World’s First Transparent Fiber–Millimeter-wave–Fiber System in 100-GHz Band

Using Low-Loss Optical Modulator and Direct Photonic Down-Conversion

【Highlights】

• A 100-GHz band fiber–millimeter-wave–fiber transparent system was constructed based on direct millimeter-wave-to-optical conversion using a low-loss optical modulator with direct photonic down-conversion.
• 70-Gbit/s high-capacity transmission over the transparent fiber–millimeter-wave–fiber system at 101 GHz was demonstrated using 64-QAM OFDM.
• This demonstration opens the door for transparent fiber–millimeter-wave systems in the field of high-capacity, low-latency, and low-power consumption communications in the 5G and beyond era.

【Abstract】

The National Institute of Information and Communications Technology (NICT, President: TOKUDA Hideyuki, Ph.D.), Sumitomo Osaka Cement Co., Ltd. (President: MOROHASHI Hirotsune), and Waseda University (President: TANAKA Aiji) jointly developed the first transparent fiber–millimeter-wave–fiber system in the 100-GHz band using a low-loss broadband optical modulator with direct photonic down-conversion. The developed broadband modulator and photonic down-conversion technology were utilized to successfully demonstrate a high-speed transmission of more than 70 Gbit/s over a wired and wireless converged system consisting of two optical fiber links and a 20 m radio link at 101 GHz.

The utilization of a low-loss broadband optical modulator for the direct conversion of a millimeter-wave signal to an optical signal^*1 significantly simplified the millimeter-wave radio receiver because it included only a radio front end and an optical modulator. In addition, by adopting direct photonic down-conversion technology*² for simultaneous detection and down-conversion of the signal to the microwave band, the fiber-radio receiver and the subsequent digital signal processing could be considerably simplified, thus rendering the proposed system a promising solution for high-capacity, low-latency, and low-power consumption fiber–wireless transmission in 5G and beyond networks.

The results of this demonstration were published as a post-deadline paper presentation at the 2021 International Conference on Optical Fiber Communications (OFC 2021).

【Background】

Fiber–wireless systems in high-frequency bands are a promising technology for inter-building connections, disaster recovery, and mobile transport networks, especially in 5G and beyond era. To date, most systems rely on the use of electronics-based receivers for radio-to-optical conversion, which generally feature less bandwidth and complicated antenna sites. Achieving fully transparent radio–optical conversion using photonic solutions is promising for increasing the transmission capacity and simplifying the antenna sites. However, the frequency of radio links in the previous systems that utilized the photonic conversion method was limited to below 90 GHz owing to the limited bandwidth of optical modulators. Recently, a plasmonic modulator was employed to realize a transparent bridge system in high-frequency bands. Generally, plasmonic modulators exhibit high insertion loss, which requires the use of optical amplifiers. However, this increases the optical noise, system cost, and the antenna site complexity.

On the other hand, most of the previous systems utilized coherent detection by using free-running lasers for signal detection at the fiber-radio receiver, which significantly increased the system complexity, frequency offset, and phase noise of the detected signal, thus requiring complicated digital signal processing algorithms for signal recovery. Therefore, employing a direct photonic down-conversion technology to simultaneously detect and down-convert the signal to the microwave band using a coherent two-tone optical signal generation*³ is promising for simplifying the system and reducing the cost and power consumption.

【Achievements】

In this work, we demonstrated the first transparent fiber–millimeter-wave–fiber system in the 100-GHz band (see Fig. 1) using two key element technologies: (i) a low-loss broadband optical modulator, and (ii) direct photonic down-conversion. For direct conversion of a millimeter-wave signal to an optical signal, we fabricated and employed a broadband modulator*⁴ for operation up to 110 GHz. This was achieved by performing Ti diffusion on the x-cut thin-film lithium niobate in the low dielectric constant layer. In addition, we employed a photonic down-conversion method based on a coherent two-tone optical signal generation technology to simultaneously detect and down-convert the signal to the microwave band. This significantly simplified the system and reduced the frequency offset and phase noise, as compared to systems utilizing coherent detection. Using the technologies developed in this study, we successfully transmitted 64-quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) signal*⁵ with a line rate of 71.4 Gbit/s over a system consisting of two fiber links and a 20 m radio link at 101 GHz.

