Secure optical interconnects using orbital angular momentum beams multiplexing/multicasting

Abstract. Orbital angular momentum (OAM), described by an azimuthal phase term exp  (  jlθ  )  , has unbound orthogonal states with different topological charges l. Therefore, with the explosive growth of global communication capacity, especially for short-distance optical interconnects, light-carrying OAM has proved its great potential to improve transmission capacity and spectral efficiency in the space-division multiplexing system due to its orthogonality, security, and compatibility with other techniques. Meanwhile, 100-m free-space optical interconnects become an alternative solution for the “last mile” problem and provide interbuilding communication. We experimentally demonstrate a 260-m secure optical interconnect using OAM multiplexing and 16-ary quadrature amplitude modulation (16-QAM) signals. We study the beam wandering, power fluctuation, channel cross talk, bit-error-rate performance, and link security. Additionally, we also investigate the link performance for 1-to-9 multicasting at the range of 260 m. Considering that the power distribution may be affected by atmospheric turbulence, we introduce an offline feedback process to make it flexibly controllable.


Introduction
The unabated exponential growth of global communication traffic has fuelled ever increasing research efforts for sustainable expansion of transmission capacity. 1 To overcome this capacity crunch, multiplexing in polarization and wavelength, known as polarization-division multiplexing (PDM) and wavelength-division multiplexing (WDM), [2][3] and different multi-level modulation such as m-ary phase-shift keying (m-PSK) and m-ary quadrature amplitude modulation (m-QAM) 4 have been used to improve the transmission capacity and efficiency.However, with the flourish of optical interconnects, those mentioned techniques fail to meet the increasing system capacity demand. One potential orthogonal modal basis set is orbital angular momentum (OAM) mode, sometimes called twisted beam, which is featured by a helical phase-front of that carries  () an OAM corresponding to per photon (where is topological charge, is azimuthal angle, and ℏ   is reduced Plank's constant). 7][10][11][12][13][14][15][16][17][18][19][20][21][22] Additionally, it is some inherent properties of OAM modes, such as orthogonality, 7 security, 5 compatibility with others multiplexing techniques, 6 that provide its great potential in optical communication both in optical fiber, 23 and free space. 6Meanwhile, the system capacity and spectral efficiency have reached laudable 1.036-Pbit/s 24 and 435-bit/s/Hz 25 in free space employing both OAM multiplexing and others techniques.However, those mentioned remarkable works were demonstrated in the labs at the range of few meters, which ignore the influence of OAM modes resulting from the real atmospheric turbulence.
Over the past decades, free-space optical (FSO) communication in practical application remained mainly confined to military applications, [26][27] inter-satellite links, 28 and deep-space links, [29][30] However, in recent years, with the development of techniques and the increasing demand of transmission capacity in optical interconnects, growing number of research institutes and companies offer solutions and products in visible, infrared (IR), and ultraviolet (UV) bands, [31][32]6 the market has begun to show future promise. Conering that multi-times increasing in system capacity and spectral efficiency by employing OAM multiplexing, it is meaningful to exploit this dimension in free-space optical interconnects.Unfortunately, there are still some challenges, for example, inhomogeneity in the pressure and temperature or the dust in the atmosphere results in variations of the refractive index along the transmission path, which can degrade the performance of optical interconnects link, especially for OAM multiplexing communication link.Some reports evaluated the performance of long-distance free-space OAM transmission link and demonstrated the information transfer.[33][34] Very recently, a laudable experiment was reported that multiplexing of 4 collocated OAM beams achieved 120-meter 400-Gbit/s free-space optical communications link.35 To the best of our knowledge, security free-space optical interconnects over hundreds of meters using OAM multiplexing and multicasting has not yet been studied.
In this paper, we experimentally demonstrate a 260-meter secure free-space optical interconnects using spatial multiplexing of 2 OAM beams, where each channel is modulated with 10-Gbaud (40-Gbit/s) 16-ary quadrature amplitude modulation (16-QAM) data signal. 36We study the OAM link performance after 260-meter propagation, including beam wandering, received power fluctuation, channel crosstalk, bit-error rate (BER), and link security.The obtained results show that the average mode crosstalk is less than -20 dB when demultiplexing by a full pattern and it degrades to ~-10 dB when demultiplexing by an angular 1/4-block pattern, which indicates the 260-meter security OAM transmission link.Furthermore, 260-meter power-controllable 1-to-9 multicasting interconnects is also demonstrated utilizing an off-line feedback process to redistribution power among 9 channels according to the demand.The BER performance shows it's only ~2 dB optical signal-to-noise ratio (OSNR) penalty compared to back-to-back (B2B) curve at enhanced forward error correction (EFEC) threshold of 2e-3.

