Controllable all-fiber orbital angular momentum mode converter

Reconfigurable and tunable twisted light laser

Twisted light, having a helical spatial phase structure and carrying orbital angular momentum (OAM), has given rise to many developments ranging from optical manipulation to optical communications. The laser excitation of twisted light in a reconfigurable and tunable way is of great interest. Here, we propose and experimentally demonstrate an OAM reconfigurable and wavelength tunable twisted light laser with achievable high-order OAM modes on a hybrid free-space and fiber platform. The excited twisted light laser is enabled by a ring resonator incorporating spatial light modulators (SLMs) and bandpass filter (BPF). By appropriately switching the phase pattern loaded onto SLMs and adjusting the BPF, twisted light laser with reconfigurable OAM and tunable wavelength is implemented. In the experiment, the OAM value is varied from -10 to +10 and the wavelength is adjusted from 1530 to 1565 nm covering the whole C band. The obtained results indicate successful implementation of a reconfigurable and tunable twisted light laser with favorable operation performance. Reconfigurable and tunable twisted light laser may open up new perspectives to more extensive OAM-enabled applications with improved flexibility and robustness.

Generating and synthesizing ultrabroadband twisted light using a compact silicon chip

Compact and broadband manipulation of spatial modes is important in applications exploiting the space domain of light waves. Here, we demonstrate chip-scale generation and synthesization of ultrabroadband orbital angular momentum (OAM) modes (twisted light) on a silicon platform. By introducing a subwavelength holographic fork grating on top of a silicon waveguide, the in-plane guided mode is converted to the free-space OAM mode. Inputs from both sides of the waveguide enable the synthesization of OAM modes. We also characterize wavelength-dependent emission efficiency, offset angle, and purity with favorable performance. The chip-scale ultrabroadband OAM generator and synthesizer may find potential applications in multidimensional optical communications and quantum key distribution.

All-fiber stable orbital angular momentum beam generation and propagation

An all-fiber scheme for stable orbital angular momentum (OAM) beam generation and propagation is proposed and demonstrated. This scheme is based on a self-designed and manufactured graded-index few mode fiber (GI-FMF) and compatible mode selection coupler (MSC). The MSC, which consists of a conventional single mode fiber (SMF) and the GI-FMF, can effectively couple the fundamental mode in the SMF to the desired OAM mode in the GI-FMF based on phase matching condition. Meanwhile, the GI-FMF breaks the degeneracy between the desired eigenmodes and neighboring vector modes, thereby allowing the preservation and propagation of the selectively excited OAM modes. As a proof-of-principle, we have implemented an all-fiber device operating in stable OAM modes with |l| = 1. The experimental results show stable propagation of OAM beams with the mode purity of ∼95% and bandwidth of 100 nm. This all fiber device could be useful for further development of wide bandwidth OAM mode division multiplexed application.

Temperature Insensitivity Polarization-Controlled Orbital Angular Momentum Mode Converter Based on an LPFG Induced in Four-Mode Fiber

We propose a novel method for generating ±1-order orbital angular momentum (OAM) in long-period fiber gratings (LPFGs) by adjusting a polarization controller (PC). An LPFG, inscribed in a four-mode fiber (4MF) using a CO₂ laser, was used to generate OAM_±1 across a broad range of wavelengths from 1530 nm to 1630 nm. Additionally, the OAM vortex phase remained stable while the temperature increased from 23 °C to 50 °C. The LPFG, as a temperature sensor, and its temperature sensitivity was measured to be 38.6 ± 0.37 pm/°C at the resonant wavelength of 1625 nm. This design offers simple fabrication and several properties which are highly beneficial for all-fiber optical communications based on the OAM mode-division multiplexing technique.

Orbital angular momentum transition of light using a cylindrical vector beam

We demonstrate that using a single cylindrical vector (CV) beam in two-mode fibers, the orbital angular momentum of light can be switched among -1, 0, and 1. The input CV beam can be a conventional radial and azimuthal polarization distribution or a generalized CV beam, and we first use and verify that a rocking-long period fiber grating generates the tunable generalized CV beam. Because of using a single CV beam as the light source, this approach not only provides an increased stability compared to the conventional superposed eigenmodes method, but also builds a bridge between the polarization singularity beams and the phase singularity beams.

