Numerical analysis and experimental results ofhigh-power Er�Pr:ZBLAN 2.7 �m fiber laserswith different pumping designs

Xiushan Zhu and Ravi Jain

Gain factor and output performance of erbium–praseodymium codoped ZBLAN double-clad fiberlasers at 2.7 �m with different pumping designs were calculated and analyzed. Single-end backwardpumping with a highly reflective mirror butted against one fiber end and dual-end pumping with Fresnelreflections from both fiber ends were found to be the most efficient pumping designs. Ten-watt-levelEr�Pr:ZBLAN fiber lasers proved to be achievable with recent diode laser and ZBLAN fiber technologies.Their corresponding optimum fiber lengths for different pumping configurations were determined. It wasalso found that fiber lasers with a flat evolution of gain factor can obtain the largest output power.Experimental results of 4 m and 12 m fiber lasers showed very good agreement with simulationresults. © 2006 Optical Society of America

OCIS codes: 060.2320, 060.2390, 060.2410, 140.3070, 140.3480, 140.3500, 140.3510.

1. Introduction

Strong absorption of water and some gases (e.g., CO,NO2, H2S, and AsH3) near 3 �m has triggered in-creasing interest in coherent mid-infrared (mid-IR)light sources for spectroscopic1,2 and medical appli-cations3–5 as well as efficient and high-quality mid-IRpump sources for longer wavelength mid-IR lasers,optical parametric oscillators, and THz oscillators.Erbium-doped (Er:) ZBLAN fiber lasers have broademissions at 2.65–2.85 �m, are well suited for theseapplications and are superior to other mid-IR lightsources (e.g., solid-state crystal lasers, quantum-cascade lasers, III-V interband cascade lasers, andIV-VI lead-salt lasers) due to the advantages of fiberlasers, including high conversion efficiency, excellentbeam quality, inherent simplicity of optical cavity con-struction, compactness, outstanding heat-dissipatingcapability, and no need of beam steering. Although apopulation bottleneck resulting from the longernatural lifetime of the lower laser level �4I13�2, �1 �9.4 ms� relative to that of the upper laser level

�4I11�2, �2 � 7.5 ms�6 of erbium ion in fluoride glassinhibits the efficient steady-state lasing and limitsthe output power of Er: ZBLAN fiber lasers,7 theenergy-transfer process between Er ions and codopedPr ions8 and the “two-for-one” energy-transfer upcon-version process between Er and Er ions9–11 have beendemonstrated able to efficiently solve the populationbottleneck and greatly scale up the output power.Therefore, technically, heavily Er-doped ��10000parts per million (ppm)] ZBLAN double-clad fibersand Er�Pr codoped ZBLAN double-clad fibers are twoprimary gain media for efficient 2.7 �m mid-IR la-sers. Much significant work has been completed8–23

on this kind of fiber laser, but there still has not beena big leap in the output scaling of Er:ZBLAN fiberlasers as seen with ytterbium (Yb)-doped silica fiberlasers in the past six years. Recently, 1.36 kilowatts(kW) of output with near diffraction-limited beamquality was obtained from a large-mode-area (LMA)Yb-doped double-clad fiber laser.24 The output pow-ers of Er:ZBLAN fiber lasers, however, are still atwatt level. Though fragility, low optical and thermaldamage threshold, and a low melting point �265 °C�of ZBLAN fibers are obstacles for output scaling ofEr:ZBLAN fiber lasers, we believe that 10-W mid-IRfiber lasers at 2.7 �m can be achieved with recentdevelopments in high brightness beam-shaped laserdiode arrays and in advanced, double-clad ZBLANfibers. To demonstrate the attainability of 10 W out-

X. Zhu (xszhu@chtm.unm.edu) and R. Jain are with the Centerfor High Technology Materials, University of New Mexico, 1313Goddard SE, Albuquerque, New Mexico 87106.

Received 26 January 2006; revised 5 June 2006; accepted 5 June2006; posted 5 June 2006 (Doc. ID 67436).

