Advances in Polymer Integrated Optics

Transcription

1 54 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 6, NO. 1, JANUARY/FEBRUARY 2000 Advances in Polymer Integrated Optics Louay Eldada, Senior Member, IEEE, and Lawrence W. Shacklette Invited Paper Abstract We report on advances in polymeric waveguide technologies developed worldwide for the telecom and datacom markets, and we describe in detail one such technology developed at AlliedSignal. Optical polymers are versatile materials that can be readily formed into planar single-mode, multimode, and microoptical waveguide structures ranging in dimensions from under a micrometer to several hundred micrometers. These materials can be thermoplastics, thermosets, or photopolymers, and the starting formulations are typically either polymers or oligomers in solution or liquid monomers. Transmission losses in polymers can be minimized, typically by halogenation, with state-of-the-art loss values being about 0.01 db/cm at 840 nm and about 0.1 db/cm at 1550 nm. A number of polymers have been shown to exhibit excellent environmental stability and have demonstrated capability in a variety of demanding applications. Waveguides can be formed by direct photolithography, reactive ion etching, laser ablation, molding, or embossing. Well-developed adhesion schemes permit the use of polymers on a wide range of rigid and flexible substrates. Integrated optical devices fabricated to date include numerous passive and active elements that achieve a variety of coupling, routing, filtering, and switching functions. Index Terms Datacom, dense wavelength-division multiplexing (DWDM), interconnects, low loss, polymers, telecom, thermooptic effect, tunable filters. I. INTRODUCTION THE RECENT surge in demand for photonic components that meet economic criteria as well as technical requirements in the telecom and datacom industries has opened the door for novel technologies that enable unique functions and/or unconventional high-yield manufacturing without sacrificing high performance. Advanced planar polymer technologies can fit the bill in every aspect. Today, glass optical fibers are routinely used for high-speed data transfer. Although these fibers provide a convenient means for carrying optical information over long distances, they are inconvenient for complex high-density circuitry. In addition to being fragile, glass fiber devices are difficult to fabricate especially when they have a high port count and as a result are quite expensive. Polymeric materials permit the mass production of low-cost high-port-count photonic circuits in parallel on a planar substrate. In addition, they provide the possibility for a much higher degree of ruggedness. Work carried out in a number of laboratories has demonstrated these advantages. Manuscript received August 19, 1999; revised October 14, The authors are with AlliedSignal Inc., Advanced Technologies, Morristown, NJ USA ( louay.eldada@alliedsignal.com). Publisher Item Identifier S X(00) What has been needed is development and characterization of suitable polymeric materials, development of high-yield manufacturing techniques for polymeric photonic devices, and full characterization of the resulting optical circuits. Each of the polymer systems available today has a unique set of properties that make it suitable for specific communication applications. II. MATERIALS Optical polymers were engineered in many laboratories worldwide and some are available commercially. Classes of polymers for use in integrated optics include acrylates, polyimides, and olefins (e.g., cyclobutene). Companies that developed such polymers include AlliedSignal [1] [9], Amoco [10], Dow Chemical [11] [13], DuPont [14], General Electric [15], Hoechst Celanese [16] [18], JDS Uniphase Photonics (formerly Akzo Nobel Photonics) [19] [21], and NTT [22] [24]. Table I lists the key properties of polymers developed at these companies. AlliedSignal has developed a wide variety of photochemically-set optically transparent polymers, which are based on combinations of multifunctional acrylate monomers and oligomers together with various additives. These polymers are particularly suitable for practical low-loss optical devices because acrylates intrinsically have relatively low stress-optic coefficients and because photochemical processing directly from monomers provides polymers characterized by relatively low internal stress. This combination of material properties allows the creation of waveguides that exhibit both low scattering losses and low polarization-dependent losses (PDL s). Upon exposure to actinic radiation (e.g., UV or e-beam), these monomer systems form highly crosslinked networks, which exhibit low intrinsic absorption in the wavelength range extending from nm. By blending and copolymerizing selected miscible monomers, the synthetic scheme allows for precise tailoring of the refractive index over a very broad range from This control of refractive index allows us to fabricate step-index or graded-index optical waveguide structures with well-defined and reproducible refractive index differences to within At the same time, the synthetic scheme allows other physical properties of the materials such as flexibility and toughness as well as such important properties as surface energy and adhesion to be tailored to meet the needs of specific applications. These materials have already passed much of the Bellcore protocols 1209 and Key material tests and characteristics are discussed below X/00$ IEEE

2 ELDADA AND SHACKLETTE: ADVANCES IN POLYMER INTEGRATED OPTICS 55 TABLE I KEY PROPERTIES OF OPTICAL POLYMERS DEVELOPED WORLDWIDE Fig. 1. Absorption losses for three AlliedSignal polymer systems. Acrylic with full CH content is solid line. Dashed line is for material with 30% CH content; dotted line is for 20% CH content. A. Optical Loss The results of spectrometry studies performed in a set of acrylates are shown in Fig. 1. Notable features in this figure are the region of transparency around the datacom/computing wavelength of 840 nm, the C-H vibrational overtones (found in all CH-containing polymers) which are most pronounced beyond 1 m, and the windows of transparency near 1.3 and 1.55 m, the wavelengths of interest for telecom. Optical absorption in polymeric materials is determined both by the energies of molecular or polymeric electronic excited states and by the locations of fundamental and overtone vibrational excitations of covalently bonded nuclei, such as carbon and hydrogen. The location of these vibrational states is determined largely by the spring constant of the bond and the reduced mass of the two nuclei. Since the vibrational energy is inversely related to the reduced mass, bonds with low reduced masses (such as C-H) have high lying vibrational frequencies extending well into the infrared communications regime (in wavelength units from 1 to 2 m). Modifying a molecule by substituting fluorine or chlorine for hydrogen in C-H bonds has the effect of greatly increasing the reduced mass, thereby lowering the energy of the fundamental bond vibration and its overtones and virtually eliminating absorption from 1 to 2 m. Fig. 2 shows the absorption strengths of different molecular bonds of concern presented in terms of calculated absorption strengths for transition from the ground vibrational state to excited vibrational states where the transition frequencies are mapped into wavelength units. Also shown in Fig. 2 are the experimental curves for PMMA and a nonhalogenated AlliedSignal polymer, denoted C20, both containing a similar large proportion of CH bonds. Replacement of CH bonds with CD bonds can reduce the intrinsic absorption by about an order of magnitude. However, synthesis of polymers based on CD bonds is difficult and expensive. Replacement of CH bonds with CF bonds or with CCl bonds can provide much greater reduction in intrinsic vibrational absorption. The solid line in Fig. 1 represents the loss of the full-ch-content acrylates; these materials exhibit loss values of 0.02, 0.2, and 0.5 db/cm at 840, 1300, and 1550 nm, respectively. The dashed and dotted lines show the results of partial halogenation (70% and 80%, respectively). This modification brings absorption losses to values as low as 0.001, 0.03, and 0.07 db/cm at 840, 1300, and 1550 nm, respectively. In the case of waveguide spectrometry, absorption was measured on slab waveguides where fabrication imperfections

