Achromatic linear polarization rotator for optical communication wavelength range.

A trend of high speed computing and communication has demonstrated incredible performance increase, 1000x in every 10yrs. Such enhancements have been realized partly due to Moore’s law based and parallelism based progresses [1]. To keep up such a trend for future, we have to integrate photonics on a chip, i.e., Si photonics, especially on-chip WDM (wavelength division multiplexing) architecture [2]. In the history of the fiber optics, WDM has brought significant increase in bandwidth of optical communication and currently the channel spacing in WDM is as small as 0.8 nm and will soon be 0.4 nm. However, Si photonics has used coarse WDM where the channel spacing is as large as 10nm. The reason for the large spacing is a large TO (thermooptics) coefficient of Si. Due to chip temperature fluctuation on a chip, wavelength is changed widely, malfunctioning WDM. This is the fundamental limit in shrinking the wavelength spacing. In other words, the key challenge to implement on-chip WDM is to lock its channel wavelength within a given channel spacing. In this talk we will focus on silicon nitride based Si photonics, i.e., medium index contrast optics, MiDex Si photonics to lock the channel wavelength within the current dense WDM (DWDM) in optical fiber communication, i.e., 0.8nm spacing [3]. Fig. 1 shows bending radii of various waveguides vs. index difference between the core and cladding of the waveguide. Generally, Si waveguide and SiO2 cladding have been used in Si photonics, where index difference is 2.0. The materials set has a high index contrast, i.e., here we will refer it to as HiDex (high index contrast). The advantage is a sharp bending of the waveguide, which reduces device footfprints and allows high density integration of optical devices on a chip. Althougt the disadvantage is large light propagation loss, Si process technology allows to minimize the roughness of waveguide to reduce the loss to ~1dB/cm. It is known however that Si has a large TO coefficient, i.e., temperature dependence of refractive index. It has been reported that chip temperature has changed about 50˚C during its operation. Thus, on-chip temperature fluctuation changes wavelength, malfunctioning of WDM. To avoid it, a large channel spacing, i.e., 10nm has been chosen. We have been focusing a slightly smaller index contrast system in the following two reasons: When we choose SiNx waveguide and SiO2 cladding, the minimum bending radius is 10 µm which is still small enough and the TO coefficient of SiNx is ~1/10 of that of Si. This indicates this materials system should be robust against temperature fluctuation on a chip. Of course Planar Lightwave Circuit based on silica fiber technology has smaller TO coefficient, here we refer it to as LoDex (low index contrast), but the bending radii should be ~1cm. Thus, we have proposed MiDex Si photonics instead of current HiDex Si photonics. SiNx (0.6-0.8 µm thick) can be deposited on SiO2 (2µm thick) using a physical vapor deposition (PVD) technique. Chemical vapor deposition techniques have widely been used, but CVD SiNx contains N-H bonds that absorb 1520nm, that is in optical communication wavelength range of 1550nm. To dissociate H from SiNx, CVD SiNx should be annealed at high temperature such as 1100˚C. To avoid such high temperature annealing, we have chosen PVD SiNx, which should be free from N-H bonds. We have demonstrated SiNx without such absorption. The present report has verified the advantage of PVD SiNx over CVD SiNx, temperature stability of filter characteristics of ring based resonators, and athermalization using polymer materials.

A Thomson scattering light source can produce polarization-controllable x rays with quasi-monochromaticity and high brightness, making it an excellent probe for x-ray imaging and nuclear physics research. In this paper, a clear physical picture of the polarization transfer process from a laser to x rays is given based on a dipole radiation model. For arbitrary interaction geometries between a laser and relativistic electrons, the electric field relation between the scattered x rays and the laser is derived analytically. The result shows that the polarization characteristics of the laser can be completely transferred into x rays, regardless of the interaction geometry. Meanwhile, the polarization of scattered x rays is dependent on the collecting angle. As the collecting angle increases, both the degree of polarization (DOP) and the bandwidth of scattered x rays will be degraded. A collecting angle confining scattered x rays of 5% rms bandwidth can guarantee a DOP higher than 97% for both linear and circular polarizations. A method for any polarization control of scattered x rays is demonstrated by using wave plates for the laser. Both the rotation of linear polarization and the switch of polarization state from linear polarization to circular polarization and from left-handed polarization to right-handed polarization can be easily realized.

