JP7792129B2 - Optical transmitter and spatial optical communication transmitter - Google Patents

Description

(1) Kick-off Symposium of the Academic Innovation Area "Comprehensive Understanding of Scattering and Fluctuation Fields and the Science of See-Through" (held via Zoom webinar), February 1, 2021, Takayama Yoshihisa, Tamagawa Ichiro, Kobayashi Tomotaka, Yamashita Yasuteru, "Measurement and Control of Scattering and Fluctuation in Free-Space Optical Propagation Communications" (2) Proceedings of Optics & Photonics Japan 2021 (PDF only), October 21, 2021, Takayama Yoshihisa, Tamagawa Ichiro, Kobayashi Tomotaka, The Optical Society of Japan, https://opt-j.com/opj2021-2/download. html (3) Optics and Photonics Japan 2021 Lecture (online lecture), October 28, 2021, Yoshihisa Takayama, Ichiro Tamagawa, Tomohisa Kobayashi, "Stabilization of receiving surface illumination by light propagating through a fluctuation field"

The present invention relates to an optical transmitter and a spatial optical communication transmission device.

In communications between satellites and ground stations or between ground stations, the use of free-space optical communications using laser beams, which enables large-capacity transmission, is expected.
As shown in Figure 20, the laser beam 61 transmitted from the optical transmitter 2 is affected by the dynamically changing absorption and scattering of atmospheric turbulence, which serves as the communication path. As a result, the laser waveform is spatially distorted and spreads at the optical receiver 5. In other words, the spatial distribution of the intensity of the laser beam 71 spreads, resulting in spotting and fluctuations in the light intensity, and scattering and movement of the focus. If the spatial distribution of the intensity of the laser beam 71 in free-space optical communications spreads, the beam may spatially deviate from the aperture of the optical receiver 5, resulting in insufficient reception of the light. Furthermore, spotting and fluctuations in the light intensity, scattering and movement of the focus, and other factors can cause momentary interruptions in the optical line, degrading communication quality. In addition, in communications with artificial satellites, the received light is used to track the satellite, and distortion also degrades this performance.

Existing techniques for solving this problem include the multi-beam method described in Patent Document 1 and the wavefront manipulation method described in Patent Document 2.
As shown in Figures 21 and 22, in the multi-beam method, multiple optical transmitters 2L and 2R are used to split a laser beam into multiple beams, which are transmitted along spatially separated paths in the atmosphere. The waveform of each laser beam is randomly distorted spatially. By superimposing these beams, the amount of light becomes somewhat uniform, and the receiving side can receive light of sufficient intensity within its aperture.

The wavefront manipulation method is a technique in which an optical receiver uses a wavefront sensor to detect the spatial distribution information of the intensity of received light, manipulates the wavefront using a deformable mirror or other device in the optical receiver, and applies feedback so that light of sufficient intensity can be received within the aperture of the receiver. Here, deformable mirrors refer to devices such as MEMS (Micro Electro Mechanical Systems) and spatial light phase modulators. In a communication system using the wavefront manipulation method, a laser beam from the other party in a two-way communication is received, and distortion of the light is detected on the transmitting side, corrected through wavefront manipulation using a spatial light phase modulator, and feedback is applied so that light of sufficient intensity can be received within the aperture of the receiving device.

Japanese Patent Application Laid-Open No. 2005-354335 International Publication No. 2020/250308

The wavefront manipulation methods include a parallel method and a cross method.
The optical transmitter 2 shown in Figure 21 is a parallel system in which two optical transmitters 2L and 2R emit laser beams in parallel. As a result, the laser beam emitted by the optical transmitter 2L and the laser beam emitted by the optical transmitter 2R pass through different spaces and are affected by different atmospheric fluctuations. In the parallel system, the effective area 8 is a long distance, so the transmitting side and the receiving side must be sufficiently far apart. The distance between the transmitting side and the receiving side is expected to be several hundred kilometers.

The optical transmitter 2 shown in Figure 22 is a crossing type in which two optical transmitters 2L and 2R transmit laser beams so that they intersect. As a result, the laser beam transmitted by the optical transmitter 2L and the laser beam transmitted by the optical transmitter 2R pass through different spaces and are affected by different atmospheric fluctuations. With the crossing type, the distance of the effective area 8 is fixed, which limits its use in mobile communications.
Furthermore, in the wavefront manipulation method, in order to separate the paths spatially, the exits of each beam must be spaced apart by several tens of centimeters to several meters, which limits the feasibility of miniaturization.

In wavefront manipulation methods, the spatial frequencies that can be detected by the wavefront sensor in the receiving system are limited. Furthermore, in wavefront manipulation methods, it takes time to drive the deformable mirror based on the wavefront detected by the wavefront sensor in the receiving system. This time is the sum of the image sensor readout speed, calculation speed, and response speed of the spatial phase modulation device. As a result, sufficient wavefront manipulation may not be possible.

In addition to this problem, the wavefront manipulation method faces the problem of the angular difference between the path at reception and the path at transmission due to feedback time delays. This makes the wavefront manipulation method particularly difficult to apply to mobile communications such as satellite communications.

