Optimal Pulse-Shaping in Actively Q-Switched Ytterbium-Doped Fiber Lasers

Abstract

In an actively Q-switched fiber laser (AQS-FL) a type of acousto-optic modulator (AOM) or (potentially) electro-optic modulator (EOM) controls the generation of output nanosecond wide pulses. An integrated Gaussian pulse shape is desirable in many applications such as material processing, microfabrication, ultrasound generation, gold photothermal therapy, etc. However, because of the system dynamics, generation of perfect Gaussian pulse shapes is not guaranteed in an AQS-FL, additionally designing the AQS-FL for a desired pulse peak and duration is an inverse problem which needs cumbersome trialerror efforts. We have developed a framework consisting of a rigorous FDM method plus a dedicated and innovative multi-objective genetic algorithm (GA) which assists the designer in achieving the desired Gaussian pulses within a reasonable time frame. The developed GA evolves the timing parameters of modulator plus the pump power and fiber length until the suitable goal is reached. To demonstrate the flexibility and design feasibility of our GA, three different single pulse and pulse train generation scenarios on a 7.5 m long Ytterbium-doped double clad fiber (YD-DCF) are examined to achieve the Gaussian,150 Wand 200 W peak power, 250 ns and 300 ns width pulses. To the best of our knowledge, it is the first implementation of an intelligent algorithm for optimizing the output pulse of an AQS-FL. It is worth noting that depending on the fiber host material and modulator specifications, much higher peak powers and different pulse durations are feasible, furthermore in case of utilizing the AOM, the pertaining limitations and feasibility are considered.

Suppression of the Stimulated Brillouin and Raman Scattering in Actively Q‐switched Fiber Lasers through Temporal Pulse Shaping

“…First order SRS Stokes can only partially benefit from the mirror reflections if the mirrors are wavelength selective, for example, FBGs can very effectively reduce the detrimental effects of SRS. [ 38 ] Furthermore, if we consider the conventional 70 nm range (1030–1100 nm) for ASE modeling [ 5,9 ] and in case of 1080 or 1064 nm lasing, the first order SRS goes beyond 1100 nm, according to extended absorption and emission cross section curves of Yb 3 + ions, [ 39 ] there is a very small emission cross section and almost zero absorption in this range. For the second order SRS, the spectral region is totally out of FBG reflection range and Yb 3 + emission cross sections can be omitted; so, the above discussion highlights the reasons that make the SRS less detrimental than the SBS in our system.…”

“…Further details on the genetic algorithm development and implementation can be found in refs. [9, 42]. Note that the GA must be run after proper fitting of its parameters (chromosomes) and mutation process; exploring the different conditions/ranges, and having the knowledge about the physics of the system is essential.…”

“…We have chosen the following parameters according to the schematic configuration of the Figure 2 and the PDE system Equations (16)–(21); a silica YD‐DCF of 4 m length, with core/inner cladding diameters of 10/125 μm, Yb 3 + doping concentration of , NA equal to 0.06 that leads to single transverse mode of operation, and switching frequency of 40 kHz for the AOM or EOM. [ 9 ] The absorption and emission cross sections are taken from refs. [39, 46] and Raman shift is performed according to Figure 4.…”

“…It is also worth pointing out that a fairly complete set of parameter details for AQS‐FL modeling can be found in ref. [9].…”

“…The difficulty of realizing this idea and method which has its roots in the complex behavior of the AQS‐FLs, can be simplified by exploiting artificial intelligence (AI) algorithms such as the genetic algorithm (GA) to find an optimal, self‐taught, and temporal behavior for generating a proper pulse shape that excites the minimum Stokes power. We have already proposed and predicted in our previous work [ 9 ] that single and pulse train generation processes for the AQS‐FL can be optimized and customized in an effective and feasible manner using GA. In this paper, we have considered a single stage, single transverse mode pulsed fiber laser, utilizing a large mode area (LMA) ytterbium‐doped double clad fiber (YD‐DCF) with the core/inner cladding diameters of 10/125 μm and featuring a combination of Q and gain switching techniques.…”