GB1598065A

GB1598065A - Laser rangefiders

Info

Publication number: GB1598065A
Application number: GB13031/71A
Authority: GB
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1971-05-04
Filing date: 1971-05-04
Publication date: 1981-09-16

Description

(54) LASER RANGEFINDERS (71) I, SECRETARY OF STATE FOR DEFENCE, London, do hereby declare the invention for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to laser rangefinders.

Lasers can be controlled to produce very short pulses at infrared or optical frequencies and therefore they can, in theory at least, be used for rangefinders given the following components: 1. A high-powered, short pulse laser; 2. A target capable of reflecting the laser radiation; 3. Means for detecting a pulse reflected from a target; and 4. Means for accurately timing the interval between the transmission of the initial short pulse and the reception of the reflected pulse.

Then, if the range to the target is r and the time interval is t, the relationship between them is given by c = 2r/t where c is the speed of light. In other words, r = ict The carbon dioxide laser for example, can be arranged to produce a high-powered, short pulse and has a reasonable working efficiency at 10.6 microns but there is no room temperature detector operating at that wavelength having a sufficiently sensitive response to re flected radiation to provide an accurate rangefinder.

It is an object of the present invention to use the principle of frequency up-conversion to provide for detection of the returned signal in a laser rangefinder.

According to the present invention there is provided a laser rangefinder apparatus in cluding a first laser arranged for the irradia tion of a given target, means for providing the first laser with a pulsed output, an optical mixing crystal arranged for the reception of radiation from the first laser reflected by the target, a second laser arranged for the irradiation of the optical mixing crystal, means for providing the second laser with a pulsed output during the period in which radiation from the first laser is expected to be incident on the optical mixing crystal after reflection by the target, wherein the optical mixing crystal, the frequency of the radiation output from the first laser and the frequency of the radiation output from the second laser are such as to allow a three-way phase matching frequency up-conversion process to occur, and means for detecting the time interval between the emission of a given part of the radiation output from the first laser and the production in the optical mixing crystal of an optical signal constituting frequency upconversion of the said part of the radiation output from the first laser after reflection by the target to the optical mixing crystal.

The first laser may be a carbon dioxide laser which may be Q-switched. The second laser may be a dye laser. The optical mixing crystal may be silver gallium sulphide, Ag Ga S,, or a material in the range Ag, As, Sb1A S3, Oxl.

Embodiments of the invention will be described by way of example with reference to the drawing filed with the Provisional Specification, which is a schematic diagram of a laser rangefinder embodying the invention.

Active rangetinders which rely on the time interval between transmitting a pulse of electromagnetic radiation and receiving a reflected pulse rely for their effectiveness on a highpower transmitted pulse and a sensitive receiver. The receiver must be sufficiently sensitive to detect a return in noise; once a signal is detected it can be amplified in order to make it of sufficient magnitude to compute the time interval between the transmitted and the received signal.

A carbon dioxide laser, operating at 10.6 microns, is one ot the most suitable lasers to use as the transmitting source for a laser rangefinder. Although there is no room tem perature detector operating at that wavelength having a sufficiently sensitive response to radiation reflected from a target to provide an accurate rangefinder, a range of photodetectors exists having responses in or dose to the visible region of the electromagnetic spectrum and which are suitable for relatively rapid and sensitive detection of optical signals above noise.The present invention utilizes the principle that reflected radiation from a target can be detected by a photodetector operating in or close to the visible region of the electromagnetic spectrum by upconverting the frequency of the reflected radiation using an optical mixing crystal and radiation from another laser so that the resultant optical signal has a frequency in the region of response of the photodetector.

A laser 1 which may be a carbon dioxide laser and which has a pulsed output beam is used as the transmitting source for irradiating a target T (the output of the laser 1 may be a single pulse or a series of pulses).

The laser 1 is shown schematically in the drawing to be controlled by means of a pulse circuit 3. The output of the laser 1 may be obtained in the form of pulses either by applying the pulse circuit 3 to the power supply (not shown) of the laser 1 or to a standard Q-switch (not shown) incorporated within the resonator system of the laser 1.

