Complete birefringence compensation is demonstrated in plasma-enhanced chemical vapor deposition waveguides by 193-nm postexposure. A single relaxation process dominates the decay in stress anisotropy, indicating that compressive stress from the substrate leads to an elastic stress anisotropy at the core.

The last two decades have witnessed a large increase in capacity in telecommunication systems, thanks to the development of high bandwidth, fiber optic based networks. Nevertheless the continuing growth of Internet data traffic, fuelled by the development of numerous services like on-line commerce, video on demand, large audio/video files downloads, demands for a significant increase in the ability of the network nodes to manage incoming and outcoming data streams effectively and fast. The different functionalities that are needed include add/drop channel multiplexing, routing, signal reshaping and retiming, electrical/optical and optical/electrical conversion. This has stimulated a large effort towards the investigation of technologies for opto-electronic integration at a wafer level, in order to cope with all the required operations, while limiting overall costs. Among the different approaches proposed, one of the most promising is the “Silicon optical bench”, which relies on the well established VLSI technology for the microelectronics part and on planar lightwave circuits (PLCs) made either with silica-on-silicon waveguide technology (low index contrast) of amorphous silicon technology (high index contrast) on the integrated optics side.

This thesis presents the development of new techniques and methodologies utilized in photonic device fabrication, which can be used to facilitate integration of temperature sensitive elements. The process is based on low temperature, plasma assisted, thick film deposition. First, a low temperature (300°C) deposition process based on Plasma assisted Chemical Vapour Deposition (PACVD) for the fabrication of silica based Planar Lightwave Circuits (PLC) is developed. The low thermal budget lends itself to monolithic integration with devices fabricated with different technologies. Absorption bands at around the wavelengths 1.48µm and 1.51µm caused by N-H and Si-H bonds within the material, respectively, had previously been thought to be intrinsic to the PACVD deposition method, when using N2O as oxidant gas of SiH4 and the other dopant precursors. The traditional method to eliminate these absorption bands was high temperature (>1000°C) annealing that seriously hinders device integration. An important achievement in this thesis is the improved suppression of these two absorption bands while keeping the whole fabrication temperature below 300°C and also having a high deposition rate. A complete fabrication process for silica planar lightwave circuits was also developed, by optimising the photolithography and etching step. Finally the effect of dopants like Ge and B on the optical properties of the deposited silica glass was investigated, with particular emphasis to the photosensitive properties of the material upon illumination in the near UV. UV trimming is shown to be a versatile method to selectively control polarization birefringence of devices. Transmission dips of above 50dB were achieved in photo-induced gratings in low temperature deposited B-Ge codoped waveguide cores, without the need for hydrogen loading or other sensitisation techniques. The application of a high refractive index like amorphous silicon is addressed for the realization of efficient Bragg reflectors, either as vertical cavity laser mirrors or as dispersive element for planar waveguides used in highly selective co-directional coupler filters. Applications of amorphous silicon as core material for photonic crystal devices are also shown. The investigations carried out in this thesis show that PACVD technology can provide low-loss and UV sensitive material suitable for realizing a variety of low cost integrated devices for future all optical networks.

A new configuration based on the coupling between a conventional low loss, weakly guiding channel waveguide and a Bragg reflection waveguide (BRW) was discussed. The strong difference between the dispersion of a Bragg reflection waveguide and a channel waveguide was used to create a narrow band coupler. The two-dimensional analysis of the BRW was generally based on the transfer matrix method. The structure consisted of a weakly guiding conventional Ge-doped silica waveguide on the top of which a BRW was stacked. The number of periods in the mirror between the BRW and the silica waveguide affected the coupling length and ultimately the bandwidth.

A new type of wavelength selective filter, based on high differential dispersion between two coupled waveguides, is presented. The Bragg Reflection Waveguide displays high effective refractive index dispersion, due to the interaction of the guided mode with the two confining Bragg reflectors. When coupled with a weakly guided buried channel silica waveguide, a very narrow bandwidth filter (< 1 nm) can be easily produced, in a shorter length, with respect to directional couplers made with standard step index channel waveguides. The complete design methodology, fabrication and characterization are presented.

