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.
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.
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.
In this paper we present the design, fabrication and characterization of arrays of boron doped polycrystalline 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 kΩ 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 μm. The measured results are in good agreement with the predicted absorbance values.
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 nm, 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.
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 characterization for the TE polarization 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.
Silica based planar technology on silicon has been identified as a very serious source of devices for optical communication
s:ystems. Low temperature fabrication of passive and active structures is of special interest as it allows monolithic
integration with temperature sensitive semiconductor components on a common silicon platform.
Standard PEC\'D (Plasma Enhanced Chemical Vapour Deposition) processing for fabrication of silica based optical
waveguides has been investigated to optimize the process parameters. We chose a high power process regime with high
ratio between nitrous oxide and silane gas flows as the best conditions. Significant improvement in optical properties of
silica-on-silicon planar waveguides for optical communication in the 1.50 -1 .55 tmwavelength range has been obtained.