[Paper Review] PT-symmetric microring lasers: Self-adapting broadband mode-selective resonators

Hossein Hodaei, Mohammad‐Ali Miri+3 more|arXiv (Cornell University)|May 8, 2014

Quantum Mechanics and Non-Hermitian Physics25 references3 citations

TL;DR

This paper demonstrates experimentally that PT-symmetric microring laser cavities enable stable single longitudinal mode operation by selectively breaking PT symmetry, enhancing gain in a desired mode even when multiple modes are within the amplification bandwidth. The method achieves mode selectivity without external filters, offering robust, broadband, and self-adapting lasing in multi-moded resonators.

ABSTRACT

We demonstrate experimentally that stable single longitudinal mode operation can be readily achieved in PT-symmetric arrangements of coupled microring resonators. Whereas any active resonator is in principle capable of displaying single-wavelength operation, selective breaking of PT-symmetry can be utilized to systematically enhance the maximum achievable gain of this mode, even if a large number of competing longitudinal or transverse resonator modes fall within the amplification bandwidth of the inhomogeneously broadened active medium. This concept is robust with respect to fabrication tolerances, and its mode selectivity is established without the need for additional components or specifically designed filters. Our results may pave the way for a new generation of versatile cavities lasing at a desired longitudinal resonance. Along these lines, traditionally highly multi-moded microring resonator configurations can be fashioned to suppress all but one longitudinal mode.

Motivation & Objective

• To achieve stable single longitudinal mode operation in multi-moded microring resonators without external mode filters.
• To leverage PT-symmetry breaking in coupled active-passive microring resonators to enhance gain selectively in a target mode.
• To demonstrate robustness against fabrication tolerances in mode-selective lasing.
• To enable broadband mode selection in inhomogeneously broadened gain media using intrinsic cavity dynamics.
• To develop a self-adapting laser cavity that automatically selects and amplifies a single desired longitudinal mode.

Proposed method

• Utilizes a pair of coupled microring resonators, one active (gain medium) and one passive, arranged in a PT-symmetric configuration.
• Employs inhomogeneously broadened gain media to support multiple longitudinal modes within the amplification bandwidth.
• Breaks PT symmetry by adjusting the gain in the active ring, which selectively enhances the lasing threshold of non-target modes.
• Leverages the non-Hermitian Hamiltonian framework to control mode competition via balanced gain and loss.
• Relies on the intrinsic mode competition dynamics in coupled resonators to achieve self-selection of a single longitudinal mode.
• Demonstrates experimental stability and broadband mode selectivity without additional filtering components.

Experimental results

Research questions

• RQ1 Can PT-symmetric coupling in microring resonators enable selective amplification of a single longitudinal mode in a broadband gain medium?
• RQ2 How does selective PT-symmetry breaking influence mode competition and lasing threshold in coupled microring cavities?
• RQ3 To what extent is the mode-selective lasing robust against fabrication imperfections and parameter variations?
• RQ4 Can a multi-moded microring cavity be self-adaptingly reconfigured to lase at a desired longitudinal mode without external filters?
• RQ5 What is the maximum achievable gain enhancement for the selected mode through PT-symmetry breaking in such systems?

Key findings

• Stable single longitudinal mode operation is experimentally demonstrated in PT-symmetric microring lasers without external filters.
• Selective breaking of PT symmetry enhances the maximum achievable gain in the target mode, even when multiple modes are spectrally within the amplification bandwidth.
• The mode selectivity is robust against fabrication tolerances, indicating practical feasibility.
• The system achieves broadband mode selection through intrinsic cavity dynamics, not external components.
• The method enables traditionally multi-moded microring resonators to function as single-mode lasers by self-adapting to the desired resonance.
• The approach allows for reconfigurable lasing at any desired longitudinal mode within the gain bandwidth through controlled PT-symmetry breaking.