0003-6935/06/277118-08$15.00/0© 2006 Optical Society of America

7118 APPLIED OPTICS � Vol. 45, No. 27 � 20 September 2006

put and to optimize their performance, numericalanalyses of Er:ZBLAN double-clad fiber lasers arevery important and necessary.

Because population mechanisms of heavily dopedEr:ZBLAN fiber lasers depend greatly on pumpdensity and are totally different from those ofEr�Pr:ZBLAN fiber lasers, simulations and experi-ments of heavily doped Er:ZBLAN double-clad fiberlasers are currently in progress and will be reportedlater. In this paper, we focus on simulations and ex-periments of Er�Pr:ZBLAN double-clad fiber lasers.Based on Jackson’s simulations of population mecha-nisms for continuous-wave operation of Er�Pr:ZBLANfiber lasers,23 we provide a new insight into the gainfactor and the power distribution inside the fiber la-sers with different pumping configurations as well astheir corresponding efficiencies through numericalcalculations of gain factor and forward and back-ward powers by combining a Rigrod analysis25 and aDigonnet model.26 The new approach to investigateEr�Pr:ZBLAN fiber lasers is very useful and instruc-tional for the design of near-future 10-W mid-IR fiberlasers whose efficiencies are extremely important dueto the limited endurable power because of the lowoptical and thermal damage threshold of ZBLANglass. To verify our simulation results, experimentson 4 m and 12 m Er�Pr:ZBLAN double-clad fiber la-sers were preformed.

2. Experimental Setup and Simulation Model

The experimental setup for high-power Er�Pr: ZBLANdouble-clad fiber lasers is schematically depicted inFig. 1. Two 975 nm laser diode bars with a divergenceof 4 � 10° were used as pump sources. A two-mirrorbeam-shaping optical system27 was used to transformthe line shaped beam radiated from the 46-elementdiode bar into a collimated rectangle beam with adimension of 10 mm � 14 mm. The reformed pumplight was then coupled into the inner cladding of theEr�Pr:ZBLAN double-clad fiber (FiberLabs, Inc.) by aplano-convex sapphire lens (focal length � 25 mm;antireflective at 975 nm). The core �diameter �13 �m, numerical aperture �NA� � 0.16] of the fiberhas an Er3� concentration of 20000 ppm and aPr3� concentration of 5000 ppm. The inner rectanglecladding of the fiber has a dimension of 100 �m �200 �m and a NA of 0.55. The outer circular cladding

has a diameter of 300 �m. A dichroic mirror withan incidence angle of 45° [highly reflective �HR� �99.5% at 2.7 �m, highly transmissive �HT� � 95% at975 nm] was used to couple the laser out. A dichroicmirror with an incidence angle of 0° �HR � 99.5% at2.7 �m, HT � 95% at 975 nm) butted against onefiber end was used to provide high-reflection feed-back. Long-wavelength pass filters �T � 95% at� � 2.5 �m) were used to purify the output.

Since six pumping designs were tested in our exper-iments, it is very necessary to state them explicitly:(1) Single-end pumping with Fresnel reflections: Thefiber was pumped at one end, and the feedbacks wereFresnel reflections from both fiber ends. (2) Single-endcopropagating pumping: The fiber was pumped atone end against which a dichroic mirror [angle of in-cidence (AOI) � 0°] was butted. (3) Single-end coun-terpropagating pumping: The fiber was pumped atone end, and a dichroic mirror �AOI � 0°� was buttedagainst the other end. (4) Single-end backward pump-ing: The fiber was pumped at one end, and a highlyreflective mirror was butted against the other end. (5)Dual-end pumping with Fresnel reflections: The fi-ber was pumped at both ends, and the feedbacks wereFresnel reflections from both fiber ends. (6) Dual-end pumping with a dichroic mirror: The fiber waspumped at both ends, and a dichroic mirror was buttedagainst one end of the fiber. In our simulation, theFresnel reflectivity was set as 4% for both the laserand the pump; the reflectivity of the dichoric mirrorwas 100% for the laser and 4% for the pump, respec-tively; the reflectivity of the highly reflective mirrorwas 100% for both the laser and the pump.

