dispersion compensation using fb fiber

7/27/2019 Dispersion Compensation Using Fb Fiber

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IEEE JO URNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 5, NO. 5, SEPTEMBER/OCTOBER 1999 1339

Dispersion Compensation UsingOnly Fiber Bragg Gratings

Paul Petruzzi, Curt Lowry, and Ponniah Sivanesan

AbstractEfficient and low-cost dispersion compensation isnecessary to achieve useful transmission distances in opticalcommunication systems at bit rates above 10 Gb/s. In this paper,we present a novel method for dispersion compensation thatuses three fiber gratings in series. This method eliminates therequirement for an optical circulator, commonly used in grating-based dispersion compensators. As a result, our device will beinexpensive to fabricate and have a low insertion loss. We presenta theoretical model that predicts how the efficiency of the devicecan be maximized. We also present experimental results showingcomplete dispersion compensation for 30-ps pulses, broadened to67 ps by propagating through 10 km of standard, singlemodefiber.

Index Terms Chirped fiber grating, cladding modes, disper-sion compensation, fiber grating.

I. INTRODUCTION

OPTICAL fiber communication systems are primarily

operated at wavelengths near 1550 nm in order to

coincide with the minimum loss point of silica fiber and

thereby maximize transmission distance. Unfortunately, at

this wavelength, there is a significant amount (typically 17

ps/nm/km) of group velocity dispersion (GVD) that limits the

achievable propagation distance. For a system that operates at a

data rate greater than 10 Gb/s, the maximum distance is on the

order of kilometers. Beyond this distance, GVD induces signalpulse broadening that causes significant overlap of neighboring

pulses and results in signal corruption. This problem can

be solved by inserting an element that imposes GVD on

the optical signal that is opposite to that imposed by the

fiber, thereby compensating for the naturally occurring GVD.

Currently, there are three common methods of performing

dispersion compensation: 1) adding dispersion compensating

(DC) fiber, 2) reversing optical signal chirp through optical

phase conjugation, and 3) inserting a chirped fiber grating to

impose compensating GVD.

DC fiber has high negative dispersion that is typically near

90 ps/nm/km. A pulse that has been broadened by the

positive GVD of a singlemode fiber (SMF) such as SMF-28 can be compressed back to its original width using the

negative GVD of DC fiber. Typical GVD values require an

approximately 5:1 length ratio of SMF to DC fiber types.

Transmission distances of a single channel over 960 km [1]

Manuscript received January 4, 1999; revised May 3, 1999.P. Petruzzi and C. Lowry are with Laboratory for Physical Sciences, College

Park, MD 20740 USA.P. Sivanesan is with Smart Structures and Materials Research Center,

University of Maryland, College Park, MD 20721 USA.Publisher Item Identifier S 1077-260X(99)07929-0.

at 10 Gb/s, and more recently, 32 channels at 20 Gb/s [2]

over 1200 km have been reported using various DC fiber

configurations. This technique has the advantage of being

passive in that it does not require applied power for operation.

It is also broadband and can be used in high bit-rate or

WDM systems. However, this technique has the disadvantage

of inducing significant power loss (typically 0.7 dB/km) and

incurring significant expense, compared to the use of SMF-28

fiber alone. These problems become more important when an

existing fiber link is being upgraded to a higher bit rate. The

high loss and long length of DC fiber required for dispersion

compensation make it necessary to add an optical amplifier tothe link which further increases link complexity and cost.

The pulse stretching induced by the positive GVD of SMF

also induces pulse frequency chirp. The effect of GVD on

a signal can be eliminated by reversing the pulse chirp and

then propagating the signal through another equal length of

SMF. Chirp reversal can be accomplished through optical

phase conjugation by using four-wave mixing in a DC fiber

or a semiconductor optical amplifier. Continued propagation

through a medium with positive GVD can then compress the

pulse back to its original width. This method has been used to

transmit signals 400 km at 2.5 Gb/s [3] and 152 km at 5 Gb/s

[4], but it typically induces 20-dB loss due the low efficiency

of four-wave mixing (FWM).Chirped fiber gratings induce different time delays for