In this study, a novel heterodyne linear polarization modulation ellipsometer (LPME) is developed. It is based on a linear polarization rotator and is integrated with a common path heterodyne interferometer for ellipsometric parameter measurement. LPME is an amplitude-sensitive interferometric ellipsometer that can not only provide a wider range for its polarization modulation frequency than that of a conventional photometric ellipsometer but also be used for measurement in real time. LPME is also insensitive to laser intensity fluctuations. The independence of the elliptical polarization to the incident laser beam induced by imperfections in the quarter-wave plate in LPME is analyzed.

The results obtained at the Institute of Atmospheric Optics, Siberian Branch, Russian Academy of Sciences, from 2010 to 2019 regarding problems on atmospheric correction of satellite images of the Earth’s surface and of optical communication based on the scattered laser radiation in the optical wavelength range in the atmosphere and under water are briefly reviewed.

Optical wireless communications have been widely studied during the past decade in short⁃range applications, such as indoor high⁃ speed wireless networks and interconnects in data centers and high⁃performance computing. In this paper, recent developments in high⁃speed short⁃range optical wireless communications are reviewed, including visible light communications (VLCs), infrared in-door communication systems, and reconfigurable optical interconnects. The general architecture of indoor high⁃speed optical wire⁃ less communications is described, and the advantages and limitations of both visible and infrared based solutions are discussed. The concept of reconfigurable optical interconnects is presented, and key results are summarized. In addition, the challenges and potential future directions of short⁃range optical wireless communications are discussed.

An LMI-based discrete time nonlinear observer for Light-Emitting Diode optical communication

Influence of width of left well in AlxGa1-xN/GaN double quantum wells (DQWs) on absorption coefficients and wavelengths of the intersubband transitions (ISBTs) is investigated by solving the Schrödinger and Poisson equations self-consistently. When the width of left well is 1.79nm, three-energy-level DQWs are realized. The ISBT between the first odd and second odd order subbands (the 1odd-2odd ISBT) has a comparable absorption coefficient with the 1odd-2even ISBT. Their wavelengths are located at 1.3 and 1.55μm, respectively. When the width of left well is 1.48nm, a four-energy-level DQWs is realized. The calculated results have a possible application to ultrafast two-colour optoelectronic devices operating within the optical communication wavelength range.

A study on a four-energy-level system in asymmetric AlN/GaN coupled double quantum wells has been performed by solving Schrödinger and Poisson equations self-consistently. It is found that the transition selection rule is recovered when the first two subband pairs resonate in the four-energy-level system. The anticrossing gap between the second excited state (2odd) and the third excited state (2even) can be up to 135 meV when the Al composition of the central barrier is 0.80. The absorption coefficient of intersubband transition (ISBT) between the ground state (1odd) and the 2even subband is approximately equal to that between the first excited state (1even) and the 2odd subband. The wavelengths of the 1odd-2even and the 1even-2odd ISBTs are 1.31 and 1.55 μm, respectively. The results give possible application to the ultrafast two-color optoelectronic devices operating within optical communication wavelength range.

In this paper, a high-efficiency and broadband reflective linear polarization rotator based on anisotropic metamaterial is proposed, which is verified by simulation and experiment. Simulated results indicate that our design can achieve 90° polarization rotation from 5.7 to 10.3 GHz with the relative bandwidth of 57.5 %, which is agreement well with experiment. The further simulated results indicate that our design can achieve linear polarization conversion or rotation by 90° under oblique incident angles with large range for both transverse electric and transverse magnetic waves. Finally, the amplitude and phase of reflective coefficients with different polarization, and surface current distribution of the unit cell structure are simulated to explain the physics mechanism of the high-efficiency and broadband polarization rotation. Our design will provide an important reference for the practical applications of the metamaterial in polarization manipulation.

We have proposed a multi-layered chiral nanostructure consisting of two sub-wavelength gratings and a T resonator sandwiched on two sides of dielectric substrates. Numerical simulation results indicate the scheme can realize large and broadband asymmetric transmission (AT) of linearly polarized waves, with huge asymmetric parameter similar to optical diode and broadband almost 90° polarization rotation in the near-infrared spectral region. Due to the delicate combination of broadband polarization conversion and Fabry–Perot-like resonance effect, the AT magnitude can reach a maximum value of 0.96 and exceed 0.9 ranging from 83 to 233 THz, exhibiting a magnitude improvement compared with previously reported chiral metamaterials. Especially, the mechanism of high AT magnitude in a multi-layered chiral structure is analyzed in detail based on the Fabry–Perot-like resonance effect. By scaling down the structure parameters properly, the structure can also achieve broadband AT in visible region. These metamaterials have a great potential to be used as linear polarization rotators or diode-like devices in the near-infrared and visible regions.

