Active Optical Applications Research Articles

Shape Control of a Unimorph Deformable Mirror for Space Active Optics under Uncertainties.

This paper focuses on the change of morphing capabilities for a unimorph deformable mirror impacted by environmental factors, which works in space for active optics applications. Various aspects of disturbing sources are considered, including complex thermal and mechanical conditions on ferroelectric behaviours of strain actuation, and influences of preconfigured initial shapes and stress-induced geometric stiffness on the structural rigidity of the mirror; changes on both the perturbed shape and the Jacobian matrix are discussed. Those variations are regarded as uncertainties in the design of control methods with both open-loop and iterative control strategies tested in the quasi-static range.

Experimental investigation on the laser backscattering characteristics of complicated targets

For active optical imaging applications where targets are irradiated by a laser beam, the backscattering characteristic influences the image quality. An outfield experimental investigation of kilometer scale was conducted in this study. Unmanned aerial vehicles with complex shapes and various surface materials were used as targets. Backscattered signals were obtained, while the targets were located at different attitudes and distances. Furthermore, the relationship between the amplitude of the backscattered signal and the target distance was deduced and summarized at close and far distances. The experimental results can be helpful in detecting complicated targets with laser illumination.

Bi-layered silicon metagratings for period tunability in the near-infrared region

Period tunability of gratings is extremely desirable for reconfigurable optics. In this study, we report a period-tunable metagrating featuring a bi-layered structure of silicon subwavelength gratings to form a Moiré pattern. Our design is based on the near field Moiré effect and employs the leaky mode resonance to enhance the far-field diffraction. The period of the metagrating can be continuously tuned to its double with the in-plane rotation of the silicon subwavelength gratings. Our simulations indicate that the metagrating exhibits efficient polarization-sensitive diffraction during period-tuning near 1550 nm. Potentially, this metagrating with a wide range of period tunability can be instrumental in developing versatile active optical applications.

Advanced Plasticized Electroactive Polymers Actuators for Active Optical Applications: Live Mirror

Herein, an advanced concept to enhance the actuation ability of electroactive polymers (EAPs) based on modified terpolymer P(VDF‐TrFE‐CFE) is proposed. Such a polymer matrix attracts a great deal of attention because of its outstanding electromechanical coupling property, particularly when doped with plasticizers, e.g., diisononyl phthalate (DINP). Herein, it is demonstrated that by optimizing the structure's multilayer design, the electromechanical coupling of the modified terpolymer is enhanced with its high dielectric permittivity, low Young's modulus, and exceptional dielectric strength. This leads to a large strain response as well as a high mechanical energy density at relatively low electric fields according to the electrostriction phenomena. The concept of stacked multilayers is demonstrated as a simple and effective technique to boost the actuation abilities. Experimental results in accordance with numerical models show actuator performance with a large electromechanical response. This technology shows feasibility for active optical surface shape control. The potential of multiple‐stack actuators is tested in a small prototype. This demonstrated mirror optical shape control and correction with a few degrees of freedoms. The proposed Live Mirror technology is useful for ground‐ and space‐based astronomy and communications telescopes.

Optical properties of Nd3+-doped TeO2-TiO2-ZnO glasses with lower hydroxyl content

This paper describes Nd3+-ions doped multicomponent tellurite glasses TeO2-TiO2-ZnO (TTZ) applicable to advanced optical devices, which were characterized by UV–Vis-NIR absorption, Raman, and photoluminescence (PL) techniques. Not only did we perform experimental characterizations but also theoretically predicted the PL properties of Nd3+-doped TTZ glasses by using the Judd-Ofelt theory. The influence of the residual hydroxyl (OH) groups and types of crucible used in the melt preparation on the structures and optical properties of TTZ glasses was investigated. We found that the use of a Pt crucible increased the network-connecting structures of trigonal bipyramidal TeO4 compared to the case of a Al2O3 crucible. Moreover, reducing the OH groups in TTZ glasses improved the optical properties of Nd3+-doped TTZ glass, and a higher spectral figure of merit of 8.71 × 10−24 s cm2 for Nd3+:4F3/2 → 4I11/2 emission at 1075 nm could be achieved, indicating that Nd3+-doped TTZ glass with lower hydroxyl content is promising for laser or active optical applications.

