Experimental Results
The MRR used here is manufactured on the platform based on Hydex glass
[30, 33, 34, 38, 39] using CMOS manufacturing process. First, use
PECVD to deposit Hydex film (n=~1.7 at 1550 nm), and
then pattern by deep ultraviolet (UV) and reactive ion etching [79]
to obtain waveguides with very low surface roughness. Finally, the upper
cladding layer composed of silicon dioxide (n=~ 1.44 at
1550 nm) is deposited. The main advantage of our platform is that it has
ultra-low linear loss (~ 0.06 dB · cm-1) and moderately
high optical nonlinear parameters (~233 W-1 · km-1).
Because the platform has ultra-low loss, our MRR has a Q factor of up to
about 1.5 million. The radius of the MRR is ~592 µm,
which corresponds to an optical FSR of 0.393 nm or 48.9 GHz. Such a
small FSR greatly increases the number of wavelengths available on the
C-band, up to 75 wavelengths, which is more than twice that of our
previous results [58].
In order to generate the micro-comb, the CW pump power is amplified to
~30.5 dBm. When the detuning between the pump wavelength
and the cold resonance of the MRR becomes small enough so that the
cavity power reaches the threshold, modulation instability will occur
(MI) driven oscillation [23]. Then the main comb is generated, the
spacing of which is determined by the MI gain peak value. As the
detuning changes, it forms a spectrum similar to that reported by the
spectral interference between the solitons that are closely packed in
the cavity, that is, the soliton crystal [27-29]. This crystal is
mainly used as the basis of radio frequency oscillators with low phase
noise [80].