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Liquid crystal tunable Bragg Gratings defined in planar substrates via a laser patterning technique exhibit complex wavelength tuning. This tuning displays threshold points and hysteresis. These tuning features are shown to be a manifestation of physical processes occurring in the confined geometry of our tunable devices. Such physical processes include the formation and removal of line disclinations and an associated wall. We discuss the effect of walls in the liquid crystal with regards to voltage tuning characteristics and whether they may allow faster wavelength tuning. [ABSTRACT FROM AUTHOR]

We report on the fabrication of bidimensional arrays of hexagonal domains with strictly periodic and quasi-periodic patterns for the implementation of nonlinear photonic crystals and quasi-crystals in z-cut congruent LiNbO3. [ABSTRACT FROM AUTHOR]

The article presents a direct grating writing technique that allows simultaneous ultraviolet (UV) definition of channel wave guides with integral Bragg gratings in a photosensitive planar media. Based on a small-spot interference pattern generated by two focused UV-writing beams, the single-step approach was developed to promote optimal use of sample photosensitivity for both the wave guide geometry and Bragg grating superstructure, with the potential for implementing many aspects of advanced grating design, such as chirp and apodisation, without the need for a phase mask.

An integrated direct UV written apodised planar Bragg grating reflector with 154 GHz (1.23 nm) of thermo-optic tuning in the C-band and a response speed 31 GHz/ms is demonstrated. The degree and speed of tuning of the fabricated device indicates potential application in dynamic optical networks, as an optical add-drop multiplexer. [ABSTRACT FROM AUTHOR]

A method for fabricating planar silica substrates via modified chemical vapour deposition (MCVD) and a modified fibre drawing technique is presented. Long lengths of planar material are generated from a single substrate offering a potentially low-cost alternative to existing planar substrate deposition processes. Buried straight and splitting waveguide channels, as well as Bragg gratings, are inscribed in the planar material using direct UV-writing technology, and the results are reported. [ABSTRACT FROM AUTHOR]

We report the infrared operation of a bulk optical Bragg modulator based on electro-optically induced refractive index gratings in z-cut periodically poled lithium niobate. Efficiencies in the first order of 45% for 1064 nm e-polarized light and 30% for o-polarized light were achieved, with maximum on/off ratios of 15:1 and 9:1, respectively. Field-induced light scattering effects due to poling are observed at higher drive voltages and compromised device performance due to these scattering effects is predicted to limit long-wavelength operation of these devices. [ABSTRACT FROM AUTHOR]

A single-step ductile dicing process capable of manufacturing optical quality facets in lithium niobate (LiNbO3) ridge waveguides with an average surface roughness of 0.29 nm is reported. This result is comparable with surface roughnesses achieved by lapping and polishing and represents an order of magnitude improvement over the prior state of the art in LiNbO3 waveguide facet dicing. [ABSTRACT FROM AUTHOR]

Presented is a study of low polarisation sensitivity 2° X-couplers with 20% coupling ratio tuning via refractive index asymmetry, attaining 50:50 power splitting at 1550 nm. The device has less than 5% coupling ratio variation over 30 nm in the C-band and <0.7 dB polarisation-dependent loss. [ABSTRACT FROM AUTHOR]

We report a waveguide-enhanced Raman spectroscopy (WERS) platform with alignment-tolerant under-chip grating input coupling. The demonstration is based on a 100-nm thick planar (slab) tantalum pentoxide (Ta2O5) waveguide and the use of benzyl alcohol (BnOH) and its deuterated form (d7- BnOH) as reference analytes. The use of grating couplers simplifies the WERS system by providing improved translational alignment tolerance, important for disposable chips, as well as contributing to improved Raman conversion efficiency. The use of non-volatile, non-toxic BnOH and d7-BnOH as chemical analytes results in easily observable shifts in the Raman vibration lines between the two forms, making them good candidates for calibrating Raman systems. The design and fabrication of the waveguide and grating couplers are described, and a discussion of further potential improvements in performance is presented. [ABSTRACT FROM AUTHOR]

