Optical Distance Meter Research Articles

Geodetic Basis of the Longest Tunnel in the Republic of Croatia

The tunnel “Mala Kapela”, 5760 m long, is the longest tunnel in the Republic of Croatia. This paper gives the presentation and the analysis of the geodetic basis in the tunnel “Mala Kapela”. The tunnel geodetic basis consists of the overhead and underground part. The overhead geodetic basis consists of:1. two geodetic rectangles (one in the vicinity of the northern, and the other in the vicinity of the southern portal)2. a precise traverse connecting positionally these two micro-networks3. a precise levelling figure connecting vertically the micro-networks of the northern and southern portals.The underground geodetic basis consists of:1. two networks in the right tunnel tube in the form of a triangle chain (one of them at the northern, and the other at the southern tunnel portal)2. two traverses in the left tunnel tube (one of them at the northern, and the other at the southern part of the tunnel) connecting the underground networks in the right tunnel tube and the overhead geodetic basis at the northern, i.e. the southern portal of the tunnel3. two closed precise levelling figures (one at the northern and the other one at the southern part of the tunnel).This is the first time that an underground geodetic basis in the form of a triangle chain has been applied in the Republic of Croatia. It was chosen because it best meets the set criteria related to the accuracy of cutting the tunnel “Mala Kapela”. The error sources in direction measurement were processed with special attention given to the error of the initial grid bearing and direction orientation. The article gives an outline of formulas used to calculate corrections and reductions of the lengths measured with electro optical distance meter. At the end, there is an illustration of the accuracy of cutting the tunnel “Mala Kapela”. The transverse deviation for the cutting point in the right tunnel tube was sQ = 0.8 cm and the longitudinal deviation sL = 2.1 cm. The transverse deviation for the cutting point in the left tunnel tube was sQ = 1.0 cm and the longitudinal deviation sL = 2.2 cm. The vertical deviation for the cutting point in the right tunnel tube was sH = 1.9 cm, and for the cutting point in the left tunnel tube sH = 2.1 cm.

High-accuracy measurement of 240-m distance in an optical tunnel by use of a compact femtosecond laser

A high-accuracy optical distance meter with a mode-locked femtosecond laser is proposed for distance measurements in a 310-m-long optical tunnel. We measured the phase shift of the optical beat component between longitudinal modes of a mode-locked laser. A high resolution of 50 microm at 240-m distance was obtained without cyclic error correction. The group refractive index of air is automatically extracted to an accuracy of 6 parts per million (ppm) by two-color measurement with the pulses of fundamental and second-harmonic wavelengths. Finally, an absolute mechanical distance of 240 m was obtained to within 8-ppm accuracy by use of a series of beat frequencies with the advantage of a wide range of intermode frequency, together with the results of the two-color measurement.

Optical Distance Meter Using a Pulsed Laser Diode and Fast Avalanche Photo Diodes for Measurements of Molten Steel Levels

An optical distance meter with a wide dynamic range, is proposed for measuring the level of molten steel in a steel mold. The meter operates by measuring the flight time of a short optical pulse that is propagated between the light source and the target. A pulsed laser diode with a fast rise time is utilized as a light source, and two avalanche photo-diodes are used as detectors. The optical distance meter performs at a measurement range and a standard deviation of the measurement error of 1 m and 1 mm for the black paper target, respectively. To test its reliability, the distance meter is used to monitor the level of molten steel during a continuous casting process. The results obtained were compared with those obtained using an eddy current sensor and a γ-ray sensor. The measurement range of the optical method is ten times those attained using other two sensors. Continuous monitoring of the steel level can be done from the initiation of the casting process with the proposed distance meter.

Optical distance meter using a short pulse width laser diode and a fast avalanche photodiode

An accurate optical distance meter that features a pulsed laser diode as a light source and two avalanche photodiodes as detectors, is reported. A cascade connection of avalanche transistors drives the laser diode to emit a periodic output pulse of 0.8 ns rise time. The photodiode rise time was found to increase whenever the incident intensity decreased below a certain threshold. Measurement errors caused by this effect were corrected by numerical processing. The optical distance meter yielded a calibration error of 0.5 mm for a white paper target placed 1 m from the laser source. The distance meter was also utilized to profile the bottom of a plastic waterpipe.

Simple optical distance meter using an intermode-beat modulation of a He-Ne laser and an electrical-heterodyne technique

A phase measurement distance meter has been devised using a frequency-stabilized intermode optical beat generated from a two-mode He-Ne laser (14-cm-long cavity) and a conventional phase meter. The beat frequency (1096 MHz) is stabilized to within 1 kHz fluctuation by a feedback control system. This frequency was converted to an electrical signal and reduced to 6 MHz by a rf-heterodyne technique in order to match the best frequency response range of the phase meter. Experimental results show that the total distance error of the optical distance meter due to the frequency fluctuation of the laser and the performance of the phase meter is 1 and 19 μm, respectively, for a mirror target located 1 m away from the laser source. Further improvement in the resolution is possible if a better phase meter is used.

Streak-camera-based long-distance range finder with 10^−7 resolution

A novel optical distance meter with a pulsed yttrium aluminum garnet laser and a pair of streak cameras is developed. To minimize timing errors, the time scale is marked by an optical-registration-clock pulse. This assures an intrinsic resolution of 3 mm. For field applications, comparison of multiwavelength and single-wavelength methods confirms that the latter method is much better if carefully corrected. By use of the equipotential temperature criterion for correction of the atmospheric parameters, such as the atmospheric boundary layer effect and the temperature along the ray trajectory, a resolution of better than 1 cm is achieved over a range of 30 km. This result corresponds to a relative resolution of 3 × 10(-7). Typical ranging results in a geophysical application are shown for monitoring of the relative translational shift of the Asian (Eurasian) and the Philippine plates; the predicted shift is found to be absent.

Ground deformations associated with the 1977–1978 eruption of USU volcano, Japan

Abstract The activity of Usu volcano situated at southern Hokkaido is characterized by the persistent occurrence of earthquake swarms and remarkable ground deformations including the formation of lava domes, both of which are due to the high viscosity of its magma. After the pumice eruptions of August 1977, the central part of the summit crater bounded by a U-shaped fault upheaved day after day at the maximum rate of 1 m/day. Although the rate has decreased, the total upheaval reached to about 180 m as of September 1981 resulting in formation of a new cryptodome. The doming at the summit has caused the northeastern rim to thrust towards the northeast about 180 m, and consequently caused corrugation of the ground at the northeastern foot of the volcano. The two kinds of the deformations, upheavals and thrusts, are closely related with each other. To examine the relationship between the earthquake occurrences and the deformations, a quasi-continuous observation of the distance from the northeastern rim of the summit crater to the foot of the volcano, was carried out by an optical distancemeter for 44 hours. It was found that the thrusts were episodic and accompanied by relatively large earthquakes. This supports that the earthquake swarms are caused by stick-slip motions of frictional sliding at the planes of the doming faults.