Abstract
We investigate three approaches to low perturbation gratings to achieve lower linewidths in filters and semiconductor lasers. The three designs, which are labeled post, sampled, and high order, are DUV lithography compatible and were fabricated on 90 nm thick Si(3)N(4) strip waveguides. Reflection and transmission spectra measurements show coupling constant, kappa, values ranging from 0.23 cm(‑1) to 1.2 cm(‑1) with FWHM values of 74 pm to 116 pm. We discuss the tradeoffs between these geometries in terms of lowest linewidth, apodization, and curved waveguide layout. These results enable long cavity single mode lasers with kHz level linewidths on a monolithic platform.
Highlights
Bragg gratings have found many applications in sensors, telecommunication filters, and semiconductor lasers
With a low loss waveguide platform, lower coupling constant, kappa ( κ ), values can be utilized to lengthen the grating, reducing the linewidth to the performance level of fiber Bragg gratings [6] and lasers utilizing those gratings [7,8]. These narrow bandwidths pave the way for sub-kHz lasing linewidths with monolithically integrated lasers, for instance, by coupling to Si/III-V active devices as previously demonstrated [9]
We demonstrate extremely low κ designs in three different waveguide perturbation geometries, and show κ values ranging from 0.23 cm−1 to 1.2 cm−1
Summary
Bragg gratings have found many applications in sensors, telecommunication filters, and semiconductor lasers. Bragg gratings are important devices for reaching passive linewidths commensurate with >500k quality factors (sub-100 pm FWHM) while being readily integrated with a waveguide coupled gain element Their single frequency nature and ability to be spatially sampled or apodized allows suppression of high order longitudinal modes in distributed Bragg reflector (DBR) and distributed feedback (DFB) lasers [4,5]. With a low loss waveguide platform, lower coupling constant, kappa ( κ ), values can be utilized to lengthen the grating, reducing the linewidth to the performance level of fiber Bragg gratings [6] and lasers utilizing those gratings [7,8] These narrow bandwidths pave the way for sub-kHz lasing linewidths with monolithically integrated lasers, for instance, by coupling to Si/III-V active devices as previously demonstrated [9]. We discuss the tradeoffs of these geometries in terms of lowest linewidth, apodization, and curved waveguide layouts
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