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Abstract
Radio-over-fiber (ROF) technology, loading microwave signal on light beams, has attracted considerable attention in wireless access network for its superiority in processing high-frequency microwave signals. Multiplexing for achieving high-capacity density, however, remains elusive in ROF communication because the optical microwave occupies large bandwidth. Here, we introduce a cylindrical vector beam (CVB) multiplexing for ROF communication with dielectric Pancharatnam-Berry phase-based metasurfaces (PBMs). CVBs, a structured light beam possessing spatially nonuniform polarization distribution and carrying vector mode, provide an additional multiplexing dimension for optical communication with the advantages of weak scintillation in free-space and low mode injure in few-mode-fiber. Exploiting the spin-orbit interaction of the PB phase, we construct PBMs to manipulate CVBs, which show broadband working wavelengths ranging from C- to L-band. After 3 m free-space propagation, two multiplexed CVBs carrying 100 GHz microwave are successfully demultiplexed, and the 100 GHz ROF communication with 12 Gbit/s QPSK-OFDM signals is realized. The crosstalk of the multiplexed CVBs is less than −15.13 dB, and the bit-error-rates (BERs) are below 3.26 × 10−5. With 5 km few-mode-fiber transmission, the CVBs are also demultiplexed with the BERs of 6.51 × 10−5. These results indicate that CVB is not only capable of free-space transmission but also available for few-mode-fiber transmission, which might pave new avenues for the multiplexing of ROF communications.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Radio-over-fiber (ROF) communication, transmitting and processing microwave signals in the optical domain, has attracted considerable attention in ultra-wideband wireless access networks [1–7]. To expand the system capacity of ROF communication, various multiplexing technologies, including wavelength-division-multiplexing [7–9] and polarization-division-multiplexing [10,11], have been demonstrated by simultaneously transmitting multiple signal channels. However, wavelength-division-multiplexing is restricted in ROF communication because the optical microwave occupies large bandwidth, where the wavelength channels are mainly determined by the system bandwidth and channel spacing. Hence, only several to a dozen of wavelength channels have been multiplexed in ROF communications [7–9]. Polarization-division-multiplexing has also been widely used to increase communication capacity, but only two orthogonal polarization states are available. One of the most enticing promises that delimit the further development of expanding the capacity of ROF communication might come from developing new multiplexing dimensions.
Recently, structured light beam (SLB), possessing spatially nonuniform phase or polarization distribution, is emerging as a powerful technique to boost the communication capacity density for their mode orthogonality [12–27]. Compared with wavelength-division-multiplexing, SLB multiplexing [13–18] does not consume frequency resource, and its mode dimensions are independent of wavelength, indicating that it is compatible with wavelength-division-multiplexing. Various efforts have been devoted to SLB multiplexing, and the transmission rate has been increased to Tbit/s by multiplexing vortex beams [15,16], which can be further enhanced via adding orbital angular momentum mode channels. However, the spiral phase wave-front of vortex beams is vulnerable to the perturbation from atmosphere turbulence [28–30] or multi-mode fiber [15,18], resulting in severe mode-crosstalk. Recently, cylindrical vector beams (CVBs) [31–34], a vectoral SLB with spatially nonuniform polarization distribution, has aroused research interests for possessing mutually orthogonal vector modes. Researchers found that CVB shows robust transmission stability in atmospheric turbulence than vortex beams because the phase distributions are more susceptible to the refractive index fluctuation [20]. Besides, CVB can exist as the eigenmodes of few-mode-fiber (FMF) [26], indicating it is available for long-haul transmission with low mode injury. These make CVBs a suitable carrier for multiplexing ROF signals. However, the lacking of broadband mode convertors with high transmission efficiency (the ratio between the transmitted- and incident power) and miniaturized size always hinder the practical application of CVB multiplexing in ROF communication. Various SLB devices [35–37] were reported for the generation of vortex beam, but few can manipulate the spatially nonuniform polarization of CVB. By using spatial-light-modulators, two vortex beams with opposite rotation direction and the same topological charge are produced [35] and coherently superposed to obtain CVBs, which has complex system and low mode purity. Due to the spin-orbit interaction, liquid crystal q-plates [36] can efficiently generate CVBs, but the large footprint and low damage threshold are not conducive to the miniaturization of the system. By changing the relative displacement between the double-layer gold nanounits, plasmonic metasurface [37] is capable of CVB generation with arbitrary linear polarization distribution. However, the large ohmic loss of Au nanostructures greatly reduces the diffraction efficiency. Besides, the narrow bandwidth of these devices is also a big obstacle for the compatibility with wavelength division multiplexing.
