Different Ports on WDM Mux/Demux

In the WDM (wavelength-division multiplexing) system, CWDM (coarse wavelength-division multiplexing) and DWDM (dense wavelength-division multiplexing) Mux/Demux (multiplexer/demultiplexer) modules are often deployed to join multiple wavelengths onto a single fiber. Multiplexer is for combining signals together, while demultiplexer is for splitting signals apart. On a WDM Mux/Demux, there are many kinds of ports for different applications. This article will discuss the functions of these ports on WDM Mux/Demux.

WDM Mux/Demux

Necessary Ports on WDM Mux/Demux

Channel port and line port are the necessary ports to support the basic function of WDM Mux/Demux to join or split signals in the data network.

Channel Port

A WDM Mux/Demux usually has several channel ports on different wavelengths. Each channel port works for a specific wavelength. Since there are 18 wavelengths of CWDM ranging from 1270 nm to 1610 nm with a 20nm interval, the number of channel ports on CWDM Mux/Demux also ranges from 2 to 18. DWDM has a more dense wavelength spacing of 0.8 nm (100 GHz) or 0.4 nm (50 GHz) ranging from S-Band to L-Band around 1490 nm to 1610 nm. The number of DWDM Mux/Demux channel ports is about 4 to 96 for high-density networks.

Line Port

Each WDM Mux/Demux will have a line port connecting to the network backbone. Combined channels are transmitted or received at the line port. In addition, line port can be divided into dual-fiber and single-fiber types. Dual-fiber line port is used for bidirectional transmission, therefore the transmit and receive port in each duplex channel must support the same wavelength. However, single-fiber line port only support one direction data flow, thus the transmit and receive port of duplex channel will support different wavelengths. The wavelengths’ order of single-fiber WDM MUX/DEMUX should be reversed at both side of the network.

Special Ports on WDM Mux/Demux

Apart from the necessary ports, some special ports can also be found on WDM Mux/Demux for particular needs.

1310nm Port and 1550nm Port

1310nm and 1550nm ports are certain wavelength ports. Since a lot of optical transceivers use these two wavelengths for long-haul network, adding these two ports when the device does not include these wavelengths is very important. CWDM Mux/Demux can add either type of wavelength ports, but the wavelengths which are 0 to 40 nm higher or lower than 1310 nm or 1550 nm cannot be added to the device. However, DWDM Mux/Demux can only add 1310nm port.

Expansion Port

Expansion port can be added on both CWDM and DWDM Mux/Demux modules. This is a special port to increase the number of available channels carried in the network. That is to say, when a WDM Mux/Demux can not meet all the wavelength needs, it is necessary to use the expansion port to add different wavelengths by connecting to another WDM Mux/Demux’s line port.

Monitor Port

Monitor port is used for signal monitoring or testing. Network administrators will connect this port to the measurement or monitoring equipment to inspect whether the signal is running normally without interrupting the existing network.

ports on WDM mux demux

Conclusion

From this post, we can know that a WDM Mux/Demux has multiple types of ports. Channel and line ports are integral ports for normal operation of the WDM Mux/Demux. 1310nm port, 1510nm port, expansion port and monitor port are used for special requests of the WDM application. Hence, you should have a thorough consideration of your project before choosing the WDM Mux/Demux module.

Getting to Know Optical Circulator

The utilization of optical circulator starts from the 1990s, and now it has become one of the important elements in advanced optical communication systems. Similar to the function of an electronic circulator, an optical circulator is used to separate optical signals that travel in opposite directions in an optical fiber. Optical circulator has been widely applied to different fields, such as telecom, medical and imaging industries. Are you ready to know more about this optical device? This article will take you to explore the secrets of optical circulator.

optical-circulator

What Is Optical Circulator?

