How EDFA (Erbium Doped Fiber Amplifier) Works

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When a normal optical fiber core is doped with trivalent ‘erbium’ ions, erbium doped fiber is formed. This erbium doped fiber act as a gain medium that amplifies an optical signal. Hence, it is named as EDFA (Erbium Doped Fiber Amplifier). The erbium doped fiber is pumped with a laser, at a wavelength of 980 nm or 1480 nm and produce optical gain in the 1550 nm region.
We can’t directly send laser light into EDFA. Before that, laser needs to be multiplexed into the erbium doped fiber. For that purpose, we use WDM (Wavelength Division Multiplexing) coupler to multiplex laser into EDFA.  After that EDFA is pumped with laser, to achieve optical gain.
EDFA uses population inversion technique to achieve optical amplification. Before we jump into EDFA, first let’s see how the actual laser works. LASER stands for Light Amplification and Stimulated Emission of Radiation. As the name suggests, laser amplifies the light by using a process called stimulated emission of radiation.
Shall we go deeper into the concept!
Every object in the universe is made up of tiny particles called atoms. However, atoms are not the smallest particles in the universe. There are some particles which are much smaller than atoms. These particles are electrons, protons and neutrons. Combining these particles make an atom. Each atom has a set of electrons, protons and neutrons. Electrons have negative charge, protons have positive charge and neutrons have no charge. Protons and neutrons always stick together because of the strong nuclear force between them.
The protons and neutrons which are stick together are known as nucleus. The overall charge of the nucleus is positive because of the positive protons (neutrons does not have charge). On the other hand, electrons have negative charge (opposite charge to protons). As we know that, there exists an attractive force between the opposite charges. So the electrons always rotate around the nucleus at different distances because of the electrostatic force of attraction between electrons and nucleus.
The electrons which are revolving at different distances from the nucleus have different energy levels associated with it. The electrons which are revolving at a very close distance from the nucleus have the lowest energy level whereas; the electrons which are revolving at a larger distance from the nucleus have the highest energy level.
The electrons at a larger distance from the nucleus have highest energy level because they reached that level by gaining additional energy from the external energy sources like light, heat and voltage. This process of gaining additional energy from the external energy sources to jump into higher energy level is called absorption of radiation.
The electrons in the higher energy level will not stay for a long time. After a short period of time, they fall back to the lower energy level by releasing energy in the form of heat or light. This process is called spontaneous emission of radiation. Laser works based on this concept.
Although, the electrons release energy in the form of light, there is no light amplification in this process. So the spontaneous emission process is not used to build a laser.
The laser works based on a special process called stimulated emission of radiation. To achieve light amplification, the number of electrons in the higher energy level (E2) must be greater than the number of electrons in the lower energy level (E1). In a two level energy system, the population of electrons in the higher energy level is always lesser than the population of electrons in the lower energy level. In some cases, the population of electrons in higher energy level will becomes equal to the population of electrons in the lower energy level. So the two level energy system is not useful for light amplification.
The light amplification is achieved by using a 3 or more energy level system. The greater the energy levels are, the greater will be the light amplification. For example, a 4 level energy system will produce more optical gain than the 3 level energy system.
The EDFA is pumped using two laser diodes (bidirectional pumping) or a single laser diode (unidirectional pumping). The EDFA pumped with a single laser diode is most commonly used. In this tutorial, EDFA pumped with a single laser diode is discussed.
EDFA is pumped with two different wavelengths of photons: 980 nm or 1480 nm.

How to determine the quality of a PLC splitter?

