Wavelength-tunable, single-mode lasers can provide great flexibility to optical networks while reducing spare-parts inventories. Laboratories worldwide have investigated dozens of approaches — many successful — to producing such lasers. Despite the successes, however, scientists still are seeking a laser that not only can serve as a tunable optical transmitter, but also is cheap to manufacture and has reliable long-term operation.Recently, researchers at the Information and Communications University in Daejeon and at Neowave Inc. in Anyang, both in South Korea, have demonstrated how a relatively simple modification of an off-the-shelf commercial laser might turn it into a low-cost, wavelength-tunable transmitter for a wavelength-division-multiplexed network.They began with a Mitsubishi ML925BF diode laser in a low-cost transistor outlook-can (TO-can) package (Figure 1). It consists of an InGaAsP laser diode coaxially packaged with an optical fiber with an aspheric lens between the two to focus the diode’s light into the fiber.Figure 1. The fiber facet of this commercial fiber-coupled diode laser is normally cut at an oblique angle to defeat an external cavity. But the researchers deliberately cleaved the fiber orthogonally to the incoming rays so that there were two cavities: the normal laser cavity between the diode’s facets, and an external cavity based on the Fresnel reflection from the face of the fiber. Because the Fresnel reflection from the cleaved face of the fiber can reflect light into the diode and destabilize its modes, the fiber is usually cleaved at an angle so that backreflected light cannot get to the diode. However, the researchers modified the fiber so that the cleaved face was orthogonal to the incident diode light and reflected it into the diode.Thus, they created an external cavity in addition to the internal laser cavity formed by the two cleaved facets of the diode. Each cavity had its own set of longitudinal modes, a comblike set of frequencies with a fixed separation between adjacent teeth of the comb. But because the separations were different for the two cavities, their modes overlapped only occasionally. It was at these occasional frequencies that the laser produced its optimal output (Figure 2).Figure 2. When the fiber facet in Figure 1 was cleaved at an oblique angle, there was little discrimination among the diode laser’s longitudinal modes (top trace). However, when the fiber was cleaved to form an external cavity, and when a single mode of the diode-laser resonator matched the frequency of an external-cavity mode, it generated ~31 dB more power than competing modes (bottom trace).When the researchers aligned the fiber facet orthogonal to the incoming light, they actually created two external cavities, but they believe that the one associated with the rear diode facet was so lossy that it had negligible impact.Similar external-cavity designs have been explored by other researchers in other laboratories but have depended on piezoelectric transducers and/or special packaging to tune the wavelength, with accompanying cost increases. The result from the Korean researchers is unique in that they tuned the laser wavelength simply by adjusting the temperature of the thermoelectric cooler beneath the laser.As they changed the laser’s temperature, the refractive index of the 300-μm-long InGaAsP resonator changed, altering the resonant frequencies of its longitudinal modes. In essence, the researchers were sliding the coarse-tooth comb — that is, the modes of the internal cavity — along the stationary fine-tooth comb: the modes of the external cavity. As they slid it, a single tooth of the coarse-tooth comb aligned with first one and then another tooth of the fine-tooth comb (Figure 3). Eventually, the single tooth of the coarse comb reached the end of the laser’s gain bandwidth, and after a little more sliding, the next tooth of the coarse comb aligned with a tooth of the fine comb. In other words, the free spectral range of the internal cavity was slightly greater than the gain bandwidth.Figure 3. The researchers temperature-tuned a longitudinal mode of the InGaAsP cavity so that it aligned first with one and then another longitudinal mode of the external cavity. When this alignment occurred, the laser oscillated in a stable single mode, as indicated by the points on this plot. However, between the discrete plotted points, when no single frequency was resonant in both cavities, the laser oscillated in multiple modes, and the side-mode separation ratio approached unity. That between-resonance behavior is not indicated in this plot.Recognizing that modulating the laser by modulating the drive current could destabilize single-mode oscillation (because the semiconductor’s refractive index is a function of the drive current), the researchers experimented with modulating the laser. They found that stable operation was possible, and that the achievable modulation depth increased as the length of the external cavity decreased. Optics Letters, Sept. 1, 2006, pp. 2586-2588.