Characteristics of the erbium doped fiber amplifier with polarization mode dispersion compensation

(1)

Characteristics of the erbium doped ﬁber ampliﬁer

with polarization mode dispersion compensation

Jeng-Cherng Dung

*

_{, Sien Chi, Chia-Chu Chen}

Institute of Electro-Optical Engineering, National Chiao Tung University, 1001, Ta Hsueh Road, Hsinchu, Taiwan, ROC Received 8 January2003; received in revised form 18 April 2003; accepted 12 May2003

Abstract

We have experimentallydemonstrated the characteristics of the reflective type erbium-doped fiber amplifier (R-EDFA) which is composed of a circulator and a Faradayrotator mirror. It is shown that the gain spectrum of the R-EDFA is shifted to longer wavelength when compared with that of the conventional single-pass EDFA. For the R-EDFA, the forward pumping is better than the backward pumping in gain and noise figure. Besides, the reflective structure can be used as a broad band light source because its ASE spectrum is wider and the output power is higher. We also show that, byusing the reflective type configuration, the polarization dependent gain is reduced and the polarization mode dispersion of EDF can be completelycompensated but the polarization mode dispersion from the circulator is still survived.

Keywords: Erbium doped ﬁber ampliﬁer; Polarization mode dispersion; Polarization dependent gain

1. Introduction

Owing to its statistical nature, the polarization mode dispersion (PMD) appears to be a complex phenomenon limiting the performance of trans-mission system. As the bit rates increase (40 Gbit/s and higher), the PMD is becoming a major system impairment [1–3]. The erbium-doped fiber amplifi-ers (EDFAs) are indispensable tools for providing optical amplification in 1.5 lm fiber transmission

systems because of its excellent characteristics such as wide band, high gain, high saturation output, and low noise. However, EDFAs still have a few drawbacks arising from the polarization nature of the light [4]. The erbium-doped fiber (EDF) exhibits a difference of refractive index for the orthogonal polarization state, called birefringence, which re-sults in the PMD of EDFA. The maximum toler-able PMD value of transmission system should remain below about 10% of bit duration. The po-larization dependent gain (PDG) is another draw-back due to the anisotropic gain saturation in EDFAs [5]. The effect originates from the micro-scopic anisotropyof the erbium-ion susceptibility. The PDG results in undesirable fluctuation of the

www.elsevier.com/locate/optcom

*

Corresponding author. Tel.: 5731824; fax: +886-3-5716631.

E-mail address:jcdung@mail.nctu.edu.tw(J.-C. Dung).

(2)

signal-noise-ratio (SNR) at the output and de-grades the performance. Therefore, PMD com-pensation and PDG reduction are important topics for enabling satisfactorytransmission in a high bit rates transmission system with EDFAs [6,7].

The use of a Faradayrotator mirror (FRM) to stabilize the state of polarization (SOP) in the re-tracing path without maintaining the SOP of sig-nal has been known to be simple and effective [8,9]. The FRM, having a 45° single-pass rotation and reflector mirror, reflects the light in the orthogonal polarization state regardless of the incident state. The reflected light then makes a second pass through the amplifier. As a result, the SOP in the backward direction is orthogonal to that in the forward direction everywhere regardless of any fiber birefringence. In this paper, we present the characteristic of a reflective type erbium-doped fi-ber amplifier (R-EDFA) employing a circulator and a FRM. The gain spectrum of the R-EDFA is shifted to longer wavelength when compared with that of the conventional single-pass EDFA. In the R-EDFA, the forward pumping is better than the backward pumping and the optimum length is about 2/3 of the optimum length of single-pass EDFA. The reflective structure can be used as a broad band light source because its amplified spontaneous emission (ASE) spectrum is wider and the output power is higher. Byusing the re-flective type configuration, the polarization mode dispersion of EDFA is almost compensated and the polarization dependent gain is reduced.

2. Numerical modeling

The EDFA can be modeled as a homoge-neouslybroadened two-level system. The spectra of the absorption cross-section (ra) and emission

cross-section (re) of the Al co-doped EDFA are

shown in Fig. 1. The ASENs are assumed to be the optical beams of eﬀective frequencybandwidth Dmkcentered at the wavelength kk ¼ c=mk to resolve

the ASEN spectrum. Under the steady-state con-dition, the equations to describe the spatial de-velopment of the pump power (PP), signal power

(Ps), and ASEN power (Pk, k¼ 1; . . . ; N ) in the

EDFA can be written as [10]:

udP P dz ¼ ðrepN2 rapN1ÞCPP P; ð1Þ udP s dz ¼ ðresN2 rasN1ÞCsP s ; ð2Þ udP k dz ¼ ðrekN2 rakN1ÞCkP k þ 2rekN2CkhmkDmk aiPPk; ð3Þ

where N1 and N2 are the population densities of

the ground level and metastable level, rej, raj, Cj,

aP, and hmj are the emission cross-section,

ab-sorption cross-section, conﬁnement factor, intrin-sic ﬁber loss, and photon energy, respectively. The superscript (Þ designates the optical beam prop-agating along z direction and u_{¼ 1. The}

coupled Eqs. (1)–(3) are numericallysolved with the absorption cross-section (ra) and emission

cross-section (re) given in Fig. 1. We use 5601

points to sample the ASEN spectrum, which cor-responds to the spacing of Dk¼ 0:025 nm.

