1. Introduction
Optical phenomena have been attracting increasing attention in view of their potential capability to handle a large volume of informa- tion, for acquisition, transmission, processing, storage and display of data. Such capability is closely connected to the fundamental physical nature of electromagnetic waves at optical frequency. The naming of ``photonics'' is widely employed to express the optical science and technology for information handling, anal- ogous to the naming of ``electronics'', which originally meant the science and technology of electrons for information handling. Among the variety of optical phenomena, the generation and control of ultrashort optical pulses have made substantial progress in the last decade. 11 For example, the generation of ultrashort optical pulses less than 5 fs (=10−15 s) has been realized. Prototypes of soliton trans- mission systems have been constructed and their performance has been evaluated. Different types of all-optical switches have been proposed and developed, with which multiplexing and demultiplexing of digital data in the time domain in the 100 Gigabit/s range and beyond have been realized optically, now known as the optical time division multiplexing (OTDM) scheme. Combined with a wavelength division multiplexing (WDM) scheme, data transmission throughput beyond 1 Tbit/s (=1012 bit/s) in a single fiber has been demonstrated. Using femtosecond optical pulse as a probe beam, high-speed measurement techniques were developed with a temporal resolution of 1 ps or less. Through the progress of related device technology, we can now handle opti- cally high-frequency electromagnetic waves up to the THz frequency region. We call such a topical field an ultrafast photonics field. In this article, the authors try to review the progress in ultrafast photonics with special emphasis on the research and devel- opment activities in Japanese research institu- tions. After summarizing the physical natures of ultrashort optical pulses, we reviewed (a) ultrahigh-bit-rate optical communication, (b) optical components for ultrafast photonics, (c) microwave photonics, (d) high speed optical measurements. Some comments on the future prospect of ultrafast photonics are also given. The research status and the prospects of ultra- fast photonics in Japan during 1990's were reviewed in ref.2. In the present article, we mainly review the progress since then. The science and technology of ultrashort laser pulses have made substantial prog- ress also in generating terawatt class optical pulses at visible spectral region, in converting them to ultrashort X-ray pulses, and in chemi- cally or physically processing materials using laser pulses. To concentrate on the informa- tion aspects, we have omitted these ultrafast power optics fields from the present review, although some of the physics are closely related.
2. Fundamental Property of
Optical Pulses and their Propa-
gation Characteristics
Ultrashort optical pulses have some unique properties over conventional optical wave- forms with slowly varying amplitude, such as (a) energy localization in a time domain, (b) a large instantaneous electromagnetic field suit- able for inducing substantial nonlinear optical effects under a modest average power condi- tion, and (c) broadening in spectral width inversely proportional to temporal pulse width. Therefore an ultrashort laser pulse can no longer be regarded as monochromatic light; hence the spectral dispersion of the medium should always be considered.
2.1   Fourier transform limit
Let E(t) be the optical electric field ampli- tude and A(ω) be the spectral amplitude, where ω is the angular frequency. They are related to each other by the Fourier transform
If the temporal waveform is expressed by the Gaussian function
the corresponding spectrum is
Here, τ and Δω are the 1/e² widths of the pulse power in time domain and angular frequency domain, respectively. The Fourier transform of E ( t ) gives the inverse proportion- ality relationship
If the amplitude E o is constant and optical carrier exp[ iωot ] is monotonic, the equality relationship holds. Such a pulse is called a ``Fourier-transform-limited'' pulse. On the other hand if the central frequency ω o drifts within the pulse duration, the pulse is called a "chirped" pulse. In this case the inequality holds. Therefore the product (τ·Δω) is a measure of the spectral stationarity of the ultrashort optical pulses.
2.2 Propagation of ultrashort pulse
through optical fiber: Soliton trans-
mission
In 1973, Hasegawa found that the inclu- sion of the third order nonlinear response (optical Kerr effect) of an optical fiber into the electromagnetic wave equation leads to a nonlinear ordinary differential equation of the soliton type for the envelope function, known as the nonlinear Schrödinger equation (NSE)
i(∂E/∂z) − k = (∂²E/∂T²) + g|E|²E = 0.  (5)
Here k" represents the linear dispersion of the fiber, and g is a coefficient which is propor- tional to the optical Kerr coefficient of the fiber. Note that the time variable t is trans- formed to T=(t-k ω ), where k 0 is the propaga- tion constant at the central angular frequency ωρ.
It was predicted that under suitable condi- tions the pulse maintains the initial waveform after propagating through the single mode optical fiber with finite spectral dispersion characteristics.³. ⁴ Under a linear approxima- tion the same spectral dispersion causes the broadening of pulse width after propagation, limiting the transmission bit rates. An impor- tant assumption in deriving the nonlinear Schrödinger equation is that the optical frequency of interest is in the anomalous dispersion region of the fiber, where it has the positive group velocity dispersion, or a nega- tive value of the second-order dispersion coef- ficient β 2 =( an/ aw ). In accordance with the prediction by Hasegawa, Mollenauer et al. of Bell Labs. and Nakazawa et al. of NTT Labs. successfully demonstrated soliton transmis- sion experiments. 5,61 In 1999, Nakazawa also achieved 1.28 Tbit/s data transmission without employing wavelength division multiplexing (WDM) scheme. 77
2.3   Dispersion-managed soliton
The soliton transmission system consists of an array of erbium-doped optical amplifiers (EDFAs) in order to maintain the peak ampli- tude to induce sufficient nonlinear response of the fiber. Due to the spontaneous emission events in EDFAs, the accumulation of inten- sity noise is inevitable. However, the nonlinear response also causes the random fluctuation of central frequency for individual pulses. If the overall dispersion of the fiber cable is nonzero, the frequency fluctuation is converted into the fluctuation of arrival time, or timing jitter, is induced. This effect was found to be a serious
Super-continuum (SC) pulse generation using dispersion-flattened fiber. (A) Low-noise SC pulse generation using normal dispersion fiber (Takushima et al. 10 ). Curve (a) 45 mW; curve (b) 140 mW; curve (c) 210 mW. The dotted curve shows the spectrum of the incident pulse. ( β ) Extermely flat SC spectrum
