Mode-division multiplexers (MDMUXs) play a pivotal role in enabling the manipulation of an arbitrary optical state within few-mode fibers, offering extensive utility in the fields of mode-division multiplexing and structured optical field engineering. The exploration of MDMUXs employing cascaded resonant couplers has garnered significant attention owing to their scalability, exceptional integration capabilities, and the anticipated low insertion loss. In this work, we present the successful realization of high-quality orbital angular momentum MDMUX corresponding to topological charges 0, ±1, and ±2, achieved through the utilization of cascaded fused-biconical tapered couplers. Notably, the measured insertion losses at 1550 nm exhibit remarkable minimal values: 0.31, 0.10, and 0.64 dB, respectively. Furthermore, the 80% efficiency bandwidths exceed 106, 174, and 174 nm for these respective modes. The MDMUX is composed of precision-manufactured high-quality mode selective couplers (MSCs). Utilizing a proposed supermode propagation method based on mode composition analysis, we precisely describe the operational characteristics of MSCs. Building upon this comprehensive understanding, we embark on a pioneering analysis elucidating the influence of MSC cascading order on the performance of MDMUXs. Our theoretical investigation substantiates that when constructing MDMUXs, MSCs should adhere to a specific cascading sequence.
We propose and demonstrate the generation of multiwavelength fiber lasers via four-wave mixing in a dual wavelength Brillouin laser cavity. Using four-wave mixing effect in highly nonlinear fiber loop, the output wavelengths with100 nm range and tunable wavelength interval is achieved. In addition, due to the stimulated Brillouin effect, the linewidth of the output Brillouin laser reaches 440Hz.
A pivotal issue of the conventional optical fiber communications networks is to meet the explosively increasing requirement in data traffic. In order to meet this ever-increasing demand, there have been a lot of research and industrial development efforts to utilize the photon in various dimensions such as wavelength division multiplexing (WDM), space division multiplexing (SDM), mode division multiplexing (MDM) and so on. Fueled by emerging bandwidth-hungry applications, orbital angular momentum (OAM) modes and their multiplexing have recently gained much attention due to its special doughnut-shaped intensity distribution, as well as its unique helical phase wavefront with the theoretically infinite topological value. The OAM modes with different topological charge values are orthogonal to each other, which has provided a new degree of freedom in MDM. In this paper, we propose and design a Ge-doped air-core ring fiber, which can support numerous OAM modes. By varying the mole fraction of GeO2 and adjusting the structure parameter, including the air-core radius and the GeO2-doped ring width, we study the influence of the different fiber parameters on the total supported OAM mode number. The hollow silica fiber with a 50-μm air core and a 1.5-μm thickness of Gedoped ring is designed in simulation to support fiber eigenmodes up to HE112,1 and EH107,1. This provides 436 OAM modes at 1550 nm while maintaining radially single mode condition. Moreover, it can support more than 400 OAM modes from 1260 nm to 1625 nm, covering O, E, S, C, and L bands.
We demonstrate a new type of Mach-Zehnder interferometer by combining a nonadiabatic microtaper with a long period
grating in the same stage of all solid photonic bandgap fibers. Meanwhile, the mode-coupling caused by the microtaper is
indirectly verified by the interference with the long period grating, which is designed for the resonance between the
fundamental core mode and LP01 cladding supermodes. Finally, the temperature response is also reported.
We report a highly tunable liquid-filled photonic bandgap fiber
(LF-PBGF) based on both bend and temperature change.
By bending the LF-PBGF and changing its temperature, the blue shift of the red edge of the bandgap resulting from bend
loss and temperature increasing is speeded up, and higher bandwidth tunability is achieved. Numerical and experimental
results are presented, and 177-nm bandwidth tunability is achieved by tuning the temperature of the LF-PBGF with bend
radius of 5 mm from 40°C to 60°C, and the average compression rate is 8.85 nm/°C.
The coupling characteristics of fluid-filled dual-core photonic crystal fibers are numerically investigated. The
transmission spectrum is obtained in different temperatures. With the help of a vector finite element method we acquire
the properties of the minimum coupling length. Finally, the coupling characteristic is experimental studied by the way of
temperature tuning.
