Passive Reflector Research Articles

Electrically pumped heterogeneously integrated Si/III-V evanescent lasers with micro-loop mirror reflector

An electrically pumped heterogeneously integrated Si/AlGaInAs evanescent laser with micro-loop mirror (MLM) as high reflectors at both ends is experimentally demonstrated. Finite-difference time-domain simulation shows that 98% reflectivity can be achieved with micro-loop mirror formed by single-mode silicon-on-insulator (SOI) waveguides. The laser based on a Si/III-V hybrid gain waveguide and passive SOI MLM reflectors is fabricated and single-mode continuous-wave (CW) lasing is achieved at room temperature with a lasing threshold current density of 2.5 kA/cm2.

Targeting the Moon

Microwave engineers and engineers in general tend to be in their profession be cause they like technical challenges. Historically, one of the biggest challenges has been how to extend radio communications, especially at microwave frequencies, beyond the horizon. One way to achieve this end at microwaves is to bounce radio signals off the Moon. This form of propagation is known as moonbounce or Earth-Moon-Earth (EME) and is considered the ultimate in long-distance 50-MHz-and-above communications. The Moon is approximately 385,000 km (240,000 mi) from the Earth, and reflecting signals from it allows communication between any two points. Because the Moon is so far away and not a particularly good reflector (about 7% efficient), communication using it as a passive reflector is not easy and considered by many a challenge. This article will discuss EME in more detail. It will cover some of the history of this form of propagation, technical obstacles to success and their solutions, and who is using the Moon today.

Application of passive acoustic reflectors to mitigate toothed whale depredation on longlines.

Indo‐Pacific longline fisheries have noted for decades that entangled fish, particularly those with swallowed hooks or with elements of metal in the entangling gear, were invariably spared from depredation attacks. To enhance the capability of fishery objects to either highlight their presence as foreign or to interfere with the clarity of returning echoes, a range of sonar reflective material logistically acceptable to the fishing industry was modeled. Small alloy spheres were shown to have target strengths equivalent to that expected of the TS of an entire tuna from a rear approach, the normal approach on hooked swimming tuna during depredation events. The performance of reflectors is governed by the ratio of the dimensions of the object to the wavelength of the toothed whale broadband echo click. Known changes in hearing and sonar focus with increasing toothed whale age must be factored into a fishery based approach. Spheres developed for the Coral Sea were trialed by Japan Fisheries Research Agency. L...

Identification of Nearshore Wave Characteristics Using Robotic Total Stations

A new geodetic method for accurate determination of nearshore wave characteristics is proposed. This method is based on a robotic total station set on stable position on the coast and aiming at a passive reflector mounted on a buoy, usually at a distance of a few hundred meters. This method permits the identification of the changing three-dimensional coordinates of the reflector/buoy in a selected, fixed coordinate system independent of the buoy with centimeter accuracy, and a rate of up to 6 Hz, and it can even determine very precisely waveforms, including those with steep sides. The effectiveness of the method is demonstrated on the basis of results of two cases studies. While this method provides exceptional quality results for small waves, it can also be used for large waves as well, and especially to calibrate other wave measurement techniques. Techniques to avoid clipping effects usually affecting the wave measurements and the possibility to measure the wave and current direction are also discussed.

