A new tunable laser using a single electroabsorption tuning super structure grating for subnanosecond switching applications

A new tunable laser structure using a single electroabsorption (EA) tuning super structure grating (SSG) section for fast wavelength switching operation is reported. Instead of index change by carrier density or by thermal variation, the tuning mechanism of this absorption change (AC) distributed Bragg reflector (DBR) InGaAsP strained QW laser is based on a localized absorption variation induced in a Franz-Keldysh EA SSG section used as Bragg reflector. According to a simulation which takes into account the absorption and refractive index variations of the SSG section versus voltage and wavelength, an optimized device theoretically allows a wide tuning range of up to 11 nm with nine regularly spaced wavelength channels. Over this whole tuning range, the variation of the AC-DBR laser threshold gain is lower than 2 cm^-1 . Compared to a standard DBR laser exhibiting an 8-cm^-1 threshold gain variation for the same wavelength shift, this AC-DBR laser would present extremely low threshold current and emitted power variations under tuning. Owing to its tuning mechanism, the switching time of the AC-DBR laser is independent of the wavelength shift, and only limited by the parasitic capacitance of the SSG ridge section. A test structure, easier to realize, with only a theoretical tuning range of 3 nm over four regularly spaced channels has been fabricated. These devices exhibiting a tuning range of 2.5 nm with three modes, present threshold current and output power variations under tuning of less than 8 mA and 1.2 dB, respectively. A switching time between two successive wavelength channels shorter than 250 ps has been observed, in agreement with typical capacitances of 0.8 pF/100 μm measured on the SSG section