VCSEL High Speed DriversWhat is a VCSEL?VCSEL CharacteristicsApplications of VCSELsHow Do You Modulate A VCSEL?Analog Laser DriverDigital Laser DriverOptical Modulation Trade-OffSlide 9Slide 10Slide 11Slide 12Slide 13Slide 14VCSEL Drive CharacteristicsTypical VCSEL DriverThe ResultsSlide 18A Wide Open EyeConclusionsVCSEL High Speed DriversBryan A. WeaverECE-E641February 27, 2003What is a VCSEL?p-contactn-contactactive regionlight outputoxideVertical Cavity Surface Emitting LaserVCSEL Characteristics•Distributed-Bragg Reflector–Periodic Variation in Refractive Index in Direction of Wave Propagation•Short Cavity - Better Modal Purity–Feedback is Frequency DependantApplications 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 THBIASMODIIIP  THBIASIIP 0Analog Laser DriverDigital Laser DriverOptical Modulation Trade-Off•Optical Power•Jitter•Extinction Ratio•Undershoot•Speed•ReliabilityOptical 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 PerformanceOptical 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 IMODOptical 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 OverdriveOptical Modulation Trade-Off•Speed–Ultimate Speed is Limited by Relaxation Resonance–Thermal Drawbacks at MHz Rates and Below•Time Constant Modulated CharacteristicsqIINgVvfthgiRmod****21Rwff *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 DriverThe ResultsEye Diagram Tr = 60 pS, BW is Approximately 100 GHz•Can You Identify the Following?–Jitter–Relaxation Resonance–Delayed Carrier RecombinationA Wide Open EyeConclusions•Low BER Requires–High Optical Power–High Extinction Ratio–Low Undershoot–Low Jitter–High Speed