General structured light generation based on programmable linearly-polarized mode synthesizer

doi:10.21203/rs.3.rs-4387352/v1

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General structured light generation based on programmable linearly-polarized mode synthesizer

https://doi.org/10.21203/rs.3.rs-4387352/v1

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Fiber-based structured light including cylindrical vector beams (CVBs) and orbital angular momentum (OAM) has gained significant interest in various areas of study for the unique properties of spatial intensity, phase, and polarization distributions. Different approaches have been proposed for the generation of these spatially structured beams such as beam reshaping or directly lasing from specific cavities. In this work, we propose the concept of programmable linearly-polarized (LP) -mode Synthesizer for general structured light generation, in which an LP-mode Pool supporting independent and selectable LP-mode output is first established, and then different CVB/OAM modes could be generated in general way through polarization and phase control according to their conversion relation with LP modes. We demonstrate a proof-of-concept LP-mode Synthesizer based on a fiber ring laser characterized by partial 5-LP mode weakly-coupled few-mode fiber (FMF) cavity and arbitrary LP-mode switch array. Various CVB/OAM beams including TE₀₁, TM₀₁, OAM₊₁, OAM_-1, OAM₊₂, and OAM_-2 modes are successfully generated. This approach provides new insights into mode manipulation methods and could enhance the performance of various structured light applications such as optical and quantum communications, optical fiber sensing, microscopy, and optical trapping.

Physical sciences/Optics and photonics/Lasers, LEDs and light sources/Fibre lasers

Physical sciences/Optics and photonics/Applied optics/Fibre optics and optical communications

Maneuvering different physical dimensions of photons including frequency, time, amplitude, phase, polarization, and spatial structure enables manifold light-related applications. In particular, fiber-based structured light beams such as CVBs and OAM modes have aroused increasing interest owing to their unique properties of spatial intensity, phase, and polarization distributions, which have promoted vigorous researches in optical and quantum communication systems [1,2], optical sensing [3,4], microscopy [5,6], optical trapping [7,8], and so on.

Various approaches have been proposed to generate these structured light in recent years. Discrete components such as spatial light modulators [9,10,11], axial birefringent components [12], specially designed laser cavities [13,14,15], spiral phase plates[16,17], and plasmonic metasurfaces [18,19,20,21] have been utilized to reshape free-space Gaussian beams into CVBs/OAMs or to directly emit CVB/OAM beam. But the bulk volume and complex optical alignment may hinder their practical applications. Meanwhile, approaches utilizing optical fiber-based components, lasing cavities, or optical circuits have also been widely investigated, such as long-period fiber gratings [22,23,24], vortex grating [25], mode-selective couplers (MSCs) [26,27], and tapered fiber [28]. However, the lacking of general method and extensibility to generate CVBs and OAMs may be their shared limitation.

Recent studies on the utilization of LP modes in weakly-coupled FMF transmission systems may provide a new way for the generation of fiber-based structured light. In these systems [29,30,31], the modal crosstalk for FMFs and matched mode multiplexers/demultiplexers (MMUXes/MDEMUXes) is significantly suppressed, so that light could independently propagate through multiple LP modes and the transmission capacity could be multiplied without inter-modal multiple-input multiple-output digital signal processing (MIMO-DSP). Because the effective index difference (|Δn_eff|) is the dominant factor of distributed modal crosstalk between LP modes, step-index FMF with high core/cladding index difference and multiple-ring-core (MRC) FMF utilizing ring index perturbations to enlarge the minimum |Δn_eff| among LP modes have been proposed for distributed modal crosstalk suppression [29,30]. Different kinds of MMUXes and MDEMUXes with low intrinsic channel crosstalk and coupling crosstalk have been adopted including photonic lanterns [32,33], multiple plane light converters [34,35], and cascaded MSCs [36,37]. Furtherly, the manipulation capability could be extended to generate fiber-based structured light by applying proper mode conversion to these independent LP modes.

