Lüthy, W.

Pollnau, M.

Weber, H.P.

English

Pollnau, Markus

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Erbium 3-µm lasers: Concepts of energy recycling

University of Twente Research Information

16:45 CTuL5 Erbium 3-pm Lasers: Concepts of Energy Recycling M. Pollnau, W Luthy. and €I P Weber Institute of Applied Physics, University of Bein, Sidlerstrasse 5, CH-30 I 2  Bern: Switzerland Tel. +41-31-631 8937,Fax+41-31-631 3 7 6 5  With rising power in cw and pulsed operation erbium 3-pm lasers work their way into a variety of applications in surgery Despite the unfavorable ratio of upper-level to lower-level lifetime cw laser opcrattori is achieved in fluoride host materials solely due to the small branching ratio on the laser transitioii 11). CW output powers exceed 1 W in LiYF4 121 and 150 mW in a ZBLAN fiber [3]. Owing to geometr). and erbium concentration the population mechanisms are different m clystals and fibers [4]. Thcsc mechanisms determine different concepts of energy recycling in the mentioned matcrials. In highly doped crystals ground-state absorption (GSA) and interionic processes play a dominant role Excited-state absorption (ESA) is weak, because the excitation density is small compared with the %round-state density [4], Interionic upconversion from the lower laser level recycles energy into the upper lascr Icvel. Energy lost by a second upconversion from the upper laser level is automatically recycled via cross relaxation. The concentration dependence of the interionic processes offers highest efficiency in Er’+( I5 %)‘LiYF4 [2] .  The experimentally obtained slope cfiaeitcy of 40 % [4] is clearly hi&er than the Stokes limit of 35 % under 973-nm pumping The theoretical limit including energy recycling is 56 % [4]. I n  fibers, in contrary, a low dopant concentration and a strong pump confinement in combmation with long Iifetmies of the laser levels lead to ground-state bleaching and a strong excitation of the laser levels. ESA dominates GSA, whereas interionic processes have only a small influence [41. Groundstate bleaching qui res  the depletion of the lower laser level by pump ESA and repopulation of the ground state via fluorcscences. However, two pump photons are absorbed for the creation of one laser photon, In addition, part of thc ESA excitation is not automaticallv recycled, but leads to competitive lasing at 850 nni and the saturation of 3-pm output power [j] Cascade lasing at 1.7 pm [3] avoids competitive lasing, recyclcs the energy into the upper laser level, and ensures that each pump photon is converted into one laser photon. Thus, the theoretical limit ofthe slope efficiency under 791-nm excitation is enhanced to 27 % 141 which IS only slightly smaller than the Stokes limit of 29 %. Experimentally a slope efficiency of 23 ”/. IS obtained [3]. The recycling concept ofcascade lasing may also be applicable to other fiber lasers. 1 1 1  11 S Quimby,W. J. MiniscaIco,Appl. opt. 28, 14 (1989) 121 l’ Jcnsen, A Diening. G. Huber, in Conference on Lam and Electrdpircs, Vol 15, OSA Technical Digest Series 131 M I’ollnau, Ch. Ghisler. G. Bunea, U Bunea. W. Luthy. H P Weher. Appl Phys Lett. 66, 3564 (1995). 141 M Pollnau, R Spring, Ch. Ghisla, S. Wittwer, W. Lathy. H. P We&, EEC I. Quanh~mElectron. 31 (4). 1996. 