Optical Transponder—an Important Component in WDM System

Introduction to Optical Transponder

Optical transponder is also referred to as WDM transponder, wavelength-converting transponder or OEO (Optical-Electrical-Optical) 3R (re-timing, re-shaping, and re-amplifying) converter, and the word “transponder” is named according to the combination between transmitter and responder. It is an important unit in WDM system which main function is to convert the wavelength and the pattern of the optical signals and amplify the optical signals for long-haul transmission. At present, the optical transponder unit is commonly used in 10G connections including SFP+ to XFP, SFP+ to SFP+ and XFP to XFP fiber connections, and 40G QSFP+ to QSFP+ connections.

Working Principle of Optical Transponder

The optical transponder is designed to automatically receive a signal, amplify it and then retransmit the signal with another wavelength, without changing the content of the signal, which enables the different system to be connected. For instance, a 10G DWDM system can be deployed on the basis of a normal 10G system if using the optical transponder to convert a 850nm signal into a 1550nm one. What’s the working principle of the optical transponder? In general, when an optical input signal passes through the optical transponder, it will be firstly converted into an electrical one. Then a logical copy of the input signal is generated that features a new amplitude and shape and is used for driving the transmitter. Finally, an optical output signal with a new wavelength would be generated, as shown in the following figure.

Wavelength Conversion Case Analysis

As mentioned above, the optical transponder unit plays an important role in WDM system, which is very welcomed when deploying a CWDM or DWDM system on the basis of a normal system. It is well known that 850nm, 1310nm or 1550nm are used in a normal system for optical signal transmission, while CWDM or DWDM wavelengths are applied in a CWDM or DWDM system. Hence, if we want to transmit the normal signals to a CWDM or DWDM system, the optical transponder should be required that enables the normal wavelengths to be converted into CWDM or DWDM ones without changing the signal data. Here shows a wavelength conversion case by using the optical transponder.

We can learn from the case that a 10G-LR 1310nm SFP+ module is connected to a 10G switch on site A, while a 10G CWDM SFP+ module working on 1610nm is used with the CWDM Mux Demux on site B. As the 10G 1310nm signal from site A is required to be transmitted to the existing CWDM system on site B, a two SFP+ ports optical transponder should be used for converting the 10G 1310nm signal into a 10G 1610nm CWDM signal. To achieve this, another 10G-LR 1310nm SFP+ module and 10G CWDM 1610nm SFP+ module should be inserted into the 10G SFP+ to SFP+ optical transponder, separately. Furthermore, fiber patch cables are required to link the two 10G-LR 1310nm SFP+ modules and two 10G CWDM 1610nm SFP+ modules together, so that a complete link for wavelength conversion can be done.

Conclusion

The optical transponder is an important component in WDM system that makes the wavelength conversion easy, so that the signal data can be transmitted from a normal system to a WDM system. For instance, with the use of the optical transponder unit, a 1310 signal from a 10G fiber optical network can be converted into a 1610 CWDM signal and transmitted to the 10G CWDM network. If you are facing the problem about wavelength conversion for connection between a normal network with a WDM network as noted above, the optical transponder is quite recommendable for you.

Why Not Using DWDM Technology to Build Your Network?

Currently, more and more users choose to deploy DWDM networks on the basis of their existing networks, as the normal network can’t afford enough capacity for their daily use. Considering that there may be some confusion for designing the DWDM networks, this paper will mainly introduce the basic knowledge of DWDM technology and analyze the difference between SDH and DWDM technology. To better understand the DWDM technology, this paper will also guide users to deploy two common kinds of DWDM network. Hope the DWDM information in the paper would be useful for deploying a smooth DWDM network with higher transmission rate and capacity.

Introduction to DWDM Technology

DWDM technology is an ideal solution to address the capacity-hungry issue, which can multiplex several wavelengths for transmission different kinds of signals through one single fiber. In principle, the network utilizing DWDM technology enables carry up to 140 channels for transmitting signals, finally achieving high bandwidth transmission. As for the DWDM components, it basically includes DWDM multi-channel Mux/Demux, dispersion compensation module, fiber optic amplifier, optical transponder, and so on.

SDH vs DWDM Technology

As we know, SDH is the technology combining more than one lower-speed electrical or optical signals into a single higher bit rate signal with a single wavelength for transmission over a single fiber or wire. In the network utilizing SDH technology, Time division multiplexing (TDM) or statistical TDM is used, which means the signals in SDH network will be received by distributed across time slots. As for the DWDM technology, it uses wavelength multiplexing method, so that the signals can arrive at the receiver simultaneously. In the DWDM network, the DWDM multi-channel Mux/Demux mentioned above is the key components that can give different wavelengths to the different optical signals and multiplex them, so that the integrate signal with different wavelengths can be transmitted over a single fiber.