The present invention aims to provide an optical transmitter and a spatial optical communication transmission device that can stably irradiate a target with light of sufficient intensity.

In order to solve the above problem, an optical transmitter according to the present invention comprises laser beam generating means for generating a plurality of laser beams that are incoherent with each other, an optical phase modulator for performing spatially different phase modulation on each of the laser beams, a combiner for combining the laser beams that have been phase-modulated by the optical phase modulators into a single laser beam, and an optical system for transmitting the single laser beam combined by the combiner to an object to be irradiated , wherein the optical phase modulator is configured to dynamically perform random phase modulation so as to follow the statistical properties indicated by a measured or estimated refractive index structure function .

The spatial optical communication transmission device comprises an optical transmitter and a signal modulator that modulates the same transmission signal onto each of the laser beams generated by the laser beam generating means.

According to the present invention, optical transmitters and spatial optical communication transmission devices are capable of stably irradiating a target with light of sufficient intensity.

1 is a configuration diagram showing an outline of a free-space optical communication system according to an embodiment of the present invention; 10 is a graph showing the relationship between the brightness of each laser beam and the normalized probability density distribution. FIG. 1 is a diagram illustrating an effective area of a free-space optical communication system. FIG. 1 is a configuration diagram showing an optical transmitter according to a first embodiment. FIG. 10 is a configuration diagram showing an optical transmitter according to a second embodiment. FIG. 10 is a configuration diagram showing an optical transmitter according to a third embodiment. FIG. 10 is a configuration diagram showing an optical transmitter according to a fourth embodiment. FIG. 10 is a configuration diagram showing an optical transmitter according to a fifth embodiment. FIG. 10 is a configuration diagram showing an optical transmitter according to a sixth embodiment. FIG. 13 is a configuration diagram showing an optical transmitter according to a seventh embodiment. FIG. 13 is a configuration diagram showing an optical transmitter according to an eighth embodiment. FIG. 13 is a configuration diagram showing an optical transmitter according to a ninth embodiment. FIG. 23 is a configuration diagram showing an optical transmitter according to a tenth embodiment. FIG. 23 is a configuration diagram showing an optical transmitter according to an eleventh embodiment. 10 is a flowchart showing a spatial phase modulation control process. FIG. 1 is a diagram showing a calculation region from a transmission system to a light receiving system. FIG. 2 is a diagram showing the input and output surfaces of each calculation domain. FIG. 10 is a diagram illustrating an example of calculation for each distance. 10 is a flowchart showing a light propagation simulation. 10 is a flowchart showing a light propagation simulation. FIG. 1 is a diagram illustrating an outline of a free-space optical communication system according to a first comparative example. FIG. 10 is a diagram illustrating an effective area of a free-space optical communication system according to a second comparative example. FIG. 10 is a diagram illustrating an effective area of the free-space optical communication system of the second comparative example.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
The free-space optical communication system of this embodiment will be outlined with reference to FIG.
The free-space optical communication system includes an optical transmitter 2 and an optical receiver 5. The optical transmitter 2 includes laser diodes 201 and 202, wavefront controllers 621 and 622, a multiplexer 63, and an optical system 3.

Laser diodes 201 and 202 are laser beam generating means that generate mutually incoherent laser beams 611 and 612. Wavefront controllers 621 and 622 perform spatially different phase modulation on these laser beams 611 and 612, respectively, and combiner 63 combines and optically superimposes them. Optical system 3 then transmits the single laser beam combined by combiner 63 to optical receiver 5, which is the target of irradiation. When the superimposed laser beams are transmitted simultaneously along the same path, the laser beams with different wavefronts overlap and reach optical receiver 5.

This results in a uniform brightness distribution on the light-receiving surface of the optical receiver 5. The wavefront controllers 621 and 622 are spatial optical phase modulators that spatially modulate the optical phase by wavefront manipulation. These wavefront controllers 621 and 622 dynamically perform random phase modulation so as to follow the statistical properties indicated by the measured or estimated refractive index structure function.

Laser beams with different wavefronts are affected randomly and differently by the same atmospheric turbulence, even if they are traveling the same path. As a result, the spatial distribution of light intensity at the receiving end, where they are superimposed, becomes nearly uniform, allowing stable reception of light of sufficient intensity within the aperture of the optical receiver. Reception can be performed using an existing, conventional optical receiver. Note that while Figure 1 shows laser diodes 201 and 202 as multiple laser beam emitting means, this is not limited to a single laser beam generating means, a splitter, and multiple polarization means; it may also include such means.

The graph in Figure 2 shows the results of a simulation of the probability density distribution of received light intensity in the optical receiver of the present invention. The vertical axis of the graph represents the normalized probability density distribution, and the horizontal axis represents brightness. The solid line on the graph represents the probability density distribution for four beams. The dashed line on the graph represents the probability density distribution for two beams. The dash-dotted line on the graph represents the probability density distribution for one beam.

