FBGs cool EDFA pump lasers
In Switzerland, researchers have demonstrated that locking the wavelength of G05-type Fabry-Perot lasers using a fiber Bragg grating (FBG) also reduces the necessity for laser cooling. Experiments performed at JDS Uniphase AG in Zurich show that high-power output at the design wavelength can be obtained at high temperatures: efficient electrical conversion produced 150 mW at 75°C and 95 mW at 100°C. By producing stable pump lasers that don't require cooling, engineers hope to reduce the cost of erbium-doped fiber amplifiers (EDFAs).
Fiber Bragg gratings are simply wavelength-dependent mirrors. They consist of periodically spaced planes of differing refractive index (for instance,
Fabry-Perot filters) that are produced holographically. Because of the way they are made through interference, they can be extremely thick and, therefore, are much more selective than their thin-film-based cousins.
Fiber Bragg gratings have been used to stabilize the wavelengths of multiple lasers through reflection back into the laser cavity. Only the design wavelength returns, which increases the number of design photons available to stimulate emission. This increases both the proportion of the beam devoted to that wavelength and, in turn, the number of photons reflected back. The feedback loop created eventually locks the output of the laser to that wavelength.
Wavelength stabilization is important to power stabilization because a changing temperature means a shifting output-spectrum. If a laser peaks around the design wavelength at room temperature, then it doesn't when it is hotter or colder. Furthermore, if the laser is being used to pump an EDFA sensitive to 980 nm, then the amplification produced is modulated by the temperature and thus the signal is degraded. As a result, such devices have had to be packaged with cooling systems.
With the FBGs, the wavelength locking works over a large range of temperatures.1 Though the overall power of the laser is still affected by any changes, the wavelength is not. Because of this, the variations in power at the desired wavelength are greatly reduced: in the JDS Uniphase system, they were smaller than 0.05 dB when the laser operated above 10 mW (see figure). For more information, contact Tomas Pliska at firstname.lastname@example.org.
- T. Pliska et al., Elect. Lett. 37, 33 (Jan. 4, 2001).
Sunny Bains is a scientist and journalist based in London, England.