Planar Waveguide Battleground

Dr. Sylvain Charbonneau and Dr. Matthew Pearson

The demand for increased communications bandwidth in North America is rapidly being replaced by demands for technology that will allow service providers to use existing optical capacity more efficiently and profitably. Consequently, a growing number of component manufacturers are focusing resources on developing cost-effective solutions for multiplexing and demultiplexing wavelengths.

Many of these companies are turning to planar waveguide devices to deliver complex optical integrated circuits that offer increased scalability and superior performance at a lower cost. As optical networks evolve to higher channel counts at lower cost, the currently popular arrayed waveguide gratings may not be able to satisfy future system demands in terms of performance, cost and manufacturability.

However, the echelle grating has quietly matured to a stage where it can outperform arrayed waveguide gratings in nearly all respects, at a fraction of the cost and while using a fraction of the power.

Although the operating principles of echelle and arrayed waveguide gratings are similar, their layouts are very different.

In an arrayed waveguide grating (Figure 1, left), light of various wavelengths enters the input waveguide (1) and travels to a slab waveguide region (2) that divides the power among the more than 100 waveguides in the grating array (3). Each waveguide has a precise length difference relative to its neighbors so that its light emerges from the end of the array section with a different phase delay. A second slab waveguide region (4) focuses the light from all the waveguides onto the output waveguide array (5). Because each wavelength has a precise differential phase tilt, each one focuses into a different output waveguide. Optical fibers then carry the wavelengths individually off the chip.

Figure 1. In an arrayed waveguide grating (left), each channel or wavelength of light enters a waveguide that delays its phase. A second waveguide focuses the result into the output waveguide array. In an echelle grating (right), reflective facets diffract and focus each channel into the output waveguide array.

Arrayed waveguide grating devices have shallow, etched ridge waveguides, which are relatively simple to fabricate on low-channel-count devices. (Additional processing steps are required to achieve polarization insensitivity.)

In an echelle grating demultiplexer (Figure 1, right), light couples into a single-mode input ridge waveguide and into a slab waveguide region similar to that found in an arrayed waveguide grating. The light then diverges in the two-dimensional waveguide until it arrives at the grating on the opposite end of the chip.

The echelle grating is vertically etched into the planar waveguide to a depth much greater than the waveguide mode width, and the grating facets are coated with a reflective material to create a series of stepped mirrors. The curved grating both diffracts and focuses the light onto one of a series of deeply etched output waveguides situated on the same side of the chip as the input waveguide. The output waveguides move the individual wavelengths off the chip and couple the light to optical fibers.

Silica-on-silicon

Several years ago arrayed waveguide gratings became the primary choice for high-channel-count dense wavelength division multiplexing applications. At that time, echelle grating manufacturing technology could not effectively etch the gratings because of the depth of the etch and the requirements for verticality and smoothness. Recent advances in semiconductor manufacturing technology and the use of silica-on-silicon are enabling echelle grating manufacturers to offer products with increased scalability and functionality.

Regardless of the planar device, silica-on-silicon is the most common material choice for optical waveguides because silica’s refractive index profile is very similar to that of optical fiber, which minimizes coupling losses. Planar device manufacturing uses two deposition techniques: chemical vapor deposition and flame hydrolysis deposition. Chemical vapor deposition, a modification of standard semiconductor processing, is compatible with cleanroom processes and with high-volume wafer production.

Figure 2. Manufacturing challenges have hampered echelle gratings in the marketplace, but new reactive ion etch technology can create deep facets with excellent verticality and smoothness.

For echelle grating devices, verticality and smoothness of the deeply etched grating facets are critical. To fabricate waveguides and gratings in silica-based materials, manufacturers use reactive ion etching. This technique can achieve grating wall verticality better than 89.8° with a root mean square roughness better than 30 nm over 30 μm on a production tool (Figure 2). However, reliability and reproducibility continue to challenge some companies that are developing and commercializing echelle grating technology.

Figure 3. A 6-in. wafer can hold more than five times as many 40-channel echelle grating demultiplexers (right) as arrayed waveguide grating demultiplexers (left).

Still, those that can manufacture these devices can offer customers some performance advantages over arrayed waveguide devices:

• Size. One 6-in. wafer can hold seven 40-channel arrayed waveguide grating demultiplexers (Figure 3, left) or 38 echelle grating demultiplexers (Figure 3, right). The echelle grating device transfers directly into a small package (Figure 4). An echelle grating mux/demux is about one-quarter the size of a comparable arrayed waveguide grating device.

