Light propagation with ultralarge modal areas in optical fibers

Power scaling of normal-dispersion continuum generation using higher-order modes in microstructured optical fibers

The use of a higher-order HE₁₂-like mode to produce weak normal dispersion over a substantial wavelength range in a microstructured optical fiber is investigated numerically. It is shown that the effective area, and thereby the pulse energy, can in this way be scaled by an order of magnitude compared to using the fundamental mode in a single-mode fiber. Multimode nonlinear simulations indicate that nonlinear mode coupling will not disturb single-mode operation in the HE₁₂ mode at least up to the threshold where polarization modulation instability sets in.

Controlled Excitation of Supermodes in a Multicore Fiber with a 5 × 5 Square Array of Strongly Coupled Cores

Coherent propagation of supermodes in a multicore fiber is promising for power scaling of fiber laser systems, eliminating the need for the active feedback system to maintain the phases between the channels. We studied the propagation of broadband pulsed radiation at a central wavelength of 1030 nm in a multicore fiber with coupled cores arranged in a square array. We designed and fabricated a silica multicore fiber with a 5 × 5 array of cores. For controllable excitation of a desired supermode, we developed a beam-forming system based on a spatial light modulator. We experimentally measured intensity and phase distributions of the supermodes, in particular, the in-phase and out-of-phase supermodes, which matched well the numerically calculated profiles. We obtained selective excitation and coherent propagation of broadband radiation with the content of the out-of-phase supermode of up to 90% maintained without active feedback. Using three-dimensional numerical modeling with allowance for a refractive index profile similar to those of the developed fiber, we demonstrated stable propagation of the out-of-phase supermode and collapse of the in-phase supermode at a high signal power.

Spectral broadening of 112 mJ, 1.3 ps pulses at 5 kHz in a LG10 multipass cell with compressibility to 37 fs

The first-order helical Laguerre-Gaussian mode (also called donut mode) is used to improve the energy throughput of nonlinear spectral broadening in gas-filled multipass cells. The method proposed in this Letter enables, for the first time to the best of our knowledge, the nonlinear spectral broadening of pulses with energies beyond 100 mJ and is suitable for an average power of more than 500 W while conserving an excellent spatio-spectral homogeneity of ∼98% and a Gaussian-like focus profile. Additionally compressibility from 1.3 ps to 37 fs is demonstrated.

Efficient low-brightness-pumped Raman amplification of a single high-order Bessel mode in 335-m of 70-µm-diameter silica-core step-index fiber

We experimentally demonstrate Raman amplification of signal pulses in a high-order Bessel mode (LP₀₆) at a wavelength of 1121 nm in a 335-m step-index fiber with a 70-µm diameter, 0.227-NA pure-silica core. This was pumped by 5-ns multimode pulses at 1065 nm from a Yb-doped fiber master oscillation power amplifier. The mode purity of the amplified pulses is well preserved to 23 dB of average-power gain, to 774 W of peak power in 2 ns pulses at a 20 kHz repetition rate, when pumped with a peak power of 942 W. The pump depletion as averaged over the signal pulses reaches 59%. We believe, to the best of our knowledge, that this is the first demonstration of stable mode propagation and Raman amplification of a single Bessel-like higher-order mode in a fiber of hundreds of meters. This shows the potential for efficient power scaling of a single signal mode with low-brightness pumping, comparable with that from continuous-wave multimode diode lasers.

Reconversion of higher-order-mode (HOM) output from cladding-pumped hybrid Yb:HOM fiber amplifier

We demonstrate operation of a cladding-pumped hybrid ytterbium-doped HOM fiber amplifier and reconversion of the HOM output to Gaussian-like beam by using an axicon based reconversion system. The amplifier was constructed by concatenating single-mode and HOM ytterbium-doped double clad fibers, and was excited by a common multimode pump source. A continuous wave (cw) input signal of 97mW was amplified to 100W at the amplifier output, which yielded a gain of more than 30dB. The LP_0,10 output of the HOM amplifier could be converted to a Gaussian-like beam with 67% conversion efficiency. We present, both analytically and numerically, the effects of scaling the beam size on axicon's apex angle, and how shape imperfections affect the mode converter's performance.