High-order mode direct oscillation of few-mode fiber laser for high-quality cylindrical vector beams

Generation of high-order modes with high quality is important for the application of cylindrical vector beams in fibers. We experimentally demonstrated high-order LP₁₁ mode generation and amplification with a broad bandwidth in an all few-mode fiber laser. A wavelength-division-multiplexing (WDM) mode selective coupler (MSC) is proposed to achieve efficient mode conversion from LP₀₁ mode to LP₁₁ mode, but also combine high-order LP₁₁ modes at the wavelengths of 980/1550 nm. To the best of our knowledge, this is the first report on the high-order mode oscillation in an all few-mode fiber laser. LP₁₁ mode and cylindrical vector beams including radially and azimuthally polarized beams are obtained with high modal purity. The purity of the generated high-order modes are all in excess of 95%.

High-order orbital angular momentum mode generator based on twisted photonic crystal fiber

High-order orbital angular momentum (OAM) modes, namely, OAM₊₅ and OAM₊₆, were generated and demonstrated experimentally by twisting a solid-core hexagonal photonic crystal fiber (PCF) during hydrogen-oxygen flame heating. Leaky orbital resonances in the cladding depend strongly on the twist rate and length of the helical PCF. Moreover, the generated high-order OAM mode could be a polarized mode. The secret of the successful observation of high-order modes is that leaky orbital resonances in the twisted PCF cladding have a high coupling efficiency of more than -20  dB.

All-fiber radially/azimuthally polarized lasers based on mode coupling of tapered fibers

We demonstrate a mode converter with an insertion loss of 0.36 dB based on mode coupling of tapered single-mode and two-mode fibers, and realize all-fiber flexible cylindrical vector lasers at 1550 nm. Attributing to the continuous distribution of a tangential electric field at taper boundaries, the laser is switchable between the radially and azimuthally polarized states by adjusting the input polarization. In the temporal domain, the operation is controllable among continuous-wave, Q-switched, and mode-locked statuses by changing the saturable absorber or pump strength. The duration of Q-switched radially/azimuthally polarized laser spans from 10.4/10.8 to 6/6.4 μs at the pump range of 38 to 58 mW, while that of the mode-locked pulse varies from 39.2/31.9 to 5.6/5.2 ps by controlling the laser bandwidth. The proposed laser combines the features of a cylindrical vector beam, a fiber laser, and an ultrafast pulse, providing a special and cost-effective source for practical applications.

Orbital angular momentum-enhanced measurement of rotation vibration using a Sagnac interferometer

We construct an experimental measurement system for rotation vibration signal detection using the orbital angular momentum (OAM) of light in a Sagnac interferometer. Inputting light beams with different OAM, we demonstrate that the measured signal and signal-to-noise ratio can be increased by the OAM mode index l. In addition, the Sagnac interferometer can further improve the vibration signal and suppress the environmental noises. Such system has potential applications in high-precision sensing and monitoring of rotation vibrations.

All-fiber second-order optical vortex generation based on strong modulated long-period grating in a four-mode fiber

We propose an effective all-fiber method to generate a high-order optical vortex (OV) via twisting a strong modulated long-period fiber grating (LPFG) written in a four-mode fiber (4MF). With a special design and optimization of the procedures of CO₂-laser irradiation, an LPFG with strong period deformation is achieved in the 4MF. Based on this LPFG, we can directly convert the linear polarization (LP) fiber fundamental mode (LP₀₁) to the high-order LP core mode (LP₂₁) with efficiency of 99.7% and then transform the LP₂₁ mode into a high-order OV mode (±2 order). This is the first time, to the best of our knowledge, that ±2-order OV modes have been experimentally generated with just one fiber grating in an all-fiber-system.

