Delivery of nanojoule femtosecond pulses through large-core microstructured fibers

We investigate femtosecond-pulse propagation through large-core microstructured fibers. Although these fibers are highly multimode, excitation of the fundamental mode is readily achieved, and coupling to higher-order modes is weak even when the fiber is bent or twisted. For prechirped input pulses with energies as large as 3 nJ, pulses as short as 140 fs were produced at the output of the fiber. Such a system could prove to be extremely useful for applications such as in vivo multiphoton microscopy and endoscopy that require delivery of femtosecond pulses and collection of fluorescence.     

塑料光纤微分模延迟的频域相移法研究

塑料光纤的微分模延迟数据对研究塑料光纤的色散特性具有非常重要的意义。采用频域相移法测量了不同长度的两种折射率分布塑料光纤：阶跃型塑料光纤(SI—POF)和渐变型塑料光纤(GI—POF)的微分模延迟曲线。实验结果表明,由于短光纤可以忽略模式耦合和模式损耗,1m塑料光纤的微分模延迟测量值与理论计算值吻合得非常好。因此,频域相移法可以简单、方便、精确地测量塑料光纤的微分模延迟。此外,通过比较不同长度塑料光纤的微分模延迟曲线的变化,可在一定程度上分析光纤中存在的模式耦合。     

光子晶体光纤中布里渊散射声波模式特性的分析

推导了光子晶体光纤中声波微小位移波动方程; 研究了泵浦波长以及纤芯折射率对声波模式的影响; 应用石英圆柱模型研究了小芯径光子晶体光纤中纤芯直径对布里渊声波模式色散的影响. 结果表明在光子晶体光纤中, 纵向声波和横向声波共同作用产生质点声场, 两者相互耦合将产生混合声波模式; 可以通过改变泵浦波长或光子晶体光纤纤芯折射率来改变参与布里渊散射(BS) 过程的声波模式的传播常数; 随着光子晶体光纤(PCF) 纤芯直径的增大, 声波模式耦合程度得到加强, 相速度呈减小趋势, 且同一传播常数下, 声波模式数呈增多趋势; 随着泵浦波频率的增大, 声波相速度减小.     

Small-core single-mode microstructured polymer optical fiber with large external diameter

A preform sleeving technique is demonstrated that allows the fabrication of single-mode polymer microstructured fiber with the smallest core and hole dimensions yet reported to our knowledge. For a fixed triangular hole pattern a range of fibers is produced by adjustment to the operating conditions of the draw tower. Numerical modeling is carried out for one of the fibers produced with a 570-microm external diameter, a core diameter of 2.23 microm, an average hole diameter of 0.53 microm, and an average hole spacing of 1.38 microm. This fiber was shown to be endlessly single mode.     

Influence of the angle-dependence of mode coupling on optical power distribution in step-index plastic optical fibers

Determination of the power distribution in step index plastic optical fibers by the power flow equation has been reported in the literature both with and without the simplifying assumption of constant coupling that is independent of the angle of light propagation. The need for this assumption is evaluated in this paper. Results with the angle-dependent coupling coefficient are compared to those derived under the simplifying assumption of constant coupling. Benchmarked to values measured experimentally, this comparison covered the coupling lengths L_c (denoting where the equilibrium mode distribution is achieved in the fiber) and lengths z_s (for achieving the steady-state distribution). Results differ slightly but only for longer fiber lengths, thus largely vindicating the simplifying assumption of constant coupling.     

Low-Loss Many-to-One Fiber Couplers with Few or Single-Moded Inputs and a Multi-Mode Output

The coupling of light from a number of few or single-moded fibers into a single multi-mode fiber is analyzed using geometric optics, and simple results demonstrating mode conservation are derived. Coupling from multiple single-mode fibers into a multi-mode fiber is investigated in detail using the overlap integral to determine coupling into each mode of the output fiber as a function of the light phase in the inputs. As well as results with practical relevance to fiber tapped delay-line filters and optical CDMA, the analysis provides pedagogic insight into light propagation and the light-gathering properties of fiber.     

