As computers get more powerful and connected, the amount of data sent and received is in a constant race with the technologies used to transmit it. Electrons are now proving insufficiently fast and are being replaced by photons as the demand for fiber optic Internet cabling and data centers grows.
Though light is much faster than electricity, in modern optical systems, more information is transmitted by layering data into multiple aspects of a light wave such as its amplitude, wavelength, and polarization. Increasingly sophisticated “multiplexing” techniques like these are the only way to stay ahead of the increasing demand for data but those, too, are approaching a bottleneck.
Researchers have developed a system that can manipulate and detect a property known as the orbital angular momentum (OAM) of light. Such “vortex” lasers, named for the way their light spirals around their axis of travel, were first demonstrated in 2016; however, they have been limited to transmitting a single, pre-set OAM mode, making them impractical for encoding more information. On the receiving end, existing detectors have relied on complex filtering techniques using bulky components that have prevented them from being integrated directly onto a chip and are thus incompatible with most practical optical communications approaches.
The new tunable vortex micro-transceiver and receiver represents the two most critical components of a system that can enable a way of multiplying the information density of optical communication, potentially shattering that looming bandwidth bottleneck. The ability to dynamically tune OAM values would also enable a photonic update to a classic encryption technique: frequency hopping. By rapidly switching between OAM modes in a pre-defined sequence known only to the sender and receiver, optical communications could be made impossible to intercept.
The researchers began with a “microring” laser, which consists of a ring of semiconductor only a few microns wide, through which light can circulate indefinitely as long as power is supplied. When additional light is pumped into the ring from control arms on either side of the ring, the ring emits circularly polarized laser light. Asymmetry between the two control arms allows for the spin angular momentum (SAM) interactions of the resulting laser to be coupled with OAM in a particular direction.
This means that rather than merely rotating around the axis of the beam, as circularly polarized light does, the wavefront of such a laser orbits that axis and thus travels in a helical pattern. Being able to multiplex the OAM, SAM, and wavelength of laser light is not particularly useful without a detector that can differentiate between those states and read them out.
The team designed a photodetector that is similarly responsive to different OAM modes. The photocurrent generated by light with different OAM modes produced unique current patterns that allowed the researchers determine the OAM of light impinging on their device.
The work holds promise for designing highly compact systems for future optical communication systems including integrated systems to demonstrate new concepts in optical communications with enhanced data transmission capabilities for classical light and, upon increasing the sensitivity to single photons, for quantum applications.