Getting More Benefits From Sensing Fiber Optic Cables

Sensing fiber optic cable is a type of optical cable that can monitor temperature, strain, acoustics and pressure. They are suitable for a wide range of applications like fire detection, vibration monitoring, industrial plant temperature sensing, temperature gradients in soil, pipeline leak and intrusion detection. Although these cables are built based on standard fiber optic cable which only operates to 85℃, the highest working temperature of them is up to 700℃. Before knowing what benefits that sensing fiber optic cable could bring to our life, let’s figure out the common types of sensing optical cable at first.

PBT Tube Temperature Sensing Optical Cable

The major difference of this optical cable is that the bare fiber inside is protected by a PBT tube. Except that, it is comprised of fiber, oil, aramid yarn and a sheath which can be made of different materials such as PVC, LSZH, PU and PE. With the unique structure, this sensing fiber optic cable is suitable for applications in subway systems, tunnels and fire protection industries to sense temperature and pressure, helping to prevent disasters and accidents.

PBT tube temperature sensing fiber optic cable

Armored Temperature Detecting Sensor Cable

Armored fiber optic cable is very common in optical communication. However, this kind of armored sensing cable is a little different. They are strengthened by both SUS spring tube and SUS braiding, which bring them very good mechanical performance of tensile resistance and pressure resistance. Therefore, these cables are widely applied to fire detecting, building health detecting and temperature detecting.

Teflon Sheathed Sensor Cable

Teflon, or PTFE (Poly tetra fluoroethylene) is used in a wide variety of high temperature applications like gas turbine and high voltage gas ignition wires due to its higher melting point. Besides, the aramid yarn and stainless steel braiding ensure good crush-resistant performance and the tensile strength of the cable. With all these characteristics, Teflon sheathed sensor cable is a good choice for high temperature resistant environment and fiber temperature sensing systems.

Teflon sheathed sensing fiber optic cable

Seamless Tube Temperature Sensing Cable

This armored fiber optic cable consists of bare fiber, oil, seamless stainless steel tube and an outer jacket. This structure brings a high tensile strength, crush resistance, a compact size and an unmatched steel performance to this fiber cable. And it features a simple structure and small size, but providing prominent transmission performance. With this sensor cable, some accidents that frequently occur in places such as tanks and mines can be avoided.

Copper Braid Armored Sensor Cable

As its name shows, there is a copper braiding around the outer jacket of this sensing cable. Copper braid is made from weaving together strands of copper wire. The flexibility of the braid is determined by the diameter of the copper wire. The smaller the wire, the greater the flexibility. And the larger the volume occupied by the braid, the more expensive the braid becomes. Apart from the copper braiding, there are stainless steel flexible tube and stainless steel braiding outside the bare fiber, which make the cable more suitable for outdoor optical fiber communication and optical fiber sensor. With the copper braiding structure, the special cable can prevent the external electromagnetic field on internal invasion, reducing its optical transmission loss.

Copper braid armored sensing fiber optic cable

Silica Gel Sensing Optical Cable

Unlike copper braid armored sensor cable, the silica gel sensing optical cable has a very simple structure, including bare fiber, Teflon tube, armored yarn and silicone jacket. As have mentioned above, Teflon is a high performance alternative insulation. With the silica gel that is another kind of good insulation material, both of them make this sensing fiber optic cable an ideal solution for high temperature resistant and high voltage environment. It can work normally even in the 250℃ high temperature environment or 6kv high voltage environment without affecting optical signals’ transmission.


The deployment of sensing fiber optic cables brings us lots of benefits in various applications. They can be applied in downhole to monitor temperatures either as DTS or datacom links to sensors; they can be deployed in power distribution networks to monitor performance of power cable systems; they can be attached to pipelines for temperature data that’s available on demand. This post introduces six kinds of sensing fiber optic cable. All of them are available on FS.COM. Welcome to contact us via

Optical Switches Overview

An optical switch is a device that can selectively switch light signals that run through in optical fibers or integrated optical circuits from one circuit to another. That is to say, optical switches can transfer light signals between different channels in communication networks. As the growing popularity of Internet and telephone, greater quantities of data managed by communication networks also expanded. Optical switching technology provides a perfect solution to fully exploit capacity of optical systems. The main focus of this post is to introduce basics of optical switches in optical communication.