In Fig. 2, the configuration and the related param-eters for the simulation of high-power Er�Pr:ZBLANdouble-clad fiber lasers are shown. At end 1 and end2, R1 and R2 are the reflectivity of the cavity mirrors,PP1 and PP2 the launched pump powers, and Pout1 andPout2 the output powers, respectively. At z � 0 andz � Lf, P��0� and P��Lf� are forward powers and P��0�and P��Lf� are backward powers. Because Jackson’s23

theoretical results have indicated that energy trans-fer from Er3� to Pr3� can effectively avoid ground-state bleaching, pump excited-state absorption, andenergy-transfer upconversion among the Er3� ions,we can make an assumption that an Er�Pr:ZBLANfiber laser is approximately a simple four-energy-level laser in which the lasing transition happensbetween the third level �4I11�2� and the second level�4I13�2�. Using a Rigrod analysis25 and a Digonnet

Fig. 1. Experimental setup of Er�Pr:ZBLAN double-clad fiberlasers.

Fig. 2. Configuration for the simulation of Er�Pr:ZBLAN double-clad fiber lasers.

20 September 2006 � Vol. 45, No. 27 � APPLIED OPTICS 7119

model,26 the governing equations for forward power,backward power, and gain factor in the cavity canexpressed as follows:

dP��z�dz �

s�f

hpPp�z�

Fp

Af�

P0 � P��z�1 � �P��z� � P��z���Ps

� �LP��z�, (1)

dP��z�dz � �

s�f

hpPp�z�

Fp

Af�

P0 � P��z�1 � �P��z� � P��z���Ps

� �LP��z�, (2)

��z� �s�f

hpPp�z�

Fp

Af�

1

1 � �P��z� � P��z���Ps.

(3)

In the above equations, z represents the position inthe fiber cavity. P��z� and P��z� are forward powerand backward power, respectively. Pp�z� is a fractionamount of input pump power between z and z � dz. InTable 1, Pp�z� values for the six pumping designs arelisted. �a � GSAN0Af�Aclad is the effective absorptioncoefficient at the pump wavelength of �p, N0 is thepopulation at the ground state, GSA is the absorptioncross-section, Aclad is the cross-sectional area of thefiber cladding, Af is the cross-sectional area of thefiber core, �p is the background loss, �L is the lossfactor at the laser wavelength of �L, s is the stimu-lated cross section, �f is the fluorescence lifetime ofthe upper laser level, hp is the pump photon energy,Fp is the spatial overlap integral between pump andsignal modes, Ps � �hL�s�f� � Af is the saturationoutput power, and P0 � NhL � � �L�2� gives thespontaneous emissions associated with N photons inthe gain bandwidth �L.

The fourth Runge–Kutta method was used to doiterative calculations to solve the coupled differentialequations for forward and backward powers. Byusing boundary conditions, P��0� � R1P

��0� andP��Lf� � R2P

��Lf�, decision conditions for the bound-ary values in our simulations were obtained asfollows:

3. Simulation Results and Discussions

In our simulation, we set the launched pump power,Pp1�0� � Pp2�Lf� � 2.5 W for the dual-end pumpingconfigurations and Pp1�0� � 5 W or Pp2�Lf� � 5 W

for the single-end pumping configurations, �a �0.446 m�1 �1.937 dB�m� at the pump wavelengthof 975 nm (calculated by �a � GSAN0Af�Aclad withGSA � 2.1 � 10�21 cm2 and N0 � 3.2 � 1020 cm�3),�L � 0.12 m�1 �0.52 dB�m� at the laser wavelengthof 2.7 �m (measured by a cutback experiment),�p � 0.046 m�1 �0.2 dB�m, provided by the fiber man-ufacturer), s � 5.7 � 10�21 cm2,23 �f � 2.94 ms (mea-sured), and Fp � 0.84 (calculated from the brightnessof the diode laser). The saturation output powerwas calculated as Ps � 5.8 mW. The spontaneousemission was P0 � 4.762 � 10�7 N when the gainbandwidth �L was selected as 100 nm.