Multimode-pumped Raman amplification of a higher order mode in a large mode area fiber

We report the first demonstration of Raman amplification in a fiber of a single Bessel-like higher order mode using a multimode pump source. We amplify the LP₀₈-mode with a 559-µm² effective mode area at a signal wavelength of 1115 nm in a pure-silica-core step-index fiber. A maximum of 18 dB average power gain is achieved in a 9-m long gain fiber, with output pulse energy of 115 µJ. The Raman pump source comprises a pulsed 1060 nm ytterbium-doped fiber amplifier with V-value ~30, which is matched to the Raman gain fiber. The pump depletion as averaged over the signal pulses reaches 36.7%. The conversion of power from the multimode pump into the signal mode demonstrates the potential for efficient brightness enhancement with low amplification-induced signal mode purity degradation.

Several new directions for ultrafast fiber lasers [Invited]

Ultrafast fiber lasers have the potential to make applications of ultrashort pulses widespread - techniques not only for scientists, but also for doctors, manufacturing engineers, and more. Today, this potential is only realized in refractive surgery and some femtosecond micromachining. The existing market for ultrafast lasers remains dominated by solid-state lasers, primarily Ti:sapphire, due to their superior performance. Recent advances show routes to ultrafast fiber sources that provide performance and capabilities equal to, and in some cases beyond, those of Ti:sapphire, in compact, versatile, low-cost devices. In this paper, we discuss the prospects for future ultrafast fiber lasers built on new kinds of pulse generation that capitalize on nonlinear dynamics. We focus primarily on three promising directions: mode-locked oscillators that use nonlinearity to enhance performance; systems that use nonlinear pulse propagation to achieve ultrashort pulses without a mode-locked oscillator; and multimode fiber lasers that exploit nonlinearities in space and time to obtain unparalleled control over an electric field.

Efficient strategy to increase higher order inter-modal stability of a step index multimode fiber

We demonstrate a novel approach to enhance the mode stability through increased effective index difference (Δneff) between the higher-order modes (LP₁₈, LP₀₉ and LP₁₉) of a multimode fiber. Fibers with large diameters have bigger effective mode areas (Aeff) and can be useful for high power lasers and amplifiers. However, a large mode area (LMA) results in an increased number of modes that can be more susceptible to mode coupling. The modal effective index difference (Δneff) strongly correlates with mode stability and this increases as the modal order (m) increases. We report here that the mode spacing between the higher order modes can be further enhanced by introducing doped concentric rings in the core. In our work, we have shown a more than 35% increase in the mode spacing between the higher order modes by optimizing the doping profile of a LMA fiber. The proposed design technique is also scalable and can be applied to improve the mode spacing between different higher order modes and their neighboring antisymmetric modes, as necessary.

Sub-MW peak power diffraction-limited chirped-pulse monolithic Yb-doped tapered fiber amplifier

We demonstrate a novel amplification regime in a counter-pumped, relatively long (2 meters), large mode area, highly Yb-doped and polarization-maintaining tapered fiber, which offers a high peak power directly from the amplifier. The main feature of this regime is that the amplifying signal propagates through a thin part of the tapered fiber without amplification and experiences an extremely high gain in the thick part of the tapered fiber, where most of the pump power is absorbed. In this regime, we have demonstrated 8 ps pulse amplification to a peak power of up to 0.76 MW, which is limited by appearance of stimulated Raman scattering. In the same regime, 28 ps chirped pulses are amplified to a peak power of 0.35 MW directly from the amplifier and then compressed with 70% efficiency to 315 ± 10 fs, corresponding to an estimated peak power of 22 MW.

Few-mode fibre-optic microwave photonic links

The fibre-optic microwave photonic link has become one of the basic building blocks for microwave photonics. Increasing the optical power at the receiver is the best way to improve all link performance metrics including gain, noise figure and dynamic range. Even though lasers can produce and photodetectors can receive optical powers on the order of a Watt or more, the power-handling capability of optical fibres is orders-of-magnitude lower. In this paper, we propose and demonstrate the use of few-mode fibres to bridge this power-handling gap, exploiting their unique features of small acousto-optic effective area, large effective areas of optical modes, as well as orthogonality and walk-off among spatial modes. Using specially designed few-mode fibres, we demonstrate order-of-magnitude improvements in link performances for single-channel and multiplexed transmission. This work represents the first step in few-mode microwave photonics. The spatial degrees of freedom can also offer other functionalities such as large, tunable delays based on modal dispersion and wavelength-independent lossless signal combining, which are indispensable in microwave photonics.

