How much optical power is lost when the signal passes through the flange, and what is the internal docking structure?
When an optical signal traverses a vacuum flange (or any fiber optic connector/adapter), the optical power lost within its internal mating structure is known in optical engineering as Insertion Loss (IL).
Here is a precise analysis at the academic and engineering level regarding its physical mechanisms, loss estimation, and design differences among various flange structures:
I. Physical Definition and Optical Power Calculation of Insertion Loss
Insertion loss refers to the attenuation amount of output optical power relative to input optical power when a device (such as a flange mating structure) is introduced into an optical fiber link, typically expressed in decibels ( \text{dB} ). The mathematical formula for calculation is:
\text{IL} = -10 \log_{10} \left( \frac{P_{\text{out}}}{P_{\text{in}}} \right)\ \text{dB}
Where:
- P_{\text{in}} is the input optical power before the mating structure.
- P_{\text{out}} is the output optical power after passing through the mating structure.
For example, if a mating structure causes an insertion loss of 0.3\ \text{dB} , it means approximately 6.7\% of the optical power is lost (converted to scattered light or thermal energy) when passing through the flange; if the loss is 0.5\ \text{dB} , the optical power loss is approximately 10.9\% .
II. Sources of Loss in Vacuum Flange Internal Mating Structures
When a signal passes through a flange mating interface, the loss of optical power mainly originates from the following four physical factors:
- Transverse Offset
This is the most sensitive source of loss in single-mode fiber mating. The Mode Field Diameter ( \text{MFD} \approx 9.2\ \mu\text{m} ) of single-mode fiber is extremely small. If there is a slight transverse misalignment (even just 1\ \mu\text{m} ) between the fiber cores on both sides, it can cause significant coupling loss. - Axial Gap & Fresnel Reflection
If the end faces of the two fibers do not achieve perfect Physical Contact (PC), the small air gap in between, due to the refractive index discontinuity at the glass (refractive index n \approx 1.45 ) and air (refractive index n \approx 1.0 ) interface, will cause Fresnel reflection. A single glass-air interface introduces about 0.15\ \text{dB} of reflection loss. Two such interfaces will result in approximately 0.3\ \text{dB} of inherent loss, and also degrade the Return Loss. - Angular Tilt
If the geometric axes of the two fibers are not parallel, forming a certain angle, it will cause the wavefront of the light field to tilt, leading to some high-order modes or fundamental mode energy being coupled into the cladding and dissipated. - End Face Quality and Surface Contamination
Roughness, scratches on the mating end faces, or attached micro-dust and oil stains will cause strong light scattering and absorption, significantly increasing insertion loss.
III. Two Types of Fiber Optic Vacuum Flange Structures and Actual Loss Values
Due to the unique requirements of vacuum sealing environments, fiber optic vacuum flanges are primarily divided into two major structural categories, exhibiting distinctly different insertion loss performance:
1. Continuous Fiber Feedthrough Type Vacuum Flange
- Structural Characteristics:** The optical fiber passes through the flange’s sealing body without interruption (e.g., hermetically sealed via special adhesives or metal welding). There are no physical mating interfaces inside the flange.
- Optical Power Loss:** Extremely low, virtually 0\ \text{dB} . In this structure, the optical signal does not undergo refractive index discontinuities or alignment deviations. Only slight micro-bending losses caused by fiber bending or encapsulation pressure occur (typically e 0.1\ \text{dB} ), making the loss essentially negligible.
2. Connectorized/Adapter Type Vacuum Flange
- Structural Characteristics:** The flange is equipped with a precision alignment sleeve (usually a Zirconia ceramic sleeve) inside. Standard fiber optic patch cords (e.g., FC/PC or FC/APC connectors) are plugged into both ends of the flange.
- Optical Power Loss:** Depends on the machining precision of the connectors and the cleanliness of the mating end faces.
- Typical values at room temperature:** When end faces are clean and properly physically connected, the typical insertion loss for a single mating is usually between 0.15\ \text{dB} and 0.3\ \text{dB} , with standard specifications marked as \le 0.3\ \text{dB} .
- Under extreme temperatures or high/low temperature cycling:** Due to the different coefficients of thermal expansion of the ceramic sleeve, metal components, and fiber ferrules, thermal mismatch can cause slight deformations. In such cases, the insertion loss may slightly increase, typically within the range of \le 0.5\ \text{dB} .
IV. Reference to Related Product Specifications
In the OFSCN® product line, standardized fiber optic vacuum flanges and adapter products are offered for demanding industrial and laboratory environments such as vacuum and high-temperature applications:
-
OFSCN® Fiber Optic Vacuum Sealed Flange
- Product Features:** Available in CF and KF vacuum specifications, with customizable female (adapter type, for quick plug-and-play) or male (continuous fiber integration, ensuring extremely low loss) ends.
- Technical Parameters:** Operating vacuum better than 1 \times 10^{-5}\ \text{Pa} to 1 \times 10^{-7}\ \text{Pa} ; standard models are for room temperature use, with special versions customizable for temperatures up to 250\ ^\circ\text{C} .
-
OFSCN® High Temperature Resistant Fiber Optic Adapter
- Product Features:** High-temperature resistant FC/APC-FC/APC type fiber optic adapters, using high-precision ceramic alignment sleeves.
- Technical Parameters:** Specially designed for extreme temperatures, capable of withstanding temperatures up to 300\ ^\circ\text{C} , maintaining excellent alignment accuracy and extremely low insertion loss across a wide temperature range.
The following are official specification images of related products:
