High Power, Ultrafast Yellow Laser using Fourth-Harmonic Generation of Ultrafast Mid-IR Cr2+:ZnS Laser in MgO:PPLN Crystal

Abstract

The ultrafast optical radiation in the yellow spectral region is of great interest for variety of scientific and technological applications including astronomy, spectroscopy and medicine, with potential to revolutionize eye surgeries through exquisite accuracy [1, 2]. However, it is relatively difficult to access the yellow region, especially when good beam quality, high power and high efficiency are required. Till date, various types of yellow lasers have been reported [3, 4] mostly in CW regime and nanosecond timescales. Here, we report on a compact, high-power, high repetition-rate, ultrafast source of yellow radiation. Using an ultrafast Cr2+:ZnS laser of average power of 4.5 W with spectral width of Δλ=138 nm centered at 2.36 μm, producing output pulses of width ∼40 fs at a repetition rate of 80 MHz, and two stage single-pass second harmonic generation (SHG), we have generated ultrafast radiation of power 1 W tunable across 577–589 nm. The schematic of the experimental setup is shown in Fig. 1(a). The single-pass SHG of Cr2+:ZnS laser in a 2 mm long 5% mol doped MgO:PPLN multi-grating crystal (Λ=33.15–35.25 μm), giving second harmonic (SH) pulses of 90 fs at 1.18 μm, with a maximum average power of 2.3 W at a single-pass efficiency as high as 65%. Further the 1.18 μm radiation is frequency-doubled in a 1 mm long and 0.5∗1 mm2 in aperture, MgO:PPLN crystal (Λ=8.9 −9.45 μm) producing yellow radiation. The acceptance bandwidth (FWHM) and temporal walk-off length (Lt) of the second crystal are calculated to be 3 nm and 288 μm, respectively. In order to optimize the focusing condition for maximum fourth harmonic generation (FHG) efficiency, we pumped the crystal at constant power of 1.3 W at 1.18 μm and recorded the fourth harmonic (FH) power as a function of focusing parameter, ξ =L/b, where, L is the length of the crystal and b=2πnw 0 2/λ, w 0 is the beam waist radius, n is the refractive index of the crystal and λ is the input wavelength.