Femtosecond Pulses Write Fiber Bragg Gratings

Breck Hitz

A group at the Communications Research Centre Canada in Ottawa has presented an alternative means of writing fiber Bragg gratings that uses femtosecond pulses of 800-nm radiation from a Ti:sapphire laser. The advantage of such an approach, according to team leader Stephen J. Mihailov, is that extremely high index modulations in the fiber can be produced with just a few laser pulses. No additional processing is required, so it has the potential to remove several steps from the fabrication of commercial fiber Bragg gratings. Moreover, although many UV-written gratings fade at elevated temperatures, no such temperature sensitivity exists for the femtosecond-written ones.

Bragg gratings usually are written in optical fiber with ultraviolet radiation, either holographically or with a phase mask. In the holographic approach, the beam from a high-power UV laser is split and directed at the fiber from two directions. The interference fringes that form where the two beams overlap write the grating into the fiber.

Figure 1. An alternative technique for writing fiber Bragg gratings uses femtosecond pulses of near-IR radiation from a Ti:sapphire laser. The photograph displays the photoinduced index modulations in the core of the optical fiber.

A zero-order-nulled phase mask is a binary transmissive grating that is designed so that virtually no radiation is transmitted in the zeroth (straight through) order, and most is diffracted into the two first-order beams. The interference fringes write the grating. The alignment and stability of the optics are critical in the holographic approach, but less so when using phase masks.

The 300-µJ, 120-fs pulses were focused through a cylindrical lens into the core of standard, germanium-doped telecom fiber, where the calculated power density was 2.9 x 10¹²W/cm². After a few minutes' exposure to the 1-kHz pulses, the fiber displayed index modulations as great as 1.9 x 10^-3 (Figure 1).

Figure 2. The researchers measured the transmission of wavelengths in the C-band through the grating.

The researchers are uncertain of the exact mechanism behind the writing process. Color-center photosensitivities involving GeO defect centers are responsible for the gratings written with UV radiation, and multiphoton absorption associated with the germanium doping may induce the formation of the femtosecond-written gratings. Another possibility is that a small amount of UV radiation, generated in the phase mask as the third harmonic of the 800-nm pulses, plays a role.