3. Experimental setup

Fig. 2(a) shows the experimental R-EDFA using forward pumping conﬁguration. An optical circu-lator places between the isocircu-lator and the wave-length division multiplexing coupler connected to one end of EDF. The optical signals generated from tunable laser input the EDF from port 1 to port 2 of the optical circulator. After optical signal has been ampliﬁed during transmission in the EDF,

Fig. 1. The absorption cross-section and emission cross-section of an Al co-doped EDFA.

(3)

the FRM at the other end of EDF reflects back the signals in the orthogonal polarization state. Then the signal is amplified in the EDF again and exits from port 2 to port 3 of the circulator. The re-flectivityof the FRM is 95%. The optical circula-tor has a loss of about 0.7 dB and an isolation of over 47 dB. About 200 mW pump power is laun-ched into the EDF from a 980 nm laser diode. The EDF we used is an Al co-doped fiber with a core radius of 1.44 lm and an NA of 0.223. The erbium concentration is 8.55 1024 _m3_{. We measure the}

signal gain and the noise figure characteristics of both the reflective type configurations and the single-path one which could be easilymodified from the reflective type ones by replacing the end FRM with an optical isolator. The backward pumping configuration is shown in Fig. 2(b). The characteristics of the backward pumping will be compared with that of the forward pumping. The initial length of the EDF we used is 15 m which is the optimum length of the single-pass with )10 dBm input signal and 200 mW pump power.

4. Results and discussion

The measured results of forward pumping by scanning the wavelength of input signal with in-tensityof )10 dBm are shown in Fig. 3. We measure signal gain and noise figure characteristics of both the reflective type configuration and the

single-path one with 15 m EDF. The numerical results of the 15 m single-path and the 11 m re-flective type are also shown in Fig. 3. We cut-back the EDF to measure the characteristics of the R-EDFA with different lengths of EDF. Among all the gain results of different lengths R-EDFA, the optimum length of EDF is about 11 m, so the amplified length is 22 m. Bycomparing the gain results of the 11 m reflective type with those of the 15 m single-path, the gain spectrum of the reflec-tive type is broader and smoother. The gain values of the R-EDFA exceed those of the single-pass EDFA at the signal wavelength longer than 1538 nm. It is noticed that the spectrum of the reflective type is shifted to longer wavelength about 8 nm. The characteristics are mainlyattributed to the longer amplifying EDF length of the reflected type. However, its noise figures of the R-EDFA are 1.2 dB higher than those of the single-pass EDFA over the whole wavelength regime because the backward stronglyamplified signals and the reflective ASEN due to FRM make the population inversion in the input of EDF low. Fig. 4 shows the gain and noise figure of the backward pump-ing EDFA. The signal gain of the backward pumping R-EDFA appears to be 1 dB lower than that of the forward pumping one. The noise fig-ure performance is higher than 2 dB over most of the measured wavelength regime. Thus, the forward pumping configuration is more suitable to

Fig. 3. The measured gain and noise ﬁgure of forward pump-ing; dotted line, numerical result of 15 m single-path EDFA; solid line, numerical result of 11 m R-EDFA; (j) measured results of 15 m single-path EDFA; (N) measured results of 11 m R-EDFA.

Fig. 2. The R-EDFA conﬁguration using (a) forward pumping, (b) backward pumping.

(4)

the R-EDFA. Besides, we also find that there is another merit for the reflective structure. With 15 m EDF and 50 mW pump power, Fig. 5 shows the ASE spectrums of the single-pass and the reflective type. The total output ASE powers are 7.04 and )0.63 dBm for the reflective type and the single-pass, respectively. Hence, the reflective structure can be used as a broad band light source because its ASE spectrum is wider than that of the single-pass. We use the Poincaree sphere method to measure the PMDs of the single-pass EDFA and the R-EDFA with a FRM [11,12]. The tunable laser of fixed polarization state is used as a light source.

We tune the wavelength of light source and mea-sure the stokes vector of polarization state after EDFA with polarimeter to obtain the PMD val-ues. Fig. 6 shows the stokes vector S2versus input

wavelength. From the stokes vector measure-ments, the PMD values are 0.97 and 0.074 ps for the single-pass EDFA and the R-EDFA with a FRM, respectively. Therefore, with a FRM in the R-EDFA, the PMD is nearlycompensated but survives 0.07 ps. If we replace a FRM with a re-flective mirror in the rere-flective EDFA, the PMD is 1.37 ps because the changes of EDF length should result in different PMD value.

