Fiber Connectors

. Connectors are mechanism or techniques used to join an optical fiber to another fiber or to a fiber optic component.

. Different connectors with different characteristics, advantages and disadvantages and performance parameters are available. Suitable connector is chosen as per the requirement and cost.

. Various fiber optic connectors from different manufactures are available SMA 906, ST, Biconic, FC, D4, HMS10, SC, FDDI, ESCON, EC/RACE, IC MT.

. Three different types of connectors are used for connecting fiber optic cables.
These are –
1. Subscriber Channel (SC) connector.
2. Straight Tip (ST) connector.
3. MT-RJ connector.

. SC connectors are general purpose connectors. It has push-pull type locking system. Fig. 1 shows SC connector.

. ST connector are most suited for networking devices. It is more reliable than SC connector. ST connector has bayonet locking system. Fig. 2 shows ST connectors.

 . MT-RJ connector is similar to RJ45 connector. Fig. 3 shows MT-RJ connector.

1. Principles of Good Connector Design
1. Low coupling loss.
2. Interchangeability – No variation in loss whenever a connector is applied to a fiber.
3. Ease of assembly.
4. Low environmental sensitivity.
5. Low cost – The connector should be in expensive also the tooling required for fitting.
6. Reliable operation.
7 .Ease of connection.
8. Repeatability – Connection and reconnection many times without an increase in loss.
2. Connector Types

. Connectors use variety of techniques for coupling such as screw on, bayonet-mount, push-pull configurations, butt joint and expanded beam fiber connectors.

Butt Joint Connectors

. Fiber is epoxied into precision hole and ferrules are used for each fiber. The fibers are secured in a precision alignment sleeve. Butt joints are used for single mode as well as for multimode fiber systems. Two commonly used butt-joint alignment designs are :

1. Straight-Sleeve.
2. Tapped-Sleeve/Biconical.

. In straight sleeve mechanism, the length of the sleeve and guided ferrules determines the end separation of two fibers. Fig. 4 shows straight sleeve alignment mechanism of fiber optic connectors.

. In tapered sleeve or biconical connector mechanism, a tapered sleeve is used to accommodate tapered ferrules. The factor end separations are determined by sleeve length and guide rings. Fig. 5 shows tapered sleeve fiber connectors.

3. Installing Fiber Connectors

. The method of attaching fiber optic connectors to optical fibers various among connector types. Following are the basic steps for installing fibers –

i) Cut the cable on inch longer than the required finished length.
ii) Carefully strip the outer jacket of the fiber with 'no nick' fiber strippers. Cut the exposed strength members and remove fiber coating.
iii) Thoroughly clean the bared fiber with isopropyl alcohol poured onto a soft, lint free cloth such as kimvipes. Never clean the fiber with dry tissue.
iv) The connector may be attached by applying epoxy or by crimping.

v) Anchor the cable strength members to the corner body. This prevents direct stress on the fiber.

vi) Prepare the fiber face to achieve a good optical finish by cleaning and polishing the fiber end.

4. Connector Return Loss

. At the connection point of optical link low reflectance levels are desired since the optical reflections can be source of unwanted feedback into the laser cavity. Due to this unwanted feedback the optical frequency response may degrade, also it generated internal noise within the source affecting overall system performance. Fig. 6 shows this connection model.

. The return loss for the index-matched gap region is given by,

       Where,
              d is the separation between fiber end.
              n₁ is index-matching material.
              R is reflectivity constant.