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The monitoring and conversion of photonic orbital angular momentum (OAM) play fundamental and important roles for both classic and quantum technologies, especially in low-loss transmission media such as ring-core fibers (RCFs), which make many OAM applications practical or vastly more flexible. However, in a RCF, the modes associated with different OAM states are highly overlapping due to the circular refractive index distribution structure, which makes it difficult to distinguish and monitor the OAM modes and in turn limits its inline conversion. Here, we report the first experimental realization of mode monitoring in a RCF using mode filters (MFs), which takes advantage of the difference in the mode adiabatic evolution and the higher-order mode cutoff conditions in tapered RCFs. Different-order OAM can be filtered using MFs with different geometric parameters, as demonstrated by the linearly polarized mode intensity. Combined the mode manipulations in RCF and single-mode fiber, the fundamental mode coupling efficiency can reach 90%, the RCF mode conversion monitoring through inline transmission spectrum evolution can be realized, and the inline fabrication of RCF grating, which couples one mode to a desired mode, can be demonstrated by the fabricating process of three long-period fiber gratings. The mode conversion efficiency between 0-order and 1, 2- or 3-order OAM modes exceeds 96%. Our work provides an efficient approach to monitor and convert OAM modes in higher-order mode supporting RCFs and even other special fibers and further promotes the improvement of the capacity of OAM transmission in RCFs.

We demonstrate 112 orbital angular momentum (OAM) modes amplification based on a designed and fabricated 7-ring-core erbium-doped fiber (7RC-EDF). The differential mode gain (DMG) of all the intra-core OAM modes in 7RC-EDF is effectively reduced. The DMG of intra-core modes can be suppressed by the high overlap between the signal modes and pump fundamental mode resulting from the designed structures of the ring core assisted by a trench. The differential gain of the inter-core can be controlled by the power of the core-pump configuration as well. With an experimentally optimized scheme of the pump power of each core, a record low-DMG of 2.8 dB among the 112 OAM modes (16-OAM-mode per core in the 7-core) is achieved at a wavelength of 1550 nm. The obtained favorable performance of the 112 OAM modes based on 7RC-EDF indicates it may promote a long-haul high-density OAM modes multiplexed transmission.

A multi-channel orbital angular momentum (OAM) mode generation and switching scheme is proposed and demonstrated based on an in-fiber mode selective interferometer (MSI), which is formed in a four-mode fiber. The MSI consists of two strong modulated long-period fiber gratings (LPFGs), which realizes the mode selected coupling between a target mode pair. With the optimized structural parameters, the MSI can couple a launched LP01 (or OAM0) into a desired higher-order azimuthal mode (HAM, LP l 1 or OAM±l, l≥1) at multiple wavelength channels and generate the HAM with high-purity. To verify this concept, we fabricate two LPFGs in a four-mode fiber with designed distance and hence realize a MSI which can generate the second-order HAM (OAM2 or LP21) at 17 wavelength channels. The mode conversion efficiency is more than 90% at 17 wavelengths and the corresponding mode purity is no less than 97%, respectively. In addition, we also demonstrate that the selected mode pair (OAM0 and OAM2) can be switched at multiple channels by changing the state of the MSI. This MSI can also be used as a wavelength band-rejection filter on different spatial modes and find potential applications in optical communications and sensing.

In this paper, we present a field-trial C-band 72Gbit/s optical on-off keying (OOK) system over 18.8km dispersion-uncompensated submarine optical cable in the South China Sea. Chromatic dispersion (CD) of 18.8km submarine optical cable causes four spectral nulls on the 36GHz bandwidth of 72Gbit/s OOK signal, which is the main obstacle for achieving an acceptable bit-error-rate (BER) performance. Decision feedback equalizer (DFE) is effective to compensate for the spectral nulls. However, DFE has a serious defect of burst-error propagation when the burst errors emerge due to the unstable submarine environment. Weighted DFE (WDFE) can be used to mitigate the burst-error propagation, but it cannot fully compensate for the spectral nulls because only a part of feedback symbols is directly decided. Fortunately, maximum likelihood sequence estimation (MLSE) can be added after the WDFE to simultaneously eliminate the resisting spectral distortions and implement optimal detection. Compared to the joint DFE and MLSE algorithm, the joint WDFE and MLSE algorithm can effectively suppress the burst-error propagation to obtain a maximum 2.9dB improvement of $\boldsymbol{Q}$ factor and eliminate the phenomenon of BER floor. In conclusion, the joint WDFE and MLSE algorithm can solve the burst-error propagation for the field-trial fiber-optic communications., Comment: Under review of Journal of Lightwave Techonlogy

