Polymer components meet performance challenges

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by Louay Eldada

After years of skepticism, polymer optical components have reached commercial viability. Switches, VOAs, and tunable-filter-based products have been introduced, and components such as modulators, lasers, and amplifiers are under development.

Monolithic integrated optics has long been the dream of the optical networking community as a vehicle for cost and size reduction, improved reliability, fiber-management simplification, and improved performance. Among the candidate material systems, high expectations have been placed on polymers as the materials choice for highly integrated optical components, mostly for economic reasons.

This anticipation has been mixed with a skepticism that has developed over time because of the spotty performance record of polymer components, mostly in the area of electro-optic modulators. Much of this skepticism is justified. The reality with polymers is that an infinite number of compositions and formulations can be conceived that exhibit a few interesting optical characteristics. However, synthesizing a polymer that meets all performance and reliability requirements for optical networking applications is a precise and demanding discipline.

MATERIALS AND WAVEGUIDES
Several material systems are being pursued as integrated optics platforms.1 State-of-the-art optical polymers are particularly attractive in integrated optics because they offer rapid processability, cost-effectiveness, high yields, and high performance (waveguide propagation loss is slightly lower than that in silica; the birefringence is smaller than that of silica by two orders of magnitude). They also offer power-efficient thermal actuation (the thermo-optic coefficient dn/dT is 10 to 40 times larger than in silica), reliability (no degradation at elevated temperatures), and compactness (because of a large refractive-index contrast).

Optical polymers have been engineered in many laboratories and some are available commercially (see table, p. 26).2 Classes of polymers used in integrated optics include acrylates, polyimides, polycarbonates, and olefins such as cyclobutene. Some polymers—most polyimides and polycarbonates—are not photosensitive, and are typically processed using photoresist patterning and reactive ion etching. Other polymers are photosensitive and as such are directly photopatternable, resulting in a full cycle time on the order of tens of minutes per multilayer optical circuit on a wafer. These materials have an obvious throughput advantage.

Optical polymers can be highly transparent, with absorption loss values below 0.1 dB/cm at all the key communication wavelengths (840, 1310, and 1550 nm). The scattering loss can be minimized in polymer waveguides by using direct photo-patterning as opposed to surface-roughness-inducing reactive ion etching.

The effect of the little roughness that is obtained can be further minimized by the use of a graded index, a natural process in direct polymer lithography where interlayer diffusion is easily achieved. The graded index profile results in weak confinement of the optical mode, causing its tails to penetrate well into the cladding, averaging out the effect of variations. The scattering loss can also be reduced by ensuring the homogeneity of the medium (for example, no abrupt refractive-index variations caused by phase separation or particles), and by minimizing intrinsic stresses.

As opposed to planar silica technologies, polymer technologies can be designed to form stress-free layers regardless of the substrate composition, which can be silicon, glass, quartz, plastic, or glass-filled epoxy printed-circuit board substrate, for instance. And these films can be essentially free of stress-induced scattering loss and polarization dependence. These favorable characteristics can be observed when operating above the glass transition temperature (Tg) in cross-linked polymer systems.

The radiation loss can be reduced by using standard integrated-optic design rules such as large radii of curvature and adiabatic modal transitions. The fiber pigtail loss can be minimized by matching the mode of the planar waveguide to that of the fiber, which can be achieved by tuning the index contrast, the index profile, and the core dimensions. It can also be minimized by optimizing the alignment of the waveguides to the fiber, and by minimizing the Fresnel reflections with appropriate index-matching materials at the interfaces.

The total insertion loss achieved in planar polymer components can closely approach the value of the material absorption loss when fabrication techniques are optimized. The polarization-dependent loss (PDL = lossTE - lossTM) varies with processing conditions. The TE loss measured in planar waveguides can be higher than the TM loss when the vertical walls of the core have a higher degree of roughness than the horizontal boundaries, and it can be lower when the vertical evanescent tails overlap with an absorptive substrate or superstrate.