It is assumed that the laser 1 irradiates the target T and some of the radiation is reflected back from the target T.

A mixing crystal 5 is arranged so as to capture some of the radiation reflected from the target T. A second laser 7 (known as the pump laser) is arranged so as to irradiate the mixing crystal 5 at the instant at which the radiation reflected from the target reaches the mixing crystal 5. The radiation from the laser 7, which is also in the form of a pulsed output beam, is deflected by conventional means 10 (for example a glass slide) into the path of radiation reflected from the target T and onto the mixing crystal 5. The reflected radiation is up-converted in frequency at the crystal 5 by the radiation from the laser 7 and the result is a visible optical signal having a frequency at the sum of the frequencies of the reflected radiation and the pump radiation from the laser 7.This frequency up-conversion occurs by a 3-wave phase matching process such as is described in United Kingdom Patent Specification No.

1,248,791 (Application No. 4927/68).

The pump laser 7 is shown schematically in the drawing to be controlled by means of a pulse circuit 9 which may be controlled by means of a control C common to the pulse circuit 3.

The output of the mixing crystal 5 contains in addition to the up-converted optical signal unconverted radiation at the frequencies of the laser 1 and the laser 7. These frequencies are filtered out in a filter 11. The output of the filter 11 is applied to a detector 13 which may be a photo-diode. A small fraction of the output of the laser 1 is extracted from the pulsed beam directed at the target T by standard means 8, such as a glass slide, and the extracted fraction is detected by means of a detector 15. The output of the detector 13 and the output of the detector 15 are applied to an electronic timing circuit 17.The timing circuit 17 is arranged in a conventional way to measure the time interval between the detection of a given part (for example the trailing edge) of a given pulse in the output of the laser 1 and the detection of the corresponding part of that pulse when reflected and up-converted.

The detector 15 may be a pyroelectric detector.

The reason why a pyroelectric detector is usable for the detection of 10.6 micron carbon dioxide laser radiation at the output of the laser 1 but not for the detection of 10.6 micron radiation reflected from the target T is that pyroelectric detectors are sensitive at 10.6 microns to the higher power output beam from the laser 1 but relatively insensitive to the lower power reflected beam.

Alternatively a small fraction of the radiation from the laser 1 may be up-converted in the mixing crystal 5 or in a separate mixing crystal and the timing circuit 17 may be arranged to be responsive to the time difference between a given part of the two corresponding pulses of visible radiation emitted by the mixing crystal 5 or by the crystal 5 and the separate mixing crystal where such is used.

One problem of using an up-converter for detecting radiation reflected from the target T is that of the power of the output beam from the laser 7. A continuous wave laser will in general have too low a power and so a laser having a pulsed output is necessary.

If the laser 7 is Q-switched it can have a very high powered output beam, but such a beam will have a very short pulse length (time). It is therefore possible but difficult to use a Q-switched laser to pump the optical mixing crystal 5 for the up-conversion of an incoming optical signal reflected from the target T. The laser 7 may be Q-switched if the precise instant at which the optical signal reflected from the target T is known to arrive.

However since that is not generally possible a Q-switched laser is not generally usable.

A laser having a pulsed output which is not Q-switched can have a longer pulse length (time) of reasonable mean power per pulse but its instantaneous output power as a function of time tends to have a spiky waveform.

Therefore an incoming signal may be incident on the mixing crystal 5 when the instantaneous output - power of the laser 7 is below the threshold for the detection above noise of a signal reflected from the target T, and so up-conversion using this method generally tends to be unreliable.

However a dye laser can have a reasonably high minimum output power over a reasonably long pulse length. To illustrate this point, consider a rangefinder working out to 5 kilometres. Radiation at say 10 microns takes some 15 microseconds or so to travel 5 kilometres, so that the time between transmission of a pulse and reception of the echo is about 30 microseconds. It would clearly be best to arrange for the pump laser 7 to irradiate the crystal 5 for the whole of this 30 microseconds but even if this is impossible it is nevertheless possible to arrange for the laser 7 to irradiate the mixing crystal 5 for such a period that there is a strong possibility of the return occurring during that period.