Two-dimensional tunable orbital angular momentum generation using a vortex fiber

We demonstrate the two-dimensional tunable orbital angular momentum (OAM) generation in a ring-core (vortex) fiber. The LP₁₁ mode generated by an all fiber fused coupler is coupled into a vortex fiber. Because the vector modes of the LP₁₁ mode group in the vortex fiber are no longer degenerate, the mode status will change between linearly polarized modes (LPMs) and complex OAM modes periodically during propagation. The generated OAM can be tuned smoothly by filtering the mixed mode with different polarization directions or changing the wavelength at a certain polarization directions. The two-dimensional tuning of OAM from l=-1 to l=+1 is experimentally demonstrated in an all fiber OAM generator.

All-fiber mode converter based on long-period fiber gratings written in few-mode fiber

We investigated an all-fiber mode converter based on long-period fiber gratings (LPFGs) written in the few-mode fiber. Mode conversion between the fundamental core mode and different higher-order core modes (LP₁₁, LP₂₁, and LP₀₂ modes) can be realized via a single LPFG with an efficiency of 99% at the resonance wavelength. Moreover, optimized mode conversion between the LP₀₁ and LP₂₁ modes can be realized by cascading two LPFGs with different grating pitches. The maximum conversion efficiency is estimated to be ∼99.5% at 1553 nm. The orbital angular momentum states with different topological charges (±1,±2) are demonstrated experimentally. The all-fiber LPFG mode converters could have promising applications in the mode-division multiplexing optical communications.

Tunable higher-order orbital angular momentum using polarization-maintaining fiber

For the first time, to the best of our knowledge, light with orbital angular momentum (OAM) of ±2ℏ per photon is produced using commercially available polarization-maintaining fiber with modal purity of 96%. Twist measurements demonstrate that the average orbital angular momentum can be continuously tuned between ±2ℏ. The authors consider beams of non-integer OAM, created using the presented method, as superpositions of integer OAM states.

Theoretical analyses on orbital angular momentum modes in conventional graded-index multimode fibre

Orbital angular momentum (OAM) modes are another mode basis to represent spatial modes. There have been increasing interests in exploiting OAM modes in specialty fibres. In this paper, we present a comprehensive characterisation of OAM modes in conventional graded-index multimode fibre (MMF). 1) We synthesise the circularly polarized OAM modes by properly combining two fold degenerate cylindrical vector modes (eigenmodes) and analyse the total angular momentum, i.e. spin angular momentum and orbital angular momentum. 2) We divide all the OAM modes of the conventional graded-index MMF into 10 OAM mode groups with effective refractive index differences between different mode groups above 10⁻⁴ enabling low-level inter-group crosstalk. 3) We study the chromatic dispersion, differential group delay, effective mode area, and nonlinearity for each OAM mode group over a wide wavelength ranging from 1520 to 1630 nm covering the whole C band and L band. 4) We discuss the performance tolerance to fibre ellipticity and bending. 5) We further address the robustness of performance against fibre perturbations including the core size, index contrast and the imperfect index profile of the practically fabricated MMFs. The obtained results may provide theoretical basis for further space-division multiplexing applications employing OAM modes in conventional graded-index MMF.

All-fiber spatial rotation manipulation for radially asymmetric modes

We propose and experimentally demonstrate spatial rotation manipulation for radially asymmetric modes based on two kinds of polarization maintaining few-mode fibers (PM-FMFs). Theoretical finding shows that due to successful suppression of both polarization and spatial mode coupling, the spatial rotation of radially asymmetric modes has an excellent linear relationship with the twist angle of PM-FMF. Both elliptical core and panda type FMFs are fabricated, in order to realize manageable spatial rotation of LP₁₁ mode within ±360° range. Finally, we characterize individual PM-FMF based spatial orientation rotator and present comprehensive performance comparison between two PM-FMFs in terms of insertion loss, temperature sensitivity, linear polarization maintenance, and mode scalability.

Low-crosstalk orbital angular momentum fiber coupler design

A fiber coupler for low-crosstalk orbital angular momentum mode beam splitter is proposed with the structure of two separate and parallel microfibers. By properly setting the center-to-center distance between microfibers, the crosstalk is less than -20 dB, which means that the purity of the needed OAM mode in output port is higher than 99%. For a fixed overlapping length, high coupling efficiency (>97%) is achieved in 1545-1560 nm. The operating wavelength is tuned to the whole C-band by using the thermosensitive liquid. So the designed coupler can achieve the tunable coupling ratio over the whole C-band, which is a prospective component for the further OAM fiber system.

