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The 2-μm optical band has gained much attention recently due to its potential applications in optical fiber communication systems. One constraint in this wavelength region is that the electrical bandwidth of components like modulators and photodetectors is limited by the immature manufacturing technologies. Here we experimentally demonstrated the high-speed signal generation and transmission under bandwidth-constrained scenario at 2-μm. It is enabled by the direct-detection optical filter bank multicarrier (FBMC) modulation technique with constant amplitude zero autocorrelation (CAZAC) equalization. We achieved a single wavelength 80 Gbit/s data rate using the 16-QAM FBMC modulation format which is the highest single channel bit rate at 2-μm according to our best knowledge. The signal is transmitted through a 100m-long solid-core fiber designed for single-mode transmission at 2-μm. The measured bit error rates of the signals are below the forward error correction limit of 3.8 × 10−3, and the 100m-fiber transmission brings negligible penalty.
© 2017 Optical Society of America
1. Introduction
Being an attractive wavelength range for biomedical and sensing applications, the two-micron spectral band has been investigated recently for the optical fiber communications [1]. As a potential supplement of 1.55 μm, the optical fiber communication in this waveband is expected to thrive due to the availability of the two micron lasers, modulators, photodetectors and low-loss optical fibers [2]. A hollow core photonic bandgap fiber (HC-PBGF) designed for 2 μm has been predicted to have 0.1 dB/km attenuation in principle and measured to have 2 dB/km loss with the state-of-art fabrication technique [3]. Besides, the Thulium-doped fiber amplifier (TDFA) is a plus that can extend the reach significantly since it covers nearly 400 nm optical bandwidth. The 2-μm transmitter can be implemented by direct modulation in a semiconductor laser [4, 5]. The Lithium Niobate modulator [6] and integrated InP modulator [7] have been used for external modulation as well. Moreover, quite a few research works on modulation at the long wavelength have been demonstrated in silicon [8], Aluminum Nitride [9], graphene [10] and etc. On the other hand, the high-speed detection at this band is achievable in ion-implanted silicon [11] and GeSn [12].
Although the 2-μm communication system can now be implemented by using the components mentioned above, its transmission performance is largely constrained by the less-mature 2-μm components. Many efforts devoted to improving the device performance are ongoing, and the data capacity can be increased by various techniques in the system level that have been developed in C-band [13–16]. The reported first transmission experiment at two micron was using 8 Gbit/s non-return-to-zero (NRZ) format with a transmission length of 300 meters [1]. Wavelength division multiplexed channels have been employed to achieve a total bit rate of 8 × 20 Gbit/s [17]. Single-λ modulations using the advanced modulation format such as discrete multi-tone (DMT) [5], pulse amplitude modulation (PAM), carrier-less amplitude and phase (CAP) modulation [6] were investigated. While the single-channel bit rate of 1.55 μm signal can reach to >100 Gbit/s easily [18, 19], it has not exceeded 52.6 Gbit/s for 2-μm waveband signal due to the limited electrical bandwidth of the components. On the other hand, most of the works mentioned above and most of the other related works [20, 21] employ the HC-PBGF. This type of fiber has low loss and dispersion, but the fabrication and splicing are complicated. In this work, we experimentally demonstrated an 80 Gbit/s single channel data rate at 1952 nm using components with <20 GHz bandwidth. The direct-detection optical filter bank multicarrier (FBMC) modulation [22] combined with the constant amplitude zero autocorrelation (CAZAC) equalization are employed to overcome the electrical bandwidth limitation. In such bandwidth constrained condition, high frequency subcarriers experience a fast SNR degradation which can be compensated by the CAZAC equalization. We showed a signal generation and transmission through a 100-m solid-core single mode glass fiber designed for 1950 nm which is easier to fabricate and splice compared with the HC-PBGF. The optical wave is encoded by 16-QAM and 32-QAM with 80 Gbit/s and 70 Gbit/s data rate, respectively. The results indicate a significant improvement under the bandwidth-constrained scenario.
