TY - GEN

T1 - Comparison of multimode LD pump beam shaping in step-index and graded-index fibers and its influence on Raman lasing

AU - Kuznetsov, Alexey G.

AU - Wolf, Alexey A.

AU - Nemov, Ilya N.

AU - Kablukov, Sergey I.

AU - Babin, Sergey A.

N1 - Funding Information: The study is supported by Russian Science Foundation (№21-72-30024). Publisher Copyright: © 2022 SPIE.

PY - 2022

Y1 - 2022

N2 - It is known that graded-index fibers (GIF) provide efficient Raman conversion of multimode radiation into a Stokes beam with improved beam quality known as Raman beam cleanup effect, whereas in step-index fibers (SIF) this effect is weak. Here we study propagation of low-coherent (~5 nm wide) highly-multimode (M2~30) pump radiation of high power 940-nm laser diodes (LDs) in GIF and SIF of ~100 µm core diameter. We measured pump beam shapes at the output of fibers with different length. It has been shown that the output beam shape is almost independent of the input pump beam shape approaching to rectangular or parabolic profile after propagation in only ten meters of SIF or GIF, respectively. The output beam shape mimics the refractive-index profile in the multimode fiber core due to random mode coupling. This means that Raman gain is nearly the same for all transverse modes in SIF and has maximum gain for the fundamental mode in GIF that is the main reason of strong Raman beam cleanup effect in GIF. We tested Raman lasing in 1-km GIF/SIF with 940-nm LD pumping of up to ~200 W power with a cavity formed by fiber Bragg gratings (FBGs). The output FBG fs-inscribed in GIF/SIF improves the output beam quality due to spatial filtering property, but in GIF the effect is much stronger.

AB - It is known that graded-index fibers (GIF) provide efficient Raman conversion of multimode radiation into a Stokes beam with improved beam quality known as Raman beam cleanup effect, whereas in step-index fibers (SIF) this effect is weak. Here we study propagation of low-coherent (~5 nm wide) highly-multimode (M2~30) pump radiation of high power 940-nm laser diodes (LDs) in GIF and SIF of ~100 µm core diameter. We measured pump beam shapes at the output of fibers with different length. It has been shown that the output beam shape is almost independent of the input pump beam shape approaching to rectangular or parabolic profile after propagation in only ten meters of SIF or GIF, respectively. The output beam shape mimics the refractive-index profile in the multimode fiber core due to random mode coupling. This means that Raman gain is nearly the same for all transverse modes in SIF and has maximum gain for the fundamental mode in GIF that is the main reason of strong Raman beam cleanup effect in GIF. We tested Raman lasing in 1-km GIF/SIF with 940-nm LD pumping of up to ~200 W power with a cavity formed by fiber Bragg gratings (FBGs). The output FBG fs-inscribed in GIF/SIF improves the output beam quality due to spatial filtering property, but in GIF the effect is much stronger.

KW - beam clean-up

KW - diode pumping

KW - FBG

KW - fiber laser

KW - graded-index fiber

KW - mode selection

KW - multimode

KW - pump depletion

KW - Raman laser

KW - random coupling

KW - step-index fiber

UR - http://www.scopus.com/inward/record.url?scp=85134465508&partnerID=8YFLogxK

UR - https://www.mendeley.com/catalogue/136686b1-d9df-3de0-907c-e5f2eaac05dd/

U2 - 10.1117/12.2622184

DO - 10.1117/12.2622184

M3 - Conference contribution

AN - SCOPUS:85134465508

SN - 9781510651562

T3 - Proceedings of SPIE - The International Society for Optical Engineering

BT - Micro-Structured and Specialty Optical Fibres VII

A2 - Kalli, Kyriacos

A2 - Peterka, Pavel

A2 - Bunge, Christian-Alexander

PB - SPIE

T2 - Micro-Structured and Specialty Optical Fibres VII 2022

Y2 - 9 May 2022 through 20 May 2022

ER -