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Abstract
Wavelength division (de)multiplexing (WDM) device is a crucial component for optical transmission networks. In this paper, we demonstrate a 4 channel WDM device with a 20 nm wavelength spacing on silica based planar lightwave circuits (PLC) platform. The device is designed using an angled multimode interferometer (AMMI) structure. Since there are fewer bending waveguides than other WDMs, the device footprint is smaller, at 21 mm × 0.4 mm. Owing to the low thermo-optic coefficient (TOC) of silica, a low temperature sensitivity of 10 pm/°C is achieved. The fabricated device exhibits high performance of an insertion loss (IL) lower than 1.6 dB, a polarization dependent loss (PDL) lower than 0.34 dB, and the crosstalk between adjacent channels lower than −19 dB. The 3 dB bandwidth is 12.3∼13.5 nm. Moreover, the device shows a high tolerance with a sensitivity of central wavelength to the width of multimode interferometer < 43.75 pm/nm.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Wavelength division (de)multiplexing (WDM) devices play an important role in the high-speed telecommunication photonic system. WDM devices multiplexes multi-wavelengths into one fiber increasing communication capacity. Different types of WDM devices have been reported on array waveguide gratings (AWG) [1–4], planar concave grating (PCG) [5,6], micro-ring resonators (MRR) [7,8] and cascaded Mach-Zehnder interferometers (CMZI) [9–12]. AWG and PCG based WDMs suffer from large footprints owing to large radius of bend. CMZI based WDMs are able to adjust central wavelength to precious wavelength by thermo-optical shifter caused by environment and fabrication process [13–15]. However, shifter with introduce undesired loss and polarization dependence. The electrical driver and package will increase the cost and make the system complex. MRR based device is ultra-compact but easily affected by environment. Recently, angled multimode interferometer (AMMI) structure, as a compact and high fabricated tolerance structure, has been reported to realize WDM function based on self-imaging effects [16–20].
Nowadays, coarse WDM (CWDM) devices have been demonstrated on various platforms including silicon-on-insulator (SOI) [6,21], silicon nitride (SiN) [22,23], thin-film lithium niobate (TFLN) [17] and silica [24,25]. Because of the high refractive index difference, SOI based devices, compatible with complementary metal oxide semiconductor (CMOS), is compact and easily realizing high-volume manufacturing. However, high thermo-optic coefficient (TOC) of silicon (1.8 × 10−4/°C) contributes strong sensitivity of temperature. The central wavelengths of the CWDM devices based on SOI shift with a rate of 70∼100 pm/°C [7,26]. Therefore, these devices should cooperate with a temperature controller module to provide a temperature compensation. Owing to low loss and a high non-linearity coefficient, the SiN platform is a very popular material in recent year. However, due to the difficulty in depositing, the thickness of SiN waveguides is 200 nm or 400 nm, which contributes high polarization dependent. TFLN is a promising solution for high-speed modulator on chip. However, the difficult fabrication process of TFLN limits its commercial application. As a commercialization available platform, the silica based planar lightwave circuit (PLC) platform has very mature production process. With the low TOC of silica (1.84 × 10−5/°C) and low transmission loss (< 0.12 dB/cm) [27], the devices based on silica show low temperature sensitivity and low loss. Therefore, it is an ideal platform to fabricate CWDM device without tuning.
In this paper, we proposed a high temperature-insensitive, fabrication-tolerant and low-loss CWDM based on AMMI structure. The WDM is fabricated on a silica platform. According to the experimental results, the fabricated devices show an insertion loss (IL) < 1.6 dB and polarization dependent loss (PDL) < 0.34 dB. The crosstalk of adjacent channels is better than 19 dB. The bandwidth of 3 dB is 12.3∼13.5 nm. We also fabricate the device proposed with different widths to verify the process tolerances. The shift rate of the variations of width of AMMI is 43.75 pm/nm. The device is packaged to measure the sensitivity to temperature. The central wavelength shift with different temperatures is 10 pm/°C. To the best of our knowledge, this is the first study of CWDM for AMMI on a silica platform operating in the O-band.
