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Abstract
Spectrally efficient optical communication with flexible capacity is required in the prospective adaptive photonic network. Optical orthogonal frequency division multiplexing (OFDM) is one of candidates to contribute to this type of photonic network. Here we report on a gate-free integrated-optic tunable filter that can demultiplex optical OFDM signals with various number and symbol rate sub-carrier channels. The filter consists of tunable couplers, an array of delay lines, and a slab star coupler-type optical Fourier transformation circuit. We can tune the number and/or the symbol rate of demultiplexed optical OFDM channels by selecting the delay lines with the tunable couplers. Optical timing gates were normally attached to the filter output ports with a view to extracting the effective time of the filtered channels. In this investigation, we removed the optical gates and substituted a high-speed photodetector for the gate with a view to achieving completely passive demultiplexing of the various capacity optical OFDM signals. Various channel symbol rate and channel number OFDM signals (5 × 10 to 20 Gsymbol/s and 3 × 20 Gsymbol/s) were successfully demultiplexed with this gate-free tunable filter. Our star coupler-type tunable OFDM filter without the gates was used to demultiplex various channel symbol rate optical OFDM signals for the first time.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Optical orthogonal frequency division multiplexing (OFDM), which employs optical orthogonal sub-carrier channels whose symbol rates are equal to their frequency spacing, is attractive because it can achieve spectrally efficient optical communication (1 symbol/s/Hz). The Fourier transformation is required for demultiplexing the OFDM sub-carrier channels [1–8]. In the prospective adaptive photonic networks, the minimal bands are adaptively allocated corresponding to the traffic and transmission distance with a view to cutting down the network resources. This kind of function is achieved by decreasing the guard bands between the used channels, and changing the number, symbol rates, and/or modulation formats of the channels [9]. The optical OFDM can contribute to the decrease of guard bands. Therefore, the research and development of optical devices, which have the function of optical Fourier transformation and can demultiplex the various capacity optical OFDM signals directly in the optical domain, are important to realize all-optical adaptive networks. Several tunable optical OFDM demultiplexers using the optical Fourier transformation were reported, which utilized a periodically poled lithium niobate (PPLN) waveguide [10], a polarization-maintaining fiber loop mirror [11], an integrated-optic filter [12,13], and a time-lens method [14].
In this paper, we report on channel demultiplexing of various channel symbol rate and channel number optical OFDM signals (5 × 10 to 20 Gsymbol/s and 3 × 20 Gsymbol/s) by utilizing a gate-free integrated-optic tunable OFDM filter, whose key component for optical discrete Fourier transformation (DFT) is a slab star coupler [15–17]. The filter comprises tunable couplers, an array of fixed delay lines, and a slab star coupler-type optical DFT circuit. We can tune the characteristics of the OFDM filter by selecting the delay lines with the tunable couplers [12,13]. A multi-mode interference (MMI) coupler-type DFT circuit was also proposed and investigated, and its footprint is smaller than the star coupler-type optical DFT circuit [18–20]. As the optical communication with large-scale channels is needed in the prospective photonic networks, we selected the star coupler-type optical DFT circuit. The star coupler is superior to the MMI coupler as regards to the channel scalability [18–21], and a 144 × 144 integrated-optic star coupler was realized [21]. We remove optical gates that we before employed to extract the effective time of the optical OFDM filter output signals [8,13,17], and alternatively utilize a high-speed photodetector (PD) to preserve the effective time in the electrical domain. We do not intend to improve the demultiplexing performance and increase the total throughput of the optical OFDM filter by removing the gates because we recognize that demultiplexing performance of the gate-free filter is inferior to the filter with the gates. But, as the gate-free filter has the merits of completely passive demultiplexing operation, simplified configuration, and consumed power and cost reduction, it is important to investigate its characteristics in detail and show its applicability to optical communication systems. We already reported an optical filter consisting of the star coupler-type DFT circuit for various channel symbol rate optical OFDM signals and demonstrated its preliminary wavelength characteristics [12]. We also have made a report on channel demultiplexing of various channel number optical OFDM signals using the optical filter with optical gates [13]. In this investigation, we demonstrate first demultiplexing experiments of various channel symbol rate optical OFDM signals with our gate-free star coupler-based tunable filter. The gate-free operation was already reported as for an integrated-optic OFDM filter, which adopted the MMI coupler-type optical DFT circuit [18]. In [18], although demultiplexing of a fixed capacity (7 × 5 Gbit/s) on-off keying (OOK)-based OFDM signal was demonstrated, specific performance comparison between with and without the optical time gating was not demonstrated experimentally or by calculation. In this investigation, we demonstrate that our filter can demultiplex the various capacity OFDM signals without optical gates, whose channel symbol rate is on the order of ten Gsymbol/s. In addition, we experimentally show the characteristics difference between demultiplexed channels with and without the gates.