We investigate the orbital angular momentum (OAM) mode-generating process of a long-period onefold chiral fiber grating (L-1-CFG) based on a helically twisted hollow-core antiresonant fiber (HC-ARF). Taking the right-handed L-1-CFG for an example, we theoretically and experimentally prove that the first-order OAM+1 mode can be generated by only inputting a Gaussian beam. We fabricated three right-handed L-1-CFG samples based on the helically twisted HC-ARF with twist rates (α) of −0.42, −0.50, and −0.60 rad/mm, where the twisted HC-ARF with α of −0.42 rad/mm can achieve high OAM+1 mode purity of 94%. Subsequently, we present simulated and experimental transmission spectra at the C-band, and sufficient modulation depths were obtained at wavelengths of 1550 nm and 1561.5 nm in the experiment.

A fastest full Mueller matrix polarimeter, to the best of our knowledge, based on optical time-stretch has been proposed and demonstrated. Thanks to the time-stretch-based ultrafast spectra detection mechanism, its measurement time could reach 10 ns. Additionally, a novel, to the best of aour knowledge, simpler method to estimate its main systematic error has been proposed and verified. With the proposed method, static measurement of polarizer and wave plate is executed with a maximum coefficient error of below 0.1. Dynamic measurement of a free space electro-optic modulator as fast-changing phase retardation has also been executed to demonstrate the feasibility of the proposed system.

We design and fabricate a dual-step-index ring-core fiber (RCF) for orbital angular momentum (OAM) mode transmission. It has been proven that the proposed novel, to the best of our knowledge, dual-step-index ring-core structure can suppress the spin–orbit coupling and cut off the radial higher-order modes when the mode number becomes very large. In experiments, we demonstrate that the fabricated fiber can support OAM8,1 with the interferometric method, where four higher-order mode groups are weakly-coupled. We also measure the loss of each mode according to the cut-back method and the loss can achieve l| = 1 to |l| = 5. The exploration of this novel optical fiber structure may provide ideas and knowledge for the improvement of the optical fiber communication capacity.

We propose a bend-insensitive grapefruit-type holey ring-core fiber to support orbital angular momentum (OAM) modes. The large air domain in the cladding, results in a large refractive index contrast between the ring-core and the cladding, which makes this fiber bend-insensitive and makes the vector eigenmodes well separated. Therefore, the proposed fiber can be adopted for the OAM mode division multiplexing transmission without the use of any multiple-input multiple-output digital signal processing (MIMO-DSP). It is also proved that the thickness of the outer silica layer around the ring-core plays an important role in confining the light. If the OAM modes in the ring-core and the cladding modes in the outer silica layer are not phase-matched, the OAM modes in the ring-core will not be coupled into the outer layer of silica and can be well confined. We fabricate such fiber in two steps by using the “stack-and-draw” method. Two kinds of fibers with different thicknesses of the outer silica layer are fabricated under different pressures and temperatures. The testing results of the fabricated fiber demonstrate the mode purity and illustrate that the loss of the OAM $_{+1,1}$ and OAM $_{+2,1}$ modes are 0.095 dB/m and 0.25 dB/m, respectively.

