Spatially resolved modal spectroscopy of Er:Yb doped multifilament-core fiber amplifier

The spatially resolved spectral (S2) imaging method is applied on an active microstructured fiber, with a multi-filament core (MFC). This type of fiber has been designed to be the last amplifying stage of a source for a long range coherent lidar. Studying the influence of the bending radius on the modal content with or without gain, we demonstrate that an upper-bound of the high-order modes content can be found by performing the S2 imaging on the bleached fiber. S2 imaging is then used to verify that the output beam of the MFC fiber can be made effectively single-mode. We also show that it can be simply adapted for measuring the fiber birefringence. Finally, a comparison of the MFC fiber mode area with that of a standard large mode area Erbium doped step index fiber illustrates the interest of the MFC structure for high power amplifiers.

Shape sensing using multi-core fiber optic cable and parametric curve solutions

The shape of a multi-core optical fiber is calculated by numerically solving a set of Frenet-Serret equations describing the path of the fiber in three dimensions. Included in the Frenet-Serret equations are curvature and bending direction functions derived from distributed fiber Bragg grating strain measurements in each core. The method offers advantages over prior art in that it determines complex three-dimensional fiber shape as a continuous parametric solution rather than an integrated series of discrete planar bends. Results and error analysis of the method using a tri-core optical fiber is presented. Maximum error expressed as a percentage of fiber length was found to be 7.2%.

Performance of large aperture tapered fiber phase conjugate mirror with high pulse energy and 1-kHz repetition rate

A large aperture fused silica tapered fiber phase conjugate mirror is presented with a maximum 70% stimulated Brillouin scattering (SBS) reflectivity, which is obtained with 1 kHz repetition rate, 15 ns pulse width and 38 mJ input pulse energy. To the best of our knowledge, this is the highest SBS reflectivity ever reported by using optical fiber as a phase conjugate mirror for such high pulse repetition rate (1 kHz) and several tens of millijoule (mJ) input pulse energy. The influences of fiber end surface quality and pump pulse widths on SBS reflectivity are investigated experimentally. The results show that finer fiber end surface quality and longer input pulse widths are preferred for obtaining higher SBS reflectivity with higher input pulse energy. Double passing amplification experiments are also performed. 52 mJ pulse energy is achieved at 1 kHz repetition rate, with a reflected SBS pulse width of 1.5 ns and a M(2) factor of 2.3. The corresponding peak power reaches 34.6 MW. Obvious beam quality improvement is observed.

Characterization of 1064nm nanosecond laser-induced damage on antireflection coatings grown by atomic layer deposition

Damage tests are carried out at 1064nm to measure the laser resistance of TiO(2)/Al(2)O(3) and HfO(2)/Al(2)O(3) antireflection coatings grown by atomic layer deposition (ALD). The damage results are determined by S-on-1 and R-on-1 tests. Interestingly, the damage performance of ALD coatings is similar to those grown by conventional e-beam evaporation process. A decline law of damage resistance under multiple irradiations is revealed. The influence of growth temperature on damage performance has been investigated. Result shows that the crystallization of TiO(2) layer at higher temperature could lead to numerous absorption defects that reduce the laser-induced damage threshold (LIDT). In addition, it has been found that using inorganic compound instead of organic compound as precursors for ALD process maybe effectively prevent carbon impurities in films and will increase the LIDT obviously.

Controllable coherent perfect absorption in a composite film

We exploit the versatility provided by metal-dielectric composites to demonstrate controllable coherent perfect absorption (CPA) or anti-lasing in a slab of heterogeneous medium. The slab is illuminated by coherent light from both sides, at the same angle of incidence and the conditions required for CPA are investigated as a function of the different system parameters. Our calculations clearly elucidate the role of absorption as a necessary prerequisite for CPA. We further demonstrate the controllability of the CPA frequency to the extent of having the same at two distinct frequencies even in presence of dispersion, rendering the realization of anti-lasers more flexible.

Inexpensive microwave leakage detectors--are they worth it? (A performance evaluation report)

Due largely to the misinformation spread by the blogosphere, many consumers are worried they may be harmed by the radiation leaking from microwave ovens. As a result, many have been purchasing and using inexpensive microwave oven leakage detectors. Several microwave engineers have tested some of these and found them to be inaccurate, often giving false-high or false-low readings. In our study, we examined a number of these selling for USD 79.95 and less, by comparison to professional equipment.

