1. Introduction

The explosive growth of global data traffic is driving an ever-increasing demand for higher data capacity and more efficient spectral usage in transmission links. To address the coming capacity crunch, it is highly desirable not only to make full use of already well-known physical dimensions (e.g. amplitude, phase, frequency/wavelength, polarization, time) but also exploit additional degrees of freedom (e.g. spatial structure) of light waves. Recently, various advanced multilevel modulation formats and multiplexing techniques, i.e. quadrature phase-shift keying (QPSK), 4-level pulse-amplitude modulation (PAM-4), m-ary quadrature amplitude modulation (m-QAM), orthogonal frequency-division multiplexing (OFDM), wavelength-division multiplexing (WDM), polarization-division multiplexing (PDM), and time-division multiplexing (TDM), have been widely used to increase the transmission capacity [1–6]. Meanwhile, space-division multiplexing (SDM) exploiting the transverse spatial structure dimension of light waves has also attracted more and more attention [7,8]. Few-mode fiber (FMF) and multi-core fiber (MCF) have gained great success in SDM for efficient increase of fiber optical transmission capacity [9–12]. In addition to FMF and MCF, orbital angular momentum (OAM), which is also related to the spatial phase structure (spiral phase front) of an electromagnetic wave [13], has also shown its possible use both in free-space and fiber transmission links to improve the transmission capacity [14–17]. An OAM beam features a helical phase front termed as exp $(\text{i}l\text{θ})$ and has discrete value of $l\hslash$ per photon, where $l$ is the topological charge number, $\text{θ}$ refers to the azimuth angle and $\hslash$ is the reduced Planck constant. The handedness of this helical phase front is determined by the sign of the topological charge number $l$ which is an unlimited value in principle, therefore, the state of OAM-carrying beams is infinite in theory. In addition, the inherent orthogonality of the various OAM states makes it possible to increase the capacity of communication systems, either by employing OAM beams as information carriers for multiplexing or by encoding information as OAM states of the beam.

In the recent years, a large number of researches have been reported in free-space and fiber transmission links using OAM multiplexing. 1) Using 20-Gbaud 16-QAM signals over pol-muxed 8 OAM modes in two groups of concentric rings (32 channels in total), free-space transmission capacity of 2.56 Tbit/s and high spectral efficiency of 95.7 bit/s/Hz were reported [14]; 2) using 17.9-Gbit/s OFDM offset quadrature amplitude modulation (OFDM/OQAM) 64-QAM signals over pol-muxed 22 OAM modes (44 channels in total) within a bandwidth of 3.2 GHz, free-space transmission capacity of 736.0 Gbit/s and high spectral efficiency of 230 bit/s/Hz were achieved [18]; 3) using 100-Gbit/s QPSK signals over pol-muxed 12 OAM modes on 42 wavelengths, free-space transmission capacity of 100.8 Tbit/s was obtained [19]; 4) using 54.139-Gbit/s OFDM-8QAM signals over 368 WDM pol-muxed 26 OAM modes, free-space aggregate transmission capacity of 1.036 Pbit/s and high spectral efficiency of 112.6 bit/s/Hz were gained [20]; 5) using 5.8-Gbaud Nyquist 32-QAM signals over pol-muxed 52 OAM modes (104 channels in total), free-space net transmission capacity of 8.16 Tbit/s and aggregate ultra-high spectral efficiency of 435 bit/s/Hz were demonstrated [21]; 6) using 20-Gbaud 16-QAM signals over two OAM modes on 10 wavelengths, 1.6-Tbit/s transmission capacity through a 1.1-km specially designed vortex fiber was realized [15]; 7) using PAM-4 and on-off keying (OOK) signals over two OAM modes through a 1.1-km FMF, fiber-based bidirectional transmissions with 2.5-Gbaud PAM-4 downstream and 2-Gbaud OOK upstream in an OAM passive optical network (PON) were accomplished [22]. Remarkably, most of these previous works with impressive performance were demonstrated by employing phase-only liquid crystal spatial light modulators (SLMs) that are polarization sensitive. Even in the pol-muxed OAM works, the pol-muxed stage is actually after the generation of OAM, which is still polarization sensitive. That is, the allowable polarization of incident light wave is limited to the working polarization of the polarization-sensitive SLMs used in the communication systems. However, polarization-insensitive operations are highly desired in practical optical communication systems [23]. In this scenario, a laudable goal would be to develop polarization-insensitive optical communication systems employing OAM modes while still using commercially available polarization-sensitive SLMs.

