This paper reviews the prospects for "femtosecond technology" which will provide an innovative and fundamentally new industrial technology based on ultrafast electronics and quantum optics occurring in the femtosecond time domain. The outline of the femtosecond technology project sponsored by the Ministry of International Trade and Industry (MITI) is also reviewed.

The characteristics of several femtosecond solid-state laser systems are described illustrating the diversity of the operational parameters of these lasers. The systems include Pr:YLF, Cr:LiSAF, Cr:Forsterite and Cr:YAG, with wavelength of operation from the visible to the near infra-red. Particular emphasis is placed upon compact, efficient pumping schemes, all-solid-state diode-pumped femtosecond oscillator configurations and newly configured, highly-efficient, tunable, femtosecond lasers pumped by high power fibre lasers.

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 (E_ph > 0. 5 keV).

This paper summarizes our recent efforts in modelocking Ti:sapphire lasers with semiconductor saturable absorber mirrors (SESAMs). We present the shortest optical pulses ever generated directly from a laser. The modelocking build-up time (T _BU) of 60 µs is, to our knowledge, the shortest reported for a passively modelocked KLM laser to date.

We extend the use of a saturable Bragg reflector to modelock a NaCl:OH^- color center laser, producing pulses adjustable from 200 fs to 2 ps, and tunable from 1. 499 µm to 1. 535 µm, with the minimum pulsewidth near 1. 51 µm. The laser is self-starting, requires no dedicated dispersion compensating optics, and maintains a highly stable, nearly transform limited output pulse train with up to 150 mW average output power.

The novel application potential of mode-locked laser diodes (MLLDs) in ultrafast optical signal processing in addition to coherent optical pulse generation is described. As the most fundamental function of MLLDs, we show that the generation of ultrashort (2 ps) coherent optical pulses with low timing jitter (<0. 5 ps) at precisely controlled wavelength and repetition frequency can be achieved by employing a rigid module configuration for an external-cavity MLLD. We then discuss new aspects of MLLDs which are functions of ultrafast all-optical signal processing such as optical clock extraction and optical gating. All-optical clock extraction is based on the timing synchronization of MLLD output to the injected optical data pulse. When the passive mode-locking frequency of an MLLD is very close to the fundamental clock pulse frequency of optical data, the former frequency is pulled into the latter frequency by optical data injection. We show that same-frequency and subharmonic-frequency optical clock pulses can successfully be extracted from optical data pulses at bit rates of up to 80 Gbit/s with very simple configurations and very low excess timing jitter (<0. 1 ps). On the other hand, optical gating is due to absorption saturation and the following picosecond absorption recovery in a saturable absorber (SA) in an MLLD structure incorporating optical gate-pulse amplification. Here, MLLDs are anti-reflection coated and used as traveling wave devices instead of laser oscillators, and small saturation energy (<1 pJ) and ultrafast recovery time (<8 ps) are demonstrated. By combining all these MLLD functions, we successfully demonstrated an experiment with 40- to 10-Gbit/s all-optical demultiplexing processing.

In this paper, we describe the properties of an external cavity modelocked semiconductor laser with a tunability of wavelength, pulse width and repetition rate. This modelocked laser generates optical pulses with pulse widths down to 180 fs and with repetition rates up to 14 GHz in a 120 nm wavelength range near 1. 55 µm or 1. 3 µm. The generated pulses are close to the transform limit and are therefore suitable for very high speed communication systems. In addition to the tunability, this pulse source is a compact and mechanically stable device. We report on two applications of this pulse source in optical time division multiplexing experiments. In the first example the modelocked laser is used as an all-optical clock recovery. In the second example the modelocked laser was used to characterize an interferometric switch by pump-probe experiments.