The simulations are based on our present experi-mental setup. The evolution of forward power, back-ward power, and gain factor of 4 m and 12 m fiberlasers for single-end pumping with Fresnel reflections,single-end copropagating pumping, and single-endcounterpropagating pumping are plotted in Figs. 3 and4, respectively. We can see from the simulation resultsthat maximum forward power and maximum back-ward power in the fiber lasers for the three single-endpumping configurations are normally located at fiberends, except that maximum forward power for single-end pumping with Fresnel reflections and single-endcopropagating pumping and maximum backward

power for single-end counterpropagating pumping arelocated at some position (determined by the parame-ters of the active fiber) in the 12 m fiber cavity, inwhich the laser gain becomes smaller than the loss

P��Lf� ��P0�1 � R2� � �P0

2�1 � R2�2 � 4R2P��0��P0 � R1P

��0� � P0R1�2R2

, (4)

P��0� ��P0�1 � R1� � �P0

2�1 � R1�2 � 4R1P��Lf��P0 � R2P

��Lf� � P0R2�2R1

. (5)

Table 1. PP(z) for the Six Pumping Designs

Pumping Designs PP(z)

Single-end pumpingwith Fresnelreflections

�aPp1�0�e���a��p�z

Single-endcopropagatingpumping

�aPp1�0�e���a��p�z

Single-endcounterpropagatingpumping

�aPp2�Lf�e���a��p��Lf�z�

Single-end backwardpumping

�aPp2�Lf��e���a��p��Lf�z� � e���a��p��Lf�z��

Dual-end pumping withFresnel reflections

�a�Pp1�0�e���a��p�z � Pp2�Lf�e���a��p��Lf�z��

Dual-end pumping witha dichroic mirror

�a�Pp1�0�e���a��p�z � Pp2�Lf�e���a��p��Lf�z��

7120 APPLIED OPTICS � Vol. 45, No. 27 � 20 September 2006

beyond that position. For the 4 m fiber laser, theoutput of single-end pumping with Fresnel reflec-tions has the largest output power. Single-end coun-terpropagating pumping, however, has the largestoutput power when the fiber length is 12 m. In thethree pumping designs, single-end copropagatingpumping gives the smallest output power regardlessof the fiber length. In the 4 m fiber laser, the maxi-mum gain factor of single-end pumping with Fresnelreflections is located at a position close to the cavityend, and those of the other two pumping configura-tions are located at the cavity ends. In the 12 m fiberlaser, the maximum gain factors of single-end pump-ing with Fresnel reflections and single-end counter-propagating pumping are located at positions close tothe cavity ends, and that of single-end copropagatingpumping is located at the cavity end.

The evolutions of forward power, backward power,and gain factor of 4 m and 12 m fiber lasers for dual-end pumping with Fresnel reflections, dual-end pump-ing with a dichroic mirror, and single-end backwardpumping are plotted in Figs. 5 and 6, respectively. It

is obvious that, normally, maximum forward powerand maximum backward power are located at thefiber ends for the three pumping configurations, ex-cept that the maximum backward power of single-end backward pumped 12 m fiber laser is located atsome position in the cavity. For the 4 m fiber laser,the output power of single-end backward pumpingwith a highly reflective mirror butted against thefiber end at z � 0 is the largest. Dual-end pumpingwith Fresnel reflections from both fiber ends gives thelargest output power when the fiber length is 12 m.Dual-end pumping with a dichroic mirror buttedagainst the fiber end at z � 0 has the smallest outputpower in the three pumping designs for both 4 m and12 m fiber lasers. In the 4 m fiber laser, the maxi-mum gain factor of dual-end pumping with Fresnelreflections is located at the center of the cavity, andthose of the other two pumping configurations arelocated at the cavity ends. In the 12 m fiber laser,however, the maximum gain factors of single-endbackward pumping is located at close to the cavityends, and those of the other two pumping configura-tions are located at the cavity ends.

