Tunable ring laser locks precisely on ITU frequencies

Yvonne Carts-Powell

Robust DWDM transmitters need backup sources in case one of the primary lasers fail, but incorporating an entire bank of backup lasers for systems with many channels is prohibitively expensive. As an alternative, Todd Haber and others at Micron Optics (Atlanta, GA) suggest a single-frequency erbium-doped fiber-ring laser that can be tuned precisely to the appropriate channel in the ITU frequency grid. The laser may also prove useful in future dynamic optical networks.1

The researchers demonstrated a discretely tunable single-frequency erbium-doped fiber-ring laser that frequency locked to the 50-GHz ITU frequency grid with an accuracy of ±0.3 GHz and stability of ±0.05 GHz over a 50-nm tuning range. The lasing linewidth was less than 126 kHz. The output power was 7 mW and extinction ratio for nonlasing wavelengths was 45 dB, making this laser useful for DWDM.

If narrowband low-loss tuning over the 50-nm range had to be continuous, it would require an extraordinary (and unavailable) filter. However, for tuning in discrete steps, the researchers can use an interferometer and filter based on fiber Fabry-Perot cavities. The laser system incorporates a fiber Fabry-Perot tunable filter and a multiwavelength reference based on a thermally controlled Fabry-Perot interferometer.

The system is a backward-propagating ring that includes a 980-nm pump source, 10 m of erbium-doped fiber of 4.3 dB/m absorption at 980 nm, a 1550/980 WDM coupler, a 50/50 output coupler, three isolators, and the fiber Fabry-Perot tunable filter and the interferometer mentioned above (see figure).

The tunable filter is an all-fiber plane Fabry-Perot interferometer with a single-mode optical-fiber waveguide segment and an air gap between two highly reflective mirrors deposited directly onto optical fibers. The filter employed in this ring cavity has a tuning range of 6250 GHz and a bandwidth of 31 GHz. The interferometer can be polished to precision lengths in order to generate multiple frequency reference lines at a given spacing (such as ITU grid spacing of 200, 100, 50, or 25 GHz). Once the interferometer order is selected to set the desired free spectral range, it can be temperature-tuned to place the peaks on all the frequencies defined by a particular ITU grid.

In practice, the laser demonstrated mode-hop-free operation for periods as long as 21 minutes. The researchers are working to improve mode stability.

REFERENCES

1. T. Haber et al., IEEE Phot. Tech. Lett. 12, 11 (Nov. 2000).

Single-frequency fiber-ring laser incorporates a fiber Fabry-Perot tunable filter and a fiber Fabry-Perot interferometer that allow the laser to tune to various preset ITU frequencies.