Gain-flattened optical limiting amplifier modules for wavelength division multiplexing transmission

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Gain-Flattened Optical

Limiting Amplifier Modules

for Wavelength Division

Multiplexing Transmission

Shien-Kuei Liaw, Sien Chi

Published online: 15 Dec 2010.

To cite this article: Shien-Kuei Liaw, Sien Chi (1999) Gain-Flattened Optical Limiting

Amplifier Modules for Wavelength Division Multiplexing Transmission, Fiber and

Integrated Optics, 18:2, 69-77, DOI:

10.1080/014680399244712

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Gain -Flatten ed Op tical Lim itin g Am p lifier

Modu les for Wavelen gth Division

Mu ltip lexin g Tran sm ission

SHIE N-KUE I LIAW

SIE N CHI

Institute of E lectro-O ptical E ngine ering, National Chiao-Tung Unive rsity,

Taiwan, R epublic of China

By in tegration of a bidirectional erbium -doped fiber am plifier and a three-port

( )

optical circulator with several fiber Bragg gratings FBG s , two gain-flattened optical

( )

lim iting am plifier OLA m odules are proposed and experim en tally dem onstrated. They rely on the u se of cen tral wavelen gth m isalignm ent an d bending loss m eth ods, respectively. They can effectively cover the whole usefu l 1.55-m m band. Both m odules can provide gain-flattening characteristics over a large input dynam ic ran ge. The FBG s] integrated OLA configuration has potential application in wavelength division m ultiplexing lightwave com m unication system s.

K eywords erbium-dope d fibe r amplifie r, fibe r Bragg grating, flattene d gain, optical limiting amplifier, wave length division multiplexing

B ecause of the feature s of wide bandwidth and high gain, e rbium-dope d fibe r

s .

amplifiers E DFA s are revolutionizing both the long-distance and distribution-based multichanne l lightwave transmission system s in the 1.55-m m band. They are ofte n use d to compe nsate for the fibe r attenuation and ne twork splitting losse s. Furthe rmore, optically amplified systems allow a manifold incre ase in total capacity

s .w x

by supporting the use of wavelength division multiplexing W DM 1 . Howe ver, the nonuniform gain curve of the E DFA is one of the m ajor problem s for multichannel lightwave transmission systems with cascading E DFAs. The strong wavelengthde -pe nde nt gain profile and saturation characteristics of E DFAs lead to rapid accu-mulation of interchannel power variation and large diffe re nce s in the optical

s .

signal-to-noise ratio SNR among channe ls. The se proble ms may cause significant system penaltie s whe n the inte rchanne l power spre ads are beyond the rece ive r dynamic range. This will re sult in the SNRs of some channels be ing too low to be de te ctable.

To date , various e xte rnal and intrinsic approaches have be e n propose d to w x

e qualize the E DFA nonuniform spectral gain curve 2] 7 . Recently, we discussed Received 27 July, 1998; acce pte d 1 Se pte mber 1998.

The authors thank Dr. Y.K. Tu, J.W. Liaw, K.P. Ho, and K.Y. Hsu for e ncourage -me nt and fruitful discussions. The re vie wer’s kind help and beneficial com-ments are gre atly appre ciate d. The erbium-dope d fibe r was fabricated at the Chung-Hwa Te le communications Laboratorie s, Taiwan, and the work was partially supporte d by Grant NSC-87-2215-E-009-012 from the National Science Council, Taiwan, Re public of China.

Addre ss corresponde nce to Shien-Kue i Liaw, Institute of Electro-O ptical E ngine ering, National Chiao-Tung University, 100 Ta-Hsueh Road, Hsin-Chu 300, Taiwan, Republic of China. E-mail: u8424802@ cc.nctu.edu.tw

69

Fiber and In tegrated Optics, 18:69] 77, 1999 Copyright Q 1999 Taylor & Francis

0146-8030r99 $12.00q .00

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S.-K. Liaw an d S. Ch i 70

the feasibility of two simple and passive techniques using a samarium-dope d fibe r sSDF. w x8 and fibe r Bragg gratings FBGss . w x9 in the E DFA as equalize rs. In this pape r, we proposed and e xperime ntally de monstrate d two gain-flattene d optical

s .