3 56 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 6, NO. 1, JANUARY/FEBRUARY 2000 Fig. 2. Calculated absorption losses for various vibrational motions. Also shown are experimental losses for PMMA and for a nonhalogenated AlliedSignal polymer, an acrylate with a comparable concentration of CH bonds. are easily avoidable. However, loss measurements were also performed on channel waveguides using the cleave-back method [1]. The results of this study on multimode and single-mode two-dimensionally confining guides produced in our full-ch-content acrylates reveal average loss values at 840 nm of 0.02 and db/cm in the multimode and single-mode cases, respectively, indicating that the loss achieved in waveguides can closely approach absorption-limited values when fabrication techniques are optimized. B. Thermal Stability A key characteristic for practical applications is thermal stability of optical properties since organic materials may be subject to yellowing upon thermal aging. Typically, such aging results from the formation of partially conjugated molecular groups characterized by broad ultraviolet absorption bands, which tail off in intensity through the visible. This yellowing is strongly influenced by the chemical structure of the original polymer. Although the AlliedSignal polymer systems described here are acrylic, the chemical characteristics of various backbone segments can vary substantially from, for example, simple aliphatic, to aromatic, and from ether, to ester, to urethane. The choice of these linkages and monomers or oligomers ultimately determines to a significant degree the characteristics of the resulting polymer including surface energy, hardness, toughness, modulus, water uptake, and stability toward aging. Because of their highly crosslinked nature, polymers derived from multifunctional acrylates are quite thermally stable. Thermal stability of the waveguide materials was studied by a variety of techniques including thermal gravimetric analysis (TGA), isothermal TGA, spectrophotometry, and in situ optical loss measurement. Fig. 3 shows the TGA curves of a nonhalogenated core polymer. The thermal decomposition temperatures, defined as the 5% weight loss temperature at a heating rate of 10 C/min, are 365 and 415 C in air and in nitrogen, respectively. The isothermal TGA curve shows negligible weight loss after 10 h of baking at 190 C under air atmosphere. These results demonstrate the superior thermal Fig. 3. Dynamic TGA of a nonhalogenated core polymer under air and nitrogen at a heating rate of 10 C/min and isothermal TGA at 190 C in air. stability of the materials against chain scission and volatilization of decay fragments. However, much more important is the retention of high optical transmission upon thermal aging. Thermal coloration is a common problem with organic polymers when exposed to elevated temperatures and must be minimized for applications as optical materials. This coloration is not always accompanied by weight loss (therefore not measurable with TGA) but causes increased optical loss. In order to investigate this phenomenon, loss measurements at 840 nm were carried out on 5-cm-long fiber-pigtailed straight waveguides made with our nonhalogenated polymers. The power loss as a function of time for a given temperature was measured by comparing the optical outputs before and after heating. Over a wide range of temperature (80 C 260 C), the degradation rates or yellowing rates (losses as a function of time and temperature) were found to be well described by an Arrhenius expression of the form rate

4 ELDADA AND SHACKLETTE: ADVANCES IN POLYMER INTEGRATED OPTICS 57 Fig. 4. Measured loss rate for pigtailed polymer waveguide versus temperature. Also shown is an Arrhenius fit. where: Boltzmann s constant; absolute temperature; activation energy for the chemical degradation process; constant. Fig. 4 shows the measured yellowing rates for a typical composition. Also shown is a fit to the Arrhenius law. The activation energy is 26.6 kcal/mole. These data show conclusively that the thermal stability of these polymer waveguides is excellent, with practical stabilities (time for thermally inducing 0.1 db/cm loss) at 840 nm of about 65 yr at 100 C, 10 yr at 120 C, one yr at 150 C, two weeks at 200 C, and one day at 250 C. Since the absorbing species from thermal decomposition are centered near the blue region of the spectrum, the thermal stability of these polymers is even greater at wavelengths longer than 840 nm, as demonstrated in Fig. 5. It was also observed that these polymeric devices are mechanically robust at high temperatures; no mechanical failure such as cracking or delamination occurs after extended treatment at 250 C. These polymer systems exhibit a level of thermal stability appropriate to demanding applications for both long term operation at 120 C and device packaging at 250 C. As for the yellowing rate of partially halogenated polymers, it is typically similar to that of nonhalogenated polymers, with some variation that depends on the synthesis process and the additives used (e.g., antioxidants). However, since yellowing is problematic mostly below 1- m wavelength and halogenated polymers are typically used in applications around 1.3- and m wavelengths, yellowing of halogenated materials is not considered to be an issue and as a result is rarely discussed in the literature. In fully halogenated materials, yellowing becomes negligible at any wavelength because the absence of hydrogen prohibits the formation of H-Halogen products, which would result in carbon double bonds, which would enable oxidation, the main cause of yellowing. C. Humidity Resistance Nonhalogenated polymers were subjected to humidity cycling tests. A sample was prepared on a flat glass substrate. Multimode 90 in-plane bends were produced. The guides were 100 m in cross section and 10.9 cm long. The test devices were Fig. 5. Loss induced in polymers by thermal exposure at 190 C as a function of wavelength. Fig. 6. Results of humidity cycling experiment showing no humidity-induced optical loss after 600 h. pigtailed with 100- m-core fiber, but were not packaged or protected in any manner. A thin cladding was printed around the core layer to give a short path for the diffusion of water. The sample was cycled between rigorously dry and humid (85 C 85% RH) conditions for over 600 h. Transmission measurements were performed at 1550 nm. The results are shown in Fig. 6, and they reveal that no increase in loss was observable. The present observations also give no evidence of mechanical failure of devices due to water incursion, such as delamination or loss of integrity at the interface with the glass fibers. The fact that humidity has very little effect on these polymers was expected. When fully cured, these materials have a very high level of crosslinking, resulting in a tightly bound network with a low level of voids, leaving little space for water to be absorbed. Halogenated polymers were found to be even more hydrophobic. D. Temperature Dependence of Refractive Index A key difference between polymeric materials and more conventional optical materials, such as glass, is that their refractive index varies more rapidly with temperature. This large difference in (about 25 times larger in most polymers than in glass) can be leveraged to produce efficient thermooptically active optical components (e.g., switches and tunable filters). The value of for different classes of halogenated and nonhalogenated cross-linked acrylate polymers varies between about 2 and 3 10 / C. When the large magnitude of