In this paper, we propose a simple configuration incorporating single polarization-sensitive phase-only liquid crystal SLM to facilitate polarization-insensitive free-space optical communications based on OAM modes. We experimentally demonstrate several polarization-insensitive optical communication subsystems employing OAM modes, e.g. polarization-insensitive single OAM mode transmission, polarization-insensitive 4 and 10 OAM modes multicasting, and polarization-insensitive 8 OAM modes multiplexing. Free-space polarization-insensitive communication links based on OAM beams carrying PAM-4 signals are demonstrated in the experiment.

2. Concept of polarization-insensitive optical communications using OAM modes

Figure 1 illustrates the concept of polarization-insensitive optical communications using space degree of photons (i.e. polarization-insensitive spatial light modulation for optical communications). In general, a randomly polarized Gaussian beam can be decomposed into two orthogonal polarizations, i.e. x-polarization and y-polarization. When it is directly delivered to the polarization-sensitive SLM, only one polarization is modulated and convert to a new beam with modified spatial structure (e.g. OAM beam with helical phase front and doughnut intensity profile) while the other polarization remains to be a Gaussian beam, which is called partially modulated as depicted in Fig. 1(a). In contrast, a randomly polarized Gaussian beam is expected to be fully modulated by using a polarization-insensitive spatial light modulation configuration, i.e. both x-polarization and y-polarization components of incident Gaussian beam with random polarization can be spatially modulated. Figure 1(b) shows the basic physical dimensions of photons including amplitude, phase, time, frequency/wavelength, polarization and spatial structure, which could be used for optical communications by modulating or multiplexing data information. OAM beam having helical phase front is related to the spatial phase structure dimension. Polarization-insensitive OAM modes generation exploiting polarization-insensitive configuration and space dimension of light waves would enable polarization-insensitive optical communications employing OAM modes.

 

Fig. 1 Concept of polarization-insensitive optical communications using space dimension of photons.

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3. Polarization-insensitive single OAM mode transmission

Figure 2 shows the experimental setup for polarization-insensitive single OAM mode transmission. The key part of the setup is the proposed polarization-insensitive configuration that incorporates a polarization beam splitter (PBS), a polarization-sensitive phase-only liquid crystal SLM, a half-wave plate (HWP), and three mirrors in a loop structure. It is noted that the polarization-insensitive configuration can be used both for polarization-insensitive modulation and polarization-insensitive demodulation. The working principle of polarization-insensitive configuration can be briefly explained as follows. For description simplicity, we assume that the polarization-sensitive phase-only SLM works only for the x-polarization while no response to the y-polarization. A randomly polarized Gaussian beam fed into the configuration is first split into x-polarization and y-polarization, which are at the output transmission port and reflection port of the PBS and then propagate along clockwise and counterclockwise directions around a loop configuration, respectively. The fast axis (i.e. the axis through which the light travels faster) of the HWP is ${45}^{°}$ with respect to the x-polarization. The x-polarized Gaussian beam propagating clockwise is modulated by the SLM, the output of which is rotated ${90}^{°}$ and change to y-polarization by the HWP. The y-polarized Gaussian beam propagating anticlockwise rotates ${90}^{°}$ to be x-polarization after passing through the HWP, the output of which is also modulated by the SLM. We insert mirrors to correct the difference of beam characteristics (e.g. beam size) caused by optical path difference along clockwise and counterclockwise after reflecting off the SLM. That is, the SLM is in the middle of the loop structure. Moreover, the number of reflections along clockwise and counterclockwise directions after SLM should have the same parity to mitigate the influence of mirror-image effect. After spatial light modulation, the light beams propagating clockwise and counterclockwise are combined again by the PBS to output spatially modulated randomly polarized beams.

 

Fig. 2 Experimental setup for polarization-insensitive single OAM mode transmission. ECL: external cavity laser; PC: polarization controller; PAM-4: 4-level pulse-amplitude modulation; AWG: arbitrary waveform generator; EDFA: erbium-doped fiber amplifier; Col.: collimator; Pol.: polarizer; HWP: half-wave plate; BS: non-polarization beam splitter; PBS: polarization beam splitter; SLM: spatial light modulator; M1-M6: mirror; VOA: variable optical attenuator; PD: photodetector. Insets show intensity profiles of modulated and demodulated OAM beam (l = 8) after the polarization-insensitive modulation and demodulation, respectively.

