Phase-locked laser arrays through global antenna mutual coupling

Abstract

Phase locking of an array of lasers is a highly effective method in beam shaping because it increases the output power and reduces the lasing threshold. Here, we show a conceptually novel phase-locking mechanism based on 'antenna mutual coupling' in which laser elements interact through far-field radiations with definite phase relations. This allows a longrange global coupling among the array elements to achieve a robust phase locking in two-dimensional laser arrays. The scheme is ideal for lasers with a deep subwavelength confined cavity, such as nanolasers, whose divergent beam patterns could be used to achieve a strong coupling among the elements in the array. We demonstrated experimentally such a scheme based on subwavelength short-cavity surface-emitting lasers at terahertz frequencies. More than 37 laser elements that span over ∼8 λ o were phase locked to each other, and delivered up to 6.5 mW (in a pulsed operation) single-mode radiation at ∼3 THz, with a maximum 450 mW A -1 slope efficiency and a near-diffraction-limited beam divergence. P hase locking of an array of lasers is a highly effective way to combine coherently the output radiations from individual lasers to achieve beam shaping and a higher output power. Moreover, the interaction between array elements could lead to a significant reduction in the lasing threshold. Recently, a new genre of laser cavity with deep subwavelength confinement in two or three dimensions, such as nanolasers or spasers 1-3 and photonic wire lasers 4 , has found various potential applications in fields such as optical-information processing, short-distance communication between integrated circuits and optical sensing 5 . However, substantial developments in reducing the excess lasing threshold and improving the coupling of light into a well-defined free-space mode are still required before these devices can be truly useful. If the phase-locking technique could be properly applied here, it will be a key method to overcome the aforementioned shortcomings.A robust phase locking requires strong couplings among the individual lasers so that otherwise-independent oscillators are forced to oscillate in sync. Currently, there are four demonstrated coupling schemes to phase-lock integrated diode laser arrayslaser ridges are coupled through exponentially decaying fields outside the high refractive-index dielectric core (evanescent-wave coupled 6 ) or through the Talbot feedback from external reflectors (diffraction-wave coupled 7,8 ), or by connecting two ridges to one single-mode waveguide (Y-coupled 9,10 ) or through lateral propagating waves (Leaky-wave coupled 11-13 ). Here we present a novel coupling scheme for phase-locking 2D laser arrays through 'global antenna mutual coupling'. This scheme is distinctly different from the other two phase-locking mechanisms for non-contacting laser elements. In contrast to the evanescent-wave coupled scheme, in which the coupling is through near-field electromagnetic fields, as in the antenna mutual coupled scheme, the coupling is establis...

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