Robustness and mode selectivity in parity-time (PT) symmetric lasers

M H Teimourpour,M Khajavikhan,D N Christodoulides,R El-Ganainy

doi:10.1038/s41598-017-10216-1

M H Teimourpour, M Khajavikhan + Show 2 more

Open Access

https://doi.org/10.1038/s41598-017-10216-1

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Abstract

We investigate two important aspects of PT symmetric photonic molecule lasers, namely the robustness of their single longitudinal mode operation against instabilities triggered by spectral hole burning effects, and the possibility of more versatile mode selectivity. Our results, supported by numerically integrating the nonlinear rate equations and performing linear stability analysis, reveals the following: (1) In principle a second threshold exists after which single mode operation becomes unstable, signaling multimode oscillatory dynamics, (2) For a wide range of design parameters, single mode operation of PT lasers having relatively large free spectral range (FSR) can be robust even at higher gain values, (3) PT symmetric photonic molecule lasers are more robust than their counterpart structures made of single microresonators; and (4) Extending the concept of single longitudinal mode operation based on PT symmetry in millimeter long edge emitting lasers having smaller FSR can be challenging due to instabilities induced by nonlinear modal interactions. Finally we also present a possible strategy based on loss engineering to achieve more control over the mode selectivity by suppressing the mode that has the highest gain (i.e. lies under the peak of the gain spectrum curve) and switch the lasing action to another mode.

Highlights

We investigate two important aspects of PT symmetric photonic molecule lasers, namely the robustness of their single longitudinal mode operation against instabilities triggered by spectral hole burning effects, and the possibility of more versatile mode selectivity
The novel concept of PT symmetry[19, 20] was introduced in optics and photonics[21,22,23,24,25], which subsequently led to intense investigations[26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43]
Our study, complementing previous works and providing insight into the operation dynamics of PT symmetric lasers, reveal the following important results: (1) PT symmetry can provide superior performance in microcavities having relatively large free spectral range (FSR), and (2) Extending the concept to millimeter long edge emitting lasers can be challenging due to nonlinear instabilities, and (3) More general loss engineering schemes can be used to achieve more control over mode selectivity

Summary

Robustness and stability of single mode PT lasers

We study the robustness of single mode PT lasers against spectral hole burning effects that might trigger multimode operation. To first demonstrate the possible different lasing regimes, we integrate equation (1) numerically for α = 1.5, κ = 1, r = 0.75 and gmax = 1 or gmax = 3.3 This set of parameters are chosen to ensures that the first lasing mode μ starts in the broken phase while the other mode is still in the PT phase (equation (2)). Note that these solutions always exist for any pump values, they might not be unique in certain regimes The stability of these steady state solutions, which determines the lasing characteristics can be found by performing linear stability analysis[2]. For completeness, we present the stability maps for a single laser cavity (having the same parameters as in Fig. 4) in Fig. 5 where we observe a noticeable expansion of the instability domain This result indicate that, at least for small r values, PT symmetric lasers exhibit superior performance over single cavity systems

Mode selectivity via dissipation engineering

Conclusion

Methods

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