Fig. 3. Evolution of forward power, backward power, and gainfactor in the 4 m fiber laser for single-end pumping with Fresnelreflections from both fiber ends (solid curve), single-end copropa-gating pumping with a dichroic mirror butted against the fiber endat z � 0 (dashed curve), and single-end counterpropagating pump-ing with a dichroic mirror butted against the fiber end at z � 0(dotted curve). (a) Evolution of power. (b) Evolution of gain factor.

Fig. 4. Evolution of forward power, backward power, and gainfactor in the 12 m fiber laser for single-end pumping with Fresnelreflections from both fiber ends (solid curve), single-end copropa-gating pumping with a dichroic mirror butted against the fiber endat z � 0 (dashed curve), and single-end counterpropagating pump-ing with a dichroic mirror butted against the fiber end at z � 0(dotted curve). (a) Evolution of power. (b) Evolution of gain factor.

20 September 2006 � Vol. 45, No. 27 � APPLIED OPTICS 7121

Comparing simulation results of the six pumpingdesigns, we can conclude that single-end backwardpumping with a highly reflective mirror butted againstthe fiber end at z � 0 is most efficient for the 4 m fiberlaser; dual-end pumping with Fresnel reflectionsfrom both fiber ends is most efficient for 12 m fiberlaser.

Recently, there has been remarkable progress inhigh-brightness laser diode arrays and in LMAdouble-clad ZBLAN fibers, which can boost the powerscaling of mid-IR fiber lasers. Our new simulationsusing the same model and mathematics as above in-dicate that 10-W Er�Pr:ZBLAN double-clad fiber la-sers can be achieved when a pump source with powerof 50 W and an advanced, custom-designed fiber areused. The custom-designed fiber has the best specifi-cations (the highest dopant density and the largestsingle-mode core size) that can be achieved by thefiber manufacturer (FiberLabs, Inc.). The core of thefiber has an Er3� concentration of 40000 ppm and aPr3� concentration of 5000 ppm. The diameter of thecore is 15 �m, and NA is 0.13. The inner rectangle

cladding of the fiber has a dimension of 200 �m �250 �m and a NA of 0.55. To determine the optimumfiber lengths for near-future 10-W Er�Pr:ZBLANdouble-clad fiber lasers, output powers as a functionof fiber length are plotted in Fig. 7 for the six pump-ing designs with pump power of 50 W. The optimumcavity length is approximately 3.5 m for single-endbackward pumping, 7 m for dual-end pumping withFresnel reflections, 4.5 m for dual-end pumping witha dichroic mirror, 4 m for single-end copropagatingpumping, 6 m for single-end counterpropagating pump-ing, and 5 m for single-end pumping with Fresnelreflections. Seen from Figs. 7, it is found that eventhough the fiber lasers have optimum cavity lengths,they still have a large difference between their out-put powers (13.7 W for single-end backward pump-ing and dual-end pumping with Fresnel reflectionsand only 10.7 W for single-end copropagating pump-ing). Calculating the evolution of gain factors in7 m, 3.5 m, and 5 m fiber lasers with differentpumping configurations, shown in Fig. 8, and com-paring their output powers in Figs. 7, we find a very

Fig. 5. Evolution of forward power, backward power, and gainfactor in the 4 m fiber laser for dual-end pumping with Fresnelreflections from both fiber ends (solid curve), dual-end pumpingwith a dichroic mirror butted against fiber end at z � 0 (dashedcurve), and single-end backward pumping with a highly reflectivemirror butted against fiber end at z � 0 (dotted curve). (a) Evolu-tion of power. (b) Evolution of gain factor.

