EVOAs continue to evolve
Electrically variable optical attenuators (EVOAs) are now ubiquitous throughout optical networks, as the desire for self-regulating networks and central-office control instead of truck rolls becomes the norm. EVOAs have been developed with a wide variety of attenuation mechanisms, but ultimately all must absorb some of the optical power and pass through the rest.
VOA designs can be divided into two main groups: those based on mechanical movement of a shutter or element and those based on other mechanisms. The variety of EVOAs is thinning based on cost, insertion loss, robustness, and integration with other functions. Mechanical solutions still dominate the market, ranging from legacy stepper-motor designs to MEMS and MOEMS designs. EVOA performance has progressed to the point where most devices are fit for use in the most common applications. Now, other factors such as price and additional functionality factor into product selection.
Virtually every implementation of an EVOA includes a sensing and feedback loop for control. This closed-loop control virtually eliminates many of the variances on the attenuation due to environmental factors and any drift in the component. When operated in closed loop mode, performance factors such as repeatability take a back seat to reliability.
Since virtually all EVOA implementations are done with active monitoring and feedback, an integrated photodiode (PD) that monitors the signal is the first level of integration that becomes useful to the system designer. Rather than using a separate tap coupler and PD to first monitor the signal as well as a separate EVOA to attenuate the signal to the proper level, all three of these bulky components can be eliminated with one compact tap VOA (TVOA). These TVOAs thus provide 3-for-1 economy of scale without sacrificing insertion loss or performance.
TVOAs are available in four basic configurations depending on the default mode of the attenuation element and position of the monitoring PD in the light path. The "shutter" in relaxed mode can be normally closed or normally open. Each of these two versions can have the PD monitoring the input signal or attenuated signal.
Due to the typical uses of EVOAs, the bandwidth of the feedback loop can be quite low. With these low-bandwidth requirements the internal monitoring photodiode can be run unbiased in photovoltaic mode, thus eliminating dark current issues at higher temperatures. For a typical indium gallium arsenide (InGaAs) PD, dark current doubles for every 7°C increase in temperature. The PD circuit is better off being operated in the unbiased mode, since bandwidth is a non-issue and the minuscule gain in top-end linearity does not make up for the dark current effects on the bottom end of the detection range. In an attempt to remedy this situation, some log amplifier manufacturers have released products that will actively change the bias. However, such devices often induce noise, and the dark current will likely still be a controlling factor at low power levels.
TVOAs that further integrate electronics and digital interfaces are now appearing on the market. A digitally controlled EVOA (DVOA) with an integrated PD to control the set point will have the photodiode monitor the attenuated signal. Having the PD on the output is one of the few common attributes that these DVOAs have. Their designs diverge depending on the range of optical powers over which the feedback loop must perform. More than 30 dB of change in optical power on the output leg and a log amplifier is the preferred application. But with less than 30-dB change, a linear amplifier will usually suit the requirement at a much lower cost.
DVOAs with internal feedback can be thought of as either regulators (constant attenuation) or active controllers (constant output). With a regulator implementation, the input must be known or assumed to be constant. As an active controller, the system can request an output level regardless of input (provided it is within the control range) and not have to revisit the device unless an alarm condition is activated indicating that the input has exceeded the control range. Output from the internal PD can be made available in analog or digital form for other control or verification purposes. As component manufacturers integrate TVOAs with the electronics required for a feedback loop, direct digital control of optical paths becomes possible.
TVOAs are widely used for output power control to protect receiver PDs in networks and test equipment from saturation and to balance the power of several wavelengths before combining them for amplification and launch. For receiver applications a monitor PD on the input is the preferred solution, since the receiver PD and circuit can supply feedback information for VOA control. This combination then produces the desired output-signal level for the receiver's circuitry while verifying that signal is present for protection switching.
VOAs are becoming more compact and cost-effective. They have evolved from three separate optical components that require an equal number of splices and a significant amount of valuable board space devoted to feedback and control circuitry to a highly integrated solution with a small footprint. As cost-cutting increases and rack space diminishes, TVOAs and DVOAs will see greater use.
William J. De Costa is director of marketing at Oluma (Carlsbad, CA). He can be reached at email@example.com.