We present our research on utilizing weak Bragg grating reflectors to assess the uniformity of zinc-doped lithium niobate ridge waveguides, aiming to optimize frequency conversion. These gratings are fabricated through ablation using a pulsed 213nm laser within a phase-controlled interferometric system, providing sub-nanometer period accuracy. By employing gratings we spectrally and spatially characterize the modal properties of our waveguides, enabling direct analysis of process variability. Through this analysis, we aim to gain a deeper understanding of the effective index variation in periodically poled lithium niobate (PPLN) waveguides, with the ultimate goal of reducing it and improving frequency conversion.
We present the use of holographic UV writing to fabricate 2D grating couplers in doped planar silica-on-silicon, enabling the creation of large (~1 cm) and efficient circular beam delivery into free space. Our fabrication process inscribes a channel waveguide with in-plane 60° blazed grating to expand a fibre-coupled beam within a planar core layer, followed by an out-of-plane 45° blazed grating to couple light out of the substrate. Our out-of-plane gratings are fabricated using a modified interferometric arrangement with a prism and index matching water layer, the arrangement and latest results will be presented here.
We have previously reported robust zinc-indiffused MgO:PPLN ridge waveguides for field applications in quantum-enhanced gravimetry and navigation, generating 2.5W of 780nm light at 74% second-harmonic generation (SHG) conversion efficiency. To tailor this process for different wavelengths and interactions, the effect of fabrication parameters on the waveguide mode shape and size from UV to MIR has been studied, with the aim to optimise mode matching between pump, SHG, and optical fibres to improve conversion efficiency, and reduce insertion loss in packaged devices.
We present the results of an extended study on the evolution of dicing blade dynamics to define the tolerances of the ductile regime for dicing optical quality facets with sub-nm surface roughness in optical materials. These results provide a route to determine the critical parameters, such as depth of cut, blade shape, and feed rate, to maintain stable ductile machining within a specific range of conditions. We will discuss our latest results and observations, including optical characterisation of waveguides in lithium niobate and other materials.
A primary constraint in the major photonic integration platform of Silica-on-Silicon, especially when combined with fabrication approaches like Direct Laser Writing is the optical waveguides' low refractive index contrast, leading thus to limitations for efficient coupling with currently available state-of-the-art single photon emitters such as semiconductor nanowires with quantum dots (NWQD). We propose and demonstrate a novel approach to drastically enhance the light coupling between silica based Laser-written channel waveguides and NWQDs, by incorporating an optical microsphere in their intermediate space. It is demonstrated that the induced photonic nanojet action of a suitably designed microsphere illuminated by the NWQD, excites efficiently the channel waveguide's modes and can enable light coupling to a degree even above 50%. The proposed method is reasonably robust to imperfections and misalignments and could be implemented by current state-of-the-art micro/nano patterning techniques. It is anticipated that the practical implementation of the method will allow the integration of multiple quantum emitters in silica based hybrid integrated circuits thus enabling their scalability towards for quantum computing and sensing applications.
We demonstrate Ultra-Violet (UV) light generation using a diode-pumped Alexandrite laser and its Second Harmonic Generation (SHG) via Zn-indiffused MgO:PPLN waveguides. A wavelength range of 375-393 nm is obtained using third order SHG in Λ = 6.1−6.9 μm poled waveguides. Up to 1.3 mW UV power is obtained from 185 mW throughput infrared power. We believe that the wavelength and transverse mode flexibility from these waveguides gives rise to a wide range of applications for an efficient and compact laser module in the UVA range.
The Networked Quantum Information Technologies (NQIT) hub was one of the quantum hubs of the first round of the United Kingdom’s flagship scheme, bringing together academia, industry, and government. The Optical Engineering and Quantum Photonics Group at the University of Southampton’s Optoelectronics Research Centre has developed an interactive demonstrator of a key device within NQIT to bring quantum photonics technologies to a wider audience. The system was exhibited at the 2017 UK Quantum Showcase in London to an audience of industry and government. It also featured as part of the Quantum City stand at the 2018 Cheltenham Science Festival, one of the United Kingdom’s leading annual science events.