5 58 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 6, NO. 1, JANUARY/FEBRUARY 2000 TABLE II VALUES OF dn=d FOR TWO ALLIEDSIGNAL OPTICAL POLYMERS AND SiO is not desirable, passive compensation schemes can be utilized, as will be discussed later in the context of the stability of grating-based filters. E. Humidity Dependence of Refractive Index The refractive index was measured to have a low humidity dependence for nonhalogenated acrylates, per % RH, and more than an order of magnitude less for 80%-halogenated polymers. The effect on guide performance is minimal since core and cladding materials undergo similar variations. F. Refractive Index Dispersion Many optical systems rely on having no wavelength-dependent optical effects other than those geometrically designed into the system. Therefore, material dispersion is generally to be avoided. The refractive indices for several polymers and a standard material (SiO ) were measured at various wavelengths in order to obtain values of. Results at key communication wavelengths are shown in Table II for nonhalogenated core and cladding polymers and the standard SiO. While the values for the polymers are comparable to those for SiO they are much lower than those for semiconductors or doped glasses. Note that Table II includes the dispersion in the refractive index due to both the primary electronic absorption in the ultraviolet and the vibrational absorption in the 1 2- m region. Kramers Kronig-type arguments were used to estimate the contribution to the dispersion from the vibrational absorption bands in this region, and a maximum contribution of 10 /nm was estimated at 1.55 m, making vibrational absorption a small, but not insignificant contribution. Plots of the refractive index as a function of wavelength for a subset of our polymers can be found elsewhere [1]. G. Birefringence A key material property impacting polarization transparency is the birefringence and its dispersion. Halogenated three-dimensionally cross-linked acrylate polymers have extremely low bireftingence - values, because they suffer little to no orientation during polymerization (processing). Conventional measurement techniques (e.g., measuring the TE and TM indices of a film with a prism coupling apparatus) do not have the resolution needed to detect the level of birefingence in these materials. A very sensitive technique of evaluating the birefingence consists of measuring the polarization splitting in the reflection spectrum of a Bragg grating made in the material. In the case when the two reflections overlap too much to be readily resolved, the grating can be heated or cooled, which causes the reflections of the two polarizations to shift at different rates, and broadening/narrowing occurs. By evaluating the difference between the minimum reflection peak width and the width at the temperature where the birefringence value is to be measured, the birefringence value can be extracted. This technique gives a birefringence value of 2 10 at 1550 nm for the nonhalogenated materials. However, with the halogenated materials, a value of zero is obtained, indicating that the birefringence value is lower than the resolution of the measurement, which is This extremely low value makes these materials particularly attractive for dense wavelength-division multiplexing (DWDM) applications [2], [3]. H. Polarization-Dependent Loss Another important property affecting polarization transparency of an optical component is the PDL. The PDL value, which is equal to loss loss, varies with processing conditions. The TE loss measured in our single-mode waveguides is typically slightly higher than the TM loss, mostly due to the fact that the vertical walls of the core have slightly more roughness than the horizontal boundaries. Measurements performed on single-mode waveguides at 1550 nm reveal that the PDL in waveguides can be reduced to below 0.01 db/cm, and the value at interfaces (due to such effects as facet roughness, glue properties, anisotropic stress, directional mode mismatch with fiber, etc.) can be below 0.01 db per interface. I. Mechanical Flexibility and Material Toughness To improve toughness and reduce stress, we have designed optical polymers on the basis of known structure-property and group-additive relationships for prospective organic polymer segments. By making appropriate choices of monomers, formulation and processing, these materials can be engineered to have relatively high levels of toughness and flexibility. These polymers behave in a manner similar to that of rubber, being both resilient and robust. Flexibility was evaluated by rolling freestanding 125- m-thick films of different formulations around mandrills of increasingly small diameters until cracking occurred. The radius of the mandrill at which the film cracked, defined as the critical radius, was used to characterize flexibility. If the sample survived the mandrill test at a radius of 0.6 mm, the film was folded onto itself and the radius of the resulting curve was compressed using a machinist s micrometer. The sample was considered to have passed the full test, if it was compressed until the ratchet of the micrometer was activated without the occurrence of cracking in the polymer. The materials discussed in this work have passed this test. III. WAVEGUIDE FABRICATION Polymeric optical waveguide devices may be fabricated in many ways. For AlliedSignal polymers, a direct photolithographic fabrication scheme is most appropriate, although other techniques such as reactive ion etching (RIE), excimer laser ablation, molding, and embossing were demonstrated to work. Photosensitive polymerization initiators are added to the monomer mixtures to provide a means for photochemically initiating the polymerization. Device patterning is typically

6 ELDADA AND SHACKLETTE: ADVANCES IN POLYMER INTEGRATED OPTICS 59 Fig. 7. (a) Top view and (b) cross-sectional view of single-mode 1 2 power splitters. achieved by either conventional mask photolithography or by laser writing. A. Contact Printing Lithography The direct lithographic production of our optical polymer waveguides using masks is a multistep process, generally involving the deposition and lithographic patterning of three polymer layers. The processing of these polymers is similar to that of negative-tone photoresists in that exposed regions become polymerized and fixed on the substrate surface. Films of optical quality from under a micron to several hundred microns can be prepared by spin casting or slot coating on a suitable substrate (which can be silicon, glass, quartz, a glass-filled epoxy printed circuit board substrate, or a flexible plastic film for example). The unreacted material is in liquid monomer form, and it is not mixed with a solvent, therefore it does require an evaporation step prior to exposure. The sample is subsequently exposed to an appropriate dose of ultraviolet radiation under a conventional Hg or Hg Xe arc lamp through a photomask. The high photosensitivity results in dose requirements of only a few tens of mj/cm at 365 nm (Hg -line). The pattern is then developed by conventional wet etch of the unreacted material with a standard organic solvent such as methanol at room temperature. This lithographic process offers very high contrast response allowing the definition of polymeric features with dimensions ranging from a few microns to a few millimeters with a high degree of accuracy and process latitude. Using the fabrication technique described above, a wide variety of optical waveguide building-block structures were produced, including straight waveguides, bends, splitters, and couplers. The micrographs of 1 2 power splitters shown in Fig. 7 illustrate the capability of the system to produce extremely sharp waveguide profiles, with a contrast level that permits removal of the unexposed monomer even at the vertex of relatively smallangle Y-branches. B. Laser Direct Writing Another approach to forming optical polymer waveguides is based on laser writing. This technique [4] has the advantage of allowing rapid prototyping, as opposed to mask-based Fig. 8. Laser-fabricated multimode optical waveguides terminated: (a) by cleaving the silicon substrate and (b) by direct laser termination. approaches in which case a mask must be designed and produced before waveguides can be fabricated. Laser writing also affords considerable latitude in power level, focusing, and writing speed, permitting the creation of novel structures that are virtually impossible to make by mask-based lithography. The process is also a discretionary one; it has the advantage of direct-writing features within restricted regions of a sample without affecting the surrounding area, as opposed to conventional techniques where the fabrication of local structures generally involves a multistep process which affects the entire surface; this discretionary property also makes possible the patterning of substrates that contain elevated structures that prevent mask contact printing. Furthermore, laser writing can play a role in a regime of large dimensions where masks cannot be produced; for instance, several-meter-long polymer waveguides can be laser-written on large substrates such as rolls of flexible plastic. It should also be noted that the very high scanning speed (of up to 5 cm/s) used in our laser writing system makes this technique a viable tool for manufacturing and for the production of large-area photonic integrated circuits, despite the serial nature of the process. Laser-delineated multimode waveguides are depicted in Fig. 8. One of the guides was terminated by cleaving the silicon substrate and the other has a 90 facet obtained by direct laser termination. Note the

