September 27, 2022

SPIM WGs with different cross-sections, allowing mode conversion. Credit: Bangshan Sun

One of the most important elements in photonic chips or quantum chips is the optical waveguide. However, due to limitations in existing fabrication methods, it is difficult to produce waveguides efficiently with high precision control of the shape and size of the 3D cross-section. To solve this challenging problem, scientists at the University of Oxford have developed a new waveguide fabrication technique that can quickly produce waveguides in a chip with precisely controlled 3D cross-sections, which also exhibit changing behavior along the waveguide. The waveguides have been shown to have very low losses and show great promise for photonic or quantum chips.


Background

With the advance of the semiconductor industry, the traditional electronic integrated circuit is approaching its limits in bandwidth and power consumption. Compared to electronic integrated circuits, photonic integrated circuits exhibit lower transmission loss, larger bandwidth and smaller time delay. On the other hand, the rapid development of quantum technology in recent decades indicates that quantum chips promise to replace some aspects of traditional electronic integrated circuits in the future.

It is well known that the basic unit of electronic integrated circuits is a semiconductor diode. Like electronic integrated circuits, optoelectronic chips or quantum chips have their own basic components. Of these basic components, the micron-scale optical waveguide is one of the most important elements. Based on volatile wave coupling, adjacent optical waveguides can realize programmable signal processing, providing indispensable functions for the quantum/photonic chips.

Because of previous limitations in fabrication technology, micron-sized optical waveguides have been limited to two-dimensional square, elliptical, and circular cross-sections. At present, there are limited technological options that can efficiently produce waveguides with both low loss and accurate 3D cross-sectional variation. This imposes many limitations on the functionalities and efficiency of photonic and quantum chips.

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The SPIM-WGs technology

In a new article published in Light science and application, dr. Bangshan Sun, Prof. Martin J. Booth and a team of scientists at the University of Oxford, collaborated with Prof. Alina Karabchevsky from Israel, Prof. Alexander Jesacher from Austria and Prof. Ian A. Walmsley of Imperial College London, have developed a new technology called “SPIM-WGs.” With this technique, optical waveguides with continuously variable 3D cross-sections can be efficiently fabricated in a chip. Optical waveguides developed on the basis of this technology not only have superior performance compared to traditional waveguides, but also bring several new features, paving the way for future photonic and quantum chips.

Based on adaptive optics, the greatest pinnacle of the technology is that it can efficiently produce low-loss waveguides with variable cross-sections, such as circular, square, annular or many other complicated shapes. The precision in controlling the cross section in each axis can reach hundreds of nanometers. For a single waveguide, the cross-sectional shape along the waveguide itself may vary. For example, they can be twisted, ranging from square to circular, or from circular to ring-shaped, and so on.

It is worth noting that the waveguide shows very low transmission losses during the precise change of morphology. Based on the glass substrate, the waveguide has a transmission loss of about -0.14 dB/cm, meaning only about 3% of the optical power is lost when transmitting 1 cm through the chip. Experimental results show that the additional transmission loss caused by cross-sectional variation is almost negligible.

The cost of time to make the waveguides is also remarkable. For example, the traditional silica-on-silicon (SoS) method takes about a month or more to produce preparatory waveguides. In comparison, SPIM-WGs can be produced in minutes, providing another level of flexibility in prototyping and fabrication.

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Application potential

The main application of SPIM-WGs is optical mode conversion. In theory, SPIM-WGs can provide optical mode conversion capabilities for arbitrary shapes, limited only by the diffraction-limited size of the fabrication laser focus. SPIM-WGs can easily convert between Gaussian light modes, elliptical light modes, dual lobe TE01 and ring TE01 modes. These modes exist in a large number of optoelectronic chips.

One of the main applications in mode conversion is between pp-KTP waveguides and single-mode fiber, bridging quantum light sources and quantum chips. At present, the pp-KTP waveguide in a quantum light source must be directly connected to a single-mode fiber, which loses about 25-30% of the light intensity. If the mode conversion waveguide made by SPIM-WGs is used for the bridging, it is expected that the loss of light intensity can be reduced to below 10%. This would significantly improve the efficiency of most quantum chips.

In addition, based on the mode conversion functionality, SPIM WGs can be connected to a single-mode fiber with a coupling efficiency of up to 95%. This allows SPIM-WGs devices to be easily combined with most existing photonic devices.

It has been found that waveguides with rectangular cross-sections rotated at an angle of 90 degrees can even be used to control the polarization of light. This is also promising for many photonics and quantum applications.


Controlling non-classical mechanical states in a phononic waveguide architecture


More information:
Bangshan Sun et al, On-chip beam rotators, adiabatic mode converters and wave plates through variable cross-sectional low-loss waveguides, Light: Science and Applications (2022). DOI: 10.1038/s41377-022-00907-4

Quote: SPIM-WGs: High Performance Waveguide Devices for Next Generation Photonic Chips (2022, July 29) Retrieved July 30, 2022 from https://phys.org/news/2022-07-spim-wgs-high-waveguide-devices- photonic .html

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