Microwave Photonic Devices and Their Applications to Communications and Measurements

Transcription

1 PIRS NLIN, VL. 4, N. 3, Microwave Devices and Their Applications to Communications and Measurements Tadao Nagatsuma 1, 2 and Yuichi Kado 1 1 NTT Microsystem Integration Laboratories, NTT Corporation 3-1 Morinosato Wakamiya, Atsugi, Kanagawa , Japan 2 Graduate School of ngineering Science, saka University 3-1 Machikaneyama, Toyonaka, saka , Japan Abstract Microwave photonics (MWP), which merges radio-wave and photonics technologies, has recently attracted increasing interest. This paper provides an overview of the status of MWP technology, focusing on a system concept and enabling devices, and describes some of the latest applications, such as high-speed wireless communications, non-invasive electric-field sensors, and spectroscopy. 1. INTRDUCTIN The recent explosive growth in communications has been brought about by wired (fiber-optic) and wireless (radio-wave) communications technologies. These two technologies have started to merge to create a new interdisciplinary area called Microwave s (MWP) [1]. In addition, viewing the electro-magnetic spectrum with wavelengths progressively decreasing to the millimeter and submillimeter-wave bands on the radio-wave side and wavelengths progressively increasing to the infrared region on the light-wave side, we see that there is a large gap in utilization on the boundary between radio waves and light waves, i.e., the frequency band between 100 GHz and 10 THz. This untapped region represents a major resource for humankind in the 21st century (Fig. 1). Figure 1: Definition of MWP technology. MWP technology aims at achieving advancement and improved functions in telecommunications systems that cannot be achieved by extension of individual technologies, mainly through the combination of radio-wave technology and photonic technology. At the same time, MWP technology is also expected to open up unused frequency bands through the fusion of different fields. The opening of new application fields other than communications is also expected. This paper describes an overview of the status of MWP device technology and their latest applications.

2 PIRS NLIN, VL. 4, N. 3, MWP SYSTM CNCPT Typical radio-wave application system is illustrated in Fig. 2. Wireless communication link consists of a transmitter (Tx) and a receiver (Rx) as shown in Fig. 2, and some kind of object is placed between the Tx and Rx in applications to measurement, testing, and sensing as shown in Fig. 2. Now, what happens when we introduce photonic technologies in Tx and Rx? Figure 3 shows a block diagram of MWP-based transmitter, that is, a photonically assisted radio-wave transmitter. First, the optical () signal, whose intensity is modulated at microwave (MW) and/or millimetre-wave (MMW) frequencies, is generated by the optical MW/MMW signal source, and is delivered through optical fiber cables, and converted to the electrical () signal by a high-frequency - converter such as a photodiode. The converted signal is followed by a power and/or a frequency multiplier, and is finally radiated into free space by an antenna. The antenna unit can be separated and remotely controlled by optical fiber cables. Transmitter (Tx) Free Space bject under sensing measurement Receiver Tx testing (Rx) etc. Rx Figure 2: Radio-wave system for communication, and measurement. ptical Fiber ptical Microwave/ Millimeter-wave Signal Source / Converter : ptical Signal : lectrical Signal lectrical (Amplifier/ Multiplier) Antenna (RF) Antenna Mixer (L) L lectrical Mixer / (L) (IF) / L Crystal /A Modulator (IF) SBD Mixer SIS Mixer (IF) Figure 3: ally-assisted MW/MMW transmitter. ally-assisted MW/MMW re- Figure 4: ceiver. Figure 4 shows two types of photonically-assisted radio-wave receivers; one employs a photonic mixer pumped by photonic local oscillator (L) signals. Typical photonic mixer is a bulk electrooptic () crystal, and optical modulator devices such as a LiNb 3 waveguide modulator and a semiconductor electro-absorption (A) modulator. Here, the optical intermediate frequency (IF) signal is converted to the electrical IF signal by a slow photodiode. The other type is based on a nonlinear electrical mixer such as a Schottky-diode mixer, and a superconducting (SIS) mixer. The L signal is generated by a high-frequency photodiode followed by the optical MW/MMW signal source, as is used in the transmitter (Fig. 3). 3. NABLING DVIC TCHNLGIS As for the optical MW/MMW source in Fig. 3, there are lots of options such as optical heterodyning using two frequency-tunable laser diodes, optical heterodyning using two modes filtered from a multi-frequency (wavelength) optical source or optical frequency comb generator (FCG), the combination of a continuous-wave (CW) laser with an external modulator, and semiconductor mode-locked lasers (Fig. 5). Low-phase-noise and frequency-tunable optical MMW generators based on the optical heterodyning technique is shown in Fig. 6 [2].

