# Statistical PT-symmetric lasing in an optical fiber network

## TL;DR

This paper demonstrates that PT-symmetry effects in optics, previously thought fragile and limited to micro-devices, are robust enough to be observed in kilometer-scale optical fiber networks, opening new avenues for fiber-based applications.

## Contribution

The study shows that PT-symmetry phenomena can persist in large-scale optical fiber cavities despite fluctuations, extending the concept beyond micro-scale devices.

## Key findings

- PT-symmetry effects observed in kilometer-long fiber cavities.
- Lasing threshold and power growth follow PT-symmetric predictions.
- A statistical symmetry-breaking point is identified with varying cavity loss.

## Abstract

PT-symmetry in optics is a condition whereby the real and imaginary parts of the refractive index across a photonic structure are deliberately balanced. This balance can lead to a host of novel optical phenomena, such as unidirectional invisibility, loss-induced lasing, single-mode lasing from multimode resonators, and non-reciprocal effects in conjunction with nonlinearities. Because PT-symmetry has been thought of as fragile, experimental realizations to date have been usually restricted to on-chip micro-devices. Here, we demonstrate that certain features of PT-symmetry are sufficiently robust to survive the statistical fluctuations associated with a macroscopic optical cavity. We construct optical-fiber-based coupled-cavities in excess of a kilometer in length (the free spectral range is less than 0.8 fm) with balanced gain and loss in two sub-cavities and examine the lasing dynamics. In such a macroscopic system, fluctuations can lead to a cavity-detuning exceeding the free spectral range. Nevertheless, by varying the gain-loss contrast, we observe that both the lasing threshold and the growth of the laser power follow the predicted behavior of a stable PT-symmetric structure. Furthermore, a statistical symmetry-breaking point is observed upon varying the cavity loss. These findings indicate that PT-symmetry is a more robust optical phenomenon than previously expected, and points to potential applications in optical fiber networks and fiber lasers.

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Source: https://tomesphere.com/paper/1706.02224