Researchers at the University of Tokyo have demonstrated a compact, self-contained supercontinuum laser based on the addition of a length of highly nonlinear dispersion-shifted fiber inside the resonator of an erbium-doped fiber ring laser. The laser’s output is continuous-wave rather than the pulsed output more common to supercontinuum sources. Because of its relative simplicity and small size, such a device might be more appropriate than conventional supercontinuum sources for applications such as optical communications and coherent tomography.Figure 1. The highly nonlinear fiber was inside the resonator, where it experienced much higher powers than in the laser’s output beam.The simple fiber-ring resonator included a 5-m length of erbium-doped fiber to provide the gain and a 2-km length of highly nonlinear dispersion-shifted fiber with a zero-dispersion wavelength of 1554 nm (Figure 1). The erbium-doped fiber was pumped with up to 2.2 W of 1480-nm radiation from an IPG Photonics Raman fiber laser. An optical isolator ensured unidirectional oscillation around the ring, and an 80/20 splitter coupled 20 percent of the circulating power out of the resonator.Figure 2. When the nonlinear fiber was removed from the resonator, the intracavity power showed virtually no spectral broadening at any pump power.Previous CW supercontinuum sources have been based on a high-power pump laser whose output beam is injected into a nonlinear fiber or other medium, where the supercontinuum is generated. By placing the nonlinear fiber inside the laser resonator, where the intracavity power is significantly greater than in the output beam, the researchers were able to generate the supercontinuum with a relatively low power laser.They measured the spectral output of their laser at different pump powers. When they removed the nonlinear fiber from the resonator, they saw little change in the spectral characteristics from threshold to the maximum pump power of 2.2 W (Figure 2). When the nonlinear fiber was included in the resonator, however, they saw dramatic spectral broadening (Figure 3). Even at low pump power, they observed that the 1562-nm erbium peak had a much broader bandwidth -- 9 nm -- than when the nonlinear fiber was absent.Figure 3. When the nonlinear fiber was in the laser resonator, the intracavity spectrum broadened significantly as pump power increased. At a pump power of 1.9 W, the spectral width (measured at the –20-dB points) was 250 nm.As the pump power increased to ~0.5 W, additional wavelengths generated by stimulated Raman scattering began to appear; at ~1 W of pump, several Raman lines were clearly visible. At even higher pump powers, a much broader spectrum began to oscillate in the laser.The researchers do not fully understand the mechanism by which the continuum built up, but they suspect that it may result at least partially from modulational instability. At a pump power of 1.9 W, they measured the supercontinuum’s 20-dB bandwidth as larger than 250 nm.They noted that the supercontinuum extended to wavelengths shorter than the erbium laser line, and they concluded that the 1480-nm pump light also played a direct role in building up the supercontinuum. At the same time, at the long-wavelength end of the supercontinuum, their measurements were limited by the spectral responses of the optical signal analyzer and the output coupler.The supercontinuum source exhibited stable operation over a period of several hours. The scientists recorded the output spectrum every 15 minutes during a two-hour period and observed no significant spectral fluctuations.