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Wireless Personal Communications

, Volume 100, Issue 4, pp 1775–1789 | Cite as

A Performance Enhancement and High Speed Spectrum Sliced Free Space Optical System

  • Aditi Thakur
  • Shaina Nagpal
  • Amit Gupta
Article
  • 45 Downloads

Abstract

A cost effective Kerr nonlinearity based spectrum sliced (SS) WDM free space optical communication system is demonstrated under different weather instabilities. The investigated supercontinuum spectrum sliced WDM FSO system is evaluated at 2.5 Gbps up to 5 km of link distance. A highly nonlinear fiber is a channel for the generation of high power broad spectrum for spectrum slicing. A dense SS-WDM is investigated at 75 GHz channel spacing among 4 channels to make system bandwidth efficient. The system is investigated for different line coding (return to zero, non return to zero) and advanced modulation format such as compressed spectrum return to zero. A major degrading factor in free space communication i.e. beam divergence is also analyzed for investigated work. Antenna diameters of receiver and transmitter play a vital role in FSO, thus various diameters performances are also studied. The approach is to cater the high-speed data demands and thus system deliberated and demonstrated from 2.5 to 10 Gbps. To strengthen the signal in this FSO system, three optical amplifiers are scrutinized such as erbium doped fiber amplifier (EDFA), semiconductor optical amplifier (SOA) and Raman amplifier in terms of bit error rate and quality factor. Results revealed that EDFA is the best amplifier in investigated SS-WDM-FSO system.

Keywords

Free space optics (FSO) Wavelength division multiplexing (WDM) Highly nonlinear fiber (HNLF) Spectrum sliced (SS) Super continuum (SC) Return to zero (RZ) Non-return to zero (NRZ) Carrier suppressed return to zero (CSRZ) Semiconductor optical amplifier (SOA) Erbium doped fiber amplifier (EDFA) Signal to noise ratio (SNR) Bit error rate (BER) Quality factor (Q) 

1 Introduction

With the rapid increase in the demand of internet connection and traffic in the variety of recently arrived new services has set a boom on conventional low-speed architecture of radio system. Free space optic (FSO) technology exhibit shared characteristics of ubiquitous technologies such as fiber optics and wireless [1]. In recent year, FSO communication system has been investigating to provide the benefits of useful properties over optical fiber communication [2]. In the past, a lot of researches have been carried out to cater the high-speed data requirements and to propose cost effective architecture [3]. Moreover, FSO renders fortify communication because of insignificant interception incorporating P2P (point to point) laser signals. Merits of FSO communications are large bandwidth and capacity, license free, operates on low power and saves the cost of trenching, permits etc. as in optical fiber communication [4]. For the geographical areas in India, where the deployment of optical fiber is not possible, FSO is a major candidate and attracted attention due to its applications and numerous advantages. The performance of free space optic are affected by the atmospheric weather conditions such as mild rain, medium rain, high rain, fog, haze etc [5, 6, 7] that enervates the link and become the cause to shut the network transmission.

Presently, wavelength division multiplexed (WDM), which permits to hold several different and autonomous optical channels which can carry Terabits per second capacities as well as it can be without difficulty incorporated with free space optical systems to significantly enhance the data speed in the system [8]. Another work was reported with the use of QPSK and it was reported that use of QPSK in WDM technique enhance the system reach [9]. To cater the demands of bandwidth hungry services, WDM-FSO is a prominent and well competent way out for broadband transmission. However, WDM technology has several disadvantages also such as more complexity and the high cost of operation. In order to overcome these limitations, a spectrum slicing technology is a good alternate to WDM. The Spectrum slicing having a less complex operation as compared to WDM, which uses several intensity sources operating at different frequencies [10]. Also, WDM system is wavelength selective and sensitive to the routing of each wavelength to the specific port. So, spectrum sliced WDM has identical advantages as wavelength division multiplexing and less complex, low cost, power efficient for future generation networks [11].

