硅光子芯片上的微波信号切换

Microwave photonics uses light to carry and process microwave signals over a photonic link. However,  light can instead be used as a stimulus to microwave devices that directly control microwave signals.  Such optically controlled amplitude and phase-shift switches are investigated for use in reconfgurable  microwave systems, but they sufer from large footprint, high optical power level required for  switching, lack of scalability and complex integration requirements, restricting their implementation  in practical microwave systems. Here, we report Monolithic Optically Reconfgurable Integrated  Microwave Switches (MORIMSs) built on a CMOS compatible silicon photonic chip that addresses  all of the stringent requirements. Our scalable micrometer-scale switches provide higher switching  efciency and require optical power orders of magnitude lower than the state-of-the-art. Also, it  opens a new research direction on silicon photonic platforms integrating microwave circuitry. This  work has important implications in reconfgurable microwave and millimeter wave devices for future  communication networks.

Te SOI wafer consists of 250nm-thick device layer and 3μm-thick buried oxide layer. During the fabrication  process, most of the silicon material is removed to form Si photoconductive patches with dimension of 16 μm  by 12 μm. Single-mode SiNx ridge waveguide with the dimensions of 800nm-width and 400nm-height are fabricated and used to guide light toward Si patches in order to activate them at diferent locations on the chip. Te  ridge waveguide and Si photoconductive patch are cladded by 1µm-thick SiO2 layer. Te Ground-Signal-Ground  (GSG) transmission lines consist of 800nm-thick Al lines with a tapered signal electrode toward the Si photoconductive patch.

Figure 1c,d show the SEM images of MORIMSs of both types. Te SiNx waveguide conformally covers the Si  photoconductive patch without any crack and discontinuity. Tis process is CMOS compatible and the details of  the nanofabrication are described in Method.

Te On/Of performances of the MORIMS are characterized by measurements  of the S-parameters. Te experimental details are described in the Characterization section. Figure 2a,b show the  measured S21 parameter of tapered- and through-type structures at On and Of state up to ~40GHz. In Fig. 2a there is a dip in S21. Tis is due to the imperfection of the transmission line. More precisely, the 21GHz frequency  corresponds to free spectral range between the probe and the gap. Te fact that this frequency shifs slightly when  the gap is illuminated testifes a change of the dielectric constant. To characterize the switches performance, the  extinction ratio Ron/of n/|S21(On)/S21(Of)| is adopted as the fgure of merit that qualifes amplitude switching  efciency for a given microwave frequency7 . Figure 2c,d show Ron/of with respect of input optical power at frequencies of 5GHz, 20GHz and 40GHz. Overall, the On/Of ratios increase linearly from 0 to ~1.5 mW before  reaching a saturation plateau. As expected, the tapered-type switch shows higher performance, with switching  efciency of ~25 dB and ~23 dB at 5GHz and 20GHz, respectively compared to ~14 dB and ~12 dB achieved at  same frequencies with the through-type confguration. Although the through-type is less efcient under same  incident optical power, the remaining energy in the waveguide can be used to control another switch as shown  next. It is worth mentioning that the switching time of the proposed device is on the order of few micro-seconds  which is compatible with beam steering and beamforming applications requirements.

Te proposed optically reconfgurable switches are a proof of concept that can be easily implemented in beamforming and beam steering microwave systems which require moderate switching time constant. Moreover,  the proposed integrated devices could also enable more advance functionalities when combining other  well-established photonic building blocks such as ring resonators, directional couplers and Mach-Zehnder modulators on the same chip (discussed in Supplementary Information Section IV). Te proposed approach can  be tailored in the future generation of ultra-high frequency communications systems which will face stringent  requirements in terms of frequency bandwidth, power consumption, size and packing density, and low-cost for  mass production. In that area, ultra-fast photoconductive switches exploiting III-V materials, with ultra-short  carrier lifetime, are required and outstanding eforts has been already made. Te proposed approach could  be exploited in sampling application that requires the combination of several switches with very accurate time  delays between them. Tis work is a real added value for developing integration technology for microwave signal  processing. Besides, in our case, the microwave signal is optically processed but in the microwave domain directly,  thus relaxing the constraint of up-converting the microwave signal to an optical carrier which leads to conversion  losses and additive noise. Accordingly, the MORIMS architecture can be directly implemented in any microwave  sub-system such tunable microwave flters of larger systems such as phased array antennas.

Fig2(c)

In summary, we have demonstrated monolithic optically reconfgurable integrated microwave switches on  a SOI chip. Our approach consists of co-integration of microwave circuits with integrated photonic devices to  form optically reconfgurable microwave switches. A single input SiNx waveguide is used to route the light toward  switches at diferent location on chip. Integrated photonics provides miniaturized Si photoconductive patches,  high confnement of light in the waveguide and high coupling efciency of light from waveguide to silicon photoconductive microwave switches. Consequently, the demonstrated engineered devices outperform their classical  analogues in term of On/Of switching efciency, footprint and optical power level requirement. We experimentally demonstrate high microwave amplitude switching performances of over 25dB around 5GHz, 23dB around  20GHz and 11dB at 40GHz, and lower optical power requirement (~2mW) by orders of magnitude lower than  the state-of-art photoconductive switches. Scalability is a challenge that has been also advanced by demonstrating  integrated multiple reconfgurable switches on the same SOI chip with high amplitude switching performance.  Moreover, phase shifs of 20° and 60° were measured for microwave signals at 20GHz and 40GHz, respectively.  Tis work is an important step in introducing photonics into direct processing of microwave signals, paving the  way towards optically reconfgurable microwave and millimeter wave devices for future ground, embedded radar  systems, and emerging 5G wireless communication networks.