1 Introduction

FSO communication systems have offered interests in the field of academia, industry, and telecommunication recently. Present scenario seeks the feasibility of FSO installation and flexibility to upgrade existing optical system mainly in PON [1]. The existing PON can be adapted to support the FSO network in built-up city areas, so the FSO/PON scenario would definitely be more advantageous since the dedicated running fiber is already set up to deliver broadband applications such as streaming internet TV or video on demand [2].

Though the fiber to the home (FTTH) is probably a promising application of PON technology, in some situations, the traditional PON might not be a suitable solution where installation of fiber is unfavorable because of geographical difficulties [3]. To overcome these shortcomings, integrated FSO/PON has been proposed, and this scheme is also known as FSO-FTTH [4, 5].

In recent years, a demand of next-generation wireless communication with the co-existence of the optical spectrum has been seen. An analog transmission over optical fiber has already been used in radio over fiber (RoF) technique, however, data-carrying RF signal recognizes the major problem in the absence of fiber infrastructure [6]. Since the FSO/PON system is a promising future generation FTTH service, it would be a favorable time for the convenient integration of the RoFSO system into the existing framework. In RoFSO, radio frequency (RF) signal is transmitted through the light beam from the central to the base station without any modification in their modulation/encoding formats and increase the frequency reuse configuration [7, 8]. Hence, it is less expensive for short links, where a conventional wireless/wired link is difficult. Additionally, RoFSO takes advantage of the reduced size of the antenna since future broadband wireless networks are tailored to push mm-wave carrier frequencies. Millimeter-wave-band, especially around 60 GHz has a huge amount of bandwidth to realize unlicensed wireless operation, it was also standardized as ‘IEEE 802.11ad’ and promoted by a trade association- the ‘WiGig’ [9]. Apart from this, the developments for harmonizing frequency band around 57–64 GHz are in progress worldwide [10]. The most common technique for generating and distributing the mm-wave signal is the intensity modulation/direct detection (IM/DD), a spectrally efficient suppressed optical double sidebands (CS-ODSB) technique used to carry RoFSO signal [11]. The only drawback of 60 GHz technology is a significant attenuation due to atmospheric gases that shrinks the radio coverage around 100 (10) m by providing a microcell (picocell) structure to establish gigabit links for home (personal) area network, the approach is also called wireless-to-the-home [10, 12].

Since FSO technology is still evolving at a relatively fast rate, a wide number of new strategies and transceiver modules have been investigated. Recently, the convergence of 60 GHz RoFSO technique with WDM-PON has raised great interests [13]. To meet the demand of high speed and bandwidth of service, DWDM becomes an essential part of optical communication, which can also allow co-existence of analog RoFSO and digital FSO-FTTH signal [14]. The development has been noticed in several directions viz. DWDM, FSO/PON, RoFSO and optical interleaving [15, 16]. An innovative technique for expanding the number of channels in a communication link is termed as interleaving. In mm-wave RoFSO systems, the spectral band available between the main optical carrier and the respective modulation sidebands of the RF signal often remains unused. By introducing wavelength interleaving, the capacity of a DWDM network can be further increased, and unused spectral space is occupied by the neighboring channels [15]. The standard channel spacing of 100 GHz is consequently reduced to 50 or even 25 GHz, which increases the number of channels per FSO link by a factor of two or four.

The standard optical wavelength grid specifies that certain frequencies can be used as WDM channels. The corresponding wavelength spacing \(\Delta \lambda\) is given by Eq. (1) [17],

$$\Delta \lambda = \frac{1}{c}\left( {\Delta \nu \lambda^{2} } \right)$$
(1)

where \(\lambda\) is center wavelength of channels, \(\Delta \nu\) is frequency interval and \(c\) is velocity of light in vacuum.

At the receiver side, to filter the spectral components from the wavelength-interleaved signal spectra, FBGs are used as demultiplexers that provide the wavelength drop separately. This scheme, however deteriorates due to unexpected power fluctuations as a result of the insertion loss, reflectivity and temperature sensitivity of individual grating [18].

In this work, we propose an interleaving concept of designing a DWDM based FSO link by merging FSO/PON and RoFSO transmission as an optical distribution system. To the best of our knowledge, such a system has been proposed for the first time. A typical example of such a system is represented by Fig. 1, where the baseband and the RF signal are optically distributed via free space atmospheric channel from a central station node to an access node of multiple access points (APs). In each AP, the signal in the central carrier frequency is used for wired service and frequency band utilized for the wireless service.

