Cost and reliability concerns dominate DWDM multiplexer design

Optical multiplexers (MUXs) and demultiplexers (DeMUXs) are fundamental building blocks of DWDM networks since they represent the on- and off-ramps for wavelengths on the fiber network. MUXs and DeMUXs are also used in advanced network elements such Optical Add/Drop Multiplexers (OADMs) and Optical Cross Connect switches. The pervasive use of these components requires constant improvement to allow operators to grow capacity while watching the bottom line and the ability to deliver services reliably. These trends have prompted the need for high channel count (greater than 16) MUXs and DeMUXs with the requisite performance characteristics.

MUX vendors have responded to cost and reliability concerns by improving three key MUX characteristics: insertion loss, filter profiles and thermal properties. It's important to understand the implications of these three key parameters on network performance, cost and reliability. Insertion losses are important because they directly impact the power budget of a DWDM link, influencing fiber reach and the need for optical amplifiers. Amplifier-free links are possible in metro and access networks since they have shorter internode distances (typically less than 80 kilometers) than long-haul networks. Reducing the number of amplifiers results in lower costs and avoids the operational and design complexities that are a consequence of their use such as noise, power balancing, gain tilt. MUX technologies that offer low insertion losses are more likely to reduce the dependence on amplifiers.

MUXs and DeMUXs also have a spectral characteristic for each channel (transmitter wavelength) that is referred to as its filter shape. Typically, MUXs have peaked or Gaussian profiles, that occur naturally from their design. A Gaussian profile, however, can be problematic in a system because drift of the transmitter wavelength off the filter peak increases insertion loss and degrades system margin. Network designers have addressed this issue by adding expensive wavelength lockers to transmitters. However, wavelength lockers do not solve thermal drift problem if the MUX characteristics themselves change with operating temperature. MUX vendors have introduced flat filter devices that relax the need to control the transmitter wavelength to a precise value. Flat filter MUXs, may require additional elements in the optical path that results in greater insertion loss for some technologies.

Another important MUX characteristic is its thermal behavior. Some MUX technologies, such as those based on Arrayed Waveguide Gratings (AWGs), are very temperature sensitive and require the use of heaters that keep the waveguide chip at a controlled temperature- approximately 75 degrees centigrade. Growing demands on network reliability, fueled by widespread use of Service Level Agreements, has prompted interest in component reliability and has favored the use of passive devices that do not require power or temperature control elements. AWG MUX vendors have been working towards modified designs that eliminate their dependence on heaters. The requirement for heaters is eliminated in MUXs based on diffraction grating technology.

The free-space diffraction grating operates here in a demux mode. Basic elements include: input and output fiber arrays, micro-optics array collimating lenses and a diffraction grating. With recent improvements, diffraction gratings can operate simultaneously in both C and L communication bands (1,520 to 1,610 nanometers) with less than 0.3 dB of PDL and absolute diffraction efficiencies of 80 to 85 percent. Source: Lightchip Optical Networking Inc.

There are four basic MUX/DeMUX technologies: Thin Film Filters (TTF), Fiber Bragg Gratings (FBG), Arrayed Waveguide Gratings (AWGs) and Free-Space Diffraction Gratings (FSDG). Thin film filters and fiber Bragg gratings were first used DWDM systems when wavelength counts were low. Although they use different physical mechanisms, they both function by filtering wavelengths serially, where individual elements are used to multiplex (or demultiplex) wavelengths on a one-by-one basis. TFFs use a concatenated set of individual interference filters, each of which has multiple dielectric coatings that pass a single wavelength and reflect all the others. TFFs work well for low channel counts but have limitations at higher channel counts (typically greater than 16) due to size and accumulated insertion losses.

FBGs rely on a grating formed in the fiber core that reflects a single wavelength and transmits other wavelengths. Each FBG is designed with a different grating period to multiplex or demultiplex a single wavelength in the system. The reflected wavelength is demultiplexed from the system fiber using an optical circulator. As was the case for TFFs, the serial process restricts the practical channel count due to size, accumulated insertion losses and costs of the individual piece parts.

