An Overview of EDFA Gain Flattening by Using Hybrid Amplifier

Int. Journal of Electrical & Electronics Engg.

Vol. 2, Spl. Issue 1 (2015)

e-ISSN: 1694-2310 | p-ISSN: 1694-2426

An Overview of EDFA Gain Flattening by Using Hybrid Amplifier 1

ShivaniRadha Sharma, 2TanviSood 2

1

M.Tech Student, Assistant Professor 1,2 ECE Department, Chandigarh Engineering College, Landran, Mohali 1

379shivanisharma@gmail.com, 2cecm.ece.ts@gmail.com

Abstract—Data communication systems are increasingly engrossing optical fiber communication system as the transmission paths for the information, the information is in the form of light pulses sending from one place to another through the optical fiber. Several types of optical amplifiers have been developed in optical fiber communication system to amplify the optical signals. The erbium doped fiber amplifier is one of the optical fiber amplifiers which are used for long distance communication. The most significant points in any optical amplifier design are gain and noise figure. They are connected to one another. The other optical amplifier, Raman amplifier has wide gain bandwidth.

The EDFA gain spectrum has variations over 1536 to 1552 nm, therefore the gain flattening is a research issue in recent years with the development of high capacity DWDM. The gain variation becomes a problem as the number of channels increases. The gain of EDFA depends on large number of device parameters such as, Erbium ion concentration, amplifier length, core radius, pump power. Raman amplifiers can be combined with EDFAs to expand the optical gain flattened bandwidth. This paper focuses on different methods used for the gain flattening.

Keywords— Optical amplifier, EDFA, Raman amplifier, Gain flattening

I.INTRODUCTION Optical fiber communication is seen as one of the most reliable telecommunication technologies to achieve consumer’s need for present and future applications. It is reliable in handling and transmitting data through hundreds of kilometers with an acceptable bit error rate. Today, optical fiber communication has been established as one of the most promising technologies within the area of medium and long distance data transmissions. Optical transmission systems are based on the principle that light can carry more information over longer distances in a glass medium, while the electrical signals can carry information over copper or coaxial cable. Light is electromagnetic waves and optical fiber is a wave-guide, in order to compensate the loss of the wave-guide, an optical amplifier is needed. Doped fiber amplifier (DFA) is an optical amplifier which uses rare-earth doping material which are: Erbium (Er3+), Praseodymium (Pr3+), Europium (Eu3+), Neodymium (Nd3+), Terbium (Te3+), Lutetium (Lu3+), Ytterbium (Yb3+), Holmium (Ho3+), Dysprosium (Dy3+), Gadolinium (Gd3+), Samarium (Sm3+), Promethium (Pm3+), Cerium (Ce3+), Lanthanum (La3+) and Thulium (Tm3+) inside the fiber. Essentially, within a transmission line the DFA is connected to a pump laser. It works on principle of stimulated emission and pump laser is used to provide energy and excite ions to an upper energy level NITTTR, Chandigarh

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(Mohammed et al., 2011a, 2011b). Then, the ions are stimulated by photons of the information signal and brought down to lower levels of energy. Subsequently, they emit photon energy exactly on the same wavelength of the input signal.. In OFCS, the active medium of DFA which has less attenuation is operating in the 1550 nm window that is created by doping a silica fiber core with the Erbium (Er3+). To date, research works are concentrating more on the Erbium-doping, particularly in silica based fibers. This is due to the emission of Er3+ ions within a set of wavelength around 1550 nm where the silica fiber exhibits the minimum attenuation on information signal. Erbium doped fiber amplifiers (EDFA) could provide gains as high as 40 dB associated with low noise, as successfully demonstrated within a pumped power range of 50 to 100 MW (Mears, 1987).

II.OPTICAL AMPLIFIERS Optical amplifier is a device that boosts the light signals in an optical fiber network. Unlike the generators which have to convert light to electricity in order to amplify it, and then convert it back again to light. The optical amplifier amplifies the light signal itself. Optical amplifiers simply strength the optical signal, optical amplifiers work without having to convert an optical signal to electrical forms and back. This feature has two great advantages over repeaters:  They support any bit rate and signal format, because they simply amplify the received signal. They are transparent to the bit rate and signal format.  They support not just the single wavelength but the entire region of wavelength.

There are three most important types of optical amplifiers: the erbium-doped fiber amplifier, the semiconductor optical amplifier, and the fiber Raman amplifier. We introduce each of these amplifiers in the following subsections.

A. Erbium-Doped Fiber Amplifier The amplifying medium is a glass optical fiber doped with erbium ions. The erbium is pumped to a state of population inversion with a separate optical input. The erbium-doped glass optical gain medium amplifies light at wavelengths that are in the neighborhood of 1550 nm – the optical wavelengths that suffer minimum attenuation in optical fibers. Erbium-doped optical fiber amplifiers (EDFAs) have low noise and can amplify many wavelengths simultaneously, making the EDFA the amplifier of choice for most applications in optical communications.

