III-V based devices have provided unsurpassed performance enabling the rapid advancement of data-communications over the past decades. Yet the integration of these components is still primitive leading to high costs due to the packaging challenges. Heterogeneous integration using controlled release of the essential device layers from individual source wafers and engineered parallel transfer to a common platform is a very promising approach as it takes the best materials and devices, produced in a conventional foundry environment, to produce powerful photonic circuits on target waveguiding platforms such as silicon-on-insulator. The devices can be pre- or post-processed and optically integrated to the silicon waveguides using butt, evanescent or potentially grating coupling. Laser devices are the most critical since they cannot easily be realized in Si. We demonstrate the transfer of lasers based on InP quantum wells where the devices are bonded by van der Waals forces. We have also demonstrated the release and transfer of silicon microcircuits, GaN materials and dielectric layers. We study the interface property between the transferred materials (e.g. InP) and the target wafer (Si). There is an improvement in device performance after the transfer due to the high thermal conductivity of Si. This approach will allow more sophisticated circuits due to the ease of including multi-wavelength lasers as well as modulating and detecting functions along with specialty materials such as potentially lithium niobate or magnetic materials. The technique enables close integration of photonics with electronics platforms and thus a route to widespread consumer applications for III-V devices.
A novel method for fabricating a single mode optical interconnection platform is presented. The method comprises the miniaturized assembly of optoelectronic single dies, the scalable fabrication of polymer single mode waveguides and the coupling to glass fiber arrays providing the I/O’s. The low cost approach for the polymer waveguide fabrication is based on the nano-imprinting of a spin-coated waveguide core layer. The assembly of VCSELs and photodiodes is performed before waveguide layers are applied. By embedding these components in deep reactive ion etched pockets in the silicon substrate, the planarity of the substrate for subsequent layer processing is guaranteed and the thermal path of chip-to-substrate is minimized. Optical coupling of the embedded devices to the nano-imprinted waveguides is performed by laser ablating 45 degree trenches which act as optical mirror for 90 degree deviation of the light from VCSEL to waveguide. Laser ablation is also implemented for removing parts of the polymer stack in order to mount a custom fabricated connector containing glass fiber arrays. A demonstration device was built to show the proof of principle of the novel fabrication, packaging and optical coupling principles as described above, combined with a set of sub-demonstrators showing the functionality of the different techniques separately. The paper represents a significant part of the electro-photonic integration accomplishments in the European 7th Framework project “Firefly” and not only discusses the development of the different assembly processes described above, but the efforts on the complete integration of all process approaches into the single device demonstrator.
In the Information and Communications Technology (ICT) sector, the demands on bandwidth continually grow due to
increased microprocessor performance and the need to access ever increasing amounts of stored data. The introduction of
optical data transmission (e.g. glass fiber) to replace electronic transmission (e.g. copper wire) has alleviated the
bandwidth issue for communications over distances greater than 10 meters, however, the need has arisen for optical data
transfer over shorter distances such as those found inside computers. A possible solution for this is the use of low–cost
single mode polymer based optical waveguides fabricated by direct patterning Nanoimprint Lithography (NIL). NIL has
emerged as a scalable manufacturing technology capable of producing features down to the hundred nanometer scale
with the potential for large scale (roll-to-roll) manufacturing.
In this paper, we present results on the modeling, fabrication and characterization of single mode waveguides and optical
components in low-loss ORMOCER™ materials. Single mode waveguides with a mode field diameter of 7 μm and
passive structures such as bends, directional couplers and multi-mode interferometers (MMIs) suitable for use in 1550
nm optical interconnects were fabricated using wafer scale NIL processes. Process issues arising from the nano-imprint
technique such as residual layers and angled sidewalls are modeled and investigated for excess loss and higher order
mode excitation. Conclusions are drawn on the applicability of nano-imprinting to the fabrication of circuits for intrachip/
board-level optical interconnect.
Polymer-based integrated optics is attractive for inter-chip optical interconnection applications, for instance, for coupling photonic devices to fibers in high density packaging. In such a hybrid integration scheme, a key challenge is to achieve efficient optical coupling between the photonic chips and waveguides. With the single-mode polymer waveguides, the alignment tolerances become especially critical as compared to the typical accuracies of the patterning processes. We study novel techniques for such coupling requirements. In this paper, we present a waveguide-embedded micro-mirror structure, which can be aligned with high precision, even active alignment method is possible. The structure enables 90 degree bend coupling between a single-mode waveguide and a vertical-emitting/detecting chip, such as, a VCSEL or photodiode, which is embedded under the waveguide layer. Both the mirror structure and low-loss polymer waveguides are fabricated in a process based mainly on the direct-pattern UV nanoimprinting technology and on the use of UVcurable polymeric materials. Fabrication results of the coupling structure with waveguides are presented, and the critical alignment tolerances and manufacturability issues are discussed.
We present designs for sharp bends in polymer waveguides using colloidal photonic crystal (PhC) structures. Both silica
(SiO2) sphere based colloidal PhC and core-shell colloidal PhC structures having a titania (TiO2) core inside silica (SiO2)
shells are simulated. The simulation results show that core-shell Face Centered Cubic (FCC) colloidal crystals have a
sufficient refractive index contrast to open up a bandgap in the desired direction when integrated into polymer
waveguides and can achieve reflection <70% for the appropriate plane. Different crystal planes of the FCC structure are
investigated for their reflection and compared with the calculated bandstructure. Different techniques for fabrication of
PhC on rectangular seed layers namely slow sedimentation; spin coating and modified doctor blading are discussed and
investigated. FCC and Random FCC silica structures are characterized optically to show realisation of (001) FCC.