Working Principles & Functions of Optical Switches

As we all know, when a light signal runs through from one computer to another in fiber optic networks, it may be required to move the signal between different fiber paths. To accomplish this, a switch is required to transfer the signal with a minimum loss. Optical switch is a technology needed. The optical switch we often see is operated by mechanical method which just moves fiber or other bulk optic components. But they can offer unprecedented high stability and unmatched low cost performance.

Optical switches are mainly deployed in establishing the light path. They feature scalability and highly reliable switching capacity. Following are the major functions that optical switches bear in optical cross networks.

  • Protection. Sometimes a failure of some single point can cause the whole network breaking down. And the protection switching is to protect the transmission data, which can avoid network fault before finding the failure causes.
  • Optical add/drop multiplexing. Optical switches must be equipped with the capability that can add or delete the wave channels without any electronic processing. This kind of optical switches is also called wavelength selective switches.
  • Optical spectral monitoring. Optical spectral monitoring is a network management operations. In this process, operators receive a small portion of optically tapped signal for monitoring power level, wavelength accuracy and optical cross talk.


Common Types of Optical Switches

As data requirements grow, the traditional electrical switches no longer meet people’s demand. There are two major types of optical switches on the market: opto-mechanical optical switches and MEMS (Micro-electromechanical Systems) optical switches.

Opto-Mechanical Optical Switches

Opto-mechanical optical switch is an old type of switches but the most widely used one. It can produce different optical path selections out of a plurality of optical path sections that are oriented in different spatial directions. Hence opto-mechanical optical switches can be used in multi-channel optical power monitoring, optical local area networks, switching multiple laser sources or optical receivers in Ethernet networks. They are also very useful in optical fiber, components or systems testing and measurement, as well as applications in multi-point fiber sensor systems. Generally, according to the number of redirecting signals, opto-mechanical optical switches have different configurations such as 1×1, 1×2, 1×4, 1×16, etc. In simple terms, the 1×8 opto-mechanical optical switch module connects optical channels by redirecting an 1 incoming optical signal into a selected signal from 8 output fibers. This kind of optical switches can achieve excellent reliability, insertion loss, and cross talk.


MEMS Optical Switches

MEMS optical switches use a micro-mirror to reflect a light beam. And the direction that the light beam is reflected can be changed by adjusting the angle of the mirror, which allows the input light to be connected to any out port. It is a compact optical switch which connects optical channels by redirecting incoming optical signals into the selected output fibers. And the switching state is highly stable against environmental variations of temperature and vibration due to its unique design. In some degree MEMS optical switch can be considered as a subcategory of opto-mechanical switches. But it is distinguished from opto-mechanical switches in many aspects such as the characteristics, performance and reliability. The most obvious is the opto-mechanical switch has more bulk compared to other alternatives, but the MEMS switch overcomes this. Besides, MEMS optical switches also have different configurations such as 1×8, 1×12, 1×16, etc.



As the increasing growth of high speed transmission demand for networks, optical networks have become the most cost-effective solution. Optical switches play a vital role in today’s optical network system. They can offer users significant power, space and cost savings. Now different optical switches are available on the market, so you can choose a suitable one based on your requirements.

Introduction to Automatically Switched Optical Network (ASON)

Optical backbone networks which based on SDH/SONET and WDM technologies are designed mainly for voice applications. However, it gradually fails to satisfy current needs triggered by rapid growth of data traffic. Thus, resources available to users often cannot be allocated properly because of the inherent inflexibility of manually provisioned large-scale optical networks. While with the advances in optical component technology, a significant amount of attentions are attached to the emerging technology of Automatically Switched Optical Networks (ASON), which enables more dynamic and diversified networking that may change the network characteristics dramatically.

Definition of ASON

As its name indicates, an Automatically Switched Optical Network (ASON) is an “intelligent” optical network that can automatically manage the signaling and routing through the network. However, in traditional network backbone, it was rather necessary to configure cross-connections in the network elements, an optical switch for example, to create a new traffic path for a customer. ASON is an optical transport network with dynamic connection capability, and this capability is achieved by using a control plane that performs the call and connection control functions. ASON aims to automate the resource and connection management within the network.

