Efficient 1.55 and 2 μm Dual‐Band SOI Grating Coupler for Light Coupling and On‐Chip Wavelength Division (De)multiplexing

Efficient fiber‐chip couplers operating at distinct wavelength bands are key components to combine or split different optical bands for emerging data transmission and nonlinear applications. Herein, a dual‐band silicon‐integrated grating coupler (GC) operated at 1.55 and 2 μm wavebands is designed and demonstrated. The proposed device can simultaneously couple 1.55 and 2 μm wavebands light into the in‐plane waveguides at the same incident angle. Numerical simulations indicate that coupling efficiencies (CEs) are −2.5 and −3.9 dB for center wavelengths at 1561 and 1979 nm, respectively. The dual‐band GC is experimentally demonstrated on a commercially available 340 nm silicon‐on‐insulator wafer. The fabricated dual‐band GC with center wavelengths of 1559 and 1968 nm obtains CEs of −4.9 and −6.4 dB, with 3 dB bandwidths of 81 and 80.4 nm, respectively. Also, the first proof‐of‐concept demonstration of 10 Gb s−1 wavelength division multiplexing transmission at 1.55 and 2 μm waveband is presented based on the fabricated dual‐band GC.

which hinders its practical applications. Although high-speed WDM data transmissions have been successfully demonstrated at telecom band [23] and 2 μm waveband [24] based on high-efficient Mach-Zehnder interferometer or arrayed waveguide grating structure-based devices separately, on-chip multiplexing and demultiplexing of optical signals of multi-wavelength bands could further improve the transmission capacity and therefore requiring integrated devices including dual-band GC operating in both telecom band and 2 μm wavebands.
In this work, we propose and fabricate a compact dual-band GC with center wavelengths located in the gain windows of erbium-and thulium-doped amplifiers (1.55 and 2 μm waveband), facilitating WDM data transmission applications in the future. The device is realized via carefully optimizing the structure with the particle swarm algorithm in terms of compactness, wide optical bandwidth, and high fabrication tolerances. The dual-band GC is fabricated on a commercially available 340 nm silicon-on-insulator (SOI) platform with simple 1D grating structure and requires single-etch step. In the experiments, the coupling efficiencies (CEs) and operation bandwidth of the dual-band GC are measured over a wide wavelength range at the designed operation waveband. Furthermore, we demonstrate for the first time that such a dual-band GC is able to demultiplex 10 Gbit s À1 nonreturn-to-zero on-off keying optical signals centered at 1560 and 2000 nm with negligible power penalties (0.5 and 1.5 dB, respectively) at a bit error rate (BER) of 3.8 Â 10 À3 . These results pave the way toward more promising feasibility of exploiting the new 2 μm spectral window.

Design and Simulation
The schematic view of the device is illustrated in Figure 1. The dual-band GC is designed for transverse-electric (TE) polarization. As a demultiplexer, 1.55 and 2 μm waveband light are simultaneously launched from a tilted single-mode fiber (SMF) and separated into opposite directions. The 1.55 and 2 μm waveband output lights are collected using two single-band GCs, respectively. On the contrary, when the dual-band GC operates as a multiplexer, light is separately coupled into the integrated waveguides via a 1.55 μm waveband GC and a 2 μm waveband GC, then combined and simultaneously coupled out into an SMF. Figure 2 shows a 2D top view and side view of our proposed dual-band GC. Generally, to implement a single GC that couples light into the waveguide in two separate spectrum ranges at the same time, a set of parameters, such as the grating pitch Λ, etch depth h e , duty cycle DC ¼ a=Λ, and coupling angle θ, should be found to meet the phase-matching conditions for two designed wavelength bands. For the 1.55 μm waveband, light with center wavelength λ 1 is coupled into the right waveguide at a positive diffraction angle θ, while for the 2 μm waveband, the light of center wavelength λ 2 is coupled into the left waveguide at a negative diffraction angle Àθ, as depicted in Figure 2b, which is illustrated in ref. [34].