generation using anomalous dispersion fiber (Igarashi et al. long of ength of the anomalous dispersion fiber. E and S denote E and S bands, respectively. ND and AD indicate restriction in the design of the system (Gordon −Haus limit). In 1995 Suzuki of KDD Labs. proposed a novel scheme to suppress this timing jitter. The new scheme employs the cascade connec- tion of positive dispersion fibers and nega- tive dispersion fibers. 81 For most of the trans- mission length, positive dispersion fibers are employed, so that the soliton effect transmis- sion is ensured. Meanwhile a small proportion of the length is filled with a strongly negative dispersion fiber, so that the overall accumu- lated dispersion is nearly compensated. With this scheme the fluctuation of the central frequency is not converted into timing jitter. More recently the group of Suzuki at KDDI Labs. performed a field test of TDM/WDM 2.52 Terabit/s throughput system along a span of 80 km by combining TDM at 40 Gbit/s with dispersion-managed soliton transmission cables and a WDM of 63 wavelength chan- nels. 99
2.4 Control of pulse shape and spec-
trum by fiber nonlinearity
With the decrease in pulse width, the spectral width becomes much broader, and more interesting phenomena appear. Firstly we summarize the nature of the higher order soliton pulse. The solution of the nonlinear Schroedinger equation is stationary or converges to a hyper-secant (sech) wave- form, if the peak power P 1 satisfies the threshold condition P 1 = 0.78λ 3 |D|(πW 2 )/(π ς η τ εν η 2 ), (6) where λ is the wavelength, D is the group velocity dispersion (GVD), ( πW o 2 ) is the effec- tive core area of the fiber, c is the speed of light, n 2 is the nonlinear refractive index, and Tewhm is the pulse width (full width at half maximum). This pulse is called the N=1 soliton. For N=1, the pulse shape is not distorted through the equilibrium mechanism between the nonlinear phase shift caused by the optical Kerr effect and the linear phase shift caused by the anomalous dispersion. At higher peak power levels, higher order solitons are excited. Waveforms for higher order solitons change periodically. For the N=3 soliton, pulse width 3/c 3/3 and z=z /3 and z=z /3 while at z=z 0 it restores to the width at z=0. Here z 0 is the soliton period. This effect can be used as a method for pulse compression. In handling the ultrashort pulses below 1 ps, the spectral broadening is enhanced. In this situa- tion higher order dispersion terms cannot be neglected as described below. Secondly we discuss the progress in the generation of a very broad spectrum using a normal dispersion fiber, namely, when β 2 is positive. In this case efficient and stable spec- tral broadening, so-called supercontinuum (SC) generation, is possible through the optical Kerr effect under a special condition that an extremely low value of normal dispersion is provided in a wide range of wavelength as discovered by Morioka et al. of NTT. 101 This latter condition is provided by a normal-disper- sion-flattened fiber. According to Takushima and Kikuchi of University of Tokyo this condi- tion has an advantage of generating a low noise supercontinuum¹¹ﾝ[Fig. 1(A)]. Thirdly, we describe the effect of higher order dispersion terms. Generally, the disper- sion function of an optical fiber has a concave curvature over the near-infrared-wavelength region. When the spectral bandwidth of an optical pulse is extremely broad, the third- and fourth-order dispersion effects are not negli- gible indicating that the method based on the standard NSE scheme is not appropriate but the generalized NSE scheme should be utilized. The latter includes the higher order dispersion effects and nonlinear effects besides the optical Kerr effect. Indeed, the phenomena affected by these higher order terms have been observed experimentally. Regarding the third-order dispersion (TOD) effects, Nishizawa¹²) reported the soliton self- frequency shift. The presence of the dispersion slope combined with the stimulated Raman scattering phenomenon causes it. Some trials have already been performed so as to apply this effect to fiber-optic pulse generation with wavelength tunability. In the meantime, Futami et al. reported the splitting of an optical pulse,¹³³ which is again due to the TOD effect. The spread of the spectrum over a zero disper- sion wavelength leads to two different behav- iors of spectral components depending on the sign of the GVD value. Regarding the fourth order dispersion (FOD), no direct evidence has been observed for standard optical fibers. The situation is different for a bunch of disper- sion-flattened fibers (DFF), where the influ- ence of TOD is minimized intentionally and the FOD nature is more emphasized conse- quently. Similar but more enhanced effects can be induced in photonic crystal fibers. For example, the supercontinuum (SC) generation in a photonic crystal fiber provides a degree of spectral expansion well beyond an octave on the basis of their enhanced nonlinearity and, therefore, wide-range dispersion character- istics and higher order dispersion effects are expected to play significant roles. 141 In such a case, however, conventional analysis on the basis of any NSE schemes can no longer be applied since the frequency range of interest is far beyond the most fundamental assumption for the NSE schemes, namely the assumption of slowly varying amplitude. Here, we focus on two simple cases where the generalized NSE scheme is still effective and, therefore, the effect of FOD can be clari- fied deductively. The first example is the fiber-optic SC generation featured by FOD in anomalous DFF. Besides the SC generation scheme in a normal DFF, which was mentioned above, more sophisticated and interesting features are given in the anomalous DFF cases. Tsuchiya, Igarashi et al. of the author's group discov- ered the parametric gain generation at a specific frequency offset which is attributed to the FOD effect¹5 [Fig. 1(B)]. A flat spectrum over 150 nm was recorded. For the genera- tion of an extremely flat SC spectrum with an extremely short fiber length, the parametric gain generation in the shorter wavelength range plays an essential role through the nonlinear phase matching mechanism origi- nating from the FOD effect. A similar obser- vation was reported for a photonic crystal fiber. 16 The second example is the achievement of the ultimate limit of fiber-optic soliton compression governed by the combination of the anomalous dispersion bandwidth of the fiber and the highest available soliton orderリアリ(Fig. 2). The former factor is again determined by the FOD characteristics.
3.  Ultrahigh-Bit-Rate  Optical
Communication
3.1 Need to increase communica-
tion capacity
There is a general trend in modern society to generate and consume as much informa- tion as possible. Having more information, one acquires more chance to create new activity, which demands acquiring further information. This positive feedback mechanism may be an underlying motivation of information society. The high bit-rate transmission systems for the intercontinental submarine network reached nearly 1 Tbit/s throughput. Also new services for Internet end users create a strong demand for a high bit-rate metropolitan network supporting more than 100 Gbit/s throughput. In Japan broadband mobile telephone services of transmitting still- and video-pictures are rapidly in progress. Both high-capacity trans- mission technologies and switching node (router) technologies are urgently demanded.
