Humio Inaba

Two-Dimensional Coherent Detection Imaging in Multiple Scattering Media Based on the Directional Resolution Capability of the Optical Heterodyne Method

This paper describes a new application of optical heterodyne detection using a laser beam for two-dimensional imaging of the internal structure of strongly scattering media in which the structure is completely obstructed from normal visual observation. The directional resolution capability for image formation due to the excellent antenna properties of the heterodyne technique is verified experimentally using a ground glass to cause strong scattering of the signal beam. Successful image detection of a test target placed in a highly scattering absorptive medium, with spatial resolution better than 400 μm in the case of our experiments, demonstrates that this Coherent Detection Imaging (CDI) method can overcome the diffuse nature of images in media such as those of biomedical interest and others to achieve scanning and tomographic imaging.

Generalized study on angular dependence of induced second‐order nonlinear optical polarizations and phase matching in biaxial crystals

The induced second‐order nonlinear coefficients in the noncentrosymmetrical biaxial crystals are formulated in a general way as functions of the their crystal symmetry and directions of propagation of optical waves in the crystal. The direction of light propagation is determined from the phase matching among three optical waves. The optimum operating condition is also derived analytically by investigating the effective nonlinearity under the phase‐matched direction. This treatment is applied to the case of the second harmonic generation of Nd : YAG laser (1.064 μm), using α‐iodic acid, lithium formate, sodium formate, and lithium‐sodium formate crystals. Experimental results were compared quite favorably with the theoretical analysis given in this paper.

PHOTON COUNTING SPECTRAL ANALYZING SYSTEM OF EXTRA-WEAK CHEMI- AND BIOLUMINESCENCE FOR BIOCHEMICAL APPLICATIONS

Abstract— The systematic study of the method for measuring the spectral distribution of extra‐weak light signals, such as from biochemical systems and living tissues involving chemi‐ and bio‐energized processes, led to the design and construction of a new type of spectrometer, called the filter spectral analyzer system, incorporated with the high‐sensitivity photon counting technique. Experimental tests to examine the operational characteristics of this filter spectral analyzer system and comparisons of the signal‐to‐noise ratio with a conventional grating spectrometer were made by accomplishing the spectral analysis of low‐level light sources. The measurement of the spectra of very weak chemilumines‐cence accompanying various kinds of chemical and biological processes, including autoxidation and enzymatic reaction were performed successfully.

Laser-Raman radar —Laser-Raman scattering methods for remote detection and analysis of atmospheric pollution

This paper reviews the recent progress of the laser radar method, providing single-ended and range-resolved means for remote detection and analysis of various molecular species in the polluted as well as ordinary atmosphere, based on the existence of their Raman backscattering; simply called the laser-Raman radar method. A brief description is given of the operational principle including the basic concept and fundamental characteristics of this scheme, along with critical discussions of their advantages. The differential backscattering cross section and the spectral distribution of Raman scattering of molecular species present in the atmosphere are discussed and summarized from the practical view-point for providing molecular density evaluation and interference-free detection. After presenting experimental approaches of the laser-Raman scheme to show the feasibility of in-field monitoring and analysis of chemical species in various air conditions, the estimation of molecular concentrations is performed from observed Raman spectra. Analytical discussions on the minimum detectable concentration and the range detectivity are also presented to indicate the present capability and limitation of this laser-Raman technique. The potentiality for further progress in resonance Raman scattering for range-resolved measurement of dispersed pollutants in ambient air is pointed out.

Generation of subpicosecond coherent optical pulses by passive mode locking of an AlGaAs diode laser

Subpicosecond coherent optical pulses as short as 0.58 ps were generated for the first time by passive mode locking of an AlGaAs double heterostructure diode laser. It was realized by a simple configuration made of a conventional AlGaAs diode laser without antireflection coatings on both facets, which was aged and exhibiting self‐pulsation, placed in an external cavity. When the external cavity length was shorter than about 60 mm, almost complete mode locking was observed over the oscillation spectrum of 3.5 nm in width. Based on the second‐harmonic generation autocorrelation measurement, ultrashort coherent optical pulses from 0.58 to 1.2 ps in duration were demonstrated by increasing the external cavity length together with the proper control of the optical feedback effect and the pumping dc current.