7 60 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 6, NO. 1, JANUARY/FEBRUARY 2000 sharp vertical walls obtained in these structures owing to the high contrast of the materials. C. Molding A distinct advantage of polymers in a manufacturing environment is their unique ability to be processed by fast turnaround techniques that are not available for more conventional photonic materials such as glass and semiconductors. These techniques include casting, molding, hot embossing, and soft embossing [25] [28]. Among the four, casting and soft embossing are the processes of choice for photochemically reactive materials, and both have been demonstrated with the acrylate materials described above. In both cases, either a UV-transparent substrate or a UV-transparent tool is required for processing. The tool comprises an inverted replica of the structure to be created (e.g., a channel on the tool will become a rib on the substrate, and vice versa). In the case of soft embossing, the tool in the form of either a roll or a platen is impressed into the liquid monomer, the monomer is cured with UV light, and the tool is removed. Waveguides are formed by subsequently either filling channels or over cladding ribs formed in the embossing process. For example, 1 8 single-mode polymeric power splitters were produced with our nonhalogenated acrylates at the Fraunhofer Institute in Jena (Dannberg and Bräuer) by embossing the core structures with metal tooling on a clear PMMA substrate to produce waveguides, which measured 6 6 m in cross section. Fig. 9(a) depicts the outputs of two guides in such a splitter and Fig. 9(b) shows the insertion loss across the eight outputs of the device when energized with 1318 nm light. The average insertion loss measured in this experiment is db (i.e., the average excess loss is 2.13 db) and the standard deviation is db. In our own facilities, we have developed techniques for embossing large area arrays of waveguides having a similar dimension and consisting of thousands of waveguides. We have used both metal tooling and soft silicone tooling for this purpose. Remarkable uniformity can be obtained over a large surface, but inevitably the replication process introduces an additional degree of roughness to the waveguides which adds a significant scattering loss contribution to the overall waveguide loss. Our studies showed that this scattering loss is typically between db/cm for wavelengths in the range from 633 to 850 nm. IV. APPLICATIONS A. Telecom Polymer components produced for the telecom industry include single-mode splitters, couplers, routers, filters, and switches. One of the earlier drivers for work with polymer waveguides was the concept of fiber-to-the-home, which raised the possible future need for low cost passive optical components such as splitters. This perception of the need for low cost devices has driven much of the early effort on the development of low cost replication technology [25]. When this opportunity did not appear as soon as expected, other higher value applications requiring greater functionality were pursued [19], [28]. The early commercialization of polymer waveguide technology came with the introduction of thermooptic switches from Fig. 9. (a) Image of the outputs of two guides in a molded 1 8 single-mode polymeric splitter and (b) insertion loss for the eight outputs of the splitter when energized with 1318-nm light. Akzo Nobel, which were made available under the Beambox name [19], [20]. This application takes special advantage of the relatively high values of that are characteristic of polymers, and was pursued by other workers [9], [13], [27], [29]. Another device application that takes similar advantage of the special properties of polymer is Bragg-grating-based filters in fixed and tunable form. We have applied cross-linked acrylate polymers to such devices and will devote the remainder of this section to a description of this work. A description of couplers and routers, which have been fabricated with these same polymers, can be found elsewhere [4], [5]. Gratings in polymers can be produced by a variety of techniques such as casting, molding, embossing, e-beam writing and photochemical processes. The first three techniques produce surface relief gratings while the last two can produce either relief gratings or bulk index gratings across the waveguide core. The photochemical fabrication process for bulk index gratings utilizes two-beam interference to induce an index modulation. This effect can be achieved either through the use of a phase mask (where two beams corresponding to the 1st and 1st diffraction orders are allowed to interfere) or through the use of direct interference of split laser beams. Both techniques were applied successfully, with the preferred approach being the use

8 ELDADA AND SHACKLETTE: ADVANCES IN POLYMER INTEGRATED OPTICS 61 Fig. 10. Reflection and transmission spectra of 2-cm-long Bragg gratings photochemically generated in planar single-mode polymeric waveguide with (a), (b) large 1n and (c), (d) small 1n. of a phase mask because it enables a fast, parallel, low-cost, robust process suitable for manufacturing. When index gratings are produced, the dynamics can be described by the photolocking effect [30]. Photolocking is the process by which an index variation can be photoinduced by causing the separation of polymeric or monomeric components that have different indices. This separation can be induced by evaporation of a fugitive species after a partial (locking) exposure, or it can occur without loss of a component as a result of a spontaneous redistribution of constituents set up by differences in reaction rates, or changes in polymer-solvent interaction parameters as a function of cure. The index modulation obtained by this process can be increased by maximizing the difference in the diffusion lengths, the polymerization rates, and the indices of the components. When gratings are printed in single-mode waveguides, the three-layer waveguiding circuit is fabricated first, but the UV exposure time is minimized so that polymerization is only partial and diffusion can still occur to a substantial degree when gratings are imprinted. A final blanket UV cure locks all the indices in place. Since this technique of inducing gratings relies on material separation instead of differential cure levels or selective photodegradation, it naturally results in an essentially constant average (or dc) index across the grating, avoiding the formation of a Fabry Perot cavity that would cause narrow peaks to appear on the short wavelength side of the main grating peak. In high contrast specialty polymers engineered at AlliedSignal for optimal grating formation, index modulation ( ) levels as high as 10 can be achieved. Fig. 10 shows reflection and transmission spectra of 2-cm-long gratings fabricated photochemically in these materials with large (a), (b) and small (c), (d). The smaller results in narrower filters, while the larger results in more efficient filters. The transmission spectrum of Fig. 10(b) reveals a reflection efficiency of 45 db and the spectrum of Fig. 10(d) reveals over 70 db; (> %). Both of these values are limited by the dynamic range of the test equipment used in characterizing the device. The grating of Fig. 10(b) has, in reality, a stronger reflectivity (estimated at about 150 db), but the spectrum of Fig. 10(d) exhibits a deeper transmission dip because the dynamic range in measuring its spectrum was boosted by the use of an amplifier. The noise level in the reflection spectrum of Fig. 10(a) reaches 20 db because the chip of the corresponding grating was pigtailed with perpendicular cuts at the interfaces, causing the broadband Fresnel reflections at the interfaces to be relatively high. In the reflection spectrum of Fig. 10(c), the noise level was reduced to below 40 db by the

