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Abstract
Implementation of the passive radio over-fiber technique permits modulating the low-frequency sub-carrier onto an optical channel for dissemination through a light-wave fiber network. Single-sideband modulation for optical signals allows the impressive utilization of channel capacity in optical fiber. Dispersion reduction techniques limit the pulse spreading of a propagated signal in any photonic scheme. To control pulse-spreading effects, the optical single-sideband modulation technique at different phase shifts is modeled, analyzed, and compared to examine the performance of a sub-carrier multiplexing system. Hence, in this paper, a quantum key distribution network using a single sideband modulation technique based on a Li-Nb Mach-Zehnder modulator has been proposed at different electrical phase shifts. In this suggested model, the Optisystem 14.2 simulator is used to analyze the nonlinear characteristics. We have designed a single-sideband contour reduction and amplification with each couplet of 120 km by increasing the distance up to 720 km, and the phase between quantum states is determined. The system performance of the suggested model is investigated and compared based on output power (dBm), quality factor, eye diagram, bit error rate (BER), extinction ratio (ER), and optical spectrum of the received signal by varying link distance (km), channel spacing (nm), input power (dBm), and fiber dispersion (ps/ns/km).
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1. Introduction
The communication industry faced tremendous growth in multimedia and data networking usage. It became the most important human requirement. The demand for a propagation scheme with a large transmission capacity is enormous nowadays [1]. With the increase in users, the time duration of online users is simultaneously increasing. It produces various nonlinear effects, like scattering, during the transmission of the signal. Therefore, the basic issue in designing a long-haul communication network is the signal loss encountered in the channel along with the dispersion loss associated with it. However, the aforementioned loss can be reduced using effective modulation techniques at the transmission side [2]. To minimize the attenuation, different optimization parameters controlling the modulation are available, such as dispersion compensation fiber, shifted fiber, optical filter, fiber Bragg grating, and hybrid modulation techniques [3]. Secure communication between sender and recipient is possible with quantum key distribution (QKD), which is based on quantum physics [4]. Ideally, information is transmitted in a single quantum state so that an eavesdropper cannot access the data without disturbing the network. Cryptographic algorithms are commonly used for secure transmission. Here, two entities let Alice and Bob share the information in the system [5,6]. It permits both sides to establish a secret key in the event of an eavesdropper (Eve). Quantum and public channels are used as the mediums. The quantum channel transmits photons to produce keys, and the public channel involves basic reconciliation, error correction, and privacy amplification protocols [7,8].
Initially, RF over fiber (RoF) is a method that provides an amazing combination of optics along with RF signals [9]. RoF technology provides photon regulation using a carrier wave propagated through an optical channel for wireless access [10]. Generally, two principal methods have been used to code the data: 1) using polarization coding, and 2) in terms of delay-coded quantum states. It is very difficult to maintain polarization over long-distance communication. But in the second case, the signal can be coded into an optical path difference [11]. To handle the chromatic dispersion issue and fading at the demodulator side, an SSB modulation is designed where a phase shift of 90° and 120°has been applied across the RF signal [12].
Different multiplexing techniques like TDM, WDM, and FDM are available, but the system performance improves in sub-carrier multiplexing (SCM) as it combats multipath fading and ISI suppresses the sideband frequency components. So, the Mach-Zehnder modulator in phase-shifting mode is used by different approaches [13]. Here, a single-sideband sub-carrier multiplexing model is proposed using a Li-Nb Mach-Zehnder modulator in phase-shifted mode along with fiber Bragg grating for secure communication. In [14], a bidirectional RoF network has been designed by applying the Brillouin scattering effect. It refers to sub-carrier multiplexing along with special broadcasting scattering (SBS) and amplitude shift keying. A comparison has been performed between the proposed model and an SBS-generated system. It is simulated for 10 km to 40 km in both directions and achieved the Q-factors of 10.6 and 6.36 for upstream and downstream. In [15], RoF propagation through 40 km single-mode fiber without any reduction method and additional 5 m propagation have been implemented for both dual-carrier modulation and orthogonal frequency division multiplexing (DCM-OFDM) ultra-wideband signals at 1.44 Gbps in a 60 GHz band. Investigational outputs have been compared with simulated data in the 60 GHz frequency band. In [16], a RoF-based passive optical network has been implemented using two electrical phase shifts with an OSSB modulation scheme. The system performance was compared for 90° and 120°phase shifts, where second-order harmonics were minimized only using 120° phase shift. Hence, this network has improved features in terms of BER, received power, and signal attenuation. In [17], a long-distance, polarization-insensitive QKD technique is proposed using a single-sideband (SSB) receiver to cover 40 km of distance. In [18], detailed comparison studies of N-channel WDM optical networks using RoF technology and analysis have been done among digital modulation to cover 100 km of link distance.
