Splitters and Couplers
What needed?
? to combine signals and/or to split them
multiple ways,
? up to 1000x1000 for WDM LANs
? three important characteristics
– Return Loss:the amount of power that is
reflected and thus lost,
– Insertion Loss:the amount of signal lost in
the total transit through the device,
– Excess Loss:additional loss of a device
over and above the loss required by theory,
Resonant Coupling
power will oscillate from one
fibre core to the other,
Single-Mode Propagation
Theory
? Equations,
dR/dz- jdR = -jKS;
dS/dz+ jdS = -jKR
? Am=R exp(-jdz);
? An= S exp(jdz)
? Detuning coefficient,d=(bn-bm)/2
? Coupling coefficient,K
Solution
)s in (
)s in ()c o s (
22
22
22
22
22
zK
K
jK
S
zK
K
j
zKR
d
d
d
d
d
d
?
?
-=
?
?
??=
Mode coupling between
two fibres
d=0,R2(z)=cos2(Kz)
S2(z)=sin2(Kz)
? coupling length,
L0=p/2K
? Coupling coefficient,
2
0
2
2
2222
0
2
1
2
1
)2(
22
1
22
1
2112;
))(1(
4
knknk
e
kD
k
KK
x
Dd
x
x
-=-=
??
== --
b?b
??b
? ?
2D
2D
d n2
n1
n2
n1
n2
Important points
? The,coupling length” is formally defined (at a
particular wavelength) as the length at which
100% of the power entering at a particular input
port is transferred to the other fibre,
? The amount of coupling and hence the coupling
length is strongly dependent on the separation
between the two single-mode cores,The further
apart they are the greater the coupling length,
Important points
? The coupling lengths are strongly
wavelength dependent! Different
wavelengths yield different coupling
lengths,
Mode coupling of two mixed
signal(of different wavelength)
Important points
? Light exiting the coupler of the opposite fibre
from which it entered is 180° shifted in phase,
Light entering on Port 1 and exiting on Port 2
is unshifted,
? If the wavelengths are the same (or close
together) and the core sizes etc,are also the
same then there is no way to couple all of the
power from both input ports to the same
output port,
X & Y- coupler
The Principle of Reciprocity
? If you try to combine two input
signals by using a y-junction,the
signals are combined but each
signal will lose half (3dB) of its
power!
Practical Couplers
3 dB Couplers
Tap
? A coupler that splits only a small part of
the light into one output and leaves
most in the other is often called a,tap”,
? For example, 99:1 coupler or 1% tap,
? you cannot use a simple coupler to take
all of the light in two fibres and merge it
onto one fibre,
Wavelength Selective Couplers
and Splitters
Power Input to an EDFA
Splitting Wavelengths for Simple WDM
Adding the Management Channel in DWDM
Star Couplers
Mixing Plate
Planar Devices
PBS:Polarization Beam Splitter
Glan-Thompson Prism
Polarisation Beamsplitter Cubes