Concept, principle and experimental setup
8][39][40][41] Figure 1 (upper row) illustrates the concept and principle of a 260-meter secure optical interconnects link employing two distinguished orbital angular momentum (OAM) channels multiplexing between two buildings.
Moreover, this link can also be used for 1-to-N OAM multicasting.Due to the whole space is full of atmospheric turbulence, the property of OAM modes will be affected (such as beam wander and phase distort) resulting in degradation of signal performance.Additionally, there is another distinct advantage that OAM can lead improved security to optical interconnects link.The eavesdropper cannot measure the accurate OAM information while wiretapping an angular of less than .As shown in the upper row in figure 1, the eavesdropper wiretaps a part of OAM beams 2π and he obtains a terrible signal performance that provides the link security.The layout of our experiment is also illustrated in Figure 1 (lower row).The OAM multiplexing/multicasting freespace optical interconnects link is exposed to the atmospheric conditions between the corridors from WNLO-E building to WNLO-H building (WNLO: Wuhan National Laboratory for Optoelectronics).The transmitter and the receiver are located in the front of the gate of WNLO-E building and the reflection mirror (M) is located at the end of the corridors.The single way distance is 130 meters thus the total distance of double-pass transmission is 260 m after reflection.The experimental setup is shown in Fig. 2. A narrow linewidth laser at 1550 nm is sent to an IQ modulator to produce a 10-Gbaud (40-Gbit/s) 16-QAM signal (refer to Fig. S1 in the Supplemental Material).The 16-QAM signal is split into two copies for two OAM channels.In one copy, the signal is delayed with a 2-km single-mode fiber (SMF) to decorrelate the data sequence and thus the two channels are decorrelated.Before connecting to collimators, the two channels are sent to erbium-doped fiber amplifier (EDFA), variable optical attenuator (VOA), and polarization controller (PC) for proper power and polarization control.The two spatial light modulators (SLMs) in two paths modulate the light beams to OAM state .After combination using a beam  = +3 splitter (BS-1) with the OAM state reversed in the reflective path (mirror image effect), two OAM beams with opposite states are multiplexed together .Meanwhile, a He-Ne laser at 632.8 ( =± 3) nm produces a clear Gaussian beam (size: 0.8 mm), which is combined with the two OAM channels using another beam splitter (BS-2).This red Gaussian beam is mainly used for easy system alignment.The two OAM channels and the red beam pass through a 1:20 expander with the beam size magnified to ~4 cm.Note that we can produce a converged beam by sliding the lens adjustment and adjust the beam waist position at the reflection site in the end of corridor (See supplementary information for more details).As a result, we can receive a ~4 cm beam in the OAM RX.
The received OAM beam size is reduced by two lens (f=300 mm and 40 mm) but still converged.
At the proper position, OAM channel can be demodulated by loading an inverse fork hologram pattern on SLM-3.After a telescope system (f = 50mm and 400mm), the converged demodulated beam is magnified and collimated and then coupled into an SMF for coherent detection assisted by off-line digital signal processing (See supplementary information Fig. S1).