Accuracy of automatic couch corrections with on‐line volumetric imaging*

The purpose of this study was to characterize automatic remote couch adjustment and to assess the accuracy of automatic couch corrections following localization with cone‐beam CT (CBCT). Automatic couch movement was evaluated through passive reflector markers placed on a phantom, tracked with an optical tracking system (OTS). Repeated couch movements in the lateral, cranial/caudal, and vertical directions were monitored through the OTS to assess velocity and response time. In conjunction with CBCT, remote table movement for patient displacements following initial setup was available on four linear accelerators (Elekta Synergy). After the initial CBCT scan assessment, patients with isocenter displacements that exceeded clinical protocol tolerances were corrected using remote automatic couch movement. A verification CBCT scan was acquired after any remote movements. These verification CBCT datasets were assessed for the following time periods: one month post clinical installation, and six months later to monitor remote couch correction stability. Residual error analysis was evaluated using the verification scans. The mean ± standard deviations (μ±σ) of couch movement based on phantom measurements with the OTS were 0.16±0.48mm,0.32±0.30mm,0.11±0.12mm in the L/R, A/P, and S/I couch directions, respectively. The fastest maximum velocity was observed in the inferior direction at 10.5 mm/s, and the slowest maximum velocity in the left direction at 3.6 mm/s. From 1134 verification CBCT registrations for 207 patients, the residual error for each translational direction from each month evaluated are reported. The μ was less than 0.3 mm in all directions, and σ was in the order of 1 mm. At a 3 mm threshold, 21 of the 1134 fractions (2%) exceeded tolerance, attributed to patient intrafraction movement. Remote automatic couch movement is reliable and effective for adjusting patient position with a precision of approximately 1 mm. Patient residual error observed in this study demonstrates that displacement is minimal after remote couch adjustment.PACS number: 87.55.Qr, 87.56.bd, 87.57.Q

Effects of incident field refraction on scattered field from vertically extended cylindrical targets in range-dependent ocean waveguides

The effect of incident field refraction on the scattered field from vertically extended cylindrical targets is investigated. A theoretical model for the total scattered field from a cylindrical target in a range-dependent ocean waveguide is developed from Green's theorem. The locally scattered field on the target surface is estimated as a function of the incident field by applying the appropriate boundary conditions on continuity of acoustic pressure and normal velocity, making the model applicable to general penetrable cylinders. The model can account for depth dependence in medium sound speed and hence refraction in the incident field along the target depth. Numerical implementation is done for a passive acoustic reflector, a long cylindrical air-filled rubber hose, often deployed vertically in experiments to provide calibration and charting consistency for wide-area active sonar systems. Analysis with the model indicates that refraction in the incident field along the target depth must be taken into account to accurately estimate the scattered field level from vertically extended cylindrical targets. It is demonstrated that the standard Ingenito waveguide target scattering model, which assumes that the incident field is planar along the target extent, can lead to significant errors of 10 dB or more in estimating the scattered field level.

A study of roadside passive reflectors for improvement in the radar reliability on curves and at intersections

The use of radar in automotive applications such as adaptive cruise control is limited to detecting target vehicles directly in front of the host vehicle. Vehicles around a curve on a highway and cross-traffic vehicles at an intersection cannot be detected by current radar systems. This is primarily due to the limited beam width angle of the radar. This paper examines and evaluates the novel concept of using passive roadside reflectors (PRRs) to increase the utility of the radar system. PRRs on the shoulder and the median of a road are designed which would enable the radar system to pick up cross traffic at highway intersections. Both simulations and experiments demonstrate that this concept could be effectively used for cross-traffic distance measurement. A limitation of this technology is that cross traffic cannot be detected when the radar-equipped vehicle is close to the intersection, owing to the lateral offset of the radar from the location of the PRR. However, the system is still valuable for providing future warning of cross traffic at rural intersections. A number of options are explored for the use of PRRs to improve radar measurements on highway curves. These include the use of tangential flat-plate reflectors, skewed flat-plate reflectors, and convex reflectors. Algorithms for distance measurement with each of these options are derived and evaluated through simulations. The use of skewed flat-plate reflectors is also evaluated experimentally and found to work effectively. The effective range of the radar on highway curves could be doubled by the use of the developed system.

Enhanced photonic band edge laser emission in a cholesteric liquid crystal resonator

A highly efficient photonic band edge dye-doped cholesteric liquid crystal (CLC) laser is demonstrated. By sandwiching an active CLC cell within a resonator consisting of two passive CLC reflectors, the lasing efficiency is dramatically enhanced. Theoretical analysis using the improved 4x4 transfer matrix and scattering matrix shows that the band edge laser mode can be supported by the external CLC resonator and its optimal output can be achieved by a relatively thin active CLC layer and thin passive CLC reflectors. Theoretical analysis agrees well with the experimental results.

Enhancing cholesteric liquid crystal laser performance using a cholesteric reflector

A high performance dye-doped cholesteric liquid crystal (CLC) laser is demonstrated by incorporating a passive CLC reflector to the active cell. The polarization-conserved CLC reflector effectively increases the distributed feedback cavity length which, in turn, results in a significant enhancement in lasing efficiency and a dramatic reduction in beam divergence. The lasing characteristics are still dominated by the circularly polarized light with the same sense as the cholesteric helix.

