Guide to Fiber Optic Attenuator

Fiber optic attenuators are devices that precisely decrease the optical power in fiber links by a fixed or adjustable amount. They can not only control the power level of optical signals, but also are used to test the linearity and dynamic range of photo sensors and photo detectors. Fiber optic attenuator has a number of different forms and is typically divided into fixed or variable attenuators. What’s more, they can be classified as LC, SC, ST, FC, MU, E2000 etc. according to the different types of connectors. This article will make a brief introduction of fiber attenuator to help you better understand it.

fiber optical attenuators

Why We Need Fiber Optic Attenuator?

As is known to all, the optical power at the receiver ultimately decides the ability of any fiber optic system to transmit data. But it isn’t the fact that the bigger signal power level is better. The truth is that either too little or too much power will cause high bit error rates. Too much power can make the receiver amplifier saturates, while too little will cause noise problems as it interferes with the signal.

Typically, the receiver power depends on two basic factors: the power launched into the fiber and the lost power by attenuation in the optical fiber cable plant. When the power is too high, fiber optic attenuator can help by reducing receive power for better performance. Generally, multimode systems do not need optical attenuators because they barely have enough power output to saturate receivers. While single mode systems, especially for short links, desperately need attenuation because they often have too much power. But nowadays, the complexity of telecommunications requires attenuation both in single mode and multimode systems.

Operating Principles of Fiber Optic Attenuator

There are many methods of power attenuation, including absorption, reflection, diffusion, scattering, deflection, diffraction, and dispersion, etc. Optical attenuators usually operate by absorbing the light, like a neutral density thin film filter. Or they work by scattering the light such as an air gap. Another type of attenuators uses the length of high-loss optical fiber, that operates upon its input optical signal power level in such a way that its output signal power level is less than the input level.

Fixed Fiber Optic Attenuators VS Variable Optical Attenuators

Fiber optic attenuators can be divided into two categories: fixed optical attenuator and variable attenuator. Both of them have unique characteristics.

Fixed Fiber Optical Attenuators

Fixed optical attenuators are compact adapter styles that can reduce signals by a specific amount. As the signal approaches a device or node in a communication link, the power is reduced to a level that is suitable for its application. They can make signal reflection less of an issue and therefore make for more accurate transmissions of data. Fixed attenuators are available with single mode, multimode and polarization maintaining fiber. And they are ideal for attenuating single mode fiber connectors in various application, such as LAN (Local Area Network), CATV (Community Access Television) and telecommunication networks.

fixed fiber optical attenuators

Variable Fiber Optical Attenuators

Variable fiber optic attenuators are rugged, hand-held devices that are used for testing and measurement, or equalizing the power between different signals. They can offer a range of attenuation values with flexible adjustment. Because variable attenuators work by directly blocking the beam, they are polarization insensitive. Like fixed attenuators, variable optic attenuators are also offered with single mode, multimode, or polarization maintaining fibers.

variable fiber optic attenuator


Fiber optical attenuators are key components in optical telecommunication systems. They can adjust optical signal levels to increase network flexibility and providing management of optical power. Besides fixed fiber optical atternuators and variable attenuators, there are many other types atternuators, such as loopback attenuators, built-in variable attenuators and so on.

Related Article:
Guideline for Fixed Fiber Attenuator 

The development direction of future of the optical attenuator

In the optical communication system, and in many occasions there need to reduce the power of the optical signal, for example, the optical power of the optical receiver is very sensitive to overloading, the input power must be controlled within the input range of the receiver to prevent saturation. Another example, optical amplifier in front of the balance between different channel input power can prevent one or some channels of the input power is too big, cause the optical amplifier gain saturation, etc. The main role of the user in accordance with the requirements of the optical signal is expected to be attenuated, the application area is absorbed or reflected by the system losses and various experiments fall out PHR, evaluation system.

Optical attenuator can be divided into fixed and variable optical attenuator attenuators, fixed attenuators can also be subdivided into a pigtail style fixed attenuators, attenuator converter, conversion formula fixed attenuator, optical attenuator fixed conversion formula, depending on the type of the interface, there may be FC, SC, ST Attenuator, FC-SC, FC-ST, SC-ST Attenuator. Inverter-type fixed attenuator can be divided into SC-FC, ST-FC, SC-ST, FC, ST, SC Attenuator.

With the development of optical communication technology, The performance of optical attenuator are: low insertion loss, high return loss, resolution, linearity and repeatability, the amount of attenuation adjustable range, attenuation of high precision, small volume during the environmental good performance. Among them, resolution linearity depends on the decay resistance of components and adopted reading display mode and mechanical adjustment mechanism to adjust the structure and mechanical methods used also depends on repeatability.