In the case of one beam, shown by the dashed line, the probability density distribution extends to areas where the reception strength is weak, and there is a certain percentage of the probability that the reception performance of the receiving device will be below its capacity. In contrast, in the case of four beams, shown by the solid line, the probability density distribution is concentrated around the average strength, and the probability of the reception performance being below its capacity is smaller. The characteristics for the two beams, shown by the dashed line, are intermediate between those for four beams and one beam.

FIG. 3 is a diagram illustrating an effective area 8 of the free-space optical communication system.
In this way, the optical transmitter 2 simultaneously transmits the light beams superimposed by the wavefront controllers 621 and 622 through the same path. Therefore, the effective area 8 is uniformly distributed regardless of the distance. In other words, the optical transmitter 2 of this embodiment can appropriately irradiate the laser beam to the optical receiver 5 regardless of the distance.

The optical transmitter 2A of the first embodiment will be described with reference to Figure 4. The optical transmitter 2A of the first embodiment includes a laser diode 20, a polarization controller 22, lenses 23L and 23R, half-wave plates 25L and 25R, wavefront controllers 24L and 24R, a mirror 26, and a polarizing beam splitter 27.

In Figure 4 and elsewhere, the laser diode 20 is simply referred to as "LD." The optical transmitter 2A also includes an optical system that transmits a single laser beam combined by the polarizing beam splitter 27 to the target, but this is not shown here.

The laser diode 20 is a laser light source that emits a laser beam (laser light) in response to an input signal. The polarization controller 22 is a splitter that splits the incident light into polarized light 4L and polarized light 4R. The combination of the laser diode 20 and polarization controller 22 corresponds to a laser beam generating means that generates multiple laser beams that are incoherent with each other. The combination of the laser diode 20 and polarization controller 22 generates polarized light 4L and 4R that are modulated with the same transmission signal. The optical transmitter 2A of the first embodiment is a spatial optical communication transmitting device that generates multiple laser beams that are modulated with a transmission signal using the combination of the laser diode 20 and polarization controller 22.

The lenses 23L and 23R can arbitrarily adjust the divergence angle of the incident light.
The wave-front controllers 24L and 24R are transmissive spatial optical phase modulators that spatially modulate the optical phase by wave-front manipulation. These wave-front controllers 24L and 24R dynamically perform random phase modulation so as to follow the statistical properties indicated by the measured or estimated refractive index structure function.

Specifically, different two-dimensional random numbers are generated for each beam according to the two-dimensional arrangement of wave-front controllers 24L, 24R, and then converted into the spatial frequency domain through a two-dimensional Fourier transform. Then, by measurement or estimation, the numbers (constants) derived from the refractive index structure function are manipulated to match the statistical properties of the intensity and phase characteristics in the spatial frequency domain, and an inverse Fourier transform is performed. This makes it possible to dynamically assign two-dimensional arrangements of wave-front controllers 24L, 24R to each beam.

The half-wave plates 25L and 25R are wave plates that impart a phase difference of π (=λ/2) between the two perpendicularly polarized components of the incident light. Here, the combination of the wave-front controllers 24L and 24R and the half-wave plates 25L and 25R corresponds to an optical phase modulator that performs spatially different phase modulation on each laser beam.
The mirror 26 reflects the incident light. The polarizing beam splitter 27 is a cube type in which rectangular prisms are combined, and corresponds to a multiplexer that multiplexes two incident polarized lights.

The laser beam emitted from laser diode 20 is modulated according to the input signal and is split into polarized light 4L and polarized light 4R by polarization controller 22. After the divergence angle of polarized light 4L is adjusted by lens 23L, the wavefront controller 24L spatially modulates the optical phase by manipulating the wavefront, and a phase difference of π is imparted between the perpendicularly polarized components by half-wave plate 25L. Polarized light 4L is then reflected by mirror 26 and enters polarized beam splitter 27.

After the divergence angle of polarized light 4R is adjusted by lens 23R, a phase difference of π is imparted between the vertically polarized components by half-wave plate 25R, and the optical phase is spatially modulated by wavefront manipulation by wavefront controller 24R. Polarized light 4R then enters polarizing beam splitter 27.

Polarized light 4L and polarized light 4R that enter the polarizing beam splitter 27 are combined and sent out simultaneously in the same direction. This allows the optical transmitter 2A to superimpose signals onto two laser beams with different wavefronts and transmit them simultaneously over the same path.

The optical transmitter 2B of the second embodiment will be described with reference to Figure 5. Note that the description of the configuration already described in the second to eleventh embodiments will be omitted or simplified. The optical transmitter 2B of the second embodiment further includes a signal modulator 21 in addition to the same configuration as the first embodiment. The signal modulator 21 modulates the intensity of the incident light according to the input signal. Note that the optical transmitter 2B also includes an optical system that transmits a single laser beam combined by the polarizing beam splitter 27 to the irradiation target, but this is not described here.

A laser beam emitted from a laser diode 20 is modulated by a signal modulator 21, and then split into polarized light 4L and polarized light 4R by a polarization controller 22. The subsequent operations are the same as those in the first embodiment.
The optical transmitter 2B of the second embodiment is a spatial optical communication transmitting device including a signal modulator 21.

