An optical module providing a semiconductor optical amplifier (SOA), and a process to assembly the optical module are disclosed. The optical module provides front and rear coupling units each optically coupled with the SOA and fixed to the housing enclosing the SOA. The housing has a slim wall fixing a lens holder soldered to the slim wall. The front and/or rear coupling unit is fixed to the lens holder by YAG laser welding after the active alignment by using a spontaneous emission of the SOA, and amplified emission of externally provided test beam.