An optical circulator is built to pass light from one optical fiber to another. It is a non-reciprocal device routing the light based upon the direction of light propagation. Both optical circulator and optical isolator can be used to move light forward. However, there is typically more loss of light energy in the optical isolator than in the optical circulator. Optical circulator usually consists three ports: two ports are used as input ports and one port as output port. A signal is transmitted from port 1 to port 2, and another signal is transmitted from port 2 to port 3. Finally a third signal can be transmitted from port 3 to port 1. Many applications only require two, so they can be built to block any light that hits the third port.

optical-circulator-structure

Technologies of Optical Circulator Components

An optical circulator includes the components of Faraday rotator, birefringent crystal, waveplate, and beam displacer. The Faraday rotator uses the Faraday effect, which is a phenomenon that the polarization plane of an electromagnetic (light) wave is rotated in a material under a magnetic field applied parallel to the propagation direction of the lightwave. The light propagation in the birefringent crystal depends on the polarization state of the light beam and the relative orientation of the crystal. The polarization of the beam can be changed or the beam can be split into two beams with orthogonal polarization states. Waveplate and beam displacer are two different forms of birefringent crystal. A waveplate can be made by cutting a birefringent crystal to a particular orientation so that the optic axis of the crystal is in the incident plane and is parallel to the crystal boundary. Beam displacer is used to split an incoming beam into two beams with orthogonal polarization states.

Categories of Optical Circulator

According to polarization, optical circulator can be divided into polarization-dependent optical circulator and polarization-independent optical circulator. The former is used for the light with a particular polarization state, and the latter is not restricted to the polarization state of a light. Most of the optical circulators employed in fiber optic communications are designed to be polarization-independent.

According to functionality, optical circulator can be classified into full circulator and quasi-circulator. As mentioned before, full circulator makes full use of all ports in a complete circle. Light passes through from port 1 to port 2, port 2 to port 3, and port 3 back to port 1. About quasi-circulator, light passes through all ports sequentially but light from the last port is lost and cannot be transmitted back to the first port. For most applications, a quasi-circulator is enough.

Several Applications of Optical Circulator
  • Duplex Transmitter/Receiver System: Optical circulators can be used to enable 2-way transmission along a single fiber. Transmitter 1 sends signal through Port 1 of Circulator 1 and through the fiber to Port 2 of Circulator 2 so that it is directed to Receiver 2. The signal from Transmitter 2 follows the opposite path to Receiver 1.

duplex-transmitte-receiver-system

  • Double Pass Erbium Doped Amplifier: This technique allows high gain amplification of a signal through an erbium doped fiber amplifier. The signal passes through optical circulator and optical amplifier, returns from the fiber optic reflector and passes through the amplifier again. This amplified signal is directed through the return port.

double-pass-erbium-doped-amplifier

  • Wave Division Multiplexing System: Optical circulators in conjunction with Bragg gratings allow specific wavelengths to be reflected and sent down different paths.

wave-division-multiplexing-system

Conclusion

From this article, you may have a basic impression about optical circulator. It is an efficient and economical solution to use optical circulator for directing light signal with minimum loss. If you are interested in the optical circulator products, welcome to visit fs.com for more information.

Technologies Used in Multiplexing

Sending email is a commonplace occurrence in our daily life. When you send an email to a friend in another city, it will firstly join up with other messages being transmitted in your city, and then get dropped off at the correct destination in the correct city. How do all of these messages get to join together and be transmitted without getting mixed up? This process is achieved through the use of multiplmexing technology, which is a method that combines multiple analog message signals or digital data streams into one signal over a shared medium. Actually, multiplexing is widely used in many telecommunications applications. This article will introduce multiplexing technology from the aspect of common technologies used in multiplexing.

Optical multiplexing filter is an essential component in multiplexing technology, which is a physical device that combines each wavelength with other wavelengths (as shown in the following figure). Many technologies are applied in multiplexing, including thin-film filter (TFF), fiber bragg grating (FBG), arrayed waveguide grating (AWG) and interleaver, periodic filter, and frequency slicer.

filter

TFF

Optical TFF typically consists of multiple alternating layers of high- and low-refractive-index material deposited on a glass or polymer substrate. This substrate is made to let only photons of a specific wavelength pass through, while all others are reflected.

