Single-mode excitation of multimode fibers with ultrashort pulses

Single-mode excitation of step-index multimode fibers with light sources with short temporal coherence lengths is demonstrated. Multimode fiber designs with reduced microbending-induced mode coupling are described that allow the propagation of the fundamental mode over long lengths with negligible mode coupling even in the presence of tight fiber bends. At a wavelength of 1.56microm a fiber with a core diameter of 45microm can preserve the fundamental mode for a propagation length of ~20m . Such fibers allow coiling with a coil diameter as small as 7cm.     

Characteristics of a Q-switched multicore photonic crystal fiber laser with a very large mode field area

We model and characterize the behavior of a Q-switched fiber laser. The fiber is a doped multicore photonic crystal fiber having six cores in a ring-type geometry. The fiber laser is Q-switched using an intracavity acousto-optic modulator. Using a mode filtering technique in the far field, a mode very close to the fundamental in-phase supermode is obtained with a mode field area of 4200 microm(2) and a divergence of 9 mrad. Pulses with energies of up to 2.2 mJ and durations of 26 ns (limited by end facet damage) at a repetition rate of 10 kHz are obtained.     

High-power photonic-bandgap fiber laser

An original architecture of an active fiber allowing a nearly diffraction-limited beam to be produced is demonstrated. The active medium is a double-clad large-mode-area photonic-bandgap fiber consisting of a 10,000 ppm by weight Yb(3+)-doped core surrounded by an alternation of high- and low-index layers constituting a cylindrical photonic crystal. The periodic cladding allows the robust propagation of a approximately 200 microm(2) fundamental mode and efficiently discriminates against the high-order modes. The M(2) parameter was measured to be 1.17. A high-power cw laser was built exhibiting 80% slope efficiency above threshold. The robust propagation allows the fiber to be tightly bent. Weak incidence on the slope efficiency was observed with wounding radii as small as 6 cm.     

Visible cw-pumped supercontinuum

We report the experimental demonstration of a visible supercontinuum in the cw pumping regime. A 20 W ytterbium fiber laser at 1.06 microm is used to pump a photonic crystal fiber whose zero-dispersion wavelength decreases along the fiber length. Visible wavelengths are generated in the fundamental mode via trapping of dispersive waves by redshifted solitons.     

Stratified waveguide grating coupler for normal fiber incidence

We propose a new stratified waveguide grating coupler (SWGC) to couple light from a fiber at normal incidence into a planar waveguide. SWGCs are designed to operate in the strong coupling regime without intermediate optics between the fiber and the waveguide. Two-dimensional finite-difference time-domain simulation in conjunction with microgenetic algorithm optimization shows that approximately 72% coupling efficiency is possible for fiber (core size of 8.3 microm and delta=0.36%) to slab waveguide (1.2-microm core and delta=3.1%) coupling. We show that the phase-matching and Bragg conditions are simultaneously satisfied through the fundamental leaky mode.     

Tunable single-mode fiber-VCSEL using an intracavity polymer microlens

We report a tunable, single-mode vertical cavity surface-emitting laser (VCSEL) format suitable for array operation, power scaling, fiber coupling, and operation in isolated environments such as those required by atom optics. The devices are fiber VCSELs, consisting of a semiconductor gain and mirror structure separated from a mirror-coated optical fiber by an air (or vacuum) gap. The gain structure has polymer microlenses fabricated on its surface, of characteristics suitable to focus the oscillating mode on both cavity mirrors, ensuring stable fundamental mode emission and high fiber coupling efficiency. We demonstrate such devices in continuous-wave operation at 1.03 microm at room temperature, with a single-mode tuning range of 13 nm, laser threshold as low as 2.5 mW, and a maximum fiber-coupled output power of 10 mW.     

具有双模特性的大模场面积微结构光纤的设计

本文提出了一种具有双模特性大模场面积的微结构光纤,通过有限元法计算其模场分布、基模有效模场面积及弯曲损耗特性,并分析了不同结构参量对限制损耗及有效模场面积的影响.计算结果表明:通过合理地改变光纤结构参量,可以使二阶模LP11、三阶模EH11截止,实现基模LP01、三阶模HE31双模传输,其中基模的有效模场面积可达700μm2.这种结构的光纤制作简单,在大容量光纤传输系统中具有重要应用.     

Light propagation with ultralarge modal areas in optical fibers

We demonstrate robust single-transverse-mode light propagation in higher-order modes of a fiber, with effective area A(eff) ranging from 2,100 to 3,200 microm(2). These modes are accessed using long-period fiber gratings that enable higher-order-mode excitation over a bandwidth of 94 mm with greater than 99% of the light in the desired mode. The fiber is designed such that the effective index separation between modes is always large, hence minimizing in-fiber mode mixing and enabling light propagation over lengths as large as 12 m, with bends down to 4.5 cm radii. The modal stability increases with mode order, suggesting that A(eff) of this platform is substantially scalable.     