In short, SDH uses time division multiplexing, while DWDM works with wavelength division multiplexing. Compared to the SDH technology, DWDM can give different wavelengths to the optical signals, which allows the signals to be transmitted with their own speed and protocol and arrive at the same time. Besides, the SDH network can transmit both electrical or optical signals, while DWDM network only supports optical signal transmission.

Common DWDM Network Designs

Generally speaking, there are many kinds of DWDM networks with topological configurations, each of them has different requirements and can be used for different applications. They are basically DWDM point-to-point network, fully connected mesh network, star network, ring network and hybrid DWDM network consisting of stars and/or rings that are interconnected with point-to-point links. The following will mainly introduce the two most common DWDM networks, point-to-point network and ring network for your reference.

DWDM Point-to-Point Network: this kind of DWDM network is always deployed for long distance transmission with fast transmission speed, high bandwidth, great reliability and path restoration capability. The numbers of fiber optic amplifier used in this DWDM network is often less than 10, while the transmission distance can be up to several hundred kilometers. If optical add-drop multiplexer (OADM) is used, channels can be dropped or added along the path of the DWDM link. To better know the DWDM point-to-point network, here offers a figure that shows a DWDM point-to-point network design with the use of DWDM multi-channel Mux/Demux, OADM and fiber optic amplifier.

DWDM Ring Network: In general, this kind of DWDM network is often applied in local or metropolitan areas that can support the DWDM network at lengths up to dozens of kilometers. A basic DWDM ring network is shown in the following figure that has many nodes fully interconnected by the fiber, and sometime there are two fiber rings in a DWDM ring network which are deployed for protecting the network. Besides, the DWDM components like DWDM multi-channel Mux/Demux, OADM and optical amplifier are also required in the DWDM ring network.

Conclusion

DWDM technology is an economical solution for transmitting multiple signals through one fiber, which can solve the problem of insufficient capacity in your network. In contrast with SDH technology, DWDM technology enables the optical signals to be transmitted fast and arrive at the receivers simultaneous, while offering much higher capacity and transmission rate. If you are interested in DWDM technology, you can visit FS.COM where the wholesale DWDM Mux Demux, OADM and optical amplifier are available. It is recommended because of the good DWDM Mux Demux, OADM and optical amplifier price and quality.

How Does EDFA Amplifier Work for Extending DWDM System?

EDFA amplifier is the most common optical amplifier, used for boosting optical signals in optical applications, especially the DWDM systems. It is the key amplification device deployed in the optical system to enhance the signal power, so that the optical transmission distance can be greatly extended. Undoubtedly, the EDFA amplifier is an ideal choice for long-haul DWDM system. But how does it work for extending DWDM system? As shown in the following figure, EDFA amplifier can be placed at the transmitting side of the DWDM link, any intermediate point along the transmission DWDM link and the receiving end of the DWDM link, separately working as booster amplifier (or post-amplifier), in-line amplifier (or optical line amplifier) and pre-amplifier for optimizing the DWDM system reach. Let’s study the related knowledge in details.

What’s EDFA Amplifier?

EDFA amplifier is a kind of optical amplifier that can directly amplify any input optical signal without the need of optical-electrical-optical conversion. It can not only save the cost for long-haul transmission, but also reduce the signal loss and unwanted noise, compared to the traditional optical-electrical-optical amplification. As the fiber attenuation limits the reach of a non-amplified fiber link to about 200 km, EDFA amplifier is an ideal choice for building wide area purely optical networks.

How Does EDFA Amplifier Work?

As mentioned before, EDFA amplifier can be deployed in three places of the DWDM link to make the power compensation, the transmitting side of the link, the intermediate point along the link and the receiving end of the link. If placed at the transmitting side, it can be called as booster optical amplifier or post-amplifier, offering high input power for the wide fiber span. If placed at the intermediate point along the link, you can call it in-line amplifier or optical line amplifier. The optical line amplifier is used for compensating the fiber loss in the transmission link. When you call it pre-amplifier, it must be deployed in the receiving end, for boosting the signal power to the the necessary receiver level. The following will introduce these three different deployments of EDFA amplifier and how does the it work in the three link.

Placed at the Transmitting Side: in this application, we always call EDFA as booster optical amplifier that features high input power, high output power and medium optical gain. It can directly amplify the aggregated optical input signal multiplexed by the DWDM Mux Demux, to achieve DWDM network transmission distance extension. By placing the EDFA amplifier at the transmitting side of the DWDM link, the transmitted signal power can be enhanced to the necessary transmitting level and the optical loss caused by the laser and optical fibers can be also compensated. Hence, the EDFA booster optical amplifier is always deployed when the DWDM Mux Demux attenuates the signal channels.