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There are five main specifications that are outlined in this standard. The following section outlines each of the specification and their importance for a fully functional optical splitter.
1. Optical Bandpass
For a fiber optic network, there are six nominal optical bandpass ranges.
A PON system has a downstream transmission (data sent from a server to a user) using the 1490nm wavelength while the upstream transmission (data sent from a user to a server) is a 1310nm wavelength. In addition, there needs to be consideration for any requirement for RF video overlay and network testing/maintenance. RF video overlay is generally transmitted through the 1550nm wavelength.
According to the ITU L.41 recommendation, the 1550nm or 1625nm wavelength is used for network for testing and surveillance. With these considerations, the required optical band needs to be determined.
The standard operating wavelength for a PON splitter is the 1260-1650nm which covers most of the optical bands.
The optical bandpass can be tested by connecting the optical splitter to an optical spectrum analyzer with a high powered light source having a central wavelength of the required bandpass. The attenuation across the required bandpass shall meet the splitter requirements.
2. Optical insertion loss
The optical splitter is the component with the largest attenuation in a PON system. The optical insertion loss is the loss of an optical signal resulting from the insertion of a component such as connector or splice in an optical fiber system. In order to conserve the power budget of a PON system, the insertion loss from the splitter needs to be minimized.
Based on the GR-1209 standard, the maximum allowable insertion loss for an optical splitter used in a PON system can be determined by using the calculations outlined in the below table.
1×N Optical Splitter 0.8 + 3.4 log2N
2×N Optical Splitter 1.0 + 3.4 log2N
Note: ‘N’ denotes the number of output ports.
The insertion loss is tested by using a light source and power meter(or) by using an insertion loss meter.The reference power level is obtained and each of the output port of the optical splitter is measured.
3. Optical return loss
The return loss is the loss of power in the light signal returned or reflected by a discontinuity in an optical fiber or transmission line. A high return loss reduces the power reflected back to the transmitting port thus minimizing noise which may result in a system power penalty.
The return loss is tested by using a return loss meter. The input port of the splitter is connected to the return loss meter and all the output ports are connected to a non-reflective index matching gel.
4. Uniformity
Uniformity is the maximum insertion loss value between one input port and any two output ports or between two input ports and one output port. This requirement ensures that for a PON system, the transmission power at each splitter output port is the same, thus simplifying the network design.
Custom optical splitters with non-uniform coupling ratio can be manufactured for specific network deployment. In such a situation, this criteria is not applicable. The usage of a non-uniform splitter in a PON system increases the complexity in testing, design and maintenance while reducing the network flexibility.
The uniformity of the splitter can be determined by referring to the results from the insertion loss test to ensure that the difference between the highest loss and the lowest loss is within the acceptable uniformity value (≤0.5 dB).
5. Directivity
Directivity is the fraction of power transferred from one input port to another input port or from an output port to another output port. For a 2×N optical splitter, when light is injected into one of the input ports, light does not only propagate out of the output ports. Some of the light propagates back through the second input port. Similarly, when light is injected into one of the output ports, some of the light propagates back through the other output ports.
In a bidirectional transmission system such as a PON, directivity is important to reduce the power back to the transmitting port to reduce signal cross talk. In addition, a high directivity value will also cause a higher insertion loss due to the loss in optical power. So it is important to reduce the directivity as much as possible.
Directivity can be measured in a manner similar to the insertion loss test. However, the light source and power meter are connected to each of the input ports of two output ports.