The Spectrum slicing technique is reported in researchers using arrayed waveguide gratings [10], semiconductor optical amplifier ASE (amplifier spontaneous noise) [12], super luminescent diodes (SLDs) [13], super continuum spectrum slicing technique (SC-SS) [14]. However, these systems support low data speed and number of channels, but FSO can support high speed and provide large bandwidth. An optimal method of SS-WDM is needed for FSO networks.

Various physical parameters of FSO also play an important role in overall transmission. Transmitter and receiver antenna diameters decide the reach and quality of reception of the system. Beam divergence is a major performance deteriorating factor and needs to be addressed along with attenuation and dispersion. Till now, many research works have been reported to generate spectrum slices [11, 12, 15], to the boost weak signal with amplifiers [16], beam divergence size [17] scintillation noise compensation [18]. Although, reported techniques are good but either supports less data rate or more complexity.

Therefore, in this research article, we accentuate on the high-speed super continuum based spectrum sliced wavelength division multiplexing (SC-SS-WDM) architecture. Clear weather, haze, fog, mild rain and medium rain are the atmospheric instabilities that are considered and investigated over investigated architecture. The utmost approach is to cater the high-speed data demands and thus system deliberated and demonstrated for 2.5 Gbps. Also, a high-speed WDM-FSO system based on spectrum slicing through supercontinuum generation is demonstrated and the system is investigated for various beam divergences, transmitter and receiver antenna diameters and modulation formats. In order to overcome the attenuation effect, we will use different optical amplifier such as EDFA, SOA and Raman amplifier are investigated over proposed architecture [19]. The utmost approach is to cater the high-speed data demands and thus system deliberated and demonstrated for 10 Gbps. Highly nonlinear fiber is incorporated in the system for supercontinuum generation and which provide high power broadened spectra for spectrum slicing. In this paper, new technique is investigated to overcome the limitation of SS-WDM system. The proposed system overcomes the SS-WDM limitation in order to enhance the system reach. The remainder of the paper is divided as follows: Sect. 2, include the system setup in which the analysis the expression and principle of generation of super continuum (SC) spectrum that involve the pulse broadening, system architecture and the various atmospheric turbulence. In Sect. 3, Results and discussion of Simulation have been presented. This section include the improved the system performance in terms of quality of the transmission. In Sect. 4, conclusion is reported.

2 System Setup

The Sect. 2.1 describes the principle of generation of super continuum (SC) spectrum, Sect. 2.2 describes the system architecture of investigated work and the Sect. 2.3 describes the value of attenuation of laser power through the different atmospheric condition in free space optic (FSO) system.

2.1 The Principle of Generation of Super Continuum Spectrum

The intensity reliant traits of optical medium or fiber are prominent cause to modulation of phase as well as self-focusing cramped pulse. In optical fiber, any other confinement that is generated by intense pulse is insignificant. Nevertheless, retardation at high pulse in contrast to the trailing and leading edge will augment in extensive communication and consequences in substantial modulation of phase. Pulse enlargement is more significant when associated with high launched power ultra short pulse and link length. Since broadening of spectrum or super continuum generation restricts the reach of system, however, this is a valuable phenomenon to save cost by providing large spectrum from single intensity source for spectrum slicing.

