Array-based VOAs offer compact signal control

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Nigel Cockroft

Variable optical attenuators are used in WDM systems to provide gain equalization in optical amplifiers, channel blanking for network monitoring, and signal attenuation to prevent detector saturation. A polymer-waveguide-based thermo-optic VOA array can offer low insertion loss and compatibility with planar processing.

Precise active control of optical signal level is essential for optimal performance of dense wavelength-division-multiplexing (DWDM) networks. Direct control of transmitter-laser drive power is undesirable and impractical in high-speed transmission systems because of the wavelength fluctuations and unpredictable performance changes when components age. Instead, variable optical attenuators (VOAs), which attenuate power in optical fiber by various means, are used to control signal levels precisely.

Many earlier optical network designs relied heavily on factory-set fixed-value attenuators. Recently, large increases in the WDM channel count, transmission speeds, total signal-power level in fibers, and the use of new optical components in networks have all led to increased demand for high-performance, electronically-controlled VOAs, in which attenuation levels may be constantly adjusted in the field using network management software.

Single-port VOAs (usually bulk-optic devices based on disturbing the transmission properties of a section of optical fiber by mechanical, thermo-optic, or filtering means) have dominated the traditional network applications in which only one or two fibers requiring attenuation control are present at one location. Such applications include attenuation control on individual line cards and total signal-level control of the optical input to erbium-doped fiber amplifiers (EDFAs). These devices tend to offer good optical performance but in relatively bulky and expensive packages.

Emerging network-system architectures require dynamic control of many signal levels on multiple fibers in a single location and demand a compact form factor. Array-based electronically controlled VOAs offer a good solution for simultaneous independent control of the signal levels of many fibers, whether they carry individual wavelength channels, bands of a few wavelengths, or a full complement of wavelength-multiplexed signals.

Array-based VOA technologies tend to offer advantages in some properties, such as compact form-factor and cost (per port), relative to fiber-based single-port devices. But these multiport devices typically have not matched fiber-based single-port VOA performance in other properties, including insertion loss, polarization independence, and power consumption. New VOA technology based on waveguide arrays in thermo-optic polymers offer a promising combination of fiber-attenuator-like performance along with the additional advantages offered by a waveguide array format.0601feat5 1

ATTENUATION EFFECTS
Variable optical attenuators are used in several ways to optimize network performance and to avoid distortions due to detector saturation and nonlinear effects in fibers and amplifiers. Important applications include signal pre-emphasis at the head of DWDM transmission systems, channel balancing after lossy network node-switching operations, and automatic gain control of amplifiers to ensure optimal and predictable spectral dependence of gain. Span balancing is used to ensure that the signal strengths at the users' site are the same, allowing the same kind of equipment to be used at the various sites. VOAs are also used to correct for ageing effects of other network components, such as lasers, modulators, multiplexers, and detectors (see figure, p. 81).

Over the lifetime of a network, the typical attenuation levels for a given VOA may continually decrease to correct for the reduction in performance of lasers and thereby maintain total network performance. But industry trends toward higher wavelength-channel count, higher-speed data transmission, greater optical transparency—including all-optical switching— and dynamic metro networks with reconfigurable add/drop nodes rapidly increase demand for small form-factor, cost-effective VOA arrays.

Many network suppliers also achieve reduced installation costs and fewer installation errors by using electronically controlled variable attenuators, which can be optimized in the field when networks are first lit or upgraded, rather than using fixed attenuation (prone to installation or factory-set errors in very complex networks). Reduced cost per port of array-based VOAs will likely increase their use in new networks to replace fixed attenuators.

VOA TECHNOLOGIES
Ideal VOAs enable precisely controlled reduction of signal level without disturbing other properties of signal light. For example, they should be polarization-independent, typically characterized by low polarization-mode dispersion and polarization-dependent loss (PDL), and deliver the same attenuation independent of wavelength over the spectral band used by the network. Insertion loss should be low so that signal transmission is maximized when no attenuation is added.

There should be a smooth and simple functional dependence of attenuation on the electronic control signal, and this function should be independent of environmental operating conditions. Generally, VOAs using low voltage or current drives are the easiest to integrate on a systems control board. A dynamic range of up to 15 to 30 dB is required for many applications and achievable with most technologies.

Trends toward increasing network capacity using less physical real estate and higher equipment-packing density drive a need to minimize VOA footprint, power consumption, and heat generation. Excellent electrical efficiency is desired to simplify power and thermal management. Thermal stabilization using heaters or thermoelectric coolers (TECs), which further add to power consumption and heat generation, is undesirable. Ideal VOAs will not require thermal stabilization.0601feat5 2

Comparing the technologies used for VOAs showcases the better models among those available. Single-port VOAs, based on discrete fiber technology, typically exhibit very low insertion losses and polarization dependence but are not easily scaled to compact multiport formats (see Table 1). The more traditional and popular means of achieving attenuation in single-fiber devices include mechanical perturbation of the beam using a stepper motor-driven filter, blade, or optical beam-path misalignment. These devices have great optical performance but are relatively slow and bulky.

