How much light is lost when two connectors are plugged together? Why must they be cleaned?
In fiber optic communication and fiber sensing engineering, the physical loss generated when two fiber optic connectors are mated through a ferrule (adapter) is called Insertion Loss (IL).
From the perspectives of physical principles and engineering practices, the following will provide a detailed explanation of “How much will be lost?” and “Why must it be cleaned?”
I. How Much Light is Lost When Two Connectors are Mated?
The degree of loss in fiber optic connectors depends on the manufacturing precision, polish type, and the alignment quality during connection.
1. Typical Loss Ranges
- Standard Single-Mode Connectors (e.g., FC/PC, FC/APC): With good alignment and clean end faces, the typical insertion loss for a single mated connection is usually \le 0.3 \text{ dB}. High-quality precision connectors can achieve \le 0.2 \text{ dB} or even \le 0.1 \text{ dB}.
- Multi-Mode Connectors: Due to their larger core diameter (typically 50\ \mu\text{m} or 62.5\ \mu\text{m}), alignment is relatively easier, and their typical insertion loss is also within \le 0.3 \text{ dB}.
2. Physical and Mathematical Expression of Loss
The formula for calculating insertion loss is:
IL = -10 \log_{10} \left( \frac{P_{out}}{P_{in}} \right) \text{ dB}
- A loss of 0.3\text{ dB} means approximately 6.7\% of optical power is lost when passing through the connector.
- A loss of 3\text{ dB} means a 50\% optical power loss, which generally indicates a serious defect in the connection in engineering terms.
Besides insertion loss, another important parameter is Return Loss (RL), which represents the light reflected back to the source. PC polish (physical contact of a micro-spherical surface) typically has an RL of \ge 45\text{ dB}, while APC polish (8^{\circ} angled physical contact) typically has an RL of \ge 60\text{ dB}.
II. Why Must It Be Cleaned? (Physical and Material Analysis)
In fiber optic engineering, “Clean First, Connect Later” is the paramount rule. Failing to clean connectors can lead to the following critical physical consequences:
1. Absolute Contrast Between Fiber Core Size and Dust Particle Size
The Mode Field Diameter (MFD) of standard single-mode optical fibers (e.g., G.652D fiber) is only about 9.2\ \mu\text{m}.
- Ordinary dust particles or skin fragments floating in the air typically range from 1\ \mu\text{m} to 10\ \mu\text{m} or even larger.
- If a dust particle just a few micrometers in size happens to land precisely on the fiber core, it acts like a wall, completely blocking, scattering, or absorbing the light signal. This can instantly cause insertion loss to skyrocket by several decibels, or even lead to a complete interruption of the optical path.