Wavelength Converter Features Three-Wave Mixing Inside a Pump Laser
A research group at McMaster University in Hamilton, Ontario, Canada, has proposed and demonstrated an intracavity approach to wavelength conversion for optical telecommunications networks that eliminates the need for high-power, tunable pump lasers. The ability to convert radiation from one wavelength division multiplexing channel to another would greatly enhance the flexibility of networks.
To this end, some researchers have investigated gain saturation in a semiconductor optical amplifier, while others have looked at various techniques involving nonlinear optics. Among the nonlinear approaches, three-wave mixing -- in which two pump photons mix with a single signal photon to generate the converted wavelength, wc = 2wp –ws-- is one of the more promising, but a drawback to this method is that at least 1 W of tunable pump power has been required to obtain a useful conversion efficiency.
Placing a nonlinear crystal inside the pump laser reduces the power required from the latter in a new approach for wavelength conversion by three-wave mixing.
The new approach overcomes this problem (see figure). The circle is a ring laser whose oscillation is restricted to the wavelength reflected by the fiber Bragg grating. The erbium-doped fiber amplifier is the gain medium for the laser, and when the circulating power builds up to a value that saturates the erbium-doped fiber amplifier's gain, the ring laser is in equilibrium. An MgO-doped, quasiphase-matched LiNbO3 waveguide at the top of the ring is the nonlinear element in which the three-wave mixing takes place. The signal wavelength from the tunable laser is coupled into the ring through the 20:80 coupler at the bottom of the ring, and the converted wavelength is coupled out through a fiber Bragg grating. Both the circulator and the coupler operate across the C-band.
This arrangement has several important advantages over mixing the outputs of separate signal and pump lasers in an external crystal. First, because the three-wave mixing takes place inside the pump laser, a much less powerful -- and less expensive -- pump laser is required. Second, most of the pump radiation is retained inside the ring laser by the fiber Bragg grating, so it is much simpler -- and, again, less expensive -- to filter the pump radiation from the wavelength-converted output. Third, the wavelength of the pump laser can be tuned by tuning the fiber Bragg grating, so no additional wavelength-tuning element is required. Finally, if polarization-maintaining fibers are used throughout the ring laser, its polarization can be automatically matched to that required in the LiNbO3 waveguide. (In the team's laboratory demonstration of the technique, this last advantage was not realized, and a separate polarization controller was required.)
In practice, when the signal wavelength injected into the ring laser was 1535 nm and the ring laser was running at 1543 nm, the setup generated a converted signal at 1551 nm, with a conversion efficiency of 230 dB. The group speculates that by reducing the intracavity losses of the ring laser or by increasing the gain of the erbium-doped fiber amplifier, the conversion efficiency could be increased to –14 dB or higher.
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