We start by explaining the configuration, operating principle, and calculated characteristics of our tunable optical OFDM filter. We then describe the experimental set-up and some experimental results relating to the optical OFDM signal demultiplexing. 5 × 10 to 20 Gsymbol/s and 3 × 20 Gsymbol/s optical OFDM signals were successfully demultiplexed with our gate-free filter. The measured lowest bit error rates (BERs) were on the order of equal to or less than 10−10 for all the demultiplexed channels. We show the potential of the filter use for the adaptive photonic networks and completely passive demultiplexing operation of the filter.
2. Configuration, operating principle, and calculated characteristics of tunable optical OFDM filter without optical gates
Figure 1 shows an integrated-optic tunable filter without optical gates for demultiplexing optical OFDM sub-carrier channels with various channel symbol rate and channel number. The filter was fabricated with silica waveguide technology with the relative index difference Δ of 1.2%. The demultiplexer is composed of nine symmetrical Mach-Zehnder interferometer (MZI)-type tunable couplers with thermo-optic (TO) phase shifters, ten fixed delay lines with TO phase shifters, and a 10 × 10 slab star coupler. The radius and waveguide pitch of the star coupler are 1.5 mm and 12.6 µm, respectively. The filter footprint is 21 mm x 44 mm. The combination of the tunable couplers, the delay lines, and star coupler enables us to realize optical DFT by the signal split, signal delay, and lightwave diffraction and interference, respectively [12,13]. The length difference ΔL between the original neighboring delay lines is designed to be 2.07 mm, and the filter free spectral range (FSR) becomes 100 GHz when all the delay lines are used or equivalently active.
Fig. 1. Configuration of gate-free integrated-optic tunable filter for demultiplexing sub-carrier channels of various channel symbol rate and channel number optical OFDM signals.
When all the delay lines are active, the input optical OFDM signal with each channel symbol period T of 100 ps is split into ten signals that are relatively delayed by one-tenth the period of T (Δt) to obtain the signals in the same time slot for the following processing at the star coupler. This operation corresponds to the serial-to-parallel (S/P) conversion of the input OFDM signal. The obtained parallel signals receive the phase shifts, which are requisite for the optical DFT, through the phase shifts on the delay lines and the diffraction at the star coupler. The TO phase shifters on the delay lines are also used to tune the center frequency of the filter. Each sub-carrier channel is demultiplexed at each filter output port after the diffraction and interference of the signals at the star-coupler. The filter thus demultiplexes the OFDM signal [8,17]. Each demultiplexed signal at the filter output is effective during only the effective time Δt, where the same symbols overlap, because the orthogonality between the OFDM sub-carrier channels is maintained within just the one-symbol period T. In this investigation, we do not use the optical gates, which we adopted to extract the effective time of the filter outputs in the past investigation [8,13,17], with a view to simplifying the device configuration and reducing the cost and power consumption. Instead we employ a high-speed PD for 40 Gbit/s signal detection, which has ability to maintain the effective time in the electrical domain, and sampling and discriminating the signal properly. The effective time is equal to each channel signal time slot divided by the channel number. Therefore, the effective time is much shorter than the channel signal time slot, and the high-speed PD is necessary for maintaining the effective time, namely not broadening the effective time region.