The main aspects of this work are concise, as follows:
- 1. A WDM propagation model is approached using the optical SSB technique, which allows the BB84 protocol.
- 2. Here, the information quantum state is encrypted in the phase deviation between the central and its side lobes. Photo detection is performed up to 720 km of link distance at a minimum signal loss.
- 3. The proposed method has been compared with previous work based on the optical spectrum, received optical power, fiber dispersion, variation in channel spacing, and laser power.
The rest of the paper is organized as a brief description of sub-carrier multiplexing with optical SSB, which is described in Section 2. Section 3 provides an explanation of the proposed model. Section 4 discusses the simulation results, and finally, the work is concluded in Section 5.
2. Concept of sub-carrier multiplexing with OSSB
Multiple RF signals are combined and propagated on a single fiber using optical sub-carrier multiplexing (SCM). The SCM technique gains acute attention because of its advantages. The advantage of SCM is that it improves the BER and Q-factor in RoF technology, improves the system capacity when in burst networks, and supports more users [19,20]. The low-phase noise of RF oscillators eases homodyne detection in the RF domain as compared to optical detection [21,22]. The basic design of an SCM optical network is shown in Fig. 1. Here, input signals are transmitted through an optical multiplexer at different carrier frequencies. These signals are combined and modulated. Then the modulated signal is transferred through the medium either through optical fiber or through air. At the Bob side, the mixed signal is separated using optical De-multiplexer. It consists of photodiode to generate the electrical signal from optical incoming signal, low pass filter (LPF) to remove the unwanted high frequencies.
In transmitting multiple signals, the signals at the receiving end suffer from group velocity dispersion (GVD). To reduce the effect of GVD, optical SSB is a suitable alternative. It also helps to improve the optical channel bandwidth. OSSB eliminates unwanted sideband using optical notch filters following stimulated Brillion scattering [23].
3. Proposed model
In the proposed design, instead of two mixers, a single dual-drive LiNb MZ modulator is used as shown in Fig. 2. The designed SSB modulation is illustrated with the help of two Mach-Zehnder modulators (MZM) [24] and a fiber Bragg grating (FBG). The FBG is to compress one of the sidebands. The intensity-modulated optical carrier from the LiNb Mach-Zehnder modulator is given to the grating through loop control.
The expression for single-sideband modulation techniques [25] is
From the above discussions, it can be seen that the MZM modulator provides two sideband components similar to those of the Hilbert transformer.
The combination of the WDM and SCM produces a high-speed optical network with low dispersion and maximum bandwidth efficiency [26]. In traditional WDM, the transmission distance can be extended across the C-band in addition to the use of erbium-doped fiber amplifiers (EDFAs). As it provides multi-wavelength propagation over a single fiber, it offers an evolutionary option to employ QKD links into existing networks [27]. Hence, in our proposed work, SCM is mixed with the WDM concept. A two-user sub-carrier multiplexing network using SSB is simulated using Optisystem 14.2 software, as shown in Fig. 3. Each subsystem has a bit sequence generator, which produces a data signal. Here, a 20Gbps SCM optic system is designed, in which 2*10 Gbps bit streams are combined into one wavelength by using a multiplexer.
As shown in Fig. 4, a pseudo-random bit sequence (PRBS) is implemented to produce random bits and given to a pulse generator such as the RZ line format. A sine wave generator at 5 GHz is given to the multiplier. The output of the multiplier is transmitted as in-phase and quadrature-phase components. The electrical phase shifter with a 90° shift is fed to one of the inputs of the dual-drive modulator. On the Alice side, the CW laser diode operating at a central frequency of 193.1 THz is applied as another input to the modulator. The intensity-modulated optical carrier from the LiNb Mach-Zehnder modulator is given to WDM-MUX. The output signal is passing through a loop control, as shown in Fig. 3. The information at Alice is set by the initial signal phase reference. She regulates the intensity with an attenuator, as in a given bit; a minimum or less than one photon can transmit through the fiber. The signal is traveling on 150 km of single-mode fiber with a span of three loops. Using EDFA in this optical system improves the transmission quality [28,29] and amplifies the light signal.
On the receiver side, as shown in Fig. 3, the modulated signals go through the WDM-DEMUX, and the output of the demultiplexer is split into two parts using a fork [30]. As shown in Fig. 5, one end of the fork is given to an optical Bessel filter, followed by a photo detector with a dark current of 10 nA and a response of 1 A/W, and another end is connected to a fiber Bragg grating. By using an optical filter, the noise produced in the used channel can be reduced [28]. The converted electrical signal is connected to a low-pass Bessel filter to eliminate the higher-frequency components. Then a regenerator and BER analyser are used to measure the system parameters, such as eye diagrams, RF spectrum, power, and bit error rate.
