We demonstrate a highly efficient and high power Ho3+-doped fluoride glass fiber laser that is resonantly pumped with a Tm3+-doped silicate glass fiber laser operating at 2.051 µm. The laser operates at 2080 nm and generated 6.66 W at a slope efficiency of 72%. We observe strong visible upconversion fluorescence centered at a variety of wavelengths including 491 nm which results from three sequential energy transfer upconversion processes; the fluorescence to pump energy ratio for this emission is one the largest reported to date.
© 2010 OSA
Direct inband or resonant pumping of Ho3+-doped fibers and bulk crystalline materials offers a low quantum defect approach to the generation of shortwave infrared laser radiation at ~2.1 µm [1–3]. For significant scaling of the output, this approach allows the use of host materials e.g., fluoride crystals  and glasses  for the Ho3+ ion that have thermomechanical properties that are comparatively weaker compared to oxide materials. Since Tm3+-based lasers in both bulk and fiber form is the preferred pump source, optical-to-optical efficiencies can be high because of the effective use of cross relaxation which can produce quantum efficiencies approaching 2. For applications requiring ~2.1 µm output and longer, inband pumping of Ho3+ appears to offer many advantages over direct diode excitation at 1150 nm  and indirect diode excitation using Tm3+ as a sensitizer .
Recently, we showed that Ho3+-doped ZBLAN fiber lasers (HFLs) can be effectively excited using a cladding-pumped Tm3+-doped silicate glass fiber laser (TFL) . Using the free-running output from the TFL this initial demonstration produced 380 mW of 2.084 µm output at a slope efficiency of 50%. The advantage that Ho3+-doped ZBLAN glass fiber supplies over Ho3+-doped silicate glass fibers is the potential for 2.94 µm and visible wavelength emission. Whilst silicate glass has the advantage of good thermomechanical stability which can result in high power performance, direct upper laser level excitation using established sources of 2 µm radiation offers the potential for significant power scaling of fluoride glass fiber lasers. The use of standard-sized axially-symmetric step index fluoride glass fiber also alleviates some of the issues associated with larger diameter complex-shaped claddings.
In this investigation, we extend this work by demonstrating 6.66 W at a slope efficiency of 72% from a Ho3+-doped ZBLAN fiber laser. We illustrate the performance of the fiber laser as a function of fiber length and demonstrate high power and efficient operation for fiber lengths many times the absorption depth. Moreover, we demonstrate the generation of efficient visible upconversion emission to wavelengths as short as 491 nm thus demonstrating one of the largest reported upconversion emission processes.
2. Experimental setup
The pump laser was an in-house TFL that used a volume Bragg grating for wavelength stabilization at 2.051 µm. A schematic diagram of the experimental setup is shown in Fig. 1 . The light from the pump laser was collimated and focused into the Ho3+-doped ZBLAN fiber using commercial triplet lenses coated for this wavelength range (99% transmission). The fiber laser resonator involved Fresnel reflection from each end of the fiber. To measure the light emitted from the pumped end of the fiber, an additional mirror was placed at a slight angle before the focusing lens and reflected 2.5% of the output from the HFL. The infrared spectra were measured using a double monochromator and the visible spectra were measured using an Ocean Optics USB4000. For the measurements of the upconversion emissions the unabsorbed TFL 790 nm pump light was removed using a mirror (highly reflecting at 0.79 µm and transmitting at 2.051 µm) and a polished Ge wafer. The area of the peaks at 2.051 µm and 2.08 µm in the infrared spectrum measured from the distal end of the fiber were used to determine the relative amount of transmitted pump and laser powers. From these measurements, we estimated that at above threshold 22% and 0% of the transmitted power from the distal end of the fiber comprised of unabsorbed pump light for the 0.56 and 1.1 m fiber lengths, respectively. The Ho3+-doped fluoride fiber (nominally ZBLAN and fabricated by FiberLabs, Japan) was doped to 5 mol.%, had a core diameter of 13 µm and a numerical aperture of 0.16. (The fiber laser was therefore emitting multimode output.) The measured background loss was 40 dB/km and 130 dB/km at 800 nm and 1400 nm, respectively.