InP-Based Single-Photon Detectors and Geiger-Mode APD Arrays for Quantum Communications Applications

To meet the increasing demand from quantum communications and other photon starved applications, we have developed various InP-based single-photon detectors, including discrete single-photon avalanche diodes (SPADs), negative feedback avalanche diodes (NFADs), and Geiger-mode avalanche photodiode (GmAPD) arrays. A large quantity of InP SPADs have been fabricated. Out of 1000 devices with a 25-μm active area diameter, operated under gated mode at temperature of 233 K, with a pulse repetition rate of 1 MHz and pulse width of 1 ns, the average dark count rate and afterpulsing probability are 30 kHz and 8 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> , respectively. Smaller (16-μm active area diameter) and larger (40-μm active area diameter) discrete devices have been fabricated as well, and their performances are presented along with the 25-μm diameter devices. NFAD devices can operate in free running mode and photon detection efficiency of 10-15% can be achieved without applying any hold-off time externally. When the temperature decreases from 240 to 160 K, the noise equivalent power (NEP) decreasesfrom1.9 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-16</sup> to 1.8 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-18</sup> WHz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1/2</sup> , with the activation energy being 0.2 eV. The very low NEP at 160 K makes NFAD devices an ideal choice for long distance, entanglement-based quantum key distributions. GmAPD arrays provide an enabling technology for many active optical applications, such as 3-D laser detection and ranging (LADAR) and photon starved optical communications. Both 32 × 32 and 128 × 32 GmAPD arrays have been fabricated with high performance and good uniformity. GmAPD focal plane arrays (FPAs) with framed readout mode have enabled very high-performance flash LADAR systems. GmAPD FPAs with asynchronous readout mode will enable high rate quantum key distributions and other quantum communications applications.

Simultaneous compensation of hysteresis and creep in a single piezoelectric actuator by open-loop control for quasi-static space active optics applications

Owing to their excellent properties piezoelectric actuators are studied as embedded elements for the quasi-statically active shape control of spatial optical mirrors. However, unwanted nonlinear effects in piezoelectric actuators, i.e., hysteresis and creep, severely limit their performance. This paper aims at developing a control methodology to compensate hysteresis and creep in a piezoelectric actuator simultaneously for quasi-static space active applications. In the methodology developed, hysteresis and creep behaviors are successively compensated by open-loop control. First, a derivative Preisach model is proposed to accurately portray the hysteresis while requiring relatively few measurements and describing the detachment between major and minor loops. The inverse derivative Preisach model is derived and inserted in open-loop to achieve hysteresis compensation. Then, the creep in the hysteresis compensated piezoelectric actuator is described by the use of a nonlinear viscoelastic model and a low pass filter is suggested to eliminate the effect of the inverse derivative Preisach model on the step reference input. To invert the creep model, the concept of “input relaxation” is implemented and an inverse multiplicative structure allows identifying the parameters of the inverse model while circumventing the difficulty of a mathematical computation. Finally, by cascading the low pass filter, the inverse model of creep and the inverse derivative Preisach model one after the other with the single piezoelectric actuator, the simultaneous compensation of hysteresis and creep is achieved. Experimental results show that in the case of step-like reference signals the hysteresis and the creep in a piezoelectric actuator can be significantly reduced at the same time. It implies that the developed methodology is effective and feasible in space active optics applications for which quasi-static distortions need to be compensated.

Preparation of high purity glasses in the Ga–Ge–As–Se system

Small additions of Ga to Ge–As–Se glasses are known to enhance rare earth ion solubility in Ge–As–Se chalcogenide glasses designed for active optical applications. The effect of variants and conditions for producing samples of an exemplar Ge16As17Se64Ga3 (atomic%) glass on optical transmission, and the content of limiting impurities, is investigated. To prepare the high-purity glass samples, chemical distillation for purification of the Ge–As–Se base-glass is used. Next, a new vapor phase transport approach of metallic Ga transfer in a GaI3 flow is developed to purify and add the batch of metallic gallium into the silica-glass reactor for the Ge–As–Se–Ga glass synthesis. A thermodynamic equilibrium based vapor phase transport model is discussed. In the best examples of these glasses, the content of residual impurities is: hydrogen – 0.15ppm, oxygen – <1ppm, and transition metals – less than 0.1ppm.

Analysis of SBS Gain Shaping and Threshold Increase by Arbitrary Strain Distributions

Fibers with a longitudinal strain distribution can be used to shape the stimulated Brillouin scattering (SBS) gain spectrum or to increase the SBS threshold power. Theoretical analysis and experimental results on the SBS gain spectrum of coiled fibers with a continuous and arbitrary distribution of longitudinal strain are presented. The concept of a critical gain factor for determining the SBS threshold power with a simple formula is reviewed for fibers with different parameters and spectral shapes. A fiber coiling machine for high coiling forces is described for realizing permanent strain distributions with a maximum strain of more than 3%. SBS spectra were broadened to a spectral width of 1.7 GHz. Measurements verify a significant SBS gain suppression by a factor of 40, which is in excellent agreement with the theoretical analysis. Finally, synthetic triangular and broadened Lorentzian SBS spectral shapes with a spectral width of 550 MHz are demonstrated. The results can be used to suppress SBS in high-power fiber lasers and amplifiers or to tailor almost any arbitrary SBS spectral shapes, which could be useful for slow-light or active optical filter applications.

