The top cladding layer in planar lightwave circuits (PLC) is more than an optical buffer. By variously doping, adjusting
the thickness of, etching patterns in and annealing the cladding layers in waveguide devices, a wide range of sensors and
photonic devices can be realized. The material properties of the cladding determine, for instance, the modal
birefringence of the waveguides; knowledge and control of these properties can be harnessed to produce polarization-independent
components. The fabrication of thermo-optically controlled switches and interferometers for tunable
filtering and optical signal processing is possible through the creation of micro heaters on top of the cladding. The
optimization of such components can benefit from engineering of the cladding, ranging from better planarization and
thickness control, to selective etching to better confine the heat distribution and provide stress relief. In addition, the
thermal properties of a given device can be radically enhanced by using a polymer layer as top cladding, which yields an
order of magnitude increase in the temperature sensitivity, an invaluable enhancement that can be harnessed for phase-tunable
waveguides or sensor structures. Long period gratings (LPGs) can be etched in the lower cladding to provide
filtering, signal processing, or sensor functions. In a borophosphosilicate cladding, typically used in silica-on-silicon
PLCs, control of the reflow properties through composition can be exploited to manufacture fillable microchannels that
are monolithically integrated with solid-core devices, enabling a unique platform for sensing, signal processing, or
nonlinear optics.
This paper presents planar long period grating (LPG) devices based on a periodic thickness variation in the waveguide
core, fabricated by etching into the lower cladding layer prior to definition of the waveguide layer. This periodic
geometric change results in a stable grating structure and a permanent refractive index modulation of 10-4 or higher,
which is comparable to the index modulation in Ge-doped silica material induced by photo irradiation techniques widely
used in fiber grating fabrication. This grating produces a strong resonance at a particular wavelength in the transmission
spectrum, enabling a range of applications from wavelength filtering to signal distribution in communication networks.
In this work, a polymer and silica hybrid architecture has been implemented in order to achieve wavelength tunability.
Using a thermally oxidized silicon layer as a lower cladding, a Ge-doped silica ridge is patterned using conventional
photolithography and reactive ion etching to form the waveguide core, which is then covered with a low index
fluorinated polymer cladding. While the silica waveguides offer a lower propagation loss and an easy processability, the
top polymer allows the device to be thermally tuned over a wide wavelength range by exploiting the opposite thermo-optic
coefficient between fluorinated polymer and silica, and the high sensitivity of the underlying LPG to the refractive
index of the cladding layer. Strong rejection bands have been demonstrated in the C+L band, in good agreement with
theoretical calculations. Corrugated structures have been defined across an extended area under multiple waveguides
resulting in coupling of light from the fundamental mode into cladding modes and back into the neighboring waveguides
located far from the evanescent coupling distance. This kind of coupler can facilitate devices that require extraction and
control of a particular waveguide mode for applications such as multiple channel signal distribution and temporal pulse
shaping. Implementation of LPGs for these applications will be discussed.
This work presents long period grating (LPG) devices based on a silica-on-silicon planar waveguide platform. All-silica
and hybrid polymer/silica device architectures are demonstrated for potential applications in wavelength filtering,
power distribution, and various types of sensors. The grating structure was realized through a periodic corrugation on a
thermally oxidized silicon layer that also serves as the waveguide lower cladding. For the hybrid architecture,
waveguide ridges were photo-patterned in a layer of low-loss fluorinated poly(arylether ketone), and covered with a
similar polymer cladding having lower refractive index. For all-silica LPGs, GeO2-doped silica waveguides were
fabricated over the grating by PECVD and reactive ion etching, and embedded in a layer of borophosphosilicate glass
(BPSG) with a refractive-index matched to that of the lower cladding material. In these structures, the corrugated silica
layer allows a stable grating structure, while the fluorinated polymer or silica waveguides offer low propagation loss and
versatile processability. Strong rejection bands have been observed in the C+L wavelength region, in good agreement
with theoretical calculations. Based on these designs, an array of waveguides incorporating long period gratings has also
been fabricated. Distribution of light at the resonance wavelength across all channels, from a single input, has been
demonstrated. These results are promising for power distribution in photonic network applications or on-chip sensors.
The sensitivities of the fabricated LPGs to temperature and to the refractive index of the surrounding medium have been
investigated and are discussed.
