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A novel bidirectional ultra-wideband over-fiber (UWBoF) system compatible with the wavelength-division-multiplexing (WDM) architecture is presented. In the proposed scheme, a 6th order Gaussian derivative is generated for UWB transmission in a downstream (DS) scenario, based on the directly modulated laser, accumulative chromatic dispersion in the transmission fiber and delay-line-interferometer (DLI). While the UWB signal is received from one of the DLI outputs, the other output is utilized to reuse the wavelength by injection locking a colorless Fabry-Perot laser diode (FP-LD). Due to the filtering effect of the FP-LD, a clear optical carrier without intensity modulation is then generated which can be used for upstream (US) baseband (BB) transmission by directly modulating the FP-LD. In order to eliminate the unwanted Rayleigh scattering induced noise in the bidirectional transmission, a dual - fiber transmission architecture is used. The principle of operation is explained. A symmetric transmission of 1.25 Gbps UWB over 60 km single mode fiber (SMF) is performed. Bit-error-rate (BER) measurements and eye diagrams for both down and upstream transmissions are presented.
© 2017 Optical Society of America
1. Introduction
The next generation of wireless local-area network (WLAN) and wireless personal-area network (WPAN) are expected to have features such as low complexity, cost and power consumption and offer wireless connectivity at a bit rate exceeding 1 Gbps in order to support bandwidth intensive services. Ultra-wideband (UWB) technology is introduced as a promising approach to fulfill these requirements due to its low power consumption, immunity to multipath fading, interference mitigation, carrier free, high data rate and capability to penetrate through obstacles [1]. The United States Federal Communications Commission (FCC) has regulated the unlicensed use of the UWB spectrum from 3.1 to 10.6 GHz, with a power spectral density (PSD) lower than −41.3 dBm / MHz [2]. Because of the low PSD, the transmission distance is limited to a few meters. UWBoF has been proposed to extend the coverage area and also integrate the UWB services into the fixed wired or wireless communication networks [3]. Generating the UWB signals directly in the optical domain is highly desired in order to avoid the use of wideband electronics. To meet the FCC regulation requirements, the desired pulse shapes are based on derivatives of the Gaussian function. The generation of the first order (monocycle) and the second order (doublet) of the Gaussian derivative has mostly been reported in the microwave photonic research, because of the sake of simplicity. However, the monocycle and doublet do not satisfy the FCC spectral rules. To meet FCC regulation perfectly, at least a fifth derivative of the Gaussian pulse is required [4]. Recently, the authors introduced and experimentally demonstrated one of the simplest and most cost efficient techniques to generate monocyle and doublet UWB pulses [5, 6]. This approach is based on the direct modulation of a semiconductor laser (DML), optical filtering and accumulative chromatic dispersion in a transmission fiber. The authors showed that the proposed transmitter can adapt to different accumulative dispersion caused by different fiber lengths installed in the optical access network [5]. They also demonstrated that the transmitter is compatible with a time division multiplexing-passive optical network (TDM-PON) and can generate non-return-to-zero (NRZ) and UWB signals at different time slots of TDM architecture [6].
On the other hand, the fixed fiber deployments for fiber to the home (FTTH) services are increasing based on PON technology. The current PON technologies (Gigabit PON and Ethernet PON) are based on TDM; therefore they cannot fulfill the requirements of the future access network, such as aggregated bandwidth, attainable reach and allowable power budget [7]. The wavelength-division-multiplexing PON (WDM-PON) has been proposed as an ultimate broadband access network due to the advantages, such as increased bandwidth per optical network unit (ONU), high security, network flexibility and protocol transparencies. However, the need for wavelength selective optical components and frequency stable light sources in WDM-PON may increase the system complexity and the cost [8].
To reduce the overall cost and the complexity of the system and improve the compatibility between WDM-PON and UWBoF, it is necessary to simplify the operation, ease the maintenance and reduce the cost. Therefore it is desired to generate the UWB signals in the optical domain at the optical line terminal (OLT) and reuse the wavelength for the upstream (US) service and realize a colorless ONU [8]. Several different techniques have been reported to achieve wavelength reuse, such as utilization of a separated optical carrier [9, 10], gain saturation of a reflective semiconductor optical amplifier (RSOA) [11, 12] and injection-locking of a Fabry-Perot laser diode (FP-LD) [13, 14]. The separated optical carrier scheme is complicated, costly and not efficient, as the sharp optical filters or interleavers are necessary in this technique and also demodulation of the signal at the ONU is complicated. The major limitation of the schemes using a RSOA or a FP-LD is that the extinction ratio of the downstream (DS) signal has to be low, which limits the network performance.