Environmentally associated signal-to-interference variability in low-frequency active sonar in Korea Strait

An analysis has been conducted of spatial and temporal variability of target echo-to-interference measurements made during Area Characterization Test III (ACT III) in 1995 in the Korea Strait. The measurements were made over a 5-day period for five fixed bistatic geometries (spatial scale of order 200 square km) using explosive sources, bottom-mounted horizontal receiving hydrophone arrays, and passive reflector targets in 100-m-depth water under downward-refracting acoustic conditions. The bottom at the ACT III site was nearly flat, of sand–silt composition. Meteorological conditions were relatively calm over the conduction of the measurements, but the main proximate oceanographic feature, the South Korean Coastal Front, did result in a relatively varied and complex sound-speed environment. The signal-to-interference fluctuation statistics associated with a single ping was determined to give a standard deviation of order 1 dB, while the observed variability of signal-to-interference over the spatial and temporal scales of the measurements gave a standard deviation of about 2 dB. The observed variability of received signal-to-interference will be discussed in terms of physical causes and measurement error. [Work supported by ONR Code OA321.]

EpiDRB - ein neues minimal-invasives Konzept für die Referenzierung im Rahmen der Computer-assistierten orthopädischen Chirurgie

Optoelectronic navigation for computer-assisted orthopaedic surgery (CAOS) is based on a firm connection of bone with passive reflectors or active light-emitting diodes in a specific three-dimensional pattern. Even a so-called "minimally-invasive" dynamic reference base (DRB) requires fixation with screws or clamps via incision of the skin. Consequently an originally percutaneous intervention would unnecessarily be extended to an open procedure. Thus, computer-assisted navigation is rarely applied. Due to their tree-like design most DRB's interfere with the surgeon's actions and therefore are at permanent risk to be accidentally dislocated. Accordingly, the optic communication between the camera and the operative site may repeatedly be interrupted. The aim of the research was the development of a less bulky, more comfortable, stable and safely trackable device that can be fixed truly percutaneously. With engineering support of the industrial partner the radiolucent epiDRB was developed. It can be fixed with two or more pins and gains additional stability from its epicutaneous position. The intraoperative applicability and reliability was experimentally tested. Its low centre of gravity and its flat design allow the device to be located directly in the area of interest. Thanks to its epicutaneous position and its particular shape the epiDRB may perpetually be tracked by the navigation system without hindering the surgeon's actions. Hence, the risk of being displaced by accident is minimised and the line of sight remains unaffected. With the newly developed epiDRB computer-assisted navigation becomes easier and safer to handle even in punctures and other percutaneous procedures at the spine as much as at the extremities without an unproportionate amount of additional trauma. Due to the special design referencing of more than one vertebral body is possible at one time, thus decreasing radiation exposure and increasing efficiency.

A focused multi-layered spherical shell with guided wave enhancements

There is a continued need for underwater reflective targets to be used as relocation and navigational aids. Traditionally, fluid-filled thin spherical shells have been used as passive reflectors, similar to cat’s eyes, and would usually be filled with liquid chloroflourocarbons (CFC’s). Other options are needed now that the production of CFC’s has been restricted. This paper looks at the relationship between a system of multi-layered concentric elastic shells and their target strength in water. The problem is formulated using the global matrix method and is applicable to any system with an arbitrary number of layers. Results show that, by using certain combinations of low loss elastic layers together, the target strength is higher than it would be for any one of the materials used in isolation. This result is explained by geometric focussing on the back surface of the sphere and guided elastic waves that circumnavigate the outer layer or shell and enhance the focussing. The results are compared with calculations and existing experiments for a stainless steel shell filled with CFC.

Passive system for speech enhancement

A passive system for speech enhancement in a motor vehicle includes a headliner adapted to be disposed in an occupant compartment of the motor vehicle. The passive system also includes at least one passive sound reflector disposed in or near the headliner and being sound-reflective and shaped to direct or focus speech between front and rear seat occupants in the occupant compartment of the motor vehicle.

