Waleed S. Mohammed

Long-period grating and its application in laser beam shaping in the 1.0 μ m wavelength region

Laser beam shaping is an important subject for industrial and medical applications since different applications may require different laser intensity distributions. We present an all-fiber beam-shaping device that could transform a Gaussian-shaped laser beam into an intensity uniform beam in the 1.0 microm wavelength region using a long-period grating (LPG). The device can be used to shape the beam of a Yb-doped high-power fiber laser. The mode structure of the LPGs made of HI1060 fiber is analyzed. The design parameters that affect the beam shaping are discussed, and the results of the beam shaping using two cladding modes, LP(03) and LP(04), are presented.

Fiber Bragg grating in large-mode-area fiber for high power fiber laser applications

Fiber Bragg gratings (FBGs) are indispensable components in the design of monolithic high-power fiber lasers. As the laser power scales up, the adoption of larger-mode-area fibers with high V numbers poses new challenges for FBG design and fabrication. In this paper, we present the simulation, fabrication, and measurement of the FBGs inscribed on large-mode-area fibers. The simulation used the T-matrix approach to calculate the spectral response of the FBG that matched well with the measured spectra. The observed fringes in the reflection spectrum are explained by the interference between the low-order modes that were also confirmed with the simulation. Some unique features of the FBG and their potential applications are discussed.

Effects of fluorescent lighting on in vitro micropropagation of Lemna minor

The vegetative in vitro propagation of Lemna minor stain SING-4 exposed to two different types of fluorescent light sources, Philips TLD 36W/54 and Toshiba FL40T8BRF/36, was studied. The liquid culture medium contained 4.43gl-1 phytohormone-free full-strength Murashige & Skoog (MS) basal medium with vitamins, 30gl-1 sucrose, and 1gl-1 MES. The results showed that both plant cultures had undergone normal asexual reproduction with an exponential increase trend. Cultures exposed to Toshiba FL40T8BRF/36 reproduced at a slightly faster rate while expressing significantly greener foliage (leaf color chart shade No.8), which indicates the presence of more chlorophyll, than cultures exposed to Philips TLD 36W/54 (leaf color chart shade No.4). The data obtained from our experiment reveals that light emitted from Toshiba FL40T8BRF/36 produces healthier and higher quality cultures.

Monte Carlo modeling (MCML) of light propagation in skin layers for detection of fat thickness

Nowadays, most activities require lesser physical actions, which could ultimately lead to accumulation of excessive body fat. The main roles of body fat are to store energy and acts as various kinds of insulators for the body. The thickness of fat layers can be measured to indicate fat-body weight ratio. Exceeding the body-mass index (BMI) could lead to many illnesses regarding obesity. Consequently, many studies have proposed various principles and techniques to measure the amount of fat within ones body. In this paper, infrared interactance in skin layers is studied for investigation of the influence of fat thickness upon photon travelling pattern in skin tissues using Monte Carlo model (MCML). Photon propagation is numerically simulated in simplified multi-layered tissues. The optical coefficients of each skin layers are accounted for different traveling paths of photons that move through random motion. The thickness of fat layer is varied, and changing in optical parameters is observed. Then the statistically obtained data are computed and analyzed for the effect of the fat layer upon reflection percentage using different wavelengths. The calculations have shown increment in the slope of change of reflection percentage versus fat thickness, when using infrared compare to visible light. This technique can be used to construct a mobile device that is capable of measuring the volume fraction of melanin and blood in the epidermis layer and dermis layer, to calculate for the necessary optical coefficients that would be necessary for measurement of fat thickness.

Numerical modeling of scanning near-field optical microscopy for fluorescence-less DNA detection

Scanning near-field optical microscopy (SNOM) has gained wide interest as a viable microscopic technique for the study of surface properties at the nanoscale. SNOM uses optical fiber to detect evanescent wave which provides a high resolution imaging beyond the diffraction limit. The collected intensity is influenced by complex refractive index of the sample. This project exploits the property of evanescent wave to discriminate between unhybridized and hybridized DNA which has a significant difference in complex refractive index. The concept could become a promising alternative since it circumvents fluorescence-labeling problems. The research focuses on numerical modeling by mean of finitedifference beam propagation and DNA hybridization based on empirical data from literatures.

Dispersion compensation for optical coherence tomography

Optical coherence tomography (OCT) is an imaging technique widely used in various applications especially in biomedical field. It constructs a high resolution 3-D image using multiple cross-sectional views. The axial resolution can be degraded if the sample is dispersive, which is usually true as most of the samples are living tissues. For time-domain OCT, this dispersion is minimized numerically by introducing a compensation filter, which is applied to the obtained signal in Wigner domain, a time-frequency domain. The filter is designed using simulated annealing optimization technique. This paper shows the design of the filter and the results of dispersion compensation.

Creating a smart environment using optical wireless

From the past until now, a radio frequency technology takes an important role in a communication system. Optical wireless technology is gaining more importance and becoming a smart alternative in the communication systems since noise and interference problem issues can be minimized. This project aims to create a smart environment by using optical wireless technology with a color multiplexing scheme. RGB Light Emitting Diodes (RGB-LEDs) are used as a visible white light source as they are becoming the next generation of lamps. A color de-multiplexing scheme is applied at the receiver port.

Pupil masks for 2-D intensity synthesis in a high numerical aperture focusing system

A high numerical aperture (NA) lens is used in many applications that require tightly focused beams including microscopy. The Debye-Wolf electromagnetic diffraction integral describes focusing by high NA lenses. Using an eigenfunction expansion of this integral, we numerically obtain a pupil mask that generates an arbitrary, within the diffraction limit, intensity distribution at the Gaussian focal plane.

All-fiber laser beam shaping at 1.0 μm wavelength region in a single-mode fiber

Laser beam shaping is an important subject in industrial and medical applications of lasers since different applications may require different laser intensity distributions. Recently we demonstrated successfully an all-fiber laser beam shaping device that could transform a Gaussian shaped laser beam into a uniform or ring-shaped beam in 1.5 μm wavelength region. In this paper we present the work of the beam shaping in 1.0 μm wavelength region to make it compatible to Yb-doped high power fiber laser. The device uses a long-period grating to couple a portion of core-mode, LP01 into a low-order mth-radial cladding mode LP0m. Interference of the two modes could reduce field at the centre and enhance the field in the first or second ring of the cladding mode to transform the Gaussian-shaped laser beam to an intensity uniform beam. The design parameters that affect the beam shaping will be discussed and the results of the interference from two cladding modes, LP03 and LP04 will be presented.