In this work, we introduce dielectric Pancharatnam-Berry phase-based metasurfaces (PBMs) to generate CVBs and investigate their application in ROF multiplexing communication. By exploiting the spin-orbit interaction of PBMs, the CVBs with vector modes of −2, +2, and +4 are produced. By virtue of the PB phase's low-dispersion, these PBMs show broadband working wavelengths ranging from C- to L-band. The measured transmission efficiency reaches 96% at the wavelength of 1550 nm. After 3 m free-space propagation, the multiplexed CVBs carrying 100 GHz microwave signals are demultiplexed with the crosstalk less than −15.13 dB, and the 100 GHz ROF communication with 12 Gbit/s QPSK-OFDM signals is realized with a bit-error-rate (BER) of $\textrm{3}\textrm{.26} \times {10^{ - 5}}$. After 5 km FMF transmission, the multiplexed CVBs are also demultiplexed with the BERs of $\textrm{6}\textrm{.51} \times {10^{ - 5}}$. These results indicate that CVB multiplexing with PBMs is successfully achieved in free-space and FMF channels, which might provide a new avenue for large-capacity ROF communication.
2. Principles and methods
2.1 Fabrication of the PBM
The PBM is fabricated by writing form-birefringent nanogratings on the fused silica glass with a femtosecond laser (it works at the wavelength of 1030 nm, and the repetition rate is about 500 kHz) [38]. Its diameter and thickness are 25 mm and 5 mm, and the diameter of the work area is 6 mm, as shown in Fig. 1. The line widths of the nanogratings are about 30 nm to 90 nm. The strip-like nanostructures are formed by the interaction of plasma and incident light [39]. The plasma of high free-electron density is produced by multiphoton ionization process under the irradiation of intense laser.
The effective refractive indices of nanograting for the ordinary- and extraordinary- light can be written as [38]
where, ${f_\textrm{m}}$ represents the filling factor, which is about 0.15. ${n_1}$ and ${n_2}$ are the refractive index of platelets constituting the grating. By writing a nanograting on the substrate, the birefringence is changed, and the local orientation of the slow axis can be controlled to be parallel to the nanograting.2.2 Generating CVB with PBMs
With the spin-orbit interaction of the PB phase [38], PBMs can manipulate the polarization distribution of light beams and produce CVBs. Its optical axis direction can be described as $g(r,\varphi ) = q\varphi + {\varphi _0}$, where $(r,\varphi )$ represents the polar coordinate, and q is a constant corresponding to the spatial rotation ratio of optical axis. $\varphi$ and ${\varphi _0}$ are the azimuthal angle and initial direction of the optical axis, respectively. Set ${\varphi _0} = 0$ and the homogeneous birefringent phase retardation of PBM as $\pi$, the Jones matrix can be expressed as [40]
When a linearly polarized Gaussian beam written as formula (4) propagates through the PBM, the CVB with vector mode of $2q$ (CVB_2q) can be generated and derived as formula (5). Here, ${E_0}$ is the simplified amplitude.
When a CVB_2q propagates through the PBM with a factor of q, it will be converted into Gaussian beam and can be expressed as
To effectively produce and demultiplex CVBs, we measured the transmission efficiencies of the PBMs (q=1 and 2) with the wavelength ranging from 1528 nm to 1612 nm. As shown in Fig. 3(a), the efficiencies remain above 85% and even reach 96% at the wavelength of 1550 nm, where the power loss is mainly induced by the material absorption and surface reflection of PBMs. Figure 3(b) shows the mode purities of CVB_+1, CVB_-1, CVB_+2, and CVB_+4, which are produced by the PBMs with q=1 and 2, and they are 88.58%, 88.48%, 85.44%, and 79.85% at the wavelength of 1550 nm (the energy ratio of the demultiplexed CVBs to the input Gaussian beam).
Fig. 3. (a) Transmission efficiencies of the PBMs with q=1 and 2 at the wavelengths of 1528 nm to 1612 nm. (b) Mode purities of CVB_+1, CVB_-1, CVB_+2, and CVB_+4 at the wavelength of 1550 nm.