different wavelengths. This rechirping can be designed to

reshape a pulse that was broadened by GVD and compress

it back to its original width. Chirped gratings can be made to

affect a broad-band [5] of optical signals or a narrow-band [6],

depending on the application. In addition, they are compact,

passive, and have low insertion loss. This method of dispersion

compensation has allowed 10 Gb/s data to be transmitted 100

[7], 500 [8], and 700 km [9] without using a repeater, and it

is coming into wide use in optical networks. However, with

this method, the optical signal is back-reflected by the chirped

grating and travels back along the input fiber. An opticalcirculator is required to separate the back-traveling signal from

the incoming light in order to allow the rechirped signal to

continue propagation along the communication link. While

the gratings are inexpensive and have low insertion loss, the

optical circulator is an expensive component. The circulator

also induces approximately 3-dB signal loss for the required

double pass. This circulator loss in turn makes greater optical

amplification necessary and further increases system cost.

Eliminating the need for an optical circulator would allow us

to take advantage of the inherent low cost and low insertion

1077260X/99$10.00 1999 IEEE


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1340 I EEE JOU RNAL OF SELECTED TO PICS IN QUA NTUM ELECTRO NICS, V OL. 5, NO . 5, SEPTEMBER/OCTOBER 1999

Fig. 1. Optical signal is reflected into a back-traveling cladding mode by

grating-2, then into a forward-traveling cladding mode by grating-1, and finallyinto a forward-traveling core mode by grating-3.

loss of chirped fiber gratings and it would permit existing

systems to be upgraded to higher bit rates without requiring

additional optical amplification. Dispersion compensation has

been done without requiring optical circulators by using long-

period gratings. However, one technique uses a fiber with a

larger core [10], compared to SMF-28, which induces loss

when the light is coupled back into standard SMF-28 fiber.

Another technique uses two long-period gratings [11] in SMF

but requires critical constraints on the grating separation.

In this paper, we present a novel method of dispersion

compensation that uses three fiber gratings and does notrequire an optical circulator or critical grating spacing. This

method induces only low insertion loss and can eliminate

the expense of both the circulator and the amplifier. We

use coupled mode theory to model and explain propagation

through our gratings. We also experimentally demonstrate

complete compensation of dispersion induced in a 30-ps pulse

by propagation through 10 km of SMF-28 fiber.

II. CONCEPT

Traditionally, fiber grating dispersion compensators use

an optical circulator to distinguish forward and backward

propagating light. In our design, we distinguish forward andbackward propagating light by using three gratings in series

to couple light into different cladding modes. Fig. 1 schemati-

cally shows the operation of the gratings. Light enters the fiber

section, propagating in the core or LP mode, and passes

through grating-1 without being affected. It is then back-

reflected by grating-2 and coupled into a backward traveling

cladding mode, LP . Because grating-2 can be properly

chirped, it can also compress the pulse by inducing negative

GVD. The compressed pulse is then reflected by grating-1

into a forward traveling cladding mode, LP . The LP

mode in turn passes through grating-2 without being affected.

Finally, the light is coupled back into the forward traveling

LP mode by grating-3.Grating-1 and grating-2 each perform two operations. Each

grating is transparent to light in one mode and reflective

of light of another mode. This is accomplished by properly

designing the period of each grating, while considering the

incoming and outgoing modes and the wavelength of the light.

The combined effect of these two operations allows light to

be twice back-reflected by the gratings and, without an optical

circulator, regain its original, forward direction of propagation.

As a result, one or more of the gratings can be chirped to create

a fiber grating system for dispersion compensation which does

not require an optical circulator.

Fig. 2. F-ber Bragg grating reflects a core mode into a core mode (largepeak) and also couples the core mode into cladding modes (small peaks).

Eliminating the need for a circulator in grating-based disper-

sion compensators significantly reduces the cost and fabrica-

tion complexity of the device. Gratings are written by placing

a phase or amplitude mask over a section of properly prepared

fiber and illuminating it with UV ( 244 nm) light [12]. If a

template is made with three adjacent masks, the entire device

can be made at one time, simplifying fabrication. Also, theoperation of the device is not interferometric, so the distance

between the gratings, and consequently the separation between

the masks, is not critical.

Important design issues for our device are the ability of

gratings to couple strongly to cladding modes and have

a bandwidth large enough to accommodate the incoming

signal. Long-period gratings inherently have a high coupling

efficiency to cladding modes and a large bandwidth [13].

