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

Transformation from Conventional Dissipative Solitons to Amplifier Similaritons in All-Normal Dispersion Mode-Locked Fiber Lasers

We report the numerical demonstration of the transformation from dissipative solitons to amplifier similaritons in an all-normal dispersion ultrafast fiber laser for the first time. Different from the strong spectral filtering as well as large spectral and temporal breathing ratios for the typical amplifier similariton fiber lasers, our case has relatively weak spectral filtering and small spectral and temporal breath ratios (<6) for amplifier similaritons. An intermediate state between the dissipative soliton and amplifier similariton is discovered, which we call 'dissipative similariton' considering its shaping mechanism and characteristics. Pulse regime dynamics is thoroughly explored. The chirp evolution together with pulse structures in time and frequency domains can be used to distinguish the three pulse regimes. This paper can enrich the pulse dynamics in all-normal dispersion fiber lasers and help one to properly design high-energy ultrafast laser configurations

Fiber Bragg grating Fabry-Perot structures under loading and their applications in switchable multi-wavelength lasers

Characteristics of fiber Bragg grating based Fabry-Perot (FBG-FP) structures under transversal loading are investigated. A novel switchable multi-wavelength fiber laser employing loaded FBG-FP is also demonstrated

Refractive-index-modified-dot Fabry-Perot fiber probe fabricated by femtosecond laser for high-temperature sensing

An optical fiber Fabry-Perot probe sensor for high-temperature measurement is proposed and demonstrated, which is fabricated by inducing a refractive-index-modified-dot (RIMD) in the fiber core near the end of a standard single mode fiber (SMF) using a femtosecond laser. The RIMD and the SMF end faces form a Fabry-Perot interferometer (FPI) with a high-quality interference fringe visibility (<20dB). As a high-temperature sensor, such an FPI exhibits a sensitivity of 13.9 pm/°C and 18.6 pm/°C in the range of 100-500 °C and 500-1000 °C, respectively. The fabrication process of this device is quite straightforward, simple, time saving, and the sensor features small size, ease of fabrication, low cost, assembly-free, good mechanical strength, and high linear sensitivity

Dynamics of soliton explosions in ultrafast fiber lasers at normal-dispersion

We found two kinds of soliton explosions based on the complex Ginzburg-Landau equation without nonlinearity saturation and high-order effects, demonstrating the soliton explosions as an intrinsic property of the dissipative systems. The two kinds of soliton explosions are caused by the dual-pulsing instability and soliton erupting, respectively. The transformation and relationship between the two kinds of soliton explosions are discussed. The parameter space for the soliton explosion in a mode-locked laser cavity is found numerically. Our results can help one to obtain or avoid the soliton explosions in mode-locked fiber lasers and understand the nonlinear dynamics of the dissipative systems

Resonant fiber grating devices and their applications

We describe our recent progress in the fabrication and applications of fiber grating based resonant devices including in-fiber Fabry-Perot etalons and distributed Gires-Tournois etalons

Apodisation of photo-induced waveguide gratings using double-exposure with complementary duty cycles

We present a novel apodisation scheme for photo-induced waveguide gratings. The apodisation is implemented with double exposures that have reversely varying duty cycles. We have successfully applied the proposed scheme to remove the sidelobes of long period gratings (LPGs). We also observed for the first time super strong sidelobes in LPGs when creating them with only a single varying-duty-cycle exposure. The strong sidelobes can be well explained with a Mach-Zehnder interference model

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

Optical load sensors utilizing fiber-Bragg-grating based Fabry-Perot resonators

We report novel optical load sensors utilizing fiber Bragg grating based Fabry-Perot (FBG-FP) structures. Each transmission peak of a FBG-FP is observed to split into two peaks due to the birefringence effect caused by the transversal loading. Transversal loading can be measured through the wavelength separation of the peak split, which shows a good linear relationship. We also demonstrate a load sensing scheme based on the measurement of the beating frequency of a dualwavelength laser employing a FBG-FP. Such a scheme also provides a new way to generate microwave signals