Fiber Ring Laser Generates 1 W in a Single Frequency in Eye-Safe Region

Breck Hitz

Fiber laser oscillators historically have not been particularly good sources of single-frequency radiation, primarily because their length makes it difficult to discriminate between closely spaced longitudinal modes. Although single-frequency outputs in excess of 100 W have been achieved from amplified fiber oscillators, these systems suffer from a high level of amplified spontaneous emission noise. In addition, they are complex and expensive.

Recently, researchers at the University of Arizona's Optical Sciences Center in Tucson generated 1 W of single-frequency power at 1.5 µm from an unamplified fiber oscillator in a ring configuration, which they say is five times the maximum output power previously obtained using this approach.

The ring configuration was one of the keys that enabled the scientists to achieve that power level. Another was the use of heavily doped phosphate-glass fiber, which allowed them to shorten the laser to ~0.6 m and thereby to increase the frequency spacing of the longitudinal modes. And a third was the addition of an extra frequency-discrimination element that selected a single longitudinal mode.

The optical layout comprised a ring of fiber, an optical circulator to ensure unidirectional oscillation around the ring, and a fiber Bragg grating that reflected light into the circulator (Figure 1). In this layout, the first grating provided the primary feedback for the resonator; the second enhanced the resonator's frequency discrimination.

Figure 1. The ring laser was cladding-pumped by four coreless silica fibers surrounding the active fiber so that the pump light was evanescently coupled into the cladding of the active fiber. Images ©OSA.

A ring configuration is advantageous for a single-frequency laser because it avoids the spatial hole burning that occurs in a linear resonator when two counterpropagating waves interfere to form a standing wave. There is no electric field at the nodes of the standing wave, and the population inversion at the nodes cannot contribute to the output power. In the fiber ring laser, a single traveling wave propagated clockwise around the ring, and no standing wave was formed. The entire population inversion was able to contribute to the laser's output.

The frequency spacing between longitudinal modes is inversely proportional to a laser's length, and fiber lasers are typically several meters long to ensure that there are enough active ions -- and hence enough single-pass gain -- to reach threshold. The close-packed longitudinal modes that result -- the separation between adjacent modes is only 50 MHz in a laser whose optical length is 3 m, for example -- make it very difficult to isolate a single mode.

To avoid this quandary, the researchers used fiber with a high concentration of active ions so they could shorten the overall length of the resonator. They spliced an 11-cm length of cladding-pumped, heavily doped (1 percent Er³⁺, 8 percent Yb³⁺) phosphate-glass fiber into their ring, and shortened the resonator length to ~60 cm, thereby increasing the mode spacing to 340 MHz.

This was still not large enough to force stable single-frequency operation. The fiber Bragg grating that reflected the light into the circula-tor provided the resonator's frequency discrimination, and with a 3-dB bandwidth of 7.5 GHz, it allowed frequent mode-hopping between the longitudinal modes. To eliminate mode-hopping, the researchers added a second fiber Bragg grating to the layout. Together, the gratings acted like a low-finesse Fabry-Perot interferometer, whose reflection peaks were separated by its free spectral range.

Figure 2. The reflectivity of the single fiber Bragg grating had a bandwidth of about 7.5 GHz (red curve). When a second grating was added, the pair acted like a low-finesse Fabry-Perot etalon whose reflection peaks were much narrower (green curve). By adjusting the spacing between the gratings with a piezoelectric transducer, the researchers could set the reflection peak on a favored longitudinal mode (blue curve). The vertical arrows represent the laser's longitudinal modes, separated by 340 MHz.

These individual reflection peaks were much narrower than the reflection from a single grating and provided adequate frequency discrimination to isolate a single longitudinal mode (Figure 2). A piezoelectric transducer placed between the fiber Bragg gratings in the experimental setup allowed the researchers to set the reflection peak precisely on the favored mode.

Figure 3. The output power at 1.5 µm scaled linearly with input power at 975 nm. The inset shows that the amplified spontaneous emission background was 60 dB below the signal level, a much cleaner result than can usually be obtained with an oscillator-amplifier configuration.

The laser's output scaled linearly with input up to a maximum of 1 W (Figure 3). At a fixed power level below about 700 mW, the laser operated without mode-hopping. Because there was no thermal stabilization, they observed slow mode-hopping at powers above 700 mW due to thermal-induced drift of the resonator length. This could be eliminated, they believe, by instituting temperature stabilization of the laser cavity.