Miniscalco, J. W.

Pedersen, B.

Quimby, R. S.

Experimental parameters have been measured and used in a quantitative model of Pr3+-doped fluorozirconate fiber amplifiers. The optimum cutoff wavelength was determined to be 800 nm and the gain for 400 mW of pump was found to increase from 12 to 34 dB if the NA was increased from 0.15 to 0.25. Lengthening the metastable state lifetime from 110 to 300 μs would significantly improve amplifier performance while concentration quenching can appreciably degrade i

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S.Published in:I E E E Photonics Technology LettersLink to article, DOI:10.1109/68.136481Publication date:1992Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Pedersen, B., Miniscalco, J. W., & Quimby, R. S. (1992). Optimization of Pr3+:ZBLAN fiber amplifiers. I E E EPhotonics Technology Letters, 4(5), 446-448. DOI: 10.1109/68.136481446 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 4, NO. 5, MAY 1992 GaAsP optical amplifiers using picosecond optical pulses,” IEEE J. Quantum Electron., vol. 25, pp. 2611-2620, Dec. 1989. [5] T. Saitoh and T. Mukai, “Gain saturation characteristics of traveling-wave semiconductor laser amplifiers in short optical pulse amplification,” IEEE J. Quantum Electron., vol. 26, pp. [6] Y. Lai, K. L. Hall, E. P. Ippen, and G. Eisenstein, “Short pulse gain saturation in InGaAsP diode laser amplifiers,” IEEE Photon. Tech- nol. Lett., vol. 2, pp. 711-713, Oct. 1990. M. Willatzen, A. Uskov, J. Mdrk, H. Olesen, B. Tromborg, and A.-P. Jauho, “Nonlinear gain suppression in semiconductor lasers due to camer heating,” IEEE Photon. Technol. Lett., vol. 3, pp. 2086-2094, Dec. 1990. [7] 606-609, July 1991. [8] B. N. Gomatam and A. P. DeFonzo, “Theory of hot carrier effects on nonlinear gain in GaAs-GaAlAs lasers and amplifiers,” IEEE J. Quantum Electron., vol. 26, pp. 1689-1704, Oct. 1990. [9] N. Ogasawara and R. Ito, “Longitudinal mode competition and asymmetric gain saturation in semiconductor injection lasers. 11. The- ory,” Jap. J. Appl. Phys., vol. 27, pp. 615-626, Apr. 1988. G. P. Agrawal, “Effect of gain dispersion on ultrashort pulse ampli- fication in semiconductor laser amplifiers,” ZEEE J. Quantum Elec- tron., vol. 27, pp. 1843-1849, June 1991. K. L. Hall, J. Mark, E. P. Ippen, and G. Eisenstein, “Femtosecond gain dynamics in InGaAsP optical amplifiers,” Appl. Phys. Lett., vol. 56, pp. 1740-1742, Apr. 1990. [lo] [ l l ]  Optimization of Pr 3+:ZBLAN Fiber Amplifiers B. Pedersen, W. J .  Miniscalco, and R. S .  Quimby Abstract-Experimental parameters have been measured and used in a quantitative model of Pr 3+-doped fluorozirconate fiber amplifiers. The optimum cutoff wavelength was determined to be 800 nm and the gain for 400 mW of pump was found to increase from 12 to 34 dB if the NA was increased from 0.15 to 0.25. Lengthening the metastable state lifetime from 110 to 300 ps would significantly improve amplifier performance while con- centration quenching can appreciably degrade it. INTRODUCTION ECENTLY, Pr 3+-doped fluorozirconate fiber amplifiers Rh ave been demonstrated to provide gain in a broad band centered on the 1300-nm window [1]-[3]. Net gains of = 30 dE? have been achieved, but the low quantum efficiency of the ‘G4 upper level in the gain transition leads to very high pump power requirements [4], [5]. Although experiments have revealed the importance of waveguide parameters in reducing the required pump power, until now there has not been a systematic analysis of the performance improvements that can be achieved through fiber design or the use of host glasses providing a longer lifetime for the metastable state. To this end we have examined the effects of cutoff wave- length, numerical aperture (NA), and ’G4 lifetime ( 7 )  on the performance of Pr +-doped fluorozirconate fiber amplifiers operated in the small-signal regime. The quantitative numeri- Manuscript received December 10, 1991; revised March 2, 1992. B. Pedersen is with the Technical University of Denmark, Electromagnet- ics Institute, Center for Broadband Telecommunications, DK-2800 Lyngby , Denmark. W. J .  