  We propose a new type of broadband and low repetition rate frequency comb
generator which has the shape of an elongated and nanoscale-shallow optical
bottle microresonator created at the surface of an optical fiber. The free
spectral range (FSR) of the broadband azimuthal eigenfrequency series of this
resonator is the exact multiple of the FSR of the dense and narrowband axial
series. The effective radius variation of the microresonator is close to a
parabola with a nanoscale height which is greater or equal to lambda/2pi*n0
(here lambda is the characteristic radiation wavelength and n0 is the
refractive index of the microresonator material). Overall, the microresonator
possesses a broadband, small FSR, and accurately equidistant spectrum
convenient for the generation of a broadband and low repetition rate optical
frequency comb. It is shown that this comb can be generated by pumping with a
cw laser, which radiation frequency matches a single axial eigenfrequency of
the microresonator, or, alternatively, by pumping with a mode-locked laser,
which generates a narrowband low repetition rate comb matching a series of
equidistant axial eigenfrequencies situated between adjacent azimuthal
eigenfrequencies.
