Output Endface Research Articles

Grating lobe-free silicon optical phased array with periodically bending modulation of dense antennas.

A grating lobe-free silicon optical phased array with large field of view is demonstrated. Antennas with periodically bending modulation are spaced at half wavelength or less. The experimental results show that the crosstalk between adjacent waveguides is negligible at 1550 nm wavelength. Additionally, to reduce the optical reflection caused by the sudden change of refractive index at the output antenna of the phased array, tapered antennas are added to the output end face so that more light will be coupled into the free space. The fabricated optical phased array shows a field of view of 120° without any grating lobes.

The effect of the pumping inhomogeneity factor on spherical aberration in a dynamic stability cavity

Based on analytical theory, the effect of the pumping inhomogeneity factor β on spherical aberration is carried out. We establish a wavefront information measurement experimental device in a plano-plano symmetry cavity to obtain spherical aberration at the crystal output end face. The results show that the magnitudes of the pumping inhomogeneity factor increase with the augmentation of crystal length or doping concentration, which can affect the numerical value of spherical aberration and the position of the zero point of spherical aberration. Finally, we analyze the output beam quality factor and get the optimal region where the zero point of the spherical aberration exists.

Design and experimental investigation on longitudinal-torsional composite horn considering the incident angle of ultrasonic wave

The composite vibration mode type of ultrasonic horn has been widely employed in ultrasonic vibration drilling. In order to explore the effect of the incident angle of the ultrasonic wave on longitudinal-torsional composite (LTC) horn, the reason for the mode-conversion and the vibrational characteristics of such horn firstly were analyzed on the basis of elastic wave-field theory. Then, a 3D model was developed with helical slots with different angle set in the conical section of the composite horn, and the vibration modal of the output end face of the horn was analyzed using the finite element analysis (FEA) method. It was found that the incident angle of ultrasonic wave exhibited a significant influence on the vibration modal at the output end face of the horn. The torsional vibration and the longitudinal vibration were changed significantly at the output end face of the horn when the incident angle was 47.6° and 67.2°, and the measured amplitude ratio of the torsional vibration to the longitudinal vibration in the former was improved by 4.9 times than that in the latter. Simultaneously, in both cases, the error of resonant frequency between the measurement and the simulation value reached 1.9% and 1.3%, respectively. It was observed from the ultrasonic vibration drilling test that the higher the amplitude ratio of the torsional vibration to the longitudinal vibration (AT/AL), the more the average drilling force was reduced and the better machined quality could be obtained. The results of this study should be well considered for further reference when designing longitudinal-torsional composite horn with different materials in ultrasonic vibration drilling.

Parametric study of fiber-optic laser-induced breakdown spectroscopy for elemental analysis of Z3CN20-09M steel from nuclear power plants

A compact laser probe of the fiber-optic laser-induced breakdown spectroscopy (FO-LIBS) system, based on a single-lens for energy focus and radiation collection, was developed in this paper. Elemental analysis of the Z3CN20-09 M steel sample, which is usually used as the structural material for main pipelines in nuclear power plants, was carried out using this laser probe. Most elements (e.g. Fe, Cr, Ni, Mn, Mo, Co, Cu, Nb, and Si) were successfully detected. The distance between the fiber output end face and the lens, which was one of the most important factors affecting the focused laser spot size and further inducing different degrees of self-absorption, was optimized. It was clear that the spectral intensities increased with the distance before decreasing. The optimal range of the distance was obtained by finding out the maximum spectral intensities and minimum spectral fluctuations. The effects of distances, which can be understood as the effects of spot sizes, were studied using plasma diagnostic methods. When the distance was shorter than the optimal range, the distinct self-absorption was observed, which was analyzed by the fast photography. Calculated changing trend of self-absorption effects was consistent with the observed spectral profiles. When the distance was longer than the optimal range, the plasma temperature and density became lower after ~1 μs. This demonstrates that it is possible for the single-lens laser probe to realize the accurate calibration and control of the focused spot size to achieve the best working conditions for a FO-LIBS system.

Optical lattice generation using vertically embedded multimode-interference square-core polymer waveguides on a silicon chip.

We demonstrate two-dimensional optical lattice generation at 1064nm wavelength using vertically embedded multimode-interference (MMI) square-core polymer waveguides on a silicon chip. We demonstrate tuning of the effective waveguide length by longitudinally offsetting the waveguide input end-face from the input beam waist. Our measurement results of the waveguides with different cross-sectional dimensions at different effective waveguide lengths exhibit lattice patterns spanning from 4 × 4 to 10 × 10 arrays at the waveguide output end-face. Our theoretical analysis reveals that the offset causes additional mode-dependent phase changes. Our numerical modeling results using the three-dimensional beam-propagation method are consistent with our experimental results and theory.

