Self-imaging of structured light in new dimensions

Researchers of photonics from Tampere University, Finland, and Kastler-Brossel Laboratory, France, have demonstrated how self-imaging of light, a phenomenon known for nearly two centuries, can be applied to cylindrical systems, facilitating unprecedented control of light’s structure with great potential for advanced optical communication systems. In addition, a new type of space-time duality is explored for powerful analogies bridging different fields of optics.

In 1836, Henry F. Talbot performed an experiment, where he observed light patterns that naturally reappear after some propagation without the use of any lenses or imaging optics – a self-imaging phenomenon nowadays of then termed the Talbot effect.

Recently, researchers interested in sculpting light from the Experimental Quantum Optics Group (EQO) in Tampere University, as well as the Complex Media Optics group at Kastler Brossel Laboratory, in Ecole Normale Supérieure, Paris, have teamed up and investigated the self-imaging Talbot effect in cylindrical systems in greater depth than ever before.  The presented interesting fundamental physics and powerful applications in optical communications have now been published in the prestigious Nature Photonics journal.

Exploring the effect of self-imaging in cylindrical geometries

Light travelling in a so-called ring-core fiber experiences a self-imaging process, albeit in angular position.

“As light enters the fiber at a specific angular position of the ring-like fiber core, it first spreads around the entire cylindrical core and then perfectly recombines to form the original field via the self-imaging process” explains Doctoral researcher Matias Eriksson, one of the leading authors of the study.

Importantly, this self-imaging in angular positions is only half of the fundamental phenomenon in cylindrical geometries. A similar interference effect also appears in a closely related property of light known as orbital angular momentum, which allows light to rotate particles around the optical axis, i.e., make them orbit on a ring-like path. Fundamentally, both properties, angular position and orbital angular momentum are considered complementary variables, which means that the precise definition of one leads to imprecision of the other property.

The team now combines the self-imaging in angle and orbital angular momentum for the first time in single experiment, for unprecedented control of the light’s spatial structure. But the study doesn’t stop there, the researchers also explore the intriguing connection to the time domain and demonstrate a powerful application for optical communication.

Bridging two popular fields in optics

A fundamental idea in optics is the so-called space-time duality, which suggests that many effects that are observed spatially, can also be seen in the light’s temporal structure. Based on this principle, the generalized self-imaging in time occurs for a periodic train of optical pulses and its corresponding frequency comb, i.e., light containing only well-defined and equally spaced frequencies.

In their work, the researchers unveil a new form of space-time duality by showing the strong link between angle/angular momentum and time/frequency.

“This means that the physical phenomena observed in these two fields are broadly connected, and the processing techniques from one may be used for the other” explains the other leading author Jianqi Hu, who was a postdoc fellow at Kastler Brossel Laboratory and is currently a researcher at École Polytechnique Fédérale de Lausanne, Switzerland.

Fundamental effect triggers application in optical communication

Benefiting from this deeper fundamental insight into self-imaging and its accompanying advanced modulation capabilities, the researchers additionally demonstrate a powerful application for optical communication.

“The generalized self-imaging effect can be cleverly tuned to encode, convert, and decode information in the light’s orbital angular momentum values such that they can act as independent communication channels,” says Eriksson.

As such, the current study shows that the theoretical promise of a loss-less and crosstalk-free operation for a vastly increased data rate is within reach, which could have a profound impact on the future of optical telecommunications.

The study on the generalized self-imaging effect in angle and angular momentum is now featured in the article Generalized angle–orbital angular momentum Talbot effect and modulo mode sorting published in Nature Photonics.