ASON uses the Generalized MPLS (GMPLS) signaling protocol to set up and monitor edge-to-edge transport connections. Switching technologies used in ASON range from single fiber switching to wavelength switching and to optical packet switching. And the components required for the switching are optical cross connects (OXCs), wavelength converters and optical add/drop multiplexers (OADMs).

Importance of ASON in Optical Network

In an optical network which is not based on ASON technology, when it comes to the need for more bandwidth, a new connection may be required. Thus the service provider must then manually plan and configure the route in the network, which is proved to be time-consuming. Moreover, it would waste a lot of bandwidth thus to cause inevitable problems to the whole network since bandwidth is increasingly becoming a precious resource. And the optical networks in the near future bear expectations to efficiently handle resources. ASON can fulfill some of these requirements for optical networks, which are listed below:

  • Fast and automatic end-to-end provisioning
  • Fast and efficient re-routing
  • Support of different clients, but optimized for IP
  • Dynamic set up of connections
  • Support of Optical Virtual Private Networks (OVPNs)
  • Support of different levels of quality of service

What should be noticed is that these requirements are not restricted to optical networks but can be applied to any transport network.

Architecture of an ASON

The layered transport plane, also referred to as data plane, represents the functional resources of the network which conveys user information between location. Transfer of information are either bi-directional or unidirectional. The transport plane can also provide transfer of some control and network management information.

Basically, the logical architecture of an ASON can be divided into 3 planes: transport plane, control plane and management plane.

The transport plane contains a number of switches, and these switches can either be optical switch or other types. Which are responsible for transporting user data via connections. These switches are connected to each other via physical interface (PI).

The control plane is responsible for the actual resource and connection management within an ASN network. It consists of a series of optical connection controllers (OCC), interconnected via network to network interface (NNIs). These OCCs have the following functions:

  • Network topology discovery (resource discovery)
  • Signaling, routing, address assignment
  • Connection set-up/tear-down
  • Connection protection/restoration
  • Traffic engineering
  • Wavelength assignment

The management plane, on the other hand, is responsible for managing the control plane. Its responsibilities include configuration management of the control plane resources, routing areas, transport resource in control plane and policy. It also provides fault management, performance management, accounting and security management functions. The management plane contains the network management entity which is connected to an OCC in control plane via the network management interface for ASON control plane (NMI-A) and to one of the switched via network management interface for the transport network (NMI-T).

ASON architecture


To sum it up, ASON can help to meet user requirements on a more realistic economical basis without resource consuming over-provisioning. Moreover, it also contributes to offering a good platform to realize a more cost-effective networking environment. I hope what presented above would help you to have a better understanding of ASON.

Related Article: A Comparison between Tee Coupler and Star Coupler

Fiberstore Passive Optical Components Solution

Passive optical components market is propelled by the accelerating bandwidth requirements coupled with the growth of passive optical network (PON). Usage of passive optical components to obtain energy efficient network solutions is gaining popularity. This article will introduce some Fiberstore passive optical components.

Optical Attenuators: an optical attenuator is a device that is used to reduce the power level of an optical signal. Optical attenuators are commonly used in fiber optic communications, either to test power level margins by temporarily adding a calibrated amount of signal loss, or installed permanently to properly match transmitter and receiver levels.

optical attenuators

Optical Circulator: an optical circulator is a multi-port (minimum three ports) non-reciprocal passive component. The function of an optical circulator is similar to that of a microwave circulator — to transmit a light wave from one port to the next sequential port with a maximum intensity, but at the same time to block any light transmission from one port to the previous port.

optical circulator

Fiber Collimator: a fiber collimator is a device for collimating the light coming from a fiber, or for launching collimated light into the fiber. It is used to expand and collimate the output light at the fiber end, or to couple light beams between two fibers. Both single-mode fiber collimators and multimode fiber collimators are available.

fiber collimator

Optical Isolator: an optical isolator is a passive optical component that allows light to propagate in only one direction. Optical isolators are typically used to protect light sources from back reflections or signals that can cause instabilities and damage. The operation of optical isolators depends on the Faraday effect, which is used in the main component, the Faraday rotator.

