FiberAttenuators.info is a resource to those seeking to learn more about fiber optic attenuators used for telecommunications applications. The information presented here is practical rather then theoretical in nature, and after reading through the information provided in this website you will be able to hold an intelligent conversation on the subject with practitioners in the field.

A fiber optic attenuator reduces the power of an optical signal traveling through the attenuator without distorting the waveform.

The applications of fiber optic attenuators are as follows:

- To reduce optical power at the optical receiver for improved performance or to stay within the dynamic range of the optical receiver.
- To reduce optical power after the optical transmitter to stay within the dynamic range of optical components in the fiber optic link or to prevent damage from the high power optical signals.
- To regulate fluctuating optical power levels in optical networks and test and measurement applications.
- To adjust the power of optical signals to very low levels in test and measurement applications.
- To test the optical power response of optical receivers.
- To regulate the output power of erbium doped fiber amplifiers.

There are several categories and subcategories of fiber optic attenuators. A brief categorization will be given in this paragraph with links to a more thorough description given for each type.

Fixed vs. Variable - Only one attenuation value is possible for fixed attenuators while the attenuation value of variable attenuators can be adjusted.

Manually Variable vs. Automatically Variable - Manually Variable Attenuators are adjusted by hand while automatically variable attenuators are adjusted by an electronic signal.

Component Size Automatically Variable vs. Handheld Size Automatically Variable vs. Instrument Size Automatically Variable - Component Size Automatically Variable Attenuators are typically used in optical networks while the remaining types are used for test and measurement and research and development applications.

Manually variable optical attenuators are one step up from the fixed optical attenuators. They are often used in the same situations as fixed optical attenuators, and function best in applications where the attenuation value needs to be adjusted infrequently and with little precision.

The simplest type of manually variable attenuator is the stepwise variable attenuator. This device can only produce a few discrete attenuation levels. The most common method for making a manually stepwise variable attenuator is to coil optical fiber in tight loops so that light can escape from the fiber. The number of coils determines the attenuation value that is produced.

OZ Optics Manually Variable Attenuator

Fiberdyne Manually Variable Attenuator

AFOP Manually Variable Attenuator

Continually manually variable optical attenuators most often use a mechanical method to produce attenuation. Typically light is coupled out of the fiber using a collimator lens and a precision screw mechanism blocks a portion of the light before it is coupled back in to the fiber. Attenuation values range between 1 and 30 dB and the cost is between $200 and $1000.

This category accounts for the largest dollar and numbers volume of all the types of attenuators. This type of attenuator, commonly referred to as a variable optical attenuator (VOA) is a basic building block of optical communications networks. There are a myriad of very specialized applications of attenuators in an optical network but almost all of them boil down to regulating optical power of an individual wavelength or the composite signal in order to produce the highest signal quality (lowest distortion, highest signal to noise ratio). Several technologies are used to produce optical attenuation:

JDSU MEMS Attenuator	Micro Electro Mechnical Systems (MEMS) - Microscopic scale optical components are manufactured using a semiconductor process. Typically a micro-scale mirror is controlled by an electrical signal to steer a light beam exiting one fiber and entering a second. Attenuation is produced by misalignment of the beam with the entrance fiber. This type of attenuator can be manufactured at a low cost and in high volume. Compact size is another advantage. An attenuation range of 40 dB is typical.
Lightwaves2020 8-channel Liquid Crystal Attenuator	Liquid Crystal - These types of attenuators are used in similar types of applications as MEMS attenuators and have similar applications and specifications. They are based on liquid crystal technology where a voltage applied to a miniature liquid crystal plate reduces the transmission power of an optical signal. Additional benefits of liquid crystal attenuators are increased reliability and resistance to mechnaical vibration due to no moving parts and a slow tuning slope with no hysteresis.
OpTun Silica-on-Silicon Integrated Attenuator and Switch Module	Integrated Optics - Several materials platforms are used for this type of attenuator, including silica-on-silicon, glass substrate planar lightwave circuit (PLC), and lithium niobate. Electro-optic and thermo-optic effects are used to produce attenuation. These types of attenuators are usually higher in cost but are more easily integrated with other functions such as switching and modulation on the same substrate to produce multi-function modules. Additionally, integrated optical attenuators that use the electro optic effect can have faster response times.

EXFO Benchtop Variable Optical Attenuator

Agilent Mainframe Optical Attenuator Module

Benchtop variable optical attenuators are primarily used for test and measurement and less frequently for network simulation and system performance test under varying levels of optical signal. These types of attenuators come in two flavors: as a stand-alone instrument and as a module in a mainframe.

Standalone instrument type benchtop variable attenuators are high-performance units with typically a mechanical mechanism with digital control. The instrument can be controlled either through the front panel or through a computer interface. Typically numerous attenuation functions are possible with this type of instrument. Attenuation values attainable are between 3 and 60 dB and prices range from USD $2000 to $12,000.

Mainframe modules typically have very similar specifications and operating mechansims with the benchtop instruments, but have a bare metal housing that slides into a mainframe. All or nearly all front panel controls are removed, and the instrument is either controlled remotely with a computer or a control pad for the entire mainframe. Prices are typically 50% lower for a mainframe unit as compared to a benchtop unit with similar specifications.