Raman-assisted optical frequency combs generated in a silica microsphere in two whispering gallery mode families

Optical frequency combs (OFCs) generated in microresonators with whispering gallery modes are attractive for different applications and basic science. The effects associated with Raman nonlinearity are constantly being discovered and investigated experimentally and theoretically expanding the capabilities of microresonator OFCs. Here we obtain experimentally Raman-assisted nested OFCs in two different mode families with soliton-like (sech²-shape) spectral envelopes for each mode family in a silica microsphere. By comparing the experimental frequencies of spectral harmonics with the calculated eigenfrequencies we verified that the generated OFC with a larger bandwidth corresponds to the fundamental TE mode family, and the OFC with a smaller bandwidth to the TM mode family. We report the regime when the pump wavelength (without an OFC nearby) is in the normal dispersion region and Raman-assisted OFCs are generated in the anomalous dispersion region. (letter)