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We propose a taper design for a silicon-core fiber for the purpose of generating a supercontinuum (SC) from a 2.1μm pulsed fiber laser. The design is tailored to maximise the conversion efficiency (CE) to the 3-4μm region, which is important for environmental sensing as it includes several key greenhouse gas absorption lines.
There is a need for compact, low-power and efficient solutions. Aluminium nitride photonic-chip waveguides have been shown to generate 0.3mW in the 3-4μm region with an 80mW input. Although this is sufficient power for some applications, the system only offers a 0.4% CE. More recently a silicon nitride planar waveguide was used to transfer energy from a commercial 2.1μm femtosecond laser to targeted wavelengths in the 3-4μm region through dispersive wave generation. To cover the entire region, it is estimated that an input of 40mW would be needed to generate ~1mW (CE of 2.5%).
Compared to these materials silicon has a higher nonlinearity and, despite multi-photon absorption, is highly efficient at transferring energy to different wavelengths with modest input powers. Moreover, silicon-core fibers can be tapered using established post-processing procedures, which can be used to control the phase-matching conditions to concentrate energy in a required wavelength range.
We have designed a silicon-core fiber taper that can take the input from a 2.1μm fiber laser and efficiently transfer the energy to cover the entire 3-4μm range.
We simulated SC generation using the generalised nonlinear Schrödinger equation including wavelength-dependent loss terms (linear, TPA and 3PA). From these simulations we estimate that ~0.8mW average power can be generated covering the entire 3-4μm region, with only 15mW input power, a CE of 5%.
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