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

2. 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)

3. Reflectivity

4. Longitudinal mode spacing

5. Laser Diode Structure and Optical modes

6. 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)

7. 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)

8. Steady-state lasing conditions

9. Turn-on delay

10. 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

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

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

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

14. Large Signal Transient Response

15. 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

16. Fabry Perot Laser Characteristics
(Hitachi Opto Data Book)

17. 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

18. 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%

19. 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 

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

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

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

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

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

25. 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

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

27. Laser Reliability and Aging

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

29. 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

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

31. 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

32. 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

33. Bragg wavelength for DFB lasers

34. 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

35. 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

36. 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