Highly wavelength selective optical filters are essential components for channel management in modern Dense Wavelength Division Multiplexed communication systems with 50GHz channel spacing and below 0.4nm channel bandwidth. We have designed, fabricated and characterized a new type of wavelength selective directional coupler, based on the high differential dispersion between a Bragg Reflection Waveguide (BRW) and a conventional buried channel silica waveguide.

The bandwidth of the device is inversely proportional to the length of the coupler as well as to the differential effective refractive index dispersion of the coupled modal fields, at the wavelength of phase matching. The BRW is made of a high index (amorphous) silicon core layer, surrounded vertically by two periodic Bragg reflectors with alternating layers of silica and silicon. The silica waveguide with a Ge-doped core, vertically stacked with the BRW, allows fiber incoupling loss below 1dB which is essentially the insertion loss of the device. The device is operating within the optical bandgap of the Bragg reflectors. Both the bandwidth and the coupling wavelength can be tuned during the fabrication process: the fields’ overlap and the coupling coefficient between the two waveguide modes are controlled by one of the Bragg reflectors (coarse control) and a spacer layer (fine control); the position of the coupling wavelength is mainly determined by the BRW core thickness.

The devices were fabricated by depositing SiO₂ and a-Si:H films on a 4” <100> oriented Si substrate, by plasma enhanced chemical vapor deposition, at a temperature of 250ºC. The 5µm wide vertical stack of BRW and silica waveguide were defined by lithography and etched in an inductively coupled plasma reactor. The 8.8µm thick coupler structure was covered with a 16µm thick silica cladding. The device can be easily integrated in a standard silica-based planar lightwave circuit.

The measured filter suppression is 14dB and the FWHM is 0.29nm for only a 1.73mm long device, which is close to the estimated value of 0.31nm, and one of the lowest ever reported for this type of coupler.

The feasibility of a full low temperature process, based on Plasma Enhanced Chemical Vapor

Deposition, for the fabrication of low loss silica-based optical waveguides is investigated.

Results from XPS, FTIR, ERDA, isochronal wet etch rate, prism coupler measurements show

that (low frequency) RF power is a critical parameter to improve microstructural properties of

as-deposited SiO

2 and minimize Rayleigh scattering. Ge doping of the silica matrix in the

core layer increases network disorder and point defects density, mainly due to the highly

reactive characteristics of the employed gas precursor (germane) and the high sticking

coefficients of its radicals. Measurements on fabricated optical waveguides show that for

relative refractive index differences between core and cladding up to 0.75%, the optical

losses are acceptable for the fabrication of high performance devices.

UV sensitivity of B-Ge codoped cores in PECVD silica waveguides has been investigated. Photoinduced refractive index changes have been introduced by KrF excimer laser irradiation at 248 rim, without any presensitization method. The effects of B codoping of Ge doped silica have been examined. It has been shown that B addition mildly increases glass network disorder, by broadening the O bridging angle distribution as from FTIR measurements, but on the other hand it does not produce point defects which may contribute to the absorption band at 5eV already generated by the presence of Ge doping. The fabricated channel waveguides show low optical loss even without high temperature annealing. Strong Bragg gratings imprinted into these waveguides confirm that in non thermally annealed Ge doped PECVD silica glass, where a small absorption band still exist at 5eV, B codoping supplies sufficient photosensitivity amplification to make hydrogen loading unnecessary.

Design, fabrication, and characterization of an optical filter based on vertical coupling between a silicon wire waveguide and a cavity in a suspended silicon photonic crystal membrane is presented for the 1550 nm wavelength spectral region.

