High-Resolution $X$ -Band Frequency Synthesizer Using SILPLL Optoelectronic Oscillators

Optoelectronic oscillators (OEOs) with their demonstrated low phase noise are popular due to the increasing demand of very clean local oscillator and clock signal sources. A forced technique of self-injection locked and phase-locked loop (SILPLL) reduces phase noise and suppresses side mode observed in long fiber delay of the standard OEO. A frequency synthesizer working at X-band is developed using an electrically coarse-tuned YIG filter cascaded with an optical wavelength fine-tuned dispersive optical transversal filter by tuning the oscillation frequency of the synthesized OEO frequency, as a replacement for fixed-frequency narrowband filter using high-Q aluminum cavities. A close-to-carrier phase noise of -115 dBc/Hz at 1 kHz is reported by the V
_π/2
operation of a Mach-Zehnder modulator (MZM) over 8-12 GHz with a frequency tuning step as small as 2.6 kHz. Phase noise introduced by a photodetector is also explored at a very close-to-carrier frequency. It shows 10-dB phase noise degradation at PD reverse bias voltage of 3 V compared with 6 V at close to the Vπ operation of an MZM. A 19-in rack mount frequency synthesizer system is also realized at X-band with a demonstrated long-term (up to 60 min) frequency drift stability of 3 kHz over whole X-band working frequencies.