Space development has attracted attention in recent years, and laser technologies will contribute to many future applications in space, including space optical communication. In this special issue, a wide range of laser applications and technologies in space are explained. This preface briefly introduces this special issue and describes some examples of laser application in space.

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This paper presents research and development trends for deep space laser communications. In particular, JAXA and NICT have initiated the collaborative research work for cislunar optical communications. Our research work aims to accomplish 2.5 Gbit/s high-speed optical communication from the Moon to the Earth. We have proposed system configuration of deep space optical links applying Geostationary Earth Orbit (GEO) data relay satellite. In view of this, we have been developing the bread board model (BBM) of key optical components onboard laser communication terminal (LCT) of the GEO data relay satellite. Our key optical components utilize high-sensitivity digital coherent optical receiver, large-aperture optical antenna, and space-grade adaptive optics (AO) module.

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In space exploration, LIDAR (LIght Detection And Ranging) is an important sensor technology for terrain observation, navigation assistance for planetary landings, and orbital rendezvous. Further development of LIDAR sensors is also expected in future space development through interactions with the growing commercial space and ground industries, which have a large market size. In this paper, the thermal vacuum environment and space radiation environment are described as environmental conditions unique to space that must be considered for use of LIDAR in space. In addition, relatively new examples of spacecraft applications are introduced, including Flash LIDAR on the HTV-X, a new resupply ship under development for the International Space Station, and LRF (Laser Range Finder) on SLIM, which made the first successful pinpoint landing on the Moon.

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For the next generation Japanese satellite navigation system, we have developed a precision microwave reference signal generation system which consists of an optical frequency reference and an optical frequency comb. The third harmonics of a 1545-nm diode laser is stabilized in reference to the iodine saturated absorption line at 515 nm. One of the longitudinal modes of an 8-shaped Er-doped fiber mode-lock laser is phase-locked to the iodine-stabilized laser, and the stable microwave frequency can be obtained as the repetition frequency of the mode-lock laser. In the present paper, we give brief introduction of our system and the environment tests applying to our fiber mode-lock lasers.

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Recently, the need for long-distance wireless power transmission has been increasing both in space and on the ground. This paper outlines the characteristics and trends of laser power transmission, as well as JAXA’s R&D roadmap for realizing the Laser-based Space Solar Power System (L-SSPS), one of the goals of the JAXA SSPS research team, and the technical challenges of the laser power transmission to a lunar rover as a milestone mission.

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In recent years, owing to demands from national security requirements and the Earth observation market, the technological development of intersatellite optical communication network systems capable of high-capacity and low-latency communications has been actively advanced in both the public and private sectors. This paper reviews the trends in projects independently developed by the governments of Japan, the United States, and Europe, as well as the movements of private companies such as SpaceX and Amazon, that surpass government projects in momentum. Additionally, this paper discusses recently developed technologies for multi-protocol optical communication networks and the multi-orbit concept.

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Digital coherent technologies were introduced in long-haul fiber communications as technologies enabling higher spectrum efficiency and compensation of waveform distortion due to fiber transmissions. Digital coherent technologies are expanding their application areas to fiber access networks and satellite communications. In this paper, we review optical satellite communications utilizing digital coherent technologies. The demonstration of multi-aperture transmissions for higher capacity and the demonstration of burst transmissions for higher sensitivity with digital coherent technologies are presented.

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Satellites are widely used not only in the field of safety and security such as weather observations, disaster situational observations, positionings, explorations, etc., but also in various fields, and satellite-based services have been essential in human life. As the application fields of satellites increase, the market needs for satellites with enhanced functionality expand, and the volume of observation data rises. Therefore, the application of COTS components is necessary to achieve higher satellite performance and high-speed data transmission. This application requires an evaluation of their compatibility for the satellite environment. This paper describes the latest technological trends and the evaluation of reliability in application of COTS components to optical communication devices.

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In this paper, we report on an on-orbit optical frequency tunable control mission using a CubeSat for realizing inter-satellite coherent optical communication using a 1550 nm narrow linewidth laser. The satellite that carried out this mission was a 3U-sized CubeSat called OPTIMAL-1. This satellite was released from the ISS (International Space Station) in January 2023 to low earth orbit. It was confirmed that optical frequency tunability exceeding ± 5 GHz, which is necessary for inter-satellite optical coherent communications, can be achieved by laser temperature control. Furthermore, the characteristics of the coherent laser source did not deteriorate even after half a year of operation in the space environment. We achieved all the success criteria for the on-orbit optical module including extra success.

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