Optical Coupling Methods for Cost Effective Polymer Optical Fiber

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IEEE TRANSACTIONS ON COMPONENTS AND PACKAGING TECHNOLOGIES, VOL. 32, NO. 3, SEPTEMBER 2009

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Optical Coupling Methods for Cost-Effective Polymer Optical Fiber Communication Jayakrishnan Chandrappan, Member, IEEE, Zhang Jing, Ramkumar Veppathur Mohan, Philbert Oliver Gomez, Than Aye Aung, Xiao Yongfei, Pamidighantam V. Ramana, John H. Lau, Fellow, IEEE, and Dim Lee Kwong, Fellow, IEEE

Abstract—In this paper, we report cost-effective light coupling methods for polymer optical fiber (POF) communication. Here, we compare the various optical coupling schemes in detail. By optical simulations, we analyze the conventional light coupling schemes, namely the direct coupling, lens coupling, and lensed fiber coupling. The simulation studies reveal that a lensed fiber tip particularly at the receiver side improves the light coupling efficiency to a great extent. The optimized lensed POF design confers an 85% coupling efficiency. Lensed POFs are realized with two low-cost fabrication methods. The characterization of the lensed POF are carried out to evaluate the lensing properties and hence to optimize the fabrication process. Index Terms—Characterization, detector, fabrication, lensed fiber, optical coupling, polymer optical fiber, simulation.

the light source. Availability of high speed, high-performance VCSELs, and resonant cavity LEDs (RCLED) at a reasonable cost keep POF networks attractive for gigabit applications. Since POFs have a large core diameter, the coupling with the transmitter does not raise any serious light coupling issues. However, the large core diameters always results in severe power coupling losses at the receiver side. This in turn reduces the maximum communication link distance achievable. Realization of additional coupling elements brings in process complexities at an added cost. All these factors necessitate a cost effective coupling method along with ease of installation, fiber plug-in modules, to keep POFs attractive in the market [6]. II. COUPLING METHODS

I. INTRODUCTION N the recent past, polymer optical fibers POFs found exciting applications as a low-cost alternative in home network, automotive, and short reach applications [1], [2]. The lower connection costs and other low cost factors made large-core multimode POFs attractive over single-mode fibers for short-distance communications [4]. POFs are large-core multimode optical fibers with very high numerical apertures. They have the advantages of optical fibers while maintaining the flexibility of copper cables in installation [3]. The major challenges for POF networks are low-cost components, their packaging, and the interfacing with the existing networks [5]. At present, both light emitting diodes (LEDs) and vertical cavity surface emitting lasers (VCSEL) are being used as the light source. They give the cost advantage over laser diode as

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Manuscript received March 19, 2008; revised August 25, 2008 and October 08, 2008. First published March 16, 2009; current version published August 26, 2009. This work was supported by the Science and Engineering Research Council (SERC) of Agency for Science, Technology, and Research (A*STAR), Singapore. This work was recommended for publication by Associate Editor F. Shi upon evaluation of the reviewers comments. J. Chandrappan, Z. Jing, P. V. Ramana, J. H, Lau, and D. L. Kwong are with the Institute of Microelectronics, A*STAR, Singapore 117685 (e-mail: [email protected]). R. V. Mohan is with the Department of Electrical and Computer Engineering, National University of Singapore, Singapore 119077. P. O. Gomez is with the School of Electrical and Electronics Engineering, Nanyang Technological University, Singapore 639798. T. A. Aung is with the School of Engineering, Ngee Ann Polytechnic, Singapore S599489. X. Yongfei is with the School of Electrical Engineering, National University of Singapore, Singapore 119077. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TCAPT.2008.2011559

