Fiber lasers generate ultrahigh power

BALTIMORE, MD--It was a battle of the fiber lasers at the post-deadline session at CLEO `97. First up was Martin Muendel from Polaroid Corp. (Cambridge, MA), who discussed a double-clad ytterbium-doped silica fiber laser (post-deadline paper CPD30-1). Using four 915-nm fiber-coupled diode bars, researchers from Polaroid pumped a laser fiber with 55.4 W to produce a pump-limited output of 35.5 W at 1100 nm, or 140 MW/cm² peak in the core. There was no damage or output instability, or other fiber-related power limitations.

According to Muendel, this result was the highest CW output from a fiber laser reported to date. Future work includes higher power generation by using brighter pumps—such as a 40-W laser from Opto Power Corp. (Tucson, AZ)—pulsed operation to produce 30-µJ pulses at 1 MHz repetition rate, and frequency doubling to cover the wavelength range from 525 to 575 nm. "This could open a whole new arena of applications," says Muendel.

Second up was Daryl Inniss from Bell Labs- Lucent Technologies (Murray Hill, NJ), who described a single-mode Yb-doped cladding-pumped fiber laser emitting from 1.065 to 1.472 µm (paper CPD31-2). The Yb-doped fiber cavities were defined by two fiber Bragg gratings. Output power generated from this system was 16.4 W at 1065 nm and 20.4 W at 1101 nm.

Inniss also reported on the construction of a cascaded Raman fiber laser with an output power of 8.5 W at 1472 nm (see photo). The Lucent fiber laser was pumped by a single Opto Power 915-nm, 1-cm-wide high-power semiconductor laser bar in a specially designed beam-shaper configuration for optimal fiber coupling. "This allows us to capture more light, enabling higher powers," says Inniss.

According to Inniss, the key to high power was the double-clad structure. Because of this, the researchers did not have to worry about coupling single-mode light into a single-mode fiber, which is the industry standard.

In a smooth transition, Inniss acknowledged the differences between the Polaroid fiber laser system and the Lucent Technologies system. Inniss says that although the Polaroid system demonstrated a higher output, it requires four diode lasers, while the Lucent system uses only one. Also, the Polaroid laser could not be used to pump a cascaded Raman laser unless it was coupled with a conventional single-mode fiber. In addition, Inniss says, the Lucent system allows greater wavelength flexibility. "In principle, the ytterbium-doped lasers can operate at any wavelength from 1050 to 1140 nm," says Inniss.

Fiber lasers have potential applications in telecommunications, materials processing, printer, and medical fields.