These gratings meet our design specifications and are currently

commercially available. There are also commercially available

reflection gratings that have reflection efficiencies greater than

30 dB. However, this efficiency refers to the coupling strength

between forward and backward traveling core modes. Ourdevice requires core to cladding mode coupling with similar

efficiencies. Methods for designing to this purpose will be

discussed in the next section.

III. THEORY

A fiber grating is typically made using a frequency doubled

argon or excimer laser and a phase mask [14]. The phase

mask creates a sinusoidally varying interference pattern that

interacts with the germanium in the core of the fiber to create

a periodic variation in the index of refraction in the fiber

core. The transmission spectrum of a grating fabricated in this

fashion is shown in Fig. 2. The large peak represents reflectionof a forward traveling core mode into a backward traveling

core mode (core mode resonance). The smaller peaks on the

short wavelength side of the large peak represent reflection

of a forward traveling core mode into a backward traveling

cladding mode (cladding mode resonance). Cladding mode

resonances like these are used in the operation of our three-

grating dispersion compensator.

In order for our device to be have low insertion loss, there

must be much stronger coupling to cladding modes than seen

in Fig. 2. This can be accomplished by blazing the grating.

In a blazed grating, the index of refraction pattern mentioned


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PETRUZZI et al.: DISPERSION COMPENSATION USING ONLY FBGS 1341

Fig. 3. Induced change in the index of refraction of a blazed grating is tilted,with respect to the axis, to increase the coupling efficiency from the incident(LP

) to the reflected (LP

) mode by the grating.

above is tilted with respect to the direction of propagation ( -

direction), as illustrated in Fig. 3. Using coupled mode theory,

the coefficient for coupling from an incident LP mode to a

reflected LP mode by a blazed grating is found to be [15]

(1)

where is related to the amplitude of induced index change,

is the transverse electric field of the LP mode, and

is the transverse electric field of the LP mode. This

equation shows that the coupling between two modes is due

to two factors: 1) the overlap between their transverse electric

fields and 2) the phase matching completed by the blaze angle

of the grating. Using this result, the maximum reflectivity at

the resonance is [16]

(2)

where is the length of the grating.

For the unblazed grating shown in Fig. 2, the wavelength,

maximum reflectivity, and bandwidth of the core mode res-

onance was measured. Then, (18), (20), and (30) from [14]

were used to calculate period, amplitude, and length of the

grating. These values were then substituted into (1) and (2)

above and the coupling of the LP mode to the LP mode

was calculated to be 47%. This is in fairly good agreement with

our measured value of 37%. Using the same grating period,

amplitude, and length, the coupling of an LP mode to an

LP mode by a grating with a blaze angle of 6 was calculated

to be 94%. In this example, introducing the proper blaze angle,

by rotating the phase mask with respect to the fiber, doubledthe coupling efficiency. The cause of the increase is not that

the LP mode has a larger portion of its field confined to the

core, compared to the LP mode, and thus has a larger overlap

with the LP mode. Rather, the large increase in coupling

is due to improved phase matching of the core mode to the

cladding mode by the blazed grating [17]. The coupling can

be further enhanced by increasing the amplitude or the length

of the grating as shown by (1) and (2). This is evident in

the transmission spectrum of a commercially available blazed

grating, shown in Fig. 4. In this grating, the coupling to the

first cladding mode is greater than 40 dB.

Fig. 4. Blazed fiber grating reduces the coupling core mode resonance( 1563 nm) and increases the coupling of the cladding mode resonances( 1561 nm).

A second design requirement for efficient operation of our

three-grating compensator is the existence of spectral regions

between the cladding modes where the grating reflection is

low. These regions are necessary because, as illustrated in

Fig. 1, the LP mode must pass through grating-1 without

being reflected and the LP mode must likewise pass throughgrating-2. Any light reflection at these points in the light

path causes scattering loss and a decrease in the efficiency of

our device. Therefore, although the blazed grating in Fig. 4

has strong coupling to cladding modes, it would lead to

high insertion loss if used in our device because no clear

transmission band is available. This problem can be avoided by

writing the grating in a fiber that has a properly designed index

profile. The W-fiber has an inner cladding region that has an

index that is less than the core, as depicted in Fig. 5(a). When

a blazed grating is written in this type of fiber, the coupling to

one cladding mode is approximately 20 dB, while most of the

other modes have been surpressed, as seen in Fig. 5(b) [18].

