Hiroyasu Sone

Generation of monocycle-like optical pulses using induced-phase modulation between two-color femtosecond pulses with carrier phase locking

Detailed numerical analysis of monocyclical optical pulse generation using induced-phase modulation between two-color carrier-phase-locked femtosecond pulses is presented. For the generation of a short pulse with a quasi-linear chirped white-continuum spectrum, it is found to be important that the large spectral broadening of each input pulse after fiber propagation is as similar as possible and the total spectral intensity of the synthesized wave at the fiber output is as homogenous as possible. As a result, it is shown that a 2.2-fs 1.3-cycle pulse is generated.

Proposal for Generation of a Coherent Pulse Ultra-Broadened from Near-Infrared to Near-Ultraviolet and Its Monocyclization

An approach for generation of an ultrabroad-band optical pulse (~550 THz width) is proposed, which is based on mutual induced phase modulation of three synchronized femtosecond pulses in a short fiber. These pulses having equidistant carrier frequencies and constant carrier-phase differences are obtained from the fundamental (ω01), the second-harmonic (2ω01) and the second-harmonic-pulse pumped parametric (3ω01/2) waves. These are generated under the phase-matched, frequency-conserved and gain-maximized conditions by one common femtosecond pulse. Computer analysis shows that chirp of the composite wave at the fiber output can be compensated for by a spatial phase modulator and a 1.76-cycle pulse is generated.

Finite-difference time-domain calculation with all parameters of Sellmeier's fitting equation for 12-fs laser pulse propagation in a silica fiber

In order to both experimentally and numerically investigate nonlinear femtosecond ultrabroadband-pulse propagation in a silica fiber, we have extended the finite-difference time-domain (FDTD) calculation of Maxwells equations with nonlinear terms to that including all exact Sellmeier-fitting values. We have compared results of this extended FDTD method with experimental results, as well as with the solution of the generalized nonlinear Schrodinger equation by the split-step Fourier method with a slowly varying-envelope approximation. To the best of our knowledge, this is the first comparison between FDTD calculation and experimental results for nonlinear propagation of a very short (12 fs) laser pulse in a silica fiber.

Extended Finite Difference Time Domain Analysis of Induced Phase Modulation and Four-Wave Mixing between Two-Color Femtosecond Laser Pulses in a Silica Fiber with Different Initial Delays

Experimental results of nonlinear propagation with the induced-phase modulation (IPM) effect of two-color 120 fs laser pulses with three different initial delays in a silica fiber were analyzed with finite-difference time domain (FDTD) calculations, including self-phase modulation (SPM), IPM, and degenerate four-wave mixing (DFWM) in the fused-silica fiber. Initial delay dependence by the FDTD qualitatively agrees with the previous experimental results. An ultrabroad spectrum induced by IPM mostly agrees with the previous experimental results that had the optimum initial delay. A Fourier transform of this spectrum yields a 4 fs optical pulse, which agrees with the previous experimental results. The spectral phases of two different pulses with three different initial delays are connected continuously. The IPM spectral phase obtained by the extended FDTD shows that the DFWM occurs. To the best of our knowledge, this is the first observation of DFWM with IPM by the FDTD simulation.

Characteristics of femtosecond supercontinuum generation in a short tapered fiber with a few micrometer diameter waist

A theoretical and experimental study of femtosecond supercontinuum generation in short tapered fibers with micrometer diameter waist has been carried out. In our numerical model, an extended nonlinear Schroedinger equation that involves the parameters of dispersion coefficient and effective core area varying along the length of tapered fiber is used to describe the evolution of intense femtosecond pulses centred at 850 nm. As a result, the supercontinuum is generated due to enhanced nonlinear effects in a narrow waist; the octave spanning spectra are found in an optimum structure with only short waist length of 8.0 mm and diameter of 2 μm for an input average power of 100mW. Experimental verification using a femtosecond pulse from a Ti: sapphire laser is then made and compared with the numerical simulations.

Numerical analysis of supercontinuum generation in a dispersion flattened /decreasing fiber

A flat and broad-band supercontinuum spectrum of soliton pulses in an optical fiber with dispersion flattened and dispersion decreasing characteristic along the axis was numerically analyzed using a split-step Fourier method and confirmed experimentally.

