Tunable X-Band Optoelectronic Oscillators Based on External-Cavity Semiconductor Lasers

Laser diodes with optical feedback can exhibit periodic intensity oscillations at or near the relaxation-oscillation frequency. We demonstrate optoelectronic oscillators based on external-cavity semiconductor lasers in a periodic dynamical regime tunable over the entire <inline-formula> <tex-math notation="LaTeX">$X$ </tex-math></inline-formula>-band. Moreover, unlike standard optoelectronic oscillators, we need not employ the time-dependent optical intensity incident on a photodiode to generate the microwave signal, but rather have the option of generating the electrical microwave signal directly as a voltage <inline-formula> <tex-math notation="LaTeX">$V(t)$ </tex-math></inline-formula> at the laser-diode injection terminals under constant current operation; no photodiode need be involved, thus circumventing optical-to-electrical conversion. We achieve a timing jitter of <inline-formula> <tex-math notation="LaTeX">$\lesssim 10$ </tex-math></inline-formula> ps and a quality factor of <inline-formula> <tex-math notation="LaTeX">$\gtrsim 2\times 10^{5}$ </tex-math></inline-formula> across the entire <inline-formula> <tex-math notation="LaTeX">$X$ </tex-math></inline-formula>-band, that ranges from 6.79 to 11.48 GHz. Tuning is achieved by varying the injection current <inline-formula> <tex-math notation="LaTeX">$J$ </tex-math></inline-formula>.