In this paper, we review advances and our works in all-solid photonic bandgap fiber gratings. Especially, we report the
fabrication and characterizations of long period fiber gratings and fiber Bragg grating arrays realized by forming
longitudinal periodic index modulation in an low-loss all-solid PBGFs with high-index Ge-doped rods surrounded by a
low index Be-doped ring lattice in the cladding. High order resonances between fundamental core mode and cladding
LP01 supermodes are demonstrated and investigated in long period fiber gratings (LPFGs) inscribed in the all-solid
photonic bandgap fiber. The fiber Bragg grating array has unusual transmission characteristics, such as large
transmission bandwidth and different transmission spectra when launching light into different Ge-doped rods, which is
very different from single fiber Bragg grating. These interesting spectral features are analyzed and interpreted.
KEYWORDS: Optical amplifiers, Optical fibers, Fiber amplifiers, High power fiber amplifiers, Fiber lasers, Pulsed laser operation, Laser systems engineering, Amplifiers, Optical isolators, High power lasers
The experimental study on an all-fiber laser pulse amplifier system using homemade double-cladding Yb-doped fibers as
gain media is presented. Secondary pulse phenomenon produced in the pulse amplifier system is analyzed and
investigated. We find that the phenomenon depends a lot on the quality of splicing between different fibers and the
amplification multiple. By controlling the pump power and amplification multiples of the amplifier and trying different
splicing parameters, we can get stable high power pulse lasing output. Finally, using the Yb-doped double-cladding
fiber(YDCF) and master oscillation power amplification (MOPA) techniques, a stable all-fiber pulse amplification
system is obtained, with lasing wavelength at 1064 nm, 20 ns line-width pulses, average power of 2.4W and repetition
rate of 50kHz.
We report on the fiber-based amplification of a commercially packaged, fiber-pigtailed-pulsed diode laser operating at
wavelength of 1064nm (bandwidth 0.45nm). The cascaded Yb-doped fiber amplifier can operate safely by using
cascaded Wavelength division multiplexing (WDM) in each single-mode amplification stage. The total isolation of
cascaded WDM can protect the laser diodes from destroyed by feedback of optical power. At repetition rate of 50kHz,
pulse duration of 20ns, and average power of 0.5mW, output power up to 1.05W And peak power up to 1.05kW are
obtained, with a high signal to noise ratio of 20dB. The whole system is a good choice for a preamplifier of high-power
short-pulse amplification. The aim of the experiment is to study the design of preamplifier for high-power short-pulse
amplifier via cascaded fiber-based amplifier.
We report on the pressure characterization of Bragg gratings in grapefruit microstructured fibers. The air holes enhance
the pressure response. The effect of air expansion in the holes on temperature response was also investigated.
Two simple and low-cost methods for achieving selective filling of air-core photonic bandgap fibers (PBGFs) are
proposed and demonstrated. In the first method, liquid paraffin was filled into a PBGF by capillary force. By a two-step
filling-cleaving process, all cladding air-holes are finally blocked but the air-core remains open. In the second method,
lateral erosion method by hydrofluoric acid was first used to make the cladding air-holes laterally open. Then, the
laterally filled liquid paraffin made all cladding air-holes blocked and left only air-core open. With these two methods,
the central hollow-core of the PBGF can be selectively filled, which allows for the fabrication of novel hybrid
functional-material-silica PBGF for various applications.
Using the plane-wave expansion method and finite element method, we investigate the properties of all-solid square-lattice
photonic bandgap fibers. The comparison of different r/Λ values in the proposed fiber is convenient to optimize
the fiber design. The simulation results demonstrate that the effective mode area of all-solid square-lattice photonic
bandgap fibers is 1.25 times larger than triangular-lattice ones and the confinement loss of the fibers is no more than
0.1dB/m within the selected bandgap.