optical isolator

Fiber Optic Sensor: a fiber optic sensor is a sensor that uses optical fiber either as the sensing element (intrinsic sensors), or as a means of relaying signals from a remote sensor to the electronics that process the signals (extrinsic sensors). Fiber optic sensors are immune to electromagnetic interference, and do not conduct electricity so they can be used in places where there is high voltage electricity or flammable material such as jet fuel.

fiber optic sensor

Pump Combiner: a pump combiner is a passive optical component built based on fused biconical taper (FBT) technique. Pump combiners are widely used in fiber laser, fiber amplifier, high power EDFA, biomedical and sensor system etc. Three types of pump combiners are available: Nx1 Multimode Pump Combiner, (N+1)x1 Multimode Pump and Signal Combiner, PM(N+1)x1 PM Pump and Signal Combiner.

pump combiner

Polarization Components: polarization is the state of the e-vector orientation. Polarization components are used to isolate and transmit a single state of polarized light while absorbing, reflecting, and deviating light with the orthogonal state of polarization. Polarization components can be utilized in high power optical amplifiers and optical transmission system, test and measurement.

polarization components

Fiberstore has all of the above passive optical components with high quality and reasonable price. You can select excellent passive optical components or other optical products for your network at

Passive Optical Network – a Superior Network Solution

With the explosive growth of Internet, the introduction of a broadband access network based on fiber-to-the-office (FTTO) and fiber-to-the-home (FTTH) has been triggered. Under this circumstance, access and metro networks should be scalable in terms of capacity and accommodation as well as flexible with regard to physical topology. Passive optical network (PON), one class of fiber access system, can deal with the various demands.


A passive optical network (PON) is a telecommunication network that uses point-to-multipoint fiber to the end-points in which unpowered optical splitters are used to enable a single optical fiber to serve multiple end-points. It consists of an optical line terminal (OLT) at the service provider’s central office and a number of optical network units (ONUs) or optical network terminals (ONTs), near end users (see the figure below). PON takes advantages of wavelength division multiplexing (WDM) and uses one optical wavelength for upstream traffic while another for downstream traffic on a single-mode fiber. The upstream signals are combined at the splitters by using a multiple access protocol (time division multiple access). The downstream signals are directed to multiple users by using passive optical splitter technology.

passive optical network

Signals in a shared fiber architecture can be split out using two methods. One is active Ethernet (AE), with which the individual signals are split out using electronic equipment near the subscriber. The other one is PON, in which the signals are replicated passively by the splitter. Compared with AE, a network based on a PON system is more superior. The advantages of PON are as below.

PON incurs lower capital expenditures because it has no electronic components in the field. Also PON lowers the operational expenditures as there is no need for the operators to provide and monitor electrical power in the field or maintain backup batteries. Besides, a PON has a higher reliability because in the PON outside plant there are no electronic components which are prone to failure. Additionally, one of the most crucial features of a PON-based access network is its signal rate and format transparency. It is much simpler for a PON to upgrade to higher bit rates. Both AE and PON require upgraded electronics in the central office (CO) and customer premises, but unlike AE, PON does not need to upgrade in the outside plant as the passive splitters are agnostic to PON speed. Lastly, a PON solution has the ability to span long distances without degrading performance. The low-loss characteristics of single-mode fiber enable PON to support a maximum physical reach of 20 kilometers.


There are some applications for which PON is well suited, such as fiber-to-the-home (FTTH) delivery of voice, Internet data, and cable access broadband video. More specifically, PON is used when the applications require anticipated system to upgrade to high-security areas or where the rerouting of cable may be difficult. Or in the cases that installations involving widely dispersed nodes require long runs of fiber. And PON is utilized for the projects where costs, especially initial deployment costs, are a key concern. At the same time, using PON can help user bandwidth to be adequately managed.

By reading the above illustration, have you got a basic understanding about the passive optical network? Fiberstore, a professional manufacturer and supplier in the optical industry, has many high-quality PON products including PON splitters, optical network units and optical line terminal. Choosing a PON product in Fiberstore can help to deploy your network more efficiently.