According to the first-order Bragg condition, the operation principle of the GC relying on the intrinsic change of radiation angle with the wavelengths λ 1 and λ 2 can be expressed as where n ef f1 and n ef f2 is the effective index of the fundamental Floquet-Bloch mode supported by the periodic structure at wavelengths λ 1 and λ 2 , respectively. n c is the refractive index of the top silica cladding, θ is the coupling angle in the top silica cladding. By combining these phase-matching conditions in Equations (1) and (2) and eliminating the grating pitch Λ, the relationship of the two allowed wavelengths can be obtained as According to the previously provided equations, the grating pitch Λ, etch depth h e , duty cycle DC ¼ a=Λ, and coupling angle θ can be determined through 2D-finite difference time domain (FDTD) calculations. First, two center wavelengths (λ 1 ¼ 1550 nm and λ 2 ¼ 2000 nm) are chosen for the dual-band GC, requiring phase-matching conditions to be satisfied over a large wavelength interval. To maximize the overlap between the fundamental mode of the SMF and the radiated fields, the width and length of the   grating are set to W GC ¼ 15 μm and L GC ¼ 10 μm, respectively. Linear tapered waveguides with length L taper ¼ 300 μm are selected for adiabatic evolution of mode from grating to singlemode waveguides. The width of the single-mode waveguide is 500 and 600 nm for 1.55 and 2 μm waveband, respectively. A figure of merit (FOM), defined as CEðλ 1 Þ ⋅ CEðλ 2 Þ, is used to describe the CEs for two wavelength bands, which was previously used for designing dual-band GCs. [35,36] Here, CEs of two target wavelengths are set as the primary optimization factor, and other parameters, such as crosstalk (CT) and back reflections, are subsequently investigated using the determined parameters of the dual-band GC. Using particle swarm optimizer (PSO) in Lumerical software, a maximum FOM of 0.3 is obtained. A total of 600 iterations are performed via PSO, where the maximum generations and the generation size are 60 and 10, respectively. Then, the parameters of the dual-band GC are set as Λ ¼ 637 nm, h e ¼ 181 nm, DC = 0.618, and θ ¼ 18.48°(corresponding to an incidence angle θ ¼ 27.15°i n the air). The calculated CEs are À2.5 and À2.7 dB for center wavelengths at 1551 and 1997 nm, respectively. However, due to the available coupling angle provided in our coupling system, we chose to keep the coupling angle to 15°(corresponding to an incident angle θ air ¼ 21.88°) for the following experimental verification. As shown in Figure 3a,b, a maximum FOM of 0.22 is obtained when the grating pitch is Λ ¼ 716 nm with DC = 0.2 and etch depth is h e ¼ 201 nm. It is worth noting that when the maximum FOM is achieved, the dual-band GC is operating slightly off the peak efficiencies of single-band operation, introducing very tiny deductions (0.1 dB for 1.55 μm waveband and 0.5 dB for 2 μm waveband) of CEs. In this instance, the minimum feature size of the designed GC is 143 nm, which can be fabricated through electron beam lithography (EBL) and is also compatible with 193 nm deep-UV optical lithography for massive fabrication. Figure 3c shows the calculated CEs and bandwidth of the coupler. The CEs are as high as À2.5 and À3.9 dB at the designed center wavelengths of 1561 and 1979 nm, respectively. In addition, the 3 dB bandwidth of 100 nm is achieved for the 1.55 μm waveband channel, whereas 126 nm is for the 2 μm waveband channel. The simulated CEs at the opposite grating port are below À20 dB for the whole operating window from 1400 to 2100 nm, and can be further suppressed by introducing the subwavelength grating structure [37] or additional Bragg gratings. [38] Moreover, the fabrication tolerances are investigated via calculating the CEs with variations in the grating pitch, DC, and etch depth. As can be seen from Figure 4, the CEs of the 2 μm waveband are sensitive to the change of DCs and the maximum CE will decrease. In addition, there is a CE penalty of 0.6 dB in addition to wavelength shift if the etch depth is changed from 191 to 211 nm. The center wavelengths of the GC exhibit a slight shift but the maximum CEs remain almost the same if there is a 20 nm variation in the grating pitch. Table 1 details the parameters of the dual-band GC and the two single-band reference GCs. Simulation details about the back reflections of the designed dual-band grating coupler (DBGC) are reported in Figure S1, Supporting Information. The back reflection can be further reduced by the continuous apodization of period and the filling factor [39] or by using the sub-wavelength structure. [37] 3