Waveguides that are well-optimized by having minimal edge roughness and a well-confining material stack can have PDL values that are immeasurably small. The birefringence (nTE - nTM) can be extremely low in polymers that undergo little molecular orientation during processing, as is common in three-dimensionally cross-linked polymers.

STABILITY, CONTROLLABILITY
The environmental stability of optical polymers—the stability of their optical and mechanical characteristics with temperature and humidity—is an important issue because most polymers do not have the level of stability required for operation in communication environments.

Organic materials can be subject to yellowing upon thermal aging due to oxidation. The presence of hydrogen in a polymer allows the formation of H-halogen elimination products, resulting in carbon double bonds that are subject to oxidation. Fortunately, the absorbing species from thermal decomposition are centered near the blue region of the spectrum, and the thermal stability can be high at the datacom wavelength of 840 nm and even greater at the telecom wavelengths of 1300 and 1550 nm.

The resistance of polymers to water incursion is critical because optical absorption results from the overtone bands of the OH-stretch of water. However, polymers that stand up to 85°C 85% RH (relative humidity) conditions have been demonstrated, and some polymers passed the Bellcore 1209 and 1221 environmental tests. Extensive materials research has yielded polymers that are highly reliable, to the extent that they are no longer the limiting factor in component lifetime (see Fig. 1).

Another important feature of polymers is the controllability of the refractive-index contrast, which can have values up to 35%, enabling high-density compact waveguiding structures with small radii of curvature. Polymers also allow simple, high-speed fabrication of three-dimensional circuits with vertical couplers, needed with high-index-contrast waveguides in which two-dimensional circuits would require dimensional control, resolution, and aspect ratios that are beyond the levels achievable with today's technologies.

Furthermore, the unique mechanical properties of polymers allow them to be processed by unconventional forming techniques such as molding, casting, stamping, and embossing, permitting rapid, low-cost shaping for both waveguide formation and material removal for grafting of elements such as active films, Faraday rotators, or half-wave plates. However, the production of commercially viable polymeric optical components is a complex task because optical polymers need to simultaneously meet many key properties.

OPTICAL COMPONENTS
Most of the work going on globally in integrated polymer components is in the areas of switches, attenuators, filters, modulators, lasers, and amplifiers.

Switches. Thermo-optic N x N switches can be interferometric switches based on directional couplers or Mach-Zehnder interferometers (MZIs), or they can be digital optical switches (DOSs) based on X or Y junctions. The most widely used switch design is the Y-junction-based DOS because of its simplicity and its digital behavior.

The 1 x 2 DOS is considered to have switched once it reaches the desired isolation value, which occurs at some level of electrical power dissipation in the electrodes, beyond which power level the device maintains the isolation, resulting in its well-known "digital" behavior. Any size MxN switch can be built out of 1 x 2 switches.

The small dn/dT in silica has kept 16 x 16 switches from being implemented with DOSs. Planar 16 x 16 switches implemented to date have been based on MZIs, which use less power but do not exhibit digital behavior. The first DOS-based 16 x 16 switch was produced in polymer. This switching matrix consists of 480 x 2 switches, interlinked with 704 S-bends that intersect at 227 locations to provide a strictly nonblocking connectivity (see Fig. 2).

The final 16 x 16 switch matrix measures 4 x 10.4 cm2. All 256 switching states can be addressed independently at the same drive power. Each path is defined by heating eight heaters; therefore 128 heaters are continuously being used. Since the crosstalk is not limited by the switches, an extinction of 15 dB per 1 x 2 stage already yields sufficient effective extinction due to the concatenation of the eight switching/combining stages. At a power dissipation of 50 mW per DOS, the insertion loss is 6 dB and the extinction is 30 dB, mainly limited by the crosstalk due to crossings in the design. The total electrical power consumption is 6.4 W.