Low-Loss Many-to-One Fiber Couplers with Few or Single-Moded Inputs and a Multi-Mode Output

The coupling of light from a number of few or single-moded fibers into a single multi-mode fiber is analyzed using geometric optics, and simple results demonstrating mode conservation are derived. Coupling from multiple single-mode fibers into a multi-mode fiber is investigated in detail using the overlap integral to determine coupling into each mode of the output fiber as a function of the light phase in the inputs. As well as results with practical relevance to fiber tapped delay-line filters and optical CDMA, the analysis provides pedagogic insight into light propagation and the light-gathering properties of fiber.     

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.     

利用超格子模型对布拉格光纤的研究

为了研究布拉格光纤的模式特征和传输特性,提出了超格子模型,利用傅里叶级数表示光纤横向折射率分布,利用平面波展开法分析布拉格光纤的能带结构,基于厄米-主斯函数的局域正交函数展开法,从全矢量耦合波动方程出发,得到关于模式传播常量和电场展开系数的本征方程,从而分析布拉格光纤的模式特征。以高折射率芯布拉格光纤为例,实现了该算法,得到基模与二次模的横向电场分布、基模色散曲线和模式双折射。基模的模式双折射可用于衡量算法的精度,结果表明该算法精度较高。超格子模型不仅可以用于研究高折射率芯布拉格光纤,而且同样可以研究低折射率区域导光的布拉格光纤。     

Propagation of femtosecond pulses in large-mode-area, higher-order-mode fiber

We demonstrate propagation of 14 nJ femtosecond pulses through a large-mode-area, higher-order-mode (HOM) fiber with an effective area of 2100 microm2. The pulses propagate stably in the LP07 mode of the fiber through lengths as long as 12 m. The strongly chirped pulses exiting the amplifier fiber are dechirped by the high-order-mode fiber, resulting in pulses with a peak power of 61 kW after propagation in 5 m of the positive-dispersion fiber. A small amount of self-phase modulation is observed in the compressed pulses and is described well by a nonlinear Schr?dinger equation model that takes into account the measured effective area and dispersion of the HOM fiber.     

Saturated absorption spectroscopy of acetylene gas inside large-core photonic bandgap fiber

Saturated absorption spectroscopy is performed on the acetylene nu(1) + nu(3) band near 1532 nm inside photonic bandgap fibers of small (approximately 10 microm) and large (approximately 20 microm) core diameters. The observed linewidths are narrower in the 20 microm fiber and vary from 20 to 40 MHz depending on pressure and power. Variations in the background light transmission, attributed by others to surface modes, are significantly reduced in the 20 microm fiber. The optimum signal for use as a frequency reference in a 0.8 m long, 20 microm diameter fiber is found to occur at about 0.5 torr for 30 mW of pump power. The saturation power is found by modeling the propagation and attenuation of light inside the fiber.     

Ultra‐large effective‐area,higher‐order mode fibers: a new strategy for high‐power lasers

This paper describes the physics and properties of a novel optical fiber that would be attractive for building high‐power fiber lasers and amplifiers. Instead of propagating light in the fundamental, Gaussian‐shaped mode, we describe a fiber in which the signal is forced to travel in a single, desired higher order mode (HOM). This provides for several advantages over the conventional approach, ranging from significantly higher ability to scale mode areas (and hence laser powers) to managing dispersion for ultra‐short pulses – a capability that is practically nonexistent in conventional fibers. Particularly interesting is the fact that this approach challenges conventional wisdom, and demonstrates that for applications requiring meter‐length fibers (as in high‐power lasers), signal stability actually increases with mode order. Using this approach, we demonstrate mode areas exceeding 3200 μm², and propagate signals with negligible mode distortions over up to 50‐meter lengths. We describe several pulse propagation experiments in which we test the nonlinear response of this fiber platform, ranging from managing dispersive effects in femtosecond pulse systems, to reducing Brillouin scattering impairments in systems operating with the nanosecond pulses.     

