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Abstract
We present the design and performances of a broadband 1 × 2 mode-independent thermo-optic (TO) switch based on Mach-Zehnder interferometer (MZI) with multimode interferometer (MMI). The MZI adopts a Y-branch structure as the 3-dB power splitter and a MMI as the coupler, which are designed to be insensitive to the guided modes. By optimizing the structural parameters of the waveguides, mode-independent transmission and switching functions for E11 and E12 modes can be implemented in the C + L band, and the mode content of the outputs is the same as the mode content of the inputs. We proved the working principle of our design based on polymer platform, which was fabricated by using ultraviolet lithography and wet-etching methods. The transmission characteristics for E11 and E12 modes were also analyzed. With the driving power of 5.9 mW, the measured extinction ratios of the switch for E11 and E12 modes are larger than 13.3 dB and 13.1 dB, respectively, over a wavelength range of 1530 nm to 1610 nm. The insertion losses of the device are 11.7 dB and 14.2 dB for E11 and E12 modes, respectively, at 1550 nm wavelength. The switching times of the device are less than 840 µs. The presented mode-independent switch can be applied in reconfigurable mode-division multiplexing systems.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
With the continuous development of information technologies such as 5 G, big data and cloud computing, the transmission capacity required for communication systems is also larger than before, and the conventional electrical interconnect technology has been unable to meet the rapidly increasing data processing speed and capacity demands. In order to increase the data transmission capacity, optical communication with various signal multiplexing methods have been developed, such as wavelength division multiplexing [1–3], time division multiplexing [4,5], polarization division multiplexing [6–8], and mode-division multiplexing (MDM) [9,10]. Among them, MDM technology has attracted increasing attention since it can expand the capacity in a single wavelength channel by utilizing the spatial dimension of light, which is helpful to reduce the power-consumption compared with other multiplexing techniques [11]. To realize MDM systems, various mode-controlling devices are needed, such as mode (de)multiplexers [12,13], mode filters [14,15], mode converters [16,17], and more. As one of the most critical building blocks in optical communication systems, optical switch can also find wide applications in MDM systems [18,19]. However, most of the presented switches are mode dependent and usually used to achieve the mode-selective switching or mode converting [20–22]. In the traditional MDM system, to implement the switching function, the high-order mode is usually first demultiplexed into the fundamental mode and, then the desired switch configuration is implemented by using a traditional single-mode switch matrix [23]. At the output of the switch, the single-mode channel is then multiplexed back into the corresponding higher-order mode. This solution will increase the switching complexity and power-consumption. In contrast, the multimode optical switch can route the multimode signals simultaneously and avoid the demultiplexing-processing-multiplexing technique to make the MDM system more flexible [24,25]. In our previous work, we have also proposed a polarization-insensitive and mode-independent thermo-optic (TO) switch based on the symmetric directional coupler formed with two identical polymer two-mode waveguides [26]. It is necessary to further develop the multimode optical switch with larger wavelength window, lower power-consumption, and flexible multi-port optical path changes.
In this paper, we present a broadband 1 × 2 mode-independent polymeric TO switch based on two-mode waveguide Mach–Zehnder interferometer (MZI) with multimode interferometer (MMI), which can add flexibility and reconfigurability to MDM systems. The use of polymer materials can offer a large TO coefficient and low thermal conductivity, which is helpful to reduce the power-consumption of the device. Moreover, because of their low index contrast, polymer waveguides are compatible with few-mode fibers in size, which allows direct butt-coupling with few-mode fibers and thus simplifies device packaging. For the same reason, the performance of polymer waveguide switches are in general polarization-insensitive [27,28]. In principle, similar devices can also be made of other material platforms [29–31]. Our proposed switch is independent for E11 and E12 modes, and the mode content of the outputs is the same as the mode content of the inputs. The proposed switch uses the Y-branch structure to realize the 3-dB power splitting of the input light, and uses the MMI structure as the coupling part of the output end. With the driving power of 5.9 mW, the extinction ratios of the switch for E11 and E12 modes are larger than 14.2 dB and 13.6 dB, respectively, at the working wavelength of 1550 nm. The insertion losses of the device are less than 14.3 dB in the C + L band for both the two modes. Our presented device with the mode-independent switching function is ideal to use in MDM systems to increase the channel capacity of the optical communication.