We propose an effective method to measure the orbital angular momentum of a perfect optical vortex (POV) beam with a hybrid angular gradient phase grating (HAG-PG), which is a gradient grating with angular changing distribution. The HAG-PG performs the transformation from the POV to the dark stripes of the diffraction intensity patterns. From the number and the orientation of dark stripes of far-field diffraction patterns formed by the HAG-PG, the modulus of TC and sign of the incident POV beam can be determined. The high scalability of this method for being unrestricted by the Fourier plane and its adaptation for measuring the POV beams with different states are demonstrated by loading the HAG-PG hologram on a spatial light modulator. And the detection sensitivity of the POV beam can be significantly improved. The experimental results agree well with the theoretical simulations.

In this Letter, a ring-core erbium-doped fiber (RC-EDF), with two-layer erbium-doped structure, supporting up to the fourth-order orbital angular momentum (OAM) mode is designed and fabricated for OAM mode multiplexed amplification. Using the RC-EDF, the third- and fourth-order OAM modes amplification with ultra-low differential mode gain (DMG) is demonstrated by observing both the modal intensity and phase distribution and measuring the modal gain under the fundamental mode core-pumping. The measured average gain of four modes (l=+3, -3, +4, -4) multiplexed amplification is higher than 19dB cover the C-band and the DMG is less than 1dB. Additionally, the gain of two conjugate OAM modes are almost the same under different pump power no matter they are amplified simultaneously or separately., 4pages,7figures,Letters

Optical coordinate transformation (OCT) has attracted widespread attention in the field of orbital angular momentum (OAM) (de)multiplexing or manipulation, but the performance of OCT would suffer from its distortion. In this paper, we quantitatively analyze the distortion of OCT from the perspective of ray optics, and explain its rationality to work under non-normal incident light. For the special case of log-polar coordinate transformation (LPCT), we use a raytracing assisted optimization scheme to improve its distortion, which is related to a Zernike polynomial based phase compensation. After raytracing optimization, the root mean square error (RMSE) of the focused rays is reduced to 1/5 of the original value and the physical optic simulation also shows great improvement. In the experiment, we use three phase masks which are realized by metasurfaces, the measured results show well consistency with the simulation. Results in this paper have great potential to improve the performance of OCT related applications., 10 pages, 5 figues, submitted to optics express

An in-fiber Mach-Zehnder interferometer (MZI) with a micro-cavity (MC) exploited for strain and temperature simultaneous measurement. The research is carried out experimentally and simulatively. The MC fabricated by splicing a hollow quartz tube between two single-mode fibers serves as a light splitter for the two arms of MZI beams and a Fabry–Perot interferometer (FPI) component at the same time. Thus both functions of the MZI and FPI were implemented in a single sensor. With aid of the different response of the MZI and FPI to the strain and temperature, this device can achieve a simultaneous measurement and the strain/temperature cross-sensitivity issue has been alleviated. Such a composite interference structure, possess the characteristics of easy making, high performance-price ratio provides a promising approach to implement strain and high-temperature simultaneous measurement.

Optical encryption has attracted attention recently as information security becomes important in modern society. For most encryption methods based on metasurfaces, however, information is encrypted into the metasurfaces of physical forms, limiting information sharing and compatibility with digital information processing technology. Here, we demonstrate an alternative scheme in which information is encrypted on optical images other than metasurfaces, releasing the constraints mentioned above. This is achieved by modulating the spatial frequencies of optical images with metasurfaces. Specifically, information encryption and decryption based on the intensity or phase of spatial frequency have been demonstrated with the developed microscopic Fourier optics technique. The abundance of spatial frequency also promises a flexible multiplexing platform for increasing information capacity, which has been verified by a 4-bit encryption experiment. These findings could enable high-security and high-capacity optical encryption platforms.