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.     

Mechanism of mode coupling in multicore fiber lasers

We present what is believed to be the first experimental demonstration of a new mechanism of mode coupling in multicore fibers (MCFs) based on their indirect interaction inside the fiber via intermediate mode, analogous to the Bragg mode, which is very sensitive to bending of the fiber. Very strong coupling between the core modes regardless of large spacing (approximately 28 microm) between them has been demonstrated in the MCF laser as well as in the probe beam schemes. 70% of power conversion from one core to another with beating length of tens of centimeters in 4-core MCF is measured.     

少模光纤布拉格光栅折射率传感的分析与测量

理论分析和模拟计算了少模光纤布拉格光栅基模及高阶模的耦合与传输特性,得到在相同外部折射率变化情况下,少模光纤基模与高阶模耦合对应的布拉格波长变化,比正、反向基模之间耦合对应的布拉格波长变化显著增大.实验上制作了少模光纤布拉格光栅,测量了基模之间以及基模与高阶模之间对应的布拉格波长随外部折射率、温度变化的情况,得到与理论分析相符的结果.而对于温度变化对折射率测量结果干扰的问题,提出了通过计算布拉格波长差来克服温度影响的方法.这些结果为采用布拉格光纤光栅测量外部折射率变化提供了一种新的途径.     

Measurement of fiber-cladding diameter uniformity by use of whispering-gallery modes: nanometer resolution in diameter variations along millimeter to centimeter lengths

Variations in fiber-cladding diameter on a nanometer scale were measured along millimeter to centimeter lengths by use of whispering-gallery modes (WGMs) in the elastic scattering of the fiber. The fiber was side coupled with a wavelength-tunable Gaussian beam. The scattered light was imaged approximately 1:1 onto a multichannel photodiode array detector. Based on the WGM wavelength shifts along the fiber, the taper of the fiber cladding's diameter was measured on a nanometer/millimeter scale. The fiber's surface roughness amplitude (in nanometers) and granular size ( approximately 100 microm) along centimeter-length fibers could also be revealed by use of higher- Q resonances.     

Ultrashort-pulse investigation of the propagation properties of the LP(11) mode in 1.55-mum communication fibers

The coupling of an ultrashort laser pulse into a single-mode optical communication fiber gives rise to two propagating pulses as a result of the excitation of two guided modes, the fundamental, LP(01) , and the leaky, LP(11) . Such a phenomenon provides a new approach to the study of the propagation properties of the LP(11) mode. An experiment with tunable 110-fs pulses at a wavelength near 1550 nm is described. Information about the group velocity, the polarization-rotation length, the attenuation coefficient, and the cutoff wavelength of the LP(11) mode is obtained in a simple and direct way for various fibers.     

Single-mode propagation in triangular tube lattice hollow-core terahertz fibers

The mode properties of hollow-core fibers with a cladding formed by a periodic arrangement of dielectric tubes in a triangular lattice for THz applications are numerically investigated. The fiber supports a high number of modes. Effectively single-mode operation can be obtain by reducing the core size, but at the expense of high fundamental mode propagation loss. Single-mode propagation can be obtained by exciting the fiber with a linear polarized gaussian beam with proper spot size.     

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.     

新型缓变锥型PCF结构接口耦合损耗特性

提出一种新型的缓变锥型光纤接口器件。该接口器件是一个长度为150mm左右的缓变锥型光子晶体光纤。利用有限元方法对模场直径差别较大的不同光纤进行拼接时的损耗问题进行研究。研究结果表明:在模场直径分别为9μm和10.4μm的光纤之间加入锥型光纤后可以有效降低耦合损耗,并且在不同波长下耦合损耗都维持在一个较低水平。因此,该接口器件能够实现光纤拼接时物理结构的过渡和不同模场直径的转换,从而使拼接损耗降到最低。