Analysis of erbium doped holey fiber using fundamental space filling mode

Erbium-doped holey fiber with hexagonal lattice was modeled by using effective index method. In order to calculate the equivalent step index of the periodic structure of the cladding holey optical fiber, all-vectorial fundamental space filling mode approach was utilized. By using EH11 mode, we have numerically solved the rate equations of a three-level pumping scheme for a fiber laser. The obtained results have shown a good agreement with the other experimental results, recently. The results have predicted amplifiers with gain efficiencies as high as 10 dB/mW.     

Single-mode fiber laser based on core-cladding mode conversion

A single-mode fiber laser based on an intracavity core-cladding mode conversion is demonstrated. The fiber laser consists of an Er-doped active fiber and two fiber Bragg gratings. One Bragg grating is a core-cladding mode converter, and the other Bragg grating is a narrowband high reflector that selects the lasing wavelength. Coupling a single core mode and a single cladding mode by the grating mode converter, the laser operates as a hybrid single-mode laser. This approach for designing a laser cavity provides a much larger mode area than conventional large-mode-area step-index fibers.     

Broadband nearly-zero ultra-flattened dispersion single mode index guiding holey fiber

This paper proposes and demonstrates a novel type of silica index guiding holey fibers (IGHFs) that has two cladding layers at the defective innermost structures. The proposed IGHFs exhibit remarkable chromatic dispersion properties such as nearly-zero and flattened dispersion over a wide spectral range and single mode guidance along with very low confinement loss. The numerical results indicate that 5 air-hole rings of nearly zero ultra-flattened dispersion single mode IGHFs can be designed with desire flattened dispersion of over a 340 nm bandwidth including the entire band of interest with low confinement loss of less than 10⁻⁶ dB/m.     

Continuous-wave, totally fiber integrated optical parametric oscillator using holey fiber

We present, for the first time to our knowledge, a cw, all-fiber optical parametric oscillator that uses a holey fiber. The oscillator operates at 1.55 microns and can yield an oscillating parametric signal that consists of a single line with a 30-dB extinction ratio and a 10-pm linewidth or that consists of multiple lines. In addition to the signal and the idler, five other pairs of spectral lines can be observed that are due to multiple parametric interactions. The source reaches threshold for a pump power of 1.28 W and saturates for pump powers in excess of approximately 1.6 W.     

Broadband single-polarization photonic crystal fiber

We demonstrate a single-mode photonic crystal fiber that supports only one polarization state in a 220-nm-broad spectral region centered at 727 nm. The fiber has a mode-field diameter of 15.5 microm and background losses of < 15 dB/km in the single-polarization region. To our knowledge, these are the broadest bandwidth and the largest mode size yet reported for a single-polarization fiber.     

Higher-order core mode resonances in a mechanically induced long-period holey fiber grating

We present the spectral analysis of higher-order core mode resonances in a long period holey fiber grating induced mechanically in an asymmetric holey fiber. Calculations based on a fast-Fourier transform mode solver shows that the mode resonances obtained experimentally correspond to the odd- and even-LP_1,1 core modes. Additionally, we analyze the twist and polarization response of these mode resonances in the long period holey fiber grating. The results obtained in this work are of great importance in the design of new all-fiber optical devices that involve couplings of higher order core modes in asymmetric holey fibers.     

Solution of the fundamental space-filling mode of photonic crystal fibers: numerical method versus analytical approaches

The accuracy of the solution of the fundamental space-filling mode of photonic crystal fibers by scalar and vectorial analytical approaches and its effect on the effective index models are investigated. Using a plane wave method as a benchmark, we show that the optimal choice of the radius of the equivalent circular unit cell used in the approximations is different for the two approaches and this value has a great effect on the accuracy of the solution of the fundamental space-filling mode. We also show that the vectorial approach with a properly defined value of the radius is highly accurate over a wide parameter range, whereas the scalar approach causes the main error in the scalar effective index model. We also confirm that a fully vectorial effective index model is accurate and efficient in the case of photonic crystal fibers with large air filling fractions. PACS 42.25.Bs; 42.70.Qs; 42.81.Qb     

Single-mode antiresonant terahertz fiber based on mode coupling between core and cladding

