Journal Articles
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Item Long haul, repeaterless, coherent optical multichannel communication systems employing hybrid multiplexing schemes wherein Wavelength Division Multiplexing (WDM) and Optical Code Division Multiplexing (OCDM) are combined is analyzed in the presence of performance limiting factors such as Inter Channel Interference (ICI), Stimulated Raman Scattering (SRS) and detector shot noise. Error control coding techniques in the form of rate 1/2 convolutional codes are employed in the system to counter these performance limiting factors. It is seen that the use of error control coding technique can enhance the system performance to such an extent that a Bit Error Rate (BER) performance of less than 10-8 is achievable without crossing the SRS power threshold for as many as 1000 to 1200 channels. Advantages of the hybrid multiplexed system over systems relying solely on WDM or OCDM are also discussed along with the advantages of using error control coding compared to optical amplification in countering the performance degrading factors.(Fachverlag Schiele und Sohn GmbH, Coding to enhance performance of hybrid multiplexed coherent optical communication systems in the presence of ICI and SRS) Kumar, M.S.; Bhat, K.N.H.; Umesh, G.2003Item Iterative or Turbo decoding of parallel concatenated binary error control codes in Binary Phase Shift Keyed (BPSK) Communication System corrupted by both Additive White Gaussian Noise (AWGN) and oscillator phase noise is discussed. As an example of application of the discussed theory, iterative decoding technique of Parallel Concatenated block Codes (PCBC) is applied to a coherent Optical Code Division Multiple Access (OCDMA) network in the presence of Multiple Access Interference (MAI), detector shot noise and laser phase noise. Remarkable improvement in performance resulting from the use of turbo decoding in the OCDMA network is also discussed.(Medknow Publications, Performance of iteratively decoded parallel concatenated error control codes in phase noise corrupted BPSK systems) Kumar, M.S.; Bhat, K.N.H.; Umesh, G.2004Item A method to generate a single sub-50-as pulse by employing a 5-fs laser pulse with intensity above the saturation intensity, was proposed. The neon target medium of 0.5 mm length and 5 Torr pressure, was placed 10 mm beyond the laser focus. Harmonic radiation from neon atoms, was obtained by solving the time-dependent Schrödinger equation coupled with Maxwell equations. The results show that the positive chirp contained in the broad continuum radiation can be compensated by an x-ray filter with a negative group delay dispersion in the spectral region of interest.(American Physical Society, Single sub-50-attosecond pulse generation from chirp-compensated harmonic radiation using material dispersion) Kim, K.T.; Kim, C.M.; Baik, M.G.; Umesh, G.; Nam, C.H.2004Item We utilize the dispersion property of an X-ray filter material for the generation of a single sub-50-as pulse from high-order harmonics. The attosecond pulse, formed by selecting the spectral range of high-order harmonic radiation, contains an intrinsic chirp corresponding to the quadratic phase variation during a half cycle of a laser pulse. We show that this chirp can be compensated by using the negative group-delay dispersion of a thin X-ray filter, compressing the attosecond pulse down to sub-50-as.(Compression of harmonie pulses by using material dispersion) Kim, K.T.; Kim, C.M.; Baik, M.G.; Umesh, G.; Nam, C.H.2004Item The theoretical model for realization of an all-fiber, polarization independent Group Velocity Dispersion (GVD) compensating module with transfer function given by Chebyshev polynomials of the second kind is proposed and investigated. The proposed realization of the GVD compensating module comprises of Mach-Zender interferometers formed by single mode fiber optic directional couplers. The realization of GVD compensating modules using single mode fiber optic couplers as proposed in this paper is superior to an earlier realization of a similar GVD compensating module, which used bulk optic components such as TiO2 birefringent crystals, polarizers and ?