Junfa Zhao

Refractive Index Fiber Laser Sensor by Using Tunable Filter Based on No-Core Fiber

A novel refractive index fiber laser sensor based on a fiber Bragg grating (FBG) integrated with a section of no-core fiber (NCF) is proposed and experimentally demonstrated. The oscillating wavelength of the fiber laser is only decided by the central wavelength of the FBG that is insensitive to the surrounding refractive index (SRI). However, the output power of the fiber laser varies with the SRI because the laser output is filtered by the edge of the bandpass filter (BPF) formed by the multimode interference (MMI) effect in the NCF, which is sensitive to the SRI. By measuring the variation of the output power, a cost-effective power detection RI sensor could be realized. The fiber laser sensor has a linear relationship with SRI and a sensitivity of 113.73 dB/RIU in the RI range of 1.333-1.4076.

Curvature and temperature sensor based on bulge-taper structures interferometer with embedded fiber Bragg grating

Abstract. A sensor head consisting of an all single-mode fiber (SMF) in-line Mach–Zehnder interferometer (MZI) with an embedded fiber Bragg grating (FBG) is proposed and experimentally demonstrated for simultaneous measurement of curvature and temperature. It is fabricated by cascading two bulge-taper fusion structures in a section of SMF including an FBG. The MZI is sensitive to fiber bending and ambient temperature with a sensitivity of −16.59  nm/m−1 in the range of 1.05 to 4.05  m−1 and 58  pm/°C in the range of 30°C to 100°C, respectively. However, the FBG is only sensitive to the latter with a sensitivity of 13  pm/°C. Simultaneous measurement of curvature and temperature is obtained and the cross-sensitivity issue can be solved. The experimental results show that the average relative error of the curvature is 0.38%, which is about 18 times better than that without temperature compensating. The average error of temperature is only 0.21°C.

Widely tunable ultra-narrow linewidth single-longitudinal-mode Brillouin fiber laser with low threshold

A widely tunable ultra-narrow linewidth single-longitudinal-mode Brillouin fiber laser (SLM–BFL) with low threshold is proposed and experimentally demonstrated by using a 205 m long high-nonlinear fiber (HNLF) as Brillouin gain media. The single-longitudinal-mode (SLM) operation is guaranteed by cascaded filters composed of a fiber ring Fabry–Perot filter and a self-induced narrow filter based on a 4 m unpumped erbium-doped fiber. Owing to the long ring cavity, the BFL has a low threshold of 12.8 mW and ultra-narrow linewidth of 0.29 kHz. The BFL can be tuned from 1532 to 1577 nm. The optical signal-to-noise ratio of the fiber laser is higher than 70 dB during the 45 nm tune range.

Double Brillouin frequency spaced multiwavelength Brillouin-erbium fiber laser with 50 nm tuning range

A 50 nm tuning range multiwavelength Brillouin-erbium fiber laser (MWBEFL) with double Brillouin frequency spacing is presented. Two separated gain blocks with symmetrical architecture, consisted by erbium-doped fiber amplifiers (EDFAs) and Brillouin gain media, are used to generate double Brillouin frequency spacing. The wider tuning range is realized by eliminating the self-lasing cavity modes existing in conventional MWBEFLs because of the absence of the physical mirrors at the ends of the linear cavity. The Brillouin pump (BP) is preamplified by the EDFA before entering the single-mode fiber (SMF), which leads to the reduction of threshold power and the generation enhancement of Brillouin Stokes (BS) signals. Four channels with 0.176 nm spacing are achieved at 2 mW BP power and 280 mW 980 nm pump power which can be tuned from 1525 to 1575 nm.

Multipoint refractive index and temperature fiber optic sensor based on cascaded no core fiber-fiber Bragg grating structures

Abstract. A multipoint fiber optic sensor based on two cascaded multimode interferometer (MMI) and fiber Bragg grating (FBG) structures is proposed and demonstrated for simultaneous measurement of refractive index (RI) and temperature. The MMI is fabricated by splicing a section of no-core fiber (NCF) with two single-mode fibers. The suitable NCF lengths of 19.1 and 38.8 mm are selected by simulations to achieve wavelength division multiplexing. The two MMIs are sensitive to RI and temperature with the maximal RI sensitivities of 429.42228 and 399.20718  nm/RIU in the range of 1.333 to 1.419 and the temperature sensitivities of 10.05 and 10.22  pm/°C in the range of 26.4°C to 100°C, respectively. However, the FBGs are only sensitive to the latter with the sensitivities of 10.4 and 10.73  pm/°C. Therefore, dual-parameter measurement is obtained and cross-sensitivity issue can be solved. The distance between the two sensing heads is up to 12 km, which demonstrates the feasibility of long-distance measurement. During measurement, there is no mutual interference to each sensing head. The experimental results show that the average errors of RI are 7.61×10−4  RIU and 6.81×10−4  RIU and the average errors of temperature are 0.017°C and 0.012°C, respectively. This sensor exhibits the advantages of high RI sensitivity, dual-parameter and long-distance measurement, low cost, and easy and repeatable fabrication.