Mode-coupling analysis of three-dimensional microdisk resonators by the finite-difference time-domain technique

Quality factor enhancement due to mode coupling is observed in a three-dimensional microdisk resonator. The microdisk, which is vertically sandwiched between air and a substrate, with a radius of 1 microm, a thickness of 0.2 microm, and a refractive index of 3.4, is considered in a finite-difference time-domain (FDTD) numerical simulation. The mode quality factor of the fundamental mode HE71 decreases with an increase of the refractive index of the substrate, n(sub), from 2.0 to 3.17. However, the mode quality factor of the first-order mode HE72 reaches a peak value at n(sub) = 2.7 because of the mode coupling between the fundamental and the first-order modes. The variation of mode field distributions due to the mode coupling is also observed. This mechanism may be used to realize high-quality-factor modes in microdisks with high-refractive-index substrates.     

Mode matching and insertion loss in ultrabroadband Cr-doped multimode fibers

We investigate the fundamental insertion loss due to mode mismatch in an optical link involving a single-mode fiber-optic (SMF) transmission with the insertion of a segment of a multimode Cr(4+)-doped fiber (MMCDF). With an MMCDF core diameter of approximately 15.5 microm that matches the SMF, we obtained coupling efficiencies of 60.3% to 87.5% in the entire transmission spectral range of 1,300 to 1,600 nm. The high coupling efficiency (or low insertion loss) makes it possible for the employment of the MMCDF as an inline ultrabroadband optical amplifier in an optical link for the entire transmission spectral range.     

新型三角芯抗弯曲大模场面积光子晶体光纤

提出了一种新型三角芯抗弯曲大模场面积光子晶体光纤.该结构采用单一尺寸的圆形空气孔, 降低了制作难度.在波长1.064 μm处, 在平直状态和弯曲半径为30 cm时, 模场面积分别为1386 μm²和1153 μm², 弯曲带来的模场面积减少量仅为16.85%. 当弯曲半径为30 cm时, 基模的损耗为0.087 dB/m, 二阶模的损耗大于1.5 dB/m, 大的损耗差有效保证了光纤单模运转.此外, 弯曲半径30 cm时, 弯曲方向角扩展至±180°, 光纤弯曲不再受弯曲方向的限制. 所设计的光纤结构具有大模场面积、小的模场面积减少量、 低的弯曲损耗以及低的弯曲方向敏感度等优势, 为小型化、集成化高功率激光传输及光纤激光器和放大器的研究奠定了基础. 关键词： 光子晶体光纤 大模场面积 抗弯曲 弯曲方向角     

七芯及十九芯大模场少模光纤的特性研究和比对分析

提出了一种新型的多芯大模场少模光纤.包含缺失空气孔的特殊结构使其具有独特的少模特性, 仅传输HE₁₁模和HE₂₁模.分析表明七芯大模场少模光纤能维持稳定的双模式运转, 且基模有效面积可达866.54 μm². 系统研究了光纤结构参数影响模式特性和基模有效面积的规律, 并分析了纤芯数目增加带来的性能相似性和差异性–-进阶的十九芯大模场少模光纤在继承少模特性的同时, 模场面积大大增加, 其基模有效面积可高达3617.55 μm². 对比已报道的少模光纤, 多芯大模场少模光纤获得了更大的有效面积, 并具有良好的弯曲特性, 有望被用于更高功率的光纤放大器、光纤激光器以及高速大容量光纤传输系统中. 关键词： 少模光纤 多芯 大模场面积 弯曲损耗     

双层孔大模场微结构光纤的模式特性分析

本文分析一种由两层低折射率孔组成的大模场光纤的模式特性.采用多极法和有限元方法数值计算并分析了内层孔与外层孔尺寸变化对光纤基模与高阶模的束缚损耗与弯曲损耗的影响,设计获得了一种具有较高的高阶模和基模损耗比同时允许一定程度弯曲的大模场光纤.结果表明:当内层孔直径为34 μm,外层孔直径为24 μm时,其基模束缚损耗为0.000 17 dB/m,而高阶模束缚损耗为1.39 dB/m;光纤的基模模场面积为2 150.9 μm²,当弯曲半径为1.2 m时,弯曲损耗为0.106 dB/m.     

