I. about6.86dB. II. INTRODUCTION The gain flattening of

I. ABSTRACT

 

There is an increasing demand of transmission bandwidth for long haul
wavelength division multiplexed optical fibre communication systems. Bandwidth
is well used by using broadband and gain flattened amplifiers. Broadband
amplification is done by combining many amplifiers having totally different
gain bandwidths. Totally different gain flattening techniques are offered for
gain flattening functions to scale back gain variation like gain flattening
filters as fiber Bragg gratings, improvement of fabric composition of fiber
amplifiers, using rare earth doped ions, hybrid amplifiers, wavelength
splitters.

The hybrid optical amplifier composed of single erbium doped fiber
amplifier and a Raman amplifier is proposed for wavelength division multiplexed
system and also investigates the impact of reduced channel spacing. The
performance has been evaluated in terms of gain and noise figure. Gain and
noise figure are important characteristics of the optical amplifier to evaluate
the performance. There are various parameters on which the gain spectrum of
EDFA and Raman amplifier depends. The effort has been made to optimize these
parameters by simulating the system of 16 channels in Opti-system software. In
this paper the length, pumping wavelength, pumping power of EDFA/Raman is
optimized. After the analysis the reduction in gain is about 2.7dB and the
noise figure is about6.86dB.

 

II. INTRODUCTION

 

The gain flattening of Erbium-doped fibre amplifiers (EDFA) has been a
research issue in recent years, with the development of high capability
wavelength division multiplexing (WDM) optical communication systems. For
single channel systems, the gain variation isn’t a problem. However, because
the variety of channels will increase, the transmission downside arises as a
result of a traditional EDFA has intrinsic non-uniform gain. They usually gift
gain peaking at concerning 1530 nm and therefore the helpful gain information
measure could also be reduced to but ten nm. The gain of EDFAs depends on
several device parameters like erbium-ion concentration, amplifier length, and
core radius and pump power. To extend the gain-bandwidth of an amplified light
wave system many ways is used, however equalising optical filters operating as
spectrally selective loss elements seem to be the most effective candidates.
This paper primarily focuses on totally different ways used for gain flattening
in EDFA.

EDFAs are widely
used in comparison to other similar amplifiers and optical devices for larger
wavelength optical communication system because of its advantages of high gain,
large bandwidth, less noise figure (NF), polarization insensitivity, low pump powers
and better performance. One difficulty in implementing a WDM system including
EDFA’s is that the EDFA gain spectrum is wavelength dependent and it doesn’t
necessarily amplify the wavelength of the channels equally. Bandwidth of EDFA
can be increased to more than 30nm with appropriate gain flattening in the
third transmission window for future development of WDM long-haul optical fiber
communication systems. Moreover, the noise performance of EDFA is characterized
by its NF which is signal to noise ratio reduction ratio of input to output of
the amplifier and should have a low value. Gain is the main characteristic to
evaluate the performance of an optical amplifier. The gain spectrum must be
uniform for the long distance transmission, but EDFA cannot amplify all the
wavelengths equally 4. Hence the gain flatness is important for EDFA’s
wavelength division multiplexing (WDM) which is important technique for long
haul optical transmission link system. Different compensation methods were
studied in past and based on these methods efforts were made to increase the
gain flatness of EDFA. Methods such as hybrid Al-co doped &Al/P co doped
EDFAs 1, hybrid EDFA/RFA amplifiers, erbium doped waveguide amplifiers
(EDWA) having gain flattened and gain clamped functions simultaneously 4, the
usage of the gain flattening filters (GFF) such as, thin film dielectric
filters, sinusoidal filters 4, chirped fiber brag gratings 5, acousto-optic
tunable filter 6. The combination of distributed Raman amplifier and EDFA
present better performance than conventional EDFA 1.

