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Self-sweeping laser could shrink 3D mapping systems

06 September 2015

A new approach that uses light to move mirrors could usher in a new generation of laser technology for a wide range of sensing and imaging applications.

This self-sweeping laser couples an optical field with the mechanical motion of a high-contrast grating mirror (Schematic: Weijian Yang). See foot of article for an explanation of this diagram

A team of UC Berkeley engineers led by Professor Connie Chang-Hasnain has used a novel concept to automate the way a light source changes its wavelength as it sweeps the surrounding landscape. They report their findings in the latest edition of the journal, Scientific Reports.

The advance could have implications for imaging technology using LIDAR (light detection and ranging) and OCT (optical coherence tomography).

“Our paper describes a fast, self-sweeping laser that can dramatically reduce the power consumption, size, weight and cost of LIDAR and OCT devices on the market today,” says Chang-Hasnain. “The advance could shrink components that now take up the space of a shoebox down to something compact and lightweight enough for smartphones or small unmanned aerial vehicles.”

In both LIDAR and OCT applications, as the laser moves along, it must continuously change its frequency so that it can calculate the difference between the incoming, reflected light and the outgoing light. To change the frequency, at least one of the two mirrors in the laser cavity must move precisely.

“The mechanisms needed to control the mirrors are a part of what makes current LIDAR and OCT systems bulky, power-hungry, slow and complex,” says study lead author Weijian Yang. “The faster the system must perform – such as in self-driving vehicles that must avoid collisions – the more power it needs.”

The novelty of the new design is the integration of the semiconductor laser with the mirror. Each laser can be as small as a few hundred micrometres square, and it can be readily powered by an AA battery.

The coupling of the laser with an ultra-thin, high-contrast grating (HCG) mirror allowed the researchers to harness the physical force of the light to move the mirror. The HCG mirror, consisting of rows of tiny ridges, was developed in Chang-Hasnain’s lab and has recently been used to create an artificial, chameleon-like skin. With an average force of just a few nanonewtons, the light exerts enough energy to cause the mirror to vibrate.

“The light acts like a kid on a swing, and the mirror is the swing itself,” says Yang, who is now a postdoctoral researcher in biological sciences at Columbia University. “If the child moves his body properly along the swinging path, he can enjoy this ‘free’ ride without any external force. This is what is happening in this self-sweeping laser.”

In their experiments, the researchers found that this opto-mechanical interaction of the laser and the mirror can sweep across a wavelength range of more than 23 nanometres in the infrared spectrum without the need for external controls.

“That wavelength range would be sufficient for a system that could resolve 50-micrometre surface profile features, even when the target is tens of metres away,” says study co-author Adair Gerke.

Moreover, the period of the sweeping cycle can be as short as a few hundred nanoseconds, enabling several million sweeps per second. This speedy sweeping rate enables 3D image capture for real-time videos and visualisation of depth change.

The study authors said the next stage of the research will be to incorporate this new laser design in current LIDAR or OCT systems, and to demonstrate its application in 3D video imaging.

Key to illustration
The self-sweeping laser in the illustration above couples an optical field with the mechanical motion of a high-contrast grating (HCG) mirror. The HCG mirror is supported by mechanical springs connected to layers of semiconductor material. The red layer represents the laser’s gain (for light amplification), and the blue layers form the system’s second mirror. The force of the light causes the top mirror to vibrate at high speed. The vibration allows the laser to automatically change colour as it scans. 

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