Proportionality constant for highly non-linear fiber is expressed as
$$\upgamma_{SPM} = \frac{{2\pi {\text{n}}^{2} }}{{\lambda A_{eff} }}$$
(1)
where γSPM is nonlinearity (rad/(Wm)) and Aeff is the effective area of highly nonlinear fiber. Major characteristics of refractive index change with the launched intensity of pulse.
$$P_{nl} = \frac{3}{4}\varepsilon_{0} \chi^{3} |E_{0} \left( {z,t} \right)e^{{\left[ {i\left( {\omega t - \beta z} \right)} \right]}}$$
(2)
where electric field amplitude is E0 (t), which fluctuate with time (t) and frequency (ω) is same to launched light. Considering the effects of polarization into the wave equation, direct change in refractive index or modification over the original n0. Where n0 is zero fields and net index is expressed as:
$$n = n_{0} + n^{\prime}_{2} |E_{0} \left( {z,t} \right)|^{2}$$
(3)
Nonlinearity in refractive index is shown as \(n^{\prime}_{2}\)
$$n^{\prime}_{2} = R_{e} \left[ {3\frac{{\chi^{3} }}{{8n_{0} }}} \right]$$
The complex field travelled through the medium can be given as:
$$E = E_{0} \left( {z,t} \right)e^{{\left[ {i\left( {\omega_{0} t - \left( {n_{0} + n_{2} I\left( {z,t} \right)} \right)k_{0} z} \right)} \right]}}$$
(4)
The instantaneous frequency is expressed as
$$\omega^{\prime} = \omega_{0} - n_{2} k_{0} z\frac{\partial I }{\partial t }$$
(5)

The effects and property of self phase modulation (SPM) on transmission of pulse can be collectively notice from the Eqs. (4) and (5). First, the positioning of extra frequency components are done on the pulse, thus raising its spectral width. A frequency sweep imposed on the pulse, which rely on the sign of ∂I/∂t. In optical fiber system, latter described feature plays an important role, since the forced frequency from self phase will accumulated or deduct from the chirp forced by linear group dispersion. The actual generation of super continuum in HNLF can be altered, reliant on fiber length, the time of pulse, maximum intensity and wavelength of pump. When short pulses are fed into nonlinear medium, broadening can be caused by SPM. There are many applications of super continuum generation include coherence tomography, cytometry, microscopy fluorescence, the description of optical devices, the generation of multiple carrier waves. It is significance to give emphasis on the coherence traits of super continua. Nevertheless, super continua produced by intense pulses can still have time coherence in the way that there can be sturdy correlations between the electric fields analogous to different signals, if the broadening is extremely reproducible. This type of coherence is incredibly significant for the generation of combs of frequency, and it may or may not be achieved reliant on parameters such as the seed duration of pulse, length of fiber, and GVD.

2.2 System Architecture of Investigated Work

The simulation software Optiwave Optisysyem is used for the investigation of spectrum sliced WDM Free space optical communication system. The wavelength division multiplexing (WDM) is a technique which is capable to support high data rate and high capacity of system. In WDM technique each wavelength is assign to the individual user. The WDM technique required N number of coherent laser source are needed to generate the N frequencies, which increase the cost of the system. The alternative technique of WDM is Spectrum-sliced on account of data rate same as aforementioned technique and transmit stream of data in similar direction at the same time. Spectrum-slicing is a technique in which wide spectrum of 1550 nm wavelength is divided into number of slices and each slice is capable to transmit data in parallel manner. Figure 1 illustrates that the Light amplification stimulated emission of radiation (LASER) is coherent light source which is operated at 30 dBm and radiate at 1550 nm. The signal is coupled into the HNLF which emphasis to generate the high power and broad spectrum for slicing with the use of nonlinearity called self-phase modulation (SPM). Molecules of HNLF move quickly and initiate the swinging that changes the refractive index of pulses. The variation in the refractive index (µ) causes the changes the velocity of pulses that further changes the phase of the pulse. So the variation in refractive index is the main reason of the broadened spectrum which is obtained at the end of HNLF and ready to be slice. The Fig. 2 show the spectrum of CW laser before and after the supercontinuum generation.
Fig. 1

System architecture of 4 channel SC-SS WDM FSO communication system

Fig. 2

A continuous wave laser spectrum a without super continuum b with super continuum generation

Figure 3 depict that the wider super continuum spectrum is further sliced into 4 channels such as 193, 193.75, 193.150 and 193.225 THz respectively. These 4 sliced channels are equally spaced with 75 GHz of channel spacing in order to avoid the interference shown in the Table 1.
Fig. 3

