Single-photon detectors record photon-detection events with high efficiency and picosecond-timing resolution allowing for lidar (ranging) with millimetre depth resolution and low light levels. However, only silicon SPADs offer high quantum efficiency without cryogenic cooling. InGaAs SPADs are limited to below a wavelength of 1800 nm with low efficiencies. We have developed an approach for high single-photon detection efficiencies in the mid-infrared (2-3.5um) using upconversion in a PPLN waveguide in combination with a silicon SPAD. Using a waveguide we confine the mode over extended lengths to increase conversion efficiency and reduce the required pump power. Using a precision-machining waveguide fabrication process we have fabricated waveguides that support modes in the mid-infrared with low loss. We show overall photon detection efficiency in excess of InGaAs SPADs. We use our new detection system for imaging and ranging applications.
KEYWORDS: Spectrographs, Waveguides, Second harmonic generation, Fabry Perot interferometers, Calibration, Supercontinuum generation, Sum frequency generation, Frequency combs
Next generation extreme precision radial velocity (EPRV) instruments such as the ANDES spectrograph of the Extremely Large Telescope will require an unprecedentedly high-precision calibration approach, particularly in the UB band region in which the most dense stellar absorption lines are present. For this purpose, astrocombs delivering thousands of atomically referenced, evenly-spaced calibration lines across a broad spectrum have the potential to be ideal calibration sources. Here, we report a novel and effective approach to generating a laser frequency comb with a multi- GHz mode spacing covering a broad wavelength range in the UB band. The approach is based on nonlinear mixing between near-infrared ultrafast laser pulses in a MgO:PPLN waveguide. The generated 1-GHz comb, spanning 390–520 nm, was filtered to a 30 GHz sub-comb using a low-dispersion Fabry-Perot etalon. The resultant UB-band astrocomb was then captured on a lab-built cross-dispersion echelle-prism spectrograph, demonstrating well resolved comb lines across the etalon bandwidth of 392–472 nm.
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 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 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.
We demonstrate a source of polarisation-entangled photons that produces pairs of entangled photons at a rate of 1.25 gigahertz. Our system is enabled by Periodically Poled Lithium Niobate waveguides that produce degenerate photon-pairs with a centre wavelength of 1560 nm and a 0.1 nm bandwidth. We measured the degree of entanglement and obtained a CHSH parameter of 2.73. From measurements of polarisation discrimination in a BB84 protocol with two mutually unbiased bases, we obtained a measurement fidelity of 98% and estimated the maximum secure key rate to be 0.633 gigabits per second.
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.
We report a nonlinear optical upconversion 3D imaging system for infrared radiation enabled by zinc indiffused MgO:PPLN waveguides. While raster-scanning a scene with an 1800 nm pulsed-laser source, we record time-of-flight information, thus probing the 3D structure of various objects in the scene of interest. Through upconversion, the 3D information is transferred from 1800 nm to 795 nm, a wavelength accessible to single-photon avalanche diode (SPAD).
We demonstrate two-photon interference and polarization entanglement at 2090 nm, constituting a crucial leap towards
free-space mid-infrared quantum communication systems in a spectral region with high atmospheric transparency and
reduced solar background.
Quantum-enhanced optical technologies operating within the 2- to 2.5-μm spectral region have the potential to revolutionize emerging applications in communications, sensing, and metrology. Currently, sources of entangled photons are available at visible, near-infrared and telecom wavelengths but are strongly underdeveloped at longer wavelengths. Here, using custom-designed lithium niobate crystals for spontaneous parametric down-conversion and tailored superconducting nanowire single-photon detectors, we demonstrate two-photon interference and polarization-entangled photon pairs at 2090 nm. These results open the 2- to 2.5-μm mid-infrared window for the development of optical quantum technologies such as quantum key distribution in next-generation mid-infrared fiber communication systems and future Earth-to-satellite communications.
There is significant interest in developing laser wavelengths between 700 and 800 nm that may then be frequency doubled to the UV for applications in spectroscopy and atomic physics. We present our most recent results on both a 739 nm AlGaAs/AlGaInP VECSEL, where we demonstrate 150 mW of CW power suitable for frequency doubling to the Yb+ cooling transition at 369.5 nm, and a 780nm AlGaAs/AlGaInP VECSEL which was utilised in a novel demonstration of second harmonic generation in a Zinc-indiffused MgO:PPLN waveguide. In the latter we have generated 1 mW of power at 390 nm.
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.
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.
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.
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.
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.
Intra-cavity frequency doubling (ICFD) of an extended cavity 49-emitters edge-emitting laser bar has been demonstrated
in a quasi-phase matching MgO-doped periodically poled lithium niobate (MgO: PPLN) bulk crystal. A maximum of 1
W of second-harmonic light at 465 nm is generated at an operating injection current of 45 A with the optimal phase-matching
MgO:PPLN temperature of 50.4 °C. To increase the efficiency further, careful design of the lens used on the
fast and slow axis beam waists and use of lower-temperature MgO:PPLN planar-waveguide array can be considered.
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.
Intra-cavity frequency doubling (ICFD) of electrically and optically surface emitting diode lasers in the near IR region
become more interesting [1-3] and will have an enormous impact in the display market. In this paper, Watts-level green
laser is generated by ICFD of multi-emitters laser bar using a MgO-doped periodically poled lithium niobate (MgO:
PPLN) bulk crystal, which has the potential to be scalable to high production volumes and low costs with immense
implication for laser-based projection displays.
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.
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.
This paper reports the fabrication of Bi2O3-based glass planar and channel waveguides using two techniques, respectively hot-dip spin-coating, and direct 244 nm UV-writing into the bulk glass. In the former, a 5 μm core glass film was achieved, which indicates a practical potential for realizing single mode operation channel guides. In the later, the laser written structures obtained showed a positive refractive index change, estimated at 4×10-4 at 633 nm, and a loss of less than 4 dB/cm.
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.
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