Referring to Figure 6, the optical transmitter 2C of the third embodiment will be described. In addition to the same configuration as the first embodiment, the optical transmitter 2C of the third embodiment further includes signal modulators 21L and 21R, a splitter 221 and a delayer 28 instead of the polarization controller 22, and does not include the half-wave plates 25L and 25R. The splitter 221 splits one laser beam into two. The delayer 28 delays the laser beam 41B split by the splitter 221, making it incoherent with the laser beam 41A. The signal modulators 21L and 21R modulate the intensity of the incident light according to the input signal. Note that the optical transmitter 2C also includes an optical system that transmits the single laser beam combined by the polarizing beam splitter 27 to the target, but this is not shown here.

Here, the laser diode 20, the splitter 221, and the delay device 28 that delays the laser beam 41B split by the splitter 221 correspond to laser beam generating means that generates multiple laser beams that are incoherent with each other. The optical transmitter 2C of the third embodiment is a spatial optical communication transmission device that includes signal modulators 21L and 21R.

The laser beam emitted from laser diode 20 is split into laser beams 41A and 41B by splitter 221. Laser beam 41A is modulated by signal modulator 21L and enters lens 23L, where the divergence angle of the light is adjusted. The wavefront of transmissive wavefront controller 24L then modulates the spatial optical phase. Laser beam 41A is then reflected by mirror 26 and enters polarizing beam splitter 27.

Laser beam 41B is delayed by delay device 28, modulated by signal modulator 21R, and then incident on lens 23R. After the divergence angle of laser beam 41B is adjusted by lens 23R, the optical phase of laser beam 41B is spatially modulated by wavefront manipulation by transmissive wavefront controller 24R. Laser beam 41B then incidents on polarizing beam splitter 27.

Laser beams 41A and 41B incident on polarizing beam splitter 27 are combined and sent out simultaneously in the same direction. This allows optical transmitter 2C to superimpose signals onto two laser beams with different wavefronts and transmit them simultaneously over the same path. Optical transmitter 2C is a spatial optical communication transmission device equipped with signal modulators 21L and 21R.

Referring to Figure 7, the optical transmitter 2D of the fourth embodiment will be described. The optical transmitter 2D of the fourth embodiment includes a reflective wave-front controller 24 instead of the transmissive wave-front controllers 24L and 24R of the first embodiment. The reflective wave-front controller 24 is a reflective type that reflects incident light and is a spatial optical phase modulator that spatially modulates the optical phase by simultaneously performing different wavefront operations using polarized light 4L and 4R with different polarizations. The optical transmitter 2D also includes an optical system that transmits a single laser beam combined by a polarizing beam splitter 27 to an irradiation target, but this is not shown here. The reflective wave-front controller 24, which is a spatial phase modulator, is, for example, a liquid crystal panel having a predetermined area. By differentiating the areas irradiated by polarized light 4L and polarized light 4R within the liquid crystal panel that constitutes the reflective wave-front controller 24, it is possible to impart different phase modulation to each irradiation area.

The reflective wavefront controller 24 may be divided into upper and lower sections, with separate phase modulation performed on the upper and lower sections. By irradiating these upper and lower sections with polarized light 4L and polarized light 4R, respectively, different wavefront manipulations can be performed simultaneously, and the optical phase can be spatially modulated.

Furthermore, the reflective wave-front controller 24 of the fourth embodiment performs phase modulation only in a specific polarization direction. For this reason, half-wave plates 25L and 25R that control the polarization direction are inserted into the optical path and combined with the reflective wave-front controller 24. However, this is not limiting, and devices that can simultaneously perform separate spatial phase modulation on beams with different polarizations may also be used.

Here, the combination of the laser diode 20 and polarization controller 22 corresponds to a laser beam generating means that generates multiple laser beams that are incoherent with each other. The combination of the laser diode 20 and polarization controller 22 generates polarized light 4L and 4R that are modulated from the same transmission signal. The optical transmitter 2D of the fourth embodiment is a spatial optical communication transmitting device that generates multiple laser beams that are modulated from the transmission signal by combining the laser diode 20 and polarization controller 22.

The laser beam emitted from the laser diode 20 is modulated according to the input signal and is split into polarized light 4L and polarized light 4R by the polarization controller 22. The divergence angle of polarized light 4L is adjusted by lens 23L, and then reflected by the reflective wavefront controller 24. The optical phase of polarized light 4L is spatially modulated by the wavefront manipulation of the reflective wavefront controller 24, and a phase difference of π is imparted between the perpendicularly polarized components by the half-wave plate 25L. Polarized light 4L then enters the polarizing beam splitter 27.

After the divergence angle of polarized light 4R is adjusted by lens 23R, a phase difference of π is imparted between the vertically polarized components by half-wave plate 25R, and the polarized light is reflected by reflective wave-front controller 24. The optical phase of polarized light 4R is spatially modulated by the wavefront manipulation of reflective wave-front controller 24. Polarized light 4R is then reflected by mirror 26 and enters polarizing beam splitter 27.

Polarized light 4L and polarized light 4R that enter the polarizing beam splitter 27 are combined and sent out simultaneously in the same direction. This allows the optical transmitter 2D to superimpose signals onto two laser beams with different wavefronts and transmit them simultaneously along the same path.

