Choosing the Right Optical Fiber for a Spectrometer: A Practical Guide

Selecting an appropriate optical fiber for a spectrometer system is essential for ensuring signal transmission efficiency and measurement accuracy. An improper choice may lead to signal loss or wasted resources.

Based on the characteristics of JINSP spectrometers, this guide covers three key fiber parameters: core diameter, operating band/material, and numerical aperture (NA). Practical tips for fiber selection are provided, along with specific recommendations for our products.

1. Core Diameter – Bigger is Not Always Better; Matching the Detector is Key

Core diameters come in various options, such as 5 μm, 50 μm, 100 μm, 200 μm, 400 μm, 600 μm, and even up to 1 mm or more. While a larger core diameter can collect more light at the input end, the spectrometer’s receiving ability is limited by the slit width and the height of the detector’s photosensitive area. Therefore, larger is not always better.

Different fiber core diameters

Fiber signal coupling into a slit

JINSP fiber optical spectromter product-specific recommendations:

• Spectrometers with a linear CMOS detector (SR50C, SR75C): A 200 μm core diameter fiber is recommended.
• Spectrometers with an area CCD detector (SR100B, SR100Z, SR100Q, ST90S, ST100S, etc.): A larger core diameter of 400 μm or 600 μm can be used, as well as multi-fiber bundles.

Multi-fiber bundles feature a circular arrangement at the input end for efficient light collection, and a linear arrangement at the output end to better match the area CCD detector.

2. Operating Wavelength & Fiber Material – Match the Material to the Band to Avoid Transmission Loss

The transmission performance of an optical fiber depends on its material. Different materials show significant differences in wavelength transmittance. The core principle is to select a fiber material that matches the actual operating band. Using an incompatible material will result in low light transmittance and severe signal attenuation.

Three main types of fiber materials are available, with distinct band suitability:

• High-OH fiber: For UV/Visible (UV/VIS) bands
• Low-OH fiber: For near-infrared (NIR) bands
• UV-resistant fiber: Specifically designed for pure UV band measurements

JINSP modular sepctrometer product-specific recommendations:

• SR50C, SR75C, SR100B spectrometers (UV-VIS-NIR range, 200–1000 nm): Use high-OH fibers.
• SR50R17, SR100N17/N25 spectrometers (NIR range, 900–2500 nm): Use low-OH fibers.

3. Numerical Aperture (NA) – Ensure Matching to Maximize Light Throughput
NA defines the light acceptance and emission angle of a fiber, directly affecting the divergence angle of light at the fiber output and influencing coupling efficiency and transmission loss. The key principle is to match the fiber NA with the spectrometer’s receiving NA, and also with any lenses or concave mirrors in the optical path to avoid wasting light energy.

Common NA values for multimode fibers include 0.1, 0.22, 0.39, and 0.5. The industry standard is NA 0.22. For this NA, the light spot diameter after a 50 mm propagation distance is approximately 22 mm, and after 100 mm it is about 44 mm. This divergence behavior guides the layout of the optical path and choice of auxiliary components.

The NA of an optical fiber determines its beam divergence angle

When an optical fiber is used together with a lens or a concave mirror, the NA values should be matched as closely as possible to avoid energy loss

A spectrometer’s NA represents the maximum acceptance angle for incoming light – effectively the NA of its internal concave mirror. Matching conditions:

• If the fiber NA ≤ spectrometer NA: All incident light is accepted and used.
• If the fiber NA > spectrometer NA: Part of the light energy cannot be received, resulting in signal loss.

The NA value of a spectrometer is the NA value of its internal concave mirror

When collecting optical signals, in addition to using fiber optic conduction, free-space optical coupling can also be employed. This method uses a lens to focus parallel light into the spectrometer. In a free-space optical path, the numerical aperture (NA) of the lens must match that of the spectrometer, and the slit of the spectrometer must be positioned at the focal point of the lens to achieve high optical throughput.

Free-space optical coupling

JINSP modular spectrometer product-specific recommendations:

JINSP transmission spectrometers (ST series) are designed with a large receiving NA of 0.25. They can fully accept light from industry-standard NA 0.22 multimode fibers without additional adaptation, ensuring efficient use of light energy.

About JINSP Company Limited

JINSP has developed a range of spectrometers, including miniature fiber-optic spectrometers, high-resolution fiber-optic spectrometers, transmission spectrometers, and miniature NIR spectrometers. We also support customisation of conventional optical parameters, size, software functions, communication interfaces, and more.