Electromagnetic Compatibility of PSK–OOK Digital Fiber Communications

This study deals with an electromagnetic compatibility (EMC) problem of fiber cable in disturbing electric field. Lightwaves that emerge from the output endface of the fiber are intensity modulated, phase shifted, or modulation frequency multiplied under special conditions. In this paper, theoretical investigation of the effect of interfering electric fields on the bit error rate of digital signals propagating originally in an optical fiber is presented for the EMC evaluation. Theoretical analysis and specific numerical examples show that depending on the required EMC level, the unshielded fiber may not be immune to interference of electromagnetic fields.

Subwavelength focusing of light by a tapered microtube

We propose a mechanism for subwavelength focusing at optical frequencies based on the use of a tapered microtube fabricated from a glass capillary tube. Using coherent illumination at 671 nm and a near-field scanning optical microscope probe which was mounted on a three-axis piezonanopositioning stage, a sequence of two-dimensional intensity profiles were obtained. Our experimental results reveal the smallest focal spot with a near diffraction-limited full width at half-maximum of ∼435 nm (0.65λ) at a distance of ∼1.47 μm (2.2λ) from the output end face of microtube. The experimental results are in agreement with our numerical simulation.

Subwavelength focusing by a micro/nanofiber array

Utilizing the diffraction property of the output light from micro/nano fibers (MNFs), a new scheme based on a MNF array to focus optical beams with subwavelength resolution in the far field is proposed. By using the three-dimensional finite-difference beam-propagation method (FD-BPM), we investigated the diffraction interference effect of the MNFs during light propagation. The numerical stimulation demonstrates a focusing spot with FWHM of 0.43 λ at a distance of 9 λ from the output endface of MNF array. The effects of the characteristics of the array, the MNFs, and the input optical field on focusing are analyzed in detail.

Subwavelength focusing by a micro/nanofiber array

Utilizing the diffraction property of the output light from micro/nano fibers (MNFs), a new scheme based on a MNF array to focus optical beams with subwavelength resolution in the far field is proposed. By using the three-dimensional finite-difference beam-propagation method (FD-BPM), we investigated the diffraction interference effect of the MNFs during light propagation. The numerical stimulation demonstrates a focusing spot with FWHM of 0.43 lambda at a distance of 9 lambda from the output endface of MNF array. The effects of the characteristics of the array, the MNFs, and the input optical field on focusing are analyzed in detail.

Radiation parameters of ridge lasers at high pump currents

Quantum-well ridge GaAs/AlGaAs heterostructure lasers are studied at high pump currents. It is shown that their catastrophic optical damage (COD) is preceded, as a rule, by a giant kink caused by the collapse of a waveguide in a horizontal plane. It is found that, by increasing the coating thickness of the output end-face of a diode by preparing an intermediate ZnSe layer between the cleaved output end-face of the diode and a coating forming the output mirror of the laser, it is possible to increase the COD threshold by a factor of 1.5–2. This optical strengthening occurs because the ZnSe layer provides an additional channel for heat sink from the overheated output face.

Studying fabrication errors of the diffraction grating on the end face of a silver-halide fiber

A technology for fabricating a diffractive microrelief on an end face of a polycrystalline IR fiber (PIR-fiber) is studied. We discuss how fabrication imperfections of a beam-splitter implemented on the fiber’s output end face could affect the specified intensity distribution of diffraction orders. Recommendations for improving the characteristics of the microrelief to manufacture are worked out.

Direct electro-optic intermodulation in fibers

He-Ne laser lightwaves in an optical fiber are modulated when a section of the fiber is subjected directly to audio-frequency-modulating electric fields. The laser light emerging from the output endface is intensity modulated, phase shifted, or modulation frequency multiplied under special conditions. We present the correlation between this observed electro-optic effect of the fiber itself and the theoretical analysis of the phenomena.