Of critical importance to the experiment was the beam quality of the 2.051 µm emission from the pump laser. The Tm3+-doped core of the TFL was surrounded by a Ge-doped annulus  that offsets the large refractive index step associated with the high concentration Tm3+ and Al3+ doped core; the numerical aperture of the 20 µm core as 0.14 and the V parameter was 4.3 at 2.051 µm. We launched the pump laser output into two commercial fibers; nLight PASSIVE-20/125 which was single mode (V = 2.1 guiding LP01 only at 2.051 µm) and Nufern MM-GSF-20-125-10A multimode fiber (V = 3.146 which guides both LP01 and LP11 modes). The Tm3+-doped silicate glass fiber was coiled to a 15 cm radius. The launch efficiency for the monomode and multimode fibers was 0.33 and 0.55, respectively. With these results, we estimated that 55% of the incident pump power was launched into the Ho3+-doped ZBLAN fiber because its V parameter was 3.27.
3. Experimental results
For the laser experiments, we tested three lengths of fiber. The output power as a function of the launched pump power is shown in Fig. 3 . For the medium length fiber, 6.66 W was generated at a slope efficiency of 72%. The threshold was 11 mW for this fiber length; this was extended to 69 mW for the shorter (0.34 m) length and to 301 mW for the longer length. The higher threshold for the 1.1 m fiber length is expected as a result of the increased amount of light reabsorption at 2.08 µm; however, whilst the 0.34 m fiber length is many times the absorption depth, the large transmission loss of the Fresnel-Fresnel resonator forced a higher threshold for this fiber length. For some fiber lengths threshold was reached when the visible fluorescence, emitted through the cladding of the fiber, was observed to reach the distal end of the fiber. For the shortest fiber, saturation of the output power was evident for launched pump powers >0.5 W.
Resonantly pumping the fiber at 2051 nm utilized approximately one third of the peak absorption cross section (i.e., 6.1 × 10 −25 m2) at 1950 nm; the calculated absorption coefficient in our fiber at 2051 µm was 1.4 cm−1. For each of the fibers we observed more output emitted from the pumped end of the fiber compared to the distal end. As a result of the large output coupling combined with the large asymmetric gain profile along the length of the fiber, we expect an unequal amount of output from each end. We also found using a single polarizer that both our pump laser and our Ho3+-doped ZBLAN fiber laser were elliptically polarized as shown in the inset to Fig. 2 . The polarization of the output from the Ho3+-doped ZBLAN fiber laser was offset by 40° from the polarization of the pump laser and exhibited stronger linear polarization.
The spectrum of the output for two fiber lengths is shown in Fig. 3. The Ho3+-doped ZBLAN fiber laser operated between 2080 nm and 2095 nm and we observed the typical wavelength shift as the fiber length was increased.
With incident pump power levels as low as a few mW, we observed fluorescence emission from the Ho3+-doped ZBLAN fiber. Figure 4 displays the spectrum of the output from the fiber laser in the visible region. We observed emission at 491 nm, 544 nm, 656 nm, 753 nm and 794 nm, corresponding respectively to the 5F3 → 5I8, 5S2 → 5I8, 5F3 → 5I7, 5S2 → 5I7 and the 5F2 → 5I6 transitions. Near infrared emission at 910 nm was also observed and corresponds to the 5I5 → 5I8 transition. It is clear from these results that high energy levels are populated despite the long wavelength of the pump laser. We postulate that three energy transfer upconversion (ETU) processes are producing excitation to the high energy levels; see Fig. 5 . Firstly, the ETU process (ETU1) 5I7, 5I7→5I8, 5I5  populates the 5I6 level after the 5I5 level partially decays as a result of multiple phonon emission. A second ETU process (ETU2) 5I6, 5I6→5I8, 5F5  continues the excitation to higher energy levels. Energy migration may play a role with these ETU processes given the relatively long lifetimes of the 5I7 and 5I6 levels which are ~12.5 ms and ~3.2 ms, respectively . Finally, a third ETU process (ETU3) 5F5, 5I7 → (5F2,5F3), 5I8  can populate the energy levels responsible for the visible emission at 491 nm, 656 nm and 794 nm. The large Ho3+ concentration combined with intense pumping at 2 µm provides the conditions necessary for the three ETU processes to work efficiently.