Silicon nitride (SiNx), has been widely regarded as a CMOS photonics enabling material, facilitating the development of low-cost CMOS compatible waveguides and related photonic components. We have previously developed an NH3-free SiN PECVD platform in which its optical properties can be tailored. Here, we report on a new type of surface-emitting nitrogen-rich silicon nitride waveguide with antenna lengths of L < 5 mm. This is achieved by using a technique called small spot direct ultraviolet writing, capable of creating periodic refractive index changes ranging from -0.01 to -0.04. With this arrangement, a weak antenna radiation strength can be achieved, resulting in far-field beam widths < 0.0150, while maintaining a minimum feature size equal to 300 nm, which is compatible with DUV scanner lithography.
We propose and numerically simulate a new and highly compact integrated 4x4 mode coupler based
on two single-mode waveguides exploiting both forward and backward propagating directions to double the number of modes. The two parallel waveguides are coupled via long and short-period gratings to the co- and
counterpropagating directions, respectively, of a single cladding mode of the device which acts as a
bus between the waveguides. By connecting all end facets to optical circulators we construct a
device with four input and output ports but only using two single-mode waveguides.
Such a device can be fabricated in a single micromachined silica ridge structure. A photosensitive
raised index layer is used for vertical confinement that supports multiple modes horizontally. We
UV-write the waveguides and the Bragg gratings and provide a tilt angle to improve coupling. We
have demonstrated this technology before for a polarizing waveguide-to-waveguide coupler and
have simulated other unidirectional devices.
We use coupled mode theory to simulate the system. By tailoring the grating parameters, we can
achieve a wide variety of coupling ratios. Analytically, we find a set of solutions in which no light
escapes via the cladding modes through the ends of the device and we have calculated device
parameters to achieve a wide range of splitting ratios including coupling light from one input port
equally into all output ports. Moreover, we derived analytically a set of parameters to implement a
Walsh-Hadamard transformation and are investigating further options to implement a universal 4x4
mode-coupler on this platform. We envisage that the device can be used for quantum information
processing where two qubits are encoded in the waveguides using a photon in each propagation
direction.
We present an investigation into the resolution of blazed chirped Bragg grating spectrometers. These are dispersive spectrometers that diffract light out of a waveguide at a wavelength dependent angle, whilst focusing the light. The spectral resolution is found to be inversely proportional to grating length; previously used fabrication schemes limited the grating length (and hence resolution) when creating compact devices. We propose and implement a solution to this problem by varying the blaze angle of a Bragg grating along its length. Initial results show the fabrication of longer gratings (while preserving focal length) and an increase in FWHM resolution from
0.57 nm to 0.52 nm. This increase in resolution is thought to be limited by the appearance of aberrations which can be corrected for in future devices.
All optical quantum information processing is an important developing field for understanding and exploiting quantum entanglement and offers opportunities in quantum computation, quantum sensing and quantum communication. The presentation will describe work on laser written devices in a silica on silicon platform. The devices can be used to create single photon sources, waveguide circuitry and number resolving detectors. Applications of the technology include boson sampling, quantum teleportation, on-chip number resolving detection and quantum simulation. The use of laser writing allows for rapid prototyping, and by including high quality Bragg gratings, it is possible to provide accurate measurements of device properties including losses and coupler variability. Furthermore, the ability to make accurate measurements of material properties such as dispersion and birefringence allows the creation of tailored devices with optimal properties for identical photon sources.
We report on the fabrication and characterization of Er:YGG films suitable for waveguide amplifiers that could in principle be used in integrated path differential absorption lidar systems. Presented is our fabrication technique, comprising pulsedlaser- deposition growth of ~10 μm-thick crystalline films, their channeling via ultraprecision ductile dicing with a diamond-blade, producing optical quality facets and sidewalls, and amplifier performance. Net gain at 1572 nm and 1651 nm is obtained for the first time in Er-doped YGG waveguide amplifiers. Additionally, in a channel waveguide a maximum internal gain of 3.5 dB/cm at the 1533nm peak was realized. Recent crystal film quality improvements promise further performance enhancements needed for the intended application for high-peak power sources in the 1.6-μm spectral region targeting Earth observation systems for monitoring greenhouse gases.
