Fiber Bragg Grating And Phase Conjugator As Dispersion Compensator

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International Journal on Advanced Electrical and Electronics Engineering, (IJAEEE), ISSN (Print): 2278-8948, Volume-1, Issue-1, 2012

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Fiber Bragg Grating And Phase Conjugator As Dispersion Compensator

Smita S. Dabhade & Savita Bhosale

MAHATMA GANDHI MISSION’S COLLEGE OF ENGG. AND TECHNOLOGY Sector-18, Kamothe, Navi Mumbai-410209.

E-mail : [email protected]

Abstract - Among the promising advancements towards cost-effective long-haul transmission are the use of Fiber Bragg Gratings (FBGs) and Optical Phase Conjugator (OPC) as the dispersion compensating module (DCM). In this paper we discuss the the performance of FBG and OPC by comparing the results of the receivers. A 10 Gb/s Non Return To Zero (NRZ) signal is launched onto a 100 km long standard single mode fiber. Comparison of eye diagrams & Bit Error Rate (BER) and quality factor show a marked improvement in the link performance due to compensation of dispersion.

Keywords : Fiber Bragg gratings (FBG), optical Phase Conjugator(OPC) , dispersion compensation.

I. INTRODUCTION

Dispersion compensating fiber (DCF) is currently used as the standard solution for dispersion compensation in long-haul transmission links, since it yields colourless, slope matched dispersion cancellation with negligible cascading impairments. However, DCF is also limited in optical input power to avoid nonlinear impairments, has a relatively high insertion loss and is bulky. Chirped FBGs could possibly replace DCF as the standard solution for in-line dispersion compensation.

Chirped FBGs have a negligible nonlinearity, low insertion loss and small size. Wavelength Division Multiplexing (WDM) systems where multiple light signals at different frequencies are simultaneously launched in an optical fiber, the highly-selective filtering capabilities of Bragg gratings combined with its all-fiber configuration and flexibility make this technology an ideal candidate. In telecommunication applications, FBG-components have already been used for purposes such as pump laser stabilizers to improve the performances of pump lasers in optical amplifiers, gain flattening filters to equalize the gain of optical amplifiers, highly selective filters for channel selection in dense WDM systems and chromatic dispersion compensators for temporal pulse shaping in long-haul and/or high bit rate systems . This potentially allows simpler erbium-doped fiber amplifier (EDFA) design by cascading the FBG transmission fiber without a mid- stage amplifier, resulting in a significant cost reduction.

The formation of permanent gratings in an optical fiber was first demonstrated by Hill et al. in 1978 at the Canadian Communications Research Centre (CRC), Ottawa, Ont., Canada, [1]. They launched intense Argon-ion laser radiation into a germania-doped fiber and observed that after several minutes an increase in the reflected light intensity occurred which grew until almost all the light was reflected from the fiber. Spectral measurements, done indirectly by strain and temperature tuning of the fiber grating, confirmed that a very narrowband Bragg grating filter had been formed over the entire 1-m length of fiber. This achievement, subsequently called “Hill gratings,” was an outgrowth of research on the nonlinear properties of germania-doped silica fiber. It established an unknown photosensitivity of Germania fiber, which prompted other inquires, several years later, into the cause of the fiber photoinduced refractivity and its dependence on the wavelength of the light which was used to the form the gratings. Detailed studies show that the grating strength increased as the square of the light intensity, suggesting a two-photon proc. In the original experiments, laser radiation at 488 nm was reflected from the fiber end producing a standing wave pattern that formed the grating. A single photon at one-half this wavelength, namely at 244 nm in the ultraviolet, proved to be far more effective. Meltz et al. showed that this radiation could be used to form gratings that would reflect any wavelength by illuminating the fiber through the side of the cladding with two intersecting beams of UV light;

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Fiber Bragg Grating And Phase Conjugator As Dispersion Compensator

International Journal on Advanced Electrical and Electronics now, the period of the interference maxima an index change was set by the angle between the beams and the UV wavelength rather than by the visible radiation which was launched into the fiber core.

