Model-based far-field alignment algorithm for Gaussian beamlike single-mode optical devices

Single-mode device-to-fiber alignment automation is usually achieved with a classical mathematical optimization approach. We present a different approach, which is based on the identification of particular intrinsic characteristics of the coupled optical power and on estimating residual axial, transverse, and angular misalignments in the far field. Such a model-based approach is based on the physical nature of the optical coupling phenomenon and can replace or be complementary to already known automated alignment methods. An alignment algorithm is described and validated experimentally using two single-mode fibers as Gaussian beam emitter and receiver.     

Low-cost optical amplitude modulator based on a tapered single-mode optical fiber

A new, to our knowledge, modulator based on a tapered single-mode optical fiber is introduced. The electro-optic device consists of a tapered optical fiber placed on a resonator made of a piezoelectric material. An electrical signal applied to the piezoelectric material makes the taper bend, and that displacement produces a modulation in the intensity of the optical signal traveling through the fiber. This device is very easy to build and is low in cost. Because of its nature, this new device might be very useful in optical fiber sensors. Its performance is analyzed, and the results are discussed.     

Dynamic thermal response of single-mode optical fiber for interferometric sensors

The dynamic temperature phase sensitivity of a three-layer optical fiber is calculated for unjacketed as well as Al- and Hytrel-coated fibers. The calculations include both the variation of the refractive index with temperature and the thermally induced axial and radial strains. The calculated phase sensitivity indicates that it is currently possible to measure a 1-microdegree C temperature change at frequencies exceeding 50 kHz with 1 cm of a metal coated optical fiber.     

Excess loss of single-mode jacketed optical fiber at low temperature

This paper describes excess loss of single-mode jacketed optical fibers at low temperature. A structural model is proposed for the jacketed fiber in order to investigate the relationship between the structure and excess loss. This model clarifies the excess loss increase mechanism that any initial irregularity existing in the jacketed fiber increases due to shrinkage of the jacket and causes excess loss. These results show that reducing the buffer diameter and controlling the fiber deformation inside the jacket are effective in suppressing excess loss at low temperature.     

Fabrication and modeling of uniform-waist single-mode tapered optical fiber sensors

We report the fabrication and modeling of single-mode tapered optical fiber sensors. The fabrication technique consist of stretching a section of fiber with an oscillating flame torch. Such a process allows controllable fabrication of lossless tapered fibers with a uniform waist. The sensor transmittance is modeled with a simple ray optics approach. In the model, all the taper parameters are taken into account. Our results indicate that sensor sensitivity can be adjusted with the taper waist diameter. As an example a gold-coated tapered fiber is theoretically and experimentally analyzed.     

Enhanced two-photon excitation through optical fiber by single-mode propagation in a large core

Multiphoton excitation through optical fibers is limited by pulse broadening caused by self-phase modulation. We show that for short fiber lengths (approximately 2 m) two-photon excitation efficiency at the fiber output can be substantially improved by single-mode propagation in a large-area multimode fiber (10-microm core diameter) instead of a standard 5.5-microm core fiber. Measurements and numerical simulations of postfiber spectra and pulse widths demonstrate that the increase in efficiency is due to a reduction of nonlinear pulse broadening. Single-mode propagation in a large-core fiber is thus suitable for multiphoton applications for which pulse recompression is not possible at the fiber end.     

Thermal characteristics of optical pulse transit time delay and fiber strain in a single-mode optical fiber cable

Thermal characteristics of optical pulse transit time delay and fiber strain in a single-mode optical fiber cable are investigated theoretically and experimentally. Measurements of the transit time delay shift are made by a spatial interference technique using a 1.5-in long fiber, six-fiber unit, and cable. Experimental results for a jacketed fiber whose fiber axis is well centered in nylon coating are in good agreement with those predicted from the theory. A jacketed fiber whose fiber axis is positioned eccentrically from the jacket center exhibits a small change in fiber strain at low temperature due to fiber buckling compared with that for the well-centered jacketed fiber. The loss increase for the off-centered jacketed fiber is explained by the buckling model. Furthermore, thermal characteristics of the unit-type cable examined here are found to coincide with those for the constituent six-fiber unit.     

Bitapered fiber coupling characteristics between single-mode single-core fiber and single-mode multicore fiber

By splicing and tapering at the fusion point of one-core single-mode fiber and three- or four-core single-mode fiber, an effective bitapered fiber coupling technique is implemented. Based on the beam propagation method, the bitapered coupling characteristics between the one-core fiber and the multicore single-mode fiber are simulated and analyzed. The theoretical prediction is confirmed by the experimental results, and the difference between the simulation and the experimental results is also discussed.     

Impulse dispersion in a multimode optical fiber from its far-field radiation pattern

A simple geometric-optical theory predicts impulse broadening in multimode fibers using the far-field power distribution of an incoherently excited waveguide with arbitrary refractive-index profile. The accuracy is comparable with that of scalar optical methods if they do not incorporate mode coupling and a differential mode attenuation model.     