This work demonstrates waveguide devices based on a novel photocrosslinkable fluorinated poly(arylene ether ketone). A new molecular design has enabled waveguide fabrication to be achieved for the first time using this kind of polymer, through direct UV patterning and a wet-etch process, which is faster and more convenient and economical than the process based on standard reactive-ion etching that has been applied to previously reported poly(arylene ether ketone) materials. High-quality waveguides with smooth, well-defined sidewalls have been produced, and optical splitter devices based on directional coupling are demonstrated. Optical characterization of these devices suggests that the waveguide quality is comparable to that of waveguides fabricated using a dry-etch process, and the experimental data obtained from the fabricated optical splitters are in excellent agreement with theoretical predictions.
Fluorinated poly(arylene ether)s are attractive for the fabrication of optical waveguide devices for photonic applications
due to their low optical attenuation and high thermal stability. A variety of waveguide devices have been fabricated in
these polymers through thermal crosslinking followed by a reactive ion etch process. This work reports on the use of a
novel fluorinated poly(arylene ether ketone) incorporating tetrafluorostyrol groups for photo-crosslinking. This new
molecular design allows sufficient photosensitivity to fabricate waveguides through a UV direct patterning and wet-etch
method, offering a fast, convenient and economic approach to making polymer photonic devices. Data concerning the
polymer molecular structure and choice of suitable photo-initiators for the fabrication of the devices are presented in this
paper. Based upon the optical and thermal properties of the resultant materials it appears that similar degrees of polymer
crosslinking can be achieved using the photo-curing process as those using a thermal crosslinking process. High quality
waveguide devices with smooth, well-defined sidewalls are demonstrated. Optical characterization of these devices
suggests that the waveguide quality is comparable to that of waveguides fabricated using a dry etch process.
A simple numerical method is developed to analyze how the intrinsic birefringence of silica-based ridge waveguides changes with waveguide dimensions and UV irradiation. Identical Bragg gratings were induced on waveguides with different widths which varied from 5 µm×6 µm to 9 µm×6 µm with ArF excimer irradiation and a phase mask. The variation of the waveguide effective index and birefringence as a function of the waveguide dimensions and UV processing are observed and quantified by monitoring the shifts in Bragg wavelength with UV irradiation. With UV irradiation, zero birefringence was easily realized in the waveguides, having an initial birefringence of <2.5×10–4. The mechanism for controlling the waveguide birefringence with UV irradiation is verified both in the theoretical analysis and experimentally. Additionally, the ±0.1-nm variation in Bragg wavelength for waveguides with the same nominal width corresponds to a ±0.1-µm dimensional change in the actual waveguides geometry. This result is used as a way for improving quality control over the waveguide dimensions obtained from the photolithographic and RIE processes.
Optical Bragg grating sensors based on side polished or etched waveguides have been demonstrated for the measurement of refractive index [1, 2, 3, 4]. However, these devices typically exhibit polarization dependent behavior for index values around 1.3. In this report, a ridge waveguide Bragg grating (WBG) sensor with high sensitivity, for refractive index measurement in liquids is presented. The device is based on a small core size silica open top cladding ridge waveguide and polarization independent Bragg gratings (PIBG) written and optimized using UV light [5,6,7]. The WBG is surrounded by a liquid analyte and is accessed via evanescent field interaction of the guided waveguide mode with the liquid layer. In the theoretical analysis, enhancement of sensitivity by optimizing waveguide structures is proposed. In the experiment, Bragg grating is induced in the open top cladding ridge waveguide using a phase mask and excimer laser radiation at 193 nm. A series of refractive index matching liquids are used to test the device. Results indicate the sensitivity is as high as 50 pm of wavelength shift for a change of the index 3×10-4. This technology can offer many advantages over previously proposed waveguide sensors, including enhanced sensitivity, and dynamic measurement range, better polarization stability, and a simpler fabrication processes.
Photonic devices based on novel bromo-fluorinated poly(arylene ether ketone)s have been prepared. In these materials, the intrinsic optical losses at 1550 nm, due to the absorption of hydrocarbon bond overtone vibration, have been minimized by replacing H in the C-H bonds with Br or F, thereby shifting the overtone absorption to a longer wavelength. Typical optical slab losses of these materials are ~ 0.5 dB/cm at 1550 nm. In addition these materials have high thermal stability (5 wt% loss at temperature greater than 450°C), and are easily processed at temperatures lower than those previously reported for other poly(arylene ether)s (< 200°C). High quality waveguides have been fabricated using standard photolithographic processes. A thin film of silicon dioxide deposited by rf sputtering or e-beam evaporation on the polymer surface was used as a mask for reactive ion etching. Data on the design, fabrication and characterization of wide-band wavelength division multiplexers are reported. The devices exhibit on-chip losses of 7 dB including the fiber to chip coupling loss, output uniformity of ± 0.5dB and central wavelength thermal sensitivity lower than 0.06 nm/oC. Optimization of devices through material properties and fabrication process parameters is discussed.
