Tunable Mode-Locked Fiber Laser in 1750–1870nm by Bending Normal Dispersion Thulium-Doped Fiber as a Distribution Filter

Due to the advance of fiber laser technologies, the bio-imaging community has been accessing the third near-infrared (NIR) optical window spanning from 1600-1870 nm, which exhibits the merits of diminished phototoxicity and reduced optical attenuation[1]. The silica-based thulium-doped fiber (TDF) laser is an attractive candidate due to the broadband emission of TDF from 1600 to 2050 nm. However, short wavelength operation below 1850 nm is restricted by the strong reabsorption due to the quasi-three-level nature [2]. To achieve short wavelength operation using TDF, amplified spontaneous emission (ASE) suppression at long wavelength is demonstrated using passive long-wavelength filters such as holmium-doped fiber, photonics crystal fiber and dispersion compensating fiber [3][4][5], however, at the expense of a system being complicated and higher pump power due to the non-negligible loss at short wavelength from passive filters. On the other hand, the pulse energies of mode-locked TDF lasers still remain at low level because of the relatively large anomalous dispersion of TDFs in the 1600-2050 nm region. Very recently, we have reported an in-house fabricated TDF with a W-type index profile, exhibiting a broadband normal dispersion spanning from 1700 nm to over 2050 nm[6]. Interestingly, the W-type index profile in the TDF can perform as a distributed long-wavelength ASE filter under bending, which improves the efficiency at the desired short wavelength without using passive filters. Therefore, high-level energy pulse in the 1600-1870 nm is envisaged by exploiting the advantages of normal dispersion (as shown in Fig. 1a) and distributed long-wavelength filtering effect in our fabricated TDF.