Y-branch
The simplest integrated optical power splitter: one waveguide forks symmetrically into two, sending half the power each way over a broad bandwidth. Run in reverse it is a combiner whose loss depends on the relative phase of its inputs, which is exactly how a Mach-Zehnder modulator turns phase into intensity.
A Y-branch (or Y-junction) is a waveguide that widens gradually and then forks into two arms. If the fork is symmetric, the input mode splits 50:50 by symmetry alone, with no interference condition to satisfy, which is why a good Y-branch is broadband and polarization-tolerant in a way a directional coupler, whose splitting ratio beats with wavelength, is not.
Fabrication reality. The ideal Y-branch ends in an infinitely sharp tip between the two arms, and no lithography can print one. The blunted tip (on the order of 100 nm) creates a small mode mismatch, radiating a fraction of the light; typical excess insertion loss is 0.1 to 0.5 dB in silicon, with optimized compact designs around 0.3 dB. The device also has a strong claim to being the most deployed photonic component on Earth: the silica splitter chips that fan one fiber out to 32 or 64 homes in a passive optical network are trees of cascaded Y-branches.
The combiner asymmetry. Run backwards, the Y-branch teaches a lesson that surprises almost everyone once. Two inputs arriving at the junction form a superposition, and the single-mode output waveguide can carry only its symmetric part. In-phase inputs combine without loss; anti-phase inputs form the antisymmetric combination, which has no output mode to enter and radiates into the chip. Combining two incoherent or arbitrary-phase signals therefore costs an unavoidable 3 dB on average: a Y-junction is a perfect splitter but not, in general, a perfect combiner, and no passive reciprocal device can be.
That radiated light is not a defect; it is the operating principle of the Mach-Zehnder modulator. At the null of an MZM, the light is not absorbed anywhere: the output Y-junction simply hands the antisymmetric field to the substrate. The extinguished signal leaves the chip sideways.
Choosing among the three standard splitters:
| Y-branch | Directional coupler | MMI | |
|---|---|---|---|
| Principle | Adiabatic symmetry | Evanescent coupling | Multimode self-imaging |
| Bandwidth | Very broad | Narrow; ratio drifts with wavelength | Moderate |
| Splitting ratio | Fixed 50:50 | Any ratio, by length and gap | Mostly fixed ratios |
| Fabrication sensitivity | Tip loss, but ratio robust | Gap-critical | Robust |
References: Chrostowski & Hochberg, Silicon Photonics Design (2015), Ch. 4; Zhang et al., Opt. Express 21, 1310 (2013).