Supercontinuum generation in a single-mode optical fiber with novel dispersion profile

A novel dispersion model different from the conventional one is introduced to enhance supercontinuum generation in dispersion-flattened/decreasing fiber. The results show that supercontinuum spectrum generated in this fiber is by far wider than that in the conventional one,whose flat width can be nearly 1000 nm. The results further show that both the fiber parameters such as peak chromatic dispersion, dispersion differential constant and dispersion decreasing coefficient and the pump pulse parameters including pulse width, soliton order, frequency chirp are crucial to the flat ultra-wideband supercontinuum spectrum generated in this fiber.     

Highly efficient wideband continuum generation in a single-mode optical fiber by powerful broadband laser pumping

By pumping a single-mode optical fiber with a powerful broadband nonselective dye laser, we obtain a high-efficiency wideband continuum (530-930 nm) with nonlinear conversion efficiency exceeding 90%. Experimental conditions for a coherent regime of broadband stimulated Raman scattering are created, which in combination with the broadband self-phase modulation and the four-photon parametric processes leads to a spectral broadening and to the continuum formation. The influence of the pump laser spectral linewidth on the nonlinear conversion efficiency is analyzed and investigated by comparative experiments at narrow-band and broadband laser excitations.     

Passive single-mode fiber depolarizer

We analyze and demonstrate a passive single-mode fiber depolarizer by using cascaded 2 x 2 couplers, recirculating delay lines, and a commercial laser diode. Design criteria and principles are discussed. We reduced the degree of polarization (DOP) to less than 20 dB by using ten cascaded couplers. The DOP can be reduced even further with more couplers. Experiments illustrate that this depolarizer is insensitive to the input polarization state and can eliminate polarization noise in a polarization-sensitive fiber-optic system.     

Surface plasmon resonance in a metal-coated single-mode optical fiber with decreased normalized frequency

We have numerically simulated the excitation of surface plasmon resonance in a bent metal-coated single-mode optical fiber with decreased normalized frequency. It is established that an optimum combination of the normalized frequency, bending radius, and film thickness favors effective energy transfer from guided radiation to a plasmon-polariton wave excited on the surface of a metal film deposited onto the optical cladding. It is shown that, using this effect, it is possible to measure the refractive index (RI) of an external medium with a spectral sensitivity of 70 δm/RI unit and a resolution of up to 3 × 10⁻⁷ RI unit.

     

Discrimination between strain and temperature by cascading single-mode thin-core diameter fibers

A simple fiber-optic sensor capable of discrimination between temperature and strain is proposed and experimentally demonstrated. The sensor head is formed by cascading two sections of single-mode thin-core diameter fibers (TCFs) that act as two different inter-modal interferometers (IMIs). Due to the different sensitivity responses of the two IMIs to strain and temperature, it is possible to discriminate temperature and strain by monitoring the resonant wavelength shifts. The experimental results indicate that the measured strain and temperature resolutions are 37.41 με and 0.732 °C within a strain range of 0-1333.3 με and a temperature range from 26.9 °C to 61.7 °C. The sensing sensitivities of strain and temperature are -1.03 pm/με and 30.74 pm/°C, respectively. The proposed sensor features the advantages of easy fabrication, low cost and high sensitivity, and it exhibits great potential in dual-parameter measurement.     

液晶光学相控阵的改进型口径拓展方法及远场特性分析

天线通光口径是液晶光学相控阵的重要技术指标,本文在多子阵并行驱动和两级器件级联方法(PAPA)的基础上,提出改进型的i-PAPA方法,通过对COM电极进行分区域驱动,在单个相控阵天线上实现大口径相控光束控制,具备单器件工作、插损低等优点。通过数值仿真分析,结果表明：相控阵天线后的近场相位分布连续;当指向角度在0°到+6°范围内,远场衍射效率和指向角度的数值关系呈现平滑单调下降,衍射效率均大于48%;当指向角度在0°到+3°范围内,衍射效率均大于80%。     

Long-period grating fabricated by periodically tapering standard single-mode fiber

We fabricated an asymmetric long-period grating (LPG) by periodically tapering a section of standard single-mode fiber using a resistive filament heating. The LPG exhibits large peak transmission attenuation of -30.31 dB with only 22 periods in a 1.0 cm long optical fiber and possesses unique characteristics for sensing applications. The bending and strain sensitivities are 1.74 nm m and 1.11 pm/mu epsilon, respectively. The polarization dependent loss is large, up to 11.65 dB, which is caused by an asymmetric index profile in the cross section of the tapered LPG.     

Effect of grading in refractive index profile on Kerr nonlinear optical processes in single-mode sub-wavelength diameter optical fibre using a straightforward method

A significant effect of nature of refractive index grading in the core of sub-wavelength diameter optical fibres on their performance characteristics in single-mode region is predicted and reported, for the first time, in connection with achievement of waveguide enhancement of Kerr nonlinearity-based optical processes. The analysis is based on a straightforward approach involving Marcuse spot size formulations for single-mode graded index fibre. Our formulations and results provide useful information and insight for optimum transverse dimensions of graded index fibre together with normalized core nonlinearity coefficient corresponding to profile exponent of the power law profile and should invite attention of system designers.