Hybrid passive components reduce size and cost while increasing control

Craig Thompson, Kathryn Li Dessau, and Yonglin Huang

New long-haul and metro networks require flexible, high-performance amplifiers. Hybrid passive components and modules provide amplifiers with more functionality, better performance, and greater efficiency, lowering costs and reducing network equipment space.

In its simplest form, the hybrid passive component is the integration of various passive component functions into a single package, such as an isolator with a polarization-beam combiner (PBC) or an isolator with a multiplexer/demultiplexer (mux/demux). Hybrid devices do not simply combine two discrete components in one package; they achieve functional integration using fewer subcomponents and more efficient designs, thus increasing the efficiency of systems by reducing costs, space requirements, and optical losses.

Value-added integration of multiple functions in a hybrid component enables new amplifier designs for the long-haul, ultralong-haul, and metro markets. For long-haul and ultralong-haul applications, this translates into Raman and erbium-doped fiber amplifier (EDFA) designs that meet the needs of next-generation systems—those that offer better overall performance and reduce the strain on pump-power budgets without increasing costs. For metro applications in which maximizing the performance-to-cost ratio is foremost, these devices are ideal for "amplets"—simpler, smaller, lower-cost EDFAs.

INTEGRATING FUNCTIONS
Until the development of hybrid modules, optical amplifiers were assembled from individual components consisting of pump laser diodes, optical isolators, monitor couplers, and—depending upon the type of amplifier—couplers and doped fiber for EDFAs or PBCs, mux/demux, and undoped fiber for Raman amplifiers (see Figs. 1 and 2). Because each component is discrete, they must be fusion-spliced during amplifier assembly, adding more losses and points of weakness with each component. While couplers and combiners are used to either link or separate signals and pump beams, isolators provide protection from unwanted back-reflections in the signal path and prevent back-reflections from returning to the laser cavity and degrading performance, which is especially important in the newer Raman amplifiers and high-power EDFA designs.

The integration of the isolator with another component such as a coupler or PBC into a single package results in several obvious advantages:

Reduction of the total packaging volume by 50%. These new devices contain fewer subcomponents than two discrete devices and are often no larger than a single discrete device, thus reducing the size of the amplifier module (see Fig. 3).
Tighter control of the component-to-component fiber interfaces, which eliminates the need for fusion splicing, improves reliability, and lowers optical losses.
Significant cost savings due to the need for fewer subcomponents.

In addition, advancements in micro-optics, component core design, and component manufacturing enable the new generation of hybrids to offer better optical performance relative to conventional discrete components. For instance, because hybrid designs use significantly fewer crystals than discrete components, they can achieve insertion losses up to 50% lower than their discrete equivalents (see table).

Advanced hybrid designs also deliver lower polarization sensitivity, a requirement for future very-long-reach and 40-Gbit/s systems. For example, advanced hybrid isolator designs improve polarization-dependent losses (PDL) and polarization-mode dispersion (PMD) by tightly controlling the walk-off and path-length difference between the two polarization states. Many discrete component designs are not suitable for higher-bit-rate and ultralong-distance applications because of inadequate insertion loss, PDL, and PMD characteristics, but integrating the isolator with other devices such as the PBC can help meet these demands.

Other key elements have also contributed to the success of integrated components and modules. These include the increased availability of high-quality subcomponents and materials and the manufacturing systems that produce these parts. As a byproduct of the growing number of component manufacturers, the knowledge and understanding of what drives performance in micro-optic parts and filters has increased. Component vendors can now choose the best performing parts for a given application from a wide variety of sources and focus on optimizing the design and performance of components for each customer. The development of advanced manufacturing systems has helped maintain high yields in the face of increasing component complexity. Component designers have achieved this yield by improved design for manufacturability, tooling, and process flow, enabling the manufacture of more complex devices.

EDFAs AND raman AMPLIFIERSWith their significant improvements in performance, real estate, and cost, hybrid devices open the door for new high-power EDFAs, Raman amplifiers, and amplets for metro applications. The trend of new amplifiers for long-haul and ultralong-haul applications is to accommodate higher channel counts—increasing bandwidth by decreasing channel spacing and spreading from the C- to the L-bands—and to operate at higher bit rates. While the power requirement of each channel at the receiver is on the order of -10 to -5 dBm (0.1 to 0.3 mW), the increasingly complex network topologies and the effect of fiber nonlinearities at higher bit rates make this requirement highly variable.

