High-precision refractive optical elements (ROEs): Unique micro-optical solutions from DUV to LWIR

ROEs are the foundation of classical optics. Unlike Diffractive Optical Elements (DOEs), which steer light via diffraction, or reflective elements (mirrors), ROEs utilize the principle of refraction. This means the direction of light changes when it passes from one medium (e.g., air) into another with a different refractive index (e.g., glass or plastic).

ROEs can be categorized in several ways: by their geometry (shape), by their optical function, or by their manufacturing technology.

The primary categories of ROEs are lenses, which modulate light propagation through convergence or divergence, and prisms, which steer light, and provide angular dispersion.

Typical lenses are:

• Lens-like: spheres, aspheres, cylinders and beam shapers
• Prism-like: refractive axicon
• MLAs (Micro Lens Arrays) systemic and randomized
• Neutral density filter and interference filter
  
  

• Focusing (bundling): Collecting parallel light into a single point (the focal point).
• Collimating (parallelizing): Converting divergent light (from a point source) into parallel light.
• Imaging: Creating a real or virtual image of an object (magnification/minification).
• Beam shaping: Changing the intensity distribution of the beam (e.g., from a Gaussian profile to a flat "top-hat" profile or a line).
• Fiber coupling is a critical and distinct function of refractive optical elements (ROEs), even though it is technically a specific application of focusing and beam shaping.

Foundry Services

Lithography

• E-Beam, projection and contact lithography
• Front to back alignment capability
• Substrate sizes: 2”-7”, 0.2 mm to 6.35 mm thickness
• Grayscale patterning

Etching

• Plasma etching
• Wet etching

• Wafer level bonding
• Optical contacting thin film deposition
• ARC, thermal oxide

Fiber coupling is the process of efficiently transferring light from a source (like a laser diode, LED, or another optical fiber) into the core of an optical fiber. The goal is to maximize the amount of light that enters the fiber and stays trapped inside it (via total internal reflection).

Why is it a special function?

• Small acceptance angle (NA): Optical fibers only accept light that enters within a specific cone angle (defined by the NA). If the light enters at a steeper angle, it escapes the fiber.
• Small core size: Fiber cores are tiny (often 2–100 µm for single-mode/multi-mode fibers). The optical element must focus the beam to a very precise spot size.
• High precision required: Misalignment by a few micrometers can cause significant signal loss.

1. Collimating lenses (at the source)

• Function: Laser diodes and LEDs emit light in a highly divergent, often elliptical pattern.
• ROE role: A small aspheric lens or GRIN lens is placed close to the source to collimate (parallelize) the light. This makes the beam easier to handle and focus.

Standard spherical lenses would introduce aberrations that make it impossible to focus the light tightly enough into the small fiber core.

2. Focusing lenses (at the fiber input)

• Function: Taking the collimated beam and focusing it into the fiber core.
• ROE role: A high-NA (Numerical Aperture) lens focuses the beam to a spot size that matches the fiber core diameter.
• Why high NA? To capture as much light as possible and ensure the focused beam fits within the fiber’s acceptance angle.

• Function: Many laser diodes emit an elliptical beam (e.g., 10 µm x 100 µm), but fiber cores are circular.
• ROE role: Cylindrical lenses or anamorphic prism pairs are used to reshape the elliptical beam into a circular one before it enters the focusing lens.
• Why? To maximize coupling efficiency. If you try to focus an ellipse into a circle, you lose a lot of light.