Moreover, the grating formation was found to be orders of-magnitude more efficient. At first, the o

photo-induced refractivity in fibers was only a scientific curiosity, but over time it has become the basis for a technology that now has a broad and important role in optical communications and sensor systems. Research into the underlying mechanisms of fiber photosensitivity and its uses is on-going in many universities and industrial laboratories in Europe, North and South America, Asia, and Australia. Several hundred photosensitivity and grating related articles have appeared in the scientific literature and in the proceedings of topical conferences, workshops, and symposia. FBG‟s are now commercially available and they have found key applications in routing, filtering, control, and amplification of optical signals in the next generation of high-capacity WDM telecommunication networks. In this paper, the performance of high speed

.FBGs are created by the exposition of a photosensitive fiber an intensity pattern of UV light. In its basic form, the resulting grating reflects selectively the light guided by the optical fiber at the Bragg wavelength given by:

λB = 2nΛ

where n and Λ are the effective index of

the fiber and the pitch of the grating in the fiber. A uniform grating can be represented by a sinusoidal modulation of the refractive index of the fiber core given by:

where ncore is the unexposed core refractive index and δn is the amplitude of the photoinduced index change.

One can see that depending on the refractive index change profile and intensity in the fiber combined with the pitch of the grating profile, numerous types of

International Journal on Advanced Electrical and Electronics Engineering, (IJAEEE), ISSN (Print): 2278-8948, Volume-1, Issue

16 now, the period of the interference maxima and the index change was set by the angle between the beams and the UV wavelength rather than by the visible radiation which was launched into the fiber core.

Moreover, the grating formation was found to be orders- magnitude more efficient. At first, the observation of

induced refractivity in fibers was only a scientific curiosity, but over time it has become the basis for a technology that now has a broad and important role in optical communications and sensor systems. Research chanisms of fiber photosensitivity going in many universities and industrial laboratories in Europe, North and South America, Asia, and Australia. Several hundred photosensitivity and grating related articles have fic literature and in the proceedings of topical conferences, workshops, and s are now commercially available and they have found key applications in routing, filtering, control, and amplification of optical signals in the next capacity WDM telecommunication networks. In this paper, the performance of high speed

optical fiber based network is analysed by using dispersion compensating module (DCM) based on FBGs. The optimal operating condition of the DCM was obtained by considering dispersion management using a 10 Gb/s Non Return To Zero (NRZ) signal by launching it into a 100 km long standard single mode fiber. Section II discusses the proposed work regarding the dispersion compensation using a FBG configuration. Results for the simulation are validated and the impact of the FBG on the receivers is compared in section III, followed by the concluding remarks in the Section IV.

II. PROPOSED WORK 2.1 FBG

A fiber Bragg grating consists of a periodic modulation of the index of refraction along the core of an optical fiber thus creating a wavelength

as presented in Following .

FBGs are created by the exposition of a photosensitive fiber an intensity pattern of UV light. In its basic form, grating reflects selectively the light guided by the optical fiber at the Bragg wavelength given by:

refraction of the fiber and the pitch of the grating in the fiber. A uniform grating can be represented by a sinusoidal modulation of the refractive index of the fiber core

unexposed core refractive index and δn is the amplitude of the photoinduced index change.

One can see that depending on the refractive index change profile and intensity in the fiber combined with the pitch of the grating profile, numerous types of

functions can be devised Wavelength Division Multiplexing (WDM) systems where multiple light signals at different frequencies are simultaneously launched in an optical fiber, the highly selective filtering capabilities of Bragg gratings combined with its all configuration and flexibility make this technology an ideal candidate. FBG technology has gained favor in wide range of applications because of its all fiber configuration, great flexibility, and highly efficient filtering functions. Fiber Bragg grating

commonly used for stabilizing pump lasers in optical amplifiers wavelength division multiplexing, filtering and chromatic dispersion compensation, because their efficiency reduces the cost of optical networking.

The design approach explores dis compensating module (DCM) discussing the Fiber Bragg Gratings (FBGs) for the dispersion compensation.

A 10 Gb/s Non Return To Zero (NRZ) signal is launched onto a 100 km long standard single mode fiber as shown in the Fig.2.

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optical fiber based network is analysed by using dispersion compensating module (DCM) based on FBGs. The optimal operating condition of the DCM was dering dispersion management using a 10 Gb/s Non Return To Zero (NRZ) signal by launching it into a 100 km long standard single mode fiber. Section II discusses the proposed work regarding the dispersion compensation using a FBG configuration. Results for the simulation are validated and the impact of the FBG on the receivers is compared in section III, followed by the concluding remarks in the Section IV.

