Micron-Scale Precision: A Linear Fiber Array for Demanding Fluorescence Spectroscopy

Six fibers. One array. Zero tolerance for error.

Fluorescence spectroscopy requires absolute precision. Every micron matters – both in how light is delivered and how background signals are suppressed. At FOS Inon Optics GmbH, we developed a linear fiber array consisting of six fibers that achieves the impossible: 600 µm core-to-core spacing, with only 50 µm of physical gap between neighboring fibers.

Technical Overview: Geometry and Design

The array is built from step-index optical fibers with the following specifications:

• Core diameter: 500 µm
• Cladding diameter: 550 µm
• Numerical Aperture (NA): 0.12
• Core-to-core spacing: 600 µm (50 µm inter-fiber gap)

This spacing ensures that each fiber is close enough for compact linear alignment, yet sufficiently separated to avoid cladding contact or unwanted optical coupling. The resulting inter-fiber gap of 50 µm is small enough to maintain a compact footprint, but large enough to guarantee mechanical stability and clean optical isolation.

Optical Considerations

The numerical aperture defines how much light a fiber can accept:

This small acceptance angle ensures low angular dispersion and minimizes stray light between adjacent fibers – a critical factor when the fibers are separated by only 50 µm.

Linear fiber arrays are especially valuable in fluorescence spectroscopy for line-scan excitation/detection and multiplexing. Here, the precision of the core-to-core alignment directly determines spatial resolution and measurement fidelity.

Manufacturing the Array: Precision in Microns

The fiber holes for the six fibers are drilled using an in-house CNC micromachining system capable of:

• Hole-to-hole spacing: exactly 600 µm
• Positioning tolerance: <±2.5 µm
• Depth precision: <1 µm
• Carrier material: synthetic quartz composite (non-fluorescent)

To control thermal drift during machining, the entire process is performed under temperature-stabilized conditions. Final inspection via high-resolution vision metrology guarantees that cumulative alignment errors across the full array remain under ±5 µm.

Eliminating Background: Low-Fluorescence Materials

Fluorescence spectroscopy is highly sensitive to background signals. Even small amounts of autofluorescent adhesive or carrier material can distort results. FOS Inon therefore uses:

• Carrier: quartz-based composite with negligible autofluorescence
• Adhesive: 2-component epoxy with measured background <5 cps/nm (450–650 nm range)
• Cleaning process: acetone + isopropanol rinses, deionized water final wash
• Curing: controlled humidity (<40% RH), followed by a 70 °C thermal cure

This process ensures extremely low background fluorescence – ideal for time-resolved and low-intensity fluorescence detection.

Application Example: Fluorescence Spectroscopy

The array was designed for a customer developing time-resolved fluorescence spectroscopy instrumentation. With linear core-to-core spacing of 600 µm, the array allows for:

• Compact integration into line-scan optical systems
• Efficient channel separation for multiplexing
• Highly reproducible excitation and collection geometries