Tag Archives: passive optical network

What is Fiber Optic Isolator?

Fiber optic isolator is a passive component used for fiber optic communications. As a magneto-optic device, the purpose of optical isolator is to allow light to be transmitted in only one direction. This helps prevent laser source from unwanted feedback which will damage the laser source or arouse unexpected laser problems, such as mode hop, amplitude modulate, frequency shift and so on. Therefore, isolator is an useful and indispensable device to reduce these effects. In the following parts, fiber optic isolator’s construction, operating principle and classifications will be discussed.


Construction of Optical Isolator

Fiber optic isolator includes three main parts of an input polarizer, a Faraday rotator with magnet, and an output polarizer. Only linearly polarized light can pass through the input polarizer into the Faraday rotator. The function of the Faraday rotator is to rotate the input light by a certain angle before it reaches the output polarizer. This allows the light in the forward direction to pass unimpeded. However, the light in the reverse direction will not be able to pass the optical isolator and is either reflected or absorbed. These three components of optical isolator skillfully work together and ensure the normal transmission of light signals.

Operation of Optical Isolator

The operation of optical isolator is based on the Faraday effect which was discovered by Michael Faraday in 1842. Faraday effect refers to a phenomenon that the plane of polarized light rotates while transmitting through glass (or other materials) that is exposed to a magnetic field. The rotation direction depends on the direction of the magnetic field instead of the direction of light transmission.

According to different light directions, there are two types of operation modes. One is the forward mode and the other is the backward mode. The forward mode enables light enter into the input polarizer and become linearly polarized. When laser light reaches the Faraday rotator, the Faraday rotator rod will rotate by 45° polarization. Thus, the light finally leaves the output polarizer at 45° polarization. However in the backward mode, the light first enters into the output polarizer with a 45° polarization. Next, as it passes through the Faraday rotator, it continues to be rotated for anther 45° in the same direction. Then the light of 90° polarization becomes vertical to the input polarizer and can not leave the isolator. As a result, the light will be either reflected or absorbed.


Types of Optical Isolator
1) Polarized Optical Isolator

Polarized optical isolator employs the polarization axis to keep light transmit in one direction. It allows light to propagate forward freely, but disallows any light to travel back. Also, there are dependent and independent polarized optical isolators. The latter is more complicated and often used in EDFA optical amplifier.

2) Composite Optical Isolator

Composite optical isolator is actually a type of independent polarized optical isolator. It is used in EDFA optical amplifier which consists of many other components, such as erbium-doped fiber, wavelength-division multiplexer, pumping diode laser and so on. Since there are many other components in EDFA module, this type of isolator is named as composite optical isolator.

3) Magnetic Optical Isolator

Magnetic optical isolator is essentially the polarized optical isolator in another expression. It stresses the magnetic part of a Faraday rotator. The Faraday rotator is generally a rod made of a magnetic crystal under strong magnetic field with Faraday effect.


In summary, fiber optic isolator guarantees the stable function of laser transmitter and amplifiers by eliminating unnecessary lights. It also ensures a higher performance of light transmission. Using fiber optic isolator is no doubt a good choice for your network.

What is Fiber Optic Attenuator?

In optical data communication, receiving either too much or too little optical power will cause high bit error rates. The receiver amplifier will saturate if power is excessive, or generate noise when interferes with the signal if power is insufficient. In order to solve the problem of too much optical power at the receiver, using a fiber optic attenuator is a good solution.

Fiber optic attenuator or optical attenuator is a passive device used to reduce the power level of an optical signal without appreciably distorting the waveform. To achieve power loss, technologies including air-gap, absorption, scattering, and interference filter are often used for the attenuator products. Fiber optic attenuator can be fixed, manually or electrically adjustable. Furthermore, according to different types of connectors, there are also various classifications of optical attenuators as LC, SC, ST, FC, MU or E2000, etc. This article will introduce some basic working principles and commonly used types of optical attenuators.

Working Principles

With the development of optical technologies, fiber optical attenuator has adopted many principles to help reduce optical power. Here are some of the working principles applied to the fiber optical attenuator:

    • Gap-loss Principle: Gap-loss principle uses an in-line configuration when inserting the optical attenuator in the fiber path to reduce the optical power level. The gap enables light to spread out as soon as it leaves the fiber end from the transmitter. Then some of the light will enter the fiber cladding before it reaches the receiver. However, optical attenuator using gap-loss principle is sensitive to the modal distribution, which means it should be placed near the optical transmitter. Otherwise, the attenuator will be less effective to get enough power loss if being put far away from the transmitter. This kind of problem can be avoided when using the absorptive or reflective principles.

Gap-loss Principle

    • Absorptive Principle: Absorptive principle or absorption reduces the light power by using the material in the optical path to absorb optical energy. This can be realized because optical fiber has the defect of absorbing optical energy and converting it into heat. It is both easy and effective to employ absorptive principle to obtain power loss.