More recent single-fiber products include those that combine a side-polished fiber section with attached polymer. Temperature variations cause controlled leakage of light from the fiber due to thermo-optic effects. The electro-optic effect is also being used in some materials to provide VOAs with <300 µsec response times, approximately 10 to 100 times faster than most other VOA technologies. Microelectromechanical systems (MEMS) technology can also be used in attenuators by making well-controlled adjustments to the alignment of light in an optical path.0601feat5 3

ARRAY-BASED TECHNOLOGIES
Various technologies have recently been developed for use in multiport VOAs, some of them based on waveguides in intrinsically wafer-scalable arrays, while others consist of several single-port devices inside a bigger package (see Table 2). These technologies can also be used in single-fiber devices but tend to offer greater advantage in their array-based versions. Many of the array-based technologies, including silica waveguides on silicon, typically use a thermo-optically driven Mach-Zehnder interferometer (MZI) to control the attenuation. The MZI technology is very temperature-sensitive and the device temperature needs to be tightly controlled with a TEC or heater.

The thermo-optic effect may also be applied in polymer waveguide devices. Because the change in refractive index with temperature is typically 100 times greater in thermo-optic polymers than it is in silica, polymer devices use much less power to achieve a given attenuation than silica-based devices. Recent developments in thermo-optic polymer engineering have enabled array-based devices with extremely low electrical-power consumption, small size, high reliability, and insensitivity to ambient operating conditions, thereby enabling the elimination of power-hungry TECs (see "Polymer VOAs provide robust high performance," p. 84).

Nigel Cockroft is director of product marketing at Gemfire, 2471 East Bayshore Road, Palo Alto, CA 94303. He can be reached at 650-849-6871 or at n.cockroft@gemfire.com.

Polymer VOAs provide robust high performance
Although many material platforms exist for VOA components, polymers are particularly advantageous because their material properties (such as refractive index, mechanical properties, thermo-optic coefficient) can be precisely tuned to produce device performance superior to other material platforms.

Custom polymers are being developed, engineered, and processed to produce high-performance eight-port VOA arrays. The thermo-optic design consists of a single resistive heater element fabricated above an embedded channel waveguide. The attenuator operates by applying a current or voltage to the heater element that creates a strong localized thermal gradient and proportionally large refractive-index gradient. Precisely controlled deflection of light from the waveguide (attenuation) occurs by adjusting the applied heater power and produces 30 dB of continuously tunable dynamic range with 0.1-dB accuracy.Acf153

A combination of high thermo-optic coefficient and device design results in a 30-dB attenuation with as little as 10 mW of heater power, compared to 250 to 500 mW for silica-based devices (see Fig. 1). The low electrical-power consumption directly results from intrinsic material properties, such as low thermal conductivity and high thermo-optic coefficient, unique to these polymers. Additionally, this design results in negligible crosstalk between the output ports because the deflected light propagates away from the heater element and into the substrate. Because the device is thermal-gradient-activated, it is insensitive to ambient temperature changes, unlike interferometric-based devices, and therefore does not require a TEC. This property results in significant power and cost savings, and eliminates a common device-failure mode.

Equally important to large dynamic attenuation range are insertion loss and PDL. Core and cladding polymers were engineered with a refractive-index difference and cross-sectional shape that provides a mode-field diameter perfectly matched to Corning SMF 28 glass fiber. With this embedded symmetric waveguide design, the insertion loss attributed to mode mismatch at the input/output fiber and polymer-waveguide interfaces is negligible.

The polymer material dispersion is similar to silica transmission fiber, thus guaranteeing single-mode wavelength operation over the entire C-band. The combination of mode-matching and short (5.0 mm) optical-path length yields a 1.0-dB fiber-to-fiber insertion loss. The embedded symmetric waveguide structure and low material birefringence (< 2 x 10-4) results in less than 0.2-dB device PDL.Acf14f

Environmental stability of materials is critical for device reliability. Specially designed ultraviolet-curable polymers resist thermal ageing in thermo-optic applications, achieve environmental stability, and enhance device performance by increasing the thermo-optic coefficient. The polymers were made from a series of miscible polymers that were blended to adjust refractive index and create core/cladding compositions with similar mechanical properties. The environmental stability of the polymers was evaluated by monitoring the refractive index of a slab waveguide as a function of time in an 85°C/85% RH temperature and humidity chamber (see Fig. 2).

After an initial densification period, the refractive index remained stable for the rest of the thermal ageing experiment and the duration of the test (>200 days) at 85/85 conditions. Even after this extended exposure, the material showed no signs of the polymer-yellowing associated with conventional polymers. Frustrated total internal reflection (FTIR) and other analytical measurements confirmed that chemical composition remained unchanged by such extreme environmental conditions.

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