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
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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:
Short-term phase stability of optical signal transmission is
sufficient to maintain the correlation amplitude.
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.
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.