2. Design and simulation
Figure 1 shows the schematic of the designed four channel CWDM device. The structure is composed of a wide multimode interference region, a tilted θ input waveguide and four tilted θ output waveguides. The width of the input waveguide accessed the multimode interference region is WTaper1. The four outputs are located on the length Li (i = 1, 2, 3, 4) of AMMI. The width on the side approaching the multimode interference region is WTaper2. The input and four output waveguides are all tapered to a narrow width of WIO to ensure the signal lights from the MMI input and output are transmitted in single-mode. Because of the tilted input waveguide and the self-imaging effect of MMI, the input signals are separated into different channel according to wavelengths. The length of self-image Li depends on the effective width We of MMI, the wavelength λi of signal light, and the effective refractive index neff of the TE0 mode. The Li can be calculated by the Eq. (1) [28]:
For the proposed AMMI, the maximum channel count Nmax and the minimum channel spacing (Δλmin) can be estimated using Eq. (4) and Eq. (5) [18]:
Equation (4) shows that the maximum number Nmax of output channels is related to the multimode interferometer width WMMI, tilt angle θ and Taper width WTaper1. To extend the channel count, we can widen the width of MMI WMMI, reduce tilt angle θ, and narrow the width of Taper. but there are some factors limit to expansion. With the WMMI increasing, the footprint will be large which will influence the production cost. Reducing tilt angle θ will cause a worse crosstalk [17], and narrowing the width of Taper will result in an increase in insertion loss.
Figure 2(a) shows the effective refractive index neff of the TE mode at the wavelength of 1311 nm for different widths of the waveguide with the height of 4 µm. To ensure the input and output waveguide transmission single-mode, the width of narrow waveguide WIO is 4 µm. The cross section of the single-mode waveguide is shown in Fig. 2(b). The calculated Ex mode profile is shown in Fig. 2(c). The effective refractive index neff for TE0 mode of single-mode waveguide is 1.463957.
Fig. 2. (a) The effective refractive index neff of the TE mode at the wavelength of 1311 nm. (b) The cross section of the single mode waveguide. (c) The Ex mode profile at the wavelength of 1311 nm.
The structure is simulated using finite-difference beam propagation method (FD-BPM). Due to the low refractive difference between core layer and cladding layer, the tilted angle θ is limited and chose to be 0.113 rad. Through simulation, we find that only when the width of the MMI is large enough, the tilted self-image points will be concentrated in one location, which can reduce the crosstalk of other channels. The width of the MMI Wmmi is set to 57.6 µm for a compact footprint. Considering the insertion loss and the crosstalk, the optimized structural parameters of the designed AMMI are summarized in Table 1. The simulated signal light propagation under different wavelengths is shown in Fig. 3. The signal light propagated into different output channels achieving wavelength division demultiplexing.
Fig. 3. The simulated light propagation in the angled MMI with TE mode of different wavelengths: (a) λ = 1271 nm, (b) λ = 1291 nm, (c) λ = 1311 nm, (d) λ = 1331 nm.
Table 1. The structural parameters of the four-channel CWDM device
Figure 4 illustrate the simulated spectra from four output channels in TE and TM mode. The ILs of TE mode from channel #1 to channel #4 are 0.63 dB, 0.59 dB, 0.57 dB and 0.57 dB, respectively. The redshift of 2 nm is observed in the central wavelength of TM mode compared with TE mode, owing to ΔnTE eff > ΔnTM eff. The simulated PDL < 0.2 dB. The 3 dB bandwidths BW3dB of each channel are about 12 nm. The crosstalk of the proposed device is < −20 dB. Since the AMMI achieves the wavelength division demultiplexing by self-imaging effect of MMI, the device does not work like the AWG, where different wavelengths are periodically output from the four output channels. However, as shown in Fig. 4, channel #1 rises rapidly from 1340 nm to 1360 nm. The phenomenon can be explained by AMMI self-imaging effect. When the signal light first enters the multimode waveguide, its diffraction angle is small. The propagation of light can be done with ray optics. In this way, there is a window in the sidewall of the multimode waveguide where the intensity of the light tends to zero, and it can be used as the access window of the output waveguides of the other channels. When the signal light at the center wavelength of the design is input to the device, the output channel of the design receives the majority of the light. The channels below the design output channels are in the window where the intensity of the light tends to zero, so these channels have lower light intensities. The channels above the design channels have lower light intensities transmitted to these channels because most of the light has been output by the design channels. When the signal light from 1340 nm to 1360 nm is input to the AMMI, the output position of the signal light is below channel #4 and the signal light will enter the second self-imaging cycle of the AMMI, at which time a window with the same light intensity close to zero as the input will also appear, so channels 2 to 4 light intensities are not receiving light. During the device design process, the total length of the four output ports is the same as the length of the window where the light intensity tends to zero, so the uppermost channel 1 will receive the light of non-self-imaging point, and fast rise. However, since the distribution of non-self-imaging light points in a multimode interferometer is not concentrated in one point, it cannot realize the function of wavelength division multiplexing at this wavelength. Similar to channel #1, channel #4 will rise before 1260 nm, at wavelengths shorter than 1260 nm, the first self-image point position is after the output channel #1, and the light intensity tends to 0 window length is not large enough, causing some of the signal light to be output from channel #4 earlier, resulting in the rise of channel #4.
