Francois Ouellette

Dispersion cancellation using linearly chirped Bragg grating filters in optical waveguides

The use of a linearly chirped Bragg grating filter for dispersion cancellation in an optical-fiber link is discussed. Numerical and theoretical calculations are made, which show that, with the proper taper function, the filter can have a high reflectivity and a quasi-constant nonzero dispersion, proportional to the inverse of the chirp. The filter can compress dispersion-broadened pulses by factors of 2-5 or more, if many filters are cascaded. Its compactness and efficiency would make it suitable for on-line implementation.

Implementation and characterization of fiber Bragg gratings linearly chirped by a temperature gradient

We propose a new method for introducing a chirp on a fiber Bragg grating. Uniformly written fiber Bragg gratings are linearly chirped through the use of a controllable temperature gradient device. This device allows the linear chirp conditions to be totally adjustable in a nondestructive repeatable manner. These conditions can be selected with great precision and are very stable, once defined. To characterize the resulting chirped gratings, we have measured their dispersion. Dispersion of the order of 200 ps/nm have been obtained for 1-cm-long fiber Bragg gratings with temperature gradients near 20 degrees C.

All-fiber filter for efficient dispersion compensation

A dispersive filter is proposed that is based on the coupling, with a chirped periodic perturbation, of two copropagating waveguide modes that have different group velocities. Numerical calculations show that a nearly constant dispersion can be obtained over the filter bandwidth. Pulse recompression by a factor of 10 or more is possible. The filter could be made with photoinduced refractive-index gratings in optical filters.

Light-induced erasure of self-organized χ (2) gratings in optical fibers

Self-organized chi((2)) gratings in optical fiber, prepared by irradiating the fiber with 1.06-microm light from a Q-switched mode-locked Nd:YAG laser simultaneously with its second harmonic, can be erased optically. This is accomplished by irradiating the prepared fiber with the 532-nm mode-locked Q-switched light alone, without the 1.06-microm light, or with intense green or blue light from an argon laser. The erasure is reversible, and the fiber can be reprepared as before for second-harmonic conversion. The nonexponential time dependence of the erasure process can be explained by a model involving charge separation during seeding and recombination during erasure.

Pulse shaping and passive mode-locking with a nonlinear Michelson interferometer

Abstract A Michelson interferometer, where one branch contains a Kerr medium with a fast response time, can shorten the duration of laser pulses and act as a passive mode-locking element. The device is analyed and experimental results, obtained with a TEA-CO 2 laser using germanium as the Kerr medium, are shown and discussed.

PERMANENT PHOTOINDUCED BIREFRINGENCE IN A GE-DOPED FIBER

We report the observation of permanent photoinduced birefringence in a Ge‐doped fiber exposed to cw mode locked at 532 nm. This effect is enhanced by increasing the Ge concentration in the core of the fiber, or by submitting it to a thermal hydrogen treatment. The photoinduced birefringence is however very weak, or nonexistent, with irradiation by 514 or 488 nm cw argon laser light.

Dynamic and orientational behavior of UV-induced luminescence bleaching in Ge-doped silica optical fiber

We report two experiments on UV light-induced bleaching of the 400-nm luminescence associated with the 240-nm absorption band in Ge-doped optical fiber. We have measured the dynamics of the bleaching for different UV intensities and show that it is well fitted by a stretched exponential function, which indicates a site-dependent bleaching rate. We have also shown that the bleaching is anisotropic, confirming a model in which the defects having a dipole moment aligned with the UV light polarization are preferentially excited and bleached.

Limits of chirped pulse compression with an unchirped Bragg grating filter

It has been suggested that the large value of group velocity dispersion (GVD) found in a Bragg grating filter written in an optical waveguide could be used for pulse compression. Three expressions are derived relating the compression ratio to the value of GVD and to the bandwidth over which GVD is nearly constant. It is shown that the maximum compression ratio that can be achieved with an unchirped grating is severely limited by the small bandwidth of the high dispersion regions in the transmission spectrum of the filter.

Correlation between ultraviolet‐induced refractive index change and photoluminescence in Ge‐doped fiber

The refractive index change arising following the UV exposure of a Ge‐doped fiber was obtained by measuring the change in the far‐field width of the fiber mode in the infrared. Simultaneous measurements showed a gradual decrease in the visible photoluminescence from the fiber which can be directly correlated to the photoinduced index change.

Remotely switched hollow-core antiresonant reflecting optical waveguide.

We describe a hollow-core antiresonant reflecting optical waveguide whose transmission can be remotely modulated by the motion of a glass substrate placed at a distance from the light-guiding core that ranges from tens to hundreds of micrometers. A maximum on-off ratio of 17 dB was observed. An analytical mathematical formalism developed to fit the data predicts the possibility of achieving on-off ratios of 30 dB.