Solid state single longitudinal mode (SLM) laser with visible light band has broad development prospects in application fields such as high-resolution Raman spectroscopy, laser detection and Brillouin microscopy due to its characteristics of SLM output, narrow spectral line-width and long coherence length [1–6], and has become a relatively hot research direction in the world. At present, the visible SLM laser usually adopts nonlinear frequency transformation method to achieve the visible light band conversion. Due to the need of one or two nonlinear crystals in the process of nonlinear frequency conversion, the laser system has high complexity, high cost, low efficiency and other negative effects. As a new type of laser crystal doped with praseodymium ion has rich emission spectrum in the red, orange, green, blue and so on, it has become an ideal material to replace nonlinear crystals to generate visible band [7]. Compared with the traditional technique of generating visible band praseodymium ion laser crystal can directly obtain the output of visible band without nonlinear frequency conversion. This not only simplifies the overall structure of visible band SLM and improves the output efficiency of SLM laser, but also enriches the application field of visible light band solid state laser. At present, the SLM Pr:YLF lasers in different visible bands have been widely researched by using different mode selection methods [8–10]. However, best to our knowledge, the dual-wavelength SLM Pr:YLF laser has not been reported.
Simultaneous dual-wavelength laser has a broad application prospect in medical treatment, terahertz source, multispectral lidar and other fields because of its output characteristics of two different simultaneous output modes [11–13]. However, with the further development of the above application fields, the performance requirements of dual-wavelength lasers are becoming more and more demanding, and the traditional dual-wavelength lasers can no longer meet the application requirements. To further expand the output performance and laser characteristics for different wavelength range of dual-wavelength lasers is a key problem to be solved urgently. Among them, dual-wavelength SLM lasers with different spectral range have attracted extensive attention due to their unique laser characteristics. Up to now, the research of dual-wavelength laser mainly focuses on the infrared band. Zhang et al reported a diode-pumped CW Tm, Ho:YVO4 microchip laser at 2041.3 nm and 2054.6 nm, and the total dual-wavelength SLM output power reaches 185 mW [14]. Ju et al reported a dual-wavelength SLM CW Tm,Ho:GdVO4 microchip Laser at 2038.9 nm and 2050.1 nm. The total output power reaches 98 mW [15]. Zhang et al reported orthogonally polarized dual-wavelength SLM CW Tm,Ho:LLF laser at 2066 nm and 2064 nm, and the total output power reached 110 mW [16]. Research on the visible dual-wavelength laser has potential value. Especially, the orange light source in the 600–610 nm band has a very important application in the treatment of epidermal pigmented lesions(EPLs) and flow cytometry [17–18]. In this study, to the best of our knowledge, it is the first time to investigate a diode-pumped orthogonally polarized dual-wavelength SLM CW Pr:YLF laser at 607 nm and 604 nm. At the absorbed pump power of 2.47 W, the maximum orthogonally polarized dual-wavelength SLM laser output power is 32 mW and the measured dual-wavelength single longitudinal mode laser spectral line-widths are 174.4 MHz and 102.2 MHz. The output power of single wavelength SLM laser at 607 nm and 604 nm is 201 mW and 81 mW and the measured single wavelength single longitudinal mode laser spectral line-widths are 52.3 MHz and 111.3 MHz, respectively.