We propose a novel design of dual-concentric-core all-solid photonic bandgap fiber (DCC-AS-PBGF). It is designed by
introducing a ring of bigger high-index rods, a new defect, in the cladding of a conventional all-solid PBGF. Using plane
wave expansion method (PWEM) and full-vector finite-element method (FEM), we study the effect of introducing such
a ring of bigger high-index rods. The numerical results show that large dispersion is gained around the wavelength where
the modes in the new defect couple with the modes in the core. More importantly, the confinement loss of the LP01
modes around the wavelengths, where large dispersion is induced, could be decreased by increasing the rings of high-index
rods for the fact that these waveband are within the bandgap.
Free-standing ZnS:Er and ZnS:Er,Yb quantum dots were prepared in the aqueous medium from readily available
precursors. The construction, morphology and luminescence properties of the ZnS:Er and ZnS:Er,Yb quantum dots were
evaluated by XRD,TEM and photoluminescence spectra. The average particle size was calculated using the Scherrer
formula to be 4nm, which is also observed from HRTEM image. The spectra of ZnS:Er and ZnS:Er,Yb quantum dots
have broad emission between 1450 nm and 1650 nm centered at 1575 nm with the excitation wavelength 980 nm, which
can be attributed to the 4I13/2→4I15/2 transition. But the intensity of ZnS:Er,Yb quantum dots significantly increases with
the addition of Yb as a sensitizing ion into ZnS:Er quantum dots. Because that Yb3+ absorbed the energy and transfer
energy from the 5F5/2 level of the 4I11/2 level (Er3+ ) and improve population accumulation on the 4I11/2 level.
The influence of lateral pressure on the propagation constant of grapefruit microstructure optical fibers is theoretically
investigated using a full-vector finite element method in this paper. With the different direction of lateral pressure, the change of
the propagation constant of grapefruit microstructure optical fibers is different. At present, the report of this aspect has not
been found out. The research has great signification in microstructure fiber sensors especially multidimensional optical fiber
sensors.
In this letter, we put forward a new kind of polarization-maintaining index-guiding photonic crystal fiber (PM-IG-PCF).
It is made up of a solid silica core, two big circular air holes near the core and a cladding with elliptical air holes. By
making use of a full-vector finite-element method (FEM), we study the modal birefringence and polarization mode
dispersion (PMD) as a function of the normalized wavelength of fundamental modes in the PM-IG-PCF we proposed.
Numerical results show that very high modal birefringence with magnitude of order of 10-3 has been obtained, which is
higher than the birefringence induced by adding two big air holes near the core or elliptical air holes in cladding
separately. Furthermore, the chromatic dispersion curves of the two orthogonal polarizations for the birefringence PCF
are presented as a function of the normalized wavelength.
Tunable photonic bandgap fibers (PBGFs) were theoretically investigated by using the vector plane-wave expansion method and the vector finite element method. The tunable PBGFs are fabricated by filling a high-index material in the air holes of index-guiding photonic crystal fibers. The wavelength dependence of leaky loss and group velocity dispersion (GVD) has been illustrated. We show the leaky loss in the tunable PBGFs can strongly depend on the refractive index of filled material due to the photonic bandgap effect. The tunable attenuator which operates at 1550 nm is designed based on this PBGFs.
Tunable photonic bandgap fibers (PBGFs) were theoretically investigated by using the vector plane-wave expansion method and the vector finite element method. The tunable PBGFs are fabricated by filling high index material in the air holes of index-guiding photonic crystal fibers. The wavelength dependence of leaky loss and group velocity dispersion (GVD) has been illustrated. We show the leaky loss in the tunable PBGFs can be strongly depended on the refractive index of filled material due to the photonic bandgap effect. The tunable attenuator which operates at 1550nm is designed based on this PBGFs.
Novel highly birefringent photonic bandgap fibers (PBGFs) are obtained by filling high index material in the air holes of total internal reflection birefringent photonic crystal fibers. The effect of filling high-index material on the transmission characteristics has been theoretically investigated. The photonic bandgap has been achieved by using the plane-wave method. Moreover, the polarization mode dispersion (PMD) has been studied by a full-vector finite-element method. Numerical results show that very high PMD with magnitude of order of 10-10 has been respectively acquired, which is much higher than those of the non-filled fibers. Furthermore, strong coupling between surface modes and the fundamental modes has been found in the bandgap of the birefringent PBGFs.
