Integrated tunable optical add/drop filter for
polarization and wavelength multiplexed signals
Yaguang Qin, Yu Yu,* Wenhao Wu, and Xinliang Zhang
Wuhan National Laboratory for Optoelectronics and School of Optical and Electrical Information, Huazhong
University of Science and Technology, Wuhan, 430074, China
*
[email protected]
Abstract: We propose and demonstrate an integrated tunable optical filter
which is promising for reconfigurable optical add/drop multiplexer
(ROADM) targeting polarization and wavelength multiplexed signals. The
proposed filter is comprised of a polarization diversity scheme and two
tunable microring resonators (MRRs). The polarization scheme is
implemented by two dimensional (2D) grating couplers which are
functioning as signals import, output and add/drop ports, while the MRRs
are signals processing units. The add/drop function can be applied on either
polarization tribute or any wavelength by controlling the resonate
wavelengths of ring resonators. For demonstration, dual polarizations and
four-wavelength signals are experimentally added and dropped with good
performance and reasonable power penalties.
©2016 Optical Society of America
OCIS codes: (130.3120) Integrated optics devices; (130.5440) Polarization-selective devices;
(130.7408) Wavelength filtering devices.
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#257890
(C) 2016 OSA
Received 21 Jan 2016; revised 13 Mar 2016; accepted 15 Mar 2016; published 24 Mar 2016
4 Apr 2016 | Vol. 24, No. 7 | DOI:10.1364/OE.24.007069 | OPTICS EXPRESS 7069
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1. Introduction
The optical add/drop multiplexer (OADM) is a key component that allows locally
adding/dropping one or more channels to/from optical paths in a dense wavelength division
multiplexing (DWDM) network [1]. Conventional OADM has fixed configuration thus
requiring manual operation to reconfigure the add/drop channels. To achieve a more flexible
and cost-efficient network, reconfigurable OADM (ROADM) is essentially desired.
ROADMs have been widely investigated and many schemes implementing various
technologies have been proposed, including planar lightwave circuit (PLC) [2], fiber arrays
[3], fiber gratings [4], liquid crystal [5], microelectromechanical systems (MEMS) [6] etc.
However, the ROADMs based on these schemes suffer from either large footprint size or
alignment and packaging complexity. The prosperous development of silicon photonics
seems to provide a feasible solution to dramatically reduce power consumption and device
size. The silicon on insulator (SOI) has become a competitive candidate for all-optical signal
processing and integration platform thanks to the complementary metal oxide semiconductor
(CMOS) compatible fabrication technology, high yields and high refractive index contrast.
The development of OADMs based on SOI platform can lead to further integration of WDM
subsystems, and several researches have been reported, such as grating-assisted
contradirectional couplers [7] and Bragg grating/MZI configurations [8]. However, a very
limited transmission isolation of ~8 dB was achieved. Being the essential building block of a
integrated ROADM, tunable add/drop filters based on integrated microring resonator (MRR)
have superior performance compared to other candidates in terms of compactness and power
consumption, and it is essentially suitable for WDM applications due to the intrinsically
periodic characteristics [9].
On the other hand, current add/drop functions are mostly performed on wavelength
domain, while multi-dimensional multiplexing is utilized nowadays to increase the
transmission capacity, for instance the WDM combined with polarization division
multiplexed (PDM) techniques. However, the polarization dependent issues for silicon nanoscale waveguide restricts the applicability of PDM signals. To tackle this problem, the
polarization diversity scheme, where the orthogonal polarization states are firstly split and
processed separately and then combined at output port, is introduced [10,11]. The twodimensional grating coupler (2D GC), which couples the two orthogonal polarization modes
from a single-mode fiber into the identical mode but two different paths of the waveguide, is
an outstanding candidate for simultaneous coupling and polarization diversity. Arising from
this, we propose a tunable add/drop filter for PDM and WDM signals, utilizing the
polarization diversity configuration comprised of 2D GCs and MRRs. The proposed filter
can be an important building block of ROADM circuit suitable for multi-dimensional system.
As demonstration, 20 Gb/s On-Off Keying (OOK) dual polarizations and four-wavelength
signals are experimentally processed. The adding and dropping functionalities can be
successfully applied to different polarization and wavelength. The bit error rate (BER)
measurements show an error free operation, indicating the good performance of the proposed
filter.
#257890
(C) 2016 OSA
Received 21 Jan 2016; revised 13 Mar 2016; accepted 15 Mar 2016; published 24 Mar 2016
4 Apr 2016 | Vol. 24, No. 7 | DOI:10.1364/OE.24.007069 | OPTICS EXPRESS 7070
2. Operation principle
(b)
(a)
(c)
Path1
4
X pol
Y pol
5
Drop
6
Path2
X pol
Thermal phase
shifter
1
2
...
x
Path1
y
3
...