Figure 4. A 40-channel echelle grating demultiplexer packaged with thermoelectric cooler and thermistor is about one-quarter the size of a comparable arrayed waveguide grating device.

The smaller form factor allows manufacturers to integrate more functionality on a chip or to integrate the device with an InGaAs detector array to produce very small optical channel monitors suitable for embedding directly in network elements. This type of functionality and integration has long been envisioned as the future of optical components; however, arrayed waveguide gratings’ size and yields have hampered its execution.

• Crosstalk. Device size hampers arrayed waveguide grating crosstalk performance because phase errors accumulate along the length of the waveguides. Echelle gratings eliminate this problem by replacing the arrayed waveguides with an etched grating. Phase errors in echelle gratings result solely from errors in the positions of the grating facets and depend only on facet fabrication accuracy.

Figure 5. Size gives echelle grating demultiplexers some performance advantages compared with arrayed waveguide grating devices. The graph shows insertion loss characteristics of a 49-channel echelle grating demultiplexer: die size, 18 x 20 mm; adjacent channel crosstalk, 35 dB; background channel crosstalk, 37 dB.

These errors are of two types: rounding errors, resulting from the finite step size of the mask, and stitching errors, which occur when small write fields are stitched together to form a large element.

Because arrayed waveguide gratings are so large, it is unlikely that they can be written in a single field. These fundamental design issues result in better performance in echelle gratings. However, all manufacturing processes must still be highly uniform and tightly controlled to achieve high isolation. Process control remains the key to high performance.

• Insertion loss. Insertion loss is the sum of the coupling and on-chip losses. Coupling losses are almost identical for arrayed waveguide and echelle gratings. Unlike discrete demultiplexers, planar waveguide demultiplexers’ insertion losses are very uniform (less than 1 dB across the whole wavelength range) and do not scale with channel count.

On-chip losses are the result of fundamental material and scattering losses due to structures on the chip. Because the optical path length in an echelle grating is typically smaller than that of an arrayed waveguide grating, its material and scattering losses are also smaller.

• Scalability. Scalability to high channel counts has led many systems manufacturers to shift from thin-film filters to planar waveguide components. Arrayed waveguide gratings have been the choice for this type of integration. However, component size restrictions and accumulated phase errors make it difficult to scale these devices to 80 channels or more. Typically, one 6-in. wafer can hold about four 80-channel arrayed waveguide gratings, but wafer nonuniformities and processing defects commonly reduce the ultimate yield to two devices — or fewer.

Echelle grating demultiplexers, in contrast, can easily scale to 80 channels, with more than 30 fitting on a 6-in. wafer. The small size of the devices also makes them less affected by wafer nonuniformities. Echelle grating demultiplexers are available in 160-channel configurations and have been demonstrated at 256 channels. Currently, the only limit for these devices is the large fan-out for fiber attach.

• Power consumption. Both devices consume power to regulate temperatures and maintain thermal stability for optimum channel tuning. Arrayed waveguide gratings are packaged with thermoelectric coolers or heaters and thermistors to monitor temperature. Echelle gratings need thermal stability only across the relatively small free propagation region. As a result, their power requirements are typically one-half to one-fifth of those of comparable arrayed waveguide gratings. Several companies are developing athermal gratings of each type. At present, manufacturing costs restrict their widespread deployment.

Integrated circuits

Perhaps the most important benefit of echelle grating technology is that it offers the ideal platform on which to pattern further functionality. The size of arrayed waveguide grating demultiplexers makes this difficult, especially at high channel counts. Yields are already so low that adding further functionality would significantly increase manufacturing costs.

On the other hand, manufacturers are coupling echelle gratings with attenuation and switching functions to develop a new form of planar lightwave circuit: the readily customizable photonic application-specific integrated circuit.

In its simplest form, such as integrated circuit would comprise a custom demultiplexer and hybrid detector arrays to create devices such as embeddable optical channel monitors. Variable optical attenuation and switching technology will bring true customization to this platform, enabling its use in fully configurable optical add/drop modules and customer-specific designs for planar lightwave circuits.

Meet the authors

Sylvain Charbonneau is chief technology officer at Optenia Inc. in Ottawa. He has a PhD in semiconductor physics from Simon Fraser University.

Matthew Pearson is a product manager at Optenia.