Waveguide Fabricated with Femtosecond Pulses Lases at 1533 nm

Breck Hitz

Researchers in Europe have operated what they believe to be the first waveguide laser fabricated with femtosecond laser pulses. Because the waveguide lasers offer excellent mode-matching with single-mode-communication fibers, have an inherently small footprint and potentially can be manufactured less expensively than semiconductor lasers, they could replace conventional diode laser transmitters in optical communications systems.

Figure 1. The waveguide laser was fabricated in Er-Yb-doped phosphate glass with 520-nm pulses from a frequency-doubled Yb:glass laser.

The scientists, affiliated with the Dipartimento di Fisica del Politecnico di Milano and the Istituto di Fotonica e Nanotecnologie del Consiglio Nazionale delle Ricerche, both in Milan, Italy, Max Planck Institut für Kernphysik in Heidelberg, Germany, and High Q Laser Production GmbH in Hohenems, Austria, fabricated the waveguide with a transverse writing geometry, which enables the creation of waveguides of arbitrary length (Figure 1). Although an amplified Ti:sapphire laser frequently is chosen as the source of femtosecond pulses for waveguide fabrication, the scientists in this case selected a frequen-cy-doubled, mode-locked, cavity-dumped Yb:glass laser oscillator.

There were several reasons for this choice. The 166-kHz repetition frequency of the cavity-dumped laser is much higher than the typical, ~1-kHz rate of a Ti:sapphire laser, enabling individual waveguides to be written in tens of seconds rather than tens of minutes.

Figure 2. A pair of 975-nm diode lasers pumped the waveguide laser from both ends.

Also, the shorter wavelength (520 nm for frequency-doubled Yb:glass versus ~800 nm for Ti:sapphire) makes the writing process more efficient. The nonlinear absorption is a three-photon process at 800 nm, but it's a simpler, two-photon process at 520 nm. That means less pulse energy is required at 520 nm, so a simple oscillator, rather than an oscillator-amplifier, can write the waveguides. This last consideration, together with the compactness and reliability of a diode-pumped laser, will simplify transferring the waveguide-writing process into an industrial environment.

The scientists fabricated waveguides in phosphate glass co-doped with erbium and ytterbium using 300-fs, 133-nJ pulses from the laser. The resulting waveguides were almost perfectly round in cross section because of the tight focusing (NA 1.4) of the pulses into the glass. The experimenters believe that the tiny focused spot, together with isotropic thermal diffusion of the pulse energy in the glass, accounts for the waveguide's circular symmetry.

Figure 3. An output coupler with 32 percent transmission resulted in better power than an output coupler with 10 percent transmission. The inset shows the relative intensity noise spectrum corresponding to 32 percent output coupling.

The 20-mm-long Er-Yb-doped glass waveguide, sandwiched between a pair of fiber Bragg gratings, comprised the tiny laser (Figure 2). Two diode lasers provided up to 420 mW of counterpropagating pump light at 975 nm. With an output coupling of 32 percent, the laser generated up to 1.7 mW, while a lower value of output coupling resulted in lower power (Figure 3). The relatively low output power was the result of ~2.1-dB insertion losses, a value larger than the output coupling losses.

Work is in progress to optimize output coupling using a fiber Bragg grating with lower reflectivity and to reduce waveguide insertion losses.