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At the transmitter, an optical 4-Gbit/s PAM-4 signal at 1550nm is prepared and then an HWP is adjusted to offer variable input polarization to the polarization-insensitive configuration. The polarization-sensitive SLM in the polarization-insensitive configuration is loaded with a forked diffraction grating (fork pattern) formed by a spiral phase distribution and a linear phase ramp to convert a Gaussian beam to an OAM beam with a topological charge number l. The distance between the lens and two SLMs in the polarization-insensitive modulation and demodulation parts is 2f to form an imaging system, in which f is the focal length of the lens. The insets in Fig. 2 show measured typical intensity profiles of OAM beam with the topological charge number l = 8 after the polarization-insensitive modulation and polarization-insensitive demodulation, respectively.

We first study the polarization-insensitive modulation function of the proposed polarization-insensitive configuration. Figure 3 depicts the measured intensity profiles of output OAM beam (l = 4) after the polarization-insensitive configuration under different angles between the incident polarization and x-polarization of (a) 0, (b) 30, (c) 45, (d) 60, (e) 90, (f) 100, (g) 120, (h) 135, (i) 150 and (j) 180 degree, respectively. No distinct changes are observed as varying the polarization of incident linearly polarized Gaussian beam. Furthermore, using another polarization-insensitive configuration placed in the experiment setup, we realize polarization-insensitive modulation and demodulation functions simultaneously. Figure 4(a)-4(e) show the measured intensity profiles of demodulated OAM beam (l = 4) with different angles between the incident polarization and x-polarization of (a) 0, (b) 45, (c) 90, (d) 135, (e) 150 degree, respectively. Clear bright spots at the center of the demodulated beam are always observed with negligible changes when adjusting the polarization of incident OAM beam. The measured eye diagrams of 4-Gbit/s PAM-4 signal for the demodulated OAM beam (l = 4) with different angles between the incident polarization and x-polarization of 20, 45, 160, 180 degree are shown in Fig. 4(f)-4(i). The eye diagram of back-to-back (B-to-B) 4-Gbit/s PAM-4 signal is also shown in Fig. 4(j) for reference. One can see clear eye-opening with negligible performance degradation after the polarization-insensitive single OAM mode modulation, transmission and demodulation.

 

Fig. 3 Measured intensity profiles of output OAM beam (l = 4) after the polarization-insensitive configuration under different angles between the incident polarization and x-polarization of (a) 0, (b) 30, (c) 45, (d) 60, (e) 90, (f) 100, (g) 120, (h) 135, (i) 150 and (j) 180 degree, respectively.

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Fig. 4 (a)-(e) Measured intensity profiles of demodulated OAM beam (l = 4) with angles between the incident polarization and x-polarization of (a) 0, (b) 45, (c) 90, (d) 135, (e) 150 degree, respectively. (f)-(j) Measured eye diagrams of 4-Gbit/s PAM-4 signal for the demodulated OAM beam (l = 4) with different angles between the incident polarization and x-polarization of (f) 20, (g) 45, (h) 160, (i) 180 degree, respectively. (j) Measured eye diagram of back-to-back (B-to-B) 4-Gbit/s PAM-4 signal.

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4. Polarization-insensitive N-fold OAM modes multicasting

In a common SDM system with multiplexed multiple channels, different data information is loaded to different channel which is called multiplexing. However, data on a single channel duplicating onto multiple channels known as multicasting is also widely used in today’s networks, e.g. teleconferencing, video distribution, interactive distance learning and so on. Analogical to the widely used wavelength multicasting in WDM systems [24–26], all-optical mode multicasting may also see potential use in mode-division multiplexing (MDM) networks [27–29].

