CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of Japanese Application No. 2020-007031, filed Jan. 20, 2020, which is hereby incorporated by reference in its entirety.

BACKGROUND

The present disclosure relates to an optical apparatus used as a light-propagation-direction controller or a selector for controlling the direction of light transmission or light reception with high accuracy in an optical transmitter or an optical receiver of free space light.

The present disclosure relates to a light-propagation-direction controller including such an optical apparatus.

Recently, with the rapid development of laser applications such as light detection and ranging (LIDAR), free-space optical communication, and an optical 3D printer, there is an increasing need for a light-propagation-direction controller to fix or sweep the propagation direction of light such as laser beams at high speed and high accuracy (error of milliradians or less).

Examples of the configuration of the main existing light-propagation-direction controller include those by angular control of a mirror, and those shifting the optical axis by lens position shifting or the like. For the former, a gimbal mirror (see Y. Kaymak et al., “A Survey on Acquisition, Tracking, and Pointing Mechanisms for Mobile Free-Space Optical Communications,” IEEE Communications Surveys and Tutorials 20, 1104-1123 (2018)), a galvanomirror system (see H. Jigang et al., “Two Dimensional Laser Galvanometer Scanning Technology for Additive Manufacturing,” International Journal of Materials, Mechanics and Manufacturing 6, 332-336 (2018)), a micro-electro-mechanical-systems (MEMS) mirror (see S. T. S. Holmstrom et al., “MEMS Laser Scanners: A Review,” Journal of Microelectromechanical System 23, 259-275 (2014)), and the like are generally used. Other examples include a spatial optical modulator that includes no mechanical movable portion and changes the light propagation direction by applying spatial modulation to the phase of light (see T. Hara, “The Latest Advance in Liquid Crystal Spatial Light Modulators,” Japanese journal of optics: publication of the Optical Society of Japan 36, 122-128 (2007)).

SUMMARY

The present inventors are considering laser wireless power transmission from a lunar lander to a lunar exploration rover. This is a system that wirelessly powers, using a laser beam (100 watts or more), the lunar exploration rover in the permanent shadow area, which may be more than 1 km away from the lunar lander in the sunshine area on which a photovoltaic power generation panel is mounted. The propagation direction control accuracy of the laser beam that is necessary here is several tens of microradians or less. Further, a light-propagation-direction controller is strongly desired to be small and light and have environmental resistance, vibration-proof, and the like as a request of the general spacecraft. Further, in the case of laser transmission exceeding 1 km, since the beam diameter is widened by the diffraction effect of light, the beam diameter on the light transmission side needs to be increased from ten and several mm to several tens mm or more. In the case where the above-mentioned existing light-propagation-direction controller is used for such a system, the following new problems arise.

(1) The optical propagation direction control accuracy of a gimbal mirror and a galvanomirror system is substantially determined by the performance of an actuator such as a motor, but it is difficult to realize the above-mentioned necessary accuracy with a light weight and a large diameter. Further, since twice the angular change of the mirror controlled by the actuator is the change in the light propagating direction as it is, the effect of vibration on the mirror is large.

(2) A MEMS mirror is difficult to accommodate large-diameter beams and high-power light because of the small-size.

(3) A spatial optical modulator is less tolerant of surrounding environmental fluctuation (temperatures, etc.) and high power, and a sophisticated optical phase monitoring/control technology is necessary for use.

(4) In general, a light-propagation-direction controller having high accuracy of the micro-radian class tends to be high-cost.

The present disclosure provides a configuration that solves these new problems. It is desired to provide an optical apparatus and a light-propagation-direction controller that have a small and lightweight optical system configuration, light-propagation-direction control accuracy of substantially micro-radians or less, and high environmental resistance and vibration resistance, and are capable of accommodating large-diameter beams and high-power beams, and low-cost.

According to an embodiment of the present disclosure, there is provided an optical apparatus including: a condensing optical part having a focal point; and an optical element that is inserted between the condensing optical part and the focal point and capable of shifting, in a direction perpendicular to an optical axis, the apparent focal point as viewed from a side of the condensing optical part, the optical apparatus being characterized in that a propagating direction angle of light in space on a side of the condensing optical part where the optical element is not provided is changed in accordance with an amount of the shifting.