limiting amplifier GF-O LA modules for multichanne l transmission. B oth are

s .

achie ved by inte gration of an E DFA, a thre e-port optical circulator O C with se veral FBGs corresponding to the transmitte d signals. The de signs of the GF-O LA module s are based on the principle s of ce ntral wave le ngth misalignment and fibe r be nding loss. Both m ethods can e ffe ctively e qualize the gain variation among channels in W DM transm ission systems. Me anwhile , they can strongly re duce the

s . w x

amplified spontane ous emission ASE 10 se veral nanom eters away from trans-mitte d signals due to the narrow-band filtering fe ature of FB Gs. In both single-channel and W DM systems, the GF-O LA can provide a nearly constant output powe r for a large input signal dynamic range and a high saturated output powe r be cause of the dual-pass amplification characteristic of the O LA. Thus, fibe r span

w x and link budge t can be increased 11 .

Ch aracteris tics of GF-OLA Mod u les

Figure 1 shows the common configuration of the propose d GF-O LA modules

s .

consisting of a thre e-port O C, a bidire ctional E DFA Bi-E DFA without any optical isolator, and seve ral FB Gs with ce ntral-refle ctive wave lengths de signe d to match the transmitte d signals. The FBGs are used to equalize the multichannel signals and to reduce the ASE . The Bi-E DFA was constructe d by a se ction of

s .

e rbium-dope d fibe r E DF and one 980

r

1550 nm W DM coupler. The E DF was pumped by an 80-mW , 980-nm laser diode and had a saturated power of 10.5 dBm .

s .

The le ngth, absorption coefficie nt at 1532 nm, and nume rical ape rture NA , core, and cladding diame te rs of the home made E DF are 10 m, 5.8 dB

r

m, 0.21, 5.0 m m, and 125.0 m m, respe ctively. The thre e-port O C provide s about 50-dB isolation for the E DFA at both the input and output ports. The Bi-E DFA with its dual-pass amplification sche me acts as an O LA. The FBGs inside the GF-O LA m odule s are operate d as refle ctive mirrors, and the input signals are amplified, filtere d, and

Figu re 1. Common configuration of the propose d GF-O LA module s, consisting of a thre

e-port optical circulator, a bidirectional EDFA, and se veral FB Gs: O LA, optical limiting amplifier; FBG s IrII, the first or the se cond module of the fiber Bragg gratings.

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G ain -Flattened Optical Lim itin g Am plifier Mo dules 71

re flecte d by the FBG chain and the n amplified again. The gain e fficie ncy of the O LA is equivalent to that of the two cascading E DFAs when the launche d power is small.

To verify the O LA characteristic, in Figure 2 we compare the output powe r characte ristic against the input powe r of the proposed single-channel GF-O LA module with a conventional E DFA at the wave le ngth of 1549.8 nm. The conve n-tional E DFA was constructed by adding two optical isolators with 55-dB isolation and 0.8-dB inse rtion loss to both e nds of the B i-E DFA. The dynam ic ranges of the propose d GF-O LA and a conventional E DFA are 30 and 9.0 dB, re spective ly, whe n they use the same pumping powe r of 80 mW . The ``optical lim iting amplifier’ ’ me ans that an amplifier can ke ep a constant saturated output powe r over a large

s .

dynamic range. The dynamic range is de fined as the input powe r differe nce in dB

s .

be twe e n the corresponding full-saturate d and half-saturated i.e., 3 dB lower output powe rs.

Op er atin g Prin cip les of GF-OLA Mod u les

Figure 3 shows an e xample of a ge neral W DM system. A W DM multiple xe r sMUX com bine s five digital channels at the transmitte r, and a W DM dem ulti-.

s .

ple xe r DMU X separates all channels at the re ceiver. The re are seve ral E DFAs in

s .

the system , with some spools of single -mode fiber SMF inse rted betwe en two E DFAs. The E DFAs are use d to com pe nsate for the fibe r loss. Five channe l signals of l 1; l 5 with the same input powe r are launche d into the W DM MUX, booste d by an E DFA, and then transmitted along the fiber link. The unflatte ned gain characteristics of saturated E DFAs will re sult in powe r le vel variation among these signals. Howe ver, whe n one of the two propose d GF-O LA modules is use d in the system, the powe r le vel of these signals can be e qualized at the output port of

s .

the GF-O LA m odule or the common port i.e ., Pt. Y in Figure 3 of the W DM DMU X .

