We previously developed a high-speed terahertz spectroscopic imaging method based on electro-optic sampling with a
noncollinear geometry of the THz beam and probe laser beam and using a multistep mirror in the path of the probe beam.
We set the incident probe laser into MAST at a 45° angle, to prevent interference between adjacent beams. However, this
produced beam vignetting, so imaging had to be performed twice, between sample movements, and this increased the
imaging time accordingly. Thus, we improved the probe-laser imaging system after reflecting from the MAST to correct
for the effects of diffraction. This prevents interference from adjacent beams and allows the angle of incidence on the
MAST to be set to 0°, enabling the entire sample surface to be imaged in one measurement. As a result, we are able to
perform measurements in 40 seconds, half the time of the previous method, and obtain a 28x28-pixel spectral image with
spatial resolution of 1.07 mm. To verify the imaging performance, we also measured test samples, showing that the
shape and thickness of items inside an opaque plastic case can be distinguished using amplitude and phase images, and
metallic foreign objects can be detected. We also evaluated the method and were able to show the validity of the spectral
imaging results by distinguishing the transmission or blocking of arbitrary frequency components.
The optical system in recordable optical disc must satisfy both a reduction in spot size for a high data capacity, and an increase in optical energy for high data transfer rate. We have devised one simple optical element that converts the Gaussian intensity distribution into the flat intensity distribution by refraction without optical energy loss or aberrations.
An optical pickup system for optical disc drives utilizes optical elements for information signal detection, focus-error detection and track-error detection. We propose an optical pickup using double holographic lenses--replacing conventional optical elements for focus-error detection and track-error detection--to reduce the pickup size. There are two important requirements when using holograms for an optical pickup. One is to compensate for chromatic aberration caused by laser diode wavelength deviation, and the other is to obtain an appropriate beam spot size for focus-error detection. To meet these requirements, we used double holograms, where one has divergent power and the other convergent power. By optimizing the double hologram pattern, the optical system is achromatized. We observed the achromatic effect, 0.07 (lambda) (RMS) for a laser diode wavelength change of +/- 10 nm. The beam position error was less than +/- 1 micrometers . We reduced the NA of the system to give a relatively large spot beam and a large depth of focus.
Since a hologram can simultaneously scan and focus a laser beam, we have investigated the use of a holographic line scanner in laser printers. However, conventional holographic scanners, suffer from wavelength shifts of laser diodes. To prevent problems caused by wavelength shifts, a conventional holographic scanner needs an expensive thermal controller for a laser diode or a frequency-stabilized laser diode. To achieve a low cost scanner, we proposed a new holographic scanner using a holoplate that compensates for large wavelength shifts. In this paper, we will discuss the wavelength shift compensation of a new scanner.