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Abstract
In this paper, based on the low loss double strip silicon nitride platform, we designed and fabricated an ultra-low loss 1×4 microwave photonic beamforming chip, which contains a 1×4 beam splitter and four 5-bit optical delay lines. Each optical delay line can achieve 32 delay states varying from 0 ps to about 130 ps, which can support 21 different beamforming angles covers from −56.42° to 56.68° for 10 GHz RF signal. A low on-chip insertion loss of about 4 dB is achieved for each 5-bit optical delay line. Furthermore, a very low loss delay ratio of about 0.0016 dB/ps is achieved and a recorded low loss fluctuation of about 0.3 dB is obtained during the 32 states delay switching. In addition, the switching speed and driving power consumptions of the proposed beamforming chip were investigated. The proposed beamforming chip could have great potential in optical controlled phased antenna arrays systems.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Optical controlled phased antenna arrays (PAAs) based on optical beamforming network (OBFN) are regarded as one promising technology in future wideband wireless communication, satellite communication, and radar systems. Compared with the electronic beamforming network (EBFN) which usually uses the electronic phase shifter or the electronic delay line, optical tunable delay line (OTDL) is adopted in OBFN to realize the so called “optical true time delay”, which can overcome the well-known “beam squint” effect [1] in the EBFN system, reduce the transmission loss, immunity to electromagnetic interferences, especially in high frequency applications [2,3]. The OTDLs based on discrete optical components have been proposed and investigated for a long time. With the fast developments of photonic integrated circuits (PICs), integrated OTDL chips which have much compact sizes, lower power consumptions, and more reliable performances are drawing more and more attentions. Until now, various types of integrated OTDLs have been reported, such as optical switched delay lines (OSDLs) [4–9], microring resonators (MRRs) [10–20], photonic crystal waveguide [21], waveguide Bragg gratings [22,23], and subwavelength grating waveguide (SWG) delay line [24]. Among them, the structures of OSDL and MRR are relative simple and easy to be fabricated on the silicon on insulator (SOI) and silicon nitride (Si3N4) platforms, which are compatible with the CMOS processes, thus have attracted a lot of researchers’ interests. It is worth noting that both the OSDL and MRR have their own advantages and disadvantages. For the OTDL realized by using MRR, continuous time delay tuning can be achieved by adjusting the MRR’s coupling coefficient. However, there is an intrinsic trade-off between the delay and bandwidth for a single MRR. The off-resonance delay tuning method can be used to expand the delay bandwidth [10,11], but the delay tuning range are relatively small, typically tens of picoseconds. Another way is cascading more MRRs to increase the delay bandwidth [12–16], but flat in-band delay is difficult to achieve by introducing multiple cascade MRRs, where complex driving methods are needed. In a word, the OTDLs based on MRRs have a trade-off among the delay tuning range, delay bandwidth, in-band delay fluctuation, and control complexity. On the other hand, except for the shortage of discrete time delay switching, OSDL has the advantages of very larger bandwidth, lower in-band delay fluctuation, and relative larger tolerances to the fabrication errors and thermal variations, which make it more suitable for the applications where large delay bandwidth and low delay fluctuation are needed. Up to date, some OSDLs have been successfully demonstrated on the SOI and Si3N4 platforms. In Ref. [4], by using a 3µm wide ridge waveguide, a single channel 7-bit OSDL chip with footprint of 7.4 mm×1.6 mm was demonstrated on the SOI platform, achieving a maximum of 1.27 ns delay with a 10 ps resolution. The on-chip insertion losses are about 6.2 dB and 16 dB for the shortest and longest delay paths, from which the loss delay ratio (LDR) of about 0.008 dB/ps can be deduced. As known, low insertion loss (including fiber-chip coupling loss and on-chip insertion loss) and low LDR of OSDL chip is highly desired for OBFN systems since the induced RF loss will be doubled when converting from optical domain to microwave domain. By using 650nm wide ridge waveguide, we have demonstrated a single channel 7-bit silicon OSDL chip with footprint of 7.4 mm×1.8 mm, 127 delay states with delay step of about 1.52ps were achieved in the delay range of 0 ∼ 191.37 ps [5]. However, the on-chip insertion losses are about 15.64 dB and 19.05 dB for the shortest and longest delay paths, from which the LDR of about 0.018 dB/ps can be deduced. Compared with SOI waveguide, Si3N4 waveguide has the merits of much lower loss and much higher optical power handling capability without two-photon absorption, which are very promising for microwave photonic applications. In Ref. [6], a single channel 4-bit OSDL chip was demonstrated on the low loss Si3N4 platform. By using the ultra-thin Si3N4 waveguide (60nm thickness) with very low propagation loss of about 0.01dB/cm, very large optical delay up to 12.35 ns with resolution of 0.85 ns were obtained. Low fiber to fiber insertion losses of about 11.8 dB and 13.5 dB were measured for the shortest and longest delays. But relative large chip size of 85 mm×45 mm is required due to the very large bending radius of 5 mm according to the weak optical confinement of the very thin Si3N4 stripe waveguide. This makes it hard to be multichannel integrated in a single chip. Besides, the obtained delay resolution is not suitable for high frequency applications.