3.2 WDM and TDM
To meet these demands there are two major approaches to generating multiplexed digital signals and sending them through optical fibers. One is wavelength division multi- plexing (WDM) and the other is time division multiplexing (TDM). In the WDM system, the system throughput is given by the product of the wavelength channel number and bit-rate per channel. In a TDM system of single wave- length, the throughput is the bit rate itself. The technical challenges for WDM are the channel spacing and the overall bandwidth in the optical frequency domain. The technical challenges for TDM are the switching speed of the multiplexer in the sender unit and the AND logic speed of the signal/clock bits of the demultiplexer in the receiver unit. The signal distortion due to fiber dispersion and nonlinear response is the common problem. To maximize the throughput a combina- tion of WDM and TDM is widely employed. Note that in the ideal WDM/TDM system the overall throughput is not dependent on bit rate, but is governed by the bandwidth of the available optical spectrum. This comes from the Fourier transform relationship between pulse width and spectral width. In order to increase bit rate, we need to use shorter pulses, which occupy a wider spectral width per cannel. Therefore the channel interval should be larger. The ideal setting for highest throughput is obtained when the channel interval is most efficiently occupied by a pulse spectrum. This system is called a dense WDM (DWDM) system. Considering the fact that throughput is defined as the product of bit rate and channel number, together with the fact that the bit rate is proportional to the channel interval, the product simply gives rise to the overall available bandwidth. The quidelines of bit rate selection come from the choice of the granularity of the information. In multi-gigabit systems, smaller numbers of packets containing larger numbers of bits are easier to handle than larger numbers of finer packets. Therefore, increasing bit rate is an attractive challenge for both elec- tronic multiplexer and for optical multiplexer. Presently the highest bit-rate available with the electronic multiplexer/demultiplexer module is bandpass filter; SMF: single mode fiber; RDF: reverse dispersion fiber; OlE: optoelectronic converter; E-DEMUX: electrical
DEMUX. (b) Measured optical spectra at the exit of 40 Gbit/s. Beyond that we need to use optical multiplexing/demultiplexing technology. This latter technology is called optical time division multiplexing (OTDM) technology. One of the most realistic implementa- tions of the DWDM transmission system was demonstrated by Miyamoto et al. of NTT. 180 In order to suppress the data quality degradation due to fiber nonlinearity, the return-to-zero (RZ) modulation format is superior to non-return- to-zero (NRZ) format. However, the RZ format has the drawback of consuming a bandwidth two times broader than NRZ. To achieve dense WDM transmission, a novel scheme called a  carrier-suppressed RZ (CS-RZ) format was proposed, where the neighboring bit has a π phase shift using cascaded electrooptic modulators. With this scheme the required bandwidth is narrowed to be 1.5 times broader than that for the NRZ format. Using this scheme a prototype 1 Tbit/s throughput system was constructed with 25 wavelength channels (1570–1590 nm) and 43 Gbit/s for each channel. A bit-error-rate (BER) less than 10 -9 was confirmed after transmission of 90 km.
3.3  Optical time division multi-
plexing by all optical switches
Most digital electronic circuits are composed of transistor switches. In each of these transistor switches electrical gate voltage controls channel current flow. The bandwidth corresponding to response speed of the switch is limited by the time constant determined by gate capacitance and load resistance, usually not exceeding 100 GHz. At a signal flow of more than 40 Gbit/s, all optical switches have better potentials. All optical switches utilize various types of photon-photon interaction, so that the optical gate pulse can control the transmittance of the signal pulse. A variety of all-optical switch principles have been proposed and investigated. These will be discussed in §4. Here, we introduce two real- istic transmission system trials using an ultra- short pulse and all-optical switches. One has been reported by Ohara (NTT), Fejer (Stanford University) et al., 191 who demonstrated a 160 Gbit/s OTDM system transmitting data over a distance of 160 km as shown in Fig. 3. They used an integrated periodically poled lithium niobate (PPLN) plate, which ensures efficient second order nonlinear wavelength conver- sion because of the quasi-phase matching effect.² ⁰ The cascaded second-order nonlin- earity (cascaded χ 2) process) acts as the AND gate for two input optical pulses (one is a periodical pulse train with a 40 GHz repetition rate and the other is a data pulse with a NRZ format). Pulses from four gates are combined with a relative delay of 6.25 ps to form 160 Gbit/s data stream. After 160 km transmission, minimum received power ensuring BER of 10-9 was at worst –20 dB m. Another example of OTDM system was demonstrated by Suzuki et al. of Femtosecond Technology Research Asso- ciation (FESTA), ²¹ﾝwho showed 8 × 160 Gbit/s (total of 1.28 Tbit/s) DWDM/OTDM unre- peated transmission over a 140 km long stan- dard fiber using semiconductor-based devices. The featured technologies for this success include mode-locked laser diodes (ML-LD), a supercontinuum generator, symmetric Mach –Zehnder (MZ) semiconductor switches, and pre-and post-dispersion compensators. Using a 25 nm optical bandwidth, the bit rate density is approximately 0.3 bit s-¹ Hz-¹.
