Femtosecond Pulses Write Grating in Nonphotosensitive Fiber

Breck Hitz

Although fiber Bragg gratings can readily be written in photosensitive fibers with ultraviolet radiation, it is far more difficult to write them in the rare-earth-doped fibers that comprise all fiber lasers. As a result, the gratings that serve as resonator mirrors in most fiber lasers today are created in photosensitive fibers and then spliced onto the rare-earth-doped fiber.

Although this approach is workable, it involves an extra step and introduces a splice loss to the laser.

Researchers at Friedrich Schiller University and at Fraunhofer Institute for Applied Optics and Precision Engineering, both in Jena, Germany, have demonstrated a new and successful technique of writing a fiber Bragg grating directly in a nonphotosensitive erbium-doped fiber. When they optically pumped the fiber, the grating’s ~99 percent reflectivity served as one laser mirror — the Fresnel reflection of the cleaved fiber served as the other — and they observed efficient laser action from the fiber.

They wrote the grating by diffracting femtosecond pulses from a Spectra-Physics Ti:sapphire laser through a phase mask and focusing the obtained diffraction pattern into the core of an aluminosilicate erbium-doped fiber from Liekki Corp. in Lohja, Finland, that was free of germanium or any other photosensitive dopants (Figure 1).

Figure 1. The interference between the phase mask’s positive and negative first-order diffracted beams produced a grating with a 1.075-μm period in the erbium-doped optical fiber, which contained no germanium or other photosensitizing dopants. Images reprinted with permission of Optics Letters.

The fused-silica phase mask had a 2.15-μm period produced by electron-beam lithography and ion etching. It diffracted 35 percent of the incident Ti:sapphire light into the positive and negative first orders.

The interference that resulted from overlapping the two first-order beams resulted in a fiber-grating period of 1.075 μm. This period in the fiber grating corresponded to the second-order reflection at 1.55 μm, which was close to the center of the gain spectrum of the erbium-doped fiber.

Figure 2. The transmission spectrum of the grating produced in the fiber showed a deep (~20 dB) and narrow valley at 1554.50 nm, corresponding to a reflectivity of 98.7 percent.

To confirm the grating’s spectral characteristics, the researchers measured its transmission when illuminated with a broadband source (Figure 2). A careful measurement of the narrow valley at 1554.50 nm corresponded to a reflectivity of 98.7 percent. It was this narrow valley that served as the rear laser mirror when the researchers optically pumped the fiber.

They coupled the 976-nm pump light into the 85-cm-long fiber with its 4-cm-long grating at the far end and separated the 1554-nm output signal with a dichroic mirror (Figure 3). They generated as much as 38 mW of output at 1554 nm from 290 mW of launched pump power, with a slope efficiency of 21.1 percent. There was no roll-off at the maximum power, indicating that higher output could be obtained if more pump power were available. The linewidth of the laser peak was much narrower than the reflection bandwidth of the grating, and the peak laser signal was 60 dB above the background.

Figure 3. Pumping the 85-cm-long fiber with 976-nm light, the researchers observed 38 mW of output from the laser from 290 mW of launched pump power (FBG = fiber Bragg grating).

The researchers believe that higher output also could have been achieved if the grating’s reflection peak matched the peak gain wavelength of the erbium-doped fiber. They concluded that their femtosecond writing technique could be used to produce gratings in other types of fibers, including double-clad and microstructured.

Optics Letters, Aug. 15, 2006, pp. 2390-2392.