M. J. F. Digonnet

Nd:MgO:LiNbO3 spectroscopy and laser devices

Laser oscillation in Nd:MgO:LiNbO3 has been demonstrated. Thresholds as low as 3.6 mW and slope efficiencies up to 39% were achieved in a resonantly pumped miniature device. The electro-optical and nonlinear-optical properties of the host were also used to make active internal Q-switched and self-frequency-doubled lasers. Photorefractive damage is shown to be greatly suppressed compared with that for non-MgO-doped material. Absorption spectra, fluorescence spectra, and lifetime measurements are also reported.

Tunable mechanically induced long-period fiber gratings

We report the experimental characterization of mechanically induced long-period fiber gratings (LPFGs) made by pressing a plate with periodic grooves against a short length of fiber. This filter, which is simple and inexpensive, exhibits transmission spectra and temperature stability similar to those of photoinduced LPFGs. It also offers the unique advantages of being tunable, erasable, and reconfigurable. Its polarization dependence also makes it useful as a polarizer, although it can also be eliminated with simple mechanical designs.

Thermal effects in doped fibers

A theoretical analysis of the pump-induced temperature change and associated thermal phase shift occurring in a pumped doped fiber is presented. Although the primary devices targeted are all-optical switches based on doped fibers, where such effects can be detrimental, this analysis is also applicable to lasers, amplifiers, and other doped fiber devices. The effects of a single pump pulse, multiple pulses and continuous wave (CW) pumping are investigated, both in the dynamic and steady-state regimes. Simple expressions are derived for the thermal relaxation time constant of a fiber, and for its steady-state temperature rise and thermal phase shift under CW pumping. This study predicts that in all-optical fiber switches utilizing a reasonably good dopant the thermal effect due to a single short pulse is negligible in all interferometers, while the steady-state effect can be sizable in a standard fiber Mach-Zehnder but is negligible in a twin-core fiber, a two-mode fiber, and a specially designed Mach-Zehnder interferometer.

EFFECTS OF CONCENTRATION AND CLUSTERS IN ERBIUM-DOPED FIBER LASERS

We report a low-threshold, high-conversion-efficiency erbium-doped fiber laser in a 100-parts-in-106 fiber pumped at 1.48 μm. The threshold and efficiency are found to deteriorate noticeably as the erbium concentration is increased. We propose that this is due mostly to rapid cross relaxation between ion pairs or clusters, and through modeling we show that the cluster content increases with concentration.

Nd:MgO:LiNbO 3 continuous-wave laser pumped by a laser diode

Diode-pumped laser oscillation was achieved in Nd:MgO:LiNbO(3). The absorbed pump power thresholds were as low as 1.9 mW for the high-gain or pi polarization and 8 mW for the low-gain polarization. A cw output power of 2 mW was obtained for the pi polarization at lambda = 1.085 microm for 9 mW of absorbed pump power. A slope efficiency of 37% was achieved. The diode-pumped Nd:MgO:LiNbO(3) lasers operated for extended periods of time without exhibiting any reduction in output power.

Analysis of nonlinear optical switching in an erbium-doped fiber

A mathematical model of the strong, resonantly enhanced nonlinear phase shift recently reported in Er-doped fibers which relates the phase shift and signal loss to the fiber parameters and the pump and signal wavelengths, is presented. Predictions are in fair agreement with the phase shift and loss measured in an experimental Er-doped fiber switch based on this effect. A strong, nearly wavelength independent contribution to the nonlinear phase shift is observed in the switch. The model suggests that this effect is due to the same nonlinear effect arising from one or more vacuum ultraviolet (VUV) transitions in Er/sup 3+/. >

20 W single-mode Yb 3+ -doped phosphate fiber laser

We report the demonstration of the first, to our knowledge, cladding-pumped continuous-wave Yb(3+)-doped phosphate-glass fiber laser. Phosphate hosts are of interest because they can be much more heavily doped than silica, and because of the possibility that they may have a higher photodarkening threshold. In an 84.6 cm double-clad fiber doped with 12 wt. % of Yb(2)O(3) and laser-diode pumped at 940 nm, nearly 20 W of single-mode 1.07 microm output power was generated with 60.2 W of absorbed pump power. The measured dependence of the output power on pump power is in excellent agreement with simulations.

Electro-optic phase modulation in a silica channel waveguide.

We report what is to our knowledge the first demonstration of electro-optic phase modulation in a fused-silica channel waveguide. The no linearity is induced through elevated temperature poling of an electron-beam-irradiated waveguide. A phase shift of 32 mrad was measured at λ = 633 nm for a device interaction length of 4.8 mm and an applied electric field of 7.3 V/μm.

Absolute measurement of the second-order nonlinearity profile in poled silica.

Measurement of the thin nonlinearity profile of poled silica by the Maker fringe technique has been impossible because of total internal reflection (TIR) at the back surface of the sample. We demonstrate that this limitation can be removed by placing a prism against each face of the sample, thus avoiding TIR. This novel technique allows, for the first time to our knowledge, the nonlinearity profile of a thin film to be inferred by the Maker fringe technique. Applied to a silica sample thermally poled under standard conditions (275 degrees C and 5.3 kV for 30 min), it suggests a Gaussian profile with a 1/e width of 8 mum and a maximum d(33) of 0.34 pm/V.

Characterization of proton-exchanged waveguides in MgO:LiNbO(3).

We report the fabrication and characterization of proton-exchanged waveguides in MgO-doped LiNbO(3), a high-optical-damage-threshold material. Results indicate waveguide characteristics similar to those of waveguides fabricated in undoped LiNbO(3) except for slower diffusion rates and freedom from etching of the y face when pure benzoic acid is used as a proton source. An optical-damage threshold of 70 kW/cm(2) was measured at 0.5145 microm in a MgO:LiNbO(3) waveguide, corresponding to a factor-of-2 improvement over undoped LiNbO(3).