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VCSEL High Speed Drivers

VCSEL High Speed Drivers. Bryan A. Weaver ECE-E641 February 27, 2003. light output. p-contact. active region. n-contact. oxide. What is a VCSEL?. Vertical Cavity Surface Emitting Laser. VCSEL Characteristics. Distributed-Bragg Reflector

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VCSEL High Speed Drivers

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  1. VCSEL High Speed Drivers Bryan A. Weaver ECE-E641 February 27, 2003

  2. light output p-contact active region n-contact oxide What is a VCSEL? Vertical Cavity Surface Emitting Laser

  3. VCSEL Characteristics • Distributed-Bragg Reflector • Periodic Variation in Refractive Index in Direction of Wave Propagation • Short Cavity - Better Modal Purity • Feedback is Frequency Dependant

  4. Applications of VCSELs Widely Used in Multi-Gigabit Optical Networks

  5. How Do You Modulate A VCSEL? • Frequency Modulation • A VCSEL is a Resonant Oscillator • Frequency can’t be Easily Varied • Amplitude Modulation • Output is Directly Proportional to Drive Current

  6. Analog Laser Driver

  7. Digital Laser Driver

  8. Optical Modulation Trade-Off • Optical Power • Jitter • Extinction Ratio • Undershoot • Speed • Reliability

  9. Optical Modulation Trade-Off • Optical Power • Increased Modulation Current Results in Increased Optical Power and Increased Dissipation • Highest Level of Optical Power will Result in Best Signal to Noise Ratio at Receiver • Bit Error Rate (BER) Ultimate Measure of Optical System Performance

  10. Optical Modulation Trade-Off • Jitter • Turn-On Delay • Data Dependant Difference in TON Between the Current Density of ITH and Absolute Zero • Carrier Density after one Zero will be Greater than after a few Consecutive Zeros • Reduced by Application of Bias Current IBIAS that is Greater than Threshold Current ITH • ITH is Determined by Difference Between Gain and Loss at the Lasing Wavelength.

  11. Optical Modulation Trade-Off • Extinction Ratio • When Commanded Off a VCSEL is Still Biased Slightly-On • Difference in Optical Intensity Between On and Off Sacrifices Extinction Ratio (SNR) • Extinction Ratio is now Controllable only by Variations in Modulation Current IMOD

  12. Optical Modulation Trade-Off • Undershoot • Relaxation Resonance • Leading Edge Pulse Ringing • Oscillatory Behavior Between Excess Carrier Generation and Recombination • Relaxation Resonance can be Reduced by Brief Period of Overdrive

  13. Optical Modulation Trade-Off • Speed • Ultimate Speed is Limited by Relaxation Resonance • Thermal Drawbacks at MHz Rates and Below • Time Constant Modulated Characteristics

  14. Optical Modulation Trade-Off • Reliability • Increased Optical Power Results in Decreased Reliability

  15. VCSEL Drive Characteristics • Current Driven • All of the Previously Discussed Drive Parameters were Current and Temperature Dependant • What is Needed is a Temperature Compensated Current Source with Bandwidth that Extends into the Millimeter Wave Band

  16. Typical VCSEL Driver

  17. The Results Eye Diagram Tr = 60 pS, BW is Approximately 100 GHz

  18. Can You Identify the Following? • Jitter • Relaxation Resonance • Delayed Carrier Recombination

  19. A Wide Open Eye

  20. Conclusions • Low BER Requires • High Optical Power • High Extinction Ratio • Low Undershoot • Low Jitter • High Speed (Bandwidth)

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