Auto-phase-locked measurement of time-gated luminescence spectra with a microsecond delay

Time-resolved techniques are widely used in measuring the spectra and lifetimes of the excited states of molecules. However, the relative apparatus always requires gated detector and phase-matching circuitry, which is expensive to implement and maintain. Herein, a novel auto-phase-locked method for time-gated luminescence (TGL) spectra measurement was developed by adjusting the exciting and detecting optical paths to pass through the same chopper wheel, which simultaneously acted as a pulse generator and detecting shutter. This low-cost system needs no phase-matching circuitry or control system. It can detect TGL spectra with a delay time of only microseconds, demonstrating a high temporal resolution for thermally activated delayed fluorescence detection.

High-order soliton evolution and pulse breaking-recovery in stretched ultrafast fiber lasers

We present a new pulse regime in a stretched ultrafast fiber laser based on numerical simulations. The pulse breaking due to high-order soliton evolution in the passive fiber is recovered to a smooth pulse in the gain fiber with normal dispersion. The new pulse regime formed by the two nonlinear processes makes the ultrafast fiber laser generate ultra-broadband, ultrashort duration, high energy and large breathing ratio pulses. Our work gives insights into the nonlinear dynamics in fiber lasers and has potential for a better design of the stretched fiber lasers.

Highly sensitive torsion sensor with femtosecond laser-induced low birefringence single-mode fiber based Sagnac interferometer

A highly sensitive optical fiber torsion sensor with femtosecond laser-induced low birefringence SMF-based Sagnac interferometer (SI) is proposed and experimentally demonstrated in this paper. A straight-line waveguide positioned horizontally with respect to the fiber core is inscribed by the femtosecond laser in the cladding of the SMF, which leads to the asymmetry stress distribution in the SMF, and then gives rise to the low birefringence in the SMF. Compared with most of the previous reported SI based torsion sensors, there is no splicing joint in the femtosecond laser-induced low birefringence SMF-based SI, which lowers the transmission loss and makes the SI based torsion sensor more robust simultaneously. The experiment result shows that the proposed torsion sensor exhibits a torsion sensitivity of up to 3.2562 nm/degree, with the high torsion resolution of 0.003 degree. In contrast, the temperature cross-sensitivity and strain cross-sensitivity of the proposed torsion sensor are low, to -0.000055 degree/°C and 0.000013 degree/με, respectively, thus overcoming the cross-sensitivity problem resulting from temperature and strain. Moreover, theoretical analysis are carried out to compare with the experimental results to demonstrate the feasibility and good consistency.

Compact assembly-free vector bend sensor based on all-in-fiber-core Mach-Zehnder interferometer

A novel low-cost, compact, assembly-free and sensitive optical fiber curvature sensor is presented. This device consists of an off-axis positive refractive index modified zone (PRIMZ), induced by a direct femtosecond laser, written in a single-mode fiber (SMF) core. The PRIMZ transforms the original SMF section into a few-mode fiber (FMF). As a result, the whole fiber forms an assembly-free "SMF-FMF-SMF" sandwich Mach-Zehnder interferometer. When the device is bent, a direction-dependent spectral shift of the interference pattern is produced. The sensitivity of the sensor is up to 2.53 and 2.24  nm/m^-1 for the 0° and 180° orientations in a wide bend range (from 0 to 4  m^-1). In addition, the device is immune to the surrounding refractive index and has a low-temperature crosstalk, which makes it very attractive for practical structural monitoring applications.

Comparison of two kinds of skin imaging analysis software: VISIA® from Canfield and IPP® from Media Cybernetics

BACKGROUND

Skin imaging analysis, acting as a supplement to noninvasive bioengineering devices, has been widely used in medical cosmetology and cosmetic product evaluation. The main aim of this study is to assess the differences and correlations in measuring skin spots, wrinkles, vascular features, porphyrin, and pore between two commercially available image analysis software.

MATERIALS AND METHODS

Seventy healthy women were included in the study. Before taking pictures, the dermatologist evaluated subjects' skin conditions. Test sites included the forehead, cheek, and periorbital skin. A 2 × 2 cm cardboard was used to make a mark on the skin surface. Pictures were taken using VISIA^® under three kinds light conditions and analyzed using VISIA^® and IPP^® respectively.

RESULTS

(1) Skin pore, red area, ultraviolet spot, brown spot, porphyrin, and wrinkle measured with VISIA^® were correlated with those measured with IPP^® (P < .01). (2) Spot, wrinkle, fine line, brown spot, and red area analyzed with VISIA^® were correlated with age on the forehead and periorbital skin (P < .05). L-value, Crow's feet, ultraviolet spot, brown spot, and red area analyzed with IPP^® were correlated with age on the periorbital skin (P < .05). (3) L-value, spot, wrinkle, fine line, porphyrin, red area, and pore analyzed with VISIA^® and IPP^® showed correlations with the subjective evaluation scores (P < .05).

