BISRAT G. ASSEFA, PHD

• Imaging-quality 3D-printed centimeter-scale lens

Three-dimensional (3D) printing of imaging-quality optics has been challenging due to the tight tolerances on surface shape and roughness. We report on manufacturing such optics with Print optical Technology, which is based on modified ink-jet printing. We demonstrate for the first time a 3D-printed singlet lens with a surface profile deviation of  ±500 nm within a 12-mm aperture diameter. Its RMS surface roughness is below 1 nm without surface polishing. The printed lens exhibits an imaging resolution of some 140 lp mm  −1 at 100-mm focal length in the visible region.

Link: https://www.osapublishing.org/oe/fulltext.cfm?uri=oe-27-9-12630&id=409154 

• Realizing freeform lenses using an optics 3D-printer for industrial based tailored irradiance distribution

Recent energy efficient illumination advancements have capitalized on using light-emitting diodes (LEDs) in combination with freeform lenses. However, the available freeform lens design methods are application specific. In addition, manufacturing of this class of optics is challenging. In this work, considering manufacturing constraints, we apply a customized algorithm to design freeform lenses for (1) transforming LED radiation into a uniform rectangular illumination pattern and (2) shaping collimated LED light beams into complex image target irradiance distributions. The algorithm is based on a numerical solution of the elliptic Monge–Ampère equation. Then, we proposed manufacturing these lenses using a modified ink-jet 3D printing technology called Printoptical technology. The demonstrated optical performance of printed lenses, exhibiting surface roughness of  RMS=10±2

“>RMS=10±2RMS=10±2

 nm, is in good agreement with the simulation. We also explored an industrial application of the 3D-printed lens matrix for low-cost illumination of a paper milling machine.

Link: https://www.osapublishing.org/osac/fulltext.cfm?uri=osac-2-3-690&id=407085

• 3D printed plano-freeform optics for non-coherent discontinuous beam shaping

The design, fabrication, and characterization of freeform optics for LED-based complex target irradiance distribution are challenging. Here, we investigate a 3D printing technology called Printoptical^® technology in order to relax or push forward both the fabrication limits and LED-based applications of thick freeform lenses with small slope features. The freeform designs are carried out with an assumption of small-sized LED source using an existing point-source-based Tailoring method, which is available in the semi-commercial software. The numerical methods of the designs are characterized by ray-tracing software. The irradiance patterns of the 3D printed freeform lenses are promising considering the average shape conformity deviation of around ± 40 µm and center area surface roughness around ± 12 nm, which is to our knowledge by far the best result reported for 3D printed freeform lenses with a thickness greater than 1 mm. Applications of freeform lenses with discontinuous target irradiance distribution patterns are expected in eco-friendly energy efficient lighting such as in zebra-cross lighting.

Link:https://link.springer.com/article/10.1007/s10043-018-0428-1

• Evaluation of freeform lens designs for specific target distributions

Freeform optics can be classified as optical components without a specific axis of rotation. The extra degrees of freedom offered by the arbitrary surface description of freeform optics allow extreme customization of the emitted irradiance distribution compared with conventional lenses . Despite the clear design advantages of freeform optics for many applications in the nonimaging area, the manufacturing of this class of optics is still difficult, expensive and time consuming. In addition, the available design methods for freeform optics are application specific, thus the choice of the method depends on the actual design tasks .

• Spectrometer optical system using diffractive grating structures

Design, fabrication and characterization of a diffractive optical elements for transmission spectrometer based on light bulb at visible spectral range

• Graphene integrated slot waveguide: Design and fabrication technique

Graphene/Waveguide geometries have been proposed in order to increase the interaction length of light with graphene. Herein, an investigation of graphene integrated slot waveguide structures with an emphasis on optical absorption of graphene is presented. The proposed structure is investigated mainly for amorphous and crystalline silicon based vertical slot waveguide materials but the same structure based on titanium dioxide and graphene on SOI slot waveguide geometry are also considered for comparison purpose. Fourier modal method (FMM) is used to model and analyze the slot waveguide modes in MATLAB software, and since the modes of electric field in the transversal direction dominates the structure only the quasi-TE mode is considered. The optimization of the structure is done by considering the slot waveguide single mode behavior and optical absorption of the graphene layer. Thus, an optical absorption coefficient of 0.8098 dB/µm and 0.7873 dB/µm are obtained for crystalline and amorphous silicon based slot waveguide/ bilayer graphene structure, respectively. In addition, the fabrication techniques that can be used for constructing the structure are presented.

• Non-uniform sampling based feature extraction for Automatic Speech Recognition

In Automatic Speech Recognition (ASR) robustness to additive noise remains a large unsolved problem. As a result selecting a proper feature extraction method has been a key research area. So many feature extraction algorithms have been proposed that are designed specifically to have a low sensitivity to background noise. However, there are still some performance problems in noisy environments. This thesis is an attempt to develop a new feature extraction method based on a combination of non-uniform sampling and mel-frequency cepstrum coefficients (MFCCs) method since MFCC works very well under clean environment. Non-Uniform sampling is used when fluctuations in sampling instants cannot be ignored or when signal samples can be obtained only at irregular or even random time intervals. It also sometimes deliberately introduced in order to see some useful effect such as the suppression of aliasing and to reduce the quantization noise which as a result improve the performance of Analog to Digital converter (ADC). Since improving ADC using non-uniform sampling method helps to increase the representation of the original signal in digital form and the non-uniformity of sampling as compared to uniform sampling efficiently improves the spectrogram which allows to determine true signal components at frequencies exceeding the half of mean sampling rate. And also the fact that spectra of the non-uniform sampled signals are not uniform in frequency domain helps to represent the non-uniform spectral sensitivity of human hearing which might helps to autofocus on most reliable part of the spectrum in noisy cases. In this thesis we deliberately introduced non-uniform sampling in order to modify the front-end analyzer to better capture the speech information and incorporate the temporal characteristics in the feature set. The step used for implementing the non-uniform sampling based speech recognition can be summarized by the following steps. The first step performs oversampling and end point detection. The second steps includes non-uniform sampling of the speech signal using sine-wave crossing method and speech segmentation by using short term temporal analysis. The third step includes finding the feature vectors using NU-MFCCs methods and vector quantizing (VQ) of the speech features. Finally, by using the means and variance of the feature calculated in VQ as an input the Gaussian Mixture Models (GMM) is used for classifier or modeling purpose. Experimental results show the average performance of the recognition system based on NU-MFCCs is around 92.18% under normal external surrounding (>35dB) and 51.27% under additive white Gaussian noise(AWGN) condition (between -5 and 35 dB) whereas in MFCC case, 92.36% and 42% respectively. But when the system was trained with a mixture of normal external surrounding and 10 dB SNR (AWGN) condition the average performance of NU-MFCCs went up to 74.84%. Similarly in MFCC case, it increased to 65.87%.

• Computer controlled home security system

Home security system is  designed and implemented  in order to improve an existing security system with some technological advancement such as using mobile telecommunication networks for notifcation purpose. A hardware plus software solution is built into the system. The hardware includes opto-coupler in which the transistor switch checks weather the system is secured; when the system is not secured, a signal will be given to the parallel port of the computer. The software, that is built using a c# programming language, it checks the status of the parallel port and send message to the intended person by accessing mobile telecommunication network incase of active signal.