Articles written in Pramana – Journal of Physics

    • Optimum parameters controlling distortion and noise of semiconductor laser under analog multichannel modulation


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      This paper presents a comprehensive modelling and simulation study on the optimum parameters that control the distortion and noise of semiconductor lasers (SLs) subject to multichannel modulation for use in analog cable television (CATV) fibre links. The study is based on numerical integration of the rate equation model of the semiconductor laser. The parameters comprise the modulation index per channel $(m/ch)$, number of loaded channels $(N)$ and fibre length $(L_{F})$. The signal distortions include the composite second-order (CSO) and composite triple beat (CTB) distortions. The noise is assessed in terms of the relative intensity noise (RIN) and carrier-to-noise ratio(CNR). In order to achieve acceptable CNR values for SL, $m/ch$ should be less than 7.5 and 2% when loading 12 and 80 channels, respectively. For the CATV fibre link with $L_{F}$ = 10 km, the increase in the number of channels from 12 to 80 corresponds to lowering the optimum value of $m/ch$ from 7 to 1%. The increase of $L_F$ to 50 km limits the optimum value of $m/ch$ between 1.4 and 1%, which corresponds to loading between 12 and 17 channels only.

    • Effects of two-tone intensity modulation on signal distortion and noise in a semiconductor laser for radio-over-fibre applications


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      This paper introduces modelling and simulation of signal distortions as well as intensity noise induced by two-tone direct intensity modulation of semiconductor lasers for use in radio-over-fibre (RoF) systems. The study develops a large-signal modulation of semiconductor laser simulations in the regime of high-frequency modulation by counting lasers with high modulation bandwidth. The temporal and spectral characteristics of the modulated laser output are investigated. The temporal characteristics include the fluctuations in the modulated signal waveforms, while the spectral characteristics include the frequency spectrum of the modulation response, second-order harmonic distortion (HD2) and second- and third-order intermodulation distortions, IMD2 and IMD3, respectively, as wellas the relative intensity noise (RIN). The investigations are performed under three bias currents I$_b$ of 2, 5 and 10 times the threshold value I$_{th}$ over a wide range of modulation depth that covers regimes of small and large-signalmodulation and at modulation frequencies as high as 8 and 25 GHz with a spacing of 10 MHz. The dynamic range of the linearity of the investigated laser is evaluated in terms of spurious-free dynamic range (SFDR). The results showthat the nonlinear distortions increase as modulation depth increases. The highest distortion levels are observed when the modulation frequency approaches the laser relaxation frequency. The low-frequency RIN increases as themodulation depth and/or bias current increase and it has levels which are smaller when the modulation frequency is 8 GHz than when it is 25 GHz.

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