For over a decade the field of quantum photonics has increasingly looked towards optical integrated platforms to perform more complex and sophisticated experiments. Silica integrated optics is an ideal material for this area, offering low propagation and fibre-coupling losses. To date many of the key on-chip experiments have been carried out in this platform, using bespoke monolithic devices. In this work we propose an alternative approach, implementing a linear network constructed from a number of identical reconfigurable modules. The modules are measured separately to produce an accurate model of the overall network. The cellular nature also allows the replacement of modules that are faulty or substandard. Each module comprises of an array of 10 Mach-Zhender interferometers. Forty thermo-optic phase shifters on each chip allows the control of both the amplitude and phase of the optical field within the devices. By cascading the modules any arbitrary NxN unitary network can be realised. The optical waveguides within the modules are fabricated by direct UV writing, where a scanning focused UV laser beam increases the local refractive index within a photosensitive germanosilicate glass layer. The resulting channel waveguides are engineered to have dimensions that are mode matched to standard optical fibre producing excellent coupling efficiency. Bragg gratings can also be simultaneously produced within the waveguides which greatly assists in the precise characterisation of the phase shifters, coupling ratios and optical losses within the modules. We will present our recent work in this area, demonstrating devices operating at telecom wavelengths for quantum information processing. We present a modular reconfigurable system for on-chip quantum optics experiments with excellent fibre compatibility and low propagation losses, implemented using direct-UV-written silica-on-silicon. The performance of fabricated devices in various configurations is reported.
The Networked Quantum Information Technologies hub (NQIT) is one of the quantum hubs of the UK's agship scheme, bringing together academia, industry and government. The Optical Engineering and Quantum Pho- tonics group at the University of Southampton's Optoelectronics Research Centre have developed an interactive demonstrator of a key device within NQIT to bring quantum photonics technologies to a wider audience. The system was exhibited at the 2017 UK Quantum Showcase in London to an audience of industry and government. It also featured as part of the Quantum City stand at the 2018 Cheltenham science festival, one of the UK's leading annual science events.
Novel research-inspired outreach activities allow scientists and members of the public to engage in a conversation, increasing the public’s understanding and interest in scientific research. This paper reviews outreach and public engagement initiatives undertaken by researchers from the University of Southampton’s Optical Engineering and Quantum Photonics Group during a 5-year research program grant entitled Building Large Optical Quantum States. The activities have been supported by a UK Engineering and Physical Sciences Research Council program grant and institutional, national and international professional organizations. The paper discusses activities and hand-outs that have been developed to increase the visibility and public understanding of integrated-photonics fabrication and testing facilities, including a cleanroom-based process for the design and fabrication of quantum outreach chips. More than 1,000 of these chips have been distributed to children, parents and government officials in public events and the activities have contributed towards the authors’ research group receiving the 2017 South East England Physics Network Highly Commended Award for Best Research Group. This paper also discusses the impact of these activities and lessons learned.
Integrated Optical Fibre (IOF) allows for robust planar integration and seamless monolithic coupling. Fabrication is achieved through an adapted Flame Hydrolysis Deposition (FHD) technique, which forms a ruggedized glass alloy between the fibre and planar substrate. It has been previously demonstrated as a low linewidth external cavity lasers diode and a hot-wire anemometer, inherently suitable for harsh environments.
This work looks at implementing the platform for harsh environment refractometry, in particular monitoring hydrocarbon fuels in the C14 to C20 range (e.g. Jet A1 and diesel). The platform uses SMF-28 fibre and direct UV written Bragg gratings to infer refractive index and thus the quality of the fuel. A challenge arises as the refractive index of these fuels are typically greater than the refractive index of the waveguide. Therefore, the guided mode operation of FBG refractometers is unsuitable. This work uniquely reports leaky mode operation and a regression analysis, inferring propagation loss through changes in amplitude of successive gratings. In effect, the proposed methodology utilises the imaginary part of the effective index as opposed to the real part, typically used by such sensors.
Initial results have shown a 350 (dB/cm)/riu sensitivity is achievable above a refractive index of 1.45. This was measured for a SMF-28 fibre wet etched to 30 µm and planarized. Considering a 0.01 dB/cm propagation loss resolution, refractive index changes of the order 10-5 can be approached.
Work will be presented on the fabrication of an IOF platform for refractometers as well as metrics for survivability in harsh environments.