The experimental setup for polarization-insensitive N-fold OAM modes multicasting is shown in Fig. 5. At the transmitter, an optical 4-Gbit/s PAM-4 signal at 1550nm is prepared and carried by a linearly polarized Gaussian mode (l = 0) with a planar phase front at its beam waist. SLM1 in the polarization-insensitive modulation dash box is loaded with a complex multi-OAM phase pattern to generate multiple collinearly superimposed OAM modes [30]. After the polarization-insensitive modulation, the information carried by the input linearly polarized Gaussian mode is copied and delivered to multiple OAM modes which are distinguishable from each other. Consequently, 1-to-N multicasting from single Gaussian mode to multiple OAM modes is available with the number of multicasting copies N determined by the complex multi-OAM phase pattern. Thus, multiple OAM modes are distributed to multiple users after free space transmission. SLM2 in the polarization-insensitive demodulation dash box is used to demodulate one of multicasting OAM modes by loading the corresponding inverted spiral phase pattern. The insets in Fig. 5 show measured typical intensity profiles of multicasted 10 OAM modes after polarization-insensitive modulation and demodulated OAM mode (l = 6) after polarization-insensitive demodulation.

 

Fig. 5 Experimental setup for polarization-insensitive N-fold OAM modes multicasting. Insets show intensity profiles of multicasted 10 OAM modes after polarization-insensitive modulation and demodulated OAM mode (l = 6) after polarization-insensitive demodulation.

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We first study the polarization-insensitive N-fold OAM modes multicasting using the polarization-insensitive modulation configuration. Figure 6(a)-6(e) show the measured intensity profiles of 4-fold OAM modes multicasting ($l=3, 6, 9, 12$) under different angles between the incident polarization and x-polarization of 0, 30, 45, 90 and 120 degree, respectively. Figure 6(f)-6(j) show the measured intensity profiles of 10-fold OAM modes multicasting ($l=-18, -15, -12, -9, -6, 6, 9, 12, 15, 18$) under different angles between the incident polarization and x-polarization of 0, 30, 45, 90 and 120 degree, respectively. The negligible changes of intensity profiles indicate the successful implementation of polarization-insensitive N-fold OAM modes multicasting.

 

Fig. 6 (a)-(e) Measured intensity profiles of 4-fold OAM modes multicasting ($l=3, 6, 9, 12$) under different angles between the incident polarization and x-polarization of (a) 0, (b) 30, (c) 45, (d) 90 and (e) 120 degree, respectively. (f)-(j) Measured intensity profiles of 10-fold OAM modes multicasting ($l=-18, -15, -12, -9, -6, 6, 9, 12, 15, 18$) under different angles between the incident polarization and x-polarization of (f) 0, (g) 30, (h) 45, (i) 90 and (j) 120 degree, respectively.

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Figure 7(a)-7(e) depict measured intensity profiles of one typical demodulated OAM mode ($l=12$) among 4 multicasted OAM modes after the polarization-insensitive demodulation under different angles between the incident polarization and x-polarization of 0, 30, 45, 90, 120 degree, respectively. Similarly, Fig. 7(f)-7(j) display measured intensity profiles of one typical OAM mode ($l=6$) among 10 multicasted OAM modes after the polarization-insensitive demodulation under different angles between the incident polarization and x-polarization of 0, 30, 45, 90, 120 degree, respectively. No distinct changes are observed in the generation of N-fold multicasted OAM modes and demodulation of the multicasted OAM modes as varying the polarization of incident beam.

 

Fig. 7 (a)-(e) Measured intensity profiles of demodulated OAM mode ($l=12$) among 4 multicasted OAM modes under different angles between the incident polarization and x-polarization of 0, 30, 45, 90 and 120 degree, respectively. (f)-(j) Measured intensity profiles of demodulated OAM mode ($l=6$) among 10 multicasted OAM modes under different angles between the incident polarization and x-polarization of 0, 30, 45, 90 and 120 degree, respectively.

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We then assess the quality of polarization-insensitive N-fold OAM modes multicasting by measuring the power distribution over all multicasted OAM channels (i.e. OAM spectrum). Figure 8(a) and 8(b) depict measured power distribution of 4-fold multicasted OAM channels ($l=3, 6, 9, 12$) under different angles between the incident polarization and x-polarization of 0 and 150 degree, respectively. The measured power distribution of 10-fold multicasted OAM channels ($l=-18, -15, -12, -9, -6, 6, 9, 12, 15, 18$) under different angles between the incident polarization and x-polarization of 60 and 90 degree are displayed in Fig. 8(c) and 8(d), respectively. As shown in Fig. 8(a) and 8(b), the observed extinction ratio (ER) for all 4-fold multicasted OAM channels, defined by the power ratio of desired OAM channel (e.g. OAM₆) to its left and right neighboring OAM channels (e.g. OAM₅ and OAM₇), is larger than 16.0 dB under different angles between the incident polarization and x-polarization of 0 and 150 degree. As shown in Fig. 8(c) and 8(d), the extinction ratio for all 10-fold multicasted OAM channels is measured to be larger than 10 dB under different angles between the incident polarization and x-polarization of 60 and 90 degree.

