Micro-Ring Based Optical Interleaver
Zhipeng Wang, Yung Jui (Ray) Chen, S.J. Chang+, and Yen Chu+
CSEE Department, University of Maryland Baltimore County, USA.
+ ITRI, Taiwan, ROC, Email: zhipeng1@umbc.edu
Abstract Design and experimental results of micro-ring assisted Mach-Zehnder Interferometer (MZI), based on
ultra-high-index-contrast (HIC) waveguide, are presented. Near rectangular spectral response is demonstrated. A
flat passband and wide stopband with extinction ratio of better than -16 dB were measured.
Introduction
Optical interleavers that combine or separate a comb
of wavelength-division-multiplexed (WDM) signals are
important
optical
components
for
optical
communications. Micro-ring assisted MZI interleaver
is very attractive because of its simple structure,
compact size and good performance [1-3]. In this
paper we present an optimal design using micro-ring
based MZI interleaver with 200 GHz free-spectrumrange (FSR). We demonstrate a good performance
ring-based interleaver and point out the promising
future of this scheme.
utilization (BWU) and passband loss. Because of the
periodic property, passband bandwidths can be
normalized with FSR. The shape factor is defined as
the ratio of normalized -1 dB and -15 dB bandwidth
with a value between 0 and 1 (1 is ideal square
passband). Stopband extinction ratio is defined as the
worst stopband value in the bandwidth requirement
range.
0
-10
c1
Input 1
Input 2
Output 1
R1
3dB
coupler
c2
3dB
coupler
Output 2
R2
Figure 1. Layout of ring-assisted MZI interleaver.
The performance of the device depends on the 3dB
coupler accuracy, the ring coupling coefficient and the
loss of the rings. The ring length and delay length
difference are thermally tuned to match each other to
produce optimum operation conditions. Fig. 2 shows
a simulated spectrum from input 1 to output 1.
Simulations were carried out to map out the device
performance versus ring coupling coefficients, based
on measured. ring loss value of 0.15 dB/ring. The
performance parameters of interest were: passband
@-1 dB bandwidth, -15 dB bandwidth, stopband
extinction ratio for 40%, 30%, 20% bandwidth
Loss (dB)
-20
Interleaver Design
Fig. 1 shows a 2-ring assisted MZI interleaver. The
interleaver consists of three sections: two 3-dB
couplers at the input and output side, and a two-arm
section with different lengths and rings on both of the
two arms. c1 and c2 are bar-state amplitude coupling
coefficients of ring 1 and ring 2, respectively. Both
rings have near identical length that is twice of MZI
delay length difference. This ring length determines
the interleaver free-spectrum-range (FSR). To
achieve large FSR, a small ring diameter is required
and in our case, an ultra-high index contrast
waveguide material (~17%) [1] was utilized to achieve
200GHz FSR for the first time.
-30
-40
-50
-60
-70
192.8
193
193.2
Frequency (THz)
193.4
193.6
Figure 2. Typical spectrum of ring-assisted MZI
interleaver
Fig. 3 shows a 2-D color map of stopband extinction
ratio as a function of c1 and c2 for the 20% BWU
case. We note that maximum stopband extinction
ratio occurs at a near linear region. The extinction
ratio can potentially reach –59 dB for a 2-ring case.
When c1 or c2 equals to 1, the structure becomes a 1ring case. If c1 (ring on short MZI arm) equals to 1,
the extinction ratio can never exceed –20 dB. It is a
poor design. While if c2 equals to 1 the extinction ratio
can reach –40 dB, not as good as the two ring case,
when c1 is around 0.37. Another interesting
observation is that, for a wide range of (c1 , c2)
combinations can be found to meet the low extinction
condition. This indicates the potential of using a
tunable ring-coupling structure for optimized
bandwidth tuning because for different bandwidth
requirements, the optimum coupling regions are
different.
1
-5
Testing results
Fig. 5 shows the distribution of amplitude coupling
coefficient of ring in 1-ring MZI interleaver from more
than 40 devices we tested. The coupling distribution
has a average value of 0.57 and spread 
0.018. This  was used in our Monte-Carlo simulation
to estimate the sensitivity of performance of our
design.
0.9
-10
0.8
-15
0.7
-20
0.6
-25
-30
0.4
-35
c1
0.5
-40
0.3
-45
0.2
-50
0.1
-55
0
0.2
0.4
0.6
0.8
1
c
2
Figure 3. Simulation of stopband extinction ratio as
function of c1 and c2 for 20% BWU.