Fig. 1
figure 1

Typical illustration of an optical distribution system by integrating wired and wireless services in hybrid free space link

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The rest of paper is organized as followed: Sect. 2 is focused on the turbulent atmosphere and its induced impairment on FSO system. Section 3 aims to the proposed design of the system and reports the performance of channel spacing in DWDM-interleaved operation. The results of the system configuration are summarized by comparative evaluation of FSO/PON and RoFSO transmission for different services in Sect. 4, and conclusions are given in Sect. 5.

2 Influence of Turbulence

Atmosphere of the earth is not an ideal propagation medium for the all-optical signal due to attenuation by gases and aerosols. Coincidently, the wavelength near the 3rd optical window i.e. 1550 nm is more transparent to the atmosphere [19]. However, due to the wind and temperature gradients, an unperturbed atmosphere becomes turbulent which results in the creation of different R.I. pockets through the air [20]. The arbitrary movement of these pockets (turbules) are not stable in time and space, and it changes the path of light through the free space channel. Therefore, random intensity fluctuations and loss of wavefront coherency of optical beam result into scintillation, impeding the correct detection of information on the receiver side.

Let \(n \left( {\rho_{1} } \right)\) and \(n \left( {\rho_{2} } \right)\) be values of the refractive index (R.I.) at the vector locations \(\rho_{1}\) and \(\rho_{2}\) respectively, therefore R.I. fluctuation in term of refractive index structure function \(D_{n}\) is defined in Eq. (2) [21]:

$$D_{n} \left( {\rho_{1} ,\rho_{2} } \right) = \langle \left| {n \left( {\rho_{1} } \right) - n \left( {\rho_{2} } \right)} \right|^{2}\rangle$$
(2)

where \(\langle\ldots\rangle\) represents statistical averaging, as \(D_{n}\) depends only on modulus of the vector separation \(\rho = \rho_{1} - \rho_{2}\), so \(D_{n} \left( {\rho_{1} ,\rho_{2} } \right)\) can be rewritten \(D_{n} \left( \rho \right)\).

\(D_{n} \left( \rho \right)\) is described by the Kolmogorov–Obukhov 2/3 power law for the optical turbule sizes bounded by the inner scale \(l_{o}\) and outer scale \(L_{o}\) [21]:

$$D_{n} \left( \rho \right) = C_{n}^{2} \left( h \right)\rho^{2/3} \;at\;l_{o} < \rho < L_{o}$$
(3)

where, the proportionality constant \(C_{n}^{2} \left( h \right)\) is called index of refraction structure parameter, and it characterizes the amount of refractive index fluctuation as a result of turbulence strength. A commonly used model to describe \(C_{n}^{2} \left( h \right)\) is the Hufnagel–Valley model given below as [22]:

$$\begin{aligned} C_{n }^{2} \left( h \right) &= 0.00594 \left( {\frac{v}{27}} \right)^{2} \left( {\frac{h}{{10^{5} }}} \right)^{10} m^{{ - \frac{2}{3}}} exp\left( { - \frac{h}{1000}} \right) \\ \quad & + 2.7 \times 10^{ - 16} m^{{ - \frac{2}{3}}} exp\left( { - \frac{h}{1500}} \right) + A \; exp\left( { - \frac{h}{100}} \right) \\ \end{aligned}$$
(4)

where, \(h\) = altitude in meter, \(v\) = wind speed in \({\text{m}}/{\text{s}}\) and \(A\) is taken as the nominal value of \(C_{n}^{2} \left( h \right)\) at the ground level \(\sim1.7 \times 10^{ - 14} {\text{m}}^{ - 2/3}\). \(C_{n}^{2}\) is also defined as:

$$C_{n}^{2} = \left( {86 \times 10^{ - 6} \times \frac{P}{{T^{2} }}} \right)^{2} C_{T}^{2}$$
(5)

where, \(P\) is pressure in millibars, \(T\) is temperature in Kelvin and \(C_{T}\) is temperature structure parameter as given by [23]:

$$C_{T} = \sqrt {\langle \delta T^{2}\rangle r^{ - 1/3} }$$
(6)

where, \(\delta T\) is change in temperature and \(r\) is the distance between temperature measured points in propagation path.

The value of \(C_{n}^{2}\) varies with the turbulence strength that categorizes the turbulence regimes. Typically, \(C_{n}^{2}\) ranges from \(10^{ - 12}\) to \(10^{ - 17} {\text{m}}^{ - 2/3}\) due to strong turbulence to weak turbulence [24].

