-
An SOI based polarization insensitive filter for all-optical
clock recovery
Jinghui Zou,1 Yu Yu,1,3 Weili Yang,1 Zhao Wu,1 Mengyuan Ye,1
Guanyu Chen,1 Lei Liu,2 Shupeng Deng,2 and Xinliang Zhang1,*
1Wuhan National Laboratory for Optoelectronics and School of
Optical and Electronic Information, Huazhong University of Science
and Technology, Wuhan 430074, China
2Network Research Department, Huawei Technologies Co., Ltd.,
Shenzhen, 518129, China [email protected]
*[email protected]
Abstract: We fabricate and demonstrate a compact polarization
insensitive filter for all-optical clock recovery (CR) based on
silicon-on-insulator (SOI), which consists of a microring resonator
(MRR) and two modified two-dimensional (2D) grating couplers. The
distributed Bragg reflectors (DBRs) are introduced to improve the
coupling efficiency of the 2D grating coupler. The MRR works as a
comb filter for CR, while the 2D grating couplers serve as the
polarization diversity unit to achieve a polarization insensitive
operation. A subsequent semiconductor optical amplifier (SOA)
performs the amplitude equalization. Based on this scheme, a good
clock signal with 970 fs timing jitter can be achieved at 44 Gb/s
from input signals with arbitrary polarization states. ©2014
Optical Society of America OCIS codes: (230.3120) Integrated optics
devices; (230.5440) Polarization-selective devices.
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#201650 - $15.00 USD Received 19 Nov 2013; revised 5 Feb 2014;
accepted 6 Feb 2014; published 14 Mar 2014(C) 2014 OSA 24 March
2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006647 | OPTICS EXPRESS
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1. Introduction
All-optical clock recovery (CR) plays a fundamental and key role
in all digital communications, and various kinds of methods based
on different physical principles for CR have been proposed and
demonstrated [1–4]. The passive CR schemes, such as the Fabry-Pérot
(FP) etalon, had received great interests in the past [5, 6], due
to the lower cost and complexity. However, a high enough finesse to
conquer the patterning effect is necessary. The critical finesse
requirement can be significantly relaxed by utilizing an additional
equalizer, for instance a semiconductor optical amplifier (SOA)
[6].
Recently, silicon on insulator (SOI) has become one of the most
promising platforms within the field of integrated optics,
attributing to the combination of the low cost, a high reflective
index contrast and the compatible fabrication processes with
complementary metal oxide semiconductor (CMOS) technology [7–9]. A
number of groups had proposed and demonstrated the SOI-based
tranceivers and other all-optical signal processing units [10, 11].
Microring resonator (MRR) is a prime example of these schemes [12].
An MRR based CR scheme has been theoretically proposed in [13]. The
MMR acts as a passive periodic filter, which can extract out the
clock information and remove the modulation information. However, a
major challenge that limits the potential applications in the
SOI-based photonic integrated circuits (PICs) is the strong
birefringence and thus polarization dependence in the silicon
nanowire waveguides. As we know, the state of polarization changes
in real optical communication networks during propagating in the
fiber due to polarization mode dispersion. Furthermore, the
utilization of polarization division multiplexing (PDM) in nowadays
transmission systems also requires the polarization insensitive
devices for switching and processing. As a result, a polarization
insensitive CR scheme is quite desirable, especially for
accommodating the transmission system utilizing the PDM
technology.
The two-dimensional (2D) grating coupler was proposed to solve
the polarization problem [14, 15]. Combining the MMR and 2D grating
coupler, a polarization insensitive CR scheme can be realized.
Generally, the coupling loss of the 2D grating coupler is a big
issue. This is partly induced by the bidirectional propagation due
to the symmetric configuration of the vertical grating coupler. By
introducing an off-normal tilt (typically 8-12°) between the
optical fiber and the surface normal of the SOI wafer, this problem
can be mitigated [16–19]. However, the angled alignment is a little
difficult to realize for a low-cost optical packaging process.
Furthermore, since the near vertical coupling, the two orthogonal
polarization states will not exhibit the same coupling efficiency,
leading to a polarization dependent loss (PDL) [20]. To improve the
efficiency of vertical coupling, the distributed Bragg reflectors
(DBRs) are introduced to conquer the symmetric configuration.
In this paper, based on an MRR and the modified 2D grating
couplers, we fabricate and demonstrate a compact polarization
insensitive filter for all-optical CR. The MRR acts as the periodic
filter for CR, while the two 2D grating couplers serve as the
polarization diversity units to achieve a polarization insensitive
operation. A subsequent SOA performs the amplitude equalization. A
good clock signal with 970 fs timing jitter can be achieved at 44
Gb/s from input signals with arbitrary polarization states.
#201650 - $15.00 USD Received 19 Nov 2013; revised 5 Feb 2014;
accepted 6 Feb 2014; published 14 Mar 2014(C) 2014 OSA 24 March
2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006647 | OPTICS EXPRESS
6648
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2. Principle and device design
Fig. 1. The proposed scheme (a) the schematic configuration and
(b) the operation principle.
