In this paper, we perform an investigation of terabit-scale data transmission in silicon subwavelength grating (SWG) waveguides for wavelength-division multiplexing (WDM) optical signals. Silicon SWG waveguide is capable of decreasing the light confinement in silicon core by engineering the geometry, leading to relatively lower optical nonlinearity compared to silicon wire waveguide. We demonstrate ultrahigh-bandwidth 2.86 Tb/s data transmission through the fabricated 2-mm-long silicon SWG waveguide over a wide range of launch powers. In the experiment, 75 WDM channels are utilized with each carrying 38.12 Gb/s orthogonal frequency-division multiplexing (OFDM) 16-ary quadrature amplitude modulation (16-QAM) signal. With the benefit of efficient reduction on optical nonlinearity, the optimum launch power is increased by 8 dB in SWG waveguide, indicating higher tolerance to the nonlinear impairments, compared to a silicon wire waveguide with identical length. With the optimum launch power, all 75 channels exhibit bit-error rate (BER) values less than 4e-5 after SWG waveguide transmission. We also evaluate the terabit-scale data transmission performance through four silicon SWG waveguides with different lengths (1 mm, 2 mm, 4 mm and 12 mm). The required optical signal-to-noise ratios (OSNRs) to achieve BER level of 1e-3 are around 15.27, 15.47, 16.66 and 20.38 dB, respectively.
© 2017 Optical Society of America
In the progress toward high performance computing systems, considerable effort has been invested into exploring the solutions to the ever increasing bandwidth and energy challenges in chip-scale interconnection networks . Optical interconnects are identified as a promising technology to improve the performance of future interconnection networks owing to its reduced transmission latency, lower power consumption and wider available bandwidth compared to the state-of-the-art copper based electrical interconnects [2,3]. Over the past decades, silicon-based photonic integrated circuits (PICs) have been developed as a competitive candidate for the chip-scale optical interconnects considering its potential to realize monolithic integration of photonics and electronics [4,5]. Monolithic electro-optical integration enables the reduction of power consumption and increment of interconnection bandwidth. The inherent properties of high refractive index contrast and complementary metal-oxide-semiconductor (CMOS) compatibility on the silicon-on-insulator (SOI) platform allow for the high-density integration and low-cost mass production. Motivated by these unique benefits, silicon photonics has grown as a very active and productive research field [6,7]. Significant progress has been made in the development of SOI-based building blocks for chip-scale optical interconnects, including light sources , modulators , detectors , and waveguides . Silicon wire waveguide which features low propagation loss and sub-micrometer dimension is generally regarded as the ideal transmission medium for the integrated silicon photonic networks . However, considering its attribute of tight light confining, the optical nonlinear effects may cause impairments to the signal pulses at high data rates when propagating in long optical links.
A potential approach to utilize unprecedented bandwidth scalability of optical interconnects is based on the wavelength-division multiplexing (WDM) scheme to enable wavelength parallelism. In 2008, transmission of a 1.28 Tb/s (32 × 40 Gb/s) non-return-to-zero (NRZ) on-off-keying (OOK) WDM data stream through a silicon wire waveguide has been demonstrated and the impairment induced by nonlinearities was evaluated . Several solutions have been investigated to alleviate the nonlinear transmission impairment. In , silicon vertical slot waveguide with high light confinement in the low-index air slot region was proposed for chip-scale data transmission. Advanced modulation format with high spectral efficiency was adopted to further increase the transmission capacity (1.8 Tb/s aggregate data rate). In spite of the reduced nonlinearities, the relatively high propagation loss demonstrated by the slot waveguides might set limitations for the data transmission [14,15]. Thus, a preferred choice is the interconnection medium with low loss and weak nonlinearity. Silicon subwavelength grating (SWG) waveguide which confines light as an index-guided structure allows for the equivalent refractive index engineering of the waveguide core by tailoring the grating geometry . Thus, an optimized SWG waveguide with decreased light confinement in the silicon core area could also be employed for the nonlinearity reduction. Additionally, this type of waveguide has been demonstrated to achieve a propagation loss comparable to that of the conventional silicon wire waveguide [16,17]. Combination of these features enables the silicon SWG waveguide to be a potential medium for the chip-scale optical interconnects.
