Back Reflection Research Articles

Travelling-wave Mach-Zehnder modulators functioning as optical isolators.

On-chip optical isolators not requiring the use of magneto-optical materials has become a long-standing challenge in integrated optics. Here, we demonstrate that a traditional travelling-wave modulator can effectively function as an optical isolator, when driven under a prescribed modulation condition. By using an off-shelve lithium niobate modulator, we achieve more than 12.5 dB isolation over an 11.3-THz bandwidth at telecommunication wavelengths with a fiber-to-fiber insertion loss of 5.5 dB, by employing only a single radio-frequency drive signal. We also verify that the proposed active isolator can be functional in a laser system to effectively prevent instability due to strong back reflections. Since travelling-wave modulators are common devices in III-V and silicon photonics, our simple but efficient architecture may provide a practical solution to non-reciprocal light routing in photonic integrated circuits.

Fabrication of hierarchical photonic nanostructures inspired by Morpho butterflies utilizing laser interference lithography

We introduce laser interference lithography (LIL) as a tool to fabricate hierarchical photonic nanostructures inspired by blue Morpho butterflies. For that, we utilize the interference pattern in vertical direction in addition to the conventional horizontal one. The vertical interference creates the lamellae by exploiting the back reflection from the substrate. The horizontal interference patterns the ridges of the hierarchical Christmas tree like structure. The artificial Morpho replica produced with this technique feature a brilliant blue iridescence up to an incident angle of 40°.

Influence of the orthogonally polarized back reflections on the power and radiation spectrum of superluminescent diodes

We have investigated the back reflections influence on the spectrum for optical radiation source of superluminescent diode type and have provided optimal operating conditions of the radiation source. The feature of the research method is the usage of a fiber polarization controller and an optical mirror coated on the end of an optical fiber. The studies were conducted with two sources of optical radiation: ThorLabs superluminescent diode series S5FC1005SXL and LED module ELED-1550-1-E-9-SM1-FA-CW. It was revealed that at the value of back reflections equal to -13 dB relative to the output power source, a negative impact on power and spectral characteristics of the source with an optical power of 2.3 µW is beginning to appear. It was also confirmed that at the increase of the radiation power by increasing the source pumping current, back reflection influence is exhibiting at a lower level of back reflections. The results obtained need to be considered when designing fiber optic sensors in order to eliminate the effect of back reflections on the sources of optical radiation having been studied in this paper.

Backward-propagating MeV electrons in ultra-intense laser interactions: Standing wave acceleration and coupling to the reflected laser pulse

Laser-accelerated electron beams have been created at a kHz repetition rate from the reflection of intense (∼1018 W/cm2), ∼40 fs laser pulses focused on a continuous water-jet in an experiment at the Air Force Research Laboratory. This paper investigates Particle-in-Cell simulations of the laser-target interaction to identify the physical mechanisms of electron acceleration in this experiment. We find that the standing-wave pattern created by the overlap of the incident and reflected laser is particularly important because this standing wave can “inject” electrons into the reflected laser pulse where the electrons are further accelerated. We identify two regimes of standing wave acceleration: a highly relativistic case (a0 ≥ 1), and a moderately relativistic case (a0 ∼ 0.5) which operates over a larger fraction of the laser period. In previous studies, other groups have investigated the highly relativistic case for its usefulness in launching electrons in the forward direction. We extend this by investigating electron acceleration in the specular (back reflection) direction and over a wide range of intensities (1017–1019 W cm−2).

Highly efficient and perfectly vertical chip-to-fiber dual-layer grating coupler.

A novel high-efficiency silicon-chip-to-fiber grating coupler is investigated here. By introducing a dual layer grating structure with an inter-layer lateral shift to mimic 45° tilted mirror behavior, perfectly vertical coupling is successfully demonstrated. Our numerical results show that a peak silicon-chip-to-fiber coupling efficiency about 70% is possible near 1550 nm. Meanwhile, for the entire telecom C-band, i.e. wavelengths from 1530 nm to 1565 nm, the coupling efficiency is > 50% and the back reflection is less than < 1%. Our proposed high-performance silicon perfectly vertical coupling structure is suitable for interfacing with multi-core fiber platform, which may play an important role in the future CMOS photonic integration technology.

