Polarized, Single-Mode Fiber Laser Generates 633 W

Breck Hitz

Although unpolarized fiber lasers have generated output in excess of 1 kW with high-quality beams, polarized outputs have been significantly lower, on the order of 300 W. Polarized outputs are important, however, in many applications, including nonlinear optics, materials processing and medicine.

Figure 1. The birefringent, double-clad ytterbium-doped fiber laser is cladding-pumped from both ends with a pair of 975-nm laser diode stacks. ©OSA.

Recently, a coalition of researchers from the University of Southampton and Southampton Photonics, both in the UK, designed and demonstrated a polarized, ytterbium-doped fiber laser that produces a polarized output of 633 W in a beam with an M² of <1.2.

Figure 2. The laser's output power increased linearly to 633 W, with no sign of rollover at the high end. ©OSA.

The heart of the laser is 6.5 m of birefringent, large-core (25-mm diameter), double-clad, ytterbium-doped fiber. This fiber is cladding-pumped from both ends by 975-nm laser diode stacks (Figure 1). The resonator is formed by a high-reflection mirror at one end and Fresnel reflection from the cleaved fiber at the output end.

A half-wave plate and Brewster window in front of the mirror enable the researchers to adjust the polarization direction to align with a birefringence axis in the fiber. These two elements impose an ~30 percent single-pass loss on the disfavored polarization, preventing it from reaching threshold in the resonator. The ends of the fiber are held in temperature-controlled V-grooves to prevent damage by stray pump or laser radiation or by heat generated in the laser.

In tests of the device, the laser output versus power pumped into the fiber increased linearly to 633 W with no sign of rollover at the high end (Figure 2). Only ~60 percent of the available pump power was launched into the fiber, however, because of the thinness of the inner cladding (380 µm) and the relatively poor quality of the beam from the diode stacks. The output power of the laser was limited only by the pump power entering the fiber.

The laser's full-power spectrum is centered at 1090 nm, with a linewidth of ~10 nm (Figure 3). Raman scattering is a major hindrance in high-power fiber lasers, and polarized lasers are more susceptible to nonlinear scattering than unpolarized ones.

Figure 4. The fiber laser has a polarized output of 633 W. Courtesy of the University of Southampton.

Nonetheless, there is no visible peak at the first Raman Stokes wavelength. The scientists estimate that the Raman gain in the laser is <2 dB, which is far below the single-pass threshold of 14 dB. Amplified stimulated emission is suppressed by as much as 50 dB (with a resolution bandwidth of 1 nm) relative to the laser output.

To compare the polarized performance of the laser with its unpolarized performance, the scientists removed the half-wave plate and Brewster window from the resonator. The increase in the laser's slope efficiency from 67 percent to 71 percent was attributable simply to the removal of intracavity loss associated with those two elements. The laser has good stability properties, with a standard deviation power fluctuation of less than 2 percent.