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Abstract
100 Gb/s NRZ-OOK transmission over 14 km standard single mode fiber in the C-band is demonstrated with a simple intensity modulation and direct detection scheme. The transmission concept utilizes single sideband modulation and comprises a single differential digital-to-analog converter with adjustable phase offset, a new dual electrode plasmonic Mach-Zehnder modulator, a laser at 1537.5 nm, standard single mode fibers, a photodiode, an analog-to-digital converter, and linear offline digital signal processing. The presented SSB concept requires no DSP and complex signaling at the transmitter. The demonstrated SSB transmitter increased the possible transmission distance by a factor of 4.6 compared to a DSB transmitter. We also investigated the equalization requirements. A T/2-spaced feedforward equalizer requires 27 taps to achieve transmission over 10 km with a BER below the HD-FEC limit. In comparison to a DSB transmitter, the SSB transmitter reduced the receiver DSP complexity by a factor of 13.7.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Short reach optical communication systems rely on low-cost and high-bandwidth electro-optic (EO) modulators. To keep cost in such systems minimal, optical transceivers with intensity modulation (IM) and direct detection (DD) are required and should utilize a single laser, a single modulator, a single electrical driver, and a single photodiode per channel. One cost- and power-efficient way to increase bandwidth per channel is to increase the symbol rate rather than to multiplex the signal onto several wavelengths [1]. Such symbol rates are predicted to exceed 120 GBd in the next decade [2]. Non return to zero with on-off keying (NRZ-OOK) is the preferable modulation format when digital signal processing (DSP) complexity needs to be reduced [1]. Yet, this requires components that have large bandwidths beyond 70 GHz. Several EO modulator technologies have been demonstrated with symbol rates of at least 100 GBd in IM/DD systems. It includes Mach-Zehnder modulators (MZM) realized in different technologies like lithium niobate [3,4], indium phosphide [5,6], polymer photonics [7,8], plasmonics [9,10] or silicon-organic hybrid [11].
However, such IM/DD systems in the C-band with symbol rates of at least 100 GBd have been limited to transmission distances of up to 5.5 km through uncompensated standard single mode fiber [12]. This limitation originates from the double sideband (DSB) modulation, which leads to chromatic dispersion induced power fading after the photodiode’s square law detection [13].
It has been shown that single sideband (SSB) modulation [14] and vestigial sideband (VSB) modulation [15–17] are techniques which can mitigate the effects of chromatic dispersion.
Both SSB and VSB modulation formats have been used for several transmission experiments of up to 80 km SMF, with symbol rates up to 90 GBd, and necessary advanced DSP techniques [18–24].
In this paper, we demonstrate for the first time the transmission of a 100 GBd signal in a dispersive SMF over 14 km in a simple IM/DD system. We keep the amount of optical and electrical components as minimal as possible and reduce the DSP complexity to achieve maximal distance under the effect of chromatic dispersion with minimum effort. The experiments are performed with a SSB transmitter, which comprises a high bandwidth plasmonic MZM with a new dual-electrode design and a differential drive signal with a phase offset. Data transmission of a 100 Gb/s NRZ-OOK signal is demonstrated over up to 14 km standard SMF in the C-Band.
2. Plasmonic MZM with dual electrodes
The single sideband transmitter includes a plasmonic-organic hybrid (POH) Mach-Zehnder modulator (MZM) with a dual-electrode design [24]. Plasmonic MZMs offer an electro-optic bandwidth of up to 500 GHz [25], operation with low voltage levels [26,27] and modulation at the micrometer scale [28,29]. The plasmonic MZM is depicted in Fig. 1 and consists of a silicon photonic (SiPh) Mach-Zehnder interferometer with two incorporated POH phase modulators [30]. The POH phase modulators are slot waveguides which are filled with an organic EO material composite (75%wt HD-BB-OH / 25%wt YLD124) [31]. The length of the active region is 15 µm and has a slot width of 100 nm. Each of the individual POH phase modulators is contacted electrically with ground (G) and signal (S) electrode contact pads. Light is coupled to and from the chip via SiPh grating couplers (GC) [32]. On the chip, the light is split and combined by SiP multimode interference couplers (MMIs). The device is fabricated as described in [29] with an additional SiO2 cladding to allow for metal waveguide crossing.
Fig. 1. Colorized microscope picture of the dual-electrode plasmonic Mach-Zehnder modulator (P-MZM). It comprises MZ interferometer with silicon photonic (SiP) waveguides (WGs) and SiP multimode interference (MMI) couplers and two plasmonic phase modulators. Light is coupled to and from the chip via SiP grating couplers (GC). The electrical signal is contacted via two ground (G) signal (S) contact pads.