Miniscalco is with GTE Laboratories Incorporated, Waltham, MA 02254. R. S. Quimby is with the Department of Physics, Worcester Polytechnic Institute, Worcester, MA 01609. IEEE Log Number 92001 18. cal model used included excited state absorption (ESA) and ground state absorption (GSA) at the signal wavelength as well as the full spectrum of amplified spontaneous emission (ASE) at the signal and pump wavelengths. The calculated small-signal gains are in good agreement with recent experi- mental results and indicate that further improvement can be obtained. MODEL FOR Pr +-DOPED FIBERS Fig. 1 illustrates the relevant energy levels and transitions included in the calculation. Level ‘G4 is excited directly by pumping from the ’“4 ground state, and gain at = 1310 nm is obtained by stimulating the ‘G4 j 3 H S  transition. How- ever, the IC4 -+ID2 transition produces an ESA band peak- ing at = 1380 nm and has a short wavelength tail down to = 1290 nm. An additional problem is the 3H4 j3F4 transi- tion that peaks at = 1440 nm and leads to ground state absorption of the signal for X 1 1290 nm. Fig. 2 shows the cross sections for the transitions indicated by arrows in Fig. 1. The GSA cross sections were determined from absorption measurements on bulk Pr3+-doped ZBLAN samples. The stimulated emission and ESA cross sections at the signal wavelength are those reported by Quimby and Zheng [6] .  The stimulated emission cross sections at the pump band (= 1050 nm) were obtained from a McCumber analysis [7] and scaled using the radiative rate of Ohishi et al. [l]. The numerical model is based on one used previously for Er3+- doped fiber amplifiers [8], which was adapted to the steady state rate equations for the excited ‘G4 state and the ground ’4 state. The resulting model is almost equivalent to the two-level model for Er3+-doped fiber amplifiers pumped at 1480 nm, except for the fact that at the signal wavelength one must consider ESA and the GSA of the signal does not contribute to the population in the upper laser level. The 1041-1135/92$03.00 0 1992 IEEE Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on November 20, 2009 at 05:32 from IEEE Xplore.  Restrictions apply. PEDERSEN et al.: Pr'+:ZBLAN FIBER AMPLIFIERS 447 l6 t 7r 12 E 1380 n m  ESA 1 GSA 1 1 Emission [Emission SA Fig. 1 .  Energy level diagram for Pr3+ with relevant transitions illustrated by arrows. 5 "E b 4  d 0 1500 Wavelength (nm) Fig. 2 .  Cross section spectra for the transitions included in the model. model includes the wavelength-dependent ASE around 13 10 and 1050 nm. To verify the model, we used the fiber parameters for a Pr3+:ZBLAN amplifier reported by Miyajima et al. [5] ,  and compared the calculated to the measured results. Fig. 3 plots the small-signal gain as function of pump power for a fiber with 2000 ppm-wt Pr3+, cutoff wavelength of 1260 nm, an NA of 0.41, and length of 8 m [ 5 ] .  