Fiber array (FAU)

A fiber array, or fiber array unit (FAU), is a block in which several fibers lie in V-grooves etched in silicon or machined in glass, fixed under a lid with adhesive and polished together to a common end face. The grooves set the fiber-to-fiber pitch, almost always 127 µm or 250 µm: 250 µm matches the coated diameter of standard fiber and of ribbon cable, and 127 µm, half of that, packs channels at the density of photonic integrated circuit I/O. The far ends of the fibers are usually terminated in individual connectors, a ribbon or an MPO connector, so that one alignment of the array connects every channel of a chip to the outside world.

End-face geometry

The polish angle depends on the coupler on the chip. For edge couplers the face is polished flat or at 8°. A flat silica face against air reflects 3.4% of the light, a return loss of 14.7 dB; the 8° angle directs that reflection out of the fiber core. The angle also refracts the beam: leaving an 8° silica face into air, it deviates 3.6° from the fiber axis, so the array is tilted to put the beam on the waveguide axis, and the deviation shrinks once the gap is filled with adhesive of similar index.

For grating couplers, which radiate out of the chip surface at an angle set by the grating period, the array is held above the chip at the design angle, often near 10° from the vertical. In permanent packages two geometries are common: an array polished so that the fibers stand at the design angle, and a block polished near 40° so that the light reflects by total internal reflection inside each fiber and leaves through its side, letting the array lie flat on the chip.

Alignment tolerance and loss

The array is aligned as a single rigid body, so its tolerances add to those of the alignment. The loss for a lateral offset $d$ between two equal Gaussian modes of radius $w$ is

$$L = -10\log_{10}\!\left[e^{-(d/w)^2}\right]\ \mathrm{dB}.$$

For standard single-mode fiber at 1550 nm the mode field diameter is 10.4 µm, so $w$ = 5.2 µm. An offset of 0.5 µm then costs 0.04 dB, 1 µm costs 0.16 dB and 2 µm costs 0.64 dB; 1 dB is reached at 2.5 µm, which matches the ±2.5 µm window usually quoted for grating couplers. Against an edge coupler with a 4 µm mode, coupled from small-core or lensed fiber with a matching 4 µm mode, the same 1 dB is reached at 0.96 µm, so the array's own errors consume most of the budget. Those errors are the groove placement, the fiber's cladding diameter and the core-to-cladding concentricity, which together put each core within a few tenths of a micrometer to about 1 µm of its nominal position, worsening with channel count. Pitch mismatch accumulates: a 0.05% difference between array and chip pitch is 0.95 µm at the sixteenth channel, 1905 µm from the first. Size mismatch between the fiber mode and the coupler mode is a separate term, covered in mode mismatch loss.

Where fiber arrays are used

Testing. On a probe station a fiber array addresses rows of grating couplers for wafer-level test of undiced wafers, and edge couplers in die-level test after dicing. Repeating one array over many dies gives coupling variation near 0.5 dB, small enough to compare devices.

Packaging. In pigtailing a chip, the array is aligned actively, with light passing through loop-back waveguides between two channels, then bonded with UV-curing epoxy. Cure shrinkage moves the array by fractions of a micrometer, which is why thin bond lines and low-shrink adhesives are used. Packaged results with standard fiber are typically 1–2 dB per facet for edge coupling with a mode converter. The procedure, adhesive data and loss budget are in Fiber Array Attach to Photonic Chips.

Dense optics. Pluggable transceivers and co-packaged optics engines bring tens to hundreds of fibers to a chip edge, and their arrays are the main optical alignment step in production.

Arrays can also carry polarization-maintaining fiber, with each slow axis aligned to the chip's TE mode, or small-core high-NA fiber matched to the chip's couplers.

Pitfalls

• A loss that ramps across the channels is pitch or yaw error; one bad channel among good ones is usually debris or a bubble in the gap.
• A 0.3° error in the polish angle of a grating-coupling array shifts the peak wavelength of every channel by a few nanometers together.
• Measured loss should be referenced to the fiber side of the array and the array's own loss removed when quoting coupling efficiency of the chip.

Common questions

What is the standard fiber array pitch?

127 µm for most photonic chips and 250 µm where the array must match ribbon fiber.

Why are fiber arrays polished at 8°?

To keep the reflection from the glass-air or glass-adhesive interface from coupling back into the fiber core, which would return light to the laser.

References: D. Marcuse, "Loss analysis of single-mode fiber splices," Bell System Technical Journal 56, 703 (1977); L. Chrostowski and M. Hochberg, Silicon Photonics Design (Cambridge University Press, 2015); G. T. Reed and A. P. Knights, Silicon Photonics: An Introduction (Wiley, 2004).