. Fabrication and Results
The dual-band GC is fabricated on a commercial SOI wafer with a buried oxide layer thickness of 2 μm and a top silicon layer thickness of 340 nm. The device layout is patterned by 100 keV EBL www.advancedsciencenews.com www.adpr-journal.com (Vistec EBPG 5000plus ES). As the etch depth is uniform for the entire device, one-step etching process is employed and subsequently a 1 μm thick silicon dioxide film is deposited using plasma-enhanced chemical vapor deposition (PECVD) to form the top silica cladding. The structural parameters of the dual-band GC are characterized with a metallurgical microscope and scanning electron microscope (SEM), as shown in Figure 5a,b. The dual-band GC is connected to two GCs, each optimized for either 1.55 μm waveband or 2 μm waveband operation, as illustrated in Figure 5c,d. A continuous-wave tunable laser (Santec TSL-710) is employed to sweep the 1480-1640 nm wavelength range and a home-built 2 μm amplified spontaneous emission (ASE) laser is used to cover 1920-2040 nm. The vertical coupling test system (OMTOOLS FA-H201M-N80) is used to input and output light to the device to be tested. The light is coupled into the chip via the dual-band GC and collected from two single-band GCs with the corresponding wavelength. The CEs of the device are characterized as CE ¼ P out À P in À CE ref , where P out is out-coupled power collected from 1.55 μm waveband or 2 μm waveband GC, P in is the input optical power, and CE ref refers to the CE of 1.55 μm waveband or 2 μm waveband reference GC. To measure the CT of the dual-band GC, an identical device was fabricated with the output 1.55 μm waveband or 2 μm waveband GC swapped. The CTs of the device are characterized as CT ¼ P out2 À P in À CE ref , where P out2 is out-coupled power collected from the non-corresponding 1.55 μm waveband or 2 μm waveband GC. The optimal input fiber tilted angle is around 22°, which is almost the same as  the designed incident angle. During the measurement, the tilted angle and position of the input SMF are kept the same for the two wavelength bands. A polarization controller (PC) is used to adjust TE polarization state. Figure 6 shows the measured CEs for 1.55 μm waveband and 2 μm waveband. As can be seen from the figure, a peak CE of À4.9 dB with a center wavelength of 1559 nm is measured and a CE of À6.4 dB with a center wavelength of 1968 nm is also recorded. The ripples on the measured CEs origin from the Fabry-Pérot effect caused by the backward reflection. When comparing with the simulations, there is a slight blueshift of the center wavelengths which we attribute to a small over-etching during the fabrication. The CT is below À20 dB at the central wavelengths of 1559 and 1968 nm, respectively. The measured CTs are smaller than the simulation results, particularly for the 1550 nm waveband, which we attribute to the normalization error due to the variability of the fabricated GCs. In addition, the measured 3-dB bandwidths are % 81 nm for 1.55 μm waveband and 80.4 nm for 2 μm waveband, respectively. Table 2 summarizes the performance of the demonstrated dual-band GCs. Compared with those GCs in previous works, our dual-band GC exhibits improved operation bandwidths and favorable CEs at both wavelength windows. The improved operation bandwidths can be attributed to the use of a thicker 340 nm SOI wafer and larger tilt angle, which can reduce effective index dispersion and increase the CE at nontargeted wavelengths. Further improvement in the operation bandwidths can be achieved by using a subwavelength grating structure or increasing the numerical aperture, as has also been demonstrated for singleband GCs. [40][41][42][43] Furthermore, this work represents the first demonstration of dual-band GC with large wavelength separation (defined as the difference between the central wavelengths of the two bands), which is capable of operating in the conventional   www.advancedsciencenews.com www.adpr-journal.com telecom band and the gain window of TDFA in the emerging 2 μm waveband. Therefore, the dual-band GC demonstrated here shows great promise as an efficient and low-cost fiber-chip coupling interface for future on-chip WDM applications. Although dual-band GC with larger wavelength separation can be obtained using SWG structures, [36] very few gain material is available at its long wavelength band which might introduce additional difficulties in signal amplification for practical applications.