All-Optical Decision Gate with an Adjustable Decision Threshold

We investigate the decision threshold adjustment in a proposed all-optical decision gate consisting of two gain-clamped semiconductor optical amplifiers (GCSOA's). The GCSOA discussed in this letter is constructed with a gain section and two passive distributed Bragg reflectors (DBR's) on each side. We derive a model for current-induced or temperature-dependent Bragg reflectance variation, with which the threshold current density and the oscillating power at DBR wavelength are described. This model allows a possible numerical prediction for decision threshold adjustment induced by current injection or temperature variation.

Asynchronous laser transponders for precise interplanetary ranging and time transfer

Satellite laser ranging (SLR) and lunar laser ranging (LLR) systems are single-ended instruments, i.e. they measure the roundtrip transit time of a laser pulse to a passive optical reflector. Since such single-ended systems are incapable of ranging beyond the Moon to the planets, we consider the feasibility of a two-way asynchronous (i.e. independently firing) interplanetary laser transponder pair, capable of decimeter ranging and subnanosecond time transfer from Earth to a spacecraft anywhere within the inner Solar System. After introducing the transponder link equation and the concept of “balanced” transponders, we describe how range and time can be transferred between terminals, and preview the potential advantages of photon counting asynchronous transponders for interplanetary applications. We then develop mathematical models for the various sources of noise in an interplanetary transponder link including planetary albedo, solar or lunar illumination of the local atmosphere, and laser backscatter off the local atmosphere. After introducing the key engineering components of an interplanetary laser transponder, we develop an operational scenario for the acquisition and tracking of the opposite terminal. We then use the theoretical models of the previous sections to perform an Earth–Mars link analysis over a full synodic period of 780 days under the simplifying assumption of coaxial, coplanar, circular orbits. We demonstrate that, using slightly modified versions of existing space and ground based laser systems, an Earth–Mars transponder link is not only feasible but quite robust. We also demonstrate through analysis the potential advantages of compact, low output power (<300 mW), photon-counting transponders, which utilize NASA's developmental SLR2000 satellite laser ranging system as the Earth terminal and offer some concluding remarks regarding future applications.

Radio wave propagation and single-cell coverage prediction of broadband radio access systems including passive reflectors

Radio wave propagation and single-cell coverage prediction of broadband radio access systems including passive reflectors

Dynamics of single mode semiconductor lasers with passive dispersive reflectors

We discuss the impact of a passive dispersive reflector (PDR) on the dynamical behaviour of a single mode diode laser. It is shown that the PDR enters the single mode equations via two specific functions of the carrier density. Depending on the parameters of these functions, the resonance frequency of the laser as well as the damping of its relaxation oscillations become modified. A bifurcation analysis yields both subcritical and supercritical Hopf-bifurcations separating regions of stationary and self-pulsating operation. On this basis the conditions for achieving high self-pulsation frequencies with a large modulation depth are derived.

Circular polarization diversity with passive reflectors in indoor radio channels

Circular polarization diversity with passive reflectors is investigated in an indoor radio channel, and its performance is evaluated with a three-dimensional (3-D) ray-tracing simulation in conjunction with a geometric-optics-based facet approximation. The simulation results show that the use of passive reflectors improves the outage probability of an indoor radio channel for base stations which exploit circular polarization diversity and the diversity gain increases with the reflector size.

Excitation of UHF radio waves in tunnels

This letter deals with the excitation of UHF radio waves in tunnels. It is found that the antenna position and the antenna gain are two major factors that affect the excitation of radio waves in tunnels. The antenna in the middle position and with high gain is preferable. Furthermore, the enhancement of the radio signal levels in the NLOS region of the tunnel by a passive reflector is simulated. ©1999 John Wiley & Sons, Inc. Microwave Opt Technol Lett 22: 408–410, 1999

Excitation of UHF radio waves in tunnels

This letter deals with the excitation of UHF radio waves in tunnels. It is found that the antenna position and the antenna gain are two major factors that affect the excitation of radio waves in tunnels. The antenna in the middle position and with high gain is preferable. Furthermore, the enhancement of the radio signal levels in the NLOS region of the tunnel by a passive reflector is simulated. ©1999 John Wiley & Sons, Inc. Microwave Opt Technol Lett 22: 408–410, 1999