In contrast to TTF and FBGs, Arrayed Waveguide Gratings and Free-Space Diffraction Grating MUXs use a parallel approach that is more conducive to high channel count applications. AWGs use silica waveguide structures to diffract light into individual output waveguides. AWGs are often designed to optimize one of the performance criteria (low loss, flat top, athermal) and do not commonly offer all three features in a single device. For instance, to achieve a flat filter profile AWG-based MUX must insert another element into the waveguide design that generally adds 2-3 dB of insertion loss. As mentioned previously, AWGs use internal heaters to keep the waveguide chip at a constant temperature-typically 75C to function properly. Heaters introduce several cost and reliability issues. From a cost perspective, system designers using AWGs must add heater control circuitry that introduces cost, size and complexity in the system. From reliability standpoint a heater failure has a significant network impact since all wavelengths will be affected. Non-heated versions of AWGs are emerging, however, these devices typically have higher insertion losses than their heated counterparts.

Free-Space Diffraction Grating (FSDG) MUXs and DeMUXs use a ruled or etched diffraction grating as the dispersion engine to separate wavelengths into individual output fibers. Diffraction gratings have been used for more than 150 years, primarily in astronomy and optical characterization equipment. FSDG MUXs use classical optics principles and operate in free space. The basic elements of the MUX are the input and output fiber arrays, micro-optics array (which can be used to control filter shape), collimating lenses and a diffraction grating. Until recently, the use of diffraction gratings in telecommunications had been limited due to their high polarization dependent loss (PDL). To combat PDL, early FSDG MUXs used polarization-conditioning elements that increased insertion loss.

Advances in the design of low PDL gratings have now enabled development of low loss MUX components. These diffraction gratings can operate simultaneously in both C and L communication bands (1520 - 1610 nm) with less than 0.3dB of PDL and absolute diffraction efficiencies of 80-85%. Through advanced optical and mechanical designs that compensate refractive index changes and mechanical thermal expansion, grating-based MUX and DeMUXs can be engineered that are insensitive to temperature variations. Also, recent improvements in multi-fiber array manufacturing have enabled high yield production of high channel count FSDG MUXs.

FSDG MUXs offer performance advantages compared with AWG-based solutions because they simultaneously achieve low insertion loss, flat filter profiles and superior thermal performance. For instance, recently designed free-space diffraction grating MUXs sacrifice only 0.5dB of insertion loss to achieve a flat filter response. FSDGs can achieve low insertion losses (less than 4dB at the passband edges) compared with flat top AWG, which have insertion losses around 7.5dB. FSDGs are also bi-directional, which means that the same device can act as a MUX or a DeMUX or can simultaneously transport wavelengths in either direction through the same device. And, FSDGs are totally passive in their operation, requiring no power for operation. There is not need for built-in heaters because their optical designs are inherently insensitive to temperature. For instance, state-of-the art FSDG designs can achieve a spectral variation as low as 0.2 picometers/C, a thermal sensitivity that is miniscule over the range of its spectral passband.

There are many MUX parameters that influence network performance. The impact of insertion loss is simple to understand. In a simple point-to-point DWDM link the insertion loss of the MUX and DeMUX can be a significant portion of the total link loss. For instance, athermal, flat top FSDG MUXs have insertion losses of 4 dB compared with flat top AWGs, which can have insertion losses in excess of 7.5 dB. Use of an FSDG MUX/DeMUX pair would result in 7 dB less attenuation compared with the AWG MUX/DeMUX pair. In networks where the link design is power limited, the use of diffraction grating technology would allow an increased link length of 35 km (based on a fiber attenuation of 0.2 dB/km). Low insertion loss MUX and DeMUX components will significantly improve the opportunities for amplifier-free designs in metro and access networks.

Quantifying the impact of filter shape and thermal behavior is more difficult and requires use of network simulation tools. Network simulations have demonstrated that the combination of low loss, flat filter profiles and athermal behavior are critical to the cost efficient design of metro networks. FSDG MUXs offer the ability to reduce the dependency on optical amplifiers and the capability to cascade many MUXs and DeMUXs without serious narrowing of the effective end-to-end passband. The ability to relax the use of amplifiers and wavelength lockers is expected to improve the reliability of the network and reduce DWDM costs to a manageable level appropriate for metro and access networks.