Polymeric materials have been widely used for the fabrication of photonic devices, in particular for applications in short haul optical networks employing coarse wavelength division multiplexing (CWDM). However, the molecular design and processing of polymeric materials to have all the properties required for the fabrication of high performance photonic devices continue to present challenges. This paper presents data on the design, fabrication and characterization of waveguide devices using novel fluorinated poly(arylene ether ketone) materials. These materials exhibit low optical loss (slab loss ~ 0.5 dB/cm at 1550 nm), high thermal stability (1 wt% loss at temperatures up to 430 °C), and are easily processed at temperatures lower than those previously reported for poly(arylene ether)s (< 200 °C). High quality waveguides have been fabricated using standard photolithographic processes. Issues affecting polymer layer and device birefringence and optical loss have been investigated, including molecular structure, processing conditions and substrate selection. Coupling devices sensitive to waveguide dimensions have been designed and fabricated, and their output compared to numerical simulations. Characterization of these devices allows further optimization of the materials and the waveguide process and assists with the design of more complex polymer photonic components.
With the development of telecommunications and high-speed computations, polymeric materials for optical applications are attracting much attention in highly integrated optical waveguides and circuits. In comparison with current inorganic waveguiding materials (e.g., silica and other III-V semiconductor materials), organic polymers offer several advantages including cheap fabrication, tunable properties, good processability and the ability for integration into large scale semiconductor circuits.
In this presentation, we describe the design and synthesis of novel crosslinkable optical polymers for use in optical waveguides based upon a bisphenol monomer containing crosslinkable tetrafluorostyrol units. The introduction of this crosslinkable bisphenol into perfluorinated poly(arylene ethers) allows the synthesis of crosslinkable fluorinated polymers with adjustable refractive index and controllable high crosslinking densities. These polymers have been shown to exhibited low optical loss at 1550 nm, low birefringence, high glass transition temperatures, good mechanical properties, and excellent processability. In the presence of a suitable initiator, these polymers can undergo rapid crosslinking either thermally or optically allowing for multilayer device fabrication.
A method of Bragg gratings written in silica-on-silicon planar waveguides to be used to monitor the overall uniformity of the waveguides and grating processing is presented. By measuring the shift of Bragg wavelength with UV exposure time, the initial effective index nOeff and birefringence B0 of the planar waveguides are measured accurately. With one phase mask, Bragg gratings induced on different waveguides with widths that vary from 4.6 to 8.8 μm, result in variations of NOeff and βO of 1.5 x 10-3/μm and 1 x 10-4/μm, respectively. The result is used as a way of improving control over the waveguide dimensions obtained from the photolithographic and RIE processes, and optimizing the design of ridge waveguide structures to compensate the waveguide birefringence. This will improve the quality of the PLCs that include symmetric Bragg grating structures: MZI-OADM etc. By writing Bragg gratings on the linear taper planar waveguide, a chirped grating response is realized.
A simple numerical method is developed to analyze changes in intrinsic birefringence of ridge waveguides with waveguide dimensions and UV irradiation. Experimentally, Bragg gratings were written on different core size ridge waveguides using the phase mask technique and ArF laser irradiation. By monitoring the shifts in Bragg wavelength with UV irradiation, the variation of the waveguide birefringence with waveguide dimension and UV processing is observed and quantified. The mechanism of the waveguide birefringence controlled with UV irradiation is verified both in the theoretical analysis and experiment.
This paper reports planar waveguide fabrication using a novel fluorinated aromatic polyether bearing cross-linkable pentafluorostyene groups. This polymer exhibits high thermal stability and low optical loss, and the crosslinker used allows thermal processing at low temperatures (<200°C). The influences of the fabrication process and the substrate choices on the refractive index and birefringence of polymer layers have been investigated. Polymer ridge waveguide structures have been fabricated and characeterized, and device optimization through molecular design, fabrication process and substrate choice is discussed.
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