Conventional Graded-index Multimode Fiber Research Articles

Design and characterization of surface relief grating on etched multimode optical fiber for refractive index sensing

We report a simple and compact optical fiber sensor based on surface relief gratings in conventional graded-index multimode optical fiber (GIMMF) for refractive index sensing. Bragg gratings on MMF are less explored compare to its single mode counterpart, even though the MMF platform possesses several advantages such as robustness, enhanced light-matter interaction and easy light coupling. In this work, surface relief gratings are directly written on one side of the cladding etched MMF core surface by using focused ion beam milling (FIB). A detailed theoretical analysis of the structure and sensing principle is analysed. The device performance as a refractive index sensor has been investigated experimentally by immersing the device in different concentration of NaCl solution. Experimental results show a sensitivity of 70 nm/RIU for a refractive index range of 1.333 to 1.345. The device is easy to fabricate with high repeatability and low cost, compact in design and competent to be used in harsh environmental conditions. The surface grating on etched MMF provides a better interaction between the sensing region and the environment which makes it a potential candidate for various chemical and biological sensing applications.

MIMO detection using a deep learning neural network in a mode division multiplexing optical transmission system

Optimum Multiple Input Multiple Output (MIMO) detector has always been a challenge in MIMO communication systems. In this paper, a novel MIMO detector has been designed using a supervised Deep Learning Neural Network (DLNN) and has been implemented successfully in a Mode Division Multiplexing (MDM) optical transmission system. A conventional Graded-Index Multi-Mode Fiber (GI-MMF) is used to design an MDM optical transmission system. We have used a DLNN for MIMO detection in MDM optical transmission system and have compared its performance with Zero Forcing (ZF) detector and Semi-Definite Relaxation Row-by-Row (SDR-RBR). The results confirm that our DLNN outruns the performance of traditional MIMO detectors.

Directly using 88-km conventional multi-mode fiber for 6-mode orbital angular momentum multiplexing transmission

Twisted light carrying orbital angular momentum (OAM), which featuring helical phase front, has shown its potential applications in diverse areas, especially in optical communications in free space and specially designed fibers, e.g. a vortex fiber. Instead of specially designed fibers extensively used in the reported OAM-based fiber transmission experiments, here we demonstrate the viability of a conventional graded-index multi-mode fiber (MMF) for OAM multiplexing transmission with less digital signal processing (DSP) complexity. We demonstrate a 120-Gbit/s quadrature phase-shift keying (QPSK) signal transmission in an 8.8-km OM4 MMF by using OAM mode multiplexing with all the modes in the first two mode-groups (OAM01L, OAM01R, OAM+11L, OAM+11R, OAM-11L, OAM-11R) with only 2×2 and 4×4 multiple-input-multiple-output (MIMO) equalization. Moreover, we demonstrate the data-carrying two OAM mode groups multiplexing transmission over the 8.8-km MMF without MIMO equalization. These demonstrations may open up new perspectives to enable the realistic use of OAM-based MMF solution in data centers and super-computers.

Passive Optical Delivering Network Using Conventional Graded-Index Multi-Mode Fiber With Mode Division Multiplexing and Sub-Carrier Multiplexing

In today's passive optical network, each optical network unit receives not only the data that belongs to its user but also the data of other users. This makes the network vulnerable to sniffing even though encryption has been done. To solve this problem, optical channels from an optical line terminal are switched directly to different user locations in such a way that the optical network unit receives only the data that belongs to its user. This paper exploits a mode-forming technique in an optical network of multi-mode fiber (MMF) using a commercially viable fiber coupler to optically route only the desired channel to the desired destination. We have successfully implemented a 2 x 2 mode-forming network, and the total channel coefficients related to each user are estimated at their locations. The feasibility of the proposed technique has been confirmed using a conventional graded-index MMF. The total channel matrix coefficients of this network are obtained by processing the transmitted training sequence in their user locations. A pre-multiple-input-multiple-output equalizer is implemented at the transmitter. The channels are successfully transmitted to their destinations, clarifying wavelength independence while maintaining data security to some extent. Furthermore, the exchange of channel in the output port is also supported by this network.

Orbital angular momentum mode groups multiplexing transmission over 2.6-km conventional multi-mode fiber.