The use of this type of grating would allow our three-gratingdevice to have a low insertion loss.

In addition to the coupling efficiency, we must also consider

the bandwidth of the cladding mode resonance. If the signal

bandwidth, which typically depends on the bit rate, is larger

than the grating bandwidth, then portions of the spectrum will

be filtered out, leading to pulse shape distortion. In our device,

the bandwidth of unchirped grating-1, similar to the grating

in Fig. 5(b), limits the bandwidth of the entire device. These

gratings have a bandwidth of around 0.2 nm, and should pass

data with a bit rate of 40 Gb/s [19] with no distortion. To

accommodate even higher bit rates, we could increase the

overall bandwidth of the device by chirping both grating-1

and grating-2.The bandwidth and dispersion of a chirped fiber grating is

dependent on the slope of the chirp and the length of the grat-

ing. The dispersion results from a wavelength-dependent time

delay induced by varying the resonant frequency of the grating

along the -axis. The slope of the chirp (units of nm/cm) is

inversely proportional to the amount of dispersion. Also, since

the resonant frequency is being changed over the length of the

grating, the grating bandwidth is dependent on the product of

the slope and the grating length. Therefore, we can design a

grating with a specific dispersion and bandwidth by properly

choosing the slope of the chirp and the length of the grating.


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(a)

(b)

Fig. 5. (a) A W-fiber with an inner cladding

that has anindex of refraction that is less than the index of the core. (b) A blazedgrating that is written in a W-fiber has strong coupling to one cladding mode,while all of the other cladding mode resonances are surpressed.

IV. EXPERIMENT

The device concept and experimental setup are shown in

Fig. 6(a) and (b), respectively. We generated 30 ps full-widthat half-maximum (FWHM) pulses with 1542-nm wavelength

using an F-center laser. After propagating 10 km in standard

SMF, the pulses were amplified by an erbium-doped fiber

amplifier (EDFA) and then passed through the grating section

where they propagate through grating-1 and grating-2, as

described in Section II. The light is brought out of the fiber

and passed through a spatial filter to separate out the cladding

mode of interest. That signal was then coupled back into a

fiber, amplified by an EDFA, and measured using a 45-GHz

optical detector.

The gratings used in our experiment were not blazed, so

their efficiencies of coupling to cladding modes were approx-

imately 37%. This caused a significant portion of the lightthat entered the grating section to pass through unaffected and

exit in the LP mode. Therefore, in our experimental setup,

we replaced grating-3 [see Fig. 6(a)] with a spatial filter [see

Fig. 6(b)] to preferentially select the cladding mode of interest

and remove the unwanted LP mode. This mode selection

along with the second EDFA in Fig. 3(b) compensated for

the low efficiency of our gratings. More efficient blazed

gratings are available commercially, and this step will become

unnecessary in future work.

The uniform heating of grating-1, shown in Fig. 6(b), was

done in order to tune the cladding mode resonance. This

(a)

(b)

Fig. 6. (a) Optical signal follows the path shown schematically in Fig. 1.(b) Experimental setup follows the schematic description, with grating-2chirped by temperature and grating-3 replaced by a mode-selecting spatialfilter.

Fig. 7. A 30 ps pulse (diamonds) is broadened by linear dispersion to 67 ps

(squares) and then compressed back to its original width (line) using onlyfiber gratings.

maximized the coupling between the output mode of grating-2

(LP in Fig. 1) and the output mode of grating-1 (LP in

Fig. 1). Grating-2 is intrinsically not chirped in that it has

a purely periodic index modulation. However, we chirped

grating-2 by heating one end of the grating and cooling the

other.

The open diamond markers in Fig. 7 show the original 30

ps FWHM pulse measured at the point in Fig. 6(b) before

the SMF section. After passing through the 10 km of SMF,

the pulse was broadened by GVD to a width of 67 ps FWHM

(square markers in Fig. 7). Passing through the grating section,by reflecting first off of chirped grating-2 and then grating-

1, the pulse was recompressed (solid curve in Fig. 7). By

properly tuning the resonance wavelengths of the gratings

as well as the chirp of grating-2, we compensated for the

dispersion induced by transmission through 10 km of SMF

and brought the pulse back to its original 30 ps FWHM.

