Author Archives: Priscilla.Luo

Introduction to WDM Transponders

With the development of wavelength-division multiplexing (WDM) technology, the network traffic volume is increasing and the demand for more network bandwidth is also on the rise. By converting the operating wavelength of the incoming bitstream to an ITU-compliant wavelength, WDM transponder serves as a key component in WDM system. As an important technology in the fiber optical network, WDM is moving beyond transport to become the basis of all-optical networking. And how to optimize WDM network has always been a hot topic. The transponder is a device to optimize the performance of WDM network, which plays an important in the whole system of WDM network. This article will introduce you the information on WDM transponders.

What Is a WDM Transponder?

Also called as an OEO (optical-electrical-optical) transponder, a WDM transponder is an optical-electrical-optical wavelength converter, which has been widely adopted in a variety of networks and applications. The picture below shows us how a bidirectional transponder works. In this picture, the transponder is located between a client device and a DWDM system. And we can see clearly that, from left to right, the transponder receives an optical bitstream operating at one particular wavelength (1310 nm), and then converts the operating wavelength of the incoming bitstream to an ITU-compliant wavelength and transmits its output into a DWDM system. On the receive side (right to left), the process is reversed. The transponder receives an ITU-compliant bit stream and converts the signals back to the wavelength used by the client device.

WDM transponder

The Application of a WDM Transponder

According to its function, the application of a WDM transponders can be classified into the following types.

  • Wavelength Conversion. It is known to us that when a CWDM Mux/Demux or DWDM Mux/Demux is added into a WDM network, there is a requirement to convert optical wavelengths like 850nm, 1310nm and 1550nm to CWDM or DWDM wavelengths. Then the OEO transponder comes to assist. The OEO transponder receives, amplifies and re-transmits the signal on a different wavelength without changing the signal content.
  • Fiber Mode Conversion. Multimode fiber optic cables (MMF) are often used in short distance transmission, while single-mode fiber optic cables (SMF) are applied in long optical transmission. Therefore, in some network deployment, considering the transmission distances, MMF to SMF or SMF to MMF conversions are needed. WDM transponders can convert both multimode fiber to single-mode fiber and dual fiber to single fiber.
  • Signal Repeating. In long haul fiber optic transmission, WDM transponder also can work as repeaters to extend network distance by converting wavelengths (1310nm to 1550nm) and amplifying optical power. The OEO converters convert the weak optical signals from the fiber into electrical signals, and regenerate or amplify, then recover them into strong optical signals for continuous transmission.
WDM Transponder and FMT Solution

At FS, OEO transponders are made into small plug-in cards to be used on the FMT platform. FMT platform makes devices like EDFA, OEO, DCM, OLP and VOA into plug-in cards and provides standard rack units as well as free software to achieve better management and monitoring. In addition, FMT series products like OEO, DCM and OLP also have higher performance than that of old ones. FMT series OEO transponder can convert optical signals into DWDM wavelengths, reducing the fault risk caused by high power consumption of DWDM fiber optic transceiver. Since the OEO transponder is made into small plug-in card in the FMT platform, it only occupies one slot in the special designed chassis when installed, thus saving a lot of space. In addition, all these FMT plug-in cards, including OEO, in a rack unit share the same power source and support hot plug & play operation. And they can be inserted or removed flexibly in the racks for DWDM networking.



Since the OEO transponder plays an important role in WDM network, such as receiving, amplifying and re-transmitting the signal on a different wavelength, adding an OEO transponder into the WDM network is very essential. The OEO transponders in our FMT series are made into small plug-in cards with high quality to ensure good transmission performance. For more information on our FMT system, please visit

Brief Introduction to EDFA

In fiber optic communication systems, problems arise from the fact that no fiber material is perfectly transparent. The visible-light or infrared beams carried by a fiber are attenuated as they travel through the material. This necessitates the use of optical amplifiers. And EDFA (Erbium Doped Fiber Amplifier) is a representative one in the optical amplifier. There is one saying that EDFA is the most popular optical amplifier in optical network communications. Next, we will begin with the definition of EDFA.

The Definition of EDFA

An EDFA, also called optical amplifier or an erbium-doped fiber amplifier or erbium amplifier, is an optical or IR (Infrared Radiation) repeater that amplifies a modulated laser beam directly, without opto-electronic and electro-optical conversion. The device uses a short length of optical fiber doped with the rare-earth element erbium. When the signal-carrying laser beams pass through this fiber, external energy is applied, usually at IR wavelengths. This so-called pumping excites the atoms in the erbium-doped section of optical fiber, increasing the intensity of the laser beams passing through. The beams emerging from the EDFA retain all of their original modulation characteristics, but are brighter than the input beams.

