Edwin Jan Klein - Universiteit Twente

Chapter 3
3.2.1 Drop port on-resonance insertion loss
The insertion loss ILDrop is a measure of the efficiency with which light of a certain
frequency can be transferred by a micro-resonator from the in to the drop port. This
parameter is especially important for micro-resonator based devices because of their
resonant nature. Depending on the field coupling coefficient even moderate cavity
losses quickly lower the power that can be dropped. Since many micro-resonator
based devices can incorporate a number of MRs in cascade configuration, such as
shown in Figure 3.3, the ILDrop can easily become the dominant factor in overall
device losses.
Figure 3.3. Drop port insertion losses can quickly add up and may largely dictate overall
device IL performance.
The insertion losses can be found by taking Equation (2.14) for a resonator on
resonance where φr=2π.
IL
⎛
−10
⋅ log⎜
⎜
⎝1
+ F
H
⋅ sin
2 2
⎞
⎛ κ ⎞
1 κ 2 χ r
⎟
= −10
⋅ log H = −10
⋅ log
⎜
⎟
( π ) ⎠
⎝ ( 1−
µ 1µ
2χ
r ) ⎠
Drop = 2
2
C
42
(3.2)
Ideally ILDrop has a value as close to zero as possible. Due to resonator losses,
however, not all power is extracted from the in-port in addition to the power lost
within the cavity.
Figure 3.4 shows the insertion loss as a function of the field coupling coefficient κ
(κ1=κ2) and the roundtrip losses αr=2.π.R.αdB in the resonator. As is evident from the
figure, ILDrop rises sharply for small coupling coefficients. In theory, the higher
insertion losses can be reduced by lowering the roundtrip losses. In practice, however,
although reduction of scattering induced losses may be possible, the bend and
intrinsic material losses pose a hard lower limit.