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Basics of Fiber Optic Attenuator


Basics of Fiber Optic Attenuator

FS Official 2014-04-18

Even though sometimes we need to strengthen the signal power to achieve long-haul data transmission, under certain circumstances, however, too much light can overload fiber optic receiver and degrade BER. This is where fiber optic attenuator come into use – to reduce the power in fiber links. This article sheds light on types, working principles and applications of optical attenuators, helping you to choose a right one.

Fiber optic attenuator is a passive device used to reduce the power level of an optical signal because too much light can overload a fiber optic receiver and degrade the bit error ratio (BER). To achieve the best BER, the light power must be reduced by using fiber optic attenuator. Generally, the optical attenuators are used in single-mode long-haul applications to prevent optical overload at the receiver.

Optical attenuator reduces signal power by absorbing the light, like sunglasses absorb the extra light energy. Or by scattering the light like an air gap. Fiber optic attenuators are commonly used in two scenarios:

 Attenuators are permanently installed in a fiber optic links to properly match signal levels at transmitter and receiver.  In fiber optic power level testing. Attenuators are used to temporarily add a calibrated amount of signal loss in order to test the power level margins in a fiber optic system.

Types of Fiber Optic Attenuators

Optical attenuator takes a number of different forms. They are typically grouped as fixed optical attenuator and optical variable attenuator.

Fixed Optical Attenuator

Fixed attenuator, as the name of which has indicated clearly, is designed to have an unchanging level of attenuation, expressed in dB, such as 1dB, 5dB, 10dB, etc. Fixed value attenuators consist of in-line type and connector type. In-line type looks like a plain fiber patch cable. It has a fiber cable terminated with two connectors which you can specify types. Connector type attenuator looks like a bulk head fiber connector, with a male connector interface on one end and a female interface connector on the opposite end. It mates to regular connectors of the same type such as FC, ST, SC and LC. Their applications include telecommunication networks, optical fiber test facility, Local Area Network (LAN) and CATV systems.

fixed optical attenuator Figure 1: Fixed Optical Attenuator

Optical Variable Attenuator

Optical variable attenuator, or variable optical attenuator, generally uses a variable neutral density filter. It has advantages of being stable, wavelength insensitive, mode insensitive, and offering a large dynamic range. Variable optical attenuator is generally used for testing and measurement, but it is also widely adopted in EDFAs (Erbium-Doped Fiber Amplifier) for equalizing the light power among different channels. Basically, there are two types of optical variable attenuator: stepwise variable attenuator and continuously variable attenuator. Stepwise variable attenuator can change the attenuation of the signal in known steps such as 0.1 dB, 0.5 dB or 1 dB. Continuously variable optical attenuator produces precise level of attenuation with flexible adjustment. Thus, operators are able to adjust the attenuator to accommodate the changes required quickly and precisely without any interruption to the circuit.

optical variable attenuator Figure 2: Optical Variable Attenuator

Optical variable attenuator can also be made as a plug-in card. It is a part of FS.COM FMT all-in-one multi-service transport system. This hot-swappable plug-in variable optical attenuator is an online attenuation adjustment device, only occupying one slot in the 1U/2U/4U chassis. It is applied to applications that optical power required strict control, such as to balance signal strengths in a DWDM network system. Card optical variable attenuator adopts MEMS technology and could continually and variably reduce the light intensity in the optical network and help simulate distance or actual attenuation in the fiber optic testing work. With the card design, this optical variable attenuator is easy to install and remove without any tool. The online attenuation adjustment also contributes to safer business.

fmt-plug-in-card-optical-variable-attenuator Figure 3: Plug-in Card Optical Variable Attenuator

Working Principles of Optical Attenuators

Optical attenuator achieves the desired power reduction according to different principle, including gap-loss principle, absorptive principle, or reflective principle.

Gap-loss Principle

The gap-loss principle reduces the optical power level by inserting the device in the fiber path with an in-line configuration. Gap-loss attenuators are used to prevent the saturation of the receiver and are placed close to the transmitter. Gap-loss attenuators use a longitudinal gap between two optical fibers so that the optical signal passed from one optical fiber to another can be reduced. This principle allows the light from the transmitting optical fiber to spread out as it leaves the optical fiber. When the light gets to the receiving optical fiber, some of the light are lost in the cladding because of gap and the spreading occurred. These attenuators should be kept close to the transmitter to ensure its effective performance. To reduce the signal farther down the fiber path, an optical attenuator using absorptive or reflective techniques is more suitable.

gap-loss principle Figure 4: Gap-loss Principle

Absorptive Principle

The absorptive principle, or absorption, accounts for a fraction of power loss in optical fiber. This is achieved due to the imperfections of the optical fiber that absorb optical energy and convert it to heat. Absorptive principle can be employed to design an optical attenuator to insert a known reduction of power. The absorptive principle uses material in the optical path to absorb optical energy. The principle is simple, but can be an effective way to reduce the power being transmitted and received.

absorptive principle Figure 5: Absorptive Principle

Reflective Principle

The reflective principle, or scattering, accounts for the majority of power loss in optical fiber and also results from the imperfections of optical fiber. But in this case, it causes the signal to scatter. The scattered light inserts interference in the optical fiber, thereby reducing the amount of transmitted and received light. This principle can be employed in the planned attenuation of a signal. The material used in the attenuator is manufactured to reflect a known quantity of the signal, thus allowing only the desired portion of the signal to be propagated.

reflective principle Figure 6: Reflective Principle

How to Use Fiber Optic Attenuators in Data Link?

For a single-mode applications, especially analog CATV systems, the most important parameter, after the correct loss value, is return loss or reflectance. Many types of attenuators (especially gap loss types) suffer from high reflectance, so they can adversely affect transmitters just like highly reflective connectors.

fiber-optic-attenuator-in-data-link Figure 7: Fiber Optic Attenuator in Data Link

Choose an attenuator with good reflectance specifications, and always install the attenuator ( X in the drawing) at the receiver end of the link as shown above. This is because it's more convenient to test the receiver power before and after attenuation or while adjusting it with your power meter at the receiver, plus any reflectance will be attenuated on its path back to the source.

Test the system power with the transmitter turned on and the optical attenuator installed at the receiver, and using an optical power meter set to the system operating wavelength. Check to see whether the power is within the specified range for the receiver.

Conclusion

Fiber optic attenuator is an essential passive component in the optical communication system. With the advancement of DWDM technology, as well as the potential to flexibly upgrade the reconfigurable optical add-drop multiplexer (ROADM), the demand for optical attenuator is sure to soar, especially for optical variable attenuator. The innovation in fiber optic industry never ceases, and fiber optic attenuator will evolve to have lower cost, faster response time and enhanced integration of hybrid with other optical communication devices.

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OTN Solution Team
Senior Telecommunication Engineers

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