This paper describes short pulse generation at over 40 GHz using monolithic mode-locked lasers integrated with electroabsorption modulators. The electroabsorption modulator using strained-InGaAsP multiquantum wells provides a pulse shortening gate at a high-repetition frequency. Pulse generation around 4 ps has been realized at a repetition frequency of 43. 5 GHz. Pulse compression using a 1. 3 µm single mode fiber is performed and a 0. 87 ps pulse is obtained.

We propose a mechanically tunable passively mode-locked semiconductor laser with a high repetition rate using a simple configuration with a moving mirror located very close to a laser facet. This scheme is demonstrated for the first time by a novel micromechanical laser consisting of an InGaAsP/InP multisegment laser with a monolithic moving micro-mirror driven by an electrostatic comb structure. The main advantage of this laser is the capability of generating high-quality mode-locked pulses stabilized by a phase-locked loop (PLL) with low residual phase noise in a wide repetition-rate tuning range. This paper describes the basic concept and tuning performances utilizing the micromechanical passively mode-locked laser in 22-GHz fundamental mode-locking and in its second-harmonic mode-locking.

We propose a novel InGaAsP semiconductor laser which theoretically exhibits a high differential gain. The proposed semiconductor laser contains an asymmetric double quantum well structure as the active region. The differential gain enhancement invokes resonant tunneling of heavy holes in the asymmetric double quantum well structure, which takes place on the way of carrier injection process. The proposed laser is expected to be far more efficient in reducing pulse width and spectral broadening (chirping) than conventional multiquantum well lasers when driven by the gain switching method.

The timing jitter of the optical pulse from a gain-switched laser diode is reduced by CW light injection. The reduction ratio of the timing jitter is 5. 5. The pulse width was compressed by a nonlinear optical loop mirror to a pedestal-free optical pulse with a pulse width of 420 fs.

Electroabsorption modulators are high speed devices that are rapidly being commercialised and finding applications in a number of areas, particularly in telecommunications. A CW laser diode modulated by an electroabsorption modulator constitutes an extremely stable, robust and simple source of high quality, high repetition rate ultrashort optical pulses. In this paper we describe the capabilities and limitations of such pulse sources, and present nonlinear pulse compression and manipulation techniques that allow one to overcome these limitations. We also present the design of a new class of comb-like dispersion-profiled fibre compressor. Such a compressor is easily fabricated from commercially available fibres and represents a simple yet powerful way of extending the range of pulse durations available. As the electroabsorption modulator is essentially a high speed switch it is also applicable to optical processing problems, and we report the application of such a device to demultiplexing.

Optimization of InGaAs/InAlAs multiple quantum well structures for high-speed and low-driving modulation, as well as polarization insensitivity and low chirp, was investigated as a function of well thickness and strain magnitude. As a result, very short optical pulses with 4-6 ps was obtained using a low driving-voltage (<2. 0 V_pp) electroabsorption modulator module operating at a 40-GHz large signal modulation. Small chirp operation for low insertion loss (<8 dB from fiber-to-fiber) with prebias was also demonstrated and the product of the pulse width and the spectral width was estimated to be 0. 39 for a 5 ps pulse width that is nearly transform-limited.

In this paper we review the stretched-pulse principle and discuss its inherent advantages for ultrashort pulse generation and transmission. An analytic theory of the stretched-pulse fiber laser is presented and shown to be in good agreement with experimental results. An extension of the stretched-pulse theory is applied to both fiber lasers and dispersion-allocated soliton transmission and then compared to numerical results. We also discuss the design and operation of an environmentally stable stretched-pulse fiber laser.

The dependence of the output characteristics of a regeneratively and harmonically FM mode-locked erbium-doped fiber laser on intracavity dispersion have been investigated by changing the group velocity dispersion (GVD) of the fiber. It is shown that a stable pulse train can be obtained only when the GVD of the cavity is anomalous in the presence of self-phase modulation (SPM). The shortest pulse obtained was 2. 0 ps at a repetition rate of 10 GHz.