One of the most distinctive features of photonic crystals (PhCs) is their unique wavelength dispersion allowing novel device concepts for enhancement of photonic functionality and performance. Here, we present examples of our design and demonstrations utilizing dispersion properties of 1D and 2D photonic crystals. This includes the demonstration of negative refraction in 2D PhC at optical wavelengths, filters based on 1D and 2D PhC waveguides, and the design of a widely tunable filter involving 1D PhC.

We present two concept examples for adding a wide range tunability to Si/SiO2 devices involving a photonic crystal element. They are based on a directional coupler filter of two different geometries, where one of the arms is a Bragg Reflection Waveguide (BRW) used for the bandwidth improvement. The tuning relies on changing the properties of the BRW core. As an illustration we consider the smectic A* liquid crystal as the core material and show that ca 100 nm tuning range is achievable by the core index variations of 0.006 under applying electric field of 5 V/mu m.

A novel and flexible technology for ultra compact AWGs based on amorphous silicon nanowires is presented. A 4×4 AWG with a total dimension of 50×50µm was fabricated. 11nm channel spacing and -10dB crosstalk was obtained.

Reported are 1.3 mu m InGaAs/GaAs vertical-cavity surface-emitting lasers (VCSELs) with a novel electrical confinement scheme based on lithographic definition and selective area epitaxial regrowth in the cavity region. More than 6 mW of output power with a record high differential efficiency of more than 70% is emitted from 10 mu m large devices.

In this paper we present the design, fabrication and characterization of arrays of boron doped polycryslalline silicon bolometers, The bolometer arrays have been fabricated using CMOS compatible wafer-level transfer bonding. The transfer bonding technique allows the bolometer materials to be deposited and optimized on a separate substrate and then, in a subsequent integration step to be transferred to the read-out integrated circuit (ROIC) wafer. Transfer bonding allows thermal infrared detectors with crystalline and/or high temperature deposited, high performance temperature sensing materials to be integrated on CMOS based ROICs. Uncooled infrared bolometer arrays with 18x18 pixels and with 320x240 pixels have been fabricated on silicon substrates. Individual pixels of the arrays can be addressed for characterization purposes. The resistance of the bolometers has been measured to be in the 50 kOmega range and the temperature coefficient of resistance (TCR) of the bolometer has been measured to be -0.52 %/K. The pixel structure is designed as a resonant absorbing cavity, with expected absorbance above 90%, in the wavelength interval of 8 to 12 mum. The measured results are in good agreement with the predicted absorbance values.

1.55 μm room-temperature continuous-wave operation of a high performance optically pumped vertical external cavity surface emitting laser is reported. The structure includes an active region with strain compensated quantum wells, and a broadband SiN^x/Si/Au Bragg reflector transferred on an Si substrate by Au/In dry bonding. Output power of up to 45 mW is achieved at 0°C, and continuous-wave operation is observed up to 45°C.

In this paper we present guidelines for optimization of process parameters for low temperature deposition of silica films for optical applications. The fabrication of optical integrated circuits based on silica deposition on silicon substrates with plasma enhanced CVD introduces different challenges in this technology, with respect to its traditional application in the field of microelectronics. The thick layers required, make deposition rate a very important parameter of merit. High accuracy of this thickness and refractive index as well as their uniformity over large area of the wafer are also very important for the functionality of the fabricated devices. Finally low processing temperature is essential as it allows monolithic integration with temperature sensitive semiconductor components. In this study a commercial parallel plate reactor has been used and the deposition parameters have been analysed to determine the process window, which allows fulfilling all these demands, so as to make a subsequent high temperature consolidation step not necessary.

