Fiber Laser Generates 1.6 W in Single Longitudinal Mode

Breck Hitz

Researchers at the University of Arizona’s College of Optical Sciences in Tucson have generated 1.6 W at 1550 nm in a single longitudinal mode from a short, cladding-pumped Er:Yb codoped fiber laser. They believe that theirs is among the highest-power single-frequency fiber lasers that have been demonstrated to date. Single-frequency fiber lasers are important in telecommunications and as the light source in many fiber sensing applications.

Figure 1. The centimeters-long, single-mode Er:Yb fiber, together with a fiber Bragg grating and the end of the multimode pump fiber, fit snugly inside a low-index glass tube that was cooled with flowing water. Images ©OSA.

The investigators built two similar lasers, one based on a 4-cm-long, single-mode fiber and the other on a 5.5-cm-long, single-mode fiber, both from NP Photonics Inc., also of Tucson (Figure 1). A fiber Bragg grating (FBG) that they fabricated in a photosensitive fiber from Nufern Inc. of East Granby, Conn., provided the frequency discrimination to force single-longitudinal-mode oscillation. The FBG had a peak reflectance of 35 percent and a full-width half-maximum bandwidth of 0.04 nm, which was sufficient to discriminate between the longitudinal modes, spaced by ~0.011 nm for the 5.5-cm length of fiber. (With a slightly longer, 7-cm fiber, the longitudinal modes were too close for the grating to force single-frequency oscillation.)

Figure 2. The shorter fiber displayed a stronger rollover than the longer one. The circles and squares are experimental data points, and the solid line is the result of a mathematical simulation for the 5.5-cm fiber.

Pump light at 976 nm was coupled into the laser’s cladding from a multimode fiber. A dielectric coating on the end of the multimode fiber reflected 98 percent of the light at the laser wavelength (1550 nm) but only 5 percent of the pump wavelength. Thus, the FBG at one end and the dielectric coat at the other defined the laser’s resonator. The effective resonator length was about 1.2 cm longer than the Er:Yb-codoped fiber because of the length of the grating.

Output from the laser was approximately the same for both fiber lengths, although the shorter fiber displayed a sharper rollover (Figure 2). Threshold for the laser with the 4-cm fiber was 280 mW; for the 5.5-cm fiber, it was 390 mW. The highest power — 1.6 W — was obtained with the 5.5-cm fiber, with a pump power of 37 W launched into the laser’s cladding.

Figure 3. The RF beat spectrum generated between the fiber laser and a tunable single-frequency diode laser showed distinct modes when the fiber laser was unpolarized (top). When enough birefringence to extinguish one polarization was introduced into the resonator, only a single mode oscillated (bottom).

The scientists believe that making relatively straightforward design modifications could improve the laser’s 5 percent slope efficiency by as much as a factor of two.

To confirm oscillation in a single longitudinal mode, they monitored the radio-frequency beat note generated between the laser and a tunable single-frequency diode laser. Because the laser was unpolarized, they observed two beat notes, corresponding to the orthogonal polarizations in the resonator (Figure 3, top). By applying pressure to one side of the FBG, they forced it to become birefringent and to discriminate between the two polarizations so that only a single beat note was present (Figure 3, bottom). This beat note could be stable for periods of minutes before thermal drift allowed the laser to hop to a different longitudinal mode.