1. Introduction

Driven by the exponential growth of the emerging big data applications and cloud services, there has been significant demand to increase the transmission capacity of current optical communication systems. Although wavelength, mode, and polarization multiplexing technology have been used to increase data capacity at conventional telecom band [1–4], exploiting new communication windows is also a straightforward and effective way to increase the communication capacity. Recently, the 2 µm wavelength band is attracting great interest for next-generation data communications since the realization of low-loss hollow-core fibers (HCFs) [5]. Besides, it is worthy to note that the thulium-doped fiber amplifier provides over 240 nm gain window in the range of 1810-2050nm [6], which can support more wavelength channels than the 1.55 µm communication band. Up to now, high-speed fiber and on-chip communication systems have been demonstrated experimentally using the proposed HCFs and integrated devices [7–11]. For an example, WDM transmission of eight-channel 100-Gbit/s at 2 µm over both 1.15 km of low-loss HC-PBGF and 1 km solid core fiber (SCF) has been demonstrated for the first time [7]. In the same year, dense WDM transmissions (8×20 Gbit/s) at 2 µm have been demonstrated based on an InP arrayed waveguide grating (AWG) [8]. In addition, a low-latency HCF short-reach optical interconnection at 2 µm is experimentally achieved with a single-lane speed of 100 Gbit/s [9]. Subsequently, the first pulse-amplitude modulation (PAM) transmission in the 2-µm band has been realized by successfully transmitting 32 Gbaud PAM-8 and 25 Gbaud PAM-16 signals through low-loss HCFs [10]. Besides, 2-µm-band coherent transmission of a Nyquist-WDM signal by on-chip spectral translation using AlGaAs-on-insulator nanowaveguides is achieved with impressive performance [11].

In the meantime, the mature silicon platform remains the distinct advantages of small footprint, high density, low loss, and reduced power consumption with improved stability at the 2-µm band. Recently, great success has been achieved for the development of 2-µm optical components on photonic integrated platform, such as laser [12], modulators [13–17], photodetectors [18,19], low-loss silicon photonic waveguides, grating couplers [20], polarization beam splitter (PBS) [21], power splitter [22], switches [23–25], filters [26], and mode multiplexers [27]. Particularly, several silicon arrayed waveguide gratings (AWGs) have been proposed and demonstrated as WDM (de)multiplexers at 2 µm [8–30]. These AWG-based WDM (de)multiplexers could provide low crosstalk, but suffer from relatively large loss. Alternatively, another promising approach for WDM (de)multiplexer is using cascaded Mach-Zehnder interferometers (MZIs). During the past few years, MZI-based WDM (de)multiplexer with low excess loss and crosstalk has been studied extensively for the O- and C-band [31–40]. It is known that the performances of silicon filters could be improved at longer wavelengths, at which the silicon waveguides are less sensitive to sidewall roughness and fabrication errors. However, high-performance MZI-based WDM (de)multiplexers have yet to be demonstrated at 2 µm, and it is highly desired for future on-chip WDM applications at 2 µm [8]. Hence, we focus on the realization of a flat-top WDM (de)multiplexer based on cascaded MZIs which could be a key element for practical 2 µm WDM systems.

In this work, we experimentally demonstrate a 1 to 8 silicon photonic WDM (de)multiplexer based on cascaded MZIs at the 2 µm waveband. The designed WDM (de)multiplexer has a channel spacing of 3.2 nm, ranging from 1955nm to 1985nm. As a trade-off between the filter shape factor and the circuit complexity, a three-stage-coupler scheme is utilized to provide flat-top passbands and reduce channel crosstalk. The TiN microheaters are exploited to control the spectra by compensating the waveguides’ phase errors. The fabrication of the device is realized by the standard silicon photonic process offered by commercial foundries. The fabricated device exhibits low insertion loss (IL< 0.9 dB) and low channel crosstalk (XT< -20.65 dB). It enables the possibility of photonic integrated circuits for WDM applications in next-generation optical communications and interconnects.

2. Device design

An illustration of the wavelength-carrier (de)multiplexing process in the WDM is shown in Fig. 1(a). It contains two main components: multiplexer (MUX) and DEMUX (multiplexer). Figure 1(b) illustrates the schematic diagram of the 8-channel (de)multiplexers with flat passbands. In the design of the WDM (de)multiplexer for 2 µm band, we select 8 channels for (de)multiplexing with a channel spacing of approximately 240 GHz (3.2 nm). In order to obtain the desired filter spectrum, three-stages binary trees are used in the structure, consist of 7 thermo-optic MZIs. The 11 relatively long delay lines are capable of accurate phase control. Every stage can deinterleave adjacent input wavelength bands to different output ports. Specifically, the 1st filter distributes light of odd and even numbering wavelengths to 2nd_1 and 2nd_2 filters. Then, 2nd_1 and 2nd_2 separate the wavelength of λ₁ and λ₅, λ₇ and λ₃, λ₂ and λ₆, λ₈ and λ₄ to 3rd_1, 3rd_2, 3rd_3, 3rd_4 filters, respectively. Finally, the 3rd filters achieve all 8-channel wavelength (de)multiplexing. In order to obtain flat passbands, the 1st filter is designed to have three serially cascaded MZIs. The shape of the transmission passband in the 1st filter plays a decisive role in the final shapes at the 8 outputs, the 2nd filter unit contains two MZIs and the last filter unit contains only one MZIs to simplify the entire design.

