Development of ultrafast optical interfaces that can operate in sub-terahertz region is important to apply superconducting electronic devices to the high-end systems. We have performed several fundamental researches to realize the ultrafast optical input interface for superconducting electronic devices. Firstly, we observed optical response of amorphous Ge thin films, and the results indicated that an amorphous Ge photoconductive switch could stably operate in a terahertz frequency range as an optical-to-electrical signal converter in the low-temperature region below Tc of YBCO. Next, we have fabricated optical-to-electrical signal conversion system with photomixing technique, and we have demonstrated the generation and the detection of high frequency signals over 50 GHz. Finally, we have observed optical responses of a Josephson vortex flow transistor under irradiation of femtosecond laser pulses, and the results suggeste that the device has high potential as an optical interface.

Ultrahigh-speed fiber lasers operating at up to 40 GHz offer a clean longitudinal comb and a narrow linewidth. This makes them suitable for applications including optical comb generation, ultrahigh-speed optical pulse transmission including PSK, and as opto-microwave oscillators. In this paper, we describe recent progress on ultrafast fiber lasers and their applications to optical metrology.

Ultrafast all optical switching using pulse trapping by 100 fs ultrashort soliton pulse across zero dispersion wavelength is investigated. The characteristics of pulse trapping are analyzed both experimentally and numerically. Using the pulse trapping, 1 THz ultrafast all optical switching is demonstrated experimentally. Arbitral one pulse is picked off from pulse train. Pulse trapping for CW signal is also demonstrated and ultrashort pulse is generated by pulse trapping. From these investigation, it is shown that ultrafast all optical switching up to 2 THz can be demonstrated using pulse trapping.

Using ultrafast nonlinear-optical response of organic dye films, a train of picosecond optical pulses can be converted into a space pattern of a mm scale. As applications of this technique we demonstrate a single-shot multichannel optical switching for 1 Tbit/s pulse trains, and a timing jitter suppression of pulse trains using a control system with femtoseconds time resolution.

We report on several photoconductive (PC) geometries for the generation of both guided-wave and free-space terahertz (THz) waveforms. It is found that guided-wave THz electrical waveforms can be produced through both PC self-switching and frozen wave generation--eliminating the need for an ultrashort carrier lifetime in the semiconductor substrate. The concept of PC switching is also applied to the generation of free-space THz waveforms, and various ZnSe detectors are investigated as potential electro-optic THz sensors.

This review paper addresses an emerging aspect of the relationship between optics and microwave electronics: the application of short pulses of laser light to the sensing and measurement of continuous-wave microwave fields. In particular, very short duration optical pulses can take on the role of ultrafast sampling gates within the framework of the electro-optic sampling technique in order to realize unprecedented temporal resolution, measurement bandwidth, and probing flexibility. As a result, in numerous instances electro-optic sampling has been demonstrated, primarily within the research laboratory, to be an effective tool in the field of diagnostic testing and the determination of the electrical characteristics of microwave components. Recently, with the emergence of new applications such as microwave electric-field mapping in wireless and radar environments, and as the ultrafast time domain has gained in importance for the area of optical telecommunications, added attention has been directed to electro-optic sampling. Herein, an abbreviated historical perspective of the history of electro-optic field mapping is presented, along with the fundamental concepts that are utilized in the technique. The effectiveness of an optical-fiber-mounted electro-optic probe in a scanning electric-field-mapping system is highlighted in several diagnostic measurements on microwave and millimeter-wave antenna arrays, and a combined electric-field and thermal-imaging capability is also introduced.

A simple system configuration was used to generate transform-limited optical pulses at 160 Gbit/s in the sub-picosecond range (625 fs). Pulse compression was achieved by broadening the spectrum using supercontinuum generation followed by a linear frequency chirping compensation.

Recent activities on ultrafast photonic device technology development in the Femtosecond Technology Project sponsored by NEDO are introduced. Topics include management and control of the higher order dispersions of optical fibers, ultrafast mode-locked semiconductor laser, symmetric Mach-Zehnder type all-optical switch, ultrafast serial-to-parallel signal converter and sub-picosecond wavelength switch. Challenges towards novel ultrafast switching material systems are also described.

