Multiwavelength Laser Utilizes Erbium-Doped Fiber Amplifier

Breck Hitz

Multiwavelength lasers are important for testing wavelength division multiplexing telecommunications systems and for other applications in metrology and sensing.

Two readily available gain media have the broad gain bandwidth necessary for a multiwavelength laser: erbium-doped fiber amplifiers and semiconductor optical amplifiers. The latter have an advantage in that they are inhomogeneously broadened, so the various wavelengths do not compete for the gain (see "Fiber Ring Laser Generates 50 Simultaneous Wavelengths," page 98). In a homogeneously broadened erbium-doped fiber amplifier, wavelengths compete, one or several wavelengths win the competition, and the others are extinguished.

Several solutions to this problem have been proposed and demonstrated. One of the more promising, reported in 2003 by a collaboration of scientists in Singapore, the US and the UK, involves the use of an intracavity phase modulator to prevent the various wavelengths from competing. Recently, a research team at the University of Electro-Communications in Tokyo improved on this technique and generated nine wavelengths within a 3-dB amplitude range at the end of the C-band (Figure 1).

Figure 1. After nullifying the effect of homogeneous broadening with an intracavity phase modulator, the researchers produced nine simultaneous wavelengths, uniformly spaced by 54 GHz, within a 3-dB amplitude band. Total power in all wavelengths was in the multimilliwatt range. Images ©2005 IEEE.

In a conventional homogeneously broadened laser -- that is, one without a phase modulator -- slightly more photons will build up at one wavelength than at another. Because all the atoms in a homogeneous population inversion can be stimulated at any lasing wavelength, the more-numerous photons stimulate more atoms and create more photons, leaving behind fewer excited atoms to provide gain at other wavelengths. After many round trips of the resonator, no gain is left for the weaker wavelengths, and they are extinguished.

An intracavity phase modulator shifts the wavelength of light passing through it, nullifying the advantage of the photons that otherwise would win the competition. During a single pass of the resonator, more photons may build up at one wavelength than at another, but before these photons can get back to the population inversion, they find themselves shifted to a different wavelength. Thus, no wavelength can dominate the stimulated-emission process.

Figure 2. Although fiber loops at both ends of the resonator serve as laser mirrors, the laser itself is linear, not a ring. Waves move in both directions through the gain medium.

The researchers in Japan improved on the earlier experiment by operating the laser in a linear configuration rather than in a ring (Figure 2). In a linear laser, waves move through the gain medium in both directions, creating a standing wave that depletes the population inversion in the spatial locations of its maxima, an effect known as spatial hole burning. But the standing wave cannot access the excited atoms at its nodes, so at least part of the population inversion remains for other wavelengths.

In a unidirectional ring laser, only one wave moves through the population inversion, and no spatial hole burning occurs. Thus, a linear resonator is more hospitable to multiwavelength operation than a ring resonator. The researchers believe that their linear resonator was largely responsible for the spectral output that was wider and flatter than what the collaboration had reported earlier.

The Lyot-Sagnac filter in the laser served the same function as the Fabry-Perot filter in the device described in the news story cited above; that is, to divide the output into a series of distinct wavelengths. Acting as a laser mirror, the Lyot-Sagnac filter reflected only those wavelengths that were resonant in it, thereby forcing the laser to oscillate only at those wavelengths. By careful manipulation of this filter, the researchers also made it serve as a gain equalizer for the lasing wavelengths.

Acting as the other laser mirror, another fiber loop comprised an optical circulator, the output coupler and the phase modulator, which was an all-fiber device driven by a piezoelectric transducer. The researchers found that any of four waveforms -- sine, sawtooth, triangular or square -- could be applied to the modulator with successful results. Moreover, they observed stable multiwavelength outputs with modulator frequencies between 500 Hz and tens of kilohertz, although not at all frequencies in that range.