Direct UV writing is a technique capable of fabricating low-loss channel waveguides, couplers and Bragg gratings in planar silica devices by translating an appropriate substrate through a tightly focused UV beam. To date direct UV written waveguides have been primarily formed using 244nm laser light, relying on the photosensitivity provided by doping with germanium and boron. To induce sufficient refractive index change, necessary for wave guiding, the substrates also require hydrogenation prior to UV writing. Not only does this require additional processing but over time the hydrogen present within the silica out-diffuses, which can cause variation of the final written structures. Deep-UV light, with a wavelength of 213 nm, has previously been used to inscribe strong fibre Bragg gratings (FBGs) in hydrogen-free Ge-doped fibres. Here we present the use of a 213 nm UV laser to write planar waveguide devices without the need for hydrogen loading.
Flame Hydrolysis Deposition (FHD) was used to deposit core and cladding layers of doped silica onto a thermally oxidised silicon wafer. Individual planar chips were diced from this wafer and a 5th harmonic Q-switched solid state laser operating at 213 nm wavelength was used to inscribe waveguides within the germanium-doped core layer of the chips without prior hydrogen loading.
We shall present our latest results of direct deep-UV written waveguides, including; the characterisation of single mode waveguides, detailed fluence and loss measurements, induced refractive index change and the first demonstration of planar Bragg gratings and photonic structures written with 213nm light.
K. Bongs, V. Boyer, M. Cruise, A. Freise, M. Holynski, J. Hughes, A. Kaushik, Y.-H. Lien, A. Niggebaum, M. Perea-Ortiz, P. Petrov, S. Plant, Y. Singh, A. Stabrawa, D. Paul, M. Sorel, D. R. Cumming, J. Marsh, R. Bowtell, M. Bason, R. Beardsley, R. Campion, M. Brookes, T. Fernholz, T. Fromhold, L. Hackermuller, P. Krüger, X. Li, J. Maclean, C. Mellor, S. Novikov, F. Orucevic, A. Rushforth, N. Welch, T. Benson, R. Wildman, T. Freegarde, M. Himsworth, J. Ruostekoski, P. Smith, A. Tropper, P. Griffin, A. Arnold, E. Riis, J. Hastie, D. Paboeuf, D. Parrotta, B. Garraway, A. Pasquazi, M. Peccianti, W. Hensinger, E. Potter, A. Nizamani, H. Bostock, A. Rodriguez Blanco, G. Sinuco-Leon, I. Hill, R. Williams, P. Gill, N. Hempler, G. P. Malcolm, T. Cross, B. O. Kock, S. Maddox, P. John
The UK National Quantum Technology Hub in Sensors and Metrology is one of four flagship initiatives in the UK National of Quantum Technology Program. As part of a 20-year vision it translates laboratory demonstrations to deployable practical devices, with game-changing miniaturized components and prototypes that transform the state-of-the-art for quantum sensors and metrology. It brings together experts from the Universities of Birmingham, Glasgow, Nottingham, Southampton, Strathclyde and Sussex, NPL and currently links to over 15 leading international academic institutions and over 70 companies to build the supply chains and routes to market needed to bring 10–1000x improvements in sensing applications. It seeks, and is open to, additional partners for new application development and creates a point of easy open access to the facilities and supply chains that it stimulates or nurtures.
We demonstrate the fabrication of a mechanically robust planarised fibre-FHD optical composite. Fabrication is achieved through deposition and consolidation of optical grade silica soot on to both an optical fibre and planar substrate. The consolidated silica acts in joining the fibre and planar substrate both mechanically and optically. The concept lends itself to applications where long interaction lengths (order of tens of centimetres) and optical interaction via a planar waveguide are required, such as pump schemes, precision layup of fibre optics and hybrid fibre-planar devices. This paper considers the developments in fabrication process that enable component development.
A pair of external cavity diode lasers are fabricated using an integrated planar Bragg grating. The planar waveguide and Bragg reflector is UV-written within a glass-on-silicon chip. Intensity isolated, continuous wavelength tuning at > 1kHz modulation rate is acheived using micro-heating elements fabricated directly over the Bragg grating. Low RIN (<140dB) and low linewidth (δν ~ 200 kHz) operation is found using a heterodyne measurement. We demonstrate the lasers operating in phase-locked loop configuration where one laser is frequency-offset locked to the other.
