Graduate Exam Abstract

Rashid Safaisini

Ph.D. Final

November 29, 2010, 2:30 pm

Engineering B2


Abstract: Increasing the modulation bandwidth and output power of vertical-cavity surface-emitting lasers (VCSELs) are of great importance in a variety of applications such as data communication systems. High temperature generated in the active region of VCSELs is one of the main limiting factors in achieving high power and high speed operation. This work is focused on investigating the effects of thermal management on improving AC and DC properties of VCSELs and achieving higher thermal performance devices. Thermal heatsinking is obtained by surrounding the VCSEL mesas with high thermal conductivity materials such as copper and also using passive heatsinking by flip-chip bonding the laser dies on a GaAs heat spreader. The research includes fabricating and characterizing 980 nm bottom-emitting and 670 nm top-emitting oxide-confined VCSELs. This dissertation is divided into three main parts: high-power, high-speed 980 nm VCSEL arrays, low thermal resistance 670 nm VCSELs, and temperature dependent dynamics of 980 nm VCSELs. Experimental work performed on fabricating and characterizing 980 nm, bottom-emitting, oxide-confined VCSEL arrays and single elements is presented first. The result of DC and AC characterization confirms the effectiveness of Cu electroplating of mesas and flip-chip bonding in reducing VCSELs’ thermal resistance to obtain lower operating temperatures. Uniformity of frequency response and operating wavelength across the arrays also motivates managing thermal issues and is an indication of uniform distribution of current and heat flux on the array. This research resulted in record VCSEL arrays with frequency response of approximately 8 GHz and operating CW power of 200 mW. These 28-element, 18 µm aperture diameter arrays represent the highest power reported for a VCSEL or VCSEL array with greater than 1 GHz modulation bandwidth. The second part of this dissertation details the fabrication steps and DC characterization of visible, 670 nm, top-emitting, oxide-confined VCSELs. Since achieving high operating temperatures is one of the main challenges in realizing improved red VCSELs, the effect of mesa heatsinking on improving their DC behavior using copper electroplating of mesas is studied. Thermal modeling of the copper plated VCSELs also facilitates better understanding and analysis of the experimental results. A photomask and process flow were designed to fabricate VCSELs with a variety of mesa diameters and inner and outer plating sizes to investigate the major direction of heat flow in the VCSELs and decrease VCSEL thermal resistance and thus increase the output power. Although copper plating significantly reduces thermal resistance, it did not substantially increase maximum operating temperature of the red devices and also put the mesas under stress that might not be desired. This study led us to analyzing the effects of stress on the VCSEL mesas which is induced by the copper films. Finally, the temperature dependence of 980 nm VCSEL dynamics is investigated using noise spectra measurement. This analysis provides some useful insights in understanding how temperature alters VCSEL properties and how these properties can be improved. A VCSEL with 7 µm aperture diameter was fabricated from the same epitaxial material and followed the same processing steps as the VCSEL arrays. Relaxation oscillation frequencies and damping factors as functions of bias current and stage temperature were extracted. These results along with the VCSEL DC measurement were used to estimate the laser differential gain as a function of temperature. The differential gain was shown to be relatively temperature independent over a temperature range of 10 °C to 70 °C with an average value of approximately 12×10-16 cm2. This research led us to the conclusion that improving the output power at elevated temperatures should yield better frequency response in this case. The VCSEL output power reduction was observed to be the major cause of bandwidth reduction at elevated temperatures for the device under test. This work is the first report on the measurement of temperature dependence of VCSEL dynamics.

Adviser: Prof. Kevin Lear
Co-Adviser: N/A
Non-ECE Member: Prof. James Sites, Physics
Member 3: Prof. Mario Marconi, ECE
Addional Members: Prof. Steven Reising, ECE

