Multi-Mode Lasers for Self-Forced Opto-Electronic Oscillators in Compact Frequency Synthesizers

Abstract

Advanced telecommunication, imaging, and remote sensing systems require reliable ultra-high stability local oscillators. Particularly, coherent detection of digital data signals relies on a low aperture jitter of local oscillators in addition to high signal to noise ratio to achieve a very low bit-error-rate (BER). Frequency stabilized local oscillators are required with phase noise of smaller than −120 dBc/Hz at 10 kHz offset carrier and low aperture jitters of below 50 fs for RF sources of 10 GHz. Self-forced opto-electronic oscillation techniques of self- injection locking (SIL) combined with self-phase locking (SPLL) employ low loss long optical delays for frequency stabilizations through comparison of instantaneous signals and delayed versions. A compact design of multi-mode laser (MML) is presented here by employing self-forced oscillation combined with self-mode locking of an MML. Proof of concept of this novel approach is reported using a custom designed InGaAsP/InP chip using Smart Photonics foundry service, where multi-quantum well semiconductor optical amplifiers (SOA), carrier injection based phase modulator (PM), front and back (a common) distributed Bragg reflectors (DBR), and external electro-absorption modulators (EAM) are designed and physically modeled based on the foundry process design kit (PDK). A compact realization of self-mode locked MML with 5 modes to a fixed inter-modal oscillation frequency is presented, where the beat notes of the inter-modal oscillation are generated over X-band to form a frequency stabilized synthesizer. The PM bias control of two MLL lasers with common DBR is used to tune this opto-electronic oscillator from 11 GHz to 13 GHz with a frequency resolution of 110 kHz. Self-forced oscillation techniques of SILPLL combined with self-mode-locking (SML) has provided frequency synthesis with measured phase noise at 10 kHz offset (and aperture jitter) improved from −36 dBc/Hz (58ps) for the free-running MML to −92 dBc/Hz (68 fs) using SILPLL and SML of only 5 correlated modes. Analytical modeling has predicted that increasing the number of self-mode-locking modes from 5 to 13 will result in 18 dB improvement in phase noise. Performance comparison of this opto-electronic frequency synthesis method is made against various commercially available X-band frequency synthesizers that use full electrical solutions.