ECOC 1975 - ECOC 2013

141
HI GH-PERFORMANCE, WIDEBAND FIBER OPTIC REPEATER AND ITS APPLICATION
J. J. Pan
Introduction:
Emerging fiber optic technologies promise to have a large impact on communication
systems, information processing, and CATV industries. However, the inherent fiber
attenuation and dispersion limit the link distance. For these future applications, the
signal level will have to be regenerated by repeaters installed at intervals of several
ki lometers to account for optical losses and dispersion.
This paper reports a wideband optoelectronic repeater utilizing commercially available
light-emitting diode (LED), ava lanche photodetector diode (APD), and low-cost, miniature,
50-ohm amplifier modules which provide system simplicity, cost reduction, and
the possibilities of IC fabrication and single fiber operation. Attention to impedcmce
matching and nonlinearity compensation are key subjects in optimizing system signalto-noise
ratio (SiN) and distortion.
Repeater Design Consideration.
The optoelectronic repeater, as depicted in Figure 1, may have a linear and flat
response from a few MHz up to 500 MHz by choosing an appropriate injection laser
diode. Presently, the repeater performance is limited by the frequency response of
commercial LED's to the vicinity of 120 MHz. An RCA C30817 APD is utilized as
the repeater input detector since it provides a low-noise equivalent power and is
optically broadband. Severa I cascaded 50-ohm amplifier modules serve as the postdetector
ampIifier. The impedance transition between APD and amplifier modules is
matched either by a wideband impedance transformer or an active field-effect transistor.
The latter can provide an optimized match ing across the frequency band with
moderate gain and low noise. In order to compensate the LED's slow response and the
fiber dispersion loss, a compensation network is utilized to equalize the signal level.
Since the LED has low resistance (3 t07 ohms), an inductive reactance at high radio
frequencies, and its modulation depth is determined by the matching network as well
as the signal level; either radial line or tapered microstrip, or a discrete transformer
has to be utilized to match the 50-ohm postdetector amplifier to the LED.
The nonlinearity of the cascaded amplifiers, APD, and LED in the repeater contribute
second- and third-order distortions; particularly, the LED' s nonlinear frequencydependent
distortion analysis, using the Volterra series approach, the phase relations
among the amplifiers, transformers, equalizers, APD, and LED dominate the distortion
performance. However, the simultaneous cancellation of both the second- and thirdorder
distortion products in the repeater will not occur owing to different constraints
on the phase relationship. Therefore, in order to reduce the second-order distortion
components and optimize the third-order distortions, balanced amplifiers and push-pull
LED configuration shou Id be used.
J. J. Pan is with HARRIS Electronic Systems Division