Path2
Arrayed fibers
Add
Y pol
Fig. 1. (a) Schematic configuration of the proposed filter, (b) 3D FDTD simulated electric
field distribution of coupled light with different input polarization via 2D GC, (c) schematic of
operation principle on both polarization and wavelength domain.
The schematic configuration and the principle of the proposed scheme are illustrated in Fig.
1. The circuit is comprised of six 2D GCs (marked from No. 1 to No. 6 and these six
corresponding ports are also marked from No.1 to No.6), two thermal tunable MRRs (marked
as MRR-1 and MRR-2) and connecting access waveguides. No. 2 and 5 2D GCs are used for
coupling main stream signals into and out of the chip. No. 1 and 4 2D GCs are functioning as
drop and add port for MRR-1, while No. 3 and 6 are for MRR-2, respectively. To be noted,
only ports 2 and 5 function as a polarizartion diversity ports, and thus 2D GCs are utilized.
The remaining ports are normal coupling ports and conventional 1D GCs are competent.
However, for a coincident performance, all the six ports are designed with 2D GCs. When
injecting into the circuit via No. 2 2D GC, the signals would be coupled into two orthogonal
access waveguide and propagate along two paths (Path 1 and Path 2), according to input state
of the polarization (SOP). As depicted in Fig. 1(b), the coupling mechanism of a 2D GC is
investigated and the simulation results are obtained utilizing 3D FDTD method [12]. When
input light is X polarized, which is orthogonal to the direction of Path 1, it tends to be fully
coupled into Path 2. If input light is Y polarized, the coupled light would propagate in Path 1.
By aligning PDM tributaries to the polarization planes of 2D GCs, it is possible to couple the
specific polarization tributary to the desired path [13]. The MRR-1 and MRR-2 are designed
to have relatively large 3dB bandwidth and free spectrum range (FSR). Figure 1(c) shows the
schematic of operation principle of proposed filter for wavelength and polarization. Since the
two polarization tributaries of PDM signal would couple into different paths on the chip, it is
possible to process them separately. By aligning the resonant wavelength of MRR-1 and
MRR-2 to the desired wavelength, the signal of specific polarization and wavelength
tributary can be added or dropped. Due to the presence of thermal phase shifter, the whole
circuit is tunable within a large wavelength range.
3. Device fabrication and experimental results
The proposed filter is fabricated on SOI platform and a commercial SOI wafer with top
silicon layer of 220 nm is used. The buried oxide layer is of 2µm thick, and the upper
cladding material is silicon oxide as well. All the waveguides are slab ones with 90 nm thick
slab to enhance the coupling strength of the MRRs. The optical micrographs and scanning
electron micrographs (SEM) of the fabricated device are illustrated in Fig. 2. Figure 2(a)
shows the top view, it is seen that all six 2D GCs are placed intentionally to match the fiber
array so that add and drop functionalities can be performed simultaneously. The red dash
boxes represent the reference MRRs with identical parameters as the ones in the proposed
filter. Figures 2(b) and 2(d) provide the details of the tunable MRRs. The radii of MRR-1 and
#257890
(C) 2016 OSA
Received 21 Jan 2016; revised 13 Mar 2016; accepted 15 Mar 2016; published 24 Mar 2016
4 Apr 2016 | Vol. 24, No. 7 | DOI:10.1364/OE.24.007069 | OPTICS EXPRESS 7071
MRR-2 are 20 µm and designed gap between the straight and bended waveguides for the
coupling region is 200 nm. The calculated coupling strength is 0.246122. The designed 3dB
bandwidth and FSR are 0.4 nm and 4.48 nm respectively. Figures 2(c) and 2(e) show the 2D
GC with a square array of round holes and an etch depth of 90 nm. The designed diameter of
the air holes are 340 nm and the lattice period is 840 nm. To improve coupling efficiency,
Bragg grating structures (BGSs) are adopted [14].
The tunable spectral response of MRRs are firstly investigated. As depicted in Fig. 3(a),
the optical spectrum at the through port of a reference MRR is measured. The FSR is
measured to be 4.9 nm. When the bias voltage varies from 0 to 5V, the resonant wavelength
shifts to longer wavelength nearly within a FSR, and the tuning efficiency is measured to be
60 mW/FSR. The extinction ratio is as large as 22 dB and the 3dB bandwidth is about 0.4
nm, indicating its capability of processing signals up to 40 Gb/s. As for the drop port, a
visible tunable spectrum is also obtained, as shown in Fig. 3(b).
