Analysis of Multimode Insertion Loss Measurements

Introduction

The increased demand for high data rates within the premises network encourages an upgrade of existing systems to 10 Gigabit Ethernet. To achieve gigabit rates laser-based technology can be utilized. While light emitting diodes can be utilized for up to 622 Mbit/s, they cannot physically support faster data rates. Today, 90% of LANs are distributed within 300 meters lengths. The installed fiber base in LAN backbones is 70% graded index multimode fiber of 62.5/125 μm, 20 percent graded index multimode fiber of 50/125 μm, and 10 percent single mode fiber of 8/125 μm. The multimode fiber is qualified at two primary wavelengths: 850 nm (short wavelength) and 1300 nm (long wavelength). The specified bandwidth for 50/125 μm fiber is 500 MHz km at 850 nm and 1300 nm. The bandwidth for TIA/EIA-568-A 62.5/125 μm fiber is 160 MHz km at 850 nm and 500 MHz km at 1300 nm. And the bandwidth for ISO/IEC 11801 62.5/125 μm fiber is 200 MHz km at 850 nm and 500 MHz km at 1300 nm.

This document provides guidance on the implementation of a test setup for the insertion loss measurements of multimode components and also answer related questions on the multimode insertion loss measurements.

Why Is Insertion Loss Testing Critical in Multimode Fiber Networks?

As enterprise networks evolve to support 10 Gigabit Ethernet, the demand for accurate testing of multimode optical components has grown. Insertion loss testing becomes essential in verifying the performance of passive components like CWDM multiplexers and demultiplexers. Multimode fibers, which transmit light through multiple paths or modes, present unique challenges due to modal dispersion and varying power distributions. The document highlights that insertion loss measurements are particularly sensitive to how light is launched into the fiber, making launch conditions a key factor in test accuracy.

What Are the Different Launch Conditions and Their Impact on Insertion Loss?

The document outlines three primary launch conditions: Underfilled Launch (UFL), Overfilled Launch (OFL), and Offset Launch. UFL, typically achieved using a laser, excites fewer modes and results in lower insertion loss. OFL, using LEDs or mode mixers, excites many modes and leads to higher insertion loss. Offset Launch introduces light at a lateral offset, producing intermediate results. These launch conditions significantly affect the modal distribution and, consequently, the measured insertion loss, making it crucial to standardize the launch method during testing.

How Is Insertion Loss Measured in Multimode Systems?

Insertion loss is defined as the ratio of transmitted optical power to received power, expressed in decibels. The measurement involves two steps: recording the reference power without the device under test (DUT), and then measuring the output power after the DUT is inserted. The insertion loss is calculated using the formula 10 log(PRef/POut). The document provides detailed test setups for each launch condition and emphasizes the importance of using calibrated equipment and consistent procedures to ensure accurate insertion loss readings.

What Do the Results Reveal About Launch Conditions and Measurement Stability?

The study shows that UFL results in the lowest insertion loss (average 0.85 dB), OFL the highest (2.92 dB), and Offset Launch falls in between (1.0–2.0 dB). These differences highlight how launch conditions influence measurement outcomes. Repeatability tests using OFL showed that stable fiber positioning and slower sweep speeds (5 nm/sec) significantly improve measurement consistency. In contrast, unstable launches or faster sweeps introduce greater variation in insertion loss, underlining the need for controlled test environments.

How Do Calibration and Connector Cleanliness Affect Insertion Loss Accuracy?

Two test cases were compared: one where the reference was measured once (Case 1), and another where it was measured before each test (Case 2). Case 1 showed better repeatability and required less time. Additionally, the document stresses the importance of connector cleanliness. Dirty connectors led to higher and more inconsistent insertion loss values, especially under OFL conditions. Clean connectors, on the other hand, maintained consistent results across repeated tests, reinforcing the need for proper maintenance during insertion loss testing.

Can Bare Fiber Connections Improve Insertion Loss Testing Efficiency?

To streamline testing and reduce costs, the document introduces bare fiber connectors as an alternative to traditional connectorized setups. These connectors allow for quick, repeatable, and accurate insertion loss measurements without the need for splicing or permanent connectors. Tests showed that bare fiber connections provided insertion loss variation comparable to conventional methods, even when repeatedly connected and disconnected. This makes them a practical solution for high-volume testing environments where speed and repeatability are essential.

For more information, please download this document and find Keysight insertion loss testing solutions.