We propose a new type of laser resonator based on imaginary "energy-level
splitting" (imaginary coupling, or quality factor Q splitting) in a pair of
coupled microcavities. A particularly advantageous arrangement involves two
microring cavities with different free-spectral ranges (FSRs) in a
configuration wherein they are coupled by "far-field" interference in a shared
radiation channel. A novel Vernier-like effect for laser resonators is designed
where only one longitudinal resonant mode has a lower loss than the small
signal gain and can achieve lasing while all other modes are suppressed. This
configuration enables ultra-widely tunable single-frequency lasers based on
either homogeneously or inhomogeneously broadened gain media. The concept is an
alternative to the common external cavity configurations for achieving tunable
single-mode operation in a laser. The proposed laser concept builds on a high-Q
"dark state" that is established by radiative interference coupling and bears a
direct analogy to parity-time (PT) symmetric Hamiltonians in optical systems.
Variants of this concept should be extendable to parametric-gain based
oscillators, enabling use of ultrabroadband parametric gain for widely tunable
single-frequency light sources
