Abstract
In this paper, we propose and demonstrate a solution to the problem of coherence degradation and collapse caused by the back reflection of laser power into the laser resonator. The problem is most onerous in semiconductor lasers (SCLs), which are normally coupled to optical fibers, and results in the fact that practically every commercial SCL has appended to it a Faraday-effect isolator that blocks most of the reflected optical power preventing it from entering the laser resonator. The isolator assembly is many times greater in volume and cost than the SCL itself. This problem has resisted a practical and economic solution despite decades of effort and remains the main obstacle to the emergence of a CMOS-compatible photonic integrated circuit technology. A simple solution to the problem is thus of major economic and technological importance. We propose a strategy aimed at weaning semiconductor lasers from their dependence on external isolators. Lasers with large internal Q-factors can tolerate large reflections, limited only by the achievable Q values, without coherence collapse. A laser design is demonstrated on the heterogeneous Si/III-V platform that can withstand 25 dB higher reflected power compared to commercial DFB lasers. Larger values of internal Qs, achievable by employing resonator material of lower losses and improved optical design, should further increase the isolation margin and thus obviate the need for isolators altogether.
Highlights
We propose and demonstrate theoretically and experimentally a simple yet fundamental solution to the problem of feedback sensitivity in semiconductor lasers
Such resonators have become possible with the advent of the Silicon (Si) photonics platform [1,2] and employ a new laser resonator design wherein the majority of the optical energy resides not in the active region, which is the legacy design [3,4,5], but in a new low-loss Si guiding layer, which is an intrinsic part of the laser waveguide, up to ∼ 99% in our case [6,7]
This paper is organized as follows: following the basic laser theory background, we describe the design details of the high-Q heterogeneous Si/III-V laser used in our demonstration
Summary
We propose and demonstrate theoretically and experimentally a simple yet fundamental solution to the problem of feedback sensitivity in semiconductor lasers. This extreme loss reduction enables the use of output-side laser reflectivity approaching unity without, surprisingly at first, sacrificing output power This is due to the orders of magnitude increase in the internal stored optical energy attendant on the high-Q resonator design which compensates, under the right conditions, described in what follows, for the reduced output coupling. These high values of output reflectivity, in turn, allow only a minute fraction,
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