Note that the measure- ments lie above the theoretical curve. This results from the experimental points being determined by adding = 6 dB of fiber loss to the net fiber gain [9]. Of this = 0.75 dB/m loss, 0.6 - 0.7 dB/m corresponds to GSA from the Pr3+ 3H4 -+ 3F4 transition, consistent with our calculated value of 0.7 dB/m at 1310 nm. As was pointed out by Ohishi et al. [4], this GSA should not be added to the observed gain to determine the fiber gain since it represents loss due to unexcited ions which is bleached in a highly excited ampli- fier. Accordingly, to obtain the true fiber gain the experimen- tal values reported must be reduced by = 6 dB, bringing them into agreement with the calculation. We also simulated the experimental results reported by Ohishi et al. [4]. The inset in Fig. 3 displays gain versus signal wavelength for 925 mW of pump power at 1017 nm. Here the fiber is doped with 500 ppm-wt Pr3+, has a cutoff wavelength of 650 nm, an NA of 0.16, and a length of 23 m [4]. The crosses are the experimental results [4], and the solid curve is the calcula- tions. The good agreement observed between calculations and measurements gives us confidence that the model can be used in a quantitative analysis of Pr3+-doped fiber amplifiers. ANALYSIS We have used the model to quantitatively examine the effects of waveguide design and 'G4 lifetime for Pr3+-doped ZBLAN fiber amplifiers. Initially the small-signal gain at 1310 nm was calculated for step-index fibers pumped codi- rectionally at 1017 nm for several pump powers. The fiber NA's ranged from 0.2 to 0.5 and the results indicated that the optimum cutoff wavelength was = 800 nm and was almost independent of the NA and pump power. Fig. 4 plots the small-signal gain versus the NA for codirectional pumping with four different pump powers and the optimum cutoff wavelength of 800 nm. Each point on the curves is for the fiber length giving maximum gain. Scattering loss in the fiber has been ignored since it is typically ~ 0 . 1  dB/m with prospects for still further improvement. The gain increases with increasing NA due to the improved overlap between the Pr3+ ions and the pump and signal modes. For 50 mW of pump power gains higher than 10 dB may not be practical since the NA would have to be >0.4. For pump powers of 100 and 200 mW the gain increases almost linearly with NA for NA < 0.35. For 400 mW of pump powers the biggest improvement is found when increasing the NA from 0.15 to 0.25, which roughly triples the gain in dB. It is observed that the gain curves bend over for high NA's. This is because the amplifier is saturated by ASE around 1310 nm. The efficiency of Pr3+:ZBLAN fiber amplifiers is limited by the low quantum efficiency of the metastable state. The 'G4 level experiences rapid nonradiative decay to the 3F4 through multiphonon emission, leading to an observed life- time of 110 ps compared to a radiative lifetime of 3.2 ms [ 11. The nonradiative relaxation rate of the 'G4 can be decreased through the use of other glass hosts with either a lower effective phonon frequency or weaker electron-phonon cou- pling. We have examined the potential improvement in am- plifier performance that could be realized through the resul- tant increase in lifetime 7 of the metastable state. Fig. 5 plots the small-signal gain as function of 7 for pump powers of 100 and 250 mW. The calculations used the cross sections for Pr3+:ZBLAN and represent the effect of changing only the nonradiative rate. For the conditions examined, the gain is very sensitive to T for 20 < T < 400 ps. If 7 could be increase to 300 ps without changing the cross sections, only 100 mW of pump power would be required to obtain 30 dB gain compared to 250 mW for Pr3+:ZBLAN at room temper- ature. As a result of cross relaxation, the lifetime of the 'G4 decreases with increasing Pr3+ concentration above 2: 1000 ppm-wt for fluorozirconate glasses [4]. Fig. 5 also illustrates how using higher concentrations to reduce the fiber length and associated scattering loss will reduce the gain. The Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on November 20, 2009 at 05:32 from IEEE Xplore.  