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We report on the demonstration of a high power, high beam quality random fiber laser. It is a half-open cavity based on 130-m-long large mode area step-index fiber with a core diameter of 20 μm and a numerical aperture of 0.08. The random laser cavity, whose central wavelength is 1120 nm, is pumped by a 1070 nm Yb-doped fiber oscillator. The maximum output power is 418 W with respect to the injected pump power of 588 W, corresponding to the optical-to-optical efficiency of 71%. The output power, spectral, and temporal characteristics are carefully presented and discussed, showing that such a cavity can export high power random laser stably and effectively. The far field beam quality is also measured, and the profile is nearly a Gaussian shape. This work provides an impactful method to advance the output power of random fiber lasers.

In this Letter, we present, to the best of our knowledge, the largest effective single-mode fiber reported to date. The employed waveguide is a passive large pitch fiber (LPF), which shows the core area scaling potential of such a fiber structure. In particular, we achieved stable single-transverse mode transmission at a wavelength of 1.03 μm through a straight passive LPF with a pitch of 140 μm, resulting in a measured mode-field diameter of 205 μm.

We investigate high brightness pumping of a multi-kilowatt Yb fiber amplifier in a bi-directional pumping configuration. Each pump outputs 2 kW in a 200 μm, 0.2 NA multimode fiber. Gold-coated specialty gain fibers, with a 17 μm mode field diameter and a 5 dB/m pump absorption, have been developed. The maximum fiber amplifier output power is 3.1 kW, limited by multimode instability, with 90% O-O efficiency and M2

The spatial modes of a 1 km elliptical core few-mode optical fiber (6 spatial modes) are analyzed by using liquid crystal on silicon spatial light modulators to measure the fiber's mode crosstalk matrix in Hermite-Gaussian, Laguerre-Gaussian, and linearly polarized spatial mode bases. It is shown that the fiber's spatial modes can be described by Hermite-Gaussian modes, which can propagate 1 km over the optical fiber with

We present a scheme to realize a controllable, scalable, low-cost, and versatile all-fiber orbital angular momentum (OAM) converter. The converter consists of a two-mode fiber (TMF) with its input terminal welded with a single-mode fiber, a mechanical long-period grating (LPG), a mechanical rotator, metal flat slabs, and a fiber polarization controller. The LPG is employed to convert the fundamental fiber mode to higher-order modes and the flat slabs are used to stress the TMF to adjust the relative phase difference between two orthogonal higher-order modes. Selective conversion from the LP(01) mode to the LP(11a), LP(11b), OAM(-1), or OAM(+1) mode is demonstrated in the experiment.

A current limit in power scaling of Yb-doped fiber amplifiers is the sudden onset of mode instabilities. We investigated this effect on a single-frequency Yb-doped photonic crystal fiber amplifier with a low mode instability threshold power. By measuring the overlap of the fiber output beam with the fundamental mode of an external cavity to be about 95%, we could exclude significant modal power transfer below a sharp power threshold. Furthermore, we directly measured the frequency resolved intensity noise spectra. No fluctuations in the overall output power were observed, but for the modal content different oscillation regimes were identified.

International audience; A design of a polarizing all-glass Bragg fiber with a microstructure core has been proposed for the first time. This design provides suppression of high-order modes and of one of the polarization states of the fundamental mode. The polarizing fiber was fabricated by a new, simple method based on a combination of the modified chemical vapor deposition (MCVD) process and the rod-in-tube technique. The mode field area has been found to be about 870 μm2 near λ=1064 nm . The polarization extinction ratio better than 13 dB has been observed over a 33% wavelength range (from 1 to 1.4 μm ) after propagation in a 1.7 m fiber piece bent to a radius of 70 cm .

We report for the first time, to the best of our knowledge, on a large mode area Er-doped photonic crystal fiber laser system. The fiber core had a diameter of 40 μm and NA of

Very large-mode-area Yb(3+)-doped single-mode photonic bandgap (PBG) Bragg fiber oscillators are considered. The transverse hole-burning effect is numerically modeled, which helps properly design the PBG cladding and the Yb(3+)-doped region for the high-order mode content to be carefully controlled. A ratio of the Yb(3+)-doped region diameter to the overall core diameter of 40% allows for single-mode emission, even for small spool diameters of 15 cm. Such a fiber was manufactured and subsequently used as the core element of a cw oscillator. Very good beam quality parameter M(2)=1.12 and slope efficiency of 80% were measured. Insensitivity to bending, exemplified by the absence of temporal drift of the beam, was demonstrated for curvature diameter as small as 15 cm.