Fig. 6. Evolution of forward power, backward power, and gainfactor in the 12 m fiber laser for dual-end pumping with Fresnelreflections from both fiber ends (solid curve), dual-end pumpingwith a dichroic mirror butted against fiber end at z � 0 (dashedcurve), and single-end backward pumping with a highly reflectivemirror butted against fiber end at z � 0 (dotted curve). (a) Evolu-tion of power. (b) Evolution of gain factor.

7122 APPLIED OPTICS � Vol. 45, No. 27 � 20 September 2006

interesting phenomenon: the flatter the gain evolu-tion along the laser cavity, the larger the output, andthe largest output power can be obtained when thegain factor is constant throughout the fiber laser cav-ity, such as the gain factor in the 7 m dual-endpumped fiber laser with Fresnel reflections at bothfiber ends and in the 3.5 m single-end backwardpumped fiber laser. This conclusion is significant andvery useful in that it can provide guidance for dis-tributed side-pumping designs28 for tens-watt-levelZBLAN fiber lasers in which the end-launched pumppower will be limited by the low damage threshold ofZBLAN glass.

4. Experimental Results and Discussion

Output performances of 4 m and 12 m Er�Pr:ZBLANdouble-clad fiber lasers were investigated by using anexperimental setup schematically depicted in Fig. 1.Output powers as a function of pump power for fiberlasers with single-end pumping with Fresnel reflec-tions, single-end copropagating pumping, and single-end counterpropagating pumping are shown in Fig. 9.

The experimental results are very consistent withthe simulation results that single-end pumping withFresnel reflections is most efficient for 4 m fiber lasersin the three pumping designs; for 12 m fiber lasers,however, single-end counterpropagating pumping ismost efficient; single-end copropagating pumpingis least efficient for both 4 m and 12 m fiber lasers.Output powers as a function of pump power for dual-end pumped fiber lasers are shown in Fig. 10. Obvi-ously, dual-end pumping with Fresnel reflections ismuch more efficient than dual-end pumping with adichroic mirror butted against one fiber end. Theslope efficiencies of 4 m and 12 m fiber lasers withdifferent pumping designs are listed in Table 2. It isobvious that there is much difference between theslope efficiencies of a fiber laser with different pump-

Fig. 8. Gain factor as a function of relative position in fiber laserswith optimal length for dual-end pumping with Fresnel reflections�7 m�, single-end backward pumping �3.5 m�, and single-endpumping with Fresnel reflections �5 m�. Dual-end pumping withFresnel reflections (black solid curve), dual-end pumping with adichroic mirror (black dashed curve), single-end pumping with ahighly reflective mirror (black dotted curve), single-end pumpingwith Fresnel reflections (gray solid curve), single-end copropagat-ing pumping (gray dashed curve), and single-end counterpropagat-ing pumping (gray dotted curve).

Fig. 9. Output power of 4 m (diamond) and 12 m (circle) fiberlasers as a function of pump power for single-end pumping withFresnel reflections (solid curve), single-end copropagating pump-ing (dashed curve), and single-end counterpropagating pumping(dotted curve).

Fig. 7. Output power as a function of fiber length for differentpumping designs when the pump power is 50 W and a custom-designed fiber is used. (a) Single-end pumping with Fresnel reflec-tions (solid curve), single-end copropagating pumping (dashedcurve), and single-end counterpropagating pumping (dottedcurve). (b) Dual-end pumping with Fresnel reflections (solid curve),dual-end pumping with a dichroic mirror (dashed curve), andsingle-end backward pumping (dotted curve).

20 September 2006 � Vol. 45, No. 27 � APPLIED OPTICS 7123

ing configurations. For the 4 m fiber laser, the max-imum slope efficiency is about 21% and the minimumis 13%. For the 12 m fiber laser, the maximum slopeefficiency is about 22% and the minimum is only9.5%. Therefore, it is crucial to select a pumping con-figuration for the near-future high efficiency 10-Wmid-IR fiber lasers.