Ultrasound nondestructive evaluation (NDE) imaging with transducer arrays and adaptive processing

This paper addresses the challenging problem of ultrasonic non-destructive evaluation (NDE) imaging with adaptive transducer arrays. In NDE applications, most materials like concrete, stainless steel and carbon-reinforced composites used extensively in industries and civil engineering exhibit heterogeneous internal structure. When inspected using ultrasound, the signals from defects are significantly corrupted by the echoes form randomly distributed scatterers, even defects that are much larger than these random reflectors are difficult to detect with the conventional delay-and-sum operation. We propose to apply adaptive beamforming to the received data samples to reduce the interference and clutter noise. Beamforming is to manipulate the array beam pattern by appropriately weighting the per-element delayed data samples prior to summing them. The adaptive weights are computed from the statistical analysis of the data samples. This delay-weight-and-sum process can be explained as applying a lateral spatial filter to the signals across the probe aperture. Simulations show that the clutter noise is reduced by more than 30 dB and the lateral resolution is enhanced simultaneously when adaptive beamforming is applied. In experiments inspecting a steel block with side-drilled holes, good quantitative agreement with simulation results is demonstrated.

Spectral sidebands on a narrow-bandwidth optical probe as a broad-bandwidth THz pulse diagnostic

Broad-bandwidth THz-domain electro-magnetic pulses are typically diagnosed through temporal electro-optic (EO) cross-correlation with an optical probe pulse. Single-shot time-domain measurements of the THz waveform involve complex setups at a bandwidth coverage limited by the probe bandwidth. Here we present an EO-based diagnostic directly in the spectral domain, relying on THz-induced optical sidebands on a narrow-bandwidth optical probe. Experiments are conducted with a 0.11-THz-bandwidth optical probe and a broadband source (0-8 THz detection bandwidth) rich in spectral features. The validity of the sideband diagnostic concept, its spectral resolution, sideband amplitude, and the effects of probe timing are studied. For probe pulses longer than the THz pulse, the sideband technique proves an accurate single-shot spectral diagnostic, with advantages in setup simplicity and bandwidth coverage no longer limited by the laser bandwidth.

A virtual sensor for online fault detection of multitooth-tools

The installation of suitable sensors close to the tool tip on milling centres is not possible in industrial environments. It is therefore necessary to design virtual sensors for these machines to perform online fault detection in many industrial tasks. This paper presents a virtual sensor for online fault detection of multitooth tools based on a Bayesian classifier. The device that performs this task applies mathematical models that function in conjunction with physical sensors. Only two experimental variables are collected from the milling centre that performs the machining operations: the electrical power consumption of the feed drive and the time required for machining each workpiece. The task of achieving reliable signals from a milling process is especially complex when multitooth tools are used, because each kind of cutting insert in the milling centre only works on each workpiece during a certain time window. Great effort has gone into designing a robust virtual sensor that can avoid re-calibration due to, e.g., maintenance operations. The virtual sensor developed as a result of this research is successfully validated under real conditions on a milling centre used for the mass production of automobile engine crankshafts. Recognition accuracy, calculated with a k-fold cross validation, had on average 0.957 of true positives and 0.986 of true negatives. Moreover, measured accuracy was 98%, which suggests that the virtual sensor correctly identifies new cases.

Shear-horizontal wave-based pipe damage inspection by arrays of segmented magnetostrictive patches

The lowest-branch torsional guided wave is very effective in pipe damage inspection because of its non-dispersive characteristics, but it cannot be used for the simultaneous identification of axial and circumferential locations of a defect in a pipe. Motivated by recent developments in magnetostrictive transducer technology, which is especially efficient in torsional and shear wave generation, the goal of this investigation is to extend this technology for simultaneous identification of the axial and circumferential locations of cracks by using shear horizontal (SH) waves. Unlike the conventional magnetostrictive patch method using a single complete patch wound around the pipe's circumference, the proposed method segments the patch into several pieces to generate SH waves propagating over the pipe surface. Accordingly, SH waves in a pipe are generated and sensed individually by a meander coil placed separately on each segment. By using two sets of segmented-patch arrays separated by some distance, the cylindrical surface of a pipe can be inspected both axially and circumferentially. After the underlying angular profile of the patch segment is investigated, experiments identifying the axial and circumferential locations of multiple cracks in a pipe are carried out to demonstrate the potential of the proposed methodology.

