Reduced temperature dependence of threshold current by broadband enhanced feedback: A new approach and demonstration

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Abstract
A new approach to reduce the rates of threshold current increase and external quantum efficiency decrease with increasing temperature in a semiconductor laser is proposed and demonstrated. This is based on the realization that the total laser cavity loss, which is normally relatively independent of wavelength, can be made to decrease with increasing wavelength. Since the gain peak of the laser shifts towards longer wavelength as temperature is increased, the rate of threshold current increase is thus reduced with increasing temperature. To implement this, we proposed and demonstrated the broadband enhanced feedback (BEF) approach using multilayer coatings, very weak or chirp gratings with peak reflectivity centered at the lasing wavelength corresponding to the maximum operating temperature desired. It was shown that the threshold-temperature coefficient T0 improved from the typical value of ∼45 K in long wavelength InGaAsP lasers (bulk active or MQW) to 85 K in BEF coated lasers. This scheme also simultaneously decreased the rate of quantum efficiency degradation with increasing temperature.