Modulation instability in high power laser amplifiers

Alexander M. Rubenchik,Sergey K. Turitsyn,Michail P. Fedoruk

doi:10.1364/oe.18.001380

Abstract

The modulation instability (MI) is one of the main factors responsible for the degradation of beam quality in high-power laser systems. The so-called B-integral restriction is commonly used as the criteria for MI control in passive optics devices. For amplifiers the adiabatic model, assuming locally the Bespalov-Talanov expression for MI growth, is commonly used to estimate the destructive impact of the instability. We present here the exact solution of MI development in amplifiers. We determine the parameters which control the effect of MI in amplifiers and calculate the MI growth rate as a function of those parameters. The safety range of operational parameters is presented. The results of the exact calculations are compared with the adiabatic model, and the range of validity of the latest is determined. We demonstrate that for practical situations the adiabatic approximation noticeably overestimates MI. The additional margin of laser system design is quantified.

Highlights

When the power of a laser beam propagating in nonlinear medium exceeds a critical value Pc, the transverse beam modulations begin to grow exponentially
The beam quality in such advanced laser facility is maintained by keeping the cascaded system elements effectively short enough to prevent the dangerous development of the modulation instability [3]
We have presented a theory that accurately describes the growth of the transversal modulations of an optical beam in a laser amplifier

Summary

Introduction

When the power of a laser beam propagating in nonlinear medium exceeds a critical value Pc , the transverse beam modulations (random or induced) begin to grow exponentially. This physical phenomenon is known as light beam modulation or self-focusing instability [1]. Every such filament experiences self-focusing up to the point at which either the high intensity produces the material breakdown, or the field collapse is arrested before breakdown by some other physical effect, depending on the specific configuration and the medium material. We present exact analytical results providing base for design guidance rules in such complex large-scale laser systems

Physical model and basic equations

The modulation instability in amplifiers

Findings

Conclusions