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SURVEY ON VARIOUS PERSPECTIVES OF RAMAN
AMPLIFIERS
Sumathy Raju
Assistant Professor, ECE, Kalasalingam Academy of Research and Education,
Virudhunagar, Tamil Nadu, (India).
E-mail: sumathyraju1110@gmail.com ORCID: https://orcid.org/0000-0001-9871-1780
Muthukumar Arunachalam
Associate Professor, ECE, Kalasalingam Academy of Research and Education,
Virudhunagar, Tamil Nadu, (India).
E-mail: muthuece.eng@gmail.com ORCID: https://orcid.org/0000-0001-8070-3475
Recepción:
05/12/2019
Aceptación:
23/12/2019
Publicación:
23/03/2020
Citación sugerida:
Raju, S., y Arunachalam, M. (2020). Survey on various perspectives of raman ampliers. 3C
Tecnología. Glosas de innovación aplicadas a la pyme. Edición Especial, Marzo 2020, 247-259. http://doi.
org/10.17993/3ctecno.2020.specialissue4.247-259
Suggested citation:
Raju, S., & Arunachalam, M. (2020). Survey on various perspectives of raman ampliers. 3C
Tecnología. Glosas de innovación aplicadas a la pyme. Edición Especial, Marzo 2020, 247-259. http://doi.
org/10.17993/3ctecno.2020.specialissue4.247-259
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ABSTRACT
Raman Amplier (RA) is the ber amplier that follows Stimulated Raman Scattering
(SRS) mechanism. For broadband amplication it is used, because of low noise and better
gain. Raman amplication was investigated in multiple views. Many research works had
focused in the views of pumping schemes, gain attening, transmission system and noise
analysis. In this paper, Raman amplication is studied in the views of varying Refractive
Index prole of core, varying core gap radius, hybrid combination of RA with Erbium
Doped Fiber Amplier (EDFA). This paper also studies the investigations of this hybrid
combination in Dispersion compensation at C and S bands, recycling of pump power and
location of EDFA.
KEYWORDS
Refractive index prole, Hybrid Ampliers, Dispersion compensation.
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1. INTRODUCTION
In long distance Optical communication system, whenever the signal strength becomes
low, we need to boost up it. At the beginning, Regenerators were used to rejuvenate the
signal. Here, the optical signal is converted into the electrical signal. After the regeneration,
the electrical signal is again converted into optical signal which is a very costly process.
Then Semiconductor Optical Ampliers (SOA) were used. From the studies (Elndash,
Mohammed & Rashed 2010; Fugihara & Pinto, 2008), these ampliers have disadvantages
like, low output power, high noise gure. In order to avoid these, Fiber ampliers were used.
Fiber ampliers are the ampliers where a ber itself becomes amplication medium by
using pumping. There are two ber ampliers are mostly used. One is EDFA and second
one is Raman Amplier. In EDFA, a rare earth element Erbium is doped. When EDFA
is compared with RA, gain bandwidth is low and wavelength of operation is limited. But
in RA, at any wavelength we can obtain amplication. So Raman amplications are now
mostly used in long distance optical communication system. The researches (Bromage, 2004;
Islam, 2002; Namiki & Emori, 2001) concluded that Raman amplication has important
advantages of having very low noise and suitable for broadband applications, particularly
in WDM systems, where simultaneous amplication is required for multi-channel light.
Raman amplication may be Distributed Raman amplication (DRA) or lumped Raman
amplication. Dhir and Gupta (2013), found that DRA has benets like high gain, high
data rate and less ber loss. Raman amplier also provides better performance when it
is combined with EDFA or SOA. Many research works had been done in various aspects
like at gain amplication, noise performance, pumping schemes, & hybrid system. Lot of
ideas to pump powers and wavelengths selection was discussed in the literatures (Ferreira,
Cani, Pontes & Segatto 2011; Neto, Teixeira, Wada, & André, 2007). Raman amplication
is happening from the process SRS. SRS is a notable non-linear eect which aects the
Signal to Noise Ratio (SNR) in a WDM system. It can also be used for amplication of
the optical signals in a long distance optical communication link. The spontaneous Raman
scattering was found by Sir C. V. Raman. In case of this scattering, a small quantity of
the incident light is changed into light signal of either low or high frequency. SRS gives
the amplication if the pump signal with suitable wavelength enters the ber (Dhir et al.,
2013; Ferreira et al., 2011; Fugihara et al., 2008). In SRS, photon form pumping source is
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absorbed by material and emits a photon with energy at its vibrational state. In fact, energy
is transmitted from a high frequency optical signal to lower frequency optical signal (Dhir
and Gupta, 2014). Raman amplication is not only possible for single mode ber but also for
multimode ber which was discussed in a research (Polley & Ralph, 2007). In this work, For
fundamental mode LP0,1, the Raman gain was compared. This multimode amplication
is mainly used in space-division multiplexing (Antonelli, Mecozzi & Shtaif 2013; Namiki
& Emori, 2001). We can make any ber into a Raman amplier by suitable selection of
pumping wavelength (Anwar & Aly, 2010). This paper investigates two important views in
RA. Section 2 investigates about RA performance in various Refractive Index Proles and
Section 3 investigates about the performance of RA with EDFA hybrid conguration.