Mode division multiplexing has attracted great attention because it can potentially overcome the limitation of single-mode fiber traffic capacity. However, it has been challenging to realize multiple modes controlling and switching due to the intrinsic overlap of the modes in the transmission waveguide. As a solution, we propose a cascaded phase-shifted long-period fiber grating (PS-LPFG) based multiple mode switching scheme. Using the PS-LPFGs, the multiple guided orbital angular momentum (OAM) modes selective controlling and switching at multi-wavelength can be achieved in few-mode fibers by regulating the grating resonance condition. In principle, a N × N mode switch matrix can be realized by cascading C N 2 gratings, where each grating acts as a mode switch element to achieve a couple selected OAM mode switching and meanwhile the other modes are under nonblocking status. As a proof of the concept, a 2 × 2 mode switching between two OAM modes at different wavelengths based on one PS-LPFG element is demonstrated in our experiments. The switching efficiency of the two modes at two wavelengths 1537nm and 1558nm are ∼98.4% and ∼98.7%, respectively. The proposed multiple OAM mode switch has potential applications in the future hybrid multi-dimensional multiplexing optical fiber communication systems.

In intensity-modulation and direct-detection (IM/DD) fiber-optic communications, it is hard to pre- or post-compensate for chromatic dispersion (CD) by digital signal processing due to one-dimensional modulation and detection. In this Letter, we propose joint optical and digital signal processing to effectively compensate for CD-caused distortions for IM/DD optical systems. As a reasonable optical signal processing, negative chirp based on self-phase modulation can suppress a part of CD to take pressure off digital signal processing. Digital signal processing is designed based on the model of a dispersive channel to accurately compensate for CD-caused distortions. To the best of our knowledge, we present a record C-band 72 Gbit/s optical on–off keying over 100 km dispersion-uncompensated link (i.e., ∼ 1700 ps / nm dispersion), achieving a 7% hard-decision forward error correction limit. We conclude that joint optical and digital signal processing is effective in dealing with CD-caused distortions to achieve a higher capacity–distance product in IM/DD fiber-optic communications.

The use of coherent optical signal processing in long-distance optical communication systems has dramatically increased data capacity enabling encoding of multiple-bit information in the amplitude and phase of a light beam. Direct detection of phase information of a high-speed modulated light remains challenging and requires an external, local oscillator for referencing, which is expensive for short-reach optical communications, for example, in datacenters. The availability of less complex integrated photonics devices for coherent signal detection would alleviate this bottleneck. On the other hand, phase information of coherent, orthogonally polarized light beams can be extracted from their polarization states and it is, therefore, possible to achieve phase measurements via fast polarization detection. Here we demonstrate an on-chip, high-speed coherent optical signal receiver enabled by spin-orbit coupling in Si-photonics circuitry. In a coherent communication experiment with up to 16 Gbaud/s rate, the high-speed quadrature phase-shift keying signals detected by a Si nanodisk based polarisation measurements at multiple wavelength in the C-band were recovered with a bit error rate below the forward error correction threshold. The proposed on-chip nanodisk coherent receiver shows promise in high-speed coherent optical communication applications where phase detection is required at low cost and small footprint.

An orbital angular momentum (OAM) fiber amplifier supporting 20 OAM modes based on a ring-core Yb-doped fiber (RC-YDF) is proposed and demonstrated. The RC-YDF we designed and fabricated has two successive Yb-doped annular layers in the ring-core and can support the amplification of OAM (|l|=1, 2, 3, 4, 5) modes at the wavelength of 1064 nm. With a core pump configuration, we characterize the amplification performance of the RC-YDF based amplifier by simulation and experiments. The amplification of each supported OAM mode is proved by the achieved gain of more than 8 dB and a low differential modal gain less than 1dB with an input signal power of about 5dBm. This is the first experimental demonstration, as far as we know, of the amplification of the OAM mode from 1- to 5-order in aYb-doped fiber.

We propose and demonstrate an all-fiber passively Q-switched laser generating a third-order orbital angular momentum (OAM) pulse by introducing a few-mode long-period fiber grating (LPFG) into the laser cavity. The LPFG with asymmetric cross structure and strong refractive index modulation overcomes the coupling issue between the fundamental and the third-azimuthal-order (LP31 or OAM3) modes and realizes their direct conversion. A homemade graphene-based saturable absorber is used to realize Q-switched operation. The laser operates at a center wavelength of 1548.2nm, with a 3 dB spectral bandwidth of 0.4nm, and the OAM+3 and OAM-3 beams can achieve the purity of 90.0% and 90.2%, respectively. This all-fiber Q-switched laser has simple and compact structure and high purity of OAM±3 beams, which has potential applications in the fields of optical tweezers and material processing.

