Two-color Laser Beam Research Articles

K-1344 ビームスキャニングによる高迎角物体はく離流れのPIV計測(S15-13 翼のはく離)(S15 噴流・はく離流れの計測と制御)

Two-color dual laser beam scanning PIV is applied to the measurements of the wake vortices behind an axisymmetric paraboloid to investigate the flows with separation over the body at high angles of attack. In order to record the image with the limited scattering efficiency of small tracer particles, laser beam scanning method was employed with a 3-CCD color digital video camera. The obtained single frame/multi-exposure color PIV image was decomposed into double frame/single exposure images according to the hue component of the pixel values. The developed technique is shown to provide sufficient velocity vectors in the plane normal to the symmetric axis of the body, with the exception in vortex cores. The agreement obtained between the PIV and flow visualization results is seen to be quite reasonable.

Investigation of 2-D concentration measurement of fuel vapor and droplets using a scanning two-color laser beam

Investigation of 2-D concentration measurement of fuel vapor and droplets using a scanning two-color laser beam

Interference effects in a two-color laser beam interaction with alkali-metal atoms

The interaction of an alkali-metal atom with two different color laser beams is analyzed. The quantum interference effect is studied in weak and strong field limits. Our theoretical results are related to the recent experimental data for the ${S}_{1/2}\ensuremath{\rightarrow}{S}_{1/2}$ two-photon transition in Na and Rb atoms. It is shown that the interference profile becomes narrow for moderately strong fields and the destruction of the interference is predicted for very strong ones. The electromagnetically induced interference effects are analyzed. The role of the relaxation processes is examined.

Two-color cross-correlation in small-angle static light scattering

We have measured by means of a charge-coupled device sensor the space correlation function between two speckle fields at different wavelengths. The fields are generated from the scattering of a two-color laser beam from a stationary, three-dimensional sample. In general, the speckle fields attain their maximum degree of correlation when the scattering angles are properly scaled according to a grating dispersion rule. The degree of correlation, however, depends both on the thickness of the sample and on its turbidity. At a given angle, the degree of cross-correlation diminishes as the thickness is increased, and it also decreases as the turbidity grows. Working formulas are derived, and we show that the dependence from the sample turbidity is related to the spread in photon paths. A comparison with the photon path spread calculated by means of a multiple-scattering Monte Carlo simulation will be presented. The connection between the present work and studies on polychromatic light diffraction from random two-dimensional transparencies and microwave transmission through thick samples will also be presented.

The Generation of Powerful Rainbow-Color Laser Emission by Four-Wave Raman Mixing for Display and Illumination.

In order to generate a multifrequency laser emission, consisting of the entire rainbow of colors, two approaches are demonstrated and compared for commercial application, such as displays and illumination. One approach uses a second harmonic emission of a Nd:YAG laser emitting at 532 nm as the main pump beam and a dye laser, which is pumped by a part of the Nd:YAG laser, as a seed beam. These beams are coaxially aligned and are focused into two Raman cells filled with hydrogen to generate multifrequency laser emission based on four-wave Raman mixing. The other approach is more straightforward and involves an elliptically polarized second harmonic emission of a Nd:YAG laser, instead of a two-color laser beam, which is simply focused into the Raman cells. The entire rainbow colors (more than 20 emission lines from 400 nm to 800 nm) are generated in both the approaches. More efficient generation of the regular (quasi-equally spaced) spectral lines with a flatter intensity distribution is achieved by the former approach. The output power obtained is 600 mW (60 mJ×10 Hz).

Generation of multifrequency laser emission quasi equally spaced throughout the entire visible region

A circularly polarized, monochromatic laser beam is focused into a Raman cell, which contains hydrogen to generate rotational stimulated Raman emission. After linear polarization, this two-color (separated by 587 cm(-1)) laser beam is focused several times into a second Raman cell that is filled with hydrogen to generate a multifrequency laser emission. Many rotational and vibrational lines are generated efficiently by this multipass effect. Eighteen colors that are quasi equally spaced with a rather flat intensity distribution are generated throughout the entire visible region. The present multifrequency laser emission may be advantageously used for illumination in a higher-grade display, such as a laser light show.

Multifrequency laser emission generated by two-color stimulated Raman effect using a single-frequency laser beam and a dye-Raman composite resonator

A single-color circularly polarized laser beam is focused into pressurized hydrogen to generate rotational stimulated Raman emission. After being corrected to a linearly polarized beam, this two-color laser beam is passed through a dye amplifier and is focused into pressurized hydrogen, again to generate multifrequency laser emission by the two-color stimulated Raman effect. The laser beam is further confined in a resonator, which enhances intracavity intensity leading to increased efficiency. In these experiments, more than 11 rotational lines are generated simultaneously at 5 atm, and many additional vibrationally shifted rotational lines are generated at 9 atm.

Efficiently generated rotational and vibrational stimulated raman emission by means of a two-color laser beam

When a two-color laser beam is introduced into pressurized hydrogen, about 40 laser emission lines are generated from the ultraviolet to the visible regions. This phenomenon is ascribed to the stimulated Raman effect due to a combination ofJ=1 →J=3 rotational andv=0 →v=1 vibrational transitions. By introducing the two-color laser beam, the threshold for generation of the rotational line is substantially reduced. The present phenomenon is attributed to four-wave mixing, which allows efficient generation of higher-order rotational and vibrational Raman lines.

Continuously tunable laser emission from the deep ultraviolet to the near infrared generated by a single optical system

A two-color laser beam separated by 587 cm(-1)is focused into a Raman cell pressurized with hydrogen. More than fifty rotational and vibrational lines are generated by a four-wave mixing-assisted stimulated Raman effect. By synchronously changing the laser frequencies, the overall tuning range is extended from the deep ultraviolet (247 nm) to the near infrared (771 nm) without any frequency gaps.

Generation of more than 40 laser emission lines from the ultraviolet to the visible regions by two-color stimulated raman effect

An intense and monochromatic laser beam consisting of more than 40 vibrational and rotational lines is generated by the stimulated Raman effect, when two-color laser beams separated by ∼590 cm−1 are tightly focused into pressurized hydrogen. This phenomenon is reasonably explained by four-wave mixing; one beam is used for two-step excitation (ω1+ω1) to an imaginary level and another beam (ω2) acts as a seed beam, generating a fourth beam (2ω1−ω2). Through cascade processes, so many laser emissions appear with a frequency difference of (ω1−ω2) in entire ultraviolet and visible regions.