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Abstract
With the rapid development of the backbone network rates, there has been a gradual increase in channel spacing and bandwidth. The C&L band ultra-broad bandwidth array waveguide gratings (AWG) of 60-channel 100 GHz channel spacing are designed and fabricated based on silica waveguide. A new parabolic design is used to achieve ultra-broad bandwidth and good spectrum. For the C band ultra-broad bandwidth AWG, the peak insertion loss, uniformity, 0.5 dB bandwidth, 1 dB bandwidth and 3 dB bandwidth are 2.98 dB, 0.36 dB, 0.614 nm, 0.721 nm and 0.937 nm, respectively. For the L band ultra-broad bandwidth AWG, the peak insertion loss, uniformity, 0.5 dB bandwidth, 1 dB bandwidth and 3 dB bandwidth are 2.91 dB, 0.27 dB, 0.560 nm, 0.665 nm and 0.879 nm, respectively. To ensure ultra-broad bandwidth AWG operation at different temperatures, a temperature control circuit is integrated into the packaging design. It has been observed that the performances remain virtually unchanged within the temperature range of −15 to 65 degree. The ultra-broadband AWGs have been successfully tested to transmit 96 Gbaud signals and can be applied to 600 G/800 G backbone network transmission. By using the C&L ultra-broad bandwidth AWGs of 60-channel 100 GHz channel spacing, the total transmission speed over a single-mode fiber can reach 72Tbps/96Tbps.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
With the rapid development of the digital economy, including big data, cloud computing, VR, Internet of Things, block chain, 5 G and artificial intelligence, there is an urgent need to accelerate the construction of next generation of information infrastructure. The backbone network, as the foundation of new generation of information infrastructure, will have a significant impact on the development of digital economy. In recent years, there has been rapid growth in big data, Internet of cars, AR/VR, mobile apps, high-definition video, telemedicine, and live online platforms. It is crucial to improve and accelerate the development of backbone network.
Since Version company laid the world's first 100 G commercial ultra-long haul optical fiber backbone network in 2007, it has been deployed worldwide. The transmission rate for 100 G coherent transmission system is 32 G baud, as shown in Fig. 1. The quad-phase shift key (QPSK) modulation is employed to achieve relay-free transmission over distances exceeding 2000km. The 200 G transmission can be achieved by using 16 quadrature amplitude modulation (QAM) modulation. The AWG used in 100 G transmission has a channel spacing of 50 GHz and a 3-dB bandwidth of more than 40 GHz. Typically, this is accomplished by combining two 100 GHz AWG chips along with an interleaver. Although a single 50 GHz AWG chip can be employed, it involves a more complex process and higher cost. By 2016, 200 G transmission have been gradually matured and commercialized. The transmission rate is 64 G baud, 200 G transmission can be achieved by QPSK modulation and 400 G transmission can be achieved by16QAM. In 200 G wavelength division multiplex transmission, the AWG has a channel spacing of 75 GHz. The AWG can be obtained by combining two 150 GHz AWG chips and an interleaver. The single 75 GHz AWG chip is becoming more popular. To ensure the 64 G baud transmission, the 3-dB bandwidth is more than 80 GHz. As transmission rate increases, AWG require higher baud rate, wider channel spacing and broader bandwidth. In the future, 600 G/800 G transmission will need a 96 Gbaud rate and a channel spacing of 100 GHz, and require an ultra-broad bandwidth AWG chip.
Since its proposal by professor M. K. Smit of Delft University in 1988 [1], AWG has been widely studied and reported in recent years. The mainly platforms are silica, silicon, and silicon nitride. The silica-based AWG is widely studied due to its low loss and low coupling loss to fiber. NTT Corporation and other groups have successfully developed silica-based AWG chips ranging from 8 to 1000 channels [2–10], with channel spacing ranging from 1 GHz to 100 GHz. In 2004, Toshiko Mako's research group at Yokohama National University proposed an ultra-small size AWG device using silicon nanowire waveguides, which has attracted significant attention due to its compact size. Silicon nanowire waveguides AWG with channel numbers ranging from 4 to 512 and channel spacing ranging from 25 GHz to 200 GHz have been reported [11–20]. However, the fabrication processes of silicon nanowire waveguides are extremely complex and have very low process tolerance. The coupling loss to fiber is big, making packaging challenging. Investigations on different wavelengths and channel spacing AWG for silicon nitride AWG have also been proposed [21–26], with a maximum of 256-channel AWG with 42 GHz spacing. Although the size is small, but the coupling loss to fiber and polarization dependent loss (PDL) is relatively bigger.