[Clinical experiences with an automatic collision warning system: instrument navigation in endoscopic transnasal surgery]

PROBLEM

The goal of this work is the extension of instrument navigation with a collision warning function. With the help of an additional distance display and warning system the performance of surgical navigation systems should be improved.

MATERIAL AND METHODS

The collision warning system (DCS) is an extension of an optoelectric navigation system (NPU, Karl Storz GmbH&Co.KG, Tuttlingen, Germany). The measurement of situation awareness was performed on phantom models of functional endoscopic surgery of the paranasal sinuses (FESS; Phacon, Leipzig). Altogether 450 measurement pairs for the analysis of surgical accuracy to the risk structure (frontal skull base, lamina papyracea, internal carotid artery) were available. To examine the influence on the clinical process, a prospective analysis of intraoperative complications was carried out. Of the 104 FESS patients, two groups, one of 56 patients with only navigation (NAV) and one of 48 patients (NAV+DCS), were examined. Efficiency was evaluated on the basis of times for system preparation and intraoperative application.

RESULTS

A significant increase in the assumed and actual distance values between instrument tip and risk structure using the collision warning system was seen at 76%. The complication rate was more favorable for the NAV+DCS group. The time needed for preparation of the navigation system with the application of the collision warning system increased on average by 48%, or 1.2 min. However, the relation between preparation time and utilization time was approximately the same at 53.5% in the NAV group and 57.4% in the NAV+DCS group.

CONCLUSIONS

This work supports the clinical use and efficiency of a collision warning system as an addition to well-known instrument navigation in endo- and transnasal surgery. The segmenting algorithm is suitable for clinical requirements.

Predicting fatigue life of a PMMA based knee spacer using a multiaxial fatigue criterion

PURPOSE

Experimental tests have played a major role in the assessment of fatigue endurance of orthopedic prostheses; however, cyclic tests on devices entail high costs. Here, a multiaxial fatigue criterion coupled with computational simulations and material properties measurements has been employed to predict fatigue life of the tibial component of a polymeric PMMA spacer. The ultimate aim is to obtain valid information on fatigue behavior avoiding fatigue tests on the device.

METHODS

First, an accurate measurement of the static and fatigue properties of PMMA samples is performed. Then, numeric simulations of the fatigue behavior of the PMMA spacer reproducing the experimental test conditions according to ISO 14879-1 were carried out in order to calculate the stress field throughout the device. Finally, a Risk Index was calculated by using a proper multiaxial fatigue criterion for brittle materials (Kakuno-Kawada) for the assessment of the device fatigue behavior by predicting the F-N curves.

RESULTS

The numeric results were validated by comparing the predictions against experimental data already published by our group. The multiaxial fatigue criterion was able to predict the most critical point on the spacer upper surface and the fatigue behavior of the device that nicely matched the experimental curves.

CONCLUSIONS

This approach represents a valuable tool to investigate the mechanical reliability of implantable devices; nevertheless, the use of advanced and specific failure criteria coupled with accurate data of the device’s material is mandatory to represent a real alternative to the experimental approach in fatigue life prediction.??Key words: Acrylic bone cement, Fatigue endurance, Finite element analyses, Knee spacer.

Contact-free fault location and imaging with on-chip terahertz time-domain reflectometry

We demonstrate in a first experimental study the application of novel micro-machined optoelectronic probes for a time-domain reflectometry-based localization of discontinuities and faults in electronic structures at unprecedented resolution and accuracy (± 0.55 µm). Thanks to the THz-range bandwidth of our optoelectronic system--including the active probes used for pulse injection and detection--the spatial resolution and precision of high-end all-electronic detection systems is surpassed by more than one order of magnitude. The new analytic technology holds great promise for rapid and precise fault detection and location in advanced (3D) integrated semiconductor chips and packages.

