Effect of femtosecond photo-treatment on inscription of fiber Bragg gratings

Abstract

Pre- and post-treatment of optical fibers is typically used to improve the fiber Bragg grating (FBG) fabrication process. Here, we investigate experimentally the effects of femtosecond photo-treatment on the ability to inscribe and erase FBGs in standard, non-sensitized, silica SMF fibers. We observe "immunity" to inscription after applying a suitable pre-treatment to the fiber and full "erasure" of the FGB after applying a suitable post-treatment. We characterize the required photo-treatment parameters and compare to FBG inscription on an untreated fiber. We believe that pre/post-photo-treatment of the fibers with fs pulses may have practical advantages such as modifying standard grating structures or observing ultrafast transient effects more clearly.

Femtosecond Transient Bragg Gratings

“…We have found that femtosecond photo pre-treatment can immunize a fiber up to a certain illumination intensity [94]. In fact, the reported multiphoton ionization and inscription threshold ($ 10 13 W=cm 2 ) can be raised so that permanent Bragg gratings first appear at a higher pulse energies.…”

Femtosecond inscription of a spectral array of four fiber Bragg gratings at the same spot, using a single uniform phase-mask

“…After the PM is translated 0.635 mm away from the fiber, a second FBG is inscribed exactly at the same spot. The second FBG is inscribed with a higher pulse energy of 330 µJ for 40 s. A higher pulse energy is needed since the refractive index is already modified at this inscription position area, and, thus, the fiber is more immune to refractive index modifications 16,17 . The result of this inscription is a second FBG with a transmission dip of −9 dB at a center Bragg wavelength of ~1553.2 nm as shown in Figure 3.…”

“…Femtosecond inscription of phase-shifted gratings has been demonstrated using this technique 15 . Finally, with pre and post fs treatments, the Bragg wavelength, λB, can be tuned by modifying the effective refractive index, neff [16][17][18] .…”

Ns-duration transient Bragg gratings in silica fibers

“…Before the experiment, we apply an "immunization" procedure, as described in [25]. We scan the fiber 20 μm around the fiber core, in a direction perpendicular to the fiber and the optical axis of the pump beam, with a uniform line pattern (8.5 mm × 6 μm) and 1 mJ pulse energy (∼6 • 10 13 W∕cm 2 ) for 1.2 M pulses.…”

“…(Color online.) Spectrum of the permanent FBGs measured at the end of the experiments (after 150 K pulses) with the fs laser pretreatment (solid red line) versus a typical grating without fs laser pre-treatment (150 K pulses, 180 μJ pulse energy; dashed blue line) taken from [25].…”