Fig. 1. (a) Experimental setup for the LITMoS test of the Yb-doped ZBLAN fiber. (b) Magnified image of the fiber holder and an illustration of the three sources of heat load: convective, conductive, and radiative.

Fig. 2. (a) Blue circles correspond to Δ(pixel) (change in the pixel value of the thermal camera image) at each wavelength, and red asterisks represent the area under the S(λ) curve. (b) Red dots represent the measurement of the cooling efficiency (ηc) for the Yb:ZBLAN fiber at different wavelengths. The solid curve shows a fitting of ηc based on Eq. (1) to the measured values, where the positive region in ηc indicates cooling.

Fig. 3. (a) Schematic of the experimental setup that is used for the MACSLA method. OSA stands for optical spectrum analyzer, LPF for long-pass filter, and MMF for multimode fiber. (b) Schematic of the propagation of the pump power in the core of the optical fiber, and collection of the spontaneous emission from the side of the Yb-doped ZBLAN fiber by two multimode passive optical fibers.

Fig. 4. (a) Emission power spectral density S(λ), measured by the optical spectrum analyzer, is plotted in arbitrary units. The inset shows the resonant absorption coefficient, which is normalized to its peak value and is calculated by using the McCumber theory. (b) The points indicate the values of r(λ) measured at different wavelengths near the peak of the resonant absorption coefficient.

Fig. 5. Schematic of the laser system and propagation of the pump power and signal in the double-cladding fiber laser. Pump power is launched at z=0, and the output signal is calculated at z=L at the power delivery port. R1(λ) and R2(λ) are the distributed Bragg reflectors at z=0 and z=L.

Fig. 6. Density plot of the optimum efficiency of the fiber laser for different pump and signal wavelengths, when the laser is pumped with 80 W of input pump power. The inset is a magnification of the density plot over the range of wavelengths, which are most relevant for an RBL system.

Fig. 7. (a) Propagation of the forward pump (FW pump), backward pump (BW pump), forward signal (FW signal), backward signal (BW signal), and temperature rise along the ZBLAN fiber for a conventional fiber laser pumped at λp=975  nm. (b) Similar graph for the RBL operation pumped at λp=1030  nm. Both lasers are optimized for the signal output power of 3 W at λs=1070  nm for αb from Table 1. Note that the fiber in the RBL design is considerably longer than in the conventional design.

Fig. 8. Similar to Fig. 7, except the ZBLAN fiber is chosen with a 10-fold reduction in the background absorption, i.e., αb′=αb/10, while maintaining the parasitic absorption of 0.01 dB/m in the cladding. The reduced value is used for both the conventional laser in (a), pumped with 3.65 W at λp=975  nm; and the RBL laser in (b), pumped with 10.2 W at λp=1030  nm. Both lasers are optimized for the signal output power of 3 W at λs=1070  nm. The RBL design has a substantially reduced temperature performance compared with the conventional laser and the trade-off of a nearly two-fold increase in the required pump power.

Fig. 9. (a) Images of the polishing fixture and the ZBLAN doped fiber, which are glued together by the Crystalbond. (b) Initial coarse polishing steps to prepare a flat surface for further polishing. From left to right, the side and facet views of the doped fiber are shown for each step of the coarse polishing. (c) Images of the facet of the ZBLAN fiber under microscope after each fine-polishing step.

Fig. 10. Thermal camera images of the laser-pumped ZBLAN fiber. Images in subfigure (a) are for pumping at the 1030 nm wavelength and sequentially improved polishing of the facets. The brighter spots indicate heating, and as the polishing quality is improved, the facet heating is reduced. When cooling-grade polishing is reached, the facets no longer are sources of parasitic heating in subfigure (b), and transition from heating to cooling is clearly observed when the pump wavelength is switched from 975 to 1030 nm wavelength.