We all know the single mode fiber has great transmission performance at long distance. And there are further requirements on the single mode fiber, such as the bending loss resistance.
HOC (Hone Optical Communications) has 19+ years experience on optical communications. Let’s have find out more about this issue.
Single Mode Fiber Type
G.657 optical fiber is also called bending-loss insensitive single-mode optical fibre. It is used in the optical cable which is thinner than ordinary telephone line.
Without G.657 fiber, there will be no large-scale application of FTTH (Fiber to The Home). It is divided into several subcategories, and G.657A2 is commonly used at present.
G. 652 optical fiber is the most widely used optical fiber in the MAN (Metropolitan Area Network). Except FTTH cable, it is basically G.652 optical fiber that’s been used in other communication network. It is divided into 4 sub categories. And the G.652D is the most generally used one at present.
So, how good is the bending resistance of G.657A2 fiber compared with G.652D fiber?
What is Bending Radius
In the optical fiber industry, the radius of curvature of optical fiber that can be bent or twisted is called bending radius. In simpler terms, the minimum allowable radius that can bend the optical fiber without damage, kink or shorten its service life is called the bending radius.
Various optical fibers have different characteristics and different physical structures, which is why the allowable bending radius between optical fibers is different. Typically, optical fibers have a specified minimum bending radius. However, it is recommended that the minimum bending radius should be kept about 10 times smaller than the outer diameter of the optical fiber. Therefore, the bending radius of a 4mm fiber optic cable should not be less than 40mm.
Standard Minimum Bending Radius
In ITU-T standards, the minimum bending radius of G.657A2 optical fiber is recommended to be 7.5mm. While the minimum bending radius of G.652 optical fiber is required to be no less than 30mm.
But the meaning of the minimum bending radius is completely different. The minimum bending radius of G657A2 optical fiber refers to that the additional attenuation measured at 1550nm/1625nm when fiber optic cable is in a loose turn of 7.5mm radius shall not exceed 0.5dB/1.0dB.
The minimum bending radius of G652D optical fiber refers to that the optical fiber is loosely wound for 100 turns with a radius of 30mm, and the additional attenuation measured at 1625nm does not exceed 0.1dB.
The additional loss caused by the bending of optical fiber with bending radius much larger than its diameter is called macrobending loss. The macrobending characteristics of G.657A2 optical fiber and G.652D optical fiber are shown in the table below.
Obviously, there is no comparability between the minimum bending radius of G.657A2 fiber and G.652D fiber.
When FTTH cable is introduced and installed, in order to facilitate the end of the FTTH cable, the installation and maintenance personnel often fuse a small section of fiber pigtail on the FTTH cable at the fiber distribution box and ONT (Optical Network Terminal).
In the picture, the optical fiber type in the pigtail is G.652D, and the single mode fiber in the FTTH cable is G.657A2.
Because the fiber pigtail and FTTH cable are in the same distribution box, and the cable size is similar (the diameter of pigtail is 2.0mm and the diameter of FTTH cable is 2.0mm / 3.0mm), it is more practical to compare the bending resistance. Therefore, it can basically reflect the difference between G.652D and G.657A2.
Since the macrobending loss of optical fiber in ODN is mainly affected by the downlink wavelength, only the additional loss of tail fiber and incoming optical cable winding one circle with different radius at 1490nm wavelength is tested. Some molds tested are shown in the figure below.
It can be seen from the test results that the incoming optical FTTH cable (G.657A2) has greater bending resistance than that of the fiber pigtail cable (G.652D), especially under small bending radius.
Due to the great difference in bending resistance between G.657A2 and G.652D single mode fiber, and the pigtail fiber patch cord itself is relatively soft, it is very easy to bend with a small radius in application.
According to the statistical data of the installation and maintenance failure in a metropolitan area network, the network failure caused by the insufficient bending radius of the fiber patch cord accounts for as much as 50%.
Therefore, it is advisable to use the fiber cable with G.657A2 to end the incoming optical cable during FTTH installation.