Performance of a wavelength-tunable erbium-doped fiber laser using a Sagnac interferometer

We experimentally demonstrate a wavelength-tunable erbium-doped fiber laser that is composed of a ring cavity and a single-mode fiber Sagnac interferometer in a new and simple arrangement. We find that the fiber laser output wavelength is tunable by adjusting the filter effect of the Sagnac fiber loop through a fiber polarization controller set there. The quasi-single-wavelength continuously tunable laser outputs could be achieved within some wavelength range. The multi-wavelength laser outputs could also be observed under some appropriate settings of the polarization controller. A theoretical demonstration of the wavelength tunability about the transmission-type Sagnac loop filter has also been achieved using the Jones calculus theory.     

Tunable microwave photonic filter based on chirped fiber gratings working with a single optical carrier at constant wavelength

This paper presents a tunable transversal filter working with a single optical carrier at constant wavelength. The filter consists of a set of chirped gratings whose time delay is tuned with respect to the emission wavelength of a fiber laser by a piezoelectric actuator; extra lengths of fiber are inserted in the filter arms in order to avoid interferences between signals reflected in different gratings. Two and three taps filters are experimentally demonstrated, the filters transfer function is electronically tuned within the free spectral range.     

扩芯光纤原理及其在光器件耦合中的应用

介绍了两种可以增大单模光纤模场直径并出射准直平行光束的扩芯光纤(ECF)的原理和制作方法。分析了单模光纤熔接渐变折射率多模光纤法通过改变渐变折射率多模光纤的长度和自聚焦参量实现模场扩大缩小的原理,制作的扩芯光纤模场直径扩大到16．6μm,出射光束平行效果较好,轴向耦合容限比单模光纤扩大了近6倍。加热扩芯光纤则是通过控制加热温度和加热时间直接使单模光纤掺杂物质发生扩散,从而实现扩束和光束准直,模场直径达到15．4μm,横向、轴向耦合容限都比单模光纤有很大提高。因此扩芯光纤可以简化单模光纤的耦合对准过程,用来制作新型的单模光纤或掺铒光纤连接器,也可以用于其它光器件中与单模光纤的准直。     

Switchable Multi-Wavelength Erbium-Doped Fiber Lasers based on a Mach-Zehnder Interferometer Using a Twin-Core Fiber

A switchable multi-wavelength erbium-doped fiber ring laser based on a compact in-fiber Mach-Zehnder interferometer comb filter at room temperature is presented. The comb filter is formed by splicing a section of twin-core fiber between two single mode fibers. By adjusting the states of the polarization controller appropriately, the laser can be made to operate in stable single-, dual- and three-wavelength lasing states. The operation principle is based on spectral hole burning induced by the saturated effect and polarization hole burning.     

Detection of liquid-level variation using a side-polished fiber Bragg grating

A liquid-level sensor based on a side-polished fiber Bragg grating (FBG) is proposed and experimentally demonstrated. The sensor can detect height variation of liquids of arbitrary refractive index (RI). For liquids with RI lower than that of fiber core, liquid-level variation can be monitored by the peak power difference of the grating segments surrounded by the liquid and air. For liquids with RI higher than that of fiber core, liquid-level information can be obtained from the influence of the shortening of the effective length of the immersed grating segment upon the reflection spectrum.     

Improving fiber to waveguide coupling efficiency by use of a highly germanium-doped thermally expanded core fiber

Possessing a gradient refractive index (RI), thermally expanded core (TEC) fibers have been studied to be able to improve coupling efficiency of waveguide-fiber or LD-fiber according to the qualitative analysis of geometrical optics. The experiments show that the mode field radius (MFR) expansion of single-mode fiber with highly Germanium-doped (HGD) is more effective than that of conventional single-mode fiber (SMF). The coupling efficiency of a fabricated TEC fiber with HGD can enhance 5%–6% compared with that of a conventional-SMF. So the TEC fiber with HGD is a good candidate for the coupling of waveguide-fiber or LD-fiber.     

Passive Q-switching of a fiber laser using a side-polished birefringent fiber with index matching gel spread on the flat side

We experimentally demonstrate the use of a side-polished birefringent fiber with index matching gel spread on the flat side as a passive Q-switch for the implementation of an all-fiberized, erbium-doped fiber (EDF)-based Q-switched laser. It is shown that Q-switched pulses with a ~2.5-μs temporal width are readily achievable from an EDF ring cavity using the side-polished birefringent fiber-based Q-switch. The tuning capability of the pulse width and the repetition rate by changing the pump power is also investigated.     

Transmission Bandwidth Tunability of a Liquid-Filled Photonic Bandgap Fiber

A temperature tunable photonie bandgap fiber （PBGF） is demonstrated by an index-guiding photonic crystal fiber filled with high-index liquid. The temperature tunable characteristics of the fiber are experimentally and numerically investigated. Compression of transmission bandwidth of the PBGF is demonstrated by changing the temperature of part of the fiber. The tunable transmission bandwidth with a range of 250nm is achieved by changing the temperature from 30℃ to 90℃.     