Confinement of light at submicron wavelengths is of great importance for highly specific sensing of bio-molecules and
for compact photonic circuits based on waveguiding. Currently this confinement can be achieved through the well
established high index contrast silicon on insulator (SOI) platform. However this material combination requires light at
wavelengths beyond 1 micron where the component cost of the InP based lasers and photodetectors are very expensive.
It is thus of great interest to develop a similar platform that could operate in the range of 850 nm where low cost lasers
(e.g. Vertical Cavity Surface Emitting Lasers as used in optical mice) and detectors (e.g., as used in camera phones) are
readily available. A possible high index material suited to this application is Gallium Phosphide which has a bandgap of
2.26 eV and refractive index of ~ 3.2 at this wavelength. For the highest index contrast, GaP should be grown on a
substrate with low index of refraction such as quartz (n=1.5) or sapphire (1.7). We report on the design and
characteristics of GaP waveguides grown on c-plane (0001) sapphire substrates using metalorganic vapour phase
epitaxy. Growth parameters such as substrate temperature and, in particular, the V:III ratio are reviewed with respect to
their effect on the nucleation, surface roughness and uniformity of the films. Modal analysis and the design of a grating
coupler at wavelengths around 850 nm have been designed for GaP on sapphire using vectorial finite element method in
order to validate the feasibility of GaP waveguides.
The use of structured metal films where electromagnetic waves are confined and manipulated as surface plasmon
polaritons (SPPs) has potential use in applications ranging from biosensing, chip-to-chip optical interconnects and in
data storage. In general, the SPPs are excited using a separate light source which compromises the compactness of any
system. As a solution the SPPs can be directly excited on a layer of gold which is deposited on the top surface of a
Vertical Cavity Surface Emitting Laser. Here, we have designed the surface of the VCSEL to include a customised
planar gold layer upon which we can excite, propagate and manipulate SPPs over distances of up to 100 microns. We
launch the SPPs using a low threshold 850 nm emitting VCSEL under continuous wave operation using a diffraction
grating etched through the gold surface. Shallow etched gratings are used to manipulate the SPPs through, for example, a
90° bend using a Bragg mirror and to out-couple the SPPs into air where the polarization dependent relative intensity of
the extracted light is measured using a CCD camera. We measure a SPP propagation length of about 50 microns. The
result paves the way to compact integrated plasmonic devices.
Lowering optical packaging costs requires developments in new technologies. In this paper, solder ink-jet process is presented for flip-chip component assembly on planar, 3D, flex and stacked submounts and substrates. Applications for this technology are presented and include linear array in-vivo dosimeters, integrated GaN LED displays, telecomm submounts and wearable ambient systems. An important aspect of developing this technology is process reliability. In this study, the reliability of the solder to bump accurately and adhere to various target bond pads was evaluated as well as MIL standard shear tests to qualify the joint strength of the bump.
We report the integration of phase gratings directly onto the surface of red vertical cavity surface emitting lasers (VCSELs) by Focused Ion Beam etching. Gratings have been used to generate quasi Bessel beams. The fabricated devices show that a diffraction limited central spot can be formed above the surface of the device. The narrow spot has a full width at half maximum of 0.5μm at a distance of 2μm above the VCSEL surface. The compact device can be formed in arrays and can be considered for a large number of sensing applications such as an optical probe for biophotonics and in optical recording systems.
We have improved the design of our red emitting VCSELs to be less sensitive to leakage induced optical losses in the
output reflector. The current designs produce in excess of 0.2mW at 652nm and 50 degrees C. We also have devices emitting
6.5mW at 668nm at 20 degrees C. We use a simple model to predict the device performance improvements of minor
modifications to the device design. By reducing the bias voltage from the current high levels, we predict that c.w.
powers in excess of 0.5mW at 80 degrees C and up to 17mW at 20 degrees C should be possible without any further design or material
We report on the use of etched curved facets to form semiconductor lasers based on unstable resonators with a real internal focus. The lasers which have a cleaved output facet operate to > 1W continuous wave power with a diffraction limited central lobe when corrected for spherical phase. The astigmatism is found to be equal to the geometric calculation and is stable throughout the operating range. Beam degradation is associated with temperature gradients. We have fabricated devices at 1200nm, 980nm and 800nm based on reactive ion etching of GaAs structures demonstrating that the technique can be considered as a platform technology. We discuss the formation of near (facet) fields from the laser and the definition of beam quality for these lasers. The technology has also been used to etch both facets of the resonator to obtain a collimated output beam. In summary, these lasers act both as a high brightness source with reduced power density on the facet as well as a versatile source for designed output and can be scaled into array format.
For optical networks, the operating life of optoelectronic components is expected to be over 20 years. Network designers therefore require components, which have been reliability tested in accordance with assured protocols, such as Telcordia Generic Reliability Assurance Practices (BellCore). In this paper, we report on the development of a system for thermal reliability studies of optoelectronic devices. The system incorporates an environmental test chamber programmed to provide differing temperature environments in the range (-180° to 300° C) as well as constant bias current or voltage to the device udner test. Case studies for preliminary screenign and temperature cycling tests on a wide range of novel active and passive devices fabricated at NMRC for short-haul networks markets are assessed and reported using this system.