3 System Configuration

The characteristics of the proposed system are achieved by using the parameters that represent in the real physical world. This FSO system is designed with the aim of producing a credible model by the analytical tools. In our work, DWDM-interleaving scheme is demonstrated for the composite signal of FSO-FTTH and RoFSO. The practical implementation of this system would definitely increase the futuristic possibilities with a new trend of optical wireless communication.

Figure 2 depicts the complete distribution process of the optical signal. It begins with the 10 MHz linewidth based continuous wave (CW) light from a tunable laser diode TLD1 of 1549.37 nm \((\lambda_{1} )\), 3 mW power (Fig. 3a), that is first modulated by a sinusoid of 30 GHz using LiNb Mach–Zehnder modulator (MZM) to generate the sidebands as subcarriers of 60 GHz RF wave. The extinction ratio of MZM is fixed at 30 dB, and the bias point is adjusted to 3 V to make the optical carriers along with the two sidebands to have the same power (Fig. 3b). All 3 frequency components are filtered by an optical bandpass filter (OBPF). As a result, the unwanted low amplitude sidebands are suppressed. Further, these are amplified by an EDFA with a gain of 5 dBm. The central optical carrier is reflected by the FBG (Fig. 3d) which is modulated by the NRZ-encoded baseband signal of 2.5-Gb/s (Fig. 3f) and the sidebands go through the FBG (Fig. 3c) for modulating another NRZ-encoded 2.5 Gb/s RoFSO signal (Fig. 3e). These two modulated outputs are further recombined by an optical coupler (Fig. 3g).

Fig. 2
figure 2

Setup configuration of DWDM-interleaved hybrid system for accessing wired and wireless users under the free space atmospheric channel (abbreviations used: TLD Tunable Laser Diode, OBPF Optical Band Pass Filter, PRBSG Pseudo Random Bit Sequence Generator, FC Fiber Collimator, OSA Optical Spectrum Analyzer, PD Photodiode, LO Local Oscillator, LPF Low Pass Filter)

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Fig. 3
figure 3

a Optical spectrum of TLD1 output b central carrier peak with two sidebands separated by 60 GHz c transmission spectrum of FBG as 60 GHz RF band d reflected Bragg wavelength from the FBG e spectrum of modulated RoFSO signal with 2.5 Gb/s data for wireless services f spectrum of modulated optical carrier with 2.5 Gb/s baseband signal for wired service g composite hybrid signal spectrum

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Resulting optical spectrums of each step assigned (a) to (g) in the schematic setup of Fig. 2 are represented in respective subplots of Fig. 3.

To summarize, the mechanism for AP1 has been described and, since the complete system is proposed for 12 users, the transmitter layout is constructed with six tunable wavelength lasers i.e. TLD1 (1549.37 nm), TLD2 (1549.53 nm), TLD3 (1549.69 nm), TLD4 (1550.10 nm), TLD5 (1550.26 nm) and TLD6 (1550.42 nm) for AP1, AP2, AP3, AP4, AP5 and AP6 respectively. A constant offset between adjacent lasers is set to 0.16 nm (20 GHz). However, channel spacing between TLD3 and TLD4 is 50 GHz in order to accommodate lower and upper sidebands generated by the intermediate lasers. For benefiting 12 end users, the reported mechanism would be replicated six times. Therefore, 6 sets of hybrid signals from coupler 1, 2, 3, 4, 5 and 6 are obtained. Subsequently, such 18 interleaved frequency components are obtained when it passes through an optical combiner, as shown in Fig. 4, indicated by the terminal (h) in Fig. 2.

Fig. 4
figure 4

Interleaving of eighteen frequency components carrying data of 12 users spanning spectral band of 200 GHz

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This spectrum itself reports successful hybrid interleaving of 18 frequency components. This composite spectrum share data of 12 users indicating the possibility of increasing the channel capacity per DWDM channel. Spectral components \(\lambda_{1}\), \(\lambda_{2}\), \(\lambda_{3}\), \(\lambda_{4}\), \(\lambda_{5}\) and \(\lambda_{6}\) are the optical carriers carrying the baseband signals for wired services while \(\lambda_{1a}\)-\(\lambda_{1b}\), \(\lambda_{2a}\)-\(\lambda_{2b}\), \(\lambda_{3a}\)-\(\lambda_{3b}\), \(\lambda_{4a}\)-\(\lambda_{4b}\), \(\lambda_{5a}\)-\(\lambda_{5b}\) and \(\lambda_{6a}\)-\(\lambda_{6b}\) are their associative sidebands carrying 60-GHz RoFSO signals for wireless services. The frequency interval between \(\lambda_{1a}\) and \(\lambda_{6b}\) is 190 GHz. To avoid the loss of the information bearing elements, an extra margin of 10 GHz is included so that the total spectrum spans on a band of 200 GHz. This single DWDM grid is utilized by adjusting existing channels of the optimized bandwidth.

