Anping Liu

The absorption characteristics of circular, offset, and rectangular double-clad fibers

Abstract The absorption characteristics of circular, offset, and rectangular double-clad fibers are investigated with phenomenological, simple, and comprehensible 2-D analysis models. For offset and reactangular fibers, almost all propagation rays are absorbable, so it is possible to achieve very high absorption efficiency. However, for circular fibers, it is not, there are few absorbable rays and absorption is lower. The dependence of absorption on fiber length is calculated with 3-D ray tracing simulations. The calculation indicates that, for circular fibers, the bend radius influences the absorption efficiency markedly. But for rectangular and offset fibers, the bend effect is negligible. The effective absorption coefficients of offset and rectangular fibers are proportional to doped concentration and area ratio of core to first cladding. The influences of offset distance for offset fibers, as well as cross section shape for rectangular ones on absorption is also discussed.

Numerical analysis and experimental results of output performance for Nd-doped double-clad fiber lasers

Numerical analysis is investigated for the high-power double-clad fiber lasers and experimental results using different microscope objectives for focusing into a Nd-doped rectangular double-clad fiber also performed. The numerical analysis includes dependence of output power on output mirror reflectivity, absorbed pump power, loss, and fiber length and pump power distribution for the cases of one-end and two-end pumps with 20 dBrkm loss. Calculated conversion efficiencies are 76.36%, 69.73%, and 63.84% for lossless, two-end pump, and one-end pump fiber lasers, respectively. Slope efficiencies from absorbed pump powerroutput powers measured using microscope objectives are 16.8%r182 mW, 53.8%r351 mW, 24.9%r1240 mW, and 13.9%r649 mW for magnifications of 5= ,1 0= ,2 0= , and 40 = , respectively. q 2000 Elsevier Science B.V. All rights reserved.

Control of the thermal lensing effect with different pump light distributions

We built a 32-laser-diode-formed virtual point source pumping system and achieved different pump light distributions from central intense to central uniform and central depressed. Continuous wave TEM(00) operations of a Nd:YAG laser were performed under these pump light distributions and their thermal lensing effects were estimated. Results show that the operation under central depressed pump light distribution has the lowest thermal lensing effect and can provide the highest output power, which agrees with the results derived from the theoretical calculation with the heat conduction equation.

Efficient operation of an intracavity-doubled nd:yvo 4 /ktp laser end pumped by a high-brightness laser diode

Using a 1-W high-brightness laser diode as the pump source, we investigated the output characteristics of an intracavity frequency-doubled Nd:YVO(4)/KTP laser. Highly efficient operation was realized based on an optimized cavity design. The second-harmonic output power at 532 nm was measured to be 286.5 mW at an incident pump power of 881.4 mW, corresponding to an optical-to-optical efficiency of 32.5%.

Simultaneous multiple-wavelength cw lasing in laser-diode-pumped composite rods of Nd:YAG and Nd:YLF

Simultaneous multiple-wavelength cw laser operations were achieved in two types of composite laser rod composed of Nd(3+):YAG and Nd(3+):YLF crystals, which were laser diode (LD) pumped within a single virtual-point-source cavity. As much as 30 W of total output at 1064- and 1047-nm wavelengths was obtained under 150-W LD input power, with approximately 25% from the 1047-nm wavelength. Different bonding methods were compared, showing that the use of an optical adhesive is effective and presents no deterioration at low and middle power levels. Simultaneous multiple-wavelength operation at 1064- and 1053-nm wavelengths was also studied.

Propagation losses of pump light in rectangular double‐clad fibers

The propagation losses of pump light in rectangular double- clad fibers are investigated both experimentally and theoretically. The dependence of propagation losses on the incidence of pump light as well as fiber lengths are measured using a simple method. It is found that for the same material compositions, the effective numerical apertures (NAs) of rectangular fibers is less than those of circular fibers and the pump light propagation loss in rectangular fibers is larger. The difference of propagation losses between both fibers is rationalized with a geometrical optics analysis.