In Fig. 6 we show the launched pump power dependence of the emissions at 491 nm, 544 nm and 656 nm emitted from the end of the fiber. Under our experimental conditions, the slope i.e., log(intensity)/log(pump power) for each emission was 0.92, 0.97 and 0.82 respectively for the 491 nm, 544 nm and 656 nm wavelengths. According to Ref , slopes of 1 correspond to emission dominated by excitation from ETU processes and a pump power that is strongly absorbed. In our experiment, some reabsorption of the emission may take place for those emissions that involve the ground state. A significant degree of ground state bleaching is expected at the pumped end of the fiber thus indicating that a more detailed theoretical analysis is required before the slopes presented in Fig. 6 can be used for more quantitative examination. Since the amount of fluorescence is strongly affected when the fiber laser is operating, i.e., we found the visible emission intensities were quite unstable after threshold was reached, the results presented in Fig. 6 relate to below threshold conditions. The unstable visible emission related to unstable 2.1 µm output which involved variable 5I7 and 5I8 population densities and hence variable rates of ETU.
The large Ho3+ concentration allowed efficient rates of ETU but it would have impeded the performance of the 2.09 µm fiber laser. The maximum slope efficiency of 72% is still quite short of the Stokes limit of 98%; it is clear that ETU and reabsorption have reduced the overall efficiency, however not significantly. No attempt was made to isolate the HFL from the TFL; an issue given the fact that the emission wavelength from the HFL is within the gain bandwidth of the TFL. Whilst no fluctuations of the output were observed using the thermal power meter, the output from the TFL was not steady state because of the long Tm3+-doped silicate glass fiber used in our non-optimized TFL; the fluctuating visible emissions also indicated non-steady state behavior of the 2.1 µm output. The use of a prism would allow the output and pump light to be separated and since the output from the HLF was linearly polarized the use of a Faraday rotator would also allow some degree of isolation.
An opportunity now exists for the gain switching of the Ho3+-doped ZBLAN fiber laser in a similar way to Ho3+-doped silicate glass fiber lasers . The use of the efficient ETU process 5I6, 5I6→5I8, 5F5 prevented a population inversion large enough for the 5I6→5I7 transition to reach threshold therefore we did not observe 2.94 µm laser emission. Lower Ho3+ concentrations will reduce the rates for all three ETU processes but with strong pumping at 2 µm a “pair-pumped” 2.94 µm fiber laser may be demonstrated. The current fiber laser would benefit from the inscription of gratings  written into the core to force output in one direction which may offer the possibility for narrow linewidth operation when constructed as a distributed feedback fiber laser.
We have demonstrated an efficient high power Ho3+-doped ZBLAN fiber laser when resonantly pumped at 2.051 µm. A non-optimized slope efficiency of 72% was demonstrated which could be further enhanced with reduced Ho3+ concentrations. The large concentration of 5 mol.% did however allow for the demonstration for triple ETU which resulted in blue emission that had fluorescent photon energies 4.2 times larger than the pump photon energy. This demonstration paves the way for a variety of new laser sources involving pulsed and narrow linewidth emission in the increasingly important 2.1 µm region of the shortwave infrared spectrum.
The authors would like to thank the Australian Research Council for financial support.
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