CONCLUSIONS

VISIA^® and IPP^® showed acceptable correlation in measuring various skin conditions. VISIA^® showed a high sensibility when measured on the forehead skin. IPP^® is available as an alternative software program to evaluate skin features.

Continuous-wave-induced resonant spectral sidebands in soliton fiber lasers

We observed the resonant sidebands similar to the Kelly sidebands, but caused by the coupling between continuous-wave (CW) and dispersive waves (DWs) in a mode-locked ring fiber laser for the first time, to the best of our knowledge. The coupling between the CW and DWs is a linear process, which is different from the previously observed parametric sub-sidebands. The regimes of the CW-induced sidebands are discussed. The experimental results agree both qualitatively and quantitatively with theoretical analysis. The results enrich the nonlinear dynamics of ultrafast fiber lasers and may have potential in optically controlling mode-locked lasers.

Sensitive refractive index sensor based on an assembly-free fiber multi-mode interferometer fabricated by femtosecond laser

We propose and demonstrate a highly sensitive refractive index (RI) sensor based on a novel fiber-optic multi-mode interferometer (MMI), which is formed with a femtosecond-laser-induced in-core negative refractive index modified line in a standard single mode fiber. The proposed MMI structure is directly written with femtosecond laser in one step, which removes the splicing process needed in conventional MMI fabrication and also significantly improves the robustness. This device exhibits a high sensitivity to surrounding refractive index, with a maximum sensitivity up to 10675.9 nm/RIU at the RI range of 1.4484-1.4513. The distinct advantages of high sensitivity, compact, robust and assembly-free all-fiber structure make it attractive for real physical, chemical and biological sensing.

Primary antibiotic resistance of Helicobacter pylori in Chinese patients: a multiregion prospective 7-year study

OBJECTIVES

To explore the characteristics of Helicobacter pylori resistance in China and the association between antibiotic resistance and several clinical factors.

METHODS

H. pylori strains were collected from patients in 13 provinces or cities in China between 2010 and 2016. Demographic data including type of disease, geographic area, age, gender and isolation year were collected to analyse their association with antibiotic resistance. Antibiotic resistance was detected using the Etest test and the Kirby-Bauer disc diffusion method.

RESULTS

H. pylori were successfully cultured from 1117 patients. The prevalence of metronidazole, clarithromycin (CLA), azithromycin, levofloxacin (LEV), moxifloxacin, amoxicillin (AMO), tetracycline and rifampicin resistance was 78.2, 22.1, 23.3, 19.2, 17.2, 3.4, 1.9 and 1.5%, respectively. No resistance to furazolidone was observed. The resistance rates to LEV and moxifloxacin were higher in strains isolated from patients with gastritis compared to those with duodenal ulcer and among women. Compared to patients ≥40 years old, younger patients exhibited lower resistance rates to CLA, azithromycin, LEV and moxifloxacin. The resistance rates to CLA and AMO were higher in strains isolated more recently, and we also found that the prevalence of resistance to metronidazole, CLA, azithromycin and AMO were significantly different among different regions of China.

CONCLUSIONS

The resistance rates to metronidazole, CLA and LEV were high in China. Patient age, gender, disease and location were associated with the resistance of H. pylori to some antibiotics. Furazolidone, AMO and tetracycline are better choices for H. pylori treatment in China.

Ultra-compact all-in-fiber-core Mach-Zehnder interferometer

Optical Mach-Zehnder interferometers (MZIs) are useful components in a variety of optical applications, including optical modulation; signal processing; and physical, chemical, and biological sensing. We introduce here a novel, assembly-free all-in-fiber-core MZI, which is directly written with a femtosecond laser. By introducing a positive refractive index-modified zone in half of the fiber core, the original single-mode fiber section is converted into a few-mode fiber section, where a strong coupling between the two lowest-order guided modes is generated, resulting in a well-defined interference spectrum in transmission. This device promises many significant advantages over existing approaches such as ease of fabrication, stability, small insertion loss, robustness extremely broad operating bandwidth, and precise and controllable cavity lengths. These advantages make this device strikingly attractive with the potential for extensive adoption in fiber communications, signal processing, sensors, and laser wavelength control.

