Diode-pumped Nd3+- and Yb3+-doped multimode fibre lasers with external cavities allow sensitive measurements of intracavity absorption in the spectral ranges of 0.9–0.945 μm and 1.03–1.15 μm. The sensitivity is characterized by the effective absorption path length and is derived from the spectral dynamics of the lasers with intracavity atmospheric absorption, and calibrating the emission spectra with HITRAN database. The highest sensitivity is achieved with a cw Nd3+-doped fibre laser; it corresponds to a 140 km effective path length of absorption. Since the achieved sensitivity decreases only weakly upon the pump rate, it seems to be restricted by the joint action of Rayleigh scattering and nonlinear mode coupling. Both multimode fibre lasers may be made the base of sensitive portable multicomponent gas detectors.

The optical nonlinearity, saturation of the lower laser level, and spontaneous emission modify the relaxation oscillations in a Nd3+-doped fibre laser. A novel four-level rate-equation model correctly describes the modified laser dynamics and allows the determination of the second-order susceptibility of the fibre, lifetimes of upper and lower laser levels, cavity loss, and the number of oscillating modes by monitoring relaxation oscillations. Measurements on five different Nd3+-doped fibres are reported.

In situ detection of the absorption of atmospheric trace gases in the near infrared is feasible by using a compact intracavity diode laser spectrometer. Provided that an effective absorber length of more than 1 km is achieved, the measurement of atmospheric peroxy radicals such as HO2 with a detection limit of 2.4 × 107 molecules per cm3 (1 ppt) is possible. During our prototype development, stable trace gases such as H2O and CO2 will be measured. This paper describes the design and development of our instrument, and first experimental results.

Novel compact and efficient larcr sources in the visible have important applications in spectroscopy, data storage, and colour displays. A promising and versatile example is the PrJ4/Yb3'doped ZBLAN fibre Ibaer. The excitation scheme of this up-convcrsion laser i a ihown in Fig. 1. The principal emiuion bands are around 717, 635, 604, 535, 520, and 490nm. So far, this fibre haa been pumped by a Ti:Mpphire luer that limits its venatility. We have demonstrated efficient pumping by d iodduer light U required for ovcrall miniaturization. The fibre of 1 . 7 ~ core diameter is doped by 0.1% PI and 1% Yb, reaultini in a low threshold. One diode h e r at 850nm sufices as the pump source. Visible luer cmiuion h u been observed around 717, 635, 604, and 520nm with 10% p e d efficiency 10 far. Fig. 2 show. the output power V8 the pump power abaorbed in the fibre. The smallest threshold, 3mW, WM obmrvcd with 635nm output; the largest tuning rangc is IOnm, and the obrrvcd cmiarion bandwidth with no frequencywlcctive clemenb is Inm.

A broadband Nd(3+)-doped fiber laser has been operated in an external cavity by pumping with an Ar(+)-ion laser. The emission spectrum of the fiber laser shows high sensitivity to intracavity absorption. The equivalent absorber path length is 15 km; with a loss-reduced setup that includes pumping by a diode laser this path length is 150 km. The spectrum of atmospheric absorption in the range from 1.065 to 1.145 microm has been recorded by stepwise tuning of the 0.3-nm-wide laser emission band by an intracavity prism.