VCSEL High Speed Drivers

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Presentation transcript:

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

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

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

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

Optical Modulation Trade-Off Reliability Increased Optical Power Results in Decreased Reliability

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

Typical VCSEL Driver

Eye Diagram Tr = 60 pS, BW is Approximately 100 GHz 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

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