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

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Conditions for gain (lasing) E 2 -E 1 <F c -F v (population inversion) g (1/L)ln(1/R)+ (net gain) =2nL/p, p an integer (phase coherence)

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Reflectivity

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Longitudinal mode spacing

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Laser Diode Structure and Optical modes

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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)

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

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Steady-state lasing conditions

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Turn-on delay

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Turn-on Delay Input Current Output Light Signal dd To reduce the turn on delay: Use a low threshold laser and make I p large Bias the laser at or above threshold I b =0 I b =0.9I th I b =0.5I th Turn on Delay (ns)

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

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

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

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Large Signal Transient Response

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

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Fabry Perot Laser Characteristics (Hitachi Opto Data Book)

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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

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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%

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Chirping 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. 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

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

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

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

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

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Threshold electron density R = 0.3152 g (1/L)ln(1/R)+ g th =1/.01849 ln(1/.3152)+100=162.4 cm -1 From figure, N=1.8x10 18 cm -3

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Threshold current J/2de = I/2deLW I th =(0.5x10 -4 )(1.6x10 -19 )(1.8x10 18 )(.01849)(4x10 -4 )/(3.7x10 -9 ) I th =29 mA

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

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Laser Reliability and Aging

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

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Problems with Average Power Feedback control of Bias Light Current Average Power Ideal L-I Characteristic Light Current Average Power L-I Characteristic with temperature dependent threshold Turn on delay increased Frequency response decreased Light Current Average Power L-I Characteristic with temperature dependent threshold and decreased quantum efficiency Output power, frequency response decreased 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 Problem: L-I curves shift with Temperature and aging

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