In order to satisfy the OBFN system requirements and increase the integration density and stability, some OBFN chips including multi-channel OSDLs have been demonstrated recently. In Ref. [7], an OBFN chip including four channel OSDLs based on SOI platform was proposed. But there is no optical beam splitter integrated on the chip, so additional optical splitter is needed which make the OBFN system complicated. Furthermore, the chip’s insertion loss and LDR were not improved compared to our previous work Ref. [5]. In Ref. [8], a silicon 1×8 OBFN chip based on OSDLs was demonstrated on SOI platform, it can support 16 GHz RF signal and has a large beamforming angle tuning range from −75.51° to 75.64°. However, the fiber-chip coupling loss is about 6.4 dB/facet, and the on-chip insertion losses are about 3.5 dB and 7.0 dB for the shortest and longest delay paths, the corresponding LDR is about 0.007 dB/ps, which needs to be further reduced. As can be seen, an OBFN chip with low insertion loss (including fiber-chip coupling loss and on-chip insertion loss) and low LDR has not been reported.
In this paper, based on the commercial low loss double strip Si3N4 waveguide platform (LioniX International using TriPleX ADS technology), which can well balance the trade-off between the waveguide bending radius and the waveguide loss, we designed and fabricated an ultra-low loss 1×4 OBFN chip based on 5-bit Si3N4 OSDLs. All the four OSDL channels can achieve 32 states of discrete delays with delay step of about 4.21 ps, which can realize 21 different beamforming angles cover from −56.42° to 56.68° for 10 GHz RF signal. The packaged OBFN chip shows good performances, including low fiber-chip coupling loss of about 2 dB/facet, low OSDL on-chip insertion loss of about 4 dB. Moreover, a very low LDR of about 0.0016 dB/ps was achieved and a recorded low loss fluctuation of about 0.3 dB was obtained during the 32 states delay switching.
2. OBFN chip design and fabrication
Figure 1(a) shows the schematic diagram of a typical optical controlled phased array antenna system using a 1×4 OBFN chip, in which the optical delay of each channel can be varied to reconstruct the wavefront and change the radiated beam direction. The beamforming angle can be expressed as:
where t is the delay between adjacent channels, d is the distance between the adjacent antennas, usually set as half wavelength of the transmitted RF signal. c is the speed of light in vacuum, θ is the beamforming angle. Figure 1(c) shows the schematic diagram of the 1×4 OBFN chip, which includes a 1×4 beam splitter and four channel 5-bit OSDLs. The 1×4 beam splitter consists of three Y branches, corresponding to about 7 dB extra insertion loss. Each 5-bit OSDL contains six 2×2 MZI thermo-optic switches, where a heater with 1000 µm length is covered on one arm to achieve thermo-optic tuning. The bottom and top waveguides between adjacent optical switches are the reference waveguides and delay waveguides, respectively. By configuring the “cross” and “through” states of all the six 2×2 thermo-optic switches, different optical delay paths can be selected in each OSDL and various optical delay differences among the four OSDL channels can be achieved.
Fig. 1. (a) The schematic diagram of optical controlled phased array antenna using 1×4 OBFN chip. LD: laser diode, MOD: modulator, PD: photodetector, EA: Electrical amplifier. (b) The cross-section of the double strip Si3N4 waveguide. (c) The schematic diagram of the 1×4 OBFN chip.
