Researchers boost cheap diode laser for precise measurements
The study by researchers from the Russian Quantum Center (RQC), the Moscow Institute of Physics and Technology (MIPT), Lomonosov Moscow State University (MSU) and the Samsung R&D Institute Russia.
“This work has two main results,” said the paper’s lead author RQC Scientific Director Michael Gorodetsky, who is also an MSU professor. “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, generating optical radiation at a million extremely stable frequencies. 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 that use non-linear materials to transform 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,” said MIPT doctoral student Nikolay Pavlov. “Microresonators open the way toward generating optical combs in a compact space and without using up much energy. Compact and inexpensive diode lasers are available for almost the entire optical spectrum. However, their natural linewidth and stability are insufficient for many prospective tasks. In this paper, 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, wh ere 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. “To narrow down the linewidth of a diode laser, it is usually stabilized using an external resonator or a diffraction grating,” said 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 — the microresonator is mere millimeters across — while also increasing monochromaticity by a factor of almost 1 billion, with the transmission band narrowed down to attometers and an optical frequency comb generated if required.