Design and Performance of an S-band Thulium doped Modified Silica Fiber Amplifier

K.P.W. Dissanayakel*, S.D. Emami4, H.A. Abdul-Rashidi, S.M. Aljamimi1, Z. Yusoff\ M.I. Zulkifli2, S.Z. Muhamad-Yassin2, K.A. Mat-Sharif- and N. Tamchek3

IFaculty of Engineering, Multimedia University, Cyberjaya, Selangor, Malaysia 2Advanced Physical Technologies, Telekom Research and Development Cyberjaya, Selangor, Malaysia

3Faculty of Science, Universiti Putra Malaysia, Serdang, Selangor, Malaysia 4Department of Physics, Faculty of Science, University of Malaya, Malaysia

*kasunprabu@gmail.com

Abstract- A new design of an S-band Thulium-doped modified

Silica fiber co-doped with aluminum is presented. The design

goal is high gain and low noise figure in the wavelength range of

1450 - 1520 nm. The optimization considers design parameters

such as the cut-off wavelength, dopant concentration, waveguide

structure, index profile and numerical aperture. These design

parameters are optimized to achieve long fluorescence lifetime,

high overlap factor and selected mode excitation. The amplifier

performance is theoretically modelled and simulated considering

the proposed design optimization. We show that the amplifier can

achieve a gain of 16 dB and 3dB noise figure.

Keywords- Fiber amplifier, S-band optical amplifier, thulium­

doped fiber amplifier (TDFA) .

1. INTRODUCTION

A tremendous increase in communication traffic in existing C-, L- communication bands in the past decade has led to utilize S-band region in Wavelength Division Multiplexing (WDM) networks and subsequently the development of S-band fiber amplifiers [1].

In this scenario, due to its high performance in Fluoride host [2], Thulium Doped Fiber Amplifier (TDF A) has emerged as a suitable candidate mainly due to its amplification bandwidth that is centered around 1470 nm [11] (which is in the S-band region). However, some downside problems have been reported in the field of fusion splicing between already developed silica based telecommunication transmission fibers and these Fluoride based TDF A. Therefore the design process has been directed towards development of TDF A in silica host [3]. However, the fluorescence lifetime of 3H4 energy level of Thulium is very low in a Silica environment due to Silica's high phonon energy which eventually leads to poor S-band gain of the amplifier. This effect could be compensated by introducing a co-dopant such as Aluminum (AI) during the doping process to reduce the phonon energy and thus, increase the upper state lifetime of 3H4 energy level for a higher amplifier gain [4]. In another approach to increase gain, efforts to suppress significant Amplified Spontaneous Emission (ASE) at both 8 �m, and 1.8 �m wavelength bands have also been made [6].

In this study, design of a new Thulium doped silica fiber is presented by co-doping with an appropriate concentration of aluminum to reduce the phonon energy of the silica host and also to achieve high gain, low noise figure and high overlap factor. In this context, we consider a 1050 run pump scheme.

978-1-4673-6075-3/13/$31.00 ©2013 IEEE

Other design parameters such as numerical aperture and core diameter are also calculated and optimized to match the above criteria.

II. DESIGN OF THULIUM DOPED FIBER

In this paper, we report the design of the Thulium-doped modified Silica fiber for S-band amplification considering the (1) concentration of modifying oxides (Ah03) and (2) refractive index profile. The first design parameter, concerns with the Thulium energy level 3H4 poor quantum conversion efficiency, high phonon energy silica host and short fluorescence lifetime. Meanwhile, the refractive index profile concerns with the cut-off wavelength (modes excited), mode field radius, signal and pump overlap factor. These design parameters are optimized to achieve high gain and low Noise Figure (NF) in the S-band window. The following sub­sections discuss these design parameters.

A. Concentration of modifying oxides For S-band amplification, the responsible stimulated

emission peak is a result of an energy transition between 3H4 and 3Fs [9]. However, the 3Hs level is very close to 3� and may lead to Non-Radiative Decay (NRD) particularly in high phonon energy glass hosts like Silica. This phenomenon is unfavourable to optical amplifiers because it leads to gain quenching. Furthermore, in high phonon energy glass hosts, the fluorescence lifetime of Thulium at 3� is as short as 14 J.ls [10]. To minimize the likelihood of NRD, we took the approach of modifying the local phonon energy surrounding the Tm3+ ions, as reported earlier by Peterka et. al. [6].

Oxide network modifiers such as Ah03, which have lower phonon energy, can be considered to lower the local phonon energy surrounding the Tm3+ ions. Based on earlier reports [4, 5, 7], introduction of 17.4 mol% of Ah03 led to an increase of Thulium fluorescence lifetime to 50 J.ls while 20 mol% of Ge02 increased the lifetime to only 28 J.ls. Based on the fabrication capability in our Modified Chemical Vapour Deposition (MCVD) and solution doping facility, the highest Al doping concentration observed was 5 mol%. For this aluminum concentration, the refractive index difference L'.n, can be estimated at 0.014.