2.3 Vector mode analysis in FMF
Optical fiber is generally employed as long-haul transmission media. Single-mode fiber can transmit fundamental modes ($HE_{\textrm{11}}^{\textrm{x/y}}$) but is not suitable for high-order vector modes. According to mode theory, vector modes are the eigenmodes of FMFs, and LP mode is a superposition of vector modes, which satisfy the weak derivative approximation [41]. The relationship between vector modes and LP modes can be written as
Fig. 5. Vector modes supported in the four-mode step-index FMF (YOFC-A7R15044CA9) at the wavelength of 1550 nm. (a1) and (a2) 1- and 2-order fundamental modes (FM) ($HE_{\textrm{11}}^{\textrm{x/y}},HE_{\textrm{12}}^{\textrm{x/y}}$). (b1) and (b2) CVB_+1 and CVB_-1 ($T{M_{01}},T{E_{01}},HE_{\textrm{21}}^{\textrm{even/odd}}$). (c1) and (c2) CVB_+2 and CVB_-2 ($EH_{\textrm{11}}^{\textrm{even/odd}},HE_{\textrm{31}}^{\textrm{even/odd}}$).
3. Experimental results and analysis
Figure 6 illustrates the experimental setup of ROF communication with CVB multiplexing. The QPSK-OFDM signals (the number of subcarriers is 256, and the length of cyclic-prefix is 32) with the transmission rate of 6 Gbit/s are generated by an arbitrary waveform generator (AWG 7122C) and intensity-modulated to signal light (1550.92 nm), which is selected based on the transmission characteristic of PBMs. At the launching end of the free space channel, the signal light and reference light (1550.12 nm) are coupled and further divided into two paths by an optical coupler (OC, 50%:50%). Here, two LDs are used to produce the signal- and reference light beams, which are employed for optical heterodyne beat-frequency [7]. After amplified by two erbium-doped fiber amplifiers (Amonics AEDFA-23-B-FA), the optical signals are de-correlated via a 10 m delay fiber. Then, CVB (de)multiplexing is performed. After demultiplexed, the light beams are amplified to match the transmitting power of millimeter-wave. A tunable optical filter (AGILTRON FTOF02512133) with a line-width of 200 GHz is used to reduce system noise, and a polarization controller is employed to adjust the polarization state of light beams to improve beat-frequency efficiency. A tunable optical attenuator located before the photo-detector (FINISAR XPDV4121R) is adopted to control the input optical power. The 100 GHz millimeter-wave is generated by heterodyne beat-frequency and radiated to free-space by the Tx_WBA antenna (SAGE SAR-2507-10-S2).
Fig. 6. Experimental schematic diagram of ROF communication with CVB multiplexing. LD: laser diode; PC: polarization controller; De-Mux/Mux: demultiplexing/multiplexing; IM: intensity modulator; AWG: arbitrary waveform generator; OC: optical coupler; Col: collimator; EDFA: erbium-doped fiber amplifier; TOF: tunable optical filter; PD: photo-detector; TA: tunable attenuator; Tx_WBA/Rx_WBA: W-band transmitting/receiving antenna; ED: envelope detector; BPF: bandpass filter; EA: electrical amplifier; DSO: digital sampling oscilloscope.
The spectrum of the 100 GHz optical microwave signal is captured by a spectrum analyzer (YOKOGAWA AQ6370D). As shown in Fig. 7, the signal light (1550.92 nm) has a wavelength difference of 0.8 nm with the reference light (1550.12 nm), which corresponds to a frequency difference of 100 GHz. At the wireless receiving end, the millimeter-wave carrying OFDM signals is received by the Rx_WBA and amplified by a low-noise amplifier before envelope detection. Finally, the received baseband signals are sampled by a digital sampling oscilloscope (Tektronix 72004C) for offline signal processing.
3.1 CVB multiplexing in the free-space channel
Figure 8 illustrates the schematic diagram of CVB multiplexing link that transmits 100 GHz microwave signals. Two Gaussian beams are converted into linearly polarized light beams by Glan lens. CVB_+2 and CVB_+4 are produced by the PBM_1 (q=1) and PBM_2 (q=2), respectively. Two beam-splitters are employed for CVB coupling and separation. After 3 m free-space transmission, CVB_+2 and CVB_+4 are demultiplexed by the PBMs with q=1 and 2.
Fig. 8. Schematic diagram of CVB multiplexing in 3 m free-space channel. Col: collimator; GL: Glan lens; PBM_1 and PBM_2: PBM with q=1 and 2; BS: beam-splitter; Mir: mirror; LPB: linear-polarized beam; GB: Gaussian beam.