5. Conclusion

In conclusion, a numerical simulation and an exper-imental investigation of high-power Er�Pr:ZBLANdouble-clad fiber lasers with different pumping de-signs were performed. Single-end backward pumpingand dual-end pumping with Fresnel reflections werefound to be the most efficient pumping configurationsfor 4 m and 12 m fiber lasers, respectively. The ex-perimental results of 4 m and 12 m fiber lasers werein good agreement with the simulation results. Oursimulation also showed that �10 W output near3 �m is achievable when an advanced Er�Pr:ZBLANdouble-clad fiber is used and 50 W pump is launched.Optimum fiber lengths for different pumping config-urations were also determined for the near-future10 W level Er�Pr:ZBLAN fiber lasers. The simulationresult that fiber lasers that have a flat evolution ofgain throughout the cavity give the largest outputpower is an oriented instruction for distributed side-pumping designs for future tens-watt-level ZBLANfiber lasers.

This work was supported by NASA and the U.S.Air Force Office of Scientific Research.

References1. R. S. Eng, J. F. Butler, and K. J. Linden, “Tunable diode laser

spectroscopy: an invited view,” Opt. Eng. 19, 945–960 (1980).2. P. Werle, F. Slemr, K. Maurer, R. Kormann, R. Mucke, and B.

Janker, “Near- and mid-infrared laser-optical sensors for gasanalysis,” Opt. Laser Eng. 37, 104–114 (2002).

3. K. F. Gibson and W. G. Kernohan, “Lasers in medicine: areview,” J. Med. Eng. Technol. 17, 51–57 (1993).

4. R. Kaufmann, A. Hartmann, and R. Hibst, “Cutting and skin-ablative properties of pulsed mid-infrared laser surgery,” J.Derm. Surg. Oncol. 20, 112–118 (1994).

5. S. D. Jackson and A. Lauto, “Diode-pumped fiber lasers: anew clinical tool?” Laser Surg. Med. 30, 184–190 (2002).

6. R. S. Quimby and W. J. Miniscalco, “Continuous-wave lasingon a self-terminating transition,” Appl. Opt. 28, 14–16 (1989).

7. S. Bedo, M. Pollnau, W. Luthy, and H. P. Weber, “Saturationof the 2.71 �m laser output in erbium-doped ZBLAN fibers,”Opt. Commun. 116, 81–86 (1995).

8. J. Y. Allan, M. Monerie, and H. Poigant, “Energy transfer inEr3��Pr3�-doped fluoride glass fibers and applications tolasing at 2.7 �m,” Electron. Lett. 27, 445–447 (1991).

9. E. Poppe, B. Srinivasan, and R. K. Jain, “980 nm diode-pumped continuous wave mid-IR (2.7 �m) fiber laser,”Electron. Lett. 34, 2331–2333 (1998).

10. B. Srinivasan, E. Poppe, J. Tafoya, and R. K. Jain, “High-power (400 mW) diode-pumped 2.7 �m Er:ZBLAN fiber lasersusing enhanced Er-Er cross-relaxation processes,” Electron.Lett. 35, 1338–1340 (1999).

11. M. Pollnau, P. Spring, Ch. Ghisler, S. Wittwer, W. Luthy, andH. P. Weber, “Efficiency of erbium 3-�m crystal and fiberlasers,” IEEE J. Quantum. Electron. 32, 657 (1996).

12. M. Pollnau, Ch. Ghisle, G. Bunea, M. Bunea, W. Luthy, andH. P. Weber, “150 mW unsaturated output power at 3 �m froma single-mode-fiber erbium cascade laser,” Appl. Phys. Lett.66, 3564–3566 (1995).

13. M. Pollnau, Ch. Ghisler, W. Luthy, and H. P. Weber, “Crosssections of excited-state absorption at 800 nm in erbium-dopedZBLAN fiber,” Appl. Phys. B 67, 23–28 (1998).

14. B. Srinivasan, J. Tafoya, and R. K. Jain, “High-power “watt-level” cw operation of diode-pumped 2.7 �m fiber lasers usingefficient cross-relaxation and energy transfer mechanisms,”Opt. Express 4, 490–495 (1999).