Optical attenuator insertion loss mainly from transmission accuracy and coupling fiber collimator technology insertion loss and attenuation unit, which focuses on technology fiber collimator production, if the fiber and self-focusing lens and two Coupling very good, you can make the spaces between the fiber collimator optical attenuator insertion loss is greatly reduced.

Amount of attenuation and insertion loss of the optical attenuator is an important technical indicator. Indicator optical attenuation amount of the fixed attenuator insertion loss is actually, in addition to the variable attenuation amount of the optical attenuator, there is a separate insertion loss indicator. Quality variable attenuator insertion loss 1.0dB less. The common variable fiber optical attenuator of the indicators can be used less than 0.3 dB.

LC APC Variable Fiber Optic VOA In-Line Attenuator 0 to 30dB

From the point of view of market demand, while optical attenuator will develope toward miniaturization, serialization, low price direction, on the other hand, because of common type optical attenuator has been quite mature, so optical attenuator research will focus on its high performance aspects in the future. Here, it should be noted that, because of a device are any reflection will cause the frequency drift and line noise sources, which affect the optical communication system, CATV networks, related to the optical communication, and even cause the entire system misuse, destruction of their normal work; while high return loss attenuator attenuator is an important direction of development.

With the continuous development of optical communications, the applications of optical attenuator is also increasing. To ensure error-free high-speed transmission line, the optical power of the respective channel signal reaches a certain value, high-precision optical attenuator return loss is essential. Using attenuated fiber optical attenuator can be a good match with common optical fiber connectors, because Ultra-wideband consistent attenuation can ensure the product is fully compatible with DWDM, CATV and other communication now and future, will be widely used.

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Variable Optical Attenuator Description

High intensity, coherent light beams are used as an increasingly common means of transmitting data. Optical fibers provide higher data rates with lower cost, weight and volume per units of length than cables relying on metallic conductors.

A variety of devices are known for controlling the light beam. Once of these is the fibre attenuator.

An exemplary optical attenuator is described and shown in U.S. NO. 4,192,573 to Brown, Jr.ct al. A flat mirror reflects an input beam of light. A focusing mirror receives the beam of light reflected from the flat mirror, so that the axis of the beam of light reflected by the focusing mirror is offset from and, parallel to, the axis of the input beam of light. A pinhole assembly receives the beam of light reflected from the focusing mirror. The pinhole assembly has a pinhole positioned on the axis of the beam of light reflected by the focusing mirror. A servo-motor actuates the flat mirror and the focusing mirror, in unison, relative to the pinhole assembly in a direction parallel to the axis of the input beam of light. The parallel movement of the mirror acts to vary the proportion of the input beam of light that passes through the pinhole. The servo mechanism is bulky and requires a relatively long period of time to move the mirrors relative to the pinhole assembly.

The present invention is a variable optical attenuator (VOA) which has a semiconductor micro-electro-mechanical device for positioning a reflecting surface in any of a plurality of positions, each providing a respectively different amount of attenuation.

The variable optical attenuator includes a Icns, a first optical waveguide, and a second optical waveguide. A semiconductor micro-electro-mechanical device is positioned on a side of the lens opposite the first and second optical waveguides. The device has a reflecting surface. The reflecting surface has a normal position in which light from the first waveguide reflects off of the reflecting surface and passes through the lens into the second waveguide. The reflecting surface has a plurality of respectively different attenuation positions in which light from the first waveguide reflects off of the reflecting surface and passes through the lens, but an amount of light entering the second optical waveguide is attenuated by respectively different amounts corresponding to the respectively different positions.

According to a further aspect of the invention, a method for controlling a beam of light includes providing a lens, first and second optical waveguides, and a semiconductor micro-electro-mechanical device positioned on a side of the lens opposite the first and second optical waveguides. The devices having a reflecting surface. The reflecting surface is pivoted to a normal position in which light from the first waveguide reflects off of the reflecting surface and passes through the lens into the second waveguide. The reflecting surface is pivoted to a plurality of respectively different attenuating positions in which light from the first waveguide reflects off of the reflecting surface and passes through the lens, but an amount of light entering the second optical waveguide is attenuated by respectively different amounts corresponding to the respectively different positions.
Click to see VOA fiber optic price

Using Fiber Optic Attenuators to Increase Bit Error Rate

Fiber optic systems transmission ability is based on the optical power at the receiver, which is reflect as the bit error rate, BER is the inverse of signal-to-noise ratio, high BER means poor signals to noise ratio. Too much power or too litter power will cause high bit error rates.

