Single-mode fiber lasers have the high beam quality required for many applications, but as their output power climbs, the intense intracavity power density in their tiny cores — typically ~10-μm diameter — causes nonlinear effects that sap the power. A solution is to enlarge the core, but that allows higher-order transverse modes to propagate and lowers the quality of the output beam. There are schemes to force only a single mode to oscillate in a large-mode-area fiber, but they are successful only up to a point. Moreover, in most cases, only the narrow fundamental mode propagates in the nominally multimode fiber, and it can access only a fraction of the population inversion across the large core. Now, X. Zhu and his colleagues in the College of Optical Sciences at the University of Arizona in Tucson have conceived and demonstrated a fiber laser in which multiple modes oscillate but interfere to generate a single-mode (M2 = 1.01) output beam. And the laser produces only slightly less output power than the same fiber would produce operated as a conventional multimode laser. The Talbot effect, which manifests itself as interference among multiple transverse modes propagating in a fiber, is the key to the new technique. First enunciated in the 19th century, the Talbot effect predicts that the electric-field profile at the input of a multimode waveguide will be periodically reimaged as the light propagates down the waveguide. The distance between reimaging planes depends on the geometry of the waveguide and on the wavelength of the light. Figure 1. For certain wavelengths, the single-mode profile input to the multimode fiberis reimaged at the far end of the multimode fiber and coupled with minimal loss into the second piece of single-mode fiber. MMF = multimode fiber; SMF = single-mode fiber. Images reprinted with permission of Optics Letters.In other words, the light propagating from a single-mode fiber into a multimode fiber will excite higher-order modes in the multimode fiber. But at periodic locations along the multimode fiber, the single-mode profile will be reimaged. Thus, it is possible to construct a wavelength filter by sandwiching a multimode fiber between two lengths of single-mode fiber (Figure 1). For certain wavelengths, the single-mode input profile is reimaged at the far end of the multimode fiber, and the insertion loss is minimized. (Theoretically there should be no insertion loss, but splice losses and fiber imperfections created ~1 dB of loss in Figure 1.)Figure 2. The fiber-laser resonator — between the dichroic mirror and the output facet of the single-mode fiber — was ~20 cm long. Although multiple transverse modes oscillated in the left half of the resonator, only a single mode oscillated in the right half, and the output was diffraction limited (M2 = 1.01). MMF = multimode fiber; SMF = single-mode fiber.The researchers fabricated their laser by splicing a precise length — 10.236 cm — of Er-Yb-doped multimode fiber to a single-mode fiber (Figure 2). They chose the multimode fiber length so that input light at the peak erbium gain wavelength was reimaged at the splice after reflecting off the dichroic mirror at the other end of the fiber. The dichroic mirror and the facet at the left end of the single-mode fiber defined the fiber-laser resonator, and multiple transverse modes oscillated in the multimode section of the resonator. But those modes were coupled with high efficiency to the single mode that oscillated in the single-mode section.The scientists pumped the laser through a multimode delivery fiber with 976-nm radiation from a diode array. The dichroic mirror in Figure 2 transmitted ~92 percent of the pump light, but had a reflectivity of ~99 percent at the 1535-nm fiber-laser wavelength. The output of the fiber laser was in excess of 1 W and was limited by the available pump power (Figure 3). Figure 3. Output from the laser in Figure 2 exceeded 1 W and was limited by the available pump power. The slope efficiency was 8.1 percent, only slightly lower than the 9.2 percent slope efficiency (SE) for a multimode laser operating without the single-mode section in Figure 2. Splice loss probably contributed to lowering the efficiency of the single-mode laser.A potential restraint on extending this concept to higher powers could be mode coupling among the oscillating transverse modes. The Talbot effect is disrupted if energy is coupled into other modes, and the investigators avoided mode coupling in their laser by keeping the length of the multimode fiber short — ~10 cm in this case. Careful mode control would have to be maintained to extend the concept to longer fibers or to those with larger cores.Optics Letters, May 1, 2008, pp. 908-910.