Polarization-maintaining, large-effective-area, higher-order-mode fiber

Higher-order-mode (HOM) fibers guiding light in large-effective-area (A_eff) Bessel-like modes have recently generated great interest for high-power laser applications. A polarization-maintaining (PM) version of HOM fibers can afford the added possibility of coherent beam combination, improved material processing, and polarization multiplexing of high-power fiber lasers. We report a PM-HOM fiber for guiding Bessel-like modes with A_eff ranging from 1200-2800  μm². The fiber modes exhibit a birefringence value that compares well with that of a conventional single-mode PM fiber (2×10^-4), and exhibit a polarization extinction ratio ranging from 13-23 dB over meter-long fiber lengths, practical for amplifier systems. This fiber presents a unique platform for next-generation high-power fiber systems, as well as for the fundamental studies on deterministically polarized Bessel-like modes in fibers.

Wavelength-agile high-power sources via four-wave mixing in higher-order fiber modes

Frequency doubling of conventional fiber lasers in the near-infrared remains the most promising method for generating integrated high-peak-power lasers in the visible, while maintaining the benefits of a fiber geometry; but since the shortest wavelength power-scalable fiber laser sources are currently restricted to either the 10XX nm or 15XX nm wavelength ranges, accessing colors other than green or red remains a challenge with this schematic. Four-wave mixing using higher-order fiber modes allows for control of dispersion while maintaining large effective areas, thus enabling a power-scalable method to extend the bandwidth of near-infrared fiber lasers, and in turn, the bandwidth of potential high-power sources in the visible. Here, two parametric sources using the LP_0,7 and LP_0,6 modes of two step-index multi-mode fibers are presented. The output wavelengths for the sources are 880, 974, 1173, and 1347 nm with peak powers of 10.0, 16.2, 14.7, and 6.4 kW respectively, and ~300-ps pulse durations. The efficiencies of the sources are analyzed, along with a discussion of wavelength tuning and further power scaling, representing an advance in increasing the bandwidth of near-infrared lasers as a step towards high-peak-power sources at wavelengths across the visible spectrum.

High-power transverse-mode-switchable all-fiber picosecond MOPA

A high-power transverse-mode-switchable all-fiber picosecond laser in a master-oscillator power-amplifier (MOPA) configuration is demonstrated. The master oscillator is a gain-switched laser diode delivering picosecond pulses with 25 MHz repetition rate at the wavelength of 1.06 μm. After multi-stage amplification in ytterbium-doped fibers, the average output power is scaled to 117 W. A mechanical long-period grating is employed as a fiber mode convertor to achieve controllable conversion from the fundamental (LP₀₁) to the second-order (LP₁₁) mode. Efficient mode conversion is demonstrated and the output characteristics for both modes are investigated. It is shown that LP₀₁ and LP₁₁ modes have nearly identical optical-to-optical conversion efficiency during amplification, but the nonlinear spectral degradation is significantly alleviated for LP₁₁ mode operation. Owing to the compact all-fiber architecture, this high-power transverse-mode-switchable fiber laser is reliable during long-term operation and thus promising for many practical applications, e.g. high-resolution laser micro-processing.

Polymer-clad silica fibers for tailoring modal area and dispersion

We demonstrate higher-order-mode (A_eff up to ∼2000  μm²) propagation in a 100 μm outer diameter pure-silica fiber with a low-index polymer jacket commonly used for fiber laser pump guidance. This simple structure obviates the need for complex designs deemed necessary for realizing large-mode-area fibers. Modes ranging from HE_1,12 to HE_1,22 were found to propagate stably over 15 m in this fiber. The index step is approximately 4 times larger than that obtained with fluorine down doping; thus the fiber supports even higher-order modes, which may have implications for building rare-earth-doped fiber lasers or achieving enhanced dispersion tunability for high-energy fiber nonlinear phenomena.

Axicons for mode conversion in high peak power, higher-order mode, fiber amplifiers

Higher-order mode fiber amplifiers have demonstrated effective areas as large as 6000 μm2, allowing for high pulse energy and peak power amplification. Long-period gratings are used to convert the fundamental mode to the higher-order mode at the entrance to the amplifier, and reconvert back to the fundamental at the exit, to achieve a diffraction limited beam. However, long period gratings are susceptible to nonlinearity at high peak power. In this work, we propose and demonstrate axicons for linear bulk-optic mode conversion at the output of higher order mode amplifiers. We achieve an M2 of less than 1.25 for 80% mode conversion efficiency. Experiments with pulsed amplifiers confirm that the mode conversion is free from nonlinearity. Furthermore, chirp pulse amplifier experiments confirm that HOM amplifiers plus axicon mode convertors provide energy scalability in femtosecond pulses, compared to smaller effective area, fundamental mode fiber amplifiers. We also propose and demonstrate a route towards fiber integration of the axicon mode convertor by fabricating axicons directly on the tip of the fiber amplifier end-cap.