There are only a few reports on ultra-broad bandwidth AWG for high-speed backbone network transmission based on silica platform. In 2007, K. Maru proposed a method for increasing bandwidth by combining multiple input AWG with cascaded MZI structure [27]. The channel spacing is 100 GHz, 1 dB bandwidth is 0.645∼0.658 nm, and 20 dB bandwidth is 0.944∼0.960 nm. However, this approach requires additional cascaded MZI structures, which increased the design and process difficulty. NTT later report a 100 GHz channel spacing MZI-synchronized AWG using a similar method [28]. The 0.5-dB bandwidth is 69 GHz and the 3-dB bandwidth is 89 GHz. In 2016, NTT proposed an ultra-compact and low loss flat top AWG multiplexer of O band LAN WDM AWG using the same method [29]. The size is 6.7mm × 3.5 mm, with an insertion loss of less than 1.9 dB and a 0.5-dB bandwidth of 410 GHz. In 2020, NTT report a multimode-output method to realize a flat top AWG [30]. For the coarse wavelength division demultiplexer (CWDM) in the C band, the insertion loss of the multimode output AWG is less than 1 dB in each passband. The 0.5-dB bandwidth is larger than 13 nm. Later in 2021, our research group report a four-channel O-BAND CWDM AWG using similar method [31], The minimum insertion loss is less than 1.07 dB, the 1-dB bandwidth is larger than 13.7 nm and the 3 dB bandwidth is larger than 16.1 nm. However, this method is only used for coupling to photodiode and data-centric communications. In 2011, W. M. Wang [32] propose a 40 channels 100 GHz AWG using multimode interference (MMI) input waveguide method. The simulated results show than the 1 dB-bandwidth and 3 dB bandwidth can be better than 0.52 nm and 0.65 nm. However, the MMI structure is very sensitive to wavelength. This compels us to search for a flat-top method that is not only simple in process but also wavelength-insensitive, enabling single-mode transmission.
In this paper, we design and fabricate two C&L band ultra-broad bandwidth 60-channel AWGs with a 100 GHz channel spacing for high-speed backbone network transmission. We have design and optimize the special parabolic types to obtain ultra-broad bandwidth performance and good crosstalk. For the C band ultra-broad bandwidth AWG, the insertion loss, uniformity, 0.5 dB bandwidth, 1 dB bandwidth, and 3 dB bandwidth are 2.91 dB, 0.36 dB, 0.614 nm, 0.721 nm, and 0.937 nm, respectively. For the L band ultra-broad bandwidth AWG, the insertion loss, uniformity, 0.5 dB bandwidth, 1 dB bandwidth, and 3 dB bandwidth are 2.98 dB, 0.27 dB, 0.560 nm, 0.665 nm, and 0.879 nm, respectively. We have achieved athermal packaging to ensure temperature independence. The ultra-broad bandwidth AWGs have been tested for transmitting the 96 Gbaud signals and can be applied for 600 G/800 G backbone network transmission. To the best of our knowledge, this is the widest bandwidth reported and the first study of high-speed verification.
2. Design of ultra-broad bandwidth AWG chip
2.1 Basic design
Figure 2 shows the structure of ultra-broad bandwidth AWG, which consists of three parts: input/output waveguides, slab waveguides and array waveguides. When the wideband signal is input from the central waveguide and enters the slab waveguide, it diffracts and form a Gaussian optical field. This field is then coupled to the array waveguide region. The adjacent channels of the array waveguide have a length difference ΔL, and which lead a phase difference Δφ, as described in Eq. (1). Where nc is the effective refractive index of the array waveguide, λ is the central wavelength in vacuum. For central input, different wavelengths of light will generate different phase differences after passing through the array waveguide region with a length difference ΔL. Following output from the right side of the array area and focused on different points in the output waveguide via the right flat slab region. The wavelength demultiplexing is finished.
The input and output waveguides satisfy the following Equation.
In Eq. (2), θi/θo is the angle between input/output and the central waveguide, ns is the refractive index of slab waveguide, nc is the refractive index of array waveguide, d is the pitch between array waveguide, m is the diffractive order. When the input is in the center, Eq. (2) can be simplified to Eq. (3).
where λ0 is the center wavelength of input, m can be obtained by Eq. (4), ng is group index. FSR is free spectrum range.2.2 Design of ultra-broad bandwidth AWG
Silica-based waveguide with a relative refractive index of 0.75% is employed in this design. The under cladding is thermally oxidized silica with a refractive index of 1.445. Similarly, the upper cladding is composed of silica doped with boron and phosphorus, maintaining the same refractive index as the under cladding. The refractive index of the core layer is 1.455919. The waveguide cross-sectional size is 6µm × 6µm to ensure the single mode transmission.
