Air-core Fiber Research Articles

Cylindrical vector beams multiplexing communication in air-core photonic crystal fiber

The data rate in a single-mode fiber is approaching the capacity limit given by the Shannon theory. Mode division multiplexing, such as few modes, orbital angular momentum, and cylindrical vector beam (CVB) multiplexing, has shown great potential to further increase data capacity in both free-space and fiber communication. We propose and demonstrate high-order CVB multiplexing communication in an air-core photonics crystal fiber (PCF). The simulation results show that the 19-cell air-core PCF supports transmission of CVB modes from ±1 to ±4 orders. In the experiment, ±1- to ±4-orders CVBs are transmitted in 8.25-m-long air-core PCF with the mode purities higher than 76.5%. We demonstrate four coaxial CVB channel communication by multiplexing the ±2- and ±3-orders CVB modes. Each CVB channel carries 10-Gbit/s on–off keying signals and the measured bit error rates satisfy the forward error correction threshold. CVB communications based on air-core PCF can be used in short-distance optical communication with high capacity and low optical latency.

Air-core fiber distribution of hybrid vector vortex-polarization entangled states

Entanglement distribution between distant parties is one of the most important and challenging tasks in quantum communication. Distribution of photonic entangled states using optical fiber links is a fundamental building block towards quantum networks. Among the different degrees of freedom, orbital angular momentum (OAM) is one of the most promising due to its natural capability to encode high dimensional quantum states. In this article, we experimentally demonstrate fiber distribution of hybrid polarization-vector vortex entangled photon pairs. To this end, we exploit a recently developed air-core fiber which supports OAM modes. High fidelity distribution of the entangled states is demonstrated by performing quantum state tomography in the polarization-OAM Hilbert space after fiber propagation, and by violations of Bell inequalities and multipartite entanglement tests. The present results open new scenarios for quantum applications where correlated complex states can be transmitted by exploiting the vectorial nature of light.

Air-core fiber or photonic-crystal rod, which is more suitable for energetic femtosecond pulse generation and three-photon microscopy at the 1700-nm window?

Energetic femtosecond pulses at the 1700-nm window are a prerequisite for deep-tissue three-photon microscopy (3PM). Soliton self-frequency shift (SSFS) in photonic-crystal (PC) rod has been the only technique to generate such pulses suitable for 3PM. Here we demonstrate through SSFS in an air-core fiber, we can generate most energetic femtosecond soliton pulses at the 1700-nm window, 5.2 times higher than that from PC rod. However, the air-core soliton pulse width is 5.9 times longer than that of PC rod soliton. Based on comparative 3PM excited with both air-core and PC rod solitons, we propose the more suitable source for 3PM. We further elucidate the challenge of generating shorter soliton pulses from air-core fibers through numerical simulation.

Orbital Angular Momentum States Enabling Fiber-based High-dimensional Quantum Communication

Quantum networks are the ultimate target in quantum communication, where many connected users can share information carried by quantum systems. The keystones of such structures are the reliable generation, transmission and manipulation of quantum states. Two-dimensional quantum states, qubits, are steadily adopted as information units. However, high-dimensional quantum states, qudits, constitute a richer resource for future quantum networks, exceeding the limitations imposed by the ubiquitous qubits. The generation and manipulation of such $D$-level systems have been improved over the last ten years, but their reliable transmission between remote locations remains the main challenge. Here, we show how a recent air-core fiber supporting orbital angular momentum (OAM) modes can be exploited to faithfully transmit $D$-dimensional states. Four OAM quantum states and their superpositions are created, propagated in a 1.2 km long fiber and detected with high fidelities. In addition, three quantum key distribution (QKD) protocols are implemented as concrete applications to assert the practicality of our results. This experiment enhances the distribution of high-dimensional quantum states, attesting the orbital angular momentum as vessel for the future quantum network.

12 mode, WDM, MIMO-free orbital angular momentum transmission

Simultaneous MIMO-free transmission of 12 orbital angular momentum (OAM) modes over a 1.2 km air-core fiber is demonstrated. WDM compatibility of the system is shown by using 60, 25 GHz spaced WDM channels with 10 GBaud QPSK signals. System performance is evaluated by measuring bit error rates, which are found to be below the soft FEC limit, and limited by inter-modal crosstalk. The crosstalk in the system is analyzed, and it is concluded that it can be significantly reduced with an improved multiplexer and de-multiplexer.

