J.L. Archambault

Er(3+):Yb(3+)-codoped fiber distributed-feedback laser.

We report what is to our knowledge the first fiber distributed-feedback laser using a single Bragg grating at 1.5 microm written directly into a 2-cm-long Er(3+)-doped fiber codoped with Yb(3+). We obtained robust single-frequency operation by either using one end reflector or locally heating the center of the grating to create the necessary phase shift.

Simple multiplexing scheme for a fiber-optic grating sensor network

A new approach for the interrogation of a large number of fiber-optic grating sensors is proposed and demonstrated for a small number of sensors in which signal recovery is achieved by matching a receiving grating to its corresponding sensor. This technique is demonstrated for both quasi-static and periodic measurands, and the resolution achieved for a single sensor–receiving grating pair for quasi-static strain is 4.12 μ∊.

Grating-frustrated coupler: a novel channel-dropping filter in single-mode optical fiber.

A low-insertion-loss, all-single-mode fiber, wavelength-division-multiplexing filter comprising a four-port polished coupler and a strong fiber grating is demonstrated. The device operated in a novel way by a process we call grating-frustrated directional coupling. A fiber grating, present in only one half of the coupler, frustrates the transference of power to the other half within a narrow wavelength range. The performance of a prototype device - a 1535-nm channel-dropping filter with 0.7-nm bandwidth, 70% peak transmission, and 13-dB isolation-shows great promise for wavelength-division-multiplexing and line-filtering applications.

Photoinduced absorption change in germanosilicate preforms: evidence for the color-center model of photosensitivity

A new technique has enabled us to resolve the strong UV absorption spectrum in UV-exposed germanosilicate preforms. Our results show, for the first time to our knowledge, that the large photoinduced absorption changes between 165 and 300 nm (~700 dB/mm at 195 nm has been observed) can account for, ~3 ×10(-4) of index change at 1.5 µm. The thermal-annealing dynamics of the photoinduced absorption changes also resemble those of fiber gratings, evidence that the color-center model of photosensitivity plays an important role in the photoinduced index changes in UV-written fiber gratings. We also find that the photoinduced absorption changes depend linearly on germania (i.e., germanium dioxide) concentrations in the preforms, and their exposure-time dependence can be fitted with a stretched-exponential function.

Efficient diode-pumped single-frequency erbium:ytterbium fiber laser

We report a 7.6-mW single-frequency fiber laser operating at 1545 nm, using for the first time an Er/sup 3+/:Yb/sup 3+/ doped fiber and a fiber grating output coupler. The laser did not exhibit self-pulsation, which is a typical problem in short three-level fiber lasers, and had a relative intensity noise (RIN) level below -145.5 dB/Hz at frequencies above 10 MHz. The linewidth of the laser was limited by the relaxation oscillation sidebands in the optical spectrum and was typically less than 1 MHz.<<ETX>>

Bragg gratings in Ce3+-doped fibers written by a single excimer pulse

Gratings with as high as 2.4% reflection efficiency written by a 20-ns single excimer pulse at 248 nm have been demonstrated for the first time to our knowledge in Ce3+-doped fibers. The grating strength is shown to increase with Ce3+ concentration under the same writing conditions. Investigation of the grating decay at elevated temperatures has revealed several different time constants, showing that several different types of mechanism may be involved. Part of the grating was found to be stable for >10 h at 600°C.

Simultaneous interrogation of interferometric and Bragg grating sensors

We propose a new method for the simultaneous interrogation of conventional two-beam interferometers and Bragg grating sensors. The technique employs an unbalanced Mach-Zehnder interferometer illuminated by a single low coherence source, which acts as a wavelength-tunable source for the grating and as a path-matched filter for the Fizeau interferometer, thus providing a high phase resolution output for each sensor. The grating sensor demonstrates a dynamic strain resolution of ~0.05 µ.epsilon/√Hz at 20 Hz, while the interferometric phase resolution is better than 1 mrad/√Hz at 20 Hz, corresponding to an rms mirror displacement of 0.08 nm.

Simple multiplexing scheme for fiber optic grating sensor networks

A novel approach for the interrogation of multiplexed fiber optic Bragg grating sensors is described. Signal recovery is achieved by matching a receiving grating to a corresponding sensor grating. As a demonstration, the technique is applied to strain sensing.

Experimental investigation of picosecond pulse reflection from fiber gratings

The dispersion of picosecond pulses on reflection from efficient photorefractive fiber gratings is explored experimentally. Unlike simple measurements of reflectivity, this approach allows both the amplitude and the phase of the grating response to be probed as a function of frequency.

Simultaneous measurement of strain and temperature using fibre grating sensors

Spectral behavior of strain and thermal sensitivities of two superimposed fiber gratings of two different Bragg wavelengths has been studied. This involves monitoring the Bragg wavelength as a function of strain and temperature on the gratings. The results show that the ratio of sensitivity at two different Bragg wavelengths (850 nm and 1300 nm) is different with strain and temperature, which can be used for simultaneous measurement of these parameters in fiber grating sensors.