Passive Optical Components and the Applications in FTTH Networks

Thanks to the optical fiber communication, we have been brought to a new internet society. Optical fibers reach more and more families. Optically that is called FTTH (fiber to the home), namely optical fibers installed directly into users’ homes. Besides optical fiber, passive components are also important to optical fiber communication. Many passive optical components are used in FTTH applications.

Passive Optical Components

In fiber optics, “passive” means the device does not require electrical-to-optical or optical-to-electrical conversion during its operation. Passive components include fiber optical connectors, adapters, attenuators, couplers & splitters, WDM filters & Mux/DeMux, optical switches, etc. First we will talk about optical splitters and filters.

Optical Splitters

An optical splitter is a device dividing optical power from an optical fiber into several optical fibers. A PLC (Planar Lightwave Circuit) splitter is usually used for fiber branches more eight, while fiber fused coupler are for the branches up to four. Fiber arrays, in which input/output fibers are arranged, are aligned to an optical circuit of a PLC chip and fixed with UV curable adhesive. To eliminate back reflection from the interface with the adhesive layer, the end faces of fiber arrays and PLC chip are designed oblique. An attenuation in an optical circuit of a PLC is less than 0.1dB/cm. So the total attenuation including coupling loss at the interface between fiber arrays and a PLC chip will be 1 dB or less. To realize such low insertion loss, the cores of the fibers and a PLC chip should be positioned in an accurate position. The accuracy of the fiber array depends on geometry of optical fibers and especially on the accuracy of processing of Vgroove.

Figure 1. 1×8 Blockless Fiber PLC Splitter SC/APC

Optical splitters’ function and application are determined by many characteristics, including the number of input and output ports, signal attenuation, the directionality of the light transmission, wavelength selectivity, single- or multimode operation, as well as the polarization sensitivity and the polarization-dependent loss of the device. In CATV networks, optical splitters broadcast video signals to thousands of subscribers through the hybrid fiber coax systems.

Optical Filter

An optical filter is a device for wavelength multi/demultiplexing with a dielectric multi-layered thin film, which can add or drop a specific wavelength in the midst of a fiber. A dielectric multilayered film consists of many layers with different refractive indexes deposited on a transparent substrate, reflecting or transmitting a specific wavelength. A thin film filter is inserted in the paralleled optics by gradient index lenses at the end faces of a twin fiber ferule and a single fiber ferule.

Figure 2. FTTH/FTTx Pass 1310nm Reflect 1550nm FWDM Filter WDM

In a fiber embedded filter, to reduce loss caused by the gap between optical fibers, the thickness of the substrate should be less than several 10 μm. For example, a fiber embedded filter uses a single optical fiber fixed in a Vgroove of a base material. If changing the fiber to a fiber ribbon, a number of filters can be made with the use of one thin film filter. By dicing a ferule of an optical connector and inserting a thin film filter, a connector with filter function can be realized.

Roles of Passive Components in FTTH Networks

In FTTH networks, PON (passive optical network) system is used. An OLT (Optical Line Terminal), transmission equipment in the central office, connects with many ONUs (Optical Network Units), transmission equipment at users. In the middle of the optical transmission line, an optical splitter is employed. With PON system, many users can share the cost of equipment in the central office and optical fiber cable, reducing the cost burden of individual users. The following will introduce two roles of passive optical components in FTTH networks.

Branching of Optical Signals

Suppose that in an FTTH network, there are 1×4 splitters in a central office and 1×8 splitters in an aerial closure near users installed. In a PON system, G-PON (Gigabit Passive Optical Network) and EPON (Gigabit Ethernet Passive Optical Network) have been standardized. The maximum branches for GPON is 64 and 32 for EPON. The signal wavelengths for down signals and upstream signals are allocated as 1490 nm and 1310 nm respectively. While for video, the band is 1550 nm. Since the wavelengths cover a wide range, so the optical splitters are needed. In the case of 64 branches, intrinsic branching loss reaches 18 dB. The number of branches is decided according to transmission distance, status of transmission lines, loss budget and so on.

Multi/demultiplexing of Video Signals

As mentioned above, 1550nm band is used for video signals. In order to multiplex this band, which locates between OLT and users’ premises, different technologies are employed based on the different positions. When video signals are are multiplexed near optical splitters, the first stage of Y-branch in a PLC type splitter is replaced to a 2 x 2 directional coupler. By launching 1490nm downstream signals and 1550nm video signals at each input port, multiplexing downstream and video signals and branching can be achieved at the same time. And the upstream signals going through the video input port will be disappeared because of an optical filter.

Upstream signals (1310 nm) enter from the common port, and go out from the transmission port through the thin film, while downstream signals (1490 nm) go in reverse. Video signals (1550 nm) come from the reflection port and go to the common port. When video signals are multiplexed in the midst of the network, optical filters are often used. It’s said that up to six optical filters have been successfully integrated on a pair of PLC-chips.

Conclusion

Passive optical components are the foundation stone of optical network systems. And they play very important role in FTTH networks. With passive components, upgrading network capacity will not cost too much without additional construction works in existing network. Additionally, they can also reduce operational expenses since adding components that monitor the network without disturbing any other signals. FS.COM provides a comprehensive line of passive components. We will continue to develop passive components and offer quality assurance program so that users can get low-cost but high quality equipment from this site.