Recently, the optical components based on SWG waveguide encountered intense research efforts in a variety of applications . However, to our best knowledge, the SWG waveguide has not yet been investigated for high-speed data transmission. In this paper, we experimentally study the transmission performance of silicon SWG waveguide for ultrahigh-bandwidth WDM orthogonal frequency-division multiplexing (OFDM) 16-ary quadrature amplitude modulation (16-QAM) signal. The transmission of 2.86 Tb/s aggregate traffic (75 WDM channels, 38.12 Gb/s OFDM 16-QAM) through the 2-mm-long silicon SWG and wire waveguides are demonstrated and compared with the launch powers varying in a wide range. The bit-error-rate (BER) tests for the comprehensive evaluation of transmission performance are conducted. We also demonstrate the WDM transmission through the fabricated 1-mm, 4-mm and 12-mm-long silicon SWG waveguides. Compared to silicon wire waveguides, SWG waveguides exhibit significant tolerance on the optical launch power.
2. Device structure
Generally, SWG waveguides are longitudinally periodic. However, unlike Bragg grating waveguides, no optically resonant behavior occurs at the wavelength of propagating light. A very intriguing feature provided by SWG waveguides is the ability to tailor the effective refractive index by tuning the width, period and duty cycle of gratings. As a result, optical mode properties of the waveguide can be adjusted and optimized for a given application. In our case, mode delocalization away from the silicon core is pursued to achieve nonlinearity reduction. However, it is also important to note that there is an upper limit on the mode delocalization considering the leakage loss to the substrate.
Figure 1(a) shows the schematic diagram of the silicon SWG waveguide, where two tapers are added at each end of the SWG waveguide to enable adiabatic mode conversion between silicon wire and SWG waveguides. Due to the large mode mismatch at the wire to SWG interface, a custom-designed taper structure is required to facilitate smooth and low-loss mode transition [18,19]. The geometrical parameters of the mode converter and SWG waveguide, including wire width ww, taper length L, SWG width wg, grating period Λ and silicon segment length a, are defined in Fig. 1(b). With ww = 400 nm, L = 50 μm, wg = 300 nm, Λ = 300 nm and a = 150 nm, the simulated propagating field along the SWG waveguide is shown in Fig. 1(c) . The grating period Λ = 300 nm is selected to avoid the formation of Bragg reflection . The width of the SWG waveguide wg = 300 nm is chosen as a tradeoff between the light confinement in the silicon core area and the leakage loss to the substrate. Using the MIT photonic bands (MPB) software, the calculated effective index value of the SWG waveguide mode is about 1.586, which is capable of providing a low substrate leakage loss . Figures 1(d) and 1(e) depict the simulated electric field distributions of the SWG waveguide at the center of a silicon segment and the wire waveguide, respectively. As seen, the optical mode is more delocalized from the silicon core in the SWG waveguide. Besides, the SWG waveguide core is discontinuous. Light also propagates in the SU-8 polymer regions between silicon segments which feature a negligible third order nonlinearity [23,24]. The above two aspects lead to the efficient nonlinearity reduction in silicon SWG waveguide. Based on the simulated mode profiles in Figs. 1(d) and 1(e), one can calculate the effective nonlinear coefficients of the SWG waveguide at silicon segment and the wire waveguide, respectively. Following the full vectorial nonlinear analysis in ref , the generalized definition of the effective nonlinear coefficient γ is given by25]26]. We define the normalized optical power asFigures 2(a) and 2(b) show the calculated normalized power distributions and effective nonlinear coefficients of the wire waveguide and SWG waveguide at the silicon segment, respectively. The SWG waveguide exhibits significantly lower power fraction in the silicon region and thus remarkable reduction of the nonlinear coefficient compared to those of the wire waveguide. Additionally, it is important to mention again that the SU-8 segment of the composite SWG waveguide core features negligible nonlinear contribution, which further decreases the effective nonlinearity of the silicon SWG waveguide.
