Suppression of cladding-mode coupling loss in Bragg grating using standard single-mode fiber

We present a technique for suppressing cladding-mode coupling loss in fiber Bragg grating fabrication. Suppression of cladding-modes down to 0.2 dB in a Bragg grating of 18dB reflectivity has been achieved in hydrogen-loaded standard single-mode fiber.

High-efficiency generation in a short random fiber laser

We demonstrate a high-efficiency random lasing in a 850 m span of a phosphosilicate fiber. Random distributed feedback owing to the Rayleigh backscattering in the fiber enables narrowband generation with output power of up to 7.3 W at the Stokes wavelength λS = 1308 nm from 11 Wof the pump power at λP = 1115 nm. The laser demonstrates unique generation efficiency. Near the generation threshold, more than 2 W of output power is generated from only 0.5 W of pump power excess over the generation threshold. At high pump power, the quantum conversion efficiency defined as a ratio of generated and pump photons at the laser output exceeds 100%. Itis explained by the fact that every pump photon is converted into the Stokes photon far from the output fiber end, while the Stokes photons have lower attenuation than the pump photons. © 2014 Astro Ltd.

Random distributed feedback fiber laser of ultimate efficiency

We demonstrate a random fiber laser of ultimate efficiency. More than 2 Watts are generated from 0.5W of pump excess over the generation threshold. At higher power, an optical efficiency corresponds to the quantum limit.

Modeling and optimization of cascaded erbium and holmium doped fluoride fiber lasers

Numerical modeling of cascade erbium-doped and holmium-doped fluoride fiber lasers is presented. Fiber lengths were optimized for cascade lasers that had fixed or free-running wavelengths using all known spectroscopic parameters. The performance of the cascade laser was tested against dopant concentration, energy transfer process, heat generation, output coupling, and pump schemes. The results suggest that the slope efficiencies and thresholds for both transitions increase with increasing Ho3+ or Er3+ concentration with the slope efficiency stabilizing after 1 mol% rare earth doping. The heat generation in the Ho3+-based system is lower compared to the Er 3+-based system at low dopant concentration as a result of the lower rates of multiphonon relaxation. Decreasing the output coupling for the upper (∼3 μm) transition decreases the threshold of the lower transition and the upper transition benefits from decreasing the output coupling for the lower transition for both cascade systems. The highest slope efficiency was achieved under counter-propagating pump conditions. Saturation of the output power occurs at comparatively higher pump power with dilute Er3+ doping compared with heavier doping. Overall, we show that the cascade Ho3+ -doped fluoride laser is the best candidate for high power output because of its higher slope efficiency and lower temperature excursion of the core and no saturation of the output. © 2013 IEEE.

Biconical-taper-assisted fiber interferometer with modes coupling enhancement for high-sensitive curvature measurement

A modal interferometer based on multimode-singlemode-multimode fiber structure built with a biconical taper for fiber curvature measurement is proposed and experimentally demonstrated. Due to the tapered singlemode fiber acting as a high-efficient mode power converter to enhance the modes coupling, curvature sensor with improved sensitivity is achieved by monitoring the defined fringe visibility of the interference spectrum. The measuring range can be tuned by changing the waist diameter of the fiber taper. Meanwhile, the sensor shows an intrinsic ability to overcome the influence of temperature cross-sensitivity and the power fluctuation of light source. The advantages of easy fabrication, high-quality spectrum with improved sensitivity, and small hysteresis will provide great potential for practical applications of the sensor. © 2013 Springer-Verlag Berlin Heidelberg.

Proposal and design of phase-modulated fiber gratings in transmission for pulse shaping

A novel approach to pulse shaping using a phase-modulated fiber Bragg grating (FBG) in transmission is proposed and designed. We show that phase-modulated FBGs can provide transmission responses suitable for pulse shaping applications, offering important technological feasibility benefits, since the coupling strength remains basically uniform in the grating. Moreover, this approach retains the substantial advantages of FBGs in transmission, such as optimum energy efficiency, no requirement for an optical circulator, and robustness against fabrication errors.

Bending and orientational characteristics of long period gratings written in D-shaped optical fiber

Long period gratings (LPGs) were written into a D-shaped single-mode fiber. These LPGs were subjected to a range of curvatures, and it was found that as curvature increased, there was increasingly strong coupling to certain higher order cladding modes without the usual splitting of the LPGs stopbands. A bend-induced stopband yielded a spectral sensitivity of 12.55 nm·m for curvature and 2.2×10-2 nm°C-1 for temperature. It was also found that the wavelength separation between adjacent bend-induced stopbands varied linearly as a function of curvature. Blue and red wavelength shifts of the stopbands were observed as the sensor was rotated around a fixed axis for a given curvature; thus, in principle, this sensor could be used to obtain bending and orientational information. The behavior of the stopbands was successfully modeled using a finite element approach.