 

III. LITERATURE
SURVEY

 

Wideband and gain flattened optical amplifiers are crucial for long haul
optical communication systems. Gain flattening of EDFA is done by numerous
techniques. In initial approach, 2 stages of EDFA are used. a brief fiber
length C-band EDFA produces ASE that, in turn, with pump power enters the
second stage to extend the output gain in long length L-band EDFA. 2 fiber brag
gratings (FBG) are placed within the second section at each ends of long fiber
to form fabric part cavity. The gain in L-band is flattened by the compressed
gain below the matched wavelength of FBG In another approach, hybrid amplifiers
are used. Fiber to amplifier couplers will work severally of others and needed variety
of amplifiers is added in step with the demand. A double pass assembly of EDFA
with hybrid gain medium- silicon and oxide is employed in parallel
configuration. Chirp fiber brag grating is employed in each stage to realize
the desired gain in Cband and L- band regions it causes the double propagation
of the signal leading to overall enhanced gain however noise figure
additionally will increase that is that the major downside. Hybrid electronic
equipment is EDFA–Raman amplifier. Raman amplifier is distributed and distinct.
The distinct Raman electronic equipment is employed in spite of style of
transmission fiber and its main purpose is to expand the usable bandwidth of
the fiber. Then again, the distributed Raman amplifier (DRA) improves the reach
of fiber span. The impact of DRA is that it decreases the noise similarly
because the non linearities. EDFA + DRA is wont to increase the gain similarly
as decrease the noise figure. EDFA includes a larger gain variation with
relevance the wavelength. Raman amplifier includes a smaller gain variation
compared to EDFA. This property is wont to kind a hybrid optical amplifier
(HOA). By increasing the quantity of pumps, the gain of HOA is enhanced .But
there’s a limit to the rise in variety of pumps otherwise gain can decrease. By
adjusting the pump wavelengths, pump power and length of fiber, amplification
and gain flattening is drained totally different wavelength regions. Erbium and
ytterbium co-doped phosphate (Er/ Yb–EDFA) and Raman phosphate fiber amplifier
is used as hybrid amplifier and has the advantage of upper transmission
capability than silicon based mostly fiber amplifiers. Phosphate glass has
larger phonon energy, greater solubility to rare earth ions, massive emission
cross section and better energy transfer efficiency. This hybrid amplifier is
used for higher transmission capability on dense wavelength division
multiplexed systems.

In another technique, Er doped wave guide Amplifier (EDWA) and Er doped
fiber amplifier is connected asynchronous to realize the gain flatness within
the C- band. EDWA is a smaller amount economical than EDFA due to higher erbium
concentration. Higher erbium concentration needs a lot of pumping power .There
is loss of wave guide within the fiber. EDWA provides high gain briefly optical
path. High gain and gain flatness is obtained by this technique however
broadband amplification can’t be obtained.

 