A dye laser emits radiation almost immediately its own pump power is provided and so the precise instant at which it starts to emit its pulse can be exactly controlled. For example when a given pulse is emitted by the laser 1 the pulse circuit 9 can be arranged so that power is delivered to the laser 7 after a given delay.

Used in this way a carbon dioxide laser having a pulse energy of 10 millijoules at a wavelength of 10.6 microns can be used to illuminate a target at 5 kilometres and a Rhodamine B dye laser, for example, having a pulse energy of 5 millijoules at a wavelength of 6200 Angstrom units can be used to pump a silver gallium sulphide (AgGaS3) mixing crystal whereby an output in the yellow part of the spectrum at 5857 Angstrom units is obtained.

Alternatively a Cresyl-violet dye laser at a wavelength of 7000 Angstrom units can be used to pump a proustite (Ag3 As S3) mixing crystal whereby an output in the red part of the spectrum at 6570 Angstrom units is obtained.

Pyrargyrite, Ag3 Sb S3, and proustitepyrargyrite solid solutions, Ag, As, Sub,, S3, 0axe1, are similar in action to proustite and may be used as the material for the mixing crystal 5.

A telescope (not shown) may be used to match the angular aperture of the mixing crystal 5 to the angular aperture of the scene it is required to accept. The laser 1 may also have a telescope (not shown) to match the output of the laser 1 to the required illuminated angle. The figures quoted above apply to a telescope-matched situation.

WHAT I CLAIM IS:- 1. A laser rangefinder apparatus including a first laser arranged for th eirradiation of a given target, means for providing the first laser with a pulsed output, an optical mixing crystal arranged for the reception of radiation from the first laser reflected by the target, a second laser arranged for the irradiation of the optical mixing crystal, means for providing the second laser with a pulsed output during the period in which radiation from the first laser is expected to be incident on the optical mixing crystal after reflection by the target, wherein the optical mixing crystal, the frequency of the radiation output from the first laser and the frequency of the radiation output from the second laser are such as to allow a three-wave phase matching frequency up-conversion process to occur, and means for detecting the time interval between the emission of a given part of the radiation output from the first laser and the production in the optical mixing crystal of an optical signal constituting frequency upconversion of the said part of the radiation output from the first laser after reflection by the target to the optical mixing crystal.

2. A laser rangefinder apparatus as claimed in claim 1 and wherein the first laser is a carbon dioxide laser.

3. A laser rangefinder apparatus as claimed in claim 1 or claim 2 and wherein the first laser is a Q-switched laser 4. A laser rangefinder apparatus as claimed in claim 1, claim 2 or claim 3 and wherein the second laser is a dye laser.

5. A laser rangefinder apparatus as claimed in claim 4 and wherein the second laser is a rhodamine B dye laser.

6. A laser rangefinder apparatus as claimed in claim 4 and wherein the second laser is a cresyl-violet dye laser.

7. A laser rangefinder apparatus as claimed in claim 1, claim 2, claim 3 or claim 4 and wherein the optical mixing crystal consists of a material in the range Ag, As, Sb,-, Sa, where 0axe1.

8. A laser rangefinder apparatus as claimed in claim 1, claim 2, claim 3 or claim 4 and wherein the optical mixing crystal consists of silver gallium sulphide, Ag Ga S,.

9. A laser rangefinder apparatus as claimed in any one of the preceding claims and wherein the means for detecting the said time interval includes pyroelectric detecting means for detecting the emission of part of the radiation output from the first laser, photodetector means for detecting the generation at the output of the optical mixing crystal of an optical signal constituting frequency up-conversion of the said part of the radiation output from the first laser after reflection by the target to the optical mixing crystal and means for detecting the time interval between the generation of an output signal from the pyrb electric means and the generation of a corresponding output signal from the photodetector means.