To accommodate the increased power requirements, next-generation EDFAs use higher-power pump lasers in each gain block, multiple pump lasers per gain block, and multistage EDFAs with a larger number of gain blocks per amplifier. Since each gain block consists of a variety of passive components—muxes/demuxes (as either pump-pump combiners or pump-signal combiners), taps, and isolators—any savings in component insertion loss has a positive effect on amplifier performance. Hybrid devices are better suited to this application than individual components because they offer better performance, less stress on the power budget, and are more cost-effective across multiple functions.

These advantages are particularly important when gain-flattening filters must be used to change the gain profile across wider bandwidths. To equalize the per-channel gain, these filters reduce the power of the most amplified channels to correspond with the lowest power. To maximize signal-to-noise ratios, it is most effective to maximize the available power through higher power lasers and low-loss components. In this instance, hybrids that reduce the number of separate components and corresponding insertion loss become invaluable.

While traditional EDFAs are now available for the L-band, they still face technical hurdles. Raman amplifiers may play an increasing role in C-, L-, and even C+L-band applications. The adoption of Raman amplification is driven by the benefits it brings in terms of lower noise and longer reach, enabling high-speed traffic to be maintained in the optical domain for longer periods and providing increased functionality at the systems level. Raman amplifier gain blocks consist of PBCs, muxes/demuxes (pump-pump combiners and pump-signal combiners), isolators, taps, and gain-flattening filters.

Because of the nature of Raman amplification, Raman amplifiers require higher pump powers, which can be achieved with high-power pump lasers or by multiplexing two pump lasers. The first is difficult to do; the second is possible using PBCs in conjunction with fused-fiber or thin-film pump combiners. Moreover, because Raman pump sources must cover both polarization states and be approximately +12 THz or about -100 nm away from the signal that they are amplifying, they must be multiplexed together using PBCs and must span a wide bandwidth to cover the ever-increasing range of signal wavelengths across the C- and L-bands.

Thus, as with EDFAs, the number of pump lasers required per Raman module is increasing from two to four to six and possibly even eight. Given the high power and wavelength requirements of Raman amplifiers, the overall pump-power budget is extremely important. Significant pump-laser savings can be recognized with hybrid passives that deliver multiple functionality with better performance than their discrete counterparts. In particular Raman amplifiers require the low PDL, low PMD, and high-power handling capability that is now possible with the hybrid passives.

AMPLETS IN METRO
As long-haul network service providers increase bandwidth, so too must the metro service providers. Metro service providers are thus turning to dense WDM technology to build high-capacity, scalable, cost-effective networks that can meet both current and future demand. Dense WDM is especially promising because it provides greater scalable bandwidth and the flexibility to enable a variety of services over a single transport platform. With increasing bandwidth usage, amplifiers will find application in metro networks, although the cost of these modules must be kept to a minimum.

Metro amplets consist of a single-stage EDFA as opposed to the multistage EDFAs commonly found in long-haul markets. While long-haul EDFAs amplify multiple channels in a single device, amplets typically act as single-channel amplifiers. Hybrids such as the array isolator combine multiple isolators in the same package dimensions of a single isolator. Real-estate savings are particularly important to providers facing expensive central office space.

To keep the performance-to-cost ratio high, it is important to maximize the power budget. The reduction in insertion loss compared with separate devices reduces liabilities in the link power budget; assuming a typical link power budget of 25 dB for metro applications, the resulting savings of 0.5 dB per component can be very beneficial.

ADVANCED HYBRIDS and MODULES
T he latest advances in hybrid devices allow more integration of passive components in one cylindrical package. The industry is already seeing two- and three-function hybrids on the market, and a range of three- and four-function and array hybrids will be available in the near future. These could include 4-in-1 array isolators, array PBCs, and bidirectional mux/demux taps with 6- or 8-port designs.

For further integration, passive amplifier modules can combine the output of several pump lasers with the signal while providing monitoring ports for both the pump lasers and output. This module could contain a number of isolator-PBCs/PBCs, pump-pump and pump-signal muxes/demuxes, or isolator-mux/demux hybrids, along with tap couplers or mux/demux-tap-isolator combinations (see Fig. 4). Indeed, the integration of both active and passive components will increase as this process continues to yield higher reliability, lower costs, increased space savings, and improved performance.

Craig Thompson is product line manager for amplifier passives, Kathryn Li Dessau is product marketing specialist, and Yonglin Huang is vice president of passives integration at New Focus, 5215 Hellyer Ave., San Jose, CA 95138. Craig Thompson can be reached at [email protected].