2. Design and experiment
2.1 Waveguide structure and simulation
Figure 1(a) shows the schematic of the 1 × 2 mode-independent TO switch based on two-mode waveguide MZI, where the Y-branch is served as the 3-dB power splitter and the MMI is employed as the coupler. The Y-branch and the MMI coupler are joined by two parallel waveguides, which are used as modulation arms. Two identical electrode heaters are provided on both modulation arms to ensure the balance of the light in them. All waveguides are two-mode waveguides which support E11 and E12 modes, except for linear tapered waveguides and multimode waveguide. The basic working principle of the investigated mode-independent 1 × 2 TO switch is as follows. For E11 or E12 mode launched into the switch, the Y-branch splits E11 and E12 modes into the two branches of MZI with equal intensity and in phase. The phase difference between the modes along the two modulation arms can be changed by heating the electrode heater. The mode-independent MMI-based 3-dB coupler operates on the general interference. When the optical signals enter the MMI coupler with equal intensity and in phase (un-modulated), the optical signals output from both O1 and O2 ports with equal intensity. When heating one of the electrode heaters, the phase difference of the light into the MMI structure will be changed and, then the interference effect of light in the multimode waveguide changes, which causes the optical power to redistribute between the two output ports. Compared with the directional coupler, both the Y-branch and MMI coupler are wavelength insensitive, which allows the device to operate with a large bandwidth.
Fig. 1. (a) Schematic 3D view of the mode-independent TO switch, and the insets show the field distribution of E11 and E12 modes; (b) The cross-section and (c) the thermal field distribution of the device.
Then we optimize the waveguide structure and dimensions of the device. We choose the strip waveguide structure on the polymer waveguide platform with a silicon layer as the substrate to implement our device. EpoCore and EpoClad are selected as the core and cladding, respectively. Their refractive indices are 1.569 and 1.559, respectively. Figure 1(b) shows the schematic cross-sectional view of the modulation arms along the propagation direction of the light. The input and output parallel waveguide cores have a uniform height H and width W1. The H and W1 are set as 9.0 µm and 4.5 µm, respectively, to support only E11 and E12 modes for the rectangular waveguide. The upper-cladding and lower-cladding are set to 16.0 µm and 7.0 µm, respectively. The insets in Fig. 1(a) show the simulated optical field distribution of E11 and E12 modes. It is clear that the two modes are well limited in the waveguide core.
To achieve the accurate and practical design, the 3D finite-difference beam propagation method (3DFD-BPM, Rsoft) was employed to perform numerical simulation for the device. The grids in the cross-section and the propagation direction of the waveguides are 0.2 µm × 0.2 µm and 5.0 µm, respectively. And the simulation domain in the cross-section and the propagation direction of the waveguides are 70 µm × 70 µm and 17700 µm, respectively. For our design, a linear tapered waveguide is adopted to connect the input waveguide and the branch waveguides of the Y-branch, which can reduce transmission loss and mode mismatch. The linear tapered waveguide is set as a gradual increase width from 4.5 µm to 9.18 µm and a length of 635 µm. The length of the modulation arms L1 is set to 5000 µm and the separation of the modulation arms D is set to 50 µm. The thermal field distribution shown in Fig. 1(c) indicates that when one modulation arm is thermally modulated, the other one will not be affected. The width and length of the MMI waveguide are denoted as W2 and L2, respectively. To determine the appropriate parameters, the value of W2 is considered as 30 µm and the value of L2 is gradually changed from 2000 µm to 2300 µm, and the relationship between the transmission and L2 is plotted in Fig. 2(a). The results indicate that L2 = 2150 µm can represent the best performance for both E11 and E12 modes, which show that the output modes from the two output ports have the almost same intensity (i.e., equal to half of the input power). To reduce scattering losses, the linear tapered waveguide should be also included at the input and output ports of the MMI waveguide to accommodate the self-imaging spot size in the multimode waveguide region and transfer it into the two-mode waveguide adiabatically. Variation of the transmission with the length of the tapered waveguide next to the multimode waveguide (L3) at a wavelength of 1550 nm is shown in Fig. 2(b), which shows that the excess losses gradually decrease with the increase of L3. Therefore, considering the excess loss and footprint of the device, the value of L3 is set as 300 µm. In addition, since the aluminum (Al) electrode heaters with a width of W3 = 12 µm are just placed above the two modulation arms, the loss and the phase shift of the light in modulation arms caused by the electrodes are neglected.
Fig. 2. The transmission of E11 mode and E12 mode as a function of (a) the length of the MMI waveguide (L2) and (b) the length of the tapered waveguide (L3).
