Simulation and analysis of 1.55μm quantum dot lasers designed for ultra-narrow spectral linewidth
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Quantum dots (QDs) are well known as active materials with remarkable properties such as high differential gain, very low linewidth enhancement factor and low threshold current density. Using a comprehensive in-house developed laser simulator based on the traveling wave method, we investigate the limitations in terms of spectral purity of quantum dot based distributed feedback lasers (DFB) for use in high bit rate optical communications. Even though quantum dot based edge emitting lasers have demonstrated linewidths below the standard quantum well and bulk lasers, by optimization of the resonant cavity we further reduce the linewidth to below 20KHz while studying the resulting operating conditions.Keywords:
Laser linewidth
Differential gain
Laser linewidth
Differential gain
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Our approach to the problem is based on the following basic concepts: using a spatially-distributed laser model; using an approximate Kovalevski-Noppe (KN) expression for effective natural linewidth for semiconductor lasers with application of generalized parameters. Physical representations of secondary spontaneous fluctuations have been formulated. These allow deriving formulas for generalized parameters in the KN expression for linewidth, on the basis of which the gain, foundation of the nonlinear system of equations, is derived. The effective natural linewidth is calculated after solving the nonlinear system of equations. Thus, the theory of natural linewidth is an integral part of the nonlinear theory on the basis of which the output power, linewidth, and threshold current for Fabry-Perot semiconductor lasers are calculated. The calculations made allow explaining some experimental measurements of natural linewidth for the Fabry-Perot semiconductor lasers.
Laser linewidth
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An experimental comparative study of the gain, index variation, and linewidth enhancement factor in 980-nm quantum-well (QW) and quantum-dot (QD) lasers structures, designed for high power applications, is presented. The gain spectra of the QW lasers at high injection level revealed three different transition energies, with a low linewidth enhancement factor (/spl sim/1.2) for E2HH2 transitions. Similar values for the linewidth enhancement factor, ranging between 2.5 and 4.5, were found for QW and QD devices, when comparing at similar values of the peak gain. This result is attributed to the contribution of excited state transitions in the measured QD lasers.
Laser linewidth
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Quantum well (QW) lasers have been predicted to have enhanced differential gain compared to their bulk counterparts—double heterostructure (DH) Quantum well (QW) lasers have been predicted to have enhanced differential gain compared to their bulk counterparts—double heterostructure (DH) lasers.' Thus, higher modulation bandwidth is expected for QW lasers due to the higher differential gain. However, the modulation bandvvidths of QW lasers do not show too much improvement over the DH lasers in experiments, especially for the case of SQW lasers. Enhanced photon density dependent gain compression,^ which leads to anomalously high damp;; ing, and carrier transport mechanism’ have been proposed to explain modulation dynamics of QW lasers. To fully understand the modulation dynamics of QW lasers it is necessary to carefully investigate the differential gain of QW lasers. The separate confinement heterostructure (SCH) QW structure is usually employed in QW lasers to support the quantum confinement of the injected carriers and confinement of the optical field. At finite temperature, due to the Fermi-Djrac statistics, the injected carriers populate not only the energy states of the QW(s) but also the large density of states in the SCH structures. The carriers populating the SCH states contribute little to the peak gain because of their nonresonance with the optical transition at the peak gain. The carrier population of SCH states makes the quasi-Fermi energy levels change slovvly with the change of injected carrier density. These lead to lower differential gain. Figure 1 shows the calculated differential gain for a typical GaAs/ AlGaAs DH laser and typical GaAs/ AlGaAs QW lasers with various number of wells. The differential gain in SQW structure is lower than that of the DH structure. There is a differential gain enhancement as the number of wells CWR5 Fig, 1. Differential gain as a function of modal gain for a typical GaAs/AlGaAs DH laser and typical GaAs/AlGaAs QW lasers with different quantum well number. increases in MQW structures. The differential gain enhancement in MQW structures is attributed to the fact that the effect of carrier population in SCH structure is distributed among the QWs and the quasi-Fetmi level is lower in MQW structures than that in a SQW structure for the same value of modal gain. These results, combined with considerations of damping effects and operating conditions, are consistent with the experimental observations. Figure 2 shows the maximum modulation bandwidth for DH laser and QW lasers with different well number. We find that some of the previous conclusions about QW lasers need to be changed by considering the unavoidable thermal population of injected carriers in the optica] SCH region. CWR5 Fig. 2. Maximum modulation bandwidth as a function of cavity length for a typical CaAs DH laser and typical GaAs QW laser.s with different well number.