OS0410-317 鋳鉄配管の減肉評価のための広帯域超音波計測システムの開発

Cast iron with inner mortal coat pipe is the most important and popular component in Japanese main water supply system. However these pipes are difficult to monitor the corrosion loss in thickness because of the lack of a nondestructive testing method. For example, the large acoustic attenuation for damaged mortal and cast iron cause the SN ratio in measurement worse. In this study, we have developed the special wideband ultrasonic transducer and also developed the effective high voltage new ultrasonic pulser for the monitoring of the corrosion loss in thickness for aging cast iron with inner mortal coat pipe. Accuracy for this measurement system for thickness sizing and the repeatability of back surface reflection echo were also investigated.

Diamond Window Diagnostics for Nuclear Fusion Applications – Early Concepts

The main problem, that has so far risen in the field of thermonuclear fusion, regards the plasma instability and the consequent disruptions that occur afterwards. This leads to pulse interruption and reaction extinguishment. The next generation power plants will therefore require an Electron Cyclotron Heating and Current Drive system (ECH&CD), capable of injecting high power microwave beams where properly needed, in order to destroy magnetic islands and instabilities inside the vacuum vessel. Such a system must be sealed to maintain the vacuum inside the vessel and to act as a barrier for tritium present during pulse operation. A nearly perfect transmission of the microwaves beams used by the ECH&CD system is mandatory: at MW power levels, even the smallest back reflections might severely damage the gyrotrons, which are responsible for the beam

Optimizing an Active Solar still for Sea Water Desalination

Sea water desalination through solar radiation distillation process can achieve low cost and sustainable fresh water for remote dry areas. In conventional passive solar stills, the solar radiation passes through the transparent cover and supplies heat to sea water with limited back reflection. The evaporative heat transfer between the water surface and the glass cover produces the distillate by means of film type condensation at the inner surface of the glass cover. In order to enhance evaporation/condensation phase changes, active solar stills were introduced. In these last, saline water is circulated and put in contact with a heat source which supplies heat to the saline water. With this extra energy, the distillate productivity is increased. In this work, heat supply is assumed to be controlled such that the temperature at the inlet of the still can be adjusted through regulation of the circulating heat transfer fluid rate. Using a modelling based on uniform temperature in each still component, a set of ordinary differential equations was derived. The input variables comprised heat transfer fluid rate, inlet temperature as well as sea water rate and basin depth. Extensive parametric studies were performed after that and optimization of the distilled water yield and rate was discussed.

Lossy waveguide design considering polarization dependency to reduce back reflection in 2 × 1 MMI combiners.

We propose a novel structure that includes two compact, simply structured, and lossy waveguides for reducing back reflection in MMI combiners. The preferred lossy waveguide consists of a bend section and a tapered section. Theoretical calculations and 2D FDTD analysis were used to confirm the properties of our proposed structure. Significantly and interestingly, for TE modes, the optimized bend radius is about 7.5 µm and the specific back reflectance depends on taper end width. For TM modes, to achieve a back reflection value smaller than -30 dB, the taper length of 30 µm is desired regardless of bend radius. Moreover, the introduction of the lossy waveguide influences neither the MMI design nor its operation.

Broadband enhancement of optical absorption in a silicon nanowire flexible thin film

A silicon nanowire (SiNW) array was embedded into a polydimethylsiloxane matrix to fabricate a flexible thin film solar cell in which a rugged metallic back surface was formed at the bottom. Superior light scattering of the randomly arrayed SiNWs significantly improved the light absorptance in a short wavelength region (λ<700nm). The rugged morphology of metallic back surface excited the surface plasmon polaritons (SPPs) along the interface between the metal and Si, which showed a plasmonic potential to enhance light absorption in a long wavelength region (λ>700nm). This feature was attributed to the three major routes for light trapping: back reflection, SPP resonance, and SPP scattering. This nanowire thin film showing the rugged back surface yielded the light absorption of ~92.6% using only ~5% of silicon required for conventional crystalline solar cells.