2. INVESTIGATIONS OF REFRACTIVE INDEX PROFILE (RIP)
Raman ampliers are used not only for amplication but also for dispersion compensation.
In Dispersion compensation, the refractive index prole of ber plays a major role.
Figure 1. Study of Raman Amplier based on Refractive Index Prole.
In the design of RA, the operating wavelength range is adjusted by correctly selecting
suitable RIP structure and inner core radius. In this paper, RIP based performance of RA
is studied as per Figure 1 Among step, parabolic and triangular refractive index proles, a
parabolic prole provides good eective Raman gain results at 20Gbps. The paper (Chan &
Premaratne 2007) also reports the change in dispersion co-ecient and gain as the function
of RIP. For obtaining better attening of Raman gain, the core gap radius of a ber is
varied with step index prole. Additional to this, large negative dispersion coecient is
achieved by varying core gap radius (Bandyopadhyay & Sarkar 2013) From this, dispersion
compensation is achieved at C and S bands. A research (Pramanik, Das, & Sarkar, 2010)
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found that Trapezoidal index prole also aects the gain performance of RA. The aspect
ratio S is modied, and same phase matching is achieved for various values of core radius.
During the design of RA, step index prole at both inner and outer cores was considered
in another research (Pramanik & Sarkar, 2014). This work says that the occurrence of
axial dip at core center is unavoidable. Up to 0.25% dip depth and 25% dip width, RA is
performing as perfect as RA without any dip or prole imperfections. But above this limit
of dip depth and dip width, the performance becomes poor. This paper concluded that step
index prole in core is most excellent for better performance of RA.
From the investigations of these research works, the Raman gain of various proles are
observed at 1550nm wavelength window for fundamental mode LP (0, 1) and tabulated in
Table 1.
Table 1. Raman Gain values at 1550nm Window.
Type of prole Raman gain (m/W)
Parabolic index 0.7x10-13
Triangular index 0.62x10-13
Step index 0.76x10-13
Trapezoidal index 1.04x10-13
From this Table 1, trapezoidal index prole gives the better Raman gain. But even with dip
at core centre step index prole gives the better performance (Pramanik et al., 2014).
3. INVESTIGATIONS ON HYBRID COMBINATION OF EDFA &
RAMAN AMPLIFIERS
Many research works have used benets from both the ber ampliers RA and EDFA.
When RA is combined with EDFA, cross talk becomes very less even for channel spacing
of 0.4nm and 0.2nm. This hybrid RA and EDFA performance is inspected for 16X10Gbps
DWDM system (Singh & Kaler, 2015). And this paper concluded that hybrid RA and EDFA
is better than other hybrid optical ampliers. Better power utilization also possible with this
hybrid RA and EDFA. It is investigated in the research (Lee, Chang, Han, Kim, Chung &
Lee, 2004) that recycling the residual Raman pump to make pumping on EDFA. Choosing
proper pump wavelength is the only thing to be considered. This gives the possibility for the
design of broadband ampliers with high gain.
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Figure 2. RA-EDFA hybrid conguration with single pump.
As shown in Figure 2, the pumping may be given either in forward direction (co–direction)
or in backward direction (counter-direction).
In hybrid optical ampliers, the site of the Erbium doped ber severely aects the
performance of the dispersion compensation of Raman/EDF and this is discussed in a
research (Ali, Abdullah, Jamaludin, Al-Mansoori, Al-Mashhadani & Abass, 2013). EDF
may be placed before RA or after RA. In this work, these are considered as two cases. A 10m
EDF and 7Km Dispersion Compensating Fiber (DCF) is considered. Here, DCF is acting
as Raman Amplication medium. In rst case, EDF is placed before RA. And in second
case EDF is placed after RA. In this research, ber has the following specications. They
are 0.55dB/Km of attenuation coecient, 98ps/nm.km of dispersion co ecient, and
15.3µm2 of eective area. And EDF has the following specications. Erbium concentration
of 440ppm, 2.2µm core radius, and eective area of 15.2µm
2
. If we place the RA rstly and
EDFA secondly, then we obtain large input signal gain, noise gure and gain variation.
