An optical attenuator is an electronic device commonly used to decrease the power level of an optical signal in a fiber optic communication system. In fiber optics, attenuation is also called transmission loss. It is the reduction of the intensity of the light signal with respect to the distance traveled by the signal in a transmission medium. Attenuation is an important element in limiting the transmission of a digital signal traveling long distances. An optical attenuator reduces this optical signal as it travels along free space or an optical fiber.
Fiber optic attenuators can employ several principles when used in fiber optic communications. A common principle is the gap loss principle. Attenuators using this principle are sensitive to the modal split ahead of the attenuator. Therefore, they must be used at or near the transmitting end. Otherwise, the dimmers could set less loss than intended. This problem is avoided with dimmers that use absorption or reflection principles.
There are three basic types of optical attenuators: the fixed attenuator, the stepper attenuator, and the continuously variable attenuator. Fixed dimmers reduce light signals by a specific amount with little or no reflection. Since signal reflection is not a problem, fixed attenuators are known for more accurate data transmission. Important elements associated with fixed dimmers include flatness over a specific frequency, range, voltage standing wave ratio (VSWR), amount of attenuation, average and peak power handling capacity, performance over a specific temperature, size, and height. . Fixed attenuators are also often used to improve matching between stages in an electronic circuit. Thorlab Fixed Attenuators are available from 5dB to 25dB. Mini-Circuits fixed dimmers are packaged in rugged plug-in and connector models. They are available in 50 and 76 ohm models ranging from 1 to 40 dB covering DC to 1500 MHz.
In variable optical attenuators (VOAs), resistors are replaced with solid-state devices, such as metal-semiconductor field-effect transistors (MESFETs) and PIN diodes. VOA attenuates the signal or light beam in a controlled manner, thus producing an output optical beam with different attenuated intensity. The dimmer adjusts the power ratio between the light beam coming from the device and the light beam entering the device at a variable rate. VOA is typically used in fiber optic communication systems to regulate optical power levels in order to prevent damage to optical receivers that can be caused by irregular or fluctuating power levels. The price of commercial VOA varies depending on the manufacturing technology used. Some of the VOA manufacturers are Timbercon and Arcoptix.
Timbercon claims that its optical attenuator units produce precision dimming levels, with the added flexibility of adjustment. Timbercon Variable Attenuators are available in singlemode and multimode versions. They have low insertion loss and back reflection. The dimmers are also compact in size and available in multiple packaging options. Arcoptix Electrically Adjustable Variable Attenuators are a liquid crystal device that allows precise control of the attenuation of beams traveling in free space. These dimmers can be adjusted in milliseconds with a simple square wave bias between 0 and 10 volts.