Open access peer-reviewed chapter

Waveguide Amplifier for Extended Reach of WDM/FSO

Written By

Bentahar Attaouia, Kandouci Malika, Ghouali Samir and Dinar Amina Elbatoul

Submitted: 03 January 2022 Reviewed: 04 April 2022 Published: 24 May 2022

DOI: 10.5772/intechopen.104790

From the Edited Volume

Multiplexing - Recent Advances and Novel Applications

Edited by Somayeh Mohammady

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Abstract

In this chapter, EYDWA (erbium ytterbium doped waveguide amplifier) is characterized for wavelength division multiplexing (WDM) approach on free space optical (FSO) transmission systems with channels being spaced at 0.4 nm interval. Moreover, in this paper, was study different characterizations of EYDWA amplifier, which depend essentially on the opt-geometric parameters, such as concentrations of ions erbium, length of the waveguide and the effect of those parameters to optimize the performance of proposed system. Furthermore, the results reveal that the EYDWA booster (post-amplification) can improve the high performance remarkably under clear rain and the acceptable transmission can be carried out up to 26 km while it get reduced to 19.5 km by using pre-amplification.

Keywords

  • free space optical
  • BER
  • EDWA
  • WDM
  • atmospheric condition

1. Introduction

FSO (free space optics) is an optical communication technology in which contains three components: transmitter, free space transmitted, and receiver. The transmitter requires light, which can be focused by using either light emitting diode (LED) or laser (light amplification by stimulated emission of radiation) to transmit information through the atmosphere. At the receiver, a photodiode converts the optical intensity signal back into an electrical signal and the information is recovered [1, 2].

The FSO communication system has the advantages of unrestricted spectrum and high-speed transmission over other wireless communication systems. This system is likely to replace other wireless communication systems in many fields and become the solution for last-mile communication. The main limitation of FSO is seen in worse weather conditions where it suffers highest attenuation [3].

Optical network that apply wavelength division multiplexing (WDM) is currently widely used in existing telecommunications infrastructures and is expected to play a significant role in FSO system supporting a large variety of services having very different requirements in terms of bandwidth capacity which ensures multiservice and multicasting opportunity [4, 5].

WDM FSO systems use a single light beam to transmit the multiplexed signal through free space [6]. A multiplexer is used at the transmitter to combine different modulated carriers and a demultiplexer at the receiver to restore each one (Figure 1).

Figure 1.

The system setup of WDM-FSO.

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2. Optical amplifier

An optical amplifier is a device that amplifies an optical signal directly, without the need to first convert it to an electrical signal. In the 1990’s, optical amplifiers, which directly amplified the transmission signal, became widespread minimizing system intricacies and cost [7]. Many techniques have been proposed to improve the performance of FSO link like the amplification of signal [3].

To maintain the integrity of information sent from one location to another, optical amplifiers, such as doped fiber amplifiers (DFA), doped waveguide amplifiers (DWA), and semiconductor optical amplifiers (SOA), are utilized to extend transmission range for the cost-effective implementation of FSO and can be used for amplification of WDM network easily [4].

2.1 Doped fiber amplifiers

Doped fiber amplifiers (DFAs) are optical amplifiers that use a doped optical fiber as a gain medium to amplify an optical signal [8]. They are related to fiber lasers. The signal to be amplified and a pump laser are multiplexed into the doped fiber, and the signal is amplified through interaction with the doping ions. Er3+is one of the most commonly used doped ions in integrated photonics and the EDFA is one effective way to amplify light signal at optical communication window between 1500 to 1600 nm.

2.1.1 Erbium doped fiber amplifier

The erbium-doped fiber amplifier (EDFA) is the most deployed fiber amplifier as its amplification window coincides with the third transmission window of silica-based optical fiber and has demonstrated high gain, low noise, and full compatibility with DWDM signals. In general, EDFA works on the principle of stimulating the emission of photons. With EDFA, an erbium-doped optical fiber at the core is pumped with a laser at or near wavelengths of 980 nm and 1480 nm, and gain is exhibited in the 1550 nm region (Figure 2).

Figure 2.

Erbium doped fiber amplifier block diagram.