Based on the index-induced mode coupling between the higher-order mode in core and the fundamental mode in cladding tubes, the single-mode operation can be realized in any antiresonant fibers (ARFs) when satisfying that the area ratio of cladding tube and core is about 0.46:1, and this area ratio also should be modified according to the shape and the number of cladding tubes. In the ARF with nodal core boundary, the mode in core also can couple with the mode in the wall of core boundary, which can further enhance the suppression of high-order mode. Accordingly, an ARF with conjoint semi-elliptical cladding tubes realizes a loss of higher-order mode larger than 30 dB/m; simultaneously, a loss of fundamental mode loss less than 0.4 dB/m.     

Characterization of single-polarization single-mode photonic crystal fiber using full-vectorial finite element method

A modal solution approach based on the powerful, finite element method (FEM) using a full-vectorial H-field formulation has been used to determine the single-mode operation of a photonic crystal fiber (PCF). The modal solution of the fundamental space-filling mode has also been obtained to identify the cutoff conditions of the waveguide modes. The FEM, with the perfectly matched layer boundary condition, has been developed and employed to characterize the leakage loss and the differential loss between the polarized modes of a PCF. The design approach for a single-polarization and single-mode PCF has also been discussed.     

Narrow-linewidth single-polarization frequency-modulated Er-doped fiber ring laser

We demonstrate a 1550-nm narrow-linewidth fiber ring laser with stable single polarization by using single-mode Er-doped fiber as active fiber and saturable absorber. A polarization-maintaining circulator is used to acquire single-polarization laser light with the degree of polarization of 99.8%--99.9%. The linewidth measured using a delayed self-heterodyne method is less than 0.5 kHz. Frequency of the fiber laser can be modulated by driving the waveguide phase modulator with proper voltage. A Mach-Zehnder interferometer with the optical path difference between two arms of about 36 km is used to study the long-distance coherent detection of the fiber laser for frequency-modulated continuous-wave application.     

Photonic crystal fiber with a flattened fundamental mode for the fiber lasers

The 1-hole-missing and 7-hole-missing photonic crystal fibers (PCFs) with flattened fundamental modes (FMs) are proposed by introducing a layer of up-doped silica into the core of the PCFs. The transverse mode competitions are compared between the 7-hole-missing PCF lasers with and without flattened-FMs. The numerical results show that the flattened-FM PCF lasers can support the single transverse mode operation, even for a large value of the ratio of air hole diameter to the spacing between holes (up to 0.53).     

Linearly polarized fiber laser using a point-by-point Bragg grating in a single-polarization photonic bandgap fiber

We present a narrow-linewidth, linearly polarized neodymium-doped fiber laser that incorporates a point-by-point Bragg grating inscribed into the core of a single-polarization all-solid photonic bandgap fiber. The Bragg grating was written within a single-polarization wavelength band of the fiber; thus, the Bragg reflection was polarized. This all-fiber laser produced 7.2 W, linearly polarized output with 25 pm FWHM and 19.6 dB polarization extinction ratio.     

Microstructured optical fiber for single-polarization air guidance

An air-core microstructured fiber design that supports a single-polarization, circularly symmetric nondegenerate mode is presented. The fiber design is modeled directly, and the microstructured cladding is analyzed by use of band diagrams to elucidate the mechanism through which polarization nondegeneracy is achieved.     

基于谐振吸收效应的单模单偏振光子晶体光纤研究

提出了一种基于谐振吸收效应的单模单偏振光子晶体光纤，阐述了其工作原理，并利用全矢量有限元法对其模场分布、限制损耗、工作带宽、消光比等基本特性进行了数值模拟. 数值结果表明这种结构可以获得很高的单模单偏振工作带宽，并能以较低的损耗代价实现极高的消光比.讨论了结构设计参量对光纤性能的影响.研究结果对设计新型的高性能单模单偏振光纤具有一定的指导意义.     

