an soi based polarization insensitive filter for all-optical clock ...an soi based polarization...

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 6647

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channel WDM RZ-to-NRZ format conversion at 50 Gbit/s based on single silicon microring resonator,” Opt. Express 18(20), 21121–21130 (2010).

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.


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.


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


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


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.


an soi based polarization insensitive filter for all-optical clock ...an soi based polarization...

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