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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

Photonic Millimeter-wave generation and transmission system applicable to the high-frequency radio interferometers

Hitoshi Kiuchi, Tetsuya Kawanishi

Published Apr 1, 2009• 3 citations
Publisher

Abstract

In the field of high-frequency radio interferometer, Atacama Large Millimeter/sub-millimeter Array (ALMA), its highest receiving frequency reaches 950 GHz. To receive such high frequencies, a higher reference frequency is required (as much as over 100 GHz). To maintain signal coherency, this reference signal has to be highly stable. To address these issues, we have developed a new method to generate and transmit a reference signal in the form of frequency difference between two coherent light waves. The two transmitted optical signals require phase stability better than 10-13(1 sec) in white phase noise in the Allan standard deviation.

DOI
10.1109/freq.2009.5168313

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In the field of high-frequency radio interferometer, Atacama Large Millimeter/sub-millimeter Array (ALMA), its highest receiving frequency reaches 950 GHz. To receive such high frequencies, a higher reference frequency is required (as much as over 100 GHz). To maintain signal coherency, this reference signal has to be highly stable. To address these issues, we have developed a new method to generate and transmit a reference signal in the form of frequency difference between two coherent light waves. The two transmitted optical signals require phase stability better than 10-13(1 sec) in white phase noise in the Allan standard deviation.
In signal transmission through optical fiber, cable length delay fluctuation accompanied by chromatic and polarization-mode dispersion affects the coherence of distributed signals. To maintain signal coherence, it is very important to generate very-high-frequency signals with minimum phase noise and transmission loss. In a photonic local signal generation/distribution system with a microwave-photonic signal generator and a real-time microwave-photonic signal phase stabilizer that we developed as an alternative photonic LO system for ALMA (Atacama Large Millimeter/sub-millimeter Array), signals are transmitted in the form of frequency difference between two coherent light waves, effectively maintaining the coherence of distributed reference signals. Through the development of the real-time phase stabilizer, we discovered that the system would be further improved with the introduction of a post-processing scheme phase stabilizer and confirmed its effectiveness by experiments.
Phase-stable radio frequency (RF) transfer attracts a growing research interest and attention in recent years, and transferring high stable RF signal to multiple places is greatly needed in many important applications such as frequency synchronization for very long base-line interferometry and connected-element interferometry in radio astronomy. [1]. For example, the Atacama Large Millimeter Array (ALMA) radio astronomy project is consist of an array of up to 64 antennas, and in order to achieve the unprecedented sensitivity and angular resolution, requires the local oscillator reference signal arriving at remote antennas with a maximum path length up to 18 kilometers must be stabilized under one picosecond [2]. The optical fiber link is a promising method for long-distance highly stable frequency distribution, however it suffers from the environment perturbations such as physical vibration and temperature fluctuations, which degrade the phase stability to be delivered [3].
We demonstrate a photonic radio-frequency transmission system via optical fiber. Optical radio-frequency signal is generated utilizing a Mach-Zehnder modulator based on double-side-band with carrier suppression modulation scheme. The phase error induced by optical fiber transmission is transferred to an intermediate frequency signal by the dual-heterodyne phase error transfer scheme, and then canceled by a phase locked loop. With precise phase compensation, a radio frequency with high-phase stability can be obtained at the remote end. We performed 20.07-GHz radio-frequency transfer over 100-km optical fiber, and achieved residual phase noise of -65 dBc/Hz at 1-Hz offset frequency, and the RMS timing jitter in the frequency range from 0.01 Hz to 1 MHz reaches 110 fs. The long-term frequency stability also achieves 8×10(-17) at 10,000 s averaging time.
Precise frequency comparisons are becoming more urgent given the recent rapid progress in next-generation frequency standards. This paper describes a new type of bidirectional optical amplifier that overcomes the fiber loss limits that have prevented accurate frequency comparisons between widely separated places; such comparison is realized by bidirectionally transmitting wavelength-division-multiplexed (WDM) signals along a single fiber. The proposed optical amplifier has an optical isolator in each two-way channel divided by wavelength filters to suppress the optical reflection that causes amplification instability. The additional insertion optical loss due to this method is about 1.5 dB. The optical gain greater than 30 dB is obtained for both signals with good optical isolation of 65 dB. A radio-frequency reference signal can directly be sent by simple intensity modulation and direct detection (IM-DD) devices in the 1550-nm region widely used in telecommunication networks. Phase comparisons of the received signals and the frequency standards at each terminal are used for frequency comparisons. The amplifier is tested in the field using two hydrogen masers. A 120-km fiber with loss of 52.5 dB is used to connect the National Metrology Institute of Japan (NMIJ) to the University of Tokyo. Because of this loss, an amplifier is needed to realize sufficient receiving power. The frequency stability of the system with a 10-MHz direct transmission is evaluated by returning the optical signal from a halfway point [55 km from the NMIJ] where the amplifier is installed. The result is 2.6 × 10-16(Allan deviation) with the averaging time of 7 × 104s. The laboratory result is 8.7 × 10-17(¿ = 4 × 104s). The amplifier's long-term stability is promising for stable frequency dissemination in addition to precise frequency comparisons.
We propose to use a parametric mixer in a passive phase correction scheme for stable distribution of a millimeter-wave signal over fiber links. The parametric mixer is realized via ultrafast four-wave mixing processes, thereby overcoming the bandwidth limit of conventional electronic devices. The mixing of round-trip probe and local pumps generates a signal immune to transmission delay fluctuations at the remote end. This scheme is validated by an experiment that distributes a 36-GHz millimeter wave over 20-km optical fiber. The remote frequency stability achieves 4.0×10−16 at 2000 s, indicating that this approach is promising for phase-stabilized delivery of high-frequency signals.
In this paper, we demonstrate stable radio frequency (RF) transfer via metropolitan optical fiber link in the Beijing area. The phase variation of the RF signal is compensated by a phase conjugation method incorporating two high-performance phase-locked loops. The wavelength conversion module extends the transmission length to 200 km with only two parallel 50 km dark optical fibers available. We optimize the configuration of dispersion compensation and optical amplification due to the high loss (0.31 dB/km) of the optical fiber link. At the same time, comparative experiments verify the short-term instability limitation that arises from the group velocity dispersion of the optical fiber link. The measured standard Allan deviation of the 2.4 GHz RF transmission system with dispersion compensation is 4.5 × 10-14/1 and 2.6 × 10-17/20 000 s, which is superior to that of the reference rubidium clock. The short-term instability of the system is deteriorated to 2.5 × 10-13/1 s without dispersion compensation.