Integrated optics is becoming increasingly important for applications in quantum information processing, quantum
sensing and for advanced measurement. Intrinsically stable and low-loss it provides essential routing and coupling for
quantum optical experiments offering functions such as interconnects, couplers, phase delays and routing. Silica-onsilicon
has particular attractions, and in this work the fabrication approaches and advantages of the technique will be
explored. In particular, UV direct writing of waveguides and Bragg gratings proves useful for its rapid-prototyping
capability and its ability to provide grating for characterization of components for loss, birefringence and coupling ratio.
This review concentrates on the fabrication of planar waveguide devices, and ways in which direct UV writing provides
important functionality. Examples of applications of silica-on-silicon waveguides include quantum enhanced
interferometry, teleportation, boson sampling as well as hybrid operation for single photon detection with transition edge
sensors directly placed onto waveguide devices.
Ductile regime dicing has been used to machine a variety of optical materials to produce waveguides for lasers, multimode
interference devices and non-linear devices. However, few papers discuss the properties of the machining, either
qualitatively or quantitatively. In this work ductile regime dicing of germanium, Yttrium Aluminum Garnet (YAG),
lithium niobate and silicon for photonic applications are reported. Machining parameters are discussed, surface
micrographs shown, and surface roughnesses are calculated for each sidewall machined. The sidewall average surface
roughnesses (Sa) were measured to be 2.1 nm for germanium, 3.5 nm for YAG, 7.9 nm for lithium niobate and 8.6 nm
for silicon.
An optomechanical dual cantilever device has been fabricated with applications as a displacement sensor and variable attenuator. A novel fabrication approach using a precision dicing saw has benefits for fabrication time, cost and energy consumption. The displacement sensor sensitivity is 0.8 dB/micron and a suppression ratio of 25 dB is obtained when the device is used as an attenuator.
The potential of a new class of multifunctional photonic circuits is demonstrated by femtosecond laser micro-machining and inscription of micro-optical structures, such as ring and disk resonators, Mach-Zehnder interferometers, and microfluidic devices, in a flexible flat-fibre chip. Additional filling of surface channels with functional materials is employed towards sensing applications. Furthermore, direct write femtosecond-laser inscribed Bragg gratings were written in the Ge-doped core of flat-fibre enabling a number of relevant applications. The flat-fibre platform offers a unique degree of freedom by allowing surface and sub-surface devices to be integrated onto an a single optical chip with the potential for straightforward incorporation into integrated photonic circuits or optofluidic devices.
The authors present a direct UV writing approach to fabricate fiber Bragg gratings (FBGs) and gratings in
photolithographic waveguides. The technique uses two coherent UV beams, which are focused to a small spot (~7μm
diameter) at the point at which they overlap. The resulting interference pattern at the foci consists of fringes which are
used to define several grating planes per exposure, giving greater design flexibility and a significantly larger accessible
spectral range compared to traditional approaches. The typical index contrast during grating fabrication is observed to be
4.7×10-3, at writing speeds of 8 mm/min.
Conventional singlemode semiconductor DFB and VCSEL lasers used in high resolution spectroscopy are often required to operate at specific, custom wavelengths, such as those associated with gas absorption lines. We present the results of work to develop alternative sources in the 1550nm and 1650nm regions, the latter coinciding with an absorption line of methane. Custom wavelength Bragg gratings have been used to stabilize the output of external cavity lasers implemented in both optical fiber and planar silica-on-silicon integrated circuits, using commercially available semiconductor gain chips, to give laser output at 1648 and 1649 nm, respectively. Thermal expansion or mechanical strain of the Bragg grating offers a suitable wavelength tuning mechanism. Results are presented including the wavelength tuning range, output power, relative intensity noise (RIN), side-mode suppression and linewidth of devices for application in high resolution gas spectroscopy. The different methods of writing Bragg gratings in optical fiber and planar silica-on-silicon allow a high degree of flexibility in the choice of emission wavelength.
We demonstrate femtosecond laser micro-machining and inscription of micro-optical structures, such as ring and disk resonators, Mach-Zender interferometers, and complex microfluidic devices, in a novel optical flat-fibre chip. In addition, we fill the channels with functional materials that can be used for sensing applications. Furthermore, direct write femtosecond-laser inscribed Bragg gratings are written in the Ge-doped flat-fibre core at 1547.8 nm. The flat-fibre chip offers a unique degree of freedom by allowing surface and sub-surface devices to be integrated onto an optical platform with the potential for straightforward incorporation into integrated photonic circuits or optofluidic devices.