Splicing: How to Properly Fuse Together Fiber Optic Cables

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Fiber optic splicing is the process of joining two or more fibers together. Whether you’re deploying a new fiber optic network or expanding an existing network, you must ensure your fibers are properly spliced to avoid network disruptions.
Splicing fibers is commonly used to rejoin fiber optic cables when accidentally broken or to fuse two fibers together to create a fiber that is long enough for the required cable run.
There are two accepted methods of splicing fibers:
Mechanical splicing
Fusion splicing
Of the two methods, a mechanical splice can be performed much quicker than a fusion splice. A mechanical splice is a junction of two or more fibers that are aligned and then held together by connectors.
Although easier to perform, mechanical splicing allows an increase in insertion loss. So, mechanical splicing is only ideal for quick or temporary restoration, not for permanent splices.
The most common method of splicing fibers together is fusion splicing, which permanently fuses fibers together using an electric arc. This method is far more popular than mechanical splicing because it provides the lowest loss, less reflectance and the strongest joint between the fibers.
FUSION SPLICING YOUR FIBERS
Fusion splicing is a very delicate process. If not properly done, your fibers may not be properly connected and your signal may suffer.
When performing a fusion splice there are generally five different steps:
1. Stripping the fiber
To start fusing your fibers together, you must remove or strip the protective polymer coating around the optical fiber. This is usually done with a mechanical stripping device, similar to a pair of wire strippers. Remember to clean the stripping tools before you start the fusing process.
2. Cleaning the fiber
After the fiber has been stripped of the coating, it’s time to clean the bare fiber. Using a 99.9% isopropyl alcohol (IPA) and lint-free wipes will keep the glass free of any contaminations.
3. Cleaving the fiber
A good cleaver is crucial to a successful fusion splice. The cleaver nicks the fiber and pulls or flexes it to cause a clean break rather then cut the fiber, which makes the end-face flat and perpendicular to the axis of the fiber.
4. Fusing the fiber
After the fibers have been cleaved, fuse them together with a fusion splicer. First, you must align the ends of the fiber within the splicer. Once properly aligned, melt the fibers with an electric arc, permanently welding the ends together.
5. Protecting the fiber
After the fibers have been successfully fused together, the bare fiber is protected either by re-applying a coating or by using a splice protector.

Difference between Gigabit optical modules and 10G optical modules

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It is easy to understand literally that the main difference between Gigabit optical modules and 10 Gigabit optical modules is that the transmission rate is different. The transmission rate of the Gigabit optical module is 1000 Mbps, and the transmission rate of the 10 Gigabit optical module is 10 Gbps. In addition to the difference in transmission rate, what are the more specific differences between Gigabit optical modules and 10 Gigabit optical modules?
Gigabit optical module
As you can know from the naming, the Gigabit optical module is an optical module with a transmission rate of 1000 Mbps, usually expressed by FE. And the Gigabit optical module generally has two kinds of Gigabit SFP optical modules and GBIC optical modules, and the transmission distance can reach between 80m and 160km. In general, Gigabit optical modules can be identified from the specification details of the product itself and the optical module naming rules provided by different companies.
The Gigabit optical module includes the 1000Base SFP optical module, the BIDI SFP optical module, the CWDM SFP optical module, the DWDM SFP optical module, the SONET/SDH SFP optical module, and the GBIC optical module.
10G optical module
The 10 Gigabit optical module is an optical module with a transmission rate of 10 G. It is also called a 10 G optical module. Generally, it is packaged in the form of SFP+ or XFP. The standards for 10G optical modules are IEEE 802.3ae, IEEE 802.3ak, and IEEE 802.3an. When choosing a 10 Gigabit optical module, we can consider factors such as price, power consumption, and space.
The 10 Gigabit optical module includes 10G SFP+ optical module, BIDI SFP+ optical module, CWDM SFP+ optical module, DWDM SFP+ optical module, 10G XFP optical module, BIDI XFP optical module, CWDM XFP optical module, and DWDM XFP optical. Nine modules and 10G X2 optical modules.
Gigabit optical modules for Gigabit Ethernet, dual-channel and bi-directional transmission Synchronous Optical Network (SONET), and 10 Gigabit optical modules for 10 Gigabit Ethernet, STM-64 and OC-192 rate standard synchronous optical networks (SONET) and 10G Fibre Channel.
In the application, you should choose Gigabit optical module or 10 Gigabit optical module. This depends mainly on the type of network you are adapting. For example, if your network is Gigabit Ethernet, you need Gigabit optical module, and 10 Gigabit Ethernet uses 10 Gigabit optical. Module.

What is the OADM Multiplexer?