From the map, we found that for 2-ring assisted MZI,
c1 equals to 0.13 and c2 equals to 0.57 is the optimum
condition for 20% BWU. If we choose 1-ring MZI, c1
should equal to 0.37. The stopband extinction ratios
are –59 dB, -41 dB, and the shape factors are 0.77
and 0.66, respectively. The passband losses are 0.26 dB and –0.22 dB, respectively.
Monte-Carlo simulations were carried out to
investigate the sensitivity of the optimum design. A
normal distribution was used to characterize the
behavior of coupling distribution, see equation (1).
The spread  of 0.02 was used, and this number was
coming from tested results of more than 40 devices.
Fig. 4 shows the extinction ratio distribution for 20%
BWU case. (a) is for 1-ring MZI, and (b) is for 2-ring
MZI. Most of the time 2-ring will have much better
extinction ratio than 1-ring case. And this is also true
for shape factors. Passband loss of 2-ring case is
only slightly bigger than 1-ring case, but still below 0.3
dB.
c  A exp( ( x  c0 ) 2 /( 2 2 ))
(1)
2500
2
2
2
y=aexp(-(x-b) /(2c )) max dev:203, r =0.976
a=2266, b=-36.0, c=3.77
Frequency
2000
1500
1000
500
0
-50
-40
-30
-20
Stopband Extinction for 20% BWU (dB)
800
2
2
2
y=aexp(-(x-b) /(2c )) max dev:117, r =0.914
a=655, b=-39.5, c=7.10
Frequency
600
400
200
0
-70
-60
-50
-40
-30
-20
Stopband Extinction for 20% BWU (dB)
Figure 4. Stopband extinction ratio distribution.
(Above) 1-ring MZI, (below) 2-ring MZI.
The length difference of the two MZI arms is 1.51 mm
and ring legnth is 3.03 mm, resulting a 200 GHz FSR
of interleaver. The dimension of the fabricated
interleaver is 12 mm  2 mm, very compact and
attactive to integration.
12
Frequency
0
2
2
y=aexp(-(x-b) /(2c )) max dev:4.52 a=6.95, b=0.566, c=0.0175
TE and TM
8
4
0
0.52
0.54
0.56
0.58
0.60
0.62
Amplitude Coupling Coefficient
Figure 5. Amplitude coupling coefficient distribution of
ring for 1-ring MZI interleaver.
Fig. 6 shows spectrum of a 1-ring MZI interleaver for
TE mode. The initial asymmetry is coming from the inbalance of ring length and MZI delay length. Fig. 6
also shows the spectrum after the interleaver was
thermally tuned of 10 GHz, and the extinction
improvement is noticeable. The relatively low
extinction ratio of –16 dB is due to the coupling
deviation (c1=0.55 in this case) of the optimum
condition (c1=0.37). When doing the tuning, the
thermal response of the heater resistance R, ring
length change LR, delay length change LD need to
be considered. Actually they were all linearly
proportional to the power applied, and the total effect
was the sum of contribution from ring heaters and
delay line heaters. The detailed explaination can be
found in another paper [4]. 3 mW of power was
needed to shift 1 GHz of central frequency.
0
must on short arm of MZI in order to achieve better
performance. Performance improvement and tunable
bandwidth can be achieved if the coupling is tunable.
Tested results show very consistent amplitude
coupling coefficient distribution. Stopband extinction
ratio of –16 dB was observed and can be greatly
improved if the coupling is designed and processed
properly.
Loss (dB)
-5
-10
-15
10 GHz shift
Initial
-20
-25
192.9
193.0
193.1
193.2
193.3
Frequency (THz)
Figure 6. 1-ring MZI interlaver spectrum before and
after 10 GHz shift.
Conclusions
Design of 1-ring and 2-ring assisted MZI interleaver
were presented. Simulation show 2-ring has a much
better performance but 1-ring is still usable when the
bandwidth is small. Simulation also shows that ring
References
1 K. Oda et al, J. Lightwave Technol., vol. 6, pp.
1016-1022, 1988.
2 C. Madsen, Photon. Technol. Lett., vol. 10, pp.
1136-1138, 1998.
3 C. G. H. Poeloffzen et al, Proc. Symposium
IEEE/LEOS Benelux Chapter, 2002.
4 Z. Wang et al, ECOC 2005, We4.P.044.