CN104601245B - Optical link capable of generating and transmitting radio frequency track angular momentum - Google Patents

Abstract

The invention discloses an optical link capable of generating and transmitting radio frequency orbital angular momentum. The diameter of the current radio frequency orbital angular momentum beam becomes significantly larger with the increase of the propagation distance, which will cause difficulty in receiving at the receiving end. The present invention includes an optical radio frequency link and a free space optical link. The optical radio frequency link includes a laser, an electro-optic modulator, an optical amplifier, and a photodetector; the radio frequency signal is modulated onto the light beam emitted by the laser through the electro-optic modulator , the light beam enters the input end of the free space optical link; the light beam coming out of the output end of the free space optical link is amplified by the optical amplifier and enters the photodetector to realize the recovery of the radio frequency signal. The present invention combines the characteristics that the radio frequency orbital angular momentum beam is not easily disturbed and the optical frequency orbital angular momentum beam is not easy to diverge, improves the propagation quality of the orbital angular momentum beam, reduces the distortion of the beam, and thus improves the transmission efficiency of the beam .

Description

An optical link capable of generating and transmitting radio-frequency orbital angular momentum

technical field

The invention relates to the field of generating and transmitting beams carrying orbital angular momentum, in particular to an optical link for generating and transmitting radio frequency orbital angular momentum.

Background technique

With the increase of communication usage and the decrease of spectrum resources, improving the capacity and speed of the communication system has become the primary issue. The angular momentum of electromagnetic waves is divided into spin angular momentum and orbital angular momentum. The spin angular momentum corresponds to the polarization state of the beam, while the orbital angular momentum has not been used yet. Therefore, exploiting orbital angular momentum is a new way to improve spectral efficiency. The electromagnetic wave carrying the orbital angular momentum has a spiral phase wave front, and the phase is related to the space azimuth and the mode number of the orbital angular momentum. The orbital angular momentum beams with different modal numbers are completely orthogonal, so they are reusable. When several beams with different orbital angular momentum modal numbers are multiplexed into one beam, where each orbital angular momentum carries independent data, the capacity and spectral efficiency of the communication system are increased several times. Theoretically, the modal number of orbital angular momentum can be taken to infinity. Therefore, using beams carrying orbital angular momentum to improve the capacity and spectral efficiency of communication systems has good development prospects.

At present, the research on orbital angular momentum focuses on optical frequency and radio frequency bands respectively. The generation methods of optical frequency orbital angular momentum beam mainly include holographic pattern, spiral phase plate, etc. The generation methods of radio frequency orbital angular momentum beam mainly include spiral phase plate, shaped antenna, array antenna, etc. Both OAM beams have their own shortcomings. Due to the small wavelength of the light wave, the phase of the optical frequency orbital angular momentum beam is easily affected by scattering and atmospheric turbulence, which makes the beam distorted. The diameter of the radio frequency orbital angular momentum beam becomes significantly larger with the increase of the propagation distance, which will cause difficulty in receiving at the receiving end. Both of these situations are not conducive to the long-distance propagation of the orbital angular momentum beam.

Contents of the invention

The purpose of the present invention is to provide an optical link designed by utilizing radio frequency on light technology and free space optics, so as to generate and transmit an optical link carrying a radio frequency orbital angular momentum beam.

The technical solution taken by the present invention to solve technical problems is:

The present invention includes an optical radio frequency link and a free space optical link. The optical radio frequency link includes a laser, an electro-optic modulator, an optical amplifier, and a photodetector; the radio frequency signal is modulated onto the light beam emitted by the laser through the electro-optic modulator , the light beam enters the input end of the free space optical link; the light beam from the output end of the free space optical link is amplified by the optical amplifier and enters the photodetector to realize the recovery of the radio frequency signal; the free space optical link includes a first standard A collimator, a second collimator, a first beam splitter, a second beam splitter, a first reflector and a second reflector; the first collimator is used as the input end of a free-space optical link, and the space light beam passes through The first beam splitter is transmitted to the first mirror, and the reflected spatial light beam is reflected to the second beam splitter through the first beam splitter, and then transmitted to the second mirror through the second beam splitter, and the second reflection The space light beam reflected by the mirror is reflected to the second collimator through the second beam splitter, and the second collimator serves as the output end of the free space optical link.