4. Simulation results and discussion
The proposed method is based on QKD-OSSB technology. As shown in Fig. 3, the two sub-carrier channels are mixed and transmitted at 193.1 THz and 193.2 THz, respectively. The frequency spectrum of the multiplexed signal for a 90° phase shift is shown in Fig. 6(a). From the figure, it can be seen that there is no upper sideband in the output spectrum. To improve the propagation quality and optimize the nonlinearity in the fiber network, it is helpful to use the grating, which can increase the system attainment [31]. The output FBG spectrum is represented in Fig. 6(b). As compared to Fig. 6(a), it provides a clear and smooth spectrum. The spectrum shows that it has low insertion loss and negligible nonlinearity when used as a dispersion compensator or for routing wavelength in WDM systems.
Fig. 6. Measured output optical spectrum at 90° phase shift showing both (a) without FBG and (b) with FBG for OSSB modulation with two sub-carrier signals.
Figure 7 implies the simulated optical spectrum using a 120° phase shift. It can be illustrated that both systems have almost the same suppression of the lower first-order sideband, but the higher second-order sideband with a 120° phase shift is suppressed by 12 dBm as opposed to a 90° phase shift. This sideband cancellation for higher-order signals minimizes the effect of CD and improves system performance [32].
Fig. 7. OSSB spectrum with 2 sub-carrier channels at 120° phase shift (a) without FBG (b) with FBG. It shows the higher second-order sideband is suppressed by 12 dBm which reduces the effect of chromatic dispersion.
The system efficiency is influenced by the nonlinearity effect, and the factors that influence the signal quality are fiber characteristics, operating wavelength, and frequency spacing between the channels [33]. A composite SCM signal is produced for two users to evaluate the influence of frequency spacing in the multiuser domain [34].
In [18], the comparison was done at various channel spacing’s and input laser power vs. output power using two digital modulation techniques, QAM and DPSK. To examine the effect of input power on the system-received power, the CW laser power is increased from 0 to 25 dBm. Hence, the frequency spacing between the two multiplexed channels is 0.1 nm, 0.5 nm, and 1 nm in the proposed model. Table 1 shows the quantitative comparison of output power with received power without FBG and with FBG.
Table 1. Variation in channel spacing
From Table 1, it can be seen that the channel spacing is directly proportional to sideband power. Hence, the nonlinearity effect is reduced. The optical power generated by a continuous-wave laser is proportional to the driving circuit input current. Table 2 shows the comparisons between our proposed model without digital modulation and the work done by [18] using the same values of input power. Again, the effect of phase shift on receiving power is analyzed.
Table 2. Comparison of laser power for previous and proposed models
From Table 2, it is found that the performance difference between QAM in [18] and the OSSB-SCM technique at input powers of 0 dBm and 10 dBm does not differ significantly. The output power is effectively increased at a higher value of LASER power [35]. It can be seen that the proposed method provides improved output power without FBG as compared to the result shown by [18]. Figure 8 shows the comparison between the input power and received power of the proposed model without FBG and with FBG for both OSSB-modulated techniques.
Fig. 8. Received power Vs input power for the proposed model. (a) Measurements at 90° phase shift and at 120° phase shift without FBG shown in black line, and (b) with FBG shown in red line. The resultant output show the received power for values of input power ranging from 0 to 25 dBm.
For the input power above 15 dBm, the received power is approximately 3 dBm above in the case of OSSB-SCM using a 120° phase shift, as shown in Fig. 8(b). For a LASER power of 10 dBm, the output power using 120° phase shift is 25.536 dBm and 19.801 dBm using 90° phase shift with FBG.
Another parameter affecting the signal transmission is the pulse widening in the transmitted signal. Therefore, the system is analysed for channel dispersion at 1 ps/ns/km and 16.75 ps/nm/km. Table 3 illustrates the comparative output in terms of fiber dispersion of the proposed model given by [18] and our proposed technique. The model by [18] has considered a fiber length of 100 km, whereas we have increased the link distance up to 720 km.
Table 3. Effect of change in fiber dispersion for previous and proposed models
It can be verified from Table 3 that our design offers an improved result with a low dispersion level as compared to [18]. The proposed design provides high received power by increasing the fiber dispersion from 1 ps/ns/km to 16.75 ps/ns/km.
To evaluate the effect of the extinction ratio (ER) on the output power, the scheme is simulated at multiple values of ER. The value is increased from 5 to 30 dB, as shown in Fig. 9. With the increase in ER, the power decreased from −18 dBm to −21 dBm in the case of a 90° phase shift. Similarly, it decreases from −18 dBm to −20 dBm in a 120° phase shift.
