In this paper, we propose a phase-conjugation-based fast radio frequency (RF) phase auto stabilization technique for long-distance fiber delivery. By phase conjugation at the center site, the proposed scheme pre-phase-promotes the RF signal with the shift which is acquired by round-trip transferring another RF whose frequency is half of the one to be sent. Such phase pre-promotion is then used to counteract exactly the following retard induced by one-way delivery. Different from the previous phase-locking-loop-based schemes, the proposed open-loop design avoids the use of any tunable parts and dynamic phase tracking, enabling a fast phase stabilization at the remote site. An end-less compensation capacity can also be achieved. Our design is analyzed by theory. Experimentally, the new scheme is verified by transferring a frequency of 2.42 GHz through a 30-km optical fiber link. Significant phase drift compression is observed. The rapid phase stabilization is verified by introducing sudden time delay change into the link. The recovery time equals to the round-trip time of the link plus the transitional duration of the delay change, which is much shorter than the traditional trial-and-error phase locking loop. Important issues of the system design are discussed.
We propose and demonstrate a novel stable radio frequency (RF) delivery system based on a radio-over-fiber link. The proposed scheme acts as a long phase-locking loop where an optical tunable delay line is involved to compensate dynamically for the time-delay variation that arises from fiber-link fluctuation. An optical carrier with variable wavelength under fiber-link dispersion results in the desired tunable delay. The tunable range is in proportion to the length of the fiber link, so a large phase-error correction capacity under long-distance delivery can be realized. The large as well as fine optical-delay tunability is experimentally demonstrated, and the RF reference of 2.42 GHz is transferred for 54 km where a time jitter compression factor of 588 is achieved.
In this paper, we propose and demonstrate a phase stabilized wideband downlink transmission scheme, which directly transmits the received radio frequency (RF) signals from remote antennas to central station. A reference RF tone is round-trip transferred between the central station and remote end to obtain the delay variation caused by the fiber link. The delay variation is then used to alter a tunable laser. Since optical carriers with different wavelengths propagate at different velocities in fiber, a tunable optical delay line is realized to cancel the delay variation of the fiber link. The tunable delay range is in proportion to the length of the fiber link, which means a very long delivery distance can be expected. Experimentally, a RF signal at frequency of 2.50 GHz has been downlink transferred through a 45 km fiber link, with stability of 3.3 x 10⁻¹³ at 1 s and 7.5 x 10⁻¹⁷ at 10⁴s.
A novel multi-band digital predistortion (DPD) technique is proposed to linearize the subcarrier multiplexed radio-over-fiber (SCM-RoF) system transmitting sparse multi-band RF signal with large blank spectra between the constituent RF bands. DPD performs on the baseband signal of each individual RF band before up-conversion and RF combination. By disregarding the blank spectra, the processing bandwidth of the proposed DPD technique is greatly reduced, which is only determined by the baseband signal bandwidth of each individual RF band, rather than the entire bandwidth of the combined multi-band RF signal. Experimental demonstration is performed in a directly modulated SCM-RoF system transmitting two 64QAM modulated OFDM signals on 2.4GHz band and 3.6GHz band. Results show that the adjacent channel power (ACP) is suppressed by 15dB leading to significant improvement of the EVM performances of the signals on both of the two bands.
Precise time delay sensing combined with stable frequency dissemination on an arbitrary intermediate point along a fiber-optic loop link is presented. Based on -dispersion-induced radio-frequency (RF) phase locking, the whole loop time delay and phase shift are first stabilized. The time and frequency signals are carried on the same optical carrier and are delivered to both the clockwise and anticlockwise directions. On the intermediate point, the instantaneous time delay from the central station to that point can be acquired by measuring the delay difference. The stable frequency standard with twice the angular frequency of the RF reference can be recovered on the intermediate point by mixing the RFs in the two directions. In the 45-km fiber loop experiment, the variation of the sensed time delay is limited in AE50-ps range, and the time deviation of the sensed time delay is measured to be 19.79 ps after 1-s averaging and 0.896 ps after 10 3 -s averaging, as compared to the real delay value. The overlapping Allan deviation of the recovered 2.42-GHz frequency reaches 2:04 Â 10 À13 and 1:71 Â 10 À16 at 1 s and 10 4 s, respectively. The loop delay tunable range is in proportion to the fiber length, giving the potential of constructing a long-distance fiber loop link.
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