The effects of an axial magnetic field applied to a fiber laser cavity on the polarization characteristics of the fiber laser output are investigated. The experimental and theoretical results show sensitive changes in the polarization mode beat frequency as a function of the applied magnetic field.

We derive a far-field diffraction integral by (vector) electromagnetic theory; the result differs from the (scalar) Rayleigh-Sommerfeld or Kirchhoff result in that it depends on polarization. For unpolarized light, we derive a factor (1 + cos(2) theta)((1/2))/ radical2 that replaces the usual obliquity factor. Omitting this factor can cause an error of the same order as the measurement uncertainty in determining the mode-field diameter of a single-mode fiber from far-field data.

A new method is proposed for the determination of the spot size of a monomode fiber based on the theory of splice losses and involving measurements of the transmitted power as a function of misalignments of the fiber axes.

A fiber-optic magnetic-field sensor is constructed by bonding a single-mode fiber to a nickel cylinder. The magnetic field, by means of the magnetostrictive effect, alters the state of polarization of light in the fiber. A sensitivity of 1.76 x 10(-2) rad/m Oe is demonstrated, permitting detection of fields as small as 4.4 x 10(-6) Oe/m of fiber.

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.

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.

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.

A new configuration is proposed for continuously transforming aspect ratios of field-intensity distributions in optical fibers. The field aspect ratio varies in proportion to the angle between the principal axes of two cascaded graded-index oval-core fibers. The highest aspect-ratio conversion is obtained at an angle of 90 degrees. The conversion effect is numerically and experimentally confirmed, showing that a circular field is successfully transformed into an elliptical one with an aspect ratio as high as 9 at a wavelength of 0.98 microm.

A kind of sensing scheme is theoretically proposed to efficiently tune the response range of a fiber-optic refractometer based on the adiabatic transmission of the higher-order LP11 mode. Near the cut-off condition, transmission of the LP11 mode is a strong function of the refractive index (RI) under detection; thus high sensitivity is achieved. The cut-off RI value is dependent on the waist diameter; therefore the response RI range with high sensitivity can be changed just by altering the waist diameter. Theoretical calculations reveal that the response range is effectively tuned from 1.43-1.438 to 1.35-1.365 when the waist diameter is reduced from 2.5 to 1 μm. The proposed fiber-optic sensor is also superior when used as an absorbing sensor since the higher-order mode LP11 has a much larger power fraction in the evanescent field compared with the fundamental mode LP01 of the same fiber.

A theory for nonreciprocal phase shift caused by cross coupling generated in a polarization maintaining (PM) fiber optic gyroscope (FOG) under the combined action of magnetic and temperature fields is proposed. The magnetic-thermal coupling in the FOG originates from the interaction of the magnetic field, fiber twist, birefringence caused by thermal stress, and the intrinsic and bending birefringence of the fiber. The cross coupling changes with temperature. When the PM fiber has a diameter of 250 μm, beat length of 3 mm, length of 500 m, twist rate of 1 rad/m, and optical source wavelength of 1310 nm, the maximum degree of magnetic-thermal coupling generated by a 1 mT radial magnetic field within the temperature range of -20°C to 60°C is -5.47%.

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.

We discuss the applicability of well-established classical optical fiber theories to holey fibers. By appropriately defining the V parameter, we can easily estimate the fundamental properties of holey fibers, such as effective index, group-velocity dispersion, mode field diameter, beam divergence, and splice loss, through simple empirical expressions without the need for heavy numerical computations. We confirm the validity of the V parameter defined here by comparing the calculated results with the earlier experimental and numerical results.

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.

We report what is to our knowledge the first experimental demonstration of a photorefractive spatial mode converter (based on mutually pumped phase conjugation) that couples light efficiently from a multimode fiber into a singlemode fiber with an extremely large degree of tolerance to misalignment. Using an argon laser (514.5 nm) and a barium titanate crystal, we have demonstrated that the laser light can be coupled from a multimode fiber (core diameter 100 microm, numerical aperture 0.37) into a single-mode fiber (core diameter 2.9 microm, numerical aperture 0.11), with an efficiency of ~15% and an alignment tolerance of ~100 microm. The coupling efficiency is more than 2 orders of magnitude, and the tolerance to misalignments is more than 30 times better than the corresponding values achievable by conventional techniques.

Suitable functions fitted to transverse offset data for measuring the mode field diameter according to the Petermann II definition are investigated with the help of computer simulation using three different model functions. The Gaussian fit to points in the vicinity of the curve maximum gives the best results.