Reduced thermal sensitivity of hybrid air-core photonic band-gap fiber ring resonator

A novel hybrid air-core photonic band-gap fiber (PBF) ring resonator with twin 90° polarization-axis rotated splices is proposed and demonstrated. Frist, we measure the temperature dependent birefringence coefficient of air-core PBF and Panda fiber. Experimental results show that the relative temperature dependent birefringence coefficient of air-core PBF is 1.42×10-8/°C, which is typically ~16 times less than that of Panda fiber. Then, we extract the geometry profile of air-core PBF from scanning electron microscope (SEM) images. Numerical modal is built to distinguish the fast axis and slow axis in the fiber. By precisely setting the length difference in air-core PBF and Panda fiber between two 90° polarization-axis rotated splicing points, the hybrid air-core PBF ring resonator is constructed, and the finesse of the resonator is 8.4. Environmental birefringence variation induced by temperature change can be well compensated, and experimental results show an 18-fold reduction in thermal sensitivity, compared with resonator with twin 0° polarization-axis rotated splices.

Encoding and decoding by the states of vector modes for vortex beams propagating in air-core fiber

In order to enhance the channel capacity and spectrum efficiency, the technology of space division multiplexing (SDM) has become research hotspot in optical communications. In this paper, a new encoding/decoding concept is proposed by the states of vector modes (polarized direction, rotational direction of phase and topological charge) for vortex beam propagating. To support encoded vector modes propagating in fiber, an OAM fiber with air-core structure is designed for encoding/decoding. Meanwhile, a new mode recognition method of judging states of the received vector modes is presented by the technology of digital image processing and digital signal processing (DSP). To verify the feasibility of encoding/decoding, an experimental platform to verify that encoded 16-QAM signal (0010_1100_1110_1010) is established. The vector modes of OAM beams can be propagated in OAM fiber with the length of 80 cm, and the received signal can be decoded to 16-QAM by image processing successfully. In addition, we also evaluate and analyze the influence factor (OAM fiber length and bit rate) on the transmitting performance in terms of BER, crosstalk and constellation figures.

Investigation of modal-interference-induced fiber optic refractive index sensor: markedly enhanced sensitivity realized by shining an optical vortex beam.

A highly sensitive multimode-interference-based refractive index sensor is reported here by shining an optical vortex beam. The sensor probe is formed by splicing a length of no-core fiber in between two air-core vortex fibers. The coupling characteristics of various modes inside the sensor and their effect on sensing properties are numerically analyzed. Simulation results show that the sensing scheme proffers a maximum sensing resolution of 7.59×10-6 and 4.18×10-6 RIU for no-core fiber length of 29.40 and 44.60mm, respectively. Because of its high sensitivity, the study has potential applications in the chemical and biological sensing fields.

Multiband supercontinuum generation in an air-core revolver fibre

Multiband supercontinuum generation in an air-core revolver fibre having a large number of transmission bands in a wide spectral range has been studied experimentally and theoretically for the first time. The fibre fabricated by us possesses unique dispersion and guidance characteristics for radiation transfer from one band to another despite the high losses at the band boundaries. In our experiments, launching 205-fs laser pulses of 110 μJ energy at 1028 nm into the fibre we have obtained a supercontinuum spanning the spectral range from 415 to 1593 nm, with 11 transmission bands. Numerical simulation suggests that, in the case of single-mode propagation of pulses with such energy in the fibre, the supercontinuum may span 14 transmission bands and have a spectral width above three octaves, with a long-wavelength edge at 4200 nm.

All-Fiber Fused Coupler for Stable Generation of Radially and Azimuthally Polarized Beams

The stable generation of TE01 and TM01 beams is demonstrated through a novel all-fiber fused coupler fabricated from a standard single mode fiber (SMF) and a custom air-core fiber. The fundamental mode in the SMF is directly coupled to the TM01 or TE01 mode by appropriately phase matching the modes in the fibers resulting in efficiency of ~67% and ~85%, and polarization purity of 70% and 82%, respectively. The phase matching is achieved through pre-tapering the SMF with precise control of the tapering ratio. The air-core fiber ensures selective excitation of the desired modes, thereby improving propagation stability and polarization quality of the TE01 and TM01 beams.

Measurement of the reflection and loss of the hybrid air-core photonic-bandgap fiber ring resonator.