Considering the high concentration of Ah03 that has priority over other design parameters, the refractive index

288

profile shall be designed around the desired Ah03 concentration. In our case, we propose the achievable Ah03 concentration to be 4.2 mol% based on our fabrication facilities.

B. Cut-off wavelength Considering a TDFA with 1050 run pump, the cut-off

wavelength is chosen based on the following: 1. To maximize the overlap factor and Thulium

absorption, the pump at 1050 run propagates in fundamental mode.

2. The S-band signal (1460 run - 1520 run) will be propagating in fundamental mode with minimal loss.

3. The 800 nm ASE propagates in higher order modes.

The number of modes at specific wavelength in a step­index fiber can be determined from the normalized frequency V, defined as:

v = (bra (NA))/A (1)

Where a is the fiber core radius, A is the signal wavelength, while the numerical aperture NA relates to the fiber refractive index profile. The fiber to operate in single mode, which means to propagate only the fundamental mode LP01, the V value should be below 2.405 [8].

Considering a TDF A with pump at 1050 run , the V parameter at 1000 run (the calculated cutoff wavelength) must be less than 2.405. This allows the pump to propagate in the fundamental mode and interact effectively with the Thulium ions doped in the core with a good overlap factor. The S-band signal will also propagate in the fundamental mode. In order for 800 run ASE to propagate in higher order modes, the V parameter at 800 run must be more than 2.405. The condition of V parameter above can be achieved by different combinations ofNA and core diameter a.

Fig.l shows the mapping of NA and core diameter a for the fiber where the V parameter conditions above are met. The value range of a and NA parameter are in accordance with the fabrication abilities of our MCVD facilities. The core diameter is set between 4 �m and 10 �m and the NA is set between 0.02 and 0.16. The shaded area in the middle of the map in Fig.l is where the combination of a and NA parameter satisfies the V parameter conditions above.

C. Overlap Factor The refractive index profile such as the index difference �n and core radius will subsequently affect the NA. Numerical Aperture is a very important parameter in Thulium doped fiber design as it determines the acceptance angle of light and coupling efficiency. NA will subsequently influence the Mode Field Radius (MFR) and also bending loss profile. NA is given by:

(2)

Eventually, MFR will influence the overlap factor, which is another dominant factor in amplifier performance. It demonstrates the percentage of mode that is covered in the core of the fiber. Overlap factor is given by:

Overlap Factor = 1 - exp {-(�)2 } (3)

Where rand ware defmed as fiber core radius and MFR of LP01 mode respectively.

10 ·

9 ..

6 ....

5 .

4 . o 0.02 0.04 0.06 0.08

NA 0.1 0.12 0.14 0.16

Fig. 1: Mapping of numerical aperture (NA) and core diameter. The shaded area indicates the NA - core diameter

pair that fulfills the design criteria

14e6

12e6

10e6 N E � 8e6

>-in lii 6e6 �

4e6

,--,-�-�-�-�-�-�-�-�---, 147 _._._._. refractive index (nght scale) -- Pump·Mode IntenSity. LP01 •••••••••. Signal.Mode IntenSity· LP01

1.468

-- ... _---

, '.

'.

Core Radius 4.3 �m 1466

MFR_Pump 393 �m

MFR_Signal 5 62 �m 1464 Overtap Factor Pump: 0 698 Overtap Faclor Signal 0443 1462

Fraction of Power in Core- Pump: 0 911 Fraction of Power in Core- Signal: OJ45 146

_._._._._._._._._._._._._._._._._._._._._._._. 1458

1.456

1454

radial position (�m)

Fig. 2: Mode Intensity and Refractive Index Profile.

III. RESULTS AND DISCUSSION

Considering the Ab03 concentration, refractive index profile, numerical aperture and overlap factor, the TDF

289

waveguide and amplifier performance was theoretically modeled and simulated.Fig.2 demonstrates the results of the fundamental mode (LPol) intensity of 1460 nm signal and the 1050 nm pump as a function of radial position. Both pump and signal were observed to be single mode. It also illustrates the refractive index profile obtained from the fiber preform, which is a graded index one though theoretically it was modeled as a step index profile.

In this study, the input signal power and TDF length are fixed at -30 dBm, and 20 m, respectively. The optimal core radius was found to be 4.3 f..lm according to the design parameters mentioned above. As shown in the figure, mode intensity of the pump was found to be higher than the signal mode intensity of LPol fundamental mode. The fluorescence lifetime of 3� energy level is 430 f..ls.

