A report is presented on the use of nanoimprint lithography for the fabrication of InP-based laterally-coupled ridge waveguide distributed feedback laser (DFB) diodes emitting around 1550 nm. At room temperature the uncoated lasers exhibited a sidemode suppression ratio of 50 dB at an output power of 6 mW. The lasers showed extremely pure singlemode spectrum with the Lorentzian part of the measured linewidth being 200 kHz. The results prove that nanoimprint lithography is suitable for the low-cost manufacturing of high performance singlemode laser diodes.Introduction: Conventional distributed feedback (DFB) lasers make use of buried Bragg gratings for achieving selective optical feedback. They require epitaxial regrowth [1], which complicates the fabrication process and can degrade device performance. Another possibility to implement the distributed feedback is to etch lateral gratings on the surface of the device enabling a regrowth-free process [2]. Typically, such surface gratings have been realised by means of electron-beam lithography. We have recently devised an approach to process high aspect-ratio surface gratings using nanoimprint lithography (NIL) and plasma etching [3]. The method consists in defining a laterally-corrugated ridge waveguide (LC-RWG) structure, in a single step, by using a cost-effective and high-throughput soft stamp UV-NIL technology. With this method a full wafer can be patterned in a few minutes at an extremely high resolution. The soft and flexible stamp accommodates locally to micro-particle presence [3] and enables large area imprinting even for wafer surfaces with curvatures caused by the internal strain of the epitaxial layers. This fabrication technique is easily applicable to different material systems; we have used it before for GaAs [3,4] and GaSb [5]. In this Letter, we present results of the first InP-based LC-RWG DFB lasers based on UV-NIL.
Two novelties have been exploited in developing high-speed directly-modulated distributed feedback and distributed Bragg reflector lasers: the use of surface gratings, to enable a single growth and processing sweep, and the use of photon-photon resonance, to enhance the direct modulation bandwidth beyond the limits set by the carrier-photon resonance.
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