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Breaking the optical bandwidth record of stable pulsed lasers

Tuesday February 28 2017

An international team of researchers including Professor David Moss, Director of the Centre for Microphotonics at Swinburne University of Technology, have redefined the limitations of ultra-fast pulsed lasers.

In the March 2017 issue of Nature Photonics, they report the first pulsed passively mode-locked nanosecond laser with a record-low and transform-limited spectral width of 105 MHz—more than one hundred times narrower than any mode-locked laser to date.

With a compact architecture, modest power requirements, and the unique ability to resolve the full laser spectrum in the radio frequency (RF) domain, the laser paves the way towards full on-chip integration for novel sensing and spectroscopy implementations.

Lasers emitting intense light-pulse trains have enabled the observation of numerous phenomena in many different research disciplines, and are the basis of state-of-the-art experiments in modern physics, chemistry, biology, and astronomy. However, high pulse intensities with low repetition rates come at the expense of mediocre noise properties.

This is where passively mode-locked laser systems come in: They are the optimal choice for generating low-noise optical pulse trains. Such systems have, for example, made it possible to create stable optical frequency references for metrology (Nobel Prize, 2005) as well as intense ultra-short pulses (i.e., single-cycle pulses in the attosecond regime) for the study of high-intensity light-matter interactions.

While many mode-locking techniques have been demonstrated, mainly aimed at creating increasingly shorter pulses with broader spectra, little progress has been achieved so far in tackling the opposite problem: the generation of stable nanosecond narrow-bandwidth pulsed sources.

In their latest publication, the international research team presents a novel laser architecture that capitalizes on recent advances in nonlinear micro-cavity optics, pushing the boundaries further. By exploiting the narrowband filter characteristics of integrated microring resonators they achieve highly nonlinear phase shifts, making it possible to generate nanosecond pulses through mode-locking.

"It is an astonishing achievement - in laser research, very rarely are records broken by two orders of magnitude in one breakthrough. We have achieved a spectral bandwidth more than one hundred times narrower than anything previously demonstrated. In fact, it is so narrow that it is impossible to measure even with state-of-the-art optical spectrum analyzers", says Professor Moss.

To characterize the laser's bandwidth, the researchers employed a novel coherent optical beating technique to achieve much higher resolution. The record-low laser bandwidth made it possible, for the first time, to measure the full spectral characteristics of a mode-locked laser in the radio frequency (RF) domain using widely available RF electronics and confirming the laser's strong temporal coherence.

These stable narrow-bandwidth nanosecond pulsed sources are highly useful for many sensing and microscopy applications, as well as for the efficient excitation of atoms and molecules (typically featuring narrow excitation bandwidths). From a fundamental perspective, the low number of optical laser modes, combined with the RF-accessibility of the associated spectrum, make the team's newly developed laser highly conducive to further study of both nonlinear mode coupling and complex mode-locking regimes.

About the publication

Published in Nature Photonics, the article “Passively mode-locked laser with an ultra-narrow spectral width” (DOI: 10.1038/nphoton.2016.271) is the result of an international research partnership involving Professor Moss at Swinburne University of Technology in Australia and researchers from Quebec, the United Kingdom, China, and Russia.

The lead author, Dr. Michael Kues, is from the Ultra-Fast Optical Processing Group at the INRS Centre Energie Materiaux Telecommunications led by Professor Roberto Morandotti. The study was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) through the Steacie, Canada Chair, and Discovery Grants programs, by the MESI PSR-SIIRI Initiative in Quebec, and by the Australian Research Council Discovery Projects scheme.

Photo: Narrow-bandwidth pulses, produced by a new scheme using a microring resonator, are characterized via a novel beating technique. Credit: Ultrafast Optical Processing Group, INRS Energie Materiaux Telecommunications Research Centre