Volume 41, Issue 4 -

A detailed performance analysis of homodyne-based coherent detection in Free-space optics (FSO) with various optical amplifiers is presented in this paper. Using homodyne detection in FSO system, the effect of scintillation is mitigated. To further enhance the performance of FSO system, various optical amplifiers such as erbium-doped fibre amplifier, semiconductor optical amplifier, gain-controlled amplifier and black box model of amplifier can be employed in the system. Improvement in the system can be validated in terms of BER, Q -factor and eye diagram.

Analysis is presented to evaluate numerically the combined effect of amplified spontaneous emission (ASE) and stimulated Raman scattering (SRS) on the bit error rate performance of a WDM transmission system with Raman amplifier using heterodyne receiver. The results are compared with those for a WDM system reported with direct detection receiver for different system parameters. It is observed that coherent heterodyne system is less affected by the crosstalk due to combined effect of ASE and SRS compared to direct detection. It is noticed that allowable transmission distance is 170 km, and 166 km of WDM channels for 8, and 16, respectively, when coherent heterodyne receiver is and with pump power of 10 mW and channel bandwidth of 12.5 GHz. The corresponding transmission distance is 60 km and 10 km, respectively, for direct detection system already reported.

In this article, a 2D photonic crystal (PC)-based optical filter has been designed using a PC ring resonator. The resonator used is of square type with a square lattice constant, which has been designed by increasing the radius of the inner rods of the resonator. The filter designed can separate the light of the wavelength of 1545.3 nm with a transmission coefficient of 98 %. The bandwidth of the above wavelength is equal to 0.5 nm and hence the quality factor of the device at this wavelength is equal to 3091. The effects of the structural parameters, such as the refractive index, the lattice constant, the radius of the dielectric rods, the radius of the inner rods of the resonator on the behavior of the proposed device, are fully investigated. To obtain the photonic band gap, the plane wave expansion method is used. In addition, the finite difference time domain method is used to examine, simulate, and to obtain the output spectrum of the structure. The designed structure has both high transmission coefficient and quality factor. Comparatively speaking, it is also simple to design which justifies its use in other photonic crystal-based optical devices.

Aiming at the characteristic of small amplitude of partial discharge current signal, all-optical fiber current sensing system is designed. Two kinds of optical fiber current sensor structures are proposed, which are linear and reflective. The optical path systems of two kinds of fiber optical current sensor are modeled based on Jones matrix. In addition, the output sensitivities of two structures are compared. The results indicate that the sensitivity of the two current sensor structures can be improved, and the output sensitivity of the reflective fiber optical current sensor is twice as high as that of the linear sensor. At the same time, the complete optical path model of optical fiber sensing system is established under the influence of fiber birefringence errors. Finally, the birefringence errors of sensing fiber are simulated. It is concluded that the reflective fiber optical current sensor can suppress the influence of linear birefringence on the accuracy of the sensing system by introducing a large amount of circular birefringence in the sensing fiber.

The effect of core infiltration in the optical properties of Photonic Crystal Fiber (PCF) is investigated. The soft glass rod infiltration provides greater refractive index contrast between the core and the cladding. This modification improves the optical properties significantly. Four structures of photonic crystal fiber (Hexagonal, Octagonal, Decagonal and Elliptical) are investigated and a comparative study has been made to observe the difference in the optical properties due to the infiltration. It is observed that, by introducing this infiltration the birefringence is improved up to the order of 10−1$10^{-1}$ and a very high negative dispersion coefficient of 7744ps/(km.nm) can be achieved. The birefringence is increased 4.82×106$4.82\times10^{6}$ times in the hexagonal PCF, 5.38×105$5.38\times10^{5}$ times in octagonal PCF, 546 times in decagonal PCF and about 8 times in the elliptical PCF at operating wavelength due to the core infiltration. The positive dispersion of the fiber is eliminated and a very high negative dispersion co-efficient of 7744ps/(km.nm) is achieved in hexagonal PCF, a relatively flattened dispersion is obtained in other cases due to infiltration at operating wavelength. The nonlinearity is increased about 73 times in case of hexagonal PCF and in other cases it is increased about 2 times. The confinement loss is reduced up to the order of 10−11$10^{-11}$ due to the infiltration at the operating wavelength of 1550 nm. Another comparative study shows that the introduced fibers outperform most of the recent works with a more simple structure, which reduce fabrication complexity. The numerical investigation of the structures is conducted using full vector finite element method.