 

Fig. 8 Measured power distribution of 4-fold multicasted OAM channels ($l=3, 6, 9, 12$) under different angles between the incident polarization and x-polarization of (a) 0 and (b) 150 degree, respectively. Measured power distribution of 10-fold multicasted OAM channels ($l=-18, -15, -12, -9, -6, 6, 9, 12, 15, 18$) under different angles between the incident polarization and x-polarization of (c) 60 and (d) 90 degree, respectively.

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We further measure the bit-error rate (BER) performance for polarization-insensitive N-fold OAM modes multicasting of 4-Gbit/s PAM-4 signal. The BER curves of 4 OAM modes multicasting are plotted in Fig. 9(a). The angles between the incident polarization and x-polarization of OAM modes $l=3, 6, 9, 12$ are 60, 90, 0 and 30 degree, respectively. The BER curves of 10 OAM modes multicasting are depicted in Fig. 9(b). We choose typical OAM channels $l=-18, -6, 6, 18$ under different angles between the incident polarization and x-polarization of 0, 90, 60 and 120 degree, respectively. The observed optical signal-to-noise ratio (OSNR) penalties are less than 1 dB both in 4 and 10 OAM modes polarization-insensitive multicasting at a BER of 2e-3 (enhanced forward-error correction (EFEC) threshold). The measured eye diagram of B-to-B 4-Gbit/s PAM-4 signal is shown in Fig. 9(c). The measured eye diagrams of demodulated OAM modes ($l=3, 12$) for 4 OAM modes multicasting with angles between the incident polarization and x-polarization of 120 and 60 degree are shown in Fig. 9(d) and 9(e), respectively. The measured eye diagrams of demodulated OAM modes ($l=-6, 18$) for 10 OAM modes multicasting with angles between the incident polarization and x-polarization of 60 and 0 degree are shown in Fig. 9(f) and 9(g), respectively. Clear eye-opening with negligible performance degradation is observed after the polarization-insensitive N-fold OAM modes multicasting.

 

Fig. 9 Measured BER curves of (a) 4 OAM modes and (b) 10 OAM modes polarization-insensitive multicasting of PAM-4 signal. Measured eye diagrams of (c) B-to-B PAM-4 signal, demodulated OAM modes ($l=3, 12$) for 4 OAM modes multicasting with angles between the incident polarization and x-polarization of (d)120 and (e) 60 degree, demodulated OAM modes ($l=-6, 18$) for 10 OAM modes multicasting with angles between the incident polarization and x-polarization of (f)120 and (g) 60 degree.

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5. Polarization-insensitive OAM modes multiplexing

Figure 10 shows the experimental setup for polarization-insensitive OAM modes multiplexing. An optical 4-Gbit/s PAM-4 signal at 1550nm is prepared at the transmitter and then divided into two paths relatively delayed with fiber. The two SLMs in the proposed polarization-insensitive configurations are loaded with a complex phase pattern to generate collinearly superimposed OAM modes. Since odd times of reflections flip the charge sign of OAM, we get 8 OAM modes ( ± 3, ± 6, ± 9, ± 12) with negative OAM charge numbers from the SLM1 path while the positive OAM charge numbers from the SLM2 path. The two paths of polarization-insensitive OAM modes are multiplexed by the BS before the lens. The distances between the lens and the three SLMs (SLM1-SLM4) are 2f to form an imaging system, in which f is the focal length of the lens. The third SLM (SLM3) in a polarization-insensitive configuration loaded with a changeable complex phase pattern is employed for polarization-insensitive OAM mode demultiplexing followed by a receiver. The insets in Fig. 10 depict measured typical intensity profiles of polarization-insensitive two paths of 4 OAM modes, 8 OAM modes multiplexing, and demodulated OAM mode.

 

Fig. 10 Experimental setup for polarization-insensitive OAM modes multiplexing. Insets show intensity profiles of polarization-insensitive two paths of 4 OAM modes, 8 OAM modes multiplexing, and demodulated OAM mode.

