Major power scaling results in the eye-safe $\sim 1.5\mu \mathrm{m}$ fiber lasers were reported with the cladding pumped Yb-Er codoped fibers pumped at $915∕980\mathrm{nm}$ into Yb-absorption band [1]. Owing to inefficiencies associated with very large $(\sim 40\%)$ quantum defect pertinent to this approach, scalability of these devices suffers from and is limited by enormous heat deposition inconsistent with major power scaling toward multikilowatt power levels. Besides, scaling beyond the current eye-safe power record of $\sim 300\mathrm{W}$ [1] is impossible with the Yb-Er codoping approach, because multihundred-watt Yb-Er fiber lasers typically carry in their output a significant fraction of competing 1 μm Yb emission [either narrowband or amplified spontaneous emission (ASE)], thus significantly compromising an eye-safe application itself. This, combined with the fact that power conversion efficiency of the Er-Yb laser systems is limited by inefficiency of Yb-Er energy transfer on top of quantum defect of Er-doped fiber pumped at 9XX nm, provides significant motivation for going back to resonantly pumped Yb-free Er-doped fiber lasers quite successfully used in telecom applications. With the resonant pumping approach Er-doped fibers should poten tially behave no different from resonantly pumped Yb-doped fibers known to be able to deliver up to $\sim 90\%$ optical-to-optical efficiency [2]. In support of that statement we have recently achieved over 85% optical-to-optical efficiency from a resonantly pumped, Yb-free, Er-doped, single-mode fiber amplifier based on commercial off-the-shelf (COTS) fiber [3]. In this case pump-induced heat deposition would predominantly be associated with low quantum defect (5% and less) of a resonantly pumped Er-doped fiber laser, which opens up significant space for fiber laser power scaling suffering no thermal management complications. As opposed to single-mode fiber lasers (only scalable toward telecom power goals), an important step toward a highly scalable resonantly pumped Er fiber laser is cladding pumping, which would accommodate highly multimode fiber-coupled 14XX–15XX nm $\mathrm{In}\mathrm{Ga}\mathrm{As}\mathrm{P}∕\mathrm{In}\mathrm{P}$ high power diodes and bars for pumping. The latter is supported by recent dramatic efficiency and power strides in the quantum-well separate confinement heterostructure lasers based on $\mathrm{In}\mathrm{Ga}\mathrm{As}\mathrm{P}∕\mathrm{In}\mathrm{P}$ systems (e.g., [4]).

So far only very few efforts were reported on resonantly cladding-pumped Yb-free Er fiber lasers [5, 6, 7]. In [5, 6] output power of $\sim 1\mathrm{W}$ was achieved (in [5] by summing up the power emitted from both fiber ends). The work of [6] was actually the first effort of successfully getting to the most scalable large-mode-area (LMA) fiber approach. The work of Dubinskii et al.[7] was the first effort to scale significantly beyond the $1\mathrm{W}$ power level. Single-frequency output power of $9.3\mathrm{W}$ was obtained in master-oscillator power-amplifier configuration from resonantly cladding-pumped Yb-free EDFA with the slope efficiency of 46% with respect to absorbed pump power [7]. Presented here are characterization results of a resonantly cladding-pumped fiber Bragg grating (FBG) laser based on Yb-free Er-doped COTS LMA fiber. To our knowledge this is the first reported resonantly cladding-pumped FBG-based Er-doped LMA fiber laser. Obtained narrowband output of $47.6\mathrm{W}$ is believed to be the highest power ever reported from a Yb-free Er-doped fiber laser. This fully integrated laser also has the optical-to-optical efficiency of 56.7%, which we believe is the highest efficiency ever reported for a cladding-pumped unidirectionally emitting Er-doped laser. No indication of power saturation effects was observed; i.e., achieved power is strictly pump limited and can be further scaled significantly.

In this work no special effort was made on laser fiber composition and design: A commercially available Liekki Er60-20/125DC double-clad (DC), Yb-free, Er-doped LMA fiber was efficiency and power tested in a simple FBG-based laser configuration for power scaling potential evaluation.

Figure 1 depicts the optical layout of the fully integrated Er fiber laser based on COTS Liekki Er60-20/125 DC fiber pumped through the FBG which was used in a function of a dichroic (WDM) pump mirror. The FBG, matching the $20∕125$ DC fiber format, was manufactured by IPG Photonics Corp. and has the $\sim 93.5\%$ reflectivity centered at $1589.4\mathrm{nm}$ with the $2.58\mathrm{nm}$ bandwidth (FWHM). The fiber laser was copumped by six fiber-coupled (into a $105∕125\mu \mathrm{m}$, NA 0.15 fiber) spectrally narrowed (bandwidth of $\sim 0.5\mathrm{nm}$ FWHM), $\mathrm{In}\mathrm{Ga}\mathrm{As}\mathrm{P}∕\mathrm{In}\mathrm{P}$ laser diode modules. The custom-made modules, spectrally narrowed by volume Bragg gratings, delivered an average of $\sim 18.5\mathrm{W}$ of fiber-coupled power each, centered at $1532.5\mathrm{nm}$. This wavelength was chosen to fit the peak of the Er60-20/125DC fiber absorption spectrum in order to minimize the required fiber length. Copumping was realized through the SIFAM six-port pump combiner matching the Er60-20/125DC active fiber. The output end of the $\sim 15\text{-}\mathrm{m}$-long fiber (the length was chosen to maximize optical-to-optical efficiency versus absorbed pump power) had a straight cleave used as an outcoupling Fresnel mirror. With this fiber length maximum of $101\mathrm{W}$ of launched power at $1532.5\mathrm{nm}$ (after the coupler and FBG) resulted in a maximum of $84.1\mathrm{W}$ of power absorbed in the fiber.

