1.5 µm asymmetric λ/4-shifted InGaAsP/InP DFB lasers, in which the λ/4-shift position was moved from the center of the DFB region toward the front side, were made in order to obtain higher output power with high single-longitudinal-mode (SLM) yield. Statistical measurements revealed that it was effective for the increase of the differential quantum efficiency from the front facet without a remarkable decrease of the SLM yield to move the λ/4-shift position to the front facet by 10–15 percent of the total DFB length. The output efficiencies of the diodes with AR coatings on the window structure were almost coincident to those expected from theoretical calculations. The main to submode ratio P0/P1 in the rear spectra of the asymmetric devices noticeably decreased with increasing asymmetry. From an experimental point of view, a criterion for stable SLM operation, for example, P0/P1 ≳ 1.5 in the rear spectrum at I = 0.9 Ith for the λ/4-shift position lf:lr = 35:65, is presented.
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