In this paper, we propose the programmable LP-mode Synthesizer as a general approach to generate fiber-based structured light. An LP-mode Pool is first established to generate independent and selectable LP modes. Then, different CVB/OAM modes could be generated in accordance with the conversion relation with LP modes. We experimentally demonstrate the programmable LP-mode Synthesizer utilizing a 5-LP mode fiber ring laser with a partial weakly-coupled FMF cavity. The output LP mode could be controlled by an arbitrary LP-mode switch array, and a polarization controller (PC) is employed for polarization and phase control. We last experimentally demonstrate the successful generation of various CVB/OAM beams including TE₀₁, TM₀₁, OAM₊₁, OAM_-1, OAM₊₂, and OAM_-2.

Concept of programmable LP-mode Synthesizer

The conversion relations between LP modes and CVB/OAM modes are the theoretical basis for the proposed LP-mode Synthesizer. As we know, the CVB modes and OAM modes are two eigen-mode sets in optical fibers under the Cartesian coordinate system [38] and the circular polarization coordinate system [39], respectively. The polarization vector mode corresponds to the CVB mode whereas the phase vortex mode conveys the OAM mode. Mathematically, the LP modes are the solutions of scalar Helmholtz equations under the weakly-guiding approximation, and the superposition of near-degenerate CVB modes can form LP states. The relation of the LP modes and CVB modes could be described by [40],

$$\left[\begin{array}{c}{HE}_{m+1,n}^{e}\\ {HE}_{m+1,n}^{o}\\ {EH}_{m-1,n}^{e}\\ {EH}_{m-1,n}^{o}\end{array}\right]={F}_{m,n}\left(r\right)\left[\begin{array}{cccc}1& 0& 0& -1\\ 0& 1& 1& 0\\ 1& 0& 0& 1\\ 0& -1& 1& 0\end{array}\right] \left[\begin{array}{c}{\overrightarrow{e}}_{x}{cos}\left(m\phi \right)\\ {\overrightarrow{e}}_{y}{cos}\left(m\phi \right)\\ {\overrightarrow{e}}_{x}{sin}\left(m\phi \right)\\ {\overrightarrow{e}}_{y}{sin}\left(m\phi \right)\end{array}\right]=\left[\begin{array}{cccc}1& 0& 0& -1\\ 0& 1& 1& 0\\ 1& 0& 0& 1\\ 0& -1& 1& 0\end{array}\right]\left[\begin{array}{c}{\overrightarrow{e}}_{x}{LP}_{m,n}^{e}\\ {\overrightarrow{e}}_{y}{LP}_{m,n}^{e}\\ {\overrightarrow{e}}_{x}{LP}_{m,n}^{o}\\ {\overrightarrow{e}}_{y}{LP}_{m,n}^{o}\end{array}\right]$$

where “e” and “o” represent the even and odd polarization states, ${F}_{m,n}\left(r\right)$ is the radial field distribution of the corresponding scalar mode (LP) solution, m is the radial order of CVB modes, n is the azimuthal order of CVB modes, r is the radial coordinate, and $\phi$ is the angular coordinate. Note that ${EH}_{m-1,n}^{e}$ is substituted by${TM}_{0,n}$and ${EH}_{m-1,n}^{o}$ is substituted by ${TE}_{0,n}$ when m = 1.

Similarly, the OAM modes having a helical wave-front phase can be formed by the superposition of CVB modes [40]. So, the OAM modes in the fiber can be expressed as linear superposition of the odd and even CVB modes with ±$\pi /2$ phase difference. Generally, in the weakly-guiding fiber, the superposition of near-degenerate CVB modes forms LP states.

