We all know that a local area network (LAN) is a computer communication network that connects an external device such as a computer, a database, and a printer in a local geographic area (such as a company, a school, a hospital, etc.). Traditional LAN usually composed of Ethernet copper cable. Now, with the increasing number of devices connected to the network, the traditional local area network (LAN) is facing bandwidth, cabling space and other bottlenecks, for solving this series of problems, the fiber-based passive optical LAN (POL) came into being.
Passive Optical LAN (POL) vs Traditional LAN (LAN)
Traditional LAN（LAN）is on the basis of copper cable which limited by bandwidth, while the passive optical LAN（POL）is deployed via fiber cable that to improve bandwidth greatly. Compared with traditional LAN (LAN), the following advantages that passive optical LAN（POL）has:
(Passive Optical LAN)
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Reduce energy consumption: According to statistics, passive optical LAN (POL) can save about 40% of the functional consumption;
Multi-services support: Single network supports WIFI, voice, data, CCTV, video monitoring, broadband Internet access, security access control and other types of business integration;
High reliability: Carrier-class equipment and a full range of redundant design to address the user’s network stability issues;
Unlimited bandwidth: fiber optic network to meet the growing needs of the future bandwidth, without the need to transform the network can be smoothly upgraded to 10G, 40G or even higher.
Components of Passive Optical LAN（POL）
As the following diagram shows, passive optical LAN (POL) consists of optical line terminal (OLT), optical network terminal (ONT), this part will be used to mainly introduce the necessary assemblies like OLT, ONT, optical splitter etc.
Optical Line Terminal (OLT)
Optical line terminal (OLT) is located between the central office and the optical network terminal (ONT), and its function is to receive the downstream signal from the central office and send it to the optical network terminal (ONT), receive the uplink signal from the optical network terminal (ONT) and send it to the central office.
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Optical Network Terminal (ONT)
Due to most of terminal network equipment used electrical port, but the data signal transmit from OLT is optical signal, so ONT is needed to turn optical signal into electrical signal. There are some ONT have the function of WIFI in today’s market.
Optical splitter is located between OLT and ONT, is used for distributing the optical signals of the optical line terminal (OLT) evenly over a plurality of end users. There are many form factors and splitting ratio for choice on the market, the most commonly used is PLC optical splitter.
Fiber jumpers, network jumpers, information panels and other connected products are essential for connecting network devices, although these products seem insignificant, they have great significance for the passive light LAN (POL) to ensure its normal operation.
The instance of office building POL
The following instance shows how to lay out a passive optical LAN (POL) in an office building:
As the figure above shown: Server room are usually set up on the first floor, fiber optic distribution box and other cable management products are essential for better manage a verity of cables. In addition, sometimes every floor of the office building need to equip with distribution box, cabinet etc. Because of the large area of the office building, optical splitter are usually used for distributing every optical network unit (ONU) evenly of server room signal, ONU are commonly connected to optical splitter via fiber optic jumpers and fiber optic information panels. Finally, terminal network equipment are connected to ONU through network jumpers.
Compared with traditional LAN, passive optical LAN (POL) uses fewer active devices that can save a lot of energy consumption and space, its maintenance costs are lower, more reliable, better able to adapt to the growing demand for future bandwidth.
Now. you’ve learned that how to build POL, but do you know that what are the functions of passive optical assemblies? Okay, maybe you have no ideal about it, the main functions of passive optical assemblies include: connecting the optical waveguide or optical path, controlling the propagation direction of the light, the distribution of the optical power, and controlling the optical coupling between the optical waveguides, the devices or between the optical waveguide and the device. The rest of tutorial will introduce the basic testing methods for these passive optical assemblies.
Basic Testing Method for Passive Optical Assemblies
About Passive Optical Assemblies
Passive optical assemblies are the devices that no need for electrical-optical or optical-electrical conversion at work, which include: fiber optic connectors, optical splitter, optical circulator, optical isolators and optical attenuators etc. These assemblies play important roles in fiber optic communication devices with the features of high return loss, low insertion loss, high reliability, stability, mechanical wear resistance and corrosion resistance, ease of operation, etc. so that have been widely used in long-distance communication, area network and FTTH, video transmission, fiber sensing etc.
(Passive optical splitter)
Commonly used testing parameters of passive optical assemblies
The purpose of device under test (DUT) determines the parameters under testing, insertion loss, isolation performance and splitting ratio are considered as the common parameters when testing passive optical assemblies.
- Insertion loss
Insertion loss is a basic testing parameter of passive optical assemblies, the following picture shows the one of basic method if insertion loss testing, its steps: firstly, measuring the optical power of the optical source via an importing fiber, so that we can obtain the input optical power of the assembly under test; secondly, we get the output optical power of the assembly under test by using the same importing fiber and optical source; finally, calculate the value of insertion loss with the formula: insertion loss (dB) = 10 log (input optical power / output optical power).
(Insertion Loss test)
The test above adopts relative testing method which the accuracy of optical power meter does not directly affect the test results, but the repeat accuracy of its stability, linearity and connection determine the stability of the test results. There are several common passive optical assemblies that require insertion loss testing:
- Fiber optic connector
- Optical splitter
- Optical isolator, forward direction
- Optical circulator, appropriate path
- Lithium niobate optical modulator, conduction state
- Optical switch, import to one port
- Optical attenuators, set the minimum value
- Filter, filter within transmission wavelength
- Waveguide equipment, proper channel and wavelength
- Isolation performance
The device under test needs to be isolation performance test if it is used for stopping the transmission of optical power. Isolation performance testing is actually an insertion loss test, but the result of insertion loss is as high as possible, so the test value is often small. There are several common passive optical assemblies that require isolation performance testing:
- Optical isolator, reverse direction
- Optical circulator, reverse direction channel
- Lithium niobate optical modulator, blocking state
- Optical switch, import to another port
- Optical attenuators, set the maximum value
- Filter, filter exceed transmission wavelength
- Waveguide equipment, near channel (crosstalk test).
- Splitting Ratio
Optical splitter is a very important part of passive optical network, the function of it is divided a bunch of input optical signals are ordered into two or more bundles of output optical signals, the range of splitting ratio is from 50/50 to 99/1. Splitting ratio are easy to be measured, now we’ll introduce one simple method for its testing:
(Splitting ratio test)
In the testing method above, we should firstly measure the output optical power of each port of the device under test (Optical splitter), then calculate the ratio. Certainly, there is no need to use optical switch if you use two optical power meters; in addition, we can directly get the splitting ration by using dual-channel optical power meter. However, the optical switch is essential when there are many ports of the device under test. Same as insertion loss testing, splitting ratio testing is also a relative testing method.
Testing tools of passive optical assemblies
As mentioned above, we usually use optical source and optical power meter to measure a verity of parameters of passive optical assemblies, moreover, we can also make use of optical source and Visual Fault Locator (VFL), Optical Time Domain Reflectometer（OTDR）that based on our needs to measure loss etc. These testing tools are the basic tools for fiber optic testing.
(Passive optical assemblies testing tools)
The testing of passive optical assemblies is very important for ensuring the performance and stability of the network, and making use of the fiber optic testing tools help you simplify the process of testing and save testing time as well.
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