Light has become one of our most important tools for carrying and transmitting information. Hundreds of kilometers of fiber optic cables link together countries and continents, with thousands of gigabytes of data being transferred along them every second.
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We rely on data transfer being both fast and accurate. This means that any information encoded in the light pulses sent needs to be carefully preserved on what may be an incredibly long journey. An important consideration of any optical component and process involved in infrastructure for information transmission is the noise figure – a measure of the degradation of the signal-to-noise ratio caused by components involved in the signal chain.
To deal with signal losses as light is transmitted, amplification steps are included in the signal chain to boost signal levels again. However, most amplification approaches lead to an enhancement in the signal levels and the noise. This leads to the cumulative effect where even the smallest noise figures in an optical design with a large number of amplifiers lead to very poor signal levels, resulting in issues with information transfer.
Research: The Use of Parametric Amplifiers for Optical Signals
One technique that has proved very promising for the amplification of optical signals is the use of parametric amplifiers.
These schemes can generate amplified signals without the addition of excess noise, particularly when designed in a phase-sensitive way.1 Parametric amplification schemes are widely used in optical physics for the generation of different frequencies of light right across the electromagnetic spectrum.
However, most work using parametric amplification has been performed using pulsed light sources.
Now, researchers at the Chalmers University of Technology have found an approach to design integrated waveguides for optical parametric amplification that operate in the telecommunication band.2 Their research has been published in the Science Advances journal. This provides a new route to noise-free amplification of optical signals that could be used for a variety of telecommunications applications.
Optical Waveguides
The team at the Chalmers University of Technology found ways to engineer silicon nitride waveguides that were nearly one and a half meters in length using electron beam lithography.
Manufacturing such long waveguides with electron beam lithography was challenging, as this technique often has very limited write areas. Machining larger shapes requires stitching together small regions of fabrication which must be done to a high degree of precision to avoid the introduction of stitching artifacts.
As well as the challenge associated with stitching various regions together for the machining, the researchers made the waveguide to follow an Archimedean spiral in its shape. This meant the curvature of the waveguide continually varies across its propagation length and S-bends must be used to connect adjacent spiral waveguide units. The reason for this choice of construction was to minimize communication between different modes of light in the waveguide, including to higher-order or radiation modes.
Despite all the complexity in its shape, the team was able to verify that its waveguide was free of any notable defects and the processing method proved relatively reliable, with 12 out of 20 waveguides made in this way proving defect-free.
Working with silicon nitride is advantageous as this material is transparent for the visible to near-infrared, so the team’s design can be used in other wavelength regions, and the entire device has a very small monolithic footprint, making for a compact amplifier.
New Applications for Long-Distance Communications
The team believes further improvements can be made by trying to limit the crosstalk between different spatial modes supported by the waveguide and improving the insulation between regions. Such improvements would also benefit the transmission efficiency, as the team thought the difference between the experimental and predicted power outputs could be due to the coupling of higher-order modes through the guides.
The new waveguide design is compact enough to be squeezed into a chip just a few millimeters big and may open new avenues for long-distance communications, even as far out as space.
A waveguide that adds virtually no noise and a significant amount of amplification means that fewer overall waveguides are needed as part of the signal chain. For communications across space where placing multiple amplification stages may not be practical, the new silicon nitride devices offer not just a more cost-effective option, but a way of making such communications feasible.
The telecommunications industry already makes extensive use of light for communications, with a significant amount of internet traffic now being transmitted by optical fibers. Again, compact, cost-effective devices may make it feasible to extend communications to hard-to-reach regions as less infrastructure work is required.
Higher signal-to-noise ratios also mean fewer losses in data transfer. For teleconferencing where the rate of packet loss directly impacts the call quality, higher fidelity information transfer could be highly appealing.
References and Further Reading
- Caves, C. M. (1982) Quantum limits on noise in linear amplifiers. Physical Review D, 26(8), 1817–1839. https://doi.org/10.1103/PhysRevD.26.1817
- Ye, Z., Zhao, P., Twayana, K., Karlsson, M., Torres-Company, V., & Andrekson, P. A. (2021) Overcoming the quantum limit of optical amplification in monolithic waveguides. Science Advances, 7(38), 1–7. https://doi.org/10.1126/sciadv.abi8150
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