At the University of Arizona in Tucson, investigators have used the modal interference in a tapered fiber to fabricate a wavelength-tunable filter inside a fiber laser and have tuned the laser’s wavelength by physically stretching the taper. They believe that the taper-tuned laser could be useful as a tunable source in optical telecom systems.A fiber taper is formed by heating the fiber and pulling it so that the softened glass narrows to a waist in the middle of the heated region. If a suitable taper is fabricated in a length of single-mode fiber, single-mode light propagating in the untapered section of the fiber is coupled into more than one mode in the tapered section. When these modes reach the far end of the taper — where the fiber expands back to its full untapered diameter — they are coupled back into a single mode. In most cases, only two modes, HE11 and HE12, are excited in the taper, and interference between them accounts for an intensity variation at the output.Figure 1. The transmission of 1550-nm light through a Corning single-mode fiber oscillated as a section of it was heated and pulled to form a taper. Starting on the left side of the trace, transmission was constant during the first ∼3 mm of pulling, then began oscillating with decreasing periods as the taper was stretched at a constant rate (100 μm/s) to 12 mm, forming an 8-μm waist. The transmission stabilized when the pulling stopped. Images reprinted with permission of Optics Letters.To demonstrate this variation, the researchers monitored the transmission at a fixed wavelength through the taper as they pulled it (Figure 1). The interference they observed resulted from the phase difference between the two modes, which in turn depended on the light’s wavelength and the length of the taper. Once the taper had cooled and hardened, they could control the intramodal interference in the taper — that is, tune the wavelength transmitted through it — by stretching the taper (Figure 2).Figure 2. The sinusoidal spectral transmission through the tapered fiber resulted from interference to two modes in the taper. Because this interference depended on the taper length, the researchers could tune the transmission by stretching the fiber.Having demonstrated that the taper acted as a tunable filter, the researchers set about using it to tune the wavelength of a fiber laser. Using 3 m of erbium-doped fiber as the gain medium, they built a ring laser that included the taper as an integral part of the ring (Figure 3). Pumping the ring laser with 1475-nm radiation from a diode laser, the researchers tuned it across 20.5 nm, between 1546 and 1566.5 nm, by stretching the taper (Figure 4).Figure 3. In the experimental ring laser, the fiber taper was mounted in V-groove clamps that could stretch or unstretch it. In effect, the intracavity taper was acting as an interferometer and, as with any interferometer, it had a free spectral range.In this case, the free spectral range was slightly less than the 20.5-nm tuning range of the laser, so when the laser was tuned near the short-wavelength end of its range, another mode of the taper-interferometer allowed a second laser wavelength to oscillate at the long-wavelength end of the range. The total power generated by the laser, whether in one wavelength or two, was relatively constant across the tuning range, which corresponded to stretching the taper by about 180 μm.Figure 4. The laser’s power was relatively constant (left axis) as it was tuned across its 20.5-nm range, corresponding to a taper stretch of approximately 180 μm (right axis).The researchers could tune the laser across four or five of its free spectral ranges (i.e., to stretch the taper by nearly a millimeter) before breaking it. But, concerned about material fatigue that might occur when the taper was repeatedly stretched and unstretched, they tuned the laser back and forth over its entire tuning range more than 5000 times.They were pleased that they observed no degradation during this experiment and concluded that the taper was not pushed beyond its elastic region by being tuned over the laser’s 20.5-nm range.Optics Letters, Aug. 15, 2006, pp. 2435-2437.