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Optics and photonics

Optics and photonics

Doughnut waveguide preserves twisted light

19 Dec 2018
Twisted light
Twisted sisters: light with OAM values of +1 (top) and 0 (bottom) before (left) and after (centre) passing through the doughnut wave guide. The column on the right shows the inference of the two signals, confirming that helicity has been preserved. (Courtesy: Yuan Chen et al/Phys. Rev. Lett.)

A new waveguide that can transmit twisted light within a photonic chip has been unveiled in China. It was created by Xian-Min Jin at Shanghai Jiao Tong University and colleagues and allows twisted light to propagate without significant degradation to its helicity. The waveguide could lead to the development of new chips for optical and quantum computing.

Characterized by its corkscrew-shaped wave fronts, twisted light carries orbital angular momentum (OAM) in a series of independent states. Each of these OAM states can be used to encode information, and this has already been used to boost the amount of data that can be transmitted using light. The quantum-mechanical nature of these states could also make twisted light useful for creating quantum computers.

An important challenge for those creating practical devices is that the effective index of refraction of a conventional solid wave guide is too low to allow the transmission of individual OAM states. Instead, states with similar OAM values will blur into each other.

Notoriously difficult

The solution has been known for some time: create a “doughnut” waveguide with an inner core and concentric rings made of materials with the appropriate indices of refraction. However, this complex structure has proven notoriously difficult to fabricate.

Jin and colleagues achieved the first practical implementation of a doughnut waveguide by using a laser-writing technique. This involves firing precisely-focused, femtosecond pulses of green light at a borosilicate glass wafer. Absorption of the pulses causes highly-localized transformations of the refractive index of the glass, resulting in a 10 µm-diameter, 20 mm-long doughnut waveguide.

Intensity profiles

The team tested their on-chip waveguide using twisted light with OAM values of +1, 0, and -1, as well as each combination of superpositions of the three states.  The researchers measured the intensity profiles of twisted light entering and exiting their waveguide, while also testing the helicity of the emerging light by interfering it with a separate twisted beam fired through free space alongside the waveguide.

They found that about 40% of the initial intensity of the twisted light was lost inside the waveguide. Even greater losses were measured for states with greater OAM. They also observed that the waveguide also filtered-out photons with poorly-defined OAM. As a result, the helicity of the light remained remarkably well-preserved as it passed through the waveguide.

Jin and colleagues believe their doughnut waveguide could be used in commercially-viable, high-capacity optical communications in the near future. The physicists will now work towards increasing the transmission rates of twisted light with higher values of OAM in their waveguide, potentially creating further opportunities in the rapidly-growing fields of quantum and optical computing.

The waveguide is described in Physical Review Letters.

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