Broad area lasers with folded-resonator geometry for integrated transverse mode selection

Abstract

AlGaInAsSb-based broad area lasers (BALs) with a monolithically integrated Fourier-optical 4f set-up in a folded-resonator geometry are realized. The two resonator branches - each one d = 0.825 mm long - are connected through a dry-etched cylindrical total-internal-reflection (TIR) mirror acting as a Fourier-transform element. Transverse mode selection (TMS) is achieved by monolithically integrated spatial-frequency filters positioned in the back focal plane of the mirror (i.e. in the Fourier-transform plane). The whole resonator is gain section (active medium) as well as part of the TMS 4f set-up at the same time. The integration of TMS within the active BAL chip is shown to be successful. All employed BAL/TMS type-II heterostructure lasers are MBE-grown on GaSb substrates, designed for an emission wavelength in the mid-infrared around 2 μm. Different laser samples without any filter elements (no-TMS) and with filters for the selection of the fundamental transverse mode (#0; TMS0) are prepared and characterized. Just for a proof of principle also samples for the selection of higher order transverse modes, here exemplarily mode #6 (TMS6) and #8 (TMS8), have been processed and investigated. The free spectral range between the longitudinal modes is found to be around 0.33 nm corresponding to the BAL's total-resonator length 2d = 1.65 mm (with an effective refractive index neff ≈ 3.8). This result strongly emphasizes that both resonator branches act together as one entity.