Silica-on-Silicon is a well established technology for the fabrication of low insertion loss planar lightwave circuits. The Ge-doped waveguide core material, deposited with low temperature plasma enhanced chemical vapor deposition and not subjected to high temperature annealing, is highly UV light photosensitive, due to residual Ge/Si-OH groups in the material that, similarly to hydrogen loading, can contribute to the formation of those defect centers responsible for the photosensitivity. Gratings have been-fabricated using a pulsed 193 nm ArF excimer laser and a phase mask. 25 mm long gratings, written on standard straight waveguides, show a record 47 dB extinction ratio and 0.2 nm rejection bandwidth for TE polarization, without hydrogen loading. Such narrow linewidth filters could find application in dense WDM systems. We designed and fabricated a compact Add/Drop multiplexer based on a high bandwidth, 2x2 multimode interference device, having a Bragg grating written in the multi-mode region. The characterisation for the TE polarisation prove the proposed Add/Drop principle, showing, in correspondence of the dropped channel, a 30dB dip at the transmitted output and a reflection peak at the drop output, this last having a larger bandwidth, and around 3dB excess loss respect to the transmitted channels.

Similarly to optical fibers silica-based planar waveguides can exhibit the property of photosensitivity when illuminated with UV light. UV-induced changes of refractive index in Ge-doped silica allow for adding new functionality to these components including wavelength filtering with help of Bragg gratings, phase adjustment, wavelength tuning, direct UV-writing of waveguides and birefringence compensation. Bragg gratings imprinted in waveguide core in the same way as done in fibers, allow for building demultiplexers, add-drop filters, dispersion compensators, frequency lockers and other grating assisted devices. In this paper we present the background of the photosensitivity in germanosilica, methods for its improvement as well as technology for fabrication of advanced low dispersion Bragg gratings for 40 Gbps and beyond. We also demonstrate some fabricated wavelength selective devices for WDM communication systems.

The development of Si-based photonics has been far behind the development of electronics for long time. There are two reasons for that. As silicon is an indirect band gap semiconductor, achieving light emission and gain is quite difficult. On the other hand, for using silicon as a light guiding material for passive devices, the main constrains until recently were relatively high propagation losses and high fiber-to-waveguide incoupling losses. The general trend towards more compact photonic devices together with progress in fabrication techniques resulted in the development of two nano-photonic technologies for next generation optical devices: photonic crystals and nanowire waveguides-based devices. To drastically increase the integration density and achieve subwavelength confinement of light along the propagation direction, plasmonic wavguides have been proposed. Surface plasmons are electromagnetic modes constituted on the interface between a metal and a dielectric. The tradeoff between the light confinement and propagation loss has here a vital importance.

Silicon-on-insulator nanowire waveguides appear to be the technology for next generation of super compact integrated devices. Due to a very high refractive index contrast and strong light confinement in the core, the waveguide bend radius can be reduced to a few micrometers and the size reduction of the functional integrated circuits can reach several orders of magnitude in comparison to standard integrated optics based on silica-on-silicon technology. Amorphous silicon is an interesting material allowing for more flexibility in nanowire structures. An array waveguide grating multi/demultiplexer that usually occupies several square centimetres in silica-on-silicon technology can be reduced to the size of 320 x 270 μm².

Silicabased planar technology on silicon has been identified as avery serious source of devices for optical communications:ystems. Low temperaturefabrication of passive and active structures is of special interestas it allows monolithicintegration with temperature sensitive semiconductor components ona common silicon platform.Standard PEC\'D (Plasma Enhanced Chemical Vapour Deposition)processing for fabrication of silica based opticalwaveguides has been investigatedto optimize the process parameters. We chose a high powerprocess regime with highratio between nitrous oxide and silane gasflows as the best conditions. Significant improvement in optical propertiesofsilica-on-silicon planar waveguides for optical communication in the 1.50 -1.55 tmwavelength range has been obtained.

Apodized waveguide Bragg gratings are written using Sagnac type interferometer. As the quality of these gratings is not enough for DWDM devices, a procedure for shaping of filter profiles was developed. Obtained 10 mm long grating gives line width of 0.4 nm and almost completely suppressed side lobes.

We present a novel practical method for group delay compensation of Bragg gratings imprinted in planar waveguides for high speed DWDM systems. Although Bragg grating-based wavelength selective devices in optical fibers have reached their maturity, similar components built on the basis of planar technology are still the research issue. We analyze an integrated Mach-Zehnder interferometer-based Add-Drop multiplexer equipped with two pairs of gratings, one designed as a wavelength selective filter and the other one as a group delay compensator.

