We discuss the dramatic development of high-power fiber laser technology in recent years and the prospects of kilowattclass
single-frequency fiber sources. We describe experimental results from an ytterbium-doped fiber-based multihundred-watt single-frequency, single-mode, plane-polarized master-oscillator power amplifier (MOPA) operating at 1060 nm and a similar source with 0.5 kW of output power, albeit with a degraded beam quality (M2 = 1.6) and not linearly polarized. Experiments and simulations aimed at predicting the Brillouin limit of single-frequency system with a
thermally broadened Brillouin gain are presented. These suggest that single-frequency MOPAs with over 1 kW of output power are possible. In addition, the power scalability of a simple single-strand fiber laser to 10 kW is discussed.
We demonstrate a high power erbium-ytterbium co-doped large-core fiber laser with narrow linewidth, an M2 value of 1.7 and a broad tuning range. The fiber was cladding-pumped by a diode stack emitting at 975 nm. The laser had a linewidth around 0.16 nm and was tuned from 1533 nm to 1566 nm by compression-tuning a fiber Bragg grating. Output powers in excess of 30 W were obtained over the entire laser tuning range which was limited by the low gain at wavelengths shorter than 1533 nm and by the grating fabrication wavelength at 1566 nm. The laser slope efficiency was ~30% and the threshold ~3.3 W. Our results underline the capability for efficient, broad-band, high-power operation of large-core Er-Yb doped fibers and demonstrate compatibility with telecom components like standard single-mode fibers and fiber Bragg gratings.
A non-destructive technique for characterising couplers by means of a local perturbation is described. A CO2 laser beam is scanned along the coupler length inducing a local perturbation to the coupler eigenmodes. Asymmetric and symmetric perturbations give respectively, accurate mapping of power-evolution and coupler-waist shape. Using this technique, both the information of the power distribution and coupling profile along the coupler waist are obtained. The method is studied theoretically and verified experimentally by characterising different types of fused fibre-couplers namely: half-cycle, full-cycle and non-uniform half-cycle couplers. Add-drop multiplexers based on the inscription of Bragg gratings in the waist of fibre couplers have attracted some attention lately. The correct mapping of the power evolution along the coupler length gives the exact positions in the coupler waist where Bragg gratings should be written in order to obtain optimum add-drop operation. Recently, it was shown that non-uniform coupler structures based on two highly coupled lateral regions and a weakly coupled central region, can provide an ideal add-drop multiplexing operation. These couplers are characterised using the described coupler characterisation technique and the weakly coupled region, where the grating should be written, is clearly identified. This non-destructive method for characterising fibre couplers can be used as a tool for accessing the uniformity of the fabricated couplers waist or the influence of the tapered transition regions in the coupler performance. Additionally it can be used to optimise the performance of add-drop multiplexers based on the inscription of Bragg gratings in the waist of fibre-couplers.