Phase-locked 1.3 µm VCSEL arrays based on patterned tunnel junction

Vertical cavity surface emitting lasers (VCSEL) are already used in many applications thanks to their unique device characteristics, particularly compact size, low power consumption, accurate wavelength setting, circular beams and on-wafer testing. The main disadvantage of VCSELs as compared to edge emitting lasers is the relatively low single mode output power (a few milliwatts) limited by their small active area. To overcome this limitation, phase-locked arrays of short wavelength (< 1 mum) VCSELs have been investigated using different fabrication approaches. This paper reports on the fabrication and lasing characteristics of phase-locked 1300 nm wavelength VCSEL arrays made using double wafer fusion. The VCSEL array structures incorporate an In(Al)GaAs/InP strained quantum wells active cavity and fused bottom and top AlGaAs/GaAs distributed Bragg reflectors. The VCSEL arrays have a maximal output power of above 9 mW, limited by thermal roll-over. Due to current spreading beyond the pixel boundaries, the average threshold current per pixel in the 1times4 and 2times4 array structures is smaller compared with a single VCSEL. Below threshold, at the pumping current of 2 mA, the near field shows an amplified spontaneous emission distribution. Above threshold the optical mode shows four emission peaks in accordance with the defined TJ pattern. Also, phase locking of the array elements is evidenced by two main lobes in the far field pattern, indicating phase shift of pi between neighboring VCSELs. Similar characteristics evidencing phase locking of the out-of-phase supermode are exhibited by the 1times8 and 2times4 arrays.