Placed at the Intermediate Points: as shown in the figure below, the EDFA in-line amplifier can be put at any intermediate point along the long transmission link. This kind of EDFA optical amplifier is designed with low input power, high output power, high optical gain and low noise figure, which are normally deployed every 80-100 km to amplify signals between any two link nodes on the main optical link, with the aim of compensating the loss caused by fiber transmission and other factors. Thereby, the optical signal level can stay above the noise floor.

Placed at the Receiving Side: EDFA optical amplifier operates at the receiving side of the link is also referred to as pre-amplifier, which has the features of medium to low input power, medium output power and medium gain. This optical pre-amplifier put before the receiver end of the DWDM link is to compensate for losses generated by the demultiplexer located near the optical receiver. With the use of pre-amplifier, the optical signal level can be enhanced before the photo detection, hence improving the receive sensitivity for a long-haul fiber DWDM link.

Conclusion

In conclusion, the EDFA optical amplifier can be deployed as booster optical amplifier in the transmitting side of the DWDM link to provide high input signal power for the wide fiber span. It can also work as in-line amplifier at the intermediate point along the link for compensating the fiber loss in the transmission link. What’s more, as the pre-amplifier deployed in the receiving end, it amplifies the signal power to the the necessary receiver level. No matter where the EDFA optical amplifier is deployed in the DWDM link, the signal power can be always enhanced for making a longer DWDM system.

How to Build an Economical Long-haul 500Gbps Metro Network?

Through years of persistent endeavor, experts have already published 40Gbps and 100Gbps solutions to address the need for higher bandwidth, which have been gradually used for bandwidth-hungry applications. In spite of this, the trend for higher bandwidth seems never ending and now 500Gbps solution for Metro network has been also put forward for thousands of kilometers transmission, in order to meet the increasing network requirements. Considering that the deployment cost for 500Gbps Metro network is very high, the following will offer a cost effective 500Gbps solution that utilizes singlemode fiber patch cable, DWDM Mux and Demux and optical amplifier to build the long-haul 500Gbps Metro network.

Singlemode Fiber Patch Cable for 500Gbps Metro Network

As we know, many fiber patch cables are required in the long point-to-point connections, which would cost high. Hence, in order to build an economical long-haul 500Gbps Metro network, it is necessary to take the fiber patch cable cost into account. Which kind of fiber should be used for the long-haul 500Gbps network? Can it save the cost? Is there any solution to save fibers? The following text will seek the answers.

Singlemode fiber patch cable is undoubtedly the best choice for long-haul 500Gbps Metro network. But its cost would takes a certain percentage in the whole network budget. How to solve it? Under this case, using DWDM technology to deploy the 500Gbps network is highly recommendable that requires only one singlemode fiber to transmit multiple signals, allowing for a big cost saving. When it comes to DWDM network, the DWDM Mux and Demux can’t be ignored. It is the most indispensable optical component to multiplex and demultiplex signals with different wavelengths, which make the 500Gbps transmission over one singlemode fiber possible. In order to save fiber cost, let’s learn how to deploy 500Gbps DWDM network over one singlemode fiber patch cable, instead of point-to-point fiber connections.

DWDM Mux and Demux for 500Gbps Metro Network

How to use the DWDM Mux and Demux to deploy a cost effective 500Gbps network? Firstly, we should choose two 40 channels DWDM Mux and Demux and insert the 10G DWDM SFP+ transceivers into the 40 channels. Hence, a total 400Gbps load is achieved. Secondly, utilizing the extra 1310nm or 1550nm port on the DWDM Mux and Demux with the 100G QSFP28 LR4/ER4 transceiver to support a total 100Gbps transmission. Hence, the whole transport 500Gbps (400G + 100G) can be finished. If you only need a 440Gbps network, you can just insert the 40G QSFP+ LR4/ER4 transceiver into the extra port. What should be noted is the wavelength of the 100G or 40G transceiver should be the same to that of the extra port. To better know how does the 500Gbps network work, here offers a figure that illustrated the network design to run such huge network load over a single fiber.

Optical Amplifier for Long 500Gbps Metro Network

Although the singlemode fiber patch cable and DWDM Mux and Demux can address the 500Gbps issue, we still need to use optical amplifiers including semiconductor optical amplifier (SOA) and erbium-doped fiber amplifier (EDFA) to enhance the signal power when the 500Gbps transmission distance is quite long. As shown in the following figure, the semiconductor optical amplifier is deployed in the extra link for boosting the 100G signals, which can extend the transmission up to 60 km. While the erbium-doped fiber amplifier should be deployed in the DWDM transmission link to enhance the 40 10G signals, enabling hundreds of kilometers transmission. Hence, the long 500Gbps Metro network can be finally built.