Representation of multiplexed spectrum-sliced (SS) channels at OSA

Table 1

Parameters of system parameters

S. no.

Quantity

Values

1

Data rate

2.5–10 Gbps

2

Weather condition investigated

Clear weather, mild rain, medium rain, haze, fog

3

Power

30 dBm

4

Wavelength

1552.52 nm

5

No. of channels

4

6

Channel spacing

75 GHz

7

Diameter of transmitting antenna (in cm)

Varied from 5, 10, 15, 25

8

Diameter of receiving antenna (in cm)

Varied from 5, 10, 15, 25

9

Beam divergence (in mrad)

Varied from 0.25, 0.5, 0.75, 1

10

Effective aperture of HNLF

10 µm2

11

HNLF attenuation

0.5 dB

12

Nonlinear index of refraction

2.6e−019 m2/W

13

Link length

More than 5 km

14

Amplifier

EDFA, SOA, Raman

15

Line coding

NRZ, RZ, CSRZ

This available spectrum is modulated with line coding and digital data of data rate of 2.5 Gbps is generated by pseudo random bit sequence generator. In addition, the speed can enhanced by incorporating advance modulation format such as compressed spectrum return to zero (CSRZ), modifier duo binary return to zero etc. the principle of generation of broad spectrum is depend upon the intensity of pulse and the nonlinearity to caused in HNLF that has very low effective area of 10 µm2. The modulated signal is multiplexed and ready to transmit from transmitter antenna into the free space optical toward the receiver with the beam divergence angle of 1 mrad. To check the performance of the system the multiplex signal amplified with different amplifier such as EDFA, SOA, and Raman. After the amplification the signal is fed to the FSO link. By the use of amplifier, this will enhance the performance of the system and also increase the speed of the system.

2.3 Atmospheric Factor in Free Space Optic (FSO)

The Free space optic is a communication system that capable to transmit the high data rate with high speed from source to destination more than 5 km. FSO communication system is immune to EMI but there are other degrading factor that affects the performance of this system such as rain, fog, and haze etc. To investigate the effect of atmospheric weather condition we need to consider the attenuation value (dB/km) of these degrading factors mentioned in [20, 21, 22] shown in Table 2.
Table 2

Atmospheric weather condition along with value of attenuation

S. no.

Atmospheric weather condition

Value of attenuation (dB/Km)

1

Clear weather

0.11

2

Haze

4

3

Mild rain

6.27

4

Medium rain

9.64

5

Fog

22

3 Results and Discussion

In order to evaluate the quality of reception under different atmospheric condition the link length of FSO communication is varied from 1 to 5 km. It is observed that maximum prolonged distance is observed in the case of clear weather condition and minimum distance in the case of fog.

The Fig. 4 depicted that Q-factor of SC-SS WDM received signal under different weather condition. The Fig. 4 revealed that with the increase of transmitter and receiver antenna distance the quality factor decreased. The Q factor for clear weather condition varies from 42.57 to 36.24 for 1 to 5 km respectively. As in case of mild rain, medium rain, haze and fog are 40.72–0, 31.72–0, 41.54–10.18, and 28.78–0 respectively. The maximum link range for this system investigated which cover for different turbulences is more than 8 km for clear weather, 6 km for haze, 4 km for mild rain, 2 km for medium rain and 1.5 km for fog. The comparison revealed that performance of SC-SS WDM FSO technique is better than the WDM technique in free space optics (FSO) communication system. The Table 3 depicts the values Q-factor of different atmospheric condition.
Fig. 4

Graphical representation of SC-SS WDM FSO system for different weather condition

Table 3

The value of quality factor for different weather condition at different link length

Distance (km)

Clear weather (0.1 db/km)

Haze (4 dB/km)