The optical transmitter 2E of the fifth embodiment will be described with reference to Figure 8. In addition to the same configuration as the fourth embodiment, the optical transmitter 2E of the fifth embodiment further includes a signal modulator 21. The signal modulator 21 modulates the intensity of the incident light according to the input signal. The optical transmitter 2E also includes an optical system that transmits a single laser beam combined by the polarizing beam splitter 27 to the target, but this is not shown here.

The laser beam emitted from the laser diode 20 is modulated by the signal modulator 21 and then split into polarized light 4L and polarized light 4R by the polarization controller 22. The subsequent operation is the same as in the fourth embodiment. The optical transmitter 2E of the fifth embodiment is a spatial optical communication transmission device equipped with the signal modulator 21.

An optical transmitter 2F of the sixth embodiment will be described with reference to Fig. 9. The optical transmitter 2F of the sixth embodiment has the same configuration as the fourth embodiment, but further includes signal modulators 21L and 21R, a splitter 221 and a delay 28 instead of the polarization controller 22, and does not include the half-wave plates 25L and 25R. The splitter 221 splits one laser beam into two. The delay 28 delays the laser beam 41B split by the splitter 221, making it incoherent with the laser beam 41A.
The signal modulators 21L and 21R modulate the intensity of the incident light in accordance with the input signal, and in this case, the same transmission signal is modulated. The optical transmitter 2F is equipped with an optical system that transmits a single laser beam combined by the polarizing beam splitter 27 to an irradiation target, but the description is omitted here.
Here, the laser diode 20, the demultiplexer 221, and the delay device 28 that delays the laser beam 41B demultiplexed by the demultiplexer 221 correspond to laser beam generating means that generates a plurality of laser beams that are incoherent with each other.

The laser beam emitted from the laser diode 20 is split into laser beams 41A and 41B by the splitter 221. Laser beam 41A is modulated by the signal modulator 21L, and the divergence angle of the light is adjusted by the lens 23L before being reflected by the reflective wavefront controller 24. After the optical phase of laser beam 41A is spatially modulated by the wavefront manipulation of the reflective wavefront controller 24, it enters the polarizing beam splitter 27.

Laser beam 41B is delayed by delay device 28, modulated by signal modulator 21R, and then incident on lens 23R. After the divergence angle of laser beam 41B is adjusted by lens 23R, it is reflected by reflective wave-front controller 24, and the optical phase is spatially modulated by the wavefront manipulation of reflective wave-front controller 24. Laser beam 41B is then reflected by mirror 26 and incident on polarizing beam splitter 27.

Laser beams 41A and 41B incident on the polarizing beam splitter 27 are combined and sent out simultaneously in the same direction. This allows the optical transmitter 2F to superimpose signals onto two laser beams with different wavefronts and transmit them simultaneously over the same path. The optical transmitter 2F is a spatial optical communication transmission device equipped with signal modulators 21L and 21R.

The optical transmitter 2G of the seventh embodiment will be described with reference to Figure 10. The optical transmitter 2G of the seventh embodiment comprises two optical transmitters equivalent to the optical transmitter 2A of the first embodiment, a mirror 263, and a polarizing beam splitter 273. The laser diodes 20 of these two optical transmitters 2A generate two laser beams modulated from the same transmission signal. The optical transmitter 2G also comprises an optical system that transmits the single laser beam combined by the polarizing beam splitter 273 to the target, but this is not shown here.

The laser beam emitted from the upper optical transmitter 2A is incident on the polarizing beam splitter 273. The laser beam emitted from the lower optical transmitter 2A is reflected by the mirror 263 and then incident on the polarizing beam splitter 273.

The four different laser beams incident on the polarizing beam splitter 273 are combined and sent out simultaneously in the same direction, allowing the optical transmitter 2F to superimpose signals onto four laser beams with different wavefronts and transmit them simultaneously via the same path.
The optical transmitter 2G of the seventh embodiment is a free-space optical communication transmitting device that generates a plurality of laser beams modulated with a transmission signal by combining a laser diode 20 and a polarization controller 22.

The optical transmitter 2H of the eighth embodiment will be described with reference to Figure 11. The optical transmitter 2H of the eighth embodiment comprises two optical transmitters equivalent to the optical transmitter 2B of the second embodiment, a mirror 263, and a polarizing beam splitter 273. These two optical transmitters 2B each comprise a signal modulator 21 that modulates the same transmission signal onto each of the laser beams generated by the laser diode 20. The optical transmitter 2H also comprises an optical system that transmits the single laser beam combined by the polarizing beam splitter 273 to the target, but this is not shown here.

The laser beam emitted from the upper optical transmitter 2B is incident on the polarizing beam splitter 273. The laser beam emitted from the lower optical transmitter 2B is reflected by the mirror 263 and then incident on the polarizing beam splitter 273.

Four different laser beams incident on the polarizing beam splitter 273 are combined and sent out simultaneously in the same direction. This allows the optical transmitter 2G to superimpose signals onto four laser beams with different wavefronts and transmit them simultaneously via the same path. The optical transmitter 2G of the eighth embodiment is a spatial optical communication transmission device equipped with a signal modulator 21.

