Preliminary Experiments of Radio Interferometer Using Fiberoptic Links Modulated in Radio Frequency

Jun Amagai (amagai(AT)nict.go.jp), Hiroo Kunimori, and Hitoshi Kiuchi

Communications Research Laboratory
4-2-1 Nukui-kita, Koganei, Tokyo 184, Japan

1. Introduction

In Very Long Baseline Interferometer (VLBI), extremely stable frequency standards are required for each station. Data recorders are also needed to record the signals detected at each observation site. Recently, real-time data transmission using ATM switches has been initiated with VLBI by the Communications Research Laboratory [Kiuchi, 1996]. Data recorders are not needed in that system, although frequency standards are still necessary.

It is possible to make a radio interferometer that has neither a data recorder nor frequency standards at each observation site by introducing fiberoptic links modulated in radio frequency. In this \pagebreak type of radio interferometer (optical-linked RF interferometer) we can utilize absolute delay, and we do not have to estimate the clock offset for baseline analysis because we use common local signals and every instrumental delay can be calibrated. According to Heki, [1990], the vertical component error of the estimated position is larger than the horizontal component error because it is difficult to distinguish the parameter for the vertical component of the station position from that for the clock offset in least-square parameter estimation. We can improve the vertical position error by introducing baseline analysis, which does not need clock offset estimation.

In conventional VLBI, it is impossible to use phase delay because an unknown phase introduced by an independent local oscillator is added to the observed phase. For the optical-linked RF interferometer, however, we use a common local oscillator for frequency conversion so that the phase difference observed by this interferometer can be treated as a phase difference caused by group delay in the case where no dispersion medium exists. We can improve delay determination accuracy considerably by introducing the phase delay.

2. Configuration of the System


Figure 1. System configuration of the optical linked RF interferometer.


Figure 1 shows the configuration of the optical linked RF interferometer. The signal of the radio frequency at 8 GHz detected by the antenna at each observation site (objective signal) is amplified by a low noise amplifier (LNA) and converted to an optical signal at the wavelength of 1.31 micro meters. The optical signals are transmitted through optical fiber to the analysis station, where the electric signals are reproduced from the received optical signals. By using a common local signal the reproduced signals are converted to video signals, which are processed by a correlation processor.

Delay changes that occur in the optical fiber are compensated for by the calibration signal, which makes a round trip between the analysis station and the observation sites. A calibration signal generated at the analysis station is converted to an optical signal at the wavelength of 1.55 micro-meters and is divided into signals by an optical power divider. The divided optical calibration signals are transmitted to the observation sites through the same optical fibers through which the objective signals pass. At each observation site the calibration signal is reproduced from the optical signal and injected into the receiving system through a directional coupler installed just before the LNA. The calibration signal returns to the analysis station via the same path through which the objective signal is transmitted.

3. Results of preliminary experiments

We investigated a radio interferometer with fiberoptic links modulated in radio frequency and found that the practicable cable length was 65 km (Fig. 2).

Figure 2. Equivalent input noise of optical link (factory data). A practicable cable length is expected to be 65 km in the case that allowable signal to noise ratio is 10 dB and optical fiber loss for unit cable length is 0.35 dB/km.


We carried out preliminary experiments with a common signal instead of the signal coming from antennas and with optical fiber cables whose length was less than 10 m [Amagai et al., 1996]. The results of the preliminary experiments suggest the following conclusions:
  1. Short-term phase stability of optical signal transmission is sufficient to maintain the correlation amplitude.
  2. Long-term delay fluctuation is less than 0.3 psec when the system is kept under the temperature condition of 1 deg peak to peak.
  3. The optical fiber delay is successfully compensated for by a calibration signal in the radio frequency which makes a round trip between the analysis station and the observation sites. We can determine the absolute delay not only by group delay but also by phase delay. To use this cable delay compensation method, however we must know the the precise ratio of the refractive indices for 1.31 micro-meters and 1.55 micro-meters(Fig. 3).

Figure 3. Relationship between the measured delay obtained by the phase delay and the cable length difference. Although the measured delay is expected to be 0, it decreases in proportion to the difference in the cable length.


We plan to solve the refractive indices problem by measuring the time necessary for a optical signal at a wavelength of 1.55 micro-meters to move back and forth between the analysis station and the observation site.

Acknowledgements

The authors would like to express their appreciation to Mr. Takeda and Mr. Kikuchi of Sumitomo Osaka Cement Co., Ltd for their help with the signal-to-noise analysis of the optical link. The authors are also grateful to Dr. Kondo of the Communications Research Laboratory for his assistance with the correlation processing analysis.

References

Amagai, J., H. Kunimori, and H. Kiuchi, Fundamental Experiments of Radio Interferometer using Fiberoptic Links Modulated in Radio Frequency, Proceedings of the Technical Workshop for APT and APSG 1996, pp.269-273, 1996.

Heki, K.,Three Approaches to Improve the Estimation Accuracies of the Vertical VLBI Station Positions,J. Geod. Soc. Japan, Vol.36, pp.143-154, 1990.

Kiuchi, H., M. Imae, T. Kondo, M. Sekido, S. Hama, T. Yamamoto, H. Uose, and T. Hoshino, Real Time VLBI of the KSP, Proceedings of the Technical Workshop for APT and APSG 1996, pp.125-129, 1996.

Updated on June 9, 1997. Return to CONTENTS