A fiber ring resonator is the key element in a resonant fiber optic gyroscope (RFOG). Both reflection and loss characteristics can severely decrease the accuracy of the RFOG. This paper first implements optical frequency domain reflectometry (OFDR) to measure the reflection and loss coefficients of the hybrid air-core photonic-bandgap fiber (PBF) ring resonator. Compared with the traditional measurement method of the resonant curve, OFDR can clearly distinguish the two junctions between the air-core PBF and the solid-core fiber. The measured reflection coefficients at the two splicing points are 1.77% and 2.65%, respectively. The excess losses are 2.28dB and 3.22dB, respectively. Thus, the measurement of the junctions in the fiber ring resonator is realized.

Multifocal array with controllable orbital angular momentum modes by tight focusing

Multifocal spots arrays with controllable three-dimensional (3D) position, orbital angular momentum (OAM) and the number of focal planes along longitudinal direction have been proposed and investigated by using a special designed phase-only multi-zone plate at the back aperture of a high numerical aperture (NA) objective. By using Debye diffraction integral, the focusing field could be rewritten as a fast Fourier transform (FFT), which is the theory and principle for generating the controllable OAM-modes focusing spots. According to this theory, a hybrid multi-zone plate at the back aperture of a high NA objective is designed for the illuminations of the radially polarized Bessel–Gaussian beam. Each focusing spot in the focal plane is generated by independent parts, so the multiplexing OAM-modes could be realized by focusing two or more spots at the same position with different orthogonal eigenstates. It is a practical and flexible OAM multi-focus technique with dynamically controllable 3D position, number of focal planes, corresponding topological charges and focal lengths to meet different requirements, such as 3D optical data storage, air-core fiber coupler and parallel optical manipulation.

13.4km OAM state propagation by recirculating fiber loop.

Enabled by an enhanced effective index separation (Δneff = 1.7 × 10-4) and low transmission loss (0.8dB/km), OAM states are propagated over 13.4km in an air core fiber using a recirculating fiber loop. We observe that intermodal crosstalk decreases rapidly with increasing effective index separation, Δneff, and an order of magnitude lower crosstalk may be achieved just by doubling Δneff. We find that, in agreement with coupled power theory, our fiber has mode coupling properties analogous to elliptical core PM fibers, which yield ~10 × or more lower crosstalk than for conventional LP fiber mode orders with the same Δneff. This confirms that, for OAM modes, birefringent perturbations rather than shape perturbations matter most. In the process of performing the loop experiment, we demonstrate that OAM states in these fibers can be preserved with low loss (≤ 0.2dB) and low crosstalk (-15dB) while splicing distinct segments of the air-core fiber. For well-designed fibers, we demonstrate that OAM modes can travel distances relevant for large-scale data centers.

Small field detector correction factors: effects of the flattening filter for Elekta and Varian linear accelerators.

Flattening filter‐free (FFF) beams are becoming the preferred beam type for stereotactic radiosurgery (SRS) and stereotactic ablative radiation therapy (SABR), as they enable an increase in dose rate and a decrease in treatment time. This work assesses the effects of the flattening filter on small field output factors for 6 MV beams generated by both Elekta and Varian linear accelerators, and determines differences between detector response in flattened (FF) and FFF beams. Relative output factors were measured with a range of detectors (diodes, ionization chambers, radiochromic film, and microDiamond) and referenced to the relative output factors measured with an air core fiber optic dosimeter (FOD), a scintillation dosimeter developed at Chris O'Brien Lifehouse, Sydney. Small field correction factors were generated for both FF and FFF beams. Diode measured detector response was compared with a recently published mathematical relation to predict diode response corrections in small fields. The effect of flattening filter removal on detector response was quantified using a ratio of relative detector responses in FFF and FF fields for the same field size. The removal of the flattening filter was found to have a small but measurable effect on ionization chamber response with maximum deviations of less than ±0.9% across all field sizes measured. Solid‐state detectors showed an increased dependence on the flattening filter of up to ±1.6%. Measured diode response was within ±1.1% of the published mathematical relation for all fields up to 30 mm, independent of linac type and presence or absence of a flattening filter. For 6 MV beams, detector correction factors between FFF and FF beams are interchangeable for a linac between FF and FFF modes, providing that an additional uncertainty of up to ±1.6% is accepted.PACS number(s): 87.55.km, 87.56.bd, 87.56.Da

Excitation and separation of vortex modes in twisted air-core fiber.