Differential Intensity Modulation Refractometer Based on SNS Structure Cascaded Two FBGs

A differential intensity modulation refractometer based on single-mode-no-core-single-mode (SNS) structure cascaded with two fiber Bragg gratings (FBGs) is proposed. The SNS structure acts as a band pass fiber (BPF), and the Bragg wavelengths of the two FBGs located at each edge of the BPF. Because the interference spectrum of the SNS is sensitive to refractive index (RI) but FBG is not, the reflective powers of the FBGs are modulated by the surrounding refractive index (SRI) and have contrary changing trends. The reflective intensity difference of the FBGs is used to describe the change of the SRI, which not only increases the RI sensitivity but diminishes the influence of the input light fluctuation as well. In addition, as the SNS and the FBGs have approximate temperature sensitivities, this sensor can accomplish temperature self-compensation. The experiments show that the RI sensitivity of this sensor is about –199.6 dB/RIU in the range of 1.3326–1.3702 and –355.5 dB/RIU in the range of 1.3702–1.4066, which is greater than the traditional SNS-FBG structure. However, its temperature sensitivity is only 0.0148 dB/°C. When the input light varies 7 mW, the shift of the intensity difference is only 0.28 dB, which is much smaller than the intensity shift of the single FBG.

A switchable single- and dual-wavelength erbium fiber ring laser in single-longitudinal-mode operation

A simple stable single-longitudinal-mode (SLM) single- and dual-wavelength erbium-doped fiber ring laser using an arrayed waveguide grating as the wavelength filter is proposed and demonstrated. SLM operation is guaranteed by a saturable-absorber-based auto-tracking filter. A 40/60 optical coupler is used to form two imbalanced loss cavities in the fiber laser initially, and single- and dual-wavelength operation can be changed by properly modifying the variable optical attenuator. The side mode suppression ratios of the laser outputs are higher than 60 dB. Through switching the two fiber optical switches, the dual-wavelength spacing is tuned from the narrowest 0.82 nm to the widest 12.05 nm, which offers potential uses in the generation of terahertz (THz) radiation.

A widely tunable double-Brillouin-frequency spaced multiwavelength fiber laser with a 110 nm tuning range

A wideband double-Brillouin-frequency spaced multiwavelength Brillouin?erbium fiber laser (MWBEFL) with 110 nm tuning range is demonstrated. The fiber laser utilizes simple compound-ring cavity structures which confine the odd-order Brillouin?Stokes (BS) signals within the right ring and couple out the initial Brillouin pump signal (BP) and even-order BS signals to generate a 0.176?nm spacing multiwavelength. A wavelength-?and bandwidth-tunable optical band-pass filter (TBF) is used to manipulate the location of self-lasing cavity and to get a wideband tuning range. All the generated output channels exhibit good stability.

Analysis of Hollow Fiber Temperature Sensor Filled with Graphene-Ag Composite Nanowire and Liquid

A hollow fiber temperature sensor filled with graphene-Ag composite nanowire and liquid is presented and numerically characterized. The coupling properties and sensing performances are analyzed by finite element method (FEM) using both wavelength and amplitude interrogations. Due to the asymmetrical surface plasmon resonance sensing (SPR) region, the designed sensor exhibits strong birefringence, supporting two separate resonance peaks in orthogonal polarizations. Results show that x-polarized resonance peak can provide much better signal to noise ratio (SNR), wavelength and amplitude sensitivities than y-polarized, which is more suitable for tempertature detecting. The graphene-Ag composite nanowire filled into the hollow fiber core can not only solve the oxidation problem but also avoid the metal coating. A wide temperature range from 22 ∘C to 47 ∘C with steps of 5 ∘C is calculated and the temperature sensitivities we obtained are 9.44 nm/∘C for x-polarized and 5.33 nm/∘C for y-polarized, much higher than other sensors of the same type.

Label-free assessment of replicative senescence in mesenchymal stem cells by Raman microspectroscopy

Here, Raman microspectroscopy was employed to assess replicative senescence of mesenchymal stem cells (MSC). A regular spectral change related to the cell senescence was found in the ratio of two peaks at 1157 cm(-1) and 1174 cm(-1), which are assigned to C-C, C-N stretching vibrations in proteins and C-H bending vibrations in tyrosine and phenylalanine, respectively. With the cell aging, the ratio I1157 / I1174 exhibited a monotonic decline and showed small standard deviations, so that it can statistically distinguish between cells having slight changes in terms of aging. We propose that I1157 / I1174 can act as a characteristic spectral signature for label-free assessment of MSC senescence.