Laser Measurement System

The present invention relates to a measurement system using laser beams.

Some of the measuring systems utilizing the high power and high directability of laser beams are rangefinders measuring distances to targets, laser radars measuring the molecular composition of the atmosphere, etc., and other systems. These systems transmit very strong laser beams. If there is a scattering surface or a man at a short distance in the path of the transmitted beam, therefore, a strongly-scattered beam is returned to the beam receiver, which could damage an optical detector and the eyes or skin of humans. Even when no damage is inflicted on the components of the receiver, the strong return beam can exceed the dynamic range of the optical detector and amplifier, etc., making it impossible to obtain specific reception signals from a target located at a short distance.

In conventional systems, accordingly, the absence of scattering surfaces or humans in the path of the transmitted beam is confirmed visually to ensure safety prior to the actual measurement, and if there is any scattering surface or the like in the path, an attenuator is put in a beam receiver unit for the measurement. This makes it difficult to carry out a rapid measurement, and it also necessitates the provision of an attachment such as nocturnal vision apparatus at night, since unaided visual confirmation is impossible then.

An object of the present invention is to provide a laser measurement system which has a high safety and enables automatic measurement.

Another object of the present invention is to provide a rangefinder which has these characteristics.

Still another object of the present invention is to provide a laser radar system which has these characteristics.

The present invention makes it possible to obtain a laser measurement system which is provided with a transmitter unit transmitting at least two kinds of laser beams into the space being searched, a receiver unit receiving reflected light in response to a first laser beam transmitted initially and converting the light into an electrical signals, a decision unit generating a first signal when the level of the reflected light signal obtained within a predetermined time after the transmission is higher than a predetermined threshold Th.sub.1, and a control unit inhibiting the transmission of a second laser beam which is different from the first beam when the first signal is generated. Actual measurement can be performed after the confirmation of safety obtained by this system, provided that the power of the second laser beam is set to be larger than that of the first laser beam.

FIG. 1 is a block diagram of an embodiment of the laser measurement system according to the present invention;

FIGS. 2A and 2B are timing charts of signals from each of the units, illustrating the operation of the embodiment of FIG. 1; and

FIG. 3 is a block diagram of a simplified part of the embodiment of FIG. 1.

FIG. 1 is a block diagram of a laser measurement system embodying the present invention, illustrating the application thereof to a rangefinder. In this figure, laser sources 10A and 10B respectively transmit relatively low-power and high-power laser beams which enable long-range measurement, and are driven by the outputs of drivers 11A and 11B operating according to transmission instruction signals S.sub.A and S.sub.B, respectively, from a controller 18. As will be described more fully below, laser source 10A is caused to transmit beam A first, and laser source 10B is caused to transmit beam B only when the reflected component of the beam A satisfies certain predetermined conditions. A reflected beam in response to the laser beam transmitted by laser source 10A is received by receiver optics 12, and is then converted into an electrical signal R by an optical/electric (O/E) converter 13. A reference-wave generator 15 generates a REF signal with a waveform analogous to the waveform of a signal of the so-called Mie or Rayleigh scattered light received immediately after the transmission. Since the molecular composition in the atmosphere is usually known, this waveform can be obtained easily from a charging waveform obtained from a CR time constant circuit consisting of, for instance, a capacitor and a resistor. The REF signal can also be obtained simply by a method wherein digital data obtained by sampling a reference waveform assumed beforehand is stored in a memory, is read out sequentially therefrom during the generation of the REF signal, and is then passed through a low-frequency filter.

A differential amplifier 14 subtracts the REF signal from the signal R from the O/E converter 13 to remove the fluctuations in the reception signal caused by Mie scattering or Rayleigh scattering.

A threshold generator 17 supplies a comparator 16 with a threshold signal TH which has two levels Th.sub.1 and Th.sub.2. Level Th.sub.1 is for detecting strongly reflected beams from obstacles located at a short distance, and it is set to be large and have a time duration T.sub.1 corresponding to that short distance. Level Th.sub.2 is for detecting signals reflected from targets located at a relatively long distance, and is set to be relatively small. The length of its duration depends on the detection range.

The comparator 16 compares a reflection signal D from the differential amplifier 14 with the threshold levels Th.sub.1 and Th.sub.2 and generates an output pulse C when D>Th.sub.1 or D>Th.sub.2.

When it receives the pulse C generated when D>Th.sub.1 within the time T.sub.1, the controller 18 reaches the decision that there is an obstacle at a short distance, and inhibits the output of the instruction signal S.sub.B for the transmission of the high-power laser beam B. When D<Th.sub.1 within the time T.sub.1, on the other hand, the controller 18 reaches the decision that there is no obstacle within a short distance, and outputs the instruction signal S.sub.B for the transmission of the high-power laser beam B.

A counter 19 is provided for measuring the distance to the target. It starts counting when the high-power laser beam is transmitted (that is, when the signal S.sub.B is generated), stops the count when the signal D exceeds the level Th.sub.2. The distance to the target is measured on the basis of this counted result.

