A vital signs monitor based on few-mode fiber (FMF) is presented in this paper. Two types of FMF, dual-mode fiber (DMF) and four-mode fiber are respectively utilized for vital signs monitoring. For DMF, core-offset distance between lead-in single mode fiber (SMF) and DMF was optimized numerically, the results of which agree very well with experiments and respiration ratio can be measured successfully with acceptable extinction ratio. For four-mode fiber, core-offset mode excitation method with optimized parameters is also utilized to excite four types of modes, LP01, LP11, LP21 and LP02, which realizes the modal interference enhancement and the extinction ratio can reach 10 dB. Breath signals can be also detected using this structure. As demonstrated in this paper, FMF-based sensor could provide a promising candidate for fiber-optic biomedical application.
KEYWORDS: Orthogonal frequency division multiplexing, Polarization, Signal detection, Digital signal processing, Photodetectors, Modulation, Single sideband modulation, Modulators, Receivers, Optical engineering
Polarization-interleave-multiplexed (PIM) with single-sideband orthogonal frequency-division multiplexing (SSB-OFDM) based on direct detection is proposed for short-reach applications transmitted up to 80 km in which the guard band can be shared for the two SSB signals with interleave electrical center frequencies. Based on two dual-drive Mach–Zehnder modulators with one single-end photodetector (PD), 100-Gb/s PIM-SSB-OFDM transmission over a 80-km standard single-mode fiber is successfully demonstrated. After 80-km transmission, the optical signal-to-noise ratio requirement is 29.1 dB with respect to the bit error rate threshold of 7% hard decision-forward error correction overhead.
We propose a fiber-based 4-LP mode (LP01, LP11, LP21 and LP02) multi/demultiplexer, which excites or separates 4-LP mode simultaneously in the integrated structure of a single optical fiber. The structure contains a 4-mode transmitting core in the center of the fiber and the other three multi/demulitplexing parts (consist of a coupling core and several assistant cores or not) which are deployed around the transmitting core with 120 degrees between each other.
The design of the fiber-based 4-LP mode multi/demultiplexer includes the following parts. Firstly, the appropriate distance between the transmitting core and the surrounding parts are below 15μm, 12μm and 10μm for multi/demultiplexing of LP11, LP21 and LP02 mode, respectively, which can guarantee the coupling loss is less than 2%. Then we design the structural parameters (core diameter and refractive index) of single-mode coupling cores to achieve phase matching between LP01 (single-mode coupling cores) and LP11, LP21, LP02 (transmitting core), respectively. The appropriate core diameter and refractive index for the coupling cores of the multi/demultiplexing parts are 5μm and 1.465703 (LP01-LP11), 2μm and 1.479793 (LP01-LP21), 1.5μm and 1.488404 (LP01-LP02), respectively, all of which can ensure that the coupling efficiency are more than 90% in the C+L band. Afterwards, by discussing the coupling crosstalk of each multi/demultiplexing part, we find that LP02-LP01 mode multi/demultiplexing part does not need any assistant core, LP21-LP01 and LP11-LP01 mode multi/multiplexing parts need to add 1 or 2 assistant single mode core as required to decrease the coupling crosstalk. Finally by adjusting the relative position of the three multi/demultiplexing parts and the transmitting core (ensure the performance of the whole system is not affected), we can ensure that the 4 mode can be multi/demultiplexed at the same time.
Above all, we put forward a fiber-based 4-LP-mode multi/demultiplexer which excites or separates 4-LP mode simultaneously with high multi/demultiplexing efficiency, low coupling loss, and low coupling crosstalk.
We propose a kind of trench-assisted graded-index ring-core fiber (TA-GI-RCF) with a low refractive index rod deployed in the center of the core, which supports three LP modes (LP01, LP11 and LP21) transmission. There are two difficulties about designing TA-GI-RCF, one is to depart LP21 mode from LP02 mode because their effective indices are too close which makes it difficult to realize only three LP modes transmission; the other one is how to make sure these three LP modes reach the receiver end with low differential mode delay (DMD), so that the computation complexity of multi-input multi-output (MIMO) digital signal process (DSP) can be reduced. At first, we realize the separation of LP21 mode and LP02 mode in TA-GI-RCF by enlarging the size of low refractive index rod. We next investigate the influence of the TA-GI-RCF structural parameters on DMD and DMD slope, and find that a graded-index core and a low refractive index rod can flexibly tune the DMD, and a trench can flexibly control the DMD slope. Through optimizing the core parameters, we find that the design region of α is 1.01~2.23 and that of Δ1 is 0.28%~0.46% at r1=30 μm, where α is the profile exponent and Δ1 is the relative refractive index difference between core and cladding. Simulation results show that TA-GI-RCF can achieve the effective area (Aeff) of LP01 mode over 2000 μm2 and the |DMD| between LP01 mode and LP11 mode is ≤100 ps/km over C+L band. Above all, we can achieve three LP modes transmission in TA-GIRCF with low DMD over whole C+L band and large Aeff.