A fiber Bragg grating consists of a periodic refraction along the core of an optical fiber thus creating a wavelength-selective mirror

ons can be devised Wavelength Division Multiplexing (WDM) systems where multiple light signals at different frequencies are simultaneously launched in an optical fiber, the highly selective filtering capabilities of Bragg gratings combined with its all-fiber configuration and flexibility make this technology an ideal candidate. FBG technology has gained favor in wide range of applications because of its all fiber configuration, great flexibility, and highly efficient filtering functions. Fiber Bragg gratings are most commonly used for stabilizing pump lasers in optical amplifiers wavelength division multiplexing, filtering and chromatic dispersion compensation, because their efficiency reduces the cost of optical networking.

The design approach explores dispersion compensating module (DCM) discussing the Fiber Bragg Gratings (FBGs) for the dispersion compensation.

A 10 Gb/s Non Return To Zero (NRZ) signal is launched onto a 100 km long standard single mode fiber

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International Journal on Advanced Electrical and Electronics Engineering, (IJAEEE), ISSN (Print): 2278-8948, Volume-1, Issue-1, 2012

17 .

. The model illustrates how to compensate fiber dispersion using the realistic fiber grating component.

The dispersion compensation is performed using advanced optical communication system simulation OptSim. The Eye diagram before compensation and after compensation shown below.

Before Compensation

After Compensation

FBG Real Before

compensation After compensation

Q factor 9.930533dB 28.486501dB BER 0.000875879 1e-040

2.2 OPC (Optical Phase Conjugator)

Group-velocity dispersion and optical nonlinearity are the major limiting factors in high-speed long- distance fiber-optic transmissions. Dispersion- compensating fibers (DCFs) have been developed to offset the dispersion effects of transmission fibers over a wide frequency band. The most advanced DCFs are even capable of slope-matching compensation, namely, compensating the dispersion and the dispersion slope of the transmission fiber simultaneously. In general, it is difficult to perfectly compensate the fiber dispersion across a wide frequency band. There are always residual dispersion and higher order derivatives, even using the best slope-matching DCFs . The significance of the residual dispersions increases as the total signal bandwidth becomes wider. It has been proposed for some time that optical phase conjugation (OPC) may be employed in the middle of a transmission line to equalize the dispersion effect of the transmission fibers.

Furthermore, theoretical and experimental studies have proved the feasibility of using OPC to compensate the fiber nonlinearities.

2.2.1 Optical phase concept

It is possible, using nonlinear optical processes, to exactly reverse the propagation direction and phase variation of a beam of light. The reversed beam is called a conjugate beam, and thus the technique is known as optical phase conjugation (also called time reversal, wavefront reversal. The most common way of producing optical phase conjugation is to use a four- wave mixing technique, though it is also possible to use processes such as stimulated Brillouin scattering. A device producing the phase conjugation effect is known as a phase conjugate mirror (PCM).

The pulse propogation equation is given by

6 0

2 ³

3 3 2 2

2 =

∂

− ∂

∂ + ∂

∂

t A t

A i Z

A β β

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International Journal on Advanced Electrical and Electronics Where A is pulse envelop amplitude. The effect of third order dispersion is included by the

β

³term. Dispersion of pulse takes place because of some phase factor acquired by spectral components of the pulse during its propagation in the fiber. All dispersion management schemes attempt to cancel this phase factor so that the input signal can be restored.

If we take complex conjugate of above equation then equation becomes

* 0 6

2 ³

3 3 2

* 2 2

* =

∂

− ∂

∂

− ∂

∂

t A t

A i Z

A β β

Above equation shows that the phase conjugated field A^* prorogates with the sign reversed for the GVD parameter

β

₂.This shows that if optical field is phase conjugated in the middle of fiber link the dispersion acquired over the first half will be exactly compensated in the second half section of the link. Since the

. fiber link. At the output of the second link the received eye is completely open. The principle of operation for OPC is phase inversion according to E_in(t)= A(t)e ^j^ϕ⁽^t⁾

And

shift j t j

out t At e e

E ( )=η () ⁻^φ⁽⁾ ^φ

Where

η

is device efficiency and φ_shiftis extra constant phase shift .Here φshift =π/2 and

η

^=1.