The above is only a description in outline. More detailed descriptions of the general construction of the rangefinder are disclosed in the article "Pulsed CO.sub.2 TEA laser rangefinder" by M. J. Taylor et al., APPLIED OPTICS, Vol. 17, No. 6, Mar. 15, 1978, pp. 885 to 889, and the article "CO.sub.2 transversely excited atmospheric (TEA) laser rangefinder" by R. J. Matthys et al., SPIE Vol. 227, CO.sub.2 Laser Devices and Applications, 1980, pp. 91 to 97.

The actual operation of the rangefinder of FIG. 1 will now be described with reference to the timing charts of FIGS. 2A and 2B.

FIG. 2A shows the timing chart of the operation when there is an obstacle at a short distance. First an instruction signal S.sub.A is generated from the controller 18 at the start of the rangefinding. Responding to the signal S.sub.A, the driver 11A drives a laser source 10A to transmit a low-level laser beam A.

A reflection signal in response to this laser beam A is passed through the receiver optics 12 and the O/E converter 13 to obtain a signal R. This signal R contains Mie or Rayleigh scattered light signal on which the signal reflected from an obstacle is superimposed. The reference waveform generator 15 generates, in synchronization with the signal S.sub.A, a REF signal prepared beforehand as described previously.

The differential amplifier 14 executed prescribed processing to remove the Mie or Rayleigh scattered-light component signal based on the signals R and REF, and the differential component of these signals is output therefrom as a signal D. The comparator 16 compares the signal D with the signal TH, and generates a signal C when D>Th.sub.1. If the signal C is generated within the time T.sub.1, controller 18 decides that there is an obstacle within a short range, and the transmission of the high-power laser beam B is inhibited. The system can subsequently perform operations such as transmitting the laser beam A again after a prescribed period of time, or transmitting the laser beam A in another detection direction, as required.

Next a description will be made of an ordinary rangefinding operation performed when there is no obstacle within a short range, with reference to FIG. 2B.

In this case, the processes as far as the removal of the scattered-light component from the reflection signal are the same as those described with reference to FIG. 2A. The signal thus obtained contains a target signal Ta. However, it does not appear in the signal C because its level is less than Th.sub.1. Since there is no signal reflected from an obstacle within the time T.sub.1 in this case, the controller 18 generates an instruction signal S.sub.B to the driver 11B at time T.sub.1. The laser source 10B, driven by the driver 11B, transmits the high-power laser beam B. Meanwhile, the instruction signal S.sub.B is transmitted as a start signal to the counter 19 to make it start counting. The reflected signal is received in receiver optics 12 and converted to an electrical signal in the O/E converter 13 to generate reflection signal R'.

The reflection signal R' in response to the laser beam B is processed by the differential amplifier 14 which takes the difference thereof from a reference signal REF' to provide a signal D' from which the scattered-light component has been removed. A target signal Ta' (of a larger level than the signal Ta since it is the response to a high-power laser beam) within the signal D' is judged to be larger than the threshold level Th.sub.2 by the comparator 16, and a target detection signal C' is thereby generated. The signal C' is supplied as a stop signal to the counter 19 to make it stop counting. The distance to the target is calculated from the count thus obtained.

The laser sources 10A and 10B and the drivers 11A and 11B are described above as separate devices. It is clear that they can be replaced by a single laser source 30 and a single power-adjusting unit 31, as shown in FIG. 3. In this case, the laser source 30 is adjusted by the power-adjusting unit 31 so that a low-power laser beam A is transmitted according to the instruction signal S.sub.A, and a high-power laser beam B according to the instruction signal S.sub.B.

The power of the transmitted laser beams is not limited to two levels, but can be three or more levels if required. It would be useful to transmit a third laser pulse of an even higher power to expand the range of detection further when the second laser pulse transmitted, after safety has been confirmed by the absence of any obstacle at short range, is insufficient for the rangefinding of an object at long range, for instance. Furthermore, the types of the laser sources may be differentiated by making not only their powers different, but also the wavelengths of the laser beams transmitted.

The differential amplifier 14 can, of course, be replaced by an adder by inverting the polarity of the REF signal from the REF signal generator 15.

The power of the laser beams transmitted can be changed by changing the peak value of the laser pulses and the widths thereof. Thus, a desirable power from the safety point of view can be obtained by reducing the peak value and increasing the pulse width when it is likely that someone is nearby. The precision of the measurement can be sufficiently good using a pulse of a narrow width and a large peak value.

It is apparent that the present invention can be applied to a laser radar measurement system. A laser radar is designed to measure, for instance, the composition of molecules (e.g. those of nitrogeous compounds) in the atmosphere, or the density of clouds, by utilizing the fact that specific materials (their molecules) are resonant with light of specific wavelengths. The construction of the laser radar is disclosed in the article "Laser radar monitoring of the polar middle atmosphere" by Iwasaki Y et al., Mem. Nat'l. Inst. Polar Res., Spec. Issue 19, 1981, pp. 178 to 187. With laser radar, laser beams are often transmitted toward the sky. Therefore it can be easily understood that the present invention is effective when there is an airborne object such as an airplane in the sky.