Demonstration of a Free-Space Optical Delay Line Using Space-Time Wave Packets

Abstract

An optical buffer having a large delay-bandwidth-product -a critical component for future all-optical communications networks -remains elusive. Central to its realization is a controllable inline optical delay line, previously accomplished via engineered dispersion in optical materials or photonic structures constrained by a low delay-bandwidth product. Here we show that space-time wave packets whose group velocity in free space is continuously tunable provide a versatile platform for constructing inline optical delay lines. By spatio-temporal spectral-phase-modulation, wave packets in the same or in different spectral windows that initially overlap in space and time subsequently separate by multiple pulse widths upon free propagation by virtue of their different group velocities. Delay-bandwidth products of ∼100 for pulses of width ∼1 ps are observed, with no fundamental limit on the system bandwidth. arXiv:1910.05616v1 [physics.optics] 12 Oct 2019The relentless increase in demand for communications bandwidth [1] has spurred developments in sub-systems that are critical for future optical networks, such as spatial-mode multiplexing [2-4] and ultrafast modulators [5]. A critical -but to date elusive -component is an optical buffer that alleviates data packet contention at optical switches by reordering the packets without an optical-to-electronic conversion [6]. Previous efforts addressing this challenge have exploited so-called 'slow light' [7,8], which refers to the reduction in the group velocity of a pulse traversing a selected material [9][10][11][12] or carefully designed photonic structure [13][14][15]. Despite the diversity of their physical embodiments [16], slow-light approaches typically rely on resonant optical effects and are thus limited by a delay-bandwidth product (DBP) on the order of unity.That is, the differential group delay with respect to a reference pulse traveling at c (the speed of light in vacuum) does not exceed the pulse width [17,18], which falls short of the requirements of an optical buffer [19]. Time-varying systems [20] can overcome this limit at the expense of increased implementation complexity. In one realization, a trapdoor mechanism in coupled resonators increases the storage time through externally controlled coupling [21] -but losses concomitantly increase in step with the delay. An alternate approach makes use of non-resonant recycling of orthogonal modes in a cavity [22]. A different strategy relies on transverse spatial structuring to reduce the group velocity in free space, but only a minute reduction below c has been detected to date [23,24]. Nevertheless, theoretical proposals suggest that pushing this approach to the limit may produce sufficiently large differential group delays for an optical buffer [25,26], but temporal spreading is associated with the propagation of these wave packets [27].Finally, a recent theoretical proposal suggests that optical non-reciprocity can help bypass the usual DBP limits [28], but doubts have been cast on this prospect [29,30].An alt...

Hybrid guided space-time optical modes in unpatterned films

“…Alternatively, multiple y modes can be excited with the exact spatiotemporal spectral structure using a single-SLM system by assigning each mode to a different spectral channel by segmenting the SLM active area and imprinting a judicious phase distribution in each, as demonstrated recently in ref. 76 .…”

“…Another exciting prospect is the study of the ST wave packets with negative group velocities 54,70,77 propagating along a waveguide. Furthermore, we have recently synthesized multiple copropagating ST wave packets in the same spatial channel while occupying the same or different spectral channels with independently addressable group velocities 76 . Combined with the demonstration reported here, these results lay the foundation for constructing on-chip optical delay lines based on confined ST wave packets, which may lead to the realization of all-optical buffers that have remained elusive to date.…”