Fiber Optic Couplers and Splitters

When working with optical fibers, it is often necessary to use fiber optic couplers for various purposes. Some examples:

Figure 1: 2×2 fiber optic coupler.

Additional isolated absorption peaks may come from some impurities. For example, silica fibers exhibit increased absorption loss around 1.39 μm and 1.24 μm if the core material is not anhydrous. Directional 2 × 2 couplers (see Figure 1) are often used for this purpose.
The same type of equipment can be used for fiber optic interferometers, and can also be used to combine two inputs. (Note that polarization problems may arise.)
Dichroic couplers can be used to combine the pump and signal inputs of a fiber amplifier, or to remove residual pump light after the amplifier.
For high power fiber lasers and amplifiers, pump couplers with multiple inputs are often required to combine the outputs of multiple high power diode bars.
The increased scattering loss may be due to irregularities at the core/cladding interface. This problem is exacerbated for fibers with large index contrast (high numerical aperture). Also, a greater index contrast usually means that the core is more heavily doped with germanium, which makes it temporarily less homogeneous. Therefore, low-loss single-mode fiber used for long-distance data transmission over telecommunication cables has a relatively small NA, even though a higher NA would provide more robust guidance.
Probably the most common operating principle for directional fiber couplers is evanescent wave coupling in a configuration where the two fiber cores are close to each other. The device can be made by heating two exposed fibers, causing the glass to begin to melt and fuse together. One may also pull on the fibers a little during the process. The refractive index profile obtained in this way is shown in Figure 1:

Figure 2: Refractive index profile of a fiber coupler.

Both waveguides are single-mode waveguides with super-Gaussian refractive index profiles. The coupling region in the middle is only a few millimeters long. Outside this region, the coupling is negligible because the mode fields do not practically touch each other.

With numerical beam propagation, it is now possible to examine what happens when light is injected into the top left input port only:

Figure 3: Amplitude distribution in a fiber coupler, obtained by numerical simulation of beam propagation, done with the software RP Fiber Power.

In this particular case, the light first couples almost completely into the lower waveguide after a short distance, then returns to the upper waveguide, where most of the power remains in the end. Since the coupling strength depends sensitively on the wavelength, for some other wavelengths, for example, almost all the power can be delivered to the lower output port. The simulated wavelength dependence is shown in Fig. 4. The somewhat odd shape of the curve in the longer wavelength region is caused by the bending loss of the waveguide, which becomes large in this region.

Figure 4: Degree of power coupling as a function of wavelength.

The wavelength sensitivity becomes stronger if the coupling is made weaker (through the waveguide distance) but can occur over longer lengths. In contrast, broadband couplers require strong coupling over short lengths.

Note that this type of coupler is a directional coupler: basically no light goes in the “backward” direction.

Of course, it is possible to inject light into the two input ports of such a fiber coupling. Then, assuming the optics are not strong enough to cause nonlinear effects, the output will be a linear superposition of the electric field magnitudes induced by the two inputs. Especially for fiber couplers made of single-mode fiber, if two coherent inputs of properly chosen power, polarization direction, and relative phase are injected, destructive interference can be obtained in one of the output ports. In this case, the other output port will interfere constructively. Of course, apart from some possible parasitic power loss, the overall power must also be preserved.

Pump couplers for high power fiber lasers and amplifiers differ in several ways. The input and output fibers are strong multimode, high core count and high numerical aperture. The coupling principle can also be different from the above examples. For example, instead of evanescent wave coupling, light can simply be injected from a smaller core into a large core for output. Care must be taken to minimize power loss—in part because light loss at high power levels can damage the coupler.