2.2 Doped waveguide amplifier

Waveguide amplifiers, in particular, are new integrated optical products well suited to metro/access applications. Some of the intrinsic benefits for using this later include their compactness, performance, flexibility, and lower-cost processing [9]. These integrated devices offer the prospect of combining passive and active components on the same substrate while producing compact and robust devices at lower cost than commercially available fiber-based counterpart. However, the way to implement all-optical network relies on the control of gain variation of amplifiers which is sensitive to total input power variation [1, 8].

2.2.1 Erbium-doped waveguide amplifier

The erbium-doped waveguide amplifier (EDWA) are planar waveguides doped with erbium ions and are excited similar to EDFAs. EDWAs integrate several functions and components onto a mass produced integrated circuit and have recently received considerable attention as a potential high-gain medium for optical amplification in the communication band (Figure 3) [8, 9].

Figure 3.

Erbium-doped waveguide amplifiers block diagram.

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3. Concentration quenching of erbium

EDWAs are less efficient than EDFAs due to higher erbium concentration in the waveguide on the substrate. Greater erbium ion concentration causes more pumping power to quench to the system. Additionally, there is greater loss in waveguides than fibers. Concentration quenching is the reduction in quantum efficiency of a erbium ion as its concentration increases. It generally manifests itself by a shortening of the measured metastable level lifetime and occurs mostly through cross relaxation or co-operative up- conversion processes.

When the concentration levels are such that the separation between two erbium ions is greater than the diameter of an individual erbium ion then the up conversion process is called “homogeneous up conversion (HUC)”. In addition to the above-mentioned effects, another important effect that needs to be investigated is the pair induced quenching (PIQ). This later is an inhomogeneous phenomenon caused by clustered ions when the Er+3 the inter-ionic distance between two erbium ions becomes less and they come much closer to each other so as to form “clusters” [10]. This issue has been addressed by co-doping the erbium by ytterbium (Yb+3) (Figure 4).

Figure 4.

Scheme of pair-induced quenching (PIQ) and up-conversion (HUC) processes in erbium-doped fiber amplifier.

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4. Co doping with ytterbium ions to inhibit PIQ

To increase the absorption cross section, ytterbium ions (Yb3+) are usually co-doped as a sensitizer. The introduction of ytterbium can effectively restrain the erbium Er3+ ion clusters, and reduce up-conversion nonlinear side effect. This can increase the total gain and the unit length gain greatly [11].

The performance of the Er3 + −Yb3+ co- doped waveguide amplifiers (EYDWA) is better than that of the EDWA, and the EYDWAs are therefore expected to be an attractive high-gain medium material for optical amplification because of their use as amplifiers in optical telecommunications and as compact light sources for eye-safe range finding in the 1.55 μm spectral range (Figure 5) [12, 13].

Figure 5.

Energy level diagram of erbium and ytterbium system.

Ytterbium offers the advantage of a high absorption cross-section and a good spectral overlap of its emission with erbium 4 I11/2 absorption, leading to an efficient energy transfer from ytterbium to erbium.

The rate equations for Er+3 and Yb+3 population can be written as [14]:

dN2dt=A21N22UupN22+N1σsaφsN2σseφs+γ32N3E1
dN3dt=N3σpeφp+N1σpaφp+PN1N6PN3N5N3σ32+γ43N4E2
dN4dt=CupN22γ43N4E3
i4N=NErE4
dN3dt=PN1N6PN3N5N5σpaφp+N6σpaφp+N6σpeφp+A65N6γ43N4E5
NYb=N5+N6E6

where, N1, N2, N3 andN4 are the Er population densities (m−3) of 4I15/2,4I13/2,4I1/2, 4I9/2 levels, respectively. The quantitiesN5,N6 are the Yb+3 population densities (m−3) of the 2F7/2 and 2F5/2 levels respectively. Whereas, φp,φs,σpa,σpe, A21,A65,P,P, Cup are defined in Table 1.

ParameterDefinition
φpThe pump photon flux
φsThe signal photon flux
σpeThe stimulated emission cross section for Er+3
σpaThe absorption cross section for Er+3
σpeThe stimulated emission cross section for Yb+3
σpaThe absorption cross section for Yb+3
A21The spontaneous emission rate of Er+3
A65The spontaneous emission rate Yb+3
K and K′The coefficient of energy transfer for the concentrations of E+3 and Yb+3
CupThe up conversion coefficient

Table 1.

Parameter definition for equations.

