Highly stable forced optoelectronic oscillators and roadmap to integrated clocks

Abstract

Low phase noise signal generated in a small structure is required for communication and high resolution imaging. Optoelectronic oscillators with their demonstrated low phase noise are popular due to the increasing demand of very clean local oscillator and clock signal sources. Forced techniques of self-injection locking phase locking (SILDPLL) applied on an OEO system reduces the phase noise and suppresses side mode observed in standard OEO with long delays. However, this novel optical feedback technique using self-ILPLL is also employed for phase noise reduction of free-running microwave oscillators. Phase noise reduction of 27 dB at 10 kHz offset has been demonstrated by applying this technique for a 10 GHz state of art DRO, achieving -137 dBc/Hz at 10 kHz offset. A phase noise prediction of this feedback technique is also presented, which very closely corroborates with experimental results. Analysis of SILPLL using the modeling has shown that further phase noise reductions could be achieved, by reducing the residual phase noise of the long delay lines and microwave amplifiers. In addition, a frequency synthesizer working at K-band is demonstrated using electronically tuned narrowband RF filter in place of fixed narrowband frequency metallic cavity based filters. The synthesizer employs optical transversal filter cascaded with YIG filter to electronically fine and coarse tune the oscillation frequency, as a replacement for fixed frequency high Q metallic filter. Second harmonic generated frequency synthesized signals with close-in to carrier phase noise of -127 dBc/Hz phase noise at 10 kHz are reported by V π operation of a Mach-Zehnder (MZM) over 18 GHz to 22 GHz with frequency tuning step as small as 20 kHz/pm. A 19’’ rack mount system is also constructed and long term stability of 4.5 kHz over 60 min is measured at K-band, while a 3.5 kHz drift is measured for a tabletop realization. A fully integrated version of this forced OEO is also being pursued to enhance size, power consumption, and cost. A fully monolithic DBR based multi-mode laser is considered with mode-locking method to build frequency stabilized and tunable RF signal generator. The number of the output modes from each laser is adjusted using reflecting bandwidth of distributed Bragg reflector and electro-absorption (EA) modulator for amplitude control, while the phase section in integrated laser system provides frequency tuning. Mode-locking of 60 laser modes results in a highly frequency stable 10 GHz RF beat-notes with a calculated phase noise of -150 dBc/Hz at 10 kHz offset frequency.