Conclusion

Due to the high cost for upgrading the current network equipment to higher data rate, it would be be more cost effective to build a long-haul 500Gbps DWDM Metro network with the use of DWDM Mux and Demux and optical amplifier (semiconductor optical amplifier and erbium-doped fiber amplifier). The DWDM Mux and Demux makes the 500Gbps network load possible through a single fiber, while the optical amplifier allows the 500Gbps signals to be transmitted longer. These two DWDM optical components are very critical to deploy the economical long-haul 500Gbps Metro network.

Why Not Use EDFA Amplifier in Long CATV Network?

When deploying a long-distance CATV network, we always worry about the link performance due to the fiber attenuation that may make the CATV signals weaker and weaker as the distance of the CATV link increases. How to deal with it? Here offers the EDFA amplifier as an ideal solution that utilizes both laser and fiber technology to amplify signals. By using the EDFA amplifier in CATV application, the low CATV signal power can be enhanced into CATV high signal power for a long-distance CATV network link. To better know the EDFA amplifier function for CATV network, let’s study the basic knowledge of EDFA amplifier and analyze a typical CATV network using EDFA amplifier.

EDFA Amplifier Overview

EDFA is an fiber optic amplifier cleverly bonding laser technology and fiber technology to enhance the signal power, which can be used for extending the CATV network. Hence, EDFA used in CATV application can be also called CATV EDFA amplifier. Basically, the CATV EDFA amplifier has two kinds of configurations, co-propagating and counter-propagating configurations, designed with different working principles.

As for the EDFA amplifier with co-propagating configuration, you can learn it from the following figure. When the optical signal with wavelength around 1550 nm comes into the EDFA amplifier, it will firstly combine with the optical pump offered by the EDFA amplifier. Then the optical signal and pump will pass through a WDM coupler, be multiplexed into the erbium-doped fiber. Thirdly, the optical signal will be amplified and the residual pump will be removed from the fiber by the second WDM coupler. An in-line optical filter to prevent the pump light from outputting with the amplified signals and an optical isolator to prevent the reflected light from entering the amplifier will be placed after the second WDM coupler. Thereby, the signal amplification can be finally finished.

As for the EDFA amplifier with counter-propagating configuration, the optical pump would firstly pass through the second WDM coupler, then multiplex with the optical signals into the erbium-doped fiber, and finally be removed from the fiber by the first WDM coupler. Compared to the co-propagating one, it will produce more noise but higher output power. Hence, combining the co- and counter-propagating amplification with bi-directional configurations to compromise is highly recommended. For more EDFA amplifier information, you can learn from EDFA wiki.

What Can CATV Network Benefit from EDFA Amplifier?

Compared to the traditional PDFA amplifier, EDFA amplifier performs better in CATV network due to its superior characteristics like low noise figure and low loss. As for the detailed characteristics of CATV EDFA amplifier, we can learn them as below:

• It has a relatively low noise figure and high stability.

• Its RS232 interface is very user friendly which is easy to control and monitor.

• It works at 1550nm, consistent with C-band. Thereby, the fiber loss can be highly reduced, which enables a longer CATV network.

• It features a wide signal gain spectrum, up to 30 nm or more. Hence, it is a good choice for broadband signal amplification, especially in WDM network.

• It designed with higher saturation output power for better performance in CATV network longer than 100 km. Besides, this feature also benefits complicated network that requires splitting optical signals into multiple fiber optic receivers.

Analysis of a Typical CATV Network with EDFA Amplifier

Here offers a figure about a basic CATV network that uses the CATV EDFA amplifier to enhance the signals with 1550 nm for a longer transmission distance. We can see at the Local CATV Provider side, multiple signals are sent and then processed and combined into an integrated CATV signal with 1550 nm. Then CATV EDFA amplifier are used for amplifying the signals, thereby the transmission distance can be extended from 50 km to 150 km. With the use of CATV EDFA amplifier, these signals can be amplified, transmitted longer and finally split into multiple signals with 1550 nm again to server the hotel rooms.

Conclusion

EDFA amplifier can serve CATV applications well because of its high output power, but low distortion and noise capability. It is an idea solution to enhance signals for extending CATV network transmission distance, especially in the long CATV network, more than 100 km. It is also useful in the CATV network where the optical signals should be finally split into multiple fiber optic receivers.