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.

Presents an efficient method for the design of complex fiber Bragg gratings. The method relies on the synthesis of the impulse response of the grating by means of a differential layer-peeling algorithm. The algorithm developed takes into account all the multiple reflections inside the grating, giving an exact solution to the inverse scattering problem. Its low algorithmic complexity enables the synthesis of long fiber gratings. The method is illustrated by designing several filters with interest for optical fiber communication systems: dispersionless bandpass filters and second- and third order dispersion compensators.

Selective excitation of spheroidal whispering gallery modes and bottle modes in a robust bottle microresonator fabricated straightforwardly from a short section of optical fiber is demonstrated. Characteristic resonance spectra of long-cavity bottle modes were obtained by using a tapered fiber to excite evanescently bottle microresonator at different points along its axis. Compared to bare-fiber cylindrical resonators, the bottle microresonator results in a 35x increase of the observed Q factor.

A new and flexible technique for producing photorefractive fibre gratings with a uniform phase mask is demonstrated. By slowly moving the fibre relative to the mask as the writing beam is scanned, wavelength shifts, pure apodisation and phase-shifted gratings can be achieved.

Transverse mode instability (TMI) is compared to thermal lensing (TL) power threshold and used to derive power scaling limits in high-power fiber amplifiers. The TMI power threshold is shown to be ~65% of the TL one and dominates power scaling. In addition to commonly used limiting effects, we introduce a bend-induced mechanical reliability criterion, which limits the maximum allowable cladding diameter to ~600μm. This also results in the introduction of a critical pump brightness, the minimum required pump brightness at which the maximum signal power is achieved. The maximum achievable power depends primarily on the choice of pumping wavelength, amplifier gain and heat coefficient. Maximum signal powers of ~28kW to ~38kW, for diode pumping (λ_p = 976nm), and ~35kW to ~52kW, for tandem pumping (λ_p = 1018nm), are predicted for single-mode fiber amplifiers operating at signal wavelength λ_s = 1070nm, when the amplifier gain is increased from 10dB to 20dB. For an amplifier gain of 10dB, the maximum achievable signal power varies from 85kW to 25kW for tandem pumping and 35kW to 20kW for diode pumping, when the heat coefficient varies from 1% to 15% and 5.5% to 20%, respectively. The corresponding critical pump brightness varies from ~0.50 W/(μm² sr) to ~0.14 W/(μm² sr) for tandem pumping and ~0.25 W/(μm² sr) to ~0.13 W/(μm² sr) for diode pumping.