Differences between CWDM and DWDM

In fiber-optic communications, WDM (wavelength-division multiplexing) is a technology which multiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths (i.e., colors) of laser light. This technique enables bidirectional communications over one strand of fiber as well as multiplication of capacity. Generally, WDM technology is applied to an optical carrier which is typically described by its wavelength.

WDM system uses a multiplexer at the transmitter to join the signals together, and a demultiplexer at the receiver to split the signals apart (see Figure 1). WDM system is very popular in the telecommunication industry because it allows the capacity of the network to be expanded without laying more fiber. By utilizing WDM and optical amplifiers, users can accommodate several generations of technology development in their optical infrastructure without having to overhaul the backbone network. Moreover, the capacity of a given link can be expanded simply by upgrading the multiplexers and demultiplexers at each end.

WDM operating principle

Figure 1

WDM could be divided into CWDM (coarse wavelength division multiplexing) and DWDM (dense wavelength division multiplexing). DWDM and CWDM are based on the same concept of using multiple wavelengths of light on a single fiber but differ in the spacing of the wavelengths, number of channels, and the ability to amplify the multiplexed signals in the optical space. Below part will introduce some differences between CWDM and DWDM system.

Wavelength Spacing

CWDM provides 8 channels with 8 wavelengths (from 1470nm through 1610nm) with a channel spacing of 20nm. While DWDM can accommodate 40, 80 or even 160 wavelengths with narrower wavelength spans which are as small as 0.8nm, 0.4nm or even 0.2nm (see Figure 2).


Figure 2

Transmission Distance

DWDM multiplexing system is capable of having a longer haul transmittal by keeping the wavelengths tightly packed. It can transmit more data over a larger run of cable with less interference than CWDM system. CWDM system cannot transmit data over long distance as the wavelengths are not amplified. Usually, CWDM can transmit data up to 100 miles (160km).

Power Requirements

The power requirements for DWDM are significantly higher. For instance, DWDM lasers are temperature-stabilized with Peltier coolers integrated into their module package. The cooler along with associated monitor and control circuitry consumes around 4W per wavelength. Meanwhile, an uncooled CWDM laser transmitter uses about 0.5W of power.


The DWDM price is typically four or five times higher than that of the CWDM counterparts. The higher cost of DWDM is attributed to the factors related to the lasers. The manufacturing wavelength tolerance of a DWDM laser die compared to a CWDM die is a key factor. Typical wavelength tolerances for DWDM lasers are on the order of ±0.1 nm, while tolerances for CWDM laser die are ±2-3 nm. Lower die yields also drive up the costs of DWDM lasers relative to CWDM lasers. Moreover, packaging DWDM laser die for temperature stabilization with a Peltier cooler and thermister in a butterfly package is more expensive than the uncooled CWDM coaxial laser packing.

To sum up, CWDM and DWDM have different features. Choosing CWDM or DWDM is a difficult decision. We should first understand the differences between them. Fiberstore has various kinds of WDM products, such as 10GBASE DWDM, 40 channel DWDM Mux, CWDM Mux/Demux module and so on. It is an excellent option for choosing CWDM and DWDM equipment.

Learn more details about CWDM and DWDM SFP+ transceivers at Everything You Need to Know Before Buying CWDM and DWDM SFP+ Transceivers

Related Article: The Advantages and Disadvantages of Multimode and Single-mode Fiber

CWDM vs DWDM: What’s the Difference?

Introduction to Fiber Optic Sensor

In recent years, fiber optic sensor has been deployed successfully in the supervision of structures. Because it is immune to electromagnetic interference and can handle extreme conditions, so it is gaining popularity as the sensor of choice for many industries. Fiber optic sensor is a sensing device that converts light rays into electronic signals. It is usually used for measuring physical quantities such as temperature, pressure, strain, voltages and acceleration etc. This blog is to introduce fiber optic sensor’s classification, characteristics and applications.


Fiber optic sensor can be mainly classified by sensing location, operating principle and applications. Depending on location of sensor, there are intrinsic and extrinsic fiber optic sensors. Considering the operating principle and demodulation technique, fiber optic sensors can be further divided into intensity, phase, frequency and polarization sensors. Based on application, fiber optic sensors can be classified in physical, chemical, bio-chemical sensors.

