Abstract
Q-switched and polarized Ho lasers are the ideal driving source for mid-infrared radiation via optical parametric oscillation. Although intra-cavity pumping is an efficient way to achieve the Ho laser oscillation at $2.1~\mu \text{m}$ , which also facilitates the direct use of common diodes in a compact structure, it was demonstrated to be not suitable for Q-switching due to the saturable effect of the Ho-doped gain medium. Here, we report a RbTiOPO4 Q-switched intra-cavity pumped laser via integrating the Tm-doped and Ho-doped gain medium into a composite structure and decoupling the Tm laser from the Ho laser before it was modulated by the RbTiOPO4 crystal. The shortest pulse of 41 ns at repetition frequency of 1 kHz was obtained with a peak power of 7.5 kW. By competing with the intensified self-pulsing, the maximum pulse repetition frequency was found to reach 7 kHz, which was half the driving frequency of the electro-optical modulator. The results pave the way for achieving regular pulses from the intra-cavity pumping mechanism, which facilities a compact, accessible and robust pulse source at $2.1~\mu \text{m}$ .
Highlights
Wavelengths above 2 μm are far away from the two-photon absorption bands of the famous non-oxide mid-infrared crystals such as ZnGeP2 and OP:GaAs [1]
EM was a plano-plano mirror coated for anti-reflection at 785 nm and high reflection (HR) at 1900 nm to 2100 nm; output coupler (OC) was a plano-concave mirror with a curvature of radius of 200 mm, which was coated for HR at Tm laser (2000nm–2020 nm) and had a transmittance of 10% at Ho laser (2090 nm–2130 nm)
This was attributed to the first oscillation in the Tm laser from the cavity consisting of EM and spectral filter (SF), since that SF had a small transmittance of 4% of the Tm laser
Summary
Wavelengths above 2 μm are far away from the two-photon absorption bands of the famous non-oxide mid-infrared crystals such as ZnGeP2 and OP:GaAs [1] These crystals made pulse Ho lasers with emission ranges from 2.05 to 2.1 μm into excellent driving sources for nonlinear frequency conversion toward the molecular fingerprint region of 3–14 μm [2]–[4]. Inserting the Tm-doped gain medium into the cavity of the Ho laser for intra-cavity pumping was recommended, which facilitated using the existing 800 nm diodes for a compact Ho laser at room temperatures [16]–[18] Such a mechanism was shown to be unsuitable for achieving active Q-switching, where irregular pulse trains occurred after the acousto-optic modulator was inserted and set with the driving frequencies of 5 to 15 kHz [16].
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