Page 351 - Optical Communications Essentials

Manufacturing Issues

Manufacturing Issues 341

TABLE 20.1. Representative Specifications of an EDWA (Specification from NKT Integration;
www.nktintegration.com)
Parameter Specification
Wavelength range 1528–1562nm
Pump wavelength 980nm
Small-signal gain 20dB @ 100-mW pump power
Output power 10dBm @ 100-mW pump power and 0-dBm input signal
Noise figure 5.0dB @ 20-dBm input power
PDL (PDG) 0.3dB
PMD 0.3ps
Size of bare chip 25 55 1mm

individual on-chip multiplexers for C-band and 980-nm pump wavelengths on
both the input and the output for either codirectional or counterdirectional
pumping. Table 20.1 gives some specifications of such an amplifier. Here the
acronyms PDL and PDG refer to polarization-dependent loss and gain, respect-
ively. This module may be used in metro applications where a few channels are
added or dropped from a high-capacity DWDM trunk line.

20.2.2. Athermal designs
The performance of a passive optical component may change significantly with
temperature. Of particular concern is wavelength drift in a DWDM application.
Among thermally sensitive components are standard arrayed waveguide gratings,
fiber Bragg gratings, and bulk-grating-based optical products. To maintain reli-
able performance of a DWDM communication link, it is essential that the wave-
length characteristics of such system components be as invariant as possible.
Conventional AWGs consist of lightwave circuits made of quartz glass wave-
guides. Since the index of refraction of quartz glass changes with temperature,
the wavelengths of light transmitted through such an AWG also change. Thus
a conventional AWG typically will have a thermoelectric cooler-based tempera-
ture controller built into the package to maintain wavelength stability.
However, by using different materials for the waveguides, an AWG can be fab-
ricated without the need for a temperature control device and, consequently,
without the need for a source of electric power. For example, an athermal AWG
made by NTT Electronics uses a special silicon resin in part of the lightwave cir-
cuit that has a different temperature coefficient from that of quartz glass. This
design cuts the temperature dependence of the wavelengths of transmitted light
to less than one-tenth of its original value, which makes using a temperature
control device unnecessary.
A different approach based on a passive temperature compensation method
can be taken for a fiber Bragg grating (FBG). In this case the FBG is attached

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