Major laser testing results obtained with a resonantly cladding-pumped, Liekki Er60-20/125DC double-clad, Yb-free, Er-doped LMA fiber laser are presented in Fig. 2 . The result indicates linear laser behavior with no saturation effects, which points to a purely pump-limited laser power scaling nature in this case. Calculated linear regression parameters for the data are indicative of optical-to-optical conversion efficiency of $\sim 56.7\%$ achieved in this laser operating in an ultralow quantum defect $(\mathrm{QD}=3.75\%)$ mode. Even though the demonstrated operation based on the COTS laser fiber is clearly not quantum defect limited, it is, to the best of our knowledge, the highest optical-to-optical conversion efficiency operation ever obtained from a unidirectionally emitting, Yb-free, Er-doped fiber laser with resonant cladding pumping. Maximum laser output power of $47.6\mathrm{W}$ was achieved at $1590\mathrm{nm}$ with the overall spectral width of the output narrower than $0.25\mathrm{nm}$ FWHM. No discernible laser output power degradation was observed after $\sim 96\text{\hspace{0.17em} h}$ of operation, which is assumed to be an indication that no major photodarkening effects are present—just as can be expected with the resonant, low-quantum defect approach. Using the optical spectrum analyzer set to a $0.05\mathrm{nm}$ resolution we found that spectral power distribution within this spectral width is not a steady-state one: There were typically two to three not quite fully resolved dynamically competing peaks observed in the laser output, but they are all always confined to a $\sim 0.25\mathrm{nm}$ spectral width. The reported $47.6\mathrm{W}$ result presents (to our knowledge) the highest power ever achieved out of Yb-free Er-doped LMA fiber with resonant cladding pumping and exceeds our previous result, with the broadband pumping diode modules, by a factor of $\sim 5$ [7].

In this FBG laser experiment the $1590\mathrm{nm}$ output was spectrally pure. It was obtained with the ASE level never exceeding the $\sim 35\mathrm{dB}$ below the signal level (see Fig. 3 ) and up to the maximum output power.

Typical FBG fiber laser output with a loosely coiled LMA fiber was multimode (two to three lower-order modes were most often seen) and unstable in time (transverse mode hopping was dynamically observed). Stable nearly diffraction-limited operation was achieved, with the penalty in efficiency of less than 5%, by partial coiling of the Er60-20/125 DC LMA fiber to a diameter of $\sim 8–10\mathrm{cm}$. The far-field pattern of the fiber laser output in this case is shown in Fig. 4 .

In conclusion, we demonstrated highly scalable, efficient, ultralow quantum defect $(\sim 3.75\%)$ operation of the resonantly cladding-pumped, Yb-free, Er-doped FBG laser based on a COTS LMA fiber. We believe this is the first reported resonantly cladding-pumped, FBG-based, Er-doped LMA fiber laser. Obtained narrowband output of $47.6\mathrm{W}$ at $1590\mathrm{nm}$ is believed to be the highest power ever reported from a Yb-free Er-doped fiber laser. The laser operating with no power saturation effects is assumed to be strictly pump limited and can be further scaled significantly. Currently achieved optical-to-optical efficiency of 56.7%, to the best of our knowledge, is the highest efficiency reported for a cladding-pumped unidirectionally emitting Er-doped laser. This efficiency is clearly far from QD-limited operation, which indicates that there are other efficiency-limiting factors likely associated with upconversion, nonradiative losses due to Er ion clustering, as well as the presence of trace water amounts in this particular COTS fiber. Yb-free Er-doped DC fibers for clad pumping are currently grossly underdeveloped. With specialty Er fibers developed specifically for the power scaling conversion efficiency can be increased to a QD-limited level, very similar to Yb-doped fiber lasers.

This work was partially supported by the High Energy Laser Joint Technology Office.

 

Fig. 1 Optical layout of the resonantly cladding-pumped, Yb-free, Er-doped fiber laser based on a COTS Liekki Er60-20/125DC fiber pumped at $1532.5\mathrm{nm}$. Laser cavity consists of the high-reflectivity FBG mirror and the straight cleave at the output fiber end.

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Fig. 2 Output versus absorbed pump-power dependence obtained for the resonantly cladding-pumped Er-doped FBG laser shown in Fig. 1. The line is the linear regression of the data points presented by open circles.

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Fig. 3 Spectral output of the FBG laser shown in Fig. 1 (ANDO AQ6370 optical spectrum analyzer, resolution $0.05\mathrm{nm}$). Indicated in Fig. 3 are: residual pump, the leaked unabsorbed pump power at $1532.5\mathrm{nm}$; ASE, amplified spontaneous emission; lasing, the underresolved (on the scale shown in Fig. 3) FBG-controlled fiber laser emission at $\sim 1590\mathrm{nm}$.

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Fig. 4 Far-field pattern of the laser output beam for the resonantly cladding-pumped, Yb-free, Er-doped FBG laser after stripping the higher-order modes by proper fiber coiling.

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