  Microresonator-based optical frequency comb (microcomb) has the potential to
revolutionize the accuracy of frequency synthesizer in radar and communication
applications. However, fundamental limit exists for low noise microcomb
generation, especially in low size, weight, power and cost (SWaP-C) package.
Here we resolve this limit, by the demonstration of an automated turnkey
microcomb, operating close to its low quantum-limited phase noise, within a
compact setup size of 85 mm * 90 mm * 25 mm. High quality factor fiber
Fabry-Perot resonator (FFPR), with Q up to 4.0 * 10^9, is the key for both low
quantum noise and pump noise limit, in the diode-pump case in a self-injection
locking scheme. Low phase noise of -80 and -105 dBc/Hz at 100 Hz, -106 and -125
dBc/Hz at 1 kHz, -133 and -148 dBc/Hz at 10 kHz is achieved at 10.1 GHz and 1.7
GHz repetition frequencies, respectively. With the simultaneous automated
turnkey, low-noise and direct-diode-pump capability, our microcomb is ready to
be used as a low-noise frequency synthesizer with low SWaP-C and thus field
deployability.