This hybrid interleaved signal is now amplified by the EDFA and after being passed through variable optical attenuator (VOA), it is transported through free space atmospheric link \((C_{n}^{2} = 10^{ - 17} {\text{m}}^{ - 2/3}\) to \(10^{ - 12} {\text{m}}^{ - 2/3}\)) of 400 m, which is having attenuation = 25 dB/km, transmitter aperture diameter = 5 cm and beam divergence = 1 mr.

As illustrated in Fig. 2, the receiver design is also configured with 6 APs, where, the transmitted signal is collected by aligned fiber collimator (FC) having an aperture diameter of 10 cm, followed by an optical splitter. To understand the concept of optical distribution at the reception node, firstly we take the case of AP1. Here, obtained hybrid signal utilizes an FBG to drop the central optical carrier of 1549.37 nm, and its associative sidebands bearing RF signal are dropped with the help of dual-peak FBG (can be replaced with cascaded OBPFs). These signals are received by the individual PIN photodiode (PD), considering the diode parameters as responsivity = 1A/W and the dark current = 0.9 nA. The 2.5 Gb/s baseband signal is directly retrieved after PD1 for the wired service. On the other hand, RoFSO signal detected by PD2, produces a 60-GHz signal, which is then radiated from the RF antenna to the wireless mobile user in a radio coverage of 50-100 m. In our scheme, channel loss for this coverage is modeled by taking an electrical attenuator of 4 dB. At the mobile end user, original data of 2.5-Gb/s is recovered by homo-detection with the help of a 60 GHz local oscillator. These electrical signals are filtered by low pass filters and then processed in data acquisition unit to evaluate their communication metrics. In a similar manner, the remaining 5 APs will provide such services to other 10 users.

4 Results and Discussions

To evaluate the performance of the proposed design, BER measurements are thoroughly tested. Keeping the turbulent atmosphere at moderate level (\(C_{n}^{2}\) = \(10^{ - 14} {\text{m}}^{ - 2/3}\)), in Fig. 5a and b, error probability is plotted as a function of the received optical power (ranging between − 28 dBm and − 18 dBm). The receiver sensitivity range for obtaining accepted BER of 10−9, is − 24 dBm to − 22 dBm for the baseband signals and − 20.2 dBm to − 18.2 dBm for RoFSO signals, respectively. This difference of ~ 3.8 dBm may be attributed to additional losses (described in Sect. 3) incurred in RF transmission.

Fig. 5
figure 5

Observed dependence of BER on received optical power for a wired services b wireless services under the turbulence regime

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The BER curves in Fig. 6a and 6b claim that, variation in \(C_{n}^{2}\) from \(10^{ - 17}\) to \(10^{ - 12} {\text{m}}^{ - 2/3}\) leads to increment in error probability of the system. It is observed that intermediate channels/bands (\(\lambda_{2}\) and \(\lambda_{5}\) / \(\lambda_{2a}\)-\(\lambda_{2b}\), \(\lambda_{3a}\)-\(\lambda_{3b}\) and \(\lambda_{4a}\)-\(\lambda_{4b}\)) are more interference-prone due to rise in crosstalk by immediate neighbors. This results in BER degradation for AP2 and AP5 for wired services, and BER improvement for AP1 and AP6 for wireless services.

Fig. 6
figure 6

Plots of BER versus \(\varvec{C}_{\varvec{n}}^{2}\) for a wired services b wireless services of the system

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5 Conclusion

FSO is a promising technology for the future optical distributing system. Our proposed integrated design (FSO-FTTH and RoFSO) caters the increasing demand on bandwidth for wired and wireless services. In this work, the interleaving effect over the range of 400 m turbulent atmosphere is carefully investigated, as the neighboring channels are likely to face the effect of crosstalk. It can be concluded that, though outside channels/bands are the good candidates for carrying user’s information, yet the intermediate channels/bands require a bit more spectral spacing among neighboring channels. In addition, for achieving the BER of 10−9, wireless services require marginal power of ~ 3.8 dBm than wired services.