JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 21, NO. 10, OCTOBER 2003 2145120-GHz Wireless Link Using Photonic Techniquesfor Generation, Modulation, and Emission ofMillimeter-Wave SignalsAkihiko Hirata, Mitsuru Harada, and Tadao Nagatsuma, Member, IEEE, Member, OSAAbstract—We present a wireless link system that usesmillimeter-wave (MMW) photonic techniques. The photonictransmitter in the wireless link consists of an optical 120-GHzMMW generator, an optical modulator, and a high-powerphotonic MMW emitter. A uni-traveling carrier photodiode(UTC-PD) was used as the photonic emitter in order to eliminateelectronic MMW amplifiers. We evaluated the dependence ofUTC-PD output power on its transit-time limited bandwidth andits CR-time constant limited bandwidth, and employed a UTC-PDwith the highest output power for the photonic emitter. As forthe MMW generation, we developed a 120-GHz optical MMWgenerator that generates a pulse train and one that generatessinusoidal signal. The UTC-PD output power generated by anarrow pulse train was higher than that generated by sinusoidalsignals under the same average optical power condition, whichcontributes to reducing the phtocurrent of the photonic emitter.We have experimentally demonstrated that the photonic trans-mitter can transmit data at up to 3.0 Gb/s. The wireless link usingthe photonic transmitter can be applied to the optical gigabitEthernet signals.Index Terms—Antenna, millimeter wave, photonic emitter.I. INTRODUCTIONGIGABIT-per-second data transmission is required forwireless communication in order to keep up with theremarkable speedup of local-area networks, such as 10-GbitEthernet. One way to increase the data rate is to use a free-spaceoptical link (FSO). The FSO has already achieved 2.5 Gb/s[1]; however, it has problems in terms of cost and size, sinceit requires a precise beam-positioning aperture to align theoptical beam. Another way is to use millimeter waves (MMWs)with a higher carrier frequency. Intensive research has beendone to develop a wireless link using the 60-GHz band, and1.25-Gb/s data transmission has achieved [2]–[4]. The occupiedbandwidth allocated to wireless links that uses100-GHzradiowave is insufficient because these frequency bands havealready been used by many systems. For example, the Japaneseand U.S. governments have allocated only 5- and 7-GHzbandwidth, respectively, to the 60-GHz band wireless com-munication system, and these bands are subdivided into pluraldifferent wireless communication systems. On the other hand,Manuscript received September 5, 2002; revised February 21, 2003. Thispaper was presented in part at the International Topical Meeting on MicrowavePhotonics 2001, January 7–9, 2002, Long Beach, CA.The authors are with the NTT Microsystem Integration Laboratories, NTTCorporation, 243-0198 Kanagawa, Japan (e-mail: [email protected]).Digital Object Identifier 10.1109/JLT.2003.814395100-GHz MMWs have not been used by any radio stationnor industrial service except for radio astronomy applications.So it is important to investigate the applicability of100-GHzMMWs in wireless communications in order to increase thedata rate to 10 Gb/s or more.The frequency region above 100 GHz remains undeveloped,mainly due to technical difficulties associated with conventionalelectronic systems. The generation, amplification, and modula-tion of electronic signals are difficult because the characteristicsof semiconductor devices deteriorateasthe frequencyincreases.Moreover, the transmission loss that occurs through metal ca-bles and the planar lines on a dielectric substrate is considerableeven when the transmission distance is very short.Recently, there have been manyattemptsto overcomethe lim-itations of electronic systems by combining them with photonictechniques. This is because optical components are more suit-able for handling high-frequency signals than electronic compo-nents. Ohno et al. applied a photonic technique to a wireless linksystem and succeeded in 1.0-Gb/s data transmission by using a40-GHz MMW photonic system [5]. For optical MMW gen-eration, many photonic techniques have been reported [5], [6].However, the effect of the difference in the optical MMW gen-eration methods on the transmission characteristics of MMWwireless links has not been evaluated.This paper describes a multi-gigabit-per-second wireless linkthat we developed using MMW photonic techniques. The linkconsists of an optical 120-GHz MMW generator, optical modu-lator, and high-power photonic MMW emitter [7], [8]. The pho-tonic emitter uses a uni-traveling carrier photodiode (UTC-PD)that features a large bandwidth and high saturation level [9].We evaluated the dependence of UTC-PD output power on itstransit-time limited bandwidth and its capacitance and resis-tance (CR)-time constant limited bandwidth, and increased theoutput power of the photonic emitter by using the UTC-PD withthe highest output power. For the MMW generation, we devel-oped another optical MMW signal generation method that usesmode-locked laser diode (ML-LD) and two-mode beating. Weapplied these two optical MMW generators to the MMW wire-less link and investigated the effect of optical MMW signal onthe photonic emitter output power and transmission characteris-tics. Finally, we experimentally demonstrated that the photonictransmitter can transmit data at a rate of up to 3.0 Gb/s andalso succeeded in transmitting optical gigabit Ethernet signalsthrough a 120-GHz wireless link.0733-8724/03$17.00 © 2003 IEEE2146 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 21, NO. 10, OCTOBER 2003Fig. 1. Schematics of optical MMW generators using (a) OTDM and (b) two-mode beating.II. OPTICAL MMW GENERATORWe have already developed an optical MMW generator thatuses optical time-division multiplexing (OTDM). Fig. 1(a) isa schematic of the generator, which employed a subharmonicmode-locked laser diode (ML-LD) [6]. The subharmonicML-LD generated an optical pulse train at the repetitionfrequency of 60 GHz by inputting electrical signals at 30 GHz.The optical spectrum of the subharmonic ML-LD is shown inFig. 2(a). The ML-LD output many modes whose frequencieswere precisely locked to a longitudinal mode spacing (60 GHz).The frequency of the pulse train was doubled by an opticalclock multiplier (OCM). In the OCM, the pulse train wasdivided into the two separate legs of a Mach–Zehnder inter-ferometer, and one leg provided a pulse delay. The two pulseswere then recombined to produce a repetition rate