EE 230: Optical Fiber Communication Transmitters From the movie Warriors of the Net
Laser Diode Structures Most require multiple growth steps Thermal cycling is problematic for electronic devices
Fabry Perot Laser Characteristics (Hitachi Opto Data Book)
Laser Diode Structure and Optical modes
Distributed Feedback (DFB) Laser Structure Laser of choice for optical fiber communication Narrow linewidth, low chirp for direct modulation Narrow linewidth good stability for external modulation Integrated with Electro-absorption modulators Distributed FeedBack (DFB) Laser Distributed Bragg Reflector(DBR) Laser As with Avalanche photo-diodes these structures are challenging enough to fabricate by themselves without requiring yield on an electronic technology as well Hidden advantage: the facet is not as critical as the reflection is due to the integrated grating structure
Thermal Properties of DFB Lasers Light output and slope efficiency decrease at high temperature Agrawal & Dutta 1986 Wavelength shifts with temperature The good: Lasers can be temperature tuned for WDM systems The bad: lasers must be temperature controlled, a problem for integration
Resonance Frequency 2 Photon Density sf ( ) s(0) f0 Laser Small Signal Frequency Response= = Excitation Current ( ) (0) 2 2 if i f f + jff where f f 0 d 1 1 gs 1 gη = = = stim 2π τ τ 2π τp 2π τp p e εs = =Damping frequency 2πτ p ( I I ) ( I I ) 2 2 fd fp= f 0 =Frequency of peak response 4 th =Resonance frequency stim g=differential gain S= η τ = photon lifetime τ = carrier lifetime = th p e 0 d 1 gs Semiconductor lasers Exhibit an inherent second order response due to energy Sloshing back-and-forth Between excited electrons And photons
Large Signal Transient Response
Thermal Properties of DFB Lasers Light output and slope efficiency decrease at high temperature Agrawal & Dutta 1986 Wavelength shifts with temperature The good: Lasers can be temperature tuned for WDM systems The bad: lasers must be temperature controlled, a problem for integration
Source-Fiber Coupling Lambertian Sources Lambertian Source radiance distribution Generalized Coupled Power
Step and Graded Index Fiber Coupling
Graded Index Fiber Coupling Continued
Source Fiber Coupling - II Schematic of a typical assembly of coupling optics Transmitters employing a) butt-coupling and b) lens-coupling designs
Chirping (Or why is my eye closed?) Current modulation causes both intensity and frequency modulation(chirp) As the electron density changes the gain (imaginary part of refractive index n i ) and the real part of the refractive index (n r ) both change. The susceptability of a laser to chirping is characterized by the alpha parameter. n r α= N where N is the electron density. Large α implies lots of chirping. n i N v(t) = α P / t + κp v αf = j + κ for P = P 4π P P 2P 0 + Pe jωt 0 α=1-3 is expected for only the very best lasers Chirping gets worse at high frequencies Relaxation oscillations will produce large dp/dt which leads to large chirping Damping of relaxation oscillations will reduce chirp Correctly adjusting the material composition and laser mode volume can reduce α
Reflection Sensitivity Problem Solution R. G. F. Baets, University of Ghent, Belgium
Laser Electrical Models Package Lead Inductance Bond wire Inductance Laser contact resistance Simple Large signal model Package Lead Capacitance Laser Pad Capacitance Laser Use a large signal diode model for the Junction laser junction, this neglects the optical reson Assume that the light output is proportional to the current through the laser junction More exactly the laser rate equations can be implemented in SPICE to give the correct transient behavior under large signal modula Small Signal Model (Hitachi)
Turn-on Delay I p τ d Input Current Output Light Signal Turn on Delay (ns) I b =0.5I th I b =0.9I th I b =0 For and applied current pulse of amplitude I the turn on delay is given by: I p d = τthln Ip Ith with a bias current I applied: τ th b I p τd = τthln Ip + Ib Ith where τ is the delay at threshold (2ns Typ.) p To reduce the turn on delay: Use a low threshold laser and make I p large Bias the laser at or above threshold
