JEOS:RP - Journal of the European Optical Society Rapid publications
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Laguerre-Gaussian Mode Division Multiplexing in Multimode Fiber using SLMs in VCSEL Arrays
No full textMode division multiplexing (MDM) is a promising technology for increasing the aggregate bandwidth of multimode fiber in conjunction with wavelength division multiplexing (WDM) as a means to alleviate the impending capacity crunch in optical fiber networks due to increasing consumer bandwidth demands. This paper models a 25-channel Laguerre-Gaussian (LG) MDM-WDM system in a multimode fiber using an integrated SLM-controlled VCSEL array. A data rate of 25Gbps is achieved at a central wavelength of 1550.12 nm. Although previous work have addressed the effect of excitation of different individual modes on the MDM performance, the implication of the separation of the mode numbers have not been investigated. Apart from the modelling of the SLM-controlled VCSEL array, another significant contribution of this paper is the analyses on the effect of the separation of radial mode numbers and separation of azimuthal mode numbers in an integrated SLM-based VCSEL MDM system. The effects ofdifferent azimuthal mode spacing and radial mode spacing of LG modes were analysed on the channel impulse response, eye diagrams and bit-error rates
Group refractive index calculation by difference approximation for length measurement
Full text linkIn this study, the possibility of employing a difference approximation to facilitate calculation of the group refractive index of air (GRA) was investigated. The forward, backward, and central difference methods were used to numerically approximate the first-order derivatives of the phase refractive index based on the Edlén empirical equations. To confirm the validity of the calculations, the calculated results were compared with the theoretical analysis results and the values in a related paper. It was found that the GRA computation could be easily approximated by the two-point central difference method with a step size of 10 nm
An analytical model for top-hat long transient mode-mismatched thermal lens spectroscopy
Full text linkIt has been shown that a top-hat excitation beam gives rise to a more sensitive signal for the thermal lens spectroscopy (TLS). Recently, a numerical model has been presented for a top- hat excitation beam in a dual-beam mod-mismatched TLS [Opt. Lett. 33(13), 1464-1466 (2008)]. In this work, we present a full analytical version of this model. Our model was based on a new solution of time-dependent heat equation for a finite radius cylindrical sample exposed to a top-hat excitation laser beam. The Fresnel diffraction integration method was then used to calculate on-axis probe-beam intensity variations due to thermal lensing by taking the aberrant nature of the thermal lens into account. The model was confirmed with experimental data of LSCAS-2 with an excellent agreement
Complex method for angular-spectral analysis of volume phase diffraction gratings recorded in photopolymers
Full text linkPhotopolymer recording materials are nowadays widely used for recording of diffraction gratings and other diffraction elements. For obtaining the best performance of these diffraction gratings for desired applications, it is important to assess these gratings from many different perspectives. In this contribution, we present an experimental and characterization approach to an analysis of diffraction gratings recorded into photopolymer materials. This approach is able to provide a complex and very illustrative description of these gratings response and, with accordance to the theory, information about some important grating parameters, such as a spatial period, slant angle, etc., as well. This approach is based on the measurement of a grating response for a wide range of angles and wavelengths and then on the construction and subsequent analysis of maps in the angular-spectral plane. It is shown that the measurements are in a good agreement with the theoretical predictions based on either approximate (Kogelnik’s coupled wave theory) or rigorous (RCWA) techniques and that this approach provides complex and detailed characterization of the grating response which can be used for additional optimization or decision of applicability of measured sample gratings
Improving three-dimensional (3D) range gated reconstruction through time-of-flight (TOF) imaging analysis
Full text linkThis paper performs an experimental investigation on the TOF imaging profile which strongly influences the quality of reconstruction to accomplish accurate range sensing. From our analysis, the reflected intensity profile recorded appears to deviate from Gaussian model which is commonly assumed and can be perceived as a mixture of noises and actual reflected signal. Noise-weighted Average range calculation is therefore proposed to alleviate noise influence based on the signal detection threshold and system noises. From our experimental result, this alternative range solution demonstrates better accuracy as compared to the conventional weighted average method and proven as a para-axial correction to improve range reconstruction in 3D gated imaging system
Liquid crystal cells based on photovoltaic substrates
Full text linkLiquid crystal cells with LiNbO3:Fe crystals as substrates, are described. The photovoltaic field generated by the substrates is able to reorient the liquid crystal director thus giving rise to a phase shift on the light propagating through the cell, as in liquid crystal light valves. The process does not require the application of an external electric field, thus being potentially useful for applications requiring a high degree of compactness. An efficient optical switch with a high transmission contrast, based on the described optically-induced electric field, is also proposed