RAMAN-SOA and RAMAN –EDFA are analyzed in the research of Upma (2015). This
research proposed the 8 channel transmitter with constant attenuation value 0.2db/Km.
And data speed of 10Gbps is considered. Under such case, Raman-EDFA gives highest Q
factor of 19.92db, RAMAN-SOA provides highest eye opening, Raman –EDFA provides
smallest jitter 0.0243. so RAMAN -EDFA is a hopeful alternative to all other hybrid
ampliers. With the help of mono pump source, dispersion compensating Raman/EDFA
hybrid amplier is achieved. Also overall power conversion eciency is increased. Here
mono pump source having two lasers is used for Dispersion Compensating Fiber (DCF)
which has Raman Amplication. The remaining power from this DCF is again recycled
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and utilized by EDFA. The laser pumps are operating at 1455nm and 1465nm. A total
pump power of 500mW is launched into a 12.6Km DCF. Eective power utilization and
larger amplier eciency is obtained in the research (Lee, Chang, Han, Chung, Kim &
Lee, 2005).
Like Erbium, Ytterbium can also be used as ber amplier. By Yb-Raman combined non
linear amplier, an improved power of 1.5kW was obtained in the wavelength of 1100nm
to 1200nm. In this work (Zhan, Tao, Zhou, Wang, & Xu, 2014), the amplier is seeded by
1070and 1120nm signal lasers simultaneously.
For 16 channel system (Lee, Oh, Lee, Lee, & Hwang, 2004), Q factor & BER for Raman-
EDFA & EDFA-RAMAN-EDFA are same for short distance. For longer distance, EDFA-
RAMAN-EDFA has largest Q value among all. For 32 channel system, RAMAN-EDFA
provides good output power; BER & Q factor. In the hybrid combination of Raman–
EDFA, EDFA may be used in parallel conguration and residual pumping is done. In
order to minimize the cost, mono pump wavelength is used in Raman amplier. Raman
amplier in the role of DCF has two benets. First one is, obtaining low loss and dispersion
compensation at the same time. Second is, amplication is done at wider band in optical
wavelength window by changing the pump wavelength and it was discussed in a research
(Singh & Kaler, 2015). In order to maximize capacity for amplication scheme and
transmission distance, a variety of combinations of three 16QAM based coded modulation
schemes with spectral eciencies 4.86/5.4 for C+L EDFA experiment and 5.45/6.14 [bits/
Hz] are used in this paper (Cai et al., 2015).
A dierent research is done in this work (Mahran, 2015). Here, the bending loss in EDFA
makes the gain of hybrid amplier to increase up to 7db more than normal EDFA/Raman.
OSNR calculations also show a better performance. In this paper bending radius is chosen
as 4mm, EDFA is taken with length 10m where, forward pump power in the range of 100-
500mW, Raman amplier length is chosen between 12 to55Km, where backward pump
power in the range of 80-200mw and input signal power is -20dBm. Even for reduced
channel spacing, this hybrid RA-EDFA provides better performance. This research (Singh
& Kaler, 2012) investigated various combination of hybrid ampliers.100 channels were
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used at channel spacing of 6.25GHz. Quality factors, acceptable bit rate, Bit Error Rate
(BER) were calculated for all the congurations.
Pump power of RA is unused in Dispersion compensating modules (DCM). This wastage
of pump power is avoided and utilized by using FBG at one side of DCM. This DCM is
designed for short or medium distance mostly from 50Km to 100Km (Nicholson, 2003).
Both the benets of dispersion compensation and eective power utilization were obtained
from this research.
4. SUMMARY
This paper assesses about various perspectives of Raman Ampliers. The eort towards
mitigating dispersion, improving gain bandwidth, eective power utilization, obtained low
noise performance, dependency of Refractive index prole using RA and corresponding
researches were described in this paper. When hybrid conguration of RA-EDFA is
considered, the innovative research papers of bending loss in EDFA for dispersion
compensation and locating EDFA for low noise were also depicted in this paper.
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