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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 12, NO. 11, NOVEMBER 2000 1501

Novel Flat Multichannel Filter Based on StronglyChirped Sampled Fiber Bragg Grating

Xiang-fei Chen, Chong-cheng Fan, Y. Luo, Shi-zhong Xie, Senior Member, IEEE, and S. Hu

AbstractIn this letter, a novel optical fiber filter basedon a strongly chirped sampled Bragg grating is proposed forwavelength-division-multiplexing (WDM) system applications. Itfeatures in multiple equalized passbands with flat-top steep-edgenearly linear phase response and high transmittance. Refractiveindex modulation amplitude with Gaussian spatial profile is usedwithin the samples, which contributes to the marked improvementof the filter performance. Furthermore, there are also multipleflat and equalized stopbands to be used as multichannel opticaladd/drop filters in WDM system.

Index TermsGratings, optical fiber communication, opticalfiber devices, optical fiber dispersion, wavelength-division-multi-plexing (WDM).

I. INTRODUCTION

MUCH attention has been drawn to the design andimplementation of compact optical devices for wave-length-division-multiplexed (WDM) systems. Among them,

multichannel transmission filters based on grating principle are

of special importance. For example, a Moir grating (MG) is

a special grating with two superimposed gratings at different

wavelengths. Usually, when a large chirp in the grating period

is included, multiple passbands may result in such a chirped

Moir grating (CMG) [1], which can be used as a multichannel

transmission filter. However, passbands of this structure are

Lorenzian with sharp peaks and broad-bottom areas. A carefuldesign by embedding some fiber sections with no refractive

index modulation in a CMG has been proposed to overcome

this disadvantage [2], [3]. Furthermore, non-Lorenzian but

rippled passbands may exist in a chirped super Moir grating

(CSMG), which is a special MG with multiple ( 2) superim-

posed chirped gratings at different wavelengths. Anyway, few

literatures discuss CSMG due mainly to the difficulties in its

precise fabrication.

On the other hand, sampled Bragg gratings (SBG) have

attracted much attention for their compactness and potential

application in WDM systems such as multichannel filtering,

multichannel optical add/drop multiplexing [4], [5] and dis-

persion compensation [6]. In this letter, we show that a SBG

Manuscript received May 23, 2000; revised June 30, 2000. This work wassupported by the Fundamental Research Foundation, School of the InformationScience and Technology, Tsinghua University, and the Foundation for VisitingScholar, State Key Labs, State Education Commission. This work was also sup-ported in part by the 863 High Tech Foundation under Contract 863-307-11-3(02) and the Natural Science Foundation of China under Contracts 69525407and 69896260-1.

The authors are with the Information Optoelectronic Technology ResearchInstitute (IOTRI) and State Key Lab on Integrated Optoelectronics, Departmentof Electronic Engineering, Tsinghua University, Beijing 100084, China.

Publisher Item Identifier S 1041-1135(00)09584-7.

can be regarded as an equivalent super Moir grating (SMG)

[6], [7]. When the grating period of SBG is strongly chirped,

multiple passbands with small ripples may appear. It is shown

that, when the distribution of the refractive index modulation

amplitude within the sample is Gaussian, the performance of

the proposed filter can be improved intrinsically: flat top with

very small ripple in peak transmittance, steep edge and nearly

linear phase response. Furthermore, the proposed device can

also have multiple flat equalized stopbands that may facilitate

the design and application of multichannel optical add/drop

multiplexers (OADM).

II. PERFORMANCE OF THE PROPOSED DEVICE

A SBG is a Bragg grating modulated by a periodic sampling

function. In terms of Fourier analysis, it can be treated as an

equivalent SMG with multiple superimposed ghost gratings [6].There are multiple stopbands corresponding to superimposed

ghost gratings and also multiple reflection peaks. When a SBG

is chirped in grating period (CSBG), all ghost gratings in the

structure are chirped with identical chirping coefficient. The

chirp of CSBG can be expressed as

(1)

wheregrating period along the structure;

grating period at the center of the grating;

chirping coefficient;

length of SBG.

When the chirp is weak, the stopbands are not overlapped, light

incident to theSBG is affectedonly by oneghost grating. In such

a situation, there is no resonance transmission in the structure.

