We present a Geometric Phase (GP)-based Sagnac Anti-Resonant Ring (ARR) interferometer mirror (GP-mirror) for achieving tunable optimum output coupling in Continuous Wave (CW) Doubly Resonant Optical parametric oscillator (DRO). The DRO is designed using a nonlinear crystal MgO: PPsLT of 30 mm length and a grating period of 7.97 μm with a GP mirror in one arm of the standing wave cavity. The GP mirror is constructed using a quarter wave-plate (λ/4), half wave plate (λ/2), and quarter wave-plate(λ/4) at +45◦, θ, -45◦ with respect to vertical polarization, respectively. The DRO output transmission can be varied continuously from 0.6% to 50%, attaining optimum output coupling of 1.4% for maximum power extraction of 2.45 W when pumped with an incident power of 5 W at 47◦C crystal temperature at signal and idler wavelength of 1054 nm and 1074 nm, respectively. The maximum pump depletion of 89% is obtained with a conversion efficiency of 49%. The transmission through a GP-based mirror delivers the tunable optimum output power across the tuning wavelength range ⪆ 90 nm. This showcased GP-mirror concept presents an avenue for enhancing the capabilities and management of coherent sources adjustable across various spectral ranges and across all time scales, ranging from continuous-wave to ultrafast femtosecond domains.
Using a 4.5-W average power Cr2+:ZnS laser having a pulse width 43 fs and a spectral bandwidth of 138 nm centred at 2360 nm with a repetition rate of 80 MHz, we have produced femtosecond pulses in yellow wavelength. Using a 1 mm long Type 0 MgO: PPLN crystal in the first stage of our experiment, we have generated a maximum of ~ 2.43 W power of ~ 60 fs pulse width and ~ 39 nm spectral bandwidth centred at 1180 nm with a maximum conversion efficiency as high as ~ 65%. In the second stage, two different crystals, MgO:PPLN and BIBO were used to generate ultrafast coherent yellow source. The 1.18 μm radiation is first frequency-doubled in a multigrating 1 mm long Type 0 MgO:PPLN crystal with grating periods Λ=8.9 - 9.45 μm. A coherent yellow source with wavelength tunability from 577- 589 nm with a spectral bandwidth of ∼ 2 nm and temporal pulsewidth of ∼ 913 fs was achieved. At optimum focussing, we obtained a maximum power of 0.92 W for 2.2 W of pump power having a conversion efficiency of 40%. In order to address the large GVM between 1180 nm and 590 nm wavelength, we used another 1.2 mm long nonlinear crystal BIBO for birefringent phase matching. With BIBO crystal, the near-IR radiation was efficiently frequency doubled into yellow range (~ 591 nm) with average power of ~ 1 W and having a maximum conversion efficiency as high as 47%. The generated beam has a pulse width of ~ 130 fs in Gaussian shaped and ~ 4 nm spectral bandwidth with a time-bandwidth product of 0.45 showing almost no chirp. The output beam is a Gaussian shaped transverse beam profile with measured M2 values of M2x ∼ 1.07 and M2y ∼1.01.
Nonlinear frequency conversion processes depend on the polarization state of interacting beams. On the other hand, vector-vortex beams have space-variant polarization in beam transverse plane. In light of these two points, it is challenging to do nonlinear frequency conversion of vector vortex beam in single-pass geometry and retain the polarization characteristics of the beam. Here, we report an experimental scheme for single-pass second harmonic generation (SHG) of vector-vortex beams. Using two contiguous bismuth borate crystals with optic axis orthogonal to each other, we have frequency-doubled the near-IR vector-vortex beam into visible vector-vortex beams with order as high as lsh=24.
We report on efficient, two stage single-pass second harmonic generation of ultrafast Cr2+:ZnS laser with spectral bandwidth of 138 nm centered at ~2360 nm and pulse width of ~43 fs at a repetition rate of 80 MHz into tunable yellow radiation across 577 - 589 nm in multi-grating MgO:PPLN crystals. A maximum average output power ~940 mW at 589 nm wavelength with a single-pass conversion efficiency as high as 41% was achieved. The yellow radiation has a spectral bandwidth of 2 nm and pulse-width of ~913 fs in absence of any pulse compression with a time-bandwidth product of 1.58.
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