Abstract

Many areas of research such as wide band frequency division multiplexed communication links, optical distribution of mm-waves for phased array radar, antenna remoting, and high speed photodiode characterization require optical generation of frequency stable mm-waves. Previously, a 6.33-GHz optically generated signal with a linewidth of <3 Hz was demonstrated,¹ and improvements in the system have enabled sub-Hz stability to be achieved. This work utilized two diode laser pumped Nd:YAG lasers frequency locked to adjacent axial modes of a high finesse interferometer to create a frequency stable heterodyne beatnote. We have recently implemented this capability for frequency locking YAG lasers to characterize our high speed photodiodes. Two 40-mW diode laser pumped nonplanar ring oscillators were locked to successive axial modes of an interferomter that had a free spectral range of 1.00 GHz. The wavelength of the lasers was 1.06 μm, and through temperature tuning we were able to generate stable mm-wave modulation of the optical beam to 57 GHz. The optical beam was focused through a fiber into the photodetector modules and, as the modulation frequency was changed, the diodes frequency response was monitored. Several of the diodes we have developed operate in the visible spectrum however, and to characterize these units the optically generated mm-wave signal needed to be frequency doubled. In this application, the same photodiode test set could be used by placing a nonlinear doubling material (KTP) into the cavity to resonantly double the fundamental with high efficiency in a fashion similar to work done elsewhere.² Locking of the lasers to adjacent axial modes of the cavity therefore enabled us to generate stable mm-waves in the visible and IR as well as generate modulation frequencies as high as 57×2 = 114 GHz.

© 1991 Optical Society of America