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arXiv:1812.01861 · IEEE Photonics Technology Letters

Temporal Cloak Without Synchronization

Zhixing Lin, Shuqian Sun, Wei Li, Ninghua Zhu, Ming Li

Published Jan 29, 2019• 4 citations

Abstract

Considered to only exist in fairy tales in the past, invisibility cloaks have been successively converted into reality no matter in the spatial domain or the temporal domain. Inspired by the spatial cloaking, time gaps are utilized to hide temporal events. However, a sophisticated synchronization for cloaking is indispensable in these cloaking techniques, therefore leaving inconvenience for the realization of the temporal cloak. Here, by exploiting the temporal Talbot effect, we propose a brand new scheme and concept to achieve a temporal cloak without any synchronization for the cloaking process, under which the intensity-modulated event is directly turned into invisibility in intensity through temporal averaging effect induced by the Talbot effect. We successfully realize the temporal cloak for periodic and pseudo-random signals. We also find that the higher order temporal Talbot effect is beneficial to the cloaking performance. Due to the transformed information on the phase, the data could be recovered from the intensity-cloaked waveform, leading to a non-full-field cloak. This method and its concept render a distinct perspective for the temporal cloak, extend temporal cloak to the pulsed-wave, and promote the development of confidential communication.

DOI: 10.1109/lpt.2019.2895859

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The pursuit of invisibility cloaks has long been a central theme in science fiction literature. One approach involves altering the waveform transmitted to the observer in the spatial domain, represented by meta-materials. Another approach is to open a window in the time domain. Specifically, researchers for the first time opened a single or periodic time gap in the probe light sent to the observer, thereby hiding an event within this time gap [Nature481, 62 (2012)10.1038/nature10695]. However, the absence of an optical carrier in the opened window prevented the recovery of the hidden event even for the cooperator. Now, we have demonstrated a novel temporal cloak that can recover hidden events. In this temporal cloak, the opening and closing of time gaps are achieved through the precise manipulation of photon density and phase synchronization in the injection-locked laser. During the process, the inevitable spontaneous emission photons are utilized to carry the event information. The cloaking window demonstrates both flexibility and reconfigurability, with its size achieving a breakthrough extension to 2.5 µs compared with dispersion-based methods. These advancements represent a significant step towards the realization of time cloaking and have a direct impact on secure communication.

A temporal cloak with a significantly extended cloaking window and spatial distribution is created using the inverse temporal Talbot effect. The continuously cloaking window and the total cloaking ratio are 196 ps and 74%, respectively, which are 5.4 and 1.6 times larger than the previous record. Moreover, the cloak is maintained over 5-km of dispersion-compensating fiber (DCF), which enables cloaking temporal events at multiple positions simultaneously. To demonstrate the cloaking performance, both message-encoded and pseudo-random temporal events are successfully concealed. Last, but not least, since our configuration does not require opposite sign of dispersion, the idea can be applied analogously to the spatial domain according to the space-time duality, thus also enriching the spatial cloaking technique.

Recent research on time cloaking has revealed a fascinating approach to hide temporal events from an interrogating optical field, by opening up and subsequently closing intensity gaps in a probe beam. Experiments thus far have demonstrated temporal cloaking of nonlinear interactions and high-speed optical data. Here we report a temporal cloak with the new capability not only to hide optical data, but also to concurrently transmit it along another wavelength channel for subsequent readout, masking the information from one observer while directing it to another. Additionally, the cloak succeeds in passing modulated data unscathed through a scrambling event, providing a new form of tampering resistance. Both examples launch a paradigm shift in temporal cloaking: instead of using time cloaks primarily to disrupt communication, we show how they can also improve data transmission, in turn greatly widening the range of possible applications in telecommunications.

Recent research has revealed a significant ability to manipulate and control a light beam to hide events in time-domain. Achieving highly functional temporal cloaking, which open up and subsequently close intensity gaps in the probe beam to conceal events from the observer with satisfied performances, are still difficult in optical fiber communication. Here, we propose and experimentally demonstrate a temporal cloak system based on ultra-short-pulse generation and time-domain fraunhofer diffraction. We succeed in hiding 83 percent of the whole time axis and cloak pseudorandom dark return-to-zero data bitrate at 10 Gbit/s. The four states of temporal cloaking and condition of cloaking off have been investigated in this paper.

Spectral cloaking, by creating a spectral hole in a probe signal within which the spectrum of an obstacle signal can hide, is an important technology for encrypted communications and signal processing. Previous studies mainly rely on electro-optic modulations to realize spectral cloaking, which, however, suffer from inherent bandwidth limitations. By constructing a nonlinear four-wave mixing (FWM) time lens, we achieve the spectral Talbot effect and apply it to realize broadband spectral cloaking. We analyze the conditions to realize integer and fractional spectral Talbot effect, whereby arbitrary scaling of the free spectral range (FSR) of an input frequency comb can be achieved. By further cascading two FWM time lenses with reversible dispersions, we achieve FSR reconstruction for the input signal via spectral multiplication and division, thus creating a broadband spectral cloak up to 40 GHz in between. Such a cloak can hide the spectral information of an obstacle signal from being probed by the signal light, thus realizing invisible functionality. We establish an all-optical approach for realizing advanced spectral cloaking through FSR control, paving also promising avenues toward high-security encryption and antijamming optical communications.