10Gbs transmission over 100 km of standard single-mode fiber with a dispersion tunable chirped fiber grating

V O ~ .42

8

NO.

SCIENCE IN CHINA (Series A)

A U ~ U S1999 ~

10Gb/s transmission over 100 krn of standard single-mode fiber with a dispersion tunable chirped fiber grating * (%T&)''2 DU, Weichong ($kE@)', ZENG Qingke (@fi$k)3, LIUNing ($1 T)', GUOQi ( @ @ ) I , LIAOChangjun (@?#@)',

QIN Zixiong

LIUSonghao ( f l 2 3 * ) ' , SUNJian CAI Lianfeng ( @

(a)

3 \ ) 4 , ~ ~ ~ ~( @X u3)4, e

& % )and 4 YU Chongxiu (&Zi?i % )4

( 1. South China Normal University, Guangzhou 510631, China; 2 . University of Science and Technology of China, Hefei 230026, China; 3 . Guangxi Normal University, Guilin 541001 , China; 4 . Beijing University of Posts and Communications, Beijing 100876, China) Received January 28, 1999

Abstract

A simple strain method is used to change uniform gratings into chirped gratings. The correspondent theory is presented. It is deduced that the product of the maximum bandwidth and the corresponding dispersion is nearly equal to a constant. The dispersion compensation distance for standard single-mode fiber G . 652,lO Gb/s systems over 100 km is realized by using this method.

Keywords : gratings in fibers, optical communication, optical dispersion

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Long distance transmission at 10Gb/s over standard telecommunication fiber G. 652 is of great interest because of the large base of such fibers already installed on the ground. The low loss of these fibers together with the ready availability of erbium doped fiber amplifiers (EDFA'S) makes the 1.55 pm window an attractive region of operation. However, the dispersion of these fibers is relatively large within this window, severely limiting transmission distances unless compensating techniques are employed. Of the various methods suggested to solve this problem, linearly chirped fiber Bragg gratings (LCFBG) as dispersion compensators are very attractive, as they are compact, passive, relatively simple to fabricate and easy to adjust. In recent years, the global information process is very rapidly developed. B-ISDN , ATM and HDTV , etc require higher rate of transmission. These ask for higher reqirements for fiber communication. Today the system at 2 . 5 Gb/s is commercially available, and the system at 10 Gb/s is being improved toward practical purposes. In China the already installed fibers are estimated to be in excess of 100 000 km. The transmission systems are mostly operated at 1.3 ,urn and the rate of transmission systems generally is the third or fourth level hierarchy. Therefore the present system capacity is not enough. The key point is whether the already installed fibers support higher system capacity. The standard single-mode fiber for the transmission signal operating at 1 -55 pm has about 18 ps/nm* km dispersion value, when the transmission rate is up to 10Gb/s or more, and its effect on system is

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* Project supported by the National 863 High Technology Research Plan of China, Guangdong Province and the National Natural Science Foundation of China.

the Key Science and Technology Research of

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very severe. Especially when EDFA' s are widely employed in fibers, the repeater spacing will be several times larger if the problem of dispersion can be solved. Therefore it is necessary to take some measures to control dispersion. It is apparent that the promotion of the already installed fiben by dispersion compensation is more economical than by dispersion-shifted fibers. In China, it is especially necessary to research and develop a kind of device which is possessed of fine working capacity of dispersion compensation and is easily popularized.

1 The configuration of the tunable chirped fiber grating and the principle of dispersion com-

pensation 1 .1 The mechanism of linearly chirped fiber Bragg gratings (LCFBG) The graings are chirped in the sense that the product of effective refractive index n," and grating pitch A,, varies linearly along the length of the structure. When light transmits through this structure, lightwave ( h D = 2neffA) of different wavelength reflects from different spatial regions along the grating, experiencing different time delay. The above model is enlightening, but oversimplistic : the reflectivity of a grating of finite length neccessarily varies along its length, leading to large oscillations of dispersion-delay characteristics . 1 .2

The stressed uniform fiber grating It is well known from the statically indeterminate structures in mechanics of materials as shown in fig. 1 that on the left of the concentrated load P , P B

