Micro Lens
A microlens (also written micro-lens or lenslet) is a miniature refractive or diffractive optical lens with a diameter typically below 1 mm—and often tens to a few hundred micrometers. In lasers and photonics they are used individually or, far more often, as densely packed microlens arrays (MLAs) to collimate, couple, homogenize, or analyze beams from compact sources such as laser diodes and VCSELs.
Because the physical size approaches or is only a few times the optical wavelength, diffraction, surface-figure errors, and fill-factor become first-order design constraints, unlike conventional macroscopic lenses.
Technical characteristics:
Operating principle: Most commercial devices are refractive: a radially varying optical-path length is produced by a curved surface (spherical, aspheric, or cylindrical) or by a radial refractive-index gradient (GRIN microlenses). Diffractive (Fresnel-type) versions exist for thinner profiles or specific wavelengths.
Typical dimensions: Diameter 10 µm–1 mm; pitch in arrays from a few tens of micrometers upward; focal lengths from tens of micrometers to a few millimeters; numerical apertures commonly 0.1–0.4 (higher values are possible but more sensitive to aberrations and fabrication errors).
Materials: Fused silica, glass, silicon, photoresists, and optical polymers (PMMA, SU-8, etc.). Surface roughness after fabrication can be sub-nanometer with optimized processes.
Fabrication routes commonly used in photonics:
Photoresist thermal reflow followed by reactive-ion etching (wafer-level optics).
Gray-scale lithography, inkjet printing, or multi-photon 3-D laser lithography.
Diamond turning or laser micromachining for aspheres.
Direct integration onto semiconductor wafers (e.g., on VCSEL or detector chips).
Fill factor (usable lens area divided by total array area) is a key figure of merit; hexagonal packing and 100 % fill-factor designs are routinely achieved for high-efficiency applications.
Principal applications:
Microlenses solve the mismatch between the highly divergent, small-mode-field output of semiconductor lasers and the needs of fibers, free-space optics, or sensors.
Laser-diode and VCSEL collimation / coupling:
A single microlens or a 1-D/2-D array collimates the fast axis of a diode bar or each emitter of a VCSEL array, raising brightness and enabling efficient fiber coupling. Fast-axis collimators (FACs) are a standard product category. Integrated microlenses on VCSELs can also enforce single-mode operation or produce doughnut-shaped beams.
Fiber coupling and photonic-integrated-circuit interfacing:
Microlenses on fiber tips or in array form match the laser mode to a single-mode fiber or waveguide array, routinely achieving coupling efficiencies of 70–90 % when anti-reflection coated. They also relax alignment tolerances in co-packaged optics and multi-channel connectors.
Beam homogenization and shaping:
Two microlens arrays plus a Fourier lens form a compact, high-efficiency beam homogenizer used in laser materials processing, lithography illumination, and medical lasers.
Wavefront sensing:
A Shack–Hartmann sensor uses an MLA to sample a laser beam’s local wavefront slope; the resulting spot pattern on a detector yields both intensity and phase information for adaptive-optics correction or beam-quality diagnostics.
Detector and imaging enhancement:
On-chip microlenses over CMOS/CCD or SPAD pixels increase fill factor and quantum efficiency. Larger arrays enable light-field (plenoptic) cameras and compact compound-eye imagers.
Other photonics uses:
Mode-field adapters, optical tweezers, lidar transmitters, and expanded-beam connectors all exploit the same miniature focusing function.
Microlenses are the workhorse micro-optic that lets high-power, highly divergent semiconductor lasers talk efficiently to fibers, waveguides, and free-space systems while remaining compatible with wafer-scale manufacturing.