Free-space beam shaping for precise control and conversion of modes in optical fiber

We consider the general problem of free-space beam shaping for coupling in and out of higher order modes (HOMs) in optical fibers with high purity and low loss. We compare the performance of two simple phase structures - binary phase plates (BPPs) and axicons - for converting Gaussian beams to HOMs and vice versa. Both axicons and BPPs allow for excitation of modes with high purity (>15 dB parasitic mode suppression), or conversion of HOMs to near-Gaussian beams (M2 < 1.25). Axicon coupling in single-clad fibers allows for lower loss (0.85 ± 0.1 dB) conversion than BPPs (1.7 ± 0.1 dB); but BPPs are compatible with any fiber design, and allow for rapid switching between modes. The experiments detailed here use all commercial components and fibers, allowing for a simple means to investigate the unique properties of multi-mode fibers.

Spatially-resolved Rayleigh scattering for analysis of vector mode propagation in few-mode fibers

We use high-resolution imaging of Rayleigh scattered light through the side of few-mode optical fibers to measure the local spatial structure of propagating vector fields. We demonstrate the technique by imaging both pure modes and superpositions of modes in the LP01 and LP11 families. Direct imaging not only gives high-resolution beat length measurements, but also records the local propagation dynamics including those due to perturbations. The imaging setup uses polarization discrimination to monitor both the transverse and the longitudinal polarization components simultaneously.

Measurement of effective refractive index differences in multimode optical fibers based on modal decomposition

We demonstrate the nondestructive measurement of the effective refractive index difference of two arbitrary modes within a multimode optical fiber by utilizing a tunable fiber grating. We use a mechanical grating of variable period to couple the respective modes and measure the mode content at the fiber output based on the correlation filter technique. From the dependence of the coupling efficiency on the grating period, the effective index difference of the modes can be extracted with high accuracy.

Sub-second mode measurement of fibers using C2 imaging

We implement cross-correlated imaging in the frequency domain (fC(2)) in order to reconstruct different modes propagating in a multi-mode optical fiber, and measure their relative powers. Our system can complete measurements in under a second (950 ms), with a maximum signal to noise ratio of 25 dB. The system is capable of group-delay temporal resolution as high as 720 fs, and this number can be tailored for a variety of modal discrimination levels by choice of apodization functions and effective bandwidths of the tunable source we use. Measurements are made on a double-clad test fiber to demonstrate simultaneous reconstruction of six guided modes. Finally, the system is used to optimize alignment into the fiber under test and achieve mono-mode purity > 95%, underscoring the utility of fC(2) imaging for near-real-time modal content analysis.

Single LP(0,n) mode excitation in multimode fibers

We analyze the transmission of a Single mode - Multimode -Multimode (SMm) fiber structure with the aim of exciting a single radial mode in the second multimode fiber. We show that by appropriate choice of the length of the central multimode fiber one can obtain > 90% of the total core power in a chosen mode. We also discuss methods of removing undesirable cladding and radiation modes and estimate tolerances for practical applications.

Materials Development for Next Generation Optical Fiber

Optical fibers, the enablers of the Internet, are being used in an ever more diverse array of applications. Many of the rapidly growing deployments of fibers are in high-power and, particularly, high power-per-unit-bandwidth systems where well-known optical nonlinearities have historically not been especially consequential in limiting overall performance. Today, however, nominally weak effects, most notably stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) are among the principal phenomena restricting continued scaling to higher optical power levels. In order to address these limitations, the optical fiber community has focused dominantly on geometry-related solutions such as large mode area (LMA) designs. Since such scattering, and all other linear and nonlinear optical phenomena including higher order mode instability (HOMI), are fundamentally materials-based in origin, this paper unapologetically advocates material solutions to present and future performance limitations. As such, this paper represents a 'call to arms' for material scientists and engineers to engage in this opportunity to drive the future development of optical fibers that address many of the grand engineering challenges of our day.

Generation of vector vortex beams with a small core multimode liquid core optical fiber

We report on the generation of vector vortex beams using a 10-μm core multimode liquid core optical fiber (LCOF) filled with CS(2). The first higher-order modes including radially, azimuthally and hybrid polarized vector modes, as well as the higher-order modes such as LP(21) mode and LP(31) mode are selectively excited by adjusting the incidence angle of the linearly polarized input Gaussian beam with respect to the fiber axis. The interferograms with single forklet verify the phase singularity of the vector beams generated. Compared to silica optical fibers, the vector vortex beams from the LCOFs have higher excitation efficiency and larger bending tolerance.