Using a full-vector finite element method, the phase modal birefringence and group modal birefringence to lateral pressure alone slow axis and fast axis versus wavelength in birefringence microstructure fiber was analyzed. In the wave band of our research, 600nm-1700nm, when different direction pressure is applied, the phase modal birefringence (B) and group modal birefringence (G) have different change to wavelength in microstructure fiber. Moreover, the results reveal that the pressure sensitivity of B and G have different change to wavelength when applying different direction lateral pressure. Our research has great signification in designing microstructure fiber and using microstructure fiber in sensing field et.al., especially using in multidimensional sensor.
We present theoretical analysis of tunable bandgap guidance in virtue of bandgap theory. By means of plane-wave method a novel tunable photonic bandgap microstructure fiber (MF) was investigated by tuning the refractive index of nematic liquid crystal crystal (NLC) filled in the holes of MFs. Moreover, by using a full-vector finite-element method (FEM) with anisotropic perfectly matched layers (PMLs), the dispersion curves of NLC filled MFs have been computed with different value of the refractive index of NLC. Moreover, the leakage loss of the fundamental modes of the NLC filled MFs has been analyzed.
In this letter, long period gratings fabricated in single-mode microstructure fibers (index-guiding MF and PBG MF) were achieved by putting periodic pressure on the cladding along the fiber length, furthermore, the characteristics of the LPGs were discussed.
A novel Moire grating in Yb3+-doped double-clad fiber is reported. It consists of two intracore Bragg reflection gratings separated by an optical phase shift; the grating was formed through double exposure phase-mask method. An Yb3+-doped double-clad fiber laser based on this grating is presented. The laser wavelengths are 1055nm and 1057nm, respectively, with less than 0.1nm line-width, over 40dB signal-to-noise ratio.
We present a numerical study of the guidance and amplification properties in an Er3+-doped honeycomb photonic bandgap fiber with down-doped core. Our analysis is based on a full-vector plane-wave expansion method and Runge-Kutta iterative algorithm. Overlap integrals between mode profiles and Er3+-doped region varies from 0.973 to 0.350 in guiding range of the fiber. The highly efficient amplifier can be designed by using this fiber.
We present a tunable multi-wavelength Yb3+-doped double-clad fiber laser based on a Fabry-Perot filter (FPF). By adjusting the FPF, the number of lasing wavelength and their spaces are controlled which is in agreement well with the numerical analysis. The laser can oscillate at different number of wavelengths from one to ten while the oscillation is quite stable. In ten-wavelength laser, the system allows us to extract more than 1.5 W with a slope efficiency of 67%.
Properties of the photonic bandgaps effect and guided modes in triangular photonic crystal fibers have been studied by means of a full-vectorial plane-wave expansion method. Photonic crystal fibers under consideration consist of a triangular array of microscopic holes forming a two-dimensional photonic crystal cladding and a defect as their core. The photonic band-structure for photonic crystal cladding structures and the field intensity distribution of defect mode with various air-filling fractions and defect size are calculated. Guidance mechanisms in photonic crystal fibers with different configuration are also investigated.
In this paper, a new digital-image correlation method-twelve variables gradient method is presented. The influence of second derivative terms of displacement is considered, the directional derivatives of correlation coefficient are obtained and variational step-length search is used in this method. Consequently, the truncation error caused by increasing of subset size and vast computation time are avoided. The measurement precision of displacement and strain are less than 0.011 pixels and 100 (mu) (epsilon) , respectively. Two test experiments and the application show that this is an efficient and accurate method.
A group of streamwise vortices in the near wall region of turbulent boundary layer are introduced by a group of parallel longitudinal heating wires to control the multi-scale eddy structures in wall turbulence. The relations between the introduced streamwise vortices and the multi-scale eddy structures in wall turbulence are studied. The influence of introduced streamwise vortices on multi-scale eddy structures in wall turbulence is analyzed by wavelet transform. The integrated kinetic energy of eddy structures at each scale is enhanced by the parallel heating wires. The characteristic time scales of different size eddy structures are changed by the introduced streamwise vortices.
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