improvement for L-band EDFA's, regardless of pump wavelength choice. Index Termsâ Erbium-doped fiber amplifiers (EDFA's), optical fiber communication ...
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 11, NO. 1, JANUARY 1999
Enhancement of Power Conversion Efficiency for an -Band EDFA with a Secondary Pumping Effect in the Unpumped EDF Section Juhan Lee, Uh-Chan Ryu, Student Member, IEEE, Seong Joon Ahn, and Namkyoo Park, Member, IEEE
Abstract—A novel structure, which utilizes detrimental backward amplified spontaneous emission as a secondary pump source is suggested for a silica-based fiber amplifier, operating at a wavelength range from 1570 to 1610 nm. By using the secondary pumping effect from the strong, wasted 1550-nm band amplified spontaneous emission power in the unpumped section of the erbium-doped fiber, it was possible to achieve a considerable improvement in power conversion efficiency, increasing smallsignal gain by more than 4 dB. The suggested pump structure was also shown to be useful in overall conversion efficiency improvement for L-band EDFA’s, regardless of pump wavelength choice. Index Terms— Erbium-doped fiber amplifiers (EDFA’s), optical fiber communication, wavelength-division-multiplexed (WDM) systems.
I. INTRODUCTION
I
N WAVELENGTH-DIVISION-MULTIPLEXED (WDM) transmission systems and their related optical networks, one of the key technological issues is the achievement of broad and flat gain bandwidth for erbium-doped fiber amplifiers (EDFA’s) [1]. This trend of fast bandwidth consumption, as a natural extension, has resulted in the latest interest in optical amplifiers operating at longer wavelengths, which a conventional EDFA cannot handle. One solution to this demand includes a new material composition for the fiber, such as tellurite-based EDFA’s [2]. Even though the telluritebased amplifier has attracted much attention, being capable of meeting those requirements, the relatively nonuniform gain spectrum and immaturity of the related research still make immediate, practical applications somewhat difficult. Other than solutions involving new material composition, most efforts have been focused on silica-based erbium-doped amplifiers with various structures [3], [4]. For these approaches, relatively longer EDF’s are commonly used with high pump power sources, to adjust the population inversion of the EDF at an approximately 30%–40% level, and correspondingly to get the amplification in a long wavelength range (1570–1610 nm, called -band). By using this -band amplifier in parallel with the conventional (called -band, 1530–1560 nm) one, Manuscript received September 3, 1998; revised October 2, 1998. J. Lee, U.-C. Ryu, and N. Park are with the Optical Communication Systems Laboratory, School of Electrical Engineering, Seoul National University, Seoul 151-742, Korea. S. J. Ahn is with the Korea Electric Power Research Institute, Yusong, Taejon 305-380, Korea. Publisher Item Identifier S 1041-1135(99)00354-7.
the silica-based EDFA can now operate with a gain bandwidth of more than 80 nm, providing a wide open window for future high capacity WDM transmission systems [5]. However, the relatively short history of the development of the -band amplifier still retains several issues for further optimization, including long EDF length and high pump power from lowconversion efficiency [6]. In this letter, we demonstrate for the first time, a novel, highly efficient EDFA structure for 1570–1610-nm band signal amplification, which uses detrimental 1550-nm band amplified spontaneous emission (ASE) as the secondary pump source for the unpumped EDF section of the amplifier. Experimental results show dramatic increases in power conversion efficiency (maximum from 11.7% to 25.7%) and small-signal gain (4 dB maximum) when compared to other EDFA structures with the same pump power and EDF length, with a relatively small penalty on noise figures. II. EXPERIMENT
AND
RESULTS
Fig. 1 shows the experimental arrangements of four types of -band silica-based EDFA’s, which were used for power conversion efficiency comparison of different structures. Fig. 1(a) and (b) is conventional forward (here called Type I) and backward pumped (here called Type II) EDFA’s, and the suggested amplifier structures with the unpumped section of EDF’s are illustrated in Fig. 1(c) and (d). For the configuration of Fig. 1(c) (here called Type III), input signals pass through the unpumped EDF section (EDF II) first, and then enter the forward-pumped EDF section (EDF I). The variant of Type III, called Type IV [Fig. 1(d)] has been arranged so that amplified signals from the backward pumped EDF section (EDF I) enter the unpumped EDF section (EDF II) later. For the purpose of objective comparison, all four configurations were constructed with identical EDF’s. The EDF used in the experiment was a commercially available, Al-codoped one which has a peak absorption coefficient of 4.5 dB/m at 1530 nm. Different lengths of EDF at 0, 5, 15, 20, 25, and 35 m were used in Section II to see the power conversion efficiency dependence on unpumped EDF section length. The length of EDF I has been fixed at 135 m for all of the experiments. To pump the EDFA’s, a 980-nm laser diode at a fixed level of pump power (90 mW) has been used. An external cavity laser, tuned at 1590 nm was used for the evaluation of the amplifier gain in conjunction with the optical spectrum analyzer (HP
1041–1135/99$10.00 1999 IEEE
LEE et al.: ENHANCEMENT OF POWER CONVERSION EFFICIENCY FOR AN
-BAND EDFA
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(a)
(a) (b)
(c)
(b)
(d) Fig. 1. Experimental arrangements for four types of L-band silica-based EDFA’s. (a) Type I: Conventional forward pump. (b) Type II: Conventional backward pump. (c) Type III: Unpumped EDF section before forward pump. (d) Type IV: Unpumped EDF section after backward pump.
