Brillouin-Enhanced Hyperparametric Generation of an Optical Frequency Comb in a Monolithic Highly Nonlinear Fiber Cavity Pumped by a cw Laser

We demonstrate self-seeded generation of a broadband comb in a highly nonlinear fiber resonator. When pumped with a cw laser, the fiber cavity generates a comb with two characteristic spacings. Hyperparametric modes spaced by $\sim 2\mathrm{THz}$ create the base structure of the comb, while commensurate Brillouin modes spaced by $\sim 10\mathrm{GHz}$ populate the intermediate frequency gaps. The frequency modes are coherent, and the repetition rate of the comb has been locked to a microwave standard.

Figure 1

(a) Photograph of a 5.2 cm-long highly nonlinear fiber (HNLF) cavity. Fiber ends are potted in ferrules, polished and mirror coated. (b) A cw laser traverses an erbium-doped fiber amplifier (EDFA) and a polarization controller (PC) before pumping the HNLF resonator. Lenses (L1 & L2) mode match between the telecom and HNLF fibers. The comb output from the cavity is divided by a beam splitter (BS); the light from one port is monitored on a high speed photodiode (PD). To lock the repetition rate of the comb, the detected beat note is mixed with an rf local oscillator (LO) and is fed back to the laser through a loop filter (LF).

Figure 2

(a) The laser is tuned across a cavity FSR. Slight polarization rotation reveals the birefringent mode shifted by $\sim 500\mathrm{MHz}$. (b) Thermal runaway as the pump laser is tuned through cavity resonance.

Figure 3

(a) Hyperparametric modes of the HNLF fiber when pumped with 550 mW near 1550 nm. To suppress SBS, the pump laser is externally modulated at 8 GHz. (b) Stimulated Brillouin modes with spacing of 9.87 GHz populate the intermediate frequency band between the terahertz modes. Each inset depicts a 1.5 nm wide region with the same vertical scale to reveal the individual modes across the spectrum. (c) Schematic of coherence measurement. Optical bandpass filters (BP1 & BP2) select different optical spectral regions that are detected on high speed photodiodes (PD1 & PD2). The rf signals are filtered and amplified before the mixer ($\otimes$). A phase shifter ($\sim$) adjusts the relative delay in optical and electrical paths.

Figure 4

Output of the HNLF cavity with pump powers of (a) 500, (b) 520, (c) 540, and (d) 560 mW.