Absorptive Principle

  • Reflective Principle: Reflective principle or scattering causes the signal to scatter which is also a deficiency of optic fiber. The scattered light interferes with fiber to reduce the signal power. Since the material in attenuator is used to reflect a known quantity of the signal, only a desired portion of signal can be transmitted.

Reflective Principle

Common Fiber Optic Attenuator Types
    • Fixed Attenuator: Fixed attenuator is able to deliver a precise power output when the desired level of attenuation is determined. It is usually applied to balance power between fibers and multifiber systems and reduce receiver saturation. Fixed attenuator is typically available in plug and inline styles for single-mode applications. Inline type looks like the ordinary fiber patch cord with the termination of two connectors. Plug type has a bulk head fiber connector with a male end and a female end.


  • Variable Attenuator: Variable attenuator delivers a precise power output at multiple decibel loss levels with flexible adjustment. The attenuation is easily modified to any level by simple adjustment controls. Variable attenuators can be further categorized as stepwise variable attenuator and continuously variable attenuator. The former changes the attenuation of signal in known steps such as 0.1 dB, 0.5 dB or 1 dB for multiple power sources applications. The latter allows attenuation to be changed on demand without any interruption to the circuit in uncontrolled environments where the input or output needs continuous change.



Fiber optic attenuator is an essential component for reducing optical power in data transmission. Signals achieve a more precise power level with the help of optical attenuators. According to different applications, you’d better choose the most appropriate type of optical attenuators for your system. Hope this article can provide some help for your future selection of optical attenuators.

PON – a Better Network Solution

When choosing a best fiber architecture for the network, many planners may choose PON today. PON, short for passive optical networks, 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. “Passive” means optical transmission has no power requirements or active electronic parts when the signal is going through the network. It is the core underpinning of fiber optical service.

Introduction of PON

Passive optical network is widely applied in fiber-to-the-curb (FTTC), fiber-to-the-building (FTTB), or fiber-to-the-home (FTTH), which is decided by the places it terminates. Commonly, it is made up of an optical line terminal (OLT) in the central office and a number of optical network units (ONU) near end users. From the picture below, we can get a brief understanding of the working process of it.

First and foremost, the data or the signals in the central office will be transmitted over a single optical fiber without interfered by each other, because encryption can prevent eavesdropping. And then the passive splitter will separate the signals into several optical network units which can be up to 64 units.


Classifications of PON

The first PON systems which is based on Asynchronous Transfer Mode (ATM, or “cell switching”) were called “APON”. It has been achieved significant commercial deployment and still be applied in someplace today. The “Broadband PON” comes after APON. Typically, these two systems both have downstream capacity of 155 Mbps or 622 Mbps and upstream capacity of 155 Mbps.

And as the technology advanced, there is a growing requirements of higher capacity. As a result, APON and BPON is gradually replaced by GPON. GPON, short for Gigabit-capable passive optical network, is the successor of APON and BPON, and also based on ATM transport. Typically, its capacity of upstream ranges from 622 Mbps to 1.25 Gbps, while the downstream capacity ranges from 622 Mbps to 2.5 Gbps. In today’s fiber-to-the-home (FTTH) networks, GPON is most widely deployed, which is generally considered suitable for consumer broadband services for the next 5 to 10 years.

EPON (Ethernet passive optical network) is the rival of GPON, using Ethernet packets instead of ATM cells. It is cheaper to deploy than GPON, but it has not garnered the level of acceptance of GPON.

And WDM PON (wavelength-division multiplexing passive optical network) is a network that combines WDM technology with PON system. It can use wavelength-division multiplexing to split each signal into different branches.

Advantages of PON

Compared with the traditional enterprise network, PON network is obvious superior to it. And there are several advantages of PON.

Energy savings – PON system does not need rack mount switches and other active devices in remote locations so that it can reduce a number of heat generating devices that must be cooled and powered, thereby generating energy savings. Also, there are reduced HVAC (Heating Ventilation Air Conditioning) requirements, since there is no radiant heat with fiber cabling.

Lower cost – Due to lower power consumption, reduction in floor space, and yearly reduced maintenance costs, the enterprise will realize significant operational expense savings over the life of the system of 45-70% over that of a traditional copper based system, as well.

Optimized bandwidth utilization – with dynamic allocation of bandwidth, the system can provide optimized network connectivity to those application and users requiring the greatest bandwidth, while facilitating future proofing.

Now, the Ethernet market becomes more and more popular, so PON is gradually getting into a bright future. With these significant advantages, PON can meet the changing demands of the enterprise network more quickly and easily. And at present, the most popular network systems are GPON and EPON. Fiberstore offers various PON products, including optical line terminals and optical network units, and if you want to deploy your network most efficiently, Fiberstore is your best choice.

Related article: http://www.chinacablesbuy.com/a-guide-for-pon.html