/4 plates. © by Fachverlag Schiele & Schön 2005.(An all-fiber polarization independent GVD compensating module) Kumar, M.S.; Bhat, K.N.H.; Umesh, G.2005Item Studies on nonlinear optical parameters of bis-chalcone derivatives doped polymer(Elsevier BV, 2006) Shettigar, S.; Chandrasekharan, K.; Umesh, G.; Sarojini, B.K.; Narayana, B.Two bis-chalcone derivatives, 1,5-[di(4-methoxyphenyl)]penta-1,4-dien-3-one and 1,5-[di(4-chlorophenyl)]penta-1,4-dien-3-one were synthesized. Their SHG conversion efficiencies are reported to be 6.0 and 5.0 times that of urea and hyper polarizabilities are 9.9×10-30 and 10.2×10-30 esu, respectively. The third-order nonlinear optical properties in PMMA matrix were studied by Z-scan technique using 7 ns laser pulses at 532 nm. The nonlinear refractive index ?, nonlinear absorption coefficient ?, magnitude of effective third order susceptibility ?(3) and the coupling factor ? have been investigated. The values obtained are of the order of 10-14 cm2/W, 1.2 cm/GW, 10-14 esu and 0.2, respectively, which are comparable with the values obtained in stilbazoleum like dyes. The experimental investigation also shows that they are very interesting optical limiting materials and their optical limiting behaviour is mainly due to two photon absorption phenomenon. © 2006 Elsevier Ltd. All rights reserved.Item ?(3) measurements and optical limiting in dibenzylideneacetone and its derivatives(2006) Kiran, A.J.; Chandrasekharan, K.; Nooji, S.R.; Shashikala, H.D.; Umesh, G.; Kalluraya, B.We investigated the third order nonlinear optical properties of dibenzylidene acetone (1,5-diphenyl-1, 4-pentadeine-3-one) and its derivatives. The nonlinear measurements were performed by using single beam Z-scan technique with Q-switched Nd:YAG nanosecond laser pulses at 532 nm. Open aperture data for dibenzylidene and its derivatives demonstrate the presence of two-photon absorption at this wavelength. The dependence of ?(3) on donor/acceptor type substituents to the basic compound clearly shows the electronic origin of nonlinearity and hence demonstrates that the mechanism by which the third order nonlinear response enhanced is not by the thermal effects, but instead due to the strong nonlinear absorption and nonlinear refraction of the compounds. The derivatives of the basic compound show very good optical limiting behavior. © 2005 Elsevier B.V. All rights reserved.Item Coding In WDM Systems to Counter Impacts of SRS and Channel Beat Noise(International Academy of Microwave and Optical Technology (IAMOT), 2006) Mruthyunjaya, H.S.; Umesh, G.; Kumar, M.S.Depletion in power due to Stimulated Raman Scattering (SRS) impairs Wavelength Division Multiplexed (WDM) system performance. This is countered usually by placing optical amplifiers to maintain required Bit Error Rate (BER) performance. Also, impact of crosstalk in N x N WDM systems causes high noise power. In this paper we propose the use of block codes for forward error correction as a possible method for improving the performance of WDM systems in the presence of SRS and crosstalk. Our results show that with 200 users and with error control coding, the detector can effectively receive the required power and attain the desired BER performance in SRS limited WDM systems. Our results also show that the multiplexer crosstalk effectively comes down to -44 dB from -34 dB. © 2006 ISRAMT. All Rights Reserved.Item Nonlinear optical studies of 1-3-diaryl-propenones containing 4-methylthiophenyl moieties(2007) John Kiran, A.; Mithun, A.; Shivarama Holla, B.; Shashikala, H.D.; Umesh, G.; Chandrasekharan, K.The third-order nonlinear optical properties of chalcone derivatives have been studied using the single beam Z-scan technique. The dependence of ?(3) on different donor and acceptor type substituents demonstrates the electronic nonlinearity of compounds. The largest value of nonlinear refractive index, n2, measured for a high electron donor substituted molecule is -2.033 × 10-11 esu. These molecules exhibit a strong two-photon absorption and interesting optical limiting of nanosecond laser pulses at 532 nm. © 2006 Elsevier B.V. All rights reserved.