3.4   Photonic network systems
In modern network systems, not only the signal transmission but also the signal routing is very important. The routers are composed Fig.4. All-optical serial-to- parallel conversion scheme using surface-reflection switches. LOTOS: low-temperature-grown
optical switch (Takahashi et al. 227 ). of digital electronics, where incoming optical data are converted into electrical signals (O–E conversion), processed electronically and then converted into optical data for output (E-O conversion). The fundamental functions of the routers are packet label recognition and redi- rection, tentative storage (buffering), and waveform reshaping. With the increased bit rate, O–E and E–O conversion become more difficult. The fluctuating time delay (latency) causes additional burden to the system. To overcome these difficulties, the idea of processing the data stream optoelectroni- cally or all-optically in the routers and mini- mizing the burden of O/E conversion is widely accepted among communication engineers as one of the next generation network tech- nologies. It is called photonic routing and the system composed of photonic routers is called a photonic network. First generation trials have been reported actively in recent years. Digital data packets are composed of contents (payloads) and labels (headers), the latter of which contain the information of the destination of the data stream to be directed. The principal issue is the way how to separate and read the header portion, and to redirect the whole packet without O/E conversion of the payload portion. One of the promising methods for such optical signal processing is all-optical serial-to- parallel signal conversion. Takahashi et al. of NTT²²-²4) developed a processing unit which can recognize the 16-bit header portion of a 1 Tbit/s data stream by converting it into a 16 channel  × 82 Mbit/s parallel data flow for O–E conversion and electronic processing as shown in Fig. 4. They developed semiconductor surface reflection optical switches based on low-temperature-grown Be-doped InGaAs– InAlAs multiple-quantum-well (MQW) tech- nology. In the recognition unit the incoming data stream is branched into 16 beams, and individual beams are given time delays of inte- gral multiples of bit interval and illuminate the array of all optical switches together with the intense clock pulse with an 82 MHz repeti- tion rate. Since the all-optical switch acts as the AND gate for the signal and the clock, the outgoing pulses contains the 16-bit slice from the incident data stream. This converter has an on/off ratio of more than 30 dB. Wada et al. of National Institute of Infor- mation and Communications (NICT, former name: CRL) demonstrated a prototype of an optical packet switching system (OPS system) based on all-optical header recognition tech- nology.² ⁵ They assume the label format of code-based labeling and bit-by-bit multiple wavelength assignment that was proposed by Kitayama and Wada of CRL. 261 The recog- nition of the header information is made by correlating it with those from the label bank. Individual bits of different wavelength are separated by the dispersive components such as the arrayed waveguide grating (AWG), the fiber Bragg grating (FBG), or ordinary diffraction grating. This wavelength separa- tion functions as serial-to-parallel conver- sion. A prototype of 40 Gbit/s OPS system was constructed and tested. The overall BER penalty of the optical processing was found to be less than 5 dB. In correlating the header signal and signals from the label bank, they also proposed a novel holographic technique, where an angular multiplexed spectral holo- gram (AMSH) and an optical Fourier trans- form correlator play key roles. 271 The usage of the optical Fourier transform correlator for header recognition was originally proposed by Konishi et al. of Osaka University.² ⁵ The operation is based on the frequency-to-space conversion principle. The design theory was recently developed by Furukawa et al. of Osaka University. 291 Another novel wavelength routing technology is studied by Uenohara et al. of Tokyo Institute of Technology using high-speed wavelength-tunable semiconductor lasers with super-structure-grating distributed- Bragg-reflector (SSG-DBR) structures.³⁰⁰ Another important function of the router is the data buffering, or tentative storage of received data. Without O–E conversion followed by storage in semiconductor memory device, Wang et al. of Northwestern Univer- sity proposed to use a combination of a nonlinear-optical-loop-mirror (NOLM) device and a nonlinear-amplifying-loop-mirror (NALM) device. 31) The operating principle relies on four- wave-mixing loading and intracavity soliton control. The signal flow of the 20 Gbit/s packet pattern was stored for more than 6 s. Takahashi et al. have proposed the scheme of optical RAM, which is again based on serial- to-parallel signal conversion. This scheme is interesting for its hybrid nature of ultrafast photonics and electronics. 320
4. Component Technology for
Ultrafast Photonic Systems
4.1 Light source
Representative ultrashort pulse sources at communication wavelength (between 1300 and 1600 nm) are semiconductor lasers and fiber ring lasers employing erbium-doped fiber amplifiers (EDFAs). A high-performance colliding pulse mode-locked (CPM) semicon- ductor laser at communication wavelength was first implemented by Arahira et al. of Oki Electric.³³ Hoshida et al. of University of Tokyo investigated jitter noise suppression by the injection of an electrical signal to the sutur- able absorber section.³⁴ Wang et al. of NEC and Arahira et al. studied the stabilization by optical injection. 35,36 More recently Arahira et al. achieved 480 GHz repetition subhar- monic synchronization of a CPM laser diode of short cavity length (174 μm) using a 10 GHz repetition hybrid mode-locked master laser. 371 Because of the strong nonlinearity of the laser medium, good stabilization was confirmed. Pulse width of 0.58 ps with a timing jitter as low as 0.14 ps was obtained. Extending this technique Arahira et al. also demon- strated the stabilization of a 160 GHz mode- locked laser was possible by injecting a coded 160 Gbit/s optical pulse stream.³ ⁸ Applied to optical clock extraction, excess timing jitter less than 0.1 ps was achieved at the injection power level in terms of a data signal of −3 dB m. A key feature of the semiconductor-based ultrashort pulse sources is their nonlinear section, which is the saturable absorber. The carrier dynamics of the saturable absorption section of a monolithic mode-locked diode laser was precisely investigated by the authors' group using a pump probe technique.³³³ It was found that hole dynamics is affected by the presence of abrupt heterointerface and its improvement is necessary. As a light source for the OTDM/DWDM system, both shortening pulse width and widening spectral width are required. The supercontinuum (SC) generation as discussed in §2.4 is suitable for this application. The system reported in ref. 21 used the SC source for generating 8 wavelength channels by slicing the spectrum with an AWG filter. A novel SC generation with extremely flat spectral characteristics was discovered by our group at University of Tokyo as described above.¹ ⁵ In contrast to the conventional SC configuration an anomalous-dispersion-flat- tened-fiber (ADFF) with the length ranging between 0.6 and 3.6 m is placed after EDFA and fiber-optic soliton compressor. 401 The optimum length of ADFF is shorter for a higher pulse energy. At 320 pJ, the optimum length was 0.6 m and the spectral flatness of ± 1 dB over a 150 nm wavelength interval was observed. This behavior was interpreted as broadband optical parametric gain generation whose spectrum is extended by fourth-order dispersion (FOD) characteristics. The stability improvement of the SC system became the next issue. Besides the dispersion profile management, one of the key factors for stability is the maintenance of polarization in the dispersion-flattened fiber. Kawanishi et al. of NTT proposed to employ the photonic crystal fiber (PCF). 411 With the proper geometrical configuration of holes at the core portion of the fiber, a strong birefrin- gence is generated. Symmetric SC generation over 40 nm wavelength interval at 25 dB m was realized.