Cladding index modulated fiber grating

A new type of fiber grating with only cladding index modulation is presented. Characteristics of both cladding index modulated short-period fiber grating (FBG) and long-period fiber grating (LPFG) are analyzed. The calculation of the modes involved in this paper is based on a model of three-layer step-index fiber geometry. Transmission of a mode guided by the core through a cladding index modulated grating when evanescent field coupling occurs is analyzed with couple-mode theory. Evanescent field coupling causes a power flowing from the core to the cladding, so the attenuation of the new grating is analyzed as well. Lower attenuation, flexible spectral characteristics are demonstrated in comparison with traditional fiber core index modulated grating.     

Electromagnetic field confined and tailored with a few air holes in a photonic-crystal fiber

Conventional and scanning near-field optical microscopy techniques are cross referenced to femtosecond nonlinear-optical measurements and finite-element numerical simulations to visualize and analyze a strong confinement of electromagnetic radiation in guided modes of a photonic-crystal fiber with only a few air holes surrounding the fiber core. A nonlinear coefficient of about 120 W⁻¹ km⁻¹ is achieved at the wavelength of 670 nm for a fused-silica fiber with a full hexagonal cycle of closely packed air holes around the fiber core. The removal of a single element from this array of air holes is shown to frustrate field confinement in guided modes, leading to mode leakage.     

Narrow spectral response of a Brillouin generator

We analyze both theoretically and experimentally the temporal response of a stimulated Brillouin scattering generator to an harmonically modulated pump beam in a 2.1 km optical fiber. It is shown that at certain frequencies (where m is an integer number and are frequencies which depends mainly on the fiber’s length L but also on the beam’s intensity via G) the first harmonic of the generated Stokes beam is relatively suppressed but never vanishes. These frequencies are considerably smaller than the Brillouin’s spectral width (∼20 MHz). Excellent agreement with the analytical model is presented.     

Influence of a holey cladding structure on spectral characteristics of side-polished endlessly single-mode photonic crystal fibers

We experimentally investigate the spectral characteristics of side-polished endlessly single-mode photonic crystal fibers of different polishing depths and radii of curvature that are important to mode field diameter, evanescent coupling, waveguide losses, higher-order mode excitation, and dispersion slopes of photonic crystal fiber components. A polished photonic crystal fiber with a greater polishing depth or a larger radius of curvature shows a more dispersive characteristic. In contrast to conventional single-mode fibers, in photonic crystal fibers the evanescent field is more strongly localized, and the propagating light can be more efficiently guided within the deformed core.     

All-solid silica-based photonic crystal fibers

An index-guiding all-solid photonic crystal fiber (PCF) composed entirely of silica material is proposed in this paper. The core of this optical fiber is composed of pure silica, and the cladding consists of doped silica rod in the background of pure silica. The dependence of confinement loss on the diameter of the doped rods, the number of doped-rod rings, and the doping level is investigated numerically. In addition, the proposed fiber possesses a shorter cutoff wavelength as compared with the air/silica PCF, which is directly confirmed by the V parameter, and explained based on a scalar approximation method. Furthermore, the bending loss for the fiber is predicted. A low-loss single-mode all-solid silica-based PCF with a large-mode-area is possible by the appropriate selection of configuration parameters.     

Nonlinear pulse propagation and supercontinuum generation in photonic nanowires: experiment and simulation

We investigate theoretically and experimentally the process of supercontinuum generated in sub-wavelength waveguides. We observe experimentally that supercontinuum generated in these photonic nanowires is increasingly blue-shifted from the pump wavelength for decreasing minimum core diameters. We also find the spectral features are sensitive to the specific nanowire profile. Numerical simulations using the nonlinear envelope equation show that accurate modeling requires consideration of the nonlinearity and full dispersion along the entire nanowire profile as well as a wavelength dependent loss. Specifically, the blue-shifting is found to result from an increasing loss for wavelengths larger than the core diameter.     

Multi-point abnormal-temperature warning sensor system with?different thresholds

We propose and experimentally demonstrate a time-division-multiplexing-based (TDM-based) multi-point abnormal-temperature warning sensor system with different thresholds. A multi-channel fiber Bragg grating (FBG) serving as the wavelength selector is employed in the fiber ring laser to generate a multi-wavelength pulsed light source. The sensor array is composed of multiple uniform sensing FBGs at different positions and with different nominal wavelengths. The warning signal is obtained by only monitoring the time slot between the injected pulse and the reflected pulse. The measurement range and resolution are theoretically discussed and experimentally demonstrated.     