Mild rain (6.27 dB/km)

Medium rain (9.24 dB/km)

Fog (22 dB/km)

1

42.57

41.54

40.72

31.75

28.78

2

40.82

36.49

32.61

1.52

0

3

39.19

29.62

20.29

0

0

4

37.67

20.27

5.72

0

0

5

36.24

10.18

0

0

0

Figure 5 revealed that the eye diagram of different weather condition for SC-SS-WDM FSO communication system. It is cleared from the eye diagram that the maximum eye opening is obtained in case of clear weather condition only which provide the idea of good quality of received signal and recognizing the effect of distortion. To demonstrate the system performance different atmospheric factor are considered such as transmitter/receiver aperture size, beam divergence, distance between transmitter and receiver, and modulation formats. A supercontinuum spectrum-sliced (SC-SS) WDM technique is used to determine the quality of the received signal by varying the free space optic link length from 1 to 5 km.
Fig. 5

Eye diagram of SC-SS WDM FSO communication system for a clear weather, b haze, c rain, d fog

The quality of received signal is depends on the distance between the transmitter and receiver of FSO. To determine the quality of received signal, the distance of FSO system is varied from 1 to 5 km. The comparison between RZ, NRZ and CSRZ modulation format is shown in Table 4. The Table 3 depict that CSRZ performs better than RZ and NRZ modulation format.
Table 4

The value of quality factor for RZ, NRZ and CSRZ at different link length

S. no.

Distance (km)

RZ

NRZ

CSRZ

1.

1

12.86

14.16

15.2

2.

2

12.86

13.91

14.56

3.

3

12.01

12.69

13.34

4.

4

11.23

11.44

11.54

5.

5

9.13

9.44

9.57

Figure 6 depicts that as the distance increases the quality of the received signal decreases. Different modulation formats are investigated over free space optical communication system and Q factor is observed for these modulation formats. It is perceived that effect of modulations on Q factor is significant. The CSRZ modulation format provides the best results than other modulation format in term of quality factor. It is evident that quality factor of CSRZ is best due to dispersion tolerance and constant power.
Fig. 6

Graphical representation of Quality (Q) factor versus distance at RZ, NRZ and CSRZ modulation format

Figure 7 shows the comparison of Q-factor at different values of beam divergence. It is observed that as the beam divergence increases the BER is also increases. It is investigated that the minimum quality of the received signal is observed at 1 mrad and maximum Q-factor is obtained at 0.25 mrad. It is observed that if the FSO transmitter’s beam divergence angle goes on increasing, it in turns decrease the received signal. It is demonstrated that lower bit error rate is obtained by using CSRZ modulation format.
Fig. 7

Graphical representation of quality factor (Q) versus beam divergence at different modulation format

The FSO system performance is analysed by various combination of aperture size of transmitter and receiver. Figure 8 shows that bit error rate increases as the link length is increasing and also the data rate. As the path length and data rate are increasing, the Q factor is decreasing. Aperture size of the transmitter and receiver are varied from 5, 10, 15 and 20, larger the diameter of the receiver antenna lager the receiver signal power is. It is cleared that by using RZ, NRZ and CSRZ modulation techniques in order to investigate the performance of system, the CSRZ technique of modulation provide the maximum received power and best quality of the signal at the receiver side. Similarly, it is observed from the Fig. 9 that Q factor of CSRZ is maximum because of high received power at receiver.
Fig. 8

Graphical representation of received power versus transmitter/receiver aperture diameter

Fig. 9

Graphical representation of Quality factor versus transmitter/receiver aperture diameter

The free space optic (FSO) communication system performance is enhanced by using different optical amplifiers such as SOA, Raman and EDFA. The investigation revealed that CSRZ modulation format provide the best quality of the received signal and maximum received power. So, CSRZ modulation format is used along with these three amplifiers. In order to evaluate the quality of received signal different amplifiers are used. So, Fig. 10 shows that maximum prolonged distance is observed in case of EDFA and minimum results are observed in case of SOA.
Fig. 10

