How to measure loss in multimode fiber

Multimode fiber loss is measured by evaluating insertion loss, splice loss, and connector loss, considering fiber alignment, modal distribution, and component characteristics.

Key Factors Affecting Multimode Fiber Loss

Fiber geometry and alignment are critical in multimode fiber loss. Misalignment of fiber cores, differences in core diameter, numerical aperture, and cleave quality can all contribute to real power loss at splices or connectors . Even when splicing identical fibers, imperfect alignment results in measurable attenuation. Mechanical splices are particularly sensitive to transverse and longitudinal offsets, while fusion splices reduce but do not eliminate these losses . Modal distribution also affects measurement. Multimode light spreads across multiple modes, and different light sources (LED vs. laser) can produce varying results due to differences in mode excitation . Using the same light source for design, fabrication, and testing ensures reproducible measurements.

Measurement Techniques

1. Insertion Loss Testing: This involves a light source and power meter to measure the total loss of a fiber link. The measured loss is compared to a loss budget, which sums expected losses from fiber, splices, connectors, and passive components . Typical multimode fiber loss is about 3 dB/km at 850 nm and 1 dB/km at 1300 nm .
2. OTDR Testing: Optical Time-Domain Reflectometry can locate high-loss points along the fiber. OTDR measurements are less precise than direct insertion loss tests due to uncertainties in backscatter coefficients and modal effects .
3. Coupled Power Ratio (CPR): CPR measures the difference in propagating loss between multimode and single-mode fibers, providing a simple metric for multimode fiber performance .

Splice and Connector Loss

Splice loss arises from both intrinsic and extrinsic factors. Intrinsic factors include core diameter mismatch and numerical aperture differences, while extrinsic factors include transverse offset, angular misalignment, and end-face quality . Typical empirical values for multimode splices are around 0.3 dB for mechanical splices and 0.15 dB for single-mode fusion splices . Connector losses vary by type, with standard adhesive/polish connectors around 0.3 dB and multifiber connectors like MPOs up to 0.75 dB .

Practical Considerations

• Always use consistent light sources to avoid measurement discrepancies due to modal effects .
• Calculate a loss budget before testing to determine acceptable loss levels for the fiber plant .
• Segment-by-segment testing is recommended if measured loss exceeds the budget, to identify high-loss points .
• Consider fiber type (50/125 µm vs. 62.5/125 µm) and wavelength when calculating expected loss . By carefully accounting for alignment, modal effects, and component losses, multimode fiber loss can be accurately measured and compared against design specifications to ensure reliable optical performance.