Based on this design, MFR of the pump and signal were found as 3.93 f..lm and 5.62 f..lm respectively. Overlap factor of the pump was calculated as 0.698 and for the 1460 nm signal, it was 0.443. Fraction of 0.911 of the pump power and fraction of 0.745 of the signal power were calculated to be propagating in the core region of the fiber.

20,---,---,----.---.----.---,---,----.---,---,

15

� 10

5

_ ..... -.­ "-'-

_._._.- Gain --- N oise Figure

" . '.

", -',

� 1� 1� 1� 1m 1m 1� 1� 1� 1m 1� Wavelength (nm)

Fig. 3: Gain and Noise Figure versus signal wavelength.

Fig. 3 shows the variation of gain and noise figure according to the signal wavelength, obtained through computer simulation. It was noticed that in S-band window of 1450nm-1500nm, with the above mentioned design parameters and a Thulium concentration of 1.56x 1025 ions/m3 , this amplifier could achieve a highest gain of 16 dB with a lowest noise figure of 3dB. The gain is highest around the signal wavelength of 1465 nm and gradually decreases towards the end of the S-band window with noise figure increasing inversely.

IV. CONCLUSION

The TDF A design is optimized based on numerical aperture, refractive index profile, overlap factor and co-dopant aluminum concentration to achieve high florescence lifetime of 430f..ls, high overlap factors of 0.674 and 0.405 for pump and signal respectively. The cutoff wavelength for the fundamental mode was decided as 1000nm to optimize the overlap factor at 1050 nm pump. The design also considered

NA and co-dopant concentration (Ah03) affects the final performance of the fiber amplifier profoundly. The TDF A was found, through theoretical modeling and simulation, a gain of 16 dB and Noise Figure of 3 dB.

V. ACKNOWLEDGEMENT

This work was funded by TMR&D through project RDTCl10766 'Design and Development of S-band Optical Amplifier Fiber'.

REFERENCES

[1] T. Sakamoto , S. Aozasa, M. Yamada, and M. Shimizu, "Hybrid fiber amplifiers consisting of cascaded TDFA and EDFA for WDM signals, "1. Lightlv. Techno!., vol. 24, no. 6, pp. 2287-2295, Jun. 2006.

[2] T. Komukai, "Upconversion pumped thulium-doped fluoride fiber amplifier and laser operating at 1.47 m," iEEE 1. Quantum Electron. ,vol. 31, no. I I, pp. 1880-1889, Nov. 1995.

[3] P. R. Watekar, S. Ju, and W. T. Han, "Analysis of 1064-nm pumped Tm­doped silica glass fiber amplifier operating at 1470 nm," 1. Lightlv.Techno!., vol. 25, no. 4, pp. 1045-1052, Apr. 2007.

[4] B. Faure, W. Blanc, B. Dussardier, and G. Monnom, "Improvement of the Tm3: 3H4 level lifetime in silica optical fibers by lowering the local phonon energy," 1. Non-Crystal!. Solids, vol. 353, no. 29, pp.2767-2773, Sep. 2007.

[5] B. Faure, W. Blanc, B. Dussardier, G. Monnom, and P. Peterka, "Thulium-doped silica-fiber based S-band amplifier with increased efficiency by aluminum co-doping," presented at the presented at the Opt. Amplif. Appl. Conf.llntegr. Photon. Res., Tech. Dig., San Francisco, CA, 2004.

[6] P. Peterka, B. Faure, W. Blanc, M. Karasek, andB.Dussardier, "Theoreticalmodelling of S-band thulium-doped silica fibre amplifiers," Opt. Quantum Electron., vol. 36, no. 1-3, pp. 201-212, 2004.

[7] P. Peterka, 1. Kasik, A. Dhar, B. Dussardier, and W. Blanc, "Theoretical modeling of fiber laser at 810 nm based on thulium-doped silica fibers with enhanced 3H4 level lifetime," Opt. Exp., vol. 19, no. 3, pp.2773-2781, 201 I.

[8] G. P. Agrawal, Fiber-Optic Communication Systems, 2nd ed. New York: Wiley, 1997.

[9] B.M. Antipenko et aI. , Soviet Journal of Quantum Electronics, 13, 558, 1983.

[IO]Siamak Dawazdah Emami, Hairul Azhar Abdul Rashid, S. Z. M. Yasin, K. A. M. Shariff, M. 1. Zulkifli, Zulfadzli Yusoff, Harith Ahmad, and Sulaiman Wadi Harun, "New Design of a Thulium-Aluminum-Doped Fiber Amplifier Based on Macro-Bending Approach", Journal of lightwave Technology, vol. 30, No. 20,pp.3263-pp.3272 October 15, 2012.

[II]P. R. Watekar and W. T. Han, "A small -signal power model for Tm­doped silica-glass optical fiber amplifier," iEEE Photon. Techno!. Lett.,

vol. 18, no. 19, pp. 2035-2037, Oct. 2006.

290