Progress in developing a thermal method for measuring the output power of medical ultrasound transducers that exploits the pyroelectric effect

Progress in developing a new measurement method for ultrasound output power is described. It is a thermal-based technique with the acoustic power generated by a transducer being absorbed within a specially developed polyurethane rubber material, whose high absorption coefficient ensures energy deposition within a few mm of the ultrasonic wave entering the material. The rate of change of temperature at the absorber surface is monitored using the pyroelectric voltage generated from electrodes disposed either side of a 60 mm diameter, 0.061 mm thick membrane of the piezoelectric polymer polyvinylidene fluoride (pvdf) bonded to the absorber. The change in the pyroelectric output voltage generated by the sensor when the transducer is switched ON and OFF is proportional to the delivered ultrasound power. The sensitivity of the device is defined as the magnitude of these switch voltages to a unit input stimulus of power (watt). Three important aspects of the performance of the pyroelectric sensor have been studied. Firstly, measurements have revealed that the temperature dependent sensitivity increases over the range from approximately 20°C to 30°C at a rate of +1.6% °C(-1). Studies point to the key role that the properties of both the absorbing backing layer and pvdf membrane play in controlling the sensor response. Secondly, the high sensitivity of the technique has been demonstrated using an NPL Pulsed Checksource, a 3.5 MHz focused transducer delivering a nominal acoustic power level of 4 mW. Finally, proof-of-concept of a new type of acoustic sensor responding to time-averaged intensity has been demonstrated, through fabrication of an absorber-backed hydrophone of nominal active element diameter 0.4 mm. A preliminary study using such a device to resolve the spatial distribution of acoustic intensity within plane-piston and focused 3.5 MHz acoustic fields has been completed. Derived beam profiles are compared to conventional techniques that depend on deriving intensity from acoustic pressure measurements made using the sensor as a calibrated hydrophone.

Recent progress in Brillouin scattering based fiber sensors

Brillouin scattering in optical fiber describes the interaction of an electro-magnetic field (photon) with a characteristic density variation of the fiber. When the electric field amplitude of an optical beam (so-called pump wave), and another wave is introduced at the downshifted Brillouin frequency (namely Stokes wave), the beating between the pump and Stokes waves creates a modified density change via the electrostriction effect, resulting in so-called the stimulated Brillouin scattering. The density variation is associated with a mechanical acoustic wave; and it may be affected by local temperature, strain, and vibration which induce changes in the fiber effective refractive index and sound velocity. Through the measurement of the static or dynamic changes in Brillouin frequency along the fiber one can realize a distributed fiber sensor for local temperature, strain and vibration over tens or hundreds of kilometers. This paper reviews the progress on improving sensing performance parameters like spatial resolution, sensing length limitation and simultaneous temperature and strain measurement. These kinds of sensors can be used in civil structural monitoring of pipelines, bridges, dams, and railroads for disaster prevention. Analogous to the static Bragg grating, one can write a moving Brillouin grating in fibers, with the lifetime of the acoustic wave. The length of the Brillouin grating can be controlled by the writing pulses at any position in fibers. Such gratings can be used to measure changes in birefringence, which is an important parameter in fiber communications. Applications for this kind of sensor can be found in aerospace, material processing and fine structures.

A sensorless drive system for controlling temperature-dependent hysteresis in piezoelectric actuators

A novel drive system is described which improves the operational characteristics of piezoelectric actuators. Electronic controls to increase the range of movement across a wide temperature range usually need a temperature sensor for maximum effect. This paper describes a novel system that uses the polarization characteristics of the piezoelectric ceramic to reverse polarize the ceramic without a temperature measurement. Significant improvements in the operating temperature range for devices such as locks or valves are achieved.