IV. PROPOSED WORK

1.
Current scenario

Erbium
doped fibre amplifiers have had a significant impact within the field of light
wave communications. Optical amplifiers have contributed to the expansion of a
fifth generation of optical communication systems. However because the demands
on the networks accrued techniques like Dense Wavelength Division Multiplexing
(DWDM) were developed. The performance of DWDM in ultra-long haul networks
improved as a result of amplifiers were accessible that might amplify the
wavelengths utilized in the network while not requiring any conversion of the
optical signal to the electrical signal. The importance of EDFA’s is because of
their compatibility with the fibre network, low insertion loss, polarization
unfitness, high gain levels and close to quantum restricted noise performance.
In DWDM transmission systems and their connected optical networks, one in all
the key technological problems is that the achievement of broad and flat gain
bandwidth for erbium Doped Fiber Amplifiers (EDFA’s). Gain variations occur
between optical channels having massive wavelength spacing (e.g. ??> 1nm).
In long electronic equipment chains, even little spectral gain variations (e.g.
?G < 0.75 dB) may result in massive variations within the received signal power, inflicting intolerably massive BER discrepancies between received signals. for some optical channels, complete power extinction will occur at the system output, as a result of depleted gain compensation along the amplifier chain. in addition, the ASE generated within the region of highest gain (i.e., near ? = 1531 nm) in un equalised EDFAs causes solid gain saturation, that affects WDM channels at longer wavelengths. 2. Proposed Technology solution The increasing demand for bandwidth in today's backbone networks makes it progressively necessary to increase the effective transmission bandwidth of the deployed fibres on the far side the widely-used C-band (1530 nm to 1610nm). because the transmission bandwidth is especially restricted by today's amplifier-of alternative, the EDFA, new ways in which of extending the amplifier bandwidth should be pursued. As an example, hybrid combinations of EDFA and Raman electronic equipment are wont to extend the seamless bandwidth of distinct amplifiers up to 80nm, a hundred nm and so on. In general, the combination of quite one optical amplifier in any configuration is termed hybrid optical amplifier (HOA). EDFA amplifies the signal however the gain spectrum isn't uniform. to scale back the gain variations hybrid combination of EDFA and Raman amplifier is the most suitable option. V.METHODOLOGY Various softwares needed for implementation of project are as follows: Optical Spectrum analyzer- An Optical Spectrum analyzer (or OSA) could be a precision instrument designed to measure and displays the distribution of power of an optical supply over a mere wavelength span. Associate degree OSA traces displays power within the vertical scale and therefore the wavelength within the horizontal scale. The increasing field of optics connected applications has created a massive style of industries and organizations that need advanced optical spectral measurements for both R and producing. These industries embrace telecommunications, consumer electronics, healthcare, life science/medical analysis, security, sensing, microscopy, and gas/chemical analysis, and environmental monitoring. Simulation Model- The basic configuration consists of 16 channels with 50GHz channel spacing, the system has a WDM transmitter with first channel is at 193.5THz and increases as the number of channels increases, the input power of WDM transmitter is 20dBm. All the channels are transmitted into the WDM multiplexer with zero insertion loss, here all the light signals are combined and transmitted over the erbium doped fiber of length 9m.The erbium doped fiber amplifier is counter pumped at 1480nm.The counter pumping scheme gives more gain than that of co-propagating pumping scheme. The pump power at which the EDFA pumped is 130Mw.The output of EDFA is then passed through the optical isolator. Optical Isolators are used to protect a source from back reflections or signals that may occur after the isolator. The output after the isolator then fed into the Raman amplifier of length 11m which is counter pumped at 1450, 1452, 1454, 1456nm with constant pump power of 450mW.The over all amplified signal is fed into the optical spectrum analyzer to analyze the optical spectrum. The dual port WDM analyzer is placed after the Raman amplifier which gives the values of gain and noise figure. The gain variations and the noise figure variations are noted down for different frequencies. At the end, it passes through the photo detector to convert the optical signal into electrical signal to note down the bit error rate value. The bit error value should be less than 10?6 dB. The simulation setup of hybrid optical is shown in the given figure 1. Fig.1 Schematic of Hybrid optical amplifier of 16 channels Figure 2 shows the results viewed from spectrum analyzer in the Opti-system software. These are power spectrum of Erbium doped fiber amplifier and the Raman amplifier when observed as inline amplifier. Fig.2 Output power spectrum (red) noise spectrum (green) for EDFA Fig.3 Output power spectrum (red) noise spectrum (green) for Raman After the analysis, RFA as an inline amplifier at pump power of 200mW provides lesser gain variations than the EDFA. Moreover the gain response of RFA shows opposite characteristics than EDFA and hence this property of RFA can be exploited for gain flattening. As the HOA is placed in the network system with reduced channel spacing, the different gain and noise figure is recorded at every step. It is found that the gain of HOA is almost