10 km-long fibre-optic remote sensing of CH4 gas by near infrared absorption

This paper reports for the first time the fully optical remote monitoring of low-level CH4 gas with a wide area coverage of 10 km in diameter realized by utilizing ultralowloss silica optical fibre link and a compact absorption cell in conjunction with highly radiant InGaAsP and InGaAs light emitting diodes (LEDs) in the near infrared region. For this novel application, the near-infrared absorption spectra of CH4 molecules were measured and studied involving the ν2+2ν3 combination band around 1.33 μm and the 2ν3 overtone band around 1.66 μm. This fibre-optic system was demonstrated to be feasible and sufficiently sensitive and safe as being a practical CH4 gas remote sensor whose detection sensitivity is normally required to be 25% of the lower explosion limit (LEL) of CH4 gas in air. This result verifies a large potential for applying extensively to various strategic points within the environment, such as industrial and mining complexes as well as urban and residential areas, with considerably increased reability, safety and marketability over the presently available techniques.

SPECTROSCOPIC DETECTION OF SO2 AND CO2 MOLECULES IN POLLUTED ATMOSPHERE BY LASER‐RAMAN RADAR TECHNIQUE

Raman backscatter of a Q‐switched ruby laser beam from SO2 and CO2 molecules were selectively detected in the polluted atmosphere by means of a laser‐Raman radar system. The spectrum of these Raman components was compared quantitatively with that for the ordinary and sulphur dioxide atmospheres, along with the estimation of relative concentrations of these molecules.Raman backscatter of a Q‐switched ruby laser beam from SO2 and CO2 molecules were selectively detected in the polluted atmosphere by means of a laser‐Raman radar system. The spectrum of these Raman components was compared quantitatively with that for the ordinary and sulphur dioxide atmospheres, along with the estimation of relative concentrations of these molecules.

Optical trapping of small particles using a 1.3-μm compact InGaAsP diode laser

We describe the noncontact optical trapping of small particles by a single-beam gradient force using a near-infrared InGaAsP diode laser operating at 1.33 μm. The feasibility and reliability of diode-laser trapping was confirmed with polystyrene latex and glass spheres as well as with yeast cells. By moving small particles vertically to the laser beam axis, we measured the horizontal component of the trapping force based on the Stokes law. Thus a linear relationship between the trapping laser power and the horizontal trapping force is demonstrated and compared quantitatively with that for the Ar laser.

Generation of picosecond optical pulses with highly RF modulated AlGaAs DH laser

AlGaAs DH lasers with strong RF modulation superimposed on the relatively low dc bias below the oscillation threshold were demonstrated to be feasible for generation of a train of approximately 30 ps optical pulses at a repetitive frequency of sub-GHz range. The pulse envelope width was measured by three different methods: a fast response photodetector, the second-harmonic generation (SHG) correlation method, and an ultrafast streak camera. The results of the SHG correlation and the streak camera agreed fairly well. In order to explain the generation mechanism and the characteristics of these ultrashort optical pulses in a highly RF modulated semiconductor diode laser, the rate equation analysis was performed and the results were generally in good agreement with the experiment. Furthermore, from the computer simulation for the analysis of the SHG correlation traces, it was inferred that an individual ultrashort optical pulse has internal substructures made of fluctuating fields whose spike widths were of the order of subpicoseconds, due to the randomness of the phases among lasing modes.

Remote sensing system for near-infrared differential absorption of CH4 gas using low-loss optical fiber link.

An all-optical remote sensing system utilizing long-distance low-loss optical fiber links in conjunction with high-radiant InGaAsP light emitting diodes and dielectric interference filters was developed for the real-time absorption measurement of molecular concentration in the near-infrared region. The highly sensitive technique was achieved employing the power-balanced two-wavelength differential absorption method in the system, which enables direct detection of differential absorption signals for the specific molecule being monitored. Based on the laboratory study of the 2ν3 overtone band of CH4 molecules at 1.66 μm, the system was used for remote detection of low-level CH4 gas using a 2-km long low-loss silica optical fiber link incorporating an InGaAsP light emitting diode. Thus we demonstrated that this method is capable of a high detection sensitivity as low as ~400 ppm, i.e., ~0.8% of the lower explosion limit of CH4 density in air. Further extensive applications of the system as well as potential improvement of its sensitivity are also discussed.