Kramers-Kronig analysis of molecular evanescent-wave absorption spectra obtained by multimode step-index optical fibers

Spectral distortions that arise in evanescent-wave absorption spectra obtained with multimode step-index optical fibers are analyzed both theoretically and experimentally. Theoretical analysis is performed by the application of Kramers-Kronig relations to the real and the imaginary parts of the complex refractive index of an absorbing external medium. It is demonstrated that even when the extinction coefficient of the external medium is small, anomalous dispersion of that medium in the vicinity of an absorption band must be considered. Deviations from Beer's law, band distortions, and shifts in peak position are quantified theoretically as a function of the refractive index and the extinction coefficient of the external medium; the effect of bandwidth for both Lorentzian and Gaussian bands is also evaluated. Numerical simulations are performed for two types of sensing sections in commonly used plastic-clad silica optical fibers. These sensors include an unclad fiber in contact with a lower-index absorbing liquid and a fiber with the original cladding modified with an absorbing species. The numerical results compare favorably with those found experimentally with these types of sensing sections.     

Direct image transmission through a multimode optical fiber

One-way transmission of a multipixel image through the multimode optical fiber based on the phase-conjugation principle is realized. Adistortion-compensating hologram for each pixel of an image to be transmitted is superposed on a photoplate. Each hologram is recorded with a reference beam of different beam incidence angle to provide proper wave-front correction for each pixel without any interference from other pixels. The reference beams are holographically generated from a photoplate in which small holographic lenslets are aligned in a matrix pattern. Images of up to 25 pixels are transmitted through the fiber experimentally.     

Coupling light from a laser diode into a multimode fiber

A parabolalike fiber end to couple the light from a high-power laser diode into a multimode fiber is presented. The formula for the coupling efficiency of such a system is given, and numerical evaluation shows that the theoretical coupling efficiency can be as high as 92% without an antireflection coating on the fiber's end face. This beam-shaping system can be fabricated easily and has a large alignment tolerance.     

Beam shaping for multicolour light-emitting diodes with diffractive optical elements

An improved particle swarm optimization method is proposed for the design of ultra-thin diffractive optical elements (DOEs) enabling multicolour beam shaping functionality. We employ pre-optimized initial structures and adaptive weight strategy in the algorithm to achieve better and identical shaping performance for multiple colours. Accordingly, a DOE for shaping light from green and blue LEDs has been designed and fabricated. Both experiment and numerical simulations have been conducted and the results agree well with each other. 15.66% average root mean square error (RMSE) and 0.22% RMSE difference are achieved. In addition, the parameters closely related to the performance of the optimization are analysed, which can provide insights for future application designs.     

Chromaticity stability of phosphor-converted white light-emitting diodes with an optical filter

The thermal stability of blue LEDs and yellow YAG phosphor for blue-pumped white LEDs is critical to the performance of white LEDs, including the color properties and luminous efficacy. To investigate the thermal effect on the color properties of emitted white light, the junction temperature of the excitation blue LED was raised by increasing the supplied current. However, by choosing an optical filter with the appropriate transmittance spectrum for processing a portion of the blue light output and reexciting the secondary yellow emission, we observed a reduction in color instability and lower color temperature across the current range of 20-200 mA. Moreover, the color temperature distribution was improved.     

Thermal influences on optical properties of light-emitting diodes: a semiempirical model

The application of LED technology to fields such as alphanumerical displays and traffic control is continuously increasing. Because the technology is used outdoors, it must be able to operate under various environmental conditions. Like all semiconductor devices, LED's have properties that change with temperature. We propose a semiempirical model, based on semiconductor solid-state theory, that predicts the changes in the emission spectrum including the effect of temperature changes on the optical properties of the LED, within a range appropriate for outdoor applications (0-40 degrees C). This model permits us to evaluate the changes in the output flux and the chromaticity coordinates of the LED. We checked this model with seven different LED's.     

通体发光光纤及其应用

通体发光光纤不仅具有导光性 ,而且具有通体发光特性。本文介绍了通体发光光纤的种类、制备方法及优越性能 ,并简单概述了通体发光光纤在照明、装饰装璜、广告及其它领域中的应用。     

Measurement of launching efficiency into multimode optical fibersusing semiconductor laser diodes

A detailed and systematic study has been carried out of the launching efficiency of light, from semiconductor laser diodes into graded-index multimode optical fiber waveguides. Four different terminations of the fibers were investigated: plane-ends, hemispherical ends, taper-with-hemisphere ends, and roof-top (RT) ends. Besides launching efficiency, the parameters of interest were the various misalignments, such as axial, lateral, and angular, between laser and fiber, and the effect of these to produce a reduction in the launching efficiency. It is shown that RT fibers increase the launching efficiency considerably but rotational optimization of the fiber about its axis is essential. It is noted that this is a considerable disadvantage as most of the other techniques considered do not require the optimization     

Photonic-crystal light-emitting diodes on p-type GaAs substrates for optical communications

Oxide-confined photonic-crystal (PhC) light-emitting diodes (LEDs) on p-type GaAs substrate in the 830 nm range are reported. The device consists of a bottom distributed Bragg reflector (DBR), quantum wells (QWs), and a top DBR, with a photonic-crystal structure formed within the n-type ohmic contact ring for light extraction. The etching depth of the PhC holes is 17-pair out of the 22-pair top DBR being etched off. The internally reflected spontaneous light emission can be extracted out of PhC holes because of lower reflectance within those areas. High-resolution micrographic imaging studies indicate that the device emits light mainly through the photonic-crystal holes and it is suitable for optical communications.     

