If you’re just stepping into the world of fiber optics, all the technical terms and abbreviations can feel overwhelming. That’s why I created this fiber glossary series — to help you understand what these terms really mean, in the simplest way possible.

TABLE OF CONTENTS

Introduction

In a previous article: How Fiber Optics Work: The Simple Science Behind Light, we explained how light travels through optical fibers and how information is transmitted over long distances with incredible speed and accuracy.

However, in real-world fiber optic systems, light transmission is never perfectly lossless. As optical signals travel through fibers — and especially when they pass through connectors, splices, or other components — a small portion of optical power is inevitably lost.

In this article, we take the next step and focus on one of the most fundamental performance indicators in fiber optics: insertion loss. We’ll explain what insertion loss is, why it happens, and how it affects the reliability and efficiency of fiber optic networks — using simple language that’s easy for beginners to understand.

What Is Insertion Loss (IL)?

Insertion loss (IL) refers to the reduction in optical power that occurs when an optical component or connection is introduced into a fiber optic link.

In simple terms: When light is “inserted” through a connector, splice, or device, not all of it makes it through. The lost portion is called insertion loss.

Insertion loss commonly occurs at:

  • Fiber optic connectors
  • Fusion or mechanical splices
  • Patch cords and adapters
  • Splitters and other passive components
  • Sharp bends in optical fibers

Insertion loss is usually expressed in decibels (dB), and lower values indicate better performance.

Where Does Insertion Loss Come From?

Insertion loss is not caused by a single factor. Instead, it results from several physical and mechanical imperfections that affect how efficiently light moves from one fiber to another.

Below are the most common causes.

Core Misalignment Between Fibers

Fiber optic insertion loss-Core Misalignment Between Fibers - diagram

When two fibers are connected, their cores must be precisely aligned. If the alignment is off — even by a few microns — part of the light will miss the receiving core and leak into the cladding.

This is one of the most common causes of insertion loss.

Simple analogy:
It’s like connecting two water pipes that don’t line up perfectly — some water will spill out instead of flowing through.

Different Core or Cladding Diameters

Fiber optic insertion loss-Different Core or Cladding Diameters - diagram

Even if fibers are well aligned, insertion loss can still occur if:

  • The core diameters are different
  • The cladding thicknesses are not the same
  • Fibers from different manufacturers or standards are mixed

These mismatches cause mode-field mismatch, meaning the light cannot fully transfer from one fiber to the other.

Air Gaps Between Fiber End Faces

Fiber optic insertion loss-Air Gaps Between Fiber End Faces - diagram

If fiber end faces do not make full physical contact, a tiny air gap may form between them.

At this interface:

  • Light encounters a glass-to-air boundary
  • Part of the light is reflected backward
  • Less light continues forward

This not only increases insertion loss but can also contribute to return loss, which is discussed in a separate article.

Angled or Poorly Polished End Faces

Fiber optic insertion loss-Angled or Poorly Polished End Faces - diagram

If fiber end faces are not perfectly flat or perpendicular:

  • Light exits the fiber at an angle
  • The coupling efficiency decreases
  • More optical power is lost

This is why connector end-face quality (PC, UPC, APC) plays an important role in controlling insertion loss.

Fiber Bending and Bending Loss

Fiber optic insertion loss-Core Misalignment Between Fibers - diagram

When optical fibers are bent beyond their minimum bend radius:

  • Light cannot stay fully confined within the core
  • Some optical energy leaks into the cladding
  • Insertion loss increases

This type of loss is known as bending loss and is especially common in tight installations and indoor wiring.

The Less Obvious Causes Engineers Run Into

Beyond the textbook causes, real installations introduce additional risks.

Connector quality varies significantly. Manufacturing tolerances, ferrule concentricity, and polishing consistency all affect how well light transfers across an interface.

Handling also plays a role. Heat-shrink sleeves that are compressed before fully cooling, excessive pulling force on patch cords, or repeated insertions can all increase loss over time.

And then there is contamination — probably the most underestimated issue in fiber optics.

A single dust particle on an end face can dramatically increase insertion loss. This is why experienced technicians are almost obsessive about cleaning. Using lint-free wipes and fiber optic cleaning pens is not optional in practice. Many high insertion loss problems are solved simply by cleaning both ends properly.

How Is Insertion Loss Measured?

Insertion loss is calculated by comparing the optical power before and after a component or connection.

The basic formula is: IL (dB) = −10 × log₁₀ (Pout / Pin)

Pin = input optical power
Pout = output optical power

In practice, engineers rarely calculate logarithms in the field. Instead, they use a shortcut:

Insertion loss ≈ Pin (dBm) − Pout (dBm)

This subtraction method is intuitive and fast. If your input power is −3 dBm and your output power is −3.5 dBm, your insertion loss is about 0.5 dB. For most practical purposes, that’s all you need to know on-site.

For beginners, the key idea is simple: The closer the output power is to the input power, the lower the insertion loss — and the better the performance.

What Counts as “Good” Insertion Loss?

Acceptable insertion loss depends heavily on fiber type and application.

Single-mode fiber systems usually have stricter requirements than multimode systems due to longer transmission distances and tighter power budgets. If you’re new to this topic, Fiber Optic: Single Mode vs Multimode – What’s the Difference? provides a helpful overview.

For standard single-fiber connectors like LC or SC, high-quality connections typically fall in the 0.2–0.3 dB range, while 0.5 dB is often considered acceptable in many networks.

Multi-fiber connectors introduce additional complexity. MPO and MTP connectors, which align multiple fibers at once, naturally have higher insertion loss due to alignment challenges across many cores. Typical values often range from 0.35 dB to 0.7 dB, depending on grade and application. For a deeper comparison, see MPO vs MTP: What’s the Difference and Which to Choose.

Because insertion loss accumulates across multiple connection points, even small improvements at each interface can make a noticeable difference in overall system performance.

Conclusion

Insertion loss isn’t a mysterious parameter — it’s simply the visible result of how light behaves when reality replaces theory.

Every connector, splice, and bend slightly reshapes the light path. Understanding where those losses come from helps engineers design cleaner routes, choose better components, and avoid common installation mistakes.

In the next article, we’ll look at return loss, which focuses on what happens to the light that doesn’t move forward — and why that backward reflection matters just as much as insertion loss.

Still Have Questions?

If you’re still unsure about something, feel free to reach out.

Want to explore more fiber optic terms? Head over to our blog section.

If the term you’re looking for isn’t covered yet, let me know — I’ll add it to the priority list!

And lastly — if you’re a telecom provider, network operator, or involved in fiber infrastructure development and looking for a reliable partner in fiber optic components — feel free to contact us.