In this paper, we summarize the fundamental properties and review the latest developments in high power fiber lasers. The review is focused primarily on the most common fiber laser configurations and the associated cladding pumping issues. Special attention is placed on pump combination techniques and the parameters that affect the brightness enhancement observed in single-mode and multimode high power fiber lasers. The review includes the major limitations imposed by fiber nonlinearities and other parasitic effects, such as optical damage, transverse modal instabilities and photodarkening. Finally, the paper summarizes the power evolution in continuous-wave and pulsed ytterbium-doped fiber lasers and their impact on industrial applications.

High Power Fiber Lasers: A Review

The authors review the history, current status, physical mechanisms, experimental methods, and applications of nonlinear magneto-optical effects in atomic vapors. They begin by describing the pioneering work of Macaluso and Corbino over a century ago on linear magneto-optical effects (in which the properties of the medium do not depend on the light power) in the vicinity of atomic resonances. These effects are then contrasted with various nonlinear magneto-optical phenomena that have been studied both theoretically and experimentally since the late 1960s. In recent years, the field of nonlinear magneto-optics has experienced a revival of interest that has led to a number of developments, including the observation of ultranarrow (1-Hz) magneto-optical resonances, applications in sensitive magnetometry, nonlinear magneto-optical tomography, and the possibility of a search for parity- and time-reversal-invariance violation in atoms.

Resonant nonlinear magneto-optical effects in atoms

Wave front shaping, the ability to manipulate light fields both spatially and temporally, in complex media is an emerging field with many applications. This review summarizes how insights from mesoscopic scattering theory have direct relevance for optical wave control experiments and vice versa. The results are expected to have an impact on a number of fields ranging from biomedical imaging to nanophotonics, quantum information, and communication technology.

Light fields in complex media: Mesoscopic scattering meets wave control

Fiber lasers have seen progressive developments in terms of spectral coverage and linewidth, output power, pulse energy, and ultrashort pulse width since the first demonstration of a glass fiber laser in 1964. Their applications have extended into a variety of fields accordingly. In this paper, the milestones of glass fiber laser development are briefly reviewed and recent advances of high-power continuous wave, Q-switched, mode-locked, and single-frequency fiber lasers in the 1, 1.5, 2, and 3 μm regions and their applications in such areas as industry, medicine, research, defense, and security are addressed in detail.

Fiber lasers and their applications [Invited]

We review the most recent progress in ultralow-noise mode-locked fiber lasers and fiber-based frequency-comb sources. With the rapid progress in theory, measurement, and control of noise in passively mode-locked fiber lasers, we have reached the point where the residual carrier–envelope-offset phase jitter (when stabilized) and pulse timing jitter performance of such laser sources can be fully optimized to the unprecedented levels of attoseconds regime. In this paper, first, major principles in building such low-noise passively mode-locked fiber lasers are reviewed. We then define noise in mode-locked fiber lasers and present the basic theoretical and numerical framework for analyzing the noise in mode-locked fiber lasers. More detailed discussions on theory, measurement methods, state-of-the-art performances, and stabilization methods of intensity noise, timing jitter, and comb-line frequency noise follow. Finally, we overview today’s most representative applications of such ultralow-noise mode-locked fiber lasers and frequency-comb sources. As an already powerful tool for various high-precision applications, ultralow-noise mode-locked fiber lasers will keep finding more exciting applications in optical science and photonic technology in the coming years.

Ultralow-noise mode-locked fiber lasers and frequency combs: principles, status, and applications

Commercial 0.5 kW Yb-doped fiber amplifiers have been characterized and found to be suitable for coherent beam combining. Eight such fiber amplifiers have been coherently combined in a tiled-aperture configuration with 78% combining efficiency and total output power of 4 kW. The power-in-the-bucket vertical beam quality of the combined output is 1.25 times diffraction limited at full power. The beam-combining performance is independent of output power.

Coherent combining of a 4 kW, eight-element fiber amplifier array.

Five 500 W fiber amplifiers were coherently combined using a diffractive optical element combiner, generating a 1.93 kW beam whose M2=1.1 beam quality exceeded that of the inputs. Combining efficiency near 90% at low powers degraded to 79% at full power owing to thermal expansion of the fiber tip array.

Diffractive coherent combining of a 2.5 kW fiber laser array into a 1.9 kW Gaussian beam.

A rate equation for pulse energy is obtained from the master equation of mode locking. This rate equation is used to investigate self-starting, steady-state pulse energy, stability, pulse quantization, simultaneous cw and pulse operation, hysteresis in mode locking, multiple-pulse operation, relaxation oscillations, and noise behavior. The theory also provides criteria for low-noise as well as high-power operation. The formulation is applied to both soliton lasers and stretched-pulse lasers, and their operations are compared.

Energy rate equations for mode-locked lasers

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<1.15. The fiber amplifier linewidth is 12 GHz.

Diode-pumped narrow linewidth multi-kilowatt metalized Yb fiber amplifier

We report direct generation of <500‐fs pulses at a 1-GHz rate from a self-starting passively mode-locked fiber laser by regeneratively synchronizing the pulses with a phase modulator. The pulses are amplified and passed through a dispersion-decreasing fiber and a normal-dispersion supercontinuum fiber. The resulting continuum is wider than 350 nm.

C.X. Yu

Papers

Coherent combining of a 4 kW, eight-element fiber amplifier array.

Diffractive coherent combining of a 2.5 kW fiber laser array into a 1.9 kW Gaussian beam.

Energy rate equations for mode-locked lasers

Diode-pumped narrow linewidth multi-kilowatt metalized Yb fiber amplifier

Gigahertz-repetition-rate mode-locked fiber laser for continuum generation.