An air-core fiber imposed by torsion is investigated in this paper. We refer to this kind of fiber as twisted air-core fiber (TAF). It has been demonstrated that the eigenstates of the TAF consist of guided optical vortex waves with different propagation constants of a different effective index. With the increase of the twist rate, TAF could separate the OAM modes which are near degenerate or degenerate in the air-core fiber. The separation of OAM modes in TAF is conductive to ultralong distance propagation with low crosstalk. TAF could be considered as an ideal candidate fiber for OAM based optical communication. Moreover, we investigated the twisted air-core photonic crystal fiber (TAPCF) which can improve the relative energy distribution of the OAM modes. Compared with TAF, more energy is located in the ring shaped core, which is conductive to ultralong distance propagation. TAF and TAPCF are of potential interest for increasing channel capacity in optical telecommunications, and the result is also of interest to the photonic crystal community.

Effect of Fresnel reflections in a hybrid air-core photonic-bandgap fiber ring-resonator gyro.

A novel hybrid polarization-maintaining (PM) air-core photonic bandgap fiber (PBF) ring resonator is firstly demonstrated by using a conventional solid-core PM fiber optical coupler formed by splicing a section of PM air-core PBF into the resonator. Due to Fresnel reflections exist at the two junctions between the air-core PBF and the solid-core fiber, the forward output signal of this hybrid ring resonator is the normal resonant curve with the superposition of the lightwaves that experienced even numbers of Fresnel reflections and the backward output signal is composed of lightwaves that experienced odd numbers of Fresnel reflections. Rigorous derivations of the forward and backward output signals are given out. The biggest resonant depth and finesse of the hybrid air-core PBF ring resonator predicted are 0.352 and 6.3 respectively by assuming a splice loss of 1.8 dB per junction. These predictions are finally confirmed by testing both the forward and backward output signals of the hybrid ring resonator. With the countermeasures against the influences of the odd numbers of Fresnel reflections, a bias stability of 0.007°/s is successfully demonstrated in a hybrid PM air-core PBF ring-resonator gyro.

Conservation of orbital angular momentum in air-core optical fibers

Light's orbital angular momentum (OAM) is a conserved quantity in cylindrically symmetric media; however, it is easily destroyed by free-space turbulence or fiber bends, because anisotropic perturbations impart angular momentum. We observe the conservation of OAM even in the presence of strong bend perturbations, with fibers featuring air cores that appropriately sculpt the modal density of states. In analogy to the classical reasoning for the enhanced stability of spinning tops with increasing angular velocity, these states' lifetimes with OAM magnitude. Consequently, contrary to convention wisdom that ground states of systems are the most stable, OAM longevity in air-core fiber increases with mode order. Aided by conservation of this fundamental quantity, we demonstrate fiber propagation of 12 distinct higher-order OAM modes, of which 8 remain low-loss and >98% pure from near-degenerate coupling after km-length propagation. The first realization of long-lived higher-order OAM states, thus far posited to exist primarily in vacuum, is a necessary condition for achieving the promise of higher-dimensional OAM-based classical and quantum communications over practical distances.

Design and measurement of all-rod terahertz photonic crystal fiber with air-core

An all-rod terahertz wave air-core photonic crystal fiber is designed and fabricated. Transmission spectra through the sample are measured by using terahertz time-domain spectroscopy system. Periodic transmission band and low loss of the fiber are experimentally proved. Measurement results show that the power loss of the present terahertz wave photonic crystal fiber is less than 1.9dB/cm in transmission bands. The wave confinement ability of the fiber is also demonstrated.

空芯光子晶体光纤谐振腔的设计与优化

空芯光子晶体光纤谐振腔的设计与优化

Thermal Sensitivity of the Birefringence of Air-Core Fibers and Implications for the RFOG

We present an analytical model of the drift in the resonant fiber optic gyroscope (RFOG) caused by thermally induced changes in the fiber beat length ${\rm L_{B}(T)}$ and find that the relevant metric for thermal-drift performance is not ${\rm dL_{B}/dT}$ but rather ${\rm(1/L_{B}^{2})dL_{B}/dT}$ . This disfavors conventional solid-core PM fibers for which ${\rm L_{B}}$ is very short and ${\rm dL_{B}/dT}$ is large. Because the air core of hollow-core fibers does not support the strains associated with differential thermal expansion in conventional PM fibers, the birefringence of hollow-core fibers should be more thermally stable than conventional fibers with a correspondingly smaller ${\rm (1/L_{B}^{2})dL_{B}/dT}$ . To identify the relative merits of different types of fibers in the RFOG, we measure ${\rm (1/L_{B}^{2})dL_{B}/dT}$ for a variety of air-core and solid-core fibers by recording the shift in the resonance frequencies of an all-fiber ring resonator when its temperature is changed. We find that the PM air-core fiber offers a 270-fold improvement in ${\rm (1/L_{B}^{2})dL_{B}/dT}$ coefficient compared to a Panda fiber.