Fiber optic splitter

A fiber optic splitter takes the optical power arriving in one fiber and distributes it among $N$ output fibers, either equally or in a fixed ratio. It has no electronics and needs no power. In reverse, light entering any output port is combined into the input fiber. Equal splitters are made from 1×2 up to 1×64 and beyond, and unequal "tap" couplers send a small fraction such as 1%, 5% or 10% to a monitor port. The fundamental cost of an equal split is $10\log_{10}N$ in decibels, 15.1 dB for the 1×32 splitters common in passive optical networks.

Split loss

Conservation of energy fixes the minimum loss. If each of $N$ outputs receives $1/N$ of the input, the loss to each port is

$$L_\mathrm{split} = 10\log_{10}N\ \ \mathrm{dB}.$$

A real device adds excess loss from waveguide propagation, fiber-to-chip coupling and imperfect branching, and its outputs are not exactly equal. For a 1×32 splitter, a measured worst-port insertion loss of 17.0 dB corresponds to about 1.9 dB above the ideal, and port-to-port uniformity of order 1 dB is typical. Datasheets quote the worst port, which is the figure to use in a link budget.

For unequal taps with coupled fraction $f$, the tap port sees $-10\log_{10}f$ and the through port $-10\log_{10}(1-f)$. A 1% tap is 20 dB down and costs the main path only 0.04 dB; a 10% tap is 10 dB down and costs 0.46 dB.

PLC and FBT splitters

Planar lightwave circuit (PLC) splitters are silica waveguides on a silicon or quartz chip, with a tree of cascaded 1×2 Y-branches or multimode interference couplers and fiber arrays attached at each end. A 1×32 device is five stages of 1×2 branching. PLC splitters have nearly wavelength-independent split ratios across roughly 1260–1650 nm, good uniformity at high port counts and a compact package, which has made them standard for high-count PON splitters.

Fused biconical taper (FBT) couplers are made by twisting two fibers together, heating them and stretching them until the tapered cores are too small to confine the light, which spreads into the fused cladding and exchanges power between the fibers, as in a directional coupler. The fraction transferred after an interaction length $L$ follows

$$\frac{P_2}{P_0} = \sin^2(\kappa L),$$

so the manufacturer stops pulling when the monitored ratio reaches the target; a 50:50 split corresponds to $\kappa L = \pi/4$. Because the coupling coefficient $\kappa$ depends on wavelength, a basic FBT coupler holds its ratio over one band, and wavelength-flattened versions widen the usable range. FBT couplers are inexpensive for 1×2 and unequal taps; higher port counts are built by cascading them, which degrades uniformity.

Variants of the fused coupler include wavelength-selective couplers, which combine for example 980 nm pump light with a 1550 nm signal, and polarization-maintaining couplers.

Where splitters are used

• Passive optical networks. A single optical line terminal port feeds 32 or 64 homes through one or two splitter stages. With a 1×32 splitter at 17.0 dB, 20 km of fiber at 0.35 dB/km (the 1310 nm upstream worst case) and 1.0 dB of connectors, the path loses 25.0 dB, within the budgets of common PON optics.
• Power monitoring. A 1% or 2% tap feeds a photodiode to monitor a laser, amplifier or channel without significant loss to the signal.
• Laboratory setups. 50:50 couplers form the beam splitter of fiber Michelson and Mach-Zehnder interferometers, fiber optic sensors and fiber-optic gyroscopes.

Pitfalls

• Combining costs the same as splitting. In the upstream direction of a PON, each subscriber's signal passes through the splitter to one input fiber and suffers the full $10\log_{10}N$ loss. Two mutually incoherent sources at the same wavelength and polarization cannot be merged into one single-mode fiber without that loss.
• Unused ports reflect. An open flat-polished port returns about 3% of its share of light toward the source; unused ports are terminated or use APC connectors.
• Polarization and wavelength. Specifications include polarization-dependent loss, typically a few tenths of a dB, and a wavelength range outside which the ratio drifts, notably for FBT types.
• Confusing excess loss with insertion loss. Excess loss is the total output relative to input; insertion loss per port includes the split. The two differ by $10\log_{10}N$.

Common questions

How much loss does a 1×8 or 1×32 splitter add?

Ideally 9.0 dB for 1×8 and 15.1 dB for 1×32 per output. Real PLC devices typically add roughly 1–2 dB to those figures, depending on port count and packaging.

What is the difference between a PLC and an FBT splitter?

A PLC splitter is a waveguide chip with a tree of branches, wavelength-flat and uniform at high port counts. An FBT splitter is made from fused, stretched fibers, cheaper at low port counts and more wavelength dependent.

Is a fiber splitter the same as a WDM?

No. A power splitter divides all wavelengths in a fixed ratio, while a wavelength-division multiplexer routes each wavelength to a different port with low loss.

References: Saleh & Teich, Fundamentals of Photonics 3rd ed. 2019, Ch. 9; G. P. Agrawal, Fiber-Optic Communication Systems 4th ed. 2010; A. W. Snyder & J. D. Love, Optical Waveguide Theory 1983.