False detection of dangerous and neutral substances in commonly used materials by means of the standard THz Time Domain Spectroscopy
Full text linkEssential limitations of the standard THz Time Domain Spectroscopy (TDS), which lead to false detection of dangerous and neutral substances in commonly used materials, are demonstrated using the physical experiment with chocolate under real conditions as well as with semiconductors under laboratory conditions. To overcome this disadvantage, we propose using the time-dependent spectrum of the THz pulse, transmitted through or reflected from a substance. For quality assessment of the standard substance absorption frequency presence in the signal under analysis, we use time-dependent integral correlation criteria. The influence of aperture placed in front of the sample on spectral properties of silicon wafers with different resistivity is demonstrated as well
Spectroscopic evidence of anthropogenic compounds extraction from polymers by fluorescent dissolved organic matter in natural water
Full text linkFDOM is one of the most important carriers of anthropogenic compounds in natural waters. It can combine with environmental contaminants and polymers to form diverse chemical structures. To this end, here a microfluidic chip was designed for the analysis of these changes in fluorescent dissolved organic matter (FDOM) fingerprints due to thermal treatment and varying time intervals of exposure. Excitation Emission Matrix Spectroscopy (EEMS) approach was utilized to detect and identify the inherent compounds in sampled FDOM. Strong direct correlations were founded, Spearman rank correlation values (Ï = 0.85 at α = 0.1, n = 4) and linear correlation R² = 0.8359 were noted between thermal treatment pattern 2 and fluorescence intensity of samples. Materials, acrylic based glue and cyclic olefin copolymer (COC) polymer, used to design the microfluidic sensor were determined to possess unique spectral features in the ultraviolet to green spectrum using EEMS. The study therefore provides an insight on methods to identify contaminants in natural waters. This underlines the potential of optical sensors providing measurements at fast intervals, enabling environmental monitoring
Deterministic measurement and correction of the pad shape in full-aperture polishing processes
Full text linkFull-aperture polishing is a significant process in fabricating large optical flats because it restrains Mid-Spatial Frequency errors and removes material quickly on the whole optic surface. Nevertheless, optical flats fabricated by full-aperture polishing generally fail to meet the stringent requirement of surface figure, which has to be corrected by sub-aperture polishing processes. Surface figure of optical flats in full-aperture polishing processes is primarily dependent on the pressure distribution uniformity which correlates intensively with the lap shape. At present, practical and precise means are urgently desired for measuring and correcting the lap shape, especially the polyurethane pad lap. In the study, we present a novel method for deterministic measurement of the pad shape. The method obtains the height of the pad at spirally distributed locations implemented by the revolution of the pad and translation of the laser displacement sensor. The pad shape in terms of matrixes whose elements representing the heights at the corresponding locations is then calculated by interpolation algorithm based on the obtained data. Further, we propose a method for deterministic correction of the pad shape utilizing a small conditioning tool. The dwell time algorithm and implementation strategy for the dwell time are provided for common full-aperture polishers. These solutions for the deterministic measurement and correction of the pad shape have been validated on a full-aperture polisher with polyurethane pad. The polishing experiments revealed that the optic surface figure was obviously improved
Light penetration in seawater polluted by dispersed oil: results of radiative transfer modelling
Full text linkThe downwelling light in seawater is shaped by natural seawater constituents as well as by some external substances which can occur locally and temporally. In this study we focused on dispersed oil droplets which can be found in seawater after an oil spill or in the consequence of intensive shipping, oil extraction and transportation. We applied our modified radiative transfer model based on Monte Carlo code to evaluate the magnitude of potential influence of dispersed oil droplets on the downwelling irradiance and the depth of the euphotic zone. Our model was validated on the basis of in situ measurements for natural (unpolluted) seawater in the Southern Baltic Sea, resulting in less than 5% uncertainty. The optical properties of dispersed Petrobaltic crude oil were calculated on the basis of Mie theory and involved into radiative transfer model. We found that the changes in downwelling light caused by dispersed oil depend on several factors such as oil droplet concentration, size distribution, and the penetration depth (i.e. vertical range of oil droplets occurrence below sea surface). Petrobaltic oil droplets of submicron sizes and penetration depth of 5 m showed a potentially detectable reduction in the depth of the euphotic zone of 5.5% at the concentration of only 10 ppb. Micrometer-sized droplets needed 10 times higher concentration to give a similar effect. Our radiative transfer model provided data to analyse and discuss the influence of each factor separately. This study contributes to the understanding of the change in visible light penetration in seawater affected by dispersed oil