Intensity profiles and beam fluctuation after 260-m transmission
Figure 3 shows the intensity profiles of generated OAM beams with topological charge of  = +3 and (a1, a2), their superposition (a3) and interferograms with Gaussian beam (a4, a5) at  = -3 the transmitter side.Correspondingly, the intensity profiles of received OAM beams after 260meter propagation are also displayed in Fig. 3(b1-b5).In fact, we recorded the intensity profiles and interferograms covering topological charge from to excluding Gaussian beam  = -6  = +6 (refer to Fig. S3 in the Supplemental Material).Meanwhile, Fig. 3(c1-c4) illustrate the demodulated beams for different patterns loaded onto SLM-3 when transmitting .It is found  = -3 that only when the demodulated pattern is inverse to the transmit OAM state the OAM beam can be converted into a Gaussian-like beam with a bright spot at the beam center (c4).Such Gaussianlike beam can be coupled into an SMF more efficient than others demodulated beams.Owing to the atmospheric turbulence, it is valuable to investigate the fluctuation of demodulated position and received power.), (c1-c2) received power of signal channel after  = +3,  = -3 SMF; (d1-d2) received power of crosstalk channel after SMF.

BER performance and security for multiplexing interconnects link
Furthermore, we demonstrate a 260-m optical interconnects link using 16-QAM carrying OAM multiplexing and also evaluate its BER performance and security (See the first section of the supplementary information for more details about BER measurement).As shown in Fig. 5(a1), we demodulate the received multiplexed OAM beam with full pattern ( and ) on SLM- = +3  = -3 3. For the BER performance, there is about 2.5 dB penalty between the B2B curve and the two multiplexing curves at EFEC threshold of 2e-3.The constellation diagram is also inserted in Fig. 5(a1) representing the system performance.interval of 1 minute.We believe that atmospheric turbulence and building sway are two major reasons leading to this phenomenon.Figure 5(a2) and (a3) display the results of BER variations over time when the system operates near the EFEC threshold.It can be observed that at certain moments, the BER exceeds the EFEC threshold.This is attributed to the fact that beam wandering can easily cause the receiver's OSNR to drop below 18 dB, resulting in a BER higher than the EFEC threshold and a degradation in communication quality.However, in practical applications of communication systems, it is typically avoided to operate the communication link near the FEC threshold.Instead, the common practice is to increase the optical power at the transmitter to elevate the receiver's OSNR, provide a certain level of redundancy, and stay well clear of the FEC threshold.In the optical communication system we have proposed, as long as the receiver's OSNR exceeds 18 dB, the BER remains below the EFEC threshold.Consequently, in practical applications, error occurrences are minimized.Moreover, we prove the link security in our 260-m OAM multiplexing optical interconnects.
For eavesdroppers, there are only two ways to obtain the transmitted information.The first method is to attempt to recover information from the light scattered by the atmosphere.However, OAM beams suffer severe phase distortion when scattered by the atmosphere, making demodulating OAM modes nearly impossible.The second method is for eavesdroppers to wiretap partial optical fields along the path of the optical beam, which is the main focus of this paper.This paper primarily discusses the security characteristics of a communication system in an extreme scenario, where the eavesdropper has already determined the topological charge of the OAM mode and the wiretapped optical field exhibits ideal angular distribution characteristics.As shown in Fig. 5 power decreases for signal channel and increases for crosstalk channel so that the inter-channel crosstalk increase rapidly (from less than -20 dB to more than -10 dB).The larger inter-channel crosstalk is, the more terrible BER performance shows.
To further analyze the mode distribution characteristics of the optical beams wiretapped by eavesdroppers, we simulated the OAM order spectra of the multiplexed OAM beams after a 260meter free-space transmission with different block pattern, as shown in Fig. 5(c1) and (c2).Figures and the power difference between adjacent orders gradually decrease.To assess the security of the communication system, we here use the power of the desired channel and the crosstalk between the multiplexed channels in the eavesdropped optical field as evaluation metrics.This is because once the eavesdropper's access to the power of the required channel is reduced, the spontaneous radiation noise introduced by the optical amplifier in receiving system will lower the OSNR of the signal, thus decreasing the accuracy of the eavesdropper in obtaining the original information.

