FTTH (fiber to the home) has become the focus of optical network construction. Asia is the fastest growing region of FTTH in the world. Many countries have promoted the development of optical fiber broadband with national strategies and made remarkable achievements.

Based on the good development trend of FTTH, this article focuses on the design and attention points of ODN optical link, common optical cable types and the selection of optical fiber.

The design and attention points of ODN optical link design and attention points

The basic components of FTTH system include FTTH optical line terminal (OLT), optical distribution network (ODN), and FTTH optical network unit (ONU).In the fiber access network, the position of ONU has great flexibility. According to the position of ONU in the access network, the fiber access network can be divided into fiber to the roadside (FTTC), fiber to the building (FTTB), fiber to the office (FTTO), fiber to home (FTTH) and other modes.

Calculation of optical link losses

Optical link loss of ODN in FTTH system includes optical loss between S/R reference point and R/S reference point, which is calculated by dB.The attenuation sum introduced by optical fiber, optical splitter, optical activity connector and optical fiber fusion joint is included. The loss calculation formula of optical link is as follows:

ODN optical link loss = optical fiber loss + optical splitter insertion loss + optical activity connector loss + optical fiber fusion loss

In the design of ODN optical link, the optical loss is usually calculated at 0.40db /km.

Optical splitter insertion loss refers to the number of dB of each output path relative to the input optical loss, and its typical insertion loss value is shown in table 1.

The insertion loss of optical active connector is usually 0.5db. Optical fiber fusion joint attenuation, split-type optical fiber joint attenuation take a two-way average of 0.08dB/ each joint;The two-way average attenuation of the optical fiber joint of the ribbon cable is 0.2db/each joint.

Optical fiber fusion loss comes from optical fiber cold connector, also known as mechanical optical fiber connector, which is suitable for occasions such as high places in the corridor, narrow space, insufficient lighting, and inconvenient power collection on site. The two-way average value is 0.15db/connector.

When designing passive optical networks, it is usually necessary to set aside a certain line maintenance margin (Mc).The selection of line maintenance margin is generally as follows:

When the transmission distance ≤ 5km, the line maintenance margin is not less than 1 dB;

When the transmission distance ≤ 10km, the line maintenance margin is not less than 2db;

When the transmission distance > 10 km, the line maintenance margin is not less than 3 dB.

Assume that chose the PON system whose ODN optical link channel allows maximum 24 dB insertion loss, assuming that the using optical splitter for 1:16, optical fiber joint loss in the PON system, as table 1 by calculating, we can know that the sum of allowed line loss and line maintenance is 7.55 dB, line loss calculation according to 0.40 dB/km, then the PON system allows maximum transmission distance of 11.3 km.

In actual situation, PON system transmission distance

The above is the calculation of optical link loss of ODN in an ideal environment. In actual system application, there are also docking loss and bending loss of different kinds of optical fibers, which will make the transmission distance of PON system shorter than the above values.

In addition, from the data of optical fiber loss, optical splitter insertion loss, optical activity connector loss, optical fiber splicing loss, we can know that optical splitter insertion loss and optical activity connector loss is bigger, in order to ensure the system enough transmission distance,  we shall pay attention to the following two points in designing system: the flexible use of different ratio of optical splitter and minimizing the number of activity connectors.

The type of optical cable commonly used in FTTH construction

1. Outdoor optical cable

The outdoor optical cable on the market at present, in addition to the traditional ordinary layer and central pipe vertical optical fiber cable or optical fiber belt optical cable, but also according to the application of special optical cable, such as: road micro-channel optical cable, drainage pipe optical cable, gas sent micro cable and small 8 character optical cable.

2 indoor and outdoor cable

Indoor and outdoor fiber optic cable is designed for optical fiber from outdoor to indoor switch to cable directly and will be more and more applied in the FTTH wiring cable, the cable mainly have the type of loose tube optical cable, tight tube optical cable and butterfly introduced cable, because of its applications, including indoor and outdoor, so it need to meet the requirements of the change of high and low temperature, waterproof, and also need to meet the requirements of indoor fire retardant.

3. Indoor optical cable

The kinds of indoor cable are various. The most widely used is the traditional structure of single-core, double-core, multi-core distribution cable, branch cable and butterfly cable etc.

The selection of fiber type in FTTH construction

According to YD/ T1636-2007 Fiber to the Home (FTTH) Architecture and Overall Requirements, the wavelength range for uplink signal is 1260-1360nm, and the wavelength range for downlink signal is 1480-1500nm. Fiber G.652 and G.657 are generally selected for this window channel. In FTTH construction, some optical cables are laid inside buildings, especially indoors.

In the construction, not only the optical characteristics of the optical cable should be considered, but also the construction convenience and decoration aesthetics should be considered. However, G652 optical fiber has better attenuation characteristics when the bending radius is more than 30mm. Therefore, the bending-insensitive G.657 optical fiber is usually used in the FTTH construction at present to make the bending radius reach or even less than 15mm.

The mode field diameter of the bending-insensitive type G657 fiber is much smaller than the conventional G.652 fiber, so when connecting the fiber, the fusion mode must be adopted instead of cold connection. In the meanwhile, it should be noted that there is a great difference between the fusion of two kinds of optical fiber and the fusion of the same kind of optical fiber. The fusion technology must be explored carefully in order to obtain better splicing effect.

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