In this paper, we propose a very simple and cost efficient wavelength reuse scheme for UWB over WDM-PON system. The downlink is based on the direct modulation of a semiconductor laser, accumulative chromatic dispersion in transmission fiber and the interference effect of a delay-line-interferometer (DLI) in the ONU to generate the 6th order derivative of the Gaussian function, whose spectrum fits perfectly to the FCC mask. The uplink is based on the injection locking, filtering effect and direct modulation of a FP-LD. While the UWB signal is received from one of the DLI outputs and propagated through the antenna, the second output of the DLI is used to injection lock the FP-LD to generate a clear optical carrier without intensity modulation for the US data transmitter. A bidirectional and symmetric transmission of 1.25 Gbps UWB signal over 60 km of single-mode fiber (SMF) is experimentally demonstrated. The transmission performance in terms of BER and eye diagrams is evaluated. An error free transmission for both down and upstream scenarios is achieved. To the best of our knowledge, such a simple and cost efficient WDM-UWBoF system is proposed for the first time, in which the 6th order derivative of the Gaussian function is generated for DS UWB transmission and a colorless ONU is realized for US transmission, using injection locked FP-LD which is not sensitive to the extinction ratio of the DS signal.
2. Proposed UWBoF-WDM-PON architecture
The proposed UWBoF over WDM-PON is illustrated in Fig. 1. In each transceiver at OLT, the desired optical pulses are generated through a chirped-managed-laser (CML) at different wavelengths. The CML is a commercially available package, comprising a DML and a passive optical filter. The optical DS signals are then multiplexed by a WDM multiplexer (MUX) and sent to the remote node (RN). In RN, the DS signal is de-multiplexed through a WDM demultiplexer (DEMUX) and distributed to the corresponding ONUs. The received signal in the ONU passes through a DLI to take advantage of the interference effect in order to shape the desired 6th order derivative of the Gaussian function. The optical UWB signal is then converted to an electrical signal by an avalanche photodiode (APD) and propagated through an UWB antenna. The second output of the DLI is redirected through a polarization controller (PC) and a circulator to a colorless FP-LD. The free running FP-LD is then injected and locked at the same wavelength as the DS in order to generate an optical carrier for the US. From the other side, the US UWB data from the wireless user is received in the ONU, down converted to the baseband (BB) by a local oscillator and a low-pass filter (LPF) and directly modulated on the selected wavelength. The US signals from different ONUs are then multiplexed at the RN using a MUX and sent to the OLT. The received US signals are de-multiplexed at the OLT utilizing a DEMUX and sent to the corresponding transceiver. At each transceiver, the optical US BB signal is converted to an electrical signal using an APD and detected by the receiver. In order to eliminate the unwanted Rayleigh scattering induced noise in the bidirectional transmission, a dual - fiber transmission architecture is used [15].
3. Principle of operation
As explained in detail in [5, 6], by applying the electrical signal to the distributed feedback (DFB) laser, not only the intensity, but also the optical frequency of the output light is modulated. The output intensity modulation (IM) is related to the photon density inside the cavity and the laser frequency modulation (FM) is due to the carrier density change. The authors demonstrated theoretically and experimentally in [6], that by locating the spectrum of the DML output on either the negative or positive slope of a tunable optical bandpass filter (OBPF), an UWB or NRZ signal can be generated, respectively. The DFB laser and tunable OBPF are commercially available in a CML package. In order to generate the required pulses for UWB generation in this work, as in the previous works [5, 6], the proper FM to IM conversion has to be performed. The difference is that here both negative and positive slopes are simultaneously taken into use. To do so, an optical signal is required, having positive and negative frequency deviation. An electrical pulse with a small distortion on the edge [Fig. 2(a)] is sufficient to generate the required optical signal. To see the optical pulse shape and its corresponding chirp after DFB laser and analyze the effect of the OBPF, the CML is modeled in MATLAB. The DFB is modeled using the laser rate equations [16, 17]. Figure 2(b) shows the intensity and chirp of the optical signal after modulating the DFB by the electrical signal. OBPF is modeled as a Gaussian filter with a 3-dB bandwidth (BW) of 0.06 nm. By tuning the OBPF and placing the spectrum of the DFB output on the proper position with respect to the wavelength of the OBPF, the desired pulse shape can be formed. The output of the OBPF is depicted in Fig. 3(a). Figure 4 explains how the FM of the signal from the DFB output [Fig. 2(b)] is converted to IM and combined with the IM of the signal to obtain the pulse shape at the output of the CML [Fig. 3(a)]. The fiber transmission is also modeled in MATLAB, using the same model as our previous work [5]. Figure 3(b) shows the pulse shape and its corresponding chirp after the fiber transmission. The impact of the chromatic dispersion on a chirped signal is fully investigated in [5]. It was shown mathematically and experimentally in [5], how the chromatic dispersion transfers the photon energy from the end to the beginning of a chirped pulse and modifies the waveform. The authors also showed that the obtained pulseshape is independent from the length of the transmission fiber and the transmitter can be adapted by controlling the laser operation point [5]. Finally, to shape the 6th order derivative of the Gaussian function, the authors take advantage of the interference effect of the DLI. The outputs of the DLI can be mathematically expressed as:
Ein is the input signal to the DLI, which is the same as the received signal after fiber transmission [Fig. 3(b)]. T indicates the time delay between two branches of the DLI. By controlling the T and choosing a proper time shift between the branches of the DLI, the desired pulse shape is achieved. From the simulation, T is found as T = 0.1 ns. The outputs of the DLI and their corresponding electrical spectrum is depicted in Fig. 5. The black mask in Fig. 5(a) indicates the FCC regulation.4. Experiment
4.1. Downstream
The experimental setup of the proposed wavelength reused UWBoF system is shown in Fig. 6. In the OLT a 10 Gbps pulse pattern generator (PPG) is programmed to generate a 1.25 Gbps electrical on-off-keying signal (PRBS 211 − 1). From the coding, a logical ‘1’ is represented by “1000 0000” (one ‘1’ bit every 8 bits), and a logical ‘0’ is represented by “0000 0000”. To generate the required electrical pulse shape as in Fig. 2(a), the positive and negative outputs of the PPG are added together using a 3-dB electrical coupler. By adjusting the duty cycle of the PPG outputs, the desired pulse shape is achieved [Fig. 7(a)]. The peak-to-peak voltage is Vpp = 1.8 V. The electrical signal is then used to directly modulate the DFB laser. The central wavelength, input impedance and threshold current of the laser module are 1538.7 nm, 50 Ω and 15 mA, respectively. By tuning the OBPF as explained in Sec. 3, the desired optical signal at the output of the CML is obtained [Fig. 7(b)]. The laser bias current, laser temperature and filter temperature are set to 50 mA, 20° C and 42° C, respectively. The 3-dB BW of the OBPF is 0.06 nm. The optical power at the CML output is measured as 2.9 dBm. Next, the generated signal is sent to ONU through a 25 km of SMF. Figure 8(a) depicts the obtained waveform after the fiber transmission. The optical power after fiber transmission is measured as −2.1 dBm. The received signal in the ONU is passed through a DLI to use the interference effect and generate the 6th order derivative of the Gaussian function. Figure 8(b) shows the obtained waveform at the first output of the DLI. The optical power at the DLI output is measured as −6 dBm. As mentioned in Sec. 3, the authors showed in [5] that this technique is independent from the length of the transmission fiber and the transmitter can be adapted by controlling the laser operation point. To prove this, the experiment is repeated for 60 km of SMF and similar results are achieved by setting Vpp to 0.75 V. The optical power at the output of the CML, after 60 km SMF and at the output of the DLI are measured as 1.8 dBm, −10.4 dBm and −14.3 dBm, respectively.
The optical UWB signal after variable optical attenuator (VOA1) is then converted to the electrical signal using an APD and propagated through an UWB antenna. After 40 cm of wireless transmission, the UWB signal is captured by another UWB antenna at the user side. The electrical pulses and their corresponding electrical spectra before and after wireless transmission are depicted in Fig. 9. The black line in Fig. 9(a) and 9(b) determines the FCC mask for indoor transmission and the red line in Fig. 9(b) shows the transfer function (S21) of the used UWB antennas. As it can be seen, the spectrum of the generated UWB signal fits perfectly into the FCC mask. To compare the pulse shapes before and after wireless transmission, the fidelity factor (F) was calculated and F = 0.9168 was reported. F is the maximum correlation coefficient of two signals and reaches unity when the two signals are identical.
Fig. 9 Electrical waveform and corresponding spectrum a) before wireless transmission and b) after wireless transmission.
After the wireless transmission, the received signal in the user side is down converted using an electrical mixer and local oscillator (LO). The frequency of the LO is tuned to each frequency component of the signal. The BB signal is then amplified and filtered to remove the high frequency components. The used electrical amplifier (Amp) and LPF have a 3-dB BW of 1.25 GHz and 950 MHz, respectively. Finally, the BER is measured versus received optical power (by changing the attenuation at VOA1) for different frequency components and the results are depicted in Fig. 10(a). The inset in Fig. 10(a) shows the observed eye diagram of the down converted signal by LO at 7.5 GHz when the received optical power is −24 dBm. As mentioned above, the experiment was repeated for 60 km of transmission fiber. Figure 10(b) compares the BER performance of the proposed setup after transmission over 25 km and 60 km of SMF for frequency components 6.25 GHz and 7.5 GHz. As can be seen, an error-free transmission (BER 10−9) for both scenarios is obtained. The experimental results are in a good agreement with the simulation results, confirming the theoretical discussion.
Fig. 10 a) Log(BER) vs. received optical power after 25 km SMF. b) Comparision of BER performance after 25 km and 60 km SMF.