The spatial positions of the reflected light beams at different azimuth angles of the first reflector or the second reflector are different, so that the optical paths of different azimuth angles on the same spatial light beam are different, thereby changing the phase of the radio frequency signal.

The spatial light beam reflected by the first mirror carries a radio frequency orbital angular momentum signal.

The first reflector and the second reflector have the same structure and complementary shapes; the second reflector compensates the optical paths of different azimuth angles, thereby restoring the phase of the radio frequency signal on the light beam and recovering the original signal.

The first reflector and the second reflector both have a stepped structure in the circumferential direction, and the height difference between each step is much larger than the wavelength of light, but is in the same order of magnitude as the wavelength of the radio frequency signal. By changing the height difference between each step The height difference between them realizes the phase shift of the radio frequency signal with the azimuth angle. The height difference between two adjacent stepped structures is D, then the delay difference Δτ of the reflected beams of adjacent stepped structures is:

Where c is the speed of light in vacuum, the difference in the phase of the radio frequency signal caused for:

where f _RF is the frequency of the radio frequency signal. Due to the characteristic that the phase of the orbital angular momentum beam changes periodically with the azimuth angle, the difference between adjacent ladder structures to the phase change of the radio frequency signal satisfies:

Among them, l is the mode of orbital angular momentum, and N is the number of ladder structures in the whole circumference.

Beneficial effects of the present invention: after theoretical analysis of optical radio frequency and optical link of free space optical design, optical beams carrying radio frequency orbital angular momentum can be generated, which can overcome the shortcoming of large divergence of radio frequency orbital angular momentum beams, and can also improve optical frequency Orbital angular momentum beams are susceptible to interference problems. Therefore, this link can improve the efficiency of orbital angular momentum communication, and can increase the communication distance at the same time.

Description of drawings

Fig. 1 Optical link for generating and transmitting radio frequency orbital angular momentum;

Figure 2 Schematic diagram of a specially designed reflector;

Fig. 3 is a schematic diagram of the phase of the radio frequency signal in the space light beam after reflection;

Figure 4 is a side view of a specially designed reflector;

detailed description

Below in conjunction with accompanying drawing, the present invention is described in further detail:

The main block diagram of an optical link that generates and transmits RF OAM is shown in Figure 1. In the figure, the laser 1-1, the electro-optic modulator 1-2, the optical amplifier 1-3, and the photodetector 1-4 form the optical radio frequency link I, which realizes the modulation and demodulation of radio frequency signals. The radio frequency signal is modulated onto the beam emitted by the laser through the electro-optic modulator, and the receiving end restores the radio frequency signal through the photodetector. In Fig. 1, the first collimator 2-1, the second collimator 2-2, the first beam splitter 2-3, the second beam splitter 2-4, the first reflector 2-5 and the second reflector Mirrors 2-6 constitute the free-space optical link II, realizing the generation and compensation of radio frequency orbital angular momentum. The light beam passing through the free space optical link is equivalent to realizing the modulation and demodulation of the radio frequency orbital angular momentum. The light beam transmitted between the first mirror and the second mirror is a light beam carrying radio frequency orbital angular momentum. Since the radio frequency orbital angular momentum in this link is carried and transmitted on the space light beam, it overcomes the disadvantage of a relatively large direction angle of the radio frequency orbital angular momentum beam. At the same time, since the radio frequency orbital angular momentum is transmitted on the space light beam, the corresponding circumferential phase change is the phase change of the radio frequency. Therefore, the tiny disturbance is negligible relative to the radio frequency wavelength, which also improves the shortcoming that the optical frequency orbital angular momentum beam is easily disturbed.

