千瓦级掺Yb3+双包层光纤激光器的热分析

针对千瓦级Yb3+光纤激光器的特殊结构,在同时考虑光纤表面的对流换热和辐射换热条件下建立了热传递模型和一维稳态传热方程.结合该稳态传热方程和光纤激光器的速率方程,数值模拟了正向泵浦1 000 W和双端泵浦两端各500 W的条件下沿光纤纵向和径向的温度分布.计算结果表明,双端泵浦的温度分布比正向泵浦的均匀,且光纤中的最高温度比正向泵浦时下降了135.9℃.将考虑辐射换热时光纤的温度分布和忽略辐射换热时的结果进行对比,后者的温度比前者高得多,表明辐射换热是光纤激光器非常重要的换热方式.最后,分析了双端泵浦以及光纤长度不变的条件下纤芯半径、包层半径以及表面传热系数对光纤内温度分布的影响.结果表明纤芯半径不会影响光纤表面温度,而加大表面传热系数以及增大包层半径可以有效地降低光纤表面及内部温度.     

Theoretical analysis and heat dissipation of mid-infrared chalcogenide fiber Raman laser

Based on the structure of mid-infrared chalcogenide fiber Raman laser, the nonlinear coupled equations and heat dissipation equations are constructed. The effects of laser parameters including pump power, fiber length, reflectance of output coupler and fiber loss coefficient on laser performance are numerically analyzed. The results show that the Raman laser pumped at 2 μm can operate at high slope efficiency using the optimized structure parameters. In addition, the output laser power decreases dramatically with the increasing of fiber loss coefficient. Moreover, the temperature distributions along the fiber radial and axial directions and the maximum temperature versus launched pump power are calculated according to the heat dissipation equations. The results show that the maximum temperature in the fiber increases dramatically with the increasing launched pump power, which is above 300 °C for launched pump power of 21 W. The above obtained results can be used for theoretical guiding and optimizing design of practical chalcogenide fiber Raman laser.     

Ultra-wideband single-polarization single-mode, high nonlinearity photonic crystal fiber

We report a novel design of photonic crystal fiber (PCF) with a rectangular array of four closely-spaced, highly elliptical air holes in the core region and a circular-air-hole cladding. The proposed PCF is able to support ultra-wideband single-polarization single-mode (SPSM) transmission from the visible band to the near infrared band. With the aid of the inner cladding formed by the central air holes, one polarization of the fundamental mode can be cut off at very short wavelengths and ultra-wideband SPSM propagation can be achieved. The inner cladding also suppresses the higher order modes and allows large air filling fraction in the outer cladding while the proposed fiber remains SPSM, which significantly reduces the mode effective area and the confinement loss. Our simulation results indicate that the proposed PCF has a 1540 nm SMSP range with <0.25 dB/km confinement loss and an effective area of 2.2 μm². Moreover, the group velocity dispersion (GVD) of the proposed PCF can also be tuned to be flat and near zero at the near infrared band (∼800 nm) by optimizing the outer cladding structure, potentially enabling many nonlinear applications.     

Simultaneous strain and temperature measurement based on a photonic crystal fiber modal-interference interacting with a long period fiber grating

An alternative all-fiber sensor for simultaneous strain and temperature measurement based on a photonic crystal fiber (PCF) spliced between single-mode fibers cascaded with a long period grating (LPG) is proposed. By collapsing the air holes at two splicing regions along the PCF, a simple but effective modal-interference (MI) is occurred between the core and cladding modes of the PCF. Due to the different responses on the changes of strain and temperature on the MI and the cascaded LPG, the strain and temperature can be measured simultaneously. Experimental results show that the sensing resolution of 9.1 με in strain measurement is experimentally achieved over a range of 2640 με, while the temperature sensing resolution is 0.27 °C within a range of 30-100 °C.     