According to the above optimized structural parameters, we further verified the function and performance of the device by using the 3DFD-BPM. The calculated normalized transmission at O1 and O2 ports as a function of the driving power are plotted in Figs. 3(a) and 3(b), respectively, for E11 mode and E12 mode at 1550 nm wavelength. While using E11 mode as input, with a heating power of 5.0 mW, the light output from O1 port is almost completely switched to O2 port, and the extinction ratios on O1 and O2 ports are 38.6 dB and 40.5 dB, respectively. For E12 mode launched into the device, the calculated extinction ratios on O1 and O2 ports are 34.8 dB and 34.2 dB, respectively, and the switching power-consumption is the same as E11 mode. Figures 3(c) and 3(d) show the optical field distribution and propagation paths of the device with the driving power set at 0 mW, 2.5 mW, and 7.5 mW when E11 and E12 modes input, respectively. From the calculated results, it is clear that the TO switch can realize mode-independent switching function with only 5.0 mW power-consumption.
Fig. 3. (a) Simulated variations of the transmission at 1550 nm with the driving power applied to electrode for (a) E11 and (b) E12 modes; The optical field distribution and propagation paths of the 1 × 2 TO switch at different driving power for (c) E11 and (d) E12 modes input at 1550 nm.
Fig. 4. (a) Simulated transmission spectrum at output ports for (a) E11 mode and (b) E12 mode launched into the input port at different driving power.
Furthermore, we investigated the wavelength characteristics of the device. Figures 4(a) and 4(b) show the simulated transmission spectrum from O1 and O2 ports of the device at three different driving powers (0 mW, 2.5 mW, and 7.5 mW) when E11 and E12 modes are launched into the device, respectively, over the wavelength range of 1530-1610 nm. It can be seen from. Figure 4(a) that the worst insertion loss and crosstalk for E11 mode are 0.2 dB (at 1610 nm) and -20.5 dB (at 1610 nm), respectively, in the C + L band, while those of E12 mode are 0.2 dB (at 1610 nm) and -19.6 dB (at 1610 nm), respectively, as depicted in Fig. 4(b). The extinction ratios of the device are larger than 19.0 dB for both two modes, in the C + L band. It can be concluded that the TO switch can operate in MDM system over a large bandwidth.
2.2 Device preparation
The 1 × 2 mode-independent TO switch was fabricated by our in-house microfabrication process, as shown in Fig. 5. Firstly, a 7.0 µm-thick EpoClad film was spin-coated on a Si substrate and prebaked at 120 °C for 5 min. Then the film was exposed to ultraviolet (UV) light from a mercury discharge lamp (peak emission wavelength, 365 nm; irradiance, 20 mW/cm2) for 17 s and hard-baked at 120 °C for 30 min to enhance the material cross-linking. After that, an EpoCore film with a thickness of 9.0 µm was spin-coated onto the coated sample and prebaked at 50 °C for 2 min and 90 °C for 4 min to remove the solvent. The standard contact photolithography with a mask was used to transfer waveguide patterns onto the EpoCore film. After post-baking, the sample was wet-etched in the developer for 50 s to remove unexposed EpoCore material, then placed in isopropyl alcohol to remove residual glue and deionized water is used to wash the reaction solution. After that, a thick EpoClad film (∼16 µm) was spin-coated onto the coated sample as the upper-cladding. Finally, thermal evaporation, UV lithography, and wet-etching processes were used to fabricate the Al heating electrodes. Figure 6(a) shows a top view of the fabricated device, the overall length of the sample is approximately 1.8 cm. The measured resistance value of the Al heating electrode is about 0.76 kΩ. Figure 6(b) shows the microscopic image of the input waveguide taken from an end face of the sample. It can be seen that the ridge wall of the waveguide is smooth and almost vertical, and the size is basically the same as our design, which is beneficial to reduce the transmission loss. Figures 6(c) and 6(d) show the top view of the Y-branch and the MMI coupler of the fabricated device, respectively, observed through an optical microscope. The edges of the Y-branch and the MMI coupler are clear, and the splitting and coupling of the beam can be realized more accurately.
Fig. 6. (a) Photograph of the fabricated switch; Microscope images of (b) the start face, (c) the Y-branch and (d) the MMI structure of the fabricated device.
