vcsel high speed drivers bryan a. weaver ece-e641 february 27, 2003
TRANSCRIPT
VCSEL High Speed Drivers
Bryan A. Weaver
ECE-E641
February 27, 2003
What is a VCSEL?
p-contact
n-contact
active region
light output
oxide
Vertical Cavity Surface Emitting Laser
VCSEL Characteristics
• Distributed-Bragg Reflector– Periodic Variation in Refractive Index in
Direction of Wave Propagation
• Short Cavity - Better Modal Purity– Feedback is Frequency Dependant
Applications of VCSELs
• Widely Used in Multi-Gigabit Optical Networks
• Frequency Modulation– A VCSEL is a Resonant Oscillator– Frequency can’t be Easily Varied
• Amplitude Modulation– Output is Directly Proportional to Drive Current– –
How Do You Modulate A VCSEL?
)(1 THBIASMOD IIIP
THBIAS IIP 0
Analog Laser Driver
Digital Laser Driver
Optical Modulation Trade-Off
• Optical Power
• Jitter
• Extinction Ratio
• Undershoot
• Speed
• Reliability
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
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.
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
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
Optical Modulation Trade-Off
• Speed– Ultimate Speed is Limited by Relaxation
Resonance
– Thermal Drawbacks at MHz Rates and Below• Time Constant Modulated Characteristics
q
II
N
g
Vvf thgiR
mod****2
1
Rw ff *55.1
Optical Modulation Trade-Off
• Reliability– Increased Optical Power Results in Decreased
Reliability– 2/1 Jyreliabilit
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
rtBW
2
Typical VCSEL Driver
The Results
Eye Diagram
Tr = 60 pS, BW is Approximately 100 GHz
• Can You Identify the Following?– Jitter– Relaxation Resonance– Delayed Carrier Recombination
A Wide Open Eye
Conclusions
• Low BER Requires– High Optical Power– High Extinction Ratio– Low Undershoot– Low Jitter– High Speed (Bandwidth)