High-Power Yb-Doped Fiber Laser Emits at 977 nm
Engineers at the University of Southampton and at Southampton Photonics Inc. in the UK have demonstrated the first high-power -- i.e., greater than 1 W -- ytterbium-doped fiber laser emitting at 977 nm. Such a device might find use as a pump for erbium-doped fiber amplifiers and for parametric devices, as well as in ultrafast and multiphoton spectroscopy.
Fiber lasers are optically pumped by coupling radiation into the fiber at one or both ends. Because the numerical aperture of the core of a single-mode fiber is low, direct optical pumping of the core requires a single-mode diode, but to take advantage of the higher powers of multimode pumps, a double-clad configuration may be employed. An outer cladding has a lower refractive index than the inner cladding, which has a lower refractive index than the core. The inner cladding, together with the core, therefore becomes a high-numerical-aperture waveguide that can be pumped with a multimode diode, and the laser radiation is confined to the core.
A mesh of thin glass strands supports the 20-µm-diameter inner cladding of an air-clad fiber that is the basis of a new high-power Yb-doped fiber laser. The 9-µm core is not visible in this image.
Because the volume of the pump waveguide is much greater than that of the core, however, the overlap of the pump radiation and gain medium is small, and a long fiber is required to absorb enough pump radiation and to enable the efficient conversion into signal radiation. In the case of a 977-nm Yb laser, a long fiber allows amplified spontaneous emission at 1040 nm to deplete the population inversion before the threshold of the 977-nm transition can be reached. Decreasing the diameter of the inner cladding increases the overlap of the pump radiation and the gain medium, allowing threshold for the 977-nm line to be achieved in a shorter fiber. Unfortunately, it also decreases its numerical aperture and makes it more difficult to couple the pump radiation into the fiber.
The solution was to use air-clad fiber. By reducing the refractive index of the outer cladding to unity, the researchers created a double-clad fiber with a small inner cladding diameter but a large numerical aperture. A mesh of thin glass strands with a wall thickness comparable to the wavelength supported the inner core of the fiber. The resulting device operated both as a 977-nm laser and as an amplified spontaneous emission source at 977 nm. In either case, a pair of fiber-coupled diode lasers supplied the 915-nm pump radiation.
The narrow (approximately 4 nm) spontaneous bandwidth of Yb enables a spectrally concentrated, high-power amplified spontaneous emission source. When the 1.2-m-long, air-clad fiber was spliced to a single-mode fiber with an angled facet to eliminate backreflection, the incoherent output was nearly unidirectional. An output of 1.2 W with a 3-nm bandwidth was obtained with a slope efficiency of 37 percent. These characteristics, along with the total random polarization, make the source an ideal pump for a distributed feedback laser in a wavelength division multiplexing system.
To convert the source into a laser, the researchers inserted a dichroic mirror that was transmissive to the 915-nm pump and reflective to the 977-nm laser light between the fiber and the pump lasers, and spliced a fiber Bragg grating reflecting approximately 10 percent at 977 nm to the output end of the fiber. In this configuration, they obtained 3.5 W of laser output from 9 W of pump power at a slope efficiency of 42 percent.
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