Russian physicists determine indices enabling prediction of laser behavior

Russian scientists, including researchers at HSE University, examined the features of fibre laser generation and identified universal critical indices for calculating their characteristics and operating regimes. The study findings will help predict and optimise laser parameters for high-speed communication systems, spectroscopy, and other areas of optical technology. The paper has been published in Optics & Laser Technology.

Erbium fibre lasers are devices that generate light within a fibre doped with ions of the rare-earth element erbium. These lasers operate at a wavelength of approximately 1.5 micrometres, making them ideal for long-distance data transmission with minimal loss. Radiation at other wavelengths requires amplification every 20-30 kilometres when passing through optical fibre, whereas radiation from erbium lasers needs 2-3 times fewer amplifiers, significantly reducing equipment and operational costs. Moreover, erbium lasers can produce radiation with a narrow spectral linewidth (less than 1 kHz), which is used in high-precision optical sensors and transducers.  

As demands for data transmission speed and capacity increase, there is a growing need to miniaturise lasers and shorten cavities without compromising their efficiency. A cavity is a component of a laser that consists of two mirrors and is responsible for amplifying light as it passes repeatedly through an active medium.

Depending on the cavity length and the concentration of erbium ions, the laser can operate in different regimes — either pulsed or continuous-wave (CW). The primary challenge is that reducing the size of the cavity requires an increase in the concentration of erbium ions. This causes the laser to operate in pulsed mode, which can result in data transmission instability, power limitations, and increased noise levels. 

A group of Russian scientists, including physicists at HSE University, prepared two types of active fibres for seven lasers and compared the effects of erbium ion concentrations (ranging from 0.03% to 0.3%) on the laser parameters. As a result, they determined the parameters of the active medium and pump power that allow for a short cavity length and CW operation simultaneously, as well as the conditions under which the switching from CW to pulsed mode occurs. 

Oleg Butov

'The transition from continuous-wave to pulsed operation regime is somewhat analogous to a classical phase transition, which follows mathematical laws and characterises processes in other systems, such as liquids and solids. Lasers with a high concentration of erbium ions exhibit two thresholds: the first is associated with the onset of pulsed mode operation, while the second marks the transition to continuous-wave mode. These laws resemble power-law dependencies and describe how the laser parameters change near the generation threshold,' explains Oleg Butov, co-author of the paper, Deputy Director and Head of the Laboratory of Fiber Optic Technologies at Kotelnikov Institute of Radioengineering and Electronics of RAS.

For the first time, researchers experimentally determined the critical indices for erbium lasers—specifically, the slopes of the logarithmic relationships between the frequency, duration, and amplitude of laser pulses and the laser radiation power. 

Alexander Smirnov

'We have established that the calculated dependencies are universal for erbium lasers, regardless of significant variations in the core composition of the active fibre, cavity length, and Q-factor (a ratio of stored energy to energy consumed in one period). The results will enable predictions of the erbium fibre lasers radiation parameters and facilitate the optimisation of their operation for various applications,' according to Alexander Smirnov, co-author of the paper and Professor at the ‘Nanoelectronics and Photonics’ Joint Department with Kotelnikov Institute of Radioengineering and Electronics (RAS) of the HSE Faculty of Physics. 

The study was supported by a grant from the Russian Science Foundation (No. 20-72-10057).