Improving the precision of multimode optical fiber attenuationmeasurements/

To improve the precision of attenuation measurements for multimode optical fibers, the different measurement errors are analyzed and an error equation is established. This is used to quantify total error by inserting relevant parameters taken from a recent Chinese nationwide interlaboratory comparison of multimode optical fiber attenuation measurements. It is suggested that the measurement error of the attenuation of a multimode optical fiber be expressed in both the error for the total loss of the entire length of the fiber, and the error of the loss per unit length, rather than the usual way of only expressing it per unit length     

Analysis of a low-finesse Fabry-Perot sensing interferometer illuminated by a multimode optical fiber

A model of the reflected fringe system for an ideal plane-parallel, low-finesse Fabry-Perot (FP) cavity illuminated by a multimode optical fiber has been developed and experimentally validated. This showed that the phase dispersion within the cavity arising from the divergent nature of the incident illumination significantly degrades the visibility of the reflected fringes. Departures from the ideal FP cavity are also examined. The effect on fringe visibility when the plane of the FP cavity is tilted with respect to the fiber axis and when the cavity surfaces are no longer perfectly parallel to each other has been explored. The analysis described is relevant to the design and the optimization of multimode optical-fiber sensors that use FP sensing cavities.     

Polychromatic light-emitting diodes with a fluorescent nanosphere opal coating

A hexagonally close-packed array consisting of fluorescent nanospheres was coated onto short-wavelength GaN light-emitting diodes to demonstrate polychromatic white light emission. The spherical particles self-assemble into ordered three-dimensional opal structures, performing the role of color conversion to generate a polychromatic spectrum with smooth and uniform emission patterns. Different ratios of green and orange-red fluorescent nanospheres were mixed and coated onto high-extraction-efficiency micro-LEDs. Four devices with different shades of white emission were demonstrated. Device A, with a high content of orange-red nanospheres, offers the highest CRI value of 80, whereas device C with a well-balanced ratio of green and orange-red nanospheres exhibits color characteristics closest to ideal white with CIE coordinate at (0.34, 0.34). At 20?mA driving current, the luminous efficacy of the devices A, B, C, and D are 40.5?lm?W(-1), 57.7?lm?W(-1), 63.1?lm?W(-1), and 67.2?lm?W(-1) respectively, while the correlated color temperatures (CCTs) of the corresponding devices are 3587, 4778, 5271, and?13?000?K.     

Flexible organic light-emitting diodes with a polymeric nanocomposite anode

Flexible organic light-emitting diodes (FOLEDs) are facing mechanical issues arising from failure of the indium-tin oxide (ITO) films fabricated on flexible substrates. Polymeric nanocomposite anodes were fabricated by including single-wall carbon nanotubes (SWCNTs) in aqueous poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). The conductivity, transmittance and flexibility of the polymeric nanocomposite anode were characterized. The polymeric nanocomposite anodes fabricated on a poly(ethylene terephthalate) (PET) substrate exhibited superior bending properties to ITO anodes on PET. The FOLEDs fabricated on the polymeric nanocomposite anodes had a low turn-on voltage and higher luminous intensity than those fabricated on ITO/PET anodes. This flexible nanocomposite polymeric anode is a very promising for fully FOLEDs and other optoelectronics.     

Impulse dispersion in a multimode optical fiber from its far-field radiation pattern

A simple geometric-optical theory predicts impulse broadening in multimode fibers using the far-field power distribution of an incoherently excited waveguide with arbitrary refractive-index profile. The accuracy is comparable with that of scalar optical methods if they do not incorporate mode coupling and a differential mode attenuation model.     

Analytical characteristics of stimulated Raman scattering in a multimode fiber obtained with an optical time-domain reflectometer

The nonlinear behavior of stimulated Raman scattering (SRS) in a low-loss optical fiber has been analytically and experimentally investigated by the assumption of certain restraint conditions. As a result, it has been found that the SRS light propagating along the optical fiber is expressed in a simple formula by the introduction of nonlinear parameters, which are experimentally identified and whose maximum value is restricted by the restraint conditions. The parameters, moreover, are available for compensating the SRS intensity in the presence of noninteracting light. Therefore the formula for SRS intensity presented is useful for investigating the SRS effect in a multimode fiber.     

In-line optical fiber sensors based on cladded multimode tapered fibers

The use of uniform-waist cladded multimode tapered optical fibers is demonstrated for evanescent wave spectroscopy and sensors. The tapering is a simple, low-loss process and consists of stretching the fiber while it is being heated with an oscillating flame torch. As examples, a refractive-index sensor and a hydrogen sensor are demonstrated by use of a conventional graded-index multimode optical fiber. Also, absorbance spectra are measured while the tapers are immersed in an absorbing liquid. It is found experimentally that the uniform waist is the part of the taper that contributes most to the sensor sensitivity. The taper waist diameter may also be used to adjust the sensor dynamic range.