3. Single sideband transmitter
Chromatic dispersion is the main reach limitation in IM/DD systems especially at large symbol rates. In standard single mode fibers, chromatic dispersion leads to a phase delay between different frequency components. At a direct detection receiver, the square law detection of the photo diode results in a cancellation of the beat signal between upper sideband and carrier and the beat signal between lower sideband and carrier. The cancellation originates from the phase difference between upper and lower sideband [33]. It results in frequency dependent power fading [13]
Fig. 2. Normalized power spectrum. The spectral response of a double sideband (DSB) signal after direct detection and 14 km transmission is shown in black. Chromatic dispersion leads to power fading. The plot in red shows the single side band (SSB) spectrum of an ideal 100 Gb/s non-return to zero (NRZ).
Utilizing single sideband modulation allows for the mitigation of the chromatic dispersion induced power fading. SSB modulation gives the advantage of reduced optical bandwidth compared to DSB and avoids the effect of cancellation between upper and lower sideband after square law detection. SSB can be simply generated by optical filtering, which requires a standard intensity modulator and an optical filter. This technique is also known as vestigial sideband modulation (VSB) [15]. A more complex implementation of the SSB transmitter is realized by the Hilbert transform, which is performed electrically and the complex analytic signal is applied on an optical IQ-modulator [18,34]. Another technique utilizes an intensity modulator followed by a phase modulator where a real-valued signal is applied to the intensity modulator and its Hilbert transform on the phase modulator [14,35]. The simplest implementation can be realized with help of a single dual electrode MZM, by applying the same signal with a $\pi /2$ phase shift on both electrodes [13]. The proposed plasmonic SSB transmitter is based on the same technique. However, a broadband $\pi /2$ phase shift would require a complex analog filter design [14]. In the case of the proposed plasmonic SSB transmitter, we utilized instead of a broadband phase shift a simple relative electrical path difference to approximate the phase shift and to keep the transmitter’s complexity at the simplest. This leads to a trade-off between transmitter complexity and ideal SSB modulation.
4. Experiments
4.1 Experimental setup
Figure 3 depicts the experimental setup to evaluate the plasmonic single sideband (SSB) transmitter in an optical IM/DD transmission scenario. At the IM transmitter, a single digital-to-analog converter (DAC, Micram, 100 GSa/s) generates the differential electrical signal. We did not apply any digital signal processing (DSP) steps beside the pattern generation of a random bit sequence. One of the differential outputs can be delayed with help of a mechanical phase shifter. The differential electrical signal is amplified and applied via microwave probes onto the chip-based POH MZM. The mean peak-voltage at the maximum eye opening is around 1.6 V which coincides with the root means square value of the signal voltage. The first inset in Fig. 3 shows the eye diagram of the electrical signal, which is digitally interpolated after being digitized with a real-time oscilloscope. The dual electrode plasmonic MZM is electrically contacted with independent $\bar{\textrm{G}}\textrm{S}$ and $\bar{\textrm{S}}\textrm{G}$ electrodes. The phase shift between the two electrodes is approximated with a mechanical phase shifter and is adjusted by optimizing the BER. Besides, the plasmonic MZM is imbalanced and its operation point can therefore be adapted by the optical wavelength. The optical carrier is generated with a tunable laser source (TLS) at around 1537.5 nm and with a power of 9 dBm. Before the transmission, an erbium doped fiber amplifier (EDFA) amplifies the optical signal to 8 dBm to compensate for the insertion loss of the modulator. To make such a transmission system applicable the usage of an EDFA should be avoided. This can be achieved for example by utilizing a larger laser power, improving the insertion loss of the plasmonic modulator, and utilizing an avalanche photodiode combined with a trans-impedance amplifier at the receiver. The optical transmission line comprises up to 14 km of standard single mode fiber (SMF) with dispersion coefficient of smaller than 18 ps/nm/km and attenuation smaller than 0.2 dB/km. At the DD receiver, the power of the optical signal can be adjusted by a variable optical attenuator (VOA). To achieve optimal BER performances, the received optical powers are adapted to values between 2 dBm and 5 dBm. A photodiode (PD) with a 70 GHz 3 dB bandwidth detects the optical signal and is directly attached to a real-time oscilloscope (RTO). The RTO samples the signal with 160 GSa/s and an electrical 3 dB bandwidth of 63 GHz. The digitized waveform is analyzed by offline DSP. The offline DSP comprises signal normalization, timing recovery, T/2-spaced feed forward equalization, symbol decision, and bit-to-error ratio (BER) calculation. The FFE is trained with a data-aided least mean square (LMS) update over the first 10% of the received symbols and is afterwards applied statically.