2.3 Measurements and discussion
To characterize the switching function of the fabricated 1 × 2 mode-independent TO switch, the light generated by a tunable semiconductor laser (TSL-510, Santec) was transmitted through a single-mode lensed fiber and, then launched into the input waveguide of the device to stimulate the required mode. At the output end of the device, the output optical signal was coupled into a receiving optical fiber, which was connected with an optical power meter (Newport 2832-C) to monitor the output power. The driving voltage was applied to the electrode heaters through the metal probes to achieve the modulation of the device. The output powers from the two output ports of the device measured at different driving power at 1550 nm wavelength are shown in Fig. 7(a), when E11 mode was launched into the device. As shown in Fig. 7(a), when the driving power of the electrode heater is 3.0 mW, the optical signal almost completely outputs from the O1 port, and the corresponding extinction ratio is 15.0 dB. When the driving power was increased to 8.9 mW, almost all the power outputs from the O2 port with an extinction ratio of about 14.2 dB. The power-consumption required to realize the switching of the optical signal between the two output ports is 5.9 mW. As shown in Fig. 7(c), the extinction ratios of E12 mode on O1 and O2 ports are 13.6 dB and 15.3 dB, respectively, with the same power-consumption of E11 mode. The near-field images taken from O1 and O2 ports of the device for E11 and E12 modes with different applied driving power are shown in Figs. 7(b) and 7(d), respectively, which were obtained by using a charge-coupled device (CCD) camera. It can be clearly seen that the two mode signals can be switched between the two output ports simultaneously. These results confirm that our device can implement mode-independent switching functions for E11 and E12 modes.
Fig. 7. Variations of (a) the transmission and (b) the near-field images taken at O1 and O2 ports with different driving power for E11 mode, at 1550 nm wavelength; Variations of (c) the transmission and (d) the near-field images taken at O1 and O2 ports with different driving power for E12 mode, at 1550 nm wavelength.
We also measured the wavelength dependence of the device. When the applied driving power is set at 0 mW, 3.0 mW and 8.9 mW, for the E11 and E12 modes launched into the device, the measured output powers from O1 and O2 ports of the device over the wavelength range from 1530-1610 nm (C + L band) are shown in Figs. 8(a) and 8(b), respectively. Over the C + L band, the insertion losses of the device are less than 12.5 dB and 14.3 dB for E11 and E12 modes, respectively. The insertion loss of E12 mode is relatively larger than that of E11 mode, which is mainly due to the large coupling loss between the optical fiber and the waveguide. Whether the light outputs from O1 port or O2 port, the extinction ratio is larger than 13.3 dB and 13.1 dB for E11 and E12 modes, respectively, which indicates that our device can operate well in the C + L band.
Fig. 8. Measured transmission characteristics of the switch for (a) E11 and (b) E12 modes, respectively, over C + L band.
To characterize the dynamic characteristics of the device, the output light was coupled into a photodetector, which converted the optical signal to the electric signal and displayed on the digital oscilloscope (RIGOL, DS4024). The electrical signal loaded onto an electrode heater is a square-wave driving signal with a frequency of 200 Hz. The input electric signal and output optical power signal waveforms for E11 and E12 modes are presented in Figs. 9(a) and 9(b), respectively. When E11 mode was launched into the device, the rise and fall times of the switch are 780 and 840 µs, respectively. While for the E12 mode launched into the device, the rise and fall times are 800 and 800 µs, respectively.
3. Conclusion
In conclusion, we have presented a broadband 1 × 2 mode-independent TO switch consisted of a mode-independent 3-dB Y-branch power splitter and a MMI coupler, which is based on polymer platform and fabricated by using UV lithography and wet-etching methods. For the E11 mode launched into the device, the measured extinction ratios on O1 and O2 ports are 14.2 dB and 15.0 dB, respectively, with the switching power of 5.9 mW applied on the device at 1550 nm wavelength. For the E12 mode launched into the device, the measured extinction ratios on O1 and O2 ports are 13.6 dB and 15.3 dB, respectively, with the same power-consumption of E11 mode. The response times of the device are less than 840 µs for both the two modes. In the wavelength range of 1530-1610 nm, the extinction ratios of E11 mode and E12 mode are larger than 13.3 dB and 13.1 dB, respectively. Our device has the advantages of simple structure and fabrication process with low cost, which is easy to integrate and expand to construct the 1×N mode-independent switch modules. Our device can play a significant role in the broadband MDM system where mode-independent switching is needed.
Funding
National Key Research and Development Program of China (2019YFB2203002); National Natural Science Foundation of China (61875069); Fundamental Research Funds for the Central Universities.
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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