9 62 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 6, NO. 1, JANUARY/FEBRUARY 2000 use of angled cuts and an optimal index-matching medium at the interfaces. Gratings with an index modulation envelope of any shape can be produced with the photochemical process. The first gratings fabricated at AlliedSignal were uniform (i.e., they had a rectangular index modulation envelope), and as a result had sidelobes as predicted from theory [2]. By locally varying the intensity of the laser beam used to produce the gratings, Gaussian-apodized gratings were produced, and the desired sidelobe suppression was obtained, as depicted in the spectra of Fig. 10. Also note the absence of all other out-of-band features, including those due to cladding mode coupling, which would appear as transmission dips at wavelengths shorter than that of the main peak by more than (where is the guide effective index, is the cladding index, and is the grating period) [31]. The elimination of coupling to cladding modes, which is key in the add/drop architecture considered here, was achieved by printing the grating with uniform strength across the three polymer layers (cladding/core/cladding) and by ensuring symmetry in the device (i.e., the grating was perpendicular to the propagation direction and the cladding was symmetric around the core). The rectangular wavelength capture of these gratings down to at least 45 db and the absence of out-of-band features allow one to maximize the utilization of bandwidth. Current DWDM systems require a usable channel bandwidth of at least 50% of the channel spacing to allow for source wavelength error and drift. The bandwidth utilization (BWU) figure of merit, defined as [channel width (transmission< 30 db)]/[minimum channel spacing (reflection< 20 db)] [32], was evaluated at 0.92, well exceeding the performance level needed for incorporation in DWDM systems. The minimum channel spacing demonstrated is 0.6 nm, making these components suitable for systems with 75-GHz channel spacing. In passive (nontuning) filtering applications, the stability of filter spectra with temperature is desirable. Although the (refractive index variation with temperature) in our polymers causes a 0.25 nm/ C shift in the reflected wavelength ( where is the Bragg wavelength), a stability of nm/ C was achieved by selection of the proper compensating substrate. This compensation scheme relies on the fact that in polymers is negative (as opposed to in glass), causing a shift to the short-wavelength side with increasing temperature, while expansion increases the grating period, thereby causing a shift in the opposite direction; the key is to select a substrate with a volumetric coefficient of thermal expansion (CTE) that causes enough expansion in the guide to compensate for the effect of of the polymer. The value of achieved with different substrates is shown in Fig. 11. The improved stability achieved in a passively compensated device allows the use of a less precise (thus less costly) heater in the final packaged product. When is nm/ C, a temperature control of 1.25 C is sufficient to ensure the wavelength registration required in a 50-GHz-channel-spacing system ( 0.05-nm stability), whereas a temperature control of 0.2 C would be needed in an uncompensated device. As for the stability with humidity, it was measured to be less than 0.2 nm over the full Bellcore humidity range in the 80% halogenated acrylates. Fig. 11. Range of d =dt achieved in a polymeric grating as a function of the substrate CTE. Bragg gratings can be printed across the arms of Mach Zehnder interferometers (MZI s) to form optical add/drop multiplexers (OADM s). Grating MZI s have been implemented mostly in glass fibers [33]. Holding a high tolerance for the 3-dB coupler is typically the most challenging requirement when fabricating a multichannel OADM (consisting of a cascade of grating MZI s each having two 3-dB couplers). This element typically comprises a directional coupler, which is very fabrication sensitive, requiring extreme precision in both thickness control and patterning resolution (the narrow channel of monomer between the guides must not polymerize during exposure and must be fully cleared during development). New more robust designs were generated to circumvent these problems. In the first design, the 3-dB couplers are directional couplers in which the separation between the guides is zero, allowing easier patterning (no material needs to be cleared between the guides). This device is also compact, reducing the length (thus the insertion loss) of the OADM. The disadvantage of this design, however, is that it is very sensitive to fabrication errors (due to the strong evanescent coupling), making it not the ideal candidate for a high-yield manufacturing process. The second and preferred design is a 2 2 multimode interference (MMI) coupler, which is more tolerant of errors. MMI couplers offer excellent tolerance to polarization and wavelength variations, and relaxed fabrication requirements when compared to alternatives such as directional couplers, adiabatic X- or Y-junctions, and diffractive star couplers, resulting in a superior yield. In addition to offering all these advantages, MMI devices are relatively compact. The key property of MMI devices is that of self-imaging [34], whereby a field profile input into a multimode waveguide is reproduced in single or multiple images at periodic intervals along the propagation direction of the guide. The input/output (I/O) guides are single-mode and so is the mixing slab in the vertical direction. In a 2 2 MMI coupler, no coupling happens between the two guides outside the MMI region. The fact that the two guides are quite far apart (typically apart, edge to edge, by at least twice the guide width) results in less problems with residual material remaining after development.

10 ELDADA AND SHACKLETTE: ADVANCES IN POLYMER INTEGRATED OPTICS 63 (a) (b) Fig. 13. Output intensity patterns of: (a) a 3-dB tapered MMI coupler and (b) a four-channel OADM based on MZI s with 3 db MMI couplers, demonstrating proper coupling. Fig. 12. (a) Simulated intensity distribution in a parabolically tapered 3-dB MMI coupler and (b) four-channel OADM based on grating MZI s with tapered 3-dB MMI couplers. Grating MZI s were cascaded to form modular multichannel OADM s. Two common device types used in wavelength filtering today are waveguide grating routers (WGR s) [35] and fiber Bragg gratings (FBG s) [36]. Our novel approach offers the advantages of both technologies by combining the integration and mass lithographic manufacture of WGR s with the performance, modularity, and channel trimming capability of FBG s. This approach results in low-cost, high-performance OADM s. In particular, four-channel OADM s were designed based on MZI s with 3-dB MMI couplers. These novel MZI s are the first to incorporate tapered MMI couplers [37]. In that design, the coupler can be shortened while its imaging properties are preserved. In MMI couplers, laterally narrow mixing regions result in short devices. These regions, however, should not be made too narrow at the input and output ends in order to avoid evanescent coupling between the I/O waveguides. The key concept in the tapered devices is to keep the MMI region dimensions (and thus the guide separation) constant at the ends, and gently decrease the lateral waist of the MMI region so that the width is minimal at the center. Parabolic tapering results in the desired effect [Fig. 12(a)]. Fig. 12(b) shows a laterally expanded CAD drawing of a four-channel OADM with parabolically tapered MMI couplers. The Bragg gratings in the four MZI s have slightly different periods. The grating closest to the input port reflects wavelength, so light at that wavelength launched at the input port exits the drop port labeled, while light at that wavelength launched into the add port labeled, exits the pass port. The device length is 4 cm, and the pitch of the input and output guides is 250 m. This integrated four-channel OADM was produced in our nonhalogenated acrylates. The first step was to produce the three-layer channel waveguide circuit (i.e., the device without the gratings), which had a 6- m square-cross-section core between two 10- m-thick cladding layers, with a refractive index difference between the core and the cladding of 0.5% of the base index (in the polymers described here, the base index is about 1.5 at 1550 nm wavelength and is ). Achieving good control over the fabrication parameters is key for repeatably obtaining proper coupling. Some simple test elements (3-dB couplers and straight guides) were printed in addition to the OADM waveguiding circuits. Fig. 13(a) depicts the output intensity pattern of a 3-dB tapered MMI coupler, revealing good uniformity in the splitting ratio (better than 49/51). Fig. 12(b) is a3 collage of five output intensity patterns from a four-channel device when different input guides are energized. For the purposes of this test, the ports labeled input and drop in Fig. 12(b) were used as input ports, and the ports labeled add and pass were the output ports [numbering is from left to right in Fig. 12(b)]. The images in Fig. 13(b) captured facing the output end of the device show output 1 on the right and output 5 on the left. Note the minimal leakage in this complex photonic circuit. Printing of gratings in the guides follows waveguide fabrication. One challenge in producing these devices was the