The system consists of the following key element technologies:

• A broadband optical modulator with a low half-wave voltage and low loss in the high-frequency band for direct conversion of millimeter-wave signals to optical signals.
• Direct photonic detection and down-conversion of signals to the microwave band utilizing coherent two-tone optical signal generation based on optical modulation technology.
• High-spectral efficiency 64-QAM OFDM signal transmission.

The optical carrier for data modulation at the antenna site was remotely generated and distributed from the fiber-radio receiver, which significantly simplified the antenna site and eased its operation and management. In addition, owing to the use of direct photonic down-conversion and detection technology at the fiber-radio receiver, the frequency offset and phase noise of the detected signal could be largely suppressed. This considerably reduced the receiver complexity and the subsequent digital signal processing. The proposed system is promising for high-speed, low-latency, and low-power-consumption communication links in 5G and beyond networks.

【Future Prospects】

In the future, we will further study the millimeter-wave-to-optical conversion device and fiber wireless technology that were developed in this study to further increase the radio frequency and transmission capacity. In addition, we will promote international standardization activities and social implementation activities related to fiber wireless communication systems.

The paper containing the results of this demonstration was published at the 2021 International Conference on Optical Fiber Communication (OFC 2021, June 6 (Sun.) to June 11 (Fri.)), one of the largest international conferences in the field of optical fiber communications. It was highly evaluated and was presented in the Post Deadline session, which is known to release the latest important research achievements, on June 11 (Fri) 2021 local time.

【References】

International Conference: Optical Fiber Communications (OFC 2021) June 2021,
paper F3C.4 (Post Deadline Paper)

Title: Transparent Fiber–Radio–Fiber Bridge at 101 GHz using Optical Modulator and Direct Photonic Down-Conversion

Authors: Pham Tien Dat, Yuya Yamaguchi, Keizo Inagaki, Masayuki Motoya, Satoshi Oikawa, Junichiro Ichikawa, Atsushi Kanno, Naokatsu Yamamoto, Tetsuya Kawanishi

【Glossary】

*1 Direct millimeter-wave to optical conversion

It is a technology that converts a wireless signal in the millimeter-wave band to an optical signal without down-conversion of frequency. On the contrary, in the electronics-based conversion method, the millimeter-wave signal needs to be down-converted to a lower frequency signal in the microwave band before its conversion into an optical signal. The direct conversion of a millimeter-wave signal to an optical signal can be realized using a broadband optical modulator or plasmonic modulator. This significantly simplifies the antenna site.

*2 Direct photonic down-conversion

It is a technology used for detecting and down-converting a millimeter-wave signal to a microwave band signal using optical signals from the same light source. In this technology, a two-tone optical signal consisting of the two optical sidebands with a frequency separation that is approximately equal to the frequency of the millimeter-wave signal is generated from a single light source. One of the sidebands is modulated by the millimeter-wave signal, and an optical double-sideband carrier-suppressed signal is generated. One of the modulated sidebands is selected using optical filtering. Finally, the modulated and unmodulated sidebands are combined and input to a low-speed photodetector to be converted to an electrical signal in the microwave band.

*3 Coherent two-tone optical signal generation

This technology generates two coherent optical signals from the same light source using optical modulation technology. In particular, an optical signal consisting of odd or even order harmonic sidebands is generated by applying a clock signal to an optical modulator and controlling the bias voltage.