The designed 5-bit OSDL can achieve 32 states of discrete delays and assuming the delay step is Δτ. For the 1×4 OBFN chip, the optical delay difference t between adjacent OSDL channels can be changed from −10Δτ to 10Δτ, which can support 21 different beamforming angles according to Eq. (1). In our design, the OBFN chip is designed for 10 GHz RF signal, thus d is set as 15 mm. The beamforming angle should cover ± 55°, according to Eq. (1), the maximum delay difference between adjacent channels is about tmax = 41 ps. Thus the delay step Δτ should be about 4.1 ps. Consider the errors caused by chip fabrication, we set the delay step Δτ as 4.2 ps, which can cover ± 57.1° beamforming angle. The reference waveguide length is set as 400 µm and the cross-section of the double strip Si3N4 waveguide is shown in Fig. 1(b), where two trapezoid Si3N4 layers with thicknesses of h1 = 175 nm and h2 = 75 nm are separated by a SiO2 layer with thickness of g = 100 nm. The etching angle is α= 82°. Furthermore, top and bottom SiO2 claddings with thicknesses of hU = hL = 8 µm are adopted to avoid radiation loss [25]. The group index (ng) of the double strip Si3N4 waveguide is about ng = 1.77, which is given in the design manual of LioniX TriPleX platform [26,27], then we can set the lengths of the five optical delay waveguides by using Eq. (2):
where ΔLn (n = 1, 2…5) is the length of the nth optical delay waveguide in the OSDL. After subtracting the reference waveguide length (400 µm), the lengths of the five optical delay waveguides in the OSDL are calculated and given in Table 1. Then, the 5-bit OSDL in the proposed 1×4 OBFN chip can achieve optical delay switching from 0 ps to 130.2 ps with a delay step of 4.2 ps.
Table 1. The lengths of the five delay paths after subtracting the reference path in the 5-bit WSDL.
In order to calibrate the driving currents of all the 2×2 thermo-optic switches, we added five optical power monitor taps based on directional coupler structures as shown in Fig. 2(a), according to the 3D finite difference time domain (FDTD) simulation results shown in Fig. 2(b), coupling length of 90 µm and coupling gap size of 2 µm were chosen to download 1.2% optical power.
Fig. 2. (a) The schematic diagram of optical power monitor taps. (b) The simulated coupling efficiency at 1560 nm with varying coupling length.
The 1×4 OBFN chip was fabricated by LioniX International using the TriPleX ADS technology, which is based on a low loss double stripe Si3N4 waveguide realized on 100 mm silicon wafers with 8 µm thermal oxide [25]. The fabricated 1×4 OBFN chip is shown in Fig. 3(a) and the packaged module is shown in Fig. 3(b). The detailed microscope image of the chip is shown in Fig. 3(c), where the 1×4 OBFN is enclosed in the white dotted box. The propagation loss of the fabricated waveguide is about 0.095 dB/cm according to our following measurements.
Fig. 3. (a) The 1×4 OBFN chip. (b) The packaged OBFN chip. (c) The optical microscope image of the fabricated chip.
3. Measurement results and discussion
First, an experimental setup shown in Fig. 4 was built to calibrate the “through” and “cross” driving currents of all the 2×2 optical switches. Take the six optical switches in the first OSDL channel as example, the laser from a tunable laser source (TLS, Santac WSL-710) with output power of 10 dBm was injected into the “in” port of the 1×4 OBFN chip as shown in Fig. 1(c), then a power meter (PM, Santac MPM-210) was used to measure the optical power dropped from optical power monitoring taps. By scanning the driving currents applied on each optical switch with an analog output module (AOM, NI PXIe-4322), the relation between the driving current and tapped optical power of each optical switch can be measured. By using this method, the first five optical switches’ responses were measured, while the last optical switch’s response was measured at the “out1” port show in Fig. 1(b).
Fig. 4. The experimental setup of switch’s extinction ratio and driving power test. TSL: tunable laser source, PC: polarization controller, PM: power meter, AOM: analog output module.