flat after the observation. The maximum gain is achieved after the hybrid optical amplifier. It means that the combination of EDFA and Raman is responsible to increase the gain. To illustrate the performance of this hybrid optical amplifier, gain and noise figure is recorded after HOA incorporated. Figure 6 shows gain as a function of frequency. The overall gain for given frequency band is increased from 12.41dB to 16.21dB. Figure 7 shows the characteristic in terms of noise figure as a function of input signal frequency. The overall noise figure is maintained below 6dB. MATLAB 7.1- The hybrid configuration has properties of each the amplifiers the wavelength used for optimizing ranges from 1569 nm -1577nm that lies within the c band of optical communication to get uniform gain with less noise figure. There are varied parameters on that gain of optical amplifier depends, by optimizing these parameters in prefer system software the maximum value of gain will be obtained. VI. BLOCK DIAGRAM     Fig.4 Schematic Diagram of EDFA VII. CONCLUSION The erbium doped fibre amplifier has high gain and may be made appropriate for optical transmission systems for long haul communications by using acceptable gain flattening technique.  The system of 16-channels are going to be designed within the range of 1532-1547nm, that lies in C-band. Within the hybrid configuration of EDFA/Raman, it's doable to scale back gain variations by optimizing erbium fibre length that is 8m within the projected model and correct selecting of pump wavelengths and injected pump powers to Raman fibre amplifier. By applying this gain equalization technique, the hybrid optical amplifier has gain worth of 15 dB. The gain variations are reduced to two.7dB and noise figure is obtained below 6 dB. There are numerous techniques used for gain flattening of EDFA. Each technique has its own merits and demerits. EDFA in C-band (1520-1570 nm) incorporates a higher gain than EDFA in L-band (1570- 1620 nm). Raman amplifier encompasses a lower noise figure, wide gain bandwidth and flexibility on choice of gain medium. A C+ L band EDFA and Raman amplifier (hybrid amplifier) are often wont to increase the general gain and scale back the noise figure at the same time. Hybrid amplifier is that the best technique because it reduces the non linearities, will increase the general gain, reduces the noise figure, price effective and doesn't have any flexibility problems. By using hybrid amplifiers, broadband amplification will be done in specific wavelength regions. VIII. RESULT   The finest amplification span length is one that facilitates the best trade-off between the low-cost requirements and stringent system performance. Long amplifier spans result in high input powers to maintain a good optical signal-to-noise ratio (OSNR), leading to increased effects of nonlinearities. In such situation a best balance between the high optical signal-to-noise ratio and nonlinear impairments is necessary. The solution of this problem is use of distributed Raman amplification (DRA). As compared to Erbium doped fiber amplification scheme, DRA improves significantly the link's OSNR. Distributed Raman amplifier in combination with Erbium-doped fiber amplifier termed as, hybrid fiber amplifier, can be used for better control of nonlinear effects. In our system reverse dispersion fiber (RDF) is used instead of dispersion compensating fiber (DCF). In DCF based Raman amplification systems pump power efficiency is very low and a significant amount of pump power is unused and wasted. This can be attributed to strong nonlinear effects in dispersion compensating fibers. A distributed hybrid Raman/Erbium-doped fiber amplifier is simulated and optimized in 1530 nm to 1565 nm wavelength range. A 60 km (30 km SMF+30 km RDF) transmission fiber is pumped by a backward Raman pumping unit (BRPU). This unit consists of pumps at wavelengths 1440 nm and 1450 nm with pump powers 120 mW and 60 mW respectively. An Erbium-doped fiber of length 8 m is forward pumped by 980 nm laser diode of pump power 12 mW. Forward pumping in Erbium gain medium and backward pumping in Raman amplifiers give better conversion efficiency and noise figure. Almost flat gain of 21 dB with gain tilt ±0.4 dB is obtained over entire wavelength range. The noise figure is well below 7 dB. The Noise figure is slightly high in lower signal wavelength region because of allocation of higher pump energy in this range. Thermal instabilities, pump-to-pump Raman interactions and power fluctuation in pumps result in more noise figure in lower signal wavelength region. Fig.5 Gain and noise figure of hybrid Raman/Erbium-doped fiber amplifier (HFA) for different signal wavelengths.   Fig.6 Flattened gain response of gain v/s frequency   Fig.7 Flattened gain response of frequency v/s noise figure   The erbium doped fiber amplifier has very high gain and can be made suitable for optical transmission systems for long haul communications by employing appropriate gain flattening technique. In this paper a system of 16-channels was designed in the range of 1532-1547nm, which lies in C-band. In the hybrid configuration of EDFA/Raman, it is possible to reduce gain variations by optimizing erbium fiber length which is 8m in the proposed model and proper choosing of pump wavelengths and injected pump powers to Raman fiber amplifier. By applying this gain equalization technique, the hybrid optical amplifier has gain value of 15 dB. The gain variations are reduced to 2.7dB and noise figure is obtained below 6 dB. In future more the 16 channels can be transmitted with reduced channel spacing by incorporating different co doping concentration, multiple pumping schemes, where the pump wavelengths and pump powers can be chosen carefully to ensure a good performance.