Growth and characterization of CaCu3Ti4O12 single crystals

The CaCu3Ti4O12 (CCTO) single crystals could be grown from the melt with the nominal composition of Ca:Cu:Ti=1:59:20 in a platinum (Pt) crucible using a self-flux method. The flux-grown CCTO single crystals have well-developed {100} habit planes, and their compositions are close to the ratio of Ca:Cu:Ti=1:3:4. Interestingly, flux-grown CCTO single crystals exhibited two different back reflection Laue patterns; one exhibited only [100] cubic Laue patterns, and the other showed not only [100] cubic Laue patterns but also the satellite spots related to the twin boundary, implying that twin-free CCTO single crystals can be grown by the self-flux method. Both the dielectric constants and losses of twinned CCTO single crystal are significantly higher than those of untwined CCTO crystal at relatively low frequency regime (<10kHz), suggesting that the dielectric property is sensitive to the twin boundary.

Effect of directional dependency of wall reflectivity and incident concentrated solar flux on the efficiency of a cavity solar receiver

Managing the optical properties of a cavity solar receiver to create spectral and directional selectivities is a solution to improve receiver efficiencies. A reduction in the incident solar power lost by reflection and by emission in a solar receiver allows the absorption of the solar flux to be maximized. This report investigates the influence of the cavity walls directional reflectivity on the thermal radiative efficiency of a cubic cavity solar receiver. A Monte Carlo ray-tracing method is used to calculate the power lost by reflections and by emission with respect to the incident radiation angular distribution and the bidirectional reflectance distribution function of the cavity walls. To study the influence of the directional dependency of the incident flux on the radiative efficiency, four patterns are considered: collimated, diffuse, focused, and Themis incidences. The directional-hemispherical reflectivity for the bottom wall (face to aperture) and lateral walls are distinguished. For diffuse walls, the absorption efficiency is primarily affected by the lateral walls reflectivity because of the back reflection losses. For specular walls, the driving parameter is the bottom wall reflectivity. In addition, the radiative efficiency with thermal emission was studied for the Themis configuration and a slightly weakest dependency of the efficiency on the lateral walls reflectivity was found.

Fully etched apodized grating coupler on the SOI platform with -0.58 dB coupling efficiency.

We design and fabricate an ultrahigh coupling efficiency (CE) fully etched apodized grating coupler on the silicon-on-insulator (SOI) platform using subwavelength photonic crystals and bonded aluminum mirror. Fabrication error sensitivity and coupling angle dependence are experimentally investigated. A record ultrahigh CE of -0.58 dB with a 3 dB bandwidth of 71 nm and low back reflection are demonstrated.

Focusing sub-wavelength grating couplers with low back reflections for rapid prototyping of silicon photonic circuits

We demonstrate fully-etched fiber-waveguide grating couplers with sub-wavelength gratings showing high coupling efficiency as well as low back reflections for both transverse electric (TE) and transverse magnetic (TM) modes. The power reflection coefficients for the TE and TM modes have been significantly suppressed to -16.2 dB and -20.8 dB, respectively. Focusing grating lines have also been used to reduce the footprint of the design. Our sub-wavelength grating couplers for the TE and TM modes show respective measured insertion losses of 4.1 dB and 3.7 dB with 1-dB bandwidths of 30.6 nm (3-dB bandwidth of 52.3 nm) and 47.5 nm (3-dB bandwidth of 81.5 nm), respectively.

Non-contact small animal fluorescence imaging system for simultaneous multi-directional angular-dependent data acquisition.

We present a novel non-contact small animal fluorescent molecular tomography (FMT) imaging system. At the heart of the system is a new mirror-based imaging head that was designed to provide 360-degree measurement data from an entire animal surface in one step. This imaging head consists of two conical mirrors, which considerably reduce multiple back reflections between the animal and mirror surfaces. These back reflections are common in existing mirror-based imaging heads and tend to degrade the quality of raw measurement data. In addition, the introduction of a novel ray-transfer operator allows for the inclusion of the angular dependent data in the image reconstruction process, which results in higher image resolution. We describe in detail the system design and implementation of the hardware components as well as the transport-theory-based image reconstruction algorithm. Using numerical simulations, measurements on a well-defined phantom and a live animal, we evaluate the system performance and show the advantages of our approach.

