Articles written in Pramana – Journal of Physics

    • Analysis of small-signal intensity modulation of semiconductor lasers taking account of gain suppression

      Moustafa Ahmed Ali El-Lafi

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      This paper demonstrates theoretical characterization of intensity modulation of semiconductor lasers (SL’s). The study is based on a small-signal model to solve the laser rate equations taking into account suppression of optical gain. Analytical forms of the small-signal modulation response and modulation bandwidth are derived. Influences of the bias current, modulation index and modulation frequency as well as gain suppression on modulation characteristics are examined. Computer simulation of the model is applied to $1.55-\mu$m InGaAsP lasers. The results show that when the SL is biased far-above threshold, the increase of gain suppression increases both the modulation response and its peak frequency. The modulation bandwidth also increases but the laser damping rate decreases. Quantitative description of the relationships of both modulation bandwidth vs. relaxation frequency and maximum modulation bandwidth vs. nonlinear gain coefficient are presented.

    • Influence of pseudorandom bit format on the direct modulation performance of semiconductor lasers

      Moustafa Ahmed Safwat W Z Mahmoud Alaa A Mohmoud

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      This paper investigates the direct gigabit modulation characteristics of semiconductor lasers using the return to zero (RZ) and non-return to zero (NRZ) formats. The modulation characteristics include the frequency chirp, eye diagram, and turn-on jitter (TOJ). The differences in the relative contributions of the intrinsic noise of the laser and the pseudorandom bit-pattern effect to the modulation characteristics are presented. We introduce an approximate estimation to the transient properties that control the digital modulation performance, namely, the modulation bit rate and the minimum (setting) bit rate required to yield a modulated laser signal free from the bit pattern effect. The results showed that the frequency chirp increases with the increase of the modulation current under both RZ and NRZ formats, and decreases remarkably with the increase of the bias current. The chirp is higher under the RZ modulation format than under the NRZ format. When the modulation bit rate is higher than the setting bit rate of the relaxation oscillation, the laser exhibits enhanced TOJ and the eye diagram is partially closed. TOJ decreases with the increase of the bias and/or modulation current for both formats of modulation.

    • 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.

    • Modelling and characterisation of the noise characteristics of the vertical cavity surface-emitting lasers subject to slow light feedback


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      This paper introduces the modelling and characterisation of the noise properties of the vertical cavity surface-emitting laser (VCSEL) coupled in lateral direction with a passive cavity. This design of VCSEL with this transverse coupled cavity (TCC) is proposed for high-speed photonics. We introduce comprehensive simulationson the influence of the induced lateral slow light feedback on the relative intensity noise (RIN) and carrier-to-noise ratio (CNR). The proposed model incorporates multiple round trips of slow light in the TCC as time delay light in the rate equations of the VCSEL. The obtained results are compared with those of the conventional VCSEL.We show that the noise performance of the TCC-VCSEL is optimised when the VCSEL exhibits stable continuous wave (CW) operation under strong slow light feedback and the TCC length is smaller than 8 $\mu$m and between 11 and 13 $\mu$m. When strong slow light induces unstable regular and/or irregular oscillations, RIN is enhanced and CNR is lowered.

    • Modelling of semiconductor laser with double external cavities for use in ultrahigh speed photonics


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      We model and investigate the dynamics and modulation performance of semiconductor laser integrated with two short external cavities facing the front and back facets with the aim to enhance the modulation bandwidth of the laser for use in high-speed photonics. The coupled cavities provide double optical feedback (DOFB) to the laser cavity through the partially reflecting facets of the laser cavity. The study is based on modifying the rate equations of the laser to include multiple reflections of laser radiations in the external cavities. Therefore, it accounts for the regime of strong OFB that causes bandwidth enhancement. We introduce correspondence between the laser stability under DOFB and the modulation response characteristics. Also, we allocate the ranges of the DOFB that induce photon–photon resonance (PPR) effect as the main contributor to the bandwidth enhancement. We show that the intensity modulation (IM) response can be tailored by varying the reflectivity of the external mirrors when the external cavities are too short to stabilise the laser output. Modulation bandwidth better than 55 GHz is predicted under strong double OFB when the external cavities are as short as 2 mm. Stronger DOFB is found to enhance the PPR effect and induce resonant modulation over a narrow frequency range around frequencies reaching 45 GHz.

    • Modelling of intensity noise, frequency noise and linewidth of semiconductor laser and their dependence on optical gain formulation


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      We introduce small-signal modelling on both relative intensity noise (RIN) and frequency noise (FN) as well as the associated linewidth of the semiconductor laser. Influence of gain suppression on the frequency characteristics of the RIN and FN spectra and the corresponding damping rate and frequency of relaxation oscillations are elucidated. Also, we investigate the dependence of laser noise on the mathematical form of nonlinear gain using three common formulas of optical gain over a wide range of injection current. We show variations of the levels of both RIN and FN with the form of optical gain in the regime of resonance frequency as a result of variation of damping rate and resonance frequency. The tolerances in predicting the RIN and FN levels are 1% and 14%, respectively, which corresponds to the tolerance of linewidth of 21.6% at low frequency of 10 MHz and current of five times its threshold value, while they are 39% and 21.6% at the relaxation frequency.

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