Twisted light carrying orbital angular momentum (OAM) is a special kind of structured light that has a helical phase front, a phase singularity, and a doughnut intensity profile. Beyond widespread developments in manipulation, microscopy, metrology, astronomy, nonlinear and quantum optics, OAM-carrying twisted light has seen emerging application of optical communications in free space and specially designed fibers. Instead of specialty fibers, here we show the direct use of a conventional graded-index multi-mode fiber (MMF) for OAM communications. By exploiting fiber-compatible mode exciting and filtering elements, we excite the first four OAM mode groups in an MMF. We demonstrate 2.6-km MMF transmission using four data-carrying OAM mode groups (OAM0,1, OAM+1,1/OAM-1,1, OAM+2,1, OAM+3,1). Moreover, we demonstrate two data-carrying OAM mode groups multiplexing transmission over the 2.6-km MMF with low-level crosstalk free of multiple-input multiple-output digital signal processing (MIMO-DSP). The demonstrations may open up new perspectives to fiber-based OAM communication/non-communication applications using already existing conventional fibers.

4×10 Gbit/s bidirectional transmission over 2 km of conventional graded-index OM1 multimode fiber using mode group division multiplexing.

We demonstrate 4 × 10 Gbit/s error-free bidirectional transmission over 2 km of conventional OM1 graded-index multimode fiber using OOK modulation and direct detection. We also perform field transmission to show reach and capacity boosts on legacy multimode infrastructure. Such transmission is enabled by selective mode group division multiplexing, based on multi-plane light conversion over 4 mode groups of the multimode fiber.

Space-division-multiplexed transmission of 3x3 multiple-input multiple-output wireless signals over conventional graded-index multimode fiber.

In this paper, we experimentally demonstrate space-division-multiplexed (SDM) transmission of IEEE 802.11ac-compliant 3-spatial-stream WLAN signals over 3 spatial modes of conventional 50um graded-index (GI) multimode fiber (MMF) employing non-mode-selective 3D-waveguide photonic lantern. Two kinds of scenarios, including fiber-only transmission and fiber-wireless hybrid transmission, were investigated by measuring error vector magnitude (EVM) performance for each stream and condition number (CN) of the channel matrix. The experimental results show that, SDM-based MMF link could offer a CN< 20dB well-conditioned MIMO channel over up to 1km fiber length within 0-6GHz, achieving as low as 2.38%, 2.97% and 2.11% EVM performance for 1km MMF link at 2.4GHz, 5.8GHz, and 200m MMF link followed by 1m air distance at 2.7GHz, respectively. These results indicate the possibility to distribute wireless MIMO signals over existing in-building commercially-available MMFs with enormous cost-saving.

SLM-Based Mode Division Multiplexing System With $6 \times 6$ Sparse Equalization

We demonstrate a mode division multiplexing (MDM) system over an 8 km conventional graded index multimode fiber. Spatial light modulators (SLMs) are used to multiplex and demultiplex three linearly polarized (LP) modes (LP01, LP11a, and LP11b) in two polarizations. A 6 $ {\times }$ 6 sparse frequency domain equalizer (FDE) is used as the channel impulse response of the SLM-based MDM system is found to be sparse due to the large crosstalk at the mode MUX/DEMUX and small coupling in the fiber. The signal transmitted on each mode is recovered with improved performance over conventional FDEs. The results indicate that this system can be used in short reach transmission applications to increase the system capacity.

Miniature optical fiber refractometer using cladded multimode tapered fiber tips

Cladded multimode tapered fiber tips with mirrored end are proposed as refractometers. The working mechanism of such devices is based on radiation of the modes guided by the cladding, which is induced by the refractive index of the sample medium. Using conventional graded-index multimode fiber, a low-power 850 nm LED, and a simple detection scheme some inexpensive, robust, and versatile refractometers are demonstrated. The length of the tips is typically 2 mm, while their diameter is less than 80 μm. The refractometers can cover a refractive index range between 1.36 and 1.46 with resolution up to 3 × 10 −5.

In-line optical fiber sensors based on cladded multimode tapered fibers

The use of uniform-waist cladded multimode tapered optical fibers is demonstrated for evanescent wave spectroscopy and sensors. The tapering is a simple, low-loss process and consists of stretching the fiber while it is being heated with an oscillating flame torch. As examples, a refractive-index sensor and a hydrogen sensor are demonstrated by use of a conventional graded-index multimode optical fiber. Also, absorbance spectra are measured while the tapers are immersed in an absorbing liquid. It is found experimentally that the uniform waist is the part of the taper that contributes most to the sensor sensitivity. The taper waist diameter may also be used to adjust the sensor dynamic range.