Yb-Fiber Laser Is Efficiently Pumped at an Oblique Angle

Breck Hitz

Fiber lasers can be optically pumped by coupling the pump light directly into the fiber core, but because the acceptance angle of the core is relatively narrow, core pumping requires a high-quality — low divergence — pump beam (Figure 1a). Because the diode lasers commonly used as pumps cannot efficiently produce beams of the required quality, scientists developed cladding-pumped fibers that are more compatible with diode pumps (Figure 1b).

Figure 1. Because the acceptance angle for core pumping (broken red lines) is small, the pump laser must provide a high-quality (low divergence) beam (a). This requirement is relaxed in a cladding-pumped laser because the cladding has a larger acceptance angle (b). A disadvantage of these two techniques is that, in their normal configurations, they require a dichroic mirror to separate the incoming pump light from the outgoing fiber-laser beam. Oblique or skew pumping is a compromise between core and cladding pumping that avoids the need for a dichroic mirror (c).

Recently, scientists at Royal Institute of Technology in Stockholm, Sweden, demonstrated, for what they believe is the first time, an off-angle pumping technique that eliminates a major drawback of other methods — the requirement of a dichroic mirror to separate the fiber-laser beam from the pump-laser beam (Figure 1c).

To wavelength-tune their laser, the investigators substituted a volume Bragg grating retroreflector for the normal back mirror and used the facet at the other end of the laser as the output coupler (Figure 2). The retroreflector assembly was based on a volume Bragg grating from Ondax Inc. of Monrovia, Calif., that had a peak reflectivity at 1063.5 nm and a FWHM bandwidth of 146 GHz. An angular adjustment of the assembly shifted the wavelength reflected without changing the angle or displacement of the reflected beam.

Figure 2. An angular adjustment of the volume Bragg grating (VBG) retroreflector tuned the wavelength of the skew-pumped fiber laser between 1022 and 1055 nm. M = mirror; L = lens. Reprinted with permission of Optics Letters.

The gain medium of the laser was a 1.7-m-long fiber doped with 1.25 ppm ytterbium by weight. The large-mode-area fiber has a core diameter of 30 μm and a cladding diameter of 250 μm. The scientists pumped it with 976-nm radiation from a fiber-coupled diode laser. They obtained similar slope efficiencies across the laser’s 30-nm spectrum from 1022 to 1055 nm, but somewhat less power at the longer wavelengths (Figure 3).

Figure 3. The laser’s slope efficiency was approximately 77 percent at all wavelengths. Reprinted with permission of Optics Letters.

The laser’s bandwidth, which the scientists measured with a Fabry-Perot interferometer, was approximately 5 GHz at all output wavelengths, significantly narrower than the volume Bragg grating’s 146-GHz bandwidth. That is because only wavelengths near the grating’s peak reflectivity were reflected strongly enough to overcome other cavity losses and reach threshold.

The investigators estimated that about 20 transverse modes were oscillating in their laser. Because high-order modes experience greater bending loss than low-order ones, they often can be extinguished by coiling the fiber.

That approach did not work with this laser, however, even though the scientists coiled the fiber as tightly as a 2-cm radius. But when they double-coiled the fiber with the two coils at a right angle to one another, they obtained close to single-mode oscillation. They measured the M-square to be less than 1.3, without power reduction across the entire 30-nm tuning range.

Optics Letters, Dec. 15, 2007, pp. 3501-3503.