Theoretical realization of holey fiber with flat chromatic dispersion and large mode area: an intriguing defected approach

We present a novel design approach for realizing holey fibers (HFs) with flat dispersion characteristics and large mode area based on the existence of an artificially defected air-hole ring in the cladding, and on the inclusion of additional defected air holes in the core of the fiber. This unique type of HF can be used for achieving remarkable flat dispersion characteristics as well as a large mode area, which are particularly useful for high-speed data transmission. The validation of the proposed design is done by adopting an efficient full-vectorial finite element method for optical characterization of HFs. The proposed fiber can be employed in reconfigurable optical transmission systems for performing wavelength division multiplexing operation. Typical characteristics of the proposed HF are a flattened dispersion of 6.3 +/- 0.5 ps/km/nm from 1.45 to 1.65 microm and an effective mode area as large as 100 microm2 in the same frequency range.     

High-energy (nanojoule) femtosecond pulse delivery with record dispersion higher-order mode fiber

Delivery of high peak-power femtosecond pulses with fibers is constrained by nonlinear distortions accumulated during pulse propagation. We address this problem with a novel, to our knowledge, fiber schematic, where the pulse propagates in a small Aeff (18 microm2) but highly dispersive (record value of approximately -900 ps/nm km) medium, enabled by transmission in the LP02 mode of a few-mode fiber. The novel fiber yields a low dispersion-to-nonlinear-length ratio (due to its large dispersion) despite its small Aeff, hence enabling mitigation of nonlinearities. This enables fiber delivery of distortion-free <150 fs, approximately 1 nJ, and 840 nm pulses--an order-of-magnitude improvement over single-mode fibers of similar Aeff.     

Bending-insensitive single-mode hole-assisted fibers with reduced splice loss

We propose and demonstrate a novel type of bending-insensitive single-mode hole-assisted fiber that has a doped core and two layers of holes with two different airhole diameters. The fiber has a 9.3 microm mode field diameter, a bending loss of 0.011 dB/turn at 1.55 microm for a bending diameter of 10 mm, and a cutoff wavelength below 1.1 microm. The fiber can be fusion spliced to a conventional single-mode fiber with low loss.     

Design of zero dispersive double clad fiber for high efficiency operation of two color light

A double clad solid silica fiber is newly designed for applications which need the high efficiency operation of two colors of light. Ultrashort pulses with a central wavelength of 800?nm are delivered by the core of the double clad fiber which can realize the transmission of the optical pulses with a net chromatic dispersion of zero. This is achieved by integrating the double clad fiber with a pair of long period gratings, which allows optical pulses to propagate in a higher order mode (LP₀₂) in the middle of the fiber as well as in a fundamental mode (LP₀₁) at the beginning and end of the fiber. The index profile of the double clad fiber is engineered so that the higher order mode has high anomalous dispersion that can be used to compensate for normal dispersion of the fundamental mode. By controlling the lengths of the fiber where pulses are in a fundamental and in a higher order modes, the fiber with total zero dispersion can be realized. The double clad fiber can collect 100?% of visible light within the numerical aperture of 0.21 with a loss of the optical pulses less than 1?%. The design of this fiber is essential for applications including fiber-optic nonlinear imaging for compactness, robustness, and low optical power loss in dispersion compensation.     

Tunable mid-infrared MHz-rate picosecond pulses generated by optical parametric amplification of white-light continuum in GaSe

The output of a long-cavity 2 MHz all-solid-state Nd:YVO(4) picosecond oscillator with nonlinear optical mirror mode locking is used for direct pumping of an optical parametric amplifier seeded with white-light continuum generated in optical fiber by the same laser to generate tunable (5-12 microm) mid-IR radiation.     

Single-mode 212 W tapered fiber laser pumped by a low-brightness source

With a tapered double-clad all-glass ytterbium fiber as a gain medium, a maximum output power of over 200 W at 1079 nm and a slope efficiency of over 70% were demonstrated. The tapered double-clad fiber concept allows for using low-brightness diode bars and results in cost-effective and efficient high-power fiber lasers. The adiabatic conical fiber gain waveguide combines improved pump absorption owing to enhanced mode mixing in the pump cladding, low-noise single fundamental mode operation with M(2)< or =1.02, and strong potential for significant power scaling.     

Structural and wavelength dependence of mode power distribution for a broadband dispersion compensating microstructured fiber

Approximate empirical analysis of mode power distribution (MPD) carried by the fundamental mode is newly investigated based on a broadband dispersion compensating microstructured fiber (MF). The fraction of modal power in the core region, η, is defined with the help of extending the applicability of well-established classical optical fiber theories to MFs. In doing so, the influences of structural parameters and wavelength on MPD characteristics of the fundamental mode are systematically analyzed in detail based on simple physically consistent concepts of conventional fibers. As a result, it is shown that for the optimum MF design in Ref. [11] as a multimode fiber, when the number of guiding modes increases, the mode power ratio of the fundamental mode constantly increases from 69.4%; in addition, we find that as wavelength increases within 1.2-1.6 μm, mode power confinement ability of the fundamental mode is lessened for single-mode propagation, whereas for multimode propagation it becomes enhanced.