2-5 μm mid-infrared lasers are of great use in many aspects such as the applications of remote monitoring, free space communication, laser surgery and infrared countermeasures. The fiber gas Raman source, combining the advantages of traditional solid core fiber lasers and gas lasers, firstly demonstrated in 2002, has aroused extensive attentions and now is still a hot topic for having better beam quality, higher damage threshold, higher peak power and narrower linewidth. By particularly designing hollow-core fibers (HCFs), selecting proper gases filled in the HCFs, mid-infrared lasers with various wavelengths can be easily obtained. So far, fiber gas Raman laser sources of different bands such as 1.5 μm, 2.0 μm, 3.0 μm, and 4.0 μm has been demonstrated through the sitmulated Raman scattering (SRS) of different gases such as methane, hydrogen and ethane. However, due to the transmission properties of current low-loss mid-infared HCFs, no efficient mid-infared fiber gas Raman laser sources beyond 4 μm have been reported up to now.
In this paper, we provides a method of generating mid-infrared laser through the SRS of gases filled in the HCFs under cascaeded sturcture. In our experiments, two kinds of HCFs with different transmission spectra are used respectively in the first stage and second stage of generating Raman laser. In the first stage, a commercial 1064 nm high peak power laser is coupled into a methan-filled HCF through reflectors and planoconvex lens, and a 1.55 μm high peak power Raman laser is achieved by the first-order vibrational SRS of methane. In the seconde stage, the output 1.55 μm Raman laser is coupled into a hydrogen-filled HCF as the pump source, and finally a 4.3 μm Raman laser is generated by the first-order vibrational SRS of hydrogen. The transmission spectra of the HCF are the key factors in determining whether to generate 1.55 μm laser by the SRS of methane in the first stage or generate 1.9 μm laser by the SRS of hydrogen in the first stage. This work provides a vital way to realize a wide wavelength range of mid-infrared, even far-infrared fiber laser sources from available commercial 1 μm lasers with appropriate HCFs and different active gases.