| Conventional studies on weak optical feedback (OFB) into a semiconductor laser (SL) have been extended to strong OFB. Three strong OFB systems are investigated either experimentally or theoretically: (1) Strong OFB to low-power SLs; (2) Self-pulsing SLs (SPSL) with strong OFB; (3) High-power broad-area SLs (BAL) with grating OFB. Strong OFB not only introduces the well-known low-frequency fluctuation (LFF) phenomenon but also produces stable single-mode operation, which some researchers call "maximum gain mode." Compared to weak OFB, strong OFB creates large intensity fluctuations from subthreshold to far-above threshold. Special power intermittency called "power jump-ups" in the LFF region has also been found for the first time. Strong OFB into a SPSL has shown interesting nonlinear dynamics. Arnold tongues and return maps have been used for analysis of this two frequency (external-cavity round-trip and self-pulsation frequencies) system. For BALs with grating OFB, the grating passband limits and narrows the effective bandwidth of the system while grating OFB actually destabilizes the spatial distribution of BALs. It is found that, for short coherence length laser diodes (gain-guided lasers or BALs), grating OFB will draw the SL into a region called "coherence collapse" and the system's bandwidth will fulfill the grating passband. Although grating OFB destabilizes BALs spatially, free-running BALs exhibit severe filamentation effect at high injection current. Thus, grating OFB into BALs at high injection current does not show strong impact on BAL's spatial coherence. Finally, applications utilizing high-power external-cavity diode lasers (HPECL) and diode-pumped power-buildup cavities (LDPBC) for Raman spectroscopy are also investigated. |