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Int. Journal of Electrical & Electronics Engg.

Vol. 2, Spl. Issue 1 (2015)

e-ISSN: 1694-2310 | p-ISSN: 1694-2426

B. Semiconductor Optical Amplifier: A semiconductor optical amplifier with gain medium is undopedInGaAsP. This material can be tailored to provide optical amplification at wavelengths near 1.3 μm or near 1.5 μm. A semiconductor optical amplifier (SOA) is pumped with electrical current. SOAs are noisier than EDFAs and generally handle less power. However, SOAs are less expensive and are therefore suitable for use in local area networks.

Energy bands of erbium ions in silica fibers[4]

C. Raman amplifier: In Raman amplifier the gain medium is undoped optical fiber. Power is transferred to the optical signal by a nonlinear optical process known as the Raman Effect.

Power to supply the optical gain is supplied by an optical pump. The wavelengths that experience optical gain are determined by the wavelength of the optical pump, so the Raman amplifier can be tailored to amplify a given optical wavelength by proper selection of the pump wavelength.

Its most popular configuration is a hybrid EDFA/Raman amplifier, a device in which the Raman amplifier compensates for the EDFA lack of gain.

After study the figure, note in particular the width of the energy bands, which determines the EDFA’s ability to amplify the range of wavelengths from 1500nm to more than 1600nm. Our goal is to achieve population inversion, which means having more ions of erbium at the intermediate level-2. To attain population inversion, we need to pump erbium ions at the intermediate level [7].

There are two ways to do this: pumping them directly at the 1480-nm wavelength or indirectly at the 980-nm wavelength.

B. Gain of EDFA:

D. Hybrid amplifiers

Gain of EDFA can be calculated as:

The combination of more than two optical amplifiers in any configuration called the hybrid amplifier.

III.ERBIUM DOPED FIBER AMPLIFIERS These fiber amplifiersare the ones normally deployed in WDM fiber –optic communication systems today. EDFA have the gift of nature that the give the output in the vicinity of 1550nm, where silica fiber exhibits minimum attenuation. This fortunate coincidence is why erbium doped fiber amplifiers are so widely used. In EDFAs pumping is done with a laser diode radiating powerful light in a wavelength other than the information signals wavelength. Specifically, an information signal is transmitted around 1550 nm but pump laser radiate either at 980nm or at 1480 nm, or both. The following figure shows the basic diagram of EDFA.

=

(L,

, )=

(

(

)

)

Where = pump wavelength, = signal wavelength, - is the ratio of pump absorption and pump emission ( / ), r = the ratio of signal absorption and signal emission( / ). The gain of EDFA is largely dependent on the parameters explained above.The gain spectrum of EDFA is shown below:

EDFA gain spectrum [5]

Here the gain variation takes place between 1530 and 1560nm.We need to have a flat gain over its range operating wavelengths. This characteristic of and EDFA is called gain flatness.Uniformity of gain involves two aspects namely: Schematic diagram of EDFA [4]



A. EDFA amplification The operation of EDFA is based on the stimulated emission mechanism, the discussion of energy –level diagram of an active medium as it pertains to this amplifier. Free ions of erbium exhibit discrete energy levels.The most important energy levels of erbium ions incorporated into a silica fiber are shown in figure below.

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

Gain equalization: Gain equalization means achieving identical gains for a discrete number of optical channels.

Gain flattening: Gain flattening means achieving a spectrally uniform gain bandwidth.

IV.GAIN FLATTENING

Erbium doped fiber amplifiers have had a major impact in the field of light wave communications. Optical amplifiers have contributed to the growth of a fifth generation of optical communication systems. But as the demands on the NITTTR, Chandigarh

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Int. Journal of Electrical & Electronics Engg.

Vol. 2, Spl. Issue 1 (2015)

networks increased techniques like Dense Wavelength Division Multiplexing (DWDM) were developed. The importance of EDFAs is due to their compatibility with the fiber network, low insertion loss, polarization insensitivity, high gain levels and near quantum limited noise performance. Several methods for the gain flattening have been presented for the last few years. Some of these are: A. Gain flattening filters 1) Passive filters: include Fiber brag gratings, Longperiod gratings (LPG) and Mach Zender interferometers. Passive filters are very popular gain equalizing devices, as they are very reliable. But there are some limitations that they do not offer flexibility and that changes in EDFA gain profile cannot be tracked. [7]. Wide band fiber brag gratings have been shown to improve the gain flatness of EDFA considerably by using optical isolator followed by FBG [8]. MZ filter is placed after the amplifier to remove amplified spontaneous emission noise[9].

2) Active filters: As the gain spectrum of EDFA is not constant but changes as EDFA conditions change, for this purpose the need of tunable gain equalizing filters takes place called the active filters. Active filters like Acousto optic filter, Tunable MZIs, Tunable LPGs were discussed [10-12].

