Investigation on thermal effects in high power Yb-doped fiber laser. Usha Chakravarty,* B. N. Upadhyaya, A. Kuruvilla, S. M. Oak

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1 Invetigation on thermal effect in high ower Yb-doed fiber laer Uha Chakravarty,* B. N. Uadhyaya, A. Kuruvilla, S. M. Oak Solid State Laer Diviion, Raja Ramanna Centre for Advanced Technology, Indore ABSTRACT We reent analyi of thermal effect in high-ower Yb-doed double-clad fiber laer. We have erformed imulation uing heat and rate equation to find the core temerature ditribution, and variation in oulation denity of energy level along the fiber length with um ower to acce the ower caling feaibility in exerimental configuration. Keyword lit: Fiber laer, Yb-doed fiber, Thermal effect, High ower fiber laer. 1. INTRODUCTION Ytterbium-doed double-clad fiber laer are making imreive contribution in high ower laer generation. They have attracted attention recently in commercial and military alication due to their high efficiency, comactne, high beam quality and large urface to volume ratio comared to their traditional counterart. Yb-doed fiber i very good candidate for the high ower alication becaue of the mall quantum defect which allow the otical to otical converion efficiency to exceed beyond 60%. Recently IPG hotonic ha reorted 10 kw of ingle mode outut ower with M 2 of 1.3 and multimode ower of 50 kw from Yb-doed fiber laer 1. In ite of fiber large urface area to volume ratio which i advantageou for cooling, at uch high average ower reidual heat become difficult to remove from the fiber. Mechanim that are reonible for heat generation inide the doing region are quantum defect, i.e. the energy difference between laer hoton and um hoton, which accumulate inide the ytem in the form of heat and quantum efficiency which i alway le than unity, o that a fraction of the excited electron decay by non-radiative relaxation and generate heat 2. The generated heat create a temerature gradient inide the fiber along the radial a well a longitudinal direction. Thi um-induced temerature gradient can caue a number of eriou roblem 3,5, uch a formation of thermal crack due to internal thermal tre and exanion, hortening of fiber lifetime due to damage of outer olymer jacket, even melting of the gla, degradation of laer beam quality due to thermal lening, and decreae of laer quantum efficiency. Heating alo affect the wave guiding and laing roertie of Yb-doed fiber due to variation of core temerature and oulation of the tark lit energy ublevel. Fig.1 how the energy level diagram of Yb ion in ilica. It oe imle atomic tructure, with only two rincile manifold 2 F 5/2 & 2 F 7/2. It i the ideal rare earth element for the laing a there i no excited tate abortion. Weak multihonon decay i the only non-radiative channel that exit and which i alo the caue of heating. Poulation of thee level follow Boltzmann ditribution. In thi aer,we have reented the exerimental reult of double end uming configuration. We have alo ued thermal conductive equation and the teady-tate rate equation to tudy thermal effect in high ower Yb-doed double-clad fiber laer in ingle (SEPC) and double end uming configuration (DEPC). 2. THEORETICAL BACKGROUND Steady tate rate equation for the linear cavity fiber laer are given a 4 N2 N = + [ P + P ] + [ P + P ]( σ Γ + σ ) hν A σaγ Γ + σa hν A hca a e Γ 1 Γ + + τ hca [ ] + ( λ) P ( z, λ) + P ( z, λ) λdλ [ ] + [ σe( λ) + σa( λ) ] P ( z, λ) + P ( z, λ) dp = Γ a 24 a e 2 λ dz [ σ N + { σ ( σ + σ )} N ] P α( z, ) P Photonic 2010: Tenth International Conference on Fiber Otic and Photonic, edited by Sunil K. Khijwania, Banhi D. Guta, Bihnu P. Pal, Anurag Sharma, Proc. of SPIE Vol. 8173, SPIE CCC code: X/11/$18 doi: / λdλ (1) (2) Proc. of SPIE Vol Downloaded From: htt://roceeding.iedigitallibrary.org/ on 02/19/2016 Term of Ue: htt://iedigitallibrary.org//termofue.ax

