Outputs of Fiber Lasers Combined Coherently
Passive phase locking results in nearly 100 percent combining efficiency.
The largest barrier to increasing the power from fiber lasers is the onset of nonlinear effects and optical damage that result from high power density in the fiber core. An obvious solution is to increase the core diameter, thereby reducing the power density, and a host of approaches involving large-mode-area fibers have emerged in recent years. An alternative, and perhaps less investigated, approach is to coherently combine the outputs of several lower-power lasers to produce a single high-power beam. Recently, researchers at the Weizmann Institute of Science in Rehovot, Israel, have demonstrated a technique of coherently combining the outputs of two fiber lasers to produce very nearly twice the power available from either laser alone.
Although the power levels achieved in these early experiments are insignificant, the highly efficient beam combining provides a potential solution to the current limitations on fiber laser power. The scientists evaluated two techniques of coherent beam combining, one in which the beams were combined inside a common resonator and one in which beams from individual resonators were combined externally (Figure 1).
Figure 1. The beams of two fiber lasers were combined inside a common resonator (a) or outside the two individual resonators (b). In either case, the reflection from the mirror labeled “output coupler” provided sufficient feedback from one laser into the other to cause phase locking, so the beams combined coherently in the combined output, and power in the loss channel was minimized. Images reprinted with permission of Optics Letters.
For intracavity combining, the output ends of the two fibers were cleaved at 8° and provided no reflection; for external combination, the ends were cleaved at normal incidence, and the resulting 4 percent Fresnel reflection defined one end of each resonator.
In both cases, the high-power combined beam existed only in free space, and only the lower-power beam from a single laser propagated inside a fiber. Although the current demonstration was limited to two lasers, presumably the technique could employ multiple beamsplitters to combine the outputs of multiple lasers.
To vary the amount of coupling between the two lasers, the scientists inserted a lens (not shown in Figure 1) to the left of the output coupler. When the output coupler was placed in the focal plane of the lens, the Gaussian beam was reflected exactly back onto itself into the lasers, so that maximum coupling — approximately the 4 percent reflectivity of the output coupler — occurred. By moving the output coupler out of the focal plane, the scientists could defocus the reflected beam and reduce the coupling between resonators.
First the scientists calibrated the amount of coupling as a function of the distance between the lens and the output coupler, and then they observed the effect of coupling on the efficiency of beam combining. Their results were different for the two configurations shown in Figure 1, but in both cases the significant result was that near-unity efficiency was achieved with less than 2 percent coupling between the two lasers (Figure 2). Because efficient combining is possible when only a small amount of light is coupled from one laser into the other, the risk of optical damage or nonlinear effects is low.
Figure 2. Whether the beams were combined inside a common resonator or outside both resonators, near-unity combining efficiency was achieved with a relatively small (~2 percent) coupling between the two lasers. The combining efficiency is defined as the ratio of power in the combined beam to the total power available from the two lasers individually.
As indicated in Figure 2, in both cases the combining efficiency decreased as the coupling decreased, but the decrease was faster for intracavity coupling. When the beams were combined externally (Figure 1b), changing the coupling between the lasers did not affect either of the lasers individually. However, when the beams were combined internally, decreasing the coupling between the lasers also increased the intracavity loss of each laser and thereby reduced the power available from each of the lasers individually.
The scientists also investigated the spectra of the combined lasers and observed the expected overlapping longitudinal modes from the combinations of coupled cavities. And, importantly, they saw that, for either internal or external combining, there were enough common longitudinal modes to indicate that coherent combining of more than two lasers should be practical.
Optics Letters, April 1, 2007, pp. 790-792.
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