11 64 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 6, NO. 1, JANUARY/FEBRUARY 2000 Fig. 14. Transmission spectrum of the pass port in a four-channel polymeric OADM, revealing proper channel spacing (3.2 nm) and location (on the ITU grid), excellent channel isolation, and uniform channel response. Fig. 15. OADM configuration consisting of a chip (which integrates a grating printed in a single-mode polymeric waveguide with a thin film heater) and two three-port optical circulators. alignment of the phase mask to the waveguides with a high level of accuracy in order to ensure phase matching of the reflections from the two arms in each MZI. The use of well-separated fiducials allowed us to achieve very accurate alignment. These devices were characterized, and they exhibited the desired 400-GHz (3.2-nm) channel spacing and ITU grid wavelength alignment, as well as excellent channel isolation and uniform channel response, as depicted in Fig. 14 which shows the transmission spectrum at the pass port when the input guide is energized. The ITU grid channels that were selected correspond to wavelength values of , , , and nm. The reflectivities in the four-channel device of Fig. 14 are weaker than those of the gratings shown in Fig. 10 because these gratings were shorter (5 mm) and their average index modulation (across the apodized length) was lower because they were not produced in the polymer system optimized for grating fabrication. The insertion loss for these 4-cm devices was about 2.5 db. Of this total loss number, about 2 db is due to absorption, about 0.3 db is caused by radiation and scattering, and about 0.2 db was due to coupling losses at the interfaces. A 250- m-pitch fiber array was utilized to characterize the device in reflection. The input guide was energized with broad-band light while the drop ports were monitored. This test confirmed that the channels missing from the pass port were exiting the proper drop ports. When tuning is desired, a simple device of the type shown in Fig. 15 can act as a narrow-band thermally tunable OADM that can cover a 15-nm wavelength window with a reasonable temperature range (e.g., 20 C 80 C). The use of two such components in series allows full coverage of the erbium C band between nm. The chip consists of a grating printed in an integrated single-mode polymeric waveguide with a deposited thin film heater. This chip, combined with two three-port optical circulators, forms a tunable OADM. In this design, the only critical requirement for the die is to have a good grating. Key characteristics of a good grating are strong and uniform reflection of a narrow band of light with no out-of-band spectral features and low insertion loss. The circulators provide the add and drop functions in a reliable fashion, circumventing the yield issues associated with 3-dB couplers in Mach Zehnder-based add/drop filters. These OADM s offer low, uniform insertion loss for add/drop paths (<2 db), efficient dropping of the reflected channel through the use of circulators with high isolation (>60 db), and low crosstalk through the use of gratings that are apodized, have uniform dc index, and have uniform strength in the core and the cladding. The general design of placing a grating between optical circulators to form an OADM has been implemented in the past with fiber Bragg gratings [38], however due to the small of glass (about 25 times smaller than that of most polymers), those filters cannot be tuned thermally over a large wavelength range. As a result, glass fiber devices are generally tuned by mechanical means. In our case, thermal tuning of polymer Bragg gratings was achieved by taking advantage of the large of polymers. The value of in these polymers is as high as 3 10 / C, resulting in a channel tuning rate as high as 0.3 nm/ C. These OADM s were built in partially halogenated polymers, which exhibited a best loss value of 0.24 db/cm at 1550 nm in single-mode waveguides, and were tuned by heating the waveguides to a series of selected temperatures. Fig. 16(a) shows the transmission spectra obtained when tuning the filter between 20 C 100 C, revealing that a wavelength window wider than 20 nm can be covered with a single tunable grating filter. Tuning over wider ranges can be achieved either by cascading these components or by integrating multiple gratings on a single chip. Fig. 16(b) shows the degree of linearity of wavelength tuning with temperature over the wide range investigated, indicating that the electrical control for the device can be quite simple. This linearity was obtained because no significant thermal transitions occur in the temperature range investigated. In particular, the glass transition temperature ( ) of the polymers used in this work has a very low value of about 50 C, well outside the temperature range of interest in tuning applications. An important aspect of crosslinked polymers is that they do not flow and are mechanically stable at temperatures above. Operation above can in fact have significant advantages, such as the minimization of stress-related effects; for example, no polarization splitting was observed in the reflected peak over the full investigated temperature range. We can estimate the loss performance of a fully optimized and packaged OADM based on polymer waveguides and filters. The transmission loss for the 2-cm-long waveguides with gratings is about 0.5 db. With each circulator having a 0.4-dB insertion loss, and with each pigtail resulting in a 0.1-dB interface loss, the total insertion loss of the OADM shown in Fig. 15 would be 1.5 db. The PDL was also investigated in these devices. The

12 ELDADA AND SHACKLETTE: ADVANCES IN POLYMER INTEGRATED OPTICS 65 Fig. 16. (a) Output transmission spectra for a tunable OADM operated between 20 C and 100 C, covering a wavelength window of more than 20 nm and (b) channel reflected as a function of temperature, revealing a highly linear dependence of wavelength on temperature. 2-cm-long guides with gratings have a PDL of 0.02 db, each circulator has a PDL of 0.03 db, and each pigtail introduces a PDL of less than 0.01 db, making the total PDL for the OADM under 0.1 db. B. Datacom Recent advances that have continued to propel optical interconnects as the wideband technology of choice in datacom include vertical cavity surface emitting laser (VCSEL) sources [39], low-cost VCSEL packaging, polymer-based optical waveguide and fiber materials, low-cost microoptical components, passive alignment, low-cost connectors, and low-cost fabrication processes (IC-like planar batch processing), allowing lowcost, large-volume manufacturing. Wafer-level processing and first-level packaging for CMOS and VCSEL technologies have a high level of similarity [6]. The development of polymer interconnect technology for datacom applications was carried out at AlliedSignal both for internal aerospace applications and under DARPA-sponsored programs (POINT, FLASH, and OBIS). The POINT program [7], [8], [15] developed high-speed optical interconnects employing polymeric waveguides. In particular, the Fig. 17. (a) An array of 32 VCSEL s embedded in a transmitter MCM. Laser-defined polymer waveguides adaptively aligned to (b) VCSEL pixels in a transmitter MCM and (c) the detecting element (dark disk) on a high-speed receiver chip in a receiver MCM. program demonstrated high-speed parallel optical data links between transmitting and receiving multichip modules (MCM s) on a board, and from board to board through a backplane. One aspect of this program is direct in situ writing of optical waveguides on these modules using the AlliedSignal technology described here. The top surface of a POINT MCM is laminated with polyimide films, to which our polymeric waveguides can be made to adhere reliably. Because of the lack of precision in the location of chips mounted in an MCM (usually with a pick-and-place machine which typically has only about one mil accuracy), the adaptive capability of our laser writing system was used to adjust the position of the waveguides based on fiducials in the chips. With this technique, devices are intercepted by the laser with a high level of accuracy. Fig. 17(a) depicts multimode waveguides directly laser-written above these VCSEL pixels. The waveguides are 100 m wide with a 140- m pitch, resulting in a highly dense circuit. Fig. 17(b) shows a laser-delineated waveguide that is centered onto the detecting element in a high-speed receiver chip embedded in a receiver MCM.

