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Threshold currents of 1.52 μm InGaAsP/InPGaAsP/InP DFB lasers with second-order gratings have been reduced to 12 mA in single-longitudinal-mode operation. External differential efficiencies exceed 25%/feet with a λ/4 Si3N4 coating on the front facet.
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Burkhard et al. (1986) studied this question.
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