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All-Fiber, Q-Switched Laser Generates 1.8 mW

Photonics Spectra
Aug 2003
Breck Hitz

A team of scientists at the University of Science and Technology of China in Hefei has designed and demonstrated an all-fiber, Q-switched laser. High-power, single-frequency oscillation is obtained with a ring resonator, which eliminates the standing waves that burn holes in conventional resonators.

In their experiments, the researchers obtained an average power of 1.8 mW at 1557 nm from the setup when the ring and seed lasers were pumped with 40 mW and 30 mW, respectively. The pulse duration was less than 500 ns, and the Q-switched pulse repetition frequency was 800 Hz. The spectral linewidth of the pulses measured 25 MHz.

Recently, high-power fiber lasers have become commercially successful and promise to be even more so as the technology matures. High-energy Q-switched fiber lasers in particular have applications in nonlinear optics, distributed sensing, lidar and laser rangefinding. Until now, however, they have employed crystal Q-switches, which display significant insertion losses and introduce coupling losses. Single-frequency oscillation also is desirable for some applications, but forcing a single longitudinal mode to oscillate in a conventional resonator causes spatial hole burning and a corresponding reduction in output power.

The layout of the new laser is shown in the accompanying figure. The ring resonator is the loop on the left side of the diagram. The structure on the right is the Q-switch -- a fiber Mach-Zehnder interferometer -- the upper arm of which is wrapped around a piezoelectric transducer. Depending on the phase change induced by the transducer, constructive or destructive interference occurs within the interferometer. If destructive, little light is returned to the ring, and the Q-switch discourages laser oscillation; if constructive, light returns to the ring, and the ring laser can oscillate at high power.

A small linear-resonator fiber laser (pictured in the upper right corner of the ring, between the distributed Bragg reflectors) is pumped with a 980-nm laser diode. The effective length of the resonator is 3 cm, and it oscillates in a single longitudinal mode. The narrow-bandwidth photons from this laser act as seeds to force the ring laser into a single frequency.

all-fiber, Q-switched laser
An all-fiber, Q-switched laser incorporates a ring resonator to yield high-power, single-frequency oscillation while eliminating the standing waves that burn holes in conventional resonators.

The ring laser, which is pumped with a 1480-nm laser diode, oscillates weakly in a single mode, even when the Q-switch is "off." When the Q-switch turns "on," this single mode quickly builds amplitude until it saturates the gain and no other mode can oscillate.

The several centimeters of fiber in the seed laser are doped with erbium at a concentration of 700 parts per million, while the 4-m fibers in the ring laser are doped at 280 parts per million. The optical isolator ensures single-direction oscillation around the ring, and the polarization controller guarantees that the correct polarization is oscillated. Output from the ring is obtained with the 30 percent coupler.


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