The conventional connector-less transceiver packages for POF communication use an aspherical lens or ball lens as an additional coupling element between the light source and POF or POF and detector [7]. The drawbacks of an external passive coupling element like a ball lens or an aspherical lens is that it requires additional structures in the package to house the coupling elements. Again, the integration of these components requires precision assembly which comes at an additional cost of manpower and time. Moreover, it becomes necessary to study the alignment requirements of the stand-alone optical coupling elements at the transmitter and receiver side in great detail. Hence, the total cost of the system will be increased by the cost of additional coupling components, the cost necessary to model and fabricate the substrate to assemble these components. At the receiver side of the POF, coupling takes place from a large core diameter to a small area photo detector. If precision subassemblies and precise component placements are not employed, optical coupling with an external element at this point becomes a tedious task. Plastic fiber with a lensed tip is a cost-effective coupling method for coupling light from the transmitter to the POF and from POF to the receiver [8], [9]. In this coupling scheme, the hemispherical lens fabricated at the tip of the fiber makes the system simple and efficient [10]. This eliminates precision subassembly and alignment, thereby lowering the manufacturing cost. In addition to this, it is seen that reflections and scattering are greatly reduced at transition surfaces in this hemispherical model since the lens and the fiber are of precisely the same material. Since POF has a large core radius, the hemispherical addendum will also have greater dimensions which will help to collect and focus light effectively at the ease of manufacturability. Besides, the device will be more mechanically stable and

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Fig. 1. Direct coupling of POF with source and detector.

less prone to shock and vibrations as external components are minimized. The conventional lensed fiber fabrication methods include fiber grinding, wet etching, hot melting, etc. [11]–[13]. Since the core diameter of the plastic fiber is very high compared to its counterparts, it is easier to fabricate the hemispherical lenses at the tip. The manufacturing cost is lower since the fabrication is simple and does not require highly sophisticated equipment. This also gives the freedom to re-engineer the lens formed as and when it is required. The fabrication methods adopted are discussed in detail in the following sections. III. POF MODELING, SIMULATION, AND ANALYSIS Geometrical image analysis in ZEMAX software is an effective tool for computing the multimode fiber coupling efficiency. This analysis can generate the irradiance at any surface from an extended source with specific size and shape at the object surface. We have simulated the parameters of MH4001 Esaka fiber from Mitsubishi Rayon. The POF we modeled has a core diameter of 980 m, cladding diameter of 1000 m, and numerical aperture of 0.3. The core material is poly methyl methacrylate (PMMA) with an index of refraction 1.49. Simulations are done for direct coupling, fiber bends, light coupling with an external lens, and lensed fiber. The direct coupling model, as shown in Fig. 1, involves a light source of 850-nm wavelength coupled to the input end of bare POF with flat ends, and the output of fiber is coupled to a detector of 50- m radius. The simulations show a coupling efficiency of only 9.38%, which reflects the need for an additional coupling optics for the fiber. At this juncture, the two possible ways of improving the coupling efficiency is to use either an external discrete lens attached to the fiber or by making lens at the tip of the fiber. To enhance the coupling efficiency, we amended the model by placing an external spherical ball lens between POF and detector. Since the diameter of the POF is large, a large ball lens is required for better light coupling to the detector, but as the size of the coupling lens increases the package dimensions also

becomes larger. Various dimensions of ball lenses are evaluated and a tradeoff between the package size and coupling efficiencies are arrived at. Based on these studies, an optimum value of 2-mm radius lens is simulated and it yielded a power output coupling efficiency of 37.45%. Fig. 2 shows that a discrete large ball lens would achieve good coupling efficiency in comparison to the bare fiber, but this coupling scheme will increase the dimensions of the transceiver module since the ball lens radius is large compared to the dimensions of the fiber, which is undesirable. On the other hand, a discrete ball lens of radius 0.48 mm, as shown in Fig. 3, would be a better fit since it will not increase the housing size required to hold the fiber-lens system. However, the difficulty is that the lenses of small radii have a shorter focal distance. A small radius lens can focus the beam to a nearer point at the expense of power output, while a large radius lens will focus at a longer distance from the lens, but maintain the power output to a high value. This can be observed with the fall in power to 18.81% in comparison to the larger lens radius coupling efficiency of 37.45%. Furthermore, using an aspherical lens to improve the coupling efficiency instead of a large ball lens, it becomes necessary to concentrate on the alignment and orientation of the lens in the module, which would require additional cost and effort, for the sake of increasing power output. The following analysis discusses the proposed coupling method, namely the hemispherical structures integrated to both side of the fiber, as shown in Fig. 4. Here, a plastic fiber of 0.49 mm radius is modeled with hemispherical ball lenses attached to both end of the fiber. The refractive indices of the fiber core and the hemispheres are made identical. Simulations are carried out to optimize the distance between the lensed fiber and detector, which would maximize the power output. The beam focusing distance from the lens at the receiver side is found to be 0.7 mm. A dummy surface with a circular aperture of 50- m radius is inserted just before the image surface to simulate the power incident on the surface of the photo detector of radius 50 m. The simulations yielded a higher power coupling efficiency of 85%. The power is concentrated in the 20- m radius, and