MA

z

v=-

Fiber grating

M = - c,z- C 2 , YM AZ Oz = - E-, I z

-I1v

where EZ is the flexural regidity of the beam; az is the normal stress; V is the deflection of the beam Fig. 1 . Diagram of forces.

pb2 C1 = - - ( L L3

+ 2a);

C2 =

-.pab2 L~

'

AZ is the elongation of the beam of length Z . In this configuration, the uniform gration is changed into chirped grating. The elongation of the grating pitch can be deduced : AA ( E ) =

aL

Z(L

+2 a )

, the grating pitch

YA 0

- -( 2 C,Z + C 2 ) . On the left of point El

Zo =

C2

- --2 CI

is compressed. On the right ( Z < a ) , it is extended.

2 Experiment 2 . 1 The experiment system 2 . l . 1 The narrow pulse source. It consists of a sinusoidal signal generator and a gain-switched laser. In the system, the narrow pulse bandwidth is in the range of 25-40 ps. 10 Gb/s NRZ pulse bandwidth and RZ ~ u l s ebandwidth are respectively 100 and 50 ps. Therefore the narrow pulse source

IOGh/s TRANSMISSION OVER STANDARD SINGLE-MODE FIBER

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meets the requirements of 10Gb/s pulses. SMF l

SMF2

Circulator

Light source EDFAl

EDFA2

LCFBG

Fig. 2 .

Experimental setup.

2 . 1 . 2 The detecting light system. It consists of a chopper, an auto correlation-measuring device and a phase-locked amplifier. In the system, the light power is hardly detected. The alternating current measuremnt is needed in order to improve SNR. While the technique of phase-locked amplifier is employed, the measurement range is extended by using a chopper to modulate light intensity. The auto correlator can be used to measure the pulses whose bandwidth is in the range of 5-200 ps. Therefore, unbroadened and some broadened pulses can be detected. 2 . 1 . 3 The dispersion compensator consists of a circulator and a LCFBG. The circulator guarantees that the grating operates in the reflective modes. 2.1.4 Two EDFAs compensate for the loss of the fiber and the dispersion compensator, ensuring normal measurements. 2.2

The adjustment of LCFBG and the measurement results A 10 cm uniform grating is embedded in a 30 cm laminated beam and is allocated in the range from 5 cm to 15 cm and near the top of the beam. The two supporting points of the beam are fmed (one point is at 1. 2 cm, the other is at 28.8 cm) , while one point can be levelly moved to adjust the central wavelength of the spectrum. The force P is applied to changing the deflection of the beam, leading to change of the bandwidth of the spectrum. The center wavelength and bandwidth of the spectrum are measured by the optical spectrum analyser. In our system, when the central wavelength is adjusted at 1 549.909 nm, M = 0.53 nm at 10 db (fig. 3 ) . Figs. 4 and 5 can be measured. The pulse shape without f h e n is shown in fig.4. The pulse shape with 104 km fiber is shown in figure 5 .

3

Discussion

The reflectivity and time delay are calculated as shown in fig. 6. In fig. 7 , using the transfer matrix techniqueL" , the grating is divided into 200 segments, each having a constant period phase and apodization, corresponding to their values at the segment mid-point. The chirp effect of the change in the average refractive index across the gratings assuming full contrast modulation is included in these simulations. In the experiment, Zo= 35.7 mm The grating is embedded in 38-138 mm .

.

an(z)

Thus ------ - 1.59 x 1 0 - ~ ( 0 . 8 9 2-~31.83), Z being in millimeter. The amount of chirp -is A dZ

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tlps Fig. 3

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The reflection spectrum of the LCFBG.

The pulse shape without fibers,

Fig. 4 .

tlps

Fig. 5 . The pulse shape with 24 km km+ EDFA+LCFBG.

+ EDFA + 80

Fig. 6 .

The reflection spectrum of the

lated values)

.