3 PIRS NLIN, VL. 4, N. 3, Figure 5: Comparison of CW optical MW/MMW sources. xample of optical heterodyning tech- Figure 6: niques. Figure 7: Structure of UTC-PD. Figure 8: xample of photonic MMW emitter. An - converter is a key device in the system. Since optical s with a high gain of over 30 db and a large bandwidth of over 1 THz are now readily available, we need a high-power - converter to boost the signal generator performance. We used an ultrafast photodiode called a unitraveling-carrier photodiode (UTC-PD), whose band diagram is shown in Fig. 7 [3]. Fig. 8 depicts an example of photonic MMW emitter, where the UTC-PD and the antenna are integrated [4]. As a good example of the photonic MMW receiver or detector, the electro-optic () sensor made of a bulk crystal offers the largest bandwidth extending to the terahertz frequency region. The operation of the sensor is analogous to that of the down-converter in the electronic mixer operation as shown in Fig. 9. Fig. 9 shows the sensor attached to the optical fiber [5]. Highly sensitive materials used at an optical wavelength of 1.55 µm are CdTe and DAST. This sensor is also applicable to microwave regions, and is proven to be useful in the specific absorption rate (SAR) measurement at cellular phone frequency (1.5 GHz 2 GHz) [5]. 4. SYSTM APPLICATINS We have applied the photonic MMW transmitter to the 120-GHz-band wireless link system to realise a 10-Gbit/s transmission capacity [6]. Fig. 10 shows a block diagram of the wireless link. A high-gain Cassegrain antenna is used for a long distance (> 1 km) transmission. The wireless link can support the optical network standards of both 10 Gb (10.3 Gbit/s) and C-192 (9.95 Gbit/s) with a bit error rate of We have also been successful in the wireless transmission of 6-channel uncompressed high-definition television (HDTV) signals using the link. The ultralow-noise characteristics of the photonically generated MMW/THz-wave signal have been verified through their application to the L for superconducting mixers in receivers used for radio astronomy. Radio-astronomical signals from the universe have been successfully observed using a GHz photonic L [7].

4 PIRS NLIN, VL. 4, N. 3, Figure 9: lectro-optic sensor as photonic MMW down-converter, block diagram, example of sensor. A great advantage of photonic Ls in spectroscopic measurement systems is their wide tunability. For this purpose, a wideband receiver has been tested with the same combination of superconducting mixers and a photonic L at frequencies from 260 to 340 GHz [8]. MMWs/THz waves generated by the optical heterodyning using the FCG and UTC-PD are successfully applied to the spectroscopy measurement [9, 10]. IN Baseband Data signal (10 Gbit/s) 125 GHz MMW signal Microwave antenna 120-GHz antenna ptical modulator PD with Receiver 125 GHz ptical signal Baseband ptical MMW signal generator ptical signal lectrical signal Data signal (10 Gbit/s) UT (c) Figure 10: Block diagram of 120-GHz-band wireless link. Photographs of field trial and (c) application scene. 5. CNCLUSINS We described a brief overview of microwave and millimeter-wave photonics systems, and key devices incorporated in the system. The fusion of wireless and optical-fiber-based wired telecommunications technologies will continue to steadily advance in a form that will support the need for high speed and ubiquity in communications. Technology for the optical generation and detection of radio waves will become essential for various fields of measurement, as it facilitates the handling of ultra-high-frequency radio waves, which has been difficult with previous technologies. ACKNWLDGMNT The authors wish to thank Drs. A. Hirata, R. Yamaguchi, H. Takahashi, N. Kukutsu, H. Togo, N. Shimizu, H.-J. Song, T. Kimura, H. Ito, T. Furuta, T. Kosugi, K. Murata, K. Iwatsuki, H. Suzuki and M. Fujiwara for their contribution and support.

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