(1) In the case where an optical parallel flat plate is used as the optical element, the accuracy and sweep pitch of the light-propagation-direction control can be adjusted by only selecting the optical thickness (=physical thickness×refractive index) of the parallel flat plate and the lens focal length, and the direction control accuracy exceeding the performance of the rotation actuator itself can be achieved. At the same time, since it is insensitive to mechanical movement of the rotation actuator as compared with the existing methods, the vibration resistance is improved in the case where the light propagation direction is controlled with the accuracy similar to that of the existing method.

(2) Since the parallel flat plate is disposed in the vicinity of the focal point, it is possible to accommodate large-diameter beams without greatly decreasing the direction control accuracy and without enlarging the area of the optical element such as the parallel flat plate. In addition, by adopting an optical parallel flat plate material that absorbs less transmission light and applying anti-reflection treatment to both surfaces of the parallel flat plate, it is also possible to ensure low optical loss and resistance to strong power light.

(3) High-accuracy control can be performed by only causing an optical element such as a simple optical parallel flat plate to pivot, and it is generally less affected by surrounding environments such as temperature and humidity.

(4) By inserting a plurality of optical elements such as optical parallel flat plates having different optical thicknesses, both high accuracy and wide range of light-propagation-direction control can be achieved.

(5) By only additionally inserting an optical element such as an optical parallel flat plate into an existing collimator or beam-diameter converter, the direction control function can be easily provided with high accuracy without greatly increasing the size and weight of the optical system.

(6) It can be configured by only adding an optical element such as an inexpensive optical parallel flat plate.

In the optical apparatus according to the embodiment of the present disclosure, the optical element includes a transmissive optical parallel flat plate that is pivotable in an axis perpendicular to the optical axis or fixed at a predetermined inclination angle and has a refractive index variable by external perturbation such as electromagnetic field application, temperature variation, and light irradiation.

In the optical apparatus according to the embodiment of the present disclosure, the condensing optical part is a collimator including one or more lenses or one or more reflecting mirrors.

In the optical apparatus according to the embodiment of the present disclosure, the reflecting mirror is a reflective telescope having a diameter of 50 mm or more.

In the optical apparatus according to the embodiment of the present disclosure, the condensing optical part is an optical relay system or a beam-diameter converter including one or more lenses, one or more reflecting mirrors, or a combination of one or more lenses and one or more reflecting mirrors.

In the optical apparatus according to the embodiment of the present disclosure, the reflecting mirror is a reflective telescope having a diameter of 50 mm or more.

In the optical apparatus according to the embodiment of the present disclosure, the parallel flat plate is pivotable in two axes perpendicular to each other.

In the optical apparatus according to the embodiment of the present disclosure, the optical element includes a first transmissive optical parallel flat plate that is pivotable about a first axis that is a rotation axis perpendicular to the optical axis or fixed at a predetermined inclination angle and has a refractive index variable by external perturbation such as electromagnetic field application, temperature variation, and light irradiation, and a second transmissive optical parallel flat plate pivotable about a second axis that is a rotation axis perpendicular to the optical axis and the first axis or fixed at a predetermined inclination angle and has a refractive index variable by external perturbation such as electromagnetic field application, temperature variation, and light irradiation.

In the optical apparatus according to the embodiment of the present disclosure, the optical element includes two or more transmissive optical parallel flat plates that are independently pivotable or fixed at a predetermined inclination angle, have a refractive index variable by external perturbation such as electromagnetic field application, temperature variation, and light irradiation, and different optical thicknesses (=physical thickness×refractive index) when there is no external perturbation.

The optical apparatus according to the embodiment of the present disclosure may further include a beam shaper disposed on a side of the condensing optical part opposite to a side where the optical element is disposed.

In accordance with the present disclosure, it is possible to realize a light-propagation-direction controller, a selector, and the like that have a small and lightweight optical system configuration, light-propagation-direction control accuracy of substantially micro-radians or less, and high environmental resistance and vibration resistance, and are capable of accommodating large-diameter beams and high-power beams, and low-cost.

These and other objects, features and advantages of the present disclosure will become more apparent in light of the following detailed description of best mode embodiments thereof, as illustrated in the accompanying drawings.

