Hiromichi Shirahata

Nonlinear IR Generation in AgGaS2

AgGaS2 has been found to be phase matchable for type-1 SHG from 0.884 to 5.55 µ m at 20.0° C. The improved Sellmeiers equations, which correctly reproduce much of the nonlinear experiments thus far reported in the literature, are presented.

Wall depletion effect of population at upper laser level on optical gain in CO2‐He waveguide lasers

The wall depletion effect of CO2 molecules at the excited (00°1) level has been taken into consideration for the calculation of the optical gain of CO2‐He waveguide lasers having a circular discharge tube and a parallel‐plate discharge tube. The simplified equations for obtaining the wall depletion rate for these two discharge tubes are presented and they are found to be in good agreement with the exact values. The numerical calculation revealed the presence of an optimum radius and height of discharge tubes as a function of gas pressure, mixing ratio of the laser gases, tubing materials, and the shape of the discharge tubes. The numerically computed small‐signal gain has been found in good agreement with the experimentally observed values.

Characteristics of Small-Signal Gain and Saturation Intensity in CO2 Waveguide Lasers

The small-signal gain, saturation intensity and volumetric output power of CO2–N2–He waveguide lasers have been analyzed theoretically by taking into account the wall de-excitation of excited CO2 molecules in both the (0001) and (0110) vibrational levels. The theoretical calculation has been performed on a BeO discharge tube in this analysis, and has shown that small-signal gain and volumetric output power have respective optimum tube radii with respect to the operational gas pressure. A new feature of saturation intensity that it is independent of the square of the gas pressure at low gas pressures and/or at small tube radii has been found. The theoretically calculated small-signal gain is in reasonable agreement with the experimental value found by Cohen.

Dependence of gain on tube radius in CO2 waveguide lasers

The wall depletion rate of the excited (0001) level of CO2 molecules was taken into consideration for the optical gain of a CO2 waveguide laser. The numerical calculation revealed that the change of the unsaturated gain has a maximum value with respect to the radius of the discharge tube, and the maximum point moves to a higher gain and a smaller radius with an increase in gas pressure.

Six band multispectral sensor using off-axis three-mirror reflective optics

This paper describes design and experimental results for the future airborne multispectral sensor with six bands between visible and infrared regions. The multispectral sensor features single optics using off-axis three-mirror reflective optics to provide same field of views for all six bands and dichroic mirrors to separate the spectral range into six bands. The off-axis three-mirror optics provides an obstruction free FOV with a wide spectral range and high spatial resolution over a wide FOV. The six bands consist of three visible bands, a near infrared band, an MWIR band and an LWIR band. The 10,000 element CCD sensors are used for the three visible bands and the near infrared band. The 960 element HgCdTe linear arrays are used for the MWIR and the LWIR bands. The results of experiments reveal that the ground-base multispectral sensor has a wide spectral range from 0.4 ^m to 10 xm, the wide FOV over 5.7° and the high spatial resolutions of less than 10 xrad for visible and near infrared bands and of less than 104 xrad for the MWIR and the LWIR bands. The overall MTF at the center of the FOV is more than 0.3 at Nyquist frequency for all bands.

Designing and testing of off-axis three-mirror optical system for multispectral sensor

Future airborne surveillance systems require high performance optics to provide high spatial resolution over wide field of view. The system for 10 km altitude needs to resolve a ground target of 10 cm over 6 degree FOV. The spectral range covers from 0.4 to 10.0 micrometer. The spectral bands of interest include the visible, near infrared, MWIR (middle wavelength infrared), and LWIR (long wavelength infrared). This paper describes the design and the experimental results of the off- axis three mirror optical system which fulfills these requirements. The three mirror anastigmat consists of two aspherical concave mirrors and a spherical convex mirror. The design is configured for a telecentric flat focal plane.

CO2 Waveguide Laser Cooled by a Peltier Thermoelectric Module

A CO2 waveguide laser cooled by a Peltier thermoelectric module was studied experimentally. Efficient cooling of the discharge tube in a compact geometry was achieved using this thermoelectric module. When the wall temperature of the discharge tube dropped to 257 K, the laser gave a small signal gain of 2.7%cm-1 at a gas pressure of 20 Torr in a flowing gas mixture of CO2/N2/He=1/0.5/3 and an output power of 2.6 W (26 W/m) at a gas pressure of 60 Torr in a flowing gas mixture of CO2/N2/He=1/1/3 with an overall electrical efficiency of 1.3%.

Novel multifunction 2-μm imaging laser radar system

A novel imaging laser radar system using a Tm, Ho:YLF laser, applicable for multi-function such as obstacle warning avoidance, range finding, range mapping and target profiling was fabricated. The maximum field of view of the laser radar image produced with Palmer scanning is 12 degree(s) by 48 degree(s). A tower at 2,500 m was observed, demonstrating its capability for range finding function. Obstacle warning avoidance function and target profiling function was tested by taking the laser radar image of the wires and a car, respectively.

Measurement Error in Saturation Parameter and Small Signal Gain Due to Beam Distortion

The measturement error in the saturation parameter and a small signal gain for a CO2. laser caused by the beam distortion is studied by numerical computation for one case utilizing the whole Gaussian beam and the other utilizing only the central portion of the expanded Gaussianbeam. The results reveal that the deviation of a stmall signal gain is very small for both cases, but a saturation parameter is measured with large deviation for the whole Gaussian beam. In addition it is verified that when the central portion of the expanded Gaussian beam is used the measurement error is greatly redtuced.