So, OAM modes can be expressed by,

$$\left[\begin{array}{c}{\overrightarrow{\sigma }}^{+}{OAM}_{+m}\\ {\overrightarrow{\sigma }}^{-}{OAM}_{-m}\\ {\overrightarrow{\sigma }}^{-}{OAM}_{+m}\\ {\overrightarrow{\sigma }}^{+}{OAM}_{-m}\end{array}\right]={F}_{m,n}\left(r\right)\left[\begin{array}{c}{\overrightarrow{\sigma }}^{+}{exp}\left(+jm\phi \right)\\ {\overrightarrow{\sigma }}^{-}{exp}\left(-jm\phi \right)\\ {\overrightarrow{\sigma }}^{-}{exp}\left(+jm\phi \right)\\ {\overrightarrow{\sigma }}^{+}{exp}\left(-jm\phi \right)\end{array}\right]=\left[\begin{array}{c}1 j j -1\\ 1-j -j -1\\ 1-j j 1\\ 1 j -j 1\end{array}\right]\left[\begin{array}{c}{\overrightarrow{e}}_{x}{LP}_{m,n}^{e}\\ {\overrightarrow{e}}_{y}{LP}_{m,n}^{e}\\ {\overrightarrow{e}}_{x}{LP}_{m,n}^{o}\\ {\overrightarrow{e}}_{y}{LP}_{m,n}^{o}\end{array}\right]$$

where ${OAM}_{\pm m}$ denote the field distributions of helical wave-front phase exp(± jmφ); the subscripts “±” of OAM separately correspond to the left-hand and right-hand helical wave-front phase. Especially, the combination of LP modes with a ±$\pi /2$ phase difference in the weak-guiding fiber can form linearly-polarized OAM modes [41],

$$\left[\begin{array}{c}{\overrightarrow{e}}_{x}{OAM}_{-m}\\ {\overrightarrow{e}}_{y}{OAM}_{-m}\\ {\overrightarrow{e}}_{x}{OAM}_{+m}\\ {\overrightarrow{e}}_{y}{OAM}_{+m}\end{array}\right]={F}_{m,n}\left(r\right)\left[\begin{array}{c}{\overrightarrow{e}}_{x}{exp}\left(-jm\phi \right)\\ {\overrightarrow{e}}_{y}{exp}\left(-jm\phi \right)\\ {\overrightarrow{e}}_{x}{exp}\left(+jm\phi \right)\\ {\overrightarrow{e}}_{y}{exp}\left(+jm\phi \right)\end{array}\right]=\left[\begin{array}{cccc}1& 0& -j& 0\\ 0& 1& 0& -j\\ 1& 0& j& 0\\ 0& 1& 0& j\end{array}\right]\left[\begin{array}{c}{\overrightarrow{e}}_{x}{LP}_{m,n}^{e}\\ {\overrightarrow{e}}_{y}{LP}_{m,n}^{e}\\ {\overrightarrow{e}}_{x}{LP}_{m,n}^{o}\\ {\overrightarrow{e}}_{y}{LP}_{m,n}^{o}\end{array}\right]$$

The schematic diagram of a programmable LP-mode Synthesizer for CVB/OAM generation is illustrated in Fig. 1, which interprets literally the mode conversion relations shown in Eqs. (1) to (3). First, the LP-mode Pool should be established to stock arbitrary LP modes and output them independently according to the need. The flexible selection of active LP mode in the LP-mode Pool is obtained by the LP-mode Selection Unit. The Polarization Component Extraction Unit is used to detach the elements of the selected LP mode. After that, the phase differences among the elements are adjusted by the Phase Control Unit. The Programmable Control Unit is responsible for the configuration of the three units according to the conversion relations.

The physical implementations of the programmable LP-mode Synthesizer may take flexible forms. The LP-mode Pool accompanied with the LP-mode Selection Unit should output switchable LP mode in a general way, which could be realized by various simple and effective approaches based on commercial optical components such as LP-mode converters or MMUXes. There are also different kinds of approaches for polarization component extraction and phase delay utilizing fiber-based or free-space micro-structured optical elements such as polarization beam splitters and optical delay lines. It should be noted that the optical circuit in the programmable LP-mode Synthesizer should be built up based on weakly-coupled FMFs and matched low-modal-crosstalk mode control components to sustain the independence among the LP modes. As a principle-of-concept example, we demonstrate the programmable LP-mode Synthesizer utilizing a compact fiber ring laser and a PC in the next section.

