Based on a semiconductor laser （SL） with incoherent optical feedback, a novel all-optical scheme for generating tunable and broadband microwave frequency combs （MFCs） is proposed and investigated numerically. The results show that, under suitable operation parameters, the SL with incoherent optical feedback can be driven to operate at a regular pulsing state, and the generated MFCs have bandwidths broader than 40 GHz within a 10 dB amplitude variation. For a fixed bias current, the line spacing （or repetition frequency） of the MFCs can be easily tuned by varying the feedback delay time and the feedback strength, and the tuning range of the line spacing increases with the increase in the bias current. The linewidth of the MFCs is sensitive to the variation of the feedback delay time and the feedback strength, and a linewidth of tens of KHz can be achieved through finely adjusting the feedback delay time and the feedback strength. In addition, mappings of amplitude variation, repetition frequency, and linewidth of MFCs in the parameter space of the feedback delay time and the feedback strength are presented.
The polarization switching（PS） and polarization bistability（PB） characteristics of a 1550-nm vertical-cavity surfaceemitting laser（VCSEL） subjected to orthogonal optical injection are systematically investigated.The simulated results show that the PS and polarization-resolved nonlinear dynamical states of the VCSEL are critically dependent on the changing paths of the injected power.The polarization dynamics for different scanning directions of the injected power is presented to explain the polarization evolution during the formation of PS.In the case of forward scanning injected power,with the increase of frequency detuning level between the VCSEL and the injected light,the injected power required for PS gradually increases for negative frequency detuning but exhibits fluctuations for positive frequency detuning.In the case of reversely scanning injected power,the injected power required for PS displays fluctuant changes within the whole frequency detuning range.Specifically,PS may disappear under certain negative frequency detuning and large bias current.Furthermore,the hysteresis width as a function of the frequency detuning is calculated,and the regions for the appearance and disappearance of PB have been determined in the parameter space of the bias current and frequency detuning.
Through employing permutation entropy and the self-correlation function, the time-delay signature （TDS） of a vertical-cavity surface-emitting laser （VCSEL） with variable-polarization filtered optical feedback （VPFOF） is evaluated theoretically. The work shows that the feedback rate η, polarizer angle Op, and filter bandwidth A have an obvious influence on the TDS. The evolution maps of the TDS in parameter space （η, A） and （ηθp） are simulated for searching the chaos with weak TDS. Furthermore, compared with a VCSEL with polarization-preserved filtered optical feedback and a VCSEL with variable-polarization mirror optical feedback, this VPFOF-VCSEL shows superiority in TDS suppression.
The time-delay signature(TDS) of chaos output in a 1550 nm vertical-cavity surface-emitting laser(VCSEL) subject to fiber Bragg grating(FBG) feedback is investigated experimentally. Autocorrelation function(ACF) and mutual information(MI) are used for quantitatively identifying the TDS of chaos. For various bias currents, the TDS evolution with the feedback strength is different, as the FBG provides wavelength-selective feedback. Furthermore,based on the TDS map of the FBG feedback VCSEL(FBGF-VCSEL) in the parameter space of feedback strength and bias current, the optimal TDS suppression regions, where the dominant polarization mode of FBGF-VCSEL locates at the edge of the main lobe of FBG reflection spectrum, have been determined. Finally, for comparative purpose,the TDS of chaos in mirror feedback VCSEL(MF-VCSEL) also has been presented, and the results show that an FBGF-VCSEL possesses better TDS suppression performance than an MF-VCSEL.