4.2  Pulse compression and shap-
ing
As discussed in §2.2, the excitation of higher soliton in anomalous dispersion fiber is an efficient method of pulse compression. Miyamoto et al. with the members of the authors' group at University of Tokyo extended this to the cascaded soliton compression scheme of LD-generated pulses. 421 Using 3 stage fiber combination they obtained a compression factor of over 400 with minimum width at autocorrelation trace of 65 fs. Recently, more systematically designed cascaded fiber has yielded a 20 fs pulse [Fig. 2(a)]. Theoretical analysis has shown that the fourth-order dispersion term is non-negli- gible.43.44) For the application of ultrashort optical pulses to real systems, the quality of optical pulses is an important issue. As pointed out by Nakazawa and his colleagues, the prop- agation of the fundamental soliton generates the least parametric noise. 451 Therefore the adiabatic compression scheme is attractive for this purpose, where the nature of the funda- mental soliton is maintained by the dispersion tailoring along the direction of propagation. 460 However, optical fibers with ideal dispersion profiles are rather hard to fabricate. Igarashi et al. of Furukawa Electric have demon- strated a comblike profiled fiber scheme to overcome the obstacle, in which the alternate combinations of (1) nonlinear fiber parts with scarce dispersion and (2) dispersed fiber part with little nonlinearity are utilized. 471 Here the compression nature can be controlled by trim- ming the lengths of fibers. Indeed, high-quality 160 GHz pulse trains have been reported by the same group. 480 Another important technique of pulse shaping was proposed by Weiner of Purdue University based on the space-to-time conver- sion principle. 491 More recently McKinney et al. of Purdue University extended the idea to direct pulse shaping for communication purposes. 500 Besides the above mentioned methods, the scheme of optical reshaping, retiming and regenerating (optical 3R) has been intensively studied, which is attractive for the all-optically regenerative transmission of ultrashort optical pulses. Watanabe et al. of Fujitsu demon- strated a 160 Gbit/s optical 3R system based on a highly nonlinear optical fiber combined with a Mamyshev filter.519 The clock recovery of the scheme is still under development. Boomholdt of HHI has demonstrated 40 Gbit/s optical 3R experiments on the basis of nonlinear semiconductor devices: injec- tion locking into 2 mode DFB-LD and cross- phase modulation in SOA. 521 The upper limits of the device operation speeds are key issues. For the simplification of the 3R scheme, 530 single device operation has been investigated by Takita et al. of University of Tokyo and by Onishi et al. of Tokyo Institute of Technology, although the results of their investigation are still preliminary. 54.55)
4.3 All-optical switches
All-optical switches are described concep- tually as four photon devices. Namely, an incident photon interacts with the switching medium, the optical properties of which are controlled by strong gate photons and deter- mine the transmittance, or probability of generating outgoing photons. The third-order nonlinear optical effect is a typical example of such a process. Now, let E 1 , E 2 and E 3 denote the optical field amplitudes of incident photon, gate photon, and outgoing photon, respec- tively. They are related to each other through the third-order nonlinear optical susceptibility χ (3) with
E 3 ∞ X (3)E 2 E 1
The outgoing photon appears only when both the incident photon and gate photon are present, so that the device functions as an AND gate. According to the quantum mechan- ical theory, the nonlinear optical susceptibility function is related to the energy levels of the interacting material via the relationship
where M i, M i, and M i, are the matrix elements of the electric dipole moment, and ερ, ε ; and  ; are the energy eigenvalues of the ground state, first excited state and second excited state, respectively. Here, ñω is the inci- dent photon energy. When one of the denom- inator factors is close to zero, resonance enhancement is expected. On the other hand, this resonance condition induces strong linear absorption. Hence, a trade-off problem exists. To achieve an ultrafast switching perfor- mance, a variety of materials have been tested. We can categorize them by their trans- parency to incident photons. When incident photon energy is far from the resonance, the material is transparent so that interaction length can be long although the nonlinear susceptibility remains modest (A). Meanwhile when the resonance condition is satisfied, the material absorbs the incident photons quickly, so that interaction length cannot be long (B). When the material is a laser gain medium under a pumped condition, it is possible that resonance coexists with no attenuation (C). A typical example of (A) is the fiber- optic nonlinear loop mirror switch (NOLM). Although the nonlinear susceptibility of silica glass is quite small, the interaction length over 1 m ensures reasonable nonlinear interaction at modest power levels. The earlier works of the Tbit/s OTDM experiment 561 utilized a NOLM switch because of its relative maturity. Another approach is the use of a high χ (2) material for cascaded second-order nonlinear signal conversion. Using a periodically poled lithium niobate (PPLN) wavequide structure, efficient optical frequency conversion into (2ω 2 – ω 1 ) is realized, where ω 1 is the angular frequency of incident light and ω 2 is the angular frequency of control light. Suhara et al. of Osaka University exten- sively studied the use of lithium niobate wave- guide devices for optical signal processing. 571 His group improved the polarization insensi- tivity by precisely controlling the phase veloci- ties of the TE and TM modes. 589 Kunimatsu et al. of FESTA demonstrated the application of this device for 100 Gbit/s demultiplexing of 250 fs pulses. 59) Semiconductor optical amplifier (SOA) acting as an all optical switch belongs to category (C). It has the advantages of a high sensitivity and a low insertion loss. Although it had been suspected that the carrier life- time limits the switching speed, Tajima of NEC demonstrated a faster response by employing the symmetric Mach Zehnder interferometer configuration. 60.61 Actually the switching speed is limited by the intraband relaxation time. For the InGaAs active region, the switching speed is approximately 2 ps. The operation principle is the carrier density dependent phase shift in SOA. More recently they showed the demul- tiplexing of 336 Gbit/s signal pulse into 10.5 Gbit/s channels using the module of a hybrid integrated symmetric Mach Zehnder semi- conductor switch (HI-SMZ). 621 Schreieck et al. of Swiss Federal Institute, Zurich, fabricated a monolithically integrated symmetric Mach Zehnder interferometer all-optical switch and they demonstrated a switching window of 1.5 ps, corresponding to a 600 Gbit/s data rate capability. 631 They also analyzed the effects of carrier heating, spectral hole burning, and two-photon absorption. Another type of all optical switch based on SOA has been studied by Sugawara's group of Fujitsu. They developed efficient quantum dot semiconductor optical ampli¬ fiers (QD-SOAs) based on InGaAs self-orga- nized quantum dot formation technology. The QD-SOA acts as an all-optical switch by the four-wave mixing (FWM) effect. The third- order nonlinear susceptibility χ (3) is caused by the sum of the carrier density pulsation effect, the spectral hole burning effect, and the carrier heating effect. In the bulk and quantum well SOAs these effects interfere differently as a function of carrier density because gain peak frequency depends on carrier density. On the other hand, in the quantum dot SOA the gain spectrum is very stable due to the spike- formed density of states. Actually Akiyama et al. of Sugawara's group demonstrated the improved performance of four-wave mixing in QD-SOA in terms of virtually symmetric wavelength conversion with positive and negative detuning. 641 This is in contrast to the conventional quantum well SOAs where FWM conversion efficiency is much larger for positive detuning than for negative detuning, which poses a problem in achieving broad- band FWM operation in WDM systems. Recently Sugawara et al. demonstrated ultrawide band (120 nm) operation using QD-SOA with a peak output power level of 23 dB m by improved material technology and showed high-quality waveform regeneration and reshaping at 10 Gbit/s. 65) The saturable absorber action of the semi- conductor offers category-B-type all-optical switching. Since the linear absorption coef- ficients of excitonic transition are approxi¬