Sensitivity enhanced fiber sensor based on a fiber ring microwave photonic filter with the Vernier effect

A temperature sensor employing the Vernier effect generated from a cascaded fiber rings based microwave photonic filter (MPF) is proposed and experimentally demonstrated. The structure of the fiber ring is used as a sensing element as well as the sampling and delaying component of the MPF in our proposed sensing scheme. The sensing characteristics of both single ring and cascaded fiber rings based sensors have been studied and compared. By employing two cascaded fiber rings of slightly different length, the Vernier effect can be generated in the frequency response of the MPF. The sensing interrogation of the cascaded fiber rings based sensor is conducted by detecting the frequency shift of the upper envelope of the measured frequency response curve. The experimental results show that the sensitivity of the cascaded fiber rings based sensor can be improved about 30 times compared with the single fiber ring based temperature sensor.

Design of an all-optical fractional-order differentiator with terahertz bandwidth based on a fiber Bragg grating in transmission

All-optical fractional-order temporal differentiators with bandwidths reaching terahertz (THz) values are demonstrated with transmissive fiber Bragg gratings. Since the designed fractional-order differentiator is a minimum phase function, the reflective phase of the designed function can be chosen arbitrarily. As examples, we first design several 0.5th-order differentiators with bandwidths reaching the THz range for comparison. The reflective phases of the 0.5th-order differentiators are chosen to be linear phase, quadratic phase, cubic phase, and biquadratic phase, respectively. We find that both the maximum coupling coefficient and the spatial resolution of the designed grating increase when the reflective phase varies from quadratic function to cubic function to biquadratic function. Furthermore, when the reflective phase is chosen to be a quadratic function, the obtained grating coupling coefficient and period are more likely to be achieved in practice. Then we design fractional-order differentiators with different orders when the reflective phase is chosen to be a quadratic function. We see that when the designed order of the differentiator increases, the obtained maximum coupling coefficient also increases while the oscillation of the coupling coefficient decreases. Finally, we give the numerical performance of the designed 0.5th-order differentiator by showing its temporal response and calculating its cross-correlation coefficient.

High-sensitivity and large-dynamic-range refractive index sensors employing weak composite Fabry-Perot cavities

Most sensors face a common trade-off between high sensitivity and a large dynamic range. We demonstrate here an all-fiber refractometer based on a dual-cavity Fabry-Perot interferometer (FPI) that possesses the advantage of both high sensitivity and a large dynamic range. Since the two composite cavities have a large cavity length difference, one can observe both fine and coarse fringes, which correspond to the long cavity and the short cavity, respectively. The short-cavity FPI and the use of an intensity demodulation method mean that the individual fine fringe dips correspond to a series of quasi-continuous highly sensitive zones for refractive index measurement. By calculating the parameters of the composite FPI, we find that the range of the ultra-sensitive zones can be considerably adjusted to suit the end requirements. The experimental trends are in good agreement with the theoretical predictions. The co-existence of high sensitivity and a large dynamic range in a composite FPI is of great significance to practical RI measurements.

Compact eccentric long period grating with improved sensitivity in low refractive index region

We demonstrate a compact eccentric long period grating with enhanced sensitivity in low refractive index region. With a period designed at 15 µm for coupling light to high order cladding modes, the grating is more sensitive to surrounding refractive index in low refractive index region. The intrinsically low coupling coefficients for those high order cladding modes are significantly improved with the eccentric localized inscription induced by the femtosecond laser. The fabricated grating is compact with a length of 4.05 mm, and exhibits an average sensitivity of ~505 nm/RIU in low refractive index region (1.3328-1.3544). The proposed principle can also work in other refractive index region with a proper choice of the resonant cladding modes.

Ultra-sensitive refractive index sensor based on an extremely simple femtosecond-laser-induced structure

We demonstrate here an extremely simple, compact, and robust refractive index (RI) probe sensor based on a femtosecond-laser induced refractive index-modified dot (RIMD) fabricated near the end face of a single-mode fiber. The RIMD and the fiber end face form a Fabry-Perot interferometer, which is highly sensitive to surrounding RI. The fabrication process of the RIMD involves only one step and takes ∼0.1  s, which is extremely short, compared with other techniques. The proposed sensor exhibits an ultra-high sensitivity of ∼2523.2  dB/RIU at an RI of 1.435, which is one to two orders of magnitude higher than that of the existing intensity-modulated RI sensors. Moreover, the proposed sensor has the distinct advantages of compact size (∼50  μm), easy fabrication, and no temperature cross-sensitivity. The advantages of the device make it a promising candidate for applications in designing highly sensitive sensors in a biochemical and environmental measurement field.