Large negative dispersion in dual-concentric-core photonic crystal fiber with hybrid cladding structure based on complete leaky mode coupling

Considering the optical stability of solution, the sugar-solution is infused into the outer core ring of dual-concentric-core photonic crystal fiber (DCCPCF). The influences of structure parameters and solution concentration on the phase and loss matching are comprehensively analyzed. By choosing the appropriate outer core mode to completely couple with the inner core fundamental mode, the large negative dispersion PCF around 1.55 μm is designed, which has the dispersion value of − 39,500 ps/km/nm as well as bandwidth of 7.4 nm and effective mode area of 28.3 μm². The designed PCF with hybrid cladding structure can effectively compensate the positive dispersion of conventional single mode fiber, and suppress the system perturbation caused by a series of nonlinear effects. Considering the mode field mismatching between the DCCPCF and the tapered fiber, the calculated connection loss around 1.55 μm is below 3 dB. In addition, the equivalent propagation constants of two leaky modes are deduced from the coupled-mode theory, and the complete mode coupling case can be well predicted by comparing the real and imaginary parts of propagation constants.     

Core temperature in super-Gaussian pumped air-clad photonic crystal fiber lasers compared with double-clad fiber lasers

In this paper we investigate the core temperature of air-clad photonic crystal fiber (PCF) lasers pumped by a super-Gaussian (SG) source of order four. The results are compared with conventional double-clad fiber (DCF) lasers pumped by the same super-Gaussian and by top-hat pump profiles.      

High birefringence and low loss circular air-holes photonic crystal fiber using complex unit cells in cladding

We propose a high birefringence and low loss index-guiding photonic crystal fiber (PCF) using the complex unit cells in cladding by the finite-element method. Results show that the birefringence and confinement loss in such PCF fiber is determined not only by the whole cladding asymmetry but also the shape of the PCF core. The maximal modal birefringence and lowest confinement loss of our proposed structures at the excitation wavelength of λ = 1550 nm can be achieved at 8.7 × 10⁻³ and 5.27 × 10⁻⁵ dB/km, respectively.     

New nonlinear and dispersion flattened photonic crystal fiber with low confinement loss

A new nonlinear dispersion flattened photonic crystal fiber with low confinement loss is proposed. This fiber has threefold symmetry core. The doped region in the core and the big air-holes in the 1st ring can make high nonlinearity in the PCF. And the small air-holes in the 1st ring and the radial increasing diameters air-holes rings in cladding can be used to achieve the dispersion properties of the PCF. We can achieve the optimized optical properties by carefully selecting the PCFs structure parameters. A PCF with flattened dispersion is obtained. The dispersion is less than 0.8 ps/(nm km) and is larger than −0.7 ps/(nm km) from 1.515 μm to 1.622 μm. The nonlinear coefficient is about 12.6456 W⁻¹ km⁻¹, the fundamental mode area is about 10.2579 μm². The confinement loss is 0.30641 dB/km. This work may be useful for effective design and fabrication of dispersion flattened photonic crystal fibers with high nonlinearities.     

Polarization maintaining large nonlinear coefficient photonic crystal fibers using rotational hybrid cladding

In this paper, we present and explore a new hybrid cladding design for improved birefringence and highly nonlinear photonic crystal fibers (PCFs) in a broad range of wavelength bands. The birefringence of the fundamental mode in such a PCF is numerically analyzed using the finite element method (FEM). It is demonstrated that it is possible to design a simple highly nonlinear hybrid PCF (HyPCF) with a nonlinear coefficient of the about 46 W⁻¹ km⁻¹ at a 1.55 μm wavelength. According to simulation, the highest modal birefringence and lowest confinement loss of our proposed structure at the excitation wavelength of λ = 1.55 μm can be achieved at a magnitude of 1.77 × 10⁻² and of the order less than 10² dB/km with only five rings of air-holes in the fiber cladding.     