Based on the above basic information, we started the ultra-broad bandwidth AWG design. Firstly, let’s calculate the value of ns, nc and ng. The wavelength range of the C band 60 channels is 1522.913nm∼1573.294 nm, the center wavelength is 1548.092 nm. The effective refractive index of the slab waveguide and array waveguide at the center wavelength is 1.453294 and 1.450792 by using the three-dimensional beam propagation method. The group refractive index at the center wavelength is calculated to be 1.47580 using Eq. (5).
Secondly, we will calculate the values of FSR, m and ΔL. The FSR is chosen to 96THz to obtain good uniformity and relatively small size. Using Eqs. (3) and (4), we can obtain that the ΔL is 22.4µm and m is 21.
Thirdly, we will calculate the values of d and Δxi/Δxo. Figure 3 illustrates the relationship between crosstalk and array waveguide pitch. The signal is input in the center channel (CH2). The result is simulated using the three-dimensional beam propagation method. From the figure, we can observe that the coupling occurs when the waveguide pitch is less than 8µm. However, for waveguide pitch larger than 8µm, there is no coupling between adjacent waveguides. Considering the chip size, the array waveguide pitch of 8µm is chosen to achieve uncoupling. With the increasing waveguide pitch, the crosstalk varies rapidly. The crosstalk can be less than −60 dB when the waveguide pitch is bigger than 20µm. Considering the discrepancies between simulation and experiment, the output waveguide pitch of 22µm is chosen to improve crosstalk performance.
Finally, we will calculate the values of the Rowland radius R and the number of array waveguide N. Using Eq. (2) we can infer the relationship between channel spacing and dispersion, as expressed in Eq. (6). From these two Equations we can obtain the Rowland radius R is 14977.42µm. In order to maximize the diffraction of light and achieve good crosstalk performance, it is crucial to carefully consider the number of array waveguides N. After careful consideration based on Eq. (7), we have chosen the number of array waveguides N to be 251.
Once the above initial design is completed, we can explore the ultra-broad bandwidth design of the AWG. For achieving a flat-top AWG design, there are three main methods: Multimode input, parabolic input, and few Rowland designs. Each method aims to generate a bimodal wave at the input waveguide, as depicted in Fig. 4. By integrating this bimodal input wave with the output fundamental mode, a flat spectral response can be obtained. In this paper, we employ the parabolic input waveguide design. Three different types of parabolic profiles are designed and optimized, using Eq. (8) and illustrated in Fig. 5(a). Where x represents the width direction, y represents the length direction, and t is the power number. The three-dimensional beam propagation method is used to simulate the spectra of different parabolic types, as shown in Fig. 5(b). From the simulation results, we can obtain that as the power number increases the 1 dB bandwidth become broader, while the 20 dB bandwidth become narrower. However, when the power number reaches 2.5, the spectral ripple worsens. Consequently, the power number is chosen as 2 for the final design.
Based on the above discussion, we can derive the final structure for the AWG design. The corresponding mask is shown in Fig. 6. The overall size of the structure is 39.3mm × 29.2 mm, with an output pitch of 127µm. For the L band AWG, the design process is similar, and we just change the center wavelength and adjust the output waveguide. The structure and size remain the same as that of the C band design.
3. Fabrication, test, and analyses
The ultra-broad bandwidth AWG is fabricated based on the 6-inch silicon substrate. The process begins with thermal oxidation, which forms a silicon dioxide under cladding layer on the silicon substrate. Next, a SiO2-GeO2 core layer of 6µm thickness is deposited using plasma enhanced chemical vapor deposition (PECVD). Subsequently, the ultra-broad bandwidth AWG circuits are transferred to photoresist through photolithography. The ultra-broad bandwidth AWG waveguides are then etched using an induced coupler plasma (ICP) system. To reduce the stress birefringence, an up-cladding layer of 15∼20µm thickness SiO2 doped with Boron and phosphorus is formed by using PECVD deposition. Once the wafer fabrication is completed, it can be cut into the bars for polishing and testing. After the bars testing is finished, the single ultra-broad bandwidth AWG chip is obtained by laser cutting. The chip picture is shown in Fig. 7.