The effects of forced phase modulation (FPM) and self phase modulation (SPM) in dispersion tuned fiber lasers (DTFL) are examined. We show that FPM, such as chirp in the modulator, plays an important role in the pulse shaping because of the important role of dispersion in the cavity. In particular, compared to the case of zero FPM, significant pulse shortening can be obtained by using up-chirp modulation. The results suggest that modulators with large chirp parameters are desirable for DTFLs. When SPM is introduced, the pulse shapes differ greatly depending on the direction of the FPM. Significant deviations from Gaussian profiles are observed.

This paper analyzes pulse characteristics of actively mode-locked fiber lasers by including the group-velocity dispersion and the Kerr nonlinearity of the fiber, both of which have not been taken into account in the conventional theory of mode locking. We show that chirped sech pulses are generated from nonlinear and dispersive fiber lasers. By considering the stability of the laser, we also derive design rules for the generation of ultra-short pulses.

We report on unforeseen results on soliton generation in mode-locked fiber lasers beyond the limit imposed by soliton resonances. We show that long (250 km) and highly dispersive (β₂=-20 ps²/km) fiber cavities permit to rule out the phase matching condition for the shedding of the soliton energy to the continuum with a consequent improvement in stability for the propagating soliton. The experiments were performed either with an erbium-doped or Raman-pumped fiber laser and the resulting widths of the generated solitons widths were always in the range 2. 88. 8 ps.

We investigate the relaxation oscillation characteristics of an actively mode-locked fiber laser and a novel stabilizing method of the laser theoretically and experimentally. The stabilizing method controls cavity length to suppress the rf power of the relaxation oscillation frequency of the laser output, and can directly monitor the stability of the laser to ensure the most stable operation. With this method, the rf power ratio between mode-locking frequency and the background noise can be kept to more than 70 dB, and highly stable transform-limited pulse generation is achieved. Bit-error-free operation at 6. 3 GHz over 10 hours is successfully demonstrated. The stability of the center wavelength of the laser output and the required accuracy of cavity control for high-speed laser operation are also discussed.

We study nonlinear pulse propagation in an optical transmission system with dispersion compensation. This is particularly important for designing an ultra-fast long-haul communication system in the next generation. There exists a quasi-stationary pulse solution in such a system whose width and chirp are rapidly oscillating with the period of dispersion compensation. This pulse also has several new features such as enhanced power when compared with the soliton case with a uniform dispersion and a deformation from the sech-shape of soliton. We use the averaging method, and the averaged equation to describe the core of the pulse solution is shown to be the nonlinear Schrodinger equation having a nontrapping quadratic potential. Because of this potential, a pulse propagating in such a system eventually decays into dispersive waves in a way similar to the tunneling effect. However in a practical situation, the tunneling effect is estimated to be small, and the decay may be neglected.

The propagation of solitons in a dispersion managed link can be mainly modeled with the evolution of two parameters γ and C, related to the spectral width and the chirp. Steady propagations are shown to be possible if the average dispersion lies in the anomalous domain. With the same conditions, periodical propagations are both theoretically and experimentally demonstrated. With the help of a perturbation theory, the jitter and the signal to noise ratio are theoretically evaluated. The latter is experimentally shown to be the low power limit of terrestrial systems based on non dispersion shifted fiber. Finally, wavelength and power margins of a single channel 20 Gbit/s soliton transmission over 11 amplifier spans of 102 km show that a 400 Gbit/s Wavelength Division Multiplexed transmission could be envisaged over the same distance.

The reduction of Soliton-soliton interaction to stabilize the soliton pulse propagation in the periodic dispersion-compensated standard fiber system using optical bandpass filter has been investigated by numerical simulation, and experimentally 10 Gbit/s soliton transmission was realized without fine tuning dispersion management over 5700 km, using appropriate optical bandpass filters and polarization scrambler.

If the fiber dispersion of soliton transmission line is optimized, the amplifiers output power becomes almost constant for different amplifier spacing and pulse width. Numerical simulations indicate the optimal dispersion can be determined, as the ratio of amplifier spacing to dispersion length is about 0. 8 for uniform dispersion line.

