Fiber Laser Generates Multiple Wavelengths
Multiwavelength fiber lasers are important for testing wavelength division multiplexing telecommunications systems and for other applications in metrology and sensing.
Unfortunately, there is a fundamental obstacle standing in the way of efficient multiwavelength fiber lasers: Fiber-based gain media are homogeneously broadened, so all wavelengths oscillating in the laser compete for the same gain. Eventually, one wavelength pulls ahead of the others, usurps all the gain available and extinguishes the other wavelengths.
During the past year, researchers at laboratories worldwide have experimented with techniques for overcoming this handicap and building efficient, robust multiwavelength fiber lasers. In one case, investigators inserted a phase modulator into the resonator to scramble the wavelengths so that no single wavelength could become dominant. In another, scientists substituted an inhomogeneously broadened semiconductor optical amplifier for the fiber-based gain medium. Now researchers at Tsinghua University in Beijing have demonstrated as many as 14 simultaneous wavelengths from a fiber laser by inserting an inhomogeneous loss mechanism into the resonator.
An inhomogeneous loss mechanism imposes a loss on each wavelength whose magnitude depends only on the power at that wavelength and is independent of the power at other wavelengths. Thus, as a given wavelength gains power, the loss it experiences in the resonator also increases, limiting the ultimate power that can oscillate at that wavelength. No given wavelength can become dominant and saturate the entire gain, so multiple wavelengths can oscillate simultaneously.
Figure 1. The narrow transmission peaks of the Fabry-Perot, together with four-wave mixing in the dispersion-compensating fiber, provided an inhomogeneous loss mechanism that allowed the laser to oscillate at multiple wavelengths. Images ©OSA.
The researchers created the inhomogeneous loss in their laser with the combination of a Fabry-Perot interferometer and a 10-km length of dispersion-compensating fiber (Figure 1). The comb of wavelengths resonant in the Fabry-Perot defined the wavelengths that could oscillate in the resonator. Four-wave mixing in the dispersion-compensated fiber broadened the higher-power wavelengths passing through it, increasing the loss they experienced at the Fabry-Perot. As the investigators increased the laser gain, they observed an increasing number of oscillating wavelengths and an increasing spectral flatness across them. With the maximum gain, 14 different wavelengths oscillated simultaneously (Figure 2a).
Figure 2. At left, with the erbium-doped gain medium in the resonator, as many as 14 wavelengths oscillated simultaneously. At right, when the gain medium was removed and the Raman gain in the dispersion-compensating fiber exceeded laser threshold, as many as 21 wavelengths oscillated simultaneously.
In a variation of the experiment, they removed the erbium-doped gain medium and injected Raman pump light into the dispersion-compensating fiber (Figure 1). The Raman gain in the fiber was sufficient to achieve laser threshold, and in this case, they observed as many as 21 simultaneously oscillating wavelengths (Figure 2b).
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