The intensity maps of CVB (de)multiplexing and mode detection are measured by CCD, as shown in Fig. 9. Figures 9(a)–(b) present the Stokes polarization distributions of CVB_+2 and CVB_+4. By rotating the polarization angle of Glan lens to 0°, 45°, 90°, and 135° with x-axis, the intensity distributions emerging as 4-lobes (CVB_+2) or 8-lobes (CVB_+4) in the cross-section, as shown in Figs. 9(a1)–(a4) and 9(b1)–(b4). Figures 9(c1)–(c3) describe the intensity distributions of CVB_+2 and CVB_+4 at the transmission distance of 0.1 m, 1 m, and 3 m in free-space. The two intensity rings are gradually overlapped with the increase of transmission distance. After demultiplexed by PBMs with q=1 or 2, the Gaussian-like beam profiles are presented at the center of light beam. CVB_+2 and CVB_+4 corresponds to Fig. 9(d1) and 9(d2), respectively. Figures 9(e1) and 9(e2) show the beam line profiles of the demultiplexed CVB_+2 and CVB_+4 in the longitudinal direction (along the green lines in Fig. 9(d1) and 9(d2)). From the normalized distribution of energy values, the demultiplexed beams present Gaussian-like distribution. These indicate that CVB_+2 and CVB_+4 are successfully (de)multiplexed in 3 m free-space channel.
Fig. 9. (a) and (b) Stokes polarization distributions of CVB_+2 and CVB_+4. (a1)–(a4) and (b1-b4) Polarizaer detection results of CVB_+2 and CVB_+4 with Glan lens rotating 0°, 45°, 90°, 135°. (c1)–(c3) Intensity distributions of CVB_+2 and CVB_+4 with a transmission distance of 0.1 m, 1 m, and 3 m. (d1) and (d2) Intensity distributions of the demultiplexed CVB_+2 and CVB_+4. (e1) and (e2) Beam line profiles in the longitudinal direction corresponding to (d1) and (d2), respectively.
After 1 m or 3 m free-space transmission, the signal (noise) power of the multiplexed CVB_+2 and CVB_+4 are −8.15 dBm (−24.12 dBm), −9.41 dBm (−24.54 dBm), or −9.56 dBm (−25.69 dBm), and −10.32 dBm (−25.87 dBm), as shown in Fig. 10. The corresponding mode crosstalk are calculated to be −15.97 dB, −15.13 dB, −16.13 dB, and −15.55 dB. The power loss is ∼1.4 dB with the transmission distance increasing by 2 m, mainly caused by beam angle divergence. The crosstalk of CVB multiplexing link remains ∼15.13 dB for 3 m free-space transmission, which indicates that the CVB multiplexing based free-space transmission possesses low mode crosstalk.
Fig. 10. Crosstalk of CVBs multiplexing based ROF communication after 1 m or 3 m free-space transmission. FSC: free-space channel.
Figure 11 illustrates the constellations of 6 Gbit/s QPSK-OFDM signals carried by Gaussian beams (B2B), CVB_+2, and CVB_+4 after 1 m or 3 m free-space channel for the optical power of 1.5 dBm. The error-vector-magnitudes (EVMs) for the CVB_+2 and CVB_+4 channels under the cases of B2B, 1 m, and 3 m free-space transmission are 18.58%, 18.68%, 19.78%, 20.01%, 21.12%, and 22.43%. The EVMs ascend with the increase of transmission distance, but the constellations remain convergences, indicating that the free-space CVB multiplexing based ROF communication is reliable.
Fig. 11. Constellations of the 6 Gbit/s QPSK-OFDM signals corresponding to B2B, CVB_+2, and CVB_+4 channels at the optical power of 1.5 dBm in free-space CVBs multiplexing based ROF communication.
The BER performance of the free-space CVB multiplexing based ROF communication are described in Fig. 12. The communication sensitivity decreases about 0.5 dB with the transmission distance increasing by 2 m. However, when the multiplexed CVBs propagate 3 m in free-space, the BERs still reach $\textrm{3}\textrm{.26} \times {10^{ - 5}}$ because of the low crosstalk among CVB channels.
Fig. 12. Measured BER performance versus received optical power of CVB_+2 and CVB_+4 in CVBs multiplexing based ROF communication. FSC: free-space channel.
3.2 CVB multiplexing in the FMF channel
To study the multiplexed CVBs propagating through a 5 km FMF channel, we further construct a CVB multiplexing based ROF communication link, as shown in Fig. 13. Here, the CVB_+2 and CVB_-2 carrying 100 GHz microwave signals are (de)multiplexed.
