Requirements of good fiber connector are as follows:
Low coupling losses:
The connector assembly must maintain stringent alignment tolerances to assure low mating losses. These low losses must not change significantly during operation or after numerous connects and disconnects.
Interchangeability:
Connectors of the same type must be compatible from one manufacturer to another.
Ease of assembly:
A service technician should readily be able to install the connector in a field environment; that is, in a location other than the connector factory. The connector loss should also be fairly insensitive to the assembly skill of the technician.
Low environment sensitivity:
Connector such as temperature, dust and moisture should have a small effect on connector loss variations.
Low cost and reliable construction:
The connector must have a precision suitable to the application, but its cost must not be a major factor in the fiber system.
High strength:
The connection should not degrade due to forces on the connector body of tension on fiber cables.
Ease of connection:
Generally one should be able to mate and unmate the connector, simply by hand.
Lensing Schemes
The source to fiber power launching analysis proves that if the source-emitting area is larger than the fiber-core area, then maximum optical power is coupled into the fiber.
However, if the emitting area of the source is smaller than the core-area, a miniature lens may be placed between the source and the fiber to improve the power-coupling efficiency.
The function of microlens is to magnify the emitting area of the source to exactly match the core area of the fiber end face.
If the emitting area is increased by a magnification factor M, the solid angle within which optical power is coupled to the fiber from source is also increased by the same factor.
Several possible lensing schemes are shown in the figure below:
These include a rounded end fiber, a small glass sphere (nonimaging microsphere) in contact with both the fiber and the source, a larger spherical lens used to image the source on the core area of the fiber end, a cylindrical lens generally formed from a short section of fiber, a system consisting of a spherical-surfaced LED and a spherical-ended fiber, and a taper-ended fiber.
Nonimaging microsphere is the most efficient lensing method used to increase power coupling efficiency.
The theoretical coupling efficiency that can be achieved is based on energy and radiance conservation principles. The efficiency is usually determined by the size of the fiber.
Although these techniques can improve the source-to-fiber coupling efficiency, they also create additional complexities. One problem is that the lens size is similar to the source and the fiber-core dimensions, which introduces fabrication and handling difficulties.
In the case of taper-ended fiber, the mechanical alignment must be carried out with greater precision since the coupling efficiency becomes a more sharply peaked function of the spatial alignment. However, alignment tolerances are increased for other types of lensing system.
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