Fig. 4. Simulated spectra of four output channels in TE mode (solid line) and TM mode (dashed line) with the parameters WIO = 4 µm, WTaper1 = 29 µm, WTaper2 = 25 µm, WMMI = 57.6 µm, θ = 0.113 rad.
The fabrication tolerance of the proposed AMMI is researched. With the different parameters of MMI width WMMI, MMI length of output channel #3 L3, height of MMI h and the input waveguide taper WTaper1, the spectral responses of output channel #3 are simulated. The result of simulations is shown in Fig. 5. The Fig. 5(a) shows the tolerance of the width of AMMI WMMI, the variation in width of AMMI causes a drift of the central wavelength at a rate of 50 pm/nm. The other parameters are insensitive to the fabrication tolerance. The changes in length of output channel #3 L3 only result in the central wavelength changes at a rate of 0.1 pm/nm as shown in Fig. 5(b). As shown in Fig. 5(c) and (d), the central wavelength shifts at a rate of 0.6 pm/nm for the changes in height of AMMI and input waveguide taper WTaper1. In conclusion, the central wavelength of the AMMI is most sensitive to the width of AMMI WMMI up to 50 pm/nm.
Fig. 5. The simulated fabrication tolerance of the proposed AMMI device with the changes (a) in Wmmi of ΔWmmi from −0.1 to +0.1 µm, (b) in WTaper1 of ΔWTaper2 from −1 to +1 µm, (c) in h of Δh from −0.5 to +0.5 µm, (d) in L3 of L3 from −5 to +5 µm.
3. Device fabrication and measurement
The width of AMMI WMMI changed by 0, ± 0.1 µm and ±0.2 µm is add in the final mask layout to study the sensitive of the width of AMMI WMMI. Finally, two spot-size converters (SSC) [25] is added to reduce coupling loss between waveguides and fibers. The AMMI based CWDM are fabricated on silica platform by the PLC foundry, SHIJIA, China. Firstly, the bottom cladding silica with lower refractive index is grown on silicon substrate by thermal oxide. Then, the 4-µm-thickness core silica with higher refractive index is deposited by Ge-doped plasma-enhanced chemical vapor deposition (PECVD). And then the wafers are annealed at temperature above 1100°C. Using ultraviolet (UV) lithography, the structures of devices is transferred from photolithography mask to photoresist. The wafer is then fully etched with inductively coupled plasma (ICP) using C4F8/SF6 gas. Finally, the top cladding silica with lower refractive index is deposited by PECVD and anneale again. The thickness of the lower cladding silica and the upper cladding silica both are 20 µm. The microscope image of the fabricated AMMI is shown in Fig. 6. The length of the AMMI is 17.48 mm and the final footprint of the device is approximately 21 mm × 0.4 mm with 3-mm-length straight waveguides considering the ease of dicing and polishing. The comparison of footprint with other silica platforms is summarized in Table 2. The footprint of AMMI is 10 times smaller than other silica plants based on the PLC platform.
Fig. 6. The microscope image of the present AMMI at the (a) input waveguides and (b) output waveguides.
Table 2. The footprints of different CWDM devices based on silica platform
We measure the proposed device using a tunable laser (TSL-550, Santec, Japan) and an optical power meter (MPM200, Santec, Japan). The polarization of the source is adjusted through polarization controller (PC). The measured spectra are shown in Fig. 7(a). The ILs from channel #1 to channel #4 are 1.22 dB, 1.55 dB, 1.02 dB and 1.22 dB including the coupling loss and the propagation loss. The uniformity of the device is 0.53 dB. The PDL is < 0.34 dB. The crosstalk at the central wavelength is < −19 dB. The 3 dB bandwidths of the device are 12.3∼13.5 nm, which is consistent to the simulation result. The central wavelengths for channel #3 of AMMIs with different width are shown and fitted linearly in Fig. 7(b). The rate of central wavelength shift is 43.75 pm/nm.
