JAXA Repository / AIREX 未来へ続く、宙(そら)への英知

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Other TitleBeam quality of phased laser array and its capability for beamed energy propulsion
Author(jpn)中川 樹生; 小紫 公也; 荒川 義博
Author(eng)Nakagawa, Tatsuo; Komurasaki, Kimiya; Arakawa, Yoshihiro
Author Affiliation(jpn)東京大学 大学院新領域創成科学研究科; 東京大学 大学院新領域創成科学研究科; 東京大学 大学院工学系研究科
Author Affiliation(eng)University of Tokyo Graduate School of Frontier Sciences; University of Tokyo Graduate School of Frontier Sciences; University of Tokyo Graduate School of Engineering
Issue Date2004-06
PublisherInstitute of Space and Astronautical Science, Japan Aerospace Exploration Agency (JAXA/ISAS)
Publication title第23回宇宙エネルギーシンポジウム 平成15年度
The 23rd ISAS Space Energy Symposium March 9, 2004
Start page153
End page157
Publication date2004-06
AbstractBeaming energy transmissions using a microwave or a laser beam in space are attracting many interests. Because laser beams are more advantageous in directionality than microwaves, high power and large-scale lasers such as free electron lasers are under development for directed energy systems and a non-diffracting single-aperture laser beam has been proposed for long-range transmission. However, there are several obstacles in scaling up of a laser oscillator: In general, with the increase in power and size of a laser oscillator, it becomes more expensive and difficult to oscillate in a single transverse mode to produce optimum beam collimation. A phased laser array is one of the solutions. It will reduce the development cost drastically. Several methods have been proposed to combine laser beams coherently, such as MOPA/injection locking, coupled oscillators and external cavities. Moreover, an arrayed beam is steerable not by a physical swing of lasers but by phase control. In addition, adaptive optics for atmospheric-scintillation compensation is achievable by phase control in the same manner as by deformable mirrors. However, spatial coherence of arrayed transmitters would be degraded because the profile becomes inevitably a cluster of multiple beams without any overlapping between each other. Therefore, it would be important to know the combined diffraction patterns and their dependency on geometric parameters of the array. In this paper, effects of spatial coherence of a laser array on a far-field diffraction pattern were numerically computed, and its capability for long-distance wireless energy transmission was evaluated. In addition, temporal coherence of a laser beam was taken into consideration to simulate realistic situations. Generally, a laser beam with a broader spectrum is more effective from the viewpoint of energy transmission because output power of a laser transmitter increases with the spectral broadening. Far-field patterns of arrays were evaluated in terms of a main lobe beam quality factor M(sub ML)(exp 2) and an energy fraction of the beam that is contained in the main lobe eta(sub ML). M(sub ML)(exp 2) and eta(sub ML) correspond to a radius of a receiver and a transmission energy efficiency, respectively. As a result, under the condition of constant aperture area of a transmitter, eta(sub ML) was insensitive to the number of array elements and its spectral broadening, and sensitive to the aperture fill factor. On the other hand, M(sub ML)(exp 2) remained constant. The arrayed beam was found focusable on the Fresnel region by phase control, without an increase of M(sub ML)(exp 2) nor a decrease of eta(sub ML). Using an array with Gaussian amplitude distribution, the main lobe would be enlarged to get more energy in the main lobe at far-field. With the consideration of random errors, initial phase differences between array elements were found critical. On the other hand, pointing angle of an array element and element-to-element variations of output power would not require high accuracy. On the basis of these results, it was suggested that 100 MW power can be transmitted from a 3.3 m x 3.3 m coherent laser diode array with the aperture fill factor f = 1.0 to a 24-m-square receiver at a distance of 40,000 km with the transmission efficiency of 73 percent.
Keywordslaser power beaming; laser array; laser beam; beamforming; spectral broadening; rectangular array; Gaussian distribution; space solar power system; レーザー送電; レーザーアレー; レーザービーム; ビーム形成; スペクトルの広がり; 矩形アレー; ガウス分布; 宇宙太陽発電システム
Document TypeConference Paper
JAXA Categoryシンポジウム・研究会

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