2 dp dz = Γ e a 2 a, e 2 0 z [{ σ ( λ) + σ ( λ) } N σ ( λ) N] P ( z λ) + Γσ ( λ) N P ( λ) α( z, λ) P (, λ) (3) Where N=N 1 + N 2 rereent the doing concentration of Yb 3+ ion in the core, with N 1 and N 2 a the lower and uer level oulation denitie; uercrit lu and minu rereent roagation along oitive and negative z-direction. σ and a σ are the abortion and emiion cro-ection with ubcrit and for the um and ignal, reectively, e and τ i the lifetime of the uer tate. Γ =A/A innerclad and Γ =P core / (P core +P clad ) rereent the ower fill factor for um and ignal, reectively. α i the cattering lo coefficient and σ 24 i the exited-tate abortion cro-ection. In eq. (3) 3 P 0( λ) = 2hc 2 / λ i the ower denity er unit wavelength correonding to the contribution of ontaneou emiion into the roagating laer mode. ν and ν are the um and ignal frequencie and A i the area of the fiber core. P and P rereent the ignal ower reectively. P ( z,λ) rereent the ignal ower denity er unit wavelength and i related to the ignal ower by P = P ( z, λ) Δλ, where Δ λ i the ignal bandwidth around the central laing wavelength. The above equation involve forward and backward roagating um and ignal ower along the fiber. Thee equation are ubject to the boundary condition: P P + ( 0 ) = R 1 P ( 0 ) + ( L ) = R P ( L ) 2 (4) Where, R1 and R 2 are the reflectivitie of the rear and outut couler mirror. Conidering the boundary condition of the laer cavity, we can numerically analyze the um and ignal ower ditribution along forward and backward direction. The um light i aborbed in the core by ytterbium ion in the core region thu there i heat ource in the core that doe not exit in the cladding region. Since the fiber length i much larger than the fiber cro-ection, the caability of heat diiation from the fiber end facet i a lot lower than that from the fiber ide. Therefore the tranvere and longitudinal temerature ditribution in the YDDC fiber at room temerature are governed by the following thermal conductive equation in the ymmetric cylindrical co-ordinate [4]. 1 Τ1 r r r r ( r, z) Q( r, z) = κ F ( 0 r r 1 ) (5) Τ r r r r 1 2 ( r, z) = 0 ( r r ) 1 r 2 (6) Where T 1 and T 2 are the temerature of the fiber core and fiber clad in the axial direction, reectively, r 1 and r 2 are the radiu of the fiber core and inner-clad, k F i the fiber thermal conductivity, Q ( r, z) i the heat diiated in the unit volume. Temerature continuity at the inner boundarie and Newton law of cooling are ued a the two boundary condition for determining the core temerature and they are given a, T1 ( ) ( ) ( r = r1 ) T2 ( r = r1 ) T1 r = r1 = T2 r = r1, = r r (7) T1 ( r = r1 ) H = [ Th T2 ( r = r2 )], r k F Where H i the convective coefficient, which i temerature deendent, T h i the temerature of the heat ink, which i taken a 298 K. Change in the refractive index due to temerature gradient i given by Δn r) = β ( T ( r) T ) ; β = I, II( I, II c dn dt (8) Clad and core region are rereented by I & II. Parameter ued in the imulation are taken from ref 3, 6. Proc. of SPIE Vol Downloaded From: htt://roceeding.iedigitallibrary.org/ on 02/19/2016 Term of Ue: htt://iedigitallibrary.org//termofue.ax

