The Tao of Lasers
By Mike Foster
Lasers conjure up images of Dr. Evil’s ultimate destructive weapon or a barrage of red dashes unleashed by Star Wars storm troopers.
Reality, of course, is a bit more mundane: lasers come in many forms and are pervasive in music and video equipment, printers, surgical devices, communications networks and barcode scanners.
Last year, Tao Zhu and his research team at Chongqing University captured attention for discovering a new way to compress the linewidth of lasers down to hundreds of hertz.
The method uses what is known as Rayleigh Backscattering (RBS), a “quasi-elastic” scattering of light that is narrower than the incident light in optical fibres, to feed the light back in a loop. The circling and amplification of RBS light via self-injection feedback results in an ultra-narrow linewidth fibre laser, as described in Tao Zhu’s paper, entitled “Rayleigh backscattering: A method to highly compress laser linewidth”, which was published on line in the Chinese Science Bulletin last September.
The work was the result of more than four years of painstaking research, which had its roots in Zhu’s time as a uOttawa postdoctoral fellow from April 2010 to May 2011, during which he was overseen by Faculty of Science Professor Xiaoyi Bao, a Canada Research Chair in Fibre Optics and Photonics at the Department of Physics’ fibre optics group.
“If I were to describe my achievement of narrow linewidth fibre laser as a little tree, my experience at the University of Ottawa would be the seed of this tree,” says Zhu.
The mechanism is like a “frequency door” which gets narrower and narrower, he says.
“Through the study of this new mechanism of Rayleigh Backscattering (RBS) to compress laser linewidth, we can promote the development of ultra-narrow, high-quality laser sources for optical measurement, coherence tomography, optical fibre sensing and for the field of communications,” explains Zhu in an email conversation from China.
“Many types of lasers featuring narrow line-widths could be developed using this mechanism. It can be used to synthesize more complex optical signals and even the THz (trillion Hz) signal and microwave signals. And it also provides technical support for high-resolution laser spectroscopy, optical atomic clocks, gravitational wave detection and low-noise, microwave signal generation.”
Born in Sichuan, China, in 1976, Zhu already had a master’s degree in science (2003) and a PhD degree in optical engineering (2008) from Chongqing University, and was a professor there, when he came to uOttawa in 2010.
Zhu was studying how to sense optical signals in optical fibres in 2010 when he noticed that the RBS light induced by different fibres had a narrower linewidth. In the years that followed, he and his team refined the experiments, using different pump powers and different fibres to reduce distortion. In 2014, he came up with the RBS linewidth compression model.
Professor Bao says: “The work of Rayleigh scattering in fibres by our group resulted in the discovery of this new mechanism of using random feedback fibre to inject light back into the main fibre configuration in order to achieve a single mode operation. Dr. Zhu, as a post-doctoral fellow working in my group, did the experiment to find the stimulated Rayleigh scattering in fibre for the first time. He made a few configurations using this principle and published a few papers associated with uOttawa. He continued this work after going back to Chongqing University and we have continued to collaborate, with two joint publications between Chongqing and the University of Ottawa.”
Today, at age 38, Zhu is looking forward to seeing further growth in the field of thin lasers.
“It’s quite a pleasure and also a challenge for me to pursue fundamental science research,” says Zhu. “Some other international groups can also fabricate fibre lasers of narrow linewidth of around 100Hz based on the mechanism. We have to further study this mechanism in detail and we plan to apply this mechanism to a semiconductor laser, both in short-wave and long-wave bands.”
Zhu says he will always be thankful for the guidance of Xiaoyi Bao and Associate Professor Liang Chen for their complete trust, as well as what he calls “an atmosphere of tireless pursuit of knowledge” at uOttawa.
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Tao Zhu took some time to learn more about Canada’s history when he was a postdoctoral fellow studying lasers at uOttawa in 2010.