Hybrid unstable resonators have been widely used in slab solid-state lasers and gas laser systems in recent years due to their high extraction efficiency and high beam quality. So far, hybrid unstable resonators have not been used in DF gas flow chemical laser devices to achieve high-power, high-beam quality mid-infrared continuous wave (CW) laser output. In addition, the beam quality in the direction of stable resonator is always poor and hard to handle, and the physical process of far-field spot from high-order transverse mode oscillation to single fundamental transverse mode oscillation should be studied theoretically and experimentally.
A kind of off-axis hybrid unstable resonator with Z-fold in the flow direction is proposed and studied. The Z-folded off-axis hybrid unstable resonator is a flat concave stable resonator in the gas flow direction, and Z-folded to reduce the gain size in the direction of the stable resonator. In the height direction, it is a positive branch confocal unstable resonator, as shown in Fig. 1. The magnification in the direction of unstable cavity is 1.2. The gain medium is generated by the DF gain generator driven by combustion. Nitrogen is used as diluent, and the static pressure of the gain air flow is generally less than 666.7 Pa. The gain size is about 30 mm in the air flow direction and 40 mm in the height direction, and the gain length of the laser single pass is 125 mm, while after Z-folding the gain length becomes 375 mm. The virtual assembly diagram of the gain generator module and the optical cavity module is shown in Fig. 2. Figure 3 shows the schematic diagram of the monitoring optical path, with which the output power, far-field spot, output spectrum and near-field spot are monitored. We calculate the oscillatory self-reproducing modes of Z-folded off-axis hybrid unstable cavities by means of fast Fourier transform based on the diffraction theory of plane-wave angular spectra. In the calculation, the seed beam used for the optical cavity oscillation is a rectangular parallel plane wave conforming to the cross section of the gain medium (30 mm×45 mm). There are four mirrors in the optical cavity, and the seed beam oscillates back and forth between the concave spherical mirror 4 and the convex cylindrical mirror 1. After the gain saturation, a self-reproducing mode with stable intensity and phase distribution is formed. The calculation process is shown in Eqs. (1)-(10).
The mid-infrared CW laser output with average power of 550 W, beam quality β value of 1.8 (the β value is 1.5 in the stable resonator direction and 1.9 in the unstable resonator direction), and spectral coverage of 3680-4089 nm is realized in a miniaturized DF gas flow chemical laser device (size of gain medium: 30 mm×40 mm×125 mm), as shown in Figs. 4-6. The calculation results of the far-field spot are shown in Fig. 4. Figures 4(a), 4(b) and 4(c) respectively correspond to the gain size in the direction of the air flow, which is determined by the rectangular apertures in front of the concave spherical mirror 4, being limited to 10 mm, 8 mm, and 6.5 mm. High-order transverse mode oscillations appear in the X direction for the far-field light spots in Figs. 4(a) and 4(b). Figure 5 shows the experimental results of the far-field spots. When the gain size in the air flow direction is limited to 10 mm and 8 mm, the intensity distributions of the near-field spot (10 mm×6.6 mm, 8 mm×6.6 mm) and the far-field spot in the experiment have the characteristics of three bright spots and two bright spots, respectively. The bright spot appears as a Hermitian-Gaussian high-order transverse mode, and the beam quality β values of the far-field spots in the air flow direction are 4.2 and 3.4, respectively. As the gain size in the air flow direction is limited to 6.5 mm, we experimentally obtained a high beam quality CW laser output with a beam quality β value of 1.8, and the beam quality in the air flow direction, i.e., the direction of the stable cavity, is 1.5.
In order to achieve high-beam quality and high-power mid-infrared laser output based on a miniaturized DF gain generator (gain volume is 30 mm×40 mm×125 mm), we propose and study a Z-folded off-axis hybrid unstable resonator in the gas flow direction as the optical resonator of DF chemical laser device, and the beam quality in the gas flow direction of the resonator is studied theoretically and experimentally. Finally, mid-infrared CW laser output with an average power of 550 W, a high beam quality β value of 1.8 (the β value is 1.5 in the stable cavity direction and 1.9 in the unstable resonator direction), and a spectral coverage of 3680-4089 nm is achieved.