3 3. EXPERIMENTAL SETUP Exerimental et-u for high ower double-end uming of Yb-doed CW fiber laer conit of an Yb-doed doubleclad fiber of core diameter 10 µm with numerical aerture (NA) of 0.075, and an inner-clad diameter of 400 µm with a numerical aerture of Thi Yb-doed fiber had an octagonal inner-clad geometry and clad-um abortion of 0.8dB/m at 975 nm. Two 30 W fiber-couled laer diode having center wavelength of 975 nm at 25 C were ued to um 25 m length of the Yb-doed fiber from both the end. Thi reulted in abortion of about 20 db or 99% of um ower. The um laer outut from both the diode, having 200 µm core diameter fiber igtail with 0.22 NA, were collimated uing a len of 20 mm focal length and then focued uing another len of 20 mm focal length to have 1:1 image of the um fiber (igtail) end onto the inut end of the doed fiber. The doed fiber wa erendicularly cleaved at both the end to utain higher damage threhold. With thi uming configuration, um couling-efficiency of more than 85% wa achieved at both end of the doed fiber; the ret could be accounted for ~4% Frenel reflection loe from fiber end and other couling loe. Two dichroic mirror were ued in thi configuration: one of them ha high tranmiion at 975 nm and high reflection in the broad range nm for normal incidence; the other mirror ha imilar characteritic at 45 angle of incidence. The cleaved um inut end of the doed fiber with ~4% Frenel reflection act a the outut couler. For further exeriment, thermal effect are tudied theoretically for further deign and develoment of high ower Ybdoed fiber laer. 4. RESULTS AND DISCUSSION A maximum outut ower of 40.5 W wa achieved at an inut um ower of 58 W, with a loe efficiency of 71.7% and an otical-to-otical converion efficiency of 69.8%. Threhold um ower for laing wa about 1.3 W. The laer outut ectrum wa eaked at 1096 nm with a FWHM of 7 nm. The laer outut wa in ingle tranvere mode with diffraction limited beam quality, and wa emitted in a full cone angle of 150 mrad, defined by the NA of the doed fiber. Temerature rie i calculated by the above mentioned equation and found to be K at the fiber inut end. Thi i too mall an it effect can be neglected. For imulation, a large mode area Yb-doed fiber having core/inner clad diameter of 20µm/400µm, with 1.5 db/m abortion at 975 nm and a length of 13 m for efficient um abortion ha been ued. Laer emiion wavelength ha been taken a 1090 nm, which i exected from thi fiber. Thermal effect have been tudied for um ower in the range of 200 W to 1000 W for 975 nm um wavelength. In thi imulation, rie in fiber core temerature ha been conidered only due to the quantum defect. Fig.3 (a) & (b) how um ower and ignal ower ditribution along the length of the fiber for SEPC and DEPC. For both cae, outut ignal ower i nearly ame which i about 830 W at inut um ower of 1000 W but the um ower ditribution i different which give rie to different temerature ditribution inide the fiber. Fig.4 (a) & (b) how the core temerature ditribution for SEPC and DEPC from 200 W to 1000 W of um ower along the longitudinal direction. It how that at 1000 W of inut um ower for SEPC temerature at the um end i ~580 K and at other end it i at room temerature, where for DEPC (500 W from each end), rie in the core temerature at the um inut end i ~ K and at the center, it i ~ 327 K. Temerature gradient for SEPC i ~280 K and for DEPC it i ~115 K. Thi indicate a high temerature gradient along the ilica fiber in cae of SEPC comared to DEPC, which can degrade the fiber quality and laer beam quality due to thermal tree and variation in the effective index of the fiber mode along the fiber length. Quantum efficiency of the fiber laer alo decreae due to the decreae in the oulation of the lower laer level, becaue of temerature. Fig.5 (a) & (b) how change in the refractive index (RI) due to the temerature gradient. For 1000 W of um ower, due to change in the RI for SEPC V number become 3.2 and for the DEPC it attain the value of 3.1. Hence the fiber become multimode from ingle mode. Change in the oulation denity due to the rie in the temerature wa alo imulated uing Boltzmann ditribution equation. It wa found that for DEPC, there i about two time rie in the oulation of the level b at the um inut end. An increae in the oulation of level b will caue tronger reabortion in the hort wavelength region ( nm). Thi will make laing more difficult in hort wavelength region and imlify laing in long wavelength region having mall emiion cro-ection. Hence, otical-to-otical converion efficiency of fiber laer will decreae. Proc. of SPIE Vol Downloaded From: htt://roceeding.iedigitallibrary.org/ on 02/19/2016 Term of Ue: htt://iedigitallibrary.org//termofue.ax

