The optimal structure of a laser diode monolithically integrated with a thickness-tapered beam expander waveguide is demonstrated by analyzing the relationship between fiber coupling efficiencies and radiation losses. It is also found that mode conversion loss is lowered in a ridge waveguide structure than in a buried hetero structure under equivalent fiber coupling. A fabricated ridge waveguide device based on this design shows threshold current as low as 16 mA and narrow beam divergences of 13° and 12°.