Effective absorption and pump loss of double-clad fiber lasers

The absorption characteristics of circular, offset, and rectangular double-clad fibers are investigated with simple and comprehensible 2D models. In the model, the pump modes in the first-cladding are divided into absorbable and unabsorbable modes. For circular fiber, a concept of absorbable power ratio is defined to characterize the effective absorption. For offset and rectangular fibers, almost all pump modes are absorbable, it is possible to achieve very high absorption efficiency. Such 2D models have been verified by 3D ray tracing simulations. The influences of offset distance for offset fibers, as well as cross sectional shapes for rectangular one on absorption are also discussed. The propagation losses of pump light in rectangular double-clad fibers are characterized both experimentally and theoretically. The measures to reduce pump loss in use of rectangular double-clad fibers are proposed. At last, the two ends of an Nd-doped rectangular double-clad fiber are bundled together to achieve efficient matching with a 10 W fiber coupled LD. With such a configuration, about 5.4 W pump light is launched into the first-cladding and 2.8 W laser output is obtained. A slope efficiency of 58 percent with respect to launched pump power has been achieved.

Numerical analysis of output performance of Nd-doped double-clad fiber lasers

Numerical analysis for the high-power double-clad fiber lasers is presented and experimental results using different microscope objectives for focusing into a Nd-doped rectangular double-clad fiber also performed. The numerical analysis includes dependence of output power on output mirror reflectivity, absorbed pump power, loss, and fiber length and pump power distribution for the cases of one-end and two-end pumps with 20 dB/km loss. Calculated conversion efficiencies are 76.36%, 69.73%, and 63.84% for lossless, two-end pump, and one-end pump fiber lasers, respectively. Slope efficiencies from absorbed pump power/output powers measured using microscope objectives are 16.8%/182 mW, 53.8%/351 mW, 24.9%/1240 mW, and 13.9%/ 649 mW for magnifications of 5x, 10x, 20x, and 40x, respectively.

Simultaneous multiple-wavelength cw operation of LD-pumped Nd:(YAG, YLF) laser

Simultaneous multiple wavelength cw laser operations were achieved in two types of composite laser rods composed of Nd3+:YAG and Nd3+:YLF crystals, which were laser diode (LD) pumped within a single virtual-point-source cavity. Up to 30 W total output from wavelength of both 1064 nm and 1047 nm was obtained under 150 W LD input power, among which about 25% was from 1047 nm wavelength. Different bonding methods were compared which shows that the use of an optical adhesive is effective and presents no deterioration at low and middle power level. Simultaneous multiple wavelength operation at 1064-nm and 1053 nm was also studied.

Simultaneous multiple wavelength CW operation of LD pumped composite Nd:(YAG,YLF) laser rod

Simultaneous lasing at multiple wavelength of solid-state lasers has been reported by a number of researchers, which may be used in double wavelength lidar, nonlinear optical study and laser medical treatment. One way to achieve multiple wavelength lasing is through utilizing the different energy levels of the active ion. Another way is by doping more that one active ions into one single host crystal. In addition, combining different laser crystals together should also be able to achieve multiple wavelength lasing and may provide a broader flexibility in choosing different wavelength combinations. One thing should be noticed is that the absorption spectrum of lasers with multiple active ions or multiple host materials will become a multiple absorption spectrum. This will be a problem when using laser diodes (LD) as a pumping source because of the narrow bandwidth of LD emission which can only match well with one absorption peak. We used two of the frequently used laser crystals: Nd/sup 3+/ doped YAG and YLF and mounted them together using two bonding methods within a single cavity. By using a LD pumping source which is a off-resonant absorption or wind-pumping system, we achieved simultaneous CW output at wavelength of 1064 nm and 1047 nm, among which output at wavelength of 1047 nm is about 25 % out of the total 30 W under 150 W LD input power. Different bonding methods are compared which shows that the use of a simple optical adhesive is effective at low and middle power presenting no deterioration.