Orbital angular momentum (OAM) conversion and multicasting using N-core supermode fiber

We propose and numerically demonstrate a conversion and multicasting scheme of orbital angular momentum (OAM) states by using N-core supermode fiber (NCSF), where the topological charges of converted OAM states mainly depend on the injected OAM state and the number of fiber cores. The conversion efficiency (CE) of the converted OAM states could be optimized by properly designing the fiber structure. Take N = 6 as an example, ~37% CE could be achieved at telecom bands. Moreover, even for a fabricated NCSF, the CE could be dynamically changed by stretching the fiber or by adjusting the refractive index of the fiber cores through external control of the environmental conditions. Meanwhile, OAM multicasting could also be realized in the designed NCSF. The crosstalk between the multicasted OAM channels and their neighboring ones are assessed to be less than -30 dB. The proposed fiber-based OAM conversion and multicasting system is compatible with the existing optical fiber communication systems, showing potential applications in the future.

Linearly polarized orbital angular momentum mode purity measurement in optical fibers

We presented a simple method for measuring the mode purity of linearly polarized orbital angular momentum (OAM) modes in optical fibers. The method is based on the analysis of OAM beam projections filtered by a polarizer. The amplitude spectrum and phase spectrum of a data ring derived from the beam pattern are obtained by Fourier transform. Then the coefficients of the mixed electric field expression can be determined and the mode purity can be obtained. The proposed method is validated and it is experimentally demonstrated in a two-mode fiber.

Generation of cylindrical vector beams and optical vortex by two acoustically induced fiber gratings with orthogonal vibration directions

Mode coupling from the fundamental vector mode (HE11x) to the cylindrical vector beams (CVBs) and optical vortex beams (OVBs) of a few-mode fiber excited by two acoustic flexural waves with orthogonal perturbations is achieved by using a composite acoustic transducer. The HE11x mode is converted to TM₀₁ and TE₀₁ modes, which have radial and azimuthal polarizations, by using the lowest-order acoustic flexural modes of F11x and F11y, respectively. Furthermore, HE11x mode can also be converted to the ± 1-order OVBs of HE21even±iHE21odd through the combined acoustic modes of F11x±iF11y. This technique provides a useful way of generating CVBs and OVBs in optical fiber with conveniently electrically-controlled mode conversion characteristics.

Simultaneous control of orbital angular momentum and beam profile in two-mode polarization-maintaining fiber

We report simultaneous control of the orbital angular momentum (OAM) and beam profile of vortex beams generated in two-mode polarization-maintaining optical fiber. Two higher-order eigenmodes of the fiber are combined to form optical vortices. Reduced coherence between the fiber modes decreases the mode purity. Varying the coherence of the fiber modes changes the average OAM while maintaining a constant annular intensity profile. Additionally, a donut mode has been shown to be insensitive to bends and twists in the fiber.

High-order optical vortex generation in a few-mode fiber via cascaded acoustically driven vector mode conversion

We propose a method to generate the high-order optical vortex in a few-mode fiber via cascaded acoustically driven vector mode conversion. Theoretical analysis showed that the vector mode conversion induced by the acoustically induced fiber grating (AIFG) could occur between two HE modes with adjacent azimuthal numbers. In the experiment conducted at 532 nm, two AIFGs were simultaneously induced in the same segment of the fiber by a radio frequency source containing two different frequency components. One AIFG was used to convert the left- and right-handed circular polarization fundamental modes to the ±1-order vortex modes, which were then further converted to the ±2-order vortex modes by the other AIFG. The topological charges of the vortex modes were verified using both coaxial and off-axial interference methods, showing typical signature patterns of spiral forms and forklike fringes, respectively.