Fig. 2. Optical micrographs of (a) the layout of proposed filter, the red dash boxes are the
reference MRRs, (b) tunable MRR, (c) 2D GC with BGSs, and the SEM images of (d)
coupling region of reference MRR, (e) air holes of 2D GC.
(b)
(a)
Fig. 3. Spectral response of reference MRR when bias voltage varies from 0V to 5V, (a)
through port of MRR (b) drop port of MRR.
#257890
(C) 2016 OSA
Received 21 Jan 2016; revised 13 Mar 2016; accepted 15 Mar 2016; published 24 Mar 2016
4 Apr 2016 | Vol. 24, No. 7 | DOI:10.1364/OE.24.007069 | OPTICS EXPRESS 7072
BPG1
DCF
DATA1
PC1 PBC
λ1
MZM1
λ2
DCF
PC2
λ3
ATT PC4 PBS
PC3
AWG
BPF
OSA
Isolator
CSA
MZM2
λ4
EDFA
DATA2
BPG2
Fig. 4. The experimental setup.
The experimental setup is shown in Fig. 4. Four CW lights at 1533.7, 1534.7, 1535.7 and
1536.7 nm are firstly combined by an arrayed waveguide grating (AWG) and are split into
two Mach-Zehnder modulators (MZMs) driven by two independent bit pattern generators
(BPGs) to obtain independent WDM OOK signals. The driven data are PRBS 231-1 at 20
Gbaud. A dispersion compensation fiber (DCF) with length of ~200 m and dispersion
parameter of −80 ps/nm/km is utilized after each MZM. Although the induced delay cannot
achieve a complete decorrelation, it is helpful to distinguish whether the proposed filter is
effective to perform the drop operation with low crosstalk and large isolation from adjacent
channels. The polarization states of the two signals are optimized by the polarization
controllers (PC1 and PC2) and combined by a polarization beam combiner (PBC), forming
the WDM-PDM signals. By vertically coupling via the 2D GC, the signals are coupled into
the chip, assisting by another PC (PC3) to align the two orthogonal polarizations in the fiber
to the polarization axes of the 2D GC in the silicon waveguide. If the circuit is not applied for
the PDM signal processing, the PC3 is not necessary and the scheme performs as a
polarization insensitive tunable filter. An optical isolator is used here to eliminate the
reflection. At the output ports, the processed signals are coupled out by another set of 2D
GCs, and the output power is optimized by the erbium-doped fiber amplifier (EDFA) and the
attenuator (ATT). Assisting by the subsequent PC (PC4) and a polarization beam splitter
(PBS), the output dual polarization signals can be detected. A band pass filter (BPF) is
further used for the wavelength demultiplexing and an optical spectrum analyzer (OSA) and
a communication signal analyzer (CSA) are also used for monitoring.
For signal demonstration, the X-polarized (as shown in Fig. 1(a)) tributary of PDM
signals is firstly processed while the Y-polarized tributary remains unchanged. This can be
done by thermally tunning the MRR-1and MRR-2, so that we can align only the resonate
wavelength of MRR-1 with the signal wavelength. The dropped signal can be detected at
No.1 2D GC while the added signal is added at No.4 2D GC.
#257890
(C) 2016 OSA
Received 21 Jan 2016; revised 13 Mar 2016; accepted 15 Mar 2016; published 24 Mar 2016
4 Apr 2016 | Vol. 24, No. 7 | DOI:10.1364/OE.24.007069 | OPTICS EXPRESS 7073
Input signal
Dropped signal
1533
1534 1535 1536
Wavelength(nm)
1533
(d)
Input signal
WL1
1534 1535 1536 1537
Wavelength(nm)
Optical Power (20dB/div)
1537
Optical Power (20dB/div)
(c)
(e)
Optical Power (20dB/div)
(b)
Optical Power (20dB/div)
(a)
1533
1534 1535 1536 1537
Wavelength(nm)
1533
WL2
WL3
WL4
1534 1535 1536
Wavelength(nm)
1537
Fig. 5. The spectral response of dropped X-polarized signal at different sigal wavelengths: (a)
wavelength 1, (b) wavelength 2, (c) wavelength 3, (d) wavelength 4; (e) the eye diagram of
four signal wavelengths.