4 5. CONCLUSION By olving the teady-tate rate equation, and heat equation temerature ditribution i calculated for the ingle end and double end umed configuration at different um ower for our fiber. Effect of the temerature gradient in the thermo-otical roertie of fiber laer i tudied. It i concluded that due to rie in the temerature, fiber become multimode from ingle mode a V number now exceed beyond Change in the oulation denity of level b i imulated for DEPC, which reult in decreae in the laer efficiency of the ytem. REFERENCES [1] IPG, hotonic [2] Sabaeian, M., Nadgaran,H., De Sario, M., Mecia, L., Prudenzano, F., Thermal effect on double clad octagonal Yb:gla fiber laer, Otical Material, vol.31, 1300(2009). [3] Brown, David C., Hoffman, Hanna J., Thermal, Stre, and Thermo-Otic Effect in High Average Power Double- Clad Silica Fiber laer, IEEE Jour.of Quant. Electronic, vol. 37, 207(2001). [4] Li Pingxue, Zhu Chen, Zou Shuzhen, Zhao Hong, Jiang Dongheng, Li Gang, chen Meng, Thoretical and exerimental invetigation of thermal effect in a high ower Yb 3+ doed double clad fiber laer, Otic & laer Technology, vol. 40, 360(2008). [5] Wong, Y., Xu, C., Po Hong, Analyi of Raman and Thermal effect in kilowatt fiber laer, Otic Communication, vol.242, 487(2004). [6] Uadhyaya, B.N., Chakravarty Uha, Kuruvilla, A., Oak, S.M., Shenoy, M.R., Thyagarajan, K., Self-uling characteritic of a high-ower ingle tranvere mode Yb-doed CW fiber laer, Otic Communication, vol.283, 2206(2010). Proc. of SPIE Vol Downloaded From: htt://roceeding.iedigitallibrary.org/ on 02/19/2016 Term of Ue: htt://iedigitallibrary.org//termofue.ax

5 g cm -1 2 F 5/2 f cm -1 e cm -1 2 F 7/2 d 1490 cm -1 c 1060 cm -1 b 600 cm -1 a 0 cm -1 Fig.1. Energy level diagram of Yb 3+ ion in ilica (a) Fig.2: Exerimental etu of Double end uming configuration Proc. of SPIE Vol Downloaded From: htt://roceeding.iedigitallibrary.org/ on 02/19/2016 Term of Ue: htt://iedigitallibrary.org//termofue.ax

6 1000 Pum ower Backward ignal Forward ignal 800 Power(W) (a) 800 Forward ignal ower 700 Backward ignal ower Backward um ower 600 Forward um ower Power(W) (b) Fig.3. Pum ower & ignal ower ditribution in (a) SEPC and (b) DEPC at 1000W of um ower. Proc. of SPIE Vol Downloaded From: htt://roceeding.iedigitallibrary.org/ on 02/19/2016 Term of Ue: htt://iedigitallibrary.org//termofue.ax

7 Temerature inide the core(k) 600 Pum Power 1000W W 600W W 200W (a) Temerature inide the core(k) Pum Power 1000W 800W 600W 400W 200W (b) Fig.4. Temerature ditribution inide the core in (a) SEPC and (b) DEPC form 200 W to 1000W of um ower. Proc. of SPIE Vol Downloaded From: htt://roceeding.iedigitallibrary.org/ on 02/19/2016 Term of Ue: htt://iedigitallibrary.org//termofue.ax

8 Change in the refractive index Pum Power:200 W Pum Power:400 W Pum Power:600 W Pum Power:800 W Pum Power:1000 W (a) Change in the refractive index Pum ower: 1000 W Pum ower: 800 W Pum ower: 600 W Pum ower: 400 W Pum ower: 200 W (b) Fig.5. Change in the refractive index inide the core in (a) SEPC and (b) DEPC form 200 W to 1000W of um ower Proc. of SPIE Vol Downloaded From: htt://roceeding.iedigitallibrary.org/ on 02/19/2016 Term of Ue: htt://iedigitallibrary.org//termofue.ax

9 Poulation denity of level b(m3) 6.50E E E E E E E E+024 Poulation denity 1000W 800W 600W 400W 200W room tem. 2.50E Fig.6. Poulation denity of level b v. fiber length in DEPC Proc. of SPIE Vol Downloaded From: htt://roceeding.iedigitallibrary.org/ on 02/19/2016 Term of Ue: htt://iedigitallibrary.org//termofue.ax