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1

The effect of cladding geometry on the absorption efficiency of double-clad fiber lasers

Javadimanesh M., Ghavami S. S., Khorsandi A.

Optica Applicata

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2016

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Vol. 46, nr 2

277--289

EN

In the present work, the inner cladding geometries of a typical double-clad fiber laser are studied and numerically simulated for different cladding shapes to obtain the maximum absorption efficiency for the pump beam. This is performed by using the ray tracing approach and dislocating the fiber core from the center to impose the asymmetry on the investigated geometries. The high absorption efficiency of ~94.5% was obtained for the optimized offset D-shaped double-clad fiber. The hexagonal shape is proposed as a new geometry for the inner cladding to attain higher absorption efficiency. It was found that the absorption efficiency of ~68% for a symmetrical hexagonal can be improved to ~95% for an asymmetrized hexagonal-shaped double-clad fiber laser. Eventually the genetic algorithm was used to enhance the performance of the investigated geometries. This resulted in the further increasing of pump beam absorption efficiency of 99.5% in the genetic algorithm optimized asymmetrized hexagonal shape.

2

Modulation index optimization for wavelength modulation spectroscopy

Mohammadi M. J., Khorsandi A., Ghavami S. S.

Optica Applicata

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2016

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Vol. 46, nr 4

639--650

EN

In this work, the second-harmonic component of wavelength modulation spectroscopy is simulated for R(22) CO2 absorption line to investigate the effect of gas temperature and pressure on the modulation index. We found that the optimum value of modulation index, that is 2.2, is not affected by temperature but gas pressure will change the optimized modulation index. Specifically, when the gas pressure decreased to lower pressures of less than 100 mbar, the modulation index is also decreased and tended exponentially to about two. Accordingly, the optimum value of modulation index is recalculated for a range of CO2 gas pressures to establish a nearly zero pressure deviation in the spectroscopy of very low pressure samples.

3

Laser-based multichannel fiber optic sensor for multipoint detection of corrosion

Shahpir R., Ghavami S. S., Khorsandi A.

Optica Applicata

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2016

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Vol. 46, nr 1

103--115

EN

We report on a multichannel sensor for multipoint corrosion monitoring of an aluminum film, which was corroded by nitric acid as a corrosive solution. The sensor head was a commercial 1x4 optical fiber coupler combined with the optical time domain reflectometry technique. The effects of the concentration and pH of acid on the corrosion rate of the aluminum film are initially studied and characterized. Then, about 100 nm of pure aluminum is coated on the end facets of each fiber channel and reduction of the back reflected signals is simultaneously monitored as a measure of corrosion rate. It is found that the fabricated sensor is very sensitive to the variation of pH as by increasing of the pH up to similar to 13, the corrosion rate of aluminum is raised to similar to 75 millimeter per year. Our experimental results have been verified by the standard immersion test, indicating very good consistency and reliability. Eventually, the sensor is used to trace the corrosion of sea water provided from the Persian Gulf, confirming the validity of laboratory results.

4

Multi-variable optimization of an ytterbium-doped fiber laser using genetic algorithm

Hashemi S. S., Ghavami S. S., Khorsandi A

Optica Applicata

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2015

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Vol. 45, nr 3

355--367

EN

We introduce the genetic algorithm for the optimization of an Yb3+-doped double-clad fiber laser based on a multi-variable scheme. The output characteristic of the laser is numerically simulated using real practical values. This is performed through solving the associated steady-state rate equation and investigating the effects of input variables such as pump and signal wavelengths and length of the fiber on the laser output. It is found that pumping of the medium around 975 nm is conducted to attain the maximum output power of ~34.8 W, while the stability of the outcoupled power is significantly improved when pumping at 920 nm, confirming good agreement with the reported experimental results. We have also found that by using genetic algorithm base multi-variable optimization, the output power can be significantly increased by about three orders of magnitude and reaches to ~28.5 W with optimum and shorter fiber length of ~57.5 m. Obtained results show that based on the genetic algorithm multi-variable discipline, fiber characteristics can be optimized according to the gaining of maximum output power.