The spontaneous emission rates of Er+3 and Yb+3 can be calculated by:

σa=hγnCB12g12γ,σe=hγnCB21g21γE7

where g12γ and g21γare the normalized emission and absorption line shape respectively, n is the refractive index of the medium and B12 and B21 are the coefficients of transition. Then the signal gain G and total noise Figure NF are given by:

G=exp0Lgzdz,
NF=10loglog101G+PASEGhγB0PSSEhγB0E8

where γ is the signal frequency, Bo is the noise bandwidth, PASE and PSSE are the power of amplified spontaneous emission, and the power of source spontaneous emission, respectively.

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5. Comparison of EDFA and EYDWA for WDM/FSO network

The FSO-WDM with eight input signals using EYDWA amplifier as a pre- or post-TT6Af an externally modulated WDM transmitter generating eight NRZ signals at 2.5 Gbit/s with input power of −10 dBm, the eight channels are multiplexed with a spacing set at 0.8 nm in the wavelength range 1550 to 1554.8 nm. Then the signal is ready to travel through 30 Km range of FSO. On the receiver’s side, the avalanche photodiode (APD) is used followed by a low pass filter and a 3R regenerator. The performance is analyzed using BER analyzer which gives the related BER, power level and eye diagrams.

Figures 6 and 7 shows the dependence of the gain and noise figure on frequency for both optical amplifiers EDFA and EYDWA, respectively. It is evident that the EYDWA amplifier also offers a better price/performance ratio (better gain of (32 dB and high NF of 11 dB) than comparable EDFA amplifier (Gain of 15 dB and better N Fof 5 dB) for WDM/FSO network applications. Most of the intrinsic advantages of EYDWAs come from their ability to provide high gain in very short optical paths than EDFA amplifier. This capability gives vendors more flexibility in the design of a compact amplifier.

Figure 6.

Gain and noise figure as a function of frequency for the erbium doped fiber amplifier (EDFA).

Figure 7.

Gain and noise figure as a function of frequency for the Er-Yb doped waveguide amplifier (EYDWA).

5.1 Concentration quenching affects

The homogeneous up conversion tends to cause more impairment in the EDFA amplifier performance than in the EYDWA amplifier. Figure 8 shows variation of gain as a function of the HUC coefficient [15]. It is observed that as this later increased; the gain spectrum decreased and showed larger variation especially for EDFA as compared as EYDWA amplifier. Furthermore for UHC coefficient higher of 2.10−22 m+3/s we can notice lowest results in term of gain for EDFA, however EYDWA amplifier provides the best results (high and flat gain). Also the maximum Q factor values occur for EYDWA amplifier and at lower HUC coefficient as compared as EDFA amplifier Figure 9.

Figure 8.

Gain as a function of up-conversion coefficient for the EYDWA and EDFA.

Figure 9.

Q factor as a function of up-conversion coefficient for the EYDWA and EDFA.

5.2 Influence of length and erbium doping

The critical turning point in the EYDWA technology is finding a compromise between the high erbium ytterbium co-doping levels, which helps create large gain in a short optical length [13]. The dependence of EYDWA performance on the length and erbium ion concentration is studied Figures 10 and 11.

Figure 10.

Curves of Q factor versus EYDWA length for WDM-FSO system under medium rain.

Figure 11.

Curves of maximum gain versus erbium ion concentration for WDM- FSO system under medium rain.

For better performance the optimization has been done and it was reported of the system amplified (WDM- FSO) under medium rain and at 2.5 Gbit/s. The EYDWA amplifier can be reached higher FSO range (over 12 km) with acceptable quality factor (Q values of 6 and BER = 10−9) by increasing the erbium concentration (up to 6.1026 ions/m3) and with optimum waveguide length (over 3.5 cm). These results proved that we can achieve high gain with a short device length.

5.3 Influence of the position of EYDWA amplifier

The performance analysis of the system amplified under clear, medium and heavy rain conditions are shown in “Table 2”, and Max Q factor has been analyzed Figures 12 and 13. It can be seen that under optimized conditions of data rate, the increase in the attenuation (respective intensity of rain) causes the decrease in the maximum transmission distance (FSO range) with acceptable quality factor Q values around of 6.

Intensity of rainSystem with EYDWA post-amplifierSystem with EYDWA pre-amplifier
Q factorBERRange (Km)Q factorBERRange (Km)
Light (clear) 3 dB5.810−9276.1210−916
Medium 9 dB610−9126.0310−98
Heavy 20 dB610−98610−94.5

Table 2.