We have demonstrated two monolithically integrated Bragg grating based accelerometers, both with the optical path and mechanical structure being made from the same substrate. The unique fabrication techniques, Direct UV Writing and precision dicing, used to create the glass microcantilevers are discussed. We show experimental results from two different Bragg grating based interrogation systems, one utilizing a single Gaussian apodized Bragg grating and the other utilizes two spectrally matched Bragg gratings forming a Fabry-Pérot interferometer. Sinusoidal accelerations were applied to both devices and their sensitivities were found to be 0.67±0.035 mV/g and 14.0±0.44 mV/g for the single Bragg grating and Fabry-Pérot interferometer respectively.
Preparation of high quality facets for low-loss coupling is a significant production issue for integrated photonics, usually requiring time consuming lapping and polishing. Recently, the development of precision dicing saws with diamond impregnated blades has allowed the achievement of optical grade surfaces in optical materials based on dicing alone. In this report we investigate the optimization dicing conditions to achieve optical quality surfaces in a silica-on-silicon planar substrate and show what can be achieved by correct selection of machining parameters.
Direct UV Grating Writing (DGW) is an attractive technique for fabricating integrated Bragg grating devices in a silica-on- silicon platform. In this work we propose and demonstrate a novel phase modulated DGW method using an Electro- Optical Modulator for planar Bragg grating fabrication that offers improved performance. This new approach has allowed us to construct Bragg gratings with versatile structures such as phase shifts and apodization profiles. Simple uniform gratings, single and multiple phase shifted gratings, apodized gratings and chirped gratings have been made in this method; using grating detuning and this new phase controlled method we have shown that planar Bragg gratings can be written among 700 nm wavelength range on a single chip exclusively using software control.
Tilted Bragg gratings (TBGs) have been shown to have a number of practical uses in planar geometries, demonstrating polarization capabilities and allowing excitation of surface plasmons. Fabrication and characterization of TBGs has been carried out in silica-on-silicon waveguides to highlight potential planar applications. An initial investigation into the coupling behaviour of TBGs has been undertaken, with greater than -20 dB coupling achieved for even small angle gratings (5 °). Experimental analysis of these TBG systems provides insight into future applications of the planarized devices.
A direct UV written single-mode planar Bragg grating element is demonstrated, with >300 GHz of applied strain tuning. The degree of tuning that the fabricated device has leads to a potential application in dynamic optical networks as an optical add-drop multiplexer. The filter bandwidth is 23±5 GHz and can be tuned over a 308-GHz range by applying transverse strain across the device's composite silica-on-silicon structure.
Integrated optical devices offer dense, multifunctional capability in a single robust package but are rarely considered
compatible with the fields of remote or distributed sensing or compete in the long-haul with conventional 'one-dimensional'
fibers. Here we aim to change that by introducing a 'flat-fiber' process that combines the advantages of
of existing low-cost fiber drawing with the functionality of planar lightwave circuits in a novel hybrid format. Adapted
from MCVD fiber fabrication, our preforms are deposited and collapsed into a rectangular geometry before drawing,
resulting in extended lengths of mechanically flexible flat-fiber material with a photosensitive germanium-doped planar
core. Direct UV writing is then used to create arrays of channel waveguides within the core layer, using a 5μm focused
laser spot that literally 'draws' refractive index patterns into the flat fiber as it moves. Having recently demonstrated
simple building blocks for integrated optical circuits in millimeter-wide flat-fibers (including; channel waveguides,
power junctions and splitters, and planar Bragg gratings), our next step is to incorporate structured windows at strategic
points along the fiber to allow fluidic access to the evanescent field for local refractive-index-based chemical
measurements. By taking this approach, we hope to extend beyond the limitations of traditional planar and fiber
substrates, allowing exotic material compositions, device layouts, and local sensing functions to be distributed over
extended distances with no coupling or compatibility concerns in highly functional distributed lab-on-a-chip devices.