151 S lkd6.M Pollnau. W. L0thy.H. P. Weber, Gpt.Commun 116.81 (1995) (Optical Saicly of America. Washington, DC, 1995), porideadllne paper CPD29 FIG I Input-output curves of Tisapphire- pumped erbium lasers the 2 SI-pm laser in ;:iw, P 2 LiYF4 71-pm (circles) t ansition pumped of the at  cascade 973 nm laser and the u1a 0  Obtained slope efficiencies versus absorbed I I ZBLAN fiber (squares) pumped at 791 nm 2 40 2oo 4oo 6w pump power are 40 %and 23 %, respcctivelv Absorbed Pump Power [mW] 17:OO CTuL6 P$+ CO-Doped 2.8-pm Erbium Fiber Laser M. Pollnau’, 1. Schneider2, U. B Unrau2, W. Luthyl, and H. P Weber1 ‘Institute of Applied Physics, University of Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland Tel +41-31-631 8937,Fax+41-31-631 3 7 6 5  21nstitute of High-Frequency Technology, Technical University of Braunschweig Schleinitzstrane 22, D-38 I06 Braunschweig, Germany The erbium 2.8-pm ZBLAN fiber laser is a promising candidate for surgical applications. Owing to pump excited-state absorption (ESA) from the lower laser level and subsequent competitive lasing at 850 nm [I] the 2.8-pm laser usually saturates at some 15 mW output power. Recently a cascade-lasing method has been demonstrated that overcomes the saturation and 150 mW output power have been achieved [ZJ However, this cascade laser is extremely difficult to adjust and requirements concerning the specifications of the mirror coatings are high. In this contribution an alternative approach is investigated. A significant quenching of the lower-level lifetime up to a factor of 30 by P?+ d o p i n g  has been reported for ZBLAN fibers 131. This quenching improves the 2.8-pm laser operation in these fibers [4]. An Er3+(50O0 ppm mol), P2+(300 ppm mol) ZBLAN fiber (length 28 an, core diameter IO mm, N.A. 0.4) is Tksapphire pumped at  792 nm. Mirror transmissions are 23 % at 2.8 pm and 72 % at 850 nm. The measured input-output curve at 2.8 pm IS shown in Fig. 1. The slope efficiency is improvd from approximately 8 % [I]  to 13 %by Pr3+wdoping. The output power, however, still saturates at 26 mW owing to competitive lasing at 850 nm. The improvement is not as significant as expected from the strong quenching of the lower-level lifetime. A calculation using the rate-equation system of [ I] and the measured Er3+ lifetimes for different Ps+ wnc. of [3] reproduces the input-output characteristics and the improvement of the laser operation when strongly quenching the lower-level lifetime, see Fig. 2. The improvement which is less than expted can be explained with Fig. 3. Although Pr3+ d o p i n g  strongly quenches the intrinsic lifetime ofthe lower laser level, its effective lifetime under lasing conditions is already reduced at high input power by pump ESA. Thus PP+ codoping additionally reduces the lifetime only slightly. The ESA is still strong enough to induce competitive Wig at 850 nm. A positive effect of PGt quenching is that laser threshold at 850 nm aad 2.8-pm output saturation occur at higher pump power. An increased Pr3+ concentration may further improve the laser performance. 111 S. Beda. M. Polhu, W. LOthy, H. P. Weber.Opr. Commun. 116,81 (1995). 121 U P o h u ,  Ch. Ghisler, 0. Bunea, M. Bun% W LUfiy, I t  P. Weber, Appl. Phys. Lett. 66,3564 (1995). 131 L. Wetenkamp, G. F. West. H T 6 b h .  I. NonCrystalllne Sol 140,25 (1992). 141 J Schneider, D. Hauschild, C. Frerichs, L Welenkhmp. Int. J. InfraredMillimkr Waves 15,1907 (1994) 0 100 200 300 4w 0 100 200 300 400 0 100 200 300 400 Launched Pump Power [mWl Launched Pump Power [mW Launched Pump Power [mw] FIG 1 Expenmental mput- FIG 2 Calculation 2 8-pm FIG 3 Effective lower-level output curve at 2 8 pm of an laser without (squares), wth lifetune wthout (squares) wth E?+ Pr3+ ZBLAN fiber with small (hamonds) and wth tugh small (dlamondr), and wth high small Pr3+ concentration (circles) Pr7+ conccntratlon (circles) ~ r 3 +  wnccntmtion Authorized licensed use limited to: UNIVERSITEIT TWENTE. Downloaded on May 28,2010 at 08:53:14 UTC from IEEE Xplore.  Restrictions apply. 

Erbium 3-µm lasers: Concepts of energy recycling

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