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Phase-locking in a 34.5 G]lz special complex cavity gyrotroxl oscillator operating at the second harmonic of the electron cyclotron frecluency was studied. Injection of the locking power was made via a quasi-optical circulator connected to the gyrotron output. Locking bandwidth was measured by comparing the phase of the injection signal and output signal using a balanced mixer. Locking was observed with input power level as low as 40 dB below the gyrotron output power. The locking bandwidth is, however, narrower than in I gyrotrons operating at the fundamental cyclotron frequency which inay be attributed to the I longer resonant cavity in the second harmonic gyrotmn and the corresponding larger value of external quality factor. The measurements are roughly in agrmment with predictions of I Adler's phase-locking equation which is given for our system in terlns of powers propagating in the output waveguide toward and away from the gyrotron cavity. I q The research described in this paper was carried out in part by the Jet Propulsion Lab-1 oratory, California Institute of Technology, under a contract with the National Aeronautics I and Space Administration; in addition, this wcjrk was supported in part by the DoD Vacuum Electronics Initiative and managed by the Air Force OfXce of Scientific Research under Grant AFOSR-91-0390.

Photonic integrated circuit (PIC) phased arrays can be an enabling technology for a broad range of applications including free-space laser communications on compact moving platforms. However, scaling PIC phased arrays to a large number of array elements is limited by the large size and high power consumption of individual phase shifters used for beam steering. In this paper, we demonstrate silicon PIC phased array beam steering based on thermally tuned ultracompact microring resonator phase shifters with a radius of a few microns. These resonators integrated with micro-heaters are designed to be strongly coupled to an external waveguide, thereby providing a large and adjustable phase shift with a small residual amplitude modulation while consuming an average power of 0.4 mW. We also introduce characterization techniques for the calibration of resonator phase shifters in the phased array. With such compact phase shifters and our calibration techniques, we demonstrate beam steering with a 1x8 PIC phased array. The small size of these resonator phase shifters will enable lowpower and ultra-large scale PIC phased arrays for long distance laser communication systems.

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