Çağlar Arpali

Flat topped beams and their characteristics in turbulent media

The source and receiver plane characteristics of flat topped (FT) beam propagating in turbulent atmosphere are investigated. To this end, source size, beam power and M(2) factor of source plane FT beam are derived. For a turbulent propagation medium, via Huygens Fresnel diffraction integral, the receiver plane intensity is found. Power captured within an area on the receiver plane is calculated. Kurtosis parameter and beam size variation along the propagation axis are formulated. Graphical outputs are provided displaying the variations of the derived source and receiver plane parameters against the order of flatness and propagation length. Analogous to free space behaviour, when propagating in turbulence, the FT beam first will form a circular ring in the center. As the propagation length increases, the circumference of this ring will become narrower, giving rise to a downward peak emerging from the center of the beam, eventually turning the intensity profile into a pure Gaussian shape.

BER evaluations for multimode beams in underwater turbulence

Abstract In underwater optical communication links, bit error rate (BER) is an important performance criterion. For this purpose, the effects of oceanic turbulence on multimode laser beam incidences are studied and compared in terms of average BER (), which is related to the scintillation index. Based on the log-normal distribution, is analysed for underwater turbulence parameters, including the rate of dissipation of the mean squared temperature, the rate of dissipation of the turbulent kinetic energy, the parameter that determines the relative strength of temperature and salinity in driving index fluctuations, the Kolmogorov microscale length and other link parameters such as link length, wavelength and laser source size. It is shown that use of multimode improves the system performance of optical wireless communication systems operating in an underwater medium. For all the investigated multimode beams, decreasing link length, source size, the relative strength of temperature and salinity in driving the index fluctuations, the rate of dissipation of the mean squared temperature and Kolmogorov microscale length improve the . Moreover, lower values are obtained for the increasing wavelength of operation and the rate of dissipation of the turbulent kinetic energy in underwater turbulence.

Effects of focused and collimated laser beams on the performance of underwater wireless optical communication links

In this paper, the optical transmittance and bit error rate (BER) of focused and collimated laser beams are experimentally examined in an underwater optical wireless communication link with different water types. The water types used are fresh water, salty water and their variations with maalox in order to obtain turbid water. In bit error rate (BER) analysis, on-off keying (OOK) is used together with Bose-Chaudhuri-Hocquenghem (BCH) and convolutional codes. Results show that salt and maalox content decreases the transmittance, the convolution codes have better BER performance than BCH codes under the same modulation scheme (i.e., OOK) and focusing improves both the transmittance and BER performance as compared to collimated beams.

Scintillation index of optical spherical wave propagating through biological tissue

Abstract Effects of the tissue turbulence on the propagation of an optical spherical wave are analysed. For this purpose, scintillation index of an optical spherical wave which is propagating in a soft tissue is formulated and evaluated in weakly turbulent soft tissue. Scintillation index of the optical spherical wave is examined against the changes in the tissue parameters which are the tissue length between the optical spherical wave source and the detector, random variations in the refractive index of the tissue and the outer scale of the tissue turbulence. According to our graphical outputs, it is observed that increase in the random variations of the refractive index of the tissue results in an increase in the scintillation index at a certain realization of the turbulence spectrum. On the other hand, larger outer scales and longer tissue lengths yield larger scintillations. The variation of the scintillation index of the optical spherical wave versus the wavelength is also investigated. It is found that at small tissue lengths, wavelength has almost no effect on the scintillations; however, when the tissue length reaches a certain value, shorter wavelengths give rise to larger intensity fluctuations.

Fluorescent on-chip imager by using a tunable absorption filter

In this work, we present a fluorescent on-chip imaging system utilizing a custom-made dye-doped-resin type filter. The filter used in the system was characterized to determine its transmission properties. Unique steep cut-off property of the filter allows to align emission wavelength of the fluorophores to provide better signal-to-noise ratio (SNR). We precisely showed that fluorescent micro-objects can be detected by using the proposed system. The system is believed to be useful especially in medical assessment where precise detection of bio-materials is necessary. Fluorescent imaging systems offer highly sensitive and specific identification and detection of various bio-molecules and cells [1]. However, due to scattering and absorbing nature of the medium (e.g. blood, serum, tissue etc.) emitted signals attenuate at detector level. These limitations demand for development of a new generation of fluorescent filters with sharp cut-off wavelengths. To improve the imaging efficiency, different methods were proposed such as interference and/or absorption based filters [2, 3]. However, such methods are often expensive due to fabrication complexity. We have recently proposed a novel filter with tunable and sharp cut-off wavelength [4]. The filter was fabricated by spin coating a thin layer of dye (Orasol Yellow) and photopolymer (NOA 60) mixture on glass substrate. By modifying the process parameters, it is possible to tune transmission rate of the filter in the range of ∼400–700 nm, which may be extended to infrared region as well. This filter was used as a part of the fluorescent on-chip imaging platform to view fluorescent polystyrene beads of 4 μm diameter suspended in a microfluidic channel. The microfluidic channel was implemented by sandwiching a scribed layer of double-adhesive tape in the form of the channel between two layers of cover glass (Fig. 1a).

Average received intensity for optical beam of arbitrary field profile after propagation in turbulent atmosphere

Employing our previously obtained formulation of the average received intensity for arbitrary optical beam in turbulent atmosphere, intensity patterns of already known and new source profiles are obtained. Arbitrary beam is defined as an incidence having arbitrary source field distribution which is produced by decomposing the source into pixels and assigning the related field to each pixel. For each source field originating from each pixel, incremental received field is found and the total received field is obtained by superposing the contributions from all the incremental received fields. Using the mentioned formula governing the arbitrary beam excitation in turbulence, average received intensity patterns for various types of beams such as cos-Gaussian, cosh-Gaussian, higher-order annular, flat-topped, general type and arbitrary beams are obtained. Our results can be applied in atmospheric optics communication links, reflection from rough surfaces, optical cryptography, optical imaging systems and propagation of partially coherent light.