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.

Phase-conjugation-based fast RF phase stabilization for fiber delivery

We report a new radio-frequency (RF) phase stabilization approach for long-haul optical fiber distribution. The phase drift of an RF signal induced by fiber-length variations can be canceled out automatically via RF mixing without using active phase discrimination and dynamic phase tracking. A key significance of our approach is that no assistant local oscillator (LO) signal is needed. Consequently, frequency estimation of the received RF signal, as well as frequency locking between the LO and the received RF signal, is no longer required, which simplifies the system. A proof-of-concept experiment shows that the phase drift of the received RF signal at 9.6 GHz is significantly reduced using the proposed method. The root mean square (RMS) timing jitter is 0.76 ps when a tunable optical delay line (TODL) inserted between the remote antenna unit (RAU) and local station is changed from 0 to 300 ps.

Stable radio-frequency phase distribution over optical fiber by phase-drift auto-cancellation.

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.

Phase stabilized downlink transmission for wideband radio frequency signal via optical fiber link.

In this paper, we propose and demonstrate a phase-stabilized radio-frequency (RF) reference transmission scheme by actively stabilizing the optical path delay of the fiber link. The reference RF tone is round-trip transferred between the central station and the remote end to obtain the delay variation. The fast and small part of the delay fluctuation is cancelled by a piezoelectric fiber stretcher, benefitting from its short response time and fine adjusting granularity. The slow and large part of the delay variation is used to alter a wavelength tunable laser, which results in a dispersion-induced tunable optical delay line with very large compensation range. Experimentally, a 2.48-GHz RF signal has been transferred through a 60-km optical fiber link with suppressed noise. The fractional frequency stability achieved is $6.5\times 10^{-14}$ at 1 s and $2.1\times 10^{-17}$ at $10^{4}\ \hbox{s}$ averaging time, respectively.

Improved Phase Stabilization of a Radio-Over-Fiber Link Under Large and Fine Delay Tuning

We describe basic principles of operation and report on implementation of a standalone photonic link stabilized using the electronic phase conjugation technique. This method has been demonstrated to improve link stability by more than 2 orders while reducing its size and power consumption compared to other systems. We have demonstrated packaged robust links that achieve a relative frequency instability of 2×10−17 (5×10−17) at 10 h averaging while the temperature of the fiber was varied with 2°C (15°C) magnitude.

Stabilized photonic links for space applications.

Stable fiber optic links have been operational in NASA's DSN for over 20 years providing distribution limited fractional frequency stability below 10-14 at one second and reaching 10-18 at approx 10,000 seconds. We summarize recent stabilized photonic link developments for DSN operation capable of high performance and reliability, and at much lower cost than previous systems. Recent research has focused on two applications, 1) actively stabilized links to transport 100 MHz metrology frequency references to sites up to 30 km from the central atomic frequency standard, and 2) stabilized links transporting broadband S, X, and Ka band signals used in deep space tracking and navigation and antenna arraying applications.

S. Huang

Papers

Stabilized Photonic Links for Deep Space Tracking, Navigation, and Radio Science Applications