Parity-time symmetry shortens laser pulses in two-ring laser

Optical systems with a balanced gain and loss are Parity-time (PT) symmetric. PT symmetry, originally the focus of non-Hermitian extensions in quantum mechanics, has recently attracted interest in optics for innovative applications that have yet to be explored. In their article, Yuan et al. analyze theoretically an actively mode-locked laser system exhibiting PT symmetry and show that pulse lengths can be shortened using a PT phase transition. They confirmed this new mechanism by modeling a two-ring laser system.

Active mode locking can generate short pulses from a laser. The system considered here consists of two coupled rings, where each ring has an amplitude modulator that allows for mode-locking. When the modulator’s phases differ by pi, the system is described by a Hamiltonian with PT symmetry. What the authors show here is that spontaneous breaking of PT symmetry in the coupled two-ring system leads to much shorter laser pulses than can be obtained with just a single ring.

The authors also showed that the two-ring system can be described by a synthetic lattice structure along the frequency axis. As the coupling strength is varied, the eigenvalues change, and the system exhibits a time window with gain that has a width much smaller than half the round-trip time for light traveling around the ring.

They go on to show that narrowing the time window in this PT symmetric system provides the capability to generate a shorter pulse than could be achieved with a normal actively mode-locked system. They expect that a PT phase transition might provide a way to further shorten pulses in the otherwise difficult mid-infrared regime.

Source: “Pulse shortening in an actively mode-locked laser with parity-time symmetry,” by Luqi Yuan, Qian Lin, Meng Xiao, Avik Dutt, and Shanhui Fan, APL Photonics (2018). The article can be accessed at https://doi.org/10.1063/1.5039375 .