Hydrogen leak detection using an optical fibre sensor for aerospace applications

An optical fibre sensor has been developed for the detection of hydrogen leakages on the cryotechnic engine of the European rocket ARIANE V. The principle of this sensor is based on the variation of reflectivity of a palladium micromirror deposited at the output end face of a multimode optical fibre. An experimental set-up using a monitored high power laser diode provides an optical heating of the palladium layer, which allows to keep up the performances in the wide range of temperatures between −196°C and +23°C. Depending on both the concentration and the temperature, detection responses between 1% and 17% with response times shorter than 5 s have been demonstrated. Fast and accurate detection of hydrogen concentrations inferior to the explosive limit has been performed.

Theory of direct modulation of lightwaves in optical fibers

He–Ne laser lightwaves in an optical fiber are modulated when a short section of the fiber is subjected directly to audio frequency electric fields. The laser light emerging from the fiber output endface is intensity modulated, phase shifted, or modulation frequency multiplied under special conditions. A theoretical explanation is presented. © 1998 John Wiley & Sons, Inc. Microwave Opt Technol Lett 18: 323–328, 1998

Split-spectrum intensity-based optical fiber sensors for measurement of microdisplacement, strain, and pressure

A self-referencing technique compensating for fiber losses and source fluctuations in reflective air-gap intensity-based optical fiber sensors is described. A dielectric multilayer short-wave-pass filter is fabricated onto or attached to the output end face of the lead-in-lead-out multimode fiber. The incoming broadband light from a white light or a light-emitting diode is partially reflected at the filter. The transmitted light through the filter projects onto a mirror. The light returning from the reflecting mirror is recoupled into the lead-in-lead-out fiber. These two reflections from the filter and the reflecting mirror are spectrally separated at the detector end. The power ratio of these two reflections is insensitive to source fluctuations and fiber-bending loss. However, because the second optical signal depends on the air-gap separation between the end face of the lead-in-lead-out fiber and the reflecting mirror, the ratio provides the information on the air-gap length. A resolution of 0.13 µm has been obtained over a microdisplacement measurement range of 0-254 µm. The sensor is shown to be insensitive to both fiber-bending losses and variations in source power. Based on this approach, a fiber-strain sensor was fabricated with a multilayer interference filter directly fabricated on the end face of the fiber. A resolution of 13.4 microstrain was obtained over a measurement range of 0-20,000 microstrain with a gauge length of 10 mm. The split-spectrum method is also incorporated into a diaphragm displacement-based pressure sensor with a demonstrated resolution of 450 Pa over a measurement range of 0-0.8 MPa.

Reduced-confinement GaAlAs tapered waveguide antennas for enhanced far-field beam directionality

Reduced-confinement tapered waveguide antennas are shown to produce reductions of >35% in the transverse far-field beam divergence for radiation emitted from GaAlAs slab waveguides, resulting in far-field beams as narrow as 8.2 degrees full width at half-maximum (FWHM) along the direction perpendicular to the wafer surface. Reduced confinement of the guided mode near the output endface is achieved using a novel molecular-beam-epitaxy growth technique to produce a longitudinal reduction in the refractive index and thickness of the waveguide film. The reduced-confinement geometry has the distinct advantage over horn antennas of being compatible with two-dimensional antenna development. A numerical simulation is used to verify and predict the performance limitations of reduced-confinement antennas. Requirements on both the amplitude and phase profiles of the electric field at the antenna output are shown to impose a stringent lower limit on the acceptable taper length for optimal antenna performance.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Reduced-confinement antennas for GaAlAs integrated optical waveguides

Monolithically integrated reduced-confinement antennas are shown to produce reductions of &amp;gt;35% in the far-field beam divergence for radiation emitted from single-mode GaAlAs slab waveguides, yielding far-field beams as narrow as 8.2° FWHM along the direction perpendicular to the wafer surface. Reduced confinement of the guided mode near the output endface is achieved using a novel molecular beam epitaxy growth technique to produce a longitudinal variation in the refractive index and thickness of the waveguide film. Unlike present horn antennas, the reduced-confinement geometry has the distinct advantage of being compatible with two-dimensional antenna development.

A new method for the detection of very small leaks in pipe systems

An optical fibre sensor has been developed for the detection of hydrogen leakages on the cryotechnic engine of the European rocket ARIANE V. The principle of this sensor is based on the variation of reflectivity of a palladium micromirror deposited at the output end face of a multimode optical fibre. An experimental set-up using a monitored high power laser diode provides an optical heating of the palladium layer, which allows to keep up the performances in the wide range of temperatures between −196°C and +23°C. Depending on both the concentration and the temperature, detection responses between 1% and 17% with response times shorter than 5 s have been demonstrated. Fast and accurate detection of hydrogen concentrations inferior to the explosive limit has been performed.