mately 10 5 cm-', an interaction length of less than 1 μm is sufficient. For application to high-speed switching, material processing to shorten the carrier lifetime while maintaining excitonic response is required. The scheme of low-temperature-grown GaAs is one repre- sentative for this purpose and, indeed, is well utilized for THz wave generation sources. However, the operation wavelength range of GaAs is around 800 nm and materials for 1500 nm operation were highly demanded. Takahashi and Kawamura of NTT found that Be-doped low-temperature-MBE grown InGaAs/InAlAs multi-quantum-well (MQW) structures satisfy these requirements. 66 ) Confining the MWQ layer in the Fabry–Perot cavity, they demonstrated the device acts as a sensitive, high-speed all-optical switch, and named it LOTOS (low-temperature-grown surface-reflection all-optical switch). 671 For practical applications in ultrafast serial-parallel converters, developing packaging technology is also important. Takahashi et al. assembled a module, which consists of a LOTOS plate, a polarization beam splitter, a planar light- wave circuit (PLC) for optical delay lines, and a micro-lens array. With this module they real- ized 40 Gbit/s 16 bit label recognition. 221 Hain- berger and members of the authors' group at University of Tokyo tried an alternative approach to assemble the saturable absorber switches in collaboration with Kodate et al. of Japan Womens University. Namely, a microop- tical platform for all-optical demultiplexing was designed, fabricated and evaluated using diffractive optics technology (Fig. 5). 68-70 Ellip- tical zone plate arrays formed on both sides of silica plate enabled the focusing of the pump and signal beams in the same position of the saturable absorber plate. The spatial walk-off effect on multiwavelength operation was studied in detail. The high speed recovery of saturable absorption was also realized by employing the inter-subband transition in quantum well structures according to Akiyama et al. of FESTA.71.72) To tune the band structure to 1500 nm transition, a coupled quantum well struc- ture of AlAsSb/InGaAs/AlAs/InGaAs/AlAsSb was designed. The use of wide-gap clad- ding and barrier materials (AlAsSb and AlAs) ensured the reduction in resonance wave- length. As expected the response time was as short as 690 fs. The pulse train of 1 ps interval (pulse width of 300 fs) was success- fully switched. Recently Mozume, Simoyama et al. of the same group improved the device performance by the introduction of an AlAs intermediate layer at the interface of the InGaAs/AlAsSb barrier. 73,74 The realiza- tion of an abrupt interface enabled to reduce the saturation energy density as low as 34 fJ/μm² and the switching of 150 fs pulses with control pulse energy of 10 pJ was confirmed. Another group of materials which exhibit sensitive absorption saturation and high speed recovery is the organic dye of J-aggregate structures. 758 Furuki et al. of FESTA demon- strated a sub-100 fs response of spin-coated squarylium dye J aggregates in the 800 nm wavelength region. 761 The group also proposed a novel scheme of ultrafast serial-to-parallel optical signal conversion and named it the femtosecond large area parallel processor (FESLAP). 771 The key technique of FESLAP is the simultaneous illumination of the signal beam and the control fs pulse beam over a large area of saturable absorber film, with the signal beam incident normal to the film, while the control beam incident obliquely to the film, so that the bleached stripe zone is scanned along the film with the speed depending on tilt angle. The pattern of the transmitted signal beam is spatially modulated with the temporal waveform. The collaboration group of FESTA, Fuji-Xerox and CRL (presently NICT) succeeded in demonstrating a pulse train with an 1 ps interval (pulse width of 100 fs) converted into a stripe pattern. They also developed a new material called BM: di(benzofranonyl) metha- nolate, whose absorption peak lies at 1100 Concept of all-optical demultiplexing module using surface reflection switch array and free-space optics employing off-axis binary zone plate array (Hainberger et al. 60 ). nm. They managed to demonstrate the ultra- fast nonlinear response of BM film at wave- lengths longer than 1300 nm.
4.4 High-speed optical memory
As mentioned in §3.4 the photonic network requires a tentative storage (buffering) function for the routers. If we do not want to convert the incident packet into an electrical signal, then we need to incorporate high speed optical memories for such a buffering purpose. There are two categories for such a photonic router storage function: frame memory and bit memory. As for the former, two examples have been shown already in §3.4. The frame memory scheme is rather more practical for the buffering function although its flexibility features are not sufficient. Regarding the bit memory, there are still serious obstacles: (1) Sufficient degrees of memory integration are not available; (2) Speed of memory opera- tion and sufficient nonlinear properties are in a trade-off relationship. Although the matu- rity of the technology is still not sufficient, a number of challenging trials are being pursued on a global scale. Similar to ferromagnetic memories, bistable optical devices have been a candidate for bit memory from the early stage of optoelectronics. Kawaguchi of Yamagata University, known as one of the pioneers of bistable semiconductor lasers, 781 has recently proposed to use vertical cavity surface emit- ting laser diodes (VCSELs) as parallel address- able optical memories, because VICSELs have two metastable lasing states of orthogonal polarization. 791 By injecting a linearly polar- ized control light pulse, the VCSEL is switched to the polarization state of the control pulse, and the state is sustained until the next pulse is incident. Takenaka and Nakano of Univer- isty of Tokyo proposed and realized a bistable integrated photonic circuit including SOA. It is a directionally coupled bistable laser diode (DC-BLD), which consists of a main cavity with a saturable absorber section and a direction- ally coupled side waveguide. 80) The injection of the control pulse to the main cavity turns the lasing on because the control pulse bleaches the saturable absorber. The injection of the control pulse to the side waveguide terminates the lasing because the nonlinear response of the directional coupler portion extracts the photons from the main cavity. Another bistable photonic circuit employing SOA was proposed by Dorren et al. of Eindhoven University of Technology.81) The planar SOA is excited axially both by the signal light and by control light. The output light is sent to the polarization beam splitter and detected. The polarization state of the signal light is tilted by 45° with respect to the junction plane of SOA, so that TE and TM modes are equally excited. The polarization controller is adjusted so that the maximum output is obtained. When a strong control pulse is incident to SOA, its gain is saturated, and the phase velocities of the TE and TM modes are changed. Due to the relative phase delay, destructive interference occurs and the output power becomes suppressed for increased control power. Now consider a pair of polarization switch units (PSW1 and PSW2) combined in a similar way as a flip-flop elec- trical circuit. Then there occur two stationary operations: one is the state where PSW1 is high and PSW2 is low, the other is the state where PSW2 is high and PSW1 is low. By the proper triggering these two stationary states can be switched between each other. Although the experimental demonstration was performed with relatively slow pulses (150 ns) the authors expect the scheme can be extended to the 10 Gbit/s regime considering the high-speed physical processes involved.