Single-Shot Phase-Shifting Digital Holography Based on the Spatial Carrier Interferometry and Its Tolerance Analysis

In digital holography (Kreis, 2005; Schnars & Jueptner, 2005), holograms are recorded using an image sensor such as CCD cameras, and reconstruction of images of the object is performed by a computer. From this, digital holography has following attractive features: A wet process for developing recording media is not required; qualitative evaluation is easy for three-dimensional images of objects; and focused images of three-dimensional objects at a desired depth can be instantaneously acquired without a mechanical focusing process. However, this technology suffers from lower resolution of image sensors than photographic materials, and therefore, object size to be recorded is restricted to be large and far from the image sensor in off-axis recording geometry. These limitations can be moderated by introducing the phase-shifting method (Schreiber et al., 2007) that enables us to derive complex amplitude of the object wave on the image sensor directly from three or more phase-shifted holograms recorded in the in-line geometry (Yamaguchi & Zhang, 1997). However, since this method requires to record at least three or four phase-shifted hologram, it is useless for instantaneous measurement of the dynamic object. In order to resolve this problem, various single-shot technologies with phase-shifting features in subsequent holograms are proposed(Awatsuji et al., 2004; Nomura et al., 2006; Toge et al., 2008; Wyant, 2003). All of these methods are based on space-division multiplexing of the phase-shifted holograms in the single recording but device and technique used are different each other. In the former three methods, special micro-element arrays of phase retarders (Awatsuji et al., 2004) or polarizers (Nomura et al., 2006; Wyant, 2003) suitable to a CCD array for recording are strictly required, and therefore, any satisfactory result for practical instrumentations has not been reported until now. On the other hand, the last method only require an introduction of off-axis plane wave illumination to an image sensor for recording single holograms (Toge et al., 2008). From its simplicity, this method seems to be one of most practical candidate for single-shot digital holography. However, it should be point out that we must align the incident angle of the reference wave plane wave so that the relative phase difference between adjacent pixels of the image sensor becomes 2π/3 or π/2 radian. This means that very rigorous alignment in reference wave illumination is required. Furthermore, this requirement may be more serious problem in the case of color digital holography where multicolor laser light is utilized. In this situation, optical alignment should be independently Single-Shot Phase-Shifting Digital Holography Based on the Spatial Carrier Interferometry and Its Tolerance Analysis 5

Spectral intensities and phase distributions of supercontinuum pulses generated in low-dispersion fibers

It was reported recently that optical pulses propagating through a tapered fiber (TF) immersed in heavy water (D2O) or photonic crystal fiber (PCF) of special design yield a broad and flat supercontinuum (SC) spectrum because the fiber dispersion characteristics are of low dispersion: around 1000 nm. This work was undertaken to investigate spectral intensities and phase distributions of SC pulses generated in low-dispersion fibers. Results show that PCF with group velocity dispersion (GVD) distributions having an extremum value of zero dispersion can be used at short fiber length for applications where the phase distribution is a concern.

Mechanism of supercontinuum spectrum generation due to cross-phase modulation in a dispertion-flattened/decreasing fiber with low birefringence

Mechanism of supercontinuum(SC) spectrum generation due to cross-phase modulation(XPM) in a dispersionflattened/decreasing fiber(DFDF) with low birefringence has been investigated theoretically and experimentally. By using a XPM, the measured SC spectum width becomes 33.6 % broader than that obtained by using only self-phase modulation (SPM). This fact implys that effectiveness and validity of the sc generation using XPM method. Furthermore, dependence of generated sc spectrum broadening on incident power, polarization angle of the input pulses, and modal birefringence are investigated. consequently, it was velifid that when a polarization angle of the input pulses is 45 degrees with regard to the principal axes of the fiber and incident average power is less than 0.147 mW, soliton trapping takes place due to a balance between soliton pulse powers and modal birefringence value. In addition, it was shown experimentally that for a polarization angle of the input pulses of 22.5 degrees, the sc spectrum is enlarged by the same amount as in the case of 45 degrees.

Enhancement of supercontinuum spectrum generation due to cross-phase modulation in a dispersion-flattened/decreasing fiber

A flat and broad bandwidth of 350 nm at -20dB for supercontinuum spectrum of soliton pulses was theoretically found to be generated due to cross-phase modulation from a dispersion-flattened/decreasing fiber of 2.4 km length.