3. Fabrication and experimental setup
The silicon SWG waveguide that we used in the experiment is fabricated on a commercial SOI wafer (SOITEC) with a 220-nm-thick silicon device layer and a 2-μm-thick buried oxide layer. Fabrication is carried out by first using electron-beam lithography (Vistec EBPG 5000 Plus) and induced coupled plasma (ICP) etching to define patterns on silicon device layer and then spin coating a 2.4-μm-thick SU-8 covering layer (n ~1.577 at λ = 1.55 μm). In addition to being the upper cladding, low-index SU-8 polymer material is interleaved with small high-index silicon segments which forms the composite SWG waveguide core. By adjusting the geometrical parameters of subwavelength gratings, the effective index of the composite waveguide core can be engineered. Additionally, the covering material itself is one degree of freedom in waveguide design. Silicon dioxide and air claddings have also been demonstrated for SWG devices [18,27]. Here, SU-8 polymer material is employed considering its fabrication simplicity and device protection ability. Figure 3 shows the SEM images of the fabricated device before SU-8 coating, including SWG waveguide (a) and SWG to wire mode converter (b). The minimum width of the silicon bridging segments in the mode converter structure is about 85 nm. The mode converter loss and SWG waveguide propagation loss are assessed to be 0.45 dB and 4.2 dB/cm. Compared with the experimental results reported previously [16,27], our fabricated SWG waveguide exhibits a higher propagation loss value, which is likely due to the imperfections in the fabrication process, such as the increased roughness of etched surfaces and larger dimensional fluctuations of silicon grating segments.
Figure 4(a) shows the experimental setup for investigating the terabit-scale WDM data transmission through the fabricated silicon SWG waveguides. At the transmitter side, three external cavity lasers (ECLs) at 1544.82, 1549.82 and 1554.84 nm are firstly divided into two sets. Then each set is fed into a phase modulator (PM), which is driven by a strong RF clock signal at frequency of 25 GHz. After passing through the PMs, each ECL is capable of generating 25 optical carriers with 25-GHz frequency spacing. A programmable wavelength selective switch (WSS) is then employed to combine the outputs of PMs and reshape the generated optical carriers. After the spectrum reshaping, the generated 75 optical carriers are amplified by an erbium-doped fiber amplifier (EDFA). The output spectrum shown in Fig. 4(b) confirms that 75 flattened optical carriers are obtained. Then all the generated carriers are modulated by an optical I/Q modulator to carry 38.12 Gb/s OFDM 16-QAM signal. The I/Q RF signal is produced by an arbitrary waveform generator (AWG) running at 12 GS/s. After the I/Q modulator, the generated 2.86 Tb/s WDM OFDM 16-QAM signal shown in Fig. 4(c) is coupled into and out of the fabricated silicon device assisted by vertical grating couplers . The optical signal propagating through the device is sent into a variable optical attenuator (VOA) and then boosted by an EDFA to adjust the optical signal-to-noise ratio (OSNR) before entering the receiver. An optical tunable filter (OTF) selects the desired single wavelength channel from the broadband signal. At the receiver side, another ECL is utilized as the local oscillator (LO) to mix with the received signal in the optical hybrid. The signal is detected by two pairs of balanced detectors. The two RF signals corresponding to the I/Q components are then acquired by a Tektronix real-time oscillator scope at 50 GS/s and processed off-line with a Matlab program.
4. Experimental results
To evaluate the data transmission performance of a 2-mm-long silicon SWG waveguide for an ultrahigh-bandwidth 2.86 Tb/s WDM data stream, we first conduct a BER versus launch power measurement to determine the optimum operating point. The carrier channel at 1549.82 nm is selected for measurement in the case of WDM transmission. As seen in Fig. 5(a), the best BER performance (about 5e-6) is achieved at the optimum launch power of 11.5 dBm for the silicon SWG waveguide. Here, the launch power value represents the total optical power of all 75 channels. Figure 5(b) shows the corresponding RF spectrum of the received OFDM 16-QAM signal after demodulation. Data transmission through a silicon wire waveguide with identical length and 400 nm width is also experimentally investigated for comparison. The results show that for the wire waveguide, the BER reaches a minimum value of 1.2e-5 at 3.5 dBm launch power and then dramatically increases due to the nonlinear impairments. An 8-dB improvement in optimum launch power when using SWG waveguide is ascribed to its efficient nonlinearity reduction compared to the wire waveguide. In the low launch power region, where nonlinear effects are not severe and benefit provided by SWG waveguide is negligible, the two types of silicon waveguides exhibit similar BER performances.
