Generating High-speed Signals by using High-speed Electronic Devices and Signal Processing and Accomplishing 250-Gbps Short-distance Optical Transmission
--Anticipated as a technology leading to 1-Tbit-class optical transmission--

Nippon Telegraph and Telephone Corporation (NTT; Head Office: Chiyoda-ku, Tokyo; Hiroo Unoura, President and CEO) has successfully performed 250-Gbps (gigabits per second) short-distance optical transmission by means of a light-intensity modulation scheme (used by Ethernet, etc.) utilizing a new technology for generating high-speed signals by combining high-speed electronic devices and signal processing.
For short-distance optical transmission such as that provided by Ethernet, an intensity-modulation^*1 scheme (which can be simply configured with optical components) is used. The speed of the CMOS^*2 circuit in the output section of a digital signal-processing chip (for switching signal patterns in order to transform information being transmitted and modulate light) is limited, and it is difficult to create an intensity-modulated signal with speeds above that limit (namely, 250 Gbps).
Utilizing our original technology--called "DP-AM-DAC"--that doubles the speed of the output signal from a digital signal-processing chip, NTT has successfully generated a 250-Gbps signal by signal-intensity modulation and demonstrated 10-km 250-Gbps transmission in an optical transmission test using that signal. This accomplishment enabled 250-Gbps short-distance optical transmission using one wavelength, and by parallelizing such signal transmissions by using four wavelengths in the future, DP-AM-DAC shows potential as an optical-transmission technology for enabling short-distance large-capacity transmission at 1-Tbps (one terabit^*3 per second), namely, a 10-times-higher transmission rate than that possible with currently standardized 100G Ethernet. The proposed technology was presented at ECOC2016, an international conference held in Dusseldorf (Germany) on September 22nd (local time), as a "Post Deadline Paper" (namely, most-recent outstanding papers).

1.Background

For short-distance transmission such as that provided by Ethernet, transceivers must be low cost and compact as well as have low power consumption. In regard to conventional short-distance optical transmission, a simple modulation scheme called "intensity modulation", which utilizes one laser-light source and one light-detector with modulating the intensity of the optical signal only, is applied. As for optical transmission in recent times, complicated waveforms are used (for efficiently transmitting information), thereforea scheme that generates and transmits a signal by means of a digital processing chip is considered to be effective. A CMOS electronic circuit called a DAC^*4, which is installed in the output portion of the chip so that it can convert the output of the digital processing chip to an analog signal, operates in a frequency band^*5 in the order of 30 GHz; consequently, it is difficult to output the high-speed signal required by 250-Gbps-class transmission (which requires a wider frequency band, namely, above 60 GHz).

2.Achievements

By means of signal processing in the chip in a manner that is unaffected by the limited speed of the DAC (preliminary signal processing in Fig.1), the proposed DP-AM-DAC or "Digital-Preprocessed Analog-Multiplexed DAC technology" outputs the input signal as a two-system signal; namely, it is converted to a low-speed signal with speed below the limited speed of the DAC output. After that, a high-speed signal is created and the above-described bottleneck is eliminated by combining into one; a compound semiconductor^*6 electronic circuit (AMUX^*7) with the digital signal-processing circuit (thereby enabling higher-speed operation than a CMOS circuit connected externally to the digital signal-processing circuit). Moreover, as for spurious signals generated by the AMUX, it is possible to create proper high-speed signals by applying a method by which a two-line signal is configured by preliminary signal processing in a reverse operation that cancels out those signals when they are about to be generated. In the present study, DP-AM-DAC was used to create a frequency band of 60 GHz, and the world's first 250-Gbps 10-km optical transmission in that band was accomplished with an optical-intensity modulation called DMT^*8, as used for ADSLs (see Figs. 2 and 3).

3.Future plans

In light of the success of this transmission test, application of DP-AM-DAC is expected to be expanded to various fields that require high-speed modulated signals, including large-capacity (e.g., 1-Tbps-class) short-distance optical communication based on multiplexing four wavelengths.

4.Technical features

Digital-Preprocessed analog-multiplexed DAC technology (DP-AM-DAC) --an original technology developed by NTT--doubles the speed of the signal output from a digital signal-processing chip. While the signal input into the digital signal-processing chip is parallelized within the chip, the input signal is crammed into the low-frequency range by shifting the signal in the high-frequency range or by a reverse operation to make the signals equal or below the speed limitation of the DAC (which is configured in the output portion of the digital signal-processing chip). As a result, degradation of the signal when it is output from the digital signal-processing chip is suppressed. Then, the parallelized output signals from the digital signal-processing chip are summed up to analog signals at high speed by the AMUX. In the frequency domain, taking the AMUX as a convolution operation of signals, the AMUX yields images of the signal on both sides of the switching clock frequency and it generates a high-frequency or high-speed signal. At that time, although spurious signals are also generated in the usual case, it is possible to obtain the desired signals only by preprocessing the digital signals so that the signals are reversely designed to cancel out the spurious signals through the AMUX operation. Since this technology uses both digital preprocessing and AMUX for digital analog conversion, we call it "Digital-Preprocessed Analog-Multiplexed DAC technology", or DP-AM-DAC.

Figure 1: Comparison of conventional technology and frequency-band-doubler technology

The digital signal-processing chip is composed of a DSP^*9 and the DAC, and the frequency stated in the middle of the figure is the frequency band. To attain a transmission speed of 300 Gbps, a frequency band in the order of 60 GHz is required. However, the frequency band concerning the DAC portion is conventionally limited to about 30 GHz, so achieving transmission at 300 Gbps is impossible as is. When Digital-Preprocessed Analog-Multiplexed DAC technology is used to generate dual parallelized signals in frequency bands of 30 GHz, and the signals are combined by means of an AMUX, an overall frequency band of 60 GHz is created, and 300-Gbps transmission is possible.

Figure 2: Configuration of transmission test

For the transmission test, the data is created by a PC simulating a digital signal-processing chip, and signals are generated by an arbitrary-waveform generator. These signals are combined by the Digital-Preprocessed Analog-Multiplexed DAC integrated with the in-house-developed AMUX, and the output is modulated by a wideband laser module (EADFB^*10 laser) and then transmitted.

Figure 3: Results of transmission tests

Measured bit error rate (BER) is plotted against transmission speed in Fig. 3. The upper limit of the transmission speed is obtained from the point of intersection of the plotted line with the upper limit of BER (namely, maximum BER correctable by error-correcting code). According to this result, maximum transmission speed is 300 Gbps; however, net transmission speed (namely, that minus the contribution of the error-correcting code) is obtained as 250 Gbps.

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