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CHANDRAPPAN et al.: OPTICAL COUPLING METHODS FOR COST-EFFECTIVE POLYMER OPTICAL FIBER COMMUNICATION

Fig. 2. POF coupling using an external ball lens of radius 2 mm.

Fig. 3. POF coupling using an external ball lens of radius 0.48 mm.

Fig. 4. POF coupling using hemispherical lenses formed at the tip.


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Fig. 6. Steel cylinder with mechanical mold. Fig. 5. Coupled power versus radius for hemispherical lensed fiber.

hence shifting of the fiber by 20 m on either side of the detector will not affect the coupling efficiency of the system. Since the POF diameter is inherently large, the hemisphere’s radius is also large, thereby improving the spatial resolution of the image. Besides this, the whole system is spherically symmetric. This eliminates the need for orientation, unlike in the case of an aspherical lens. An optimization algorithm is run for optimizing the radius of curvature of the lens. The radius of the integrated hemispherical lens is varied from 0.40 to 0.55 mm, to identify the optimum radius of the lens for the maximum output power coupling. A graph plotted with radius of lens curvature versus coupled power is shown in Fig. 5. The results shown here reveal that the best case scenario for maximizing the power output is when the radius of the half lens at both ends is equal to the radius of the fiber. On either side of the radius, the output power drops sharply. In the second optimization step, we analyzed whether the power output can be improved with a change in the refractive index of the hemispherical lens. It is observed that when the hemispherical lens is of the same material as that of the fiber, output power coupling is 85.445%. By increasing the refractive index 20% higher than that of the fiber, the power output can be increased to 88.67%. IV. FABRICATION METHODS Various methods have been reported in the literature for the fabrication of light coupling elements for POFs [14]. We have selected two fabrication methods based on the ease of fabrication and lower manufacturing cost. The first method involves formation of the lens at the fiber edge by pressing the end face on a hot lens forming mold. The second method involves dipping the polished fiber tip in an optically transparent organic liquid [15]. The process of fabrication of a lens by hot melting method onto a fiber of plastic or glass origins would include the mechanical process of injection molding or compression [16]. The physical property of either a PMMA or perfluorinated polymer fiber and their lower fabrication temperatures have made the fabrication of lenses onto the fiber much easier. PMMA material is less dense with density

Fig. 7. Lensed tip POF with unpolished edges.

range from 1150–1190 kg/m . This is less than half the density of glass which ranges 2400 to 2800 kg/m . More importantly, PMMA has higher impact strength than glass and does not shatter but instead breaks into large, dull pieces. The hot plate method requires melting the fiber tip at its material melting point temperature and molding it with suitable cavity to form the lens. Based on the simulation results, we fabricated a die in steel material by precisely machining the required geometries, as shown in Fig. 6. Apart from this, the setup requires a hot plate to heat the mold and an arrangement to hold the POF vertically to the cavity. This simple fabrication setup and highly repeatable process makes the fabrication method cheap and attractive. The steel mold is heated to the required temperature initially. The temperature of the mold is cross checked with the help of a thermocouple to reaffirm the required temperature level. Once the temperature reaches an approximate level of 150 C–160 C, the fiber is lowered towards the mold and allowed to take the shape of the mold. The fiber is subsequently allowed to cool to normal room temperature for 3–4 min. The fiber under microscopic view clearly shows the formation of a lens at the fiber tip, as shown in Fig. 7. However, due to the small geometry of the mold cavity and the large diameter of the POF, the POF comes in contact not only with the semispherical cavity of the mold but also with the entire top surface of the steel structure. This makes the edges of the POF melt outwards, and it requires additional polishing.



