A Silicon-Based Monolithic Optical Frequency Comb Source
Recently developed techniques for generating precisely equidistant optical frequencies over broad wavelength ranges are revolutionizing precision physical measurement [1-3]. These frequency "combs" are produced primarily using relatively large, ultrafast laser systems. However, recent research has shown that broad-bandwidth combs can be produced using highly-nonlinear interactions in microresonator optical parametric oscillators [4-11]. Such devices not only offer the potential for developing extremely compact optical atomic clocks but are also promising for astronomical spectroscopy [12-14], ultrashort pulse shaping [15], and ultrahigh-speed communications systems. Here we demonstrate the generation of broad-bandwidth optical frequency combs from a CMOS-compatible integrated microresonator [16,17], which is a fully-monolithic and sealed chip-scale device making it insensitive to the surrounding environment. We characterize the comb quality using a novel self-referencing method and verify that the comb line frequencies are equidistant over a bandwidth that is nearly an order of magnitude larger than previous measurements. In addition, we investigate the ultrafast temporal properties of the comb and demonstrate its potential to serve as a chip-scale source of ultrafast (sub-ps) pulses.