The broad spectral linewidths of fiber lasers make them attractive candidates for versatile, wavelength-tunable sources, but the wavelength-tuning mechanisms — often involving multimode fiber Bragg gratings or other complex components — are neither inexpensive nor simple. Recently, scientists at Centro de Investigaciones en Óptica in León, Mexico, and at the University of Central Florida in Orlando developed a straightforward technique that they believe can lead to very cost-effective wavelength-tunable fiber lasers. Figure 1. Light emerging from the single-mode fiber is imaged by the multimode fiber. A mirror reflects the light back through the two fibers. But different wavelengths are imaged at different locations, so the mirror can reflect only one wavelength through the fibers perfectly. Images ©OSA. The technique involves placing a simple wavelength filter — in reality, a multimode interferometer — on one end of the fiber laser. The filter consists of a length of single-mode fiber followed by a multimode fiber and a broadband mirror (Figure 1). The multimode fiber images the exit face of the single-mode fiber to a location slightly beyond the end of the multimode fiber. If a mirror is placed precisely at this location, the light is perfectly reflected back on itself through the fibers.But because the multimode fiber is dispersive, different wavelengths are focused to different distances beyond its end. Thus, for any given mirror position, one wavelength is reflected perfectly through the filter, and all others encounter a greater loss. When the device in Figure 1 replaces one of the fiber laser’s mirrors, it selects the wavelength oscillating in the laser.Figure 2. By replacing one of the cavity mirrors with the wavelength filter shown in Figure 1, the scientists created a simple and inexpensive wavelength-tunable fiber laser.To demonstrate the concept, the scientists spliced their filter onto the end of a 16-m length of double-clad, ytterbium-doped fiber (Figure 2). The filter served as one of the laser’s mirrors, and the cleaved end of the double-clad fiber served as the other. They pumped one end of the fiber with 3 W of 915-nm light from a fiber-coupled diode laser. A dichroic mirror separated the incoming pump light from the emerging fiber laser beam. They tuned the laser over a range of 1088 to 1097 nm by translating the mirror between positions 100 to 230 µm from the end of the multimode fiber (Figure 3).Figure 3. The laser could be tuned with minimal variation in output powerbetween 1088 and 1097 nm. Although only four distinct wavelengths are shown here for clarity, the wavelength was continuously tunable. The tuning range was limited by the reflectivity of the mirror (inset), which did not provide a good match to the laser’s bandwidth.They believe that the 8-nm tuning range was limited primarily by the reflectivity of the translatable mirror, which shrank from ~93 percent to ~73 percent for wavelengths from 1070 to 1100 nm (Figure 3, inset) and thus did not provide a good match to the ytterbium fluorescence, which peaks at ~1082 nm. They estimate that a larger tuning range, perhaps as much as 30 nm, could be achieved with a mirror that better matches the laser’s bandwidth.Another improvement the researchers want to implement is self-alignment of the broadband mirror with the multimode fiber via a capillary tube. Manual alignment was tedious to maintain in their experiments to date, and they hope it will be much simpler with an automated system.