Operation of a strained-layer distributed-feedback surface-emitting laser

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Objective: To develop high-power InGaAs/GaAs strained-layer DFB surface-emitting (GSE) lasers and laser arrays. Background: Most GSE laser arrays fabricated to date, have been distributed-Bragg reflectortype lasers with the second order grating fabricated into the surface of the cladding layer (outside the gain section) at the air interface. In general, these devices tend to have higher threshold currents than comparable edge-emitters, and the far-field patterns can have significant side lobe structure. Improvements in these characteristics should be possible by extending the grating into the gain section and using a transparent substrate to transmit the grating-coupled light output. Technique: We report on InGaAs/GaAs strained-layer DFB-GSE lasers where a continuous second order grating is buried under the p-cladding layer, as shown in Fig. 1. The gain sections are distributed-feedback (DFB) lasers that are terminated by unpumped DBR waveguide sections. This type of GSE structure should have a lower threshold than DBR-GSE lasers. The wafers for these devices are grown by a two-step OMCVD process. In the first growth step, the n-clad, separate confinement heterostructure and strained InGaAs quantum well, undoped AlGaAs barrier layer, and GaAs grating layer are deposited. The wafer is removed from the reactor and the second order grating is fabricated by ion-beam etching into the GaAs layer over the entire surface of the wafer. Fabrication of the grating on a such a planar surface reduces nonunifonnities. The wafer is returned to the reactor to grow the p-clad and p-cap layers. Surface-emitting lasers are then fabricated as described in reference 1. Results: Undkr pulsed operation DFB-GSE lasers showed stable, single-lobed far-field patterns with drive current with almost no measurable change in the 0.16 degree beam divergence as shown in Fig. 2. The spectral output was also very stable with drive current, as shown in Fig. 3. The operating wavelength variation was less than 1 a over the range of operation. At certain operating levels, the two modes of the DFB-GSE laser can be seen in the spectrum (as shown in Fig. 3). The grating coupling constant can be determined from the separation of these two modes. In Fig. 4, the power-current curve is shown for a ten-element injection-coupled DFB-GSE array. The threshold of 25 mA per gain section corresponds to a threshold current density of about 200 A/cm2. This is about a factor of two lower than that measured for comparable DBR-GSE laser arrays. Impact: This is the first demonstration of an InGaAs/GaAs strained-layer DFB-GSE lasers and laser arrays. Single element devices show very stable far-field and spectral outputs with drive current. The ten element DFB-GSE array has the lowest threshold current density reported so far for a GSE array.