Fiber optic sensor offers unique characteristics that make it very popular and sometimes become the only viable sensing solution. Some inherent characteristics of fiber optic sensor are shown as following:

  • Harsh environment stability to strong electromagnetic interference immunity, high temperature and chemical corrosion, as well as high pressure and high voltage etc.
  • Very small size, passive and low power.
  • Excellent performance such as high sensitivity and wide bandwidth.
  • Long distance operation.
  • High sensitivity.
  • Multiplexed or distributed measurements – which are used to offset their major disadvantages of high cost and end-user unfamiliarity.
Fiber optic sensor has a variety of applications that can be found in equipment from computers to motion detectors. Several applications are specifically shown as following:

  • Mechanical Measurement – such as rotation,acceleration, electric and magnetic field measurement, temperature, pressure, acoustics,vibration, linear and angular position, strain, humidity, viscosity etc.
  • Electrical & Magnetic Measurements
  • Chemical & Biological Sensing
  • Monitoring the physical health of structures in real time.
  • Buildings and Bridges – concrete monitoring during setting, crack monitoring, spatial displacement measurement, neutral axis evolution, long-term deformation monitoring, concrete-steel interaction and post-seismic damage evaluation.
  • Tunnels – multipoint optical extensometers, convergence monitoring, shotcrete vaults evaluation, and joints monitoring damage detection.
  • Dams – foundation monitoring, joint expansion monitoring, spatial displacement measurement, leakage monitoring, and distributed temperature monitoring.
  • Heritage structures – displacement monitoring, crack opening analysis, post-seismic damage evaluation, restoration monitoring, and old-new interaction.
  • Detection of Leakage

By this blog, we have learnt some basic knowledge about fiber optic sensor by its classification, characteristics and applications. However, it is not just enough, more knowledge is waiting for us to learn. For more detailed information about fiber optic sensor, welcome to visit Fiberstore or contact us over

Differences Between CWDM and DWDM

Wavelength division multiplexing (WDM) is a technology or technique modulating numerous data streams, i.e. optical carrier signals of varying wavelengths (colors) of laser light, onto a single optical fiber. The goal of WDM is to have a signal not to interfere with each other. It is usually used to make data transmission more efficiently. It has also been proven more cost effective in many applications, such as WDM network applications, broadband network application and fiber to the home (FTTH) applications and so on. According to channel spacing between neighbored wavelengths, there are two main types of WDM: Coarse WDM (CWDM) and Dense WDM (DWDM). Though both of them belong to WDM technology, they are quite different. We can differentiate them from the definition, data capacity, cable cost and transmission distance.

CWDM is defined by wavelengths and has wide-range channel spacing. DWDM is defined by frequencies and has narrow channel spacing.

  • CWDM is a method of combining multiple signals on laser beams at various wavelengths for transmission along fiber optic cables, such that the number of channels is fewer than in DWDM but more than in standard WDM. “Course” means the channel spacing is 20 nm with a working channel passband of +/-6.5 nm from the wavelengths center. From 1270 nm to 1610 nm, there are 18 individual wavelengths separated by 20nm spacing.
  • DWDM is a technology that puts data from different sources together on an optical fiber, with each signal carried at the same time on its own separate light wavelength. “Dense” refers to the very narrow channel spacing measured in Gigahertz (GHz) as opposed to nanometer (nm). DWDM typically uses channel spacing of 100 GHz with a working channel passband of +/-12.5 GHz from the wavelengths center. It uses 200GHz spacing essentially skipping every other channel in the DWDM grid. And it has also gone one step further using an Optical Interleaver to get down to 50GHz spacing doubling the channels’ capacity from 100GHz spacing.
Data Capacity

In fiber optic network system, DWDM system could fit more than 40 different data streams in the same amount of fiber used for two data streams in a CWDM system. In some cases, CWDM system can perform many of the same tasks compared to DWDM. Despite the lower transmission of data through a CWDM system, these are still viable options for fiber optic data transmission.