Millimetre Wave with Rotational Orbital Angular Momentum

Orbital angular momentum (OAM) has been widely studied in fibre and short-range communications. The implementation of millimetre waves with OAM is expected to increase the communication capacity. Most experiments demonstrate the distinction of OAM modes by receiving all of the energy in the surface vertical to the radiation axis in space. However, the reception of OAM is difficult in free space due to the non-zero beam angle and divergence of energy. The reception of OAM in the space domain in a manner similar to that in optical fibres (i.e., receiving all of the energy rings vertical to the radiation axis) is impractical, especially for long-distance transmission. Here, we fabricate a prototype of the antenna and demonstrate that rather than in the space domain, the OAM can be well received in the time domain via a single antenna by rotating the OAM wave at the transmitter, i.e., the radio wave with rotational OAM. The phase and frequency measured in the experiment reveal that for different OAM modes, the received signals act as a commonly used orthogonal frequency division multiplexing (OFDM) signal in the time domain. This phase rotation has promising prospects for use in the practical reception of different OAMs of millimetre waves in long-distance transmission.

Optical vortex generation with wavelength tunability based on an acoustically-induced fiber grating

We presented a method to actualize the optical vortex generation with wavelength tunability via an acoustically-induced fiber grating (AIFG) driven by a radio frequency source. The circular polarization fundamental mode could be converted to the first-order optical vortex through the AIFG, and its topological charges were verified by the spiral pattern of coaxial interference between the first-order optical vortex and a Gaussian-reference beam. A spectral tuning range from 1540 nm to 1560 nm was demonstrated with a wavelength tunability slope of 4.65 nm/kHz. The mode conversion efficiency was 95% within the whole tuning spectral range.

Tunable orbital angular momentum generation in optical fibers

We present a method in this Letter to generate optical vortices with tunable orbital angular momentum (OAM) in optical fibers. The tunable OAM optical vortex is produced by combining different vector modes HE2,meven (HE2,modd) and TE_0,m (TM_0,m) when l=1 or combining HEl+1,meven (HEl+1,modd) and EHl-1,modd (EHl-1,meven) when l>1 with a π/2 phase shift. The vortex can be regarded as a result of overlapping two orthogonal optical vortex beams of equal helicity but opposite chirality with a π/2 phase shift. We have experimentally demonstrated the smooth variation of OAM from l=-1 to l=+1 by adjusting a polarizer at the output end of the fiber.

Generation of linearly polarized orbital angular momentum modes in a side-hole ring fiber with tunable topology numbers

A refractive index (RI) tunable functional materials infiltrated side-hole ring fiber (SHRF) is proposed to generate 10 LP OAM states with 6 topology numbers. On the basis of perturbation theory, the basis of the SHRF is demonstrated to be the LP modes. After a fixed propagation distance of 0.03 m, 0.009 m and 0.012 m, the phase difference between the odd and even LP₁₁x, LP₂₁x,y, LP₃₁x,y modes in the SHRF accumulate to ± π/2 respectively with na ranging from 1.412 to 1.44. Correspondingly, the output states are OAM _{± 1}x, OAM _{± 2}x,y, OAM _{± 3}x,y with a bandwidth of 380 nm, 100 nm and 80 nm respectively. The proposed fiber is easy to be fabricated with the mature fiber drawing technology and could facilitate the realization of all fiber based OAM system.

Cylindrical vector beam generation in fiber with mode selectivity and wavelength tunability over broadband by acoustic flexural wave

Theoretical analysis and experimental demonstration are presented for the generation of cylindrical vector beams (CVBs) via mode conversion in fiber from HE₁₁ mode to TM₀₁ and TE₀₁ modes, which have radial and azimuthal polarizations, respectively. Intermodal coupling is caused by an acoustic flexural wave applied on the fiber, whereas polarization control is necessary for the mode conversion, i.e. HE11x→TM₀₁ and HE11y→TE₀₁ for acoustic vibration along the x-axis. The frequency of the RF driving signal for actuating the acoustic wave is determined by the phase matching condition that the period of acoustic wave equals the beatlength of two coupled modes. With phase matching condition tunability, this approach can be used to generate different types of CVBs at the same wavelength over a broadband. Experimental demonstration was done in the visible and communication bands.