2. Experimental setup
The schematic diagram of the experimental setup and the block diagram of the digital signal processing (DSP) are depicted in Fig. 1. A narrow linewidth semiconductor laser, emitting a continuous wave (CW) with the wavelength of 1952.52 nm, is used at the transmitter. The output power of the laser can reach to >20dBm. Thus no amplifier is needed for short reach applications. A good signal quality is expected at the receiver since no additional amplified spontaneous emission (ASE) from the external boost-TDFA is induced. The polarization of the CW light is aligned by a home-made polarization controller. To reduce the loss, all the jumped fibers and the pigtail fibers in the setup are single mode fibers designed for 2-μm. Then a Lithium Niobate Mach-Zehnder modulator (MZM) designed for 2μm is used for modulation. The modulator is in push-pull configuration with an extinction ratio of 20 dB and an insertion loss of ~3.5 dB. The nominal electrical bandwidth of the MZM is 18 GHz. The electrical signal is generated by an arbitrary waveform generator (AWG: Keysight M8195A) with 25-GHz bandwidth working at 40 GSa/s. An electrical amplifier is used to drive the MZM. The modulator is biased at the quadrature point for small signal modulation. The optical signal is coupled out from the modulator through a polarization maintaining fiber and then is sent into a 100-m solid core single mode fiber (SCSMF) designed for 1950 nm. The fiber has a core diameter of 7 μm with a cladding diameter of 125 μm. The numerical aperture of the fiber core is 0.2 and the mode field diameter of the fiber is ~8 μm at 1950 nm. The dispersion parameter is ~36 ps/km/nm at 1952 nm. The attenuation of this fiber is 10 dB/km while the measured link loss of the 100-m fiber is ~1.5 dB including the connection loss. Though the SCSMF has higher loss than the HC-PBGF, it is easier to fabricate and splice. As a result, it should be more suitable for the short reach application which is highly cost-sensitive. The signal after transmission is received by a photodetector with a responsivity of ~1 A/W and a bandwidth of ~22 GHz at 2 micron wavelength regime. The electrical signal is sent to a real-time oscilloscope (Lecroy LabMaster MCM-Zi-A) without amplification. Offline signal processing is then applied to calculate the bit error rate (BER).
Fig. 1 Block diagram of the experimental setup and digital signal processing. MOD: modulator, EA: electrical amplifier, SMF: single mode fiber, PD: photodiode.
As shown in Fig. 1, the transmitter DSP includes the serial-to-parallel (S/P) conversion, symbol encoding with data formats of 16-QAM and 32-QAM, CAZAC equalization, inverse fast Fourier transform (IFFT), poly phase network (PPN) filtering, parallel-to-serial (P/S) conversion and digital-to-analog conversion (DAC), respectively. The received signal is first converted from analog-to-digital domain and the AD conversion is followed by the synchronization. Then, serial-to-parallel (S/P) conversion is completed before the PPN filtering. The channel estimation is also included after the fast Fourier transform (FFT). The inverse CAZAC transform is used for equalization and the final step is symbol decoding. It should be noted that the Hermitian symmetry is applied to the PPN-FFT process in order to obtain the real valued staggered multi-tone (SMT) symbols for real transmission. In the experiment, the transmitted signal is generated from a 215-1 pseudorandom binary sequence (PRBS). The signal is modulated into a SMT frame with the length of 139 and 11 of which are used for synchronization and channel estimation. The total subcarrier number is chosen to be 128. Due to the Hermitian symmetry, the real data is carried on 63 of the subcarriers while the first subcarrier is abandoned to eliminate noise near the direct current component. As introduced above, the 2-μm components do not have enough electrical bandwidth. To achieve the high data rate under the bandwidth-constrained condition, the CAZAC precoding is introduced here to improve the transmission performance [23].