For the first kind of GF-O LA module, as shown in Figure 4 a, the m

isalign-s . s .

me nt value D l i nm for signal i whe re is 1, 2, 3, 4, or 5 is defined as the ce ntral

s .

wavelength differe nce be twe en the spectrum of distributed fe e dback lase ri DFBi

and the re flective pe ak of the FBG . In this case, the misalignme nt values arei

adjuste d according to the relation of D l 5 ) D l 4) D l 3) D l 2 ) D l 1, assum ing that the FB Gs have the same transfer shape and re fle ctivity. Thus the wave le ngth

Figu re 2. O utput powe r versus input power for

the propose d GF-O LA module and the conve n-tional EDFA.

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Figu re 3. Ge ne ral WDM syste m: DFB , distribute d fee dback laser; MUXrDMUX, multi-plexe rrde multiple xer; MO D, modulator; SMF, single -mode fibe r.

s .

misalignme nt induce d signal attenuation dB for l 5 is large st and reduce s subse -quently for those of l 4, l 3, l 2, and l 1. The misalignme nt value s can be

dynami-s .

cally adjuste d simply by te mperature tuning of the DFB lase r s or straining the s .

FB G s . Using this method, some W DM channels are likely to pass through the ``e dge ’’ region instead of the ce ntral re flective region of the FB Gs. An FBG was use d, for e xample , to me asure the signal attenuation ve rsus wavelength m

isalign-s .

Figu re 4. O pe rating principle s of the two propose d GF-O LA module s, one based on the a

s .

central wavelength misalignment and b be nding loss method.

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me nt be twe e n a DFB lase r and an FB G, and the re sult is shown in Figure 5 a. The FB G used here has a ce ntral re flective wavelength at 1549.8 nm with 95% re flectivity, and 3-dB bandwidth of 0.25 nm that is slightly unsymmetrical. The

s .

slope y

r

x value of the curve will change from sm ooth to ste e p for D l ) 0.2 nm. B ecause the transfe r function of many com mercial or homem ade FBGs is not flat and pe rfe ct around their ce ntral re flecte d wavelength, too large a wave le ngth misalignme nt value D l may induce unavoidable powe r penalties, which is m ainly due to the imperfe ct FBG filtering and the ASE .

Figure 4 b indicate s the second kind of the propose d GF-O LA module by using the bending loss m ethod for signals gain-equalization. B y using this approach, the FB G , FBG , FB G , FBG , and FB G should be locate d from left to right inside1 2 3 4 5 the GF-O LA module, as shown in Figure 1. The bending attenuation of each se ction of SMF located betwe en e very two adjace nt FBGs can be dynamically

s .

adjuste d. Figure 5 b shows the signal attenuation dB ve rsus the fiber be nding loss sdB for a spe cific FBG. The slope of the curve is about 2, since the propose d. GF-O LA module is a dual-pass amplification configuration for the transm itte d signals. In other words, these signals are attenuated twice at e ach bending point of SMF, whe re it

r

the y passe d back and forth. For instance , the be nding attenuation value be twe e n B FG and B FG is about 2.0 dB if a 4.0-dB powe r le vel variation3 4 should be compensate d betwe en l 3 and l 4.

Exp er im en tal Res u lts an d Dis cu s s ion

The GF-O LA modules are applied to a five -channe l W DM system to dem onstrate the functions of the GF-O LA module s. Figure 6 a shows the spectrum of the five input channe ls at the input port of the GF-O LA modules. The five input channe ls

Figu re 5. FB G used for an example to measure

s . s .

the signal atte nuation dB versus a wave le ngth

s . s .

misalignme nt nm and b fiber bending loss sdB ..

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s . s .