When the power is too high as it often is in short single-mode systems with laser transmitters, you can reduce receiver power with an fibre attenuator. Attenuators can be made by introducing an end gap between two fiber, angular or lateral misalignment, poor fusion splicing, inserting a neutral density filter or even stressing the fiber. Both variable and fixed attenuators are available.

Variable attenuators are usually used for margin testing, it is used to increase loss until the system has high bit error rate. Fixed attenuators may be inserted in the system cables where distances in the fiber optic link are too short and excess power at the receiver causes transmission problems.

Generally, multimode systems do not need attenuators. Multimode source, even VCSELs, rarely have enough power output to saturate receivers. Single mode system, especially short links, often have too much power and need attenuators. For a single mode application like analog CATV systems, the return loss or reflectance is very important. Many types of attenuators suffer from high reflectance, so they can adversely affect transmitters just like highly reflective connectors.

Attenuators can be made by gap loss, or a physical separation of the ends of the fibers, including bending losses or inserting calibrated optical filters. Choose one type of attenuator with good reflectance specifications and always install the attenuator at the receiver end of the link. It is very convenient to test the receiver power before and after attenuation or while adjusting it with your fiber optic meters at the receiver, plus any reflectance will be attenuated on its path back to the source.

When testing the system power, turn on the transmitter, install the attenuator a the receiver, use a fiber optic power meter set to the system operating wavelength. Check to see whether the power is within the specified range for the receiver. For accurate measurements, the fiber attenuators connector types much match the lanch and receive cables to be tested, e.g. LC fibre optic attenuators is needed to work with the LC fiber patch cable, it work in 1250-1625nm range with optional attenuation value from 1dB to 30dB.

If the appropriate attenuators is not available, simply coil some patch cord around a pencil while measuring power with your fiber optic power meter, adding turns until the power is in the right range.

MEMS Based Variable Optical Attenuators

It is commonly known that fiber optic attenuators are used in fiber optic communications, as fiber optic tester tools to test power level margins by temporarily adding a calibrated amount of signal loss, or installed permanently to properly match transmitter and receiver levels. According to its stability, it divided into fixed fiber optic attenuators and variable optical attenuators. Variable fiber optic attenuators generally use a variable neutral density filter, with advantages of being stable, wavelength insensitive, mode insensitive, it offers a large dynamic range.

With the rapid increases in traffic on optical telecommunications systems, there is an active program for developing transmission devices for use in wavelength division multiplexing (WDM), which is becoming mainstream technology for providing higher transmission speeds and a larger number of signal channels. It has been suggested that in the WDM systems of the future, variation in power due to the wavelength could be reduced a the quality of transmission improved by adjusting the power after demultiplexing into individual signals wavelengths. It is envisaged that the current method, in which the power of all the multiplexed optical signals is adjusted by a single variable optical attenuators (VOA) would give way to a method in which one VOA is used for each wavelength. Given the number of multiplexed wavelengths, this change will require VOAs that are considerably more compact. Against this background, There have developed a VOA using micro-electromechanical system (MEMS) technology with loss characteristics that have low wavelength dependence.

Single-mode fiber was used as the input and output of the VOA developed here, with a graded index fiber having the same diameter, 125um, as the SMF fusion spliced for a specified length, to form an optical coupling with a lens function. An anti-reflection coating is applied to the tip of the GIF (graded index fiber). GIF tip is polished at an angle so that the light beam emitted from the end of the GIF is not aligned with the optical axis of the fiber, but is at an angle to it. This angled optical beam is interrupted by means of a shutter that has been formed by inductively-coupled plasma deep reactive ion etching. The MEMS chip uses a silicon-on-insulator wafer, with the shutter, actuator and fiber grooves formed simultaneously on the chip by ICP-DRIE, followed by metal vapor deposition over the whole chip.

The actuator of the MEMS chip is of the comb type, and the GIF is held in the fiber grooves by means of adhesive. The MEMS chip with this GIF optical coupling system is fixed by adhesive within a casing, which is hermetically sealed.

MEMS variable optical attenuators are variable in three different configurations. The VA series works in transmission, whereas the VP series uses reflection to modulate the attenuation. The VX series is the VP or the VA series in mint plastic packing. In terms of performance, the VP series achieves lower insertion loss and better Polarization dependent loss characteristics. Whereas the VA series allows for an easier array integration and is the lower cost.

FiberStore offers a full line of optical attenuator variable testers, they are often combined with an active system component to maintain optical power on a network even if the power changes in the input signals. Our automatical variable optical attenuators are specifically designed for use in DWDM networks with individual channel source elements such as add/drop transmitters. The cost and performance characteristics of our automatically variable optical attenuators are specifically targeted to allow for the use of these devices in volume as principal DWDM channel stabilization components.