Experimental setup

Figure 2 shows the experimental configuration of a fiber ring laser to verify the feasibility of the programmable LP-mode Synthesizer, the cavity of which is composed of both single-mode fiber (SMF) and FMF sections. In the FMF section, the weakly-coupled MRC-FMF supports independent light propagation through 5 LP modes (LP₀₁, LP₁₁, LP₂₁, LP₀₂, LP₃₁) with very low modal crosstalk, whose structure is shown in Fig. 6a in the Methods. The matched MMUX/MDEMUX consists of 5 cascaded MSCs with the structure shown in Fig. 6e in the Methods. It should be noted that each MSC could only perform mode conversion between the fundamental mode in the SMF and one of a pair of degenerate LP modes in the FMF. An 80:20 few-mode optical coupler (FM-OC) is utilized as the output coupler of the laser, which is a free-space wave-plate beam splitter and achieves better mode insensitivity than fused-type FM-OC [41]. A conventional single-mode pump scheme is adopted in the SMF section, consisting of 980-nm laser diode (LD), 980/1550-nm wavelength multiplexer (WMUX) for pump/signal light combination, and a piece of single-mode Erbium-doped fiber (SM-EDF) with the length of 5-m as the gain medium. The optical isolator (ISO) enables unidirectional light propagation. The single-mode optical coupler-1 (SM-OC-1) is used to split the light into 5 branches. The higher-order modes are converted back to LP₀₁ mode by the MDEMUX and then combined to the SMF through another SM-OC-2 for the next round of signal circulation.

Compared with the schematic diagram of the programmable LP-mode Synthesizer shown in Fig. 1, we can see that the whole fiber ring laser could act as the LP-mode Pool with selectable output light from one of the 5 LP modes, while the SM-OC-1 and the Optical Switching Array could bear the function of LP-mode Selection Unit. The PC is realized by wrapping multiple turns of MRC-FMF around a 3-paddle adjusting device. For the operation to non-degenerate LP₀₁ and LP₀₂ modes, the PC could modify the polarization components and apply phase difference between them by twisting the 3 paddles to reallocate and select electric fields. While for the case of operation to degenerate LP₁₁, LP₂₁ or LP₃₁ modes, the PC could reallocate the optical power and apply phase difference among all the 4-fold eigenmodes even if only even or odd mode is excited by the MMUX consisting of spatial-orientation-selective MSCs. So, the PC could perform the functions of both polarization and phase control. The output of the 3-dB SM-OC-3 as a reference Gaussian beam is utilized to verify the generation of OAM lasing. The spectrum properties of the laser are measured by an optical spectrum analyzer (YOKOGAWA, AQ6370C) with a resolution of 0.02 nm. The output mode profiles are observed by a charge-coupled device (CCD) camera (Xenics, Bobcat-5316). The output power is measured by an optical power-meter (EXFO, FPM-302X-FOA-22).

Experimental results for LP generation

The independent lasing for each LP mode of the proposed fiber ring laser is first investigated. The Optical Switching Array is adjusted to excite each LP mode one by one, and the output power characteristics of output light at the 20% output port of the FM-OC are measured by the optical power-meter (EXFO, FPM-302X-FOA-22), while the intensity profile is measured by the CCD camera. The output signal power versus the pump power for five lasing modes is plotted in Fig. 3a. We can see that the lasing output power for different LP modes increases linearly with the pump power when working above the lasing threshold. The slope efficiencies of 1.17%, 0.82%, 0.56%, 0.35%, and 0.13%, and the pump power thresholds of 35, 42, 46, 48, and 53 mW are obtained for LP₀₁, LP₁₁, LP₂₁, LP₀₂, and LP₃₁ lasing modes, respectively. The different values of slope efficiencies, and pump power thresholds among the five LP lasing modes are mainly influenced by different insertion losses for the five LP modes in the partial weakly-coupled FMF cavity.

