fiber-optic attenuators (original) (raw)

Definition: optical attenuators for use in fiber optics, usually used with fiber connectors

Category: fiber optics and waveguides

• optical elements

  • optical attenuators
    * variable optical attenuators
    * fiber-optic attenuators
    * fixed or variable fiber-optic attenuators, gap loss attenuators

• optics

  • fiber optics
    * fibers
    * fiber connectors
    * fiber-optic adapters
    * fiber couplers
    * fiber-optic pump combiners
    * fiber bundles
    * fiber endface inspection
    * cleaving of fibers
    * fiber cleavers
    * fiber joints
    * fiber splices
    * fiber Bragg gratings
    * fiber cables
    * fiber coatings
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    * fiber recoaters
    * fiber coils
    * fiber collimators
    * fiber launch systems
    * fiber lenses
    * fiber loop mirrors
    * fiber patch panels
    * fiber shuffles
    * fiber-optic attenuators
    * fixed or variable fiber-optic attenuators, gap loss attenuators
    * fiber-optic plates
    * fiber-optic tapers
    * (more topics)

Related: optical attenuatorsfibersinsertion loss

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Contents

What is a Fiber-optic Attenuator?

Fiber-optic attenuators are a specific type of optical attenuators which are used in fiber optics, e.g. for achieving a suitable signal level for a data receiver in a telecom system.

Usually, such attenuators either have a housing equipped with some type of fiber connectors (e.g. FC/PC or LC/APC) for easy connection with fiber patch cables, or they are integrated into patch cables (in-line attenuators). Connectorized attenuators often have a quite compact housing, essentially looking like a fiber-optic adapter.

Fixed or Variable Attenuation

Some of these devices provide a fixed level of attenuation, quantified as the insertion loss in decibels. One may, for example, have a couple of attenuators with 1 dB, 5 dB and 10 dB, and by properly combining those one can realize a wide range of attenuation levels — the decibel values are simply additive.

Other devices are variable optical attenuators, providing an amount of insertion loss which is adjustable within some range (e.g. 2 dB to 50 dB), e.g. with some adjustment wheel or screw. In some cases, the attenuation can only be adjusted in steps of e.g. 1 dB, e.g. by exchanging absorber glasses.

Different methods of adjusting the attenuation level may differ a lot in terms of convenience. It is relatively cumbersome to exchange filters, easier to just turn a screw, and most comfortable to select attenuation levels interactively on an electronic device with a few keys and a digital display showing the current loss setting.

Ideally, the attenuation would be precisely adjustable in a wide range, stay stable over long times and exhibit a negligible influence of wavelength and polarization, and, in the case of multimode devices, also a negligible mode dependence. However, available devices may differ quite a lot in such respects.

Working Principles

Many different working principles can be applied; some examples:

• An attenuator may contain an air gap (possibly adjustable in width) between two fiber endfaces, so that only some of the light leaving the input fiber gets into the core of the output fiber. That is the principle of the so-called gap loss attenuator.
• Similarly, one may use an intentionally misaligned fiber splice. Here, however, the insertion loss depends quite critically on the degree of misalignment.
• An attenuator may contain two lenses for collimating the beam coming from one fiber and launching it into the second one. Between those lenses, there can be some kind of blocking device, e.g. a movable blade. That approach is not well suited for multimode devices (because of the strong mode dependence of the losses), also not ideal for broadband signals because of the wavelength dependence.
• As an alternative, attenuation may be achieved with a more sophisticated setup as used in some bulk-optical variable attenuators. For broadband and mode-independent operation, it is preferable not to use any spatial dependence. For example, one may use a neutral density filter (inserted at some angle to avoid parasitic reflections) for fixed attenuation, or an apparatus with two or more moving optical components for variable attenuation.
• One may also exploit bend losses. Such devices may simply be a plastic housing through which a fiber cable can be led with one or more tight turns, possibly with different radii. Apart from the simplicity, an advantage of that approach is that any air–glass interfaces and critical alignments are avoided. That approach, however, introduces a substantial wavelength dependence: light at longer wavelengths is normally attenuated more strongly.
• Some devices contain a piece of fiber where the fiber core is doped with a material which provides a suitable amount of absorption.
• Another possibility is to use a fiber coupler with evanescent wave coupling, exploiting the fact that some (typically fixed) fraction of power is sent to another output port.
• There are also attenuators based on MEMS (micro-electro-mechanical systems). For example, this may involve movable micromirrors, shutters or microlenses.

Various Aspects of Importance

A wide range of details can be relevant for different applications. The most important of those are discussed in the following.

