Electronic variable optical attenuators advance optical networking

The evolution of worldwide communications networks will hinge on the development of more reliable and cost-efficient optical components.

Marcella R. Backer

Corning Inc.

Demand for higher bandwidth in communications networks, driven by the Internet and wider adoption of broadband services, is propelling fiber-optic technology development toward higher-capacity solutions that make economic sense. A key element in this evolution is the development of reliable, low-cost optical components.

This point is particularly valid in the area of optical amplification. Simplicity (and its ultimate promise of lower cost and higher reliability) has fueled the adoption of erbium-doped fiber amplifier (EDFA) technology. As the number of channels increases in communication networks, EDFA designers must find ways to adapt the technology without sacrificing simplicity. Recent advancements in variable optical attenuator (VOA) technology are a good example of how an emphasis on simple design contributes to the overall development of the optical layer.

The need for VOA technology

Increasing the number of channels in a communications network affects how optical amplifiers and other parts of the system work. EDFAs, for example, are sensitive to optical input power, which can be a function of the number of channels. As the number of channels increases or decreases, so does the optical power entering the EDFA. The variation in optical input power can have several undesirable effects on the EDFA, such as the introduction of spectral gain tilt. Gain tilt is defined as non-uniform gain charges for changing inversion conditions. Mainly to reduce gain tilt and keep the average inversion power constant, VOAs were introduced into the design of wideband amplifiers. These devices are used to mitigate the chromatic effects in the fiber as well as stimulated Raman scattering, both of which add to gain tilt in the channel spectrum.

VOAs also allow for varying span losses between EDFAs. Regulating span loss between optical amplification is important for the same reasons already stated, namely controlled optical input power to the EDFA. Before the adoption of VOAs, system designers attempted to devise systems with constant span, sometimes deploying manual optical attenuators. These attenuators had to be set at the initial installation of the system for optical amplification and tuned over time as the system aged or as the system link was modified.

Improving the technology

Despite the advantages of VOAs, many of the current designs require the optical path to be collimated, manipulated, and then collimated again. This complexity of materials and subcomponents can lead to degraded optical performance over time. It can also lead to high insertion loss and polarization sensitivity, which must be considered in the overall optical performance of the amplifier.

One way to address these concerns is through the design of electronic VOAs. One result of the technology pursuit was the application of patented MultiClad coupler technology. This coupler is a device in which two or more optical fibers are encased within an optical glass sleeve, heated, and fused. The coupler attenuation is designed to be uniform over wavelengths, allowing for a very low insertion loss and spectral non-flatness of the VOA.

This design also offers an extremely strong fused region due to the additional cladding layer glass. Unlike other coupler designs, such as Fiber Biconic Taper, which is relatively fragile, the strong fused region of the MultiClad coupler is an advantage in VOA applications. The result is an optical attenuator that does not include free-space collimating optical elements, meaning the optical path is free from interruption and completely contained within the optical fiber of the 1ٴ device. This design offers high reliability, low insertion loss, and low polarization-dependent loss.

Operating principle

The 1ٴ MultiClad coupler carries all of the optical power in one leg. As the coupler is precisely bent (1-mm range of motion), it diverts an increasing fraction of the input power to the tap leg of the device. The bending of the coupler is accomplished through a precision stepper motor. The stepper motor is controlled using TTL voltages for increment, decrement, and reset. When the stepper motor is signaled electronically to increment, it will precisely bend the coupler, thereby increasing the attenuation of the system leg of the 1ٴ coupler (see Fig. 1). As the number of steps is increased, the amount of attenuation increases. The optical tap leg receives the diverted system power (see Fig. 2).

The power that is transmitted to the monitor (tap) leg can be used as a means to predict the optical attenuation of the output leg, eliminating the need for optical tap splitters downstream of the VOA. The optical attenuation of the VOA is required over a wide wavelength range for many of today`s applications. Figure 3 represents the attenuation of the system output at different levels between 1510 and 1570 nm.

Latching, a feature defined as the ability to hold the attenuation level with or without electrical power to the attenuator, is important in certain systems. Many optical system and EDFA designers require the control of the optical signal to be stable even during power outages. It is also required to hold the attenuation level when the power returns. The latching in the electronic VOA is due mainly to the stepper motor chosen in the design of the device. The stepper motor is magnetically stabilized when power is eliminated, holding the position of the coupler and, therefore, the attenuation. This feature is not found in many VOA designs today.

System and EDFA designers also require thermal stability. The performance of the VOA needs to be stable over a temperature range of 0 to 70C in most applications. The design of the MultiClad coupler package accomplishes this through a passive athermalization approach where the materials incorporated into the package compensate for expansion over temperature. The result of this type of design is an attenuation change over the full temperature range to less than 0.4 dB.

Alternative VOA approaches

Many VOA technologies on the market today can be classified into two categories: devices with moving parts and devices with non-moving parts. In the latter category, the devices are based on thermal-optic (planar and fiber), electro-optic, and magneto-optic technologies. These devices can have difficulty meeting the latching requirements of many optical network designers when power loss occurs in the system. VOA devices with non-moving parts also struggle with high power consumption, higher insertion loss, polarization sensitivity, and long-term reliability.

Typically mechanical in nature, devices in the first category use moving parts that can meet the key requirements of optical network system designers. Many of these designs manipulate the optical path with collimators both at the entrance and exit of the VOA. Between the collimators is a beam of light, which is often physically blocked with an eccentric cam. Another method is to move the collimators in and out of focus with a small motor. Both of these methods result in optical power attenuation.

Another widely used design is the application of a variable neutral-density filter or optical mirror. Depending on the physical position of the filter/mirror, it either absorbs light or deflects portions of the light out of the collimated beam, which results in system attenuation.

Most of these VOA designs use small motors to move a portion of the device into and out of the collimated beam or the collimators themselves. The motor design allows the VOA to hold position upon loss of power depending on the motor specified. A number of difficulties, however, can arise with these types of devices:

The design involves complicated materials.

Micro-machining is required for manufacturing assembly and alignment.

Complex designs can lead to high electrical power consumption.

The sensitivity of all of these materials makes them vulnerable to various environmental conditions, including temperature, humidity, and vibration.

Finally, many of these opto-mechanical devices have difficulty meeting the minimum insertion-loss requirements as well as other optical performance characteristics.

After comparing technologies, it`s clear that a simpler design makes a substantial contribution in the effectiveness of VOA technology. Implementing a coupler such as the MultiClad device as the optical medium can offer low insertion loss, thermal stability, and an environmentally robust package. Another advantage of the design is an inherent optical monitor port without the signal loss and expense normally incurred from splitters. Because the device incorporates a latching mechanism, the output signal through the VOA will remain constant even with electrical power loss. Using a self-compensating passive athermalization approach limits the insertion loss drift over the full temperature range. The coupler is also inherently low in polarization sensitivity over the attenuation ranges and wavelengths required by the optical system and EDFA designers.

Recent improvements in VOA technology are essential in the development of wideband amplification. Continuing emphasis on the reliability of optical components contributes to the overall development of the optical layer. u

Marcella R. Backer is the manager of optical components, products, process, and development for Corning`s Photonic Technologies Div. (Corning, NY). Other authors who contributed to this article are David O`Brien, Scott Hellman, Bill De Boynton, Eric Dippolito, and Matthijs Broer.