Most of the current large mode area (LMA) fibers are few-moded designs using a large, low numerical aperture (N.A.) core, which promotes mode coupling between core modes and increases bending losses (coupling with claddingmodes), which is undesirable both in terms ofmode area and beamquality. Furthermore, short LMA fiber lengths and small cladding diameters are needed to minimize nonlinear effects and maximize pump absorption respectively in high-power pulsed laser systems. Although gain fiber coiling is a widely used technique to filter-out unwanted modes in LMA fibers, coupling between modes can still occur in component leads and relay fibers. In relay fiber, light coupled into higher-order modes can subsequently be lost in the coiling or continue as higher-order modes, which has the overall effect of reducing the effective transmission of the LP01 mode and degrading the beam quality. However, maximum transmission of the LP01 mode is often required in order to have the best possible beam quality (minimal M2). Launching in an LMA fiber with a mode field adapter (MFA)1 provides an excellent way of ensuring maximum LP01 coupling, but preservation of this mode requires highmodal stability in the output fiber. Small cladding, low N.A. LMA fibers have the disadvantage of being extremely sensitive to external forces in real-life applications, which is unwanted for systems where highly sensitive mode coupling can occur. In this paper, we present a detailed experimental and theoretical analysis of mode coupling sensitivity in LMA fibers as a function of fiber parameters such as N.A., core diameter and cladding diameter. Furthermore, we present the impact of higher N.A. as a solution to increase mode stability in terms of its effect on peak power, effective mode area and coupling efficiency.
With the rapid development of Fiber Bragg Grating (FBG) sensing during recent years, FBG sensors are used in
many fields; applications involving impact and vibration measurement require a high-speed interrogator. We have
developed and prototyped a high-speed FBG interrogation system with a sampling rate up to 5 kHz. We show that FBG
sensor optical spectral deformation may affect the performance of interrogators, such deformation can be introduced
from non-uniform strain field or during FBG sensor packaging. This paper reports the experimental investigation of the
impacts of the FBG spectral deformation on the interrogator accuracy, sensors with different optical spectral shapes are
tested and analyzed, furthermore, we show how this high-speed interrogator is more tolerant to such deformation than
peak tracking instruments.
We have developed and prototyped C+L band erbium-doped fiber ASE source by making use of both forward and
backward ASEs with double-pass configuration. Simulations and experiments are performed for different pump powers
and fiber lengths to optimize the design. The spectrum bandwidth of 80.6nm (1526.7~1607.3nm) with flatness of 5.22dB
is realized. Pumping-conversion efficiency of about 18.9% is reached. This C+L band light source will be used to extend
illuminating bandwidth of the high-speed interrogator which has been prototyped and been used by different clients. The
sampling rate can be up to 5 kHz. The broadband light source allows the interrogator no moving part that enables high-speed
FBG sensor interrogation.