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settle down. Once the required shape is reached, the fiber tip is irradiated with UV light to make a proper adhesion. Many experiments have been carried out to optimize the required profile by using various viscous liquids and variable time for settling. The lens formed is as shown in the Fig. 9. V. RESULTS AND DISCUSSIONS

Fig. 8. Lensed tip POF with polished edges.

Fig. 9. Hemispherical lens formed by polymer dipping.

Fig. 10. From flat polished POF.

Fig. 8 shows the fiber tip after polishing. Another phenomenon of the plastic’s behavior towards the heating and cooling process is the shrinkage of the fiber. Due to the lack of a surmountable pressure on the POF, the fiber warps and shrinks, this prevents the fiber in assuming the exact shape of the mold. The second method describes a way of forming a spherical convex contour at the tip of POF by immersing the fiber in an optically transparent organic liquid and bonding this properly to the end face soon after the optical fiber is lifted from the liquid. The refractive index of the polymer material is matched to the refractive index of the POF core. In this process, the fiber tip is polished initially by mechanical polishing. Once the polishing is done, the fiber ends are cleaned and dipped to the polymer liquid. Now the fiber is taken out from the liquid and allowed to form the lens shape under surface tension. The shape of the lens depends on the viscosity of the liquid and the time given to

The lenses formed by various methods are being characterized to evaluate its performance. The evaluation is done based on their effectiveness to converge the large spot size of POF output to a considerably smaller spot size and hence to improve the coupling efficiency. Initially, the POF with lensed tip is illuminated with the light at the 850-nm wavelength. The distance from the beam profiler is adjusted in such a way that the output beam spot is reduced to the minimum size possible. Now the lensed POF is replaced with a flat polished POF, and the output spot is captured at the same distance using beam profiler. The output spot size profile for a flat polished POF and lensed POF formed by polymer dip method are as shown in Figs. 10 and 11. The captured spot sizes at 1.6-mm distance from the profiler are compared, and it clearly shows a reduction in the spot size of the order of one-fourth of the original dimension. The coupling efficiency has improved by 27% with the lensed fiber fabricated. The experimental result shows that there is a major variation from the output spot size predicted by simulation. The light beam is not converging to the smaller dimension, and hence the coupling efficiency is much below the expected value. As per our understanding, the minor variations in the lens parameters cannot be responsible for such a larger deviation in the coupling efficiency. Closely analyzing the results, it is realized that the initial simulations are done assuming that the fiber is straight, an ideal state. It did not include any fiber bends, but in the real case, the fiber has random bends. In addition, the lens structure at the input side generates a restricted mode launch situation under center launch conditions. Due to these factors, straight fiber status and restricted mode launch conditions, light is confined to the lower order modes lying at the center of the core which results in a lower divergence beam exit [17]. Under the random bends, all possible fiber modes are excited, and this leads to larger divergence and hence a larger beam size at the exit. In order to verify this fact, we modeled the same lens under bend fiber conditions. This resulted in a lower coupling efficiency of 34%, which very agrees well with the experimental results under the fabrication limitations. The lenses fabricated with the hot melting method did not show a better convergence, as the lens curvatures could not be controlled since the whole fiber tip was getting deformed during the fabrication process. A new mold with wider aperture is being considered to avoid these problems. Another concern is that, if the molded portions are not allowed to cool down properly, instead of a perfect smooth spherical surface, a lot of fiber “hairs” will be formed. This reduces the lensing effect and further polishing is required to make the surface perfect. In this case, the polymer dip method seems to be much better for the fabrication of lensed POFs. The variables for optimization vary with the liquid properties and the final lens shape to be fabricated.


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Fig. 11. From the lensed tip.

