Elsevier

Optics Communications

Volume 475, 15 November 2020, 126196
Optics Communications

Dispersion pre-compensation of 25.6 Tbps waveforms using an optical frequency comb synthesizer/analyzer

https://doi.org/10.1016/j.optcom.2020.126196Get rights and content

Highlights

  • Ultra-fast (25.6 Tbps with 312.5 fs bit duration) arbitrary waveform propagation.

  • 8-ary amplitude and 32-ary phase modulation using optical frequency comb synthesizer.

  • Dispersion pre-compensation using optical pulse synthesizer.

  • Dispersion spectra measurement using scanless dual-heterodyne mixing.

Abstract

In this study, we propose and demonstrate a dispersion compensation system for an ultrafast 25.6 Tbps waveform using multilevel 8-ary amplitude and 32-ary phase modulation. The waveform bit period was 312.5 fs, which was controlled and compensated by a 200 GHz optical frequency comb (OFC) synthesizer with a 6.4 THz bandwidth. Dispersion spectra were measured in parallel and simultaneously (within 1 ms), based on single-shot dual-heterodyne mixing by introducing an OFC and arrayed waveguide grating to separate sidebands. An optical pulse synthesizer (OPS) can individually control the phase and amplitude spectra of the OFC. To control transmitted waveforms in a dispersed media, the OPS can be oppositely biased to feed the measured dispersion back into the source waveform when it generates an arbitrary waveform. In this study, a 25.6 Tbps dispersion-free waveform was successfully transmitted through a 10.55 km fiber.

Introduction

Over the past few decades, the demand for high-speed optical signals has rapidly increased in numerous applications ranging from optical imaging, spectroscopy, laser processing, and optical communication to pure science and technology applications [1], [2]. Even higher speeds will be required in the future [3]; however, pulse spreading caused by dispersion is a significant high-speed systems limitation [4]. Therefore, the need for dispersion management or dispersion pre-compensation increases with data rates and transmission distances [5].

The ultra-high-speed waveform observation techniques reported in the past include autocorrelators, frequency-resolved optical gating (FROG), and spectral phase interferometry for direct electric field reconstruction (SPIDER) [6]. Although these methods can provide high femtosecond resolutions, autocorrelators have issues related to narrow time windows and unobservable asymmetric waveforms [7]. Because FROG and SPIDER use nonlinear effects, they encounter an issue where the intensity of input signals is limited, making these methods unsuitable for single-shot measurement [8], [9], [10], [11], [12], [13]. Furthermore, indirect signal processing outside the time domain (e.g., response speeds of electronic circuits) has reached its limits, meaning that signal processing technology that is not limited by response speed is necessary to realize further use of high-speed optical signals [14], [15].

Multilevel modulation formats are a promising solution for ultra-high-speed signals beyond 100 Gbps because they can transmit additional information in the amplitude, phase, or polarization domains, or a combination of all domains. As the number of levels increases, the complexity of modulation algorithms also increases. To date, the highest fiber capacity demonstrated in a transmission experiment is 32 Tbps [16], [17].

To meet the ever-growing demand for higher data capacity, multi-core fibers represent the most common solution for future bandwidth requirements. Such fibers can achieve a bit rate up to the petabit range [18]. However, interference between optical signals leaking from cores increases when the core diameter is increased to facilitate multimodal propagation. Additionally, methods for forming connections with existing optical fibers are so complex that expensive technology is required [19]. Furthermore, no existing techniques consider the total bandwidth measurement and are limited to optical communication applications.

Dispersion compensation is typically performed in the optical domain using dispersion-compensating fibers (DCFs) or fiber Bragg gratings [20], [21], [22]. However, electronic dispersion compensation (EDC) can process complex optical fields in the electronic domain without requiring expensive DCFs. Additionally, the amount of compensation can be rapidly and simply modified by adjusting the parameters of compensation algorithms [23], [24]. However, EDC requires sufficient bandwidth for processing optical signals and causes severe signal distortion and a loss of optical phase information, which cannot be effectively compensated [25], [26]. Furthermore, post-dispersion compensation processing requires multiple samples to ensure sufficient data for implementing equalization techniques. The higher the number of samples, the higher the quality of an equalized sequence. Furthermore, in the vicinity of high-speed dispersive media, samples suffer from significant distortion. To resolve this problem, we propose a dispersion pre-compensation system for high-speed signals.

