λ/4 shitted distributed feedback laser diodes (DFB LDs) have been developed as highly stable single-longitudinal-mode (SLM) light sources because of the large threshold gain difference between the main mode and submode.1 However, recently it was pointed out that an unevenly distributed optical field along the cavity might degrade SLM stability. The mechanism is as follows: Spatial hole burning, Induced by such an optical field, causes the refractive index to increase at around the phase shift region.2 A derivative phase shift is substantially added to the λ/4 phase shift built in the grating. As a result, an equivalent phase shift larger than λ/4 is induced. Therefore, the threshold gain difference between the main mode and the short-wavelength mode nearest the main mode (+1 mode) decreases. The +1 mode is likely to oscillate with the optical field increase. From this point of view, a DFB LD with a grating whose phase shift value is <λ/4 is expected to show more stable SLM operation up to high output power by suppressing the +1 mode oscillation. This paper reports stable SLM properties of a 1.55-μm DFB-DC- PBH LD3 with the phase shift value of A/8.
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