FBG

A bragg grating is made of a small section of fiber that has been modified by exposure to ultraviolet radiation to create periodic variations in the refractive index of the fiber. And the process of creating periodic variations will generate wavelength-specific dielectric mirrors. Thus, the FBG can reflect particular wavelengths of light and transmit all others.

AWG

AWG devices can multiplex a large number of wavelengths into a single optical fiber. These devices are designed on the fundamental principle of optics that light waves of different wavelengths interfere linearly with each other. That’ to say, if each channel in an optical communication network makes use of light of a slightly different wavelength, then the light from a large number of these channels can be carried by a single optical fiber.

Interleaver, Periodic filter, and Frequency Slicer

Interleaver, periodic filter and frequency slicer are often used together to perform the function of multiplexing. The following figure shows how interleaver, periodic filter and frequency slicer work together to make a multiplexer device. Periodic filter is in stage 1, which is an AWG. Stage 2 represents the frequency slicer which is another AWG. The interleaver is at the output part, which is provided by six bragg gratings. Six wavelengths (λ) are received at stage 1 which breaks the wavelengths down into odd and even wavelengths. Then the odd and even wavelengths go to stage 2 respectively. Finally, they are delivered by the interleaver in the form of six discrete, interference-free optical channels.

interleaver, periodic filter and frequency slicer

All in all, the usual goal of multiplexing is to enable signals to be transmitted more efficiently over a given communication channel rather than save bandwidth. Nowadays, the most popular multiplexing technology is wavelength division multiplex (WDM), which can be divided into coarse wavelength division multiplexing (CWDM) and dense wavelength division multiplexing (DWDM). It is hoped that multiplexing technology would offer significant gains in bandwidth efficiency.

EPON VS GPON

A passive optical network (PON) is a fiber network that only uses fiber and passive components like splitters and combiners. It starts from the optical line terminal (OLT) in the central office and ends at the optical network unit (ONU) at the customer’s home (as shown in the following figure).

PON

Ethernet passive optical network (EPON) and gigabit passive optical network (GPON) are two popular versions of PONs. The most dramatic distinction between EPON and GPON is a marked difference in architectural approach. EPON employs a single Layer 2 network that uses Internet Protocol (IP) to carry data, voice, and video. While GPON provides three Layer 2 networks: ATM for voice, Ethernet for data and proprietary encapsulation for voice. Moreover, they also vary from each other in terms of bandwidth, per-subscriber cost, efficiency, management system and encryption.

  • Usable Bandwidth

EPON generally delivers 1 Gbit/s symmetrical bandwidth. And its Gigabit Ethernet service actually constitutes 1 Gbit/s of bandwidth for data and 250 Mbit/s of bandwidth for encoding. GPON, however, promises 1.25 Gbit/s or 2.5 Gbit/s downstream and upstream bandwidths scalable from 155 Mbit/s to 2.5 Gbit/s.

  • Per-subscriber Costs

EPON lowers the costs of subscribers by allowing carriers to simplify their networks and to eliminate complex and expensive asynchronous transfer mode (ATM). While the costs of EPON equipment are approximately 10 percent of the costs of GPON equipment.

  • Efficiency

According to the IEEE 802.3 protocol for Ethernet, data transmission occurs in variable-length packets of up to 1518 bytes in EPON. The use of variable-length packets makes Ethernet to carry IP traffic, which significantly reduces the overhead relative to ATM.

In GPON, data transmission occurs in fixed-length 53 byte cells as specified by the ATM protocol. This format makes it inefficient for GPON to carry traffic formatted according to IP, which calls for data to be segmented into variable-length packets of up to 65,535 bytes. This process is time-consuming and complicated.

  • Management systems

EPON requires one single management system, which means EPON results in a significantly lower total cost of ownership. In addition, it does not require multi-protocol conversions, and the result is a lower cost of silicon. In GPON, there are three management systems for the three Layer protocols. Thus it is more expensive. Furthermore, GPON does not support multi-cast services. This makes support for IP video more bandwidth-consuming.

