Low-cost 1.3um InGaAlAs/InP laser integrated with laterally tapered SSC in reverse mesa shape

There is a big difference between the spot sizes of diode lasers and those of fibers or Si based waveguide, which lead to a high cost related to the assembling and packaging of lasers. To reduce the cost of optical transmitter module, it is promising to integrate monolithically a spot size converter (SSC) which expands spot size with LDs. Up to now, a number of different types of SSC have been proposed. Among them, laterally tapered SSC fabricated in the InP cladding layer of a LD needs only a simple fabrication process, helping to lower the device cost. However, it is difficult to obtained a narrow waveguide tip through conventional photolithography, which is key for high quality SSC. In this paper, we report the fabrication of a laterally tapered InP SSC in a reverse mesa shape. With an optimized wet etching condition, a reverse ridge waveguide for which the top width is significantly larger than the bottom width can be obtained. With this process, a SSC having a waveguide tip as narrow as 200 nm can be obtained with a high yield through conventional photolithography. A 1.3 μm InGaAlAs/InP laser integrated with the laterally tapered SSC has been fabricated. The threshold current of the 1000 μm long device is 25 mA and 44 mW single facet optical power can be obtained at 300 mA current. The lateral and vertical divergence angles are as low as 11° and 13°, respectively.