Cable Cost

CWDM system carries less data, but the cabling used to run them is less expensive and less complex. A DWDM system has much denser cabling and can carry a significantly larger amount of data, but it can be cost prohibitive, especially where there is necessary to have a large amount of cabling in an application.

Transmission Distance

DWDM system is used for a longer-haul transmission through keeping the wavelengths tightly packed. It can transmit more data over a significantly larger run of cable with less interference. However, CWDM system cannot travel long distances because the wavelengths are not amplified, and therefore CWDM is limited in its functionality over longer distances. If we need to transmit the data over a very long range, DWDM system solution may be the best choice in terms of functionality of the data transmission as well as the lessened interference over the longer distances that the wavelengths must travel. As far as cost is concerned, when required to provide signal amplification about 100 miles (160 km), CWDM system is the best solution for short runs.

Polarization Dependent Isolator VS. Polarization Independent Isolator

Connectors and other types of optical devices on the output of the transmitter may cause reflection, absorption, or scattering of the optical signal. These effects on the light beam may cause light energy to be reflected back at the source and interfere with source operation. In order to reduce the effects of the interference, an optical isolator is usually used. Optical isolator allows a beam of light to stream through a single one way direction. At the same time, it prevents the light from going back in the opposite direction. According to the polarization characteristics, optical isolators can be divided into two types, including polarization dependent isolator and polarization independent isolator. The polarizer-based module makes a polarization dependent isolator, and the birefringent crystal-based structure makes a polarization independent isolator. You may be very confused about them as you find that there is only a little difference via their names. So, what are they and what are the differences between them? This paper will give you the answer.

Polarization Dependent Isolator

The polarization dependent isolator consists of three parts, an input polarizer , a Faraday rotator, and an output polarizer. Light traveling in the forward direction becomes polarized vertically by the input polarizer. The Faraday rotator will rotate the polarization by 45°. The analyser then enables the light to be transmitted through the isolator. Light traveling in the backward direction becomes polarized at 45° by the analyser. The Faraday rotator will again rotate the polarization by 45°. This means the light is polarized horizontally. Since the polarizer is vertically aligned, the light will be extinguished.

principle of polarization dependent isolator

The picture shows us a Faraday rotator with an input polarizer, and an output analyser. For a polarization dependent isolator, the angle between the polarizer and the analyser, is set to 45°. The Faraday rotator is chosen to give a 45° rotation. Because the polarization of the source is typically maintained by the system, polarization dependent isolator is widely used in free space optical systems.

Polarization Independent Isolator

The polarization independent isolator also consists of three parts, an input birefringent wedge, a Faraday rotator, and an output birefringent wedge. Light traveling in the forward direction is split by the input birefringent wedge into its vertical (0°) and horizontal (90°) components, called the ordinary ray (o-ray) and the extraordinary ray (e-ray) respectively. The Faraday rotator rotates both the o-ray and e-ray by 45°. This means the o-ray is now at 45°, and the e-ray is at −45°. The output birefringent wedge then recombines the two components.

principle of polarization independent isolator

Light traveling in the backward direction is separated into the o-ray at 45, and the e-ray at −45° by the birefringent wedge. The Faraday Rotator again rotates both the rays by 45°. Now the o-ray is at 90°, and the e-ray is at 0°. Instead of being focused by the second birefringent wedge, the rays diverge. The picture shows the propagation of light through a polarization independent isolator. While polarization dependent isolator allows only the light polarized in a specific direction, polarization independent isolator transmit all polarized light. So it is usually widely used in optical fiber amplifier.

Comparison of Polarization Dependent Isolator and Polarization Independent Isolator

In fact, you have already understood these two types of isolators according to the contents above. We can see their similarities and differences through the comparison of their definition, working principle and applications. Both of them consist of three parts and have a same principle based on Faraday effect. However, to overcome the limitation of polarization dependent isolator, polarization independent isolator has been developed. Regardless of the polarization state of the input beam, the beam will propagate through the isolator to the output fiber and the reflected beam will be isolated from the optical source. If the extinction ratio is important, a polarization dependent isolator should be used with either polarization maintaining fibers or even regular single-mode fibers. If the system has no polarization dependence, a polarization independent isolator will be the obvious choice.