VI. CONCLUSION We have analyzed various cost-effective coupling methods for POF communication by optical simulations. The simulations showed an improved light coupling efficiency with the lensed POF. The lensed tips are realized using hot blowing and polymer dip fabrication techniques. The experimental evaluation of optimized lens structures resulted in an improved coupling efficiency of 27%. It is also eminent that the fiber bends play an important role in deciding the actual divergence of the beam at the exit end. The variation of light coupling efficiency from the simulation results are attributed to the higher order mode excitation and divergence associated with the fiber bends. Therefore, it is advised to take care of this fact while designing the coupling optics. Comparing the two fabrication methods, the polymer dip method is preferred over the hot blowing technique. Another added advantage of this method is that it can be used to fabricate lenses of any refractive index. In general, the use of lensed POF improves the coupling efficiency, reduces assembly costs, and also helps to maintain the package dimensions to minimum. They also offer ease of installation and altogether become a cost-effective solution for POF communications

[7] Y.-K. Lu, Y.-C. Tsai, Y.-D. Liu, S.-M. Yeh, C.-C. Lin, and W.-H. Cheng, “Asymmetric elliptic-cone-shaped micro lens for efficient coupling to high-power laser diodes,” Opt. Express, vol. 15, no. 4, pp. 1434–1442. [8] J. K. Kim, D. U. Kim, B. H. Lee, and K. Oh, “Arrayed multimode fiber to VCSEL coupling for short reach communications using hybrid polymer-fiber lens,” IEEE Photon. Technol. Lett., vol. 19, no. 13, pp. 951–953, Jul. 2007. [9] G. Shu, M. P. Bozeman, R. S. Hays, D. P. Robinson, W. K. Wright, and A. D. Kuan, “Design and evaluation of fiber tip lenses for fiber optic transmitter and receiver applications,” in Proc. IEEE Avionics, Fiber-Opt. Photon. Technol. Conf., Oct. 2–5, 2007, pp. 46–47. [10] G. D. Khoe, J. Poulissen, and H. M. de Vrieze, “Efficient coupling of laser diodes to tapered monomode fibers with high-index end,” Electron. Lett., vol. 19, pp. 205–207, 1983. [11] Y. Ohtsuka and Y. Hatanaka, “Preparation of light focusing plastic fiber by heat drawing process,” Appl. Phys. Lett., vol. 29, no. 11, pp. 735–737, Dec. 1976. [12] D. F. Merchant, P. J. Scully, and N. F. Schmitt, “Chemical tapering of polymer optical fiber,” Sens. Actuators A, vol. 76, pp. 365–371, 1999. [13] L. C. Ozcan, V. Treanton, F. Guay, and R. Kashyap, “Highly symmetric optical fiber tapers fabricated with a CO laser,” IEEE Photon. Technol. Lett., vol. 19, no. 9, pp. 656–658, May 2007. [14] K. L. Mittal and K.-W. Lee, Polymer Surfaces and Interfaces: Characterization, Modification and Application. Utrecht, The Netherlands: VSP, Jul. 1997. [15] K.-R. Kim, S. Chang, and K. Oh, “Refractive micro lens on fiber using UV-curable fluorinated acrylate polymer by surface-tension,” IEEE Photon. Technol. Lett., vol. 15, no. 8, pp. 1100–1102, Aug. 2003. [16] H. Sakata and A. Imada, “Lensed plastic optical fiber employing concave end filled with high-index resin,” J. Lightw. Technol., vol. 20, no. 4, pp. 638–642, Apr. 2002. [17] C. K. Asawa and H. F. Taylor, “Propagation of light trapped within a set of lowest-order modes of graded-index multimode fiber undergoing bending,” Appl. Opt., vol. 39, pp. 2029–2037, 2000.

Jayakrishnan Chandrappan (M’07) received the M.Tech. degree in optoelectronics and laser technology from the International School of Photonics, Cochin University of Science and Technology, Cochin, India, in 1999 and the M.Sc degree in physics from Cochin University of Science and Technology in 1997. He has been with the Micro Module Components (MMC) Laboratory, Institute of Microelectronics (IME), Singapore, since January 2007. Prior to that, he was with SAMEER, R&D Lab, Government of India, Indian Institute of Technology (I.I.T), Mumbai.

Zhang Jing, photograph and biography not available at the time of publication.

