Muneyuki Adachi

Quartz revisits nonlinear optics: twinned crystal for quasi-phase matching [Invited]

The pioneering material utilized in first optical mixing revisits nonlinear optics with the cutting-edge polarity-control technology stress-induced twinning. Periodically twinned quartz with modulated polarity demonstrates quasi-phase-matched SHG emitting vacuum UV light at 193 nm.

Pulse compression using direct feedback of the spectral phase from photonic crystal fiber output without the need for the Taylor expansion method

Characterization and compensation of the complex spectral phase and the temporal profile of output pulses from a photonic crystal fiber (620-945-nm spectral broadening) were performed using a computer-controlled feedback system that combines a modified spectral-phase interferometry for direct electric-field reconstruction apparatus and only a 4-f chirp compensator having a spatial light modulator. These pulses were adaptively compressed from 12-fs input pulses to 6.8-fs. In addition, the compressed pulse profile showed excellent agreement with results measured independently with fringe-resolved autocorrelation.

Spectral-Phase Characterization and Adapted Compensation of Strongly Chirped Pulses from a Tapered Fiber

A computer-controlled feedback system that combined a modified spectral-phase interferometer as a direct electric-field reconstruction apparatus and a 4-f pulse shaper with a spatial phase modulator as a programmable chirp compensator was developed. The system enabled us to successfully characterize the complex spectral phase and temporal intensity profile of a weak peak-intensity pulse from a tapered fiber with a high sensitivity comparable to that of the conventional interferometric autocorrelator. In addition, we were able to compress for the first time 185 fs fiber input pulses to 16 fs with subpulses originating from the beat between two splitting-spectral components of the fiber output.

Sub-5-fs Pulse Compression of Laser Output Using Photonic Crystal Fiber with Short Zero-Dispersion Wavelength

Output pulses from a photonic crystal fiber (PCF) with a zero-dispersion wavelength (ZDW: 853 nm) were compressed to 5.8 fs by feedback spectral-phase compensation. Furthermore, for another PCF with a shorter ZDW (744 nm), whose output spectral-phase measurement and pulse compression are generally thought to be difficult because of coherence degradation between pulses, we measured successfully their phases in the entire spectral region (480 to 1000 nm) by a suitable selection of input pulse and fiber parameters. The pulse compression experiment using an improved feedback system showed that in the short-wavelength region, pulses can be compressed to 4.9 fs, whereas in the entire spectral region, pulses cannot be compressed. This is because the visibility of the spectral interferogram in the spectral-phase interferometry for direct electric-field reconstruction (SPIDER) signal in the longer-wavelength region corresponding to the phase-sensitive soliton-breaking-up region is degraded by large phase modulation using a spatial light modulator.

Deep ultraviolet light generation at 266 nm by quasi-phase-matched quartz

We demonstrated the finest twin structure ever reported in crystal quartz with a period of 17.8 mum. Second harmonic 266 nm light of 0.10 mW was obtained by the third-order QPM in quartz, from a ns-pulsed doubled Nd:YVO4.

Fine twin structure in crystal quartz for quasi-phase-matched deep ultraviolet generation

We succeeded in fabrication of the finest twins in crystal quartz with a period of 11.9 mum and generation of 266 nm light with 2.2 mW by 2nd-order-QPM through precise control of the temperature and pulsed stress.

Complete Automatic Phase Compensation for the Generation of A-few-cycle Pulses

An automatic feedback spectral-phase compensation system using the wavelet transformation method for the M-SPIDER signal analysis has been developed. The system is completely free from the manual operation and generates 9.6-fs pulses after chirp compensation.

Vacuum ultraviolet light generation at 193 nm by quasi-phase-matched quartz

Coherent all-solid-state light source of a wavelength below 200nm is attracting a lot of attention for industrial applications such as semiconductor processing, eye surgery, and micro machining. Multi-stage wavelength conversion from a high power infrared solid-state laser is a promising solution. We have developed a technology for quasi-phasematching (QPM) in crystalline quartz that utilizes stress-induced twinning. In the present paper, we report a novel stressmaintaining module that suppresses back-switching of twinning and enables QPM-SHG in the vacuum ultraviolet (VUV) region. We demonstrated the fabrication of finest periodic twins with a period of 9.6 μm and performed QPM-SHG experiment. Vacuum ultraviolet 193.4 nm light of 17 mW was generated from 177 mW fundamental light. To the best of our knowledge, this is the shortest emission wavelength ever obtained with QPM technology.

VUV 193 nm emission from micro-twinned crystal quartz

VUV light at 193 nm was generated by second harmonic generation in quasi-phase-matched crystal quartz. Specially developed mechanical module stabilized a micron-scale twin structure realizing stable QPM wavelength converter to 193 nm.

Orange fiber laser for ophthalmology

For the light source of photocoagulators for ophthalmology, orange laser is more suitable than green laser because of low scattering loss by the crystalline lens, and low absorption by xanthophylls in the retina. We developed two orange fiber lasers (580 nm and 590 nm) to investigate the effect depending on the difference in the range of orange. The 580nm laser is composed of a 1160 nm fiber laser and a Periodically Polled Lithium Niobate (PPLN) crystal for second harmonic generation. The 1160 nm fiber laser beam is focused into the MgO-doped PPLN crystal whose length is 30 mm with 3-pass configuration. Continuous-wave 1.3 W output power of 580 nm was obtained with 5.8 W input power of 1160nm for the first time. The conversion efficiency was 22%. The band width of the second harmonic was 0.006 nm (FWHM). The 590 nm laser is almost the same as 580 nm laser source. In this case we used a Raman shift fiber to generate 1180 nm, and the output power of 590 nm was 1.4 W. We developed an evaluation model of photocoagulator system using these two laser sources. A 700 mW coagulation output power was obtained with this orange fiber laser photocoagulator system. This is enough power for the eye surgery. We have the prospect of the maintenance-free, long-life system that is completely air-cooled. We are planning to evaluate this photocoagulator system in order to investigate the difference between the two wavelengths at the field test.