Comparison of EYDWA post- and pre-amplifier.

Figure 12.

Quality factor as a function of FSO range for EYDWA post- amplifier under clear (light), medium and heavy rain.

Figure 13.

Quality factor as a function of FSO range for EYDWA preamplifier under clear (light), heavy and medium rain.

We note that EYDWA as post-amplifier can be carried the link range up to 26 km at and 8 km at BER 10−9 under clear and heavy rain, respectively. However EYDWA preamplifier limits this distance of FSO range to 16 Km and 4.5 km at BER 10−9 under clear and heavy rain conditions, respectively.

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6. Gain flatness of EDFA for a WDM FSO system

An amplifier does not have a flat gain curve, that is, it does not generally provide equal amplification for all wavelengths of signals transmitted over the same transmission line (WDM). This disparity is an important limitation for wavelength division multiplexed systems. Good gain flatness requires continuous control of the input power to adjust it to its optimum value [15]. There are several methods for the design of a flat spectral EDFA gain e.g. by combining in-line amplifiers with gain equalizing filters or by controlling some internal EDFA parameters such as the length of the doped fiber, the concentration and the pump power [16].

6.1 Equalization of gain with optimization of EDFA parameters

The figures bellows show the power and noise spectrum as a function of wavelength at the output of the EDFA for different concentrations of erbium ions (from 1.10 +24/m−3 to 1.10+25/m−3) and different lengths of the doped fiber (from 2.5 m to 12.5 m), eye diagrams of the simulated system are also shown. According to the graphs obtained, the gray wave represents the noise that decreases as the length and concentration of the doped fiber decreases, while the red symbol in the graphs indicates the sample wavelength (eight wavelengths).

The results obtained show that the concentration of de 5,5.10+24/m−3 and the length of the doped fiber of 7.5 m give the best results in terms of maximum gain (around 26.63 dB) and equalization of the amplified optical spectrum.

It is noticeable that the eye aperture is well open which the quality factor is between 9 and 10 which are higher than 6 that mean that the system works correctly. The BER is higher than 10−12 which expresses that the transmission is error-free (Figures 1418).

Figure 14.

(a) Power spectrum and noise figure, (b) eye diagram for concentration and length of the doped fiber (C = 5.10+24/m−3 and L = 7, 5 m).

Figure 15.

(a) Power spectrum and noise figure, (b) eye diagram for concentration and length of the doped fiber (L = 5 m and C = 3,5.10+24/m−3) .

Figure 16.

(a) Power spectrum and noise figure, (b) eye diagram for concentration and length of the doped fiber (C = 1.10+24/m−3 and L = 2.5 m).

Figure 17.

(a) Power spectrum and noise figure, (b) eye diagram for concentration and length of the doped fiber (C = 7,5.10+24/m−3 and L = 10 m).

Figure 18.

(a) Power spectrum and noise figure, (b) eye diagram for concentration and length of the doped fiber (C = 1.10+25/m−3 and L = 12, 5 m) .

The table below summarizes the simulation results at the output of the WDM analyzer for different values of the erbium ion concentration and the length of the doped fiber, where the term RGrepresents the difference between the maximum and minimum value of the EDFA gain (maximum ratio), while RNFindicates the variation of the noise figure.

Comparison of the five graphs leads to the conclusion that the gains are flattened with a RG=0.7in the band 1537 nm to 1545 nm wavelength around a gain of 26.3 dB with a noise Figure (NF) of less than 4 dB for 8 transmission channels for a concentration of 5, 5.1024/m−3 and a doped fiber length of 7.5 m the worst case (gain less equalized with aRG=1.54) is obtained with a fiber length of 12.5 m (Table 3).

GainNFRG = Gmax – GminRNF = NFmax – NFminConcentration “N”L
2.95 dB1.95 dB0.08 dB0.46 dB1.1024/m32.5 m
19.44 dB3.43 dB0.47 dB1.63 dB3.5.1024/m35 m
26.63 dB3.65 dB0.70 dB2.15 dB5.5.1024/m37.5 m
23.59 dB4.47 dB0.98 dB2.04 dB7.5.1024/m310 m
14.71 dB4.66 dB1.54 dB2.06 dB1.1025/m312.5 m

Table 3.

Optimization results obtained by the WDM analyzer.