V. DISCUSSION

Heating a silica fiber uniformly creates a uniform shift in

the index of refraction. When this occurs across the length


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PETRUZZI et al.: DISPERSION COMPENSATION USING ONLY FBGS 1343

of a grating, the average index of refraction decreases and

the grating resonances shift equally to longer wavelength in

proportion to the change in index. Similarly, if a temperature

gradient in the -direction is created along the length of a

grating, the index has a -dependence. Therefore, the resonant

wavelength depends on , and this results in different time

delays for different wavelengths. This time delay is used to

compensate for the pulse dispersion induced by GVD in an

optical fiber.

The temperature tuning described above was necessary

in our experiment to maximize the output power from the

gratings and properly compensate for the GVD imposed on

the pulse. However, the output signal was still too weak to

detect, so it was amplified by an EDFA. This amplification

introduced timing jitter and noise into the compressed pulse

that prohibited averaging of the data and made the compressed

pulse noisier than the original pulse.

Proper design of the gratings can eliminate the need for

temperature tuning and optical amplification and allow this

method of dispersion compensation to be applied to many

optical communication systems. The bandwidth of the gratingsused in our experiment were 27 GHz, which supports a

transform limited pulse width of 16 ps. Therefore, these

gratings could be used in a system that has a bit rate up

to approximately 24 Gb/s. This bit rate can be increased

by varying the grating length or the slope of the chirp, as

discussed in Section III.

Our technique is also applicable to WDM systems. Gratings

are currently available that have only one resonance that

couples to a cladding mode [20]. Such gratings would allow

our device to select only one channel out of many in a

WDM system. Therefore, one device for each channel can

be placed in series and only the desired channel will interact

with its corresponding grating compensator and the remainderwill be unaffected. Alternatively, because the spacing between

gratings is not important, gratings-1 and 2 could be composed

of multiple gratings that would then compensate a wide band

of WDM channels with the same device.

VI. CONCLUSION

We combined a chirped fiber grating with a cladding mode

coupling grating to perform dispersion compensation without

the use of an optical circulator, while maintaining the forward

propagation direction. This device would be inexpensive to

fabricate and readily added to an existing system for upgrade

to a higher data rate or longer transmission distance. In the

future, we will use blazed gratings to increase the efficiency

and selectivity of the cladding mode coupling. We will also

use commercially available chirped gratings to increase the

amount of attainable dispersion compensation.

REFERENCES

[1] D. Breuer, K. Jurgensen, F. Kuppers, A. Mattheus, I. Gabitov, and S.K. Turisyn, Optimal schemes for dispersion compensation of standardmonomode fiber based links,Opt. Commun., vol. 140, pp. 1518, 1997.

[2] D. Le Guen, A. OHare, S. Del Burgo, D. Grot, F. Favre, and T. Georges,Narrowband 640 Gb/s soliton DWDM transmission over 1200 km ofstandard fiber with 100 km 0 21 dB amplifier spans, Electron. Lett.,vol. 24, pp. 23452346, 1998.

[3] R. M. Jopson, A. H. Gnauck, and R. M. Derosier, Compensation offiber chromatic dispersion by spectral inversion, Electron. Lett., vol.29, pp. 576578, 1993.

[4] S. Wanatabe, T. Naito, and T. Chikama, Compensation of chromaticdispersion in a single-mode fiber by optical phase conjugation, IEEEPhoton. Technol. Lett., vol. 5, pp. 9295, 1993.

[5] R. Kashyap, H. G. Froehlich, A. Swanton, and D. J. Armes, 1.3 m longsuper-step-chirped fiber Bragg grating with a continuous delay of 13.5ns and a bandwidth of 10 nm for broadband dispersion compensation,

Electron. Lett., vol. 32, pp. 18071809, 1996.

[6] K. O. Hill, S. Theriault, B. Malo, F. Bilodeau, T. Kitagawa, D.C. Johnson, J. Albert, K. Takiguchi, T. Kataoka, and K. Hagimoto,Chirped in-fiber Bragg grating dispersion compensator: Linearization ofdispersion characteristic and demonstration of dispersion compensationin 100 km, 10 Gb/s optical fiber link, Electron. Lett., vol. 30, pp.17551756, 1994.

