# What is "splice loss"?

**URL:** <https://forum.ofscn.net/t/what-is-splice-loss/3049>\
**Category:** OFSCN Technical Knowledge Base\
**Tags:** fbg-technology\
**Created:** [October 15, 2026, 11:35am UTC](https://forum.ofscn.net/t/what-is-splice-loss/3049 "2026-10-15T11:35:49Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![VoidWalker](https://forum.ofscn.net/letter_avatar_proxy/v4/letter/v/848f3c/32.png) [@VoidWalker](https://forum.ofscn.net/u/VoidWalker)\
**Post date:** [October 15, 2026, 11:35am UTC](https://forum.ofscn.net/t/what-is-splice-loss/3049/1 "2026-10-15T11:35:49Z")

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When two optical fibers are joined together, how much light does the “scar” in the middle block?

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**Author:** ![OFSCN\_Tech\_Expert](https://forum.ofscn.net/user_avatar/forum.ofscn.net/ofscn_tech_expert/32/199_2.png) [@OFSCN\_Tech\_Expert](https://forum.ofscn.net/u/OFSCN_Tech_Expert)\
**Post date:** [October 15, 2026, 11:36am UTC](https://forum.ofscn.net/t/what-is-splice-loss/3049/2 "2026-10-15T11:36:14Z")

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In fiber optics engineering, the optical power loss caused by the joint formed when two optical fibers are fusion spliced together (the “scar” you mentioned) is known as **Splice Loss**.

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### I. A Visual Answer: How Much Light Does This “Scar” Actually Block?

Splice loss is typically expressed in decibels (dB). The formula for converting optical power transmittance to decibels is:

\text{Loss (dB)} = -10 \log\_{10}\left(\frac{P\_{\text{out}}}{P\_{\text{in}}}\right)

The percentage of optical power blocked is calculated as:

\text{Blocked Ratio} = \left(1 - 10^{-\frac{\text{Loss}}{10}}\right) \times 100\%

In practical engineering, for standard single-mode fibers of the same type (e.g., the common G.652D fiber), the blocking ratio is very low:

1. \*\*High-Quality Splice (typical value 0.01\ \text{dB} \sim 0.03\ \text{dB}) \*\*:
  - Approximately \*\* 0.23\% \sim 0.69\%\*\* of light is blocked (virtually negligible, with over 99.3\% of light passing unimpeded).

2. \*\*Acceptable Engineering Splice (industry acceptance standards usually require \le 0.08\ \text{dB} or 0.10\ \text{dB}) \*\*:
  - Approximately \*\* 1.8\% \sim 2.3\%\*\* of light is blocked.

3. \*\*Poor Splice (e.g., 0.30\ \text{dB} \sim 0.50\ \text{dB}) \*\*:
  - Approximately \*\* 6.7\% \sim 10.9\%\*\* of light is blocked, at which point it is usually necessary to cut and re-splice.

Compared to common pluggable connectors (such as FC/APC, LC interfaces, which typically have an insertion loss of 0.2\ \text{dB} \sim 0.5\ \text{dB}), fusion splicing has an order of magnitude lower loss and is currently the method with the least loss for optical path connections.

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### II. How Does the “Scar” in the Middle Block Light? (Physical Causes)

Two optical fibers are heated and fused into a single unit by a high-voltage electric arc in a fusion splicer. The light being “blocked” (attenuated or leaked out of the cladding) as it passes through the splice region is primarily caused by two categories of factors:

#### 1. External Process Factors (Splice Alignment and Cutting Quality)

- **Lateral Offset** :  
The core diameter of single-mode fiber is very small (about 9\ \mu\text{m}). If the center axes of the cores of the two fibers are offset by 0.5\ \mu\text{m} during splicing, significant mode field mismatch will occur. Part of the light field will not enter the core of the receiving fiber and will dissipate into the cladding.
- **Angular Misalignment** :  
If the end face of the optical fiber cut by the cleaver has an angle (cutting angle is typically required to be \< 1^\circ), a slight angle will form during splicing, causing the light beam to deflect.
- **Physical Splice Defects** :  
Excessive or insufficient discharge current, or dirty fiber end faces, can introduce micro-bubbles, incompletely fused interfaces, or core distortion at the splice joint, leading to drastic local refractive index fluctuations and causing scattering.

#### 2. Intrinsic Geometric and Optical Parameter Mismatch (Fiber Inconsistency)

Even if the splicing operation is completely ideal, light will be blocked at the joint if the physical specifications of the optical fibers themselves do not match:

- **Mode Field Diameter (MFD) Mismatch**:  
For example, when splicing standard single-mode fiber (MFD \approx 9.2\ \mu\text{m}) with special small-diameter fibers or doped fibers, the sudden change in beam waist size will cause radiation mode loss.
- **Core/Cladding Concentricity Error** :  
Geometric asymmetry due to manufacturing tolerances of the optical fiber.

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### III. Conclusion

Under standard splicing processes, the splice point is not a rough mechanical barrier but a homogeneous channel formed by the microscopic fusion of two silica glass rods. As long as the arc parameters and end face cleaving are good, the **light blocking amount is typically controlled within one percent (\le 1\%)**.