Design Guide

Custom Glass Optical Fiber Design Guide

Custom fiber design begins with the path light must take and the physical environment the glass must survive. Core and cladding indices, numerical aperture, diameter, open area ratio, geometry, absorbing glass, and bundle architecture all affect transmission and image quality.

1. Total Internal Reflection

A glass fiber guides light when its higher-index core is surrounded by lower-index cladding. At the boundary between the two glasses, a ray striking above the critical angle is reflected back into the core rather than refracted into the cladding. Repeated total internal reflection allows the ray to propagate along the fiber.

Snell's law describes the relationship at the interface:

n_1 \sin \theta_1 = n_2 \sin \theta_2

(1)

At the critical condition, the refracted ray travels along the boundary (θ₂ = 90°). For a core index n₁ greater than the cladding index n₂:

\sin \theta_c = \dfrac{n_2}{n_1}

(2)

2. Numerical Aperture

Numerical aperture (NA) describes the range of incident angles that can enter the fiber from air and remain guided. It is the sine of the half-angle of the acceptance cone relative to the fiber axis. For a simple step-index fiber in air:

\mathrm{NA} = \sin \theta_a = \sqrt{n_1^2 - n_2^2}

(3)

A higher NA accepts light over a wider angular range. A lower NA produces a narrower acceptance cone and may better match applications that require angular selectivity. CHI can produce NA values from approximately 0.1 to 1.0, depending on available core and cladding glasses.

3. Diameter and Flexibility

A preform may begin at approximately 5–60 mm in effective diameter and be reduced through drawing. Finished individual optical fibers or drawn glass parts can range in cross-sectional size from around 50 µm to several mm or larger, and custom structures can fall outside that range.

Below roughly 1.5 mm, an individual glass fiber or rod becomes semi-flexible. Near 50 µm, it can be comparable in flexibility to a human hair. Flexibility is not the same as bend-insensitivity: allowable bend radius depends on glass, diameter, geometry, surface condition, coating, handling, and service life.

4. Fiber Geometry

Circular cross-sections are the simplest and most common. Square and hexagonal fibers can pack efficiently into coherent structures, while custom cross-sections can support alignment, field shaping, geometry conversion, or non-optical functions. The macroscopic preform geometry is generally well preserved after drawing, though corners, wall thickness, and symmetry can sometimes change as the glass flows.

5. Glass Selection

The core and cladding must have compatible thermal behavior while maintaining the refractive-index difference needed for optical guidance. CHI has a wide variety of glasses in stock and available through our raw material supplier network, including common optical glasses, borosilicate, lead-oxide, and soda-lime glasses as well as radiation-resistant, etchable, colored, dark, and scintillating formulations. Custom glasses may be developed with glass suppliers or evaluated from customer-provided material.

6. Open Area Ratio in Fiber Optics

For a single fiber, open area ratio is often used to describe the fraction of the cross-section occupied by the light-conducting core:

\mathrm{OAR} = \left(\dfrac{d_{\mathrm{core}}}{d_{\mathrm{total}}}\right)^2

(4)

A larger core fraction increases active area but leaves less cladding between neighboring cores. In very small fibers, insufficient cladding thickness can increase leakage and cross-talk. Reducing OAR may improve isolation, but it also reduces the fraction of the structure that carries useful light. The correct balance depends on fiber diameter, wavelength, index contrast, and bundle architecture.

7. Extra-Mural Absorption Glass

Extra-mural absorption glass (EMA) is a dark, light-absorbing glass introduced into a fiber matrix to capture photons that escape the intended cores. Reducing this stray-light path improves contrast and limits cross-talk between neighboring fibers. The tradeoff is lower overall transmission proportional to the percentage of cross-sectional area occupied by the opaque EMA glass.

EMA Configuration

EMA Configuration	Description	Typical Design Effect
Statistical	Some light-conducting fibers are replaced by small rods of absorbing glass distributed statistically through the matrix.	Adds distributed absorption with a moderate loss of active area.
Interstitial	Absorbing glass occupies spaces between neighboring fibers.	Targets stray light in the matrix while preserving individual fiber construction.
Circumferential	Each fiber receives a complete absorbing jacket outside the optical cladding.	Provides strong fiber-to-fiber isolation but increases non-transmitting area and fabrication complexity.

8. Multi-Fibers

A multi-fiber contains from a few to hundreds of thousands of individual fibers packed and fused into a larger coherent structure. Standard pack geometries include square and hexagonal arrangements inside circular, square, or hexagonal exterior shapes. Constituent fibers may range from micrometers to millimeters, depending on the required resolution and optical behavior.

Multi-fibers form the building blocks of image conduits, faceplates, remapping devices, coherent bundles, and some illumination components. They can be designed with custom constituent-fiber diameter, numerical aperture, OAR, EMA configuration, and exterior dimensions.

9. Important Tradeoffs

Changing one optical design variable usually affects several others. The table below summarizes common tradeoffs CHI evaluates with customers.

Variable	Increasing It Can...	But May Also...
Numerical aperture	Accept light from a wider angular range	Increase mode range, mismatch a source, or reduce angular selectivity
Core OAR	Increase transmitting area	Reduce cladding thickness and isolation
EMA volume	Improve contrast and reduce cross-talk	Reduce active area and total output
Constituent fiber diameter	Increase output per fiber and ease fabrication	Reduce image resolution or spatial sampling
Overall diameter	Increase field size or light capacity	Increase rigidity and routing constraints
Index contrast	Increase guidance and NA	Narrow glass-compatibility choices and alter dispersion or losses

10. Specification Checklist

Use this checklist when preparing an inquiry or RFQ.

Function: illumination, image transfer, light collection, angular filtering, sensing, radiation conversion, or another use
Wavelength range and source angular distribution
Required numerical aperture or acceptance angle
Fiber diameter, core size, OAR, and exterior geometry
Transmission, contrast, resolution, or cross-talk target
EMA requirement and preferred configuration
Length, bend, routing, and packaging constraints
Temperature, vacuum, chemical, radiation, sterilization, or bonding environment
End-face polish, coating, shaping, mounting, and integration requirements
Prototype and production quantity

Not sure which optical variables should be specified?

Start with the source, detector, wavelength, geometry, environment, and performance goal. CHI can help turn those system conditions into a fiber design worth testing.

Discuss a fiber design