5(c1
On the other hand, an increase in the crosstalk between the multiplexed channels will enhance the interference experienced by the eavesdropper when trying to obtain other signals within the required channel.This can severely affect the accuracy of the eavesdropper in obtaining the original information.Therefore, in a communication system, if the power of the required channel in the portion of the optical field energy obtained by the eavesdropper is low and the crosstalk between the multiplexed channels is high, it indicates that the communication system has a higher level of security.Conversely, it is considered insecure.We then simulate the normalized power of In practical optical communications, it is challenging for eavesdroppers to obtain most of the optical fields demonstrated in the experiment.This is because wiretapping too much optical field energy would result in significantly reduced energy at the receiver's end of the original communication system, leading to a deterioration in communication quality and raising the receiver's awareness.Furthermore, it is difficult for eavesdroppers to guarantee that the wiretapped optical field has the ideal angular distribution characteristics demonstrated in the experiment, which increases the difficulty of demodulating OAM modes and the crosstalk between different channels.Considering that eavesdroppers also need to account for the trial-and-error difficulty of attempting different mode numbers when demodulating OAM modes, it becomes challenging for eavesdroppers to obtain the desired information in a free-space optical communication system based on OAM mode multiplexing.Therefore, the experiment results validate the possibility of eavesdroppers to obtain communication link information in an extreme ideal scenario, confirming the high security of the free-space optical communication system based on OAM mode multiplexing.

260-m power-controllable 1-to-9 multicasting interconnects link
When coming to the situation that one-to-many or many-to-many data distribution in some remote areas, war zones and temporary stations, hundreds of meters of multicasting in free-space optical interconnects is always the best option.Here we also demonstrate a 260-meter powercontrollable 1-to-9 multicasting interconnects link.
Considering different influence for different OAM orders resulting from the atmospheric turbulence and other reasons such as slight displacement at the receiver side, the power loss of different topological charge is quite different and the relationship is non-linear.In fact, we ignore the exact relationship among the OAM orders and various possible reasons, but just care about the output power and input power like a black box.So, we establish an off-line feedback process between the output and input and change the complex power-controllable multicasting pattern [42][43] on the transmitter side continually to achieve the desired power target (refer to Fig. S4 in the Supplemental Material).When we transmit 1-to-9 multicasting in same power designed on pattern, the nine channels ( ) produce different power loss resulting from real- = 0, ± 1, ± 2, ± 3, ± 4 time atmospheric turbulence and other reasons, as shown in Fig. 6(a1).The power distinguishes between largest multicasting channel and least one is about 9 dB while it is about 6 dB between the least power multicasting channel and undesired channel.For examining the power-controllable off-line feedback process, we demonstrate how to correct each channel to achieve nearly the same power distribution.In figure 6(a2), the inter-channel power distinguishes reduce to about 2.4 dB and it remains about 6 dB between the least power multicasting channel and undesired channel.
The intensity profiles of 1-to-9 OAM multicasting before and after such off-line feedback process are presented in Supplemental Material Fig. S5.In many practical scenarios, the different users (different channels) may require different power according the distance or other reasons, the power-controllable off-line feedback process we demonstrated can not only correct the power distribution affected by atmospheric turbulence but also provide flexible control on power distribution according practical applications.We also investigate the BER performance (Fig. 6(b)) for power-controllable 1-to-9 multicasting interconnects link as the power distribution shown in Fig. 6(a2).All the nine BER curves (including Gaussian) almost huddle together and the average OSNR penalty is about 2 dB at the EFEC limit of 2e-3.
For OAM beam multicasting interconnects, we also need to analyze its security.In OAM beam multicasting systems, since each multicasting channel carries the same information, there is no crosstalk-induced interference between different multicasting channels.The eavesdropper only needs to detect the multicasting channels carrying the information.In this case, we use the average power of the multicasting channels obtained by the eavesdropper as the evaluation metric.The lower the average power of the multicasting channels obtained by the eavesdropper, the more susceptible the signal is to noise interference, making it easier to reduce the accuracy of the eavesdropper's retrieval of the original information, indicating higher security for the multicasting interconnect system.Figures 6(c1) depicts the simulated OAM order spectra obtained by eavesdroppers for nine multicasting channels ( ) corresponding to  = 0, ± 1, ± 2, ± 3, ± 4 different angular block patterns.It can be seen that as the area of the beams wiretapped by eavesdroppers decreases from 31/32 (1/32-block) to 1/32 (31/32-block), the power of desired multicasting channels gradually decreases.We then simulate the average power of the multicasting channels when the eavesdropper wiretaps on different beam sizes, as shown in Fig. 6(c2).One can indicate that as the area of the beams wiretapped by eavesdroppers decreases from 63/64 (1/64block) to 1/64 (63/64-block), the average power of the multicasting channels decreases from -14.66 dB to -38.44 dB.We then evaluate the security of the multicasting interconnect system in a general scenario as shown in Figs.6(d1) and (d2), where the aperture radius of eavesdropper r e is equal to that of OAM beam with r b .Figures 6(d1) shows the OAM order spectra obtained  = +4 by eavesdroppers for nine multicasting channels at different aperture offsets.Figure 6(d2) displays the simulated average power of nine multicasting channels as functions of eavesdropper's aperture offset.One can indicate that as the eavesdropper's aperture radius decreases from 1.5r b to 0.5r b , and the eavesdropper's aperture offset increases from 0 to 2r b , the average power of nine multicasting channels rapidly decreases.This result indicates that once the eavesdropper obtains only a portion of the optical field distribution, the average power of nine multicasting channels acquired by the eavesdropper decreases, making it difficult for the eavesdropper to accurately recover the original information, validating the security of the multicasting system.

