Monolithically Integrated Dual-Comb Generator Based on a Generic Platform Photonic Integrated Circuit

Dual-comb interferometry is a powerful measurement technique with a wide range of applications including spectroscopy, fiber sensing and ranging. It is based on the measurement of the interference pattern produced by the beating in a photodetector of two mutually coherent Optical Frequency Combs (OFCs) with slightly different repetition frequencies. The beating produces a compressed spectrum in the electrical domain, which consists of an RF comb whose repetition frequency is determined by the difference of the repetition frequencies <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\delta f$</tex> between both OFCs. This process maps the optical spectrum into the RF domain, allowing the absorption spectrum of a sample to be measured at high speed, with high resolution and relatively low bandwidth. The amount of compression is determined by the difference between the repetition frequencies or the compression factor, defined as <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$CF= f_{R 1} / \delta f$</tex>, where <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$f_{R1}$</tex> is the repetition frequency of OFC1 thus being <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$f_{R 2}=f_{R 1}+\delta f$</tex> ‘. Hence, an optical bandwidth of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\Delta v$</tex> is effectively mapped into an electrical bandwidth of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\Delta v / C F$</tex>. Therefore, the resolution of dual-comb measurements is determined by the comb line spacing.