Single-mode chirally-coupled-core fibers with larger than 50 µm diameter cores

In this paper, we report an advance in increasing core size of effective single-mode chirally-coupled-core (CCC) Ge-doped and Yb-doped double-clad fibers into 55 µm to 60 µm range, and experimentally demonstrate their robust single-mode performance. Theoretical and numerical description of CCC fibers structures with multiple side cores and polygon-shaped central core is consistent with experimental results. Detailed experimental characterization of 55 µm-core CCC fibers based on spatially and spectrally resolved broadband measurements (S(2) technique) shows that modal performance of these large core fibers well exceeds that of standard 20 μm core step-index large mode area fibers.

Broadband parametric wavelength conversion at 1 μm with large mode area fibers

Fiber-optic parametric wavelength conversion (PWC) below the zero-dispersion wavelength of silica is typically constrained by the requirement of a small, tightly confined mode with anomalous dispersion to achieve phase matching. This limits the ability to power scale PWC at arbitrary wavelengths. However, the constraint is lifted for higher-order modes. We demonstrate PWC in the 1 μm band via degenerate four-wave mixing pumped in a large effective area (>600  μm²) LP(0,7) mode of a double-clad fiber. We obtain up to 25% conversion in to the Stokes line with 0.5 ns pump pulses, corresponding to ~20  kW peak power at the converted wavelength.

Higher-order mode fiber enables high energy chirped-pulse amplification

Energy scaling of femtosecond fiber lasers has been constrained by nonlinear impairments and optical fiber damage. Reducing the optical irradiance inside the fiber by increasing mode size lowers these effects. Using an erbium-doped higher-order mode fiber with 6000 µm(2) effective area and output fundamental mode re-conversion, we show a breakthrough in pulse energy from a monolithic fiber chirped pulse amplification system using higher-order mode propagation generating 300 µJ pulses with duration <500 fs (FWHM) and peak power >600 MW at 1.55 µm. The erbium-doped HOM fiber has both a record large effective mode area and excellent mode stability, even when coiled to reasonable diameter. This demonstration proves efficacy of a new path for high energy monolithic fiber-optic femtosecond laser systems.

Recent Advances in Fiber Lasers for Nonlinear Microscopy

Nonlinear microscopy techniques developed over the past two decades have provided dramatic new capabilities for biological imaging. The initial demonstrations of nonlinear microscopies coincided with the development of solid-state femtosecond lasers, which continue to dominate applications of nonlinear microscopy. Fiber lasers offer attractive features for biological and biomedical imaging, and recent advances are leading to high-performance sources with the potential for robust, inexpensive, integrated instruments. This article discusses recent advances, and identifies challenges and opportunities for fiber lasers in nonlinear bioimaging.

Ultimate efficiency of spectral beam combining by volume Bragg gratings

Spectral beam combining (SBC) by volume Bragg gratings (VBGs) recorded in photo-thermo-refractive (PTR) glass is a powerful tool for laser applications that require higher radiance than a single laser unit can achieve. The beam-combining factor (BCF) is introduced as a tool to compare various beam-combining methods and experiments. It describes the change of radiance provided by a beam-combining system but is not affected by the initial beam quality of the combined lasers. A method of optimization of VBGs providing the maximum efficiency of SBC has been described for an arbitrary number of beams. An experiment confirming the proposed modeling for a two-beam SBC system by a single VBG has demonstrated a total combined power of 301 W with a channel separation of 0.25 nm, combining efficiency of 97%, close to diffraction limited divergence with M(2)=1.18, BCF of 0.77, and spectral radiance of 770 TW/(sr·m(2)·nm), the highest to date for SBC.

Bend compensated large-mode-area fibers: achieving robust single-modedness with transformation optics

Fibers with symmetric bend compensated claddings are proposed, and demonstrate performance much better than conventional designs. These fibers can simultaneously achieve complete HOM suppression, negligible bend loss, and mode area >1000 square microns. The robust single-modedness of these fibers offers a path to overcoming mode instability limits on high-power amplifiers and lasers. The proposed designs achieve many of the advantages of our previous (asymmetric) bend compensation strategy in the regime of moderately large area, and are much easier to fabricate and utilize.

First multi-watt ribbon fiber oscillator in a high order mode

Optical fibers in the ribbon geometry have the potential to reach powers well above the maximum anticipated power of a circular core fiber. In this paper we report the first doped silica high order mode ribbon fiber oscillator, with multimode power above 40 W with 71% slope efficiency and power in a single high order mode above 5 W with 44% slope efficiency.

