Fiber Laser Is Wavelength-Tunable for the S-Band

Breck Hitz

A group at the National Chiao-Tung University in Hsinchu, Taiwan, has capitalized on recent successes with S-band erbium-doped fiber amplifiers to design and demonstrate a wavelength-tunable fiber laser for this telecommunications band. Wavelength- tunable fiber lasers are attractive alternatives to the expensive, distributed-feedback semiconductor lasers currently used as transmitters in wavelength division multiplexing telecom systems. A tunable fiber laser would also be useful in calibrating telecom components and in a variety of optical sensing applications.

Considerable effort has been applied to the development of such lasers, using existing erbium-doped fibers as the gain medium. Most of this work has been in the spectral region above 1530 nm -- within the C- and L-bands -- because the bandwidth limitations of erbium-doped fiber amplifiers prevent their use at shorter wavelengths. Photonics researchers recently overcame this limitation, however, by using erbium-doped silica fiber with a depressed cladding and 980-nm pumping to create erbium-doped fiber amplifiers in the 1450- to 1530-nm S-band.

Figure 1. Researchers in Taiwan have developed a wavelength-tunable fiber laser for the S-band. The device utilizes an S-band erbium-doped fiber amplifier as its gain medium, represented inside the dotted box.

The Taiwanese scientists built their laser in a ring configuration with an S-band erbium-doped fiber amplifier serving as the gain medium (Figure 1). A piezoelectrically tunable fiber Fabry-Perot interferometer narrows the laser's bandwidth and provides the mechanism for tuning the wavelength. The optical isolators within the amplifier suppress amplified spontaneous emission and ensure that the radiation travels in only one direction around the ring resonator. Ten percent of the circulating power is coupled out of the resonator with a 90/10 coupler and is fed to a power meter and optical spectrum analyzer.

Figure 2. The laser's output spectra are shown for different voltages applied to the intracavity, piezoelectric tuning element.

The researchers analyzed the laser's output power spectra for several different voltages applied to the fiber Fabry-Perot's piezoelectric transducer (Figure 2). The laser can be tuned from 1480 to 1522 nm with a side-mode suppression ratio better than 30 dB. When they varied the pump power to obtain a constant, –2.3-dBm power from the laser, they could tune the laser from 1482 to 1517 nm with less than 0.05-dB power variation.