However, when the chirp is strong, the stopbands are widely

broadened and several neighboring stopbands related to the cor-

responding ghost gratings may be overlapped. Incident light

with wavelength located within the overlapped region will be

affected by several ghost gratings, and may transmit through the

CSBG without impediment. Therefore, when the chirp is strongenough, there may be multiple passbands in such a CSBG. It is

should noted that passbands produced by the resonance related

to multiple superimposed gratings (not two superimposed grat-

ings in CMG); that is to say, the optical response in a CSMG is

the interference of the response of multiple CMGs. In such a

situation, the passbands are non-Lorenzian. The passbands can

be flat (with small ripples) as shown in Fig. 1(a), where the

sample number , sampling ratio (sample length di-

vided by sampling period) , mm and

m. The average refractive index of the grating is

10411135/00$10.00 2000 IEEE

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1502 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 12, NO. 11, NOVEMBER 2000

Fig. 1. Transmission spectrum of a strongly chirped sampled Bragg gratingmm, mm , , . (a)and . (b) and .

1.485. Please note the index modulation amplitude is constant

in each sample.

In order to further improve the filter performance (further re-

duce the ripple), a Gaussian, rather than constant, spatial profile

of the refractive index modulation amplitude is applied within

each sample

(2)

where

refractive index modulation amplitude within

each sample;

and parameters;

sample length (in the order of millimeter).

Performance of the proposed CSBG filter is shown in Fig. 1(b)

for . In Fig. 2, a peak from Fig. 1(b) is replotted, showing

flat passbands with very small ripples, steep edges and high

transmittance.

The phase response of CSBG is also investigated, wherein the

group delay of the filter is also plotted in Fig. 2, showing verysmall variance. This means that the dispersion in the passbands

is nearly zero.

As an example, a CSBG appropriate for 32-channel filtering

with 50-GHz channel spacing is shown in Fig. 3. The averaged

transmittance is 99.7% and the ripple is less than 0.1 dB. The

0.1 dB, 3 dB, and 20 dB bandwidths are 0.23, 0.27, and

0.31 nm, respectively. Although similar performance of pass-

bands can be obtained with the idea shown in [ 2], there are

some distinct differences in between: 1) mechanismthe CSBG

proposed by this work is based on an equivalent CSMG, not

a CMG [2], consequently the passbands is intrinsically non-

Lorenzian; 2) channel spacingin CSBG, the spacing between

Fig. 2. Transmittance (solid line) and group delay (dotted line) of a peak inFig. 1(b).

Fig. 3. Transmission spectra of a strongly chirped sampled Bragg gratingmm, mm , , , , and

.

neighboring passbands can be precisely controlled because it

varies with the sampling period, which is in the order of mil-

limeters; 3) fabricationCSBG can be fabricated by single ex-

posure of a single linearly chirped phase mask [6], not by dual

exposure [1][3]. Furthermore, the bandwidth of the passbandscan be individually adjusted by the parameter . All these fea-

tures will facilitate the applications of CSBG in WDM systems.

Low-cost and high-performance grating filters with multiple

reflection wavelengths is also important for OADM applica-

tions. In an unchirped or lightly chirped SBG, the dispersion is

large in the stop-band edge although the filter performance can

be almost identical between channels with careful design [8].

This disadvantage can be minimized in a strongly chirped SBG,

which can be seen in Fig. 4. The filter performance is near-iden-

tical among channels. Fig. 5 shows an enlarged view of a stop

band as well as its group delay. In addition to nearly flat spec-

trum response, the in-band difference of the group delay is less

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CHEN et al.: MULTICHANNEL FILTER BASED ON STRONGLY CHIRPED SAMPLED FIBER BRAGG GRATING 1503

Fig. 4. Reflection spectrum of a strongly chirped sampled Bragg gratingmm, , mm , , , and

.

Fig. 5. Reflectance (solidline)and groupdelay(dotted line) ofa peak in Fig. 4.

than 30 ps. These features can be used to fabricate high-per-

formance multichannel add/drop multiplexer filters for WDM

applications.

III. CONCLUSION

In this letter, we have proposed a novel multichannel trans-

mission filter based on a strongly CSBG. The principle is re-

lated to an equivalent CSMG mechanism and is different from

published works. By simply chirping a conventional SBG that

can be fabricated with a single exposure of a single linearly

chirped phase mask, passbands with small ripples can be ob-tained. When the spatial profile of the refractive index modula-

tion amplitude within each sample is Gaussian, the performance

of the proposed filter is improved: nearly flat passbands with

very small ripples in peak transmittance, steep edges, nearly

linear phase responses and high transmittance. Similar flat stop-

bands can also be obtained in the proposed device. With shorter

length and fewer number of samples, the proposed device can

be used as multichannel filters and multichannel OADM filters

in WDM systems.

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