The spectral features and the stabilities of the lasing outputs are also investigated. Figure 3c shows the optical spectra of the lasing output of five LP modes measured by the optical spectrum analyzer when the pump power is fixed to be about 400 mW. The central wavelengths of 1561.23, 1561.41, 1561.18, 1561.25 and 1561.62 nm, the 3-dB linewidths of 0.028, 0.02, 0.028, 0.024 and 0.024 nm, the side-mode suppression ratios (SMSRs) of 59, 53, 53, 46 and 37 dB are achieved for the five LP modes from low to high orders, respectively. The wavelength differences come from the different effective lengths of lasing cavities for different LP modes. The output stabilities for the five lasing LP modes are measured during the 60-min period at room temperature, and the output optical spectra are shown in Fig. 3d. The data are recorded at a 5-min interval over a 60-min period. The fluctuations of peak wavelength and average output power for each lasing LP mode are less than 0.032 nm and 0.055 mW, respectively.

Experimental results for CVB generation

Selectable generation of CVBs is demonstrated utilizing the proposed LP-mode Synthesizer. According to the conversion relationship between LP modes and CVB modes by Eq. (1), the LP₁₁ mode is formed by the superposition of near-degenerate CVB modes including radially-polarized TM₀₁, azimuthally-polarized TE₀₁, and ${\text{H}\text{E}}_{21}^{\text{e}}/{\text{H}\text{E}}_{21}^{\text{o}}$ modes. So, adjusting the Optical Switching Array to only enable the lasing of LP₁₁ modes, and then adjusting the PC to induce the rotation and extrusion, we could obtain the TM₀₁ or TE₀₁ mode. The intensity distributions of the output light are recorded by the CCD camera, and the results are shown in Fig. 4a & 4b. The Fig. 4a.i & 4b.i show the doughnut-shaped intensity profiles of both TE₀₁ mode and TM₀₁ mode, respectively. In order to distinguish the polarization distributions of both CVB modes, a rotatable linear polarizer is placed in the light path between the lasing output and the CCD camera. Figure 4a.ii ~ v & 4b.ii ~ v show the intensity distributions with different polarization orientations (represented with white arrows) for the polarizer. The intensity patterns with two-lobe shape perpendicular to the orientation of the linear polarizer in Fig. 4a.ii ~ v show that the output lasing beam is azimuthally-polarized TE₀₁ mode, while the intensity patterns with two-lobe shape parallel to the orientation of the linear polarizer in Fig. 4b.ii ~ v indicate that the light beam is radially-polarized TM₀₁ mode.

Figure 4c presents the optical spectra of CVB outputs at the fixed pump power of about 400 mW. The central wavelengths of 1561.38 and 1561.45 nm, the 3-dB linewidths of 0.024 and 0.024 nm, and the SMSRs of 55 and 54 dB are measured for the TE₀₁ and TM₀₁ lasing modes, respectively. Figure 4d shows the output stabilities for TE₀₁ and TM₀₁ lasing modes at a 5-min interval during the 60-min period. The peak wavelength fluctuation and the average output power fluctuation for TE₀₁ lasing mode are less than 0.044 nm and 0.042 mW, respectively. The fluctuations of peak wavelength and average output power for TM₀₁ lasing mode are less than 0.040 nm and 0.030 mW, respectively.

