Packaging improves filter performance

Cylindrical, axially symmetric packaging and hard-coated thin-film elements create stable, low-loss filters, isolators, and wavelength-division multiplexers.

Dec 1st, 1997
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Packaging improves filter performance

Cylindrical, axially symmetric packaging and hard-coated thin-film elements create stable, low-loss filters, isolators, and wavelength-division multiplexers.

David Polinsky

The demand over the past two years for fiber amplifiers in wavelength-division-multiplexing (WDM) systems has led to tighter performance specifications for the passive components used within such amplifiers. In addition, WDM networks designed for dense channel spacing and long-haul systems with multiple amplifiers have created the need for new types of passive components such as wavelength lockers and voltage controlled attenuators--used within service provider terminal equipment--as well as gain-flattening filters and multiplexer/isolator hybrids--used within the amplifiers.

System requirements have fostered packaging-technology improvements that come close to meeting theoretical performance limits for maximum insertion loss, insertion-loss stability, and wavelength stability. Performance in these devices is now limited primarily by the material properties of the elements in the optical path, rather than by the package design itself.

Prior to the implementation of dense wavelength-division multiplexing (DWDM), an erbium-doped fiber amplifier (EDFA) consisted of a length of erbium-doped fiber pumped by a single laser operating at 980 or 1480 nm, coupled to the fiber in either the forward or reverse direction using a fusion-biconic-taper (FBT) coupler. In long-haul systems that contained multiple amplifiers, a single-stage isolator integrated into the EDFA input and output prevented backreflected and backscattered signals from destabilizing the upstream terminal equipment transmitter, the EDFA, or the pump laser.

In some cases FBT tap couplers were included for monitoring the input and output signal levels. Unfortunately, FBT tap couplers, pump couplers, and single-stage isolators have relatively narrow passbands for loss and isolation. High gain with low noise can thus only be guaranteed for a single carrier signal operating within a relatively narrow (䔮 nm) window about 1550 nm.

Dense WDM systems, introduced in 1995, require passive components to exhibit low-loss characteristics over a much wider window--1535 to 1565 nm for EDFAs, broader in the case of fluoride-doped fiber amplifiers. For the tap filter and pump coupler, these requirements can only be met by cylindrically packaged, thin-film components. The bandpass shapes for such components have wide, flat tops, which create low-loss transmission regions for the individual DWDM channels.

The basic construction consists of a thin-film-coated glass substrate sandwiched between two gradient-index (GRIN) collimating lenses, with ferrules coupling the signal into the component (see Fig. 1). Commercially available versions of these components measure just 4.5 mm in diameter and less than 40 mm in length (see photo above). The axial symmetry of the basic design, as well as use of index-matching epoxy between the GRIN lenses and the filters, produces devices with extremely low insertion loss and excellent environmental stability. Devices manufactured by DiCon Fiberoptics (Berkeley, CA), for example, typically achieve a 0.4-dB loss, with a thermally induced variation of less than 0.1 dB over the temperature range from -20°C to +75°C, and a center wavelength shift of less than 0.1 nm over the same range. Device performance is stable in high humidity, even for packages that are not hermetically sealed (see Fig. 2).

The low-loss performance is noteworthy, considering that each of the eight or more optical surfaces in the path introduces loss at the interface caused by small differences in the index of refraction. The GRIN lens material, thin-film coatings, and filter substrates also introduce fixed absorption and scattering losses.

Designs for different devices

Depending on the number of fiber ports in the two ferrules and the filter element used, cylindrically packaged WDM components can be assembled as two-port amplified-spontaneous-emission (ASE) filters or gain-flattening filters; as three-port pump couplers, bandsplitters, taps, or service-channel wavelength-division multiplexers; or as four-port add/drop multiplexers. Amplified-spontaneous-emission filters preserve the largest portion of the erbium spectral window above 1535 nm for DWDM channels while blocking ASE noise, which peaks at 1532. Gain-flattening filters minimize the effects of differential amplification of channels with a passband profile that approximates the inverse of the shape of the amplifier gain curve, equalizing the gain for all wavelengths.

In single-stage amplifiers, bandsplitters route bidirectional signals in the same direction through the gain medium; in dual-stage amplifiers, the bandsplitter divides the carriers into two bands. Service-channel wavelength-division multiplexers add and drop a telemetry signal at each amplifier. Add/drop filters can be used to statically or dynamically alter the content of each channel.

Often, seven or more cylindrically packaged passive components are used in each EDFA designed for DWDM channels (see Fig. 3). The signal loss incurred by so many elements provides strong incentive to develop integrated hybrid components that use thin-film technology to combine the functionality of an isolator, pump coupler, and/or tap. Such hybrids have the potential to significantly reduce network signal loss--and potentially component cost--by eliminating the need for additional GRIN lenses and discretely packaged components.

For example, traditional single-stage isolator designs begin with the same packaging geometry as above, but replace the filter element with two polarizers and a Faraday rotator. Adding a thin-film element results in a hybrid between an isolator and a wavelength-division multiplexer. For both hybrids and single-function components, the cylindrical geo metry and packaging maintains environmental stability while providing a rigid mounting structure for optical elements. Hy brid components that use thin-film technology to combine the functionality of an isolator, pump coupler, and/or tap will revolutionize next-generation amplifier designs.

For DWDM systems, the International Telecommunication Union (ITU; Geneva, Switzerland) has defined a grid of nominal transmitter wavelengths that are evenly spaced at 100 GHz within the 1550-nm window. Hence, such systems need to control this center wavelength to within a small fraction of this spacing over a temperature range of 0°C to 70°C. To achieve this control, DiCon has developed a new generation of wavelength lockers, based on a patent-pending Fabry-Perot design. The wavelength lockers incorporate a spacer element, rather than a filter element, mounted between two GRIN lenses. The free spectral range of the devices matches that of the DWDM 100-GHz ITU grid, with an accuracy of 5 GHz. This accuracy requires submicrometer stability of the optical thickness of the spacer, which can only be achieved with a cylindrical packaging scheme.

Growth in demand for passive components within amplified WDM systems has led to significant packaging innovation for micro-optic designs with cylindrical symmetry. This packaging approach has reduced component losses by more than 0.5 dB over the past two years and correspondingly improved operating conditions and failure rate per million operating hours. These latest advances in cylindrical component packaging are now being incorporated in a wide variety of products from isolators, filters, and wavelength-division multiplexers to wavelength lockers and hybrid components. o

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Cylindrically packaged filter wavelength-division multiplexer delivers low insertion loss with high environmental stability. The device is approximately 40 mm long by 4.5 mm in diameter.

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FIGURE 1. Cylindrically packaged filter consists of a thin-film-coated glass substrate sandwiched between two gradient-index (GRIN) lenses. Depending on the filter element and fiber configuration, components can serve as filters, pump couplers, bandsplitters, taps, or multiplexers.

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FIGURE 2. Signal loss introduced by cylindrically packaged filter remains consistently low for conditions of 85% relative humidity and 85°C.

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FIGURE 3. Amplifier for long-haul wavelength-division-multiplexing applications typically include isolators to prevent backreflection, ta¥couplers to monitor the signal, a coupler for the pum¥beam, and filters to condition the output over multiple spectral channels. cylindrical packaging enhances the environmental stability and loss characteristics of these components.

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