1725nm all-fiber SWIR CW laser using W-type Tm:Ge doped fiber

Demand for high power fiber lasers at short wavelength infra-red (SWIR) region of 1.7μm has increased in recent years due to their usefulness in various fields such as bio-imaging, material processing, laser treatment, liposuction etc [1] , owing to strong C-H bond absorption and reduced water peak. Tm doped fiber is one of the major contenders for gain medium as it has wide emission region of 1.65μm to 2.1μm with peak ~1.9μm and hence covers 1.7μm. Although typical high power Tm fiber lasers work at 1.8 μm-2.1 μm region, shorter side of emission band can be accessed by direct pumping using high brightness Er-doped laser but needs fiber Bragg grating for wavelength selection [2]. A simpler way to select shorter wavelength is by utilizing distributed short-pass filtering effect in a fiber with W-type index profile by applying bend loss [3]. In this method, losses at longer wavelengths are increased by bending, so that only shorter wavelength region sees more gain than loss. Since the peak of a typical Tm:Al doped fiber gain medium is ~1.9μm, gain at 1.7μm is quite low for efficient lasing at shorter wavelength via bending technique. However, it has been proven that choosing germanium (Ge) as co-dopant can blue-shift the absorption- and emission wavelength in a Tm doped fiber [4]. In this work, we report a SWIR laser at 1.725μm using a novel W-type Tm:Ge active fiber which takes advantage of short-pass filtering effect and blue-shift in absorption/emission cross-section.