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Narrow-Linewidth DFB and DBR Lasers with Surface Gratings Fabricated Using UV-Nanoimprint Lithography

Research output: Book/ReportDoctoral thesisCollection of Articles

Details

Original languageEnglish
PublisherTampere University of Technology
Number of pages70
ISBN (Electronic)978-952-15-4071-4
ISBN (Print)978-952-15-4048-6
Publication statusPublished - 24 Nov 2017
Publication typeG5 Doctoral dissertation (article)

Publication series

NameTampere University of Technology. Publication
Volume1512
ISSN (Print)1459-2045

Abstract

This thesis presents the design, modeling, and fabrication of high performance narrowlinewidth edge-emitting semiconductor distributed feedback (DFB) and distributed Bragg reflector (DBR) lasers employing surface gratings fabricated without epitaxial re-growth. The re-growth free fabrication method mitigates the risks of contamination and atmospheric oxidization in the active and waveguide layers, improving the device performance, reliability and yield. Compared to conventional buried gratings, in the surface gratings the injected carriers have much less interaction with the defect prone processed interfaces, which decreases the non-radiative recombination and the probability of degradation, increasing the reliability, particularly at high injection currents.

In this work soft-stamp ultraviolet (UV)-nanoimprint lithography (NIL) has been used to define the etch masks. Unlike in conventional photolithography, the UV-NIL’s resolution is not limited by diffraction and scattering effects. Moreover, UV-NIL has low fabrication costs and high throughput, which makes it ideal for a large scale mass production.

The thesis discusses how the device design can influence the emission linewidth and how this affects other laser characteristics, some of which require contradictory design parameters. For example a decreasing mirror loss reduces the linewidth but also reduces the slope efficiency, limiting the maximum output power. Also novel device design elements have been introduced to alleviate the technological limitations of the fabrication process. A laterally coupled (LC)-ridge waveguide (RWG) grating with lateral protrusions alternating on the sides of the central ridge has been employed to enable the fabrication of gratings having wider trenches than the grating period and reduced lateral current leakage. Under the technological restrictions given by the achievable etching aspect ratio, the wider trenches enable the fabrication of lower order gratings, reducing radiation losses.

The fabricated devices achieved state-of-the-art characteristics: 30 mW to 40 mW output power with ∼10 kHz full width at half maximum (FWHM) linewidths at 300 mA bias current for 780 nm DFB lasers and ∼500 mW output power with <250 kHz FWHM linewidths at 1630 mA bias current for 1180 nm DBR lasers. Also monolithic master oscillator power amplifier (MOPA) lasers have been fabricated to avoid the compromise between achieving a narrow linewidth and a high power as well as to enable independent control of the emission wavelength and output power. ∼7 W output power was obtained for a 780 nm MOPA laser with a 3 mm long DFB master oscillator (MO) section and a 4 mm long tapered power amplifier (PA) section under 500 mA continuous wave (CW)mode bias for the MO section and 15 A pulsed-mode bias (1 µs pulse width and 1% duty cycle) for the PA section. The pulsed-mode operation of the PA section, which did not enable accurate emission linewidth measurement for the MOPA lasers, was employed because the p-side up mounting did not provide good enough thermal management.

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