13 66 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 6, NO. 1, JANUARY/FEBRUARY 2000 of 45 mirrors is the direct laser termination of waveguides as they are laser delineated (the desired angle is achieved by a careful balance of the scanning speed, laser power, and the photolithographic contrast of the material formulation). An array of laser-written waveguides directly terminated with 45 mirrors is shown in Fig. 18(b). The curvature of these mirrors can be controlled from convex, to straight, to concave, while achieving the desired angle [8]. The convex design is particularly useful since it helps focus the beam in addition to bending its path. A natural role for low-loss polymeric interconnects in computing cabinets is in backplane waveguides where run lengths can be large (approximately in) and where state-of-the-art polymer loss values are needed to produce the desired long waveguides while keeping the total insertion loss low. Arrays of in-long multimode waveguides were produced as part of POINT, and were shown to have good guide-to-guide and array-to-array loss uniformity with losses over a large number of samples lying between db/cm at 840 nm. In the final POINT demonstration, four such arrays were joined together to form a one-yard-long array of 200 guides. These flexible ribbons were attached to backplanes allowing 90 bending to accommodate connection of daughter boards. V. CONCLUSION Fig. 18. Arrays of waveguides with 45 mirrors: (a) ablated with an excimer laser and (b) directly laser-written. Fig. 17(a) and (b) demonstrates the accuracy of the adaptive patterning technique. A key element of optical interconnects is to achieve 90 light-path bending to couple light from VCSEL s into waveguides and from guides into detectors. 45 mirrors have the desired compactness. A conventional approach would involve incorporating bulk-optic prisms in the waveguiding circuitry to achieve the desired path bending. That approach lacks the accuracy and simplicity desired in a manufacturing environment. A preferred approach is completely lithographic and consists of shaping the facets of the waveguides to form reflectors. This result can be obtained using a variety of processing techniques. The process used was based on excimer laser ablation and was developed in the POINT program in collaboration with GE. This process produced mirrors with a smooth finish that achieved reflection efficiencies as high as 80% without the use of a reflective coating. Fig. 18(a) shows an array of waveguides terminated with 45 mirrors using this technique. An alternate process to excimer ablation We have reviewed the key properties of polymeric systems developed worldwide for the telecom and datacom industries, and have given a detailed description of AlliedSignal s crosslinked acrylate photopolymer waveguide technology. These materials are characterized by low optical loss, thermal stability, humidity resistance, low refractive index dispersion, low PDL, low birefringence, flexibility, mechanical robustness, high photolithographic contrast, capability for precise tailoring of the refractive index, and ability to be cast in a wide range of thicknesses. These unique material properties enabled fabrication of a variety of optical waveguide devices, such as straight waveguides, bends, splitters, directional couplers, MMI couplers, star couplers, wavelength filters, and long, high-density waveguide arrays, that match the dimensions and numerical aperture of conventional optical fibers and meet practical application requirements. In particular, high-performance, single-mode DWDM components were fabricated. Gratings with % reflection efficiencies were produced, apodization was used to eliminate sidelobes, grating presence in the cladding was ensured to avoid cladding mode loss, and uniform dc index was achieved to avoid Fabry Perot oscillations. The BWU factor of these components was as high as 92%, with a minimum channel spacing of 75 GHz. Integrated multichannel grating-mzi-based OADM s were produced. Channel dropping was demonstrated with uniform response and excellent isolation, and further, the desired spacing and alignment to the ITU grid was achieved. High-performance tunable OADM s based on Bragg gratings and circulators were also fabricated. Thermal solid-state tuning over a range of 20 nm was achieved with a single device. Polymeric circuits were also produced on chips, MCM s, boards, and backplanes. Path bending of 90 was achieved

