Forced Oscillation in Integrated Opto-electronic Circuits for Realization of Stable RF Synthesizers
Authors:   , Sun Tianchi , Daryoush Afshin S. , Nabet Bahram 
Published: July 2021

Highly stable and small size local oscillators is a very important component of any remote sensing, imaging, and telecommunication systems. The key factor influencing the frequency stability is quality factor of the resonance structure. Related close-in to carrier phase noise for offset frequencies above active device corner frequency reduces at least at a rate of 20 dB/decade while offset below corner frequency increases at rate of 30 dB/decade. A new approach in realization of the electrical oscillators is to augment electrical quality factor by optical energy storage components (e.g., fiber optic delay lines and ring resonators) to enhance effective quality factor (Q) to 10^4-10^6, well beyond available electrical Q of 2000 in dielectric resonator oscillators (DRO). Moreover, stability of resonance frequency has been impacted by the environmental induced (e.g., piezo and pyro properties of material) sensitivities. In addition, micro-scale design approaches are reported using multi-mode (both semiconductor and solid state) lasers for low phase noise intermodal oscillations. Finally, forced oscillation techniques of injection locking, phase locking, and mode-locking are attractive to further stabilize electrical and optical oscillation methods. In this thesis, development of broadband stable frequency sources is reported covering both X- and K-band using forced opto-electronic oscillator technique. The broadband frequency tuning is achieved using coarse electrical tuning of YIG bandpass filter combined with fine-tuned optical transversal filter using dispersive delay component and wavelength control of a fiber laser, as optical source. Forced opto-electronic oscillation techniques of self-injection locking (SIL), self-phase locked loop (SPLL), and self-injection locked phase locked loop (SILPLL) are optimized for further frequency stabilization of a frequency synthesizer. In SILPLL application, long delay line is crucial for substantial phase noise reduction; therefore, kilometer long optical fiber delay lines have been used to construct the optical feedback since their loss is extremely low compared to electrical delay lines. Meantime, open loop gain is another key parameter to achieve low phase noise in these feedback systems, as the phase noise reduction is proportional to phase locking loop gain. The phase noise and pull-in time can be greatly reduced in SILPLL as opposed to SIL or SPLL alone. A forced oscillation of fixed frequency DRO at 10 GHz system provide -137 dBc/Hz phase noise at 10kHz offset with under 10 fs aperture jitters, while OEO based X-band and K-band synthesizers provide phase noise of -137 dBc/Hz (<10 fs jitters) and -127 dBc/Hz (<12 fs jitters) at offset 10 kHz respectively in a computer controlled 19" rack-mountable OEO systems. After realization of the desired modular frequency synthesizer, development a compact opto-electronic oscillator system is demonstrated in my PhD study. Monolithic design and testing of multi-section quantum well lasers are reported on 5x2 mm2 InP chip using a commercial foundry service (i.e., SmartPhotonics). The designed lasers consist of 4 major sections of semiconductor optical amplifier (SOA), optical phase modulator (PM) and distributed Bragg reflector (DBR) mirrors. The DBR section provides wavelength selectivity while phase section and gain medium provide output wavelength tuning. An electro-absorption modulator (EAM) is prepared for mode number control using its wavelength sensitive absorption coefficient. The experiments supported predicted performance of this multi-mode laser structure and impact of combined forced oscillation of intermodal oscillations with mode locking method to realize a frequency stabilized and tunable RF signal source. Meantime, forced technique SILPLL and optimum mode locking techniques are applied in order to achieve improved phase noise performance. The free-running inter-modal oscillation frequencies suffers from poor phase noise of -5 dBc/Hz at 1 kHz and -32 dBc/Hz at 10kHz offset around 11.54 GHz, while under forced oscillation a -58 dBc/Hz at 1 kHz offset and -98 dBc/Hz at 10 kHz offset are achieved. A frequency tuning range of 800 MHz is also measured. In mode locked laser operation, the RF beat-notes forced with SILPLL topology provides a better performance. Improved phase noise of -79 dBc/Hz at 1kHz, -110 dBc/Hz at 10 kHz is measured with under 50 fs aperture jitters, while a broadband frequency tuning from 11 GHz to 13 GHz range is measured. The tuning range is potentially to be further improved, if the shared DBR laser pair symmetry could be improved. In summary, this thesis provides the solution for building RF synthesizer using forced opto-electronic oscillator and laser beat-notes oscillation output. The experimental results are corroborated with detailed analytical and numerical modeling to optimize oscillator phase noise performance. The modular design solution shows great stability and low phase noise, while the compact monolithic design solutions are quite attractive for a smaller size. Potential for full integration of multi-mode multi-section MQW laser with optical delay lines are also presented as a logical development path towards a compact frequency synthesizer.