We examine effects of higher order dispersion and optical non-linearity, including linear attenuation on intense short pulses propagating along solid and Noble-gas-filled wave-guides. In order to simulate realistic pulse and spectral shapes, we have taken into account quadratic and cubic dispersion as well as Raman (solid wave-guide) and shock nonlinear terms in the generalized Schrodinger equation of propagation. The phonon oscillation corresponding to 13. 3 THz in SiO₂ gets superimposed on to the exit pulse in the normal dispersion regime of solid fiber for lengths exceeding 6L_DL_NL. The super continuum spectrum from hollow Noble-gas-filled fibers shows remarkably deep modulations.

An ultra-broadband optical parametric amplification can be attained by a noncollinear phase-matching. The group-velocity matching of the signal and idler reduces the signal-pulse width to 14-fs in an optical parametric amplifier based on a β-BaB₂O₄ crystal pumped by a second harmonics of a Ti: sapphire regenerative amplifier. This simple novel method shows the potential light source of a tunable sub-10-fs pulse in a visible region.

The effect of sampling-pulse pedestals, generated by pulse compression, on the temporal resolution in electro-optic (EO) sampling is studied both theoretically and experimentally. Analysis is made on how the pedestals degrade a measurement bandwidth and a temporal waveform. Based on the analysis, a practical guideline on the suppression of pedestals is also given. Gain-switched laser diode (LD) pulses adiabatically soliton-compressed using a dispersion decreasing fiber are used to confirm the theoretical results, and are successfully applied to high-temporal-resolution (>100 GHz) EO sampling measurements.

We demonstrate an electrooptic synthesis technique for generating arbitrarily shaped short optical pulses from a CW narrow linewidth laser. For the optical pulse shaping, a large-amplitude electrooptic phase modulator is specially fabricated by employing the quasi-velocity-matching. The phase modulated light having sidebands as wide as 1 THz is separated and phase-only-controlled spatially by a liquid crystal modulator array. After composing the light by using a grating, nearly 1. 2 ps of Fourier-transform-limited optical pulses is obtained.

Widely tunable coherent terahertz (THz)-wave generation was successfully demonstrated based on the laser light scattering from the lowest A₁-symmetry polariton mode by using a Q-switched Nd:YAG laser pumping. This method exhibits multiple advantages like wide tunability (frequency: 0. 9-2. 2 THz), coherency and compactness of its system. In this paper, the general performances of this THz-wave generator, as well as the recent development of the system and its application are reported. Measurements of tunability, coherency, power, polarization, radiation angle, and divergence are shown. The cryogenic cooling of the crystal was performed in addition, and a more than one hundred times higher THz-wave output was observed. A spectroscopic application of our wave source is demonstrated by measuring the water vapor absorption.

The development of a far-infrared imaging system based on ultrafast THz time-domain spectroscopy has opened a new field of applications of femtosecond technology. We describe the principle of this new imaging technique and report recent progress to augment the possibilities of "T-ray" imaging. These include sub-wavelength-resolution near-field imaging and three-dimensional tomographic reconstruction of a samples refractive index profile.

This paper describes a low-skew clock distribution technique for multiple targets. An automatic skew compensation circuit, that detects the round-trip delay through a pair of matched interconnection lines and corrects the delay of the variable delay lines, maintains clock skew and delay from among multiple targets below the resolution time of the variable delay lines without any manual adjustment. Measured results show that the initial clock skew of 900 ps is automatically reduced to 30 ps at a clock frequency of up to 250 MHz with 60 ps of clock jitter. Moreover, they show that the initial clock delay of 1500 ps is cancelled and 60 ps of clock delay can be achieved. The power dissipation is 100 mW at 250 MHz.

