Fiber Optic Essentials

Chapter 12.4.2 - Fiber Optic Components: Fiber Optic Couplers - Applications

Applications

Power Dividers   As discussed earlier, one of the most important applications of
a fiber directional coupler is as a power divider. In many applications, such as in
local area networks or in fiber optic sensing, it is necessary to split or combine
optical beams. Such a fiber optic directional coupler forms an ideal component
since it is compact and possesses low loss. One of the important issues in such
couplers is the wavelength dependence of the coupler.Wavelength-flattened couplers
are characterized by coupling ratios, which remain almost constant for a given band
of wavelengths, such as the C-band (1530 to 1565 nm). If power is to be divided
into many ports, 3-dB couplers can be concatenated to split from one input port into
multiple ports, as shown in Fig. 12.6.

Wavelength-Division Multiplexers/Demultiplexers   Another very important
application of such couplers is in wavelength-division multiplexing and demultiplexing.
As discussed earlier, fiber directional couplers are in general wavelength
sensitive, and we may have a coupler for which if light beams at two different wavelengths
(say, one at 980 nm and the other at 1550 nm) are launched simultaneously
in one of the ports, one of the wavelengths will exit port 2 and the other wavelength
will exit from the other port, as shown in Fig. 12.7a. If we use the same coupler
in the reverse direction, the two wavelengths incident on the coupler appear at the
same port, as shown in Fig. 12.7b; such a coupler is used in an erbium-doped fiber
amplifier as shown in Fig. 9.6.

Coupler Fabrication

Fiber couplers are fabricated using a variety of techniques; however, the method
used most extensively is fusion. Fused fiber couplers are fabricated by first slightly
twisting two single-mode fibers (after removing their protective coating) and then
heating and pulling them so that the fibers fuse laterally with one another and are
also tapered (Fig. 12.8). Heating can be accomplished by using an oxybutane flame
or miniature electrical heating element. The coupling ratio is monitored online as the
fibers are fused and drawn, and the process of fusion and tapering is stopped as soon
as desired. Figure 12.9 shows the cross section of a fused fiber coupler composed of
single-mode fibers. One can see the two cores that lie close to each other.

 

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