REFERENCES [1] A. Polley and S. E. Ralph, “100 m, 40 Gb/s plastic optical fiber link,” in Proc. Opt. Fiber Commun. Conf. Expo., National Fiber Opt. Eng. Conf., OSA Tech. Dig. (CD) (Opt. Soc. Amer., 2008), 2008, paper OWB2. [2] S. Junger, W. Tschekalinskij, and N. Weber, “Multimedia and multiservice systems for home, office and industrial networks,” in Asia-Pacific Opt. Wireless Commun. Conf., Wuhan, China, 2003, 4.11.2003. [3] C. Koeppen, R. F. Shi, W. D. Chen, and A. F. Garito, “Properties of plastic optical fibers,” J. Opt. Soc. Amer. B, vol. 15, pp. 727–739, 1998. [4] G. Giaretta, W. White, M. Wegmuller, and T. Onishi, “High-speed (11 Gbit/s) data transmission using perfluorinated graded-index polymer optical fibers for short interconnects,” IEEE Photon. Technol. Lett., vol. 12, no. 3, pp. 347–349, Mar. 2000. [5] D. Hanson, “Wiring with plastic optical fibre,” IEEE LTS, vol. 3, no. 1, pp. 34–39, Feb. 1992. [6] N. Fujimoto, H. Rokugawa, H. Watanabe, A. Ishizuka, and T. Sukegawa, “Photonic access connector: A compact, gigabit optical link for plastic optical fiber,” in Proc. ECOC, Sep. 15–19, 1996, vol. 2, pp. 59–62.

Ramkumar Veppathur Mohan completed six month internship during his undergraduate studies at the Institute of Microelectronics in 2007–2008. He graduated with Honors in Bachelor of Electrical Engineering from the National University of Singapore in 2008 with Optoelectronics Specialization. He is currently employed with Credit Suisse.

Philbert Oliver Gomez graduated in 2008 from School of Electrical and Electronics Engineering, Nanyang Technological University, Singapore. He completed a six-month internship during his undergraduate studies at the Institute of Microelectronics in 2007–2008. Currently, he is working with Economic Development Board, Government of Singapore.



Than Aye Aung, photograph and biography not available at the time of publication.

Xiao Yongfei, photograph and biography not available at the time of publication.

Pamidighantam V. Ramana, photograph and biography not available at the time of publication.

John H. Lau (F’94) received three M.S. degrees in structural engineering, engineering physics, and management science and the Ph.D. degree in theoretical and applied mechanics from the University of Illinois, Urbana Champaign. He has been the Director of the MMC Laboratory of the Institute of Microelectronics (IME), A*STAR, Singapore, since January 2007, and prior to that he was with HP and Agilent. With more than 30 years of R&D and manufacturing experience, he has authored or coauthored more than 300 peer-reviewed technical publications and more than 100 book chapters, and given more than 250 presentations. He has authored and coauthored 16 textbooks on advanced packaging solder joint reliability and lead-free soldering and manufacturing. He is an elected ASME Fellow.

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Dim Lee Kwong (F’09) is the Executive Director of Institute of Microelectronics (IME), Agency for Science, Technology and Research (A*STAR), Singapore, a Professor of Electrical and Computer Engineering at National University of Singapore, an Adjunct Professor of Electrical and Computer Engineering at the University of Texas at Austin, and Distinguished Scientist at Republic Polytechnic, Singapore. He was Earl N. and Margaret Bransfield Endowed Professor at The University of Texas at Austin from 1990 to 2007, and the Temasek Professor of the National University of Singapore from 2001 to 2004. He is the author of more than 900 referred archival publications (500 journal and 400 conference proceedings), has presented more than 70 invited talks at international conferences, and has been awarded with more than 23 U.S. patents. He has been a consultant to government research labs, semiconductor IC manufacturers, and materials and equipment suppliers in U.S. and overseas. More than 55 students received their Ph.D. degrees under his supervision. His areas of current research interest include CMOS-compatible Si/Ge nanowire devices and circuits for future CMOS technology and applications, silicon electronic–photonic integrated circuit technology and applications, Si-based biosensors and integrated lab-on-chip, and sensors, actuators, and integrated microsystems. Prof. Kwong received the IBM Faculty Award in 1984 to 1986, the Semiconductor Research Corporation (SRC) Inventor Award in 1993 and 1994, the General Motors Foundation Fellowship in 1992 to 1995, the Halliburton Foundation Excellent Teaching Award in 1994, the Engineering Foundation Award in 1995, and IEEE George Smith Award in 2007.