Fig. 9. Variation of extinction ratio vs received power. Received power is calculated for the various extinction ratio ranging from 5 to 30 dB.
The proposed system is analyzed at different lengths of optical fiber and compared with others in terms of Q-factor and BER. The Q-factor is an estimation of OSNR for any transmitted binary sequence. It is the ratio between the dissimilarity of the intensity of the light signal at high and low levels in an eye pattern [36]. The quality factor is calculated at multiple link distances in a range of 120 km to 720 km with a span of three, and the result is given in Table 4. This can be well noted graphically through the eye diagram shown in Fig. 13. It is observed that, on varying the optical link, the value of the Q-factor decreases. The value deteriorates from 9.2674 to 5.4466 with increasing length for various channel characteristics as shown in Table 5. For example, because of noisy channels, non-linear effects, intra-channel FWM, and any polarization, the signal quality degrades and the value of the Q-factor is reduced [37,38].
Table 4. Effect of fiber Bragg grating on link distance with Q-factor of the received signal
Table 5. Effect of fiber Bragg grating on link distance with Q-factor of the received signal
To analyze the reliability of the modulated signals, their performance is investigated based on their Q-factor vs. distance, as in Fig. 10 for a single-user system and in Fig. 11 for the multi-user SSB system. It can be seen from Fig. 10 that the effect of nonlinear distortion can be minimized by implementing the FBG.
Fig. 10. Comparison graph between Q-factor and distance for a single user system. Red line and dot marks are the simulation results of Q-factor without FBG and black line with square marks are the results of Q-factor with FBG at the link distance ranging from 120 to 720 km.
It is observed from Fig. 11 that the system quality decreases with an increase in distance. It happens due to the overlaying of multiple data points, the nonlinearity effect, and the devices used in the system. Therefore, the maximum Q-factor in the multi-user system is 7.621 with FBG.
Further, the BER analysis of the link has been analysed in Fig. 12. BER is the basic entity assessing the performance of the model. The error rate automatically reduces when the quality factor is high by increasing the quality of the system. The comparison graph in Fig. 12 shows the minimization of BER at the application of fiber Bragg grating at the receiver side.
Fig. 12. BER as a function of link distance. Measurements of BER without FBG are shown with a sky line, while the brown line shows the values with FBG which represents the bit error rate is almost constant up to 300 km.
From the above figure, it can be seen that nonlinearity is higher without the FBG and BER maximum after 120 km, which decreases the system performance. However, by using FBG at the receiver side, the minimum BER is almost constant up to 300 km. The eye diagram is achieved at different optical fiber lengths, and in Fig. 13, we have obtained the eye pattern for a distance of 720 km. At a distance of 720 km, the SSB system has a quality factor of 5.7954, whereas the SSB with sub-carrier multiplexing techniques has a quality factor of 6.257. From both values, we can verify that the SCM method provides a high quality factor.
Fig. 13. Eye diagram at QKD receiver output. The and black diagram shows the result with SCM and the sky lines shows the result without SCM technique. The eye pattern is obtained at a link distance of 720 km with Q-factor of 6.257 with SCM and of 5.7954 without SCM technique.
5. Conclusion
The proposed work shows the acceptance of optical single-sideband modulation at different channel lengths and its use in quantum distribution. In this work, an optical SSB network has been designed and analyzed in terms of Q-factor and bit error rate. And the result of single-user OSSB is compared with another design of OSSB sub-carrier multiplexing (SCM), in which compensation and amplification of the signal have been implemented after 120 km for every span of the loop, as compared to past analysis where optimization has been done at a distance of 100 km. Again, the utilization of FBG at the receiver side makes the system cost-effective. The achieved maximum quality factor is 6.257, and the minimum BER is 8.03353e-012. Future work will address a much more reliable and sturdy propagation network by suppressing side harmonics through an optical comb generator.
Acknowledgment
The authors would like to acknowledge the research funding to the Innovative Technologies Laboratories (ITL) from King Abdullah University of Science and Technology (KAUST).
Authors Contribution. Conceptualization: Bandana Mallick & Priyadarsan Parida; methodology: Bandana Mallick, & Bibhu Prasad; validation: Bandana Mallick & Chittaranjan Nayak; investigation: Chittaranjan Nayak, Priyadarsan Parida & Yehia Massoud; resources: Priyadarsan Parida, & Amit Kumar Goyal; data curation: Gopinath Palai & Bibhu Prasad; writing—original draft preparation: Bandana Mallick, Amit Kumar Goyal & Priyadarsan Parida; writing—review and editing: Priyadarsan Parida.; visualization: Priyadarsan Parida; supervision: Priyadarsan Parida, Chittaranjan Nayak & Yehia Massoud. All authors have read and agreed to the published version of the manuscript.
Disclosures
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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