The performances of the ultra-broad bandwidth AWG chips are evaluated using a tunable laser, polarization controller and optical power meter. The resulting optical spectrum is shown in Fig. 8, and the parameters are analyzed in detail.
In the calculation of the ultra-broad bandwidth 60-channel 100 GHz channel spacing AWG performance, a passband of ±0.11 nm is used. The insertion loss characteristics are illustrated in Fig. 9. For the C band, the peak insertion loss is 2.98 dB, the uniformity is 0.27 dB. The worst passband loss of all 60-channel is 3.87 dB. For the L band, the peak insertion loss is 2.91 dB, the uniformity is 0.36 dB. The worst passband loss of all 60-channel is 3.47 dB.
Figure 10 displays the bandwidth characteristics of the ultra-broad bandwidth AWG. For the C band, the 0.5 dB bandwidth is 0.614 nm, the 1 dB bandwidth is 0.721 nm, and the 3 dB bandwidth is 0.937 nm (117 GHz). The ultra-broad 3 dB bandwidth guarantees the data transmission performance of 96 Gbaud. These bandwidth values surpass those reported in previous Refs. [27,28,32–35] as shown in Table 1. The ultra-broad bandwidth is obtained by using the high-power parabolic type in the input waveguide. Additionally, the bandwidth remains highly uniform across all 60 channels, primarily due to the varying output widths corresponding to different wavelengths. For the L band, the 0.5 dB bandwidth is 0.560 nm, the 1 dB bandwidth is 0.665 nm, and the 3 dB bandwidth is 0.879 nm(109 GHz). The ultra-broad 3 dB bandwidth guarantees the data transmission performance of 96 Gbaud. The bandwidths for the L band are slightly narrower than those in the C band, primarily due to the longer wavelengths. However, by adjusting the input and output widths, it is possible to achieve optimal values for the desired performance.
Table 1. The reports of flat top AWG
Figure 11 shows the crosstalk of ultra-broad bandwidth AWG. For the C band, the adjacent crosstalk (AX), non-adjacent crosstalk (NX) and total crosstalk (TX) are more than 17.743 dB, 39.029 nm, and 14.880 dB, respectively. For the L band, the AX, NX and TX are more than 24.739 dB, 36.856 nm and 23.526 dB, respectively. From the results we can see that the adjacent crosstalk for L band is better than that for C band, which is caused by the narrower bandwidth.
Figure 12 illustrates the PDL and ripple of ultra-broad bandwidth AWG. The worst PDL values for both the C band and L band are below 0.27 dB. These excellent results are achieved through the utilization of square waveguides and ensuring good stress matching during the fabrication process. From Fig. 12(b) we can observe than the worst ripple among all channels is less than 0.42 dB. This low level of ripple is attributed to the ultra-broad bandwidth design and maintaining consistency throughout the fabrication process.
4. Package and temperature performance
Because the center wavelength of AWG is sensitive to temperature, and AWGs are common packed into module using two different packaging forms. One form is athermal packaging, which achieved temperature-independent characteristics through a mechanical carrier, but it is relatively expensive. The other form is thermal packaging, where the temperature independent characteristics are achieved through circuit control, resulting in lower costs. In this paper, the thermal packaging is used, and the corresponding structures can be observed in Fig. 13(a). The AWG chip is placed on the TEC controller, allowing it to operate within a temperature range of 68 to 85 degree, as controlled by the circuit. To monitor the input power, a coupler with the splitting ratio of 95:5% is added before the input. The final module is depicted in Fig. 13(b).
After the completion of the module, its performance was tested and verified at different temperature. The performance changes at different temperature are illustrated in Fig. 14, from the figure we can see that the performances remain almost unchanged within the temperature ranging of −15 to 65 degrees. The variations in center wavelength, 0.5 dB bandwidth, 1 dB bandwidth, 3 dB bandwidth, ripple, AX, NX and TX are less than 0.013 nm, 0.017 nm, 0.011 nm, 0.004 nm, 0.097 dB, 1.37 dB, 0.289 dB and 1.060 dB, respectively as shown in Table 2. These results indicate that the AWG module exhibits high temperature insensitivity.
