Filter technologies vie for DWDM system applications

Oct 14, 2024

By: Jerry Bautista and Robert Shine, Wavesplitter Technologies Inc.

Contents Market perspective Comparing the technology Approaches to mux/demux Reliability testing Interleaving technology Sidebar: Coarse wavelength division multiplexing (CWDM)

To meet bandwidth demand and make optimum use of existing amplifier bandwidth, dense wavelength division multiplexing (DWDM) systems must offer ever increasing channel counts at more and more narrow channel spacings. Systems deployed roughly two years ago features 16 channels with 200 GHz channel spacing in the C-band (1530 nm to 1560 nm). Systems to be deployed this year, on the other hand, use 40 channels or more with 50 GHz channel spacing.

Although new amplifiers operate over a broader spectral range, channel density will not be any less in these new wavelength regions. Thus, multiplexer/demultiplexer (mux/demux) components are key elements in a successful network. A number of different DWDM technologies exist to meet the needs of system designers, but each forces a design tradeoffs in terms of narrowness of channel spacing, cost, reliability, and manufacturability. Now interleaver technology allows designers to achieve narrow channel spacing with mature technology.

Recent market data indicates several trends (see Figure 1). First, channel counts are indeed increasing. Further, there is a migration from 16 channels and below to 32 channels and above, predominantly driven by the long-haul market. At the same time, increased deployment of DWDM systems in the metro/access market ensures the continued health of the 16 and below channel count market. Finally, the obvious trend of increasing overall system revenues year upon year is clear.

Primary performance points of comparison are

Polarization dependent and dispersion effects as well as channel-to-channel insertion loss uniformity are also often parameters of interest. Finally, compliance with Bellcore GR 1209 or 1221 is often specified to meet reliability concerns.

In order to provide such performance, 200 or more layers of material are deposited in a carefully controlled manner on a glass substrate in large deposition chambers. Chips are diced, polished, and precision mounted in metallic housings along with collimators to yield a wavelength-specific, free-space device. The primary challenge for this technology is to decrease channel spacing to 100 GHz and below with good yield as well as increasing channel count beyond 16.

The primary challenge is to control layer thickness, composition, and defect inclusions for devices that are typically much larger than those produced in standard IC facilities. These devices lend themselves to high integration and consequently large channel counts. Channel spacings are typically 100 GHz, although 50 GHz devices are also available. Temperature stability is often an issue, requiring active heating to bring the devices above ambient temperature. Insertion loss is often compromised to some degree as well although careful waveguide design can minimize this disadvantage.

The technology landscape is summarized for these devices in Figure 3. As is clear from the brief descriptions above, there is no clear winner and the choice of technology often depends on the channel count and channel spacing used in the DWDM system. The selection criteria for today's DWDM applications may often require a hybrid approach due to limitations of a particular technology platform or to provide a graceful path for bandwidth expansion.

Unfortunately, very few DWDM mux/demux filter suppliers are able to provide such a technology hybrid and customers must perform the integration themselves. Alternately, this sets the stage for a situation where prior competitors must cooperatively supply a need for a given customer and perform the integration jointly.

At present two major standards set reliability guidelines for optical components, these are Bellcore GR1209 and GR1221. Perhaps the most critical individual test is the high temperature/high humidity storage test. The device must be able to withstand 85º C and 85% relative humidity with less than 0.5 dB variation at the end of 500 hours and information at 2000 hours for a controlled office environment. The suggested sampling plan is for zero failures in 11 samples, 1 failure in 18 and 2 in 25.

A number of other tests are required involving storage and temperature cycling as well as various mechanical tests such as impact and vibration. Often in addition to specifications, customer negotiations include allowable variations in optical properties for various reliability tests. System considerations provide allowable variation and failure rates for each major element in the assembly.

This approach allows a technology that performs better at a wider channel spacing to address a narrower one. By using interleavers, manufacturers can avoid long design cycles required to fabricate components with technologies suited to narrow channel spacings. The resulting assembly may be more economical, driven by the maturity of technologies for wide channel spacing components, which can lead to higher yields and consequently lower cost.