Variable optical attenuators. With the increasing complexity of WDM optical networks comes an increasing need for reliable, low-cost VOAs that adjust the power level of optical signals with high accuracy and repeatability. Variable optical attenuators can be based on any switching principle, including interferometry, mode transition, or mode confinement.

An interferometric approach uses an MZI where heat can be applied to at least one of the arms to induce a phase shift between the two optical signals before they recombine, thereby controlling the level of optical power exiting the output guide. The PDL achieved in polymeric VOAs is under 0.2 dB across the entire attenuation range, a value that is lower than that achieved in any other material system.

Tunable filters. By periodically alternating the refractive index in a waveguide around an average effective refractive index n, an in-line series of weakly reflecting mirrors (a Bragg grating) of spacing L can be created. The cumulative effect of the mirrors is to maximally reflect wavelengths λ, equal to 1/N multiples of 2 • n • Λ, where N ≥1 is an integer indicating the order of the grating period.

Gratings in planar polymers can be produced by a variety of techniques such as casting, molding, embossing, e-beam writing, and photochemical processes.3 The first three techniques produce surface-relief gratings while the last two typically produce bulk-index gratings. Photochemical fabrication processes utilize two-beam interference to induce an index modulation. This effect can be achieved through the use of either interference of split laser beams or a phase mask (where two beams corresponding to the +1st and -1st diffraction orders interfere).

The grating is tuned by actuating a heater. The value of dn/dT in the polymers used is -3.1 x 10-4/°C (about 30 times larger than in glass), resulting in a tuning rate of -0.36 nm/°C, therefore permitting tuning across the entire erbium C-band (1528 to 1565 nm) with a temperature range of about 100°C.

Modulators. Some polymer formulations have been designed to have a large electro-optic coefficient (as large as 200 pm/V, the largest value achieved in any material system). These formulations are typically composed of standard polymers (for example, polycarbonate) impregnated with specialty chromophores (such as CLD-1). They exhibit a large electro-optic effect once subjected to poling, a process in which large electric fields (~200 V/μm) are applied to the material to orient the molecules. However, the effect of the poling process has been disappointing in that the result is not stable with time or environmental conditions, limiting the applications in which polymer electro-optic modulators can be used.

Click here to see a PDF of the Key Properties of Optical Polymers Developed Globally

Lasers and amplifiers. Rare-earth doping is widely used to produce lasers and all-optical amplifiers that are simple, reliable, low-cost, and have a wide gain bandwidth. Although rare-earth doping has been typically used in silica (mostly in fiber, and to a lesser degree in planar waveguides), there is ongoing research to develop viable and stable rare-earth-doped polymer lasers and amplifiers. The main rare-earth ions used are thulium (1450 to 1510 nm) and erbium (1530 to 1570 nm). The main issue with rare-earth-doped polymer lasers and amplifiers has been the inefficiency of the pumping due to the de-excitation of the excited states caused by the infrared absorption in the polymer. A high level of pump power is then required, creating stability issues in the polymer.

Laser dyes such as Rhodamine B are highly efficient gain media that can be used in liquids or in solids to form either laser sources with narrow pulse width and wide tunable range, or optical amplifiers with high gain, high power conversion, and broad spectral bandwidth. Laser dyes captured in a solid matrix are easier and safer to handle than their counterpart in liquid form. Dye-doped polymers are found to have better efficiency, beam quality, and optical homogeneity than dye-doped silica. The photostability is one of the main concerns in solid-state dye-doped gain media, because the high pump intensity can cause a quick degradation of the dye molecule.

REFERENCES
1. L. Eldada, Opt. Eng. 40, 1165 (2001).

2. L. Eldada et al., IEEE J. Select. Top. Quant. Electron. 6, 54 (2000).

3. L. Eldada, OSA TOPS WDM Components 29, 105 (1999).

Louay Eldada is founder, CTO, and vice chairman of Telephotonics, 100 Fordham Road, Wilmington, MA 01887. He can be reached at leldada@telephotonics.com.

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