Why are manually polished heads usually less stable than machine polished ones?
In fiber optic communication and fiber optic sensing engineering, the quality of the end-face physical contact of fiber optic connectors (such as FC/PC, FC/APC) directly determines the insertion loss (IL) and return loss (RL) of the system.
First, it needs to be clarified that fiber optic polishers (and related polishing jigs) are general-purpose precision processing equipment for fiber optic end-faces and do not belong to the core product line of Beijing Dacheng Yongsheng Technology Co., Ltd. (OFSCN®). However, from the perspective of optical engineering and surface processing physics, manually polished fiber optic connectors typically perform significantly worse in terms of performance and stability compared to machine polishing. The core reasons can be rigorously analyzed from the following dimensions:
1. Precision Control Capability of Key Geometric Indicators (3D Geometric Parameters)
According to International Electrotechnical Commission standards (such as IEC 61755-3 series), physical contact connectors with high reflection performance must strictly control three microscopic geometric indicators. The high uncertainty of these indicators in manual polishing is the fundamental reason for its instability:
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Apex Offset:
Apex offset refers to the radial distance between the apex of the end-face spherical surface and the geometric center of the fiber core. To ensure perfect physical contact between the cores of two fibers, the apex offset is typically required to be \le 50\ \mu\text{m}.- Manual Polishing:** Due to the inability of the operator’s hand force to be absolutely symmetrical and the difficulty in ensuring that the ferrule is absolutely perpendicular (90^\circ) to the polishing disk surface, the polishing angle becomes skewed. The apex offset often far exceeds the standard, easily leading to a small air gap during mating.
- Machine Polishing:** Using precision polishing jigs (Jigs) with rigid mechanical constraints, each ferrule is locked perpendicular, allowing the apex offset to be stably controlled within \le 30\ \mu\text{m}.
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Radius of Curvature (ROC):
The radius of curvature determines the contact elastic deformation and contact pressure during connector mating. For PC connectors, the standard ROC range is typically 10\ \text{mm} to 25\ \text{mm}; for APC connectors, it is 5\ \text{mm} to 12\ \text{mm}.- Manual Polishing:** Limited by fluctuations in manual pressing force and local non-uniform deformation of the polishing pad, the ROC distribution is extremely wide. If the ROC is too large (too flat contact surface), insufficient plastic deformation will occur during mating to eliminate air between the contact surfaces; if the ROC is too small (too sharp contact surface), the contact area is too small, and repeated mating and unmating can easily cause pressure damage to the fiber end-face.
- Machine Polishing:** By setting constant mechanical pressure, standardized polishing pads, and fixed rotational speeds, the ROC can be stably controlled at the center of the target process window.
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Fiber Undercut / Protrusion:
Refers to the height difference between the silica (fiber optic material) end-face and the zirconia (ceramic ferrule) end-face, typically required to be within \pm 50\ \text{nm}.- Manual Polishing:** Due to the significant difference in hardness and wear resistance between zirconia and silica, slight deviations in polishing time, tangential speed, or polishing force can lead to excessive fiber undercut (deteriorating reflection loss) or excessive protrusion (easily crushing the fiber core during mating, causing permanent physical damage).
- Machine Polishing:** Through multiple polishing films with different grit sizes (from microns to nanometers), and strict control of motion parameters for each step, a material removal rate controlled at the nanometer level can be achieved.
2. Consistency of Dynamics (Consistency of Dynamics)
- Consistency of Polishing Trajectory:
Machine polishing typically employs a dual planetary trajectory or a compound “figure-8” motion. This multi-axis compound motion ensures that the abrasives on the polishing film slide isotropically on the ferrule end-face, avoiding directional deep scratches. Manual polishing, due to poor mechanical repeatability, is prone to leaving localized scratches of varying depths on the fiber core surface, increasing light scattering. - Constancy of Applied Pressure:
Machine polishing uses pneumatic or precision spring mechanisms to apply extremely constant force to each ferrule (e.g., approximately 4\ \text{N} to 6\ \text{N} per connector). Human muscles cannot maintain such high-precision constant force, leading to significant pressure differences in different polishing stages, even within the same batch or the same connector, making end-face quality irreproducible.
3. Angle Precision of Angled Physical Contact (APC, Angled Physical Contact)
For FC/APC connectors, which require suppression of high reflections and extremely low return loss, the end-face needs to be polished with a precise 8^\circ angle:
- In a manual polishing environment, it is almost impossible to grind an 8^\circ \pm 0.1^\circ angle on the ceramic end-face by hand without the rigid constraint of a precision angle jig. If the angle deviation exceeds 0.3^\circ, the fiber cores will not achieve physical contact when two fibers are mated, and the return loss will drop sharply from a normal \ge 60\ \text{dB} to \le 30\ \text{dB}.
- Machine polishing jigs ensure a fixed angled structure through machined inclined grooves, thereby ensuring consistency and interchangeability of the angle.
Conclusion
Due to the inability to overcome the mechanical and geometric uncertainties arising from the physiological limitations of the human hand, the fiber optic connectors produced by manual polishing cannot meet the requirements for industrial-grade communication and high-precision fiber optic sensing (such as Fiber Bragg Grating sensing, distributed fiber optic sensing, etc.) in terms of both the mean value and the standard deviation (dispersion) of optical indicators (IL, RL). This is also why high-performance fiber optic sensors (such as the OFSCN® series of Fiber Bragg Grating sensors from Beijing Dacheng Yongsheng Technology Co., Ltd.) default to using industrial-grade machine-polished high-precision connectors, aiming to ensure the optical stability of the entire measurement system at the source.