The Temperature Distribution in Longitudinally Pumped Ti:sapphire Lasers

l.IntroductionItiswe1lknownthatthetemperaturcofaso1id-statelasermediawillgethighwhenitispumPedbyflash-lamporlaser,x`hichwi1l1eadtohigheroscillationthersholdandlowerefficiency.Duetotheinhomogeneouspumpandtheinhomogeneouscool,thetem-Peraturedistributioninthemediabecomesinhomogeneoussothattherefractiveindexofthemediachangesinhomogeneouslyandtheheatlenseffectwillbegenerated,whichwillaffectontheosci11ationthreshold,efficiency,andotherlaserparameters.Itisessentia]andimportanttoknowthetemPeraturedis…     

Cladding mode coupling in long-period gratings formed in photonic crystal fibers

We extend the improved effective index method (IEIM) to analyze the cladding mode of the photonic crystal fiber (PCF) and to predict cladding mode coupling in gratings. By introducing a new step-index fiber model for the PCF, the cladding mode coupling between a LP₀₁ core mode and HE₁₁ cladding mode in long-period gratings formed in PCF is accurately predicted by the IEIM. The fiber model works well for the PCF used here not only in predicting resonant mode coupling, but also in analyzing core and low-order cladding modes.     

Characterization of specially designed polarization maintaining photonic crystal fiber from far field radiation patterns

We report the development of theory and experiment for the characterization of polarization maintaining photonic crystal fiber (PM PCF) from far field intensity measurements. In this specially designed photonic crystal fiber, the air hole diameters along orthogonal axes adjacent to the core region are different, and hence create an effective index difference between the two orthogonal polarization modes. From the experimental measurements of the far field radiation pattern, we obtain transmission characteristics of PM PCF in terms of mode field diameters (MFD), V-values along major and minor axes, mode field area, birefringence, core radius, effective refractive index of cladding and numerical apertures along major and minor axes of PM PCFs. It is shown that the experimentally obtained geometrical and wave guiding parameters of the PM PCF match with the manufacturer/simulation/scanning electron microscope (SEM) data within the experimental limits.     

Modeling photonic crystal fiber for efficient soliton pulse propagation at 850 nm

We numerically investigate the dynamics of soliton propagation at 850 nm in chloroform filled liquid core photonic crystal fiber (LCPCF) by using both finite element method (FEM) and split step Fourier method (SSFM). We propose a novel chloroform filled PCF structure that operates as a single mode at 850 nm featuring an enhanced dispersion and nonlinearity for efficient soliton propagation with low input pulse energy and low loss over small distances. We adopt the projection operator method (POM) to derive the pulse parameter equations which clearly describes the impact of fourth order dispersion on the pulse propagation in the proposed PCF. To analyse the quality of the pulse, we perform the stability analysis of pulse propagation numerically and compare our results of the newly designed chloroform filled PCF with that of standard silica PCF. From the stability analysis, we infer that the soliton pulse propagation in modified chloroform filled PCF is highly stable against the perturbation.     

Neodymium-doped phosphate fiber lasers with an all-solid microstructured inner cladding

We report on high-power fiber lasers based on index-guiding, all-solid neodymium-doped (Nd-doped) phosphate photonic crystal fiber (PCF) with a hexagonal-shaped inner cladding. The optimum fiber laser with a 36 cm length active fiber, generated up to 7.92 W output power at 1053 nm, which benefited from a high absorption coefficient for pump power due to its noncircular inner cladding. The guiding properties of the all-solid PCF were also investigated. A stable mode with a donut-shaped profile and a power-dependent laser beam quality have been observed experimentally and analyzed.     

New high negative dispersion photonic crystal fiber

A new high negative dispersion photonic crystal fiber is proposed. It has double-core structure. The inner core has a circle germanium-doped region. The outer core is formed by removing the 3rd ring air-holes around the core. There are two ring air-holes between the two cores, Diameter of the 1st ring air holes is bigger than that of the 2nd ring air-holes, this can make mode coupling between inner mode and outer mode and showed that the high negative PCF is the result of this structure characteristics. There are honeycomb photonic lattice in the PCF's cladding. The influence of the structure parameters deviated from the design those on the chromatic dispersion are evaluated. When the structure parameters Λ=1.50 μm, d_core=2.10 μm, d₁=0.90 μm, d₂=0.44 μm and d₃=1.04 μm, the dispersion coefficient D is −1320 ps/(nm·km) at 1550 nm. This is a new kind of chromatic dispersion compensation PCF.     

