EE 230: Optical Fiber Communication Lecture 10

1. EE 230: Optical Fiber Communication Lecture 10 Light Sources and Transmitters From the movie Warriors of the Net

2. Light Emitting Diodes An Introduction to Fiber Optic Systems-John Powers

3. LED Output Characteristics • Typical Powers • 1-10 mW • Typical beam divergence • 120 degrees FWHM – Surface emitting LEDs • 30 degrees FWHM – Edge emitting LEDs • Typical wavelength spread • 50-60 nm An Introduction to Fiber Optic Systems-John Powers

4. 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

5. Bragg wavelength for DFB lasers

6. Thermal Properties of DFB Lasers Light output and slope efficiency decrease at high temperature • 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 Agrawal & Dutta 1986

7. VCSELs • Much shorter cavity length (20x) • Spacing between longitudinal modes therefore larger by that factor, only one is active over gain bandwidth of medium • Mirror reflectivity must be higher • Much easier to fabricate • Drive current is higher • Ideal for laser arrays

8. Choosing between light sources • Diode laser: high optical output, sharp spectrum, can be modulated up to tens of GHz, but turn-on delay, T instability, and sensitivity to back-reflection • LED: longer lifetime and less T sensitive, but broad spectrum and lower modulation limit • DFB laser: even sharper spectrum but more complicated to make • MQW laser: less T dependence, low current, low required bias, even more complicated • VCSEL: single mode and easy fabrication, best for arrays, but higher current required

9. Laser Diode Transmitter Block Diagram

10. Source-Fiber Coupling – Lambertian Sources Lambertian Source radiance distribution Generalized Coupled Power

11. Step and Graded Index Fiber Coupling

12. Graded Index Fiber Coupling Continued

13. Source Fiber Coupling - II Schematic of a typical assembly of coupling optics Transmitters employing a) butt-coupling and b) lens-coupling designs

14. Turn-on delay

15. 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 ¥ 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

16. Traditional Laser Transmitter Approaches Use a transmission line and impedance match Keep it close and don’t worry about the match

17. B i a s A d j u s t M o n i t o r P h o t o d i o d e M o n i t o r P h o t o d i o d e L a s e r L a s e r - - - -5V B i a s V r e f V r e f A v e r a g e + + P o w e r + I n t e g r a t o r D a t a D a t a M o d u l a t e d P o w e r A d j u s t D a t a D a t a - - -5V - + M o d u l a t i o n + -5V P e a k - p e a + I n t e g r a t o r -5V P o w e r P e a k D e t e c t o r Laser Driver Stabilization Average Power and Mark Density Compensation Average Power, Mark Density and Modulation 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

18. Packaging Drawing of Packaging Approach Optical Module (a), Electrical module (b) • 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 Close-up of assembled module Completed module integrated on test board Bostica et. al., IEEE Transactions on Advanced Packaging, Vol. 22, No 3, August 1999

19. Example Commercial Transmitter Module Palomar Technologies

20. DFB-HEMT OEIC Laser Transmitter • Transistor Technology • InGaAs-InAlAs HEMT • 1.5 mm 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 • l/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

21. Polarization • In molecules, P=μ+αE+βE2+γE3+… • In materials, P=X(o)+X(1)E+X(2)E2+X(3)E3+… If multiple electric fields are applied, every possible cross term is generated. At sufficiently high values of E, quadratic or higher terms become important and nonlinear effects are induced in the fiber.

22. Electro-Optic Coefficient r (Pockels Effect)

23. Electro-Optic Material Figures of Merit Phase shift efficiency n3r; favors lithium niobate in most cases Bandwidth per unit power n7r2/ε; favors organic materials