Harvard University has developed an optical isolator that can seamlessly integrate with optical chips

Recently, a group of researchers from the John Paulson School of Engineering and Applied science (SEAS) of Harvard University developed a method to create an efficient integrated isolator that can be seamlessly integrated into an optical chip made of Lithium niobate. Their research results were published in the journal Nature Photonics. It is reported that the optical isolator developed by SEAS can greatly improve optical systems in many practical applications.

Optical micrograph of a thin film Lithium niobate electro-optic isolation chip composed of four devices with different modulation lengths. (Image source: Loncar Lab/Harvard SEAS)

All optical systems used in telecommunications, microscopy, imaging, quantum Photonics and other fields rely on lasers to generate photons and beams. To prevent damage and instability of these lasers, these systems also require isolators (i.e. components that prevent light from propagating in unwanted directions). Isolators also help reduce signal noise and prevent unrestricted light rebound. However, traditional isolators have a relatively large volume and require multiple materials to be connected together, making it difficult to achieve enhanced performance.

Marko Loncar, the Electrical engineer who led the above team, pointed out that they had built a device that allowed the light emitted by the laser to propagate unchanged, while the reflected light reflected back to the laser changed its color and was rewired from the laser. He said: "This is achieved by sending an electrical signal in the direction of the reflected optical signal, thus taking advantage of the excellent electro-optical characteristics of Lithium niobate. In this feature, voltage can be applied to change the characteristics of the optical signal, including speed and color."

We want to create a safer working environment for lasers, and by designing this one-way street, we can protect equipment from laser reflections, "said Mengjie Yu, one of the first authors of the paper and a former postdoctoral researcher at Langkar Laboratory, To our knowledge, compared to all other demonstrations of integrated isolators, this device has the best optical isolation performance in the world. Apart from isolation, it has the most competitive performance in all indicators, including loss, energy efficiency, and adjustability

The uniqueness of this device is that its core is very simple - it actually only has one modulator. All previous similar engineering studies required multiple resonators and modulators, and they were able to do this because of the nature of Lithium niobate itself.

Another reason behind this high-performance and high-efficiency performance is related to the size of the device - the team manufactured a chip with a thickness of 600 nanometers at Harvard's Nanoscale Systems Center, and etched (using specified nanostructures to guide light) to a depth of 320 nanometers.

The smaller size and ultra-low loss characteristics of the platform also increase its optical power. Due to the fact that light does not need to travel that far, there is less attenuation and power loss.

The team demonstrated that the device can successfully protect the laser on the chip from external reflections. It is reported that they are the first team to demonstrate the stable operation of laser phase under the protection of optical isolators. This progress represents a significant leap in practical and high-performance optical chips, which can be used in conjunction with a range of laser wavelengths and can achieve the desired results with only backpropagation of electrical signals. Integrating all aspects of optical systems onto a single chip can replace many larger, more expensive, and less efficient systems.

Source: OFweek