Download VCSEL High Speed Drivers

VCSEL High Speed Drivers
Bryan A. Weaver
ECE-E641
February 27, 2003
What is a VCSEL?
Vertical Cavity Surface Emitting Laser
light output
p-contact
oxide
active region
n-contact
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
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
P1  ( I MOD  I BIAS  ITH )
P0   I BIAS  ITH 
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
f w  1.55 * f R
1
 g I mod  I th
fR 
i * v g * *
*
2
V N
q
Thermal Drawbacks at MHz Rates and Below
Time Constant Modulated Characteristics
Optical Modulation Trade-Off
Reliability
Increased Optical Power Results in Decreased
Reliability
reliabilit y  1 / J 2
VCSEL Drive Characteristics
Current Driven
All of the Previously Discussed Drive Parameters
were Current and Temperature Dependant
BW 
2
tr
What is Needed is a Temperature Compensated
Current Source with Bandwidth that Extends into
the Millimeter Wave Band
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)