Fiber optic connectors

Fiber- optic connector (optical connector, connector) - a device for terminating fiber ends, designed to quickly mechanically connect and disconnect optical fibers , compared to termination via a splice plate . The connectors mechanically combine the centers of the fibers, allowing light to pass through. Higher-quality connectors give extremely small losses of the light signal from reflection or skew of the fibers. In total, there are about 100 types of fiber optic connectors on the market ^[1] .

Design

The detachable connection of optical fibers consists of three parts: two connectors and the corresponding socket type. The connectors are inserted into the socket from opposite sides (in the opposite direction) until they are in full contact with each other and are fixed. The method of fixing the connectors in the socket can be threaded, bayonet , using the locking mechanism and is determined by its type.

Fiber Optic Connector FC / PC

A ferrule made of ceramic ( zirconium oxide) having a coefficient of thermal expansion similar in value to the coefficient of thermal expansion of glass from which optical fibers are produced is embedded in a plastic or metal case of an optical connector (connector). This ensures a stable optical connection in the temperature range from −40 to + 80 ° C.

Connector Types

By type (design) of the casing

Existing types (form factors) of connectors (differ in shape, size and connection method): FC, SC, ST, LC, MTRJ, MPO, MU, SMA, FDDI, E2000, DIN4, and D4.

By tip end type (ferrule)

PC- connectors (Physical Contact) - the tip end is strictly perpendicular to the longitudinal axis.

APC Connectors (Angled Physical Contact) - tip end has a slope of 8 or 9 degrees from the perpendicular.

SC / APC Fiber Optic Connectors with Socket and End Cap

In order to unambiguously describe the type of fiber-optic connector, the type of the housing and the type of the tip, for example FC / PC, SC / APC, are indicated through a fraction.

Fiber Connector Installation

The optical fiber is fixed in the inner hole of the tip (ferul) using epoxy glue or by mechanical crimping. After that, the fiber is cleaved at a certain distance from the end face of the tip and polished. Since the end face of the tip has a rounded shape (with a radius of curvature of the order of 10-25 mm for RS and 5-12 mm for APC connectors), the end of the optical fiber takes this rounded shape as a result of the grinding process. Grinding the end of the optical connector is a complex multi-stage process, in which significant geometric parameters are strictly controlled, such as:

End vertex offset (Dome Offset or Apex Offset) - describes the deviation of the top point of the rounded end of the tip from the longitudinal axis of the fiber. According to the IEC specification, a tolerance of not more than 50 microns is permissible.

Depth (Undercut) - shows how deep inside the tip is polished the fiber itself. With an optimal penetration value, the pressing force is evenly distributed between the fiber and the ceramic tip. If this parameter is exceeded - as a result of longer grinding, the fiber goes deeper into the tip and thus physical contact is lost. However, the reverse situation is also not desirable when the fiber extends beyond the end surface of the tip due to insufficient grinding time. In this case, most of the pressing force (8-12 Newton) is applied to the fiber, which can lead to the appearance of a “fatigue effect ” of the epoxy adhesive and the fiber is pressed into the tip. As a result, there is a catastrophic increase in insertion loss.

Optical Link Options

In the connected state, the ends of the mating connectors are pressed against each other with a force of 8-12 Newton. Arising in ceramic tips (ferrules), elastic deformation leads to the appearance of the so-called physical contact (physical contact - PC) and a decrease in the influence of the Fresnel reflection . However, the luminous flux passing through the point of contact twice passes through the boundary of two media with different refractive indices : glass / air and air / glass. Thus, the connection of optical fibers is characterized by two indicators:

- Insertion loss - attenuation of the signal at the connection, it is especially important to consider when connecting single-mode optical fibers .

- Return loss (back reflection) is the ratio of the power of the direct signal to the return (reflection of the beam at the contact point of the optical fibers). It is necessary to minimize its value, as any reflection leads to malfunctions of high-speed digital transmission systems.