Physics of the conical broadband terahertz emission from two-color laser-induced plasma filaments

We propose a comprehensive physical model explaining the conical character of broadband terahertz generation from femtosecond two-color laser-induced air plasma filaments. We show, that, in contrast to other models, emission is always conical, resulting from phase matching of the radiation produced inside the filament combined with a partial back reflection of the generated terahertz field from the filament itself due to the frequency-dependent critical plasma density. The obtained conical angle varies from 2${}^{\ensuremath{\circ}}$ to 10${}^{\ensuremath{\circ}}$, depending on the plasma density distribution and filament length. Unlike previously proposed models, our model shows good agreement with our experiments as well as a wide range of experimental findings from the literature.

Fast operational wavelength initialization technique of wavelength division multiplexed‐passive optical network based on tunable laser diode using back reflection of tunable filtered light

ABSTRACTA new wavelength initialization technique for wavelength division multiplexed‐passive optical network system is proposed and verified experimentally in this article. The proposed system is implemented using the back‐reflection of tunable filtered light over optical access link. At first, a wavelength band for allocated channel is determined using the power difference between two filtered lights with the help of two different optical tunable filters. Next, an accurate operational wavelength is obtained in the determined wavelength band using the back‐reflection of dedicated light path in the determined wavelength band. Experimental results show that the required time for wavelength initialization is measured below 3 s using the proposed technique. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:1084–1087, 2014

Polarization-Controlled TIRFM with Focal Drift and Spatial Field Intensity Correction

Total internal reflection fluorescence microscopy (TIRFM) is becoming an increasingly common methodology to narrow the illumination excitation thickness to study cellular process such as exocytosis, endocytosis, and membrane dynamics. It is also frequently used as a method to improve signal/noise in other techniques such as in vitro single-molecule imaging, stochastic optical reconstruction microscopy/photoactivated localization microscopy imaging, and fluorescence resonance energy transfer imaging. The unique illumination geometry of TIRFM also enables a distinct method to create an excitation field for selectively exciting fluorophores that are aligned either parallel or perpendicular to the optical axis. This selectivity has been used to study orientation of cell membranes and cellular proteins. Unfortunately, the coherent nature of laser light, the typical excitation source in TIRFM, often creates spatial interference fringes across the illuminated area. These fringes are particularly problematic when imaging large cellular areas or when accurate quantification is necessary. Methods have been developed to minimize these fringes by modulating the TIRFM field during a frame capture period; however, these approaches eliminate the possibility to simultaneously excite with a specific polarization. A new, to our knowledge, technique is presented, which compensates for spatial fringes while simultaneously permitting rapid image acquisition of both parallel and perpendicular excitation directions in ∼25 ms. In addition, a back reflection detection scheme was developed that enables quick and accurate alignment of the excitation laser. The detector also facilitates focus drift compensation, a common problem in TIRFM due to the narrow excitation depth, particularly when imaging over long time courses or when using a perfusion flow chamber. The capabilities of this instrument were demonstrated by imaging membrane orientation using DiO on live cells and on lipid bilayers that were supported on a glass slide (supported lipid bilayer). The use of the approach to biological problems was illustrated by examining the temporal and spatial dynamics of exocytic vesicles.

Thermally reconfigurable device for adaptive reflection suppression on a silicon-on-insulator platform

We demonstrate a compact, thermally reconfigurable reflection suppressor on a silicon-on-insulator (SOI) platform, without reliance on nonreciprocal mechanisms. A reflection suppression ratio of 40 dB is achieved with a footprint of 105 μm in length. The insertion loss of the device is below 0.15 dB, and its total power consumption stays below 20 mW. The operation bandwidth depends on the frequency dependence of the back reflection going into the suppressor, which is predominantly determined by the distance between the device and the source of reflection. In this work, a 20 dB reflection suppression bandwidth of 20.7 GHz was achieved.

Transmission enhancement of sharply bent nanoplasmonic slot waveguides

The bending performance of standard nanoplasmonic slot waveguides is substantially improved by reducing the leakage loss and reflection in a systematic way. The out-of-plane power leakage is suppressed by using square metallic patches above and below the bend. The in-plane power leakage is reduced by elimination of the asymmetry between the electrical length of the inner and outer corners of the bend. The unwanted reflection is decreased by changing the shape of the outer corner of the bend. The partial reflections caused by the indentations of the modified shape destructively interfere with each other and thereby decrease the unwanted back reflection.