New laser offers high-power amplifier pumping
By YVONNE CARTS-POWELL
A new fiber laser design introduced at the Conference on Lasers and Electro-Optics, held recently in Baltimore, MD, provides significantly higher power for pumping erbium-doped fiber amplifiers (edfas), which could simplify amplifier and system design. The laser could also be used for pumping new amplifier designs or for providing other wavelengths from 1100 to 1700 nm.
Daryl Inniss of Lucent Technologies? Bell Laboratories, Murray Hill, NJ, explains that the edfas for 1550-nm signals are pumped by diode lasers operating at 980 or 1480 nm. If one fiber laser provided enough pump power for several amplifiers, then more signals could be amplified with fewer components. Similarly, if one fiber laser pumped an amplifier with more power, fewer amplifiers might be needed, which would also reduce the number of components in a system. At 1472 nm, the experimental singlemode fiber laser system produced 8.5W. In comparison, most commercial edfas are now pumped with about 120 mW.
The system for producing this wavelength consists of a single diode array, a cladding-pumped ytterbium-doped fiber laser, and a cascaded Raman laser (see Fig. 1). The diode array, from Opto Power Corp. in Tucson, AZ, is a 1-cm-wide laser bar emitting at 915 nm, with beam-shaping optics that allow it to couple more than 40W of power into a multimode fiber with a numerical aperture of 0.22 and an outer diameter of 400 microns (see Fig. 2).
The light is coupled from the multimode fiber into the first fiber laser, which is doped with ytterbium in the singlemode section. This fiber has an unusual polymer with an index of refraction lower than silica, which means that pump light doesn?t have to be coupled directly into the core of the fiber. The light launched in the cladding is absorbed by the ytterbium ions in the core. Because the core?s cross-sectional area and numerical aperture are so much smaller than those of the cladding, this pumping system Oincreases the brightness,O explains Inniss, Oby more than a thousand times.O Thus, the core of the fiber is a laser pumped from the fiber?s cladding.
Like other fiber lasers, Bragg gratings in the fiber form the ends of the laser cavity. In this case, the gratings were formed in undoped fiber and fusion-spliced to the ytterbium-doped fiber. Experimental results showed the singlemode laser emitting 16.4W at 1065 nm or at 20.4W at 1101 nm.
The cascaded Raman fiber laser is then used to convert the output to longer wavelengths. In this case, the researchers chose 1472 nm to demonstrate the feasibility of pumping edfas with the system. Such cascaded Raman lasers could efficiently convert the cladding-pumped laser output to wavelengths from 1100 to 1700 nm.
If slightly different fiber amplifiers replaced the current standard edfas, then the cladding-pumped laser could provide even more power to the amplifiers. Inniss explains that amplifiers made from fiber doped with both erbium and ytterbium and pumped at wavelengths from 1050 to 1080 nm could be more efficient than amplifiers doped only with erbium. The cladding-pumped laser emits more than 16W within this range, at 1065 nm.
Although the laser is a research system now, its components should be commercially available from Lucent Technologies and Opto Power within the next several years. The cascaded Raman cavity is already commercially available, says Inniss, and he predicts that the cladding-pumped laser cavity will be available in about 18 months. Inniss?s colleague in this project, Deepak Boggavarapu of Opto Power, expects the semiconductor laser bar, with fiber coupling optics, to be available within the year. q
Yvonne Carts-Powell writes from Belmont, MA.