The waveguide structure made of lithium niobate crystal (LiNbO3, LN) can further improve the device integration. It has been widely used in electro-optical modulator, frequency conversion, acousto-optic Q-switching and other optoelectronic devices. It has important application prospects in optical fiber communication, photoelectric sensing, lidar, aerospace and other fields. The LN waveguide made by traditional Ti diffusion method has poor resistance to photorefractive damage in short wave applications. The LN waveguide made by annealing proton exchange method only support TM mode (transverse magnetic mode) single polarization transmission, and its application field is limited. In this paper, a new Zn diffusion method is proposed to fabricate magnesium doped LN ridge waveguide. By establishing the diffusion model and simulation of the waveguide, the technological conditions were explored and tested. The lowest transmission loss of LN waveguide is 0.86 dB/cm, and the photorefractive damage threshold can reach 184 kW/cm². It will provide a better preparation way for the research and development of high power lithium niobate waveguide integrated optoelectronic devices. [ABSTRACT FROM AUTHOR]

We demonstrate a new management of multi-stage optical parametric generator (OPG) and amplifier (OPA) to obtain high-energy picosecond sources with high beam quality. The setup of multi-stage OPG-OPA requires mode-matching between the pump beam and the stable mode of the OPG-OPA. In a proof-of-principle experiment, the single-pass multi-stage OPG-OPA consists of three walk-off compensated KTP crystal pairs and two lenses, pumped by an 86 ps, 1064 nm 10 kHz picosecond laser. The signal light at ∼ 1.77 μ m has an average output power of 502 mW with record energy up to 50.2 μ J. The beam quality factor of the signal light can be improved to M x 2 × M y 2 = 1.87 × 2.16 after filtering out about 40% signal power. To the best of our knowledge, it is the first picosecond single-pass multi-stage OPG-OPA pumped at kHz regime. [ABSTRACT FROM AUTHOR]

Presents the 2020 subject/author index for this publication. [ABSTRACT FROM AUTHOR]

Glass photonics are widespread, from everyday objects around us to high-tech specialized devices. Among different technologies, sol-gel synthesis allows for nanoscale materials engineering by exploiting its unique structures, such as transparent glass-ceramics, to tailor optical and electromagnetic properties and to boost photon-management yield. Here, we briefly discuss the state of the technology and show that the choice of the sol-gel as a synthesis method brings the advantage of process versatility regarding materials composition and ease of implementation. In this context, we present tin-dioxide-silica (SnO2-SiO2) glass-ceramic waveguides activated by europium ions (Eu3+). The focus is on the photorefractive properties of this system because its photoluminescence properties have already been discussed in the papers presented in the bibliography. The main findings include the high photosensitivity of sol-gel 25SnO2:75SiO2 glass-ceramic waveguides; the ultraviolet (UV)-induced refractive index change (Δn ~ -1.6 × 10-3), the easy fabrication process, and the low propagation losses (0.5 ± 0.2 dB/cm), that make this glass-ceramic an interesting photonic material for smart optical applications. [ABSTRACT FROM AUTHOR]

Summary: Glass-ceramics (GCs) are constituted by nanometer-to-micron-sized crystals embedded in a glass matrix; usually, their structural or functional elements (clusters, crystallites or molecules) have dimensions in the 1 to 100 nm range. As the name says, GCs must be considered an intermediate material between inorganic glasses and ceramics; in most cases the crystallinity is between 30 and 50%. GCs share many properties with both glasses and ceramics, offering low defects, extra hardness, high thermal shock resistance (typical of ceramics) together with the ease of fabrication and moulding (typical of glasses). The embedded crystalline phase, however, can enhance the existing properties of the matrix glass or lead to entirely new properties. GCs are produced by controlled crystallization of certain glasses, generally induced by nucleating additives; they may result opaque or transparent. Transparent GCs are now gaining a competitive advantage with respect to amorphous glasses and, sometimes, to crystals too. The aim of the present paper is to introduce the basic characteristics of transparent glass-ceramics, with particular attention to the relationship between structure and transparency and to the mechanism of crystallization, which may also be induced by selective laser treatments. Their applications to the development of guided-wave structures are also briefly described. [ABSTRACT FROM AUTHOR]

