Abstract
We investigate the phase noise performance of optoelectronic oscillators when the optical energy storage elements are in the following three configurations: a high-Q whispering gallery mode resonator, an optical delay-line and a combination of both elements. The stability properties of these various optical elements are first characterized, and then systematically compared in the optical and in the microwave frequency domains. Subsequently, the spectral purity of the oscillator is theoretically and experimentally examined for each case. When the resonator is used as both delay and filtering element inside the delay-line based oscillator, the generated spurious modes are highly rejected. A spur rejection by more than 53 dB has been demonstrated for the first-neighboring spur.
Highlights
Microwave sources with high spectral purity are currently becoming increasingly important for various applications
We present different studies performed on an opto-electronic oscillator (OEO) based on two different optical stability elements, first considered individually, and later on conjointly: a magnesium fluoride (MgF2) crystalline disk-shaped whispering gallery mode resonators (WGMRs) and a 4km long optical DL
10 kHz offset frequency from the different generated carriers, as follows: below −97 dBc/Hz for the 6.07 GHz carrier generated by the WGMR based OEO and below −124 dBc/Hz for both the 6.25 GHz carrier generated by the DL based OEO and the 6.07 GHz carrier generated by the DL-WGMR based OEO
Summary
Microwave sources with high spectral purity are currently becoming increasingly important for various applications. The first OEO, proposed in 1994 [4], was based on an optical delay-line (DL; a long, low-loss optical fiber) and features today ultra-low phase noise performance (e.g. a 16km long optical DL based OEO was used to obtain the highest achieved spectral purity: −163 dBc/Hz at 7 kHz from a 10 GHz carrier, [5,6]) In such case, the microwave signal is carried to the optical domain using a laser lightwave (e.g. λlaser ~1559nm) via optical modulation. The main problem in using DL-based OEOs is the presence of spurious modes in the generated signal’s spectrum, whereas these modes need bulky and/or complex configurations to be reduced [7,8,9] Another alternative optical stability element that can be used in OEOs is an optical resonator featuring ultra-high optical quality factor (QOpt > 109) within relatively small dimensions. The measured optical, RF and phase noise spectra of the various signals are presented and discussed in detail
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