The number of used delay lines corresponds to the upper limit number of processable OFDM sub-carrier channels N, and the length difference ΔLu between the used delay lines determines the FSR ΔfFSR of the tunable filter. Therefore, N and/or the channel spacing of the filter Δf can adaptively be changed by selecting the delay lines, where the lightwaves pass, with the tunable couplers [12,13]. The product of N and Δf is expressed as the following equation when the delay lines are selected every J.
where v is a light speed in the waveguide. Figures 2(a), 2(b), 2(c), and 2(d) indicate the calculated relation between Δf and N when ΔL values are set at 2.07 mm, 1.04 mm, 0.52 mm, and 0.26 mm, respectively. The value of J is utilized as a calculation parameter. The number of the fixed delay lines assumed to be fifty, and we postulate the use of the silica waveguide at 1.55 µm. Figure 2 is useful for designing the parameters of the star coupler-based tunable optical OFDM filter. The center frequency of each filter channel can flexibly be tuned within the FSR by changing the TO phase shifts for the delay lines within 2π. We can set each channel at an arbitrary center frequency by combining the tuning characteristics with the FSR-based periodicity. The accuracy and response time of the TO phase shift are better than 0.01π and of the millisecond order, respectively [22]. Therefore, we can set the center frequency of each channel with the accuracy of better than GHz.Fig. 2. Calculated relation between filter channel spacing Δf and upper limit number of processable OFDM sub-carrier channels N when ΔL values are set at (a) 2.07 mm, (b) 1.04 mm, (c) 0.52 mm, and (d) 0.26 mm.
3. Experimental set-up
Figure 3 shows an experimental set-up we utilized for measuring and evaluating the characteristics of the gate-free integrated-optic tunable OFDM filter. We generated 10 or 20 GHz-spaced optical frequency combs by using a LiNbO3 (LN)-type phase modulator (optical bandwidth: 32.5 GHz, half-wavelength voltage Vπ: 4.1 V) and extracted the requisite number of flat combs with a tunable rectangular-shaped filter consisting of a bulk-optic grating. We modulated interleaved even and odd channels with different LN intensity data modulators to decorrelate the adjacent channels. The interleave filter was composed of a bulk-optic tunable asymmetrical MZI. The modulators were driven with two sequences of non-return-to-zero 10 or 20 Gbit/s OOK data [pseudo-random bit sequence (PRBS): 27-1], which were generated from two synchronized pulse pattern generators. We adopted this PRBS length because our investigation was at the preliminary stage. We utilized the data modulators for 40 Gbit/s signal generation to produce the near-rectangular optical pulses suitable for the optical OFDM communication. The optical bandwidth and half-wavelength voltage Vπ at 40 Gbit/s relating to the data modulator 1 or 2 were 30.3 GHz and 2.9 V or 36.9 GHz and 2.6 V, respectively. The two sets of modulated lights were combined after adjusting their symbol timing for generating 5 × 10 to 20 Gbit/s and 3 × 20 Gbit/s optical OFDM signals. The generated optical OFDM signal was introduced into the gate-free tunable optical OFDM filter, and the BERs of all the demultiplexed sub-carrier channels were evaluated at an error detector. We employed a high-speed PD with the widest bandwidth (26.5 GHz) that we had, which was usually utilized for 40 Gbit/s signal detection. The bandwidths of optical and electrical sampling oscilloscopes used for the signal measurement were 65 GHz and 40 GHz, respectively.
Fig. 3. Experimental set-up for measuring and evaluating characteristics of gate-free integrated-optic tunable OFDM filter.
4. Experimental results
We show experimental results, which we obtained by using the set-up shown in Fig. 3, in Figs. 4–7.
Fig. 4. Measured transmittance of gate-free tunable optical filter for demultiplexing (a) 10 × 10 Gbit/s and (b) 5 × 20 Gbit/s optical OFDM signals.
