Thin-film filters give flexibilityto OADMs

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By Scott DeMange

Fiber Bragg gratings have been the technology of choice in 50-GHz optical add/drop multiplexers. Advances in deposition techniques and design have created competitive 50-GHz thin-film filter-based OADMs for long-haul and four-skip-zero banded OADMs for metro applications.

Network flexibility hinges on the ability to efficiently drop a wavelength or add one at various points in the network, a function carried out by optical add/drop multiplexers (OADMs) in both long-haul and metro networks. Predominant OADM technologies have been thin-film filters to achieve 100-GHz and wider channel spacing, and fiber Bragg gratings (FBGs) to achieve 50-GHz and narrower channel spacing. As long-haul networks have progressed toward 50 GHz, the FBG has been the only existing technology in OADMs to facilitate that channel spacing.

In metro networks, a band of four or more wavelengths is often added or dropped at a time. Ideally, a "band OADM" would add or drop the multiple wavelengths without attenuating the nearest adjacent channels. In practice, it is challenging to produce a filter wide enough to pass four channels and steep enough to have acceptable isolation from adjacent channels. The typical solution has been the "four-skip-one" filter, which skips over the channel adjacent to the four dropped channels, resulting in a loss of bandwidth.

Recent advances in thin-film deposition have enabled better filter coatings that meet the desired performance for OADMs in both the long-haul and metro networks. For the long-haul market, these coatings can produce 50-GHz and narrower filters that offer an alternative to FBGs in the 50-GHz OADM market. In the metro market, these high performance coatings are capable of producing filters known as "four-skip-zero" filters, enabling OADMs to add or drop four consecutive channels without affecting the nearest adjacent channels.

Thin-film filters are composed of quarter-wave-thick (λ/4n) layers of high-index (H) and low-index (L) material deposited on a glass substrate. Tantala (Ta2O5) and silica (SiO2) are frequently used for the high index and low index material respectively. Thin-film filters work on the principle of stacking multiple resonant cavities and coupling them in a manner that produces a "flat top" filter shape. Filters are manufactured using three main structures: the reflective stack, the spacer, and the transition layer (see Fig. 1).

The reflective stack is composed of alternating quarterwave layers of H and L that form a dielectric mirror. The spacer is composed of an even number of quarter-wave layers of either H or L. A spacer sandwiched between reflective stacks forms a resonant Fabry-Perot cavity. The transition layer is composed of a single quarter-wave- thick layer of either H or L. The transition layer is placed in between the resonant cavities, coupling them to produce the flat-top filter shape (see Fig. 2). Adding more layer pairs to the reflective stack or increasing spacer thickness will narrow a filter's bandwidth and increase filter slope. Adding more cavities to a filter will "square up" its filter shape, slightly widening the flat top and dramatically increasing its filter slope.

DEPOSITION DIFFICULTIES
Film stress and film loss are two of the main challenges in producing narrow (50 GHz or less) thin-film filters and wide, steep filters like the four-skip-zero. Film stress results from the high density of the silica and tantala molecules on the glass substrate, a property that is inherent to the thin-film deposition process. Film loss can result from impurities in the film itself as well as from other sources such as discrete defects. Filters such as the 50-GHz and four-skip-zero require many cavities, and thus a large number of layers to meet required bandwidth and adjacent-channel isolation specifications. Unfortunately, both stress and loss increase with the number of layers. This makes it difficult to design and produce 50-GHz and four-skip-zero filters.

A recently developed "advanced energetic deposition" process reduces film stress and impurities, enabling an increase in the number of cavities for thin-film design, and thus an increase in thin-film filter performance. By changing vacuum-chamber mechanics and using a more precise monitoring system during film growth, this process produces purer films with fewer discrete defects. The resulting filters attain 50- and even 25-GHz spacing.

FEWER CHANNELS
In long-haul networks, typically only a small number of wavelengths or channels are dropped or added from the backbone at one time because the geographic region is less populated than a metropolitan area. In low-channel OADM applications, thin-film filters and FBGs are the two dominant technologies.

A typical single-channel thin-film OADM module consists of two three-port packages spliced together to form a four-port device (see Fig. 3). Multiple channels enter the input port of the first three-port package. One channel is transmitted through the three-port device while all other wavelengths (known as "express channels") are reflected to the second three-port device. At the second three-port device, the express channels are once again reflected, but a new channel at the same wavelength as the dropped channel may be added.

A common type of single-channel FBG OADM consists of a fiber Bragg grating device connected to three-port circulators on either side (see Fig. 4). Multiple channels enter the OADM through the first circulator and encounter the Bragg grating. At the Bragg grating, one channel is reflected back while the rest transmit through the grating. The reflected channel then goes back through the circulator and exits the device at a different port than it came in on. This is the dropped channel. The rest of the channels (the express channels) go through the second circulator to the output port. The added channel comes in through the second circulator on its input port, reflects off the grating, and is multiplexed with the express channels.

Thin-film OADMs have both loss and cost advantages over such FBG OADMs. These advantages are mainly due to the need for the circulator components, which account for most of the loss and cost in the FBG OADM architecture. In some implementations, thin-film OADMs can have half the loss of FBG OADMs and can cost half as much.

MORE CHANNELS
The metro network has two segments: metro core and metro edge. Metro core consists of ring networks that primarily connect central offices and other points of presence to the terrestrial long-haul network. Metro edge consists of ring networks connecting business, large buildings, campuses, and smaller access rings to the metro core network.

Banded OADMs are important in the metro core and metro edge rings. These OADMs allow smaller rings to access some, but not all of the bandwidth from larger rings. Single-channel OADMs can then further route bandwidth to even smaller rings and feeders. Two of the key parameters in accessing bandwidth from a larger ring network are channel loss and bandwidth efficiency.

One method of making a banded OADM is to cascade many single-channel OADMs. The problem with cascading is that express channels incur approximately 1 dB of loss with each pass through a single-channel OADM. A four-channel OADM composed of four single-channel OADMs will cause 4 dB of loss for the express channels, which is unacceptable for a metro network where amplification is to be avoided.

A second method for a banded OADM is to use a filter with a passband that transmits four channels instead of one (see Fig. 5). In this configuration, four channels are dropped and added, with the express channels experiencing a total loss of 1 dB. Thin-film filters are a viable solution in this architecture with their high performance, especially when wider bandwidths are needed. In the past, limitations in filter technology could not produce such a filter cost-effectively. This filter has bandwidth for four channels but does not provide isolation from adjacent channels. Because the adjacent channels cannot be used, the bandwidth efficiency of this OADM configuration is limited.

High-performance four-skip-zero filters have bandwidth for four channels as well as sufficient channel isolation. The four-skip-zero banded OADM can add/drop four channels without losing the available bandwidth in the adjacent channels. These advances will increase bandwidth utilization and offer a choice in the 50-GHz OADM market.

Scott DeMange is product manager at Cierra Photonics, 3640 Westwind Boulevard, Santa Rosa, CA 95403. He can be reached at scott.demange@cierraphotonics.com.

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