Fourier synthesis of ultrafast optical-pulse trains was demonstrated using a simplified experimental configuration consisting of three independent continuous-wave lasers and a semiconductor optical amplifier (SOA) used as a four-wave mixer. When the three lasers were phase-locked, ultrafast optical-pulse trains were successfully generated at repetition frequencies ranging from 504 GHz to 1.8 THz with high waveform stability.

A newly developed hybrid-integrated Symmetric Mach-Zehnder (HI-SMZ) all-optical switch is reported. For integration, we chose the Symmetric Mach-Zehnder (SMZ) structure rather than the Polarization-Discriminating Symmetric Mach-Zehnder (PD-SMZ) structure which is similar to SMZ but more often used in experiments using discrete optical components. We discuss advantages and disadvantages of SMZ and PD-SMZ to show that SMZ is more suitable for integration. We also discuss about the use of SOAs as nonlinear elements for all-optical switches. We conclude that, although the ultrafast switching capability of SMZ is limited by the gain compression of SOAs, the very low switching energy is more important for practical devices. We then describe the HI-SMZ all-optical switch. This integration scheme has advantages which include low loss, low dispersion silica waveguides for high speed operation and ease in large scale integration of many SMZs with other optical, electrical, and opto-electrical devices. We show that a very high dynamic extinction ratio is possible with HI-SMZ. We also examine HI-SMZ with 1 ps pulses to show its ultrafast capability. Finally, we describe a 168 to 10.5 Gbps error-free demultiplexing experiment which is to our best knowledge the fastest experiment with an integrated device.

A novel electrical gating circuit is proposed for ultrafast applications in electronics. The circuit employs a two-conductor coupled line, and does not have any active devices such as transistors or diodes. Hence, the ultimate speed of the circuit is limited only by the cutoff frequency of the lines employed. The authors describe the circuit theory and discuss the results of experiments that involved ultrafast measurement using electro-optic sampling techniques. The latter suggests the potential of the circuit to achieve the gatings of at least 80-Gbit/s.

This paper reports on time-space conversion-based differential processing of optical signals using a high-resolution arrayed-waveguide grating (AWG) and a spatial filter at a wavelength of 1.55 µm. We clarify the advantages of the AWG device and show where it is applicable. In order to reduce loss at the spatial filter, we propose a new phase-only filter that functions as a differential filter. The difference between the exact differential filter and the proposed phase-only filter is calculated theoretically. We confirm experimentally that the optical pulse can be differentiated by the proposed filter. For application of differential processing, we also proposed a phase modulation to amplitude modulation (PM-AM) conversion and demonstrated the PM-AM conversion at 10 Gbit/s signals using a PSK-non-return-to-zero (NRZ) format.

This paper reports on time-space conversion-based differential processing of optical signals using a high-resolution arrayed-waveguide grating (AWG) and a spatial filter at a wavelength of 1.55 µm. We clarify the advantages of the AWG device and show where it is applicable. In order to reduce loss at the spatial filter, we propose a new phase-only filter that functions as a differential filter. The difference between the exact differential filter and the proposed phase-only filter is calculated theoretically. We confirm experimentally that the optical pulse can be differentiated by the proposed filter. For application of differential processing, we also proposed a phase modulation to amplitude modulation (PM-AM) conversion and demonstrated the PM-AM conversion at 10 Gbit/s signals using a PSK-non-return-to-zero (NRZ) format.

We experimentally demonstrate the ultrafast and high-repetition capabilities of a polarization-discriminating symmetric Mach-Zehnder (PD-SMZ) all-optical switch. This switch, as well as an original symmetric Mach-Zehnder (SMZ) all-optical switch, is based on a highly efficient but slowly relaxing band-filling effect that is resonantly excited in a passive InGaAsP bulk waveguide. By using a mechanism that cancels out the effect of the slow relaxation, ultrafast switching is attained. We achieve a switching time of 200 fs and demultiplexing of 1.5 Tbps, showing the applicability of the SMZ or PD-SMZ all-optical switches to optical demultiplexing of well over 1 Tbps for the first time. High-repetition capability, which is another important issue apart from the switching speed, is also verified by using control pulses at a repetition rate of 10.5 GHz. We also discuss the use of nonlinearity in a semiconductor optical amplifier to further reduce the control-pulse energy.

