Fiber-optic attenuators are a specific type of optical attenuators which is used in fiber optics. Usually, such devices 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 in 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 law 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, in the case of multimode devices also a negligible mode dependence. However, available devices may differ quite a lot in such respects.
Many different working principles can be applied; some examples:
- An attenuator may contain an air gap (possibly adjustable in width) between two fiber ends, 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 suitable for multimode device ( because of the strong mode dependence of the losses), also not ideal for broadband signal 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 lead 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.
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.
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.
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
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.
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. Example cases, however, one may require a higher precision.
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 two use attenuators with particularly high return loss, i.e., weak back reflection, e.g. in order to avoid parasitic lasing.
Maximum Optical Power
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.
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,
- if excessive nonlinear 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.
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