The schematic configuration and the principle of the proposed
scheme are illustrated in Fig. 1. As we know, the light propagating
in the fiber can be divided and mapped into two orthogonal
polarization states: X- and Y-polarization. The signals are
vertically coupled into the 2D grating coupler, with the X- and
Y-pol inputting into the corresponding two orthogonal waveguides
both in TE modes as shown in Fig. 1(a). The two output waveguides
are connected and coupled with an MRR. As a result, one MRR can
process the two signals with clockwise and counter clockwise
propagating directions simultaneously and identically. A same
structure is designed at the drop port of the MRR, coupling the
clockwise and counter clockwise signals out from the MRR. Because
of the symmetry and identity, the transmission profiles of X- and
Y-pol will be the same. In this sense, the transmission profile
will remain the same regardless of the input polarization states,
in other words the device is polarization insensitive.
The operation principle of CR is based on the passive periodic
filtering. The MRR with periodic transmission profile removes the
modulation information while preserving the clock tones, as
illustrated in the schematic diagram in Fig. 1(b). The free
spectral range (FSR), which determined by the group refractive
index (ng) and radius (R) of the MRR through Eq. (1) should match
with the input bit rate.
0=2 g
cFSRRnπ
(1)
where c0 is the velocity of light in vacuum. As for the ng, it
can be calculated by the following Eq. (2) based on the effective
reflective index (neff),
( )
( ) ( ) effg effn
n nλ
λ λ λλ
∂= − ⋅
∂ (2)
To achieve an accurate ng, the material dispersion of silicon
for wavelengths around 1.55μm is taken in account when we calculate
the neff for varied wavelengths, and the refractive index of
silicon (nSi) can be expressed by Eq. (3), where the unit of
wavelength is micrometer [21, 22].
2( ) 3.476 0.07805( 1.55) 0.082( 1.55)Sin λ λ λ= − − + − (3)
Figure 2(a) shows the cross section and transverse field
distribution of the fundamental
TE0 mode. The waveguide is a strip waveguide with the height of
220nm and width of 500nm to realize a single-mode transmission. The
calculated neff and ng are shown in Fig. 2(b) as a function of
wavelength and the ng at the wavelength of 1550nm is 4.359. The FSR
of the MRR is design to be 44GHz (the maximal available bit-rate we
can obtain is 44Gb/s) and the radius is thus calculated to be
250μm.
#201650 - $15.00 USD Received 19 Nov 2013; revised 5 Feb 2014;
accepted 6 Feb 2014; published 14 Mar 2014(C) 2014 OSA 24 March
2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006647 | OPTICS EXPRESS
6649
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Fig. 2. Simulated results of MRR design.
Another key parameter for the MRR is the Q factor. To achieve a
perfect CR result, the Q factor should be large enough to overcome
the patterning effect in case the input signals have long
consecutive zeros, resulting in the locking time will be increasing
correspondingly. We will however have to face two issues: a large
power loss and a narrow locking range, which will degrade the
recovery performance in terms of the optical signal to noise ratio
and limit the operation range. Furthermore, fabricating an SOI
based MRR with very high Q is full of challenge. As mentioned, an
SOA working in saturation region can release the critical Q
requirement and mitigate the amplitude fluctuation. As a result, an
MRR with moderate Q will be designed in this work. The coupling
which depends on the gap between the straight waveguide and the
ring is the key to achieve a proper Q [12]. In our scheme, the gap
is designed to be 280nm and the corresponding coupling efficiency
is κ2 = 0.02 as illustrated in Fig. 2(c).
Fig. 3. Simulated results of 2D grating coupler.
As mentioned above, the DBRs are utilized to improve the
coupling efficiency in the situation of vertical coupling. There
are two kinds DBRs: shallow etched and fully etched. The shallow
etched DBRs have the same etching depth as the 2D grating area, so
they can be etched in one step and no more etching step will be
taken. However, it takes more than tens periods to achieve a high
reflection. On the contrary, the fully etched DBRs only need less
than ten periods to acquire a reflection higher than 90%. Figure
3(a) shows the simulated reflection spectra of the fully etched
DBRs with different periods (P), fill factors (FF) and etching
depths (ED) by 2D Finite-Difference Time-Domain (FDTD) simulation.
Results indicate that DBRs with 340nm period, 50% fill factor and
220nm etching depth (the thickness of the top silicon layer is
220nm) show a reflection higher than 90% from 1500 to 1600nm range.