Fig. 14. The performance of ultra-broad bandwidth AWG at different temperature (a) 1524.885 nm (b) 1572.063nm
Table 2. The performance changes at different temperature
5. High-speed transmission performance tests
The ultra-broad bandwidth AWG of 60-channel 100 GHz channel spacing is designed for 600 G and higher speed backbone network transmission. The transmission speed at single wavelength need to achieve 96 Gbaud. The high-speed transmission is tested by the national optoelectronics innovation center, and the test principle is shown in Fig. 15. The test setup involves a tunable laser source entering the modulator via the ultra-broad bandwidth AWG. After passing through the AWG, a single wavelength signal light is obtained. Simultaneously, a high-frequency signal generated by an arbitrary waveform generator is sent through the driver and modulator to obtain the required signal. The resulting signal is then fed into an oscilloscope after being amplified by an EDFA. Figure 16 presents the eye diagrams for 96 Gbaud transmission. Figure 16(a) shows the eye diagram without the ultra-broad bandwidth AWG, where the signal noise ratio (SNR) is 5.34, and extinction ratio (ER) is 6.154 dB. The Fig. 16(b), Fig. 16(c) and Fig. 16(d) depict the eye diagrams with the ultra-broad bandwidth AWG at the wavelength of 1553.329 nm, 1555.747 nm and 1563.863 nm. At these wavelengths, the SNR is 4.81, 4.81 and 4.94, respectively. The ER is 4.098 dB, 4.098 dB and 4.258 dB, respectively. It is evident that the introduction of the ultra-broad bandwidth AWG has minimal impact on the performance, highlighting its effectiveness. In order to compare the difference between our ultra-broad bandwidth AWG with conventional AWG, the high-speed transmission is also tested using the conventional AWG, as shown in Fig. 17. While there is minimal change observed with the conventional AWG at 50 G baud transmission, the ER and jitter of the eye diagram degrade significantly at 96 Gbaud transmission. This demonstrates that conventional AWGs are not suitable for high-speed transmission.
Fig. 16. the eye diagrams of 96 Gbaud (a) without AWG (b) at the wavelength of 1553.329nm(with AWG)(c) at the wavelength of 1555.747nm(with AWG)(d) at the wavelength of 1563.863nm(with AWG)
Fig. 17. the eye diagrams (a) 50 G baud without AWG (b) 50 G baud with AWG at the wavelength of 1553.329 nm (c) 96 Gbaud without AWG (d) 96 Gbaud with AWG at the wavelength of 1553.329nm
Base on above discussion, it is evident that the ultra-broad bandwidth AWG, with 60-channel 100 GHz channel spacing, can successfully support 96 Gbaud transmission. From Fig. 1, we can obtain that this AWG can be used for 600 G and 800 G transmission by 16QAM and 64QAM modulation. By employing the C and L ultra-broad bandwidth AWGs with 60-channel 100 GHz channel spacing, the total transmission speed for a single-mode fiber can reach 72Tbps (120*600Gbps) and 96Tbps (120*800Gbps).
6. Conclusion
In conclusion, we have successfully designed and fabricated C&L band ultra-broad bandwidth AWGs of a 60-channel 100 GHz channel spacing based on a silica-based 0.75% relative refractive index. Special parabolic types were designed and optimized to achieve ultra-broad bandwidth and good crosstalk performance. For the C band, the on-chip insertion loss, uniformity, 0.5 dB bandwidth, 1 dB bandwidth and 3 dB bandwidth were measured as 2.91 dB, 0.36 dB, 0.614 nm, 0.721 nm and 0.937 nm, respectively. For the L band, the on-chip insertion loss, uniformity, 0.5 dB bandwidth, 1 dB bandwidth and 3 dB bandwidth were measured as 2.98 dB, 0.27 dB, 0.560 nm, 0.665 nm and 0.879 nm, respectively. To ensure stable performance at different temperature, a temperature control circuit was employed for packaging, and the AWGs demonstrated consistent performance within a temperature range of −15 to 65 degrees.
We further verified that the ultra-broadband AWGs are capable of transmitting 96 Gbaud signals, making them suitable for use in 600 G/800 G backbone network transmission. By utilizing the C&L ultra-broad bandwidth AWGs with a 60-channel 100 GHz channel spacing, the total transmission speed for a single-mode fiber can reach 72Tbps/96Tbps. Overall, our designed and fabricated ultra-broad bandwidth AWGs exhibit excellent performance characteristics, making them highly promising for next-generation backbone network applications.
Funding
National Key Research and Development Program of China (2021YFB2800201), Strategic Priority Research Program of Chinese Academy of Sciences (XDB43000000).
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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