Fig. 7. (a) Measured spectra of four output channel devices in TE mode (solid line) and TM mode (dashed line). (b) The measured central wavelength positions (red point), and the linear fit (red dotted linear) for channel #3 with different WMMI. The central wavelengths shift at the rate of 43.75 pm/nm
The proposed device is packaged as shown in Fig. 8(a). The temperature sensitivity of the present CWDM is measured by using temperature controller module (TCM). The central wavelength of the device is measured at 20°C, 25°C, and 35°C. As shown in Fig. 8(b), the centra wavelength shifted at the rate of 10 pm/°C. The spectra of the proposed device at −40°C and 90°C are shown in Fig. 8(c). At extreme temperature, the device works still normally. The insert loss is < 3.1 dB (−40°C) and < 2.7 dB (90°C). There are two reasons which caused the insert losses increase. Firstly, the SSC used is designed to be used at room temperature, and when the temperature is far from room temperature, its operating state changes, resulting in increased coupling losses between the chip and the fiber. Secondly, the refractive index of the curing adhesive used in our packaging devices changes when the temperature changes drastically, leading to unexcepted loss between fiber array and chip.
Fig. 8. (a) The image of packed device. (b) The measured central wavelength of different temperatures (red point), and the liner fit (red dotted linear), the central shift at the rate of 10 pm/°C. (c) The measured spectral response of four output channels devices at −40°C and 90°C.
The comparison with other WDM devices is summarized in Table 3. The AWGs based on silica show lower IL and larger bandwidth. Comparing the data in Table 2 and Table 3, it can be seen that the device proposed in this paper can achieve similar performance to silica-based AWGs with a smaller footprint. In [31], an 8 channels WDM device with two AMMI and one MZI is demonstrated on SOI. The sensitivity to the fabricated process is 100 pm/nm. The sensitivity to temperature of 72.4 pm/°C [3] for AWG and 46 pm/°C [32] for grating are reported. A four channel WDM devices with four MRR is reported, with the sensitive to fabricated process of 800 pm/nm and the sensitive to temperature of 70 pm/°C [33]. Since the high refractive index difference of SOI, the devices have mentioned can achieve WDM function in a small footprint. However, because of the high refractive index difference and TOC, the WDM devices based on SOI have a strong dependence on process and temperature. Compared with the device in this paper, the Δλ/ΔW for this fabricated device based on silica platform is about 18 times better than the MRR fabricated on SOI, and is about twice better than the similar structures based on SOI. The Δλ/ΔT for this device is 4 times better than the devices prepared on the SOI platform. The SiN has lower refractive index difference and TOC than SOI and is used to prepare WDM devices. The AMMI devices based on SiN have been reported with the sensitive to fabricated process of 100 pm/nm and the sensitive to temperature of 11∼44 pm/°C [18,20]. And the CMZI fabricated on SiN shows sensitive to temperature of 18.7 pm/°C [34]. The devices based on SiN platform show excellent insensitive to temperature, but the fabricated device in this work the tolerance is twice better than these devices. Because lithium niobate is an anisotropic material, the compactness of on-chip routing for TFLN waveguides is limited. Due to less bend than other routing devices, AMMI device on TFLN has been reported [17]. The device is worked at IL < 0.72 dB, XT < 18 dB and has a sensitivity of 100 pm/nm to the fabricated process. The device shows good performers and tolerance, while compared with device on silica based on PLC, the fabricate process of TFLN is more difficult.
Table 3. The performance of different types WDM
4. Conclusion
In a conclusion, we have designed and prepared a four-channel CWDM device based on a silica platform using AMMI structure. Due to the low TOC of silica, the device has a low temperature sensitivity of 10 pm/°C. Compared with AWG based CWDM on silica platform, the device has a smaller footprint of 21 mm × 0.4 mm due to its fewer bending structure. Measurement results show that the CWDM has a low insertion loss of < 1.6 dB, a low crosstalk of < −19 dB and large 3 dB bandwidths about 12.3∼13.5 nm. We conduct the fabrication tolerance experiment with different width of AMMI. The width sensitivity of AMMI is 43.75 pm/nm. The device work in a high-performance condition at the extreme temperature of −40°C and 90°C. We believe that it has great potential for future applications on large-scale integration and mass production.
Funding
National Key Research and Development Program of China (2021YFB2800202).
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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