Fast Axis Collimation Lens (FAC Lens)
Fast Axis Collimation Lens (FAC lens) is a small cylindrical (usually acylindrical/aspheric) microlens placed immediately in front of an edge-emitting laser diode or diode bar. Its job is to collimate only the highly divergent “fast axis” of the diode emission so the beam can be handled by later optics. It is one of the most important beam-shaping elements in high-power diode laser modules.
Fast axis vs. slow axis:
An edge-emitting laser diode has a very thin active layer (typically ~1 µm) perpendicular to the junction and a much wider emitter (tens to hundreds of micrometers) parallel to the junction.
Fast axis: the thin direction. Diffraction produces large divergence, often 30°–45° (FWHM) or a numerical aperture of ~0.5–0.8.
Slow axis: the wide direction. Divergence is much smaller, typically 5°–12°.
A single spherical lens cannot correct both axes well and would have to sit extremely close to the facet. A FAC solves the fast-axis problem first, with a short-focal-length cylindrical lens whose power is only in that plane.
Technical characteristics:
Form: plano-acylindrical or biconvex acylindrical glass microlens. The aspheric (acylindrical) profile reduces spherical aberration so collimation can approach the diffraction limit.
Numerical aperture: commonly 0.5–0.8 so nearly all of the fast-axis light is collected.
Effective focal length: typically 0.15–1.5 mm (sometimes a few millimeters). Back focal length is even shorter, so the lens sits tens to a few hundred micrometers from the facet.
Materials: high-index optical glasses (n ≈ 1.8 or higher) or fused silica; AR-coated for the diode wavelength (common bands 400–480 nm, 790–990 nm, 9xx nm, 14xx–16xx nm). Transmission can exceed 99 %.
Residual divergence: a few milliradians after a good FAC.
Formats: single-emitter lenses, long bars that cover an entire diode bar, or arrays for stacks. Some designs include a 90° deflection prism.
A slow-axis collimator (SAC)—a longer-focal-length cylindrical lens or lens array—is usually added farther downstream to collimate the remaining slow-axis divergence. Together, FAC + SAC produce a quasi-collimated, more symmetric beam that can be focused into a fiber or combined with other emitters.
Why it matters:
Without a FAC, most of the diode power would miss subsequent optics or couple poorly into a fiber.
The FAC:
Captures nearly the full fast-axis NA.
Reduces beam parameter product in that axis.
Lets the rest of the optical train sit at a practical working distance.
Improves brightness and coupling efficiency of diode bars and stacks.
Alignment and “smile” (bow of the emitter line on a bar) must be tight; a poorly placed FAC leaves residual divergence or astigmatism.
Applications:
High-power diode laser modules for material processing (welding, cladding, hardening).
Pump sources for fiber lasers, disk lasers, and end-pumped solid-state lasers.
Fiber coupling of single emitters and bars.
Beam combining (spatial, wavelength, or polarization) of many diodes.
LiDAR, medical/aesthetic devices, and illumination systems that start from diode bars.
Blue diode systems used in copper welding and display applications (special FAC glasses and coatings are used at short wavelengths).
The FAC is the first and most critical optic that turns the raw, highly asymmetric output of an edge-emitting diode into a usable beam.