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This paper describes the physics and properties of a novel optical fiber that would be attractive for building high-power fiber lasers and amplifiers. Instead of propagating light in the fundamental, Gaussian-shaped mode, we describe a fiber in which the signal is forced to travel in a single, desired higher order mode (HOM). This provides for several advantages over the conventional approach, ranging from significantly higher ability to scale mode areas (and hence laser powers) to managing dispersion for ultra-short pulses - a capability that is practically nonexistent in conventional fibers. Particularly interesting is the tact that this approach challenges conventional wisdom, and demonstrates that for applications requiring meter-length fibers (as in high-power lasers), signal stability actually increases with mode order. Using this approach, we demonstrate mode areas exceeding 3200 μm 2 , and propagate signals with negligible mode distortions over up to 50-meter lengths. We describe several pulse propagation experiments in which we test the nonlinear response of this fiber platform, ranging from managing dispersive effects in femtosecond pulse systems, to reducing Brillouin scattering impairments in systems operating with the nanosecond pulses.

It is demonstrated that the lateral resolution of a non-linear scanning multi-photon microscope can be improved significantly by using the light from an LP{in02} mode instead of a Gaussian-like beam.

Modelocking in an Yb-doped figure-eight fiber laser is demonstrated utilizing anomalous dispersion from an LP(02) higher-order-mode fiber for dispersion management. Outside the laser cavity, the pulses were re-compressed to 95 fs using a second HOM module, the shortest demonstrated pulses to date from an Yb-doped figure-eight fiber laser. Operation of the laser with HOM fiber in the cavity is compared to an Yb figure-eight laser that utilizes all-normal dispersion fibers.

Fiber devices that amplify light propagating in a single fundamental mode face several challenges in extracting ever-increasing powers. Here we explore the advantages and present results showing amplification of light in a single higher order mode.

Femtosecond fiber lasers are currently of great interest due to their small size, stable operation, long lifetime and low cost compared to bulk lasers. However, for operation in the 1 µm wavelength range of Yb lasers, a major obstacle has been the lack of suitable fibers with anomalous dispersion that can compensate for the normal dispersion of the conventional active and passive fibers used. However, a new promising fiber device using a higher order mode (HOM) with anomalous dispersion in the 1 μm range has recently been demonstrated. The device comprises integrated all fiber mode converters based on long period gratings (LPG), and hence has the potential to be low loss and easy to splice, while offering a large effective area, and the possibility of third order dispersion compensation. In this paper, optimization of HOM fibers with anomalous dispersion in the 1 μm range has been investigated theoretically and experimentally. Fibers with dispersion coefficients ranging from +50 to +300 ps/(nm·km) at 1060 nm have been fabricated and devices including integrated LPG mode converters have been characterized. Modeled and measured properties of the modules, such as dispersion, grating bandwidth etc., are found to correlate well. It is shown that there is a tradeoff between a high dispersion coefficient and the bandwidth of LPG mode converters. The characteristics of such HOM devices have been studied in a linear, passively mode-locked laser-cavity using SESAM as saturable absorber.

We present a mode-locked ytterbium fiber laser with a higher-order mode fiber compensating the group-velocity dispersion and partially the third-order dispersion of the single-mode fiber at a wavelength of 1 microm. The generated pulses had an energy of 0.5 nJ and could be dechirped externally to a pulse duration of less than 60 fs. The power spectrum shows a spectral full width at half-maximum of 57 nm.

We present a mode-locked ytterbium fiber laser with a higher-order mode fiber compensating the GVD and partially the TOD of the SMF. The pulse duration is 52 fs with a spectral FWHM of 57 nm.

We demonstrate robust single-transverse-mode light propagation in higher-order modes of a fiber, with effective area A(eff) ranging from 2,100 to 3,200 microm(2). These modes are accessed using long-period fiber gratings that enable higher-order-mode excitation over a bandwidth of 94 mm with greater than 99% of the light in the desired mode. The fiber is designed such that the effective index separation between modes is always large, hence minimizing in-fiber mode mixing and enabling light propagation over lengths as large as 12 m, with bends down to 4.5 cm radii. The modal stability increases with mode order, suggesting that A(eff) of this platform is substantially scalable.

Light propagation in higher-order modes of fibers yields a promising schematic for arbitrarily increasing modal areas. We demonstrate this with modes of record A eff ranging from 2100 to 3100 μm2, which experience negligible loss or mode coupling.

We demonstrate the first all-solid (non-holey), silica-based fiber with anomalous-dispersion (+60ps/nm-km) at wavelengths (1080nm) where silica material dispersion is negative. We demonstrate its functionality as a critical enabler for an all-fiber, Yb-based, femtosecond ring laser.

We demonstrate a higher-order-mode fiber-based platform, to achieve low-loss, bend-resistant, mode-coupling-free light transmission over meter-length fibers, in a record A/sub eff/ of 2100 /spl mu/m/sup 2/. Its nonlinearity tolerance is demonstrated by compressing fiber-laser pulses to 64 fs, yielding 52-kW peak powers.

We present a technique for measuring the chromatic dispersion of the higher order modes in kilometer lengths of fiber. The LP/sub 02/ dispersion is measured for several different fibers, and accuracies better than 1% are achieved.

We demonstrate propagation of 14 nJ femtosecond pulses through a large-mode-area, higher-order-mode (HOM) fiber with an effective area of 2100 microm2. The pulses propagate stably in the LP07 mode of the fiber through lengths as long as 12 m. The strongly chirped pulses exiting the amplifier fiber are dechirped by the high-order-mode fiber, resulting in pulses with a peak power of 61 kW after propagation in 5 m of the positive-dispersion fiber. A small amount of self-phase modulation is observed in the compressed pulses and is described well by a nonlinear Schrödinger equation model that takes into account the measured effective area and dispersion of the HOM fiber.

Modelocking in Yb-doped figure-eight fiber lasers is demonstrated utilizing dispersion compensation from a higher-order-mode module with anomalous dispersion. Pulses were compressed to 91 fs. the shortest demonstrated pulses for a Yb-doped figure-eight fiber laser.

Higher order modes for large-mode-area high power fiber amplifiers are reviewed. Results from amplification in both Yb-doped HOM fiber at 1083 nm and Er-doped HOM fibers at 1550 nm are presented.