Flexible high-performance carbon nanotube integrated circuits

Carbon nanotube thin-film transistors are expected to enable the fabrication of high-performance, flexible and transparent devices using relatively simple techniques. However, as-grown nanotube networks usually contain both metallic and semiconducting nanotubes, which leads to a trade-off between charge-carrier mobility (which increases with greater metallic tube content) and on/off ratio (which decreases). Many approaches to separating metallic nanotubes from semiconducting nanotubes have been investigated, but most lead to contamination and shortening of the nanotubes, thus reducing performance. Here, we report the fabrication of high-performance thin-film transistors and integrated circuits on flexible and transparent substrates using floating-catalyst chemical vapour deposition followed by a simple gas-phase filtration and transfer process. The resulting nanotube network has a well-controlled density and a unique morphology, consisting of long (~10 µm) nanotubes connected by low-resistance Y-shaped junctions. The transistors simultaneously demonstrate a mobility of 35 cm(2) V(-1) s(-1) and an on/off ratio of 6 × 10(6). We also demonstrate flexible integrated circuits, including a 21-stage ring oscillator and master-slave delay flip-flops that are capable of sequential logic. Our fabrication procedure should prove to be scalable, for example, by using high-throughput printing techniques.

Real-time pulse length measurement of few-cycle laser pulses using above-threshold ionization

The pulse lengths of intense few-cycle (4-10 fs) laser pulses at 790 nm are determined in real-time using a stereographic above-threshold ionization (ATI) measurement of Xe, i.e. the same apparatus recently shown to provide a precise, real-time, every-single-shot, carrier-envelope phase measurement of ultrashort laser pulses. The pulse length is calibrated using spectral-phase interferometry for direct electric-field reconstruction (SPIDER) and roughly agrees with calculations done using quantitative rescattering theory (QRS). This stereo-ATI technique provides the information necessary to characterize the waveform of every pulse in a kHz pulse train, within the Gaussian pulse approximation, and relies upon no theoretical assumptions. Moreover, the real-time display is a highly effective tool for tuning and monitoring ultrashort pulse characteristics.

One-laser-based generation/detection of Brillouin dynamic grating and its application to distributed discrimination of strain and temperature

This paper presents a novel scheme to generate and detect Brillouin dynamic grating in a polarization-maintaining optical fiber based on one laser source. Precise measurement of Brillouin dynamic grating spectrum is achieved benefiting from that the pump, probe and readout waves are coherently originated from the same laser source. Distributed discrimination of strain and temperature is also achieved with high accuracy.

Measurement of the effective nonlinear and dispersion coefficients in optical fibers by the induced grating autocorrelation technique

The induced grating autocorrelation technique, a technique based on temporally resolved two-beam coupling in a photorefractive crystal, was used to measure the nonlinear coefficient γ of three photonic crystal fibers (PCFs): a 30-cm long highly nonlinear PCF, and two large mode area PCFs of 4.5-m and 4.9-m lengths. The measurement used intense 2-ps, 800-nm (850-nm in one case) pulses from a Ti: sapphire laser that experienced self-phase modulation and group velocity dispersion as it travels inside the fibers. This technique was also expanded to measure γ and the dispersion coefficient β2 simultaneously.

Fiber-based cryogenic and time-resolved spectroscopy of PbS quantum dots

PbS quantum dots are promising active emitters for use with high-quality Si nanophotonic devices in the telecommunications-band. Measurements of low quantum dot densities are limited both because of low fluorescence levels and the challenges of single photon detection at these wavelengths. Here, we report on methods using a fiber taper waveguide to efficiently extract PbS quantum dot photoluminescence. Temperature dependent ensemble measurements reveal an increase in emitted photons concomitant with an increase in excited-state lifetime from 58.9 ns at 293 K to 657 ns at 40 K. Measurements are also performed on quantum dots on high-Q (>10(5)) microdisks using cavity-resonant, pulsed excitation.

Low-power SoC design for ligament balance measuring system in total knee arthroplasty

A design of a low-power wireless System-on-Chip (SoC) for the Ligament Balance Measuring System (LBMS) in Total Knee Arthroplasty (TKA) is presented in this paper. It includes a signal conditioning circuit that can support up to 15 force sensors, a 433 MHz RF front-end for data transmission, an 8-bit low-power microprocessor, and a FIFO with a digital filter. Idle and wake-up modes are well designed to reduce the power consumption since the device should be used for the whole surgical procedure. Test results show that the signal conditioning circuit with 16-bit single line output can operate under a wide voltage range, which is from 1.2V to 3.6V. The minimal power consumption is 139μ.W@1.2V with a 200 KHz clock. Experimental results demonstrated in static and body tests are given in the paper also. The chip will be used in an aided monitoring system for Total Knee Arthroplasty in the future work.