2nd-order random lasing in a multimode diode-pumped graded-index fiber.

Area of Science:

• Photonics and Optics
• Fiber Lasers
• Nonlinear Optics

Background:

• Graded-index fibers (GIFs) offer potential for high-power laser diode conversion.
• Raman clean-up effect in GIFs can improve beam quality.
• Cascaded Raman generation is a key process for wavelength conversion and beam improvement.

Purpose of the Study:

• To investigate cascaded Raman generation of the second Stokes order in a long GIF.
• To compare linear and random cavity configurations for second-order Raman lasing.
• To achieve high-quality, high-efficiency beam generation using diode-pumped GIF.

Main Methods:

• Utilized a 1.1-km long GIF with a 100-μm core, pumped by 915-nm diodes.
• Employed fiber Bragg gratings (FBGs) to form linear cavities for first and second Stokes orders.
• Investigated random distributed feedback via Rayleigh backscattering for second-order lasing.

Main Results:

• Achieved first Stokes order generation (950-954 nm) with beam quality improved from M²≈30 to M²≈2.3.
• Second-order random lasing yielded a nearly diffraction-limited beam (M²≈1.6) with up to 27 W power.
• Random lasing configuration demonstrated superior beam quality and higher slope efficiency compared to linear cavities.

Conclusions:

• Second-order random lasing in diode-pumped GIF with FBGs is highly effective for generating high-quality beams.
• This method provides high efficiency and broad wavelength tunability within the Raman gain spectrum.
• The results highlight the potential for advanced fiber laser systems with improved beam characteristics.