What are Fiber Optic Attenuators?

Fiber optic attenuators, also called optical attenuators, are passive devices used to reduce the power level of an optical signal. Since too much light may saturate the fiber optic receiver, optical attenuators are often installed in the system to reduce the light power and achieve the best fiber optic system performance. Fiber optic attenuators are used in applications where the optical signal is too strong and needs to be reduced. Like in a multi-wavelength fiber optic system, where one needs to equalize the optical channel strength so that all the channels have similar power levels. This means reducing stronger channels’ powers to match with the lower power channels.

Optical attenuators are typically classified as fixed or variable attenuators which are described below. Fixed attenuators have a fixed optical power reduction number, such as 1dB, 5dB, 10dB, etc. Whereas variable attenuators’ attenuation level can be adjusted, such as from 0.5 dB to 20dB, or even 50 dB. Generally, multimode systems do not need attenuators as the multimode sources, even Vertical-cavity surface-emitting lasers, rarely have enough power output to saturate receivers. Instead, for single-mode systems, especially long-haul Dense Wavelength Division Multiplexing network links, fiber optic attenuators are necessary therefore to balance the optical power during the transmission.

Fiber Optic Attenuators Working

Optical attenuators achieve the desired attenuation in optical fiber links in three different principles which are discussed below

Gap-loss Principle

In the principle of gap loss, power reduction is achieved by inserting fiber optic attenuators in the fiber path. Gap-loss attenuators are placed near the transmitter to avoid the saturation of the receiver. 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, part of the light is lost in the cladding because of a gap and the spreading occurred. To reduce the signal farther down the fiber path, an optical attenuator using absorptive or reflective techniques would be more suitable.

Absorptive Principle

The absorptive principle accounts for a fraction of power loss in optical fiber. This is because optical fiber absorbs optical energy and converts it to heat. The absorptive principle can design an optical attenuator with 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.

Reflective Principle

The reflective principle, accounts for a fraction 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. The material used in the fiber optic attenuator is manufactured to reflect a known quantity of the signal, thus allowing only the desired portion of the signal to be propagated.

Types of Fiber Optic Attenuators

One can find many optical attenuator types in the market with different classification perspectives such as the connector type, cable type, etc. Generally, they are widely accepted to be grouped as fixed optical attenuators (FOA) and optical variable attenuators (VOA). While considering the types of cables, they can also be divided into single-mode and multi-mode attenuators.

Fixed Optical Attenuator

Fixed attenuator, is designed to have an unchanging level of attenuation in optical fiber, expressed in dB, typically between 1dB and 30dB, such as 1dB, 5dB, 10dB, etc. These optical attenuators often use either doped fibers, or misaligned splices, or total power while non-preferred attenuators often use gap loss or reflective principles.

The above picture shows an example of a fixed optical attenuator. The attenuation level is fixed at 1 dB, which means it reduces the optical power by 1dB. This attenuator has a short piece of fiber with metal ion doping that provides the specified attenuation. These fixed value attenuators consist of in-line type and connector type. The in-line type looks like a plain fiber cable. The in-line type optical attenuators are incorporated into cables. The connector type attenuator looks like a bulkhead fiber connector. Usually, it has a male plug connector at one side to allow the fiber attenuator to be plugged directly into receiver equipment or adapters, and at the other side, there is a female-type fiber optic adapter to allow the cords to plug in. There are also female-to-female optical attenuators, which can be used as adapters and attenuators at the same time. Their applications include telecommunication networks, optical fiber test facilities, Local Area Networks (LAN), and CATV systems.

Optical Variable Attenuator

Optical variable attenuator/ variable optical attenuator (VOA), uses a variable neutral density filter. VOA is generally used for testing and measurement, but it is also widely adopted in Erbium-Doped Fiber Amplifier (EDFA) for equalizing the light power among different channels. It has the advantages of being stable, wavelength insensitive, mode insensitive, and offering a large dynamic range.

There are two types of optical variable attenuators: stepwise variable attenuators and continuous variable attenuators. Stepwise variable attenuators can change the attenuation of the signal in known steps such as 0.1dB, 0.5dB, or 1dB. Continuously variable optical attenuators can provide a precise level of attenuation through flexible adjustment. Thus, operators can adjust the attenuator to accommodate the changes required quickly and precisely without any interruption to the circuit.

The picture shown above depicts the mechanism used in one type of variable attenuator. Here variable means the attenuation level can be adjusted, for example, it could be from 1 dB up to 20dB. Here the incoming light from the input fiber is expanded into a larger beam by the first collimating lens, then a blocking device, which could be a neutral density filter, is inserted into the light path to partially block the light, so only part of the light can pass through. Then the second collimating lens is used to focus the light back into the output fiber. When you move the blocking device inward or outward, you get different attenuation levels.

Single Mode and Multimode Fiber Optic Attenuator

Since fiber optic attenuators can be used with two types of fiber cables, single mode and multimode, optical attenuators can be classified into single mode type and multimode type. Fiber optic attenuators are usually used in single-mode long-haul applications. Accordingly, the commonly used type is the single mode type. Although fiber optic attenuators are normally used for single mode, there are also multi-mode fiber optic attenuators available to mate with multi-mode fiber cables. When choosing one type of optical attenuator over another one, it is necessary to consider the attenuation range and the wavelength.

Application of Fiber Optic Attenuators

Fiber optic attenuators are usually used in two scenarios. One is when fiber optic attenuators are used to reduce the receiver power. There are sometimes when the signal arriving at the receiver is too strong and may overpower the receiving elements. Usually, the receiver power depends on two factors: how much power is launched into the fiber and how much power is lost by the attenuator. Too much receiver power can be mainly caused by the mismatch between the transmitters/receivers or caused by the use of media converters designed for a much longer distance. In this case, optical attenuators can be permanently installed in the fiber optic link to reduce the signal power and properly match the signal level.

The other one is when the attenuators are used for testing the power level margins. When testing the optic power level, the attenuators are used to temporarily add a calibrated amount of signal loss to test the power level margins in the fiber optic system. With the transmitter turned on and using a fiber optic power meter which is set to the system operating wavelength, the attenuator can be used to test the system power.

Use Fiber Optic Attenuators in Data Link:

For single-mode applications, especially analog community antenna television (CATV) systems, the most important parameter second to the correct loss value is return loss or reflectance. Many types of optical attenuators especially gap loss types have the common problem of high reflectance, so they can adversely affect transmitters just like highly reflective connectors. 

For use of optical attenuators in link data, first, one needs to choose an attenuator with good reflectance specifications. And second, always install the attenuator 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.

For testing the system power, the transmitter is turned on and the optical attenuator is installed at the receiver. And check whether the power is within the specified range for the receiver. If the optical power is higher or lower than the configuration required, the optical attenuators should be changed to adjust the power again.