Based on this research, we propose a two-wavelength simultaneous heterodyne detection method [27], [28] to restore real-time waveforms by measuring the phase and amplitude spectra in the frequency domain using high-speed time-frequency conversion to overcome the problems discussed above. With the proposed method, it is possible to realize waveform observation without the limitations imposed by the response speeds of the detection circuits [27]. We demonstrate the observation of a 25.6 Tbps digitally multilevel modulated optical waveform using an optical receiver with a frequency band of 6.4 THz and an optical frequency comb (OFC) interval of 200 GHz. Additionally, by using an acousto-optic modulator (AOM) following the OFC as a reference light and introducing an arrayed waveguide grating (AWG) on the detection side, we implement a multi-wavelength simultaneous heterodyne detection method that enables parallel processing and faster optical waveform measurement.

Additionally, a high-speed transmission waveform leads to signal degradation based on the wide-band spectrum, leading to an increased error rate. Therefore, we realized the collective measurement of dispersion values on the frequency axis by maximizing the signal processing performance in the optical frequency domain, which is the core concept of the dual-wavelength simultaneous heterodyne detection method. In the past, the relative phase has been measured in parallel using an OFC (holographic OFC analysis), and the dispersion values related to wavelength have been collectively measured.

When handling broadband signals, waveforms deteriorate based on the chromatic dispersion of the optical transmission media. However, chromatic dispersion can be measured using the proposed OFC analyzer because dispersion appears as a relative phase spectral transformation [29], [30].

In our previous study, an ultrafast optical arbitrary waveform synthesis/analysis technique was demonstrated using 2 Tbps waveforms by manipulating the amplitude and phase of a 400 GHz OFC [31].

In this study, a 25.6 Tbps optical arbitrary waveform was generated using multilevel amplitude (8-ary) and phase (32-ary) modulation with a 200 GHz OFC based on single-shot dual-heterodyne mixing. Furthermore, dispersion measurements were conducted via parallel measurement of the relative phase spectrum using the proposed OFC analyzer. Additionally, we developed a dispersion pre-compensation system that can eliminate the effects of chromatic dispersion in optical transmitting media and control optical waveforms by feeding the measured dispersion spectra back into the phase control signals of an OPS. Furthermore, we measured high-speed signals simultaneously and in parallel using single-shot dual-heterodyne mixing within 1 ms.

Section snippets

Dispersion pre-compensation

Fig. 1 presents a schematic of the proposed dispersion compensation system. A signal synthesized by the optical frequency synthesizer passes through the dispersion media, which alters the OFC phase spectrum by an amount corresponding to the chromatic media dispersion and deteriorates the waveform. To compensate for dispersion, the amount of phase spectrum change is fed back into the driving currents for the phase modulators integrated with the optical frequency synthesizer. Each current

Experimental method

Fig. 4 presents the experimental system used in this study. A distributed feedback laser diode (DFB-LD) is used as a light source for the signal light OFC. Its output is incidental on an OFC generator driven by a radio-frequency signal with a frequency of 25 GHz, resulting in an OFC with a frequency interval of 25 GHz. The signal then passes through a 2 × 2 optical coupler. The coupler output is passed through an AOM in which the incident light frequency is shifted to 148.8 MHz and used as a

25.6 Tbps waveform generation

To perform spectral control using an optical synthesizer and generated waveform, a real signal waveform is generated, and a synthesized signal waveform is created using a computer, as shown in Fig. 7(a). Specifically, we used 25.6 Tbps 128-bit digital signals. To achieve a capacity of 25.6 Tbps, we used eight amplitude levels (0.125 to 1) and 32 phase levels (2π32 to 2π) with 128 or 16×3+5 bit signals. The signal in Fig. 7(a) has been generated using the amplitude and phase information explain

Summary

Waveform generation and measurement were conducted for signal waveforms with intensity-phase composite modulation using a high-speed optical signal waveform generation and measurement system combining an optical synthesizer and multi-wavelength simultaneous heterodyne detection method. We confirmed the validity of our experimental results by comparing measured results to calculated spectra and waveforms for 8-ary amplitude modulation and 32-ary phase modulation. The proposed method was able to

Declaration of Competing Interest

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.

Acknowledgment

The authors are grateful to Masanori Nishiura and Leona Yuda at the Saitama University for their technical support. A part of this work was financially supported by JSPS KAKENHI Grant Nos. JP16H03879 and JP17K19069.

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