Wavelength selective Bragg grating filters in form of periodic modulation of the refractive index along the waveguide can be laser-imprinted in fibers and planar lightwave circuits (PLC)s utilizing UV photosensitivity of the Ge-doped silica core material. Such gratings have a potential to be extensively used in dense wavelength division multiplexing (DWDM) systems in many optical components including add/drop multiplexers. As a bit rates in DWDM systems continuously increase, these components must have low group delay dispersion as well as steep filter characteristics. In this paper we present fabrication technology and optical characteristics of PLC Bragg gratings and grating assisted Add/Drop multiplexers (ADM)s developed for 40 Gbps DWDM systems. Mach-Zehnder interferometer (MZI)-based ADM structures were fabricated with silica-on-silicon planar technology using Plasma Enhanced Chemical Vapor Deposition and subsequent Reactive Ion Etching. The MZI consisted of two 3dB couplers and two identical Bragg gratings UV-imprinted in both arms of the interferometer. For imprinting of gratings in (PLC)s a computer controlled interferometer with special configuration was designed and fabricated. The interferometer allows writing gratings with periods corresponding to any wavelength within C-band. Gratings as short as 4 mm can give over 30 dB suppression of the reflected channel. If needed, group delay compensation can be introduced by programmable phase perturbation during grating writing. The fabricated ADMs have been tested and shown 0.4 nm flat top transmission bandwidth measured in the Drop port. Clear eye openings at 40 Gbps have been obtained, when tested with SHF 5005A multiplexer and Agilent 86100B digital sampling oscilloscope.

Micro-ring resonators have been fabricated in hydrogenated amorphous silicon on silica structure. The intrinsic quality factor is estimated as 56,000 and the notch depth is ̃ 30dB. The intrinsic loss per unit length is 15.3dB/cm, comparable to 9.16dB/cm in the single-crystalline silicon ring of the same geometry.

Resonance-splitting and enhanced notch depth are experimentally demonstrated in micro-ring resonators on SOI platform as a result of the mutual mode coupling. This coupling can be generated either by the nanometer-scaled gratings along the ring sidewalls or by evanescent directional coupling between two concentric rings. The transmission spectra are fitted using the time-domain coupled mode analysis. Split-wavelength separation of 0.68 nm for the 5-mu m-radius ring, notch depth of 40 dB for the 10-mu m-radius ring, and intrinsic Q factor of 2.6 x 10(5) for the 20-mu m-radius ring are demonstrated. Notch depth improvement larger than 25dB has been reached in the 40-39-mu m-radius double-ring structure. The enhanced notch depth and increased modal area for the concentric rings might be promising advantages for bio-sensing applications.

We have recently demonstrated an optical filter based on side coupling between silicon wire waveguide and two-dimensional photonic crystal surface-mode cavity. The design is first optimized numerically by parallel three-dimensional finite-different time-domain simulations. The device is then fabricated using electron-beam lithography and plasma etching on amorphous silicon-on-silica structure. The drop wavelength is observed around 1580nm. The extinction ratio of the filter is larger than 10dB and the intrinsic quality factor of the surface-mode cavity is approximately 2,000. The quality factor can be improved by optimization of fabrication procedures as well as cavity design to eliminate multi-mode behavior.

Design, fabrication, and characterization of an optical filter based on side coupling between silicon wire waveguide and photonic crystal surface mode cavity in silicon on silica structure is presented for the 1550 nm wavelength spectral region.

An optical filter based on side coupling between silicon wire waveguide and two-dimensional photonic crystal surface-mode cavity is presented. The design is optimized numerically by parallel three-dimensional finite-different time-domain simulations. The device is then fabricated on amorphous silicon-on-silica structure. The drop wavelength is observed around 1580 nm. The extinction ratio of the filter is larger than 10 dB and the intrinsic quality factor of the surface-mode cavity is approximately 2000.