Quantum teleportation is a fundamental concept in quantum physics that now finds important applications at the heart of quantum technology, including quantum relays, quantum repeaters and linear optics quantum computing. Photonic implementations have largely focused on achieving long-distance teleportation for decoherence-free quantum communication. Teleportation also plays a vital role in photonic quantum computing, for which large linear optical networks will probably require an integrated architecture. Here, we report a fully integrated implementation of quantum teleportation in which all key parts of the circuit-entangled state preparation, Bell-state analysis and tomographic state measurement-are performed on a reconfigurable photonic chip. We also show that a novel element-wise characterization method is critical to the mitigation of component errors, a key technique that will become increasingly important as integrated circuits reach the higher complexities necessary for quantum enhanced operation. [ABSTRACT FROM AUTHOR]

Yb:fiber lasers have shown excellent performance in the 980–1100 nm wavelength range. To extend the range below 980 nm, it becomes difficult to reach the transparent condition due to the smaller ratio between emission and absorption cross sections. As a result, a high demand of population inversion is needed, and the selection of pump wavelength as well as other intra-cavity parameters are crucial for lasing in the 920–960 nm wavelength range. To find a feasible solution, the pump wavelength, fiber length, and laser cavity transmittance were systematically studied. Based on the experimental result of a 960 nm Yb:fiber laser, the re-absorption loss and temperature dependent gain can be reliably modeled. The result shows promise in the development of a Yb:fiber laser at a wavelength as short as 920 nm. [ABSTRACT FROM AUTHOR]

The efficient production of high-quality facets for low-loss coupling is a significant production issue in integrated optics, usually requiring time consuming and manually intensive lapping and polishing steps, which add considerably to device fabrication costs. The development of precision dicing saws with diamond impregnated blades has allowed optical grade surfaces to be machined in crystalline materials such as lithium niobate and garnets. In this report we investigate the optimization of dicing machine parameters to obtain optical quality surfaces in a silica-on-silicon planar device demonstrating high optical quality in a commercially important glassy material. We achieve a surface roughness of 4.9 nm (Sa) using the optimized dicing conditions. By machining a groove across a waveguide, using the optimized dicing parameters, a grating based loss measurement technique is used to measure precisely the average free space interface loss per facet caused by scattering as a consequence of surface roughness. The average interface loss per facet was calculated to be: −0.63 dB and −0.76 dB for the TE and TM polarizations, respectively. [ABSTRACT FROM AUTHOR]

We present several approaches to make integrated optic filters, which include either liquid crystals or composite structures including polymers. The presence of liquid crystals allows tunability of the filters controlled by means of low power electric signals. All-optical tuneable filters are also presented in which optical nonlinear properties of liquid crystals are deployed to drive the device. [ABSTRACT FROM AUTHOR]

We present our recent experimental results on guided wave devices that can be switched and tuned by optical driving signals. Two different waveguide structures recently implemented are reported. The first one uses liquid crystals (LCs) as a core in a SiO2/Si V-groove and optical signals whose power is in the order of few milliwatts to control propagation. The second structure is an all-optical tunable filter that includes photosensitive composite materials. The latter combines the simple and low-cost ion exchange waveguide technology with a composite LC methyl red azo-dye photosensitive compound to obtain full optical tunability. [ABSTRACT FROM PUBLISHER]

A refractive index profile and a mode index of single-mode LiNbO3 channel waveguides are simultaneously estimated from the measured near-field intensity distributions. A third-order low-pass Butterworth filter is used to filter the measured amplitude and its double derivative to remove high-frequency spatial noise and impulses. The mode index or effective index of the propagating mode is extracted by matching the index change profile equal to zero at the waveguide boundary. The proposed technique is applicable to any slowly varying, single-mode, integrated-optic waveguide for TE and TM modes and for any operating wavelength within the single-mode regime. [ABSTRACT FROM AUTHOR]

The dielectric properties of near-stoichiometric LiNbO3:Fe and LiNbO3:Ce single crystals have been investigated using terahertz time domain spectroscopy in a frequency range of 0.7-1.6 THz at room temperature. When coupled with an applied external optical field, obvious photorefractive effects were observed, resulting in a modulation of the complex dielectric constant for the crystals. The variation in refractive index, |Δn|, had a linear relationship with the applied light intensity, accompanied by a step-like decrease at high intensity. The findings were attributed to the internal space charge field of the photorefraction and the light-induced domain reversal in the crystals. [ABSTRACT FROM AUTHOR]