4.2. Upstream
As shown in Fig. 6, for establishing the US, the second output of the DLI is injected to a colorless FP-LD through the second VOA, an optical circulator and a polarization controller (PC) in the ONU. Consequently, the optical carrier for the US is generated at the same wavelength as the DS. Figure 11 shows the spectrum of the used FP-LD before the injection-locking. The used FP-LD is designed for optical C-band with a channel range from 1531.12 nm to 1562.23 nm [Fig. 11(a)]. The enlarged spectrum is illustrated in Fig. 11(b) and 11(c) to see the modes and the channel spacing which is about 50 GHz. The spectrum of the injecting signal from the second output of the DLI is shown in Fig. 12(a). To see the successful injection locking, Fig. 12(b) outlines the power spectral density (PSD) with respect to wavelengths in terms of different injection power. The maximum injection power to the FP-LD is −5 dBm. The injection power is changed from −6 dBm to −28 dBm using VOA2. Figure 12(c) shows the side mode suppression ration (SMSR) versus the injection power.
From Fig. 11 and Fig. 12 it can be seen that FP-LD is locked to the same frequency as the DS and all other modes are suppressed. Figure 13(a) shows the waveform and electrical spectrum of the injecting signal before the PC. The applied signal to FP-LD would experience the FP etalon effect in the FP cavity. This leads to reshaping the spectrum and the waveform consequently. The waveform and electrical spectrum after the injection locking at the third output of the circulator are illustrated in Fig. 13(b). It can be seen that the frequency components below 1.25 GHz are suppressed by the filtering effect of the FP-LD. As the US signal is a 1.25 Gbps BB, this suppression is very desirable as the noise level is minimized. The high frequency components will appear as an out-of-band noise and can be easily removed by a LPF in the receiver. The generated carrier is then sent to the OLT through a 25 km of SMF. In the receiver, the optical signal is converted to an electrical one using an APD and the high frequency components are filtered out utilizing a LPF with a 3-dB BW of 950 MHz. Figure 14(b) shows the electrical spectrum of the received signal after the filtering. The temporal waveform of the optical carrier after photodetection and filtering in the OLT is shown in Fig. 14(a), showing that the DS data is completely erased from the US carrier. The clear optical carrier is then used for US data transmission.
Fig. 13 Waveform and electrical spectrum, a) before injection locking and b) after injection locking.
Fig. 14 a) The temporal waveform of the US carrier after photodetection and low-pass filtering. b) Electrical spectrum of the US carrier.
A PPG generates a 1.25 Gbps BB on-off-keying signal (PRBS 215 – 1) in the ONU, emulating the down converted US UWB signal. The injection-locked FP-LD is then directly modulated. The injection-locked US signal is separated from the DS signal using the circulator and sent to the OLT through 25 km of SMF. The optical power after the injection-locking at the third output of the circulator and after the fiber transmission are measured as 1.4 dBm and −3.7 dBm, respectively. In the OLT, the received US signal is converted to an electrical signal utilizing an APD and passed through a LPF with a 3-dB BW of 950 MHz to remove the higher frequency components and BER versus received optical power is finally measured (using VOA3). As mentioned before, the experiment is repeated for a symmetric 60 km of fiber transmission. As the injection power to the FP-LD for symmetric transmission over 60 km is −14.3, the optical power after the injection at the third output of the circulator and after 60 km of SMF are measured as, 0.2 dBm and −13.1 dBm, respectively. The BER measurement results for US transmission over 25 km and 60 km of SMF are shown in Fig. 15. The inset in Fig. 15 shows the eye diagram after 25 km of fiber transmission, when the received optical power is −31 dBm. As can be seen, an error-free transmission (BER 10−9) for both scenarios is obtained.
5. Conclusion
A novel, simple and cost efficient UWBoF system over WDM-PON was proposed. The principle of operation was explained in detail. A precise simulation was provided in order to explain the theory. The 6th order derivative of the Gaussian function was generated experimentally for UWB transmission in the DS scenario, based on the direct modulation of a semiconductor laser, optical filtering, accumulative chromatic dispersion in transmission fiber and the interference effect of DLI. The experimental results show a good agreement with the simulation, confirming the theoretical explanation. A 1.25 Gbps UWB transmission was performed and error free transmission was achieved. For the US scenario a colorless FP-LD was injection locked, by the second output of the DLI. Due to the filtering effect of the FP-LD, a clear optical carrier was generated. Data erasing of the US carrier was proved. A 1.25 Gbps BB transmission was performed by directly modulating the injection locked FP-LD and error free transmission was obtained. The experiment was repeated for symmetric transmission distance over 25 km and 60 km of SMF, proving that in the proposed technique the transmitter can be adjusted with respect to the fiber length by controlling the laser operation point. The proposed technique is a promising candidate for the future optical access networks.
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