  • Encryption

EPON uses an advanced encryption standard (AES) based mechanism, which is supported by multiple silicon vendors and deployed in the field. Furthermore, EPON encryption is both downstream and upstream. While the encryption in the GPON is part of the International Telecommunication Union (ITU) standard and GPON encryption is downstream only.

All in all, both EPON and GPON have their advantages and disadvantages. It is hard to say which one will be prevailing. But one thing is clear: PON deployment will continue expanding. Fiberstore launches a series of integrated, high reliability and affordable EPON/GPON system solutions for its customers to meet the fast growing demand of PON deployment.

TWDM-PON: Taking Fiber to New Wavelengths

Time and Wavelength Division Multiplexed Passive Optical Network (TWDM-PON) technology is the telecommunications industry’s chosen solution for implementing NG-PON2, the next-generation fiber access technology. For network operators, TWDM-PON opens up new avenues toward increased revenue, reduced cost and lower risk.

From Bandwidth Booster to Value Generator: Fiber’s Changing Role

Operators are using Gigabit Passive Optical Network (GPON) technology to extend fiber access to consumers. GPON will satisfy the bandwidth needs of residential customers for years to come. But to get the most from their fiber networks, operators need to go beyond the capabilities of GPON and find new ways to monetize and maximize their fiber infrastructure investments.

Many operators want to have one flexible network that can support many revenue-generating services, make efficient use of existing assets, and decrease the cost involved in deploying ultra-broadband on a large scale.

TWDM focuses on all facets of this goal. Currently being standardized by the Full Service Access Network (FSAN) and International Telecommunication Union (ITU), TWDM offers an efficient upgrade path for operators that currently invest in GPON infrastructure.

Fiber Evolution Roadmap

FSAN and ITU envisage two phases for fiber network evolution: NG-PON1 (medium term) and NG-PON2 (long term). NG-PON1 is based on XG-PON1 technology. It offers 10Gbps in downstream and 2.5Gbps in upstream. While available for some time, the XG-PON1 technology has generated little traction in the market. There is limited near-or medium-term demand for 10Gbps residential service that will justify investment in new technology. Meanwhile, emerging technologies offer capabilities beyond higher speeds, so the market focus is shifting to NG-PON2.

For NG-PON2, FSAN evaluated several solutions. Three main contenders emerged:

  • TDM evolution is conceptually similar to current PON systems, using much faster electronics and optics. It supports very efficient bandwidth sharing between users. But it requires every ONT to operate at the full 40Gbps line rate, a rate that far exceeds the foreseeable needs of individual users. FSAN dropped TDM-PON from consideration because of high costs and uncertainly associated with some challenges, such as resolving the problem of chromatic dispersion for very high line rates.
  • Dense Wavelength-Division Multiplexing PON (DWDM-PON) supports many wavelengths on one fiber. It offers each user on the PON a dedicated wavelength with 1Gbps symmetrical bandwidth per user (10Gbps in the future). Ultimately, FSAN discounted DWDM-PON for residential market use because of its high cost, its inability to support bandwidth sharing between users, and because of the operational complexity involved in terminating and managing one wavelength per user. However, DWDM-PON has potential for niche applications: a DWDM wavelength can be overlaid on GPON to support applications like mobile fronthaul with a fixed constant bandwidth. The annex to the proposed ITU standard describes this special DWDM use case, which is also called point-to-point WDM.
  • Time Wavelength Division Multiplexing PON (TWDM-PON) provides four or more wavelengths per fiber, each of which is capable of delivering symmetrical or asymmetrical bit rates of 2.5 Gbps or 10 Gbps. In 2012, FSAN named TWDM-PON technology as its solution of choice for implementing the NG-PON2 architecture.
TWDM-PON: the Right Evolution Path

Each of the NG-PON2 candidate technologies delivers more capacity or flexibility than current GPON technology. But these improvements come with additional cost. The challenge is to identify the degree of capacity and flexibility that can deliver the target benefits at the lowest possible cost.

