Impact of the saturable absorber on the linewidth enhancement factor of hybrid silicon quantum dot comb lasers (Student paper)
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Laser linewidth
Saturable absorption
Laser linewidth
Differential gain
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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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The quantum dot laser is a complex nonlinear system in which light fields dynamically interact with the charge carriers in the dots and the embedding quantum well medium. In real laser systems, typical dot-to-dot variations in size, energy levels and material parameters exist. In addition, the dots are not equally positioned on a grid within the layers. The respective variance in quantum dot parameters and dot-to-dot distance depends on the material system and the epitaxial growth process of the particular quantum dot system. To elucidate the influence of spatial fluctuations, we calculate the temporal light field dynamics of quantum dot lasers with variable fluctuations in the characteristic dot parameters. The simulations on the coupled ultrafast spatio-temporal light-field and carrier dynamics in quantum dot lasers are based on a two-level Multi-Mode Maxwell-Bloch description. The constituent equations consist of coupled spatio-temporally resolved wave equations and Bloch equations for the carriers within each quantum dot of a dot ensemble constituting the active gain medium of a quantum dot laser. It is shown that the light field dynamics and the emission spectra are strongly determined by the nonlinear coupling between the light fields and the charge carrier plasma, spatially varying material properties of the quantum dot ensemble as well as device geometry and carrier injection.
Charge carrier
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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 apply the self-mixing method for the measurement of the linewidth enhancement factor of several types of semiconductor lasers. The α-factor value above threshold is determined by analysing the small perturbations that occur to the laser when it is subjected to moderate optical feedback, relying on the well-known Lang-Kobayashi equations. The method is applied to Fabry-Perot, VCSEL, External Cavity Laser (ECL), DFB, Quantum Cascade Laser. It is found that for some lasers the α-factor varies with the emitted power, and these variations can be correlated with variations in the laser linewidth.
Laser linewidth
Vertical-cavity surface-emitting laser
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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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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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The requirement of the linewidth of semiconductor lasers used in coherent optical communication systems is very stringent. DFB semiconductor lasers operating in the free-running state in the longer-wavelength region typically have lens of megahertz linewidths, which are still too wide to be useful. The electrical negative feedback control of the laser frequency has been proposed as one method for narrowing the linewidth. A 200-kHz linewidth of 1.5-μm DFB lasers has been achieved by using this scheme.1
Laser linewidth
Semiconductor optical gain
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