Photonic crystal fiber for fundamental mode operation of multicore fiber lasers and amplifiers

A mode-selection method based on a single-mode photonic crystal fiber (PCF) in the multicore fiber (MCF) lasers is presented. The designed PCF has a central core region formed by a missing air-hole, and three air-hole rings. With an appropriate choice of the design parameters of the PCF, the power coupling between the fundamental mode (FM) of the PCF and the fundamental MCF mode can be much higher than those between the FM and the other supermodes. As a result, the fundamental MCF mode has the maximum power reflection coefficient on the right-hand side of the MCF laser cavity, and dominates the output laser power. Since the maximum power of the fundamental MCF mode will lead to the desired laser beam profile, higher the fraction of the fundamental MCF mode power contained in the total output power contributes to higher beam quality. The numerical simulations show that the effectiveness of the fundamental MCF mode-selection is higher in the MCF lasers with the PCF as a mode-selection component than in the MCF lasers based on the free-space Talbot cavity method. Additionally, for the MCF amplifiers, an approach is presented to decrease the sensitivity of the amplifier performance to the variation of Gaussian beam waist utilizing the coupling between the Gaussian beam and the FM of the PCF. The numerical results show that this method can effectively increase the design flexibility for a broad range of the Gaussian beam waist.     

Leakage of the fundamental mode in photonic crystal fiber tapers

We report detailed measurements of the optical properties of tapered photonic crystal fibers (PCFs). We observe a striking long-wavelength loss as the fiber diameter is reduced, despite the minimal airhole collapse along the taper. We associate this loss with a transition of the fundamental core mode as the fiber dimensions contract: At wavelengths shorter than this transition wavelength, the core mode is strongly confined in the fiber microstructure, whereas at longer wavelengths the mode expands beyond the microstructure and couples out to higher-order modes. These experimental results are discussed in the context of the so-called fundamental mode cutoff described by Kuhlmey et al. [Opt. Express 10, 1285 (2002)], which apply to PCFs with a finite microstructure.     

W型单模单偏振光纤基模截止归一化频率

用数值计算了 W型单模单偏振光纤基模截止归一化频率 ,分析了基模截止归一化频率与光纤结构参数的关系 ,得出了在外包层远离纤芯的情况下 ,基模截止归一化频率仅与内包层折射率深度有关 ,同时给出了其相应的近似函数关系和曲线     

宽带空芯THz单模单偏振光纤

传统的THz单模单偏振光纤大部分都是基于实芯光纤设计的,增加了THz波的传输损耗。最近几年,也有一些关于空芯THz单模单偏振光纤的报道。但是,他们的带宽太窄,往往只能在单一频率附近工作,影响了单偏振器件在THz范围内的使用价值和技术优势。为了解决这些问题,设计了一种新颖结构的空芯THz单模单偏振器件。它拥有超过0.1 THz的带宽,工作频率范围从1.63 THz到1.73 THz。这种器件对于THz通信网络的建设和应用具有重要意义。     

用相位恢复方法测量多孔光纤的三维折射率分布

结合计算机断层成像方法，提出了一种新的相位恢复显微测量实验技术.应用该技术测量了多孔光纤的三维折射率分布.这种技术是基于强度传递方程的理论，对相位的测量是直接的，测量对光纤无损害.这是一种测量多孔光纤的三维折射率分布的有效方法. 关键词： 断层法 相位恢复 显微测量 多孔光纤     

UV generation in a pure-silica holey fiber

We report supercontinuum generation extending to 300 nm in the UV from a pure-silica holey fiber. The broad spectrum was obtained by launching ultra-short pulses (150 fs, 10 nJ at 820 nm) from an amplified Ti:sapphire laser. The extension of holey-fiber-based supercontinuum generation into the UV should prove to be of immediate application in spectroscopy. By slightly detuning the launch conditions we excited a higher order spatial mode, which produced a narrower supercontinuum, but with enhanced conversion efficiency at a series of blue/UV peaks around 360 nm. We present numerical simulations, which suggest that differences in the dispersion profiles between the modes are an important factor in explaining this enhancement. In a related experiment, using the same laser source and fiber, we demonstrate a visible supercontinuum from several subsidiary cores, with distinct colours in each core. The subsidiary cores were excited by an appropriate input coupling. Fabrication of a fiber with a range of core sizes (dispersion profiles) for tailored supercontinuum generation can therefore be envisaged for practical applications. PACS 42.72.Bj; 42.79.Nv; 42.81.Dp