Experimental results for OAM generation

Selectable generation of OAMs is also verified utilizing the proposed LP-mode Synthesizer. According to the conversion relationship by Eq. (3), the OAMs can be obtained by the superposition of a specific pair of degenerate LP modes in the weakly-guiding FMF under proper polarization and phase control. For examples, the ${OAM}_{\pm 1}$could be obtained utilizing the ${LP}_{m,1}^{e}$ and ${LP}_{m,1}^{o}$, while the ${OAM}_{\pm 2}$ could be obtained utilizing the ${LP}_{\text{2,1}}^{e}$ and ${LP}_{\text{2,1}}^{o}$. So, the Optical Switching Array should be adjusted to obtain the LP₁₁ or LP₂₁ lasing output for the generation of the ${OAM}_{\pm 1}$or ${OAM}_{\pm 2}$, respectively. The PC should be carefully adjusted to rotate the lobe orientation in addition to polarization and phase control. Figure 5a shows the interference setup used to determine the charge number (± 1, ± 2) of the generated OAM beams by observing their characteristic fork patterns with a CCD camera. The reference Gaussian beam at the output of SM-OC-3 is used for the interference setup to analyze the generated OAM beams. A free-space beam combiner combines both light to form the interference pattern and a variable optical attenuator balances the power of the two optical paths. A half-wave plate is used to adjust the polarization state of the reference Gaussian beam. By properly adjusting the PC, annular intensity profiles and their corresponding patterns could be observed. Figure 5b presents the measured intensity distributions for LP modes and OAM beams, and fork intensity patterns at the laser output. The number of forks represents the topological charge of the OAM beams, which are |m|=1,2.

Figure 5c shows the optical spectra of the OAM_+ 1, OAM_-1, OAM_+ 2, and OAM_-2 outputs at the fixed pump power of about 400 mW. The central wavelengths of 1561.39, 1561.43, 1561.18, and 1560.44 nm, the 3-dB linewidths of 0.020, 0.028, 0.032, and 0.028 nm, the SMSRs of 51 dB, 53 dB, 53 dB, and 55dB, are measured for the OAM_+ 1, OAM_-1, OAM_+ 2, and OAM_-2 lasing modes, respectively. Figure 5d shows the output stabilities for all the four OAM lasing modes at a 5-min interval during the 60-min period. The fluctuation of peak wavelength for the OAM_+ 1, OAM_-1, OAM_+ 2, and OAM_-2 lasing modes are less than 0.048, 0.056, 0.072 and 0.060 nm, respectively, while the fluctuation of average output power are 0.072, 0.060, 0.041 and 0.058 mW, respectively.

We have proposed the concept of LP-mode Synthesizer as a general way to generate CVB/OAM structured light according to their conversion relationship with LP modes. Based on a compact fiber ring laser generating switchable one of 5 LP modes and a PC for polarization and phase control, we successfully demonstrate the generation of TE₀₁, TM₀₁, OAM_+ 1, OAM_− 1, OAM_+ 2, and OAM_− 2 modes. The proposed synthesizer could make full advantage of mature LP mode manipulation schemes based on weakly-coupled FMFs and related optical components and is promising to be extended to wide applications.

Weakly-coupled MRC-FMF

The MRC-FMF in the experiment is a customized weakly-coupled FMF fabricated by plasma chemical vapor deposition (PCVD) technique with the index profile shown in Fig. 6a [43], and the n_eff distribution at the wavelength of 1550 nm is also plotted. Because the |Δn_eff| is the dominant factor for modal crosstalk between LP modes and the propagation performance of the FMF is limited by the worst mode channel, the weakly-coupled MRC-FMF is designed by applying two index perturbation regions to the core area of an initial step-index FMF to enlarge the minimum |Δn_eff| among all LP modes. The diameters of the central low-index region, the upper boundary of the high-index region, the fiber core, and the cladding are 8.05, 12.45, 16.5, and 125 µm, respectively. The relative index differences for the core of the initial step-index FMF, the low-index region, and the high-index region to the index of cladding are 0.748%, 0.688%, and 0.827%, respectively. The normalized frequency V is 5.95 and 6 LP modes are contained in the weakly-coupled MRC-FMF, among which the highest-order LP₁₂ mode has large propagation loss and acts as a guarding mode. The minimum |Δn_eff| of the weakly-coupled MRC-FMF is 1.49×10^− 3 lying between LP₂₁ and LP₀₂ modes. The photo of the cross section of the fabricated fiber is shown in Fig. 6b.