We report evaluation of first higher order modal field for dual mode optical fiber having step and parabolic index profiles. The study is carried out both in absence as well as in presence of Kerr nonlinearity. The analysis is based on a simple iterative method involving Chebyshev formalism. Taking some typical step- and parabolic-index fibers as examples, we show that our results agree excellently with the exact results which can be obtained by applying rigorous methods. Thus, our simple formalism stands the merit of being considered as an accurate alternative to the existing cumbersome methods. The prescribed formalism provides scope for accurate estimation of different propagation parameters associated with first higher order mode in such kinds of fibers in presence of Kerr nonlinearity. The execution of formalism being user friendly, it will be beneficial to the system engineers working in the field of optical technology.

An enormously low loss symmetrical hybrid decagonal porous core spiral photonic crystal fiber (SH-PCF) has been proposed for terahertz (THz) wave guiding. The modal characteristics of the fiber and its mathematical analysis have been numerically completed using a full-vector finite element method (FEM). Simulation results show an ultra-low material loss of 0.0167 cm −1 and large effective area 1.95×10 6 µm 2 which is 91.6 % of bulk absorption material loss at controlling frequency f=1.0 THz with a core porosity 42 %. Additionally, proposed structure establishes the comparatively higher core power fraction maintaining lower scattering loss about 1.8×10 −15  dB/cm at the same operating frequency. It promises the aforementioned advantages for efficient THz wave propagation.

Elastic optical networks (EONs) are new generation optical networks that provide flexible bandwidth and spectrum assignment characteristics to accommodate diverse demand range over traditional dense wavelength division multiplexing (DWDM)-based networks. While overcoming spectrum contiguity and continuity constraints, Routing and Spectrum Assignment (RSA) is a challenging task in EONs. In this article, we have proposed an EON network model. Using existing RSA techniques we have analyzed the performance of the proposed model on the basis of Network Blocking Probability (NBP) and Network Utilization Efficiency (NUE) under variable load conditions. It has been found to be working optimally even at a load of 200 Erlangs.

In this paper, the performance of one-coincidence frequency hopping code/quadratic congruence code (OCFHC/QCC) is analysed and compared with the existing codes. The impact of variation in code weight, code length and number of wavelengths on error probability is evaluated. It is observed that for the same code weight, code length and number of available wavelengths, error probability of OCFHC/QCC is better than the existing codes under certain conditions. The large cardinality, simple design methodology and good performance suggest its use in local area network, multi-code keying and multi-rate multimedia applications.

In this work, a maintenance issues resolved suppression method of four-wave mixing (FWM) is proposed using optical phase conjugation (OPC) modules and different placements of OPCs are investigated in ultradense wavelength division multiplexing (WDM) system with coherent orthogonal frequency division multiplexing (OFDM) incorporation. Moreover, comparison of the placements of OPC in six different cases has been done and compared with conventional WDM (no-OPC) system in terms of Q-factor, BER and induced FWM. Analysis has been done at different launched power levels. It is observed that placement of OPC after each laser source performs exceptionally well and suppresses FWM with ease of maintenance.

The gigantic demand on high bandwidth and capacity has triggered the need to use the radio over optical fiber (RoF) systems. RoF systems offer several attractive features such as low attenuation loss, large bandwidth, less interference, etc., which can improve the communication system reliability and achieve high data rates. However, the RoF system suffers from linear and nonlinear fiber impairments, which result in signal distortion. Several solutions have been proposed to overcome linear impairments; on the other hand, the nonlinear impairments issue is still a challenging problem in RoF communication systems. Four-wave mixing (FWM) is the most dominating nonlinear impairments that impacts the performance of optical fiber communication systems and the RoF systems as well. In this article, we present a strategy to mitigate the FWM effect in wavelength division multiplexing (WDM) RoF systems. Specifically, we study the effect of FWM under high transmit power levels over long transmission distances. This proposed strategy aligns the optical and radio link parameters to alleviate the FWM effects. To achieve this objective, the impact of WDM RoF system parameters such as wavelength (λi$\lambda_i$), transmitted power, etc., on system performance is investigated. The main performance metrics are the total signal-to-noise ratio ( SNR ), optical SNR ( OSNR ), Q-factor, and bit error rate ( BER ). Given these metrics, our proposed strategy defines an optical threshold level that applies to the optical threshold component to get rid of the undesired signals generated by the FWM effect.