Fig. 13. Experimental schematic diagram of the multiplexed CVBs propagating through 5 km FMF. Col: collimator; GL: Glan lens; PBM_1: PBM with q=1; BS: beam-splitter; Mir: mirror; LPB: linearly polarized beam; GB: Gaussian beam PC: polarization controller; Mir: mirror.
Figure 14 illustrates the intensity distributions of the multiplexed CVBs before and after 5 km FMF. From Figs. 14(a1)–(a4)/14(b1)–(b4)/14(c1)–(c4)/14(d1)–(d4), the intensity distributions of CVB_+2 and CVB_-2 show 4 lobes, which vary with the rotation angles between Glan lens and x-axis, indicating that the CVBs are successfully transmitted through the 5 km FMF. Figures 14(a) and 14(b) present the intensity distributions of the CVBs before 5 km FMF, which have a good circular symmetry. However, under the influence of channel noise, the circular symmetries are deteriorated because of the crosstalk among adjacent vector modes, as shown in Figs. 14(c) and 14(d). The intensity distributions of the demultiplexed CVBs before and after 5 km FMF transmission are shown in Figs. 14(a5)–(b5) and 14(c5)–(d5). From these figures, the Gaussian spots grow smaller after the 5 km FMF, which mainly attributes to the insertion loss of PCs, and the transmission loss and channel noise of FMF.
Fig. 14. Intensity distributions of the CVB_+2 and CVB_-2 (a)–(b) before 5 km FMF, (c)–(d) after 5 km FMF. Polarizer detection results of the CVB_+2 and CVB_-2 (a1)–(a4) and (b1)–(b4) before 5 km FMF, (c1)–(c4) and (d1)–(d4) after 5 km FMF. Intensity profiles of the demultiplexed CVB_+2 and CVB_-2 (a5)–(b5) before 5 km FMF, and (c5)–(d5) after 5 km FMF.
The signal- and noise optical powers of the CVBs before and after 5 km FMF transmission are described in Fig. 15. The signal powers of CVB_+2 and CVB_-2 before and after 5 km FMF transmission are −8.21 dBm, −9.05 dBm, −18.43 dBm, and −19.54 dBm, respectively. The corresponding noise powers are −23.09 dBm, −24.37 dBm, −29.64 dBm, and −30.7 dBm. Hence, the mode crosstalk of CVB_+2 and CVB_−2 before or after 5 km FMF are −14.87 dB, −15.32 dB, −11.21 dB, and −11.16 dB, indicating that the signal crosstalk among CVB channels increases by about 4 dB after 5 km FMF transmission, which is mainly caused by channel noise.
Fig. 15. Signal- and noise powers of CVB_+2 and CVB_-2 before/after 5 km FMF transmission. Bef./Aft.: before/after. FMF: few-mode-fiber.
The QPSK-OFDM constellations before and after 5 km FMF transmission at the optical power of 1.5 dBm are described in Fig. 16, and their EVMs are 19.95%, 20.15%, 21.80%, and 22.66%, respectively. After 5 km FMF transmission, the EVM increase ∼2% with the crosstalk increasing about 4 dB, but the BER remains 10−5, indicating that the CVB multiplexing based ROF communication system has good performance after 5 km FMF transmission.
Fig. 16. Constellations of the QPSK-OFDM signals carried by CVB_+2 and CVB_-2 before and after 5 km FMF transmission.
The BERs of the multiplexed CVBs before and after 5 km FMF transmission are illustrated in Fig. 17. The 12 Gbit/s QPSK-OFDM signals transmitted by CVB multiplexing ROF communication are demodulated with the BERs of $6.51 \times {10^{ - 5}}$ at the optical power of 2.5 dBm, and the communication sensitivity decreases about 1.5 dB due to the channel noise.
Fig. 17. Measured BER performance versus received optical power of CVB_+2 and CVB_-2 before and after 5 km FMF transmission. Bef./Aft.: before/after. FMF: few-mode-fiber.
To analyze the signal damage introduced by transmission channels, we have measured the power-spectrum-densities of QPSK-OFDM signals after 1 m, 3 m free-space, and 5 km FMF transmission, which are illustrated in Fig. 18. The positive frequencies of the power-spectrum-densities are displayed in Figs. 18(a)–(d), which are -83 dB/Hz, −120 dB/Hz, −125 dB/Hz, and −120 dB/Hz for the transmitted signals, received signals after 1 m, 3 m free-space transmission, and 5 km FMF transmission, respectively. The power-spectrum-density loss caused by 5 km FMF is about 5 dB/Hz, which is lower than 3 m free-space transmission. These indicate that the FMF channel introduces a lowish electrical signal consumption for CVB multiplexing communication.
