Page 176 - Optical Communications Essentials

Active Optical Components

166 Chapter Ten

Figure 10.3. Typical VOA package for moderate optical power handling capabilities. (Photo courtesy
of Bookham Technology; www.bookham.com.)

TABLE 10.1. Some Representative Operational Parameter Values for a Typical VOA
Parameter Specification

Insertion loss 1.8dB
Attenuation range 25dB (up to 60dB possible)
PDL @ 25-dB attenuation 0.3dB
Maximum optical power per channel 150mW (up to 500mW possible)
Optical return loss 42dB
Insertion loss 0.7dB

10.4. Tunable Optical Filters

Tunable optical filters are key components for dense WDM optical networks.
Several different technologies can be used to make a tunable filter. Two key ones
are MEMS-based and Bragg-grating-based devices. MEMS actuated filters have
the advantageous characteristics of a wide tuning range and design flexibility.
Fiber Bragg gratings are wavelength-selective reflective filters with steep
spectral profiles and flat tops. This is shown in Fig. 10.4 for a 100-GHz filter
that has a reflection bandwidth of less than 0.8nm at 25dB below the peak.
Standard uniformly spaced fiber gratings have large sidelobes, which lie only
about 9dB below the central peak. However, these sidelobes can be reduced to
be at least 30dB below the peak by using a special apodization mask to precisely
control the UV beam shape in the Bragg grating fabrication process.
Apodization is a mathematical technique used to reduce ringing in an interfer-
ence pattern, thus giving a large central waveform peak with low sidelobes.
As Sec. 9.4 describes, one way of creating a fiber grating is by using ultravio-
let light to set up a periodic interference pattern in a section of the core of a ger-
mania-doped silica fiber. This pattern induces a permanent periodic variation in
the refractive index along the core. Tunable optical filters based on fiber Bragg
gratings involve a stretching and relaxation process of the fiber grating. Since
glass is a slightly stretchable medium, as an optical fiber is stretched with the

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