In this paper, we propose a fiber link delay mismatch detection method for the fiber-optic phase-aligned transmission system of broadband microwave signals. Different from the conventional pulse time-of-flight method to solve the problem of periodic ambiguity for a sine signal, an adjustable frequency signal is adopted as the reference in the process of phase discrimination. The main frequency is employed to extract the fiber delay difference and its fluctuation within the corresponding period. Meanwhile, the auxiliary frequency is introduced to deal with the periodic ambiguity for the delays beyond one period. A proof of principle system with an equivalent 5 km outdoor transmission length is implemented to verify the feasibility and effectiveness of the proposed method. Broadband microwave signals received at eight antenna ends are transmitted to the central station through wavelength division multiplexing. At the central station, each link delay is obtained in turn from the incorporated reference frequency signal. Then, the phases of each broadband microwave signals are set in line by the corresponding adjustable optical delay lines. According to the experimental demonstration, the phase consistency is better than 8.68° for a 10 GHz transmitted frequency.
In this paper, we develop and demonstrate a proof-of-principle OEO, which features ultra-low phase noise in a Ka frequency band. The prototype of the whole OEO is in a cylindrical form. The optical fibers are wound on the outside, while all the optoelectronic devices are in the center. The fiber transmission noise is suppressed via phase modulation for the power redistribution. The spur-level improvement and steady state operation is guaranteed by dual-loop structure with 8.7 km and 11.6 km fiber spools, respectively. The optical power loss is reduced by the dual-output electro-optical intensity modulator (DEOM) instead of another 50:50 optical coupler. The noise floor for the fiber link from laser intensity and phase noises is suppressed by the balanced photodetector (PD) with specialized working conditions. Performance is investigated in detail. The OEO operates at the frequency of 30 GHz with the spur suppression of 74.6 dBc. The phase noise of -130.7 dBc/Hz (-149.1 dBc/Hz) @1 kHz (10 kHz), respectively, are achieved. The spectral purity is much higher than the current commercial signal source and equipment. Further, the developed OEO is applied to the frequency conversion. The RF signal, to be converted with a frequency of 7 GHz, is coupled into the OEO. Each beat results with OEO are observed clearly. All these results show that OEO has broad prospects in high precision infrastructure and projects.
Microwave photonic (MWP) based radio frequency (RF) channel is usually composed of three parts: microwave pre-processing part, electro-optical interconversion part and microwave post-processing part. The nonlinearity of active components causes serious harmonics at the output of MWP based RF channels. This paper mainly analyzes the harmonic distortions produced in the external-modulated optical transmission link and studies the variation of output harmonic power in the RF channel by changing the modulator bias point. According to the experiment, it is verified that the second harmonic can be effectively suppressed when the modulator is biased at high power point.
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