An optical attenuator is an electronic device commonly used to decrease the level of power of an optical signal in a fiber optic communication system. In fiber optics, attenuation is also called transmission loss. It is the reduction in light signal intensity with respect to the distance traveled by the signal in a transmission medium. Attenuation is an important element to limit the transmission of a digital signal traveling in large distances. An optical attenuator reduces this optical signal as it travels along a free space or an optical fiber.
Optical fiber attenuators may employ several principles when used in fiber optic communications. One common principle is the gap loss principle. Attenuators using this principle are sensitive to the modal distribution ahead of the attenuator. Thus, they should be utilized at or near the transmitting end. If not, the attenuators could establish less loss than intended. This problem is avoided by attenuators that use absorptive or reflective principles.
There are three basic types of optical attenuator: the fixed attenuator, step-wise attenuator and the continuously variable attenuator. Fixed attenuators reduce light signals by specific amount with negligible or no reflection. Because signal reflection is not an issue, fixed attenuators are known for more accurate data transmission. Important elements associated with fixed attenuators include the flatness over a specified frequency, range, voltage standing wave ratio (VSWR), amount of attenuation, average and peak power-handling capability, performance over a specific temperature, size and height. Fixed attenuators are also often used to enhance interstage matching in an electronic circuit. Thorlab’s fixed attenuators are available from 5 dB to 25 dB. Mini-Circuits’ fixed attenuators are packaged in rugged plug-in and connector models. They are available in both 50- and 76-ohm models ranging from 1to 40 dB spanning DC to 1500 MHz.
In variable optical attenuators (VOA), resistors are replaced with solid state devices such as the metal semiconductor field effect transistor (MESFETs) and PIN diodes. VOA attenuates light signal or beam in a controlled manner, thus producing an output optical beam with different attenuated intensity. The attenuator adjusts the power ratio between the light beam coming from the device and the light beam entering the device over a changeable rate. VOA is usually used in fiber optic communication systems to regulate optical power levels in order to prevent damages in optical receivers which may be due to irregular or fluctuating power levels. The price of commercial VOA varies depending on the manufacturing technology used. Some of manufacturers of VOA are Timbercon and Arcoptix.
Timbercon claims that its optical attenuator units produce precision levels of attenuation, with the added flexibility of adjustment. Timbercon’s variable attenuators are available in single mode and multi-mode versions. They have low insertion loss and back reflection. The attenuators are also compact in size and available in multiple packaging options. Arcoptix’s electrical adjustable variable attenuators are a liquid crystal device which allows the precise control of the attenuation of beams traveling in free space. These attenuators can be adjusted in milliseconds with a simple square wave bias between 0 and 10 volts.
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