Graphical representation of Quality factor versus distance by using different optical amplifiers

It is clearly observed that by using the amplifier we can enhance the quality of the received signal and increase the link length between the transmitter and receiver antenna. It is clearly noticed that with the increase of distance, Q-factor decreases. It is evident that the maximum Q-factor is observed in case of EDFA amplifier. Values of Q factor are 24.32–10.26 at the range from 1 to 5 km, for Raman amplifier it is 24.86–8.63 and for SOA amplifier it is 16.54–4.29 respectively. Figure 10 depict that the maximum link length is covered by EDFA amplifier (10 km), Raman amplifier covered 8 km and SOA amplifier covered 3.5 km. So, it is observed that by using different amplifiers, the best performance is obtained in case of EDFA amplifier.

Figure 11 depicts that graphical representation of received power with the increase of distance. So, it is clearly noticed that with the increase in link length, received power of the signal decreases. By comparing the performance of different amplifiers, it is observed that maximum eye opening and received power of signal is observed in case of EDFA and in case of Raman amplifier the receiver power is less as compared to EDFA. Minimum received power is observed in case of SOA amplifier (Fig. 12).
Fig. 11

Graphical representation of received power versus distance of EDFA, Raman and SOA

Fig. 12

Eye diagram of SC-SS WDM FSO system for a EDFA b RAMAN c SOA amplifiers

After transmission through FSO channel, signal is de-multiplexed and taken at definite ports which are fixed according to the transmitted frequencies. At the received side the photodiode detect the light and convert it into electric form. In this PIN photodiode with 10 nA and responsibility is 10 A/W is used. The photo detector is followed by Low Pass Filter (LPF) Bessel filter which remove the undesired signal from the received signal. A 3R regenerator is used after LPF in order to reshaping, retiming and re-amplification of the signal at the end BER analyser is used in order to analysed the value of Q-factor, BER, Eye diagram etc. In order to access the spectrum of frequencies the optical spectrum analyser is used.

4 Conclusion

In this paper, a 2.5 Gbps SS WDM FSO communication system is investigated which is based on Kerr nonlinearity self phase modulation. Analysis has been carried out for different weather conditions such as clear weather, mild rain, haze, and fog. For clear weather condition maximum link length observed is more than 8 km and minimum link length 1.5 km in case of fog. Secondly, the investigation has been carried out for the purpose to check the system performance by varying the beam divergence angle, transmitter/receiver antenna apertures diameter, different modulation formats. So, by varying such parameters, we observed that SC-SS WDM technique provides the best performance in case of CSRZ modulation format as compared to other modulation formats. It is observed that the by choosing the small value of beam divergence angle, the Quality of signal increases and BER decreases. However, larger aperture size of transmitter and receiver antenna also enhances the performance of the system. Finally, investigation has been carried out for different optical amplifiers are used such as EDFA, Raman and SOA along with CSRZ modulation technique. So, it is observed that by comparing the performance of these amplifiers, the best results are obtained in case of EDFA amplifier. By using the EDFA amplifier, the system is capable to transmit the data rate up to 10 Gbps for 10 km. Such investigated system promising the low cost architecture for high speed free space optic communication system. Result revealed that the performance of SC-SS WDM FSO system using EDFA amplifier is better than Raman and SOA amplifier. The result suggested that, there is always a requirement of high data rate in FSO systems, in this work 2.5 and 10 Gbps data rate is achieved. But more data rate can be supported in future FSO systems.

Notes

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Copyright information

© Springer Science+Business Media, LLC, part of Springer Nature 2018

Authors and Affiliations

  1. 1.Electronics and Communication EngineeringChandigarh UniversityGharuanIndia
  2. 2.Electronics and Communication EngineeringIKG PTUJalandharIndia

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