An optical attenuator is a passive device that is used to reduce the power level of an optical signal. The attenuator circuit will allow a known source of power to be reduced by a predetermined factor, which is usually expressed as decibels. Fiber attenuators are generally used in single mode long-haul applications to prevent optical overload at the receiver.
Fiber Optical Attenuators typically come in two forms of packaging. The bulkhead optical attenuator can be plugged into the receiver receptacle. The inline attenuator resembles a patch cord and is typically used between the patch panel and the receiver.
The Principles of Optical Attenuators
Optical attenuators use several different principles in order to accomplish the desired power reduction. Fiber attenuators may use the gap-loss, absorptive, or reflective technique to achieve
the desired signal loss. The types of attenuators generally used are fixed, stepwise variable, and continuously variable.
The principle of gap-loss is used in optical attenuators to reduce the optical power level by inserting the device in the fiber path using an in-line configuration. Gap-loss attenuators are used to prevent the saturation of 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 is attenuated. 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 will be lost in the cladding because of gap and the spreading that has occurred.
The gap-loss attenuator will only induce an accurate reduction of power when placed directly after the transmitter. These attenuators are very sensitive to modal distribution ahead of the
transmitter, which is another reason for keeping the device close to the transmitter to keep the loss at the desired level. The farther away the gap-loss attenuator is placed from the transmitter, the less effective the attenuator is, and the desired loss will not be obtained. To attenuate a signal farther down the fiber path, an optical attenuator using absorptive or reflective techniques should be used.
Keep in mind that the air gap will produce a Fresnel reflection, which could cause a problem for the transmitter.
The absorptive principle, or absorption, accounts for a percentage of power loss in optical fiber. This loss is realized because of imperfections in the optical fiber that absorb optical energy and convert it to heat. This principle can be employed in the design of an optical attenuator to insert a known reduction of power.
The absorptive principle uses the 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.
The reflective principle, or scattering, accounts for the majority of power loss in optical fiber and again is due to imperfections in the optical fiber, which in this case cause the signal to scatter. The scattered light causes 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.
Now that we have looked at the principles behind the attenuator theories, we will discuss some of the types of fiber attenuators. We will examine fixed, stepwise variable, and continuously variable attenuators and when they should be used.
Types of Attenuators
Fixed attenuators are designed to have an unchanging level of attenuation. They can theoretically be designed to provide any amount of attenuation that is desired. The output signal is
attenuated relative to the input signal. Fixed attenuators are typically used for single-mode applications.
Stepwise variable attenuators
A stepwise variable attenuator is a device that changes the attenuation of the signal in known steps such as 0.1dB, 0.5dB, or 1dB. The stepwise attenuator may be used in applications dealing with multiple optical power sources—for example, if there are three inputs available, there may be a need to attenuate the signal at a different level for each of the inputs.
Conversely, the stepwise attenuator may also be used in situations where the input signal is steady, yet the output requirements change depending on the device that the signal is output to.
The stepwise attenuator should be used in applications where the inputs, outputs, and operational configurations are known.
Continuously variable attenuator
Continuously variable attenuator is an attenuator that can be changed on demand. These attenuators generally have a device in place that allows the attenuation of the signal to change as required. A continuously variable attenuator is used in uncontrolled environments where the input characteristics and output needs continually change. This allows the operator to adjust the
attenuator to accommodate the changes required quickly and precisely without any interruption to the circuit.
Calculating the attenuation value
In summary, there are many types of attenuators and many principles on which they work. The key to choosing the appropriate one is to understand the theory on which each operates and the application that the attenuator will be applied to. Of course, you also need to be able to determine the attenuator value in decibels required for your application.
In this example let’s assume that the maximum optical input power a fiber optic receiver can operate with is -6dBm. If the input power exceeds this power level, the receiver will be overloaded. The transmitter, which is located 10km from the receiver, has an output power of 3dBm. The loss for the 10km of optical fiber, including interconnections, is 5dB.
To calculate the minimum attenuation required to prevent the receiver from being overloaded, we need to subtract all the known losses from the output power of the transmitter as shown here:
Transmitter power (TP) = 3dBm
Receiver maximum optical input power (MP) = –6dBm
Total losses (TL) = 5dB
Minimum attenuation required = MP + TL – TP–6dBm + 5dB – 3dBm = –4dB
At a minimum, a 4dB attenuator is required. However, an attenuator with a larger value could be used as long as it did not over-attenuate the signal.
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