vcsel high speed drivers bryan a. weaver ece-e641 february 27, 2003

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)