Nanophotonic wavelength routers, which can separate and steer different incident optical wavelengths into different output ports, play a key role in many applications of integrated photonic devices. We design and experimentally demonstrate ultrasmall broadband wavelength routers using an intelligent algorithm that combines a genetic algorithm and the finite element method. The size of the device is only 1.4 μm×1.8 μm, around the optical communication range, and is the smallest one demonstrated experimentally. The maximum transmission is 98% in the simulation and 71% in the experiment. Moreover, we show that various wavelength routers with different materials (both dielectric and metal), different structures, different output ports, and different operation bands can be conveniently designed using the intelligent algorithm. The average position error tolerance for each cell structure is about ±20 nm for all the wavelength routers designed by our intelligent algorithm, which is possible with current nanofabrication technology. This work provides a universal platform for the realization of nanophotonic wavelength routers, and enables the design and integration of nanophotonic devices.

11 - Rare-earth-doped chalcogenide glass for lasers and amplifiers

Infrared emitters are reasonably rare in solution-processed materials. Recently, research into hybrid organo-lead halide perovskite, originally popular in photovoltaics,1-3 has gained traction in light-emitting diodes (LED) due to their low-cost solution processing and good performance.4-9 The lead-based electroluminescent materials show strong colorful emission in the visible region, but lack emissive variants further in the infrared. The concerns with the toxicity of lead may, additionally, limit their wide-scale applications. Here, we demonstrate tunable near-infrared electroluminescence from a lead-free organo-tin halide perovskite, using an ITO/PEDOT:PSS/CH3NH3Sn(Br1-xIx)3/F8/Ca/Ag device architecture. In our tin iodide (CH3NH3SnI3) LEDs, we achieved a 945 nm near-infrared emission with a radiance of 3.4 W sr(-1) m(-2) and a maximum external quantum efficiency of 0.72%, comparable with earlier lead-based devices. Increasing the bromide content in these tin perovskite devices widens the semiconductor bandgap and leads to shorter wavelength emissions, tunable down to 667 nm. These near-infrared LEDs could find useful applications in a range of optical communication, sensing and medical device applications.

All-angle negative refraction of electromagnetic waves [1, 2] has generated great interest because it provides the foundation for a wide range of new electromagnetic effects and applications, including subwavelength image formation [2] and a negative Doppler shift [1], as well as novel guiding, localization and nonlinear phenomena [3, 4]. There has been tremendous progress in achieving negative refraction in recent years using either dielectric photonic crystals [5–9] or metallic meta-materials [10–17]. For either approach, however, there is an underlying physical length scale that sets a fundamental limit [18]. Below such a length scale, the concept of an effective index no longer holds. For photonic crystals, it is the periodicity, which is smaller than but comparable to the operating wavelength of light [8]. For metallic meta-materials, it is the size of each individual resonant element. In the microwave wavelength range, constructing resonant elements that are far smaller than the operating wavelength is relatively straightforward. As one pushes towards shorter optical wavelengths, however, it becomes progressively more difficult to construct resonant elements at a deep subwavelength scale [15]. Moreover, in the optical wavelength range, the plasmonic effects of metals become prominent. The strong magnetic response of metallic structures, as observed in microwave and infrared wavelength ranges, may be fundamentally affected. It is therefore very desirable to accomplish all-angle negative refraction using structures that are flat at an atomic scale.

With the advent of new optical materials a host of services can be provided through optical device technology which include voice, data, and visual communication. The realization of low loss optic fibers provide the option of long distance optical communication and the growth of information processing through computer technology by optical means is bound to occur. Heavy metal fluoride glasses have proved to be excellent hosts for both rare earth and 3d transition metal ions. At present the potential of the heavy metal fluoride glasses for ingrared transmission light guides is unexcelled with ultralow losses of the order of l0 db/km a distinct possibility if trace impurities can be removed. One potential complication of the utilization of these materials is the effect of radiation damage. Irradiation could produce defects detrimental to long range optical communication or laser action. Therefore a detailed study of irradiation effects in these materials is important. The purpose of this paper is to review some of the excellent work that has been done in this area and to provide a background on radiation effects in heavy metal fluoride glasses.