Blue-green laser,with its high optical transmittance and low transmission loss in seawater,is currently the only spectral band capable of penetrating the air-sea interface and seawater to achieve transmission at great depths.In recent years,with the rapid development of oceanic laser remote sensing and underwater communication,there is an increasingly urgent demand for compact and high frequency repetition rate blue-green pulse laser that can be combined with airborne platform to achieve information transmission and high-precision detection of oceanic vertical profile.The blue light in the spectral band of 450 nm to 490 nm has stronger penetrating ability for open ocean than green light,making it the optimal operating spectral band for oceanic lidar.To our knowledge,the only report of quasi-three-level frequency-doubled∶YLF laser is about its continuous operation.This study presents a blue pulse laser output at 454 nm for marine applications by intracavity frequency doubling of a pulse diode-pumped Nd∶YLF laser at 908 nm based on the quasi-three-level transition.Using 806 nm LD end-pumped Nd∶YLiF4(Nd∶YLF)crystal,a L-shaped laser resonator is built to suppress parasitic oscillation at 1047 nm,and successfully realize the quasi-three-level Nd∶YLF laser emitting at 908 nm.The resonator of the fundamental frequency laser consists of three mirrors:a concave input coupler with a curvature radius of 200 mm,a plane mirror inserted at 45° with respect to the optical axis,and a 908 nm output coupler with output transmission of 2.7%at 908 nm.This 45° mirror is high-reflection coated at 908 nm and 454 nm,and anti-reflection coated at 1 047 nm.Thus,the resonator is singly resonant at 908 nm.Two methods,electro-optic Q-switching and gain-switching,were proposed to obtain the fundamental frequency pulse laser.Thus,for the experimental setup of Q-switching method,a polarizer and a double-RTP Pockels were inserted in the resonator as a voltage-decreased electro-optical Q-switcher.A type-Ⅰ critical phase-matched LiB3O5(LBO)crystal with a size of 3 mm×3 mm×10 mm and phase matching cut angles of θ=90° and φ=19.3° is used to generate frequency doubled laser at 454 nm.And another 45° placed mirror with high-reflection coated at 454 nm and anti-reflection coated at 908 nm is used for the 454 nm laser output.For the gain-switched method,at a repetition rate of 100 Hz,the maximum output pulse power of the intracavity frequency-doubled 454 nm laser is 3.16 mW,with a pulse width of 418 ns.The frequency doubling efficiency is 1.1%,which is close to the theoretical calculation value of 1.3%.At different distances behind a focusing lens,the blue output laser beam diameters corresponding to the x and y direction were measured.The laser beam quality factors are calculated to be Mx2=1.07 and My2=1.12,with the beam divergence angels are measured to be 2.26 mrad in the x direction and 1.99 mrad in the y direction.For the Q-switching method,at a repetition rate of 100 Hz,the maximum output pulse power of 454 nm laser reaches 23.62 mW,with a pulse width of 66 ns,and the maximum frequency-doubling efficiency is around 3.7%.The laser beam quality factors are Mx2=1.6 and My2=1.3,and the beam divergence angels in x and y directions are separately 2.89 and 2.96.The central laser wavelength is measured to be 454.02 nm,and the FWHM of the spectral bandwidth has reached the limit of the spectrometer measurement resolution,which is less than 0.05 nm.In this paper,we present a blue pulse laser output at 454 nm by intracavity frequency doubling of a pulse diode-pumped Nd∶YLF laser at 908 nm based on the quasi-three-level transition.This study provides a new technical route for compact all-solid-state blue pulse laser,which can be used as a reference for the new laser source of ocean laser detection systems and underwater wireless optical communication systems.
Nd∶YLF laserBlue laserIntracavity frequency doublingGain-switchQ-switch
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