In this paper, for the first time, we propose a large-broadband orbital angular momentum (OAM) mode converter based on helical long period fiber grating (HLPG) working at turning point (TP). Owing to the combination of dual-resonance peaks at TP, an OAM mode converter with 3-dB bandwidth of ~287 nm is readily obtained, and a high coupling efficiency of ~100% is achieved. We believe that this OAM mode converter will play a key role in fiber communication and other fields.
We propose a novel strain sensing based on fiber-optic Mach-Zehnder Interferometer. A vortex beam is generated by a vortex fiber in the reference arm, and interferes with the fundamental Gaussian beam in the sensing arm to generate an interference spiral pattern. A camera is used to record the sequential frame of the spirals, and digital image processing technique is employed to recognize the rotation angle of the spirals induced by the strain under measurement. By this method, a strain resolution over 10-9 can be reached easily with a small measurement deviation. Besides, we compare the sensing characteristics for the cases of the optical vortex carrying different topological charges, and prove that the optical vortex with charge one fits our sensing application best.
In this paper, we study spectral characteristics of long-period double-helix chiral fiber gratings fabricated by a
mechanical stepping system. By discussing the effects of the step of the stepping system on the spectral characteristics,
we suggest the selection of the step in the fabrications.
We develop coupled-mode formulism for multimode couplings of LP01 core modes to LP1n cladding modes in singlehelix chiral long-period fiber gratings (S-CLPG). Then mode-coupling characteristics of the structure, such as coupling coefficients and relative bandwidths of dips are studied and compared with those of the conventional long-period fiber grating (LPG). Subsequently, transmission spectral characteristics in response to the liquid level are investigated for SCLPG partially immersing in a liquid with refractive index lower than that of the fiber cladding. Simulation results indicate that the dips corresponding to the multimode couplings in the transmission spectrum will shift continuously and monotonically as liquid level changes. Thus S-CLPG could be used for liquid level sensing based on wavelength interrogation, which is beneficial to avoid sensitivity deterioration caused by power instability in power interrogation for most conventional LPG-based liquid level sensors. In addition, we identify that the sensitivity can be enhanced with a thinner fiber cladding or by utilizing higher-order cladding mode resonance. The optimization result indicates that the wavelength shift will be over 1.03 nm per millimeter liquid level change.
We propose a way to enhance the temperature sensitivity of single-mode-multimode-single-mode (SMS) fiber structure,
by replacing the cladding of silica multimode fiber with a polymer coating. Based on the mode expansion method, we
analyze transmission characteristics of the polymer-coated SMS fiber structure and optimize design parameters for
sensing. Then we do experiments and verify theoretical analysis. By using the optimized parameters in sample
fabrication, a temperature sensor is obtained with an easily detected and demodulated transmission spectrum; and then
by monitoring valley wavelength shifts, a measuring sensitivity of about 706 pm/°C is achieved.
A simple temperature sensor based on a bent singlemode–multimode–singlemode (SMS) fiber structure fastened on
a polymer base plate is proposed and experimentally investigated. The surrounding refractive index (RI) is higher than
that of the silica fiber and RI changes with temperature will not lead to wavelength shift. This SMS fiber structure
utilizes changing of temperature to control the curvature of SMS fiber which is induced by expanding of polymer base
plate. The shifts of central wavelength are measured at temperature range from 59 to 82 °C. The proposed temperature
sensor offers sensitivity of 3.9 nm/°C, which is significantly higher than that of a normal straight SMS structure or a
grating-based fiber structure.
A bidirectional single-longitudinal-mode Er-doped fiber ring laser was proposed. Based on it, a fiber laser gyroscope was
demonstrated. With circularly polarized lights in the Er-doped fiber, and linearly polarized lights in the rest part of the
ring, a reciprocal structure was obtained, and the spatial hole burning effect was suppressed. In the experiments, the
lock-in threshold rotation rate was 6 o/s. When the rotation rate was beyond 6 o/s, a beat RF signal of over 30-dB noise
was observed and a good linear relation between the beat frequency shift and cavity rotation rate were obtained. Finally,
the beat frequency stability was tested for 20 o/s clockwise cavity rotation.
Mode couplings of the core modes to cladding modes with both lower and higher angular orders in chiral long period
gratings (CLPGs) are analyzed. The sensing sensitivity of the resonant wavelength in response to strain and temperature
are demonstrated, respectively. Both single- and double-helix CLPGs formed by silica and polymer fiber are considered.
This provides useful guidance for practical designs of CLPGs to enhance the optical sensing sensitivity of the strain and
temperature, or to decrease the environmental influence on spectral characteristics of CLPGs.
A novel long-period fiber grating (LPG) refractive index sensor is presented for the measurements of ambient refractive
indices which are higher than that of the fiber cladding. As the measurement parameter, the transmission power of the
core mode is interrogated at a single wavelength where the core mode and a leaky mode are phase-matched. Firstly, the
couplings of the core mode to leaky modes in the novel structure are analyzed by using complex coupled-mode theory,
and then classified into three cases analogous to those in the damped oscillations. The power evolutions of the core mode
in the couplings are thus intuitively understood. Based on these, for the first time, we demonstrate, with optimized design
parameters the transmission power of the core mode is rather sensitive to the change of a higher ambient refractive index
at resonant wavelengths. Then we focus on two optimization objectives. One is to enlarge the operational range while
keeping given sensitivity, the other is to enhance the sensitivity within a given operational range. Finally, we demonstrate
the operational range of the LPG refractive index sensor can be from 1.46 to 1.7 with a sensitivity of ~10-4, while for a
given operational range from 1.455 to 1.465, the sensitivity can be ~10-6, if assuming the dynamic range of the power is
less than 30dB and the measurement resolution is 0.01dB for both cases.
A highly efficient and accurate full-vector finite-difference mode solver is developed for calculation of both guided and
leaky modes of circular symmetric optical fibers with high index contrast. To demonstrate and validate the mode solver,
several examples, such as the guided modes of a silicon wire with a large index difference, the vector properties of
guided modes in step-index fibers, as well as the quasi-guided and leaky modes in a Bragg fiber, are simulated.
A masterly two-stage structure for Erbium-doped superfluorescent fiber source (EDSFS) with high output power and broadband amplified spontaneous emission (ASE) spectrally flattened in both C-band and L-band is presented in this paper. Thereinto, the L-band ASE is generated in the first forward-pumping double-pass stage, and amplified in the second backward-pumping single-pass stage, where a high-power C-band ASE is simultaneously generated. The output ASE with a power of 16dBm and 3dB bandwidth over 61nm is obtained by simulations with the simple structure without any filter, by suitably choosing the structure parameters of the two-stage EDSFS, such as the lengths of Erbium-doped fibers (EDF) in the two stages and the beam splitting ratio of pump powers of the two stages.