Erbium Doped Fiber Amplifier Gain Flattening Using Long-Period Fiber Gratings

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1999 Chinese Phys. Lett. 16 513

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CHIN.PHYS.LETT. Vol. 16, No. 7 (1999) 513

Erbium Doped Fiber Amplifier Gain Flattening Using Long-Period Fiber Gratings *

CHEN Gen-xiang(%@@), JIAN Shui-sheng(fl$$&), LI Tang-jun($&%), LIU Chun-ning(%J@T), XIE Zeng-hua(#q@)

Institute of Lightwave Technology, Northern Jiaotong University, Beijing 100044

(Received 17 December 1998)

Based on theoretical and experimental studies on the optical properties of long-period gratings (LPGs), an erbium doped fiber amplifier (EDFA) gain equalizer was fabricated by writing three long period gratings with designed properties in hydrogen-loaded standard singlemode fibers. T h e LPGs were written by a KrF excimer laser at 248 n m with amplitude masks. The gain variation of f 3 d B in the EDFA is reduced t o f 0 . 3 d B with a bandwidth of 3 0 n m and the total insertion loss o f the device is less than 0.3 dB.

PACS: 42.81. Dp, 42.81. Wg

Erbium doped fiber amplifiers (EDFAs) are now an integral part of long-haul high bit rate fiber-optic communication systems and are finding applications in wavelength division multiplexing (WDM) systems for the realization of ultimately large capacity op- tical fiber transmission which requires wide band- width amplification.' One of the major issues with EDFAs is that the gain is not constant over the entire amplifying bandwidth, which generally causes an imbalance among the WDM channels in cascaded EDFA chains and limits the usable bandGidth. Tech- niques for EDFA gain flattening have aroused great interest recently and several approaches have been proposed.2-12 Since fiber gratings with a period of hundreds of microns have very low extra loss and negligible back reflection and their transmission spec- trum shapes may be well controlled by changing the grating parameters during their fabrication, many theoretical and experimental efforts have focused on long period fiber gratings for EDFA gain flattening app1ication.'-l2

In this paper the optical properties of long pe- riod fiber gratings were studied theoretically, based on which an EDFA gain flattening filter was fabri- cated by using three long period fiber gratings. The fiber gratings were written in hydrogen-loaded stan- dard single-mode fibers using a 248nm KrF excimer laser with amplitude mask technology. The 6 dB gain variation of the EDFA is reduced to 10 .3dB over a bandwidth of 30 nm by using a long-period grating (LPG) filter and the total extra insertion loss of the device is less than 0.3 dB.

For a simple step-index single-mode fiber in which an LPG is photo-inscripted with a period denoted by A , the coupling between the fundamental mode (LPol) and a co-propagating azimuthally symmetric cladding mode (LPo,) in the grating region can be described

by12

- = i 6 R + i ~ S , dR dz - dS = - i6S+ in*Rl dz

where R ( z ) and S(z) are slowly varying amplitudes of the fundamental mode and the mth cladding mode (LPo,), respectively; K is the coupling coefficient be- tween the modes and 6 the phase detuning, being writ- ten as

(3) 7r

K = -An,,r], x 1 7T

2 s = - ( p 01 - P&) - (4)

Here X is the wavelength of the incident light, An,, the photo-induced index change in the core of the fiber, r ] the overlap integration of the coupled modes, and pol and @&, are the propagation constants of the funda- mental mode and mth cladding mode, respectively.

The appropriate initial conditions of the coupled modal Eqs. (1) and (2) can be specified by assuming only the LPol mode is incident from z = -00, that is

R(0) = 1, S(0) = 0. ( 5 )

Thus, the closed-form solutions of the coupled modal equations can be found and the power transmission can be deduced to be

sin2 ( d w L ) , (6) + 1 + ti2/62

where L is the grating length. From Eq. (6), we can write the peak loss of the grating as

P ~ ( x , ) = sin2(nL), (7)

*Supported by the National Natural Science Foundation of China under Grant No. 69777010, the National 863 High Technology Project of China, and the Paper Publication Foundation of Northern Jiaotong University.