Fig. 3. Experimental setup. The single side band intensity modulator comprises a dual-electrode plasmonic-organic hybrid (POH) Mach-Zehnder modulator (MZM), tunable laser source (TLS), single digital-to-analog converter (DAC) with differential output (p,n), a mechanical phase shifter (PS), and RF amplifiers (RF-Amp). An erbium doped fiber amplifier (EDFA) amplifies the optical signal. The transmission line exists of standard single mode fiber (SMF) of up to 14 km. The direct detection receiver comprises a variable optical attenuator (VOA), a photodiode (PD), and a real-time oscilloscope (RTO). The offline digital signal processing (DSP) utilizes normalization, timing recovery, T/2-spaced feed forward equalization (FFE), symbol decision, and bit error ratio (BER).
4.2 Experimental results
Figure 4(a) shows two optical spectra of a 100 Gbit/s NRZ-OOK signal with double sideband modulation (DSB, black) and a single sideband modulation (SSB, red) measured as normalized optical power versus wavelength. The optical carrier is located at 1537.5 nm. One can also see the clock tones spaced apart by 200 GHz. The SSB modulation has a sideband suppression ratio of around 10 dB.
Fig. 4. (a) Optical spectrum of a 100 Gbit/s NRZ-OOK signal for double sideband modulation (DSB) and single sideband modulation (SSB) at an optical carrier wavelength of 1537.5 nm. The two clock tones are spaced apart by 200 GHz. (b) Experimental results for 100 Gbit/s NRZ-OOK. Bit-to-error ratio (BER) as a function of transmission distance for double sideband (DSB) modulation (red, circle) and single sideband (SSB) modulation (black, square). As a reference the HD-FEC limit at 3.8×10-3 (dashed grey) is shown.
The bit-to-error ratio performance of 100 Gbit/s NRZ-OOK signals for different transmission distances through standard single mode fiber (SMF) at the C-band is shown in Fig. 4(b). We compare DSB modulation with SSB modulation and apply only linear FFE equalization in the offline DSP. DSB modulation allows for transmission of up to 3 km below the HD-FEC limit. In contrast, SSB modulation enables data transmission of up to 14 km below the HD-FEC limit. Thus, the SSB transmitter extended the possible transmission distance by a factor of 4.6.
Furthermore, we analyzed the necessary receiver complexity for successful data transmission of both DSB and SSB transmission links at various distances. Figure 5(a) shows the BER performance as a function of T/2-spaced feed forward equalizer filter taps for the DSB modulated signal. It can be seen that 41 taps are required for data transmission over 3 km below the HD-FEC limit. Data transmission through 5 km and 10 km reveals an error floor, which prevents BER performance below the HD-FEC limit even with 201 filter taps. Figure 5(b) presents the BER performance as a function of T/2-spaced feedforward filter taps for SSB modulated 100 Gbit/s NRZ-OOK signals. It can be recognized that that 3, 27, 27, 41, and 141 filter taps are required for successful data transmission of 3 km, 8 km, 10 km, 12 km, and 14 km below the HD-FEC. We did not apply any non-linear or feedback based equalizer. By considering a transmission distance of 3 km, the SSB transmitter reduces the receiver DSP complexity by a factor of 13.7.
Fig. 5. (a) BER as a function of feed forward equalizer filter taps required for double sideband modulation (DSB) and different transmission distances. As a reference the HD-FEC limit at 3.8×10-3 (dashed grey) is shown. (b) BER as a function of feed forward equalizer filter taps required for single sideband modulation (SSB) and different transmission distances. As a reference the HD-FEC limit at 3.8×10-3 (dashed grey) is shown.
5. Conclusion
We demonstrated the transmission of a 100 Gb/s NRZ-OOK signal over 14 km standard SMF in an IM/DD scheme without chromatic dispersion compensation. It has been enabled by a plasmonic SSB transmitter comprising a broadband dual electrode plasmonic MZM and an electrical differential drive signal with a $\pi /2$ phase offset. The demonstrated SSB transmitter increased the possible transmission distance by a factor of 4.6 and reduced the DSP receiver complexity by a factor of 13.7 for a transmission distance of 3 km.
These results of the plasmonic SSB transmitter show that the combination of broadband electro-optic modulation with a chromatic dispersion tolerant SSB format may – in the future - offer a solution for radio over fiber applications, which shift towards frequencies of up to 100 GHz and beyond [36].
Funding
European Research Council (670478, 688166); National Science Foundation (DMR-1303080); Air Force Office of Scientific Research (FA9550-15-1-0319).
Acknowledgments
This work was partly carried out at the BRNC and FIRST of ETH Zurich. We thank Aldo Rossi for technical support.
Disclosures
B.B., C.H., W.H. are involved in activities toward commercializing high-speed plasmonic modulators at Polariton Technologies Ltd.. The authors declare no conflicts of interests.
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