SIST EN 3745-302:2004

Overview

EN 3745-302:2002, published by CEN as part of the Aerospace series, specifies a test method to determine the effective numerical aperture (NA) of optical fibres and cables intended for aircraft use. The standard defines specimen preparation, required apparatus and two primary measurement methods (far‑field rotating arm and CCD camera), plus reporting and precautionary requirements for aerospace qualification and quality control of optical interconnects.

Key topics and technical requirements

• Scope: Method for determining effective NA of optical cables/fibres used in aerospace applications.
• Specimen preparation:
  • Follow EN 2591-100 and EN 3745-100 for general and environmental/test-condition requirements.
  • Specimens must be stabilized at standard test conditions for 24 hours before measurement.
  • Test details to record include cable/fibre type, length (if not 2 m) and acceptable bend radius during measurement.
• Launch conditions: Adjust launch to specified settings (e.g., NA = 110% NA_nom and fCR = 100% fCR_nom as given in the method).
• Apparatus:
  • Light Launch System (LLS), mechanical chopper, and appropriate detection system.
  • Method A (“far field rotating arm”): rotating detector on a goniometer driven by step motor; measures radiant intensity vs. emission angle.
  • Method B (“CCD camera”): fixed set-up with CCD imaging the output pattern; per‑pixel intensity analysis with cos(θ) correction for oblique angles.
  • Measurement resolution relates to detector aperture and fibre‑to‑detector distance (resolution tg ≈ detector aperture / fibre‑detector distance).
• Measurement and calculation:
  • Plot radiation intensity i = f(θ) and determine NA as NA = sin(θm) where θm corresponds to the angle at 5% of Imax.
  • Typically three curves are recorded and averaged; standard deviation reported.
  • Precision note: an angular uncertainty of ±0.5° typically corresponds to NA accuracy of about ±0.01.
• Precautions: Maintain constant bend radius; avoid altering specimen curvature during testing.

Applications

• Qualification and acceptance testing of aerospace optical fibres and harnesses.
• Design verification and supplier quality control for aircraft communication and sensor links.
• Laboratory characterization for optical component selection in avionics and airborne platforms.
• Support for procurement specifications and certification activities where controlled NA affects coupling, loss budgets and modal behavior.

Who uses this standard

• Aerospace OEMs, avionics designers and electrical/optical systems engineers
• Cable and fibre manufacturers supplying the aerospace sector
• Test laboratories performing compliance, qualification and R&D measurements
• Quality assurance and procurement teams specifying optical interconnect performance

Related standards

• EN 2591-100 - Elements of electrical and optical connection: test methods, general.
• EN 3745-100 - Aerospace fibres and cables, optical: general test conditions.

Keywords: EN 3745-302:2002, numerical aperture, optical fibre test method, aerospace optical cable, NA measurement, CCD method, far-field rotating arm.