Conclusion and discussion
We demonstrate a 260-m secure optical interconnects link employing 10-Gbaud (40-Gbit/s) 16-QAM carrying orbital angular momentum multiplexing and power-controllable 1-to-9 multicasting link is also investigated.Firstly, we study the beam wandering, received power fluctuation and channel crosstalk after 260-meter propagation.It's obvious that the beam displacement and power fluctuation after beam reduction remain stable even suffer 260-meter atmospheric turbulence.For multiplexing, the average inter-channel crosstalk is less than -20 dB for and .Meanwhile for multicasting the average crosstalk for desired channel and undesired channel is less than -6 dB.Furthermore, we study the BER performance and link security.The results show that our 260-m optical interconnects link works well for both security OAM multiplexing and power-controllable 1-to-9 multicasting.The eavesdropper cannot recovery the correct data information when he wiretaps just a part of OAM beams.The more OAM beams blocked; the more errors received by the eavesdropper.Only complete reception of OAM beams corresponds to the best BER performance.As for multicasting application, off-line feedback process is proved works for improving the link property and also flexible control the power distribution for practical application.
For some practical application such as inter-building interconnects, such few-hundred freespace optical link plays an important role and attract more and more interest.What's more, with introducing orbital angular momentum as orthogonal SDM modal basis set, the system capacity and spectral efficiency have dramatically increased which will provides an alternative option for solving the capacity crunch bring by booming of optical interconnects.Security is another advantage for employing OAM multiplexing and it is hard to wiretap the information from interconnects link between WNLO-E building and WNLO-H building.WNLO: Wuhan National Laboratory for Optoelectronics.