This publisher’s note contains corrections to Opt. Lett. 45, 758 (2020)OPLEDP0146-959210.1364/OL.384168.

Supplementary Material

We discuss the mode properties for orbital angular momentum (OAM) modes supporting multi-mode ring-core fiber (MM-RCF). For strongly-guiding MM-RCF, by introducing one more layer to the ring-core, a two-layer ring-core fiber (TL-RCF) is proposed. It is proven that this TL-RCF can increase the separation degree between the vector eigenmodes and meanwhile weaken the spin–orbit coupling. For mode-group supporting MM-RCF, the maximum mode number is limited by the thickness of ring-core and the cut-off of radial higher-order mode. We also present the tradeoff relationship between the mode group effective index difference and inter-eigenmode differential mode delay in one mode group. The discussion in this paper can give guidance for the design of MM-RCF.

We here reported a single wavelength second order vortex mode lasing in optical fiber system. For this, we implemented a high efficient mode conversion between the fundamental guiding mode(LP01 mode) and the second azimuthal order guiding mode(e.g.LP21 mode), and an appropriate mode modulation that converted the mode from LP21 mode to the second order optical vortex(OV) mode. The high efficient mode conversion was realized by utilizing a single intra-cavity strong modulated long-period fiber grating (SM-LPFG) that enables a higher than 20 dB( > 99 % ) direct mode conversion efficiency over a bandwidth range of 3 nm. And the mode modulation was fulfilled with high-order mode polarization controller that control the polarization state of the resonating mode to transform the LP21 mode to the 2nd order OV mode. Experiment results showed that a single wavelength of 1542.88 nm of the 2nd order OV mode laser was obtained with a 3 dB bandwidth 0.22 nm and over 27 dB signal to noise ratio. As far as we know, this was the first report on the implementation of the 2nd order OV mode lasing in optical fiber system by only one intra-cavity SM-LPFG in the laser cavity.

The cylindrical vector beams (CVBs) compose two circularly polarized vortex beams carrying orbital angular momentum (OAM) states with the opposite topological charge. We derived the probabilities for OAM of high-order CVB propagation through turbulent atmosphere. The calculated probabilities of OAM are based on the polarization insensitive element detection and phase perturbation coherence function assumption. The numerical calculation results show that the probabilities of OAM state spreading to neighbor states of CVBs through turbulent atmosphere increase with the propagation distance increase. Under the same conditions, the spreading of OAM for CVBs are weaker than Laguerre-Gaussian beam propagation through turbulent atmosphere. The results show that the OAM state of CVBs could improve the performance of optical communications systems in atmospheric turbulence.

We propose and demonstrate an optical router by selecting data encoded in massive orthogonal orbital angular momentum (OAM) states carried by collinear optical vortex (OV) beams. By switching the OV grating on a digital micromirror device, we achieve information exchange and multicasting in 49 OAM channels with 1.37 Tbit/s aggregated data capacity. The time-domain characterization of the OAM router shows a fast-switching time of 6.9 μs. Both the analytical derivations and experimental demonstrations show that the router has signal-to-noise ratios (SNRs) better than 10.1 dB for all tens of OAM channels. The OAM-based optical interconnect technique is a promising solution for networking multiple users with ultrahigh data capacity density in datacenters.

We propose a new method to generate orbital angular momentum (OAM) mode by superposing multiple LP11 modes with microphase difference distributed along fiber propagation. This way, OAM mode can be generated by superposing a series of LP11 mode with phase difference much less than π/2, which is necessary for schemes using two degenerated LP11 mode superposition but is difficult to generate and hard to keep stable. We demonstrate this principle experimentally by utilizing few-mode fiber long period grating (FMF-LPG), in which a series of LP11 modes with microphase difference distribution can be generated by twisting FMF-LPG. This scheme provides a potential all-fiber, compact, and low-attenuation scheme to generate stable OAM modes.