Differential gain
Gain compression
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A theory of the low-frequency phase fluctuations in the output of a semiconductor laser due to spontaneous emission is developed. The theory can be used as a tool to numerically calculate the linewidth of complicated laser structures, e.g., by use of the transfer matrix formulation. The results for the single Fabry-Perot laser are shown to be in exact agreement with the most accurate treatments published so far. Results are then presented for both DFB and F-P lasers with external optical feedback showing how the linewidth varies with the threshold gain, with the coupling coefficient, and with the external feedback conditions.
Laser linewidth
Semiconductor optical gain
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Quantum dots (QDs) are well known as active materials with remarkable properties such as high differential gain, very low linewidth enhancement factor and low threshold current density. Using a comprehensive in-house developed laser simulator based on the traveling wave method, we investigate the limitations in terms of spectral purity of quantum dot based distributed feedback lasers (DFB) for use in high bit rate optical communications. Even though quantum dot based edge emitting lasers have demonstrated linewidths below the standard quantum well and bulk lasers, by optimization of the resonant cavity we further reduce the linewidth to below 20KHz while studying the resulting operating conditions.
Laser linewidth
Differential gain
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We investigate numerically influence of linewidth enhancement factor and injection current on the route to chaos and associated operation states of semiconductor lasers subject to optical feedback. Numerical solutions of a time-delay rate equations are employed to construct bifurcation diagrams. The simulation result shows that changes in the linewidth enhancement factor cause important changes in the route to chaos and the laser states. The state of the laser is identified into six distinct regimes, namely, continuous wave, periodic oscillation, period doubling or two, period-three, period-four oscillations and chaos, which is depending on the linewidth enhancement factor, optical feedback strength and injection current level. Decreasing the injection current and increasing the linewidth enhancement factor stimulate the laser to be more stable and operate in continuous and periodic oscillations.
Laser linewidth
Oscillation (cell signaling)
Optical chaos
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We report linewidth properties of active-passive coupled monolithic InGaAs semiconductor ring lasers with various length of passive waveguide. It is experimentally confirmed that the linewidth of the lasers is proportional to the square of the ratio of the length of active part of the cavity over the total length of the cavity. The lasers are applicable for communication and sensing devices, which need the narrow linewidth.
Laser linewidth
Active layer
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Higher speed short-wavelength (850 nm) VCSELs are required for future high-capacity, short-reach data communication links. The modulation bandwidth of such devices is intrinsically limited by the differential gain of the quantum wells (QWs) used in the active region. We present gain calculations using an 8-band k·p Hamiltonian which show that the incorporation of 10% In in an InGaAs/AlGaAs QW structure can approximately double the differential gain compared to a GaAs/AlGaAs QW structure, with little additional improvement achieved by further increasing the In composition in the QW. This improvement is confirmed by extracting the differential gain value from measurements of the modulation response of VCSELs with optimized InGaAs/AlGaAs QW and conventional GaAs/AlGaAs QW active regions. Excellent agreement is obtained between the theoretically and experimentally determined values of the differential gain, confirming the benefits of strained InGaAs QW structures for high-speed 850-nm VCSEL applications.
Differential gain
Modulation (music)
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A simple expression for the spectral linewidth of passive-coupled-cavity (PCC) semiconductor lasers is presented. Experimentally observed linewidth narrowing and periodic variation of the linewidth as functions of the injection current and the gap between the active cavity (laser) and the passive cavity (graded index, or GRIN rod) can be satisfactorily explained by this formula.< >
Laser linewidth
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