UV radiation in the spectral region beyond ∼320 nm is strongly absorbed by lithium niobate single crystals. The strong absorption and the consequent localized heating of the crystal triggers physical processes which can affect the state of the material not only by changing the refractive index, as shown in the past, but also by changing its ferroelectric disposition. UV laser irradiation of the polar surfaces in particular can induce or inhibit the inversion of ferroelectric domains. A summary of these UV light induced effects and their utility in the microstructuring of this very important optical ferroelectric crystal will be presented here. [ABSTRACT FROM AUTHOR]

Nonlinear optics has produced a wealth of spectacular scientific results, opening new fields such as harmonic generation, parametric amplification and oscillation, optical solitons, phase-conjugation, stimulated Raman scattering. Since the early days, it was clear that third-order nonlinear optical phenomena had the potentiality for the implementation of all-optical integrated devices for applications related to information and communication technology, but the accomplishment of such a program was made difficult by the lack of appropriate materials and realization schemes. The new trend emerging in the last few years is based on the utilization of cascaded interactions in second-order materials. The consideration that cascaded effects can give rise to large effective third-order nonlinearities with ultrafast response times and perfect transparency of the nonlinear medium, associated with important technological advances in the fabrication of microstructured waveguides, has triggered an enormous interest in both academic and industrial laboratories. In particular, it has been shown that the technique of optical frequency conversion, with the associated spectral inversion, can have important applications in optical communication networks based on wavelength-division multiplexing. The results described in this article, all of very recent publication, show that significant steps have been made in the direction of understanding nonlinear optical interactions in a confined geometry and of developing integrated optical devices. [ABSTRACT FROM AUTHOR]

Properties of the optoelectronic microwave oscillator (OEO) based on an optical whispering gallery mode lithium niobate disc cavity are studied theoretically. We show that the threshold of the oscillation of this device may be as low as 1 mW, and its frequency stability (Allan deviation) may be as small as 10 -15 / t 1/2 for reasonable experimental conditions. The properties of resonant and non-resonant OEOs are compared. Preliminary experimental results on the whispering gallery mode OEO are reported. [ABSTRACT FROM AUTHOR]

Fabrication capabilities of high optical quality hexagonal superstructures by chemical etching of inverted ferroelectric domains in lithium niobate platform suggests a route for efficient implementation of compact hexagonal microcavities. Such nonlinear optical hexagonal micro-resonators are proposed as a platform for second harmonic generation (SHG) by the combined mechanisms of total internal reflection (TIR) and quasi-phase-matching (QPM). The proposed scheme for SHG via TIR-QPM in a hexagonal microcavity can improve the efficiency and also the compactness of SHG devices compared to traditional linear-type based devices. A simple theoretical model based on six-bounce trajectory and phase matching conditions was capable for obtaining the optimal cavity size. Furthermore numerical simulation results based on finite difference time domain beam propagation method analysis confirmed the solutions obtained by demonstrating resonant operation of the microcavity for the second harmonic wave produced by TIR-QPM. Design aspects, optimization issues and characteristics of the proposed nonlinear device are presented. [ABSTRACT FROM AUTHOR]

With the rapid progress in fiber technologies, femtosecond fiber lasers, which are compact, cost-effective and stable, have been developed and are commercially available. Studies of optical parametric oscillators (OPOs) pumped by this type of laser are demanding. Here we report a femtosecond optical parametric oscillator (OPO) at 79.6 MHz repetition rate based on MgO-doped periodically poled LiNbO3 (MgO:PPLN), synchronously pumped by the integrated second harmonic radiation of a femtosecond fiber laser at 532 nm. The signal delivered by the single resonant OPO is continuously tunable from 757 to 797 nm by tuning the crystal temperature in a poling period of . The output signal shows good beam quality in TEM00 mode profile with pulse duration of 206 fs at 771 nm. Maximum output signal power of 71 mW is obtained for a pump power of 763 mW and a low pumping threshold of 210 mW is measured. Moreover, grating tuning and cavity length tuning of the signal wavelength are also investigated. [ABSTRACT FROM AUTHOR]

