EE 230: Optical Fiber Communication Lecture 15
WDM Components
From the movie
Warriors of the Net
ITU Grid
• Wavelengths for CWDM and frequencies for
DWDM defined by International
Telecommunication Union, a part of the
United Nations located in Geneva
• Central frequency is 193.1 THz, equivalent to
1552.52 nm
• Frequencies for 50 GHz channel spacings
are thus defined as 193.1 + 0.05n THz where
n is a positive or negative integer
Active vs. Passive Devices
• Passive: requires no electrical power
and transfer function cannot be modified
by user
• Active: allows user to manipulate what
it does to light pulses. Requires power.
Platforms for WDM components
• Discrete optics: thin-film filters,
microelectromechanical systems
(MEMS), isolators, circulators
• All-fiber components: couplers, MachZehnder interferometers
• Planar lightwave circuits (PLC):
arrayed-waveguide gratings (AWG),
couplers, MZs, etc.
Coupler parameters
Splitting ratio: P2/(P1+P2)
Excess loss: 10 log (P0/P1+P2)
Insertion loss: 10 log (Pin/Pout)
Crosstalk: 10 log (P3/P0)
Coupling as function of length
P2  P0 sin z e
2
z
Mach-Zehnder Interferometer
 
c
2neff L
where neff is determined from the Pcore/P
graphs
Multiplexing/demultiplexing criterion
1 1
2neff   L  
 1 2 
where L is the path length difference
between the two arms
Wavelength dependence of MZ output
For wavelengths 1 entering at input port 1, and
2 entering at input port 2,
 nL 
2  nL 
  cos 

PO1  sin 
 1 
 2 
2
Wavelength adjustment (“trim”)
• Coarse adjustment possible with fiber
MZs by heating and pulling shorter arm
to increase channel spacing
• Fine adjustment for both fiber and PLCs
done with UV irradiation to line
transmission peaks up with ITU grid
Example
To multiplex four wavelengths separated
by 50 GHz (0.4 nm)
How many stages needed?
2. (log2 W). How many total MZs?
3. Two in one stage, one in the next.
What is L for each stage?
Example, continued
If first frequency is ITU center, what are
other three, and their wavelengths?
193.10, 193.15, 193.20, and 193.25 THz
1552.52, 1552.12, 1551.72, and 1551.32
nm
If neff=1.45, determine L values
Example, continued
• First stages have 100 GHz channel
spacing, one for even-numbered
wavelengths and one for odd. L
equals c/2n(100x109)=1.0 mm
• Second stages have 50 GHz channel
spacing. L =c/2n(50x109)=2.1 mm
• As channel spacing gets smaller, it gets
easier to make MZs (larger L)!
General MZ expression
For a multiplexer or demultiplexer with N
wavelengths, you need n=log2N stages where
the path length difference for stage i is
c
Li  n i
2 n
Arrayed-Waveguide Grating
AWG channel spacing
cdxns nc
 
2
mc L f ng
where ns=input/output waveguide index,
nc=central waveguide array index, and
dnc
ng  nc  
d
Tuning an AWG
Each input waveguide corresponds to a
different center wavelength and channel
spacing. Several waveguides around
the center one will correspond to the
correct channel spacing within the
tolerance, and the peak wavelengths
will vary from one waveguide to another.
WDM Muxes and Demuxes
Grating Based Demultiplexer
Optical Filters
Interference Filter Based WDM
Thermal drift in waveguide devices

   nL     L
n 
1 n 
 1 L 1 n 


L



n
  
    

T nL  T  nL  T
T 
n T 
 L T n T 

n/T for silica=7.5x10-6 per degree
 for silicon=2.63 ppm per degree
d/dT = 12 pm per degree (red shift)
2/3 due to thermooptic effect, 1/3 to CTE
Effect of thermal drift
Channel spacing=100 GHz=0.8 nm=800 pm
DWDM device completely transparent every
800 pm, opaque between
Silica-on-silicon drifts 12 pm/
Device becomes a beam stop if temperature
changes by ?
33! “Passive” devices routinely T stabilized;
customers unhappy
Athermalization Techniques
• Mechanical compensation: flex entire
chip, adjust point at which signal
injected into device
• Materials compensation: design
waveguide to be inherently athermal