Figures 4(a) and 4(b) indicate measured transmittance of the gate-free tunable optical filter for demultiplexing 10 × 10 Gbit/s and 5 × 20 Gbit/s optical OFDM signals, respectively. The indicated channel number corresponds to the filter output channel in Fig. 1. The measured fiber-to-fiber losses of the filter were 5.0 dB and 7.6 dB in Figs. 4(a) and 4(b), respectively. The filter loss was optimized to indicate the minimum value in the 10 × 10 Gbit/s operation mode. The used delay lines for obtaining the characteristics of Figs. 4(a) and 4(b) were all the ten delay lines and D3 to D7 in Fig. 1, respectively. We targeted five channel demultiplexing when each channel bit rate was 10 Gbit/s. However, we set the filter characteristics so that the maximum processable channel number was ten. The effective time of the filter output signals, namely demultiplexed channel signals is 10 ps or 20 ps when the maximum processable channel number is ten or five in the 10 Gbit/s channel operation mode, respectively. We intentionally adopted shorter effective time, which corresponded to a more severe demultiplexing condition, with a view to indicating the merits and usefulness of our gate-free tunable optical filter clearly. We used the characteristics in Fig. 4(b) when both 5 × 20 Gbit/s and 3 × 20 Gbit/s OFDM signals were demultiplexed because, under the filter parameters in Fig. 1, we could not set N at three in the 20 Gbit/s channel operation mode.
Fig. 5. Measured BERs of all demultiplexed sub-carrier channels when optical OFDM signals were (a) 5 × 10 Gbit/s, (b) 5 × 20 Gbit/s, and (c) 3 × 20 Gbi/s.
Figures 5(a), 5(b), and 5(c) show measured BERs of all the demultiplexed sub-carrier channels when the OFDM signals were 5 × 10 Gbit/s, 5 × 20 Gbit/s, and 3 × 20 Gbit/s, respectively. The BERs without notes indicate the characteristics in the gate-free experiments using the high-speed PD for the 40 Gbit/s signal. In Fig. 5(a), with a view to clarifying properties and merits of our tunable OFDM filter, we showed two measured BERs relating to CH7 when we used a PD for a 10 Gbit/s signal (bandwidth: 10.0 GHz) as substitute for the 40 Gbit/s PD in the gate-free experiment and we added the time gating procedure for extracting the effective time of the filtered signal. In the latter measurement, we used an LN intensity modulator-based optical gate (optical bandwidth: 26.5 GHz, Vπ: 3.1 V), which was driven with 10 GHz electrical short pulses from a comb generator [13]. In addition, the PD for the 10 Gbit/s signal was employed. As shown in Fig. 5, we obtained BERs in the order of equal to or less than 10−10 for all the demultiplexed channels. We thus achieved first demultiplexing of the various channel symbol rate and channel number optical OFDM signals by using our gate-free integrated-optic filter and verified its operation. In all the figures of Fig. 5, the middle CH9 showed the worst characteristics when the gate-free experiments using the 40 Gbit/s PD were carried out, because the CH9 sustained the largest crosstalk from other channels.