With the launch power fixed at the optimum operating point, we then carry out the BER measurement for all 75 WDM channels through the fabricated 2-mm-long SWG waveguide. The result is shown in Fig. 6(a), from which one can observe that all 75 channels achieve BER values less than 4e-5, indicating the suitability of the SWG waveguide for ultrahigh-bandwidth WDM transmission. Figure 6(b) shows the measured output spectrum after the SWG waveguide transmission. Next, we study the BER performance as a function of received OSNR for three wavelength channels at 1544.82, 1549.82 and 1554.84 nm. The BER curves shown in Fig. 6(c) exhibit almost negligible differences between these WDM channels. As we can see, the required OSNR for the BER level of 1e-3 is around 15.5 dB.
Finally, we examine the BER performances for the 2.86 Tb/s WDM signal transmission through four silicon SWG waveguides with different lengths (1 mm, 2 mm, 4 mm and 12 mm) at 11.5 dBm launch power. Single channel measurement for the optical carrier at 1549.82 nm is conducted. From the experimental results shown in Fig. 7(a), one can observe that the OSNR sensitivities to achieve BER of 1e-3 are about 15.27, 15.47, 16.66 and 20.38 dB for the data transmission through four SWG waveguides, respectively. Figure 7(b) shows the measured constellations for the four SWG waveguides around the BER level of 1e-3.
Although efficient nonlinearity reduction can be realized in the SWG waveguide, the small part of guided mode existing in the silicon region still contributes to the nonlinear interactions. The observed increment in OSNR penalty under a long SWG waveguide is most likely attributed to the transmission impairments induced by optical nonlinearity. However, it should be noted that the signal transmission performance through the longest SWG waveguide (12 mm) at 11.5 dBm launch power is still superior to that of the 2-mm-long wire waveguide, which can be observed from the Fig. 5(a) and Fig. 7(a), indicating the benefit of silicon SWG waveguide as the medium for optical interconnects. In addition to decreased mode confinement in the silicon core region, the reduced free-carrier lifetime might contribute to the subdued nonlinear distortion in silicon SWG waveguide. The presence of interface states at increased etched surfaces accelerates the recombination of free carriers . Shortening of carrier lifetime is profitable for alleviating the free-carrier absorption (FCA) and dispersion (FCD) effects [30,31].
For the silicon SWG waveguides, one possible concern is the fabrication with the facilities in a CMOS line. Recent report demonstrates the reliable fabrication of SWG waveguides by using 193 nm stepper lithography . Based on the subwavelength gratings, extremely low-loss waveguide crossings and broadband power splitters with wavelength-flattened response have been experimentally demonstrated, which are capable of facilitating the chip-scale optical interconnects based on the SWG waveguides [19,33].
In summary, we experimentally investigate ultrahigh-bandwidth terabit-scale data transmission performance through silicon SWG waveguides. Using the 2.86 Tb/s optical signal composed of 75 WDM channels each carrying 38.12 Gb/s OFDM 16-QAM signal, we demonstrate the data transmission through the fabricated 2-mm-long SWG and wire waveguides. Compared to the wire waveguide, an 8 dB higher launch power is achieved in the SWG waveguide, which indicates higher nonlinearity tolerance. With the optimum launch power, BER values less than 4e-5 are acquired for all 75 channels after SWG waveguide transmission. We also evaluate the signal transmission performance through four silicon SWG waveguides with different lengths (1 mm, 2 mm, 4 mm and 12 mm). To achieve the BER level of 1e-3, the required OSNRs are about 15.27, 15.47, 16.66 and 20.38 dB, respectively. The experimental results show that silicon SWG waveguide could be a promising enabler of chip-scale optical interconnects.
National Natural Science Foundation of China (NSFC) (61335002); Major State Basic Research Development Program of China (2013CB632104, 2013CB933303); National High Technology Research and Development Program of China (2015AA016904); 2015 Key Projects of Natural Science Foundation of Hubei Province (2015CFA056).
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