PURPOSE: To obtain the stable fiber light output by reducing a slip of optical axes even by effecting an environmental test by making a linear thermal expansion coefficiency of a metallic substrate stuck to the optical fiber by a low- temperature solder approximate to that of the solder within a range of ±10%.
CONSTITUTION: A semiconductor laser element 1 and a single mode fiber SMF2 are connected to a metallic substrate 4 with their optical axes being in accordance. The SMF2 is stuck to the substrate 4 by a low-temperature solder 3 whose melting point is 150°C or under. Then the substrate 4 is composed of the metal whose linear thermal expansion coefficiency is in accordance with that of the solder 3. By such constitution, as the internal residual stress consequent on the bond of the substrate 4 with the solder 3 is small, there is no creep produced by a temperature cycle test or even by leaving for a long time. Accordingly, there is no slip of optical axes consequent on positional displacement of the SMF2 and the stable fiber light output can be obtained.