B. Hybrid Optical Amplifier (HOA) Cascading EDFAs with different gain spectra leads to the formation of hybrid optical amplifier (HOA)[1]. Before the formation of hybrid optical amplifier it is necessary to evaluate the transmission performance. The transmission performance is evaluated by transmission capacity.

Transmission capacity = bit rate × no. of wavelength Where,no. of wavelengths= optical signal BW × channel density[13].

C. Input signal power Adjustment of input signal powers can also reduce the gain variations[8].

IV. GAIN FLATTENING USING HYBRID AMPLIFIERS As mentioned earlier there are various techniques to reduce the gain variations. The use of filters reduce the gain variations to some extent but they do not offer flexibility means they do adapt the changes which takes place in the EDFA gain spectrum. One LPG along with other phase shifted LPG reduced the gain ripple to approximately 0.9dB. But by the use of hybrid amplifiers, means the by the use of dual stage EDFA with bidirectional pumping can be used along with this the Raman amplifier with counter pumping scheme is used. The use of Raman amplifier reduces the automatic spontaneous emission.

e-ISSN: 1694-2310 | p-ISSN: 1694-2426

amplifier and adjustment of different parameters. These all have some drawbacks, but by the use of Raman amplifier which reduces the effective span loss can be combined with EDFA to expand the optical gain flattened bandwidth.

After cascading these two amplifiers the use of a passive filter can be considered as future work.

REFERENCES [1] Sohn, Ik Bu, Jang Gi Kim, Hyo Sang, Seok Hyun Yun, HyangKyun Kim, Namkyoo Park, Byoung Yoon Kim, “Actively gain-flattened erbium-doped fiber amplifier over 35nm by using all-fiber acoustooptic tunable filters,” 1998 [2] A. M. Vengsarkar, “Broad-band erbium-doped fiber amplifierflattened beyond 40 nm using long-period grating filter,” IEEEPhoton. Technol. Lett., vol. 9, pp. 1343–1345, Oct. 1997 [3] Gu, X. J. "Wavelength-division multiplexing isolation fiber filter and light source using cascaded long-period fiber gratings,” Optics letters vol. 23, no.7, pp. 509-510, 1998 [4] Masuda, Hiroji, “Review of wideband hybrid amplifiers,” In Optical FiberCommunication Conference, vol. 1, pp. 2-4. Optical Society of America, 2000 [5] Kim, Seung Kwan, Moo Jung Chu, and Jong Hyun Lee, “Wideband multi wavelength erbium-doped fiber ring laser with frequency shifted feedback,” Optics communications vol. 190, no. 1, pp. 291302, 2004 [6] Nam-Kwon Lee, Hyung Woo Kwon, Jae-Won Song, “Gain flattened and improved EDFA using microbending long-period fibre gratings,” Electronics Letters vol. 38, no. 22, pp. 1324-1325, 2002 [7] Surinder Singh, and R. S. Kaler, “Gain flattening approach to physical EDFA for 16× 40 Gb/s NRZ-DPSK WDM Optical Communication System,” Fiber and integrated optics vol. 25, no. 5, pp. 363-374 2006 [8] Liaw, S. K., C. K. Huang, and Y. L. Hsiao, “Parallel-type C+ L band hybrid amplifier pumped by 1480 nm laser diodes,” Laser Physics Letters vol. 5, no.7, 2008 [9] Bae, Jun Kye, JinhoBae, Sang Hyuck Kim, Namkyoo Park, and Sang Bae Lee, “Dynamic EDFA gain-flattening filter using two LPFGs with divided coil heaters,” Photonics Technology Letters, IEEE vol.17, no. 6 ,pp.1226-1228, 2005 [10]Martin Lopez, Sonia, Miguel Gonzalez Herraez, Pedro Corredera, María Luisa Hernanz, A.Carrasco, “Gain-flattening of fiber Raman amplifiers using non-linear pump spectral broadening,” Optics communications vol. 242, no. 4, pp. 463-469, 2004 [11]Cho, Y.T Alahbabi, M.N, Brambilla, G.Newson, “Distributed Raman amplification combined with a remotely pumped EDFA utilized to enhance the performance of spontaneous Brillouin-based distributed temperature sensors,” Photonics Technology Letters, IEEE , vol.17, no.6, pp.1256-1258, June 2005 [12]Singh, Simranjit, and R. S. Kaler, “Flat-gain L-band Raman-EDFA hybrid optical amplifier for dense wavelength division multiplexed system,” Photonics Technology Letters, IEEE vol.25, no. 3, pp. 250252, 2013 [13]S. Singh, R.S. Kaler, “Investigation of hybrid optical amplifiers for dense wavelength division multiplexed system with reduced spacings at higher bit rates,” Fiber and Integrated Optics vol. 31, no. 3, pp.

208-220, 2012

V.CONCLUSION AND FUTURE SCOPE This review paper presents the variations takes place is the gain spectrum of EDFA and various techniques to reduce the gain variations. These techniques include use of active and passive filters, cascading EDFA with different optical NITTTR, Chandigarh

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