70 952B), and power meter (HP 8153A). Two different levels of input signal powers at 20 and 0 dBm were fed to the amplifier in order to accurately measure the small-signal gain, noise figure, saturating output power, and power conversion efficiency. Insertion loss for the input section of the EDF was precisely measured, and found to be less than 2 dB in all cases. Fig. 2(a) shows the comparison of small-signal gain measured from the four types of -band silica-based EDFA’s illustrated in Fig. 1, for various lengths of EDF II. -band EDFA’s of Type III and Type IV with an unpumped EDF section showed considerable increase in small-signal gain, with strong dependence on the length of EDF II, when compared to the cases of conventional forward (Type I) and backward pump (Type II) EDFA’s. Power conversion efficiencies, measured with 0-dBm saturating tone for Types III and IV amplifiers were also observed to be better than Types I and II, as illustrated in Fig. 2(b). The highest small-signal gain and power conversion efficiency were obtained from the Type III amplifier with 35-m-long EDF II, at corresponding values of 21.83 dB and 25.7%, which is an improvement of over 4 dB and 14.0% when compared to the worst Type I EDFA under the same operating conditions. From a viewpoint of increasing trend of gain and power conversion efficiency, it is also believed that an unpumped EDF section with the length of more than 35 m can be adopted to achieve more performance improvement. From these results, it was evident
0
Fig. 2. Comparison of (a) small-signal gain at input power of 20 dBm and (b) power conversion efficiency at input power of 0 dBm for various lengths of EDF II, in the four types of L-band silica-based EDFA’s that were shown in Fig. 1.
that a very efficient use of pump power can be achieved by employing an unpumped EDF region before (for forward pumped EDFA) or after (for backward pumped EDFA) a pump laser diode. We attribute this efficiency improvement to the reuse of wasted ASE power propagating in the opposite direction of the pump light (here called backward ASE power regardless of a forward or backward pump scheme) as a 1550-nm pumping source for an unpumped EDF section, generating photons in the 1600-nm band. To confirm the existence of a backward ASE strong enough for conversion efficiency improvement, we measured the backward ASE spectrum in a conventional forward pumped EDFA of Type I without EDF II, using a circulator which was located just before the WDM coupler. Fig. 3 shows the backward ASE spectrum at 0 dBm input power, measured with an optical spectrum analyzer at 0.2-nm resolution bandwidth. We attribute the peak at the wavelength of 1590 nm to a Rayleigh back-scattering from the input signal. The wavelength range with more than 25 dBm/0.2 nm optical power extends from 1520 to 1565 nm, resulting in a large amount of total power as much as 20.59 mW when integrated. With a smaller input signal ( 20 Bm), even stronger backward ASE power, as much as 28.9 mW was observed as expected, approaching approximately a 32% level of the total pump power [7]. These levels of ASE powers are considered to be sufficient for band amplification, from a viewpoint of the previous result on -band amplification which used much weaker 1550-nm band signals as the pump source [3]. Independent experiments
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 11, NO. 1, JANUARY 1999
pump power at the input end, from the increased pump power absorption with longer lengths of an erbium fiber. III. DISCUSSION
Fig. 3. Measured backward ASE spectrum with input power of 0 dBm in a conventional forward pumped EDFA of Type I (without EDF II. Resolution bandwidth = 0:2 nm).
AND
CONCLUSION
To summarize, we have demonstrated a novel pump scheme for the improvement of power conversion efficiency, which can be used for signal amplification in a wavelength range from 1570 to 1610 nm. It was previously reported that the suppression of backward ASE is required to obtain sufficient -band signal gain because it induces amplifier saturation [4]. On the contrary, we were able to improve signal gain and pump power efficiency of an -band EDFA by using the detrimental backward ASE power as a pumping source for the unpumped EDF section. The experimental results showed a considerable improvement in power conversion efficiency from 11.7% to 25.7% as well as a small-signal gain increase at a maximum of 4 dB, with less than 1 dB of noise figure penalty. Performance improvement was also observed in the case of 1480-nm pumping (from 43.5% to 62.5%, with 35-mlong unpumped EDF), which implies that the suggested pump scheme is applicable to any pump wavelength. Therefore, we believe that this approach will play an important role in the development of practical -band EDFA’s from the perspective of economical usage of pump power. REFERENCES
Fig. 4. Comparison of noise figure for various lengths of EDF II in the four types of L-band silica-based EDFA’s, which were shown in Fig. 1.
also confirmed the generation of 1600-nm band ASE from this unpumped section of the EDF [8]. To see the noise figure penalty from two-level pumping by a 1550-nm band, we also investigated the noise figures for different types of amplifiers. For fairness, -band EDFA’s of Types I and III which employ forward pump schemes were compared to each other in separation with backward pumping cases of Types II and IV (Fig. 4). As expected, a noise figure penalty for the Type III EDFA was observed when compared to Type I, believed to be due to the limited ASE pump power as well as the existence of large emission cross section at backward 1550-nm band ASE wavelengths which were used for 1600-nm pumping in the unpumped EDF section. For the Type IV amplifier, an improvement in the noise figure was observed when compared to Type II. The Type II EDFA also showed a rapid increase in the noise figure as the EDF II length increased, due to the lower inversion state by smaller
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