DWDM-PON would provide maximum flexibility, although with limited real additional capacity, if each wavelength were individually routed through a fiber cross-connect panel and terminated on discrete optics with discrete fibers. But this flexibility would require up to twice as much CAPEX as current GPON and significantly higher OPEX, in the form of bigger central offices, additional Optical Distribution Frames (ODFs), more operational complexity and greater outside plant impact. Terminating all the wavelengths on an integrated multi-wavelength optical module would reduce costs but remove flexibility. It is therefore difficult to realize the target benefits with DWDM.

By contrast, TWDM allows for higher bandwidth (up to 10Gbps for any user with a total of 40Gbps) and optimal flexibility relative to bandwidth per user, fiber management, service convergence and resource sharing. These improvements come at 30% less CAPEX and with less operational complexity than DWDM. In effect, TWDM optimally combines the benefits of TDM with the benefits of DWDM systems.

Turn Flexibility into Revenue Growth

The increased bandwidth capacity and flexibility of TWDM-PON opens up a new range of possibilities for fiber networks. For example:

  • TWDM can supply, manage and evolve to the right (or best-fitting) bandwidth for different services—for instance, high-bandwidth symmetrical for business or high-bandwidth asymmetrical for mobile backhaul. It can also support the convergence of more services and users on the same infrastructure. These capabilities will bring CAPEX savings and additional revenue.
  • TWDM-PON adds flexibility by supporting the overlay of multiple services, user groups or organizations on the same fiber. For example, an operator can simplify its operations by using dedicated wavelengths to isolate applications provided by its different business divisions, as shown in the figure below. TWDM-PON allows different wavelengths to be allocated to different servicesPrint.
  • TWDM-PON can accelerate the process of bringing more bandwidth to more users. Increasingly, operators are deploying deep fiber micro-nodes so that they can quickly and cost-efficiently cover more users. These deployments demand high-capacity backhaul. As operators add more micro-nodes, the need for TWDM will surge.
  • TWDM-PON enables operators to add flexibility to the customer experience by letting users share the unused common part of bandwidth capacity. For example, an operator could guarantee that 1 Gbps of bandwidth will always be available to all users. Using TWDM-PON, it could allow users to burst above their assigned bandwidth up to 10Gbps to accelerate actions such as downloading HD movies or backing up data to cloud storage.
  • With TWDM, different wavelengths can be assigned to different operators. This flexibility will facilitate co-investment in common infrastructure, encourage investment and promote sharing of cost and risk.
Get More Value from Existing Fiber

One of the key requirements for NG-PON2 is to preserve operators’ current PON investments and allow them to reuse their most expensive component: the outside plant. TWDM-PON addresses this requirement in three key ways:

  1. GPON compatibility: TWDM-PON can coexist with, and expand on, current GPON deployments. This coexistence ensures that operators’ fiber investments will keep providing value in the long term.
  2. Zero outside plant impact: TWDM-PON typically has no cost or operational impact on existing passive components, including the outside plant. As part of this no-impact approach, TWDM-PON uses the same splitters to simplify fiber management, facilitate tuning and maximize compatibility with network components and end-user equipment.
  3. Easy introduction: TWDM-PON can be introduced smoothly and gradually into existing Fiber To The X (FTTX) deployments. Although integrated multi-wavelength solutions are feasible, some operators may find it easier to start with one wavelength and add more as bandwidth demand grows. Upgrades can be introduced on a fiber-by-fiber basis. They are as simple as adding and configuring new cards at the central office.
Reduce Cost and Risk

TWDM-PON promises to reduce cost and risk by promoting efficient operations. Specifically, it supports wavelength management techniques that can allow operators to move end users from one wavelength to another to rebalance bandwidth, reconnect users to other operators, or decrease downtime during software upgrades. These techniques can help operators forgo expensive truck rolls and in-field fiber reconnections in favor of efficiently reassigning users to specific wavelengths from the central office.

Co-investment offers additional opportunities to reduce cost and risk. TWDM-PON brings successful co-investment within reach by providing an environment in which several operators can effectively share a common fiber infrastructure. With TWDM-PON, individual wavelengths can be associated with and controlled by different operators. Each co-investor benefits from lower capital and maintenance costs, added flexibility to support new services, and new opportunities to establish partnerships and business models.

