Optical attenuator
Encyclopedia
An optical attenuator is a device used to reduce the power
level of an optical signal, either in free space or in an optical fiber
. The basic types of optical attenuators are fixed, step-wise variable, and continuously variable.
, either to test power level margins by temporarily adding a calibrated amount of signal loss, or installed permanently to properly match transmitter and receiver levels.
Sharp bends stress optic fibers and can cause losses. If a received signal is too strong a temporary fix is to wrap the cable around a pencil until the desired level of attenuation is achieved. However, such arrangements are unreliable, since the stressed fiber tends to break over time.
Inline style attenuators are incorporated into patch cables. The alternative build out style attenuator is a small male-female adapter that can be added on to other cables.
Non-preferred attenuators often use gap loss
or reflective principles. Such devices can be sensitive to: modal distribution, wavelength, contamination, vibration, temperature, damage due to power bursts, may cause back reflections, may cause signal dispersion etc.
A manual device is useful for one-time set up of a system, and is a near-equivalent to a fixed attenuator, and may be referred to as an "adjustable attenuator". In contrast, an electrically controlled attenuator can provide adaptive power optimization.
Attributes of merit for electrically controlled devices, include speed of response and avoiding degradation of the transmitted
signal. Dynamic range is usually quite restricted, and power feedback may mean that long term stability is a relatively minor issue. Speed of response is a particularly major issue in dynamically reconfigurable systems, where a delay of one millionth of a second can result in the loss of large amounts of transmitted data. Typical technologies employed for high speed response include LCD, or Lithium niobate
devices.
There is a class of built-in attenuators that is technically indistinguishable from test attenuators, except they are packaged for rack mounting, and have no test display.
They may be either manually or motor controlled. Motor control give regular users a distinct productivity advantage, since commonly used test sequences can be run automatically.
Attenuator instrument calibration is a major issue. The user typically would like an absolute port to port calibration. Also, calibration should usually be at a number of wavelengths and power levels, since the device is not always linear. However a number of instruments do not in fact offer these basic features, presumably in an attempt to reduce cost. The most accurate variable attenuator instruments have thousands of calibration points, resulting in excellent overall accuracy in use.
Power (physics)
In physics, power is the rate at which energy is transferred, used, or transformed. For example, the rate at which a light bulb transforms electrical energy into heat and light is measured in watts—the more wattage, the more power, or equivalently the more electrical energy is used per unit...
level of an optical signal, either in free space or in an optical fiber
Optical fiber
An optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...
. The basic types of optical attenuators are fixed, step-wise variable, and continuously variable.
Fiber-optic attenuator applications
Attenuators are commonly used in fiber optic communicationsTelecommunication
Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...
, either to test power level margins by temporarily adding a calibrated amount of signal loss, or installed permanently to properly match transmitter and receiver levels.
Sharp bends stress optic fibers and can cause losses. If a received signal is too strong a temporary fix is to wrap the cable around a pencil until the desired level of attenuation is achieved. However, such arrangements are unreliable, since the stressed fiber tends to break over time.
Fixed fiber-optic attenuators
Fixed optical attenuators used in fiber optic systems may use a variety of principles for their functioning. Preferred attenuators use either doped fibers, or mis-aligned splices, since both of these are reliable and inexpensive.Inline style attenuators are incorporated into patch cables. The alternative build out style attenuator is a small male-female adapter that can be added on to other cables.
Non-preferred attenuators often use gap loss
Gap loss
Gap loss is a type of signal strength loss that occurs in fiber optic transmission when the signal is transferred from one section of fiber or cable to another....
or reflective principles. Such devices can be sensitive to: modal distribution, wavelength, contamination, vibration, temperature, damage due to power bursts, may cause back reflections, may cause signal dispersion etc.
Built-in variable fiber-optic attenuators
Built-in variable optical attenuators may be either manually or electrically controlled.A manual device is useful for one-time set up of a system, and is a near-equivalent to a fixed attenuator, and may be referred to as an "adjustable attenuator". In contrast, an electrically controlled attenuator can provide adaptive power optimization.
Attributes of merit for electrically controlled devices, include speed of response and avoiding degradation of the transmitted
signal. Dynamic range is usually quite restricted, and power feedback may mean that long term stability is a relatively minor issue. Speed of response is a particularly major issue in dynamically reconfigurable systems, where a delay of one millionth of a second can result in the loss of large amounts of transmitted data. Typical technologies employed for high speed response include LCD, or Lithium niobate
Lithium niobate
Lithium niobate is a compound of niobium, lithium, and oxygen. Its single crystals are an important material for optical waveguides, mobile phones, optical modulators and various other linear and non-linear optical applications.-Properties:...
devices.
There is a class of built-in attenuators that is technically indistinguishable from test attenuators, except they are packaged for rack mounting, and have no test display.
Variable fiber-optic test attenuators
Variable Fiber Optic Test Attenuators generally use a variable neutral density filter. Despite relatively high cost, this arrangement has the advantages of being stable, wavelength insensitive, mode insensitive, and offering a large dynamic range. Other schemes such as LCD, variable air gap etc. have been tried over the years, but with limited success.They may be either manually or motor controlled. Motor control give regular users a distinct productivity advantage, since commonly used test sequences can be run automatically.
Attenuator instrument calibration is a major issue. The user typically would like an absolute port to port calibration. Also, calibration should usually be at a number of wavelengths and power levels, since the device is not always linear. However a number of instruments do not in fact offer these basic features, presumably in an attempt to reduce cost. The most accurate variable attenuator instruments have thousands of calibration points, resulting in excellent overall accuracy in use.
Test automation
Test sequences that use variable attenuators, can be very time consuming. Therefore, automation is likely to achieve useful benefits. Both bench and handheld style devices are available that offer such features.See also
- Attenuation (electromagnetic radiation)
- Optical fiber cableOptical fiber cableAn optical fiber cable is a cable containing one or more optical fibers. The optical fiber elements are typically individually coated with plastic layers and contained in a protective tube suitable for the environment where the cable will be deployed....
- Optical fiber connectorOptical fiber connectorAn optical fiber connector terminates the end of an optical fiber, and enables quicker connection and disconnection than splicing. The connectors mechanically couple and align the cores of fibers so that light can pass...
- Gap lossGap lossGap loss is a type of signal strength loss that occurs in fiber optic transmission when the signal is transferred from one section of fiber or cable to another....
Attenuation sources and causes - Optical power meterOptical power meterAn optical power meter is a device used to measure the power in an optical signal. The term usually refers to a device for testing average power in fiber optic systems...