The measured responses of all the six optical switches are shown in Fig. 5(a)∼(f), respectively. As can be seen, over 20 dB optical extinction ratios (ERs) are guaranteed for all the six optical switches. With the driving currents to maintain the “through” states and the “cross” states (denoted as It and Ic) and the resistances of the six heaters, the driving power consumptions of all the 32 delay states for the first OSDL channel were obtained as shown in Fig. 6(a). By using the same procedure, the driving power consumptions of other three OSDL channels were also measured and shown Fig. 6(b)∼(d).
Fig. 5. (a)∼(f) The measured transmissions at the optical power taps with varying driving currents in the first OSDL channel. IT: the driving currents to maintain the “through” state, IC: the driving currents to maintain the “cross” state, ER: extinction ratio.
Fig. 6. Power consumptions of all the 32 delay states in (a) channel 1, (b) channel 2, (c) channel 3, (d) channel 4.
As known, the optical switch’s response time is another important characteristic since it determines the switching speed of the OBFN chip. An experimental setup shown in Fig. 7(a) was used to measure the switching time of the Si3N4 thermo-optic switch. Laser (10 dBm) output from the TLS (Santac WSL-710) was injected into the first optical switch in the OSDL, whose heaters’ driving voltage was controlled by the arbitrary waveform generator (AWG, Keysight 33500B) and the AWG’s output was also sent to the oscilloscope (Agilent, MSO7104B) for reference. At the power monitor tap, a photodetector (Thorlabs DET01CFC) was adopted to convert optical signal into electronic signal, which is sent to the oscilloscope to obtain the switch response. By applying a 1 KHz square wave signal with high level voltage of 12.6 V (corresponding to 45 mA, IT) and low level voltage of 9.1 V (corresponding to 32.5 mA, IC) to the thermo-optic switch, the time response of the thermo-optic switch was measured and show in Fig. 7(b), in which the rise time (heating, 10%∼90%) and fall time (cooling, 90%∼10%) are 102 µs and 109 µs, respectively.
Fig. 7. (a) The switching time measurement setup. TSL: tunable laser source, PC: polarization controller, PD: photodetector, AWG: arbitrary waveform generator. (b) The measured switching time response.
After calibrating the “cross” and “through” driving currents of all the twenty-four optical switches in the 1×4 OBFN chip, all the delay states of the four channel 5-bit OSDLs were measured by using the optical vector network analysis (OVNA) method [28], as shown in Fig. 8. The laser from a TLS (Santac WSL-710) with 15 dBm output power was injected into the MZM (EOSpace AX 0MSS 20 PFA PFA LV), which was biased at the quadrature point and driven by the RF signals from the vector network analyzer (EVNA, Agilent N5242A). Then the light was injected into the OBFN chip. A PD (Finisar XPDV2120RA) was adopted to retrieve the RF output signal after the OBFN chip, the RF output signal was amplified by low noise amplifier (MWLA1966), which was sent back to the EVNA to measure the delay. By controlling the driving currents applied on the switches, the delay values of the 32 delay states of each OSDL can be easily measured utilizing the OVNA method.
Fig. 8. The delay measurement setup based on the OVNA method. TSL: tunable laser source, PC: polarization controller, MZM: Mach-Zehnder modulator, PD: photodetector, LNA: low noise amplifier, EVNA: electronics vector network analyzer, AOM: analog output module.
Figure 9(a)∼(d) show the measured 32 delay states (from 0Δτ to 31Δτ) of OSDL1, OSDL2, OSDL3, OSDL4, respectively. The black dots denote the measured delays, while the red dots denote the designed delays. As can be seen, relatively small delay errors, which are the differences between the measured delays and the designed delays, are achieved for all four OSDLs. The average delay errors of the four channels are −0.57 ps, −0.50 ps, 0.06 ps, 0.67 ps, respectively, which mainly come from the measurement errors. In addition, relative good linearities are obtained for all the OSDLs and the corresponding delay slopes are about 4.20 ps/state, 4.21 ps/state, 4.23 ps/state, 4.20 ps/state, respectively, which are very close to the design value of 4.20 ps/state.
