Optical Attenuators and where they can be used in a Fiber Optic Link

Optical attenuator is used to reduce the optical power level of an communication signal, in an optical fiber used in a fiber optic communication link. Optical attenuators use different principles of functioning like gap-loss, absorptive or reflective techniques. Three basic types of optical attenuators are step wise variable, continuously variable and fixed.

One customer asked for 6 fiber cable for use in CATV purpose. Instead of inquiring over phone or sending a fax or e-mail, he came personally to our manufacturing division and was interested to get the cable as soon as possible. He needed around 8.2 kilometers of Central tube design 6 Optical fiber cable that will be installed on poles.

As his requirement was urgent and it will take at least one day for manufacturing new cables for him, he was asked if it is possible for him to select cables from our stock. Over a period of more than 3 years there was an accumulated stock of around 23 kilometers of 6F central tube design optical fiber cables with two FRPs embedded in the PE sheath. This is the common design employed in India for CATV. The fiber count will vary, but the basic construction up to 12 fibers will be same.

The reason for accumulation of those cables was the high attenuation at both the wavelengths of 1310nm and 1550nm. There was a pressure from the management to reduce such Non-conformed products as it occupied space and unwanted inventory. The arrival of our new customer will be a new step for clearing that stock.

When talked about his application area and what systems he will employ in CATV network, interestingly he mentioned about the Optical attenuators. He told he use Optical attenuators near the transmitting end to increase the attenuation in case the line length is short and enough attenuation is not there in the total fiber optic link for the system to detect optical signals. He was shown the attenuation test results for those cables in the stock. Many of them had attenuation in the range of 0.8 dB/km at 1310nm and more than 1.0 dB/km at 1550nm.

Clearly this was due to the bend loss and manufacturing problems like unequal shrinkage of central tube and Polyethylene outer sheath material. After calculating the fiber optic link loss for his CATV network systems he agreed to accept those cables. Thanks to the Optical attenuators which helped to reduce our Not OK product stock.

Optical attenuator is used to reduce the optical power level of an communication signal, in an optical fiber used in a fiber optic communication link. Optical attenuators use different principles of functioning like gap-loss, absorptive or reflective techniques. Three basic types of optical attenuators are step wise variable, continuously variable and fixed.

Optical attenuators using gap-loss principle shall be used at or near the transmitter end as they are sensitive to modal dispersion ahead of the attenuator. Attenuation is a limiting factor in optical communication and different modules like EDFAs are developed to counter those loss effects. It is strange to hear that some applications require low power level in order to the receiver detects the signal. This may be to avoid a situation where a module would not work as expected because of saturation of high power levels or when one channel starts influence other channels in a DWDM system due to non-linear effects.

The power levels of signal channels vary considerably from each other. Variations in the transmitter output power levels, different transmission lengths or the wavelength dependence of the optical components and modules are said to be the reasons for the above mentioned power level variation. Fixed optical attenuators are made by an air gap, a doped fiber, a filter glass introduced into the transmission path as absorption filters, or by bending an optical fiber in the transmission path. Doped fiber or filter glass-based fixed attenuators are coming in various connector styles by integrating them in the adapter housings.

Fixed type optical attenuators can be added as in line attenuators behind transmitters or amplifiers. Dynamic or variable type optical attenuators are preferable in order to have more control as the performances of the optical components and modules changes with respect to variables like time and temperature. A mechanism to bend a multiclad coupler will dynamically adjust optical attenuation.

Use of thermo-optic effect with Mach-Zehnder interferometers in planar lightwave circuit technology is another method as shown in the diagram below. Temperature dependence of the refractive index of the glass is used in this method. The waveguides are heated by the electrodes and lengthen the optical length of one arm of the interferometer. Depending on the induced phase shift, the interferometer at the output of the Mach-Zehnder interferometer results in a specific attenuation spectrum. A small portion of the light is coupled out and measured by a detector for controlling the heating current.

With planar lightwave circuits, we can integrate many variable optical attenuators on one chip in parallel, thereby allowing reduction in size and number of devices. The power levels of several DWDM channels can be adjusted dynamically in one device with this method.

Author: samirbhai

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