John Wallace
Wavelength-tunable and switchable lasers are becoming more important to WDM. Because rare-earth-doped fiber lasers have a large gain bandwidth, they make ideal candidates for tunable sources; indeed, tunability has been demonstrated in actively modelocked fiber lasers. But existing techniques for changing wavelength in fiber lasers require changing the modelocked frequency, adding complexity to the system. Recently, researchers at The Chinese University of Hong Kong (Hong Kong, China) developed a compensated dispersion-tuning technique that combined a section of standard single-mode fiber and a section of dispersion-compensating fiber; unfortunately, the technique required long sections of fibers that introduced instabilities in the optical output.
Now, the same researchers have bettered themselves. Their newest switchable fiber laser has a relatively short 60-m cavity and is wavelength-switched electrically at a constant repetition frequency. The trick is the use of two modelocking LiNbO3 amplitude modulators driven at the same 1-GHz frequency. Cascaded fiber Bragg gratings in the laser have differing and incremental cavity round-trip times. Appropriate delay times between the signals sent to the two modulators select the proper wavelength. Peak powers of five switchable wavelengths at or near 1550 nm are 10 mW, with extinction ratios of 40-45 dB. Modelocking can be performed with a delay tolerance of 40 ps, resulting in a potential 25 channels for 1-GHz operation. For details contact C. Shu at [email protected].