Low phase-noise mid-infrared frequency combs based on microresonators

Summary form only given. The mid-infrared spectral range (λ~2-20 mm) is of particular importance as many molecules exhibit strong vibrational fingerprints in this region. Optical frequency combs [1, 2] - broadband optical sources consisting of equally spaced and mutually coherent sharp lines - are creating new opportunities for advanced spectroscopy [3, 4, 5]. Here we demonstrate a novel approach to create mid-infrared optical frequency combs via four-wave mixing [6, 7] in a continuous-wave pumped ultra-high Q crystalline microresonator made of magnesium fluoride [8]. Careful choice of the resonator material and design made it possible to generate a broadband, low-phase noise Kerr combs at λ=2.5 mm. With its distinguishing features of compactness, efficient conversion, large mode spacing and high power per comb line, this novel frequency comb source holds promise for new approaches to molecular spectroscopy and is suitable to be extended further into the mid-infrared.Crystalline MgF2 resonators are shaped and polished on an air-bearing spindle with diameters ranging from 500 μm to 5 mm, corresponding to a free spectral range of 10 to 100 GHz. Optical quality factors are typically on the order of Q~109. The resonators are pumped by several hundred milli-Watts of continuous-wave (CW) light at 2.45 μm and the comb lines spanning over more than 200 nm (~10 THz) are observed. Low phase-noise of the generated mid-infrared combs is confirmed by the heterodyne beat note measurement between a narrowlinewidth CW laser and the comb lines, as well as by transmission noise measurement. Intrinsically low phasenoise combs can be expected across the entire mid-infrared region. Combining with quantum cascade lasers (QCLs), this may facilitate a new class of frequency combs for molecular spectroscopy.