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We first study the polarization-insensitive multiplexing and demultiplexing of OAM modes. Figure 11(a)-11(c) depict measured intensity profiles of superimposed OAM modes (−3, −6, −9, −12) generated from the SLM1 path under different angles between the incident polarization and x-polarization of 0, 30 and 60 degree, respectively. The measured intensity profiles of superimposed OAM modes (3, 6, 9, 12) generated from the SLM2 path are displayed in Fig. 11(d)-11(f) under different angles between the incident polarization and x-polarization of 90, 120, and 150 degree, respectively. Figure 11(g)-11(i) show the multiplexing of superimposed 8 OAM modes from two paths under different angles between the incident polarization and x-polarization of 180, 45, and 135 degree, respectively. The demultiplexing of linearly polarized OAM modes (−6, −12, + 6) with varying polarization states (50, 110, 70 degree) are displayed in Fig. 11(j)-11(l). The red dotted rings mark the demodulation of OAM modes with bright spots at the beam center.

 

Fig. 11 (a)-(c) Measured intensity profiles of superimposed OAM modes (−3, −6, −9, −12) generated from the SLM1 path under different angles between the incident polarization and x-polarization of 0, 30 and 60 degree, respectively. (d)-(f) Measured intensity profiles of superimposed OAM modes (3, 6, 9, 12) generated from the SLM2 path are displayed in Fig. 11(d)-(f) under different angles between the incident polarization and x-polarization of 90, 120, and 150 degree, respectively. (g)-(i) Measured intensity profiles for multiplexing of superimposed 8 OAM modes under different angles between the incident polarization and x-polarization of 180, 45, and 135 degree, respectively. (j)-(l) Measured intensity profiles for demultiplexing of OAM modes (−6, −12, + 6) with varying polarization states (50, 110, 70 degree).

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We then evaluate the quality of 8 OAM modes multiplexing by measuring the power distribution over all multiplexed OAM channels. Figure 12(a) and 12(b) depict the OAM spectrum of all multiplexed OAM channels under different angles between the incident polarization and x-polarization of 30 and 90 degree, respectively. As shown in Fig. 12(a) and 12(b), the observed extinction ratio for all 8 multiplexed OAM channels is larger than 14.0 dB. The extinction ratio is related to the mode crosstalk. The sources of the crosstalk are mainly dependent on the employed (de)multiplexing techniques. With further improvement, the crosstalk could be reduced by increasing the OAM channel spacing or using other efficient OAM (de)multiplexing techniques such as cylindrical lens mode converters and OAM mode sorters.

 

Fig. 12 Measured power distribution of 8 OAM modes multiplexing under different angles between the incident polarization and x-polarization of (a) 30 and (b) 90 degree, respectively. (c) Measured BER performance of PAM-4 signal for polarization-insensitive 8 OAM modes multiplexing.

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We further measure the BER performance of PAM-4 signal for polarization-insensitive OAM modes multiplexing. The BER curves of 8 OAM modes multiplexing are plotted in Fig. 12(c). The angles between the incident polarization and x-polarization of OAM modes $l=-12, -9 ,-6, -3, 3, 6, 9, 12$ are 0, 30, 90, 60, 120, 135, 150, 180 degree, respectively. The observed OSNR penalties are less than 1 dB for polarization-insensitive OAM modes multiplexing at a BER of 2e-3 (EFEC threshold).

6. Conclusion

In summary, by exploiting a simple polarization-insensitive configuration incorporating single polarization-sensitive phase-only liquid crystal SLM in a loop structure, we report polarization-insensitive free-space optical communications employing OAM beams. We demonstrate multiple polarization-insensitive optical communication subsystems in the experiment, including polarization-insensitive single OAM mode transmission, polarization-insensitive 4 and 10 OAM modes multicasting, and polarization-insensitive 8 OAM modes multiplexing. Free-space polarization-insensitive optical communication links based on 4-Gbit/s PAM-4 carrying OAM modes are experimentally demonstrated. The observed OSNR penalties are less than 1 dB in both polarization-insensitive N-fold OAM modes multicasting and polarization-insensitive multiple OAM modes multiplexing at a BER of 2e-3 (EFEC threshold). The obtained results indicate successful implementation of polarization-insensitive optical communications using OAM modes with favorable operation performance.