[7] M. J. Cole, H. Greiger, R. I. Lammings, S. Y. Set, M. N. Zervas, W. H.Loh, and V. Gusmeroli, Broadband dispersion compensating chirpedfiber Bragg gratings in a 10 Gb/s NRZ 110 km nondispersion shiftedfiber link operating at 1.55 m, Electron. Lett., vol. 33, pp. 7071,1997.

[8] W. H. Loh, R. I. Lamming, N. Robinson, A. Cavaciuti, F. Vaninetti, C.J. Anderson, M. N. Zervas, and M. J. Cole, Dispersion compensationover distances in excess of 500 km for 10 Gb/s systems using chirpedfiber gratings, IEEE Photon. Technol. Lett., vol. 8, pp. 944946, 1996.

[9] W. H. Loh, R. I. Laming, A. D. Ellis, and D. Atkinson, 10 Gb/stransmission over 700 km of standard single-mode fiber with 10-cmchirped grating compensator and duobinary transmitter, IEEE Photon.Technol. Lett., vol. 8, pp. 12581260, 1996.

[10] N. H. Ky, H. G. Limberger, R. P. Salathe, and F. Cochet, Efficientbroadband intracore grating LP

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mode converters for chromatic-dispersion compensation, Opt. Lett., vol. 23, pp. 445446, 1998.

[11] B. Gisin, N. Gisin, and F. Cochet, Transmission gratings for chromaticdispersion compensation, Opt. Lett., vol. 21, pp. 686688, 1996.

[12] M. Douay, W. Xie, T. Taunay, P. Bernage, P. Niay, P. Cordieret al., Densification involved in the UV-based photosensitivity ofsilica glasses and optical fibers, J. Lightwave Technol., vol. 15, pp.13291342, 1997.

[13] A. M. Vengsarkaret al., Long-period fiber gratings as band-rejectionfilters, J. Lightwave Technol., vol. 14, p. 58, 1996.

[14] M. Douay, W. Xie, T. Taunay, P. Bernage, P. Niay, P. Cordieret al., Densification involved in the UV-based photosensitivity ofsilica glasses and optical fibers, J. Lightwave Technol., vol. 15, pp.13291342, 1997.

[15] T. Erdogan, Tilted fiber phase gratings,J. Opt. Soc. Amer. A., vol. 13,

pp. 296309, 1997.[16] , Fiber grating spectra, J. Lightwave Technol., vol. 15, pp.

12771294, 1997.[17] S. J. Hewlett, J. D. Love, G. Meltz, T. J. Bailey, and W. W. Morey,

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[18] C. W. Haggans, H. Singh, W. F. Varner, Y. Li, and M. Zippin,Narrow-band rejection filters with negligible backreflection using tiltedphotoinduced gratings in single-mode fibers, IEEE Photon. Technol.

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Paul Petruzzi received the B.S. degree in electrical engineering from thePennsylvania State University, University Park, in 1996.

He is currently a Graduate Research Assistant at the Laboratory for PhysicalSciences, the University of Maryland, College Park. His research includesfiber Bragg grating applications and polarization properties of vertical cavitysurface emitting lasers (VCSELs).


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Curt Lowry received the B.S. degree in electrical engineering from BrighamYoung University, Provo, UT, in 1985, and the Ph.D. degree in optical sciencesfrom the University of Arizona, Tucson, in 1993.

He joined the Laboratory for Physical Sciences, University of Maryland,College Park, in 1993, and has applied nonlinearities in optical fibers andsemiconductors to develop methods for signal header recognition, opticaldispersion compensation, and high-speed switching for optical networks. Heis the author or coauthor of ten journal papers and 14 conference papers, andholds one patent.

Ponniah Sivanesanreceived the B.S. degree in physics from the University ofJaffna, Sri Lanka, in 1989, and the M.S. degree in physics from the Universityof Maryland, College Park, in 1997.

He is currently a Graduate Research Assistant at the Smart Materials andStructures Research Center, the University of Maryland, College Park. Hisresearch interests include fiber Bragg grating applications in sensors and fiberlasers.

Mr. Sivanesan is a student member of the Optical Society of America(OSA).

dispersion compensation using fb fiber

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