This paper describes recent trends of high-precision optical measurements. First, interferometric length-measuring methods are reviewed, wich inclnde fringe-counting methods, optical heterodyning techniques, synthetic-wavelength methods and two-color methods for an air refractive index. Secondly, the outline of measuring methods are presented of dimensions, length, gravity, water-density, fluxoid quantum, Avogardro's number, and earth strain in physics.

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Key techniques for optical frequency measurements including laser stabilization, frequency conversion using nonlinear devices and a phase loc kare presented. Also described are conventional frequency comparison systems which link the ¹³³Cs frequency standard to the infrared or the visible by a heterodyne or an interferometric method. Novel schemes proposed for optical frequency divisionand synthesis are reviewed. Applications of optical frequency measurements are discussed.

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Generating mechanisms and measuring methods of the intensity noise and the frequency noise of semiconductor lasers are reviewed. Both types of noise originates from the field flucturation inherent in the spontaneous emission which works as a source of a lasing field. The noise level and its property are determined with lasing mechanism and related physics. Several topics on the noise problem are also shown presented.

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Various methods of ultrashort optical pulse waveform measurement are reviewed. They are classified into two categories; (1) real-time measurement methods including a PIN-PD followed by a sampling oscilloscope and a streak camera, and (2) correlation measurement methods, i. e., an SHG (Second Harmonic Generation) autocorrelation such as IRIS (iterative pulseform reconstruction from interferometric signals) and an SFG (Sum Frequency Generation) crosscorrelation. Features and future prospectives of these methods are compared with each other from the point of view of pulse waveform evaluation required for the optical fiber communications. Also, for novel applications to optical fiber communications, some measurement methods utilizing ultrashort optical pulses are described, including an optical-optical sampling method used for evaluation of modulated optical signals and E-0 modulators and a BOTDA (Brillouin optical-fiber time domain analysis) method used for he distributed strain measurement of optical fibers.

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Great progress in the practical application of fiber optic sensors is described. The first topic is a fiber optic gyroscope. Noise sources have been studied thoroughly for an interferometer type. High performance that can meet even a specificationfor aircraft navigation has already been realized. A resonator type has a possibility to shorten the lenth of a sensing fiber coil drastically. A fiber ring laser utilizing fiber Brillouin scattering is investigated as a new type of fiber gyroscope. A fiber optic interferometric sensor has also shown a high sensitivity as an acoustic sensor. Distributed temperature sensor based on temperature dependence of fiber Raman scattering has been developed and is used to monitor the distribution along a high voltage power transmission line. Brilloin scattering can also be applied to the measurement of temperature distribution. Methods to diagnose sensor systems and devices are required, which is also discussed.

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Lidar remote sensing techniques for measuring atmospheric parameters are reviewed. The topics include observation of atmospheric structures by Mie scattering lidars, measurement of atmospheric pollutants with differential absorption lidars, observations of stratospheric ozone, aerosols and temperature by ground based lidars, and wind measurements by Doppler lidars. Lidar observations from airplanes and from the space are also discussed.

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Semiconductor lasers for the optical disk heads are selected based not only on their good spacial coherence but also on the broader spectral width and by the worse temporal coherence. The geometrical wave-fronts aberrations of a laser beam are easily degraded through various kinds of optical components from the window of the laser to a disk infomation storage layer, such as a collimator, beam splitter, prisms, objective, and disk substrate. It is very important that the total rms wave-fronts aberrations degraded from the laser to the disk should be within λ/14 in order to get a diffraction limitted minimum beam waist size at the focal point in the disk. To keep this value the rms wave-fronts aberrations for both the objective and the collimator must be less than 0.03λ and 0.025λ, respectively.

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In this paper, we report and discuss the achievment of laser absorption computed tomography of actual in vivo/in vitro biological objects using the Coherent Detection Imaging (CDI) method. This CDI scheme offers the feasible and reliable means for the computer-assisted optical tomographic imaging based on the optical heterodyne technique that has excellent directionality and very high sensitivity. We have experimentally demonstrated the fulfillment of fundamental conditions for the establishment of laser absorption tomography by our novel CDI method in highly scattering and absorptive media such as actual biological tissues and objects. Thus we have realized the tomographic image reconstruction of several biological objects including plant stems, chicken tissue, whole egg, struma specimen and new-born mouse brain employing the projection slice theorem.

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Detection of gravitational waves is expected to open a new window on the universe. Laser interferometers with an armlength of several kilometers are proposed to detect the wave from extragalactic sources. Laser technology plays an important role in successful construction of the interferometers. Following an introduction of gravitational waves, a conceptual design of a laser interferometric detector for gravitational waves is described in view of technical requirements on lasers, mirrors and other optical components.

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The authors' studies on photon scanning tunneling microscope (PSTM) are reviewed. Normal and lateral resolutions achieved by a transmission-type PSTM are estimated as high as 2-nm and 5-nm, respectively. To improve the performance of the system, several methods are proposed, e. g., a novel reflection-resonance-type PSTM is advantageous to improve the resolution. Advanced application systems are also proposed, and especially, a possibility of using the PSTM system as a single-atom-level crystal growth is discussed.

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