Introduction of the Transients in Optical WDM Networks

A systems analysis continues to be completed to consider dynamical transient effects in the physical layer of an Optical WDM Network. The physical layer dynamics include effects on different time scales. Dynamics from the transmission signal impulses possess a scale of picoseconds. The timing recovery loops in the receivers be employed in the nanoseconds time scale. Optical packet switching in the future networks will have microsecond time scale. Growth and development of such optical networks is yet continuing. Most of the advanced development work in optical WDM networks is presently focused on circuit switching networks, where lightpath change events (for example wavelength add/drop or cross-connect configuration changes) happen on the time scale of seconds.

It is focused on the dynamics from the average transmission power associated with the gain dynamics in Optical Line Amplifiers (OLA). These dynamics may be triggered by the circuit switching events and have millisecond time scale primarily defined by the Amplified Spontaneous Emission (ASE) kinetics in Erbium-Doped Fiber Amplifiers (EDFAs). The transmission power dynamics will also be influenced by other active components of optical network, for example automatically tunable Optical Attenuators, spectral power equalizers, or other light processing components. When it comes to these dynamics, a typical power of the lightpath transmission signal is recognized as. High bandwidth modulation from the signal, which actually consists of separate information carrying pulses, is mostly ignored.

14-nodes Ring WDMRing WDM networks implementing communication between two fixed points are very well established technology, in particular, for carrying SONET over the WDM. Such simple networks with fixed WDM lighpaths happen to be analyzed in many detail. Fairly detailed first principle models for transmission power dynamics exist for such networks. These models are implemented in industrial software allowing engineering design calculations and dynamical simulation of these networks. Such models could possibly have very high fidelity, but their setup, tuning (model parameter identification) and exhaustive simulations covering a variety of transmission regimes are potentially very labor intensive. Adding description of new network components to such model could need a major effort.




14-nodes Mesh WDMThe problems with detailed first principle models is going to be greatly exacerbated for future Mesh WDM networks. The near future core optical networks will be transparent to wavelength signals on a physical layer. In such network, each wavelength signal travels through the optical core between electronic IP routers around the optical network edge using the information contents unchanged. The signal power is attenuated in the passive network elements and boosted by the optical amplifiers. The lightpaths is going to be dynamically provisioned by Optical Cross-Connects (OXCs), routers, or switches independently on the underlying protocol for data transmission. Such network is basically a circuit switched network. It might experience complex transient processes of the average transmission power for every wavelength signal at the event of the lightpath add, drop, or re-routing. A mix of the signal propagation delay and channel cross-coupling might result in the transmission power disturbances propagating across the network in closed loops and causing stamina oscillations. Such oscillations were observed experimentally. Additionally, the transmission power and amplifier gain transients could be excited by changes in the average signal power because of the network traffic burstliness. If for some period of time the wavelength channel bandwidth is not fully utilized, this could result in a loss of the average power (average temporal density of the transmitted information pulses).

First circuit switched optical networks are already being designed and deployed. Fraxel treatments develops rapidly for metro area and long term networks. Engineering design of circuit switched networks is complicated because performance has to be guaranteed for all possible combinations of the lightpaths. Further, as such networks develop and grow, they potentially need to combine heterogenous equipment from a variety of vendors. A system integrator (e.g., Fiberstore) of such network might be different from subsystems or component manufacturer. This creates a necessity of developing adequate means of transmission power dynamics calculations which are suitable for the circuit switched network business. Ideally, these methods should be modular, independent on the network complexity, and use specifications on the component/subsystem level.

Fiberstore has technical approach to systems analysis that’s to linearize the nonlinear system around a fixed regime, describe the nonlinearity like a model uncertainty, and apply robust analysis that guarantees stability and gratifaction conditions within the presence of the uncertainty. For a user of the approach, there is no need to understand the derivation and system analysis technicalities. The obtained results are very simple and relate performance to basic specifications of the network components. These specifications are somewhat not the same as those widely used in the industry, but could be defined from simple experimentation using the components and subsystems. The obtained specification requirements may be used in growth and development of optical amplifiers, equalizers, optical attenuators, other transmission signal conditioning devices, OADMs, OXCs, and any other optical network devices and subsystems influencing the transmission power.