The eye diagram before transmission, after

18 Where A is pulse envelop amplitude. The effect of third

term. Dispersion pulse takes place because of some phase factor acquired by spectral components of the pulse during its propagation in the fiber. All dispersion management schemes attempt to cancel this phase factor so that the lex conjugate of above equation then

0

Above equation shows that the phase conjugated prorogates with the sign reversed for the GVD .This shows that if optical field is phase conjugated in the middle of fiber link the dispersion acquired over the first half will be exactly compensated ion of the link. Since the

β

₃^term

does not change sign on phase conjugation ,OPC cannot compensate for the third order dispersion. Actually OPC compensates for all even order dispersion term while leaving the odd order term unaffected.

2.2.2 Optical Phase Conjugator as dispersion and non-linearity compensator

The schematic setup is given below and it contains transmitter, a fiber span consisting of two links, a mid span phase conjugator, and receiver. A 10 Gb/s NRZ data stream is sent over a first lossless fiber link.

transmitted optical power is substantially higher dBm). This power increase in optical power causes non linearity effects become important.

After 100 km the eye diagram shows a completely closed eye, due to the accumulation of chromatic dispersion and non-linearity. Then the signal goes through an OPC, and then to another 100 km

.

fiber link. At the output of the second link the received eye is completely open. The principle of operation for OPC is phase inversion according to

is extra constant

The eye diagram before transmission, after 100km,after

optical phase conjugator and after 100km at the receiver is shown below

Before Transmission

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does not change sign on phase conjugation ,OPC cannot compensate for the third order dispersion. Actually OPC compensates for all even order dispersion term while leaving the odd order term unaffected.

2.2.2 Optical Phase Conjugator as dispersion and

The schematic setup is given below and it contains transmitter, a fiber span consisting of two links, a mid- span phase conjugator, and receiver. A 10 Gb/s NRZ

ver a first lossless fiber link. The substantially higher (+9 dBm). This power increase in optical power causes non- After 100 km the eye diagram shows a completely accumulation of chromatic linearity. Then the signal goes an OPC, and then to another 100 km long

after 100km at the receiver

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International Journal on Advanced Electrical and Electronics After 1000km

After Phase Conjugator

After 1000km

OPC Non- linear

Before

Transmission After 100km After Phase Conjugator

Q factor 40dB 12.438639dB 12.438639dB BER 1e-040 2.06025e-005 2.06025e-005

and after 100km transmission we get Q factor of 12.438639dB and after phase conjugator and 100km transmission the Q factor is 30.672950dB.The BER before transmission and at the receiver is same that is 1e-040.

19

After Phase

Conjugator After 100km

12.438639dB 30.672950dB 005 1e-040

and after 100km transmission we get Q factor of and after phase conjugator and 100km Q factor is 30.672950dB.The BER before transmission and at the receiver is same that is

III. CONCLUSIONS

It is shown in this paper that the recent advances in fiber Bragg grating technology and Optical phase conjugator now allow the realization of a high performance, high speed optical fibers with good in line dispersion compensation. The system

evaluated for a 10 Gb/s system using FBG and OPC for the in line dispersion compensation. The results demonstrate that the Q factor increases by near about three times in FBG and in OPC by 18Db

REFERENCES

[1] www.fiberoptics4sale.com/wordpress/what fiber-bragg-grating

[2] http://en.wikipedia.org/wiki/Fiber_Bragg_grating [3] https://globaljournals.org/GJRE_Volume11/4

Impact-of-Fiber-Bragg-Grating Compensator.pdf

[4] http://www.rp-

photonics.com/fiber_bragg_gratings.html [5] http://en.wikipedia.org/wiki/Nonlinear_optic [6] http://sharp.bu.edu/~slehar/PhaseConjugate/

PhaseConjugate.html

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is shown in this paper that the recent advances in fiber Bragg grating technology and Optical phase conjugator now allow the realization of a high- performance, high speed optical fibers with good in line dispersion compensation. The system performance is valuated for a 10 Gb/s system using FBG and OPC for the in line dispersion compensation. The results Q factor increases by near about three times in FBG and in OPC by 18Db.

www.fiberoptics4sale.com/wordpress/what-is- en.wikipedia.org/wiki/Fiber_Bragg_grating

globaljournals.org/GJRE_Volume11/4- Grating-As-Dispersion-

photonics.com/fiber_bragg_gratings.html http://en.wikipedia.org/wiki/Nonlinear_optic http://sharp.bu.edu/~slehar/PhaseConjugate/