This paper describes a method for analyzing active impedance, i. e. equivalent resistance and equivalent reactance, of a narrow-band transistor Colpitts crystal oscillator. This oscillator, employing an AT-cut resonator filter, has a very narrow-band width and an achievement of extremely low phase-noise characteristics is expected. The analysis proposed is based on an algebraic formula, which employs a nonlinear approximation for transistor g_m, and a simplified circuit model. Calculated results are compared with the experimental results in the frequency characteristics of the oscillator active impedance with changing the driving signal current. Good agreement between the calculation and experimental results shows that the proposed technique is suitable for designing Colpitts crystal oscillators with resonator filters. In addition we apply this technique to the analysis of dual-mode crystal oscillators.

The semiconductor lasers operating with self-sustained pulsation are under developing to be lasers which are less disturbed by the optical feedback from a surface of optical disk. Structures setting saturable absorbing regions utilizing the multi-layer configuration become popularly used for giving stronger pulsation. However, the quantum (intensity) noise in these lasers tends to be enhanced. The ridge stripe structure, of which almost self-sustained pulsation lasers consist, seems to give a leak current flowing along plane of the cladding region. Such leak current also increases the quantum noise. In this paper, theoretical calculations of operating characteristics, such as the self-sustained pulsation, the optical output, the quantum noise as well as the transverse filed profile, are theoretically analyzed by including the above mentioned several phenomena.

Recently fiber optic links have been applied to radio signal distribution networks and also to signal feeder networks for phased array antennas, because they are able to offer wide bandwidth for achieving the high bit-rates and large capacity needed in the multimedia age. In these networks, a great many modules are needed to convert optical signals to radio signals. In order to reduce the complexity and cost of these modules, direct optical control techniques, which inject optical signals directly into microwave circuits, are very attractive. Thus, this paper proposes a novel optical control technique using tunable inductance circuits. This technique employs direct illumination as a means of optically tuning the inductance. Since the inductance value is inversely proportional to the square of the transconductance, it varies widely when the FET is directly illuminated. With direct illumination, the measured inductance variation in an experimental inductance circuit built with Pseudomorphic AlGaAs/InGaAs/GaAs HEMTs is more than 20 % from 0.5 to 2 GHz. As an application, a direct optically controlled oscillator was fabricated. The measured optical tuning range of the oscillation frequency is more than 19 % with an output power of -51 dBm. This is a promising technique for a variety of devices, including optically controlled oscillators, filters, phase shifters, and active antennas.

We present a finite-difference time-domain (FD-TD) method with the perfectly matched layers (PMLs) absorbing boundary condition (ABC) based on the multidimensional wave digital filters (MD-WDFs) for discrete-time modelling of Maxwell's equations and show its effectiveness. First we propose modified forms of the Maxwell's equations in the PMLs and its MD-WDFs' representation by using the current-controlled voltage sources. In order to estimate the lower bound of numerical errors which come from the discretization of the Maxwell's equations, we examine the numerical dispersion relation and show the advantage of the FD-TD method based on the MD-WDFs over the Yee algorithm. Simultaneously, we estimate numerical errors in practical problems as a function of grid cell size and show that the MD-WDFs can obtain highly accurate numerical solutions in comparison with the Yee algorithm. Then we analyze several typical dielectric optical waveguide problems such as the tapered waveguide and the grating filter, and confirm that the FD-TD method based on the MD-WDFs can also treat radiation and reflection phenomena, which commonly done using the Yee algorithm.

It is shown from the Hilberts theory that if the real function Π(θ) has no zeros over the interval [0, 2π], it can be factorized into a product of the factor π⁺(θ) and its complex conjugate π^-(θ)(=). This factorization is tested to decompose a real far-zone field pattern having zeros. To this end, the factorized factors are described in terms of bicomplex mathematics. In our bicomplex mathematics, the temporal imaginary unit "j" is newly defined to distinguish from the spatial imaginary unit i, both of which satisfy i²=-1 and j²=-1.