UV written planar waveguide sensors provide an integrated solution to refractive index sensors. The high sensitivity of the devices originate from their use of Bragg gratings which provide an accurate means of interrogating the local effective index. Conventionally the optical mode is made sensitive to an external refractive index by etching away the cladding and exposing it to an analyte. These devices have been used to sense liquid/solid phase changes and have displayed their potential for use as biological and chemical sensors. Recent results demonstrate sensitivities rivaling that of the highest specification Surface Plasmon Resonance (SPR) techniques. Here we introduce a new geometry which embraces the benefits of planar technology to realise new integrated devices. The geometry relies upon the use of a vertical trench or groove to produce an interface of optical quality which provides lateral access for an optical mode. The evanescent field interacts with the material within the groove and a Bragg grating in the region provides the means for interrogation. This reorientation of the sensor geometry provides additional flexibility to UV written devices, allowing several different sensors to be defined on the single substrate without multiple etching processes. These multiple sensors may offer complementary information such as the effective index as a function of penetration depth and interrogation wavelength for dispersion analysis. The paper also outlines the inherent feature benefits and fabrication advantages, including a reduction in return loss, spectral artefacts and stress induced birefringence.
Novel liquid crystal-based integrated optical devices with >140GHz electrical tuning are presented. Initial results with Bragg wavelength tuning covering five 25GHz WDM channel spacing have been achieved with 170V (peak-to-peak) sinusoidal voltages applied across electro-patterned ITO-covered glass electrodes placed 60μm apart. These prototype devices were fabricated using direct UV grating writing, with an evanescent field coupling into a liquid crystal overlay through an etched window. Two distinct threshold conditions are observed, manifesting only during the increase of supply voltage and forming a hysteretic tuning curve. The secondary threshold which takes place at higher voltages has never been reported before. We believe these threshold points are related to the formation and bleaching of disclination lines. Geometric and effective index consideration could not explain the similar tuning behaviour displayed by both TE and TM polarised light.
Second harmonic generation via periodically-poled nonlinear materials offers an efficient means of generating high-quality
visible light at wavelengths that would be otherwise unattainable with traditional laser sources. While this
technology has the potential for implementation in many mass-industrial applications, temperature stability
requirements, often as restrictive as 0.1°C, can make packaging with a pump source problematic. In this work we are
investigating the use of synthesised response PPLN gratings to create crystals that are better suited to visible SHG. Our
route towards addressing this issue is to convert the standard sinc-shaped temperature-tuning response of a uniform
grating to a flat-top temperature tuning function with widths of up to several degrees. We have achieved a
computationally efficient means of designing such gratings with a required temperature tuning profile based on a
simulated annealing algorithm using repeated local changes of grating layout and subsequent Bloembergen-style
analysis of the second harmonic, successive iterations of which quickly lead to the desired temperature tuning profile.
Using our high fidelity poling technique we have fabricated synthesised response PPLN with precise placement of poled
domains in Lithium Niobate based on the designs from our mathematical models. Measurements on these initial devices
provide more than 4°C flat-top temperature stability, albeit with a corresponding loss in operational efficiency. Our aim
is to implement improved designs in magnesium-doped Lithium Niobate for packaging with near-room temperature
diode-based pump sources, as could be applied towards RGB TV and projector applications.
We report on the experimental demonstration of electrically tunable Bragg grating using liquid crystals. Such concept and device have been modelled in the past, but not realised experimentally. Using Direct UV writing technique, a waveguide and Bragg grating are simultaneously patterned in a planar silica on silicon substrate. Overcladding in the vicinity of the grating can be replaced by liquid crystals to exploit their electro-optic properties. The interactions of the evanescent field with the variable refractive index of the liquid crystal allow the tuning of the centre wavelength of the filter. In this way a 35 GHz tunable planar Bragg grating using liquid crystal and electric field was demonstrated.
A planar Bragg grating in silica is used to form an integrated optical refractive index sensor. The device, inherently suited to remote sensing using single mode transmission fibre, is shown to clearly detect phase transitions in a nematic liquid crystal and in water. Transitions from ordered to isotropic, gas to liquid and liquid to solid as well as the reverse transitions can all be clearly identified. The sensor also allows supercooled liquid to be easily identified, a task previously found challenging by other sensor technologies.