FIG. 2 is a structural schematic diagram of a specially designed first reflector 2-5 and a second reflector 2-6. Spatial light from the collimator is incident on the center of the mirror. It can be seen that the specially designed mirror has a stepped structure along the circumference, and the height difference between each step is much larger than the wavelength of light, but it is in the same order of magnitude as the wavelength of the radio frequency signal, which is achieved by changing the height difference between each step Phase shifting of RF signals with azimuth angle. Fig. 3 is a schematic diagram of the phase of the radio frequency signal in the reflected spatial light beam.

Figure 4 is a side view of a specially designed mirror. As shown in the figure, the height difference between two adjacent ladder structures is D, then the delay difference between the beams irradiating the upper half and the lower half is

Where c is the speed of light in vacuum, and the difference in the phase of the radio frequency signal is

where f _RF is the frequency of the radio frequency signal. Due to the characteristic that the phase of the orbital angular momentum beam changes periodically with the azimuth angle, the difference between adjacent ladder structures to the phase change of the radio frequency signal should satisfy

Where l is the mode of orbital angular momentum, and N is the number of ladder structures on the entire circumference.

According to the above principles, if a free-space optical link with a specially designed mirror is added to an optical radio frequency link, the light beam between the two specially designed mirrors will carry the radio frequency orbital angular momentum of the phase vortex distribution, thus This hybrid optical link can simultaneously realize the generation and transmission of light beams carrying radio frequency orbital angular momentum.

Claims (1)

1. An optical link that can generate and transmit radio-frequency orbital angular momentum is characterized in that: it includes an optical radio frequency link and a free space optical link, and the optical radio frequency link includes a laser, an electro-optic modulator, an optical Amplifier, photodetector; the radio frequency signal is modulated onto the beam emitted by the laser through the electro-optic modulator, and the beam enters the input end of the free space optical link; the beam coming out of the output end of the free space optical link is amplified by the optical amplifier and enters the photoelectric detection The device realizes the restoration of the radio frequency signal; the free space optical link includes a first collimator, a second collimator, a first beam splitter, a second beam splitter, a first reflector and a second reflector; The first collimator is used as the input end of the free space optical link, the spatial light beam is transmitted to the first mirror through the first beam splitter, and the reflected spatial light beam is reflected to the second beam splitter through the first beam splitter , and then transmitted to the second mirror through the second beam splitter, the spatial light beam reflected by the second mirror is reflected to the second collimator through the second beam splitter, and the second collimator acts as a free-space optical link the output terminal; The spatial positions of the reflected beams of the first reflector or the second reflector at different azimuth angles are different, so that the optical paths of different azimuth angles on the same spatial light beam are different, thereby changing the phase of the radio frequency signal; The spatial light beam reflected by the first reflector carries a radio frequency orbital angular momentum signal; The first reflector and the second reflector have the same structure and complementary shapes; the second reflector compensates the optical paths of different azimuth angles, thereby restoring the phase of the radio frequency signal on the light beam and restoring the original signal; The first reflector and the second reflector both have a stepped structure in the circumferential direction, and the height difference between each step is much larger than the wavelength of light, but is in the same order of magnitude as the wavelength of the radio frequency signal. By changing the height difference between each step The height difference between them realizes the phase shift of the radio frequency signal with the azimuth angle; the height difference between two adjacent ladder structures is D, then the delay difference Δτ of the reflected beams of adjacent ladder structures is: $\mathrm{<span\; class="notranslate">\; \&Delta;</span>}\mathrm{<span\; class="notranslate">\; \&tau;</span>}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; -</span>\frac{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; D.</span>}}{\mathrm{<span\; class="notranslate">\; c</span>}}$ Where c is the speed of light in vacuum, the difference in the phase of the radio frequency signal caused for: where f _RF is the frequency of the radio frequency signal; due to the characteristic that the phase of the orbital angular momentum beam changes periodically with the azimuth angle, the difference between the phase change of the adjacent ladder structure to the radio frequency signal satisfies: Among them, l is the mode of orbital angular momentum, and N is the number of ladder structures in the whole circumference.