High-efficiency ～2 μm laser in a single-mode Tm-doped lead germanate composite fiber

Abstract

A highly Tm-doped lead germanate glass fiber is developed using the rod-in-tube method. The ∼2 μm laser beam quality of the fiber is ∼1.5. The lead germanate composite fiber jumpers are homemade for all the fiber laser investigations. When core is pumped by a 1590 nm Yb/Er fiber laser, a maximum laser output of 313 mW is achieved at a 670 mW pump power, and the corresponding slope efficiency is ∼52.8%. Moreover, by using a 2 cmlong lead germanate fiber as the gain medium, a 33 mW 1942 nm Tm laser is also demonstrated.OCIS codes: 160.5690, 160.2290, 140.3510. doi: 10.3788/COL201614.081601.Fiber lasers operating in the 2 μm eye-safe region have been studied extensively owing to their potential applications, including environmental sensing, laser radar, space communication, and efficient mid-infrared (IR) light generation [1][2][3][4][5] . In the past decade, rapid progress has been made with the development of ∼2 μm Tm-doped fiber lasers. Compared with silica glass, multicomponent glass as a fiber host has several advantages, including a lower phonon energy, resulting in a wider IR transmission range, high rare earth solubility, resulting in high gain per unit length, and a larger absorption/emission cross section, and it has attracted considerable attention [6][7][8][9] . In particular, germanate glasses, which have not only better mechanical strength and chemical durability than fluoride glasses but also higher thermal stability than tellurite glasses for output scaling, are promising materials for realizing 2 μm fiber lasers. In 2007, a 104 W ∼2 μm laser produced by a large-core-area single-mode germanate fiber was reported [10] , and it is the highest output power among various multicomponent glass fibers. Furthermore, some efforts for developing germanate fiber glasses have also been made. For example, a 0.75 W laser output with a slope efficiency of 28.7% was obtained in a Tm-doped tellurium germanate glass fiber pumped by a 793 nm diode laser [11] . With an in-band pump at 1568 nm, an ∼2-μm Tmdoped fiber laser in a barium gallo-germanate glass system was demonstrated with a slope efficiency of 7.6% [12] . In comparison, lead germanate glass systems have relatively large glass-forming regions, low softening temperatures, and good formability while maintaining high IR transmissions [13][14][15] , which make them potential candidates for mid-IR fibers. A Tm-doped lead germanate fiber laser at 1.88 μm, pumped by a 794-nm Ti:sapphire laser and with a slope efficiency of 13%, was first reported in 1992 [16] . In our previous work [17] , we demonstrated an ∼2 μm continuous-wave laser and a passively pulsed laser based on a lead germanate fiber. Commercial lead silicate glasses, which possess the appropriate thermal properties and viscosities, can be used as fiber cladding materials to reduce the cost. In these reports, the large difference between the refractive indices of the germanate glass core and the silicate glass cladding leads to a multimode laser operation. However, a single-mode fiber is needed for deve...

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