Peer Reviewed Journal Articles: 1. Rashid Safaisini, John R. Joseph, and Kevin L. Lear, "Scalable high-CW-power high-speed 980-nm VCSEL arrays," IEEE Journal of Quantum Electronics, vol. 46, no. 11, pp. 1590-1596, November 2010. 2. R. J. Yan, S. P. Mestas, G. Yuan, R. Safaisini, and K. L. Lear, "Response of local evanescent array-coupled biosensors to organic nanofilms," IEEE Journal of Selected Topics in Quantum Electronics, vol. 15, no. 5, pp. 1469-1477, September-October 2009. 3. Rashid Safaisini, John R. Joseph, Gerard Dang, and Kevin L. Lear, "Scalable high-power, high-speed CW VCSEL arrays," Electronics Letters, vol. 45, no. 8, pp. 414-415, April 9, 2009. 4. R. J. Yan, S. P. Mestas, G. Yuan, R. Safaisini, D. S. Dandy, and K. L. Lear, "Label-free silicon photonic biosensor system with integrated detector array," Lab on a Chip, vol. 9, no. 15, pp. 2163-2168, 2009. 5. Rashid Safaisini, John R. Joseph, Duane Louderback, Xiaojun Jin, Ahmad N. Al-Omari, and Kevin L. Lear, "Temperature dependence of 980-nm oxide-confined VCSEL dynamics," IEEE Photonics Technology Letters, vol. 20, no. 14, pp. 1273-1275, July 15, 2008. Conference Papers: 1. Rashid Safaisini, Klein Johnson, Mary Hibbs-Brenner, and Kevin L. Lear, "Stress analysis in copper plated red VCSELs," IEEE Photonics Society, 23rd annual meeting, Denver, Colorado, Nov. 9, 2010. 2. Rashid Safaisini, Klein Johnson, and Kevin L. Lear, "Thermal resistance reduction in 670 nm vertical-cavity surface-emitting lasers," in Proc. SPIE (Photonics West), vol. 7615, 76150L, 2010. 3. John R. Joseph, Rashid Safaisini, Gerard Dang, and Kevin L. Lear, "Non-mechanical beam steering of high speed VCSEL arrays," in Proc. SPIE (Photonics West), vol. 7615, 76150B, 2010. 4. R. Safaisini, J. R. Joseph, G. Dang, and K. L. Lear, "Uniform high bandwidth, high CW power VCSEL arrays," Conference on Lasers and Electro-Optics, CLEO, 2009. 5. K. L. Lear, RJ. Yan, S.P. Mestas, R. Safaisini," An Integrated Photonic Waveguide Biosensor System on a Chip", IEEE Semiconductor Conference Dresden, 2009 (IEEE), Dresden, Germany, Apr. 29-30, 2009. 6. R. Yan, G. Yuan, S. Mestas, R. Safaisini, and K. L. Lear, "Demonstration of local evanescent array coupled biosensors with organic nanofilms," IEEE Lasers and Electro-Optics Society, LEOS, 21st Annual Meeting, Nov. 9-13, 2008. 7. (Invited) K. L. Lear and R. Safaisini, "Oxide optics in VCSELs", International Symposium on VCSELs and Integrated Photonics, paper F-2, pp. 84-86, Tokyo, Japan, Dec. 17-18, 2007. 8. R. Safaisini, J. R. Joseph, D. Louderback, X. Jin, and K. L. Lear, "Temperature dependence of 980 nm VCSEL dynamics," IEEE Lasers and Electro-Optics Society, LEOS, 20th Annual Meeting, Oct. 21-25, 2007. 9. R. Safaisini, A. N. Al-Omari, J. R. Joseph, and K. L. Lear, "Dynamics of 980 nm VCSELs characterized using temperature dependent RIN spectra," American Physical Society, APS March Meeting, Mar. 5-9, 2007. Other Presentations: 1. Rashid Safaisini, John R. Joseph, and Kevin L. Lear, "Vertical-cavity surface-emitting laser (VCSEL) arrays," poster presented at Colorado Photonics Industry Association (CPIA), Annual Meeting, Boulder, CO, Nov. 12, 2009. 2. Rashid Safaisini, John R. Joseph, and Kevin L. Lear, "Temperature analysis of 980nm VCSELs," poster presented at Colorado Photonics Industry Association (CPIA), Annual Meeting, Boulder, CO, Nov. 14, 2007. Award: SPIE educational scholarship in optical science and engineering in 2009

Program of Study:
BIOM 680
ECE 525
ECE 536
ECE 672
ECE 799
PH 521