Stable, single-mode, dual-wavelength lasers have important applications in fiber sensors, in optical instrumentation and in communications based on wavelength division multiplexing. Recently, Shilong Pan and his colleagues at Tsinghua University in Beijing demonstrated a dual-wavelength laser in which each wavelength oscillates in only a single longitudinal mode. Moreover, the pair of wavelengths can be step-tuned between 1533 and 1565 nm.The laser was configured as a fiber optic ring, with the gain provided by both an erbium-doped fiber amplifier and a semiconductor optical amplifier (Figure 1). The erbium-doped fiber amplifier’s gain is homogeneous, which means its entire population inversion can be drained away by a single wavelength. But the inhomogeneously broadened semiconductor optical amplifier is more socialis-tic in nature: Any given wavelength can access only a portion of its population inversion.Figure 1. The fiber ring resonator included gain from both an EDFA and an SOA. Low-noise, dual-wavelength lasing resulted from balancing the two gain media. OC = optical coupler; FFPF = fiber Fabry-Perot filter; PC = polarization controller; SOA = semiconductor optical amplifier; OBPF = optical bandpass filter; EDFA = erbium-doped fiber amplifier. Images reprinted with permission of Optics Letters.Dual-wavelength operation would be extremely difficult with only the erbium-doped fiber amplifier because one wavelength would usurp all the population inversion. On the other hand, the semiconductor optical amplifier alone would provide insufficient gain for a good signal-to-noise ratio. Hence, the scientists had to carefully balance the two gain media to obtain the stable, low-noise operation they observed.The fiber Fabry-Perot and the bandpass filter, working together, provided the laser’s wavelength selectivity. The Fabry-Perot’s free-spectral range was 40 GHz, and the bandpass filter’s full width at half maximum was 100 GHz, wide enough to allow two Fabry-Perot wavelengths through. So two wavelengths separated by 40 GHz oscillated in the ring resonator. And by adjusting the bandpass filter, the scientists could shift which pair of Fabry-Perot modes oscillated, tuning the laser in 40-GHz steps across a spectral range of more than 20 nm (Figure 2).Figure 2. By adjusting the bandpass filter’s pass wavelength, the scientists forced the laser to hop from one pair of wavelengths to another.The mode discrimination to force each wavelength to oscillate in only a single longitudinal mode was provided by the three fiber rings in Figure 1, whose free spectral ranges were 3.1, 34 and 238 MHz, respectively, from the largest to the smallest rings. When both the small rings were disconnected from the large master ring, the beat notes among many oscillating longitudinal modes of the master ring were very evident (Figure 3a). Reconnecting one of the small rings quieted these beat notes considerably (Figure 3b), and the third ring extinguished them completely (Figure 3c).Figure 3. When neither of the small rings in Figure 1 is connected to the master ring, beat notes among the master ring’s longitudinal modes are very noisy (a). Adding one ring quiets them somewhat (b), and connecting the second ring extinguishes them (c).Optics Letters, April 15, 2008, pp. 764-766.