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

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

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. Eye Diagram Tr = 60 pS, BW is Approximately 100 GHz
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. High Speed (Bandwidth)
Conclusions
Low BER Requires
High Optical Power
High Extinction Ratio
Low Undershoot
Low Jitter
High Speed (Bandwidth)