From molecular precursors in solution to microstructured optical fiber: a Sol-gel polymeric route

Solid-core photonic crystal fibers with the core derived from non-doped or Erbium-doped sol-gel silica rods are fabricated. The results demonstrate that the direct polymeric sol-gel route constitutes a promising method to prepare large high quality glass pieces that can be integrated into microstructured optical fibers suitable for passive and active optical fiber applications.

Enhancement of luminescence of Rhodamine B by gold nanoparticles in thin films on glass for active optical materials applications

Fluorescent dyes in solid matrices have many potential applications provided that their high optical efficiencies are achieved. We present here gold nanoparticles formed and incorporated together with fluorescent dye Rhodamine B into a film of polyvinyl alcohol (PVA). The increase of fluorescence of the dye results from its interaction with surface plasmons. The electric charge on the gold nanoparticles and the distance between them and the dye molecules has a significant effect on the fluorescence intensity. Fluorescence enhancement of 74% was achieved for the negatively charged particles. Dynamic measurements reveal decrease of fluorescent lifetimes of the dye in presence of gold nanoparticles. Our findings enable utilization of films with enhanced fluorescence in optical materials such as luminescence solar concentrators, solid state tunable laser and active waveguides.

A parametric study of Er 3+-ions doped Phospho-tellurite glass thin films by pulsed laser deposition

Phospho-tellurite glass thin films have been deposited on silica substrates using an ArF laser ( λ = 193 nm, τ = 20 ns) at a laser fluence of 3.2 J/cm 2 and for various oxygen pressures, substrate temperatures and target-to-substrate distances. The measured propagation losses of the phospho-tellurite glass thin films are in the range of 0.13–0.75 dB/cm, whilst the refractive indices of the glass thin films were in the range of 1.623–1.649 which is very close to that of the target glass, showing nearly stoichiometric transfer of the material from target to substrate. The results show significant promise for integrated active optical device applications using pulsed laser deposition (PLD) of this erbium doped phospho-tellurite glass material.

Synthesis, Ferroelectric and Electrooptic Properties of Transparent Crystallized Glasses with SrxBa1−xNb2O6 Nanocrystals

Crystallized glasses consisting of SrxBa1−xNb2O6 (SBN) nanocrystals (x=0.5, 0.61, and 0.62) with a tetragonal tungsten–bronze structure are synthesized using a conventional glass crystallization technique in xSrO–(32−x)BaO–32Nb2O5–36B2O3 glasses with x=16, 19.5, and 24. Nanocrystals of Sr0.5Ba0.5Nb2O6, Sr0.61Ba0.39Nb2O6, and Sr0.62Ba0.38Nb2O6 are found to have rod‐like shapes with an average diameter of 20–40 nm as determined from X‐ray diffraction analyses and transmission electron microscope observations and to exhibit ferroelectricities as deduced from the presence of hysteresis in polarization–electric field curves. Although the optical transparency of crystallized glasses in the visible region decreases along with the formation of SBN nanocrystals, optical transparencies (sample thickness: ∼1 mm, transmittance: ∼80%) are observed at the wavelength of 1.55 μm. Electrooptic activity was confirmed for the crystallized glasses with SBN nanocrystals, indicating that the present study would be a step for active optical device applications in glass‐related materials consisting of SBN nanocrystals.

Optimization of Low-Loss Al2O3 Waveguide Fabrication for Application in Active Integrated Optical Devices

In this paper we will present the fabrication and properties of reactively co-sputtered Al2O3 layers, being a very promising host material for active integrated optics applications such as rare- earth ion doped laser devices. The process optimization towards a reactive co-sputtering process, which resulted in stable, target condition-independent deposition of Al2O3 layers with high optical quality will be discussed in detail. The loss value of as- deposited optical waveguides sputtered by the optimized process has been measured. The loss in the near infrared wavelength range was 0.3 dB/cm. Furthermore, Al2O3 material hosts fabricated by sputtering techniques are compatible with Si-based integrated optical technology and allow for uniform deposition over a large substrate area.

Distortion optimized focusing mirror device with large aperture

A new concept of a MOEMS based active focusing device has been developed. The device consists of an electrostatically driven membrane-mirror with almost perfect parabolic shape over a very large area and a ring shaped counter electrode, which is part of the chip package. The device has been optimized for a parabolic deflection shape over a large surface area by coupled field FEM simulation. Using SOI-technology, the realization of stress free membranes with a diameter up to 10 mm has been proven. The device can be used in active optical applications where large numerical apertures are needed. Potential applications are e.g. confocal microscopy or scanning applications for focus control.

