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Fiber Optic Attenuators: What They Are and When to Use Them

Article by Henry Martel, Field Application Engineer, Antaira Technologies.

Fiber Optic Attenuators: What They Are and When to Use Them

Proper management of optical power levels is crucial in fiber optic communication systems to ensure reliable data transmission. Signal levels must be strong enough for data interpretation but not so strong as to damage the circuits in the receiver.

Fiber optic signal strength exceeding the specified operating range of a fiber optic receiver can overload it, causing high bit error rates or worse. In these situations, network administrators will install fiber attenuators to reduce optical power levels. Attenuators enable the fine-tuning of adjustable signal power and ensure that it reaches the receiver within its dynamic range, preventing saturation and maintaining the signal-to-noise ratio. Also, by preventing overloading, attenuators can increase the lifespan of network components, while improving overall network cost efficiency.

TOO MUCH OF A GOOD THING
If power is the silver bullet in fiber transmission, why not install a more powerful transmitter well beyond the requirements of the power budget? Although on the surface this approach sounds like it makes sense, in the real world of fiber optics, it simply doesn’t work. When too much light passes through fiber cables and reaches a fiber optic receiver it will overload. Overloads are usually evident in distorted signals, intermittent data, or in many cases, no operation at all. Common culprits are a mismatch between the transmitters/receivers or because the media converters used are designed for a much longer distance.

Overloading occurs more frequently in single-mode fiber optic applications than in multimode fiber optic since stronger lasers are employed. VCSEL (Vertical Cavity Surface Emitting Laser) sources that typically supply multimode fiber optic systems rarely have enough power output to overwhelm receivers. However, due to the interplay of the different light paths propagated in multimode fiber systems, an attenuator may be installed to decrease modal noise.

UNDERSTANDING ATTENUATION
Attenuation (reduction) is a natural and unavoidable phenomenon in fiber optics. Attenuation refers to the amount of light lost as light pulses travel through the fiber. Several factors can influence attenuation such as the length of the fiber optic cable as the distance increases, the light signal wavelength, bend radius, the quality and configuration of connectors and splices, and the composition of the fiber.

An attenuator device mechanically creates attenuation by absorbing, scattering or diverging light until the signal strength is within the operating range of the receiver, ideally not too close to either its sensitivity limit or the overload level. Absorption involves the attenuator converting optical power to heat, while scattering is a process that causes light to scatter in different directions due to non-uniformities in the path, therefore reducing power. The divergence method lowers the power density of the signal by using beam-shaping optics. Which method is best for your optical network depends on its operating wavelength (1310nm, 1550nm, 850nm), the amount of attenuation needed, gain used, connector compatibility, and the acceptable levels of signal distortion, among other factors.

INSTALLING ATTENUATORS
Installing common plug-style (buildout) male-to-female attenuators involves mounting them on one end of a fiber optic cable so that the cable may be inserted into a patch panel, or connected to receiving equipment. Female-to-female (bulkhead) attenuators are used to join two fiber optic cables or to mount in patch panels. The female-to-female design is sometimes referred to as “fiber optic adapter” type attenuators since the device serves as both adapters and attenuators.

As for placement, installing the attenuator at the receiver end of the link makes it more convenient to measure and adjust the power level with a meter. Plus, it ensures that reflectance will not affect the transmitter.

DIFFERENT TYPES OF ATTENUATORS
There are two basic types of attenuators: fixed and variable. Fixed attenuators are ideal for networks with constant signal strength, while variable attenuators are helpful in networks where the input signal strength varies.

FIXED: As the name implies, a fixed attenuator reduces the signal to a fixed amount, normally between 1 and 30dB. Fixed attenuators are specified by their attenuation value and their connector type. For instance, an “LC 10db” signifies a fixed attenuator for LC connections that reduces the power level by 10dB. Besides LC, fixed attenuators are available for SC, ST, FC and MU cables with a choice of APC or UPC polish. Two ideal scenarios for fixed attenuators are where high-power light signals are being used, such as telecom systems, or for balancing a network that needs power levels to be equalized across multiple paths.

VARIABLE: A variable attenuator uses a variable regulator filter. This permits adaptation and flexibility in adjusting how much signal you want to attenuate. Being variable allows this type of attenuator to be more precise than a fixed attenuator. Another benefit is that attenuation levels can be changed as needed without interrupting the flow of the network. Common applications for variable attenuators are for fine-tuning networks by controlling the power levels in different channels, for lab testing of optical devices under various signal intensities, for calibrating optical power meters ensuring accuracy, and for adjustments of signal strength in long-haul telecom lines to prevent damaging expensive receiving equipment. Variable attenuators are more expensive and complex to integrate than fixed attenuators.

FINAL CONSIDERATIONS
Perform a system test to make sure the attenuator is operating as intended after installation. The signal may be too strong or too faint if the attenuator isn't set to the proper attenuation level, which could result in signal distortion or data loss.

Another important consideration is the power budget, which is the difference between the transmitter power and the required attenuation for receiver sensitivity. Make sure the attenuation level does not cause the signal power to fall below the receiver’s minimum power requirement.

Performance will be impacted negatively by a lower quality attenuator or amplifier. To avoid signal loss and degradation, buy the best quality device within your budget from a reputable manufacturer. Always adhere to the installation guidelines provided by the manufacturer.

Fiber optics are the backbone of modern networks and telecommunications, and have ushered in unprecedented speed, reliability, and capacity. Planning and designing a fiber optic network is a complex process that includes many decisions. The fiber optic experts at Antaira stand ready to help you meet your fiber optic network requirements with scalability and future growth in mind.

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