Fig. 4. The measured gain and noise ﬁgure of backward pumping; dotted line, numerical result of 15 m single-path EDFA; solid line, numerical result of 11 m R-EDFA; (j) measured results of 15 m single-path EDFA; (N) measured results of 11 m R-EDFA.

Fig. 5. The ASE spectrums of (a) single-pass and (b) reﬂective type with 15 m EDF and 50 mW pump power.

Fig. 6. The measurement of stokes vector S2 versus input

(5)

Employing a circulator and a FRM in EDFA, it has been demonstrated that the PMD can be compensated both in the circulator and in the EDF [9]. We will investigate it further. We mea-sure the PMD of the R-EDFA with different EDF lengths. The results are shown in Fig. 7, the PMD values for different EDF length are around 0.07 ps. For comparison, we measure the PMD of a cir-culator, from port 1 to port 2 and reflecting by FRM then exiting from port 3, and the value is 0.07 ps. Therefore, the survived PMD value of the R-EDFA is all from the circulator. It is apparent that the reflecting signal byFRM will not pass through port 2 to port 1 again. So the input and output paths of the circulator are different and the PMD of a circulator cannot be compensated. Thus, using a FRM in the R-EDFA, the PMD of EDF is completelycompensated and the PMD from the circulator is still survived. Furthermore, when we find a circulator which has the same PMD value and the parallel principal states within both port 1) port 2 and port 2 ) port 3, the PMD value of the circulator with a FRM will be zero.

We also demonstrate a R-EDFA that incorpo-rates a FRM to passivelycancel the PDG pro-duced bythe gain anisotropy. The cancellation of PDG was demonstrated bymeasuring the depen-dence of gain on the polarization states of a sat-urating signal beam. The polarization states are set byplacing a linear polarizer after tunable laser and

using polarization controller to scan the states of linear polarization. The input probe signal beam, 0 dBm at 1550 nm, is detected at the amplifier out-put using a power meter. The measured gains versus the input polarization states are shown in Fig. 8. The measured data points are the average values of six measurements. The PDGs of the single-path EDFA and the R-EDFA with a FRM are 0.08 and 0.017 dB, respectively. It shows clearlythat the effect of reducing PDG bythe FRM which reflects the signal with a 90° rotation in the EDFA.

5. Conclusions

In conclusion, we have presented the charac-teristics of gain and noise figure of the R-EDFA employing a circulator and a FRM. Due to the longer amplifying length of the R-EDFA, the signal gain is better but the noise figure is worse than the conventional single-pass EDFA. The gain spectrum is shifted to longer wavelength and the forward pumping is more suitable to the R-EDFA. Besides, the reflective structure can be used as a broad band light source because its ASE spectrum is wider and the output power is higher. Using a FRM in the R-EDFA, the PMD of EDF is com-pletelycompensated and the PMD from the cir-culator is still survived. The reflective method of suppressing PMD is an easyimplement and is beneficial to reduce the PDG in EDFA lightwave transmission systems.

Fig. 7. The PMD values for R-EDFA with diﬀerent EDF length.

(6)

Acknowledgements

This research was supported bythe National Science Council of ROC under Contract NSC 91-2215-E-009-054 and the Academic Excellence Program of ROC. Ministryof Education under Contract 91-E-FA06-1-4-91X023.

References

[1] C.D. Pool, J. Nagel, in: Optical Fiber Telecommunications IIIA, Academic Press, New York, 1997.

[2] D. Sobiski, D. Pikula, J. Smith, C. Henning, D. Chowdh-ury, E. Murphy, E. Kolltveit, F. Annunziata, Electron. Lett. 37 (2001) 46.

[3] N.Y. Kim, D. Lee, H. Yoon, J. Park, N. Park, IEEE Photon. Technol. Lett. 14 (2002) 104.

[4] H. Taga, N. Edagawa, S. Yamamoto, S. Akiba, J. Lightwave Technol. 13 (1995) 829.

[5] M.G. Taylor, IEEE Photon. Technol. Lett. 5 (1993) 1244. [6] C.R. Giles, Electron. Lett. 30 (1994) 976.

[7] C. Francia, F. Bruyeere, J.P. Thieery, D. Penninckx, Elec-tron. Lett. 35 (1999) 414.

[8] A.D. Kersey, M.J. Marrone, M.A. Davis, Electron. Lett. 27 (1991) 518.

[9] S. Yamashita, K. Hotate, M. Ito, J. Lightwave Technol. 14 (1996) 385.

[10] E. Desurvire, Erbium-Doped Fiber Ampliﬁers, Wiley, New York, 1994.

[11] D. Andresciani, F. Culti, F. Matera, B. Danino, Opt. Lett. 12 (1987) 844.

[12] Y. Namihira, K. Nakajima, Electron. Lett. 30 (1994) 1157.