(b)
Input signal
Filtered signal
Optical Power (20dB/div)
Optical Power (20dB/div)
(a)
1533 1534 1535 1536 1537
1533 1534 1535 1536 1537
Wavelength(nm)
Wavelength(nm)
(d)
Optical Power (20dB/div)
Optical Power (20dB/div)
(c)
1533 1534 1535 1536 1537
Wavelength(nm)
1533 1534 1535 1536 1537
Wavelength(nm)
Fig. 6. The spectral response of X-polarized signal after the filtering at different signal
wavelengths: (a) wavelength 1, (b) wavelength 2, (c) wavelength 3, (d) wavelength 4.
#257890
(C) 2016 OSA
Received 21 Jan 2016; revised 13 Mar 2016; accepted 15 Mar 2016; published 24 Mar 2016
4 Apr 2016 | Vol. 24, No. 7 | DOI:10.1364/OE.24.007069 | OPTICS EXPRESS 7074
(a)
(b)
Optical Power (20dB/div)
Optical Power (20dB/div)
1533
1537
(d)
WL1
1533
1534 1535 1536
Wavelength(nm)
1537
Optical Power (20dB/div)
Optical Power (20dB/div)
(c)
1534 1535 1536
Wavelength(nm)
(e)
Input signal
1533
1534 1535 1536
Wavelength(nm)
1537
WL2
WL3
WL4
1533 1534 1535 1536 1537
Wavelength(nm)
Fig. 7. The add functionality of X-polarizaed tributary at four signal wavelengths via bias
voltage tuning, the spectral response of different signal wavelengths: (a) wavelength 1, (b)
wavelength 2, (c) wavelength 3, (d) wavelength 4; (e) the eye diagram of four signal
wavelengths.
The input PDM signals are fed into chip via No.2 2D GC and detected at No.1 2D GC.
The drop functionality of the proposed filter for X-polarized signals is illustrated in Figs. 5(a)
to 5(d). The axes of input signal and 2D GC are aligned by adjusting PC3. Consequently, Xpolarized signal is guided into MRR-1 and Y-polarized signal is guided into MRR-2. The
black curves are the spectra of input signals, while the red ones represent the dropped signals
at four different wavelengths, respectively. The insertion loss at drop port, which is defined
by the power dissipation between input signal and dropped signal at specific wavelength, is
measured to be ~18 dB. It originates mainly from the coupling loss of two 2D GCs and might
be improved by further optimizing the design of 2D GC [15]. The drop loss is 3.6 dB and the
measured isolation from adjacent channel (considering 1 nm channel spacing) is 15 to 20 dB,
for different channels. Figure 5(e) shows the eye diagrams of four dropped signals at
different wavelengths (WL1 to WL4). To be noted, the inputs are PDM signals but the eye
diagram is only single polarization, since the oscilloscope can only be triggered by either
pattern generator. It can be seen that all the eye diagrams are clear and open, indicating the
effective dropping functionality of signals. Then, the output is switched to No.5 port instead
of No.1 to investigate the optical spectrum after filtering by MRR-1. Figures 6(a) to 6(d)
show the spectral evolution of the signals processing. The black and red curves represent the
input and output signals after dropping. The input signals can be fully downloaded one by
one according to the results in Fig. 6. The measured crosstalk between dropped and
undropped signal at same signal wavelength, which is defined as through crosstalk, is less
than −14 dB. The insertion loss at through port (defined as the power attenuation between
input and unfiltered signal at same wavelength) is measured to be ~15 dB.
Finally, another input at different wavelength is fed into the chip via No.4 2D GC and
detected at No.5 2D GC to test the add functionality, with results shown in Fig. 7. WL1 to
WL4 represent the added signals at different wavelengths, recording from the output port 5
assisted by the polarization and wavelength demultiplexing. The measured optical spectrum
and clear eye diagrams reveal the good adding performance.
#257890
(C) 2016 OSA
Received 21 Jan 2016; revised 13 Mar 2016; accepted 15 Mar 2016; published 24 Mar 2016
4 Apr 2016 | Vol. 24, No. 7 | DOI:10.1364/OE.24.007069 | OPTICS EXPRESS 7075
We then foucus on the feasibility of Y-polarized tributrary of PDM signals processed by
the MRR-2. Similar results on spectra and eye diagrams are obtained, as shown in Figs. 8, 9
and 10. The drop functionality of the proposed filter for Y-polarized signals is illustrated in
Figs. 8(a) to 8(e), including the spectra and eye diagrams. Figures 9(a) to 9(d) show the
spectral evolution of the signals processing. The black and red curves represent the input and
output signals after dropping. The add functionality for Y-polarized signals is presented in
Fig. 10, for different wavelength cases. The drop loss and isolation from adjacent channel is
4.2 dB and 13 to 18 dB for different channels. The through crosstalk is measured to be less
than −12 dB. It is seen that results for Y-polarized signal is consistant with the ones for Xpolarized, verifying the capability of handling dual polarization and multiple wavelengths. In
order to quantitative investigate the performance of the proposed scheme, the BER
measurements are perform for the adding and dropping functions, respectively.The measure
results are depicted in Fig. 11. For simplification, only results at one wavelength (WL2) are
presented, while the dispersion for different wavelengths are less than 0.7 dB. The power
penlty is 1.2 and 1.5 dB for adding and dropping operation, respectively.