LCFBG ( calcu-

proportional to the beam deflection, which provides an easy way to adjust the grating to meet the actual system requirements. Thus the grating not only provides equalisation for the designed 100 km at 10Gb/s but is also suitable for different combinations of fiber length and bit rate, where the product is less than or equal to lo3 Gb/s km . Fig. 3 accords well with fig. 6 considering that the values have been averaged in the practical measurements. In the time delay spectrum (fig .7), the distance between two dash lines is defined as the maximum dispersion compensation bandwidth. In the experi,, = 0 . 3 7 , and the grating dispersion D = 1 550 ps/nm. If the dispersion value 18 ps/nm ment Ah , km for the fibers of G .652 is assumed, 86 km compensated distance is calculated. The experiment tallies well with the theoretical calculations, because the weak fiber nonlinearity and the oscillations in the time-delay spectrum allow the system length to be extended by an enhancement factor of 0.167 for the grating reflectivity 50 % -30 % ['I . In the experiment, 10 db bandwidth Ah = 0.53 nm when the energy efficiency of gain-switched laser is synthetically considered . But, in fig. 7, the maximum dispersion compensation bandwidth Ah ,,, = 0.37 nm, which is not enough to completely compensate for the bandwidth 0 . 5 3 nm . The relation between the dispersion of 10 cm LCFBG and Ah,,, is shown in fig. 8 by numerical value simulations, similar to AA, x D = constant. For a conventional 10 Gb/s binary system, the minimum acceptable grating bandwidth is required to be 3 0.lnm, which limited the maximum compensation distance achievable with 10 cm gratings to about 350 km. When the bandwidth is 0.53 nm, a 10 cm

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grating only compensates for 64 km fiber. Dispersion copmensation of 100 km fiber needs more gratings. The grating center wavelength is hard to be controlled exactly. The most convenient ways are the temperature and strain . The strain method employed in the experiment is relatively easy to be operated, practical and very flexible.

0l 0.05 0.15

0.25 0.35

0.45

0.55

0.65 0.75

Maximum dispersion bandwidthlnrn Fig. 7 .

The time delay spectrum of the LCFBG (calculated

values) .

Fig. 8 . The maximum dispenion bandwidth of compensation-dispersion characteristics for the tunable grating.

It is worth noticing that 60% energy transmits and only 40% energy reflects, according to the measured transmission spectrum. It is lower considering 10 db bandwidth. High reflectivity and large dispersion compensation bandwidth are desirable to the system. However, when the reflectivity is high, multiple reflections occur within the grating, leading to the oscillations and to bit-pattern dependency, while the large bandwidth shortens the compensation distance. The tradeoffs are of important practical significance. How to reduce the oscillations in the time-delay characteristics needs deeper research(discussed in another paper). The location of the grating in the system (particularly when several gratings are concatenated to increase the transmission length) , the inexact dispersion compensation and fiber nonlinearity are worth discussing.

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Conclusion

The way to change uniform gratings to chirped gratings is innovatory as we have known. The way to shift the center wavelength and to change the chirp degree is simple to operate, and is practical and flexible. The correspondent theory is concise and accords well with the experiment. Dispersion compensation over distances in excess of 100 km for 10 Gb/s systems using a chirped fiber grating has been realized when the pulse bandwidth A)\ = 0.37 nm , which takes the lead in China.

References Yamada, M. , Sakuda, K . , Analysis of almost-periodic distributed feedback slab waveguides via a fundamental matrix approach, Appl . Opt. , 1987, 26( 16) : 3474. 2 Atkinson, D , Loh , W . H . , 0' Reilly , J .J . et a1 . , Numerical study of lOem chirped-fibre. grating pairs for dispersion compensation at 10Gb/s over 600km of nondispersion shifted fiber, ZEEE. Photon. Technol. Lett. , 1996, 8(8) : 1085. 3 Hill, P. C . , Eggleton, B. J . , Strain gradient chirp of fibre Bragg gratings, Electron Lett. , 1994, 30( 14) : 1172. 4 Garthe, D . , Epworth , R .E . , Lee, W .S et a1 . , Dispersion equaliser for 10 and 20Gb/s over distance up to 160km, Electron Lett. , 1994, 30(25) : 2159. 1

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