Restrictions apply. 448 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 4, NO. 5 ,  MAY 1992 0 50 100 150 200 250 300 Pump Power (mw) Fig. 3. Gain versus launched pump power. Crosses are measurements reported by Miyajima et al. [5]. The solid curve is simulations. Inset: gain versus signal wavelength. Crosses are measurements reported by Ohishi et al. [4], and the solid curve is simulations. degradation is seen to be quite severe: cutting the lifetime in half will cut the gain in dB by a factor of two. As in Fig. 4, the gain curve bends over due to ASE. CONCLUSION A model has been developed for Pr3+-doped fiber ampli- fiers that provides small-signal gains in good agreement with reported experimental results. It has been used to optimize waveguide design and predicts an optimum cutoff wave- length of 800 nm for step-index fibers that is insensitive to NA and launched pump power. For 50 mW of pump power, gains > 10 dB can be obtained only for NA’s >0.4. For pump powers I 200 mW and NA’s < 0.35, the small-signal gain in dB increases linearly with the NA. For 400 mW of pump power the gain can almost be tripled by increasing the NA from 0.15 to 0.25. Increasing the metastable state life- time to 300 ps would reduce to pump power required to achieve 30 dB gain from 250 to 100 mW. ACKNOWLEDGMENT We wish to thank B. T. Hall and R. C. Folweiler for synthesizing the Pr 3+-doped ZBLAN samples. REFERENCES [l] Y. Ohishi, T. Kanamori, T. Kitagawa, S. Takahashi, E. Snitzer, and G. H. Sigel, “Pr3+-doped fluoride fiber amplifier operating at 1.3 p n , ”  in Techn. Dig. OFC’91, San Diego, CA, 1991, postdeadline paper PD2. Y. Durteste, M. Monirie, J .  Y. Allain, and H. Poignant, “Amplifica- tion and lasing at 1.3 pm in praseodymium-doped fluorozirconate fibres,” Electron. Lett., vol. 27, pp. 626-628, 1991. S. F. Carter, D. Szebesta, S. T. Davey, R.  Wyatt, M. C. Brierley, and P. W. France, “Amplification at 1.3 pm in a Pr3+-doped single-mode fluorozirconate fibre,” Electron. Lett., vol. 27, pp. [2] [3] 628-629, 1991. 50 Cut-off wavelength 800 nm 0.1 0.2 0.3 0.4 0.5 NA Fig. 4. Small-signal gain versus NA for step index fibers and codirectional pumping. The signal wavelength is 1310 nm and the pump wavelength is 1017 nm. _ _ _ - - - -  10 100 loo0 7 (PS) Fig. 5. Small-signal gain versus the observed lifetime, 7, for the IC4 level. The launched pump powers are indicated. [4] Y. Ohishi, T. Kanamori, T. Nishi, and S. Takahashi, “A high gain, high output saturation power Pr 3f-doped fluoride fiber amplifier operating at 1.3 pm,” Photon. Technol. Lett., vol. 3, pp. 715-717, 1991. [5] Y. Miyajima, T. Sugawa, and Y. Fukasaku, “38.2 dB amplification at 1.31 pm and the possibility of 0.98 pm pumping in Pr3+-doped fluoride fiber,” in Proc. Optical Ampl. Appl., Snowmass Village, CO, 1991, postdeadline iapers, paper PDl. R. S. Quimby and B. Zheng, “Excited state absorption in Pr3+-doped fluorozirconate glass,” Appl. Phys. Lett., Mar. 1992. W. J. Miniscalco and R. S. Quimby, “General procedure for the analysis of Er3+ cross sections,” Opt. Lett., vol. 15, pp. 258-260, 1991. [8] B. Pedersen, A. Bjarklev, J. H. Povlsen, K. Dybdal, and C. C. Larsen, “The design of erbium-doped fiber amplifiers,” J. Light- wave Technol., vol. 9, pp. 1105- 11 12, 1991. [6] [7] [9] Y. Miyajima, Private communication. Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on November 20, 2009 at 05:32 from IEEE Xplore.  Restrictions apply. 

Optimization of Pr3+:ZBLAN fiber amplifiers

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