Amplification of higher-order modes by stimulated Raman scattering in H2-filled hollow-core photonic crystal fiber

We report a method for amplifying higher-order guided modes, synthesized with a spatial light modulator, in a hydrogen-filled hollow-core photonic crystal fiber. The gain mechanism is intermodal stimulated Raman scattering, a pump laser source in the fundamental mode providing amplification for weak higher-order seed modes at the Stokes frequency. The gain for higher-order modes up to LP31 is calculated and verified experimentally.

Mode area scaling with all-solid photonic bandgap fibers

There are still very strong interests for power scaling in high power fiber lasers for a wide range of applications in medical, industry, defense and science. In many of these lasers, fiber nonlinearities are the main limits to further scaling. Although numerous specific techniques have studied for the suppression of a wide range of nonlinearities, the fundamental solution is to scale mode areas in fibers while maintaining sufficient single mode operation. Here the key problem is that more modes are supported once physical dimensions of waveguides are increased. The key to solve this problem is to look for fiber designs with significant higher order mode suppression. In conventional waveguides, all modes are increasingly guided in the center of the waveguides when waveguide dimensions are increased. It is hard to couple a mode out in order to suppress its propagation, which severely limits their scalability. In an all-solid photonic bandgap fiber, modes are only guided due to anti-resonance of cladding photonic crystal lattice. This provides strongly mode-dependent guidance, leading to very high differential mode losses. In addition, the all-solid nature of the fiber makes it easily spliced to other fibers. In this paper, we will show for the first time that all-solid photonic bandgap fibers with effective mode area of ~920?m² can be made with excellent higher order mode suppression.

Mode evolution in long tapered fibers with high tapering ratio

We have experimentally studied fundamental mode propagation in few meters long, adiabatically tapered step-index fibers with high numerical aperture, core diameter up to 117 μm (V = 38) and tapering ratio up to 18. The single fundamental mode propagation was confirmed by several techniques that reveal no signature of higher-order mode excitation. It can be, therefore, concluded that adiabatic tapering is a powerful method for selective excitation of the fundamental mode in highly multimode large-mode-area fibers. Annular near field distortion observed for large output core diameters was attributed to built-in stress due to thermal expansion mismatch between core and cladding materials. The mechanical stress could be avoided by an appropriate technique of fiber preform fabrication and drawing, which would prevent the mode field deformation and lead to reliable diffraction-limited fundamental mode guiding for very large core diameters.

Semi-guiding high-aspect-ratio core (SHARC) fiber amplifiers with ultra-large core area for single-mode kW operation in a compact coilable package

A new class of optical fiber, the SHARC fiber, is analyzed in a high-power fiber amplifier geometry using the gain-filtering properties of confined-gain dopants. The high-aspect-ratio (~30:1) rectangular core allows mode-area scaling well beyond 10,000 μm2, which is critical to high-pulse-energy or narrow-linewidth high-power fiber amplifiers. While SHARC fibers offer modally dependent edge loss at the wide "semi-guiding" edge of the waveguide, the inclusion of gain filtering adds further modal discrimination arising from the variation of the spatial overlap of the gain with the various modes. Both methods are geometric in form, such that the combination provides nearly unlimited scalability in mode area. Simulations show that for kW-class fiber amplifiers, only the fundamental mode experiences net gain (15 dB), resulting in outstanding beam quality. Further, misalignment of the seed beam due to offset, magnification, and tilt are shown to result in a small (few percent) efficiency penalty while maintaining kW-level output with 99% of the power in the fundamental mode for all cases.

Measurement of effective refractive-index differences in a few-mode fiber by axial fiber stretching

A method for measuring the effective refractive-index differences in a few-mode fiber by applying axial fiber stretching is described. This method represents a straightforward technique for characterization of few-mode fibers. Interference between LP01 and LP11 and in some cases also between LP11 and LP21 are observed in a fiber designed for support of LP01 and LP11. The relative strength of the coupled modes depends on specific splicing characteristics, and in some cases only two modes are seen. The results agree well with theoretical predictions for the fiber under investigation.

Mode conversion in rectangular-core optical fibers

Mode conversion from the fundamental to a higher-order mode in a rectangular-core optical fiber is accomplished by applying pressure with the edge of a flat plate. Modal analysis of the near and far field images of the fiber's transmitted beam determines the purity of the converted mode. Mode conversion reaching 75% of the targeted higher-order mode is achieved using this technique. Conversion from a higher-order mode back to the fundamental mode is also demonstrated with comparable efficiency. Propagation of a higher-order mode in a rectangular-core fiber allows for better thermal management and bend-loss immunity than conventional circular-core fibers, extending the power-handling capabilities of optical fibers.

