Fiber Optical Attenuator

There are four different types of fiber optical attenuator and they can take a number of different forms and are typically classified as fixed or variable attenuators. What's more, they can be classified as LC, SC, ST, FC, MU, etc. according to the different types of connectors.

Fixed 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 misaligned splices, or total power 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 onto 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.

Loopback Attenuators: Loopback fiber optic attenuator is designed for testing, engineering and the burn-in stage of bfiber optic adapterrds or other equipment. Available in SC/UPC, SC/APC, LC/UPC, LC/APC, MTRJ, MPO for single mode application.

Built-in Variable 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.
The 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 liquid crystal variable attenuator (LCVA), 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.

Variable Optical Test Attenuators: This type generally uses a variable neutral density filter. Despite the 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 control. Motor control gives regular users a distinct productivity advantage since commonly used test sequences can be run automatically.
Attenuation calibration could be a real issue for the fiber infrastructure. 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 devices 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.