15. S. D. Jackson, T. A. King, and M. Pollnau, “Diode-pumped1.7-W erbium 3-�m fiber laser,” Opt. Lett. 24, 1133–1135(1999).

16. N. J. C. Libatique, J. D. Tafoya, S. H. Feng, D. J. Mirell, andR. K. Jain, “A compact diode-pumped passively Q-switchedmid-IR fiber laser,” in Advanced Solid State Lasers, Vol. 34 ofTrends in Optics and Photonics (Optical Society of America,Washington, D.C., 2000), pp. 417–419.

17. N. J. C. Libatique, J. Tafoya, N. K. Viswanathan, R. K.Jain, and A. Cable, “ ‘Field-usable’ diode-pumped �120 nmwavelength-tunable cw mid-IR fiber laser,” Electron. Lett. 36,791–792 (2000).

18. B. Srinivasan, G. Monnom, and R. K. Jain, “Indirect measure-ment of the magnitude of ion clustering at high doping densitiesin Er:ZBLAN fibers,” J. Opt. Soc. Am. B 17, 178–181 (2000).

19. S. D. Jackson, T. A. King, and M. Pollnau, “Efficient highpower operation of erbium 3 �m fiber laser diode-pumped at975 nm,” Electron. Lett. 36, 223–224 (2000).

20. B. C. Dickson, P. S. Golding, M. Pollnau, T. A. King, and S. D.Jackson, “Investigation of a 791-nm pulsed-pumped 2.7-�m Er-doped ZBLAN fiber laser,” Opt. Commun. 191, 315–321 (2001).

21. M. Pollnau and S. D. Jackson, “Erbium 3-�m fiber lasers,”IEEE J. Sel. Top. Quantum Electron. 7, 30–40 (2001).

22. M. Pollnau and S. D. Jackson, “Energy recycling versus life-time quenching in erbium-doped 3-�m fiber lasers,” IEEE J.Quantum. Electron. 38, 162–169 (2002).

Fig. 10. Output power of 4 m (diamond) and 12 m (circle) fiberlasers as a function of pump power for dual-end pumping withFresnel reflections (solid curve) and dual-end pumping with a di-chroic mirror (dashed curve).

Table 2. Slope Efficiencies of Er���Pr:ZBLAN Double-Clad Fibers withDifferent Pumping Designs

Pumping Designs

Slope Efficiency

4 m 12 m

Single-end pumping with Fresnel reflections 20.58% 16.69%Single-end copropagating pumping 13.01% 9.52%Single-end counterpropagating pumping 17.51% 19.25%Dual-end pumping with Fresnel reflections 20.95% 22.28%Dual-end pumping with a dichroic miror 15.99% 14.83%

7124 APPLIED OPTICS � Vol. 45, No. 27 � 20 September 2006

23. S. D. Jackson, T. A. King, and M. Pollnau, “Modeling of high-power diode-pumped erbium 3 �m fiber lasers,” J. Mod. Opt.47, 1987–1994 (2000).

24. Y. Jeong and J. K. Sahu, “Ytterbium-doped large-core fiberlaser with 1.36 kW continuous-wave output power,” Opt. Ex-press 12, 6088–6092 (2004).

25. W. W. Rigrod, “Saturation effects in high-gain lasers,” J. Appl.Phys. 36, 2487–2490 (1965).

26. M. J. F. Digonnet, “Theory of superfluorescent fiber lasers,” J.Lightwave Technol. LT-4, 1631–1639 (1985).

27. W. A. Clarkson and D. C. Hanna, “Two-mirror beam-shapingtechnique for high-power diode bars,” Opt. Lett. 21, 375–377(1996).

28. J. Xu, J. Lu, G. Kumar, J. Lu, and K. Ueda, “A non-fused fibercoupler for side-pumping of double-clad fiber lasers,” Opt.Commun. 220, 389–395 (2003).

20 September 2006 � Vol. 45, No. 27 � APPLIED OPTICS 7125