Characterization of optical properties of nanocrystalline doped PZT thin films

High optical transmittance and refractive index together with strong electro-optic Kerr effect of the ferroelectric lead-zirconate-titanate (PZT) films can be utilized in various active optical applications like optical shutters, modulators and waveguides. Pulsed laser deposition (PLD) with XeCl-excimer laser with a wavelength of 308 nm and Nd-modified PZT target were used for the optical thin film fabrication. The films were deposited on single-crystal MgO (1 0 0) substrates. The crystal structure and grain size distribution were studied using XRD technique. The optical transmission spectra of the films were measured at UV–vis–NIR wavelengths, which was utilized to obtain the refraction index n dispersion, extinction coefficient k, and the value of the band gap E g. Electro-optic coefficients were determined by ellipsometric technique. In the case of polycrystalline films, mean grain size was between 9 and 20 nm. At the wavelength of 633 nm the refractive index varied from 2.28 to 2.46 as a function of mean grain size. Also, the electro-optic coefficient showed dependence on grain size distribution increasing from 0.37 × 10 −18 to 2.49 × 10 −18 m 2/V 2 with increasing mean grain size.

Novel micromachined membrane mirror characterization and closed-loop demonstration

Micromachined electrostatic membrane mirrors have been in use for several years in active and adaptive optics applications. We introduce a deformable mirror with small static aberrations and resistant to damage during electrostatic snap-down in an architecture designed so that a multi-layer dielectric coating that can be applied before the mirror is released. We also make laboratory performance measurements and produce a closed-loop control algorithm.

A STUDY OF NONLINEAR COUPLING BETWEEN TWO IDENTICAL PLANAR WAVEGUIDES

A study of coupling characteristics between two optically nonlinear planar waveguides has been performed in terms of their individual analytic solutions. It is shown that with an appropriate choice of guide separation and proper redefinition of effective propagation constants, the commonly adopted dual waveguide coupled equations can be formally retained even when the optical nonlinearity in each guide is fully taken into account. The coupling coefficients are determined explicitly in terms of the individual mode fields and the associated intensity-dependent effective index of refraction. A specific numeric illustration of the power flow pattern was given on the basis of its analytical expression derived from the coupled equations. The result describes the detailed coupling characteristics and its variation with respect to input optical power, demonstrating its viability for active optical device applications.

Sol–gel derived germanium sulfide planar waveguides

In active integrated optics applications for 1.3 μm involving Pr 3+ doping, a low vibrational energy sulfide-based glass matrix is desirable, in order to increase the quantum efficiency of the Pr 3+ emission. This paper demonstrates the feasibility of preparing germanium sulfide planar waveguides via sol–gel processing. A two-step method was used: (1) germanium oxide gel films were deposited by spin-coating onto single-crystal silicon wafers or silica glass substrates, using germanium ethoxide as the precursor; (2) the gel films were then heat treated in a H 2S atmosphere, under different conditions. Infrared and Raman spectroscopies, X-ray photoemission spectroscopy (XPS), X-ray diffraction and scanning electron microscopy (SEM) were used, in order to characterize the purity and structure of the films. The residual oxide content was found to be low (∼2%). Film thickness and refractive index were measured by mechanical profilometry and ellipsometry. The occurrence of propagating modes and their respective optical losses were also investigated at 633 nm, using either prism-coupling, or butt-coupling methods.

Spectroscopic and laser properties of Nd3+ in fluoride glasses

Abstract In the last few years an increasing interest has arisen in the study of heavy-metal fluoride glasses related to passive and active optical applications materials. Up to now all glass lasers have used trivalent lanthanides as active ions. Neodymium-doped glasses are by far the most widely investigated systems. The spectroscopic properties of Nd3+ are dependent upon the chemical composition of the host glass. In this work we present a brief review of the optical properties of Nd3+ ions in heavy metal and transition metal fluoride glasses. This includes radiative and nonradiative transition probabilities, emission cross-sections, peak wavelengths and linewidths of the 4 F 3 2 → 4 I 11 2 laser transition, and Nd-Nd energy transfer. This is followed by a short discussion on the site-selective spectroscopy of Eu3+ in zirconium-barium-lanthanum and barium-indium-gallium fluoride glasses. Finally, we report a study of the laser properties of Nd3+ doped fluoride glass BlGaZLuTMn (30BaF2-l8InF3-l2GaF3-20ZnF2-9LuF3-6ThF4 -4MnF2-lNdF3 mol% composition) pumped by flashlamps. As far as we know this is the first time laser action has been obtained in bulk in this glass. From these studies the influence of site-dependent effects on the time-resolved stimulated emission properties of Nd3+ ions is also discussed.