Three Major Applications for Optical AmplifierThree Major Applications for Optical Amplifier

The above picture illustrates the three major applications for optical fiber amplifiers: booster, in-line amplifier, and pre-amplifier. These applications are described in more details below:

Booster Amplifier

Booster amplifiers are placed directly after the optical transmitter. In this application, booster amplifier is adopted to compensate for the losses of optical elements between the laser and optical fibers so that the increased transmitter power can be used to go further in the link.

In-line Amplifier

In-line amplifiers or in-line repeaters are placed along the transmission link to compensate for the losses incurred during propagation of optical signal. They take a small input signal and boost it for re-transmission down the fiber. Here it should also be pointed out that to control the signal performance and the noise added by the EDFA is important, because noise added by amplifier will limit the system length.


Pre-amplifiers are placed just before the receiver to increase the signal level before the photodetection takes place in an ultra-long haul system so as to improve receiver sensitivity. By placing a pre-amplifier, a much larger signal can be presented to the receiver, thus easing the demands of the receiver design.

Top EDFA Products Overview

By now, you should have a basic idea of what an EDFA is and what it is used for, next I will introduce you some truly excellent EDFA products on the market.

22dBm Output Booster DWDM EDFA C-band 24dB Gain, 1U Rack Mount
16dBm Output Mid-stage DWDM EDFA C-band 26dB Gain, Plug-in Card for FMT Multi-Service Transport System
17dBm Output Mid-stage DWDM EDFA C-band 17dB Gain, Plug-in Card for FMT Multi-Service Transport System

Of the various technologies available for optical amplifiers, EDFA technology is the most advanced, and consequently the vast majority of optical amplifiers are designed based on this technology. In addition, the combination of reliable performance and relatively low cost allows EDFA to be widely deployed in modern optical networks.

The Rise of White Box Switch

White-box switching is nothing new to us. ODMs (original design manufacturers) have been building hardware for well-known vendors for many years. These vendors take the ODM hardware, install their operating system, and sell the unit as a bundle, often attaching a support contract. Until now many companies like FS are also getting into the game of producing white-box switches. White-box switches look just like any other switch, which are gaining increasing attention in next generation data center deployments, with many software-defined networking (SDN) startups offering solutions that include them. Enterprises are wondering how white-box switches will impact their data center plans. So what is a white box switch?

What Is a White Box Switch?

white box switch

A white box switch is a network switch which comes with an installed operating system. It can be used as a standard for the base of hardware system elements. In the case of operating systems, white box switches are generally preinstalled on the system, or can be installed later. Loading of the white box switch is not difficult and can be done in a short period of time. They are generally used with SDNs and particularly useful in terms of a networking approach where the control is generated from the physical infrastructure after decoupling it. It can act as an efficient open-source tool for management of materials and information on a device.

And the major difference between traditional black box switch and white box switch is that the first one can’t be programmed but the later one can be programmed. With white box switches, a service can be programmed by using switch controller like ONOS while traditional black box switch provides very limited features and every time when you need to update something you have to log into switches and then change the rules. The white box switches are flexible, fast and inexpensive, which is why many opt for this type of switch.

Reasons for Buying White Boxes

Although white box switches have been around for years, the adoption has been limited to niche companies that have large engineering departments. The rise of software-defined networking (SDN) has brought them into the public eye, though, as a lower-cost alternative to traditional network hardware. In fact, some of the early messaging around SDN revolved around using white boxes as a complete replacement for all network hardware. Besides, many improvements have been made in white boxes during the past few years. So if you ask me why it is the time of white box switches and why you should buy white box switches. Here I’ve got a number of reasons for you:

  • 3-year ROI. A low-cost product can get ROI (return on investment) in less time and be replaced sooner. Faster hardware rotation equals more innovation/feature adoption.
  • Software bugs. Vendors take months to locate, accept, and fix bugs, which has enormous impact on your business. With OCP-compliant white boxes, you can switch software and keep your business alive, or work around slow vendor support.
  • Self-sparing. For some/most use cases, self-sparing is better than relying on vendor inventory. When products are cheap, you can hold inventory in your data center and bring MTTR down to hours instead of days.
  • Cost and reliability. What the customer is often paying for is the software that rides on top of the hardware and the logo. From a reliability standpoint, white boxes are on par with brand-name systems because they are actually the same hardware.
  • SDN. Move your operational focus from a vendor-specific CLI to an SDN solution. If you’re concerned about having multiple vendors to operate, then buy a SDN solution that is device independent.
  • Network operations. Many engineers may ask questions like “Do I have to write my own operating system?” “How do I install a network operating system” “What do I buy?” when considering a white box switch. Now, they can be reassured because white boxes can now be purchased from mainstream network vendors such as FS and HP. Also, when one purchases a white box, those suppliers will offer the kind of technical support most engineers need.

White boxes are certainly ready for mainstream adoption. Although they aren’t for every use case, but in the right situation, like an SDN deployment, they can be as good or better than traditional switches with a much lower price point and equivalent operational costs. If you want to purchase one, you may visit FS.COM where you can find the best-value and cost-efficient white box switch.