Direct UV-writing is an ideal technique for rapid prototyping and small batch fabrication of integrated optical circuits. Based on the refractive index increase of a glass from exposure to a tightly focused UV beam. The translation of this beam relative to a suitable substrate allows the definition of 2-d waveguide structures such as s-bends and power couplers without the need for subsequent processing.
Our alternative technique, Direct Grating Writing retains the advantages of Direct UV writing for channel definition but allowing both the grating and channel structure to be formed in the same process. Using this new technique, we present the fabrication of conventional channel waveguides and Bragg channel waveguides. We demonstrate the independence of the Bragg grating strength from the strength of the channel waveguide, the sensitivity of this process as a characterization technique, and the ability to use this technique to fabricate more complex 2-D structures for integrated optical circuits. We finally present the fabrication of a range of gratings spanning the entire wavelength span commonly used for optical communication with no change in the equipment.
LiNbO3 and LiTaO3 are commonly used ferroelectric crystal materials. Since the first reports of successful single domain crystal growth in 1965, these materials have found increasing use in optoelectronics, laser systems, Q- switching and frequency conversion, holographic data storage, surface acoustic wave devices, integrated optics and modulator use, and most recently, microwave telecommunications. In single domain format these ferroelectrics are photorefractive, pyroelectric and piezoelectric, and possess usefully large nonlinear optical and electro-optical coefficients. If domain engineering or micron/nano-scale bulk or surface modification is performed however, greater functionality is introduced, leading to additional uses such as phase-matched frequency conversion, grating and photonic structures, and the recently proposed use in MEMS and MOEMS devices. We discuss here a range of techniques for domain engineering and domain selective etching, as well as the use of light in poling and etching modification, and illustrate this potential with several devices that we have constructed by these routes.
Ferroelectric materials such as LiNbO3 and LiTaO3 offer many potential advantages over silicon for MEMS structures and self-actuating miniature devices. These materials possess numerous useful intrinsic properties such as piezoelectricity, pyroelectric and electro-optic coefficients, enabling the construction of micro-scale cantilevers, membranes, tips and switches. So far however, reliable and accurate methods for machining and microstructuring LiNbO3 single crystals have been lacking. We have recently been exploring several such methods, which are sensitive to ferroelectric domain orientation. A sample that has been domain-engineered shows a large difference in etch characteristics: the +z face does not etch at all, whereas the -z face etches normally. Microstructured devices can be fabricated therefore, via spatially selective domain poling followed by etching. The extreme sensitivity of the etch process to domain orientation has enabled us to fabricate ridge waveguides for electro-optic modulator applications, alignment grooves for efficient fibre pig-tailing to LiNbO3 modulators, and micro-cantilevers using a novel technique of contact bonding of dissimilar ferroelectric hosts.
Self-guided waves that can be excited in quadratic nonlinear media have been extensively studied for their potential applications in ultra-fast all-optical processing. We have previously reported the use of solitary waves collision in a KTP crystal to experimentally demonstrate all-optical switching of IR picosecond pulses. Up to now, the intensity required to obtain self-trapping of a beam remained at a high level. This has been due to the lack of nonlinear crystals which combine the attributes of a large nonlinearity and phase-matching capability at experimentally convenient wavelengths. The availability of Periodically Poled Lithium Niobate can circumvent this difficulty. 2D spatial solitary waves in PPLN have been predicted theoretically and simulated numerically. In this communication we will report their experimental observation and for the first time their interaction in a 15mm long crystal. Then we will compare solitary wave behavior in KTP and PPLN, in particular self-trapping intensity threshold versus phase mismatch. We will also compare experimental data with the reslut of our computations modeling. In a last part we will show our first experimental result about 2D quadratic soliton collision in a PPLN crystal. Finally we will discuss the advantages of choosing PPLN to realize all- optical devices using solitary wave interactions.
We investigate the use of combined optical and electrical techniques to control domain formation in ferroelectric SBN, and examine the periodic structures induced by spatially modulating the light intensity through the crystal during the electrical poling process. The role of photoexcited charges in compensating and stabilizing the induced domain structures is summarized, and the importance of thermal effects established. The process of domain re-ordering is shown to be particularly sensitive to temperature changes close to a domain freezing point of SBN, which occurs near room temperature. The resulting light-induced domain re- ordering is assessed using current monitoring during the repoling process, and photorefractive two-beam coupling of the resulting structures.
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