2021 Ultimate Guide of Fiber Optic Cable Adapter

In the optical fiber communication system, in order to meet the needs of flexible connection between different equipment and systems, there must be a kind of active connection device between the optical fiber and optical fiber, optical fiber and device (including active device, passive device, and optical fiber sensor), optical fiber and instrument, optical fiber and the system, which enables the optical signal to be transmitted according to the required channel. The device that can achieve this function is called a fiber optic connector adapter, which is the most indispensable optical passive device in the optical fiber communication system.

 

With the further development of optical fiber communication, different countries and different manufacturers have successively developed different fiber connector adapters. According to incomplete statistics, up to now, there are about 30 types used internationally. Generally, fiber optic adapters can be divided into single mode fiber connectors and multimode fiber connectors according to different transmission media. According to the structure of the connector, it can be divided into FC, SC, ST, LC, MTRJ, DIN, MU, MT, etc. According to the shape of the fiber end face, there are FC, PC (including SPC or U generation), and A generation. According to the number of fiber cores, there are simplex (single-core) and duplex (dual-core) types. There are many types and varieties of matching adapters, each with its own technical characteristics. But their functions are the same, that is, to connect the optical fiber (optical cable) plugs together to connect the optical fiber.

 

The requirements for the adapter are mainly low insertion loss, high reflection loss, good repeatability, stable environment, and good mechanical performance. The cladding diameter of a single-mode fiber is 125 um (equivalent to the thickness of a hair), and the diameter of its core (light-transmitting part of the fiber) is only 9 um. The task of the optical fiber connector is to accurately butt the 9um cores together so that 98% of the light can pass through. The technical requirements are very precise. There are many kinds of adapters, this article only makes a simple analysis of FC-type adapters.

 

The basic structure of FC type adapter

 

The FC-type adapter uses a sleeve structure, and this connector consists of a pin and a sleeve. The pin is a precision sleeve, and the optical fiber is fixed inside the pin. The sleeve is also a precision-machined sleeve (with opening and non-opening). The two pins are connected in the sleeve and ensure the alignment of the two optical fibers. The principle is: the outer cylindrical surface of the pin is used as the reference plane, and the pin and the sleeve are closely matched. When the coaxiality of the outer cylindrical surface of the fiber core, the outer cylindrical surface and the end surface of the pin, and the inner hole of the sleeve are processed very precisely, the two pins are butted in the sleeve to achieve the alignment of the two optical fibers.

From the rationality of this structural design, the feasibility of batch processing, and the use effect, the processing technology can achieve the required accuracy. At present, this structural design is the most widely used, and it is the mainstream of connector development today. FC, SC, ST, and other adapters all adopt this structure.

 

Sleeve

In the use of the optical cable coupler, the sleeve is the same important part as the pin. Its function is to maintain the precise alignment of the pin and the fiber core to ensure 90% light transmission. There are two main structures:

① Slip on the opening. The open sleeve is the most commonly used in connectors. Its main dimensions and technical indicators are: length (L) 11.4 ±0.1mm; outer diameter (D) 3.2 ±0.1mm; inner diameter (d) 2.49mm; inner hole finish is 14. The elastic deformation is less than 0.005mm; the insertion loss is ≤-0.2dB. The open sleeve is made of materials with good elasticity, such as phosphor bronze, bronze, and oxidized ceramics. When the pin is inserted into the sleeve, the supporting force of the sleeve to the pin should remain constant. However, the amount of sleeves made of phosphor bronze and bronze is very small. Most of them are sleeves made of zirconia ceramics. Because zirconia ceramics have superior properties in smoothness, elastic deformation, and coupling with ceramic pins than phosphor bronze and bronze.

② The sleeve is not open.

This kind of sleeve is rarely used in connectors, but it is more used in the connection of optical fibers and active devices. Its shape and size are basically the same as the open sleeve. The difference is that its inner hole diameter is 2.5 + 0.005mm, which is 1um larger than the outer diameter of the pin. Not only can the pins be inserted smoothly, but the gap should not be too large so that when the optical fiber is connected to the luminous tube and the detector, the repeatability and interchangeability meet the required indicators. The sleeve is a very important part of the optical adapter, which directly affects the insertion loss of the fiber optic cable ends.

 

Mechanical parts of the fixed sleeve

The design of the mechanical parts of the fixed sleeve is determined by its machining process. Different machining processes can produce different structures. Therefore, different companies have designed the appearance structure that conforms to the process and is simple to process. It can be divided into the one-piece type and split type; and the shape includes square, round, rhombus, combined shape, etc. The appearance is mainly designed and produced according to the needs of users.

 

1. Split type

Generally, the mechanical parts of the split fixed sleeve can be divided into 5 components, which are usually called 5-piece fiber optic cable adapters. The 5 parts are respectively a pair of flanges, 2 small metal sleeves, and inner cavity metal rings. Two small metal sleeves are assembled together, and the open sleeve is installed inside for fixing. The flange is the shape of the adapter, the protruding part is a nut, and its base is to ensure that the optical cable coupler is fixed where needed and to fix the mechanical parts inside.