6.2 Gain equalization through the use of a GFF filter

Gain Equalizing Filters, also known as GFF (Gain Flattening Filters), are integrated into optical systems and are usually combined with optical amplifiers in the transmission chain to ensure good gain flattening. This process provides a solution to the problem of equalizing the amplifier output power of a WDM multiplexed system. This part refers to a process of combining the EDFA amplifier with a GFF filter and visualizing the power and noise spectrum.

The tables below show the results of the WDM analyzer at the output of the EDFA “Table 4”and the output of the gain equalizing filter “Table 5” in terms of wavelength, gain, noise figure and the deference between the maximum and minimum values of these figures. Comparison between the tables below leads to the conclusion that the gains are flattened with a RG=0.77in the band 1537 nm to 1545 nm wavelength around a gain of 26.97 dB with a noise Figure (NF) of less than 2.15 dB for 8 transmission channels.

Wavelength (nm)Gain (dB)Noise Figure (dB)
1537.427.8594.017
1535.8229.4483.368
1534.2531.7143.913
1532.6832.9903.382
1531.1232.7643.144
1529.5531.2652.382
1527.9929.0962.971
1526.4426.2982.063
Gain (dB)Noise Figure (dB)
Min value26.1972.0631
Max value32.9944.026
Total30.7120
Ratio Maxmin6.6981.954
nmnm
Wavelength at1526.441526.44
Wavelength at1537.401537.40

Table 4.

Results of the WDM analyzer at the EDFA output.

Wavelength (nm)Gain (dB)Noise Figure (dB)
1537.426.9763.697
1535.8226.6873.991
1534.2526.8644.026
1532.6826.6343.297
1531.1226.3643.169
1529.5526.4642.928
1527.9926.8032.597
1526.4426.1971.871
Gain (dB)Noise Figure (dB)
Min value26.1971.871
Max value26.8644.026
Total26.6280
Ratio Maxmin0.7782.155
nmnm
Wavelength at1526.441526.44
Wavelength at1537.401537.40

Table 5.

WDM analyzer filter GFF output results.

The figures below show a schematic representation of the spectrum emitted at the output of the EDFA and output of the gain equalizing filter GFF. In Figure 19, the different wavelengths of the multiplex at the output of the EDFA (at the input of the filter) can be recognized, represented by red symbols from 1 to 8. It can be seen that the spectrum received at the output of the EDFA has a different total power for the different wavelengths while the spectrum received at the output of the filter has a total power which is substantially equal and in particular the gain is flat over the amplification band Figure 20.

Figure 19.

Power spectrum and noise figure at EDFA output.

Figure 20.

Power spectrum and noise figure at filter FGG output.

According to the eye diagrams for both cases we can clearly see the aperture of diagram Figure 21 compared to diagram Figure 22; this confirms that the use of the gain equalizing filter GFF improves the quality of the amplified WDM system.

Figure 21.

Eye diagram of the WDM/FSO with GFF filter.

Figure 22.

Eye diagram of the WDM/FSO without GFF filter.

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7. Conclusion

This chapter summarizes the simulation results and their interpretations of the comparison of two configurations of EYDWA (post-and preamplifier) in the WDM/FSO system. The mentioned amplifiers were evaluated based on values of BER and the quality -factor. WDM over FSO communication system is very suitable and effective in providing high data rate transmission with low bit error rate (BER). Therefore, WDM – FSO system has achieved very good results, it has many problems, such as heavy attenuation coefficient.

For the heavy rain condition the maximum link range about 8 km at BER = 10−9. Also, It can be noticed from simulation that the best results achieved EYDWA post- amplifier configuration, this later was able to reach transmission distance of WDM/FSO up to 26 Km however the worst results in terms of FSO range are obtained with EYDWA preamplifier and the distance of transmission is limited at 16 Km.

Furthermore the gain flatness and the noise figure of EDFA have been studied. The gain non-uniformity for each channel using the optimization of erbium doped and length fiber doped in order to equalize the amplitude gain in the WDM-FSO system have been simulated. The simulation results prove that the proposed method by optimization of erbium doped and length of fiber doped offers and improved the best performance in terms of the gain flatness GF = 0.7 for fiber length L = 7.5 m and erbium concentration C = 5,5.10+24/m−3.

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Written By

Bentahar Attaouia, Kandouci Malika, Ghouali Samir and Dinar Amina Elbatoul

Submitted: 03 January 2022 Reviewed: 04 April 2022 Published: 24 May 2022