ECOC 1975 - ECOC 2013

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The SiN of the repeater is determined by the individual contribution of APD, LED,
and transistor amplifiers. The noise analysis of the transistor amplifier can be found
elsewhere. The APD noise consists of thermal noise, quantum noise, and shot noise.
The contribution from these noise sources depends upon the operation bandwidth,
resistance, and signal level. The dark current noise, leakage current noise, and background
noise are neglected in the analysis. The noise currents of the LED may be due
to a recombination process, admittance thermal noise, bulk material outside the diode
junction, and the light field and its intensity fluctuations. All of these noises except
the latter are device-oriented and cannot be reduced by circuit compensation. The
beat noise of the LED can be neglected in the multimode fiber communication link since
the beat noise is inversely proportional to the number of spatial modes of the fiber as
well as the LED's spectral width. Poor VSWR and admittance mismatching in the circuit
design at radio frequencies will degrade the noise performance considerably.
Repeater Performance.
Both SiN and intermodulation (1M) distortions were measured to evaluate the repeater
performance. The test was performed in a general laboratory environment without any
screen room shielding in order to simulate a practical field condition. The lOO-foot
Corning, low-loss, 19-fiber cables (with a measured loss of 17 dB/km at 9100 A) were
used in this experiment. The cable ends were terminated by a set of SMA connectors
and precisely machined ferru les wh ich are compatible with the SMA connectors on the
APD input and LED output. The modal dispersion loss and modulation transfer function
of the fiber have been considered for equalization computation. At a modulation depth
for the LED of nearly 50 percent, the measured th ird-order 1M products and SiN versus
frequency for the repeater were as plotted in Figure 2.
Repeater Application.
Besides the repeating relay application in communication links and CATV systems, the
fiber optic repeater also can be utilized to construct spectral transformers, fiber modal
transformers, active couplers, an optoelectronic oscillator, combiner, and divider as
well as to reduce noise, nonlinearity, and temperature instabilities of the optical system.
Especially, the active coupler has many superior advantages over the conventional
T- or star-coupler. When a tunable LED is employed in the design, programmable
couplers, dividers, or spectral transformers can be realized. A three-way active
divider and a spectral transformer are illustrated in Figure 3 and Figure 4, respectively.
Conclusion.
The low-cost, compact, optical repeater shown in Figure 3 and Figure 4 is a breadboard
used to demonstrate its high performance as well as its applications. Employing
chip forms for the APD, LED, and amplifiers, IC modules can be constructed which
would reduce the repeater size considerably and also provide an application to single
fiber operation. When the clamping, peak detection, timing coincidence, and
regenerator circuits are included in the design, the repeater may be applied to highspeed
digital applications.
Acknowledgement.
The author would like to express his gratitude to A. Mazzara, L. Lavendol, and
L. R. Allain for their encouragement and support, and M. P. Arnold for his assistance
in the measurements.