EE 230: Optical Fiber Communication Lecture 9 Light Sources From the movie Warriors of the Net

Conditions for gain (lasing) E2-E1<Fc-Fv (population inversion) g(1/L)ln(1/R)+ (net gain) =2nL/p, p an integer (phase coherence)

Reflectivity

Longitudinal mode spacing

Laser Diode Structure and Optical modes

Conditions for continuous lasing (steady state) Net rate of change of density of conduction band electrons is zero (injection minus recombination and depletion) Net rate of change of density of photons created is zero (stimulated emission minus leakage and spontaneous emission)

Laser Electrical Models Simple large signal model Package Lead Inductance Bond wire Inductance Laser contact resistance Package Lead Capacitance Laser Junction Use a large signal diode model for the laser junction, this neglects the optical resonance Laser Pad Capacitance More exactly the laser rate equations can be implemented in SPICE to give the correct transient behavior under large signal modulation Assume that the light output is proportional to the current through the laser junction Small signal model (Hitachi)

Steady-state lasing conditions

Turn-on delay

Turn-on Delay d Input Current Output Light Signal Ib=0 Ib=0.9Ith Ib=0.5Ith Turn on Delay (ns) Input Current Output Light Signal d To reduce the turn on delay: • Use a low threshold laser and make Ip large • Bias the laser at or above threshold

Relaxation oscillation Decays as e-t/2, where and with a freqency , where

Modulation frequency Difference between optical output at modulation frequency m and steady-state output is proportional to

Resonance Frequency Semiconductor lasers exhibit an inherent second order response due to energy “sloshing” back-and-forth between excited electrons and photons

Large Signal Transient Response

Effects of current and temperature Applying a bias current has the same effect as applying a pump laser; electrons are promoted to conduction band. Fc and Fv get farther apart as well Increasing the temperature creates a population distribution rather than a sharp cutoff near the Fermi levels

Fabry Perot Laser Characteristics (Hitachi Opto Data Book)

Quantum efficiency Internal quantum efficiency i, photons emitted per recombination event, determined empirically to be 0.650.05 for diode lasers External quantum efficiency e given by

Total quantum efficiency Equal to emitted optical power divided by applied electrical power, or he/qV For GaAs lasers, TQE  50% For InGaAsP lasers, TQE  20%

Chirping Current modulation causes both intensity and frequency modulation(chirp) As the electron density changes the gain (imaginary part of refractive index ni) and the real part of the refractive index (nr) both change. The susceptability of a laser to chirping is characterized by the alpha parameter. 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 

Reflection Sensitivity Problem Solution R. G. F. Baets, University of Ghent, Belgium

Example A GaInAs diode laser has the following properties: Peak wavelength: 1.5337 m Spacing between peaks: 1.787x10-3 m J/Jth=1.2 What are the turn-on delay time, the cavity length, the threshold electron density, and the threshold current?

Turn-on delay time =3.7 ln(1.2/1.2-1) = 6.63 ns

Cavity length L = (1.5337)2/(2)(3.56)(1.787x10-3) = 184.9 m

Threshold electron density g(1/L)ln(1/R)+ gth=1/.01849 ln(1/.3152)+100=162.4 cm-1 From figure, N=1.8x1018 cm-3

Threshold current J/2de = I/2deLW Ith=29 mA Ith=(0.5x10-4)(1.6x10-19)(1.8x1018)(.01849)(4x10-4)/(3.7x10-9) Ith=29 mA

Laser Diode Structures Most require multiple growth steps Thermal cycling is problematic for electronic devices

Laser Reliability and Aging

Power degradation over time Lifetime decreases with current density and junction temperature

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 dependent threshold Ideal L-I Characteristic - + Data Laser Monitor Photodiode Vref -5V Output power, frequency response decreased Light Average Power Current 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 L-I Characteristic with temperature dependent threshold and decreased quantum efficiency

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

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

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

Bragg wavelength for DFB lasers

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

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