Distributed Bragg reflector structures based on PT-symmetric coupling with lowest possible lasing threshold

A new approach towards the design of optimized distributed Bragg reflector (DBR) structures is proposed by taking advantage of recent developments related to the concept of parity-time (PT) in optics. This approach is based on using unidirectional gratings that provide coupling between co-propagating modes. Such couplers with PT symmetric gratings can provide co-directional mode coupling occurring only in one direction. This specific coupling property is achieved through a combined contribution of superimposed index and gain/loss modulations with same grating periodicity, but shifted with respect to one another by a quarter periods. Based on the transfer matrix approach, the transmission and reflection properties of the structure are modeled. One of the unique characteristics of the structure is very low lasing threshold. Such low threshold can be achieved by 100% reflectivity of the both Bragg grating mirrors, and by releasing the amplified signal in one single direction through a PT symmetric grating assisted co-directional coupler. Besides the lasing applications, the proposed structure can be implemented as an optical memory unit of replicating any input optical waveform.