Large mode area fibers with asymmetric bend compensation

Fibers with asymmetrical bend compensation offer to completely remove the tradeoff between mode area and single-modedness, with potentially huge impact on high-power amplification. These fibers would be difficult to fabricate, but are the only fundamental-mode strategy that can remove the bend-distortion limitations on mode-area scaling. Here, we show that even imperfect fibers can achieve essentially complete HOM suppression for areas of 2000 square microns or larger. Ultimate performance limits due to finite cladding size and fabrication imperfections are calculated.

Semi-guiding high-aspect-ratio core (SHARC) fiber providing single-mode operation and an ultra-large core area in a compact coilable package

A new class of optical fiber is presented that departs from the circular-core symmetry common to conventional fibers. By using a high-aspect-ratio (~30:1) rectangular core, the mode area can be significantly expanded well beyond 10,000 μm2. Moreover, by also specifying a very small refractive-index step at the narrow core edges, the core becomes "semi-guiding," i.e. it guides in the narrow dimension and is effectively un-guiding in the wide mm-scale dimension. The mode dependence of the resulting Fresnel leakage loss in the wide dimension strongly favors the fundamental mode, promoting single-mode operation. Since the modal loss ratios are independent of mode area, this core structure offers nearly unlimited scalability. The implications of using such a fiber in fiber laser and amplifier systems are also discussed.

Fiber laser with combined feedback of core and cladding modes assisted by an intracavity long-period grating

We present a fiber laser made in a single piece of conventional doped-core fiber that operates by combined feedback of the fundamental core mode LP((0,1)) and the high-order cladding mode LP((0,10)). The laser is an all-fiber structure that uses two fiber Bragg gratings and a long-period grating to select the modes circulating in the cavity; the laser emits at the coupling wavelength between the core mode LP((0,1)) and the counterpropagating cladding mode LP((0,10)) in the Bragg gratings. This work demonstrates the feasibility of high-order mode fiber lasers assisted by long-period gratings.

Large-mode-area multicore fibers in the single-moded regime

The effectively single-mode regime is analyzed for a class of large mode area multicore fibers. The performance tradeoff between bend loss, single-modedness, and mode area for these fibers is shown to be at best equivalent to step-index fiber.

The influence of index-depressions in core-pumped Yb-doped large pitch fibers

Rare-earth doped photonic crystal fibers rely ideally on an index matching of the doped core to the surrounding glass to work properly. Obtaining a perfect index matching is technologically very challenging, and fiber manufacturers opt for targeting an index depression instead, which still ensures the influence of the photonic structure on the light propagation. In this paper the analysis of the influence of this core index depression on the higher-order mode discrimination and on the beam quality of the fundamental mode of different designs of core-pumped active large pitch photonic crystal fibers is discussed. The most promising design is evaluated in terms of mode area scaling with a view to mode field diameters above 100 µm. Detailed requirements on the accuracy of the core index matching are deduced.

Fiber lasers and amplifiers: an ultrafast performance evolution

The first rare-earth-doped fiber lasers were operated in the early sixties and produced a few milliwatts at a wavelength around 1 mum. For the next several decades, fiber lasers were little more than a low-power laboratory curiosity. Recently, however, fiber lasers are entering the realm of kilowatt powers in continuous as well as in pulse operation with diffraction-limited beam quality. In this article we review this power evolution.

A higher-order-mode erbium-doped-fiber amplifier

We demonstrate the first erbium-doped fiber amplifier operating in a single, large-mode area, higher-order mode. A high-power, fundamental-mode, Raman fiber laser operating at 1480 nm was used as a pump source. Using a UV-written, long-period grating, both pump and 1564 nm signal were converted to the LP(0,10) mode, which had an effective area of 2700 microm(2) at 1550 nm. A maximum output power of 5.8 W at 1564 nm with more than 20 dB of gain in a 2.68 m long amplifier was obtained. The mode profile was undistorted at the highest output power.