10. A laser rangefinder apparatus as claimed in claim 1, claim 2, claim 3, claim 4, claim 5, claim 6, claim 7 or claim 8 and wherein the means for detecting the said time interval

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. noise of a signal reflected from the target T, and so up-conversion using this method generally tends to be unreliable. However a dye laser can have a reasonably high minimum output power over a reasonably long pulse length. To illustrate this point, consider a rangefinder working out to 5 kilometres. Radiation at say 10 microns takes some 15 microseconds or so to travel 5 kilometres, so that the time between transmission of a pulse and reception of the echo is about 30 microseconds. It would clearly be best to arrange for the pump laser 7 to irradiate the crystal 5 for the whole of this 30 microseconds but even if this is impossible it is nevertheless possible to arrange for the laser 7 to irradiate the mixing crystal 5 for such a period that there is a strong possibility of the return occurring during that period. A dye laser emits radiation almost immediately its own pump power is provided and so the precise instant at which it starts to emit its pulse can be exactly controlled. For example when a given pulse is emitted by the laser 1 the pulse circuit 9 can be arranged so that power is delivered to the laser 7 after a given delay. Used in this way a carbon dioxide laser having a pulse energy of 10 millijoules at a wavelength of 10.6 microns can be used to illuminate a target at 5 kilometres and a Rhodamine B dye laser, for example, having a pulse energy of 5 millijoules at a wavelength of 6200 Angstrom units can be used to pump a silver gallium sulphide (AgGaS3) mixing crystal whereby an output in the yellow part of the spectrum at 5857 Angstrom units is obtained. Alternatively a Cresyl-violet dye laser at a wavelength of 7000 Angstrom units can be used to pump a proustite (Ag3 As S3) mixing crystal whereby an output in the red part of the spectrum at 6570 Angstrom units is obtained. Pyrargyrite, Ag3 Sb S3, and proustitepyrargyrite solid solutions, Ag, As, Sub,, S3, 0axe1, are similar in action to proustite and may be used as the material for the mixing crystal 5. A telescope (not shown) may be used to match the angular aperture of the mixing crystal 5 to the angular aperture of the scene it is required to accept. The laser 1 may also have a telescope (not shown) to match the output of the laser 1 to the required illuminated angle. The figures quoted above apply to a telescope-matched situation. WHAT I CLAIM IS:-

1. A laser rangefinder apparatus including a first laser arranged for th eirradiation of a given target, means for providing the first laser with a pulsed output, an optical mixing crystal arranged for the reception of radiation from the first laser reflected by the target, a second laser arranged for the irradiation of the optical mixing crystal, means for providing the second laser with a pulsed output during the period in which radiation from the first laser is expected to be incident on the optical mixing crystal after reflection by the target, wherein the optical mixing crystal, the frequency of the radiation output from the first laser and the frequency of the radiation output from the second laser are such as to allow a three-wave phase matching frequency up-conversion process to occur, and means for detecting the time interval between the emission of a given part of the radiation output from the first laser and the production in the optical mixing crystal of an optical signal constituting frequency upconversion of the said part of the radiation output from the first laser after reflection by the target to the optical mixing crystal.

3. A laser rangefinder apparatus as claimed in claim 1 or claim 2 and wherein the first laser is a Q-switched laser

4. A laser rangefinder apparatus as claimed in claim 1, claim 2 or claim 3 and wherein the second laser is a dye laser.

includes directing means for directing a fraction of the radiation output from the first laser onto the optical mixing crystal, photodetector means for detecting the generation at the output of the optical mixing crystal of a first optical signal constituting frequency up-conversion of a part of the radiation output from the first laser after direction onto the optical mixing crystal by the directing means and of a second optical signal constituting frequency up-conversion of the said part of the radiation output from the first laser after reflection by the target to the optical mixing crystal and means for detecting the time interval between the generation by the photodetector means of an output signal corresponding to the first optical signal and an output signal corresponding to the second optical signal.

11. A laser rangefinder apparatus substantially as hereinbefore described with reference to the drawing filed with the Provisional Specification.