Figu re 6. a Five -channe l input be fore the GF-O LA module s. b Transfe r function of the FB G chain.

are from 1548.5 to 1559.5 nm for a range of about 11 nm with unequal input powe rs of 16.0-dB variation. Figure 6 b is the transfer function of the FBG chain with re flectivity ranging from 93; 99.99% , designed to m atch the ce ntral wave-lengths of the five channe ls. As shown in Figure 6 c, the GF-O LA module provide s almost the same output power for all five W DM channe ls with a variation of le ss than 0.6 dB at the output port of the module when the me thod of ce ntral wavelength misalignme nt was used. In that case , the misalignment values are 0.0, 0.12, 0.15, 0.2, and 0.25 nm for l 1, l 2, l 3, l 4, and l 5, re spectively, for e qualizing the signals. As shown in Figure 6 d, when the bending loss m ethod was used to e qualize the five signals, the output spectrum is similar to that of Figure 6 c with a

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s . s .

Figu re 6. cont. Five-channe l outputs using the methods of c ce ntral wavele ngth misalign-s .

me nt and d bending loss.

gain variation of only 0.5 dB. The re sults confirm the fe asibility of the GF-O LA module s. In Table 1 we summarize and compare the fe ature s of these two GF-O LA modules.

Con clu s ion

B y integration of a B i-E DFA and a three -port O C with seve ral FBGs, two GF-O LA m odule s are proposed and expe rim entally studied for gain flattening the multichanne l signals in W DM systems containing cascading E DFAs. The se two

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Table 1

Summary feature s of two GF-O LA modules

Ce ntral wave le ngth Fibe r be nding m isalignm ent loss

s .

Measure d unit X axis nm dB

Simple design ye s yes

Te mpe rature tuning of DFB s

r

FBGs ye s no

Dynamically adjustable ye s yes

FBG se que nce necessary no yes

ASE filtering fe ature ye s yes

Dual-pass amplification ye s yes

s .

DFB , distributed fe edback lase r ; FBG , fibe r Bragg gratings; ASE , amplified sponta-neous emission.

module s re ly on the use of central wavele ngth misalignme nt and fibe r bending loss. E ithe r of the GF-O LA m odule s can gain flatten the W DM channels and provide a larger input dynamic range to incre ase the link budge t and fiber span than can a conve ntional E DFA. The GF-O LA m odule s can also find im portant applications in

s .

high-spe ed G 10 Gb

r

s per channe l W DM transmission systems while re placing w x FB Gs with chirpe d fibe r gratings for fibe r chromatic dispersion com pe nsation 12 as we ll as for signals gain-flatte ning purpose s.

Refer en ces

1. Tonguz, K., and F. A. Flood. 1997. Gain-e qualization of EDFA cascade s J. Lightwave s .

Tech nol. 15 10 :1832] 1840.

2. Park, S. Y., H. K. Kim, G. Y. Lyu, S. M. Kang, and S. Y. Shin. 1998. Dynamic gain and output power control in a gain-flattene d e rbium-dope d fibe r amplifier. IEE E Photon . Tech nol. Lett. 10:787] 789.

3. Hwang, S. T., J. Nilsson, S. Y. Yoon, J. M. Kim, and S. J. Kim. 1998. Gain-flattened e rbium-dope d fiber amplifier with a wide dynam ic range in the 1542] 1560 nm band. In 1998 Conference on Optical Fiber Com m unication, OFC’98. Pape r WG3.

4. Kim, H. S., H. Yyun, H. K. Kim, N. Park, and B. Y. Kim. 1998. Actively gain-flattene d e rbium-dope d fibe r amplifier acoustic tunable filters. IEE E Photon . Tech nol. Lett. 10:790] 792.

5. Yoshoda, S., S. Kuwano, and K. Iwashita. 1995. Gain-flatte ne d E DFA with high Al conce ntration for multistage re pe ated WDM transmission syste ms. Electron. Lett. 31:1765] 1767.

6. Barart, D., B. Clesca, L. Hamon, and J. L. Be ylat. 1994. Expe rime ntal inve stigation of the gain flatness characteristics for 1.55-m m e rbium-dope d fluoride fibe r amplifiers. IEE E Photon. Tech nol. Lett. 6:613] 615.