The adapter flanges of this structure are generally coupled together by screws, which is convenient for disassembly. This facilitates the replacement of other damaged parts, and it is also an advantage that other structures do not have. However, due to too many parts, the flanges are difficult to completely overlap during assembly, so a new type of split structure design is available. To improve on the previous basis, a small metal sleeve is directly generated on a piece of the flange. Another small metal sleeve is also directly generated on the other flange. The inner cavity metal ring is directly generated at the bottom of a flange. The assembly process of this structural adapter is simple. The sleeve is placed in a small metal casing, the two flanges are fastened together and then riveted with rivets. This is more convenient and quicker, accurate in alignment, low cost, beautiful in appearance, and good in the hand. But it cannot be disassembled after riveting.

 

2. All-in-one

The one-piece type means that the two flanges are connected into one body, and there is no inner cavity in the internal structure, and there is no need to fix the inner cavity metal ring parts of the small sleeve, which is less and simpler than the split type. One-piece adapters of this structure are now widely used. Its assembly process is first press the small metal sleeve into the flange connecting body, then put the sleeve in the small metal sleeve, and then press the small metal sleeve in. The two small metal sleeves have different dimensions. However, two sleeves need to be pressed into the assembly of this fiber optic cable ends, and the size of the sleeve is strict, and the tolerance accuracy is high, otherwise, the flange cannot be pressed. There are also integrated adapters that only press one sleeve. It uses the latest machining methods to completely connect the small metal casing and the flange together. When assembling, just put in the sleeve and press on the small metal sleeve. This kind of conjoined structure design avoids the accumulated tolerances of individual parts in the processing process, fundamentally guarantees the accuracy and tolerance of the various dimensions required by the adapter, ensures the accuracy of the coupling of the pin body in the fiber connector adapters, reduces additional loss, and improves return loss.

 

 

Performance of fiber optic adapter

The optical performance of the fiber cable adapters should be considered, as well as interchangeability, repeatability, tensile strength, temperature, and the number of plug-ins.

 

1. Optical performance

The requirements for the optical performance of the fiber optic cable coupler are mainly the two most basic parameters of insertion loss and return loss. Insertion Loss is the connection loss, which refers to the loss of the effective optical power of the link caused by the introduction of the adapter. The smaller the insertion loss, the better, and the general requirement should not be greater than 0.5dB.

Return Loss optical power, and its typical value should not be less than 25dB. In practical adapters, the pin surface has been specially polished, which can make the return loss greater, generally not less than 45dB.

 

2. Interchangeability and repeatability

Optical connector adapters are universal passive components. Generally, the same type of fiber optic adapters can be combined arbitrarily and used repeatedly. The additional loss introduced by this is generally less than 0.2dB.

 

3. Tensile strength

For a finished optical cable coupler, it is generally required that its tensile strength should not be less than 90N.

 

4. Temperature

It is generally required that the optical fiber adapter must be able to be used normally at a temperature of -40 ℃ ~ + 70 ℃.

 

5. Mating times

The fiber optic cable ends currently used can generally be plugged and unplugged more than 1000 times.

 

 

Insertion loss generation

 

Insertion loss refers to the optical power attenuation caused by the connected adapter to the system (that is, the relative decrease in the output power of the optical adapter relative to the input power). The insertion loss is mainly caused by the deviation between two consecutive optical fibers. If the two optical fibers are arranged in a straight line and the deviation is zero, the insertion loss caused is minimal. But the possibility of realizing the actual docking of the fiber optic cable adapter is very small. Because of the different cores of the fiber core and the fiber cladding, the misalignment of the fiber cladding and the pinhole, and the concentricity error between the pinhole and the outer diameter, etc., will cause the lateral deviation between the fibers. The longitudinal gap and the quality of the end face in the optical fiber connector are also one of the factors that cause the insertion loss. The UPC plug contact method commonly used in recent years has better solved the longitudinal gap problem. In this way, the pin and the fiber end face are spherically polished, so that the two opposing pins have meshed together under the action of external force so that the apex of the meshed fiber is deformed and flattened to form a sufficient butt of the fiber and reduce the fiber joint the longitudinal gap.

 

The mechanism of insertion loss is:

1. Mismatch of core (or mode field) size (see Figure 1)

Figure 1

2. Numerical aperture mismatch (see Figure 2)

Figure 2

3. Mismatch of refractive index distribution (see Figure 3)

Figure 3

4. End face clearance (see Figure 4)

Figure 4

5. Axis inclination (see Figure 5)

Figure 5

6. Lateral offset or concentricity (see Figure 6)

Figure 6

 

In the actual connection of the optical fiber system, the influence of these factors may exist at the same time, and the total loss is the superposition of each loss. In order to reduce the insertion loss, the influence of the above-mentioned various factors should be avoided in the design and manufacture of the optical fiber adapter.

 

 

With the continuous development of optical fiber communication technology, especially the development of high-speed local area networks and optical access networks, fiber optic cable adapters will be more widely used in optical fiber systems. At the same time, more and higher requirements have been put forward for optical cable couplers. The main development direction is miniaturization and low cost, but the requirements for performance are getting higher and higher. In the next period of time, various newly developed fiber optic cable ends will form a pattern of "each has its own strengths and each has its own use" together with traditional FC, SC, and other adapters.