Long distance transmission in few-mode fibers

Using multimode fibers for long-haul transmission is proposed and demonstrated experimentally. In particular few-mode fibers (FMFs) are demonstrated as a good compromise since they are sufficiently resistant to mode coupling compared to standard multimode fibers but they still can have large core diameters compared to single-mode fibers. As a result these fibers can have significantly less nonlinearity and at the same time they can have the same performance as single-mode fibers in terms of dispersion and loss. In the absence of mode coupling it is possible to use these fibers in the single-mode operation where all the data is carried in only one of the spatial modes throughout the fiber. It is shown experimentally that the single-mode operation is achieved simply by splicing single-mode fibers to both ends of a 35-km-long dual-mode fiber at 1310 nm. After 35 km of transmission, no modal dispersion or excess loss was observed. Finally the same fiber is placed in a recirculating loop and 3 WDM channels each carrying 6 Gb/s BPSK data were transmitted through 1050 km of the few-mode fiber without modal dispersion.

Near-diffraction-limited operation of step-index large-mode-area fiber lasers via gain filtering

We present, for the first time to our knowledge, an explicit experimental comparison of beam quality in conventional and confined-gain multimode fiber lasers. In the conventional fiber laser, beam quality degrades with increasing output power. In the confined-gain fiber laser, the beam quality is good and does not degrade with output power. Gain filtering of higher-order modes in 28microm diameter core fiber lasers is demonstrated with a beam quality of M(2)=1.3 at all pumping levels. Theoretical modeling is shown to agree well with experimentally observed trends.

Modeling of inter-modal Brillouin gain in higher-order-mode fibers

Finite element calculations of inter-modal Brillouin gain between LP(0n) modes in acoustically-inhomogeneous higher order mode (HOM) fibers are presented. When the pump beam is launched in the LP(08) mode, the LP(01) mode of the Stokes beam experiences the highest gain, approximately 6.7 dB higher than the peak LP(08)-LP(08) gain. An LP(01) Stokes beam experiences successively more Brillouin gain when pumped by higher-order LP(0n) modes.

Extremely large mode area optical fibers formed by thermal stress

We report a strictly single-mode optical fiber with a record core diameter of 84 microm and an effective mode area of approximately 3600 microm(2) at 1 microm. We also demonstrate fundamental mode operation in an optical fiber with a record core diameter of 252 microm and a measured mode field diameter (MFD) of 149 microm at 1.03 microm, i.e. an effective mode area (Aeff) of approximately 17,400 microm(2) at 1.03 microm, an Aeff of 31,600 microm(2) at 1.5 microm. All these fibers have near parabolic index profiles with a peak refractive index difference DeltaN approximately approximately 6 x 10(-5), i.e. a record low numerical aperture (NA) of approximately 0.013 in an optical fiber. This low refractive index difference was achieved by frozen-in thermal stress as a result of two different types of glass in the fibers. When the fundamental mode was excited in the 252 microm core fiber using a 1.03 microm ASE source, the output beam was measured to have M2x = 1.04 and M2y = 1.18.

Highly ytterbium-doped silica fibers with low photo-darkening

Phosphorus co-doping is known to reduce clustering levels of rare earth ions in silica hosts. In this paper, ytterbium-doped silica fibers with approximately 8.9 wt% Yb(2)O(3), up to approximately 4700 dB/m peak core absorption at 976 nm, and low photo-darkening are demonstrated using high phosphorus co-doping. Measured gain as high as approximately 7 dB/cm is demonstrated in the fiber.

Complete elimination of self-pulsations in dual-clad ytterbium-doped fiber lasers at all pumping levels

We have demonstrated suppression and elimination of self-pulsing in a watt-level, dual-clad, ytterbium-doped fiber laser. The addition of a long section of passive fiber in the laser cavity makes the gain recovery faster than the self-pulsation dynamics, allowing only stable cw lasing. This scheme provides a simple and practical method for eliminating self-pulsations in fiber lasers at all pumping levels.

Modeling of transverse mode interaction in large-mode-area fiber amplifiers

We model the transverse mode interaction in a large-mode-area fiber amplifier by solving the Fresnel wave equation including local gain saturation. In order to calculate the electric field distribution we apply a finite difference beam propagation method, which is followed by the derivation of the modal powers and modal polarization states. A polarization dependent mode amplification is found that is in good agreement with recent experimental results.

Diffraction limited amplification of picosecond pulses in 1170 microm2 effective area erbium fiber

Robust fundamental mode propagation and amplification of picosecond pulses at 1.56 microm wavelength is demonstrated in a core-pumped Er fiber with 1170 microm2 effective area. Record peak power exceeding 120 kW, and 67 nJ pulse energy are achieved before the onset of pulse breakup. A small increase in input pulse energy results in a temporal collapse of the pulse center to 58 fs duration, with peak powers approaching 200 kW.