Fig. 1
Fig. 1 Concept and principle.Upper row, Concept of an inter-building optical interconnects employing orbital angular momentum (OAM) multiplexing/multicasting and shows its security.Lower row, Layout of a 260-meter security OAM multiplexing/muticasting free-space optical interconnects link between WNLO-E building and WNLO-H building.WNLO: Wuhan National Laboratory for Optoelectronics.

Figure 4 (
a1) and (a2) depict the center displacement of the received demodulation beam.It is observed that the maximum displacement is around ~0.45 mm for  = +3 and ~0.5mm for (See supplementary information for more details).Figure 4(b1) and (b2)  = -3 present the space light power fluctuation after the beam reduction (the inverse telescope consists of Lens-1 and Lens-2 at the OAM receiver site) and the power is stable with slight difference for two channels due to the atmospheric loss.The fluctuations of the received power of signal channel and crosstalk channel after SMF are shown in Fig. 4(c1) (c2) and (d1) (d2).The received power fluctuations up to ~8 dB for signal channel demultiplexed by , ~10 dB for crosstalk channel  = +3 demultiplexed by , ~4 dB for signal channel demultiplexed by , and ~6 dB for  = +3  = -3 crosstalk channel demultiplexed by are observed within a 50% probability distribution  = -3 range (See supplementary information for details).The crosstalk between two channels is ~-20 dB ( ) and ~-24 dB ( ) which provides the possibility for employing OAM multiplexing  = +3  = -3 and others advanced modulated techniques.All the received data are recorded in 200 seconds at the interval of 1 second.