Fabrication of low OH loss holey fibers with varying air hole sizes and their optical properties

We experimentally investigate a flexible fabrication technique for low OH and transmission losses holey fibers with a Ge-doped core and air holes in a silica cladding region. Versatile holey fibers of different size, pitch, and shape of air holes were achieved by controlling the temperature and heating time of the holey fiber preform. In addition, we suppress the OH loss of less than ∼0.323 dB/km at 1383 nm. After fabricating holey fibers, we measure their optical properties including cut-off wavelength, mode field diameter, splicing loss, dispersion, bending loss, and polarization dependent loss based on the size of air holes. The total transmission loss was measured to be ∼0.226 dB/km at 1550 nm by improving the fabrication process. After fabricating optical patch cord based on holey fibers, we measured the long-term stability of the fabricated holey fiber by using the temperature cycling technique for 24 and obtained low power fluctuation of 0.2 dB. We achieve the high quality holey fiber with a low bending loss of ∼0.04 dB/turn under a bending radius of 2.5 mm at 1550 nm. We also obtain a tunable band rejection filter with a number of bending turns.     

Design of defect-core in highly birefringent photonic crystal fibers with anisotropic claddings

The influence of defect-core on the birefringence and confinement losses of rectangular-lattice photonic crystal fibers are investigated numerically by applying the multipole method. Numerical results illustrate that the birefringence in such fibers is determined not only by the arrangement of air holes in the cladding but also the shape of the core. It is found that asymmetry of the core represented by its rectangular shape implies a higher effective index of the mode that is parallel with the longer side of the rectangle, whereas the anisotropic rectangular-lattice cladding gives rise to just the opposite effect and thus the resulting birefringence can be controlled by a proper combinations of both mechanisms. In particular, effect of the asymmetry of the core on the birefringence is dominant for shorter wavelength. Increased birefringence and reduced confinement loss can be achieved, if we form the core by the omission of several air holes in a row to reduce its negative effect on the birefringence. On the other hand, when asymmetry is increased in the other direction, a negative birefringence at shorter wavelength can be achieved. This occurs due to the fact that asymmetry of the core at higher frequencies overcomes the effect of the asymmetric cladding. As a result, its possible to achieve zero birefringence in anisotropic cladding photonic crystal fiber with an asymmetric core.     

Design and fabrication of an intrinsically gain flattened Erbium doped fiber amplifier

We report design and subsequent fabrication of an intrinsically gain flattened Erbium doped fiber amplifier (EDFA) based on a highly asymmetrical and concentric dual-core fiber, inner core of which was only partially doped. Phase-resonant optical coupling between the two cores was so tailored through optimization of its refractive index profile parameters that the longer wavelengths within the C-band experience relatively higher amplification compared to the shorter wavelengths thereby reducing the difference in the well-known tilt in the gains between the shorter and longer wavelength regions. The fabricated EDFA exhibited a median gain ?28 dB (gain excursion below ±2.2 dB within the C-band) when 16 simultaneous standard signal channels were launched by keeping the I/P level for each at −20 dBm/channel. Such EDFAs should be attractive for deployment in metro networks, where economics is a premium, because it would cut down the cost on gain flattening filter head.     

Designing dispersion-compensating photonic-crystal fibers using a genetic algorithm

A genetic algorithm is combined with a fully vectorial finite-element solver to design photonic-crystal fibers (PCFs) for a broadband dispersion compensation in a generic stretcher-compressor system of an ytterbium fiber laser. Two types of PCFs are compared in terms of their dispersion-compensation capability, optical nonlinearity, and confinement loss. Fibers of the first type are standard PCFs where a solid core is surrounded by a triangular uniform lattice of identical air-holes. In PCFs of the second type, the solid core is surrounded by a dual-scale cladding, where the inner part comprises air-holes of different diameters, while the outer cladding consists of large-diameter air-holes. Second-type PCFs are shown to provide a much more accurate dispersion compensation. The influence of fiber-fabrication tolerances on the precision of dispersion compensation in short-pulse fiber laser systems is examined.     

Indoor coverage improvement of MB-OFDM UWB signals with radio over POF system

A radio over fiber system using the fluorinated based polymer multimode fibers (PMMF) is presented in this paper for the enhancement of the indoor coverage of the multiband orthogonal frequency division multiplexing ultra-wideband standard (MB-OFDM UWB) inside a building. A preliminary part related the cost analysis owing to glass and polymer multimode fiber deployment inside a fiber network is reported. The study of the physical properties of the polymer optical fibers (core diameter, numerical aperture, differential mode delays, modal bandwidth…) is firstly performed in order to effectively exhibit the potentialities and the robustness of such fibers to be used in a low cost radio over fiber system. The DMD measurements of four fluorinated based polymer optical fiber are reported. The designed system operates at 850 nm with commercial off the shelf (COTS) devices combined to the intensity modulation/direct detection technique. The opportunity of using polymer fibers and COTS components to improve the indoor coverage of the MB-OFDM UWB standard is so reported by the measurement of the Error Vector Magnitude or the Relative Constellation Error variation as a function of the system parameters (RF power, optical attenuation, fiber length…) as well as the compliance of the eye diagram with the mask testing. By the way, the transmission performance of both 200 and 480 Mbps signals is demonstrated over up to 200 m link length of polymer multimode fibers: transmission penalties are quantified by relative constellation error with values under the standard requirements. A comparative study with classical OM2 50 μm based glass multimode fiber having the same bandwidth/length product than the PMMF is done.