14 ELDADA AND SHACKLETTE: ADVANCES IN POLYMER INTEGRATED OPTICS 67 by forming 45 mirrors in the waveguides. Structures were produced by conventional mask photolithography, adaptive laser writing, and soft embossing, and their adhesion to a variety of rigid and flexible substrates was demonstrated. The combination of conventional lithographic manufacture and a broad range of advanced specialty polymers indicate the potential of this technology to be a solution to the problem of cost-effective production of high-performance integrated optical circuitry. ACKNOWLEDGMENT The authors have reviewed a broad range of work to which many have contributed. They wish to thank R. Blomquist and M. Maxfield for materials development, D. Pant for device fabrication, C. Poga for environmental studies, D. Nguyen and P. Ferm for embossing process development, A. Nahata for his work in producing backplane structures for POINT, G. Boudoughian for strip heater development, and R. Norwood and J. Yardley for their numerous technical contributions. REFERENCES [1] L. Eldada, S. Yin, R. A. Norwood, and J. T. Yardley, Affordable VTDM components: The polymer solution, in Proc. SPIE, vol. 3234, 1997, pp [2] L. Eldada, S. Yin, C. Poga, C. Glass, R. Blomquist, and R. A. Norwood, Integrated multi-channel OADM s using polymer Bragg grating MZIs, IEEE Photon. Technol. Lett., vol. 10, pp , [3] L. Eldada, R. Blomquist, M. Maxfield, D. Pant, G. Boudoughian, C. Poga, and R. A. Norwood, Thermo-optic planar polymer Bragg grating OADM s with broad tuning range, IEEE Photon. Technol. Lett., vol. 11, pp , [4] L. Eldada, C. Xu, K. Stengel, L. Shacklette, and J. T. Yardley, Laserfabricated low-loss single-mode raised-rib waveguiding devices in polymers, J. Lightwave Technol., vol. 14, pp , [5] J. F. Viens, C.L. Callender, J. P. Noad, and L. Eldada, Polymer-based waveguide devices for WDM applications, in Proc. SPIE, vol. 3799C, [6] L. Eldada, Polymer optical polymers interconnects, in Future Trends in Microelectronics: Off the Beaten Path, S. Luryi, J. Xu, and A. Zaslavsky, Eds. New York: Wiley, [7] L. Eldada, A. Nahata, and J. T. Yardley, Robust photopolymers for MCM, board, and backplane optical interconnects, in Proc. SPIE, vol. 3288, 1998, pp [8] L. Eldada and J. T. Yardley, Integration of polymeric micro-optical elements with planar waveguiding circuits, in Proc. SPIE, vol. 3289, 1998, pp [9] L. Eldada, R. Norwood, R. Blomquist, L. W. Shacklette, and M.J. Mc- Farland, Thermo-optically active polymeric photonic components, in Proc. Optical Fiber Communication, [10] K. D. Singer, T. C. Kowalczyk, H. D. Nguyen, A. J. Beuhler, and D. A. Wargowski, Cross-linked polyimides for integrated circuits, Proc. SPIE, vol. CR68, pp , [11] C. R. Kane and R.R. Krchnavek, Benzocyclobutene optical waveguides, IEEE Photon. Technol. Lett., vol. 7, pp , [12] G. Fischbeck, R. Moosburger, C. Kostrzewa, A. Achen, and K. Petermann, Singlemode optical waveguides using a high temperature stable polymer with low losses in the 1.55 m range, Electron. Lett., vol. 33, pp , [13] R. Moosburger and K. Petermann, 4 x 4 digital optical matrix switch using polymeric oversized rib waveguides, IEEE Photon. Technol. Lett., vol. 10, pp , [14] B. L. Booth, Low loss channel waveguides in polymers, J. Lightwave Technol., vol. 7, pp , [15] Y. S. Liu, R. J. Wojnarowski, W. A. Hennessy, J. P. Bristow, Y. Liu, A. Peczalski, J. Rowlette, A. Plotts, J. Stack, J. T. Yardley, L. Eldada, R. M. Osgood, R. Scarmozzino, S. H. Lee, and V. Uzguz, Polymer optical interconnect technology (POINT): Optoelectronic packaging for board and backplane interconnect, in Proc. SPIE, vol. CR62, 1996, pp [16] C. C. Teng, Traveling-wave polymeric optical intensity modulator with more than 40 GHz: of 3-dB electrical bandwidth, Appl. Phys. Lett., vol. 60, pp , [17] C. C. Teng, M. A. Mortazavi, and G. K. Boudoughian, Origin of the poling-induced optical loss in a nonlinear optical polymeric waveguide, Appl. Phys. Lett., vol. 66, pp , [18] M. C. Oh, S. S. Lee, S. Y. Shin, W.Y. Hwang, and J. J. Kim, Polymeric waveguide polarization splitter based on poling-induced birefringence, Electron. Lett., vol. 32, pp , [19] M. H. M. Klein Koerkamp, M. C. Donckers, B. H. M. Hams, and W. H. G. Horsthuis, Design and fabrication of a pigtailed thermo-optic 1 x 2 switch, in Proc. Integrated Photonics Research Conf., vol. 3, 1994, pp [20] M. B. J. Diemeer, P. M. C. De Dobbelaere, and M. C. Flipse, Polymeric thermo-optic waveguide switches for optical communications, in Proc. Plastics in Telecommunications Conf., vol. 8, 1998, pp [21] M. B. J. Diemeer, L. H. Spiekman, R. Ramsamoedj, and M. K. Smit, Polymeric phased array wavelength multiplexer operating around 1550 nm, Electron. Lett., vol. 32, pp , [22] R. Yoshimura, M. Hikita, S. Tomaru, and S. Imamura, Low-loss polymeric optical waveguides fabricated with deuterated polyfluoromethacrylate, J. Lightwave Technol., vol. 16, pp , [23] J. Kobayashi, T. Matsuura, S. Sasaki, and T. Martino, Single-mode optical waveguides fabricated from fluorinated polyirnides, Appl. Opt., vol. 37, pp , [24] M. Usui, M. Hikita, T. Watanabe, M. Amano, S. Sugawara, S. Hayashida, and S. Imamura, Low-loss passive polymer optical waveguides with high environmental stability, J. Lightwave Technol., vol. 14, pp , [25] A. Neyer, T. Knoche, L. Moller, and R. Klein, Low-loss passive polymer waveguides at 1300 and 1550 nm, in Proc. Polymer Optical Fibers Conf., 1994, pp [26] H. Kragl, R. Hohmann, C. Marheine, W. Pott, and G. Pompe, Low cost monomode, integrated optics polymeric components with passive fiber-chip coupling, Electron. Lett., vol. 33, pp , [27] G. Pompe, M. Jolinck, S. Kalveram, S. Lehmacher, S. Rudolph, and A. Neyer, Technology of hot embossed thermo-optic switches in plastics with passive fiber coupling, in Proc. Plastics in Telecommunications Conf., vol. 8, 1998, pp [28] A. Brauer and P. Darmberg, Polymers for passive and switching waveguide components for optical communications, in Proc. SPIE, vol. CR63, 1997, pp [29] N. Keil, H. H. Yao, and C. Zawadski, Integrated optical switching devices for telecommunications made on plastics, in Proc. Plastics in Telecommunications Conf., vol. 8, 1998, pp [30] E. A. Chandross, C. A. Pryde, W. J. Tomlinson, and H.P. Weber, Photolocking a new technique for fabricating optical waveguide circuits, Appl. Phys. Lett., vol. 24, pp , [31] T. Erdogan, Fiber grating spectra, J. Lightwave Technol., vol. 15, pp , [32] T. Strasser, P. J. Chandonnet, J. DeMarco, C. E. Soccolich, J. R. Pedrazzani, D. J. DiGiovanni, M. J. Andrejco, and D. S. Shenk, UV-induced fiber grating OADM devices for efficient bandwidth utilization, in Proc. Optical Fiber Communication Conf., 1996, pp [33] D. C. Johnson, K. O. Hill, F. Bilodeau, and S. Faucher, New design concept for a narrowband wavelength-selective optical tap and combiner, Electron. Lett., vol. 23, pp , [34] L. B. Soldano and E. C. M. Pennings, Optical multi-mode interference devices based on self-imaging: Principles and applications, J. Lightwave Technol., vol. 13, pp , [35] C. Dragone, An NxN optical multiplexer using a planar arrangement of two star couplers, IEEE Photon. Technol. Lett., vol. 3, pp , [36] K. O. Hill and G. Meltz, Fiber Bragg grating technology fundamentals and overview, J. Lightwave Technol., vol. 15, pp , [37] D. S. Levy, R. Scarmozzino, Y. Li, and R. M. Osgood Jr., A new design for ultracompact multimode interference-based 2 x 2 couplers, IEEE Photon. Technol. Lett., vol. 10, pp , [38] C. R. Giles and V. Mizrahi, Low-loss add/drop multiplexers for WDM lightwave networks, in Proc. IOOC Conf., 1995, pp

15 68 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 6, NO. 1, JANUARY/FEBRUARY 2000 [39] R. A. Morgan, J. Lehman, M. Hibbs-Brenner, Y. Liu, and J. Bristow, Vertical cavity surface emitting lasers: The applications, in Proc. SPIE, vol. 3004, 1997, pp Louay Eldada (S 88 M 91 SM 00) received the B.S., M.S., and Ph.D. degrees in electrical engineering from Columbia University, New York, in 1989, 1991, and 1994, respectively. His Ph.D. research was in the area of integrated optics. Since 1994, he has been with AlliedSignal, Morristown, NJ. He is a Principal Engineer responsible for device development including device design, fabrication, packaging, and characterization. His early work at AlliedSignal was in the electronic and optical materials division, where he worked on optical interconnects and optoelectronic packaging for datacom and performance computing applications while developing a single-mode polymeric photonic technology. That technology later became the basis of the Telecom Photonics business, where he is currently Manager of the Device Development group. He is an author of more than 70 technical papers and book chapters. He has received three patents in the area of polymer integrated optics. Dr. Eldada is a member of the Institute of Electrical and Electronics Engineers, the Lasers and Electro-Optics Society, the Optical Society of America, and the International Society for Optical Engineering. Lawrence W. Shacklette received the B.S. degree in physics from Brown University, Providence, RI, in 1967 and the Ph.D. degree in materials physics from the University of Illinois, Urbana, in He is Manager of the Materials Physics and Engineering group within AlliedSignal s Telecom Photonics business, Morristown, NJ. He is responsible for optical materials supply, new materials development, process innovation, prototype fabrication, and design for manufacturing. He previously worked on the development of fabrication techniques for optical interconnects for use in datacom with particular application to the aerospace environment. During his time at AlliedSignal, he has used his background in materials physics and polymers to play a leadership role in the development of a variety of new electroresponsive polymers along with novel applications for these materials ranging from batteries to corrosion preventive coatings to optical displays. Before joining AlliedSignal, he was a member of the Physics Department of Seton Hall University, South Orange, NJ, as Assistant Professor ( ) and as Associate Professor ( ). He joined AlliedSignal, Inc. (then Allied Chemical Corp.) in 1979 as a Member of the Polymer Laboratory. He has 35 issued U.S. patents to his credit and has authored more than 100 scientific and engineering articles and books. Dr. Shacklette is a Fellow of the American Physical Society and a member of the SPIE.