The OADM, optical add drop multiplexer, is a gateway into and out of a single mode fiber. In practice, most signals pass through the device, but some would be “dropped” by splitting them from the line. Signals originating at that point can be “added” into the line and directed to another destination. An OADM may be considered to be a specific type of optical cross-connect, widely used in wavelength division multiplexing systems for multiplexing and routing fiber optic signals. They selectively add and drop individual or sets of wavelength channels from a dense wavelength division multiplexing (DWDM) multi-channel stream. OADMs are used to cost effectively access part of the bandwidth in the optical domain being passed through the in-line amplifiers with the minimum amount of electronics.
OADMs have passive and active modes depending on the wavelength. In passive OADM, the add and drop wavelengths are fixed beforehand while in dynamic mode, OADM can be set to any wavelength after installation. Passive OADM uses WDM filter, fiber gratings, and planar waveguides in networks with WDM systems. Dynamic OADM can select any wavelength by provisioning on demand without changing its physical configuration. It is also less expensive and more flexible than passive OADM. Dynamic OADM is separated into two generations.
A typical OADM consists of three stages: an optical demultiplexer, an optical multiplexer, and between them a method of reconfiguring the paths between the optical demultiplexer, the optical multiplexer and a set of ports for adding and dropping signals. The optical demultiplexer separates wavelengths in an input fiber onto ports. The reconfiguration can be achieved by a cross connection panel or by optical switches which direct the wavelengths to the optical multiplexer or to drop ports. The optical multiplexer multiplexes the wavelength channels that are to continue on from demultipexer ports with those from the add ports, onto a single output fiber.
Physically, there are several ways to realize an OADM. There are a variety of demultiplexer and multiplexer technologies including thin film filters, fiber Bragg gratings with optical circulators, free space grating devices and integrated planar arrayed waveguide gratings. The switching or reconfiguration functions range from the manual fiber patch panel to a variety of switching technologies including microelectromechanical systems (MEMS), liquid crystal and thermo-optic switches in planar waveguide circuits.
CWDM and DWDM OADM provide data access for intermediate network devices along a shared optical media network path. Regardless of the network topology, OADM access points allow design flexibility to communicate to locations along the fiber path. CWDM OADM provides the ability to add or drop a single wavelength or multi-wavelengths from a fully multiplexed optical signal. This permits intermediate locations between remote sites to access the common, point-to-point fiber message linking them. Wavelengths not dropped, pass-through the OADM and keep on in the direction of the remote site. Additional selected wavelengths can be added or dropped by successive OADMS as needed.

What are the Main parameters of the optical transceiver modules?

Main parameters of the optical modules
1. Transmission rate
The transmission rate refers to the number of bits transmitted per second in units of Mb/s or Gb/s. Main rates: 100M, Gigabit, 2.5G, 4.25G and 10G, 25G, 40G, 56G, 100G, 120G, etc.
Therefore, based on different data rate, our optical transceiver modules arrange 100M, 1G/2G/4G SFP module, 10G SFP+/XFP, 16G SFP+, 25G SFP28, 40G QSFP+, 56G QSFP+, 100G CFP/CFP2/CFP4/QSFP28 Module.
2. Transmission distance
The transmission distance of the optical module is divided into short distance, medium distance and long distance. It is generally considered that a short distance is 2 km or less, a medium distance of 10 to 20 km, and a long distance of 30 km, 40 km or more.
■ The transmission distance of the optical module is limited, mainly because the optical signal has a certain loss and dispersion when transmitted in the optical fiber.
note:
• Loss is the loss of light energy due to absorption and scattering of the medium and leakage of light as it travels through the fiber. This energy is dissipated at a certain rate as the transmission distance increases.
• Dispersion is mainly caused by the unequal speed of electromagnetic waves of different wavelengths propagating in the same medium, which causes different wavelength components of the optical signal to reach the receiving end at different times due to the accumulation of transmission distance, resulting in pulse broadening and thus inability to distinguish Signal value.
• Therefore, users need to select the appropriate optical module according to their actual networking conditions to meet different transmission distance requirements.
3. Center wavelength
• The center wavelength refers to the optical band used for optical signal transmission. Currently, there are three main types of optical wavelengths commonly used in optical modules: the 850 nm band, the 1310 nm band, and the 1550 nm band.
• 850nm band: mostly used for short distance transmission of ≤2km
• 1310nm and 1550nm bands: mostly used for medium and long distance transmission, more than 2km transmission.
In addition, users also need to confirm which brand of their equipment, which will decide the compatiblity of the SFP transceiver modules.