Orthogonal Frequency Division Multiplexing (OFDM) is a potential transmission approach for high capacity communication systems. Despite the many advantages of OFDM, the major downside is the high peak-to-average power ratio (PAPR) which increases the system complexity, reduces the efficiency of the system, causes degradation in BER performance, and makes OFDM sensitive to nonlinear distortion in the transmission. Various methods have been proposed to deal with the PAPR problem, including the partial transmit sequence (PTS) that has attracted considerable attention. Hence, this paper presents a hybrid approach combining an enhanced PTS technique with Mu-Law companding. The PTS technique was enhanced through improving its sub-block partitioning scheme, where the enhanced partitioning scheme consolidated a conventional interleaved partitioning into an adjacent partitioning scheme. This incorporation of Mu-Law characteristic in time domain for PAPR reduction in OFDM essentially enhances the PAPR reduction performance, based on using numerical simulation results. Consequently, though the pseudorandom sub-block partition method obtains better PAPR reduction more than the other sub-block partition schemes (interleaved and adjacent) of ordinary PTS, it is quite difficult to be designed. The findings show that the enhanced PTS technique with Mu-Law companding, while maintaining low computational complexity, performs significantly better than the pseudorandom partitioning PTS on various types of modulation formats and subcarriers.

Orthogonal Frequency Division Multiplexing along with the coherent detection and spectrally efficient modulation formats is a promising solution for long haul and high speed communication systems. Although they offer significant advantages which make them suitable for high speed and long haul communication systems they all are susceptible to phase noise. In this paper a combination of RF-pilot-based approach followed by pilot-based equalization approach is used for laser phase noise compensation. The various parameters such as drive voltage, bias voltage and extinction ratio are optimized to get the optimum performance from the proposed scheme. The designed system is then analyzed in terms of Symbol Error Rate (SER), constellation diagrams and error vector magnitude using various types of Quadrature Amplitude Modulation (QAM) formats.

In this paper, an analytical approach is presented to evaluate the bit error rate (BER) of a free space optical (FSO) link with radio frequency (RF) multiple subcarrier modulation, taking into account the effect of weak atmospheric turbulence considering a direct detection optical receiver followed by RF synchronous demodulator for each sub-channel. Analysis is carried out to find a closed-form expression for conditional BER at the output of the RF demodulators conditioned on a given value of atmospheric turbulence-induced fading and intermodulation distortion (IMD). The average BER for each sub-channel is then found by averaging the conditional BER over the probability density function of the atmospheric turbulence modeled as log-normal distribution. Degradations of BER due to atmospheric turbulence are evaluated for several values of system parameters like number of RF subcarrier, turbulence variance, link distance, data rate and power penalty suffered by the system due to atmospheric turbulence and IMD. For a given system bandwidth, it is found that the maximum power penalty occurs when the subcarrier number is around four and the performance gradually improves with increase in the number of subcarrier. For example, at a system bandwidth of 20 GHz with subcarrier number 4, the power penalty at a BER of 10 –9 is found to be 30 dB for a link distance of 3.6 km, whereas the power penalty reduces to 13 dB when the number of subcarrier is increased to 32.

Free space optics (FSO) links provide an effectual and efficient way of transmission of data throughout free channels due to its higher bandwidth and inbuilt capability of security. In this research analysis, the FSO link is evaluated on the basis of Q value for different modulation formats and weather conditions. Gamma–Gamma and log-normal scintillation models have been analyzed for different index of refraction in terms of Q factor versus a range of a proposed FSO link. Attenuation increases during the occurrence of different fog conditions, and there might be complete link failure in the case of dense fog and for some of the cloud conditions.