Haruyasu Itoh

Spherical aberration correction suitable for a wavefront controller

We propose a simple method to correct a large amount of spherical aberration caused by a refractive index mismatch. The method is based on inverse ray tracing and can generate correction phase patterns whose peak-to-valley values are minimized. We also demonstrated spherical aberration correction in a transparent acrylic block using a liquid-crystal-on-silicon spatial light modulator (LCOS-SLM). A distorted focal volume without correction was substantially improved with correction. This method is useful in cases where a large phase modulation is needed, such as when employing a high-NA lens or focusing a beam deep inside a sample.

Optimization of resonant two-photon absorption with adaptive quantum control

Adaptive quantum control is applied to the resonant two-photon absorption of Rb atoms, which includes an intermediate state between the initial and final states. The phase distribution of incident laser pulses in the frequency domain is decided adaptively free from a priori knowledge on the phase distribution with the help of a simulated annealing algorithm to achieve the maximum luminescence intensity from the state associated with the excited state. The optimized phase pattern acquired in our system matches a theoretical prediction based on the perturbative treatment of the semiclassical model on the absorption of light.

Femtosecond Pulse Delivery through Long Multimode Fiber Using Adaptive Pulse Synthesis

We report on a delivery technique for 164-fs optical pulses with a peak power of 1.1 kW through a long multimode optical fiber and a glass block using an adaptive pulse-shaping feedback loop. We used two devices to optimize the input pulse; a pulse stretcher and a pulse shaper. 382-ps chirped pulses are compressed to 370-fs pulses at the output end of a glass block joined to a standard graded-index multimode fiber, 96 m in length and with a core diameter of 50 µm (the output end of the system). The adaptive pulse shaper compensates for the remaining high-order phase dispersion, which results in 164-fs pulses at the output end of the system. Our work shows that an adaptive pulse synthesis technique provides a powerful and convenient technique for programmable fiber dispersion compensation over a broad optical bandwidth.

Character recognition by feature extraction using cross-correlation signals from a matched filter

A hybrid pattern recognition system is described. It is based on a matched spatial filter (MSF) synthesized not by conventional template-matching patterns but by feature-extracted reference patterns. Some features are extracted from target objects used to recognize unknown objects. The optimum reference patterns for the filter are selected according to computer simulations. Through more than 30 simulations, four best patterns are chosen for recognition of 10 kinds of digits by the MSF. Recognition is performed not by conventional autocorrelation peaks but by cross-correlation signals because the reference patterns are not wholly the same as the input. The proposed system shows feasibility for feature-extracted pattern recognition using the multiplexed MSF.

Stable and flexible multiple spot pattern generation using LCOS spatial light modulator

The LCOS spatial light modulator (LCOS-SLM) can generate desired multiple spot patterns (MSPs) via the application of suitable computer-generated-holograms (CGHs), but the MSP intensity distribution varies because ambient temperature affects the phase modulation characteristic and causes wavefront distortion. To generate high-optical-quality MSPs we use our hardware-compensated (with a Peltier system to even out phase modulation) and software-corrected (via multiplication of the CGH by temperature correction coefficients) LCOS-SLMs. Experimental results with a 14 × 14 MSP generation show that the hardware-compensated LCOS-SLM provides stable MSPs between 9 to 32 °C. The software-corrected LCOS-SLM provides uniform spots over twice the temperature range obtained with conventional SLM method. We confirm that our methods are highly efficient for use in two-photon excitation microscopy application such as multifocal mulitphoton microscopy.

Compact geometry for diode-pumped Cr:LiSAF femtosecond laser

A novel geometry for dispersion compensation in femtosecond lasers without specific cut of optical parts in a flat/Brewster angle or Gires-Tournots mirrors is demonstrated for attaining a compact diode-pumped Cr:LiSAF femtosecond pulse laser. In this geometry, the laser medium lies between the dispersion-compensation prism pair. This approach enables the laser to operate at variable repetition rates from 163 to 235 MHz keeping the pulsewidth less than 90 fs, where the value of the time-bandwidth product are not larger than 0.34. An 89 fs pulse duration generated at the 235 MHz repetition rate is, to our knowledge, the shortest pulse achieved in an all-solid-state Kerr-lens mode-locked laser operating above the 200 MHz repetition rate.

Shaping tight-focusing patterns of linearly polarized beams through elliptic apertures

We propose elliptic deformation of apertures for shaping focusing patterns of tightly focused linearly polarized beams. Numerical integration of the vectorial Rayleigh-Sommerfeld diffraction formula predicts the formation of symmetric focusing patterns by shrinking the aperture shape in the direction perpendicular to the polarization of the beams. The elliptic deformation is also applied to a focused linearly polarized beam through an annular aperture to demonstrate the formation of smaller symmetric focal spot than that through a simple oval aperture.

Transform-limited 30-fs pulse generation from a diode-pumped kerr-lens mode-locked Cr:LiSAF laser

The generation of transform-limited 30-fs pulses from a laser-diode pumped, Kerr-lens mode-locked Cr:LiSAF laser is reported. Good mode-locking was achieved by use of a simple cavity configuration with a laser diode pumping source which had a separate section of a laser diode amplifier. The 30-fs optical pulse width was confirmed by use of an autocorrelator having little internal dispersion due to its composition of only reflecting mirror optics. A time-bandwidth product of 0.318 was obtained for the laser output pulse by assumption of a sech2 waveform.

All solid-state Cr:LiSAF laser

Many kinds of experiments about all solid-state Cr:LiSAF lasers have been reported. A tunable all solid-state cw laser having tunability over more than 100 nm was developed by using the spectrum selection self-injection locking (SSSIL) method in 1992. The authors realize tunable picosecond radiation from an all solid-state laser with 146-200 ps pulses in an 88 nm range. Also, 70 fs pulse generation was achieved using a Kerr-lens mode-locking pumped with an Ar/sup +/ laser. >

Improvement of laser dicing system performance I: high-speed, high-quality processing of thick silicon wafers using spatial light modulator

In the laser wafer dicing technique of stealth dicing (SD), a laser beam that is tightly focused inside a silicon wafer is scanned multiple times at different depths. The focused beam creates multilayered cracks that allow dry, debris-free dicing. To reduce the dicing time, it is desirable to produce longer cracks with each scan. However, when the laser beam is focused in a deep region of the wafer, the beam is blurred, and its power density decreases owing to spherical aberration caused by a refractive index mismatch between air and the wafer. Consequently, the generated cracks become shorter. We present an approach to making longer cracks deep within the wafer by correcting the spherical aberration. This correction is made using an SD machine incorporating a phase-only spatial light modulator to apply aberration correction patterns, which are calculated by a method based on inverse ray tracing. Experimental results using 300-µm wafers show that, when the aberration was corrected, the cracks formed during multidepth scans became longer even deep within the wafer and that the dicing speed with correction is more than twice that without correction. This is because each scan produced longer cracks, so fewer scans were necessary. We also demonstrated that the quality of dicing was improved.