Abstract
We present a detailed analysis of a semiconductor hybrid laser exploiting spectral control from an external photonic waveguide circuit that provides frequency-selective feedback. Based on a spatially resolved transmission line model (TLM), we have investigated the output power, emission frequency, and the laser spectral linewidth. We find that, if the feedback becomes weaker, the spectral linewidth is larger than predicted by previous models that are based on a modified mean-field approximation, even if these take a strong spatial variation of the gain into account. The observed excess linewidth is caused by additional index fluctuations that are associated with strong spatial gain variations.
Highlights
Tunable, narrow linewidth diode lasers are of significant relevance, ranging from terrestrial applications such as fiber-optic communications [1] or optical sensing [2], to space-based applications such as laser cooling [3] and atomic clocks [4] in global positioning systems (GPS)
The benefits to be gained with low-loss waveguide feedback circuits are that: a) flexible filtering schemes can be applied which allow for wide wavelength tunability, b) narrow linewidths can be achieved due to increased photon lifetime that is associated with an extended cavity length [7], and that c) such lasers are ideal for integrating into subsequent waveguide circuitry fabricated on the same waveguide chip
We present the first modeling of external cavity diode lasers (ECDLs), called extended cavity diode lasers, that takes into account the complex feedback obtained with waveguide resonator circuits such as in [11] and [14] and that simultaneously takes into account the detailed spatial variation of the diode-internal intensity and carrier density, such as in [12] and [13]
Summary
Narrow linewidth diode lasers are of significant relevance, ranging from terrestrial applications such as fiber-optic communications [1] or optical sensing [2], to space-based applications such as laser cooling [3] and atomic clocks [4] in global positioning systems (GPS). -called hybrid diode waveguide lasers, which have been subject of recent, extensive research, can offer much longer photon lifetimes, and narrower linewidths, in addition to wider tunability The concept of such hybrid lasers is based on optically coupling a semiconductor gain medium to a low-loss passive waveguide circuit that provides a significantly extended resonator length and a highly frequency selective feedback. In order to cover different regimes of interest, here we systematically vary this power coupling strength, β, between the InP gain section and the Si3N4 waveguide chip This variation brings the diode internal field and carrier distribution from spatially high uniformity (β ≈ 1 and strong feedback from the waveguide resonators, where the MFA remains justified) towards a strong spatial variation (β ≈ 0 or weak resonator feedback) where a detailed modeling has not been performed so far. For each setting of the chip-to-chip coupling efficiency we calculate the power spectral density of the frequency noise from which we derive a value for the laser linewidth, to compare with values obtained with a modified MF theory
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