3. Results and discussions
3.1 16-QAM
The 16-QAM format has been considered as a promising candidate for metro and even local area networks due to its high spectral efficiency and lower signal to noise ratio (SNR) requirement, compared with other higher level modulation formats. We first encode the data with 16-QAM. The baud rate is set to be 40 Gbaud which results in a total bit rate of 80 Gbit/s. However, according to the measured electrical spectrum shown in Fig. 2(a), a flutter fading is observed when the signal bandwidth exceeds 15 GHz. This is because the overall frequency response of the 2-μm system is not good enough due to the limited bandwidth of the components. To overcome this problem, the CAZAC precoding is used for equalization. We calculate the SNR of each subcarrier from the demodulated signal and the results are shown in Fig. 2(b). It can be seen that the SNR curve is significantly flattened by the CAZAC equalization. To further confirm the performance improvement and the capability of data transmission at 2-μm using 40 Gbaud 16-QAM, we measured the BER of the back-to-back (B2B) signal and the signal after 100-m SCSMF transmission. The BER curves are shown in Fig. 3(a). The BERs of the B2B and 100-m SCSMF transmission cases are measured to be below the FEC limit of 3.8 × 10−3 with 7% overhead. However, the required receiver power is close to 2.4 mW which is a bit higher for a direct detection scheme. This is because no boost amplifier is used in the receiver and we believe the sensitivity could be significantly enhanced by using a transimpedance amplifier (TIA) after the photodetection. When increasing the received power further, the error floor is observed and the performance does not improve further. The constellation diagrams of the 16-QAM signals are also measured and are shown in Figs. 3(b) and 3(c). From both the BERs and constellations, there is no much difference between the B2B and 100m-SCSMF. This is because the dispersion and attenuation of the 100m fiber are negligible.
Fig. 2 (a) The normalized spectrum of the received electrical signal for the b2b 16-QAM. (b) The measured SNR of each subcarrier for the b2b 16-QAM signal.
Fig. 3 (a) Measured BER curves for b2b and 100m SCSMF transmission of 16-QAM-FBMC signals. The measured constellation diagram for (b) the b2b and (c) the 100m SCSMF transmission case.
3.2 32-QAM
Since the 16-QAM results have shown an obvious bandwidth constraint, an alternative is to generate the high speed signal with lower bandwidth but higher level modulation format. Higher spectral efficiency can be achieved with 32-QAM which encodes the data with 5 bits per symbol. Thus we also investigate the transmission using 32-QAM and the baud rate is set to be 28 Gbaud which results in a total bit rate of 70 Gbit/s. From the measured electrical spectrum shown in Fig. 4(a), the bandwidth constraints have been greatly relaxed compared with the 16-QAM format. There is only a slight fading at those frequencies near 14 GHz. The SNR of the signal without precoding is calculated and plotted as a blue line in Fig. 4(b). The CAZAC equalizer is used here as well. The SNR performance after the equalization is depicted by the red line in Fig. 4(b). The measured BERs of the B2B and 100m-SCSMF transmissions are shown by the curves in Fig. 5(a). The BERs are below the FEC limit with a received power of ~2.6 mW for the B2B and 100m-SCSMF case. This power level is a bit higher than the 16-QAM signals which is due to the higher SNR requirement of the 32-QAM signal. No significant degradation of the 100m-SCSMF transmission is observed. The constellation diagrams of the 70 Gbit/s 32-QAM signals are shown in Fig. 5(b) and no distortion is present.
Fig. 4 (a) The normalized spectrum of the received electrical signal for the b2b 32-QAM. (b) The measured SNR of each subcarrier for the b2b 32-QAM signal.
Fig. 5 (a) Measured BER curves for b2b and 100m SCSMF transmission of 32-QAM-FBMC signals. The measured constellation diagram for (b) the b2b and (c) the 100m SCSMF transmission case.
4. Conclusion
In summary, we have demonstrated a 100-m SCSMF fiber transmission link at 2-μm waveband using a direct-detection FBMC scheme, assisted by the CAZAC precoding equalization. This technique improves the system performance under the bandwidth-constrained condition due to the limited electrical bandwidth at this wavelength region. We have relied on the <20 GHz components to achieve up to 80 Gbit/s data rate using 16-QAM and 70 Gbit/s using 32-QAM. This is so far, the highest single channel bit rate achieved in 2-μm. The BERs are measured to be well below the FEC limit by using a photodiode without TIA, and the 100-m SCSMF has induced negligible penalty. The results reveal the possibility of high data rate for single channel and the potential for optical interconnects with large capacity, operating at the 2-μm wavelength region.
Funding
This work is supported by National Natural Science Foundation of China (NSFC) (61505039, 61331010); the 863 High Technology plan Grants 2015AA016904, Shenzhen Municipal Science and Technology Plan Project (JCYJ20150403161923530), and the Program for New Century Excellent Talents in University (NCET-13-0235).
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