Design strategy for broadband MECSELs

Abstract

Membrane external-cavity surface-emitting lasers (MECSELs) [1] are made of a semiconductor laser-active membrane solely sandwiched between two transparent heat spreaders [2] . This gain element is then inserted in a laser cavity. Monolithically integrated distributed Bragg reflectors (DBRs), which are typically used in conventional vertical-external-cavity surface-emitting lasers (VECSELs) [3] are not needed. Additional benefits are the double-side cooling, as the active region is directly embedded between two heat spreaders, and the possibility of double-side pumping [4] . Besides that, the epitaxial growth of the semiconductor gain structure is drastically simplified, as there is no need to grow the thick DBRs. Since the DBR has typically a certain bandwidth limiting the wavelength tunability, the absence of the DBR allows to exploit the full gain bandwidth of the implemented gain structure (quantum wells or quantum dot layers). External dielectric mirrors with broadband coatings are used instead to build the cavity, which allows the phase of the standing light wave to not be fixed by the DBR [5] . This makes it significantly easier to match the antinodes of the standing light wave with the resonant periodic gain structure. When looking at the tuning ranges of vertically emitting semiconductor lasers, those of VECSELs but especially MECSELs, are amongst the best results originating from semiconductor lasers, but do not yet reach the bandwidth of solid state lasers based on Titanium or Chromium doped crystals (for example Ti 3+ :Al 2 O 3 , Cr 3+ :La 3 Ga 5.5 Nb 0.5 O 14 ) [6] . The approach of combining different quantum wells in a single semiconductor structure [7] and applying this to the MECSEL concept opens new paths towards semiconductor lasers with an extremely large bandwidth.