Researchers Demonstrate Mode-Locked Fiber Laser

Breck Hitz

A fiber laser capable of producing a train of picosecond, wavelength-tunable pulses at gigahertz frequencies would be a useful source for wavelength division multiplexing telecommunications and could find application in fiber-sensor systems and optical phased-array radars. An approach to realizing such a laser has been proposed and demonstrated at the Chinese University in Hong Kong.

The laser's gain medium is not the fiber itself, but the semiconductor optical amplifier in the fiber ring (see figure). Mode-locking is accomplished by modulating the gain of the amplifier at 2.5 GHz, the 119th harmonic of the fiber ring. The gain is modulated when it is saturated by the signal from the amplified distributed feedback laser. The laser signal is coupled into the ring with a circulator and dumped at the isolator after a single pass through the semiconductor optical amplifier.

Scientists at the Chinese University have demonstrated a mode-locked, fiber-ring laser that incorporates a semiconductor optical amplifier as a gain medium and a mode locker. Adjusting the optical delay line tunes the lasers to a wavelength reflected by the fiber Bragg gratings.

The radiation circulates clockwise in the fiber ring (the boxlike structure in the bottom half of the diagram). A string of nested fiber Bragg gratings extends from the lower circulator into the center of the ring. Each reflects a different wavelength between 1549 and 1555 nm. The laser can be tuned to any of these wavelengths by adjusting the optical delay line incorporated into the Regardless of the wavelength, each pulse has to make a single pass around the ring in 47.6 ns (119 times the reciprocal of 2.5 GHz) to remain in phase with the modulated gain of the semiconductor optical amplifier. Because different wavelengths must travel different distances into the string of fiber Bragg gratings, a given wavelength will oscillate in the ring only when the delay line is adjusted so that its transit time is 47.6 ns. The nonresonant wavelengths are suppressed by 30 dB The researchers have observed stable mode-locking of the laser, with no mode hops or other instabilities, for as long as two hours. The duration of the mode-locked pulses, which exit through the 10 percent coupler at the bottom of the figure, is approximately 43 ps.

An optical spectrum analyzer revealed that the 3-dB bandwidth of the pulses was approximately 0.22 nm, so the bandwidth-pulse-width product is 1.17. This large value indicates a severely chirped pulse, which probably results from time variation of the charge density in the semiconductor optical amplifier.