Since there is no resonant structure introduced in the OSDL, and wide microwave band corresponds to very narrow optical band, the greatest advantage of OSDL is the intrinsic very large delay bandwidth, which can easily handle large instantaneous bandwidth required in microwave and even Terahertz beamforming. Here we measured the optical transmission spectra of the shortest (0Δτ) and the longest (31Δτ) optical delay path in the OSDL, which is flat in 1550 nm ∼ 1570 nm as shown in Fig. 7(a), in which the enlarged transmission spectra near the optical carrier wavelength is shown as the inset. Only about 1 dB fluctuation is observed, which may be caused by the residual unbalanced MZI interference due to the optical switches’ “not perfect” crosstalk and the residue Fabry-Perot interferences in the optical packaging. On the other hand, the measured total fiber to fiber insertion loss of the 1×4 OBFN chip is about 15 dB, which includes about 2 dB/facet fiber-chip coupling loss, about 7 dB on-chip insertion loss of the 1×4 beam splitter and about 4 dB on-chip insertion loss of the OSDL. It is worth noting that only about 1.5% loss are induced by the optical delay waveguides, which indicates that much longer optical delay can be easily achieved by using this Si3N4 waveguide platform. In addition, according to Fig. 10(b), the insertion loss variations among all the delay states is only about 0.3 dB, which is well agree with the 0.22 dB propagation loss difference between the shortest and longest delay paths. By linear fitting of the insertion losses under different delay states, a very small LDR of about 0.0016 dB/ps was achieved, which is an order of magnitude lower than typical SOI based OSDL.
Fig. 10. (a) The transmission spectra of the shortest (0Δτ) and longest (31Δτ) delays. (b) The insertion losses under different delay states and the linear fitting.
In order to evaluate the beamforming performance of the proposed 1×4 OBFN chip, the far-field radiation patterns were simulated as the summation of all the radiations from each antenna, which can be expressed as [9]:
Fig. 11. (a) The simulated radiation patterns based on the measured delay values and (b) the designed delay values.
Finally, we compare our work with the state-of-the-art integrated OSDLs chips as illustrated in Table 2. Our OBFN chip based on the double strip Si3N4 waveguide shows low fiber-chip coupling loss, low on-chip insertion loss and very low LDR, which can greatly reduce the optical power fluctuations among different delay states of the OSDLs. Comparing with the low loss OSDL using single ultra-thin Si3N4 stripe waveguide [6], our OSDL based on the double stripe Si3N4 waveguide can greatly reduce the OSDL’s footprint, which is 8.5 cm×4.5 cm in Ref. [6]. The very large bending radius of 5 mm makes it hard to be multichannel integrated in a single OBFN chip. Besides, the driving power of our OSDL could be much lower than Ref. [6], since thinner SiO2 cladding thickness are used, which also can support much faster switching speed. Compared to the OSDL based on SOI show in Ref. [4,5,7,8,9], our Si3N4 waveguide based OSDL shows a lower insertion loss and LDR. On the other hand, the main disadvantages of the proposed Si3N4 waveguide based OBFN chip is the much larger power consumption and much lower switching speed comparing with the SOI waveguide based OBFN chips. In future, these disadvantages could be alleviated by combing spiral phase shifter and optimized pulse driving as shown in our previous work [29]. Moreover, Si3N4 waveguide combined with piezoelectric electrode could be another effective way to greatly reduce the driving power [30].
Table 2. Performance comparison of the OSDL based ODLs and OBFN chips.
4. Conclusion
In this paper, we designed and fabricated an ultra-low loss OBFN chip based on 5-bit OSDLs on the double strip Si3N4 waveguide platform. The measured total fiber to fiber insertion loss of the 1×4 OBFN chip is about 15 dB, which includes about 2 dB/facet fiber-chip coupling loss, about 7 dB on-chip insertion loss of the 1×4 beam splitter and about 4 dB on-chip insertion loss of the OSDL. Moreover, a very low loss delay ratio of about 0.0016 dB/ps was obtained, which is an order of magnitude lower than the traditional SOI based OSDL. A recorded low loss fluctuation of about 0.3 dB was obtained during the 32 states delay switching. In future work, spiral phase shifter and optimized pulse driving can be adopted to improve the power consumption and switching speed of OBFN chip.