We experimentally demonstrate the ultrafast and high-repetition capabilities of a polarization-discriminating symmetric Mach-Zehnder (PD-SMZ) all-optical switch. This switch, as well as an original symmetric Mach-Zehnder (SMZ) all-optical switch, is based on a highly efficient but slowly relaxing band-filling effect that is resonantly excited in a passive InGaAsP bulk waveguide. By using a mechanism that cancels out the effect of the slow relaxation, ultrafast switching is attained. We achieve a switching time of 200 fs and demultiplexing of 1.5 Tbps, showing the applicability of the SMZ or PD-SMZ all-optical switches to optical demultiplexing of well over 1 Tbps for the first time. High-repetition capability, which is another important issue apart from the switching speed, is also verified by using control pulses at a repetition rate of 10.5 GHz. We also discuss the use of nonlinearity in a semiconductor optical amplifier to further reduce the control-pulse energy.

By sacrificing approximately ten percent of the transmission speed, ultra-high speed optical time division multiplexed network can be fully operatable by the use of currently available electrical switches. The network utilizes dispersion managed quasi-solitons and transmits TDM packet which comprises of ATM cells that are introduced from a gateway through bit compression to match to the ultra-high speed traffics. The network can provide flexible bandwidth and bit on demand at burst rate of the maximum LAN speed.

Recent advances in solid-state laser technology and ultrafast optics led to the generation of optical pulses as short as 5 femtoseconds with peak powers up to the subterawatt level from a compact kHz-repetition-rate all-solid-state laser. This source significantly pushes the frontiers of nonlinear optics. Exciting new possibilities include the investigation and exploitation of reversible nonlinear optical processes in solids at unprecedented intensity levels, the development of a compact laser-driven coherent soft-X ray source at photon energies near 1 keV, and the generation of attosecond xuv pulses. First, a brief review of recent milestones in the evolution of ultrafast laser technology is given, followed by a description of the high-power 5-fs source. The rest of the paper is devoted to applications in previously inaccessible regimes of nonlinear optics. We demonstrate that wide-gap dielectrics resist intensities in excess of 1014 W/cm2 in the sub-10 fs regime and the extension of high-harmonic generation in helium to wavelengths shorter than 2. 4 nm (Eph > 0. 5 keV).

The electric field intensity of the THz radiation from YBCO thin films excited by ultrashort laser pulses has been enhanced by a factor of 3 using a-axis oriented films instead of c-axis oriented ones used previously under the same excitation conditions. This corresponds to the enhancement of a factor of 10 for the radiation power. From the transmittance measurements of the millimeter wave for a-and c-axis oriented films, the origin of the enhancement is attributed to the increased fraction of the THz electromagnetic wave power transmitted from the YBCO film to free space. This result indicates that the utilization of the anisotropic properties of high-Tc superconductors is effective to enhance the radiation power.

New THz radiation devices made of high-Tc superconductors are fabricated and their characteristics are studied in detail. Ultrashort electromagnetic pulses with 0.5 ps width have been radiated into free space from current biased devices made of superconducting YBa2Cu3O7 (YBCO) films by exciting with femtosecond laser pulses. The Fourier spectrum of them extends up to 3 THz. The radiation mechanism is ascribed to the ultrafast supercurrent modulation by the optical pulses. The THz waveform is analyzed using rate equations describing the relaxation of photoexcited quasiparticles. By the improvement of the device structure and the collecting optics, the radiation power can be increased up to 0.5 µW. A new type THz radiation from YBCO films under an external magnetic field without a transport current is also reported.

Ultrafast MR imaging (e.g., echo-planar imaging) acquires all the data within only several tens of milliseconds. This method, however, is affected by static magnetic field inhomogeneities and chemical shift; therefore, a high degree of field homogeneity and water and fat signal separation are required. However, it is practically impossible to obtain an homogeneous field within a subject even if in vivo shimming has been performed. In this paper, we describe a new ultrafast MR imaging method called Ultrafast Single-shot water and fat Separated Imaging (USSI) and a correction method for field inhomogeneities and chemical shift. The magnetic field distribution whthin the subject is measured before thd scan and used to obtain images without field inhomogeneity distortions. Computer simulation results have shown that USSI and the correction method can obtain water and fat separated images as real and imaginary parts, respectively, of a complex Fourier transform with a single-shot scan. Image quality is maintained in the presence of field inhomogeneities of several ppm similar to those occurring under practical imaging conditions. Limitations of the correction method are also discussed.