With reference to Figure 12, the optical transmitter 2I of the ninth embodiment will be described. The optical transmitter 2 of the seventh embodiment includes two optical transmitters equivalent to the optical transmitter 2D of the fourth embodiment, a mirror 263, and a polarizing beam splitter 273. The laser diodes 20 of these two optical transmitters 2D generate two laser beams modulated with the same transmission signal. Note that the optical transmitter 2I also includes an optical system that transmits a single laser beam combined by the polarizing beam splitter 273 to the target, but this is not shown here.

The laser beam emitted from the upper optical transmitter 2D is incident on the polarizing beam splitter 273. The laser beam emitted from the lower optical transmitter 2D is reflected by the mirror 263 and then incident on the polarizing beam splitter 273.

The four different laser beams incident on the polarizing beam splitter 273 are combined and sent out simultaneously in the same direction, allowing the optical transmitter 2I to superimpose signals onto four laser beams with different wavefronts and transmit them simultaneously via the same path.
The optical transmitter 2I of the ninth embodiment is a spatial optical communication transmitting device that generates a plurality of laser beams obtained by modulating the same transmission signal using the laser diodes 20 of two optical transmitters 2D.

With reference to Figure 13, the optical transmitter 2J of the tenth embodiment will be described. The optical transmitter 2J of the tenth embodiment comprises two optical transmitters equivalent to the optical transmitter 2E of the fifth embodiment, a mirror 263, and a polarizing beam splitter 273. The laser diodes 20 of these two optical transmitters 2E generate two laser beams modulated with the same transmission signal. Note that the optical transmitter 2J also comprises an optical system that transmits a single laser beam combined by the polarizing beam splitter 273 to an object to be irradiated, but this is not shown here.

The laser beam emitted from the upper optical transmitter 2E is incident on the polarizing beam splitter 273. The laser beam emitted from the lower optical transmitter 2E is reflected by the mirror 263 and then incident on the polarizing beam splitter 273.

The four different laser beams incident on the polarizing beam splitter 273 are combined and sent out simultaneously in the same direction. This allows the optical transmitter 2J to superimpose signals onto four laser beams with different wavefronts and transmit them simultaneously via the same path. The optical transmitter 2J of the tenth embodiment is a spatial optical communication transmission device equipped with a signal modulator 21.

An optical transmitter 2K according to the eleventh embodiment will be described with reference to Figure 14. The optical transmitter 2K according to the eleventh embodiment includes multiple laser diodes 20a-20n, multiple signal modulators 21a-21n, a spatial phase modulation unit 29, a combiner 63, and an optical system 3. The spatial phase modulation unit 29 includes multiple spatial phase modulators 292a-292n and a spatial phase modulation control unit 291 that controls these.

In order to transmit the power of the transmitted light to the receiving side as efficiently as possible while taking into account the optical propagation characteristics, the optical transmitter 2K of the eleventh embodiment dynamically performs random phase modulation so that the phase modulation follows the statistical properties indicated by a numerical value (constant) derived from the refractive index structure function by measurement or estimation. Methods for deriving a numerical value from the refractive index structure function by measurement or estimation are described in the following documents, for example.
"Atmospheric Fluctuation Measurement Techniques for Realizing Optical Space Transmission to Moving Objects" Laser Research, Vol. 47, No. 12, pp. 688-692 (2019)
It should be noted that even if the phase modulation is simply mutually spatially random, the same effect can be obtained, although the power efficiency of the transmitted light will be lower in consideration of the optical propagation characteristics.

The operation of the optical transmitter 2K of the eleventh embodiment will be described below. The multiple laser diodes 20a-20n generate multiple laser beams that are incoherent with each other and are modulated with the same transmission signal. The multiple signal modulators 21a-21n modulate the same transmission signal onto each laser beam. The multiple spatial phase modulators 292a-292n dynamically perform random phase modulation so as to conform to the statistical properties indicated by the measured or estimated refractive index structure function. In other words, the spatial phase modulation control unit 291 controls the multiple signal modulators 21a-21n to dynamically perform random phase modulation so as to conform to the statistical properties indicated by the measured or estimated refractive index structure function. This operation will be described with reference to the flowchart in Figure 15.

The spatial phase modulation control unit 291 first accepts input of a numerical value derived from the refractive index structure function (step S30). Then, the spatial phase modulation control unit 291 assigns a different random number to each laser beam according to the two-dimensional arrangement of the spatial phase modulation units (step S31).

The spatial phase modulation control unit 291 converts the random numbers associated with each laser beam into the spatial frequency domain by two-dimensional Fourier transform (step S32). Then, the spatial phase modulation control unit 291 performs calculations to match the converted spatial frequency domain data with the statistical properties of the intensity and phase characteristics in the spatial frequency domain given by the input refractive index structure function (step S33). This calculation is described in the following document.
DL Fried, "Statistics of a geometric representation of wavefront distortion," Journal of Optical Society of America, 55, 11, pp.1427-1435 (1965)

The spatial phase modulation control unit 291 performs an inverse Fourier transform on the calculated spatial frequency domain data to convert it back into different two-dimensional data in real space (step S34). The spatial phase modulation control unit 291 then converts the different two-dimensional data in real space into modulation data for the spatial phase modulator for each laser beam (step S35), and ends the processing of Figure 15.