Funding
National Key Research and Development Program of China (2018YFB2201800); National Natural Science Foundation of China (62171118); National Natural Science Foundation of China (62105061).
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.
References
1. I. Frigyes and A. J. Seeds, “Optically generated true-time delay in phased array antennas,” IEEE Trans. Microwave Theory Techn. 43(9), 2378–2386 (1995). [CrossRef]
2. J. Capmany and D. Novak, “Microwave photonics combines two worlds,” Nat. Photonics 1(6), 319–330 (2007). [CrossRef]
3. J. Yao, “Microwave Photonics,” J. Lightwave Technol. 27(3), 314–335 (2009). [CrossRef]
4. J. Xie, L. Zhou, Z. Li, J. Wang, and J. Chen, “Seven-bit reconfigurable optical true time delay line based on silicon integration,” Opt. Express 22(19), 22707–22715 (2014). [CrossRef]
5. P. Zheng, C. Wang, X. Xu, J. Li, D. Lin, G. Hu, R. Zhang, B. Yun, and Y. Cui, “A Seven bit silicon optical true time delay line for Ka-band phased array antenna,” IEEE Photonics J. 11(4), 1–9 (2019). [CrossRef]
6. R. L. Moreira, J. Garcia, W. Li, J. Bauters, J. S. Barton, M. J. R. Heck, J. E. Bowers, and D. J. Blumenthal, “Integrated ultra-low-loss 4-bit tunable delay for broadband phased array antenna applications,” IEEE Photon. Technol. Lett. 25(12), 1165–1168 (2013). [CrossRef]
7. X. Wang, L. Zhou, R. Li, J. Xie, L. Lu, K. Wu, and J. Chen, “Continuously tunable ultra-thin silicon waveguide optical delay line,” Optica 4(5), 507–515 (2017). [CrossRef]
8. P. Zheng, X. Xu, D. Lin, P. Liu, G. Hu, B. Yun, and Y. Cui, “A wideband 1 × 4 optical beam-forming chip based on switchable optical delay lines for Ka-band phased array,” Opt. Commun. 488(1), 126842 (2021). [CrossRef]
9. C. Zhu, L. Lu, W. Shan, W. Xu, G. Zhou, L. Zhou, and J. Chen, “A silicon monolithic integrated 8-channel microwave photonic beamformer,” Optica 7(9), 1162–1170 (2020). [CrossRef]
10. N. Tessema, Z. Cao, J. Zantvoort, K. Mekonnen, A. Dubok, E. Tangdiongga, A. Smolders, and A. Koonen, “A tunable Si3N4 integrated true time delay circuit for optically-controlled K-band radio beamformer in satellite communication,” J. Lightwave Technol. 34(20), 4736–4743 (2016). [CrossRef]
11. W. Shan, L. Lu, X. Wang, G. Zhou, Y. Liu, J. Chen, and L. Zhou, “Broadband continuously tunable microwave photonic delay line based on cascaded silicon microrings,” Opt. Express 29(3), 3375–3385 (2021). [CrossRef]
12. L. Zhuang, C. Roeloffzen, R. Heideman, A. Borreman, A. Meijerink, and W. van Etten, “Single-chip ring resonator-based 1×8 optical beamforming network in CMOS-compatible waveguide technology,” IEEE Photonics Technol. Lett. 19(15), 1130–1132 (2007). [CrossRef]
13. J. Xie, L. Zhou, Z. Zou, J. Wang, X. Li, and J. Chen, “Continuously tunable reflective-type optical delay lines using microring resonators,” Opt. Express 22(1), 817–823 (2014). [CrossRef]
14. Y. Liu, A. Wichman, B. Isaac, J. Kalkavage, E. Adles, T. Clark, and J. Klamkin, “Tuning optimization of ring resonator delays for integrated optical beamforming networks,” J. Lightwave Technol. 35(22), 4954–4960 (2017). [CrossRef]
15. Y. Liu, A. Wichman, B. Isaac, J. Kalkavage, E. J. Adles, T. R. Clark, and J. Klamkin, “Ultra-low-loss silicon nitride optical beamforming network for wideband wireless applications,” IEEE J. Select. Topics Quantum Electron. 24(4), 1–10 (2018). [CrossRef]
