Narrowing the Spectrum of Diode Lasers

Russian physicists have developed a method for dras­tically narrowing the emission spectrum of an ordinary diode laser, like that in a laser pointer. This makes their device a useful replace­ment for the more complex and expensive single-frequency lasers, enabling the creation of compact chemical analyzers that can fit into a smart­phone, cheap lidars for self-driving cars, as well as security and structural health monitoring systems on bridges, gas pipelines, and elsewhere. The device was developed by researchers from the Russian Quantum Center (RQC), the Moscow Institute of Physics and Tech­nology (MIPT), Lomo­nosov Moscow State Univer­sity (MSU), and Samsung R&D Institute Russia.

Physicists have developed a method for drastically narrowing the emission spectrum of an ordinary diode laser, like that in a laser pointer, for use in compact chemical analyzers that can fit into a smartphone and cheap lidars for self-driving cars. (Source: N. G. Pavlov et al., Nat. Phot.)

“This work has two main results,” said RQC Scientific Director Michael Gorodetsky. “First, it serves to show that you can make a cheap narrow-linewidth laser, which would be single-frequency yet highly efficient and compact. Secondly, the same system with virtually no modi­fications can be used for generating optical frequency combs. It can thus be the core component of a spectro­scopic chemical analyzer.”

The optical frequency comb technique underlies laser-based spectro­scopy, pioneered by the 2005 Nobel laureates in physics, John Hall from the U.S. and Theodor Hänsch from Germany. The two developed a laser device that generates optical radiation at a million extremely stable frequencies. The radiation in the gain medium of such lasers bounces between mirrors and is ulti­mately emitted as a continuous train of brief pulses of light of a million different colors. Each pulse lasts mere femto­seconds. The emission spectrum of such a laser consists of a great number of evenly spaced narrow spectral lines, the “teeth” of the optical comb.

An optical laser frequency comb can be used as a ruler to accurately measure light frequency and therefore make precise spectro­metric measurements. Other appli­cations include satellite navi­gation, accurate time data transfer, and the radial velocity method for detecting extrasolar planets. It turned out that there is an easier way to generate frequency combs, which relies on optical micro­resonators. These are ring- or disk-shaped transparent components. By virtue of their material’s non­linearity, they transform pump laser radiation into a frequency comb, also referred to as a microcomb.

“Optical micro­resonators with whispering gallery modes were first proposed at MSU’s Faculty of Physics in 1989. They offer a unique combi­nation of submilli­meter size and an immensely high quality factor,” explained MIPT doctoral student Nikolay Pavlov. “Micro­resonators open the way toward generating optical combs in a compact space and without using up much energy.” But not any laser can be used to pump optical frequency combs in a micro­resonator. The laser needs to be both powerful and mono­chromatic. The latter means that the light it emits has to fall into a very narrow frequency band.

The most common and cheap lasers nowadays are diode lasers. Although they are compact and convenient, in spectro­scopy they fall short of more complex and expensive devices. The reason is that diode lasers are not suffi­ciently mono­chromatic: The radiation they emit is smeared across a 10-nanometer band. “To narrow down the line­width of a diode laser, it is usually stabi­lized using an external resonator or a diffrac­tion grating,” explained Gorodetsky. “This reduces the linewidth, but the cost is a major decrease in power, and the device is no longer cheap, nor is it compact.”

The researchers found a simple and elegant solution to the problem. To make laser light more mono­chromatic, they used the very micro­resonators that generate optical frequency combs. That way they managed to retain nearly the same laser power and size while also increasing mono­chromaticity by a factor of almost 1 billion. That is, the trans­mission band is narrowed down to attometers and an optical frequency comb is generated, if required.

“As of now, compact and inexpensive diode lasers are available for almost the entire optical spectrum,” added Pavlov. “However, their natural linewidth and stability are insuf­ficient for many pro­spective tasks. We show that it is possible to effec­tively narrow down the wide spectrum of powerful multifrequency diode lasers, at almost no cost to power. The technique we employ involves using a micro­resonator as an external resonator to lock the laser diode frequency. In this system, the micro­resonator can both narrow the linewidth and generate the optical frequency comb.”

The proposed design has many possible appli­cations. One of them is in telecommu­nications, where it would considerably improve the bandwidth of fiber optic networks by increasing the number of channels. Another sphere that would benefit is the design of sensors, such as reflecto­meters used as the basis of security and moni­toring systems. For example, if a fiber optic cable runs along a bridge or an oil pipeline, the light in the cable will respond to the slightest distur­bances or variations in the geometry of the object, pinpoin­ting potential problems.

Single-frequency lasers can be used in lidars, or optical radars, which are installed on self-driving cars, among other uses. Finally, the tech­nology enables highly precise ana­lyzers, such as those measuring the composition of air or running medical diag­nostics, that could be integrated into smart­phones or watches. “The demand for such lasers would be really high,” said Gorodetsky. (Source: MIPT)

Reference: N. G. Pavlov et al.: Narrow-linewidth lasing and soliton Kerr microcombs with ordinary laser diodes, Nat. Phot. 12, 694 (2018); DOI: 10.1038/s41566-018-0277-2

Link: Russian Quantum Center, Skolkovo, Russia

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