Frequency combs make their way to real-world use
An optical frequency comb consists of a light source with equally spaced sharp spectral lines, which can be seen as the teeth of a comb. As the spectral lines' frequencies are known, the comb is used to determine unknown frequencies by measuring beat notes. The latter reveal the difference between the unknown and the comb frequencies. Optical frequency combs have been realised using pulsed, mode-locked lasers in which the equal spacing between the many lines is defined by the repetition rate of the lasers. Recently, a new class of monolithic frequency comb generators has emerged by coupling a continuous wave laser to a high-finesse silica microcavity. This new approach promises unprecedented high efficiency and compactness. Researchers working on the EU-funded project μCOMB (Microresonator based frequency comb generators) have made several steps towards the realisation of such devices. Making use of crystalline resonators, they developed microresonator frequency combs in the mid-infrared – a wavelength range that is of particular importance for molecular sensing. To this end, the μCOMB team explored and refined various fabrication techniques to create what are also known as Kerr combs in different host materials and structures. Among these, lithographically defined micro-rings are complementary metal-oxide semiconductor compatible, allowing the integration of resonators and coupling waveguides in a monolithic structure. One obstacle had to be overcome to bring such frequency combs closer to applications. Where generated in crystalline or silicon nitride-based resonators, they exhibit phase noise appearing as linewidth broadening. As it turned out, the reason for this noise is not of external nature – as it was assumed before the μCOMB project – but intrinsic to the comb formation process. The explanation of phenomena not previously understood led to efficient measures to overcome this obstacle. The conditions for low-phase noise performance were also identified. A breakthrough achievement was the realisation of mode-locked states in microresonators, providing an output that combines short pulses with high repetition rate. These low-noise states paved the way for fully stabilised Kerr combs that are now within reach. Only a few days after the end of the μCOMB project, unprecedented high temporal coherence was demonstrated for octave-spanning spectra generated with a microresonator frequency comb.
Keywords
Frequency combs, spectral lines, continuous wave laser, microresonator, molecular sensing
