Time Interval Measuring Device

This invention relates to a time interval measuring device for measuring the unknown time interval between optical pulses.

In a conventional long distance measuring technique, or typically in a surveying technique, optical interference is utilized for accurate measurement. That is, in the method, a laser beam modulated with an electrical signal is caused to go and return for a distance to be measured, and the laser beam returned is demodulated, so that the phase of the modulating signal is compared with that of the original one. The difference in phase is converted into a time interval, from which the distance is calculated.

Furthermore, a method is also known in the art that an optical pulse is allowed to go and return for a distance to be measured, and the distance is calculated from the period of time required for the optical pulse to go and return for the distance.

However, the conventional method in which the phase comparison is carried out is disadvantageous in that, in the case where the weather conditions of the light transmitting space change with time, the light beam returned changes in amplitude, which results in an error in phase, thus limiting the measurement.

In the latter method in which the time required for an optical pulse to go and return for a distance to be measured is detected to measure the distance, the measurement is substantially free from the change of the amplitude which is caused by the variations of the weather conditions. However, heretofore the method is not sufficiently utilized because there has not been provided means for accurately measuring the difference between the time required for an optical pulse to go for a distance to be measured and the time required for the optical pulse to return for the distance.

If a photo-diode is used as in the conventional method, the time interval between two optical pulses can be measured with an error of about 100 ps. If a streak camera is used, then the time interval can be read with an error of about 1 ps. However, the time required for an optical pulse to go and return for a long distance is longer than that, and for instance a period of time of the order of 100 .mu.s cannot be measured with that accuracy. The reason for this resides in that, in an electrical circuit for determining the time positions of electrical pulses obtained through photo-electric conversion with gate circuits therein and accumulating for the time required till an end of measurement, the error in time determination or the timing fluctuation of the gate circuit readily reaches about 100 ps, and these errors are accumulated with the gating operation.

Accordingly, an object of this invention is to eliminate the above-described difficulties accompanying a conventional time interval measuring method.

More specifically, an object of the invention is to provide a time interval measuring device with high accuracy which, for instance, can measure a period of time of 100 .mu.s with an error of 10 ps.

The foregoing objects and other objects of the invention have been achieved by a time interval measuring device according to the invention for measuring the time interval between a first optical pulse and a second optical pulse, comprising a reference optical clock pulse generator for producing a train of reference optical clock pulses; short time interval measuring means for measuring the time interval between the first optical pulse and a first one of the train of reference to cock pulses which lies close to the first optical pulse, and the time interval between the second optical pulse and a second one of the train of reference optical clock pulses which lies close to the second optical pulse; and means for calculating the time interval between the first and second optical pulses according to the outputs of the short time interval measuring means and the time interval between the first and second reference optical clock pulses.

The nature, principle and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.

FIG. 1 is an explanatory diagram, partly as a block diagram, showing the arrangement of one example of a time interval measuring device according to this invention which is utilized for the measurement of a distance by way of example; and

FIG. 2(A), (B) and (C) are time charts for a description of the operation of the device according to the invention.

By way of example, a time interval measuring device according to this invention will be described with reference to the case where it is utilized for a distance measuring device.

As shown in FIG. 1, an atomic clock 1 and a laser diode 2 form an optical clock pulse train generator for producing a reference optical clock pulse train. The laser diode 2 produces a train of optical clock pulses with an accurate pulse interval (period) T with the aid of the atomic clock 1 as shown in FIG. 2(A).

The optical clock pulses thus produced, being partly reflected by a half mirror 7, are applied to a first streak camera 5. The optical clock pulses passed through the half mirror 7, being reflected by a reflecting mirror 9, are applied to a second streak camera 6.

A single optical pulse generator 3 produces an optical pulse, which is applied to another half mirror 8, where it is split into two parts comprising an optical pulse reflected by the half mirror and an optical pulse passed through it. The former optical pulse, namely, a first optical pulse b.sub.1 is applied to the first streak camera 5 which forms first short time interval measuring means. The first streak camera 5 is adapted to measure the short time interval (Tb.sub.1) between the first optical pulse bl and the optical clock pulse (O in FIG. 2(A)) of the train of optical clock pulses which is closest to the first optical pulse b.sub.1.

The optical pulse passed through the half mirror 8 is reflected by the surface 4 of an object under measurement which is located at an unknown distance, and again reflected by a reflecting mirror 10, so that the optical pulse is applied, as a second optical pulse b.sub.2, to the second streak camera 6 which forms second short time interval measuring means. The second streak camera 6 is adapted to measure the short time interval Tb.sub.2 between the second optical pulse b2 and the optical clock pulse i of the train of optical clock pulses (FIG. 2(A)).

The optical pulses and the optical clock pulses appear on the output phosphor screen of the streak cameras as shown in FIG. 2(C).

The time interval Tm between the first optical pulse b.sub.1 and the second optical pulse b.sub.2 is:

where i is the integer, T is the reference optical clock pulse interval, Tb.sub.1 is the short time interval between the reference optical clock pulse (O) and the first optical pulse, and Tb.sub.2 is the short time interval between the reference optical clock pulse (i) and the second optical pulse.

The time interval Tm is the period of time which elapses from the time instant when the optical pulse passes through the half mirror 8 until it returns to the half mirror 8 after being reflected from the surface 4 of the object positioned at the unknown distance l. Therefore, Tm=2 l/C where C is the velocity of light. Accordingly, the distance l is:

Reference numeral (11) represents a calculating means for determining the time interval Tm and the distance l according to the equations (1) and (2) which is electrically or optically connected to the streak cameras (5) and (6).

In the above-described embodiment, two streak cameras are utilized. However, the short time interval can be measured with one streak camera. That is, the above-described embodiment is based on the fact that there is a time difference between the arrival of the first optical pulse to the first streak camera and that of the second optical pulse to the second streak camera, and therefore the two streak cameras can be replaced by one streak camera with suitable gate means.

Further, in the above-described embodiment, a streak camera is employed as short time interval measuring means. However, this invention is not limited thereto. For example, an equivalent fast temporal registration device to the streak camera may be used as short time interval measuring means.

In the measurement of distances, the refractive index of the air in which light propagated depends on the variations of temperature, humidity and atmospheric pressure, and therefore sometimes the light velocity varies, thus adversely affecting the measurement. However, this difficulty can be overcome by employment of two or three kinds of optical pulses different in wavelength.

The laser diode according to this invention is driven by the output electrical pulses of the atomic clock or accurate crystal oscillator, and therefore the reference optical clock pulses are generated with considerably high accuracy, further, as the streak cameras are employed as the short time interval measuring means, the short time interval between the first optical pulse and the reference optical clock pulse, and the short time interval between the second optical pulse and the reference optical clock pulse can be read with considerably high accuracy.

The total error in measurement of the device according to this invention is extremely small, because it comprises only the time error of the atomic clock and the measurement error of the streak cameras which have extremely small errors.

In the measurement of the time interval with the electrical circuit according to the conventional method, the operating errors of the circuit itself, being accumulated, make it impossible to achieve the measurement with high accuracy. However, in the case where it is required to measure the time interval with an error of about 100 ps or less, as described above, the method of the invention is much superior to the conventional method.

The device of the invention is applicable to a wide range of technical fields such as for instance the measurement of a time interval. Thus, the industrial effects and merits of the invention should be highly appreciated.