4.5 Photodetectors
As shown in previous sections, it is not possible to manipulate large-capacity informa- tion flow only with optical technology, even though the advantage of ultrafast photonics has recently been recognized. The majority of information processing including compu- tation, storage, pre-and post-processing in communication systems, and routing relies on the VLSI technology. Therefore the devel- opment of high-speed photodetectors in the wideband photonic system is important. In the conventional pin photodiode, there is a trade-off relationship between speed and quantum efficiency, because thinning the absorption layer is favorable for shortening transit time, while only some of the incident photons are absorbed. To overcome this diffi- culty, waveguide-type pin photodiodes were developed. 821 By edge coupling the quantum efficiency increases to nearly 100% and a cutoff frequency exceeding 50 GHz were real- ized. Another requirement is the heavy-duty operation characteristics. When the photode- tector is used together with the optical pream- plifier, the input power levels may be relatively high. In conventional photodiodes the carrier accumulation at the active region substan- tially deteriorates the frequency characteristics. Ishibashi et al. of NTT invented uni-traveling carrier photodiodes (UTC-PDs), for which carrier accumulation at input power levels of 50 mW is negligible and cut-off frequency of over 100 GHz is not deteriorated. 83.841 This was made possible by introducing an asym- metric heterostructure, so that only electrons are mobile and holes are blocked. UTC-PDs are also very useful in microwave photonic applications for the optical generation of high power and high frequency electromag- netic wave. Indeed, it is advantageous for the generation of higher power sub-THz signals, whose frequency range is beyond the cutoff of the device.
5. Fusion of Optical and Mi-
crowave Technologies: Micro-
wave Photonics
Although the ultrafast nature of photo- nics provides scientifically and technologically attractive opportunities as described above, the wavelength ranges for efficient frequency conversion are limited because ultrashort optical pulses, intense pulse source and trans- parent nonlinear media or frequency conver- sion methods are not always available. That is, ultrafast photonics are not provided to all the wavelength range of lightwave. However, there is a means of conferring the ultrafast nature of photonics to its base band, which is one of the most important frequency bands, through fast interactions between light- waves and electromagnetic waves. Here, the frequencies of interest range from GHz to THz. Also, frequency-down-conversion tech- niques are effective. Together with advantages in the low-loss and high-bandwidth fiber- optic transmission, the technical field based upon the fusion of photonics and microwave technologies is growing and is called micro- wave photonics. 851 This technological field is expected to cultivate the interdisciplinary area between two major communication technolo- gies: fiber-optic and wireless communications. Recently, another related technical field is also growing rapidly, which is called THz photonics. This field has attracted considerable amount of interest for its application to environmental sensing, 86 security, 87 ) and important futuristic frequency resources. Another notable aspect of microwave photonics is high-speed optical signal genera- tion on the basis of lightwave modulation, that is, optical sideband syntheses by micro- wave or millimeter-wave signals. Here, the optical modulation techniques are key issues: representative techniques include directly modulated semiconductor lasers, EO modula- tors, EA modulators and integrated modules of modulators and lasers. The frequency multi- plication techniques are important when one wants to achieve higher frequency compo- nents beyond the device bandwidths. Yama- moto and Kawanishi of NTT have demon- strated an optical local oscillator, with which one can generate a wide range of electromag- netic waves quasi-continuously: 100 GHz– 1 THz. They use the fiber-optic nonlinearity and the SC generation to multiply the avail- able frequencies.881 Kawanishi of NICT has demonstrated another method to multiply the input frequency by electro-optical modulation. The multiple phase modulation of reciprocally traveling light waves within an integrated LN modulator gives rise to the frequency multipli- cation. 899 What is particularly interesting here is the fact that the advanced modulation formats developed in the field of microwave tech- nology are being applied in a practical manner to the fiber-optical transmission of digital signals currently. 901 The phase shift keying techniques are good examples. This recent trend has been supported by the continuous development of integrated high speed LN modulators. 91 ) One of the high-end techniques in micro- wave photonics is extremely broad band wire- less communication. A wireless transmission system with 10 Gbit/s has been demonstrated by Hirata and Nagatsuma et al. of NTT, where an optically generated 120 GHz carrier is utilized.921 Here, the high-bandwidth nature of photonics gives rise to the successful trans- mission of signals with the highest bit rate in air (Fig. 6). Sasai et al. of Panasonic have provided a representative microwave photo- (Sasai et al. 82 ). nics approach in a rather practical manner, where the frequency range is not necessarily high but indispensably important in the real world. 938 The essence of the work is the flex- ibility provided to a wireless LAN system by the transparency of photonic links (Fig. 7). Indeed, the flexible nature and light-weight properties of fiber-optic radio wave links have attracted considerable attention from the commercial viewpoints. Through radio-on-fiber techniques, problems in radio-wave-insensitive areas such as underground shopping areas, subways, submarine tunnels and high-rise hotel build- ings can be solved. The combination of microwave photonics and nonlinear photonics has led to a consid- erably attractive scheme. By this technique the locking of absolute optical phase to the highest-stable microwave signal was realized. This has already been applied to ultra-accurate clocks. 940 Let us look forward to further devel- opment.