Moreover, the reflection stays over 90% when the period, the fill
factor and the etching depth variate ± 20nm, ± 0.1 and −20nm
respectively, indicating a high fabrication tolerance. As the
symmetric configuration of the vertical 2D grating coupler, the
bidirectional
#201650 - $15.00 USD Received 19 Nov 2013; revised 5 Feb 2014;
accepted 6 Feb 2014; published 14 Mar 2014(C) 2014 OSA 24 March
2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006647 | OPTICS EXPRESS
6650
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propagation light is equal, so if the reverse directional
propagation light is fully reflected back assisting by the DBRs,
the coupling efficiency will have a 3dB improvement. However, an
improvement less than 3dB was observed due to the fact that
reflected light will go through the grating area again and part of
it will be coupled out of the chip. Furthermore, an incompletely
reflection by the DBRs will lead to partial loss. As a result, ~1dB
improvement can be achieved compared with the traditional 2D
grating coupler without DBRs. The insets in Fig. 3(b) show the
field distributions of 2D grating couplers without and with DBRs
respectively. It can be clearly seen that part of the lights will
be coupled into the reverse direction due to the symmetric
configuration. However, the situation will be quite different when
two DBRs are added at the two idle ports. The light will be
reflected back and the coupling efficiency will be improved
significantly.
Fig. 4. (a) The SEM top view of the device and (b) the measured
spectrum of MMR.
The layout of the device is shown in Fig. 4(a). The device is
fabricated based on an SOI wafer with top silicon layer of 220nm
and SiO2 layer of 3µm. The electron beam lithography (EBL) and
inductively coupled plasma (ICP) etching are used. The left insets
of Fig. 4(a) show the zoom-in 2D coupler region, which is a square
array of holes with an etch depth of 90nm and the lattice period is
580nm. Two distributed Bragg reflectors (DBRs) are used to improve
the coupling efficiency, by reflecting the light from the two idle
ports of the 2D grating coupler. Each DBR consists of six fully
etched silicon slabs with 340nm period and 50% fill factor. The
right insets present the zoom-in SEM pictures of the coupling
region and the holes of the 2D grating. The radius of MRR is
designed to be 250µm and the gap between the MRR and the straight
waveguide is designed to be 280nm, which is a tradeoff between CR
performance and the locking range. The measured transmission
spectrum of the proposed filter is given in Fig. 4(b), showing an
FSR of 44GHz and an extinction ratio of more than 15dB and the Q
factor is 31000. The measured coupling loss of the modified 2D
grating coupler is 5.5dB, which is 1.5dB better than the case of
traditional 2D grating without DBRs in [14].
3. Experimental setup and results
Fig. 5. The experimental setup.
The experimental setup is shown in Fig. 5. A CW light at
1549.09nm is coupled into a modulation unit with two cascaded MZMs,
which are driven by a 44Gb/s data signal(PRBS 231-1) and a 22GHz
clock signal, to obtain the return-to-zero (RZ) optical signal at
44Gb/s. A polarization scrambler varying the input polarization
states arbitrarily is introduced to validate the polarization
insensitivity of the device. The signal is then coupled into the
MRR, which will remove the modulation information while preserving
the clock tones. After that, the
#201650 - $15.00 USD Received 19 Nov 2013; revised 5 Feb 2014;
accepted 6 Feb 2014; published 14 Mar 2014(C) 2014 OSA 24 March
2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006647 | OPTICS EXPRESS
6651
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processed signal injects into an SOA, which acts as a high pass
filter and works in saturation region to remove the fluctuation and
perform the amplitude equalization. The bias current is 210mA. The
output clock signals are analyzed by the communication signal
analyzer (CSA) and the optical spectrum analyzer (OSA),
respectively.
Firstly, the scrambler is turned off and the measured spectra
and eye diagrams are shown in Figs. 6(a)–6(c), indicating the
evolution how the MRR removes the modulation information and the
subsequent SOA equalizes the signals with fluctuations. Then, we
turn on the polarization scrambler and record the eye diagrams as
shown in Fig. 6(d). The results show that the eye diagram only
deteriorates slightly when the input polarization states are
arbitrary, indicating the device is indeed polarization
insensitive. The measured spectra under three different input
polarizations are also measured and compared in Fig. 6(d). It can
be seen the spectra remain almost the same. This further reveals
the polarization insensitivity of the proposed scheme.
Fig. 6. The measured spectra and eye diagrams of (a) original
RZ-OOK signals (b) signals after MRR but before SOA (c) signals
after SOA (d) varying polarization states signals after SOA.
4. Conclusions
In conclusion, we have proposed and fabricated an SOI based
polarization insensitive device consisting of an MRR and two 2D
grating couplers. The DBRs are introduced to improve the coupling
efficiency by reflecting the reverse coupled light. Based on this
device, polarization insensitive clock recovery from 44 Gb/s RZ
signal has been achieved successfully assisting by an SOA. The
device is potential for on-chip clock recovery.
Acknowledgments
This work was supported by the National Basic Research Program
of China (Grant No. 2011CB301704), the National Science Foundation
for Distinguished Young Scholars of China (Grand No.61125501), the
National Natural Science Foundation of China (Grant No. 61007042
and 61275072), New Century Excellent Talent Project in Ministry of
Education of China (NCET-13-0240), and Huawei Technologies Co.
Ltd.
#201650 - $15.00 USD Received 19 Nov 2013; revised 5 Feb 2014;
accepted 6 Feb 2014; published 14 Mar 2014(C) 2014 OSA 24 March
2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006647 | OPTICS EXPRESS
6652