As the world’s appetite for data transmission grows, established ways of sending multiple simultaneous, independent signals down a single optical fibre – a process known as multiplexing – are falling behind. This week, US researchers report progress towards an alternative multiplexing method that could vastly increase the capacity of existing fibre-optic networks. Their technique relies on controlling the orbital angular momentum (OAM) of light using a chip-based microlaser. In a separate paper, they also demonstrate, for the first time, that they can detect the OAM of this “twisted” light electronically. The two papers together mark a significant step towards OAM multiplexing in fibre-optic communications and could also have implications for quantum communication.
Photons can carry two types of angular momentum. The first is spin angular momentum (SAM), which arises from the rotation of the polarization of the electric and magnetic fields of light as the wave propagates. Separate signals can be encoded in these polarization states, and while such “polarization division multiplexing” faces technical issues, it has found niche commercial use and several companies are developing it further. However, since photons have only two orthogonal polarization states, this type of multiplexing can, at best, merely double an optical fibre’s capacity.
The other type of angular momentum, OAM, arises when the wavefronts themselves curl around the axis of propagation like pasta spirals. OAM is quantized – the wavefront must appear identical after each full wavelength — but there is no limit to how large it can be. Better still, each OAM state is orthogonal to the others. In principle, this means that every optical fibre could transmit an infinite number of signals at each wavelength with no interference.
That’s the theory. In practice, states of light with OAM values of up to 100 have been produced, but controlling them requires physically manipulating optical components in a way that would be impractical in a working data transmission system. Several research groups have therefore developed ways to modulate the OAM of laser light before it is emitted. However, this approach faces limitations, says optical engineer Liang Feng of the University of Pennsylvania, US. “You need an external laser to feed the light in,” he explains.
Worse, the OAM of light is undetectable with a traditional photodetector. “In light with OAM, all the information is in the phase of the waves,” says Ritesh Agarwal, an optical engineer and Feng’s colleague at Pennsylvania. “All detectors are basically counting the number of photons impinging on the material at that point and producing a photocurrent based on that. The phase information is gone.”
Momentum control
The Pennsylvania researchers and colleagues at other institutions have now published back-to-back papers in Science presenting ways to overcome these obstacles. The first paper builds on work by Liang Feng’s group at the University of Buffalo, US, in 2016 in which he and members of his lab isolated a single chiral (clockwise or counter-clockwise) mode in a circular micrometre-scale indium gallium arsenide phosphide laser cavity. This advance meant that the laser’s output light travelled in only one direction and was emitted with a precisely defined OAM.
In the latest work, the researchers show how to switch a similar laser between different OAM modes. Adding two microscopic “control arms” around the cavity allowed them to control the SAM of the photons, and thereby to select the chiral mode, which is locked with the SAM. Modifying the cavity itself with a set of “gear teeth” allowed them to transform SAM to OAM. They could therefore increase the light’s OAM further by injecting additional SAM from the control arms and utilizing the requirement that total angular momentum (the sum of OAM and SAM) be conserved.
The result is a micrometre-scale laser that can dynamically switch between high-purity OAM states anywhere from +2 to -2 – potentially in picoseconds – without altering the output wavelength from a telecom-friendly 1493 nm. For these experiments, the researchers pumped the microlaser with a 1064 nm laser, but Feng, the work’s senior author, says this should not be necessary. “For practical applications, we can in principle change the optical pumping to electrical pumping,” he says.
Non-local detection
In the second paper, Agarwal and colleagues identify an “orbital photogalvanic effect” by which light can transfer OAM and energy simultaneously to electrons. Crucially, Agarwal explains that detecting the OAM has to be non-local – meaning that it can be made only by comparing values at several different locations. “In local detection, you measure a corresponding photocurrent based on the intensity at that point,” he says. “In light with OAM, all the relevant information is in phase because the light is swirling around.” This information cannot be detected at any single point, Agarwal says, but it is contained in the electric field gradient, and can therefore produce a photocurrent.
Agarwal and colleagues therefore designed and fabricated a detector that uses U-shaped electrodes made from tungsten ditelluride – a special class of material called a Weyl semimetal – to pick up this photocurrent. “We have to come up with very interesting device geometries to extract the information about the phase and ensure that other things we don’t want get cancelled out,” Agarwal explains.
The researchers focused a laser beam of constant frequency and intensity on the centre of their electrode setup and varied its OAM between +4 and -4. The current their detector measured varied in discrete steps, matching their theoretical predictions. Agarwal predicts that if the detector were cooled to superconducting temperatures, it could be used to detect single photons – a capability with significant implications for quantum communication and quantum computing protocols involving “qudits”, or photons with multiple possible states beyond 0 and 1.
Seminal achievement
Miles Padgett, a physicist at the University of Glasgow, UK, who specializes in OAM, is impressed by both papers, and believes the second may prove seminal. “The first paper represents the state of the art in solid state laser generation of these vortex beams – no question about that – but it builds on what’s gone before,” he says. “As far as I’m aware, the ability to detect [the OAM quantum number] based on the nature of the photocurrent is – well, it’s the first time anybody’s been able to do that.”
Alan Willner, an electrical engineer at the University of Southern California, US, who made one of the first demonstrations of OAM multiplexing back in 2012, concurs. “We were building things with big, expensive devices on optical tables, and to a large extent we still are,” he says. “The idea that one could, in principle, build a future system in a cost-effective, reliable, high-performance way requires these types of building blocks. I consider this to be a wonderful step forward.”
Padgett and Willner would now like to see the researchers combine the two technologies to see whether their detector can pick up the OAM variations of their microlaser. “I’d love to see a transceiver, where you have the transmitter and receiver integrated together,” Willner says.
