Lensed fiber
An optical fiber with a precision micro-lens formed or attached at its tip that focuses the emerging beam to a smaller spot than the fiber mode field. The standard input for coupling between fibers and small-mode chip waveguides or laser facets.
A lensed fiber has a micro-lens at its tip that focuses the divergent beam emerging from the fiber core down to a spot smaller than the fiber's mode field. This focused spot can then be coupled efficiently to small-mode targets (chip waveguide tips, laser facets) that have mode-field diameters much smaller than the bare fiber.
Why bare fiber doesn't work for chip coupling. Standard SMF-28 has mode field diameter (MFD) of 10.4 μm at 1550 nm. A typical silicon photonic inverse taper tip has effective mode diameter μm. Coupling a 10.4 μm beam to a 2.5 μm target produces about 7 dB of mode-mismatch loss. A lensed fiber that produces a 2.5 μm spot can match the target precisely, dropping loss to dB. The fiber coupling calculator computes the loss for any spot and mode size, with offset and tilt.
Lens geometries.
| Type | Construction | Spot size | Working distance |
|---|---|---|---|
| Cleaved + polished cone tip | Mechanically polished | 5 – 8 μm | 8 – 15 μm |
| Wedge-tip lensed | Polished asymmetric wedge | 2 – 4 μm (cylindrical asymmetric) | 5 – 15 μm |
| Hyperbolic / hemispherical | Photothermal melting + shaping | 2 – 5 μm | 5 – 30 μm |
| GRIN (graded-index) fiber lens | Spliced GRIN segment + cleave | 4 – 12 μm | mm-scale |
| Drawn taper + spliced ball | Tapered fiber with spherical tip | 1.5 – 3 μm | 5 – 20 μm |
| Optically printed micro-lens | 3D-printed by 2-photon polymerization (research) | μm | varies |
Specifying a lensed fiber. Standard parameters:
- Working distance (WD): distance from fiber tip to spot waist; usually 5 – 30 μm
- Spot diameter at waist: 2 – 8 μm typical
- Numerical aperture: 0.15 – 0.45, set by the spot diameter and the wavelength
- Polarization-maintaining or single-mode: PM lensed fibers are available for polarization-sensitive measurements
- Anti-reflection coating: typically 0.25% or 1% AR at the design wavelength
Tradeoffs in lens design. Smaller spot tighter focus + higher coupling to small targets, but:
- Shorter Rayleigh range tighter Z-position tolerance (for 0.5 dB of loss against a matching mode, about μm for a 2.5 μm spot at 1550 nm, against about 38 μm of gap between two bare SMF-28 fibers)
- Tighter lateral tolerance: 1 dB of loss at a lateral offset of about 0.6 μm for a 2.5 μm spot at 1550 nm, against 2.5 μm between two standard fibers; the angular tolerance moves the other way, about 11° against 2.6°, because a smaller spot diverges faster
- More sensitive to lens-tip damage: sub-μm scratches or contamination destroy performance
Lifetime. A lensed tip degrades with handling, contamination and accidental contact with the chip, so lab practice is to track individual fibers and replace each one when its coupling efficiency has fallen by a set amount.
Use in pigtailing. Lensed fibers are the standard input for active-alignment pigtailing of laser diodes and silicon photonic edge couplers. The high coupling efficiency reduces the loss budget needed for the actively-aligned fiber position, allowing faster optimization and tighter mechanical positioning specifications.
Alternative: cleaved single-mode fiber with no lens is used when:
- Coupling to large-MFD targets (other SMF-28 fibers, large-mode-area fiber amplifiers)
- Coupling through a separate beam-shaping optic (free-space lens or aspheric lens)
- Low-cost applications where 5–10 dB coupling loss is acceptable
References: H. M. Presby and C. A. Edwards, "Near 100% efficient fibre microlenses," Electronics Letters 28, 582 (1992); R. Marchetti et al., "Coupling strategies for silicon photonics integrated chips," Photonics Research 7, 201 (2019).