Intensity modulated torsion sensor based on optical fiber reflective Lyot filter

We proposed and experimentally demonstrated a highly sensitive optical fiber torsion sensor based on a reflective Lyot filter for the first time to our knowledge. The reflective Lyot filter is constructed by inserting a section of polarization-maintaining fiber (PMF) between a fiber linear polarizer and a 3dB coupler based fiber loop reflector. Based on the intensity modulation, the proposed torsion sensor exhibits a high torsion sensitivity of up to 20.336 dB/rad, one order of magnitude higher than the achieved in state-of-the-art. In contrast, the temperature cross-sensitivity and strain cross-sensitivity of the proposed torsion sensor are low to -2.0 × 10^-4 rad/°C and -6.39 × 10^-6 rad/με, respectively, thus overcoming the cross-sensitivity problem resulting from temperature and strain. Moreover, we perform the theoretical simulation of the proposed torsion sensor, and the simulation result obtained agrees well with the experiment results, vividly confirming the viability of the fiber Lyot filter based torsion sensor. Such fiber Lyot filter may find potential applications of highly sensitive torsion sensors in the field of modern smart structure monitoring.

Numerical simulations of fast-axis instability of vector solitons in mode-locked fiber lasers

We demonstrate the fast-axis instability in mode-locked fiber lasers numerically for the first time. We find that the energy of the fast mode will be transferred to the slow mode when the strong pump strength makes the soliton period short. A nearly linearly polarized vector soliton along the slow-axis could be generated under certain cavity parameters. The final polarization of the vector soliton is related to the initial polarization of the seed pulse. Two regimes of energy exchanging between the slow mode and the fast mode are explored and the direction of the energy flow between two modes depends on the phase difference. The dip-type sidebands are found to be intrinsic characteristics of the mode-locked fiber lasers under high pulse energy.

Small-period long-period fiber grating with improved refractive index sensitivity and dual-parameter sensing ability

We UV inscribe and characterize a long-period fiber grating with a period of 25 μm. A series of polarization-dependent dual-peak pairs can be seen in the transmission spectrum, even though only the symmetrical refractive index modification is introduced. The fabricated grating exhibits a lower temperature sensitivity compared with standard long-period gratings and an enhanced refractive index sensitivity of ∼312.5  nm/RIU averaged from 1.315 to 1.395, which is more than four-fold higher than standard long-period gratings in this range. The full width at half-maximum of the fabricated grating is only about 0.6 nm, allowing for high-resolution sensing. Moreover, the grating period is so small that the attenuation dip corresponding to a high-order Bragg resonance can also be seen, which can act as a monitor of the unwanted perturbation to realize dual-parameter sensing.

Ultra-compact strain- and temperature-insensitive torsion sensor based on a line-by-line inscribed phase-shifted FBG

A novel temperature- and strain-independent optical fiber torsion sensor based on a phase-shifted fiber Bragg grating (PSFBG) inscribed by the line-by-line (LbL) technique in a standard single-mode fiber with a femtosecond laser has been proposed and experimentally demonstrated. The strong birefringence created by the LbL inscription technique leads to the significant polarization splitting of the transmission peak of the PSFBG. By simply monitoring the variation of the amplitude difference between the two polarization-peaks, the fiber torsion angle and the fiber torsion direction can be simultaneously deduced without temperature and strain confusion. The torsion sensor exhibits a high torsion sensitivity of up to -1032.71 dB/(rad/mm), with the distinct advantages of low manufacture cost, small dimension (just ~1.72mm), and extremely robust and simple structure, which make it very attractive for practical applications. To the best of our knowledge, this is the smallest torsion sensor ever reported.

Fabrication of surface nanoscale axial photonics structures with a femtosecond laser

Surface nanoscale axial photonics (SNAP) structures are fabricated with a femtosecond laser for the first time, to the best of our knowledge. The inscriptions introduced by the laser pressurize the fiber and cause its nanoscale effective radius variation. We demonstrate the subangstrom precise fabrication of individual and coupled SNAP microresonators having the effective radius variation of several nanometers. Our results pave the way to a novel ultraprecise SNAP fabrication technology based on the femtosecond laser inscription.

All-fiber passively mode-locked laser based on a chiral fiber grating

A novel passively mode-locked all-fiber laser using a chiral fiber grating as a polarization-selective element is demonstrated for the first time, to the best of our knowledge. The chiral fiber grating serves as a key component to form an artificial saturable absorber to realize mode locking through nonlinear polarization rotation in the cavity. The laser generates stable short pulses with energy of 0.34 nJ, a fundamental repetition rate of 3.27 MHz, and an FWHM bandwidth of 28 nm. We also show that harmonic mode-locked pulse trains of different orders can be obtained by increasing the pump power.