"Light propagation simulation"
16 and 17, a light propagation simulation for a single laser beam will be described. The atmosphere exists between the optical transmitter 2 of the transmission system and the optical receiver 5 of the light receiving system. Here, the distance between the optical transmitter 2 of the transmission system and the optical receiver 5 of the light receiving system is divided into small regions 81a to 81c at appropriate distances, and light propagation is calculated sequentially. The marks on the small regions 81b and 81c indicate that light is being scattered by the atmosphere.

Here, the inventor discretizes space, which is essentially a continuous quantity, for computer processing. The inventor determined the upper limit of the discretization distance based on various conditions, such as the angle at which light is diffracted and the dimensions of the area illuminated by the diffracted light after propagation. The inventor then selected a predetermined distance less than the upper limit to set the calculation area and run the simulation.

The calculation of small region 81a will be explained based on Figure 17. In the light propagation simulation, this small region 81a is calculated by dividing it into an input surface, an output surface, and the wavefront of the intermediate region connecting them. In this case, the computer calculates Fraunhofer diffraction, with the optical axis as Z, the direction opposite to gravity as Y, and the horizontal direction as X.

Here, the computer performs calculations so that the statistical properties of the wavefront conform to the statistical properties indicated by the refractive index structure function. The computer configures the wavefront as a two-dimensional array and assigns computer-generated random numbers to each element of the array. The computer performs a two-dimensional Fourier transform on the two-dimensional array configured with random numbers and performs calculations in the spatial frequency domain. The computer then performs an inverse Fourier transform on the calculated spatial frequency distribution to obtain the shape of the wavefront.
When the calculation for the small region 81a is completed, the calculation for the next small region 81b is started. The output surface of the small region 81a becomes the input surface of the small region 81b.

Figure 18 shows example calculations for each distance. This simulates the propagation of light according to the distance from the transmission system, and shows that the laser beam expands as the distance increases. Here, we assume that the aperture of the transmission system has a Gaussian beam waist with a diameter of 5 cm. Therefore, the wavefront at the aperture is calculated as a plane. Here, the propagation distance and diameter of the transmitted light are set to 5 cm, the aperture to 1.5 μm, and the square of the refractive index structure function Cn, which represents the influence of the atmosphere, is calculated as 10 to the -14th power. The refractive index structure function Cn determines the magnitude of the change in refractive index (i.e., turbulence) in the atmosphere; as the value increases, the fluctuation of the received light increases.

The structure function is an index that shows the characteristics of a random quantity, and is given as the mean square of the difference between the quantity between two points. These two points are specified in terms of time or position (distance), depending on the nature of the random quantity in question. It is known that the mean square of the temperature difference between two points in the atmosphere is proportional to the 2/3 power of the distance between the two points. If the proportionality coefficient is taken as CT squared, it has dimensions of the 2/3 power of the distance. The change in the refractive index of the atmosphere is inversely proportional to the atmospheric temperature. Therefore, the mean square of the refractive index difference between two points in the atmosphere, like the mean square of the temperature difference, is proportional to the 2/3 power of the distance between the two points. If this proportionality coefficient is expressed as Cn squared, Cn squared has dimensions of the 2/3 power of the distance.

For a simulation of light propagation in the case of a single laser beam, please refer to the following literature.
Katsuji Imashiro et al., "Development of a simulator for laser light propagation in atmospheric turbulence," Proceedings of the 25th Laser Sensing Symposium, pp. 159-160, (2007)

19A and 19B, a flowchart showing a light propagation simulation for the present invention in which different spatial optical phase modulations are applied to multiple incoherent laser beams, which are then combined and transmitted, will be described based on the light propagation simulation for the case of a single laser beam. The processing of this light propagation simulation can be performed by a computer.
First, the following calculation conditions are set in the computer: the number of beams, wavelength (1.5 μm), propagation distance (1 km), initial light wavefront shape (Gaussian distribution with a waist beam of 5 cm), refractive index structure function representing the influence of the atmosphere, and the length of the small regions dividing the transmission region (step S10). This allows the computer to perform the following simulation. Note that the values set here are not limited to those mentioned above.

In steps S11 to S19, the computer repeats the calculation of the received wavefront for all beams.
The computer constructs the wavefront of this transmission beam as a two-dimensional array, associates different random numbers with the array, and superimposes them on the shape of the initial light wavefront to give a different output wavefront shape of the transmission system for each beam (step S12).The computer then calculates the phase change that the atmosphere causes to the optical wavefront from the refractive index structure function (step S13).

Steps S14 to S19 are repeated for all small regions in the beam. The computer adds a phase change to the wavefront of the light transmitted to the small region in the beam (step S14). The computer generates random numbers in a two-dimensional array and performs two-dimensional frequency domain calculations on the two-dimensional array using the random numbers so that its statistical properties are equivalent to the statistical properties indicated by the refractive index structure function.