7. Seikai, S., T. Tohi, and Y. Kanaoka. 1994. Erbium-dope d fibre amplifier circuit having a subsidiary erbium-dope d fibre use ful for bidirectional optical transmission syste ms, Electron . Lett. 30:1877] 1878.

8. Liaw, S.-K., and Y.-K. Chen. 1996. Passive gain-equalization wide-band e rbium-dope d fiber amplifier using samarium-dope d fiber. IEEE Photon . Tech nol. Lett. 8:879] 881. 9. Chi, S., and J.-C. Dung. 1998. Gain flattening for WDM syste m by use of fibe r Bragg

gratings in EDFA. In 1998 Conference on Optical Fiber Com m un ication. OFC’98. Pape r WG2.

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10. Le bre f, R., E. De levaque , B. Landousie s, H. Poignant, M. Guibert, and T. Ge orges. 1996. An advance amplifier structure for WDM transmission: The multichanne l equal-ized and stabilequal-ized gain amplifier. In 1996 Proceedin gs, Optical Am plifier and Th eir Application s, OAA, pp. 83] 86.

11. Liaw, S.-K., and S. Chi. 1998. Expe rime ntal inve stigation of a fibe r Bragg grating inte grate d optical limiting amplifie r with high dynam ic range. SPIE Opt. Eng. 37: 2101] 2103.

12. Loh, W. H., R. I. Laming, N. Robinson, A. Cavaciuti, F. V anine ttim, C. J. Ande rson, M. N. Z ervas, and M. J. Cole . 1996. Dispersion compensation ove r distance in e xce ss of 500 km for 10 Gbrs syste ms using chirpe d fibe r gratings. IEEE Photon . Tech n ol. Lett. 8:944] 946.

Biograp h ies

Sh ien -K u ei Liaw re ce ived his BSE E degree from the National Taiwan University and

his MSE E degre e from the National Tsing-Hwu University, Taiwan, in 1988 and 1993, respective ly. From 1993 to 1997 he was a membe r of the te chnical staff at Applied Research of Chung-Hua Telecommunication Laboratories in Yang-Me i, Taiwan. In 1996 he was a reside nt visitor at Be llcore , Re d Bank, Ne w Jersey, for a pe riod of 6 months. Currently, he is pursuing his Ph.D. de gre e at the National Chiao-Tung Unive rsity, Taiwan. His re search inte re sts include optical fiber communications, fiber amplifiers, and fiber Bragg gratings and the ir re lated applications.

Sien Ch i re ceived his BSE E de gre e from the National Taiwan University and his MSE E

degree from the National Chiao-Tung Unive rsity, Taiwan, in 1959 and 1961, re spe ctively. He receive d his Ph.D . degree in ele ctrophysics from the Polytechnic Institute of Brooklyn in Ne w York in 1971, after which he joined the faculty of the National Chiao-Tung University, whe re he is currently a profe ssor at the Institute of E lectro-O ptical Engine ering. From 1972 to 1973 he chaired the De partment of Electrophysics; from 1973 to 1977 he directed the Institute of Ele ctonics; from 1977 to 1978 he was a resident visitor at Bell Laboratories, Holmdel, Ne w Jersey. From 1985 to 1988 he was the principal advisor with the Hua-E ng Wire and Cable Company, the first manufacture r of fibers and fibe r cables in Taiwan, deve loping fiber making and cabling te chnology; and from 1988 to 1990 he dire cte d the Institute of Electro-O ptical Enginee ring. He was the symposium chair of the International Symposium of O ptoe le ctronics in Computers, Communications and Control in 1992, which was co-organized by National Chiao-Tung University and SPIE . In 1993 he receive d the Distinguishe d Re se arch Award sponsore d by the National Scie nce Council, Taiwan, RO C. His rese arch interests are optical fiber communications, optical solitons, and optical fibe r amplifiers. Dr. Chi is a me mber of the Chinese O ptical E ngine ering Socie ty and a fe llow of the O ptical Socie ty of Ame rica and the Photonics Socie ty of Chine se -Americans.