Fig. 5
Fig.5 BER performance for multiplexing.(a1) Measured BER performance for 260-meter 10-Gbaud 16-QAM OAM multiplexing free-space optical interconnects link.(a2) (a3) BER fluctuation with and on SLM-3  = +3  = -3 respectively at the OSNR of ~18 dB.(b1) Measured BER performance for 260-meter security OAM multiplexing free-space optical interconnects link.(b2) (b3) Average received power distribution according to various angular block parts.(c1) Simulated OAM order spectra of OAM beams with wiretapped by the eavesdropper with angular  = +3 block.Inserts are intensity profiles wiretapped by the eavesdropper.(c2) Simulated OAM order spectra of multiplexed OAM beams with and wiretapped by the eavesdropper with angular block.(c3) Simulated normalized  = +3  = -3 power of OAM beam with and crosstalk between OAM beam with and wiretapped by the  = +3  = +3  = -3 (b1), we respectively load the 1/16-block, 1/8-block and 1/4-block pattern onto SLM-3 and present their BER performance demultiplexed by and .The patterns used here serves the dual  = +3  = -3 purpose of enabling the eavesdropper to wiretap a portion of the optical field and to demodulate the OAM modes, for the sake of simplifying the experimental setup.The portion of the patterns with the fork holograms correspond to what the eavesdropper captures.When the eavesdropper wiretaps a majority of the OAM beams with 1/16-block, the BER curves can be still below the EFEC limit of 2e-3 but with significantly increased OSNR penalty (~4.5 dB) compared to the case with full pattern.Such phenomenon can be ascribed to the increased OAM channel crosstalk when blocking a part of OAM beams, which enhances the security of OAM multiplexing transmission link.As the blocked part of OAM beams increases, the BER curves degrade rapidly.OSNR penalty increase to ~7 dB when loading 1/8-block demodulated pattern.When 1/4 part of OAM beams is blocked (1/4-block pattern), the BER performance of the optical interconnects link cannot be below the EFEC limit, i.e., the eavesdropper loses lots of correct data information.The insert constellation diagram can also explain the degraded tendency.As a consequence, the eavesdropper wiretapping a part of OAM beams fails to get correct data information.The more OAM beams blocked; the more errors received by the eavesdropper.Only complete reception of OAM beams corresponds to the best BER performance.From another perspective, the obtained results shown in Fig. 5(b2) and (b3) also indicate successful demonstration of 260-meter security free-space optical interconnects link using 16-QAM carrying OAM multiplexing.As the blocked part increase from 0 to 1/4-block for both two demodulated patterns ( and ), the received  = +3  = -3 ) and (c2) respectively depict the OAM order spectra obtained by eavesdroppers for individual OAM beam () and two OAM multiplexed beams ( and ) corresponding to = +3  = +3  = -3different angular block patterns.It can be observed that as the area of the beams wiretapped by eavesdroppers decreases from 31/32 (1/32-block) to 1/32 (31/32-block), the desired OAM orders the required OAM beam with and the crosstalk between OAM beams with and  = +3  = +3  when the eavesdropper wiretaps on different beam sizes, as shown in Fig. 5(c3).One can = -3 indicate that as the area of the beams wiretapped by eavesdroppers decreases from 63/64 (1/64block) to 1/64 (63/64-block), the normalized power of the desired OAM beam with  = +3 decreases from -0.19 dB to -35.92 dB.At the same time, the crosstalk between OAM beams with and increases from -35.84 dB to -0.11 dB.To further evaluate the security of the  = +3  = -3 communication system, we conduct an analysis of the eavesdropper in a general scenario, as shown in Figs.5(d1) -(d3).In this scenario, we assume that the eavesdropper has a finite circular aperture and wiretaps at different positions across the beam cross-section.Figures 5(d1) and (d2) respectively show the OAM order spectra obtained by eavesdroppers for individual OAM beam ( ) and two OAM multiplexed beams ( and ) at different aperture offsets, where = +3  = +3  = -3 the aperture radius of eavesdropper r e is equal to that of OAM beam with r b .It can be seen  = +3 that as the eavesdropper's aperture offset increases from 0.5r b to 2r b , the power of the required OAM beam with and rapidly decreases, and the power difference between adjacent  = +3  = -3 channels sharply decreases.Figure 5(d3) show the simulated normalized power of OAM beam with and crosstalk between OAM beams with and as functions of  = +3  = +3  = -3 eavesdropper's aperture offset.It can be observed that as the eavesdropper's aperture radius decreases from 1.5r b to 0.5r b , and the eavesdropper's aperture offset increases from 0 to 2r b , the power of the OAM beam of the required channel rapidly decreases, and the crosstalk valuesbetween two multiplexed channels increase sharply.From the results above, we can see that whether the eavesdropper is co-axial with the OAM beam or situated in a general off-axis scenario, once the eavesdropper only obtains a small part of the light field distribution, the power of the required channel and the crosstalk values between different multiplexed channels make it difficult for the eavesdropper to recover the original information, thus confirming the high security of the communication system.

Fig. 6
Fig.6 BER performance for multicasting.(a1)(a2) Multicasting power of pattern-designed and experiment before and after off-line feedback process respectively.(b) Measured BER performance for 260-m power-controllable 1-to-9 multicasting interconnects link.(c1) Simulated OAM order spectra of multicasting OAM beams wiretapped by the eavesdropper with angular block.Inserts are intensity profiles wiretapped by the eavesdropper.(c2) Simulated average power of multicasting channels wiretapped by the eavesdropper with angular block.(d1) Simulated OAM order spectra of multicasting OAM beams wiretapped by the eavesdropper in a general scenario.Inserts are intensity profiles wiretapped by the eavesdropper.r e : eavesdropper's aperture radius.r b : OAM beam radius of transmitted over  = + 4 260 m. (d2) Simulated average power of multicasting channels wiretapped by the eavesdropper in a general scenario.

Fig. S2
Fig. S2 (a)The layout between corridors which is exposed to the atmospheric conditions from

Fig. S3
Fig. S3 OAM intensity profiles and interferograms at (a) transmitter side and (b) at receiver side

Fig. S5
Fig. S5The intensity profiles of 1-to-9 multicasting at Tx and after 260-m propagation at Rx with

Fig. S6 3 Video 1
Fig. S6 Gif figure of recorded 200 intensity profiles at the interval of 1 second for . = +3