Sailing to the stars with photonics
PERSPECTIVES imaging or spectroscopic equipment and a communication laser [ 3 ]. A massive, kilometre-sized Earth-based laser array with a mind-boggling 100 GW of power accelerates the sail to 20 % of the speed of light over ten or so minutes( see abstract graphic). The lightsail then cruises to Proxima Centauri over 20 years, takes a few pictures of its exoplanet and sends them back to Earth one photon at a time. It may sound farfetched, but an increasing number of scientists believe the many challenges of this mission can be overcome over the next several decades.
SENDING AND RECEIVING: THE LASER AND COMMUNICATIONS The primary obstacle to the mission is the laser array, with the main concern being not only its power, but its size: the laser must be capable of focusing onto the small sail area until it reaches its target velocity. The sail acceleration, limited by material strength, leads to a predicted acceleration distance of order 10 10 m, almost 10 % of the distance to the Sun, while the laws of diffraction impose a laser-aperture diameter on Earth of order one kilometre. The entire array must be coherent with phase control to compensate for atmospheric turbulence and be able to compensate for the rotation of the Earth. Researchers have proposed hierarchical modular fibre-laser arrays, which work with internal phase sensing, or could be adapted with orbiting lasers for a coherent phase reference compensating the atmospheric fluctuations. To minimise atmospheric absorption, the laser wavelength is proposed to be in the near-infrared( NIR, e. g. 1064 nm— 1550 nm). With current fibre-laser technology, the cost of such a laser would exceed one trillion
The fundamental principle enabling solar sails and lightsails is radiation pressure. Light incident upon a perfectly reflecting mirror at an angle θ experiences a radiation pressure force F = 2Pcos 2 θn^ / c, where P is the power intercepted by the mirror and n^ is the normal to the mirror surface. This equation holds in the ray-optics regime, where the characteristic size of features on the sail is much larger than the light wavelength. For more general scattering, wave optics can be applied to calculate the radiation pressure force. This often applies to sail structures, such as metasurfaces, that scatter light non-specularly, i. e. with components transverse to the incident light propagation. Transverse scattering is necessary for lightsails to remain centred within a propelling laser beam, maximizing propulsion and ensuring accurate transport to the sail destination. Indeed, the sail’ s scattering profile and laser beam’ s intensity profile can be designed in tandem such that small displacements from the beam’ s propagation axis change the proportion of power intercepted on sections of the sail [ 4 ]. This affects the light scattering in the transverse plane, enabling the sail to restore itself to the beam centre.
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