(b)
Input signal
Dropped signal
1533
1537
WL1
1533 1534 1535 1536
Wavelength(nm)
1537
Optical Power (20dB/div)
1533 1534 1535 1536
Wavelength(nm)
WL2
(d)
Optical Power (20dB/div)
(c)
1534 1535 1536
Wavelength(nm)
(e)
Input signal
Optical Power (20dB/div)
Optical Power (20dB/div)
(a)
1537
WL3
WL4
1533
1534 1535 1536
Wavelength(nm)
1537
Fig. 8. The spectral response of dropped Y-polarized signal at different sigal wavelengths: (a)
wavelength 1, (b) wavelength 2, (c) wavelength 3, (d) wavelength 4; (e) the eye diagram of
four signal wavelengths.
#257890
(C) 2016 OSA
Received 21 Jan 2016; revised 13 Mar 2016; accepted 15 Mar 2016; published 24 Mar 2016
4 Apr 2016 | Vol. 24, No. 7 | DOI:10.1364/OE.24.007069 | OPTICS EXPRESS 7076
(b)
Optical Power (20dB/div)
Optical Power (20dB/div)
(a)
Input signal
Filtered signal
Optical Power (20dB/div)
1534 1535 1536 1537
1533
Wavelength(nm)
(d)
1533
1534 1535 1536 1537
Wavelength(nm)
Optical Power (20dB/div)
1533
(c)
1534 1535 1536 1537
Wavelength(nm)
1533
1534 1535 1536 1537
Wavelength(nm)
Fig. 9. The spectral response of Y-polarized signal after the filtering at different signal
wavelengths: (a) wavelength 1, (b) wavelength 2, (c) wavelength 3, (d) wavelength 4.
(b)
(e)
Input signal
Optical Power (20dB/div)
Optical Power (20dB/div)
(a)
1533 1534 1535 1536
Wavelength(nm)
1537
1533
(d)
1534 1535 1536
Wavelength(nm)
1537
Optical Power (20dB/div)
Optical Power (20dB/div)
(c)
WL1
1533
1534 1535 1536
Wavelength(nm)
1537
WL2
WL3
WL4
1533 1534 1535 1536 1537
Wavelength(nm)
Fig. 10. The add functionality of Y-polarizaed tributary at four signal wavelengths via bias
voltage tuning, the spectral response of different signal wavelengths: (a) wavelength 1, (b)
wavelength 2, (c) wavelength 3, (d) wavelength 4; (e) the eye diagram of four signal
wavelengths.
#257890
(C) 2016 OSA
Received 21 Jan 2016; revised 13 Mar 2016; accepted 15 Mar 2016; published 24 Mar 2016
4 Apr 2016 | Vol. 24, No. 7 | DOI:10.1364/OE.24.007069 | OPTICS EXPRESS 7077
-3
-4
X-pol drop
Y-pol drop
X-pol add
Y-pol add
BtB
Log (BER)
-5
-6
-7
-8
-9
-10
-15
-14
-13
-12
-11
-10
-9
-8
-7
-6
-5
Received power (dBm)
Fig. 11. The measured power penalty.
4. Conclusion
In conclusion, we have propose and demonstrate a tunable add/drop filter that can be used for
both wavelength and polarization multiplexed signals. 20 Gbit/s WDM-PDM signals can be
successfully added and dropped with clear eyediagrams. Arising from this, many other
polarization handling devices for PDM and WDM signal processing can be achieved by
changing the configuration of this proposed scheme, and we believe that can be advantageous
in monolithic integrated circuit for signal processing in the future optical transport networks
utilizing multi-dimensional multiplexing.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (NSFC)
(Grant No. 61475050 and 61275072), the New Century Excellent Talent Project in Ministry
of Education of China (NCET-13-0240), the Fundamental Research Funds for the Central
Universities (HUST2015TS079), and Huawei Technologies Co. Ltd..
#257890
(C) 2016 OSA
Received 21 Jan 2016; revised 13 Mar 2016; accepted 15 Mar 2016; published 24 Mar 2016
4 Apr 2016 | Vol. 24, No. 7 | DOI:10.1364/OE.24.007069 | OPTICS EXPRESS 7078