Kazunori Naganuma

Group-delay measurement using the Fourier transform of an interferometric cross correlation generated by white light

A new method and apparatus for measuring the group delay in optical components and laser cavities are described. Cross-correlational fringes are fully recorded with a Michelson interferometer, in one of whose arms the optics to be measured are inserted. The path difference of the interferometer is calibrated to subwavelength accuracy, and the group delay is calculated from the phase of the Fourier transform of the measured fringe. The group delay for the entire visible-wavelength region is evaluated after a single measurement in approximately 10 min, using white light.

50-fs pulse generation directly from a colliding-pulse mode-locked Ti:sapphire laser using an antiresonant ring mirror

50-fs pulses were directly generated from a colliding-pulse mode-locked Ti:sapphire laser. To achieve the collidingpulse mode locking, a miniature antiresonant ring containing an organic saturable dye jet was employed as the end mirror for the linear cavity laser. Based on measured dispersion of intracavity elements, a prism pair was implemented to control the cavity dispersion. The generated pulses have no linear chirp but do exhibit parabolic instantaneous frequency owing to third-order dispersion introduced by the prism pair.

Compact diode-pumped all-solid-state femtosecond Cr(4+):YAG laser.

We have developed a compact, all-solid-state self-mode-locked Cr(4+):YAG laser. The laser is pumped by a cw laser-diode-pumped Nd:YVO(4) laser and produces highly stable femtosecond pulses near 1.50 microm. Measurements of noise power spectra for frequencies below 10 kHz show that the diode-pumped laser system greatly improves the output energy fluctuation and pulse timing jitter compared with a laser system that uses a conventional arc-lamp-pumped cw Nd:YAG laser.

Characteristics of femtosecond pulses near 1.5 μm in a self-mode-locked Cr 4+ :YAG laser

The operating characteristics of a self-mode-locked Cr(4+) :YAG laser that produces sub-100-fs pulses are described. Stable, nearly transform-limited pulses are obtained at a central wavelength longer than 1.50 microm, and the spectrum of the wavelength is extended past 1.60 microm, over the net gain bandwidth of the laser material. The group-velocity dispersion and the third-order dispersion of the Cr-doped YAG rod are measured by a Fourierindicating that the group-velocity dispersion vanishes at a transform interferometric cross-correlation technique, wavelength of 1.59 microm.

A Novel Real-Time Chirp Measurement Method for Ultrashort Optical Pulses

Rapid measurements of ultrashort optical pulse chirping were realized with a newly developed real-time chirp monitor. The application of a moving coil translator and electrical filters enabled the rapid measurement and decoding of fringe-resolved second-harmonic-generation (FRSHG) autocorrelation. The prototype system displayed chirping characteristics at a repetition rate higher than a few hertz, which demonstrates the usefulness of this monitor for dye laser system alignment.

Fast Response Variable Focal-Length Lenses Using KTa1-xNbxO3 Crystals

We fabricated variable focal length lenses with fast responses by using the strong Kerr effect of KTa1-xNbxO3 single crystals. We observed focus shifts of about 40 mm with the assistance of a 250-mm-focal-length lens, which corresponds to a focus shift from infinity to 1.56 m without the assisting lens. The time response was as fast as 1 µs.

Semiconductor laser cavity dispersion measurement based on interferometric crosscorrelation of amplified spontaneous emission

The Fourier transform of the interferometric crosscorrelation of amplified spontaneous emission (ASE) is proposed as a general‐purpose method for measuring cavity dispersion. The electric field correlation of the ASE from a cavity shows subfringes displaced from the ordinary interferogram by the cavity round‐trip time. The cavity transfer function is derived from the subfringe using the Fourier transform. This method is demonstrated on semiconductor devices. It provides a very quick way to measure round‐trip group delay dispersion as well as gain across a device’s whole gain band.

Time direction determination of asymmetric ultrashort optical pulses from second‐harmonic generation autocorrelation signals

A general procedure for determining pulse asymmetry including time direction is described. Fundamental and second‐harmonic interferometric autocorrelations are measured before and after passing through a glass block. Pulse form is reconstructed iteratively for each pulse, yet time direction is ambiguous. Time direction for both pulses is then simultaneously determined by phase curvature comparison in the frequency domain, taking the known dispersion of the glass block into account. This results in complete evaluation of ultrashort optical pulses. This method is demonstrated successfully for passively mode‐locked dye laser pulses.

Fast response varifocal lenses using KTa 1−x Nb x O 3 crystals and a simulation method with electrostrictive calculations

We fabricated cylindrical varifocal lenses with fast responses by using the strong Kerr effect of KTa(1-x)Nb(x)O(3) (KTN) single crystals. We observed focus shifts of up to 87 mm with the assistance of a 250 mm focal length lens, which corresponds to a focus shift from infinity to 720 mm by the KTN lens itself. The response time was as fast as 1 μs. We also present a simulation method for calculating refractive index distributions in KTN single crystals, which is essential when designing the lens. The method is characterized by the strain contribution, which has not conventionally been typical of electro-optic simulations. We used this method to explain the refractive index modulations that are characteristic of the varifocal lenses.

Generation of Broadly Tunable Subpicosecond Light Pulses from a Synchronously and Passively Mode-Locked CW Dye Laser

Transform-limited pulses with a minimum width as short as 0.30 ps have been obtained from a synchronously pumped rhodamine 6G dye laser with a dye solution of mixed gain and saturable absorber (DODCI) media. Stable subpicosecond pulses (0.30–0.85 ps) were generated in a wide tuning range of 574–611 nm using a broad-band wedged-etalon tuning element. With the frequency tuning element inside the cavity, the cavity-length detuning range to maintain perfect mode-locking was ±10 µm which is less critical than for synchronous mode-locking alone, and the range extended to ±1.5 mm without the frequency tuning element.