16. D. Lin, X. Xu, P. Zheng, H. Yang, G. Hu, B. Yun, and Y. Cui, “A tunable optical delay line based on cascaded silicon nitride microrings for Ka-band beamforming,” IEEE Photonics J. 11(5), 1–10 (2019). [CrossRef]
17. M. Burla, D. Marpaung, L. Zhuang, A. Leinse, M. Hoekman, R. Heideman, and C. Roeloffzen, “Integrated photonic Ku-band beamformer chip with continuous amplitude and delay control,” IEEE Photonics Technol. Lett. 25(12), 1145–1148 (2013). [CrossRef]
18. C. Xiang, M. Davenport, J. Khurgin, P. Morton, and J. Bowers, “low-loss continuously tunable optical true time delay based on Si3N4 ring resonators,” IEEE J. Select. Topics Quantum Electron. 24(4), 1–9 (2018). [CrossRef]
19. G. Choo, C. Madsen, S. Palermo, and K. Entesari, “Automatic monitor-based tuning of an RF silicon photonic 1×4 asymmetric binary tree true-time-delay beamforming network,” J. Lightwave Technol. 36(22), 5263–5275 (2018). [CrossRef]
20. J. Cardenas, M. Foster, N. Sherwood-Droz, C. Poitras, H. Lira, B. Zhang, A. Gaeta, J. Khurgin, P. Morton, and M. Lipson, “Wide-bandwidth continuously tunable optical delay line using silicon microring resonators,” Opt. Express 18(25), 26525–26534 (2010). [CrossRef]
21. J. Sancho, J. Bourderionnet, J. Lloret, S. Combrié, I. Gasulla, S. Xavier, S. Sales, P. Colman, G. Lehoucq, D. Dolfi, J. Capmany, and A. De Rossi, “Integrable microwave filter based on a photonic crystal delay line,” Nat. Commun. 3(1), 1075 (2012). [CrossRef]
22. M. Burla, L. Cortés, M. Li, X. Wang, L. Chrostowski, and J. Azaña, “Integrated waveguide Bragg gratings for microwave photonics signal processing,” Opt. Express 21(21), 25120–25147 (2013). [CrossRef]
23. I. Giuntoni, D. Stolarek, D. I. Kroushkov, J. Bruns, L. Zimmermann, B. Tillack, and K. Petermann, “Continuously tunable delay line based on SOI tapered Bragg gratings,” Opt. Express 20(10), 11241–11246 (2012). [CrossRef]
24. J. Wang, R. Ashrafi, R. Adams, I. Glesk, I. Gasulla, J. Capmany, and L. R. Chen, “Subwavelength grating enabled on-chip ultra-compact optical true time delay line,” Sci. Rep. 6(1), 30235 (2016). [CrossRef]
25. R. G. Heideman, A. Leinse, M. Hoekman, F. Schreuder, and F. H. Falke, “TriPleX: The low loss passive photonics platform: Industrial applications through multi project wafer runs,” in Proc. IEEE Photon. Conf., 224–225, (2014).
26. Lionix International, “silicon nitride waveguide technology TriPleX,” https://photonics.lionix-international.com
27. C. G. H. Roeloffzen, M. Hoekman, E. J. Klein, et al., “Low-Loss Si3N4 TriPleX Optical Waveguides: Technology and Applications Overview,” IEEE J. Sel. Top. Quantum Electron. 24(4), 1–21 (2018). [CrossRef]
28. S. Pan and M. Xue, “Ultrahigh-resolution optical vector analysis based on optical single-sideband modulation,” J. Lightwave Technol. 35(4), 836–845 (2017). [CrossRef]
29. D. Lin, W. Chen, S. Shi, et al., “Improving performance of silicon thermo-optic switch by combing spiral phase shifter and optimized pulse driving,” IEEE Photonics J. 14(3), 1–7 (2022). [CrossRef]
30. N. Hosseini, R. Dekker, M. Hoekman, M. Dekkers, J. Bos, A. Leinse, and R. Heideman, “Stress-optic modulator in TriPleX platform using a piezoelectric lead zirconate titanate (PZT) thin fifilm,” Opt. Express 23(11), 14018–14026 (2015). [CrossRef]