Wavelength Dependence

Generally, the obtained insertion loss has some dependence on the optical wavelength. Some attenuators have a relatively strong wavelength dependence and are made for working in narrow wavelength regions, e.g. with a bandwidth of only 20 nm around a center wavelength of 1550 nm. Others are optimized for a weaker wavelength dependence, making them usable for broadband light, e.g. for the full C band, filled with a lot of DWDM channels.

Polarization Dependence

As light in fibers often does not have a well-defined polarization state, it is important that a fiber-optic attenuator exhibits only a minimum amount of polarization dependence.

Single-mode and Multimode Attenuators

Most fiber-optic attenuators (e.g. for telecom applications) are connected to single-mode fibers. Others can work with multimode fibers.

For multimode attenuators, the possible dependence of the insertion loss on the modes may be an issue to observe. For example, if an attenuator is realized with a moving blade, blocking a free-space beam to some extent, it will generally have a substantial mode dependence. That implies that the effective attenuation will depend on the distribution of optical powers over the fiber modes — which is generally undesirable. However, with some other principles of attenuation used, such a mode dependence can be largely avoided.

Reciprocity

For single-mode devices, the insertion loss can not depend on the direction of propagation, as long as no non-reciprocal parts are used, as e.g. in a Faraday isolator. For multimode devices, however, some loss difference is possible in conjunction with a mode dependence.

Precision of Loss

For many applications, it will not be a problem if the obtained insertion loss slightly deviates from the specification (e.g. by 1 dB), or if it slightly changes over time. In some cases, however, one may require a higher precision.

Return Loss

Most fiber-optic attenuators exhibit a relatively high return loss (at least several dozens of decibels), i.e., there is not much light which is reflected back into the input fiber. For some sensitive applications, e.g. when using an attenuator before or after a high-gain fiber amplifier, one may have to use attenuators with particularly high return loss, i.e., weak back reflection, e.g. to avoid parasitic lasing.

Maximum Optical Power

Generally, the removed light is converted to heat in the device. Therefore, only a limited amount of optical power (e.g. 200 mW) can be handled; otherwise, the attenuator may be damaged.

Substantially higher power levels (e.g. several watts) are usually not possible, and are also usually not required in the context of fiber communications, the main application area. In the area of high-power fiber lasers and amplifiers, fiber-optic attenuators are hardly usable.

Custom Versions

Although the basic function of a fiber-optic attenuator may seem quite simple, characterized by a single number (the insertion loss), quite a few additional parameters may have to be properly chosen for a certain application — for example, the operation wavelengths, the fiber or connector type, and the length of attached fiber (in the case of pigtailed devices). Therefore, it may be necessary to use custom versions to meet all requirements.

Applications of Fiber-optic Attenuators

Fiber-optic attenuators are used throughout the field of fiber optics. For example, they are common in the area of optical fiber communications. Here, one may e.g. use such an attenuator

• if an optical signal would otherwise overload a fiber-optic signal receiver (thus deteriorating the quality of the received signal, or even damage the receiver),
• if excessive nonlinear optical effects in a fiber-optic link would result,
• if the balance of channel powers in a WDM system would otherwise not be achieved, or
• to test the bit error rate of a telecom system as a function of signal power level at the receiver.

The examples show that some attenuators are required only for temporary tests, while others are permanently integrated into telecom systems.

Frequently Asked Questions

This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).

What is a fiber-optic attenuator?

A fiber-optic attenuator is a passive device used in fiber optics to reduce the power level of an optical signal. It is often used in optical fiber communications to adjust the signal to a suitable level for a receiver.

What are the main applications of fiber-optic attenuators?

They are used to prevent optical receivers from being overloaded, reduce unwanted nonlinear optical effects in fiber links, balance channel powers in WDM systems, and for testing the performance of telecom systems.

What is the difference between fixed and variable fiber-optic attenuators?

Fixed attenuators provide a constant, specified level of insertion loss (in decibels). Variable optical attenuators allow the attenuation to be adjusted within a certain range, for example with an adjustment screw or electronically.

How do fiber-optic attenuators work?

Common principles include creating an air gap between fiber ends (gap loss), inducing controlled bend losses, using an absorptive doped fiber, or employing a fiber coupler to divert a portion of the light away from the main path.

What is return loss and why is it important for attenuators?

Return loss measures how little light is reflected back towards the source. A high return loss is important to avoid instabilities, such as parasitic lasing, in systems containing components like fiber amplifier.

Can fiber-optic attenuators handle high optical powers?

Generally, they cannot. They are designed for low optical powers, typically at most for a few hundred milliwatts, as the attenuated light is converted into heat. They are unsuitable for high-power fiber lasers and amplifiers.

Do fiber-optic attenuators affect all wavelengths equally?

Not always. The attenuation often has some wavelength dependence. While some attenuators are designed for narrow wavelength bands, others are optimized for broadband use, e.g., across the C band in telecom systems, by minimizing this dependence.

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