*4 Broadband optical modulator using thin-film lithium niobate

A broadband Mach–Zehnder modulator (MZM) can be fabricated using a thin substrate. In this work, we fabricated a broadband MZM, in which Mach–Zehnder interferometer waveguides were fabricated by Ti diffusion on the x-cut thin-film lithium niobate in the low dielectric constant layer. This was done to achieve ripple-free operation and maximized electro-optic responsivity up to 110 GHz. By thinning the substrate, as shown in Figs. 3(a) and (b), the frequency ripple due to mode coupling between the coplanar guided mode and substrate mode can be suppressed. The electrodes were also optimized to reduce electrical propagation loss to attain high sensitivity. The optical insertion loss, including fiber pigtails, is approximately 4.6 dB at 1550 nm. The half-wave voltage at 100 GHz is approximately 6.7 V, demonstrating a sufficiently low value for high-sensitivity conversion of a millimeter-wave signal to an optical signal at the antenna site.

*5 OFDM 64-QAM signal

OFDM is a digital multi-carrier modulation scheme that uses multiple subcarriers within the same single channel. Instead of transmitting a high-rate data stream using a single subcarrier, OFDM uses a large number of closely spaced orthogonal subcarriers that are transmitted in parallel. In this work, subcarriers are modulated with 64 QAM symbols, each of which consists of six input data bits.

Appendix

1. Configuration of the proposed system

Fig. 4 shows a schematic diagram of the proposed system, which includes six main parts: a fiber-radio transmitter, a millimeter-wave radio transmitter, a millimeter-wave radio receiver, a fiber-radio receiver, millimeter-wave-to-optical conversion, and signal down-conversion and detection.

(1) Fiber-radio transmitter

This block generates and modulates signals. A two-tone optical signal with a frequency separation of 91 GHz was generated using optical modulation technology. The two optical sidebands were separated, and one of them was modulated by a 10 GHz radio signal. The bias voltage to the modulator was controlled to generate only the upper modulation sideband. The modulated and unmodulated sidebands were combined to form a 101-GHz radio-over-fiber (RoF) signal.

(2) Millimeter-wave radio transmitter

After transmitting over a 20-km single-mode fiber, the RoF signal was up-converted to a 101-GHz millimeter-wave radio signal using a high-speed photodetector. The generated radio signal was emitted into free space using a millimeter-wave antenna.

(3) Millimeter-wave radio receiver

The millimeter-wave signal was received by another millimeter-wave antenna, amplified, and converted to an optical signal using the developed high-speed optical modulator.

(4) Fiber-radio receiver

Another two-tone optical signal with a frequency separation of 84 GHz between the two sidebands was generated. One of the sidebands was transmitted to a millimeter-wave radio receiver for data modulation.

(5) Millimeter-wave-to-optical conversion

The optical carrier signal generated at (4) was modulated by the 101 GHz millimeter-wave signal obtained from (3), and the bias voltage to the modulator was controlled to generate a double-sideband suppressed carrier signal. The modulated signal was transmitted to the fiber-radio receiver using a 10-km single-mode fiber link.

(6) Signal down-conversion and detection

One of the modulated sidebands from (5) was selected using optical filtering and combined with the unmodulated sideband of the generated two-tone optical signal from (4) to form an RoF signal with a center frequency of 17 GHz (= 101–84GHz). The signal was converted to a microwave band signal using a low-speed photodetector.

2. Experimental results

In the demonstration, an OFDM signal at 10 GHz was generated and transmitted over the system. The performance measured in terms of the error vector magnitude (EVM) for the 64-QAM OFDM signal is plotted in Fig. 5(a) for different signal bandwidths. Considering a forward error correction overhead of 20 %, which requires an EVM value of 11.2 %, a satisfactory transmission performance was experimentally confirmed for the OFDM signal with a bandwidth of 14 GHz or smaller. This confirmed that a line rate of 71.4 Gbit/s could be attained when transmitting a 14-GHz bandwidth signal that consisted of 4096 subcarriers, of which, 15 % were inactive at the band edges. The superior performance of the system using a 20 m radio link could be attributed to the better power adjustment of the fiber-radio transmitter. An example of the received signal constellation is shown in Fig. 5 (b).

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