 

Fig. 1. (a) An illustration of the wavelength-carrier (de)multiplexing process in the WDM. (b) Schematic layout of 8 wavelength channels (de)multiplexer with flat passbands based on a binary tree of wavelength splitting filters.

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Five different power cross-coupling coefficients (κ₁/κ₂/κ₃/κ₄/κ₅= 0.5/0.29/0.2/0.08/0.04) are used in the power splitting ratios of directional couplers (DCs) to flatten transmission passbands, which was widely used in previous MZIs-based WDM (de)multiplexer [31]. Compared with multimode interference (MMI) couplers, DCs have lower excess loss [34]. The DCs are designed via 3D finite-different-time-domain (FDTD) method. In the simulations, two coupled 650-nm-wide strip waveguides with a gap of 200 nm and the bending radius of 15 µm leading waveguides are used to support the fundamental TE mode at the designed wavelength range. First, we analyze the variation of coupling ratio with the change of coupling length at the wavelength of 1970nm, which is shown in Fig. 2(a). The simulation result shows that the corresponding coupling lengths of DC are 9.37/6.17/4.46/1.76/0.58 µm for the chosen power cross-coupling coefficients. Then, simulated coupling ratios of the DC are plotted as a function of wavelength in Fig. 2(b). As can be seen from the figure, the coupling ratio varies slowly in the whole operating wavelength range.

 

Fig. 2. Simulated coupling coefficients as a function of (a) length and (b) wavelength.

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The theoretical transmission spectra of three splitters are calculated and shown in Fig. 3, from which one can see the shape of the transmission passbands evolve from a standard sinusoidal to a flat spectrum with two and three delay stages. The delay waveguide length imbalances ΔL in Fig. 3 is formed at:

 

Fig. 3. Calculated filtering curves of (a) 1st (b) 2rd and (c) 3nd splitters.

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Table 1. Parameters for the calculation of the delay line lengths of the Mach-Zehnder wavelength filters

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The transmitted spectra of the (de)multiplexers in Fig. 1 (b) are calculated using the transfer matrix method. The simulation parameters are set to Δλ = 3.2 nm, n_g = 4 and n_eff = 2.33. Note that 11 delay lines should be carefully selected to improve the device performance, as provided in Table 1. The calculated results are shown in Fig. 4. The transmitted spectra exhibit the expected 8 flat passbands with < -26.5 dB the channel crosstalk for all channels, thanks to suitable delay waveguide lengths. Therefore, this designed WDM filter could meet the requirements of error-free communication in practical applications in point-to-point optical links, Ethernet, and data center, which require the crosstalk of < -15 dB [38].

 

Fig. 4. Calculated filtering curves of the 8-channel (de)multiplexers with flat passbands.

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3. Fabrication and measurement

The designed device was fabricated by utilizing the commercial silicon photonics multi-project wafer (MPW) service using 248 nm deep-ultraviolet (DUV) lithography with 180 nm node size. Standard lithography and etch processes are used to fabricate the device on an SOI wafer with 220 nm top silicon and 2 µm buried oxide layer. The width of strip waveguides is set to 650 nm to fulfill the single-mode operation condition. The top-view microscope images of the device are shown in Fig. 5 (a). Light is coupled into the device using the 4th grating on the left and collected and measured using the 8 grating couplers on the right. The front 11 pads will apply different voltages from left to right, and the 12th pad is connected to the ground. The footprint of the 1 to 8 (de)multiplexers is only 0.5×0.9 mm² (without the electrodes), respectively. The DCs are shown in Fig. 5 (b). It is known that phase errors are inevitable for practical silicon waveguides and thus 11 TiN heaters with a length of 100 µm on top of delay line waveguides are added to accurately control phase shifts. The heaters are connected to the pads by metal aluminum. Figure 5 (c) shows the zoomed-in image of the TiN heater. In the experiment, the average resistance is measured to be about 248 Ω.

 

Fig. 5. (a)Top-view microscope images of the 8-channel (de)multiplexers with flat passbands. (b) Zoomed-in image of the DC. (c) Zoomed-in image of the TiN microheater. (d) Micrograph of a grating coupler for the 2 µm wave band. (e) Measured 2 µm grating coupling efficiency as a function of wavelength.

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The experimental setup consists of a 2-µm broadband light source, fiber-chip coupling stages, and an optical spectrum analyzer (OSA, Yokogawa AQ6375B) to measure the transmission spectra of the device. Using the multi-channel current source (HAMEG HMP4040) and 16 array probes with a 150 µm cycle, we can independently adjust the heating power on each microheater to compensate the phase errors of 11 delay waveguides. The fiber grating couplers are used to couple light into and out of the chip, as shown in Fig. 5(d). The grating coupler has a period of 926 nm and a duty cycle of 0.5 with an etch depth of 70 nm. As shown in Fig. 5(e), the coupling efficiency is measured to be ∼7 dB/facet. In the experiment, the temperature of the chip is kept at 20 °C by TEC to ensure the stability of the test.

