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

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

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

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 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 Distributed FeedBack (DFB) Laser Distributed Bragg Reflector(DBR) Laser

Bragg wavelength for DFB lasers

Thermal Properties of DFB Lasers Agrawal & Dutta 1986 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 integratio n

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

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

Laser Diode Transmitter Block Diagram

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

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

Turn-on delay

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

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

Laser Driver Stabilization Average Power and Mark Density Compensation Average and Peak Power Stabilization 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

Packaging Bostica et. al., IEEE Transactions on Advanced Packaging, Vol. 22, No 3, August 1999 Drawing of Packaging Approach Optical Module (a), Electrical module (b) Close-up of assembled module Completed module integrated on test board 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< km transmission with no BER degradation

Example Commercial Transmitter Module Palomar Technologies

DFB-HEMT OEIC Laser Transmitter Transistor Technology InGaAs-InAlAs HEMT 1.5  m 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 /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

Polarization In molecules, P=μ+αE+βE 2 +γE 3 +… In materials, P=X (o) +X (1) E+X (2) E 2 +X (3) E 3 +… 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.

Electro-Optic Coefficient r (Pockels Effect)

Electro-Optic Material Figures of Merit Phase shift efficiency n 3 r; favors lithium niobate in most cases Bandwidth per unit power n 7 r 2 /ε; favors organic materials