As is common for a new class of devices, there are a number of different technical approaches to interleavers with no clear technical winner as of yet. The general principal behind interleavers is an interferometric overlap of two beams. The interference creates a periodic, repeating output as different integral multiples of wavelengths pass through the device, and controlling the fringe pattern sets the desired channel spacing of the device.

Manufacturers today use fused-fiber interferometers, liquid crystals, birefringent crystals and other more exotic technical approaches to build interleavers. Probably the simplest design in terms of raw material and technologies is a fused-fiber, Mach-Zehnder interferometer. In this design, an unequal fiber path length between two 3-dB coupler creates the interference. By carefully controlling the path length difference, the channel spacing can be set to the desired value and matched very well to the ITU grid. Because of the all-fiber design, this technology has very low loss, uniform response over a wide wavelength range, very low dispersion and minimal polarization dependence effects.

The logical extension of interleaving technology is a 1 x 4 front-end device (see Figure 5). In this example, dielectric thin film or planar array waveguide devices with 200 GHz spacing can be made to address a 50 GHz application. This approach allows channels to be easily added in banks of four. This scaleable approach to increasing bandwidth is of particular interest to the metro/access market allowing a "pay as you grow" approach and the potential for increased flexibility in optical network provisioning.

Whether as a 1 x 2 or 1 x 4 interleaver, fiber-based devices provide an excellent balance of cost, reliability, and high performance. These devices allow for extremely low insertion loss with very good isolation and bandwidth flexibility. Furthermore, very narrow channel spacings—well beyond those of today, can be achieved with an all-fiber, Mach-Zehnder device. And finally, the fiber-based devices have excellent dispersion performance, a critical feature for high bit rate systems.

Today's DWDM system deployments require a premium on performance, cost, flexibility and reliability. No single technology appears to provide the optimal solution for all applications. A careful technology selection or the application of a hybrid approach such as the paring of fiber-based interleavers and dielectric or planar waveguide components can be a very attractive solution.

About the authors…Jerry Bautista is CTO and Senior vice president of Technology for WaveSplitter Technologies in Fremont, California. He received his BS from Stanford University in 1979 and PhD from Princeton University in 1984 - both degrees in chemical engineering. Prior to joining WaveSplitter Technologies, he was with Lucent Technologies for 14 years. The first seven years involved basic research in Bell Labs – Murray Hill with a focus on fiber and optical components technology while the later 7 involved marketing, sales and technology implementation – from inception through manufacturing. In his last assignment he was responsible for Lucent's planar waveguide business for DWDM applications.

Bob Shine is Director of Marketing for WaveSplitter Technologies. He received his BS from Harvard University in 1990 and Ph.D. from Stanford University in 1995. Prior to joining WaveSplitter Technologies, he was with New Focus for five years, involved in marketing and product management roles.

Coarse systems offer increased capacity while addressing the metropolitan area network need for economical components.

While the emphasis today for long-haul DWDM systems is on very narrow channel spacings, not all applications require the associated cost and complexity of DWDM network management. In the metro and access environments, first installed cost, ease of provisioning and the ability to grow bandwidth are major considerations in system development.

The bandwidth need falls between the original 1310/1550 nm coarse WDM systems and the long-haul DWDM systems. To meet this need, a number of system providers are exploring the development of CWDM systems in which four channels may be spaced as far as 10 nm apart in the 1530 to 1560 nm window (see Figure 1s, below).

By using wide channel spacings, less expensive lasers with relatively wide bands and reduced center wavelength precision can be employed. For cost reasons as well, these systems are typically designed without the use of optical amplifiers, placing a premium on component insertion loss.

For the mux and demux components, relatively simple technologies such as narrow band fused fiber couplers can be used. These devices are entirely passive, small, and have very low insertion loss. The arrangement of a demux device is shown schematically in Figure 7. Two 10 nm narrow band couplers follow a 5 nm coupler. This device offers excellent performance and is consistent with the emphasis on cost and simplicity.

There is a significant potential in the metro and access market for such simplified and cost-reduced systems. It should be noted that a simpler system does not also imply a reduction in reliability.

Source: Fiber Optics Online.com, sister Website to Photonics Online.com