Download Lecture 10: Light Sources

EE 230: Optical Fiber Communication Lecture 10
Light Sources and Transmitters
From the movie
Warriors of the Net
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
2 n
B 
k
  B 
 m  1 / 2
2
B
2nL
Thermal Properties of DFB Lasers
Light output and slope efficiency decrease
at high temperature
Agrawal & Dutta 1986
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
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
Lambertian Source
radiance distribution
Generalized
Coupled Power
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
 J  Jb 

td   N ln 
 J  J th 
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:
P 
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 
1/2

1  r 
h Q i 2
1  r  q 
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
Bonding Inductance
Matching Resistor
Bonding Inductance
Transmission Line
Junction
Capacitance
Pad
Capacitance
Keep it close and don’t worry about the match
Contact
Resistance
Laser
Junction
Bonding
Inductance
Drive
Transistor
Contact
Resistance
Laser
Junction
Pad
Capacitance
-Vee
Junction
Capacitance
-Vee
Packaged Laser Driver
Thin Film
Resistor
Laser Diode
Laser Driver
Transmission Line
Packaged Laser Driver
Packaged Laser
Laser
Bondwire
Laser Driver
Laser Diode
Laser Driver Stabilization
Moni t or Phot odi ode
Laser
Moni t or Phot odi ode
Laser
-
Vr ef
-
Vr ef
+
Lav
+
Dat a
Vr ef 1
Vr ef 2
Vr ef 1
Dat a
Dat a
Dat a
Dat a
-
Vr ef 2
Dat a
-
+
-5V
+
Lpp
Dut y Cycl e
Measur ement
-5V
Peak Det ect or
Average Power and Mark Density Compensation
Bi as Adj ust
Laser
Average Power, Mark Density and Modulation
Monit or Phot odi ode
-
Bi as
+
-
-5V
Average
Power
+
Int egrat or
Modul at ed Power Adj ust
Dat a
Dat a
-5V
Modul ati on
-5V
+
Int egrat or
+
+
Peak Det ect or
Average and Peak Power Stabilization
Peak- pea
Power
A variety of feedback
approaches are available to
compensate for laser
imperfections and the
consequences of temperature
variation and aging
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
Example Commercial Transmitter Module
Palomar Technologies
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
• /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+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.
Electro-Optic Coefficient r
(Pockels Effect)
4

n
r
2 
 
2
 1 
 2   rE
n 
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