Traditional Laser Transmitter Approaches Use a transmission line and impedance match -Vee Bonding Inductance Pad Capacitance Transmission Line Junction Capacitance Bonding Inductance Matching Resistor Contact Resistance Laser Junction Keep it close and don t worry about the match Drive Transistor Pad Capacitance -Vee Packaged Laser Driver Bonding Inductance Junction Capacitance Laser Contact Resistance Laser Junction Laser Driver Thin Film Resistor Laser Diode Laser Driver Bondwire Laser Diode Transmission Line Packaged Laser Driver Packaged Laser
Laser Reliability and Aging
Example Commercial Transmitter Module Palomar Technologies
Laser Diode Transmitter Block Diagram
Laser Driver Stabilization Laser Moni t or Phot odiode La s e r Monit or Phot odiode Vr ef - + Vr ef - + La v Vr ef 1 Vr ef 1 Vr ef 2 Dat a Dat a Da t a Da t a - Vr ef 2 - Da t a Da t a -5V -5V + + Lpp Peak Det ect or Dut y Cycle Measurement Average Power and Mark Density Compensation Average Power, Mark Density and Modulation Dat a Modul at i on -5V Dat a Laser Bias - + I nt egr at or BiasAdjust - -5V + I nt egr at or Modul at ed Power Adj ust -5V - + Moni t or Phot odi ode - + - Aver age Power + Peak- pea Power Peak Det ect or A variety of feedback approaches are available to compensate for laser imperfections and the consequences of temperature variation and aging Average and Peak Power Stabilization
Problems with Average Power Feedback control of Bias Problem: L-I curves shift with Temperature and aging Turn on delay increased Frequency response decreased Light Average Power Light Average Power Current Current L-I Characteristic with temperature Ideal L-I Characteristic dependent threshold Output power, frequency response decreased Laser Monitor Photodiode Light Current Average Power L-I Characteristic with temperature dependent threshold and decreased quantum efficiency Data -5V Vref Average number of 1s and Os (the Mark Density ) is linearly related to the average power. If this duty cycle changes then the bias point will shift - +
OEIC Transmitters must compete with more traditional approaches Drawing of Packaging Approach Close-up of assembled module 10 Channels 12.5 Gb/s aggregate bandwidth 1300 nm commercial laser array 50/125 Multimode fiber ribbon 130 mw/channel CMOS Driver Array BER<10-14 1.2 km transmission with no BER degradation Optical Module (a), Electrical module (b) Completed module integrated on test board Bostica et. al., IEEE Transactions on Advanced Packaging, Vol. 22, No 3, August 1999
DFB-HEMT OEIC Laser Transmitter Transistor Technology InGaAs-InAlAs HEMT 1.5 µm gate length Laser Distributed Feedback Laser Self-Aligned Constricted Mesa (SACM) 7 MHz linewidth at 3 mw output power 19 GHz 3db frequency 8 ma average threshold Fabrication λ/4 shifted cavity fabricated by e-beam 2-step MOCVD OEIC Performance: Clean output eyes for all pattern lengths up to 5 Gb/s Operation at shorter patterns up to 10 Gb/s Demonstrated link operation over 29 km at 5 Gb/s Lo et. al. IEEE Photonics Technology Letters, Vol. 2, No. 9, September 1990
Extinction Ratio Penalty If the transmitter does not turn all the way off during the transmission of a zero then the extinction ratio r ( the ratio to a power transmitted during a 0 to that during a 1 ) will cause a bit error rate penalty and a reduction in sensitivity. For a PIN receiver the peak power required for a given signal to noise ratio will become: r=0 if the optical signal is completely extinguished during a logical 0 r=1 if the optical power during a 0 equals that during a 1 in this case the power required approaches ηp = 1+ r hν Qi 1 r 2 1/2 q For APD detectors with gain the effect of the multiplied noise during the 0 is more severe, this case is shown in the graph to the left. k is the ratio of the hole and electron ionization coefficients and is a property of the material in the avalanche multiplication region