Low modal-crosstalk MMUX/MDEMUX

The fabricated MMUX and MDEMUX have the same structure composed of 5 cascaded MSCs but operate in opposite directions, as shown in Fig. 6e. Each MSC performs mode conversion between the fundamental mode of SMF and a specific LP mode in the MRC-FMF. The MSCs are fabricated by tapering parallel-placed MRC-FMF and SMF-28e fiber using a fused biconical taper platform (OSCOM, XQ-LZJ-B02) according to the phase-matching condition [44], as shown in Fig. 6d. At the coupling region, light from the SMF could be coupled to a specific LP mode with high selectivity. Figure 6c illustrates the changes of n_eff for tapered SMF and MRC-FMF with different fiber diameters. The phase-matching conditions could be satisfied at the cross points in the curves, for which the corresponding values on the x-axis are the proper diameters for the tapered fibers. Different pre-tapering lengths for the MRC-FMF or the SMF should be applied to fabricate different MSCs. We can see that the n_eff of the LP₀₁ mode in FMF is consistently higher than that of the LP₀₁ mode in SMF. Therefore, pre-tapering the FMF is necessary for the LP₀₁ MSC, while the SMF needs to be pre-tapered for the other MSCs.

The manufacturing of the MSCs involves two steps: pre-tapering the SMF or FMF to establish the initial diameter ratio of the two fibers, and then tapering the two fibers together maintaining the diameter ratio constant. The two fibers are heated using the hydroxide flame. Continuous-wave light at 1550 nm is injected into the SMF, and the power and mode field pattern at the FMF output should be monitored during the second taper process. The process is stopped when the output coupling power reaches its first maximum value. After the tapering process, the coupling region is pre-sealed and placed in a quartz groove, with the two ends fixed using thermosetting glue. Finally, a heat shrink tube is placed on the outer side of the quartz groove and heated to shrink it.

The modal-crosstalk matrix of the MMUX/MDEMUX in back-to-back (B2B) configuration is also measured. In this configuration, the MMUX and MDEMUX are connected directly without passing any component. The mode crosstalk is measured by injecting 0-dBm optical power at 1550 nm into the input port of the MMUX and measuring the optical power at each output port of the MDEMUX. For a 5 LP mode MMUX/MDEMUX, the result is a 5×5 matrix with the diagonal elements being none and each of the remaining elements is calculated by the ratio of the measured output power of each LP mode to that of the target LP mode. The modal crosstalk for all five modes can be less than − 11.23 dB as shown in Fig. 6f.

Acknowledgements

This work is supported in part by National Nature Science Foundation of China (U20A20160 and 62101009) and Pengcheng Zili Project (PCL2023AS2-4)

Author details

¹State Key Laboratory of Advanced Optical Communication Systems and Networks, School of Electronics Engineering and Computer Science, Peking University, 100871 Beijing, China. ²School of Computer and Communication Engineering, University of Science and Technology, 100083 Beijing, China. ³Peng Cheng Laboratory, 518055 Shenzhen, China.

Author Contributions

F.R. and J.H.L conceived the idea. W.C. and X.F.L. conducted the experiment. D.W.G. contributed to the design and characterization of MRC-FMF. Y.Y.G. contributed to the design and characterization of MMUX/MDEMUX. W.C., R.F., and J.H.L. wrote the manuscript. F.R. and J.H.L. supervised the project. All the authors discussed the results and commented on the manuscript.

Conflict of Interest statement

The authors declare no competing interests.

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General structured light generation based on programmable linearly-polarized mode synthesizer

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Abstract

Figures

Introduction

Results

Concept of programmable LP-mode Synthesizer

Experimental setup

Experimental results for LP generation

Experimental results for CVB generation

Experimental results for OAM generation

Conclusions

Methods

Weakly-coupled MRC-FMF

Low modal-crosstalk MMUX/MDEMUX

Declarations

References

Additional Declarations

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