6.  Optical Probing
The electrooptic (EO) effect is indispens- able for optical modulation techniques. It is also very useful as the ultrafast optical measurement technique. The technique called EO sampling (EOS) was first demonstrated by Valdmanis et al. 951 and is now a standard method for the characterization of high speed devices and circuits with 100 GHz operation frequencies or more. 951 The introduction of the concept of desk-top style EO probing system had fairly intense impact was brought on its practical usage. 97-99) Such as system is more convenient than the conventional EO sampling system of laboratory sizes. As its extension, a 300 GHz EO network analyzer was demon- strated by Sahri and Nagatsuma. 100) Another trend has been brought about by optical probes based on pig-tailed LN modula- tors with probing metal elements attached. 101 ) These are attractive for far field measurement of electromagnetic wave radiation and, hence, are expected to be effective for the electro- magnetic compatibility problems. However, the presence of metal parts significantly degrades the noninvasive property, which is one of the most advantageous features of optical probing techniques and is indispensable for precise analyses of very near field distribution. Almost contiguous measurement is advantageous for the diagnoses of microscopic-scale integrated circuits of high frequency and for highly dense packages. This is possible only when a metal- free nature is conferred to optical probes. In order to minimize the electromagnetic inva- siveness, the use of a fiber-edge electrooptic probe is effective, where an EO crystal is glued to the edge of the optical fiber. 1021 The optical fiber works not only as an optical waveguide but also as a very fine metal-free EO crystal holder, providing real metal-free circumstances and accessibility to densely packaged parts on a crowded printed-circuit board. 103 ) Maqneto-optical (MO) probe is new compared with EO probes. The difference between them may originate from the fact that the magneto-optical process is gener- ally slow, which is correct if one considers the magnetic domain inversion phenomena only. Yamazaki et al. of the authors' group at University of Tokyo noticed the fast nature of rotation magnetization (RM), 1049 and imple- mented the first RM-based optical probe with a bandwidth of more than 10 GHz. 105) Here, the fiber-edge optical probe scheme is again utilized. Figure 8 shows the magnetic field images obtained above a GHz band-pass filter made of microstrip lines. 106) The standing wave nature of GHz current is clearly seen, with which GHz current profiler action has been demonstrated. Note that the MO probing scheme is complementary with the EO probing scheme and that the simultaneous combina- tion of these could lead to clarification of local impedance in a complicated circuit. Further- more, highly sensitive MO probes are effec- tive to measure the variation of current distri- butions in low impedance patterns such as ground/power lines. 107)
7. Summary and Future Pros-
pects
In summary, the progress in the applica- tion of ultrafast photonics to information tech- nologies is reviewed with special emphasis of the activities in Japanese research institutions. Broadband optical communication system trials employing the combination of optical time division multiplexing (OTDM) and wavelength division multiplexing (WDM) have become real- istic, with electrical–optical interface data flow of 40 Gbit/s and total throughput reaching 1 Tbit/s. To meet the demand of routing high- bit-rate data flow without O–E conversion, optical header recognition and rerouting tech- nologies are investigated thoroughly. As pulse sources for OTDMWDM optical communica- tion, both optically stabilized mode-locked semiconductor lasers and supercontinuum generators using the nonlinearity of disper- sion flattened fibers are promising. A variety of all-optical switches for demultiplexing and other all-optical signal processing have been developed, including semiconductor optical amplifier (SOA) switches, cascaded second- order frequency converters using periodically poled lithium niobate (PPLN), semiconductor saturable absorber switches, organic dye satu- rable absorber switches, and bistable semicon- ductor laser switches. As a measurement probe, ultrashort pulse generation and control technology are important. By cascaded soliton compression, the authors realized a semiconductor-laser- based pulse source of 20 fs width. Employing high speed uni-traveling carrier-photodiodes (UTC-PDs), novel signal processing procedures for microwave/photonic systems are devel- oped. Electro-optic sampling technologies were extended to high-speed electromag- netic field imaging technologies. Using a small magneto-optical probe chip on the fiber end, the author was successful in the rf magnetic field imaging of microwave components at 10 GHz. These measurement technologies are important in precisely controlling the equip- ment of opto-wireless systems. It was shown that there is steady progress in the research and development of ultrafast photonics for information systems. A number of pico- and femto-second devices are close be realized as commercial products, and novel devices and promising ideas are contiuously added. Thus, the ultrafast photonics field is providing technical seeds to many information handling areas as powerful and versatile tools. However, we are still staying at some distance from the point where the technology is widely deployed to meet real world demands. Impor- tant issues should be investigated from stand- points such as reliability physics, mass produc- tivity, interface compatibility, and system flexibility. 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Tsuchiya: Jpn. J. Appl. Phys. 41 (2002) L864. 106) E. Yamazaki, S. Wakana, H. Park, M. Kishi and M. Tsuchiya: IEICE Trans. Electron. E86-C (2003) 1338. 107) M. Iwanami, S. Hoshino, M. Kishi and M. Tsuchiya: in preparation. Takeshi Kamiya studied Applied Physics at the University of Tokyo and received his B.E., M.E., and Ph. D. degrees in 1963, 1965, and 1970, respectively. From 1965 till 2000 he served at the University of Tokyo as research assistant, lecturer, associate professor, and professor. In 2000 he was awarded professor emeritus from University of Tokyo. Since 2000 he is affiliated as professor with National Institution for Academic Degrees and Uni- versity Evaluation (NIAD-UE), where he acts as Dean of Faculty for the Assessment and Re- search of Degrees since 2004. His current research interests are ultrafast photonics, semi- conductor optoelectronics and engineering education. Masahiro Tsuchiya received his B.E., M.E., and Ph. D. degrees from the University of To- kyo, Japan, in 1983, 1985, and 1988, respectively, all in electronic engineering. He was a postdoctoral fellow with the University of California at Santa Barbara from 1988 to 1990, and a research staff member at the Research Development Corporation of Japan from 1990 to 1991. In 1991, he joined the Department of Electronic Engineering, the Univer- sity of Tokyo as a lecturer, and became an associate professor in the same department in 1993. From 1996 to 1997, he spent his sabbatical leave as a visiting researcher at Bell Laboratories, AT&T/Lucent Technologies, Holmdel, NJ. In fall 2003, he moved to the Com- munications Research Laboratory, which is presently the National Institute of Information and Communications Technology (NICT). Now, he belongs to the Photonic Information Technology Group of NICT as its group leader. His current research interests are on ultrafast photonics, microwave photonics, nonlinear photonics, silicon photonics, optical networks, resonant tunneling devices, and applications based on optoelectoronic devices, circuits and systems. He is also interested in the social and environmental aspects of information and communications technology. Dr. Tsuchiya is a member of the Japan Society of Applied Phys- ics and the Institute of Electronics, Information and Communication Engineers, Japan. He also belongs to IEEE LEOS and MTTS.