Figure 6 shows measured spectra of the optical OFDM signals and their filtered channels [(a) 5 × 10 Gbit/s, (b) 5 × 20 Gbit/s, and (c) 3 × 20 Gbit/s OFDM signals]. Some demultiplexed channels are showcased [(a) CH0, (b) CH7, and (c) CH7]. Figures 7(a), 7(b), and 7(c) show measured eye diagrams of the original and filtered 5 × 10 Gbit/s, 5 × 20 Gbit/s, and 3 × 20 Gbit/s OFDM signals, respectively. The eyes of the OFDM signals indicated typical noise-like waveforms. Figures 7(a) and 7(b) also contain eye diagrams of the filtered optical signals in the gate-free experiments. The eye diagram of each filtered channel was measured when the lowest BER was obtained. Figure 7(a) also includes electrical eye diagrams when the 10 Gbit/s PD was used in the gate-free experiment and the timing gating was adopted. In Fig. 7, the filtered eye diagrams demonstrate that the effective time was maintained not simply in the optical domain but in the electrical domain. In the gate-free experiments using the 40 Gbit/s PD, the power penalties compared to the best-case BER characteristics [CHs7, 3, and 7 in Figs. 5(a), 5(b), and 5(c), respectively] were 0.03 dB to 0.9 dB, 0.3 dB to 1.1 dB, and 0.3 dB and 0.6 dB at BERs of 10−9 in Figs. 5(a), 5(b), and 5(c), respectively. In our investigation on the 5 × 10 Gbit/s OFDM signal, the demultiplexed CH7 characteristics without the gate were again evaluated after replacing the 40 Gbit/s PD with the 10 Gbit/s PD. When we investigated the 5 × 10 Gbit/s OFDM signal, we evaluated the characteristics with the LN modulator-based optical gate as for CH7. The gate was installed in the CH7 output of the filter in Fig. 1 through the optical fiber. In Fig. 5(a), the demultiplexed CH7 characteristics in the gate-free experiment using the 40 Gbit/s PD showed 3.1 dB power penalty compared to the characteristics with the gate at the BER of 10−9. In Fig. 7(a), the CH7 eye diagram detected by the 10 Gbit/s PD became deteriorated compared to the CH7 eye detected by the 40 Gbit/s PD as for the gate-free experiments. In Fig. 5(a), the power penalty of the CH7 characteristics detected by the 10 Gbit/s PD was 1.8 dB compared to the CH7 characteristics detected by the 40 Gbit/s PD at the BER of 10−9. These obtained results conclude that our gate-free optical filter could function as the OFDM signal demultiplexer and the filter characteristics depended on the bandwidth of the receiver PD we used. Although the characteristics of the gate-free optical OFDM filter were inferior to the filter with the gates, the gate-free filter has the benefit of completely passive demultiplexing operation, simplified configuration, and consumed power and cost reduction.
Fig. 6. Measured spectra of optical OFDM signals and their filtered channels (a) 5 × 10 Gbit/s, (b) 5 × 20 Gbit/s, and (c) 3 × 20 Gbi/s OFDM signals.
Fig. 7. Measured eye diagrams of original and filtered (a) 5 × 10 Gbit/s, (b) 5 × 20 Gbit/s, and (c) 3 × 20 Gbit/s OFDM signals.
5. Conclusion
We reported on a gate-free integrated-optic tunable filter for demultiplexing various capacity optical OFDM signals, which was composed of tunable couplers, an array of fixed delay lines, and a star coupler-type optical DFT circuit. The number and/or the symbol rate of demultiplexed sub-carrier channels can be tuned by selecting the delay lines with the tunable couplers. In this investigation, we replaced an optical gate, which was normally attached to the filter for extracting the effective time of the filtered output signals, with a high-speed photodetector with a view to maintaining the effective time in the electrical domain and realizing completely passive demultiplexing of various capacity optical OFDM signals. We could show the first channel demultiplexing experiments of various channel symbol-rate and channel number OFDM signals (5 × 10 to 20 Gbit/s and 3 × 20 Gbit/s) with our gate-free integrated-optic tunable filter, and we thus showed its effectiveness. The obtained bit error rates of all the demultiplexed channels with our filter were on the order of equal to or less than 10−10. In the 10 Gbit/s channel-based experiments, we clarified the channel demultiplexing characteristics of our gate-free filter employed in conjunction with a 40 Gbit/s photodetector by comparing them with the characteristics of the gate-free filter used in conjunction with a 10 Gbit/s photodetector and the characteristics using an optical gate. The demonstrated tunable OFDM filter has the benefit of the completely passive demultiplexing operation, simplified configuration, and consumed power and cost reduction, and we think that it has possibilities to contribute to the realization of the prospective adaptive photonic networks.
Funding
Japan Society for the Promotion of Science (19H02144).
Disclosures
The authors declare no conflicts of interest.
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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