Fig. 18. Power-spectrum-densities of the QPSK-OFDM signals carried by CVB_+2 and CVB_-2 after 1 m, 3 m free-space and 5 km FMF transmission.
4. Discussion
Radio-over-fiber technique is proposed to produce and remotely transmit microwave signals with fiber in the optical domain. To increase system capacity and spectral efficiency, conventional ROF communication typically focuses on wavelength-division-multiplexingand polarization-division-multiplexing. However, optical microwave occupies large bandwidth, which makes wavelength-division-multiplexing severely restricted, and only two orthogonal polarization states are available for polarization-division-multiplexing. We proposed CVB multiplexing for 100 GHz ROF communication. Because the vector mode is independent of wavelength, CVB multiplexing is compatible with wavelength-division-multiplexing, which can significantly improve the communication capacity density. Besides, the vector modes carried by CVBs are eigenmodes in FMF, which indicate that the multiplexed CVBs can achieve long-haul FMF transmission. In this work, we have employed the spin-orbit interaction characteristics of PBM to generate and restore CVBs. Profiting from the transmittance-type design and low ohmic loss of fused silica glass, the transmission efficiencies of the PBMs reach 96% at the wavelength of 1550 nm, and the mode purity of the generated CVB (e.g. q=1) is over 88%. The dispersion-free property of the PB phase rewards PBM with a natural broadband working wavelength ranging from C- to L-land. As a proof-of-concept, we have experimentally demonstrated a CVB multiplexing based 100 GHz ROF communication with two CVBs. The 3 m free-space and 5 km FMF channels are both verified, respectively. The experimental results show that the CVB multiplexing based ROF links have effectively transmitted 12 Gbit/s QPSK-OFDM signals. The communication capacity of ROF system can be exponentially improved by adding the vector modes of CVBs. Furthermore, the order of CVBs can be flexibly adjusted by designing the q-value of PBM, which indicates that the PBM can be further used as a passive optical processing device to realize all-optical information processing in ROF multiplexing communication, such as mode up-conversion/down-conversion, mode routing, and mode adding/dropping. By integrating coupler, gratings, lens, etc. into the PBM, multifunctional optical devices can also be constructed based on the dielectric PBM. Hence, the investigation of CVB multiplexing using PBM provides a feasible solution for the application of miniaturized photonic integrated devices with more functions in ROF system.
5. Conclusion
In summary, we have proposed and experimentally demonstrated a CVB multiplexing based ROF communication with PBMs. By writing form-birefringent nanogratings on the fused silica glass with a femtosecond laser, the PBMs with q=1 and 2 are fabricated to produce the CVBs with the vector modes of +2, −2, and +4, and the transmission efficiencies are over 85%, which even reach 96% at the wavelength of 1550 nm. By virtue of the low-dispersion of the PB phase, these PBMs show broadband working wavelengths ranging from C to L-band. After 3 m free-space propagation, two multiplexed CVBs carrying 100 GHz microwave are successfully demultiplexed, and the 100 GHz ROF communication with 12 Gbit/s QPSK-OFDM signals is realized. The mode crosstalk of the multiplexed CVBs are less than −15.13 dB, and the demodulated BERs are below $\textrm{3}\textrm{.26} \times {10^{ - 5}}$. With 5 km few-mode-fiber transmission, the CVBs are also demultiplexed with the BERs are below $\textrm{6}\textrm{.51} \times {10^{ - 5}}$. These indicate that CVB multiplexing with PBMs may have a good prospect in large-capacity ROF communications.
Funding
National Natural Science Foundation of China (61805149, 61805087); Natural Science Foundation of Guangdong Province (2020A1515011392, 2019A1515111153, 2018A030313368); Program of Fundamental Research of Shenzhen Science and Technology Plan (JCYJ20200109144001800, JCYJ20180507182035270, GJHZ20180928160407303); Science and Technology Planning Project of Guangdong Province (2016B050501005); Science and Technology Project of Shenzhen (ZDSYS201707271014468); International Collaborative Laboratory of 2D Materials for Optoelectronics Science and Technology (2DMOST2018003).
Disclosures
The authors declare no conflicts of interest.
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