength tuning effects involved in these asymmetric three-section DFB lasers. Under both experimental conditions, pulsed and continuous biasing, comparably large modejump-free tuning ranges are obtained. Possible explanations of the enhanced wavelength tunability in these devices are discussed such as threshold gain modulation, plasma effect enhancement controlled by strong spatial hole burning, residual end facet phase effects, and asymmetric geometrical conditions for all section lengths and the position of the phase shift of the grating.
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