Open, government-owned fiber networks can also provide opportunities to reduce cost and risk. TWDM-PON permits each operator to use one (or more) wavelengths on the common PON, deployed on either common or separate central office equipment. With their own wavelengths, operators gain the flexibility and independence to offer, manage and upgrade quality of service according to their business priorities.

Technical Enablers and Optimization

Operators need efficient solutions that can help them capitalize on the potential and benefits of TWDM-PON. Areas requiring technical enablers and optimization include wavelength tuning, optical signal amplification and scalability.

Wavelength tuning

To simplify operations, all TWDM ONTs must be colorless. The ONT transceiver must be able to tune to the correct wavelengths in upstream and downstream during end-user provisioning.

Operators can use a variety of techniques to control the wavelength in the upstream direction from the ONT to the OLT. Temperature is the primary parameter of control. For example, thermo-electric controlled lasers can use heating and cooling to accurately set the wavelength. These lasers are available today and can be used in the ONT, but they are expensive.

A heat-only laser could provide a more cost-effective and power-efficient alternative for addressing the needs of the mass market. However, a heat-only approach can only set the laser to wavelengths that correspond to the ambient temperature or above—never below. Therefore, a cyclic Wavelength Multiplexer (WM) must be used at the OLT to route sets of wavelengths, rather than individual wavelengths, to each OLT port. In this way, the laser can be heated above ambient temperature until its wavelength is part of a given wavelength set associated with the destination OLT port. The wavelength sets are analogous to the sets of piano notes with the same name (i.e. “C”, “D”, “E” etc.) and which repeat cyclically at every octave. This results in a simple and low-cost tuning scheme.

In the long-term, it is hoped that photonic integrated circuits will provide other economical solutions to wavelength control.

In the downstream direction, thermo-controlled thin-film filters can be integrated into the ONT receiver to allow tuning. Photonic integrated circuits may provide a future solution.

At the OLT side, thermoelectric-controlled lasers can accurately hold the downstream wavelengths at the correct wavelength. The upstream wavelengths are routed to the correct OLT receiver by way of a WM, using either Thin-Film Filter (TFF) technology or Arrayed Waveguide Gratings (AWGs).

Optical signal amplification

To transmit higher bit rates with a low error rate, it is necessary to improve the sensitivity of the receivers or increase the level of the received signal. Amplification is needed to transmit these higher-rate optical signals through existing fiber networks, which have a fixed attenuation. The need for amplification is even greater given that NG-PON2 is simultaneously targeting an increased split ratio, greater fiber distances and higher speeds.

Some amplification technologies can be used in both the upstream and downstream directions. These include semiconductor optical amplifiers (SOA) and Erbium Doped Fiber Amplifiers (EDFA). With SOAs, the optimal amplifier location is immediately after the laser in the downstream direction and before the wavelength demux in the upstream direction. All of these components are typically located in the central office.

Scaling and evolution

Operators will need to choose the equipment growth and fiber evolution strategy that best addresses their business needs. For many, the key decision will be whether to adopt a pay-as-you-grow strategy with modular optical components or deploy a fully integrated solution in anticipation of future demand. Others will need to decide whether it makes more business and operational sense to go it alone or embrace a co-investment or network sharing strategy.

Most technical issues can be solved with solutions and advice from knowledgeable vendors. Vendors can also help with business issues, but operators will ultimately need to choose the strategies that best align with their business goals.

Unlock network potential with TWDM

Today’s GPON-based networks are bringing fundamental change to many different industry sectors. However, GPON is tapping a fraction of fiber’s potential. New technologies like TWDM will unlock this potential by adding capacity and flexibility. These improvements will help operators generate more revenue by converging multiple services onto one fiber. They will help operators lower cost by increasing operational efficiency and providing access to viable co-investment options. TWDM will also help operators preserve their existing fiber investments: The upgrade path to TWDM uses the same fiber and passive components as current GPON networks and simply overlays new active components.


From Alcatel-Lucent