- Good Splices Make Good Lasers
Intracavity losses of a fraction of a percent can cut the output power of a low-gain laser, such as an erbium-doped fiber laser, by a factor of two or more. A significant source of loss in an erbium-doped fiber laser is the splice between the normal, single-mode fiber and the erbium-doped fiber, which results from the differing mode patterns in the two types of fiber. Now researchers at National Chiao Tung University in Hsinchu and Chunghwa Telecom Co. Ltd. in Taoyuan, both in Taiwan, have demonstrated a technique to reduce this loss by shaping the single-mode fiber before it is fusion-spliced to the erbium-doped fiber.
Figure 1. Researchers investigated four techniques of splicing single-mode fiber to erbium-doped fiber: a direct splice (a), the insertion of a length of fiber with a 5.5-µm core diameter (b), the insertion of fiber with a 6.6-µm core diameter (c) and the insertion of a tipped single-mode fiber with a tapered core and cladding (d).
The group considered the standard approach to splicing as well as three alternatives in which another fiber structure is inserted between the fibers to be spliced (Figure 1). The single-mode fiber could be spliced directly to the erbium-doped fiber, the two fibers could be connected by an intermediate fiber with a 5.5-µm core or by one with a 6.6-µm core, or a tapered single-mode fiber could be fusion-spliced between the two.
The scientists used a ring laser resonator to evaluate these splicing techniques (Figure 2). A laser diode pumped the 6-m erbium-doped fiber with up to 75 mW at 976 nm. Two optical isolators ensured single-direction oscillation around the ring, and the laser output was obtained through a 10 percent output coupler. The experimental fiber section was spliced into the resonator (on the right side of the figure). When the laser was off, a distributed-feedback laser diode could be switched into the optical circuit to measure the relative losses of the fibers.
Figure 2. The scientists evaluated the splicing techniques by monitoring their respective effects on the optical efficiency of a ring fiber laser.
To create the best possible splices in every case, the researchers turned on the ring laser before making the splice and aligned the fiber ends in the fusion splicer to produce the maximum output from the ring laser. They then would turn on the splicer's arc for several seconds.
They evaluated each splicing technique by observing its effect on the laser's optical efficiency; that is, the ratio of input pump power at 976 nm to output laser power at 1.55 µm. The best optical efficiency obtained by other teams with similar lasers was 1.2 percent, approximately the same as the 1.3 percent the scientists observed with the direct splice. For the 5.5-µm connecting fiber, however, the optical efficiency was 1.75 percent, and it increased to 2.25 percent with the 6.6-µm fiber. With the tapered single-mode fiber insert, moreover, the efficiency was 2.75 percent.
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