Operation of a laser diode (LD) module for a single‐mode fiber (SMF) is affected by reflection from coupling elements, in particular, from the SMF endface. For an LD module employing a cylindrical lens method, it has been determined that the phase of the reflected beam fluctuates with temperature, which causes fluctuation in the modulated waveform. It was also found that LD modules are easily subject to fluctuation if an external cavity constructed by the LD and SMF endface has a larger mode spacing than that for the LD cavity. LD modules with anti‐reflection characteristics have been fabricated using a combination of two confocal‐lenses with large separation between the LD and the SMF. These modules exhibit stable operation against temperature variation.
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Sugie et al. (1982) studied this question.