Measurements of polarimetric sensitivity to hydrostatic pressure, strain and temperature in birefringent dual-core microstructured polymer fiber

We experimentally characterized a birefringent microstructured polymer fiber of specific construction, which allows for single mode propagation in two cores separated by a pair of large holes. The fiber exhibits high birefringence in each of the cores as well as relatively weak coupling between the cores. Spectral dependence of the group and the phase modal birefringence was measured using an interferometric method. We have also measured the sensing characteristics of the fiber such as polarimetric sensitivity to hydrostatic pressure, strain and temperature. Moreover, we have studied the effect of hydrostatic pressure and strain on coupling between the cores.

Bending and orientational characteristics of long period gratings written in D-shaped optical fiber

Long period gratings (LPGs) were written into a D-shaped single-mode fiber. These LPGs were subjected to a range of curvatures, and it was found that as curvature increased, there was increasingly strong coupling to certain higher order cladding modes without the usual splitting of the LPGs stopbands. A bend-induced stopband yielded a spectral sensitivity of 12.55 nm · m for curvature and 2.2 × 10-2 nm°C-1 for temperature. It was also found that the wavelength separation between adjacent bend-induced stopbands varied linearly as a function of curvature. Blue and red wavelength shifts of the stopbands were observed as the sensor was rotated around a fixed axis for a given curvature; thus, in principle, this sensor could be used to obtain bending and orientational information. The behavior of the stopbands was successfully modeled using a finite element approach.

Low thermal sensitivity grating devices based on ex-45° tilting structure capable of forward-propagating cladding modes coupling

The authors describe a detailed investigation on tilted fiber Bragg grating (TFBG) structures with tilted angles exceeding 45°. In contrast to the backward mode coupling mechanism of Bragg gratings with normal and small tilting structures, the ex-45° TFBGs facilitate the light coupling to the forward-propagating cladding modes. The authors have also theoretically and experimentally examined the mode coupling transition of TFBGs with small, medium, and large tilt angles. In particular, experiments are conducted to investigate the spectra and far-field distribution, as well as temperature, strain, and refractive-index sensitivities of ex-45° devices. It has been revealed that these ex-45° gratings exhibit ultralow thermal sensitivity. As in-fiber devices, they may be superior to conventional Bragg and long-period gratings when the low thermal cross sensitivity is required.

Side-detection of out-coupled core light from a microfluidic fiber microslit

The interactions of the core-propagating light with an intersecting microslit within a conventional single-mode fiber are investigated. Orientation-dependent out-coupling of core light was utilized to create side-detection, miniature fiber rotation sensors.

Side-detection of out-coupled core light from a microfluidic fiber microslit

The interactions of the core-propagating light with an intersecting microslit within a conventional single-mode fiber are investigated. Orientation-dependent out-coupling of core light was utilized to create side-detection, miniature fiber rotation sensors.

New fiber laser architecture with transform-limited nonlinear spectral compression

We numerically demonstrate a new fiber laser architecture supporting spectral compression of negatively chirped pulses in passive normally dispersive fiber. Such a process is beneficial for improving the energy efficiency of the cavity as it prevents narrow spectral filtering from being highly dissipative. The proposed laser design provides an elegant way of generating transform-limited picosecond pulses. © 2012 IEEE.

Design of an ultrafast all-optical differentiator based on a fiber Bragg grating in transmission

We propose and analyze a first-order optical differentiator based on a fiber Bragg grating (FBG) in transmission. It is shown in the examples that a simple uniform-period FBG in a very strong coupling regime (maximum reflectivity very close to 100%) can perform close to ideal temporal differentiation of the complex envelope of an arbitrary-input optical signal.

Amplifier similariton fiber laser with nonlinear spectral compression

We propose a new concept of a fiber laser architecture supporting self-similar pulse evolution in the amplifier and nonlinear spectral pulse compression in the passive fiber. The latter process allows for transform-limited picosecond pulse generation, and improves the laser’s power efficiency by preventing strong spectral filtering from being highly dissipative. Aside from laser technology, the proposed scheme opens new possibilities for studying nonlinear dynamical processes. As an example, we demonstrate a clear period-doubling route to chaos in such a nonlinear laser system.