To verify the performance of the fabricated dual-band GC for WDM transmission at 1.55 and 2 μm waveband, a proof-of-concept demonstration is performed using the experimental setup exhibited in Figure 7. For 1.55 μm waveband channel, light from an external cavity laser centered at 1560 nm is modulated externally by a C-band commercial intensity modulator (Fujitsu FTM7937). An erbium-doped fiber amplifier is used to amplify the modulated signal. The 1.55 μm light is connected to CH1 of the WDM, and the optical power is controlled by a variable optical attenuator (VOA). The 2 μm transmitter is based on a directly modulated semiconductor laser (EP2000-DM-HAA). The insertion loss of the dual-band GC is increased to 9.3 dB near 2000 nm, and therefore, a high-power TDFA (HP-TDFA) is used after a preamplifier to compensate for the additional loss. The 2 μm light is connected to CH2 of the WDM, and the optical power is controlled by another VOA. The combined light is then injected into and coupled out the chip by carefully adjusting the incident angle of the SMFs. At the receiving end, an optical coupler (99:1 or 90:10) is used to split the light for power measurement and signal detection. As a free space coupled 2 μm PD (EOT ET-5000) is used here, a pair of lenses (L1 and L2) are employed to collimate and focus the light into it, which introduces % 3dB loss. An arbitrary waveform generator (Tektronix AWG 70 002) is used to generate a 10 Gbit s À1 electrical on-off keying (OOK) signal, and the recovered electronic signal is analyzed via a real-time sampling oscilloscope (Keysight DSA-Z 204A) operating at 80 GSa s À1 with a bandwidth of 20 GHz. Figure 8a,b shows the measured spectra of unmodulated and modulated light at around 1560 and 2000 nm. One can clearly see that the modulated optical spectra of both wavelengths exhibit a wider bandwidth, indicating successful signal modulation. The sidebands in Figure 8b origins from the Fabry-Pérot based 2 μm laser diode. The BER curves are measured as a function of the received optical power (1560 nm back to back, CH1, 2000 nm back to back, CH2), as plotted in Figure 8c,d. The received optical power is reduced by VOA until reaching the BER of 3.8 Â 10 À3 (7% hard-decision forward error correction [HD-FEC] threshold). As shown in Figure 8d,f, clear and open eyes are obtained for both wavelengths, which suggests this dual-band GC can successfully multiplex the 1.55 and 2 μm waveband signals. The 1.55 μm waveband channel shows a negligible 0.5 dB power penalty while the power penalty of the 2 μm waveband channel increases to 1.5 dB at the 7% HD-FEC threshold. Due to the additional noises introduced by the HP-TDFA, a larger power penalty and slightly deteriorated eye diagram are obtained for the 2 μm waveband. The minimum optical power for zero BER operation of the 2 μm waveband is around 11 dB higher than that of 1.55 μm, which we attribute to the lower sensitivity of the 2 μm PD. It is worth noting that 10 Gb s À1 OOK signal used in our experiment is constrained by the bandwidth of the available 2 μm directly modulated lasers. We anticipate that successful high-speed data transmission of more advanced signals could be demonstrated in our dual-band GC with reduced insertion loss. Moreover, the CEs of the dual-band GC could be improved by optimizing the etching depth, increasing the thickness of the silicon layer, and introducing a complex apodized period.
Although the 1.55 and 2 μm communication system can now be implemented by using the components mentioned earlier, its transmission performance is still limited by the immature 2 μm components. It is believed that the performance including data transmission rate and capacity can be further enhanced by optimizing essential optoelectronic devices such as a directly modulated laser, TDFA, and PD. [44] Moreover, the dual-band GC demonstrated in this work can be integrated with dense WDM devices at both 1.55 and 2 μm waveband, and paves the way for large-capacity optical fiber communications at multi-wavebands. Figure 7. The experimental setup of 10 Gbit s À1 fiber-chip transmission using the dual-band GC device at 1.55 and 2 μm waveband. AWG, arbitrary waveform generators; RF Amp, radio frequency amplifier; IM, intensity modulator; VOA, variable optical attenuator; WDM, wavelength division multiplexer; PC, polarization controller; OPM, optical power meter; PD, photodetector; HP-TDFA, high-power thulium-doped fiber amplifier; DSA, digital signal analyzer.

Conclusion
In conclusion, we propose and experimentally demonstrate a dual-band GC that can be operated as a (de)multiplexer at 1.55 μm waveband and the emerging 2 μm waveband on 340 nm SOI platform. Our device is optimized via a particle swarm algorithm and only requires one shallow etching step. Peak CEs of À4.9 and À6.4 dB are measured with center wavelengths of 1559 and 1968 nm, respectively. In addition, the designed dual-band GC implements a large 3-dB bandwidth with 81 nm for 1.55 μm waveband, whereas 80.4 nm for 2 μm waveband. The experiment results are in good agreement with 2D-FDTD simulations and the performance of the device can be further enhanced with apodization. Moreover, we experimentally verify the performance of our device for successful data transmission at 1.55 μm waveband and 2 μm waveband. These results open the potential of employing WDM technology with ultrabroad wavelength separation in emerging mid-infrared applications such as on-chip optical interconnects and optical data processing.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.
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