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Figure 5. Acoustic circulator based on angular-momentum biasing through spatiotemporal modulation. Top: Three cylindrical cavities are coupled together through small channels. The cavities are also coupled to three external waveguides through small channels. The volume of the cavities is dynamically modulated with the same am- plitude and frequency and with a phase difference of 120° between adjacent cavities. Bottom: By tailoring both the amplitude and fre- quency of the volume modulation, it is possible to obtain an acoustic circulator( full-wave simulations, complex pressure field). Adapted with permission from Fleury et al. (2015). © 2015 American Physical Society.
Outlook for Nonreciprocal
Acoustic Devices
The realization of nonreciprocal acoustic systems based on existing theoretical foundations and experimental valida- tion can undeniably extend our ability to manipulate acous- tic signals. They therefore have a strong potential for broad industrial impact, as is the case for their electromagnetic counterparts. This section therefore provides a technologi- cal outlook for this nascent field based on a few possible ap- plications for which nonreciprocal acoustical devices may offer unique capabilities.
Circulators are widely used in electromagnetics in radar and radiocommunication systems as a way to realize full duplex operation with a single antenna (Figure 6, top). Full duplex operation means that a given transducer is capable of emit- ting and receiving at the same time on the same frequency channel. If one does not have a circulator, the functional- ity can be realized using two distinct transducers (Figure 6, middle), one for the receiving circuit (Rx) and the other one for the transmitting circuit (Tx). This, however, requires the use of two well-isolated transducers, and suitable, quite con- voluted signal-processing tricks to distinguish the two signal flows.
If one wants to use a single transducer, full duplex opera- tion is no longer possible because the transducer cannot dis- tinguish between incident and outgoing signals at the same frequency. A solution is to use time-multiplexing techniques and time gating in the form of a switch that connects the transducer to Rx or Tx depending on whether one is receiv- ing or transmitting (Figure 6, bottom). Half duplex is typical in sonar, underwater acoustic communication systems, and ultrasound imaging devices. These systems send short pulses or use signal-processing techniques such as pulse compres- sion to separate the incident wave from echoes. In these cas- es, the output power is limited by the duration of the pulse, which ultimately limits the sampling rate and the signal-to- noise ratio. In underwater acoustic communication systems, half duplex means that two communicating systems cannot talk at the same time and have to take turns sending infor- mation, which ultimately limits the communication speed. Using different frequency channels to transmit and receive is another option, but this also implies an inefficient use of the available spectrum. In other words, the absence of acoustic circulators inherently limits current acoustic imaging and communication systems. Interestingly, coupling an electro-
Figure 6. Nonreciprocal acoustic devices such as isolators may be used to enable full duplex operation in underwater acoustic com- munications or sonar systems.
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