Due to the considerable phase errors accumulated inside the silicon waveguides, voltages applied on the 11 thermo-optic microheaters are manually tuned to compensate the phase errors for spectra controlling according to the output spectra. For MZI-based filters with more channels, phase recovery algorithm based on fast and accurate phase control can be used to ease the processes [34]. The power consumption of the microheaters is summarized in Table 2. The total power consumption of 11 microheaters is 196.4 mW.

Table 2. Power consumption of 11 heaters

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The normalized transmission spectra at 8 output ports of the device are shown in Fig. 6. We note that the transmission spectra of device without phase correction are chaotic and have narrow pass-band width, low channel crosstalk due to the fabrication errors. Using thermal phase shifters, top-flat transmission spectra can be obtained. Compared to the simulations, the measured spectra show that the transmission channels of this device are slightly shifted about 1.5 nm toward the shorter wavelengths. The wavelength channels for a higher order appear on the right side of the spectra, which is marked by “λ1⁺¹”. The measured ILs and XTs are shown in Table 3. Measured ILs of 8 channels are 0.84 dB, 0.78 dB, 0.7 dB, 0.68 dB, 0.8 dB, 0.89 dB, 0.72 dB and 0.8 dB, respectively. The XTs are measured to be XT1 < -22.65 dB, XT2 < -21.95 dB, XT3 < -21.44 dB, XT4 < -20.65 dB, XT5 < -21.9 dB, XT6 < -21.34 dB, XT7 < -21.93 dB, XT8 < -21.54 dB, respectively. Overall, the worst channel crosstalk of −20.65 dB occurs at the λ₄ channel. At the extinction ratio level of 15 dB, the width of the stop-band of the suppressed channels is more than 1.3 nm. The 1-dB and-3 dB bandwidths of the passbands are about 2.3 nm and 3 nm, respectively. Compared to the simulated results, the measured ILs and bandwidth only exhibit slightly degradation, which suggests that moderate waveguide propagation loss and phase errors are introduced during the fabrication processes.

 

Fig. 6. Measured transmission spectra of the 8-channel flat passbands (de)multiplexer (a) without phase correction (b) after phase correction.

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Table 3. ILs and XLs of 8 wavelength channels (de)multiplexer

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In Table 4, we summarize the performances of the reported demultiplexers at the 2 µm wavelength band. Although the first demonstration of WDM data transmission is successfully achieved using AWG on the InP platform, the IL of the WDM device is 12.8 dB [8]. Based on SOI platform with 500 nm thick Si layer, the IL is reduced to 2.4 dB [28]. Subsequently, using standard fabrication on 220 nm SOI in a commercial foundry, high-performance 64 channels AWG has recently been demonstrated with IL < 5 dB and XT < -10 dB [30]. Compared with these previously reported (de)multiplexers at 2 µm, our devices could provide lower IL (< 0.9 dB) and comparable XT (< -20.6 dB). For this MZI-based filter, it is possible to reduce the crosstalk by introducing fabrication-tolerant DCs to eliminate the phase error caused by fabrication errors. The channel number could be further improved by cascading more stage couplers, and the channel spacing could be designed by tailoring the delay waveguide length imbalances. In addition, the phase error correction and wavelength tuning could be achieved by applying additional voltages to the phase shifters on the delayed arms [34,35].

Table 4. Performance comparison of (de)multiplexers at 2 µm band.^a

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4. Conclusion

In conclusion, we have experimentally demonstrated an 8-channel WDM (de)multiplexer based on cascaded thermos-optic MZIs for the 2 µm waveband. Two and three delay lines are introduced to wavelength splitters to achieve flat transmission pass-bands and strong suppression of the drop channels. The measurement results show insertion loss lower than 0.9 dB, channel crosstalk lower than -20.6 dB, and 1-dB bandwidth of 2.3 nm. It is believed that the demonstrated device can find potential applications for next-generation low-cost and power-efficient optical communication networks at the emerging 2 µm waveband.

Appendix: fabrication tolerance analysis for the directional couplers

Figure 7 shows the simulated variations of coupling coefficients with waveguide-width deviation (ΔW=±15nm). Coupling ratios can vary slowly with the waveguide width errors in the fabrication. Furthermore, fabrication-tolerant DCs can be used in our device to further improve performance using bent DCs novel two-stage-coupler scheme [40,41].

 

Fig. 7. Simulated variations of coupling coefficients with different waveguide width (a) κ₁ = 0.5 (b) κ₂ = 0.29 (c) κ₃ = 0.2 (d) κ₄ = 0.08 (e) κ₅ = 0.04.

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Funding

National Natural Science Foundation of China (62175080, 51874301, 62105028); Key Research and Development Program of Hubei Province (2021BAA005); China Postdoctoral Science Foundation (2021M690390).

Disclosures

The authors declare no conflicts of interest.

Data availability

Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.

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