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High-speed directly modulated Fabry-Perot and distributed-feedback spot-size-converted lasers suitable for passive alignment, unisolated operation, and uncooled environments up to 85°C

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13 Author(s)
Klotzkin, D. ; Univ. of Cincinnati, OH, USA ; Kojima, K. ; Jordache, N. ; Chand, N.
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Semiconductor lasers with narrow far fields are highly desirable because they can be directly coupled to fiber with high-coupling efficiency, eliminating the cost of lenses and extra packaging. Because of their smaller size, they facilitate more compact transmitters, and since they can be used unpackaged, they enable complex integrated optoelectronic devices. We describe the design, fabrication, and performance characteristics of our family of spot-size-converted (SSC) devices with narrow far fields designed for directly modulated (2.5 and 10 Gb/s) uncooled (85°C) use. The design consists of a conventional active region buried heterostructure laser coupled to an expander region (consisting of a loosely confining waveguide) through a laterally tapered etch of the active layers. This basic design achieves far fields of ∼16×10 degrees, suitable for coupling ∼50 percent of the emitted light into a flat cleaved fiber. Distributed feedback (DFB) 1.3- and 1.5-μm devices, and 1.3-μm Fabry-Perot (F-P) lasers have been implemented in this technology. The devices have dc thresholds from 8-12 mA at 25°C and 35-45 mA at 85°C, with peak power of >15 mWs over temperature, all similar to non-SSC devices with the same active regions. Both SSC F-Ps and DFBs demonstrate bandwidths of >7 GHz and wide-open eyes at 85°C, and reliability suitable for uncooled use. With the DFB device, we also demonstrate a wide open 10-Gb/s eye pattern at room temperature. The laser on a submount demonstrates coupling to a fiber in a v-groove of ∼25% using passive alignment, and sufficient tolerance to back reflection to enable transmission over at least 15 km with typical drive circuitry. Analysis shows that these lasers also have a factor of two improvements in alignment tolerance compared to standard devices in a typical lensed system. The expander impacts the dc and dynamic characteristics of the SSC F-P through the increase in cavity size. The dc and dynamic characteristics of SSC DFBs and standard DFBs are very similar, demonstrating that expander-related absorption and mode transition loss has been almost eliminated in this structure. The SSC-DFBs are, thus, the preferred device for high-speed applications. To ou- r knowledge, the 1.3-μm lasers described are the first SSC devices fully suited for use as an uncooled 2.5-Gb/s transmitters up to 85°C. The combination of a tailored narrow far field with an edge emitting structure rivals vertical cavity lasers in alignment tolerance for low-cost packaging with superior dynamic and thermal performance.

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Lightwave Technology, Journal of  (Volume:21 ,  Issue: 1 )