A bidirectional bend sensor based on U-shaped Fabry-Perot interferometer (UFPI) is experimentally demonstrated. The UFPI is constructed by eccentrically splicing a section of microfiber between two cleaved standard single mode fiber end faces serving as mirrors. A preliminary theoretic analysis of bending response of UFPI is presented. The bend and temperature sensing properties are measured. The theoretical and experimental investigation demonstrates the bending direction discrimination and the monotonous sensing characteristic in a single direction along or opposite to the cavity opening direction. The proposed bend sensor with more compact (~105 μm in length) and easier fabrication presents a high contrast of the interferometer fringes and low temperature sensitivity of 1.21 × 10 -3 nm/°C.

Polarization topological order of a cylindrical vector beam (CVB) can be defined as the repetition number of 360° polarization state change along the azimuthal axis, with its sign denoting the rotating direction of the polarization. In order to detect CVBs with different order, a miniaturized polymer grating is proposed and demonstrated. The grating is almost pure phase modulated and polarization independent. The CVB modes with different orders or with the same order but opposite sign can easily be separated by adding a quarter wave plate and a polarizer. The experimental results are consistent with the theoretical predictions, thus validating our proposed method.

Compared to glass walls with a positive curvature, those with a negative curvature have been proven to have stronger confinement of light. Therefore, we change the multi-layered air holes in a photonic crystal fiber into several negative curvature tubes. As a result, the confinement medium is shifted from a low-index cladding material into a special structure. The theoretical analysis shows that each vector eigenmode has a corresponding threshold value for the outer tube thickness. It means that we can confine the target modes and filter the unnecessary modes by shifting the outer tube thickness. After substantial investigation on this fiber, we obtain the appropriate values for each structural parameter and then fabricate this negative curvature ring-core fiber under the guidance of the simulation results. Firstly, we draw the central cane under vacuum condition, then stack the cane and six capillaries to form the preform, and finally draw the ring-core fiber by using vacuumization method. The fiber test experiment indicates that the fiber length should be at least 15 m∼20 m to form the donut facula, and the tested losses of OAM+1,1, OAM+2,1, OAM+3,1, and OAM+4,1 are 0.30 dB/m, 0.36 dB/m, 0.37 dB/m, and 0.42 dB/m, respectively.

This paper firstly, to the best of our knowledge, proposed two-dimensional (2D) encryption based on the Arnold transformation for implementing a secure DC-biased optical orthogonal time-frequency multiplexing (DCO-OTFM) in optical-wireless communications (OWCs). The encrypted data is transformed to the particular 2D matrix and decrypted by the only key to get the correct information. Meanwhile, the number of keys in 2D encryption is enormous, which prevents eavesdroppers from exhaustively searching secret keys rapidly to find the right decryption. Numerical results demonstrate that the secure DCO-OTFM based on 2D encryption can effectively prevent signal decryption from the eavesdropper, which has good secure performance for applying in OWC.

We propose and demonstrate an optical vortex Dammann axicon grating to simultaneously detect coaxial high-order Bessel beams. The coaxial superposition of multiple Bessel beams are spatially separated and converted into light spots by the grating according to their carried orbital angular momentum (OAM) states. The self-healing property of the Bessel beams is verified for single and multiple Bessel beams in terms of the OAM states detection. We also quantitatively measure that the OAM has a mode purity of 96% with obstacle.

The third-order orbital angular momentum ( O A M ± 3 ) guided mode generation is demonstrated for the first time, to the best of our knowledge, by employing an asymmetric long-period fiber grating (AS-LPFG). The proposed AS-LPFG is modeled by coupled local-mode theory, which is extended to the coupling of core modes and is fabricated by multicycle scanning ablation with increasing power in a six-mode fiber. The experiments demonstrate that one fabricated AS-LPFG can convert the L P 01 mode to the third-azimuthal-order (3AO, L P 31 or O A M ± 3 ) guided mode with efficiency of ∼ 99.8 % . The model and the method presented, in principle, can be used to generate any other high-order modes.