The advent of multiplexing interferometric schemes for fibre Bragg grating sensor interrogation has enabled the combination of largescale multiplexing and high-resolution measurement. Described is a simple technique to add absolute wavelength measurement capabilities to any passive-interferometric multiplexer. Through simulation a wavelength accuracy of < 10 pm using common system specifications is demonstrated. The technique allows high-resolution interferometric interrogation to be applied to many key industrial applications requiring precise knowledge of absolute values, such as static temperature or strain, without trading-off high interrogation rate or resolution. [ABSTRACT FROM AUTHOR]

Over the last 20 years quantum walks (QWs) have gained increasing interest in the field of quantum information science and processing. In contrast to classical walkers, quantum objects exhibit intrinsic properties like non-locality and non-classical many-particle correlations, which renders QWs a versatile tool for quantum simulation and computation as well as for a deeper understanding of genuine quantum mechanics. Since they are highly controllable and hardly interact with their environment, photons seem to be ideally suited quantum walkers. In order to study and exploit photonic QWs, lattice structures that allow low loss coherent evolution of quantum states are demanded. Such requirements are perfectly met by integrated optical waveguide devices that additionally allow a substantial miniaturization of experimental settings. Moreover, by utilizing the femtosecond direct laser writing technique three-dimensional waveguide structures are capable of analyzing QWs also on higher dimensional geometries. In this context, advances and findings of photonic QWs are discussed in this review. Various concepts and experimental results are presented covering, such as different quantum transport regimes, the Boson sampling problem, and the discrete fractional quantum Fourier transform. [ABSTRACT FROM AUTHOR]

Direct UV writing is a planar fabrication process capable of simultaneously defining waveguides and Bragg gratings. The technique is fully computer controlled and uniquely uses a small focused spot ~7 μm in diameter for direct writing exposure. This work investigates its use to achieve phase trimming and Bragg grating definition in silica-on-silicon lithographic waveguides. It is observed that birefringence control using direct UV writing can be made independent of exposure fluence with this technique through tailoring substrate stress. The result is demonstrated experimentally and supported theoretically using finite element analysis. [ABSTRACT FROM AUTHOR]

Integrated photonics is a proven platform for physical and chemical sensing. It offers miniaturised solutions that are suited for use in extreme environments, including strong EM-fields, EM-pulses and contact with flammable materials, often far exceeding electronic sensors in this regard. This review looks into direct UV-written planar Bragg grating technology and its application to integrated photonic sensors. The platform has been demonstrated widely for measurement of physical properties such as temperature, pressure and strain. In addition, by using an evanescent interaction, refractive index can be measured allowing for chemical and biochemical detection. Further to this, the platform has recently been utilised in quantum information processing, where quantum gate operations and single photon detection has been shown. [ABSTRACT FROM AUTHOR]

A study of polymer optical fiber microstructuring by use of deep ultraviolet excimer laser radiation at 193 nm wavelength is performed. The ablation characteristics of the fiber cladding and core materials are analyzed comparatively. The laser irradiation effects are dynamically studied by on-line monitoring of the laser ablation induced waveguiding losses, the latter being correlated with the spatial structuring parameters. The fiber surface is modified to incorporate cavities, which are subsequently employed as sensitive material receptors for the development of customized photonic sensors. The sensing capability of the microstructured plastic optical fibers is demonstrated by ammonia and humidity detection. [ABSTRACT FROM AUTHOR]

The dielectric properties of near-stoichiometric LiNbO3:Ce (near-SLN:Ce) single crystal have been investigated by using terahertz time domain spectroscopy (THz-TDS) in a frequency range of 0.7-1.6 THz at room temperature. When coupled with an applied external optical field, a photorefractive effect was observed, resulting in the modulation of the complex dielectric constant for near-SLN:Ce. The variation of the refractive index |Dn| has a linear relationship in scale with the applied light intensity accompanied with an abrupt decrease. These findings were attributed to the internal space charge field of photorefraction and the light-induced domain reversal in the crystal. [ABSTRACT FROM AUTHOR]