@by the Chinese Physical Society

514 CHEN Gen-xiang et al. Vol. 16

where the peak resonant wavelength A, is deduced from Eq. (4) with the phese matching condition 6 = 0, and can be written as

where Neff,~l and N:k,Om are the effective indexes of the fundamental mode and LPom cladding mode, re- spectively.

Normalized wavelength X/X,

Fig. 1. Calculated power transmissions of three different LPGs with nL = ~ / 2 ; solid line for L = 100mm, dashed line for L = 40 mm, dotted line for L = 20 mm.

Excimer laser

Amplitude mask - \ I

Cylindrical lens Electronic f shutter

E w i b z r r o D t i c a l ...I.. I*,̂ ” I

Fiber holder Program controlled1

or and shutter I motor system I controller I I

Fig. 2. Setup for long-period fiber grating fabrication us- ing amplitude masks.

Figure 1 shows the typical power transmission spectra for three uniform gratings of different lengths with nL = ~ / 2 .

From Eqs. (4) and (6), we can deduce the band- width between the first zeros on both sides of the main peak for LPGs with moderate coupling strength (nL < T ) as

where N is the total number of grating periods. In order to obtain a long period fiber grating with

desired transmission properties, such as peak loss, res- onant wavelength and rejection bandwidth, we must properly design the grating parameters according to

Eqs. (7)-(9). The center wavelength can be set by choosing the appropriate grating period while the fil- ter bandwidth and depth are controlled by adjusting the photo-induced index change and the total number of grating periods.

Figure 2 shows the experimental setup for long- period fiber grating fabrication using amplitude masks and a properly designed program controlled scanning system. A large number of experimental results shows that the transmission spectrum of LPGs can be well controlled by carefully adjusting the energy density per pulse of the excimer laser. the scanning speed and the electronic shutter during the fabrication of the LPGs. and the annealing process after the LPGs have been written in hydrogen-loaded standard single- mode fibers. The results are in good agreement with the theoretical analysis derived from Eqs. (7)-(9).

Because LPGs are very unstable after their ul- traviolet writing even at room temperature, which is mainly due to hydrogen diffusion in the fibers, it is necessary to anneal the LPGs at high temperature. All the LPGs used in our experiments were annealed at 152°C for 6 h

~

1520 1530 1540 1550 1560 1570

W’avelength (nm)

Fig. 3. Transmission spectrum of the fabricated EDFA gain flattening filter assembled from three long period fiber gratings.

Gain flattening over the full bandwidth of the EDFA requires the fabrication of LPG filters with complex shapes. which accurately match the gain rip- ple of the EDFA gain spectrum. An EDFA gain- flattening filter assembled from three LPGs has been fabricated. The fiber length between the adjacent LPGs is about 0.5m, and the whole fiber length of the filter is about 1.2m. The designed peak loss and the rejection bandwidth is obtained by carefully ad- justing the energy density per pulse of the excimer laser. scanning speed and the electronic shutter. The choosing of the resonant wavelength is performed by properly designing the amplitude masks for each LPG. The characteristic of the gain-flattening filter is shown in Fig. 3. The optical gain spectra of an EDFA without and with the gain-flattening filter are shown in Figs. 4

No. 7 CHEN Gen-xiang et al. 515

and 5 , respectively. From these figures, we can note that the gain ripple of the EDFA has been reduced to f0.3dB over a bandwidth of 30nm. The measured result shows that the total extra insertion loss is less than 0.3 dB.

-2 I

-2 2- 1020 1530 1540 1550 1560 1570

Wavelength (nm)

Fig. 4. Gain spectrum of an EDFA without the gain flat- tening filter.

-2 I 1

-22 1 1520 1530 1540 1550 1560 1570

Wavelength (nm)

Fig. 5. Gain spectrum of an EDFA with the gain flatten- ing filter.

In summary, theoretical and experimental results show that the transmission spectrum of an LPG can

be well controlled by carefully adjusting the grating period, index change and the grating length during the LPG fabrication, and complex low-loss EDFA gain flattening filters can be produced using three porperly designed and fabricated LPGs. This technique is suit- able for the assembly of low gain ripple EDFAs with low insertion loss for WDM .applications.

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