Design and analysis of multi-ring microstructured-core optical fibers

A novel design of single-mode large-mode-area optical fiber is presented. The core is composed of alternate high and low-index regions to form an effectively low-index contrast between the core and the cladding. The proposed fiber is investigated by the finite-element method with anisotropic perfect matched layer boundary conditions. In addition, the bending losses of the fibers are calculated and compared with those of the step-index optical fibers. In particular, numerical simulations demonstrate that single-mode operation can be achieved in one such fiber with mode area larger than 600 μm² at the wavelength of 1.55 μm and bending loss lower than 0.02 dB/m for bending radius greater than 20 cm.     

Numerical investigation and optimization of a photonic crystal fiber for simultaneous dispersion compensation over S + C + L wavelength bands

In this paper, we numerically investigate and optimize the profile of a photonic crystal fiber (PCF) that can eliminate the residual dispersion from the telecom link as well as can provide identical dispersion compensation over S + C + L bands. A full-vectorial finite element method combined with genetic algorithm is used to optimize the fiber’s profile as well as to accurately determine its modal properties. The optimized PCF exhibits a dispersion of −98.3 ps/nm/km with a variance of ±0.55 ps/nm/km from 1.48 μm to 1.63 μm (i.e., over 150 nm bandwidth) and a zero dispersion slope. Macro-bending loss performance of the designed PCF is also studied and it is found that the fiber shows low bending losses for the smallest feasible bending radius of 5 mm. Further, sensitivity analysis has been carried out for the proposed fiber design and it has been found that a ±2% change in the fiber parameters may lead to a ±8% shift of the dispersion from its nominal value.     

A polymer photonic crystal fiber with high and flattened birefringence

A novel high birefringence polymer photonic crystal fiber (PCF) is proposed in this work. This PCF is composed of a polymer core and a cladding with elliptical air holes and squeezed triangular lattice. The high birefringence is introduced on the combined effect of elliptical air holes and the squeezed lattice. Our numerical results based on the supercell lattice method indicate that the birefringence can reach as high as 0.0018 at 650 nm wavelength with a properly designed cladding structure. We also analyze the dependence of the birefringence on structure parameters. And we design a PCF that has high and flattened birefringence.     

Bending dual-core photonic crystal fiber coupler

In this paper, we systematically study a designed structure of a bending dual-core photonic crystal fiber (PCF). We propose the controllable wavelength-selective coupling PCF. This coupler allows highly accurate control of the filtering wavelength. The different wavelengths can be selected by controlling the bending radius of the fiber. Coupling characteristics of novel bending wavelength-selective coupling PCF are evaluated by using a vector finite element method and their application to a multiplexer demultiplexer (MUX–DEMUX) based on the novel coupler is investigated. When the fiber length is 4168 μm, the bending radius of PCF couplers for 1.48/1.55 μm, 1.3/1.55 μm, 0.98/1.55 μm, and 0.85/1.55 μm is calculated, respectively, and the beam propagation analysis is performed. Different from the traditional wavelength-selective coupling PCF, the dual-core PCF is bent and it can realize the separation of multiple wavelengths.     

Gain and gain-flatness improved photonic crystal fiber Raman amplifier based on a dual-pass amplification configuration with single pump laser

A gain and gain-flatness improved L-band dual-pass Raman fiber amplifier (RFA) utilizing a photonic crystal fiber (PCF) as gain medium is demonstrated. By introducing complementary gain spectra of typical forward and backward pumping single-pass RFA using the same PCF, we finally achieve average net gain level of 22.5 dB with a ±0.8 dB flattening gain in 20-nm bandwidth from 1595 nm to 1615 nm, which is rare in RFAs with only one single pump and no flattening filter. Compared with the single-pass pump configurations, gain level, flatness and bandwidth are greatly improved by using the dual-pass amplification configuration. The limitation of this configuration caused by multi-path interference (MPI) noise and stimulated Brillouin scattering (SBS) is also discussed.