The computer determines the propagation of light in the small region using Fraunhofer diffraction (step S15).

The computer then determines whether calculations for all small regions have been completed (step S16). If calculations for all small regions have been completed (Yes), the computer proceeds to step S18 in FIG. 19B. If calculations for all small regions have not been completed (No), the computer sets the output wavefront shape of that small region as the input wavefront for the next small region (step S17) and returns to step S14.

In step S18, the computer saves the shape of the received wavefront of the beam. In step S19, the computer determines whether the calculation of the received wavefront has been repeated for all beams. If there is a beam for which the calculation of the received wavefront has not yet been performed, the computer returns to step S11.

Once the computer has repeated the calculation of the received wavefront for all beams, it proceeds to step S20 and calculates the intensity distribution at the receiving surface for each beam from the stored amplitude and phase at the receiving surface for each beam. The computer then superimposes the intensity distribution at the receiving surface for each beam to determine the intensity distribution at the receiving surface when all beams are superimposed (step S21). The computer then calculates the average value and variance of the waveform shape at the receiving surface to determine the probability density distribution of the intensity when all beams are superimposed (step S22), and ends the processing of Figure 19B.

The graph in Figure 2 shows the simulation results of the probability density distribution of the received light intensity in the optical receiver of the present invention for each number of beams, calculated according to the flowcharts shown in Figures 19A and 19B under the conditions of wavelength (1.5 μm), propagation distance (1 km), initial light wavefront shape (Gaussian distribution with a waist beam of 5 cm), and refractive index structure function ( ^Cn2 = ^10-14 [m ^-2/3 ]) which is the influence of the atmosphere. The vertical axis of the graph represents the normalized probability density distribution, and the horizontal axis represents the brightness. The solid line of the graph represents the probability density distribution for four beams. The dashed line of the graph represents the probability density distribution for two beams. The dashed line of the graph represents the probability density distribution for one beam.

<<Variation>>
The present invention is not limited to the above-described embodiment, and modifications can be made without departing from the spirit of the present invention, for example, the following (a) to (d) are possible.
(a) The combination of optical transmitters is not limited to the sixth to tenth embodiments, and any type of optical transmitter may be combined, and is not limited thereto.

(b) The means for generating a laser beam is not limited to a laser diode; gas laser, solid-state laser, or liquid laser medium may also be used. Furthermore, discharge, flash lamps, or chemical reactions may also be used as excitation sources.

(c) The combiner that combines multiple laser beams into a single laser beam is not limited to a polarizing beam splitter.
(d) The optical transmitter may emit a laser beam without any superimposed signal, so that, for example, an artificial satellite can be tracked by the light emitted from the optical transmitter on board the satellite.

2, 2A to 2K, 2L, 2R Optical transmitter 20 Laser diode (laser beam generating means)
21, 21L, 21R: signal modulator 22: polarization controller (demultiplexer)
23L, 23R Lens (Optical System)
24L, 24R Wavefront controller (optical phase modulator)
24 Reflective wavefront controller (optical phase modulator)
25L, 25R Half-wave plate (polarization means)
26, 263 Mirror 27, 273 Polarizing beam splitter (combiner)
28 Delay device (optical phase modulator)
3 Optical system 4L, 4R Polarized light 41A, 41B Laser beam 5 Optical receiver 61, 611, 612 Laser beam 621, 622 Wavefront controller 63 Combiner 71 Laser beam 8 Effective area 81a to 81c Small area

Claims (8)

a laser beam generating means for generating a plurality of mutually incoherent laser beams;
an optical phase modulator that performs spatially different phase modulation on each of the laser beams;
a combiner that combines the laser beams that have been phase-modulated by the optical phase modulators into a single laser beam;
an optical system that transmits the single laser beam combined by the combiner to an irradiation object;
and
the optical phase modulator dynamically performs random phase modulation to conform to statistical properties exhibited by a measured or estimated refractive index structure function;
1. An optical transmitter comprising: the optical phase modulator is a reflective wavefront controller;
2. The optical transmitter according to claim 1. the optical phase modulator is a transmission type wavefront controller;
2. The optical transmitter according to claim 1. The laser beam generating means includes a plurality of laser generators.
4. The optical transmitter according to claim 1 , wherein the optical transmitter comprises: a first optical fiber; the laser beam generating means is configured to include a single laser generator and a splitter that splits the laser beam by polarization ;
4. The optical transmitter according to claim 1 , wherein the optical transmitter comprises: a first optical fiber; the laser beam generating means comprises a single laser generator, a branching filter, and a delay means for delaying one of the two laser beams branched by the branching filter;
4. The optical transmitter according to claim 1 , wherein the optical transmitter comprises: a first optical fiber; An optical transmitter according to any one of claims 1 to 6 ;
a signal modulator for modulating the same transmission signal onto each of the laser beams generated by the laser beam generating means;
A spatial optical communication transmitting device comprising: The optical transmitter according to any one of claims 1 to 6 ,
the laser beam generating means generates a plurality of laser beams modulated with the same transmission signal;
A free-space optical communication transmitting device.